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Test methods for pressure-sensitive labels

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FINAT's technical committee provides globally recognized standardized testing methods (FINAT Test Method or FTM) for the PS label industry

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After the label has been used, it needs to be recycled or otherwise safely disposed of, and FINAT has developed test methods for this phase in the life of the label, either to control the process or to make sure that materials are suitable for recycling.

RELEASE LINER AND FACE LABEL

The testing requirements for silicone release coating on a paper or film allow manufacturers to control the thickness of silicone, release level and the consistency.

The FINAT test method allows evaluation of silicone coverage on a paper substrate by using a dye stain test. The exact applied amount of silicone on either filmic or paper base liners can be determined according to the silicone coat weight test method. (Figure 8.4).

A sample of the finished laminate is put into an oven with a weight attached and the release force over time is measured. Laminators can also check resistance to UV light and the adhesive coat weight can also be determined.

This may be an effective tool to ensure proper functionality of the label and identify any potential risk such as adhesive bleeding from the label roll.

CONVERTER-LEVEL TESTS

The label converter might firstly want to check the quality of the laminate at goods-in. Another check to make at this stage is the surface tension of a filmic face material, as this relates closely to the adherence of printing inks and whether corona treatment needs to be specified on press (Figure 8.6).

INK ANCHORAGE AND LABEL APPLICATOR TESTS

The release properties of the laminate need to be checked before the label applicator stage. This is particularly critical when high speed automated label applicators are being used, and the label has to lift off correctly each time. This requires a delicate balance.

The label should not release too easily from the backing liner and at the same time should not stick on the liner too long, otherwise the dispensing will not work properly.

So, measuring the release force at this point gives a good indication of how the label will perform at the dispensing stage (Figure 8.8).

This may be an effective tool to ensure proper functionality of the label and identify any potential risk such as adhesive bleeding from the label roll.

CONVERTER-LEVEL TESTS

The converter will also measure the fluorescence and white point of paper substrates for color measurement purposes.

The latest test method is intended for UV curing of transparent lacquers and UV white inks. This method is based on a color-reaction to test for proper curing of UV inks.

After printing, there are tests to check and control die-cutting quality, particularly to ensure the die-cut has not gone too deep (die strike) and that the laminate’s release properties are matched with requirements for high speed matrix stripping (Figure 8.7).

Measurement of adhesion properties are important to ensure the label sticks firmly and instantly to the container surface. Typically this uses the loop tack measurement test which gives an idea of how ‘sticky’ the adhesive is.

Measurement of the adhesive is also a sign of good curing performance of the silicone, since there are no components migrating to the adhesive.

In this sense the loop tack measurement test is a good method for testing correct silicone cure.

Dimensional stability of the label can also be tested, along with a test method for chemical resistance in harsh environments.

Critical to final label quality is to test that the ink has adhered properly. Ink adhesion tests include rub and scratch resistance, and there are further tests for over laminating a protective film.

There are specific rub and scratch resistance tests for UV inks (Figure 8.9).

For testing final adhesion properties, we have already mentioned the loop tack measurement test. But because the label has to stick on a range of different surfaces, the peel adhesion test is critical to specify the label and adhesive on the final application. Peel adhesion tests are carried out at both 90 and 180 degrees peeing angle at speeds of 300mm/min.

Sheer resistance might also be of interest depending on the final application. If a label has to hold something together, or if there is likely to be any force applied against the label, then sheer resistance, or dynamic sheer testing is an important part of the final specification.

Low temperature adhesion tests are required to ensure the proper functionality of a label being dispensed in a cold environment to a container for any liquids or foodstuff.

If labels are going to be applied to smaller diameter curved containers, such as small pharmaceutical vials, then the labels should not be too stiff and must adhere quickly to the container, and the Mandrel Hold test covers this (Figure 8.10).

RECYCLABILITY

Label recyclability can also now be tested under the FINAT Test Methods. We can measure the wash-off properties of a label where labels need to be washed off by water in a separation tank (Figure 8.11).

Another recently added test protocol identifies any ‘stickies’ such as remaining adhesive or plastic left in the pulp which may contaminate the recycling process (Figure 8.12).

Intro section

The technical committee of FINAT, the European label industry association, provides globally recognized standardized testing methods (FINAT Test Method or FTM) for the PS label industry.

FINAT test methods are dedicated to the whole label production chain (Figure 8.1, 8.2).

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Identification and characteristics of PSA label materials

Feedimporter — Thu, 26 Nov 2020 16:14:00 +0000

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Identification and characteristics of PSA label materials

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Identifying the characteristics of self-adhesive materials is an essential if we are to fully understand their uses and applications

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As opposed to other forms of labeling such as wet-glue and shrink sleeves, there is no requirement for application of an external adhesive force, and this creates a great advantage for labelstock material – the ability to handle in a clean room environment.

Pressure-sensitive labels are extremely versatile in terms of their ability to function across a vast range of applications and environmental conditions, thanks to a wide choice of adhesives and release liners, and to be dispensed at very high speeds on automated label application lines.

As we have seen, pressure-sensitive label laminate starts with the release base paper or film as a base, which is then coated with silicone, followed by an adhesive coating and then the face material. This face material can be top-coated or primer-coated depending on the print process which follows.

What is the criteria for choosing a face material? It depends, first of all, on the type of printing – thermal printing, digital printing or flexo printing, for example. Based on that, the face material will be different. What finish is required of the label? That, again, dictates the type of face material, whether opaque, clear, metalized or matt finish.

At this point it is worth considering material thickness, particularly as regards film. Different types of container require different material thicknesses to give the required bulk, strength and stiffness characteristics. Thus, for example, PE labels for ampoules are typically in the range 30-50 microns, while PE labels for large containers such as lubricants would be up to 110 microns.

Material thickness, strength and stiffness are also important factors during label conversion. A certain amount of strength in the paper or the film is essential for correct die-cutting matrix removal, as well as dispensing.

There is a complex relationship between material thickness and price. It seems common sense that when the thickness of the material goes down, the price will also go down. But this is not always the case. The price may actually increase. In the case of paper a higher percentage of pulp may be required to give the required stiffness or strength, and similarly the tensile strength and stiffness required in thinner films could also make the cost of the material higher.

Examining materials for adequate strength and stiffness requires firstly checking the GSM and thickness of the material, whether a paper or a film.

In case of paper, checks can also be made for pulp content. Other important factors for paper include roughness, opacity, gloss/matt, brightness, whiteness, shade, tensile strength, whether it contains OBA or not and – very important in today’s climate – whether it is FSC/PEFC certified or not.

In films, characteristics to look for include GSM, thickness, stiffness, gloss/matt finish, brightness, whiteness (for opaque films), shade and tensile strength.

FACE PAPER CHARACTERISTICS

a. Uncoated paper. Paper manufactured in its raw form using pulp. It has a very thin layer on top to allow for printing and to deliver some degree of smoothness. The paper is porous, meaning penetration of ink is very fast. It is mainly used for blank labels and thermal transfer printing. One or two color printing is sometimes used where brand identification is required.

b. Semi-gloss papers. Papers coated with clay, which allows for excellent printability. This is most commonly used for thermal transfer-printed barcodes as well as for prime product labels where thermal transfer over-printability is required.

c. Cast coated paper. Cast coated paper is also known as Mirror Coat paper because of its high gloss. It has a heavy deposition of clay and is highly calendared to give it the required level of gloss. The thickness of the clay coating makes the paper impermeable, meaning the ink stays on the surface, giving a rich printing effect. For this reason, these label papers are widely used in the cosmetics and liquor industries as well as other similar high end FMCG products.

d. Metalized paper. Metalized papers are formed by the deposition of vaporized aluminum onto paper. This can be done either directly, or by a process of transference. Transfer metalizing is carried out by depositing the aluminum vapors onto a polyester film, which is laminated to the paper then delaminated, giving the paper a more glossy look compared to direct metalizing.

Direct metalizing tends to be used for applications like beer labels, while transfer metalizing is used more in the high-end consumer markets such as cosmetics and premium liquors.

e. Direct thermal papers. Direct thermal paper has a heat-sensitive layer impregnated on the paper which turns black on exposure to heat. Thermal printers impart this heat in a particular pattern to give the barcode or variable text. Thermal papers can be both top coated and non-top coated. Non-top coated papers are the most economical grade and they are typically used for various logistics and pricing applications for example in supermarkets. Thermal linerless labels are one of the fastest growing trends in supermarket logistics and pricing labels.
Top coated thermal papers are used in applications where resistance to chemicals, water, heat and abrasion is required. Shelf life can be up to 12 years.

If a longer shelf life is required, thermal transfer ribbons are usually chosen over a direct thermal paper.

FILM CHARACTERISTICS

a. Polyethylene. PE is used in applications where flexibility, durability and resistance to abrasion and impact are required.

On the other hand, PE does not have any resistance to oxidization; it is not 100 percent clear, being somewhat hazy, even in the clear format; it has no resistance to chlorinated hydrocarbons or to harsh outdoor conditions.

Its calliper has to be generally greater than 70 microns for safe dispensing, since PE is mostly used mostly for larger containers.

PE is available in clear and opaque white finishes, as well as metalized, although metalized PE is not so widely used today as there are more cost-effective alternatives.

To identify a PE, separate the laminate and stretch the film, which will deform but will not tear easily. PE, like PP and co-extruded film, floats on water.

b. Oriented Polypropylene. Polypropylene film has several key performance advantages: resistance to tearing, abrasion and chemicals; good outdoor UV stability; and excellent die-cutting and printing characteristics, particularly where ink adhesion is enhanced by corona treatment or a top coating. Its key limitation is less resistance to heat. PP finishes include clear, opaque and metalized.

How do you identify PP film? Take the film, put a notch to it and cut it through. A knife will cut through easily.

c. Polyester. Polyester film has high resistance to heat, high resistance to tearing, high resistance to abrasion, good resistance to chemicals, excellent dimensional stability, resistance to UV – making it excellent for outdoor applications – and resistance to solvents.

Due to its stiffness, it has less conformability than a PE and its cost is higher than PP/PE due to the higher density of the film. Its density is 1.4 as compared to a PP, which about .85 or .9 and PE with a density of 1.

Polyester is available in clear, opaque white and both gloss and matt metalized finishes. How do you identify a Polyester film? Take the film, put a notch to it, try to tear it apart, and it breaks in a zig-zag manner.

d. PVC. PVC may be open to environmental objections, but it still has key applications which cannot easily be replicated by other films.

Its key advantages are a high degree of durability for both indoor and outdoor use and a high degree of flexibility for a semi-rigid material. In addition, it is relatively easy to convert, as corona treatment or top coating is not required for printing to a high standard.

General applications include outdoor advertising and battery labels.

The downside of PVC is undesirable environmental characteristics, as it forms toxins when incinerated. In addition, plasticiser migration has the potential to kill the adhesive in a PS laminate, presenting challenges in adhesive selection.

How do you identify PVC? Separate the laminate and stretch the film and observe how it tears and it will tear differently from the other films.

VISUAL IDENTIFICATION

Another way of identifying face materials is by their gloss and brightness levels and how they change when the labels are printed (Figure 7.1).

Samples of materials printed with the same ink (and on the same press under identical conditions), but with different surface characteristics can look completely different.

CONCLUSION

Given the huge range of choices available to the end user and the label converter, how does one select the right labelstock in totality? Factors that need to be taken into account include:

The bond required, whether permanent or removable.

The shape of the container or labeled surface, whether circular or flat, and if circular, what is the diameter and co-diameter of the product?

The size of the label required will help decide the GSM, or thickness, of the material.

What is the texture of the surface where the label will be applied? Rough, smooth, porous or a mixture of these characteristics.

What is the chemical composition of the substrate to be labeled? Glass, HDP, LDP, corrugated etc. They all have different surface tension and the way they create bonds will be different, requiring different kinds and GSMs of adhesives.

What are the printing and converting requirements?

What finish is required from the label

What is the application temperature and end use temperature of the label?

After all these technical considerations have been taken into account, the economics required of the label have to be looked at.

These are among the factors which help decide which label product is appropriate for a particular application. The pace of technology change shows no sign of slackening, giving end users and converters ever greater choice as we move forward.

RECLOSEABLE LABELS CASE STUDY

An excellent case study of materials selection is reclosable labels, an ever-more popular choice for wet wipes, rice and other reclosable packs. Why is PP chosen over PE and Polyester for this application?

The key performance requirement is a label which is flexible enough to adjust to the changing shape of the pack as the number of wipes or the volume of rich, for example, decreases in the flexible pack.

On the surface, one might expect PE to be specified for its flexibilty, but PE would actually stretch too much.

Polyester, meanwhile is too stiff, and PVC is not considered environmentally friendly enough despite having potentially the correct performance characteristics.

For these reasons, PP is chosen. But to ensure it functions correctly, the thickness needs to be increased towards 90 more microns.

Intro section

Identifying the characteristics of self-adhesive materials is an essential if we are to fully understand their uses and applications. Identification is also closely linked to handling of samples and printability testing.

Labelstock material, as we have seen, consists of a pressure-sensitive material which immediately creates a bond with the substrate with the application of a slight pressure.

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Self-adhesive labels: filmic face labels

Feedimporter — Thu, 26 Nov 2020 16:06:00 +0000

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Self-adhesive labels: filmic face labels

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An look into different types of filmic labelstocks, their properties and market applications

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Polyester and PVC are used for some specialty applications (Figure 6.1).

Polyethylene. Polyethylene has a non-oriented structure – the molecules do not line up in either a longitudinal or transverse direction. This gives PE great flexibility and conformability (the ability to adapt to a container’s shape), making PE films suitable for squeezable containers such as tubes but also complex container shapes.

Polypropylene. Polypropylene films can be stretched in one direction (OPP) or two directions (BOPP), usually in three layers. This makes the film very rigid, and suitable for labeling of rigid containers such as glass or polyester bottles. Excellent clarity makes PP ideal for ‘no label’ look (clear-on-clear) labels. The material offers good film flatness, excellent dispensability even for thin films, and good die-cutting and printing characteristics.

Co-extruded. Semi-conformable co-extruded facestocks are multilayer films that can be a mix of polyethylene and polypropylene, depending on the properties required. These are suitable for semi-squeezable containers and bottles, often used for shampoo or sauces.

Polyester. Polyester films are very rigid films with high durability and clarity, and also high temperature resistance. Their rigidity allows these materials to be used for label-grade facestocks as thin as 12 microns. Some polyester films are flame retardant, and are used for applications such as electric cable labeling.

PVC (polyvinyl chloride). PVC films are highly conformable and flexible, and one key application is graphic wraps for automobiles. These materials also have very good outdoor resistance and ageing properties, along with high durability and resistance to UV light, temperature and chemicals.

PVC-based labels are suitable for applications such as drum labeling, farm applications, and durable indoor/outdoor graphics.

MATERIALS CHOICE

Choosing the right labeling material grade depends on container type, required film appearance and end-use performance requirements.

As we have seen, containers can be anything from very rigid to fully conformable. Polypropylene suits rigid containers, because of its clarity, dimensional stability and rigidity. Polyethylene is flexible, and is therefore the main industry choice for conformable films.

The appearance of a film – opaque, colored or clear – depends not only on the material chosen, but also the production process employed (Figure 6.2).

A filmic face label has three layers: a core, a print ‘skin’ and an adhesive ‘skin’. When all of these layers are clear, the final facestock is also clear. An opaque white film can be made by adding titanium dioxide to the core layer.

