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Managing a self-adhesive materials inventory

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Setting up an automated stock management system has become critical as label printers are increasingly asked by brand owners to provide traceability of all self-adhesive materials for every batch

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Test methods for pressure-sensitive labels

Feedimporter — Thu, 26 Nov 2020 16:27:00 +0000

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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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5 years 8 months ago

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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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5 years 8 months ago

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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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