Printing technologies and inks for shrink sleeve decoration

While the majority of shrink sleeves are printed on mid- to wide-web presses, there has also been significant growth in sleeve printing using the narrow-web process, especially using UV flexo, and to a lesser extent, water-based flexo. Whatever the sleeve printing process, and whether it is wide- or narrow-web, it is important to have an understanding of the inks used for sleeves. This requires us to look at the ink chemistries and the different types of raw materials used.

SHRINK SLEEVE INK CHEMISTRIES

The primary ingredients in inks transcend the different printing technologies. Every ink has a pigment, a resin, a diluent or a solvent, as well as various additives that are incorporated to improve performance. These ingredients are all summarized in Figure 4.1, and each will be examined in greater detail.

Pigments

The first and most important aspect of ink to consider is the pigment, the coloring matter that goes into the ink to give it, say, a red, blue or green color. The different pigments used in shrink sleeve printing inks may appear the same color when first printed, but they can perform quite differently after they are processed in any way – for instance, when they have passed through a heated shrink tunnel, are filled or used in a chemical environment, or are exposed to strong sunlight. In these examples, the pigments may change color or 'bleed' to differing degrees. An understanding of pigment chemistries and how they are defined can therefore be an important factor in ink sourcing.

Pigments are identified, globally, by a Colour Index™ (CI) number that defines the particular chemistry of the pigment, enabling it to be classified along with other products whose essential colorants are of the same or different chemical constitutions. For example, Figure 4.2 shows the CI numbers for Red 57.1, Red 184 and Red 177. These three different Red CI numbers will each give a magenta or rubine shade, but the chemistry of each of the pigments is different: differences that can affect performance in terms of light fastness, chemical resistance or heat fastness. It is worthy of note that the use of incorrect pigments in shrink ink can result in the ink changing color upon shrinking in the heat tunnel.

Chemical resistance of the chosen pigment is also important. For example, in the case of a household chemical where the container is being filled after sleeving, it is imperative that the ink be resistant to the chemicals to avoid bleeding, should the chemical make its way between the sleeve and the container.

Consideration and understanding of ink pigments, and the end-user requirements of the ink, are always important. Take, for example, the requirement for light fastness. The four yellow colorants shown on the chart in Figure 4.3 are all Pantone® yellow. If they were printed on a sheet of white paper, they would all appear as Pantone® yellow. Place them in a light fastness tester or expose them outside to light, however, and the top one, which is a dye-based material, will fade to the point of disappearing after a 24-hour time period. Again, depending on the choice of pigments, degradation of the color will occur over time with exposure to UV light. 

It is also possible to select pigments with chemistries that hardly degrade at all. Despite these pigments being more expensive, they may be an absolute requirement, depending on the application in question. For example, if the sleeves are for a chemical that will be stored in a greenhouse or outside at a hardware store, then the ink's light fastness is a requirement that cannot be compromised. The sleeve printer needs to make sure that the pigments in the inks are appropriate for the application.

Resins. Resin chemistry forms the backbone of any ink, and it is with resins that pigment manufacturers, ink suppliers, and chemists select from a range of options. Some of the most common are:

• Nitrocellulose
• Polyamide
• Acrylic
• Phenolic
• Epoxy Acrylate – used in UV/EB inks and coatings
• Urethane, Polyester – used in solvent and UV/EB inks and coatings

Resins provide the main basis of the ink and will affect adhesion, flexibility, resistance properties, drying speed/cure, and overall end performance of the printed material.

Nitrocellulose, urethane and polyamides are commonly used in solvent systems. Acrylics can be used in water-based systems, and there are different epoxy acrylate, polyester acrylate, and urethane acrylate chemistries that can be used in radiation curable systems.

Each chemistry provides a different performance. With shrink sleeves, the most fundamental requirements of the ink are that it provide good adhesion, that it be flexible, and that it can follow the shrink profile of the film in the shrink tunnel.

Other properties that ink suppliers consider are resistance properties, running speed, and ink drying-speed requirements. Ink suppliers will select different chemistries based on these requirements to optimize performance.

Diluents. Another raw material component of inks are diluents. These are used to reduce the viscosity of the ink. For water-based ink, the diluent is primarily water, and for solvent-based ink, the diluent can be alcohol/acetate blend. UV systems use monomers. Diluents are always selected with the shrink application in mind. With solvent rotogravure systems, it is not possible to use ethylacetate as a diluent as it impacts the chemistry of the shrink film. Since every solvent system has its preferred solvent, should that solvent be unusable with shrink film, then it follows that not all solvent systems can be used with shrink film.