Another type of PP is a ‘cavitated’ white. Air is entrapped in the middle white layer, adding a pearlescent effect to the white color.

As well as white and clear filmic facestocks, silver filmic facestocks are available. The silver layer is applied either to the print skin or to the adhesive skin when the facestock is made. When it is applied on the print skin, the absence of an intermediate material makes the film appear high gloss.

When applied on the adhesive skin, the material has a more moderate gloss look.

SURFACE TREATMENT

After manufacture, uncoated filmic facestocks require a surface treatment (corona, plasma or topcoating) to ensure proper ink anchorage and good printability.

During corona treatment, the film manufacturer increases the surface energy of the film by exposing the surface to a high voltage discharge.

Because the effect wears off over time, it is recommended that converters boost the corona effect using an inline treater on the press infeed press, to guarantee good ink anchorage.

Topcoating is a different form of surface treatment by adding a chemical top layer to the print skin, a coating designed to improve ink and toner anchorage (Figure 6.3). Universal topcoats, suitable for any printing technique, have been unavailable in the past, but are now coming to the market. This avoids the need to warehouse different topcoated materials for different print processes.

Topcoated films have some advantages over corona-treated films, such as a more premium look (improved inch anchorage), compatibility with high speed printing, and in some cases suitability for both conventional and digital (UV Inkjet) print technologies.

PAPER OR FILM?

When are filmic labels generally preferred over paper labels?

One material type is not generally ‘better’ than the other. Instead, the requirements of a specific application will point the way towards paper or film.

One benefit of using film rather than paper is that it allows for the premium ‘no label’ look, often used in craft beer, spirits or personal care applications.

Film is also the preferred choice where flexibility, conformability, weather, moisture and tear resistance are required. Polyester provides extreme durability, for example coping with high levels of UV, heat, chemical or abrasion resistance.

The principal advantages of paper are lower cost, a wider variety of facestock options and higher performance with printing technologies that suit porous material structures. Film is a smooth material, and ink cannot anchor without either a topcoating or corona treatment. Paper has a structure that means it can more readily absorb inks such as water-based inks.

Depending on the application, paper has its own disadvantages. These include lower durability, no (or less) water resistance, lower tear resistance, and a possible tendency to wrinkle in the presence of moisture.

FILMS STRENGTHS AND WEAKNESSES:

Strengths

High mechanical strength

Heat-sealable

Transparency – ‘no label’ look

Shrinkability

Variety of polymers

Flexibility/squeezability

Gloss with additional lamination

Good barrier/product resistance

Compatibility with plastic containers

Suited to high speed application

Weaknesses

Lack of stiffness

Higher cost

Poor heat resistance

Unsustainable resource

Low degradability

POLYETHYLENE VERSUS POLYPROPYLENE

As noted earlier, the two main types of filmic materials used in facestocks are PE and PP. We now look in more detail at their performance characteristics and how this affects application choice.

Polyethylene (Figure 6.4)

Standard PE, typically used for labeling home & personal care products, comes in a standard thickness of 80-85 microns.

Semi-conformable films (co-extruded) are used for squeezable packaging, typically in the food and personal care sectors (e.g. shampoos and ketchups). They are available in the 50-65 micron range.

Thinner grades of PE (from 30-60 microns) can be used for direct labeling of food products such as kiwi fruit.

For industrial applications where increased durability is required – such as petrochemicals, lubricant packaging or drum labeling – thicker PE facestocks in the range 100-120 microns are used.

Polypropylene (Figure 6.5)

Standard PP (between 50-60 microns) is used mainly on rigid surfaces such as glass and polyester containers (bottles, jars, trays), for beer & beverages, wine & spirits and food applications.

Thin PP (between 20-40 microns) is typically used for small diameter pharmaceutical devices such as syringes and injectors. It can also be used as an overlaminating film to protect print from scratching – a common practice for glass bottles.

Thick PPs in the range 100-120 microns are, for example, used in wine and spirits applications where requirements such as high opacity and ice-bucket performance play an important role.

PP films with a direct thermal coating are available for variable information labels in food and retail, creating opportunities for a premium ‘no label’ look in these segments as well.

ENSURING SUSTAINABILITY

As the environmental debate around the place of plastics in the packaging supply chain continues, what is the likely future for filmic labels?

Globally, government legislation and controls around plastics are increasing. In 2018, the European Commission announced the EU’s plastics strategy, stating that by 2030 all plastics packaging put into the EU market must be reusable, or able to be recycled ‘in a cost-effective manner.’

Plastic waste recycling targets were set at 55 percent of plastic packaging waste by 2030.

Single-use plastics are also being targeted in developing economies. India, for example, banned all single-trip plastics from October 2019, while ruling out the use of any plastics films below 50 microns.

This will directly impact polyester liner suppliers, because using a polyester liner above 50 microns is considered economically unviable, as well as going against the trend of light-weighting.

Alongside this legislative trend, younger consumers in particular have started to focus more on products available in what is perceived to be sustainable packaging. The major global brands have responded with their own sustainability pledges.

For example, Unilever has pledged that by 2025 all its plastic packaging will be designed to be fully reusable, recyclable or compostable.

Coca-Cola has pledged that by 2025 all its packaging will be fully recyclable globally.

What does all of this mean for the label industry? Suppliers of filmic PSA labelstocks are clearly focused on sustainability issues, including:

Increasing the recycled content of plastic label materials. It is estimated that PET liners with 30 per cent recycled content save 14 percent of greenhouse gases, up to 5 percent of water usage and up to 11 percent of energy consumption.

Reducing the amount of material used in face stocks and liners (down-gauging). Note that there are limits to what can be achieved with face materials, due to the bulk performance requirements noted above.

Improving PET recycling. For single-use PET bottles, new ‘switch off’ adhesives deactivate in the presence of liquids used in the recycling process, so that PP label materials can be separated from PET flakes in a flotation tank.

Label solutions that enable reuse of returnable glass bottles in the beer & beverage market. These labels detache easily and clean from the bottle in a conventional bottle washer.

Switching to renewables. Filmic labels can be made from renewable and bio-based sources.

In summary, the overwhelming majority of the filmic label market consists of PP (for rigid containers), PE (for conformable) and PE/PP co-extrusions (for semi-conformable). Polyester and PVC facestocks are used for specialist applications.

Surface treatment is always required for film stocks. This can be either a corona treatment or a topcoat. In performance terms, filmic facestocks give better flexibility, conformability, tear resistance, water resistance and durability when compared with paper labels. Paper has its own advantages, as discussed above, principally cost savings and ease-of-use with some printing technologies.

Intro section

Pressure-sensitive adhesive (PSA) labels made from films are the fastest growing category within the self-adhesive label sector. This article looks at the different types of filmic (non-paper) labelstocks and their properties. We also look at different market applications for filmic PSA labels, and at sustainability issues.

Filmic facestocks can be made from a range of different materials, but the main ones used today are polypropylene, polyethylene, polyester and PVC. Polypropylene (PP), polyethylene (PE) are by far the most widely used, along with co-extrusions of both materials.

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Self-adhesive labels: paper face materials

Feedimporter — Thu, 26 Nov 2020 15:51:00 +0000

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Paper face materials

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Pressure-sensitive labels consist of an adhesive-coated face material, which can be either be paper or film, laminated to a release liner

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For uncoated papers, ink absorption is most affected by the gap between the fibres as well as the absorption properties of the fibres themselves.

Synthetic papers were developed to combine the printing properties of papers with the durability and performance benefits of films.

Synthetic papers are coated on both sides, and the ink absorption, as with coated papers, is defined by microporosity in the coating layer.

A. Uncoated Papers: Let’s take a closer look at uncoated papers. As mentioned above, uncoated papers have a high degree of roughness, with more peaks and troughs than coated papers (Figure 5.1). This requires specific print techniques to achieve good printing quality.

For uncoated papers the most stable printing methods are thermal transfer and dry toner, and SOHO (small and home office) printers, inkjet or laser printers. They are printable with conventional methods like flexography, offset and letterpress, but we have to remember that due to high ink absorption, print quality – the resolution achievable – will be not be high.

Despite not having a top coating, there are still possibilities to modify the surface of an uncoated paper to get better printing quality. One method is called sizing, which makes these kind of papers printable to a higher quality with laser or water-based inkjet.

The nature of these papers and the available printing methods limit the range of typical end uses for uncoated papers. So typically they are used for logistics labels, A4 labels, office documentation and address labels.

B. Coated papers: Coated papers offer many more possibilities when it comes both to high quality printing and to end use applications. Thanks to the coating, which can be matt, semi-gloss or high gloss, the surface of the paper is much smoother and has a lower roughness than uncoated papers.

The coating affects not only the roughness of the surface and the visual appearance of the paper, but also acts as a barrier to protect against external conditions (Figure 5.2).

Coated papers can be printed with excellent results by flexography, offset, letterpress, screen or digital, and can also accept hot foil stamping.

They can be printed with dry toner and are suitable for thermal transfer print. Top coats can also be designed specially for particular digital printing methods, including liquid toner, UV or water-based inkjet.

Typical end uses for coated papers include food, beverages, and home and personal care products.

While papers for commercial printing may be coated both sides, in the case of the labels most papers are coated only on one side. This is to avoid possible interactions with the adhesive coating layer.

DIRECT THERMAL PAPERS

One type of specialist coated papers are direct thermal papers. These are constructed by coating multiple layers on top of the base paper, each of which makes the surface smoother for the additional layers, as well as protecting the base paper from the heat energy which is generated during printing.

These layers are, in order, a pre-coating, followed by a thermal layer, followed by an optional top coating and a reverse side barrier (Figure 5.3).

The thermal layer has three main components: the color former, developer and sensitizer.

In some cases, a top coating is applied on the top of the thermal layer to protect the thermal layer from external conditions, UV light and chemicals.

But this top coating also makes possible the printing of thermal direct papers with conventional methods like flexography. It is important to remember that for this kind of pre-printing, inks which are compatible with the direct thermal printing heads should always be used.

On the bottom side of the direct thermal papers, there is always an additional coating. This is because these papers are very sensitive to the possible migration of different components from the adhesive and from the package the label is applied to.

SYNTHETIC PAPERS

Synthetic papers are coated on both sides and have all the performance advantages of films. This means they are flexible, resistant to tearing, and resistant to water, a lot of chemical products and oily substances. These characteristics make synthetic labels ideal for use in industrial labeling applications, such as chemical drums. With suitable top coatings, both conventional and digital printing methods can be used.

The use of synthetic papers for labeling in niche markets where moisture, most contaminants and harsh environmental conditions would severely damage paper labels, has been undertaken for many years.

Synthetic papers generally resist tearing, water, grease and certain chemicals, can withstand extremes of heat and cold, have good UV-resistance and weathering characteristics, are non-toxic and, depending on the specific synthetic material, are FDA approved for food contact.

Synthetic papers are designed to incorporate the best attributes of natural paper and plastic to form a material that delivers strength and durability, yet are printable by most mechanical or electronic printing processes.

Collectively, they can be perforated, sprocket- punched and come in reels or sheets, with or without an adhesive coating, while top coatings on synthetics produce high-quality printing characteristics similar to paper.

Applications for synthetic paper labels and tags include the labeling of products which are shipped or stored outdoors; outdoor bar-coding; products that are used on pallets or for garden supplies; for luggage, airline baggage and horticultural tags; as hospital patient identification tags; for chemical drum labels; slaughterhouse meat and carcass tags; tags for outdoor signs or notices; and for in-mold labeling of blow-molded HDPE bottles for under-the-sink products.

Price will always be a deterrent to wider use of synthetic papers, but for the right applications it is a very cost-effective product.

CONVERTER PERFORMANCE REQUIREMENTS

What are the converter performance requirements when it comes to paper face materials? Printing requirements depend on the printing method used, the characteristics of the face materials, optical properties of the paper and coating and the requirements of color consistency.

To ensure consistent print results, delta-E variation, measured using LAB color profiles, should be minimal from batch to batch. Then the roughness or smoothness of the paper will define which printing methods are recommended.

What are the key characteristics defining printability?

1. SURFACE COMPRESSIBILITY

When printing paper by any contact method, higher levels of surface compressibility give better contact with the printing plate and, ultimately, better printing quality. Different printing methods also require varying levels of ink absorption.

2. SURFACE STRENGTH

Surface strength is very important when high viscosity inks are used. High surface strength also minimizes the risk of the coating surface breaking down. This can be affected by any surface treatment, which also affects absorption properties.

COATING CHEMISTRY

We now turn to coating chemistry, and how this relates to different print methods.

With water- and UV-based flexography, papers should have a high degree of smoothness, so there is a good contact between the printing plate and the paper.

It is also better for the paper to have low absorption properties for better ink hold out.

In the case of offset, where high viscosity inks are used, a high surface strength for the paper is critical to avoid breaking of the paper surface by the viscous ink and fountain solution. With offset the coating also needs to be adjusted for its absorption properties. The coating must absorb relatively quickly and evenly. Too slow and uneven absorption will cause mottling.

In thermal transfer a high degree of paper surface smoothness is required to achieve good contact between the ink and paper. But – the opposite to water-based and UV flexo – high absorption properties are required to ensure good ink coverage and anchorage.

Digital, as we know, includes a range of different technologies – water-based inkjet, UV inkjet, laser/dry toner and liquid toner – and each of them demands very different properties from the paper.

The papers to be printed on laser printers require optimized electrical and thermal properties and the surface chemistry should be adjusted for this type of technology to achieve sufficiently high levels of toner transfer. These kind of papers typically have high surface porosity to ensure the toner anchors.

In the case of water-based inkjet, it is very important that the paper has optimized absorption to achieve correct density levels during printing. And the surface coating chemistry should be appropriately adjusted to avoid ink bleed.

For both UV inkjet and liquid toner, it is very important is to have the correct surface energy, along with optimized absorption properties and porosity. Paper printable with UV inkjet will not always be printable with liquid toner.

DIE-CUT AND MATRIX STRIPPING

The process which typically follows printing at the convertor is die-cutting and matrix stripping. From the point of view of the face paper, what is important here?

Factors include the type of the paper – paper-based or synthetic – paper strength and coating formulation.

It is important to note that the face paper is only one element in assessing an optimum die-cutting strategy. Also important are the characteristics of the release liner, such as thickness and variations in thickness, density and compressibility, as well as the release force built into the silicone coating, and the properties of the adhesive layer. Here we focus on the face paper and how it affects the die-cutting process.

The most important factor is to design the die-cutting tool with the proper cutting blade angle, which is usually between the 70-110 degrees for paper face materials.

Paper’s break point is at 60 or 65 percent compression of the laminate as a whole (Figure 5.4). Synthetic papers should be treated as a film material, so the blade angle should be between 40 up to 70 degrees. The break point of foils is at 90-95 percent compression (Figure 5.5).

The tensile strength and tear resistance of the paper in both the machine and cross-direction is another key factor if we are to avoid the matrix breaking. High tensile strength and high tear resistance are required to run the material and separate out the matrix at high press speeds.

Even paper formation during manufacture in the paper mill is important to minimize die-cutting problems. If there are too many long fibres on the back side of the paper it will make die-cutting much more difficult. The coating formulation is also important because some pigments contained in the coating are much more abrasive than others and can cause much faster die wear. This can be a particular issue with matt coatings.

END USER PERFORMANCE REQUIREMENTS

Paper face materials are used in a wide range of different applications, and each one puts different demands and requirements on the face paper (Figure 5.6). For food applications food approval certificates are required.

