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.
Figure 5.1 Surface of an uncoated paper
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).
Figure 5.2 Surface of a coated paper
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).
Figure 5.3 Structure of a direct thermal paper
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).
Figure 5.4 Paper face material die-cutting characteristics. Break point at 60-65% compression
Figure 5.5 Synthetic paper shares the break point of film at 90-95% compression
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.
Figure 5.6 Main PS label end use applications
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.