Moisture resistance will be required for humid, cold-store or ice bucket environments, and for industrial applications a wide range of chemical resistance – to oil, for example – will be required, as well as the ability to survive hostile environmental conditions.

The following properties are important:

1. STIFFNESS.

A high degree of stiffness is required to allow operation on high speed automatic label application lines. But in the opposite case – small tubes and other curved surfaces for example – stiffness should be kept low to keep the face adhered to the container surface.

2. OPTICAL PROPERTIES

Optical properties need to be optimized for specific applications in terms of gloss levels, whiteness and opacity. Stability – resistance to external environmental conditions such as UV light, moisture and chemical resistance – will also require correct paper specification.

3. TEAR STRENGTH

Tear strength of the paper is important not only during the application process, but also to minimize the risk of the label being destroyed during transport or useage. High internal strength of the paper is required to avoid de-lamination, particularly with applications like reclosable labels or when the face material is used with a removable adhesive.

3. SAFETY AND REGULATORY REQUIREMENTS.

These will all depend on the final application, but examples would include Declaration of Conformity for FDA, BfR, Toy Safety etc.

4. SUSTAINABILITY

Label papers may contain a percentage of recycled pulp or be manufactured 100 percent from recycled pulp. These come in both coated and uncoated grades and have exactly the same properties as the virgin fibers they are made from. They can be printed with the same methods and can be approved to the same regulatory standards.

The only thing to note is that for some types of recycled papers it is possible to see the impurities which come from the recycling process.

Paper labels applied to cardboard or corrugated boxes can be recycled without any problems because the purity demands of the cardboard recycling process are lower than for paper pulp.

Another key issue for the label industry is responsible sourcing, and there has been a major move to adopt face papers from sustainable sources covered by either FSC (Forest Stewardship Council) or PEFC (Program for the Endorsement of Forestry Certification) qualifications.

Intro section

Pressure-sensitive labels consist of an adhesive-coated face material, which can be either be paper or film, laminated to a release liner.

We begin with paper – a category which includes synthetic (filmic) paper materials. Label papers are manufactured from wood pulp and may be either coated or uncoated. With coated papers the main influence on how ink is absorbed is the presence or absence of a top coat and its micro-porosity.

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Pressure-sensitive adhesive technologies

Feedimporter — Thu, 26 Nov 2020 15:26:00 +0000

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Pressure-sensitive adhesive technologies

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What is adhesive, how it works and why it can fail

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Not all adhesives are designed to be permanent. Some applications require the label to be removed from the surface at some point after application. Other applications may require semi-permanent adhesion - something between permanent and removable – or labels might need to be repositionable or recloseable. Each of these makes different demands on the adhesive technology.

Adhesion characteristics

High tack

Permanent

Semi-permanent

Removable

Repositionable

Reclosable

Adhesive requirements

Low temperature

Ice bucket resistance

Oil and chemical resistance

BS 5609 sea water

Pharmaceutical

Cost (in use)

Food contact

Transparent

Non water-whitening

Tamper evident

Once we know the application and what the adhesive or the label should be capable of doing, we can specify the adhesive chemistry and formulation required.

ADHESIVE TYPES – AN OVERVIEW

Pressure-sensitive adhesives (PSAs) differ from other types of adhesives in that they are able to form a bond at any time, are permanently tacky, are capable of bonding to almost any surface and are adhesive above their glass transition (Tg) temperature. No activation by water, solvent or heat is required to exert a strong adhesive bond on materials as diverse as paper, plastic, glass, wood, cement and metals.

Four well-established pressure-sensitive adhesive technologies are currently used, with a fifth beginning to find commercial applications:

Acrylic solutions. Solvent-based acrylic PSA solution formulations have been widely displaced by water-based and hot-melt systems for economic as well as ecological reasons. Solvent recovery and/or incineration are essential to meet clean air legislation requirements.

Such equipment is expensive and can only be justified for large output operations. Acrylic solution adhesives are therefore not widely used for labels today, except for speciality applications such as durable labels that require chemical resistance and/or compatibility with materials that cause adhesive failure due to plasticizer migration.

Rubber/resin solutions. These are also less frequently used, because they are coated in solvent, except for high performance ‘peelable’ labels and specialities such as oil can labels.

Hotmelts. Hot-melt PSAs are a fast-growing sector, because their conversion performance is now very good and they are competitive. They can be used where an aggressive permanent adhesive is needed with high tack and some recipes have excellent performance in cold and wet conditions, particularly when labeling plastic surfaces. Hot-melts are easy to coat on compact equipment and are also the preferred choice for most in-house converters and printers.

Acrylic dispersions. Water-based acrylic PSA dispersions (emulsions) now represent the dominant technology for the labelstock producer and can also offer a practical option for in-house coaters. The large labelstock suppliers formulate their own adhesives, but ready-to-use formulations are commercially available for small and medium-sized coaters.

Radiation cured. Adhesives can be cross-linked (cured) by electron beam or ultra-violet radiation. This enhances some characteristics such as high temperature performance of hot-melt adhesives. UV Curable adhesives are now used in some speciality tape applications and are also beginning to be used in limited applications by label or forms converters.

Pressure-sensitive label adhesives successfully meet an enormous range of demands. They can provide a permanent bond or can be removable. Some substrates are easy to stick to, for example paper or board, but PSAs can also be formulated to stick to ‘difficult’ surfaces such as plastic containers or moist glass bottles.

Label service temperatures normally range from -20°C (-4°F), or lower for freezer packs up to +200°C (392°F) or more for car engine parts.

The cohesive strength of PSA adhesives can be reduced to allow labels to disintegrate to discourage tampering. The key properties of the main pressure- sensitive adhesives are shown in Figure 4.1.

Before looking in more detail at these different adhesive technologies, we need to ask a more fundamental question. What actually is a pressure-sensitive adhesive and what makes it different from other adhesives?

A pressure-sensitive adhesive is a soft, permanently tacky material which is capable of making an instantaneous bond to almost any surface within a certain temperature range without any additional force being applied.

A pressure-sensitive adhesive has an infinite open time, meaning it is always tacky and always sticky. This is the opposite of adhesives that dry and once dry cannot then be stuck any more. A pressure-sensitive adhesive can be peeled off and still feels sticky.

To achieve this effect, a PSA combines the properties of a solid with the ability to ‘flow’, which makes it possible for the material to form a bond. (Figure 4.2).

The flow is not the same as a liquid, since liquids are not able to form an adhesive bond. Liquid honey, for example, exhibits ‘stickiness’, but will not form a bond, as it does not have the properties of a solid.

Combining the properties of a liquid and a solid in the same material creates a ‘viscoelastic’ property which is the key feature of a pressure-sensitive adhesive.

The study of a material’s response to different modes of flow and deformation is the ‘rheology’ of a material.

Pressure-sensitive materials have an instantaneous tackiness which ensures a material will form some kind of bond within a short time. Raw materials can be selected to be more or less tacky, but for correct bonding the material must also be fluid enough to cover the material surface onto which you want to bond.

The other important property is the shear of an adhesive, otherwise known as the cohesion. This is the resistance required to tear these bonds apart and is a property of a solid.

We know that the ‘solid’ property of a PSA ensures good cohesion, so it is easy to think that to maximize this property requires designing a product with high molecular weights.

That will, for sure, deliver higher shear values, but the downside is that the viscosity of the material becomes so high that processing equipment will no longer properly work. So there is a trade-off involved between cohesion and viscosity.

Certain chemistries offer inherently higher molecular weights. With self-curing solvent acrylics, for example, the curing mechanism used to take out the solvents can itself boost molecular weight.

Similarly, with UV acrylics the chemical cross-linking process delivers a very high molecular weight.

MODULUS

A material’s stiffness properties are known as its ‘modulus’, a property which changes with temperature. At low temperature, materials have a high modulus, which means the material is stiff, and at very low temperatures the material is brittle, with no flexibility.

When the materials are heated, the modulus will start to drop. Starting from a certain temperature, the material has enough energy to show some mobility at a small scale. When polymer chain segments between entanglements have some mobility, the material has reached its glass transition temperature (Tg) range.

By optimizing PSA design to have the Tg at a low temperature, it is possible to have PSA characteristics needed for a deep freeze label, for example.

Pressure-sensitive materials have been found empirically to have a modulus between set values. This is known as the Dahlquist (1966) theorem.

When the modulus is within that range, or window, the material has pressure-sensitive properties. By selecting certain raw materials processed in the right way, the designer ensures that this window corresponds to the temperature at which the adhesive should work. This allows labels to adhere at both extremes of the temperature range.

To take the example of rubber hotmelts, these are made pressure-sensitive by using a blend of elastomers, tackifiers and other additives that result in a mix that has the correct modulus. The individual components – for example the elastomers – have too high a modulus to be pressure-sensitive in their own right. It is the combination of all the materials that makes the final product pressure-sensitive.

An acrylic adhesive is different in that the polymer itself is designed and built to be pressure-sensitive. The raw materials selected –the molecular weight and type of acrylic monomers – ensure that the end product is a pressure-sensitive polymer.

The same applies to water-based acrylics, with the difference that they are made in a process called heterogeneous polymerization. This means that you end up with particles dispersed in water, rather than an organic solution of long polymeric chains that look like entangled spaghetti, which is what a solution acrylic looks like.

UV curable hotmelt is similar in design to a solvent acrylic with the key difference in molecular weight before curing and the curing mechanism itself.

CURING MECHANISMS

When we examine the modulus of rubber hotmelts, above a certain temperature they fall apart (they ‘melt’), a property which allows them to be processed into an adhesive film. But when they become a liquid, they are no longer pressure-sensitive – they are simply a mix of ingredients that has become liquid.

This process is reversible, so when the hotmelt materials cool down these mixes become pressure-sensitive again.

By contrast solvent and UV acrylics have a curing mechanism such that when they are heated, the modulus does not drop at a certain point. This means the adhesive film does not become a melt, but keeps its pressure-sensitive properties at higher temperatures. This makes this class of adhesives the first choice for really demanding applications where the label must withstand high temperatures in combination with, for example, a high mechanical load.

Designers configuring acrylic polymer chains have a wide choice of functional monomers which form ‘asset groups’ with metals like aluminum to ensure that when the adhesive is cured, a strong chemical bond forms between the polymer chains.

Although UV acrylics work with photo-chemical rather than asset group mechanisms, the end result is the same: a chemical bond between the polymer chains, which means that above a certain temperature the adhesive does not fall apart.

These chemical bonds do not exist in a rubber hotmelt mix – and that is the key difference in high temperature performance.

PROCESSING

Of all the available adhesive chemistries, rubber and UV acrylic hotmelt is the easiest to process. It starts as a solid product which is put in a melter, and processing can begin straight away. There are, of course, safety risks associated with hot and sticky surfaces, but equally there are no flammable solvents to handle and no drying process. This makes it a compact setup and explains why rubber hotmelts are very popular from a converting point of view.

Solution acrylics or emulsion acrylics start out as ‘wet’ adhesives that need to be transformed into an adhesive film, and the water or solvents removed. These operations take up a lot of space and require higher levels of knowhow and expertise to ensure the adhesive is properly cured.

Testing protocols exist to ensure these standards are met, but it is a very different setup to hotmelt production.

The adhesive as supplied comes with certain characteristics guaranteed by the manufacturer.

Optical appearance can be transparent, yellow or hazy depending on the end use properties required.

Depending on the technology of the PSA, the manufacturer has to measure and control specific characteristics like a specified solid (non-volatile matter) content, Brookfield viscosity, kinematic viscosity, pH, particle size distribution, etc.

These factors must all be tightly controlled from the manufacturing side to ensure quality of the adhesives batch to batch (Figure 4.3).

Each PSA technology comes with its own processing challenges (Figure 4.4). Rubber hotmelt has strict viscosity limits and requires protection from oxidation.

For emulsion acrylic key challenges are wetting out of the adhesive and foaming.

For solvent acrylics the challenge lies in handling flammable solvents and ensuring correct cure. For UV acrylics correct usage and maintenance of the UV source to deliver reliable and repeatable cross-linking is essential.

TESTING AND TROUBLESHOOTING

FINAT test methods provide the industry-standard way of measuring factors such as peel, tack and shear (Figures 4.5 and 4.6).

The FINAT test methods offer a reliable standard for measuring key properties of PSAs. They enable suppliers and users of PSA to compare values measured according to these standards. As well as the numerical test values,the observed failure mode of, for example, a shear test, can help understand how an adhesive performs successfully, or fails.

In the case of a removable label, for example, where you do not want the adhesive to stay on the surface permanently, both a shear test and a peel test can distinguish a proper removable PSA from an adhesive that has low adhesion but is not a cleanly removable PSA.

While peel, shear and tack (see information box) are the basic methods for testing adhesives, a range of more general label-specific test methods are also used, for example accelerated aging tests, migration and penetration tests.

Migration can be a particular issue when working with certain paper types and rubber hotmelts, which contain oils and plasticizers that will migrate.

Papers with a closed structure will generally be more resistant to migration. The final application will help decide how much of a problem this is likely to be.

There are also chemical and photochemical ways to test the bond of a label or tape, which all help ensure a better understanding of the integrity of the bond throughout a specified lifetime.

One specific method for labels worth mentioning is mandrel performance, which tests specifically for smaller diameter vials and bottles, ensuring the adhesive will stay stuck and will not start ‘flapping’.

A typical laboratory test is to bond labels onto a small glass or plastic test tube and observe what adhesives are best suited for that application, remembering it is the specific balance of adhesion and cohesion that makes the label work.

SUSTAINABILITY

The role of the PSA in contaminating recycling streams is now widely recognized. If, for example, an adhesive remains on the PET bottle surface after the label is removed, the PET flake recycling stream can be contaminated.

Adhesive manufacturers have responded with adhesives that ‘switch off’ in the presence of the alkaline solution found in recycling wash systems.

The adhesive stays with the label film leaving the polyester clean for recycling.

Adhesive manufacturers are also working on systems which allow cleaner recovery of pulp in the paper recycling process. These include screenable adhesives, which are designed with bigger molecular components.

When paper labels are pulped and soaked in water during the recycling process the adhesive separate from the paper and the larger adhesive particles can then be screened off, allowing smaller paper pulp particles to be separated.

On the sustainability front, compostable PSA adhesives are now available, along with alkali-soluble PSA adhesives and bio-based.

Intro section

Adhesive is at the core of the pressure-sensitive label. This article looks at what adhesive is, how it works, why it sometimes fails and how to make sure that it keeps working in often hostile end use environments.

ADHESIVE PROPERTIES

Descriptions of permanent adhesives talk about how well they stick well to certain surfaces – for example ice bucket resistance, BS 5609 sea water resistance, or adhesion at low or very high temperatures. Also important is the type of application – for example a requirement for clarity on a no-look-label construction, or for regulatory compliance on pharmaceutical or indirect food contact applications.

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Silicone release liner technology

Feedimporter — Thu, 26 Nov 2020 15:08:00 +0000

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Silicone release liner technology

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This article examines the process and materials used for release liners, then looks at performance characteristics and how these are measured

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The silicone coating is extremely thin, typically of the order of one micron thick. It is applied as a liquid and then transformed to a silicone elastomer or rubber. (Figure 3.3).

PAPER SUBSTRATES

Both surface and mechanical properties are critical when specifying paper-based release liners.

Paper substrates for use as release liner base are selected to be as smooth as possible, and with a ‘closed’ surface to significantly reduce any silicone from penetrating inside the paper, minimizing the amount of coating required. The surface treatment of paper is also important as some chemicals used in certain paper grades can ‘poison’ the platinum catalyst – which is often used as a critical component of many silicone release coatings – preventing the silicone curing or cross-linking as it is transformed from a liquid into an elastomer.

The paper surface properties must also be optimized to encourage robust anchorage of silicone to the paper surface – especially important where high coating speeds are being used.

In terms of mechanical properties of the paper, the release liner has to carry the weight of the laminate, act as the base for die-cutting, and carry the die-cut label through a high-speed label applicator, as well as being able to withstand the stresses of the silicone coating process itself. All of these requirements mean that the paper must have a high degree of mechanical strength and tear resistance to prevent snapping or tearing during coating, die-cutting or label application processes.

In addition to this the caliper (or thickness) must be closely controlled to a high degree of consistency, or problems will be encountered in the die-cutting process.

Paper stiffness is also important since die-cutting problems can arise if the material is too soft, as the cutting blade will tend to penetrate into and deform the paper rather than actually cutting the face material.

By far the most commonly used paper materials are glassines or super-calendar krafts (SCK). Whilst there are some differences between the two types in terms of how they are made, super-calendared Kraft has become primarily the paper of choice in the US, while glassine is the choice in in Europe and Asia. (The reason for this is historical rather than technical – US manufacturers simply carried on making super-calendar kraft rather than changing to glassine).

Both glassine and SCK are characterized by their very smooth surfaces and high level of surface refinement (closed), along with their excellent mechanical and chemical properties.

The other commonly used paper substrates are the clay coated krafts (CCKs). These are essentially standard kraft papers on which a combination of clay and a latex is applied to form a sealed surface. As well as a highly closed surface they have excellent lay flat properties.

Other variants include polyethylene-coated kraft (or PEK), which is more common in Asia than Europe or the US. This has a very smooth and highly closed surface, as well as excellent mechanical properties.

FILM SUBSTRATES

A major trend in recent years has been the move towards filmic release liners, with PET (polyethylene terephthalate) the film most commonly used, mainly due to its mechanical properties. PET is very ‘hard’ and can survive relatively high temperatures, which is why it tends to be used in preference to other films. Its very smooth surface means lower silicone consumption is possible, and a high degree of transparency makes it ideal for ‘no-label-look’ labels.

There is some limited use of BOPP and HDPE substrates for special applications.

Overall, in terms of percentage useage in the label industry, glassine and super-calendar kraft account for roughly 50 percent of all substrates used. Clay coated, polyethylene coated and PET account for roughly equal shares of the remaining 50 percent (Figure 3.3).

RELEASE LINER PROPERTIES;

Glassine & SCK (Super-Calendared Kraft)

Very smooth surface, High level of surface
refinement (closed)

Excellent mechanical and chemical
properties.

CCK (Clay-coated Kraft)

Highly closed surface, Excellent lay-flat
properties

PEK (Polyethylene coated Kraft)

Very smooth surface, High level of surface
refinement (closed)

Excellent mechanical properties.

PET film

Very smooth surface (lower silicone
consumption), ideal for ‘no-label look’ labels

Excellent mechanical properties (and
transparent)

Others

Some limited use of BOPP substrate.

HDPE for special applications

SILICONE RELEASE TECHNOLOGY

The function of the thin layer of silicone release coating is to release something that is ‘sticky’, meaning it has to have anti-adhesion properties. In PS labels this means protecting the surface of the base substrate from a pressure-sensitive adhesive.

To understand how silicone works as a release coating, it is necessary to know how a pressure-sensitive adhesive works and then what it is about silicones that enable them to stop the PSA from doing its job.

The function of pressure-sensitive adhesives is to bond two surfaces together and stop them from separating (Figure 3.4).

When trying to peel apart such a laminate which has been bonded together, we are essentially trying to get a crack to propagate between the two surfaces. To prevent or slow down the propagation of this crack we either need to form chemical bonds between the two surfaces – an adhesive force that must be overcome before separation – or we need to absorb/dissipate energy within the layers to prevent their separation.

In the specific case of PSAs, their performance as adhesives is largely based on their ability to absorb/dissipate energy when they are being deformed, rather than any chemical bonding. This unique rheology is the reason, for example, that PSA labels can adhere to a polyethylene bottle despite its very low surface energy and the difficulty of chemically bonding with the surface.

SILICONE CHEMISTRY

Although silicones are typically found in liquid form, they can be modified by crosslinking polymer chains to form silicone elastomers (rubber), which is the basis of label release coatings (Figure 3.5).

In terms of their architecture, silicones are quite unusual for polymer structures. They are made up of polymers based on a backbone of Silicon and Oxygen surrounded by ‘organic’ groups (typically methyl groups).

This gives silicones both a very low surface energy – which means they are difficult to ‘wet’ – and a very stable backbone, which means they are quite unreactive. In terms of their surface energy (referred to as PDMS), silicones are typically in the range of 22-23 dyn/cm. This is significantly lower that many ‘organic’ polymers such as PE, although there are a few specialized polymers, such as PTFE, with an even lower surface energy (see Figure 3.6).

The low surface energy of silicone will already mean that an adhesive coated onto this surface will not easily ‘wet out’ on the surface, and so will not easily bond with the silicone surface.

But low surface energy is not enough to explain why silicones work so well as release coatings, otherwise PTFE ought to perform even better than silicone, which is not the case in practice. The other aspect of silicones which is important for their performance as release coatings is the way that their surface still behaves like a liquid even when the silicone polymers are crosslinked together to form an elastomer. So even when cured into an elastomer, the silicone polymer chains are actually still mobile.

This has the effect that an adhesive coated onto this surface will still be able to ‘slide’ across the silicone surface (if we look at it at the ‘nano-scale’). This effect of allowing surface ‘slippage’ of the PSA on the surface of the release coating means that it will stop the PSA from absorbing energy as we try to separate the two layers – essentially stopping the PSA from doing what it is designed to do.

As an analogy, imagine you are having a ‘tug-of-war’ with somebody (the PSA), much stronger than you, but who is standing on ice.

Normally, their strength would mean they should win the contest, but because they are standing on ice and you are not, it doesn’t matter how strong they are: the slippery nature of the ice means that they cannot make use of their strength. This is effectively what the silicone release coating is doing: stopping the adhesive from using its built-in strength to absorb energy and stick to a surface.

The combination of low surface energy and highly flexible polymer chains means that the force needed to remove a PSA from the surface of a silicone release coating is low enough to make them ideal for use in label manufacture.

MANUFACTURING PROCESS

The manufacturing process for release liners consists of taking the base paper or film and applying the silicone in liquid form, which can be as an emulsion, a solvent dispersion or solvent-free silicone. The liquid silicone coating is then transformed through the action either of heat – the most common technique in the pressure-sensitive label industry – or UV radiation, to create a cross-linked silicone elastomer (Figure 3.7).

Regardless of the nature of the liquid or the type of crosslinking (heat or UV), the final silicone release coating is always in the form of a silicone elastomer.

The choice and design of the silicone release coating is very much influenced by the process requirements of the equipment and the materials making up the label laminate.

For cost reasons the silicone is coated as a very thin layer, typically just one micron thick, and at high speeds of up to 1,000 m/min. It is critical to completely cover the surface of the substrate with silicone, because wherever there is no silicone, the PSA will be able to ‘stick’ to the substrate underneath.

At these very high line speeds, the time allowed for the silicone to be transformed from a liquid to an elastomer is typically no more than 1-2 seconds. In this short space of time not only does the silicone need to ‘crosslink’ but it must also ‘stick’ to, or react with, the surface of the film or paper. If not, the silicone coating can be easily abraded from the surface of the substrate. This is why it is so important that the surface of the substrate is of sufficient quality.

The most common silicone technology used for labels today is thermally cured solventless. While this is generally the most cost-effective process, it does require an expensive precious metal catalyst based on platinum to provide the very fast cure speeds.

As a result, there is a lot of focus in the industry on reducing the amount of platinum required as far as possible. There are still a few applications where emulsion-based and solvent-based are used, most typically in Asia, but these are quite small and specific to unusual combinations of materials. An example is PVC release liners, where solvent based systems are still used. There is also a portion of self-adhesive labels where UV-cured solventless silicones are used.

This tends to be the technology of choice where UV silicones are coated on narrow web presses.

COATING TECHNOLOGY

Solventless silicones, the most commonly used release coatings for pressure-sensitive applications, are usually applied in one of two ways: either a multi-roll coating head or a 3-roll offset gravure system (Figures 3.8 and 3.9).

The difference between the two types of coating head is a combination of cost vs desired line speed.

The most expensive system is multi-roll coating head. This consists of either five or six rolls pressed together under high pressure in a stack, with the individual rolls turning at different speeds relative to one other.

They are run up to 1,000m/min, although trials have shown that the technology can reach speeds of 1,600m/min and still provide an even silicone coating. An efficient cooling system is required to prevent heat build-up, and this adds even more to the cost.

The older, and cheaper, offset-gravure technology consists of a gravure cylinder that transfers silicone to an applicator roll (which is turning at a different speed), and then onto the substrate when pressed against a backing roller. This achieves the same effect but is limited to speeds of around 300 m/min. At such slow speeds, though, there are fewer challenges in terms of heat build-up.

Faster coating speeds means more output from the coating line in the same production time, but at very high speeds there is also the challenge of ‘misting’.

This is an unfortunate side-effect of trying to coat a liquid at high speed where transferring a coating from one surface to another. A mist of small droplets is formed (in this case silicone), as the film is transferred from one roll to another, and especially from the final roll onto the paper or film surface (Figure 3.10).

At lower speeds this is not a major issue. But at speeds in excess of 600-800m/min it needs to be dealt with in order to prevent a ‘fog’ or ’mist’ of silicone droplets appearing around the coating hall and covering every surface as well as getting into the drying ovens.

Misting can be reduced by mechanical modifications to the coating head, but there are also chemical solutions through the use of additives in the silicone coating.

RELEASE FORCE

The whole purpose of a silicone release coating is that it should ‘release’ the PSA. The release performance of a release coating is characterized in terms of its release force – the force required to peel a self-adhesive label away from the surface of the release liner.

The way the release force is measured is essentially a simulation of the way in which the label is dispensed using a label dispensing head, and the force required is related to the angle at which this happens.

Typically, the industry performs tests at 180 degrees (Figure 3.11), but it could equally be 90 degrees or another angle if needed.

The measurement of the release force using the classic ‘peel adhesion test’ is not just a measurement of the ease of removing the adhesive from the silicone surface, but also a measurement of the flexibility of the PSA layer, the face stock and even, to some extent, the base substrate.

This is important since the strength of the release force is not only related to the silicone release coating, but also to the characteristics of the PSA (thickness and type), and the stiffness of the face stock and base substrate.

In terms of the silicone release coating, the main factors that influence the release performance are the quality and coverage of the coating – how completely the surface of the base paper is covered by the silicone – and the silicone cure, which is determined by how well the silicone is crosslinked.

A silicone that has not been fully crosslinked can potentially interact with the PSA surface it is in contact with, giving unstable release force and even a release force that rises over time as the label laminate is aged. In extreme cases, if the level of silicone cure is poor, then there can potentially be un-reacted silicone present which may migrate to other surfaces and impact the performance of other materials.

This could include migration to the PSA surface, leading to a loss of ‘tack’ or adhesion. It could also include migration to the surface of the label where it could affect the printing performance of the label laminate. It is therefore of key importance that the silicone has been completely transformed/cured to a silicone elastomer.

What is also important with the release force measurement is the speed at which the peel test is performed: the release force will actually vary depending on the peel speed being used. This is important as there may be different processes where the laminate needs to be peeled apart (de-lamination for example), and these processes may be run at very different speeds.

As a simple example, hand-applying a label would be done at a relatively low peed speed, while machine-applying a label on a high speed bottling line would be at a higher peel speed, and die-cutting/converting a label – where the matrix needs to be removed – would be at an even higher speed.

The change in release force with different peel speeds is known as the ‘release profile’ of the laminate, and is an important factor in the choice of the silicone release coating being used.

The graph at Figure 3.12 shows a typical release profile of a label laminate and how much the release force can change depending on the peel speed being used. It shows how the different processes handling the laminate can equate to different peel speeds.

In the past, when the materials used within the label industry were relatively thick, and labeling and converting processes were slow, this change in release force with changing release speed (the release profile) was not so important.

In today’s industry, however, there is a never-ending drive to become more efficient in terms of materials and processes, which means that label materials are constantly being downgauged to save on material and costs, and production processes are constantly being speeded up.

The effect of these changes is that the release profile of a self-adhesive label laminate is now very important in determining how well a given laminate will perform across the different processes. Controlling the release force at a specific peel speed is very important in how that laminate will perform.

If we look at label dispensing in a machine-applied bottle labeling line as an example, if the release force is too high at the point the label should be dispensed, there is a risk that the label will simply remain on the release liner.

If it is too low, then the labels may fly off the liner within the labeling machine before they reach the bottle. Only if the release force is within a narrow range will the labels properly dispense onto the bottle.

The release profile of a given label laminate need not be a ‘fixed’ set of values that cannot be changed. By modifying the silicone – specifically be modifying the rheology of the cured silicone rubber – it is possible to change the way in which it ‘releases’ the PSA and thus change the release profile.

This makes it possible to modify the release profile to suit the requirements of where the labels are to be used, as well as to suit the characteristics of different types of PSA, such as hotmelt vs water-based, acrylics vs rubber based and so on.

Typically the target is to reduce the release force at high peel speeds, making it easier to convert the laminate, and this can be achieved by ‘flattening’ the release profile. Note that when the release forces at higher peel speeds are reduced, this often coincides with an increase in release force at lower peel speeds.

This effect is shown in Figure 3.13, where modification of the silicone release coating has led to a change in the release profile of the laminate.

SILICONE TESTING – COVERAGE AND CURE

A. Coverage: As mentioned earlier, an important factor in determining how a silicone coating may influence the release performance of a given laminate is the silicone coverage. This is simply a measure of how well covered the paper or film surface is by silicone. The reason that silicone coverage is so critical is simple: wherever there is no silicone, the PSA will come into contact with the base substrate and will happily stick to it.

In the case of a paper base substrate this can be particularly challenging since the PSA may be mobile enough not only to come into contact with the base paper but even to penetrate into the paper structure and bond even better to the paper. This can even lead to situations where there is no longer any ‘release’ at all and only by tearing the face or base paper can we separate the laminate, meaning a sticky mess which will no longer release at all.

Since the release force of a laminate can be so sensitive to silicone coverage, it is an important quality check during the production of silicone release liner to make sure that the surface is fully covered by silicone.

The simplest solution would be of course to coat much more silicone, but this would be an expensive solution, so the focus is always on trying to optimize the coating process as far as possible to use the least amount of silicone and still maintain excellent coverage.

FINAT testing methods for silicone coverage include stain tests and optical measurements (Figure 3.14).

The stain test is one where a colored stain or dye solution is applied to the silicone coated surface, designed to color the substrate but not the silicone. These stain/dye solutions are ideal for SCK and Glassines which are easily stained, but have only very limited use for clay coated papers (CCK), and are not suitable at all for filmic substrates or even PEKs.

Specialist optical techniques are available which use polarized light to differentiate between silicone and substrate to help identify defects in the silicone coating and variations in silicone coat weight. This method is better suited to filmic substrates and PEKs, due to their specific optical properties. These systems have the advantage that they can be set up for in-line process measurements on a moving web.

B. Cure: The other important silicone-based factor that can affect release force is how well cured the silicone release coating is. Not fully cured silicone can mean problems with release stability over time as well as migration of unreacted silicone polymers into other materials such as the PSA or the label surface prior to printing.

Measurement of silicone release coating cure can be achieved either directly or indirectly.

Direct testing involves submerging the silicone coated substrate into a solvent (MIBK) and extracting any of the silicone polymers that are not cross-linked. If the level of extract is below around five percent, this normally indicates a stable coating in terms of silicone cure.

Indirect testing is where we measure the impact of migration of unreacted silicone from the release coating into a PSA that has been in contact with the silicone surface. This is referred to as the ‘Subsequent Adhesive Strength’ test (SAS test).

According to the FINAT test methods the Subsequent Adhesive Strength test involves taking a self-adhesive tape and applying it to the silicone release liner for a certain period. The strip of PSA-tape is then peeled off the silicone surface and applied to another, standardized surface such as glass, steel or PET.

The PSA tape is then peeled away from the ‘standard’ surface and the peel force compared, as a percentage, to that of a freshly applied strip of PSA-tape that has not been in contact with silicone. If the ‘SAS’ value falls below a level of 85 percent, it indicates that there has been some contamination of the adhesive by unreacted silicone, showing that the silicone cure was insufficient.

Intro section

The silicone release liner is an integral part of a self-adhesive label. This article examines the process and materials used for release liners, then looks at performance characteristics and how these are measured.

A pressure-sensitive label release liner consists of a paper or film coated with a very thin layer of silicone which is laminated to a face material backed by an adhesive coating (Figures 3.1 and 3.2). The silicone coating allows the die-cut adhesive-backed face paper or film to ‘release’ from the liner at the point where the label is applied to a container or other surface.

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Self-adhesive laminate constructions

Feedimporter — Thu, 26 Nov 2020 14:37:00 +0000

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Self-adhesive laminate constructions

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Self-adhesive label materials are complex multi-layer laminate constructions in which each layer of the laminate has a specific purpose and function

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There may also be legislative requirements that the applied label has to comply with, for example marine applications where up to three month immersion in sea water and chemical resistance are required.

These different requirements place significant demands on the label and, in particular, the label face material and adhesive used in the construction.

So how is a pressure-sensitive label constructed? The multiple layers of the laminate include a label face material of paper, film, synthetics or foil; a pressure-sensitive adhesive; and a silicone coated backing paper or film, called the release liner.

This structure can be seen in the following diagram (Figure 2.1).

Some laminate constructions may also have an additional top coating to make them suitable for subsequent operations, such as HP Indigo or inkjet digital printing, or for thermal printing.

WHAT IS THE FUNCTION OF EACH LAYER IN THE PS SANDWICH?

The backing or release liner layer. The release liner protects the pressure-sensitive adhesive during handling, printing, converting, die-cutting and re-winding of the labels and right up to the point of dispensing and application. At this stage, the backing is peeled away from the adhesive immediately prior to its application.

The release liner must also stop the adhesive sticking to the backing or carrier material during the whole printing and converting process.

Depending on the particular requirements of the laminate, the release liner may be paper (super-calendered, coated, polymer coated, glassine, Kraft) or a plastic film (PET, OPP, LDPE or HDPE). Whatever the material used it must be smooth and consistent in caliper right across the coating machine web width. Some liner materials may need to be transparent, have different strength or stiffness requirements.

The silicone coating layer. Silicone coatings, which may be solventless, water-based, solvent-based or radiation curable, have long been the most widely used release agent for pressure-sensitive adhesive applications, dominated by the label market. The silicone is coated on the backing material to provide all the essential features of a typical self-adhesive label construction.

The specific adhesion between the silicone surface and the adhesive being used can be fine-tuned by the use of controlled release additives to the silicone coating to ensure the laminate is held together, yet provide appropriate release levels to suit labels that need to be applied automatically at great speed, or that can withstand the turns and flexing in the paper patch of a laser printer or copier without the labels dispensing inside the machine.

The silicone release system needs to allow the backing to be removed from the label, regardless of the tack level of the adhesive.

Silicon coating weights and coating performance have improved significantly in recent years and are discussed in more detail in the following chapter.

The pressure-sensitive adhesive layer. Pressure-sensitive adhesives are adhesives which in their dry state at room temperature are always tacky and will adhere or stick to a wide range of surfaces by simple contact under light pressure.

The simplest method of categorising pressure-sensitive adhesives is to class them according to their permanence or removability. Within these two categories there are a range of different adhesive types – natural rubber, synthetic rubber and acrylic polymers – which in turn may be solvent-based adhesives, hotmelts or dispersion (emulsion) adhesives.

The choice of adhesive will be dictated, for example, by the nature of the face material and the influences to which the label is exposed. There are also regulations and standards that must be observed.

A full explanation of pressure-sensitive adhesive technology can be found in chapter 4.

The label face material layer. Label face materials are those that finally appear on the product being labeled. The choice of material depends on many different performance, printing, converting, application, storage, handling, distribution and usage requirements. Selection of a label face material may depend on a great many factors, such as visual appearance and image, surface texture, degree of transparency, durability, long life, production flexibility, environmental considerations, volumes required, speed of application, the performance needs of the label (chemical or water resistance, sterilizing).

Because so many different factors and requirements may be involved in the choice of face material, there is invariably a wide range of different face materials to choose from. These include coated and uncoated papers, woven or laid papers, filmic substrates, non-paper synthetics, metalized papers and films and metalic foils.

Some materials may have additional top coatings added for specific performance requirements.

PAPER MANUFACTURING

Paper is a fiber pulp-based material and can be printed with a variety of printing techniques. Spruce and birch wood from managed forest is often used as a basic raw material. Sustainably managed forests are certified by either PEFC or FSC.

The wood is separated from the bark and chaffed into chips. The wood chips are then pulverized in presses or in grinders with the addition of water. The particles are finally filtered and cleaned in several successive baths in order to achieve a homogenous fiber pulp. The pulps of today are generally a mixture of wood fibers and paper, to which a binding agent is added for the better formation of the final paper.

Modern paper production is undertaken on machines which can be over 100 meters long and up to 10 meters wide. The paper webs are produced at speeds of up to 1,800 m/min (Figure 2.3).

When the pulp reaches the machine, it is dissolved in water, generally at a ratio of 5 percent pulp to 95 percent water. The dissolved pulp flows into a reservoir tank at the beginning of the paper machine which creates a uniform paper pulp to run through the paper machine. This is known as the ‘wet’ end of the process line.

The mixture at this stage is called slurry and is transferred to an endless rotating polyester screen where the water content of the fiber pulp is reduced from 95 percent to 80 percent. Any fibers which escape this screening process are captured and recycled. By the end of the screen, the paper is strong enough to be transferred from the wire to the wet press section of the paper machine.

The wet paper is then fed through a succession of pressure rollers which are equipped with absorbent felts. At the end of this section, the paper has lost some of its weight and has a water content of just 60 percent.

The final phase, during which the paper attains its final water content of five percent, is the drying section. This consists of a sequence of steam-heated drying cylinders which are arranged on top of each other. Their temperature reaches up to 120 degC as a result of which the moisture evaporates. The temperature gradually decreases from one cylinder to the next. After the drying section the basic paper has been created.

Depending on the final specification of the paper, extended processes like coating or calendering can follow in-line or off-line.

The calendering process makes the paper surface smooth and glossy. To better understand calendaring, it can be compared to ironing textiles, where the textile is pressed on by a warm iron. The same happens with paper. The paper is fed through a pile of heated chrome cylinders and the paper comes out smoothed, or ‘ironed’ at the end.

Instead of making the paper appear glossy, it can also be made matt by replacing certain chrome rollers with ceramic rollers at this stage of the process. The pressure applied to the paper will affect the paper thickness.

FILM MANUFACTURING

Films are generally made from granulate. Granulates are small plastic particles which will melt at a certain temperature and during the film manufacturing process the granulate becomes liquid.

PE manufacturing. Polyethylene (PE) is a type of polymer that is thermoplastic, meaning that it can be melted to a liquid and re-molded as it returns to a solid state. The technology makes use of PE granulates as a basic raw material. There are two main processes to manufacture polyethylene films : casting and blowing technology.

Casting PE is manufactured by melting the PE granulate, pressing it through an extruder and guiding the film in a horizontal direction via rollers to the winding unit.

Blown PE is a process where PE granulates are melted and pushed out of the extruder in a vertical direction in the form of a bubble. At the end of the bubble the film is collected into double folded film webs and transported to slitter and winder units.

To make the films receptive to ink, corona treaters are installed at the end of the film production lines. The surface tension of the film should be 38 dyne or higher for most printing technologies, and because surface tension reduces over time, corona re-treatment is often required on the printing press. Alternatively the film can be top coated, which has the same effect of increasing ink anchorage.

PP manufacturing. The manufacturing of PP is very similar to the production of cast PE (Figure 2.4). The PP granulate is melted, pressed through an extruder and the film is stretched and cooled down by rollers in the horizontal direction to the winding unit.

PET manufacturing. Manufacturing PET is in its basics very similar to cast PP production (Figure 2.5).

SELF-ADHESIVE LAMINATE PRODUCTION

The manufacture of self-adhesive materials today is predominately undertaken on wide-web high-speed coating and laminating lines incorporating sophisticated process monitoring and control systems to ensure that quality, coat weights, tension, rewind, slitting, etc, are all within demanding production and performance tolerances (Figure 2.6).

The most sophisticated of these lines takes a backing material, coats with a silicone release coating, dries or cures the coating, coats with adhesive, dries or cures this then applies the required face material, before rewinding and slitting to the required web widths for the converter (see next chapter for more details on coating technology).

With die-cutting at the label converting plant being so critical for high-speed applicator lines, all materials and coating are processed within tight tolerances.

HOLOGRAM PRODUCTION

Holograms can be divided into two groups:

Paper-based or filmic substrates with a web-wide holographic image, repeating in the machine direction. These are for general use, mostly for decorative labels.

Single paper-based or filmic holograms with an individual brand logo and selected security levels for high end applications like security labeling.

Holograms covering the web and repeating in the machine direction are produced in a rotary process. Depending on the diameter of the original transfer roller, there will be frequent image interruptions in the cross direction. These join-lines are called ‘shim’-lines, and are part of the hologram production process. Shim line specifications should be considered when holograms are involved.

ALUMINIUM FACE LABELS

Durable face labels (with serial numbers, UL approval references, etc) can be manufactured from pure aluminium in combination with a strong adhering solvent-based or UV acrylic based adhesives. For high quality printing, the aluminium needs to have a top coating.

Die-cutting of aluminium requires a suitable die-cutting tool. A typical example of a solid aluminium label is that found on whiskey bottles, often combined with embossing.

Intro section

Self-adhesive label materials are complex multi-layer laminate constructions in which each layer of the laminate has a specific purpose and function. Each layer has to be carefully monitored and controlled during the laminate manufacturing process if optimum web handling, printing, die-cutting, waste stripping, dispensing and application are to take place.

In addition, the label needs to meet a wide range of specific requirements including: the application line (speed, label size, environmental conditions, etc); the labeling conditions (hot, cold or moist container); the surface of the product/container being labeled (porous, such as paper or board; solid, such as glass, metal or plastic containers); the product storage conditions (hot or cold, inside or outside, wet, chilled or frozen); the handling and distribution process (rubbing, scuffing, refrigerated trucks, etc); and the specific requirements of the end-user.

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Introduction to the self-adhesive label market

Feedimporter — Thu, 26 Nov 2020 14:28:00 +0000

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It seems unthinkable in today’s world that we could do without self-adhesive labels, yet they are a relatively modern phenomenon

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These first self-adhesive labels were not technically die-cut but rather were die-punched using a male die which came up through a guiding plate and which die-cut two 3⁄4 inch round discs of adhesive coated paper. Carrying them through a female die and sticking them side by side on a strip of backing paper, Stan Avery made his 3⁄4 inch Kum-Kleen price labels for antique and gift shops before expanding to other retail establishments.

When the limitation of the die-punched method began to restrict both the production and volume sales of self-adhesive labels, he had the idea to make a sandwich of the backing, the face paper and the adhesive and then, as a separate process, die-cut the required label shape through the face paper only.

To create the first cutters, Stan Avery used the edge of a very thin strip of watch spring. Supported on edge in a thin metal plate with only a fraction of an inch or so exposed for cutting, Avery was able to make dies of any desired shape with uniformly thin cutting edges. It was then only necessary to work the die between absolutely flat ground steel surfaces to make perfect cuts every time.

By 1937 Avery had developed the first synthetic-based pressure-sensitive adhesive and, using a second-hand dough mixer purchased for $10, started his own adhesive production. It was many years later before he purchased his first high-speed mixer. Then, in 1938, Avery Adhesives as it was renamed, suffered a fire that destroyed all of its equipment – except a stock of labels. It was while rebuilding the factory that Stan Avery implemented the changes in his die-cutting machinery.

In 1938, Sessions of York started converting the first Avery Kum-Kleen labels in England under license. These were manufactured on the company’s roll-label seal presses which were used to stamp out printed labels from slit-back Kum-Kleen tape purchased in 41⁄2 inch (112mm) rolls – the full width of Stan Avery’s original tape-based plant.

During World War II Avery Adhesives received wartime government contracts for self-adhesive labels that replaced metal identification tags and also for instructional labels for assembly-line workers. Among the products being produced at this time were waterproof labels bearing ‘SOS’ in Morse code that were stuck on rescue radios.

When World War II ended, Avery Adhesives changed its business focus and markets from retail labels to one that encompassed much broader markets. The war economy had without doubt hastened market acceptance of pressure-sensitive labels.

In Europe, the post-war years saw UK-based Samuel Jones – which had built its business on gummed labels – building experimental coating equipment and selling self-adhesive laminate. These self-adhesive laminates were based on non-silicone release coatings using a castor oil and shellac varnish.

Early silicone coatings were initially water-based, moving rapidly to petroleum-based – a far cry from today’s solvent-free silicones. Adhesives were solvent-based and, judging by the formulation, were not especially aggressive, which was just as well bearing in mind the absence of silicone release coatings.

It was researchers at Dow Corning, formed in 1943, who first saw the potential of silicones for releasing self-adhesive materials, and the company built this business from the 1950s onwards.

The next significant advance in self-adhesive label technology came about in 1949. Killing time on a train journey, Stan Avery used a wooden matchbox to satisfy himself that if the backing paper of a roll of labels is pulled away at a sharp angle, the labels on the roll will always detach themselves. Here the first automatic on-roll label dispensing system was born. Today’s sophisticated label dispensing equipment still uses that basic simple principle developed by Avery.

Another key development took place in 1951when Heinrich Hermann developed a process for coating adhesive paper, setting up Herma to commercialize the technology.

Meanwhile, other players were emerging. In Germany, Werner Jackstädt had joined his father’s wholesale paper business in 1947 producing self-adhesive sheets and postcard materials. By 1954, the Jackstädt business had started to dispatch the first sample rolls of self-adhesive paper to printers in Europe and the Jac organization eventually grew to become the world’s largest privately-owned manufacturer of self-adhesive papers, films and labels before being acquired by Avery Dennison in 2002.

Today’s other dominant player, UPM Raflatac, traces its origins to 1972, when Juhani Stromberg, a young chemist at a company called Raf. Haarla, based in Tampere, Finland, developed a water-based adhesive as an alternative to the solvent-based adhesives which prevailed in the label industry at that time. Raf Haarla’s first laminating machine was built in 1976, the same year Raf. Haarla merged with United Paper Mills Ltd., with label production becoming a separate unit called Raflatac. UPM then acquired the self-adhesive operation of the Kymmene-Stromberg Corporation (Kymtac Oy), to further strengthen the position of the combined Raf Haarla and Sterling (now UPM Raflatac) business within Europe.

In 1985 UPM Raflatac began to globalize its operations with production and sales in the US.

There is not enough space in this here for a full corporate history of the self-adhesive label industry, which saw the emergence of companies like Ritrama, Herma, Mactac and others, and the interested reader is directed to Mike Fairley’s excellent and comprehensive book The History of Labels, available through the Label Academy.

As a general rule, the percentage of PS against wet-glue labels increases with the level of economic development. In the US, for example, wet-glue labels now represent less than 20 percent of overall label demand. In Europe, that figure is closer to 50/50, but this conceals major differences between the developing economies of Eastern and Southern Europe – where wet-glue remains for now the predominant technology – and Western, Central and Northern Europe, where PS labels long ago eclipsed wet-glue.

The fastest growing label technology is sleeving, predominantly heat-shrinkable sleeves, which now account for almost one fifth of the world market by volume and growing by 8 percent year on year. But this has to be put into perspective. Individual shrink sleeve labels are bigger than PS labels since they have to wrap 360 degrees around a container, so volume comparisons need to take this into account (Figure 1.5).

It is noteworthy that PS growth anywhere in the world and in any decade, closely tracks the rate of GDP growth. Indeed, analysis conducted by FINAT demonstrates that PS labels are a leading indicator, or predictor, of economic growth trends. This reflects the fact that label growth is itself highly sensitive to trends in wider consumer and industrial markets. This can be seen by looking at the main end user markets for PS labels: (in rough order of growth) beverage, pharma, industrial chemicals, personal care, food, household chemicals, transport and logistics, automotive and retail (Figure 1.7)

Another measure of PS penetration is per capita consumption. The region with the highest PS consumption, 17 sqm per capita, is Scandinavia, and this figures plummets to below 1 percent per capita in India and China, demonstrating the huge potential for future volume growth in developing markets.

We have already noted that PS represents 40 percent of global label consumption. But less than half of this volume goes to the prime label market.

Around 44 percent of global PS production is in fact accounted for by VIP (variable information printing) labels. Typical applications would be tracking labels for parcels, pallet labels, address labels and so on.

A further six percent is accounted for by functional or security labels, and five percent are promotional labels. This leaves some 45 percent of global PS production going to the prime label market.

For the prime label market then, wet-glue is still, by far, the biggest label technology globally, accounting for 46 percent of the market, compared to 24 percent for PS and 24 percent for sleeving (Figure 1.8).

In terms of PS market growth, we see a distinct difference between lower growth rates of 1-3 percent in developed markets and growth of anything up to 10 percent in developing markets such as China, India and ASEAN, with an average global growth of 5 percent.

Once again, we need to be cautious about seeing Europe as one market. While lower growth rates have characterized Europe’s developed economies, markets like Poland and Turkey have been growing until recently in double digits (Figure 1.6).

FILM VS PAPER

Another key trend is the increased use of filmic materials compared to paper. The market for which we have the best statistics is Europe, where FINAT (and prior to that EPSMA) has been collecting detailed data on the pressure-sensitive label market for over 50 years. FINAT’s figures show that in 2018 of a total European consumption of 7.49 billion sqm, non-paper roll label materials accounted for 27 percent of the total, up from 15 percent in 2000. This represents a growth rate almost four times that of paper face materials and demonstrates a continued trend towards high value-added applications in the consumer FMCG markets and an increased demand for clear-on-clear products.

SUSTAINABILITY

With growing global awareness of sustainability issues, PS label technology has come under the spotlight on three fronts: non-removeable label adhesive can contaminate a plastics container waste recycling stream; waste created when the label is die-cut and the matrix skeleton is stripped away is non-recyclable; and the liner waste left when the label is applied to the container, whether paper or plastics-based, is still mostly thrown into landfill.

Although currently incineration for energy is still an allowable route for dealing with matrix and liner waste, increasingly legislatures and bodies like the EU are adopting ‘circular economy’ policies, where raw material used to create a product must be ‘upcycled’ and used to recreate new versions of that product, or a product of equivalent value. This is opposed to ‘recycle’, where processed reclaimed waste usually ends up in secondary applications such as insulation, plastic park benches and traffic cones.

Of all the process waste produced in the PS life cycle, filmic liners are the easiest to upcycle in a way compatible with circular economy thinking.

Providing they are properly sorted, recycled PET liners provide excellent feedstock for new PET liners, and this value allows recyclers to pay for collection and reprocessing.

Paper-based liners present challenges, since the silicone coating must be removed before they can be reprocessed. The technology does exist and has been commercialized, but there are only a handful of sites with the appropriate technology.

A bigger issue for both film and glassine liners is the lack of large-scale sortation and collection of liner waste by major end user companies.

Label organizations in both Europe and the US have sponsored collection and recycling systems and some industry suppliers and global brands have been proactive in making such systems work. But there is a long way to go before the industry as a whole can be counted as sustainable.

Matrix waste provides an even more intractable problem if the goal is a circular economy. Currently it is not possible to cleanly separate the laminate components after they have been tightly wound into a roll. The only feasible non-landfill disposal path remains pelletization as a feedstock for industrial furnaces and waste-to-energy schemes.

Adhesive contamination of PET containers means that after the label is removed, a layer of adhesive remains on the surface which does not allow clean separation of the PET material. This is an issue which has received attention from materials suppliers. Avery Dennison’s CleanFlake is an excellent example of this kind of a ‘switchable’ adhesive which deactivates in the presence of the fluids found in container recycling systems.

In terms of sustainable PS face materials, the two schemes focused on label papers have proved very successful. These are run by the PEFC (Program for the Endorsement of Forest Certification) and FSC (Forest Stewardship Council) organizations. They ensure the papers are sourced from biodiverse and sustainable plantations and are not sourced from old growth forests.

For PS face films there are a growing number of biomass-based non-fossil fuel alternatives, as well as bio and photo-degradable films.

VIP VS PRIME LABELS

There are two main categories of self-adhesive labels from an end user point of view: Prime (or Primary) labels and VIP (variable information print, sometimes called Variable Data Printed, or VDP) labels.

Prime labels are found mainly in fast moving consumer goods (FMCG) applications, generally placed in a prominent position on the top or front of a product. It is usually decorative and eye-catching, and includes only the most important pieces of information about the product. There may also be a back label where legal, nutritional, recycling and other information will be found.

Prime labels can be either filmic or paper, and can incorporate a range of surface printed effects. The main printing methods for prime labels include flexography offset, letterpress, gravure, Screen and digital (toner and liquid electrophotography and water-based and UV inkjet).

VIP labels are found mainly in the supply chain where products and packages need to be identified for track and trace or addressing purposes.

Many in-plant logistics systems – warehousing, distribution, shipping, storage, tracking – use labels that are printed with variable data. This variable information printing may be in the form of variable text, barcodes, sequential numbers, batch codes, date codes, etc. Variable information or data printing on labels is undertaken with non-impact laser, thermal transfer, or inkjet printers, or with impact printing systems such as dot-matrix printers.

It is important to understand the imaging processes involved as they directly impact the properties required in the self-adhesive laminate (Figure 1.10).

The main VIP print processes include:

Laser printing. This electrophotographic printing process, which is also widely used in photocopiers, uses fine toner particles to provide the image.

The laser beam in the printer creates the image, point by point, controlled by a computer, into a charged pattern on to a pre-sensitized belt or drum.

This pattern has opposite polarity to the toner powder and so attracts it, forming the image.

Paper or label material held against the photoreceptor collects the toner image which is then passed through a fusing system to bond the toner to the labelstock.

Most fusing systems use heat in a pressure nip, but variations include radiant heat fusing and flash fusion using halogen and xenon lamps. The latter are also called ‘cold’ or ‘cool’ lasers because only the dark toner is heated during the flash fusion process.

Variations of how the image is created from the computer’s memory include magnetography and ion deposition (now renamed as Electron Beam Imaging or EBI. technology).

Direct thermal printing. The main process used for adding price-weight information, product description and barcodes to supermarket frozen and fresh produce labels – meat, fish, cheese, fruit, vegetables, etc. – which are weighed and priced at food packers, remote from the supermarket, but also in the store for delicatessen, bread and produce labeling.

The print head for direct thermal printing consists of numerous elements in the form of a grid or matrix that are heated and cooled selectively by a microprocessor controller. A special heat-sensitive coated paper is required which, when heated by these elements changes color within the areas of contact to form the required letters, words, numbers or codes.

As the special thermally printable coating is heat- sensitive, direct thermal printing is primarily used for fresh and chill cabinet products that have a short shelf-life in store of several days. It is not normally used for long shelf-life labeled products or in warm or hot conditions.

Direct thermal labels can be unprotected, or have both a top coating and a barrier coating to protect the image from contamination from both sides.

They are still only recommended for short term labeling as the thermal coating is sensitive to both heat and light so the coating will darken with age and this can adversely affect the print contrast signal (PCS) of printed barcodes, so they cannot be scanned.

Thermal transfer printing. The most commonly used variable data printing process and again makes use of elements which are heated and cooled selectively. However, this time, rather than using a special thermally-sensitive coated paper; the elements come into contact with a filmic one-pass ribbon (of which there are different types), which carries a heat-activateable ink coating.

The required image is therefore created by transferring the heat-activated ink coating from the film carrier to the substrate according to the pattern or shape of the heated elements.

Thermal transfer printing is used for variable information printing of batch codes, date codes, sequential numbers, text, diagrams and barcodes onto pallet, carton or box end labels, for warehousing and distribution requirements, for bakery labels and for DIY and industrial labeling. Printers may be incorporated into packaging and/or weighing lines or be stand-alone. Some are also print-and-apply systems.

Inkjet printers. There are two inkjet printing systems, continuous inkjet (CIJ) and impulse, also called ‘drop on demand’ (DOD) inkjet printing.

Continuous inkjet printing is widely used for printing batch numbers and ‘sell by’ dates or barcodes directly onto products or in-line, on label printing and continuous stationery machines. This inkjet printing method uses minute droplets of solvent-based inks, usually MEK, which are activated and fired at a label surface by means of electrical charges to form the desired image.

Continuous inkjet printing utilizing piezo-electric technology can print in multiple colors at press speed. ‘Impulse’ or ‘Drop on demand’ inkjet is used for office A4 printers with both thermal ink-jet and piezo electric methods used to create the ink droplets.

In thermal inkjet (Canon ‘Bubble-jet’ and Hewlett Packard ‘Desk-jet’) each ink drop is generated when it is needed, by the activation of an electric current to a resistor in the wall of each ink chamber. This heats water in the ink causing it to vaporize and expand. A bubble is created, which forces ink out of the chamber nozzle as the pressure increases.

Dye based inks are being replaced by pigmented inks, where improved water resistance and light fastness are needed.

Piezo electric inkjet systems can utilize either water-based or hotmelt/solid inks. This technology is also fired by an electrical pulse which is applied to a piece of piezo crystal along the wall of the ink chamber. The pulse causes the crystal to deform and reduce the area inside the chamber, thereby forcing an ink droplet out through the nozzle.

It is important to note that film labels must be provided with a special coating in order to ensure optimum ink absorption and ink keying properties, as well as sharpness of image, this being an essential requirement for the printing of barcodes.

METALLIZED MATERIALS

Prime labels in higher value-added markets often make use of metallized materials, and can be either filmic or paper. They have been coated on one side with a very thin layer of metal (about 1 micron thick), usually aluminum. Metallizing is produced by melting and vaporizing the aluminum in a vacuum while passing a web or paper or film around a chilled roller and over the point of vaporization. The vaporized molecules then collect on the cool web, so providing the paper or film with a metallic finish.

Metallizing may be carried out by direct metallizing onto the material surface, or by transfer metallizing where the vaporized metal particles are attracted in the vacuum chamber to a very smooth plastic carrier web and then transferred to the chosen substrate under pressure. This gives a higher finish than direct vacuum metallizing.

Metallic foil is a thin, flexible layer of metal – most commonly aluminum – which is used as a label face material. Some thinner gages are often laminated to paper for improved strength.

Alternative ways of metallizing self-adhesive face materials include metallic inks and cold and hot foiling.

LINERLESS LABELS

Linerless labels consist of a roll of self-wound material, most commonly a direct thermal coated and top coated label. The surface to be printed is coated with a release coating and the reverse side with a pressure-sensitive adhesive. When the roll is wound up the face stock functions as the release surface. The labels are then butt cut – or may have limited shapes cut to the top or bottom – and applied on a print-apply label applicator or for prime labels by a proprietary applicator system.

Linerless labels are most commonly found in the form of pressure-sensitive labels for the blank label industry, as well as thermal labels used in print and apply weigh-price label dispensers and applicators for meat, poultry, and seafood packaging.

Linerless pressure-sensitive labels for prime label applications first came to the fore in the early 1980s when Waddingtons in the United Kingdom developed its Monoweb coating technology to produce linerless labels which were used by major brands including Heinz.

A specially designed applicator system die-cut and applied the label in one pass on the production line. Today a number of companies offer proprietary linerless technology and applicator systems for both primary and secondary product decoration labels.

Intro section

It seems unthinkable in today’s world that we could do without self-adhesive labels, yet they are a relatively modern phenomenon. As recently as 1970 self-adhesive made up less than ten per cent of total label usage in Europe, with label markets dominated by wet-glue applied labels (70 percent) and gummed paper labels (20 percent). Since then self-adhesive labels have experienced consistent and rapid growth worldwide of between 4-7 per year – generally keeping 1-2 percentage points above GDP growth - to become the dominant label technology. (Note that in this series of articles the terms ‘self-adhesive’, ‘pressure-sensitive’ (PS) and ‘pressure-sensitive adhesive’ (PSA) labels are used interchangeably).

Key to this growth has been the flexibility of the self-adhesive label format, with end users able to specify bespoke constructions using a vast range of different materials, top coatings, adhesives and liners. This has allowed end users to meet the challenges of labeling products and containers even in the harshest environments from deep freezers to chemical drums on oil rigs, and from automobile and aerospace to the FMCG retail shelf.

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Inks, coatings and varnishes – safety explored

Feedimporter — Thu, 26 Nov 2020 09:13:00 +0000

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Inks, coatings and varnishes – safety explored

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Without ink it is not possible to reveal what is inside a pack, instructions regarding usage and of course who made the product

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Of all the security related print devices available for brand protection and securing packaging and labeling from threats of counterfeiting, tampering and alteration of important information such as sell-by dates, identification references and variable supply chain data, inks are the most versatile component in the armory of the printer and converter.

Regardless of the printing process, whether it is litho, letterpress, flexo, gravure or silk screen, there is a whole range of inks available to act as precursors of authenticity, indicators of alteration and covert messengers that can be used to conceal hidden information that is only accessible to those with the knowledge and equipment to reveal their secrets.

In itself, ink is a versatile commodity that can be adapted to carry security features that are sensitive enough to provide reactivity to external stimuli such as light and heat. If used correctly, these responses deliver an indication that a pack can be trusted or that a label is not all it purports to be.

In order to protect security inks from unauthorized use, it is necessary for the industry to establish that it is dealing with bona-fide printers that can be trusted before supplying product to any printer or converter.

This is a necessary prerequisite of any of the materials utilized in product protection and applies to paper, board and foils as well. Safeguarding such supplies whilst they are in stock or on the print shop floor awaiting conversion is an important responsibility for the printer too.

Since there is a tendency for packaging and labeling converters to generate product using the four color printing process it is necessary to point out that in order to provide security it is obligatory to deliver a security ink using spot color and this requires extra print stations to be available depending upon the number of colors needed in excess of cyan, magenta, yellow and black (CMYK).

This in itself is not a hindrance for most suppliers since they will already have equipment capable of delivering a number of additional colors over and above CMYK, but for those with a basic four color limit, or those with only digital print facilities, security Inks for primary validation work will require additional runs through the press or the utilization of alternative security devices if only digital methods of production are available.

As hinted at earlier, there is a wide choice of security inks, all designed to deliver specific reactions in order to safeguard against a whole range of potential threats. These threats range from unauthorized replication right through to detecting alteration and substitution attacks.

Instances include printed codes that may be erased or replaced in order to disguise an out of date product, or to mislead consumers by indicating that a component is suitable for a particular use when it is not.

An illustration of ‘change of use’ would be where a code on an electrical product label specified it was safe for industrial use when it had only been authorized for domestic applications. Such changes would immediately increase the ‘value’ of a product by just changing or altering the identification code on the label.

Before deciding on the suitability of ink as a method of adding a security feature to print it will be necessary to evaluate the protection from potential risks that such a solution will provide. Decisions should be made relating to whether protection is required at point of sale or right through the supply chain.

Additionally, it will be essential to establish whether an ink will be used as a primary, secondary or forensic identification feature, or maybe all three. Designers should appreciate that if they are also to incorporate supplementary devices such as holograms or serialized codes then consideration must be given to how these all complement each other in the system and whether any unnecessary duplication of function is encountered.

It is, for instance, a duplication of function if a hologram is used for primary recognition and combined with another optically based feature such as a visible color change ink. For low to medium type security protection, such combinations can add considerably to the cost of the final product with little additional benefit.

There follows an inventory of inks and vanishes that are suitable for security packaging and labeling applications. This list is non-exhaustive since new inks and pigments for such uses are under continual modification and development.

INKS THAT MAY BE USED FOR PRIMARY VALIDATION PURPOSES

These inks are designed to supply an initial indication of provenance that may be obtained visually within a few seconds.

Clear or optically variable varnishes

The use of varnish in print provides a matt/gloss effect that is highly resistant to scanning and attack through the use of ‘home office’ laser and ink jet printers that are often used for low end replication of counterfeit labels and packaging such as small cartons.

By adding a color shifting pigment to the varnish security can be increased even further. In the illustration (Figure 4.1) it should be noticed that braille is also present on the carton, as a prerequisite of pharmaceutical requirements for those that require tactile confirmation on the pack as they may be visually impaired.

Because of the size of the color shifting pigment particles in the ink it may not always be possible to use some printing techniques such as litho and flexo to deliver optically variable varnishes on some materials.

Optically Variable inks

As their name implies, optically variable inks (OVI’s), visibly change color when tilted by the observer to deliver an easily recognizable shift in appearance.

The color change is delivered by millions of small light reflecting platelets distributed within the ink. These shiny substances provide a very definite change in color when tilted and are widely used in product protection as can be seen from Figure 4.2.

Such inks were originally formulated for banknote protection and understandably they are under continual refinement and development.

Basic OVI’s for brand protection are restricted to specialized suppliers and only the highest security products are used in currency applications.

Because such inks carry high levels of pigmentation to deliver their optically shifting effects it may be necessary to use printing processes that are designed to carry heavy ink weights such as silk screen. A high degree of consultation with the ink manufacturer is recommended before OVI work is undertaken.

Iridescent inks

Similar in visual results to optically variable varnishes, iridescent inks deliver a multitude of colors in an effect that is similar to that observed on bird’s feathers or the wings of a butterfly.

Such inks are widely available and used predominantly to decorate cosmetic and body care products. Unless an ink of this type is specially developed to provide a unique visual appearance it should only be considered for decoration and very low level protection applications.

Thermo-chromic inks

These products react to specific variations in heat and are mainly used to temporarily indicate a certain degree of chill has been reached before a beverage such as beer is consumed, or that conversely a warm drink such as coffee is still too hot to consume. Communication is achieved by a brief color change from clear to deeper shades of blue in the case of chilling and towards orange and red in the event of a heat warning.

Thermo-chromic products are also useful where permanent records of temperature exposure are required. In these instances color changes are designed to point out that a product pack or label has been exposed too long to a specific temperature threshold that makes the product unsafe or unusable. In medical applications it is useful as a tool to measure correct autoclaving temperatures have been reached.

In product security applications, thermo-chromic inks are used as an indication of genuineness since they can be formulated to react to body heat in the form of pressure from a thumb or finger and change from color to clear or the other way round from clear to color. Alternatively such inks can also react to heat created by the friction generated by scratching the ink with a rough object such as a coin or a thumbnail. (note: some ink encapsulation processes deliver a similar result)

It is also possible to embed thermo-chromic properties into plastic containers and closures which enhances their use even further.

Ultra violet reactions are a form of photoluminescence. But this reaction ceases the moment the UV light source is switched off.

An additional reaction, known as phosphorescence can be made to deliver luminescence that continues to fluoresce after the UV light source has been extinguished. The intensity of the luminescence displayed by the printed image then decays over a period of time that it can be measured accurately. For this reason such inks are very useful to security printers because they can be tuned in both color and the period of time they take to decay. These properties make ideal authentication devices.

Metameric inks

Metamerism is a phenomenon that is used to describe color changes that occur in certain specially formulated inks when they are viewed under different light sources. For instance, under natural daylight a pair of color-matched inks will appear to be exactly the same. Expose the inks to an alternative light source and a very different image is observed.

Therefore metameric inks need to be ‘paired’ to work effectively. A metameric pair can be alternatively defined as two colors with different spectral compositions that generate the same color stimuli under certain conditions such as lighting, size and angle of viewing or the chromatic sensitivity of observers. In fact, we talk about a metameric pair because this effect is evident when comparing at least two color samples.

Such inks are available in a variety of combinations and since it is not always evident that such inks are metamerically paired on a piece of secure print it is possible to deliver a highly covert authentication feature that can act as an effective counterfeit detection measure.

Photochromic inks

Photochromism is the ability of a chemical to respond to light and display this response in the form of a color change. In the case of sunglasses, these chemicals react to light intensity by changing to a darker shade as the sun gets brighter.

The security-related print industry has long toyed with such reactions and tested these compounds on a number of occasions. When carried in inks, the photochromic chemicals (spiropyrans) can so far only be made to react to high intensity light such as a camera flash, and further limitations are that they are affected by daylight which causes loss of color fastness over time.

The color change reactions of photochromic inks are reversible and once a change develops it lasts much longer than the changes that are observed when phosphorescent light excitation is removed.

Work is currently underway at a university in the UK to improve the range of colors available and the results so far promise an early solution to these problems.

If successful, expect photochromic reactive inks to be more widely used in future.

Conductive inks

These products are widely used in the printed electronics market which is way beyond the terms of reference for this study.

However certain developments in this area are of interest to product security inasmuch as work carried out recently in Germany has delivered a conductive ink that can interface with the screen of a smartphone.

An invisible printed pattern is applied to the tag, label or board used in packaging for promotional purposes and authentication.

This pattern acts as a ‘pointer’ or trigger so that when the surface of the printed item is touched to the smartphone or tablet screen it interferes with the capacitive touch functions and acts in the same way as a finger is used to tap or navigate the system.

Codes can vary and when activated can ‘virtually deliver’ the user to a webpage where product authentication can take place. They need to work in conjunction with an app which needs to be installed before touching the print to the tablet or smartphone screen.

Machine readable inks

In order to remove any ambiguity that may exist when authenticating inks and deciding their genuineness manually, a number of automated technologies now exist in the form of small hand held gadgets that recognize specific chemical signatures and respond with an audio or visual signal that confirms or rejects provenance of the ink.

Unique chemical signatures are formulated from a number of ‘rare earth’ materials that are then embedded in minute amounts within a printing ink or varnish.

The particles, which are invisible to the naked eye, glow brightly when lit up with specific frequencies of light. These specks can easily be manufactured and integrated into a variety of crystallized materials, and can withstand extreme temperatures, sun exposure, and heavy wear.

These crystals are doped with elements such as ytterbium, gadolinium, erbium, and thulium, which then emit visible colors under say, near-infrared light. By altering the ratios of these elements, it is possible to tune the crystals to emit a number of colors in the visible spectrum.

Such materials are then engineered to deliver highly specific reactions that can be used in millions of disparate applications, thus protecting the integrity of each system from reverse engineering.

Readers that recognize these signatures, range from small inexpensive hand held devices that are powered from a battery, through to high speed automated machinery that is utilized to validate batches of banknotes before they are placed back into circulation after being received over the counter.

INKS THAT ARE DESIGNED TO DETECT AND DETER TAMPERING

Tamper evident inks have been used for decades to protect financial documents from alteration and erasure attacks. They have also found wide use in the construction of tamper evident labels so that messages can be displayed such as ‘opened’ and ‘void’ to reveal that a label used as a closure can display a warning message and be destroyed during the unfastening process.

More recently, with the popularity of product coding it has become necessary to protect products that carry date and product coding, serialization and batch specific data from alteration and/or erasure assaults.

This is because there is hidden value in these codes as they are used to identify when products should be withdrawn from sale because they have reached their ‘use by’ date. There is also value in protecting the codes used to identify the performance of products and there is money to be made in remarking those with low end specifications in order to disguise them as offering higher functionality. An illustration of this point is the remarking of computer processor components or graphic cards so that they pass off as more highly priced articles.

Inks used for coding then, need to be permanent and they also require a secure platform or base on which to reside.

Placing a screen of erasable ink under the code offers a degree of protection, which can be enhanced further by including a degree of chemical sensitivity in the ink to protect against solvent or bleach attacks that are aimed at erasing the code so that new data may be placed down or the original code altered to deliver a different message.

Ultra violet reactions are a form of photoluminescence. But this reaction ceases the moment the UV light source is switched off.

Metameric inks

Photochromic inks

The color change reactions of photochromic inks are reversible and once a change develops it lasts much longer than the changes that are observed when phosphorescent light excitation is removed.

Work is currently underway at a university in the UK to improve the range of colors available and the results so far promise an early solution to these problems.

If successful, expect photochromic reactive inks to be more widely used in future.

Conductive inks

These products are widely used in the printed electronics market which is way beyond the terms of reference for this study.

An invisible printed pattern is applied to the tag, label or board used in packaging for promotional purposes and authentication.

Machine readable inks

Unique chemical signatures are formulated from a number of ‘rare earth’ materials that are then embedded in minute amounts within a printing ink or varnish.

Such materials are then engineered to deliver highly specific reactions that can be used in millions of disparate applications, thus protecting the integrity of each system from reverse engineering.

INKS THAT ARE DESIGNED TO DETECT AND DETER TAMPERING

Inks used for coding then, need to be permanent and they also require a secure platform or base on which to reside.

Likewise, codes that are used for product authentication are often protected by a layer of scratch off ink that is removed during the verification process, ensuring that the same code cannot be reused again later.

For additional security the scratch off panel can be overprinted with a tamper evident design.

Forensic inks and taggants

For very high security applications it is necessary to add a form of forensic security to the ink. Marking an ink or other print related material such as paper or label adhesive with a forensic signature requires the addition of a tag or taggant.

These tags are microscopic particles that are embedded in the material and are distinguishable either through their chemical or biological signature or through high power magnification. They are sometimers referred to as nano-markers.

Intro section

Ink is, of course, an indispensable component of any label or packaging application. Without ink it is not possible to reveal what is inside a pack, instructions regarding usage and of course who made the product. A blank label is essentially useless.

Since ink is used to open a visual communications channel between every party in the process of product distribution and final use it follows that it can also offer a handy method of communicating provenance at the same time.

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The importance of printing substrates in brand security

Feedimporter — Wed, 25 Nov 2020 16:22:00 +0000

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The importance of printing substrates in brand security

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Learn more about how substrates play a key role in protecting a brand

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So that security is not compromised, it is essential that such protected materials are not available widely and that supplies are limited to bona-fide security-related printers and converters only. In order to achieve this objective, recognized and trusted suppliers of security materials and substrates will endeavor to ensure that such security materials are only made available to trustworthy converting partners within the industry.

PROTECTION FOR NON-ORGANIC PACKAGING MATERIALS

Initially, this article will focus on non-organic materials that are used for packaging applications.

Metals, in the form of aluminum and tin plated steel are the primary materials we are dealing with here. Both are inherently difficult to secure (in raw material form) since they are not able to carry easily accessible markers that would be useful in assuring provenance. Therefore most security features need to be added further down the supply chain in the form of labels, inks or serial marking placed using inkjet print heads or laser engraving.

The one notable exception to this theme is the use of holographical, decorative and security embellishment to the raw material that can be applied before it is converted into cans or box format.

Holographic designs can be applied using a laminate or varnish onto which the optical effects are engraved using high definition shims and very heavy pressure. This gives the material a diffractive finish that can be formatted to provide a continuous image that delivers similar effects to those seen on elementary holograms (see Figure 3.1).

It should also be remembered that metal is the most widely used material for the twist off lids used on glass jars. Tamper evidence on these components is delivered through vacuum sealing the container after filling and installing the closed twist cap.

The vacuum draws down the closure and allows for a ‘button’ mechanism built into the closure to remain firmly in place when pressed. In this form the cap is safe as the vacuum is only released on opening. After opening the button moves up and down when pressed and provides an audible warning. A message on most closures advises to ‘reject if button clicks on pressing’.

Metal, such as aluminum in thin rolled form, is also used as a lidding closure although metalized polyester type products are more popular in food and drink applications since tamper evidence is obtained through the use of heat sealing the film to the container to form a protective bond that guards against tampering and keeps the product fresh at the same time. It is possible to produce both these materials with a difficult to copy optically variable design that offers the twin benefit of tamper detection and authentication through the application of one device.

The combination of such films with blown or vacuum molded containers can also be seen in use for protecting non-food items such as printer cartridges and engine oil packaging. Both products are vulnerable to counterfeit and refilling attacks.

Finally, metal is a material that lends itself to rolling and tactile forming so that cans may be produced. Special opening mechanisms such as pull caps and the traditional use of a can opener make these components pilfer proof without further need for elaborate anti-tamper features.

The other inorganic packaging material that is difficult to protect at base level is glass. It should be recognized that glass containers can be colored and formed in many shapes and sizes and this requires a high degree of skill and expensive plant and equipment which is generally outside the reach of most counterfeiters.

For this reason most glass vessels of individual design, such as perfume bottles (Figure 3.3) and such like are relatively safe from the attention of copycat attacks. This does not mean to say that they are safe from refilling assaults though.

PROTECTION FOR ORGANIC PACKAGING AND LABELING MATERIALS

The use of organic materials such as paper, board, plastics and more recently organically derived polymers and synthetics form the most widely used materials in packaging and labeling applications.

Paper

This material is by far the most popular for labeling applications because it is economic and can be easily transformed into labels.

Paper is easily printed and can be converted into both wet glue and pressure sensitive format in a variety of styles such as sheets, rolls and die-cut packs for onward automated application to containers, jars, cans and bottles.

Furthermore paper is endlessly flexible inasmuch as it may be manufactured in a range of colors and finishes and can be combined with other materials in order to provide decoration, instruction and of course security.

A further benefit of labels in paper format is that they are not robust enough to survive refilling attacks, since they are easily degraded and destroyed during attempts at removal, either before recycling or for re-applying to containers or bottles of fake product.

Paper is also the most trusted of materials since it is extensively used for high security print products such as passports and currency.

Therefore it should be no surprise that security labels manufactured from this resource mimic many of the safety features found in paper money and traveler identity documents.

The most predominant security feature that is used in security paper is the watermark. This feature is added to the paper at the ‘wet end’ of the papermaking machine and through the use of a specially constructed meshed roller the fibers in the wet wood-pulp are arranged into a form delivers a mono-tonal image that can be viewed in transmitted light.

The most commonly used process to make watermarked paper is the Fourdrinier process and the distinctive marks are formed by wires in a cylinder known as a ‘dandy-roll’. Because papermaking is such a complex operation and requires high investment as well as highly skilled operators, adding security features to the material is an effective method of thwarting copy attacks since these economies of manufacturing scale are not easily available to the counterfeiter.

Watermarks may be placed in discrete register with labels so that they always appear in the same position or alternatively they can be incorporated in a continuous band in line with the machine direction of the web.

If discrete watermarks are chosen as a method of securing the material then the sheet will need to be registered with the print during the set up process on the printing press.

If a continuous web of material is being converted, as you would expect for roll label pressure sensitive production, it is necessary to fit a web guiding and register device to the printing machine in order to keep the mark in constant register with the printing web.

Established suppliers of pressure sensitive watermarked label material will be able to assist with the design and placement of a watermark on a self-adhesive substrate. Since discrete personalized watermarks can be expensive to originate and material will only be supplied in volumes high enough to set off the costs incurred with setting up a papermaking machine, it may be more practical to adopt a ‘stock’ design for shorter runs.

A number of alternative security features can be added to paper in order to check its provenance and make it difficult for counterfeiters to copy.

In its raw wet pulp form the material is fluid and it is possible to add visible and invisible ‘markers’ to the substrate at this time. Markers can take the form of colored fibers, small polyester disks (planchettes) or microscopic hi-lites. The latter are UV light reactive particles that shine like stars when irradiated with UV light.

Fibers can be added in various colors and at various lengths so that they can be observed on the surface of the substrate either by looking for them with the unaided eye or by exposing them to UV light. Measuring the length, distribution and color of these fibers allows for an individual ‘fingerprint’ to be embedded in the material, thus making it almost impossible to replicate by anyone wishing to copy this security feature.

It is also possible to embed narrow, two or three millimeter wide, security threads made from printed or metalized polyester in the security paper used for packaging and labeling requirements. Identical features can be viewed in banknotes and these threads can be deeply implanted in the material or made to alternate between the surface and interior of the paper in conjunction with the watermark (see Figure 9.1 on page 83).

There is also potential to add further security to these threads by applying coatings that react to thermal stimulation such as body heat from a finger or from friction through rubbing the surface of the thread.

Finally, it should be recognized that optically dull paper or board should be chosen for security applications where invisible UV security inks are to be applied during the printing process. This advice is applicable to inks that react to UV light and should not be confused with UV ink curing systems which is an entirely different process and used for drying inks and not for authentication purposes.

It will be discovered later in this training module, security paper used in conjunction with other complementary security devices, can be a resilient defense against both copy and tamper assaults.

Board

Carton board for security applications consists of both pasteboard and pulp board. A slightly different approach is required when adding security features to these materials. Together, these substrates provide an excellent primary packaging material as well as offering a handy platform for important supply chain information such as route to market, manufacturing source and expiry data to be added.

Board products are also an ideal material for swing and hang tickets that can be used to provide useful guidance on a product’s capabilities, contents and size. In swing ticket and hang tag format both products can carry their own individual security features or be further embellished with security inks, foils or RFID labels.

Paste board, as its name implies, consists of a number of paper plies which are bonded together to form a thicker sheet that can be used to make boxes or cartons that protect products in transit and in storage before use.

These individual paper plies can carry their own authentication features in the form of forensic markers or more cost effectively offer a low grade security feature through mixing colors within each ply so that a ‘sandwich’ of colored material is created. Authentication is simply a matter of tearing the tag to reveal a colored core.

It is also possible to metalize board products so that they display a variety of colored shades and also to emboss the metal coating to deliver an optically variable image or holographic decoration, both of which offer certain low grade security devices by making the material difficult to replicate by scanning or by attempting to copy the design using publishing software and a desk top printer.

PROTECTION FOR ORGANIC PACKAGING AND LABELING MATERIALS

The use of organic materials such as paper, board, plastics and more recently organically derived polymers and synthetics form the most widely used materials in packaging and labeling applications.

Paper

This material is by far the most popular for labeling applications because it is economic and can be easily transformed into labels.

Paper is also the most trusted of materials since it is extensively used for high security print products such as passports and currency.

Therefore it should be no surprise that security labels manufactured from this resource mimic many of the safety features found in paper money and traveler identity documents.

Sophisticated shrink wrapping films aided by specially marked tear tape and colour changing effects are now being combined with coding technology to protect bottles and similar containers from refilling, counterfeiting and dilution attacks. These take the form of shrink sleeves that are applied over the closure in order to provide protection and indication of first opening.

Developments in disabling RFID tags in an attempt to ‘kill’ them after a product is opened to prevent re-use also offers a dual tamper protection benefit to users.

Material science continues to make headway in this important area with a number of universities in the US, Europe and China now involved in researching and developing films and adhesives to meet the threats of the future.

Shape memory polymers

One further technology that is beginning to set foot in this market is SMP (Shape Memory Polymer). The material changes shape and form at specific temperatures thus providing an indication of authenticity and temperature change in the supply chain.

SMP materials can be converted into labels and tags and therefore provide a new range of opportunities for the brand protection industry as a whole.

Shape Memory Polymers are smart materials that have the ability to return to their original shape from a temporary state through the introduction of a trigger event – in this case heat.

Such label materials can be constructed to ‘store’ 3D shapes in the form of embossed text or logo’s and then programmed to release these when exposed to a specific temperature change, thereby creating unique massages that can be utilised as brand protection confirmations or specific safety alerts.

If discrete watermarks are chosen as a method of securing the material then the sheet will need to be registered with the print during the set up process on the printing press.

A number of alternative security features can be added to paper in order to check its provenance and make it difficult for counterfeiters to copy.

It will be discovered later in this training module, security paper used in conjunction with other complementary security devices, can be a resilient defense against both copy and tamper assaults.

Board

Other fraud deterrence techniques include surface embossing to create discrete patterns and film coating which delivers various optical effects.

Material biometrics

At this point is worthwhile exploring some new and interesting technologies that utilize the surface properties of material at a nano-level to deliver an assurance that the material is original and not a fake.

Whilst these technologies are not per se print related they can be used to authenticate the very material on which the print resides.

This may be a label, a carton or a plastic container.

At a microscopic level each square millimeter of material is different. As an illustration of this fact a piece of paper is composed of millions of discrete fibers, all interlocked to form a continuous sheet.

If it was possible to ‘grab’ an image of say a specific square millimeter of the surface of a label – each label’s top right hand corner for instance – it would be observed at a nano-surface level that each was different and carried its own individual mat of fibers arranged in a dissimilar pattern.

Technology now exists that can record a ‘digital fingerprint’ of a predetermined, minuscule piece of material on every label, making it possible to identify the provenance of each one accurately. The process is also referred to as random feature identification since it allows items to be identified through the random changes that each item possesses at a microscopic level.

Sometimes these may be imperceptible changes in a letter of type – which will be addressed later – or imperfections in the material surface.

In biometric terms this technique is similar to the fingerprint recognition methods used by the police to identify suspected felons at a scene of crime.

A search for one fingerprint in a database of many millions takes time but this can be shortened through the use of complex software and high power computers.

Material biometrics works in a similar way but can deliver a positive or negative result much more quickly if the ‘fingerprint’ from the material is linked to a sequential barcode. This allows the barcode to act as an identifier and as a link to the surface material scan and its corresponding record in the database.

Various other methods exist that include the recognition of visible fibers randomly distributed in the material mat and laser illumination of the material’s surface and analysis of the reflected light from each predetermined area of a label. For speedy recognition both these methods also rely upon a barcode or other machine readable reference from which to match the image.

This whole fascinating field of material biometrics is changing constantly as imaging and computer processing and data management skills progress. Technology enablers such as cloud computing, smart phone apps and scanners as well as 4G communication systems are all conspiring to provide more on-the-spot identification methods for both the police and those investigators involved in brand protection.

Plastics and synthetics

These materials are very hard to protect from counterfeit attack. The best method of defense is to follow similar procedures to those used in glass containers; stylish, registered designs that are specific to each product should be considered since having to copy discrete designs and molded inlays can be restrictive when it comes to try to knock-off a particular brand.

This technique is not sophisticated enough to deter the determined counterfeiter though.

Further protection can be achieved through the selective use of micro-taggants which are uniquely identifiable particles embedded in base material and identifiable through proprietary readers or laboratory analysis.

Metalized films

Metalization adds a layer of complexity to films that can act as a barrier to counterfeit attack. Again this process is unlikely to discourage the determined counterfeiter. Opportunist attackers however will find it off-putting and may well try to experiment with an alternative target that takes less effort.

Metalization is mainly used as a barrier coating but also for decoration. In decorative form as will be discovered later, can be useful as a base for creating difficult to copy optically variable features such as holograms and high resolution multi-diffractive effects that are resistant to copy attacks and counterfeiting in general.

Tamper evident adhesives and tapes

Common usage in the labeling and packaging industry of the terms tamper evident, tamper resistant and tamper proof understandably lead to a good deal of confusion.

This is because these terms are used to describe a variety of similar functions that are designed to combat some very different threats or risks.

Tampering with labels to remove or mask fraudulent activity has been a threat ever since the introduction of self-adhesives over a half century ago.

The benefits that are available, for instance on removal of price marking labels, allow perpetrators to exchange the labels on low cost items for those carrying a higher price. This is why the majority of price marking labels carry distinctive cuts and indentations around their circumference.

Attempted removal leads to destruction of the label and immediate evidence that an attack has occurred.

Such benefits may be viewed as negligible now since price marking labels have all but disappeared with the growth of universal product coding and item scanning at check outs. Nevertheless this acts as an easily recognisable use of security cuts on a self-adhesive label.

Trends in tamper evidence

Tamper evidence has since evolved into a number of distinct forms, all aimed at protecting products from unwanted and often dangerous activity such as refilling, product spiking – which is linked to extortion attacks – and pilfering which involves taking part of the contents out of a sealed pack and then resealing it to conceal the fraud.

Primarily the technology provides visual confirmation that a product that has been sealed has not been previously opened. This is achieved through the application of tape or a label over the closure or vulnerable points in the pack or at point of contact between the lid and body of the container.

However, as such tamper evident indicating products have evolved so have the techniques to compromise them.

Sophisticated stress-indicating materials and adhesives that are resistant to temperature changes are now being combined with authentication devices in an attempt to combine these essential functions.

The pressure sensitive materials industry has developed a range of ‘VOID’ substrates that separate when removal is attempted and the word ‘VOID’ appears when the top of the label is peeled away after fixing down. Otherwise a bespoke approach can be taken using a brand name that becomes tamper evident when opening a carton holding valuable goods such as a computer or smart phone.

Other methods involve the use of specially selected traditional paper face stock with good resistance to tear. It is necessary to also have a good understanding of the packaging material that the tamper evident label will be applied to. Therefore it is often necessary to work closely with an adhesive supplier and the pressure sensitive adhesive supplier in order to achieve the best results of ‘initial tack’ and permanence. This is especially important where labels are being applied during an automated container filling process.

An alternative approach is to use ‘destructible vinyl’ face material for the tamper evident label construction. Vinyl face materials are high performance filmic products that are frangible on removal. Such products are utilized in heavy duty applications where resistance to moisture, heat, cold, dirt and grease would interfere or degrade paper.

Evidence of tampering is provided by the material itself fracturing into minute pieces as soon as removal is attempted. As removal involves a certain degree of ‘picking’ at the edges of the label the material fracturing becomes evident thus visually drawing attention to the attempts. Further security can be added at no cost by printing a solid colored border around the edge of the label making tampering even more evident.

By adding holograms plus track and trace technology, and in some cases RFID, these tamper evident devices will become an important component in future brand protection applications where anti-theft properties as well as protection from refilling and counterfeiting are important attributes (Figure 3.10).

The secret behind such materials lies within their molecular structure and their ability to change state rapidly when exposed to the correct trigger – in the case of the materials illustrated above this is 65°c.

Whilst some work is being done on triple shape memory materials that change shape and then revert back to their original shape after the trigger event, most SMP materials are irreversible after activation.

Since there is no need for specialist activation/authentication equipment the materials are ideal for consumer verification since all that is required to set the change in motion is a cigarette lighter or hair dryer.

It is too early to appraise the attributes of such a material yet but it shows promise in tamper protection when combined with the functions mentioned previously.

Forensic markers and taggants

Anyone who watches popular crime scene investigation TV and direct streaming programs will be familiar with forensic analysis that relies upon the principle that where we go we all leave microscopic traces of our passing along the way. This may be in a trace of DNA left on a cup or glass or a fiber from our clothes. These traces can be used in evidence should any of our misdemeanors be challenged or brought before a court of law.

Forensic markers and taggants are unique artificial and natural compounds that can be placed in suspension within a liquid or coated on to a solid object and even incorporated, at infinitesimal levels in the material used to make packaging and labeling substrates such as inks, paper, plastics and also, adhesives.

These substances consist of naturally available, but rare compounds that are mineral based or artificial organically produced ingredients that mimic DNA in their construction.

This topic will be revisited in more detail later in this study but at this time it is sufficient to know that many products today carry these forensic markers as a definitive measure in order to prove provenance and also purity.

In defining provenance we are referring to authenticity and a check against unauthorized copies being made. In terms of purity it should be recognized that where there is a risk of dilution attack, then covertly added ingredients that can be recognized in proportion to their mix, within a liquid or powder, are useful devices to measure unsanctioned dilution.

This measure is obtained through minute levels of taggant that are placed in (say) a liquid at a ratio of one part per billion. This is a quantitative amount so it should be present at all stages of the distribution system. If later analysis provides data that shows (say) one part per ten billion then it is provable that unauthorized dilution has occurred.

Likewise forensic taggants can be placed in inks, paper or board and then measured and recognized downstream in the distribution cycle in order to detect counterfeit products that may have infiltrated the supply chain.

It should be noted in passing that some sources claim that synthetic label materials are more easily recycled than their paper equivalents, since synthetic labels are separated during the recycling process and recovered for use in various polyolefin compounds.

Paper labels, especially those used on glass or plastic containers break down and create a mushy pulp that has to be sent to landfill.

Intro section

In the introduction to security and product protection reference was made to the requirement for packaging and labels to protect, contain and inform as well as manage risk on behalf of the brand owner. Since materials such as metals, glass, paper, board, plastic and flexible films are the basis for all materials used in containing and protecting products, it follows that these resources offer an ideal platform on which security features such as tamper evidence and authentication systems may be based.

Such materials are widely available in their basic form for onward conversion in to common forms of wrappings and containers. In order to make these constituents more secure, so that they can act as indicators of provenance, it is necessary to modify them in some way so that they may be easily distinguishable and convey information that can be useful in establishing whether a pack has been opened previously, or the product labeling should be regarded as unauthentic in some way.

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