Additives. There is a wide range of additives added to inks to achieve various performance characteristics. Among the most common are: defoamers (or anti-foaming agents), waxes and silicones, matting agents, photo-initiators, adhesion promoters, surfactants, and optical brighteners. Among the most important additives for shrink sleeve applications are waxes and silicones that are added to achieve the appropriate coefficient of friction on the last-down white or slip coating.

From the ink manufacturer’s perspective, corona treatment of shrink films by the printer is important. A light bump treatment of almost any film with a corona treater – whether shrink labels, a flexible packaging construction, or a pressure-sensitive film label – is always recommended. As films age, they will lose treatment. As film is handled, or as it sits in the warehouse . with temperature and humidity variations . the treatment level will diminish. A light bump treatment will bring the dyne level back to where it was intended to be. Shrink films are very printable in terms of print performance, but achieving ink adhesion is not always so easy. PVC and OPS are generally very good for adhesion, while PETg is not as good.

Polyolefin can also be challenging for some chemistries, such as water-based or UV.

Solvent chemistries, whether for rotogravure or flexo, generally offer the best adhesion to shrink films. With water-based, radiation curable UV systems, and digital, films will generally need to be treated. In some cases, it may be necessary to use a primer.

If manufacturers recommend treatment, be careful not to over-treat films because this can create issues of blocking in the roll. Basically, corona treatment changes the surface characteristics of the film. The level of treatment is measured in dynes. The printer can undertake some easy dyne tests to make sure that the right dyne levels are being achieved. Corona treating puts a carboxyl or hydroxyl functionality on the film surface so that the ink chemistry can chemically bond to that film surface. With the proper resin selection, the right chemistry in the ink and the proper application and drying or curing, it should be possible to adhere to these materials. Nevertheless, a light treatment during the printing process is generally recommended.

CHALLENGES FOR SHRINK SLEEVE INKS

What are the market requirements, and what are the challenges for shrink sleeve inks? As Figure 4.4 illustrates, shrink sleeve inks require a very high color strength ink that can cure quickly and maximize press speeds. The presses run fast and inks are needed that can print, cure and dry quickly in order to be as productive as possible, achieve a good print quality, achieve good adhesion characteristics, and have a low odor. It is also necessary to meet various environmental regulations like Proposition 65, Nestlé and the Swiss Ordinance on printing inks, which has become the standard for food packaging inks in Europe, or FDA regulations in North America.

Ideally, the shrink profile of shrink film will not change when ink is applied to it. Nevertheless, a few layers of white or black ink coverage will surely start to change the shrink profile of the raw film stock - it likely will not shrink as far or as fast. Ink systems are challenged to be as robust as possible to adhere to the variety of films available.

Inks also need to exhibit good surface slip properties, especially for the seaming and application stages. The coefficient of friction (COF)  is important for the last down white which, due to the sleeve being reverse printed, is the layer of ink that comes in contact with the tooling on seamers, applicators, and ultimately the surface of the container upon which the sleeve will be shrunk.

Hazing can be a challenge in situations where there is a clear, i.e., unprinted, area incorporated into a graphic design. While a coating may be desirable for COF reasons, the recommendation is generally to leave the window uncoated (or unprimed) to avoid the hazing that can occur. If the film must be primed or coated, then avoid using UV chemistry, which will get hazy as it shrinks. If it is necessary to put slip coatings or primers as a last down from a seaming standpoint, it is recommended to use a solvent-based or water-based chemistry.

Another phenomenon that frequently challenges ink in a shrink sleeve is called the ‘wet look’. In fact, the ‘wet look’ can be divided into two totally unrelated effects. The first has become known as the ‘wet T-shirt’ effect, which is caused by moisture that gets trapped between the sleeve and the container as it passes through a steam tunnel. The ‘wet T-shirt’ effect is temporary and disappears as soon as the moisture evaporates. The second ‘wet look’ is more complex in its cause, and more complex to resolve. If a wet look develops, and it is not caused by moisture (as would certainly be the case if it was not shrunk in a steam tunnel, for example) then we refer to this as the ‘Sellotape’ effect. The ‘Sellotape’ effect is caused by the refraction of light when two very smooth surfaces come in contact with each other. In fact, it is observed when two panes of glass are put together, or, if the smooth inner surface of a shrink film is forced tightly against the smooth outer surface of a glass or PET container. 

The ‘Sellotape’ effect is really a nuisance as it diminishes the overall look and finish of the container. Fortunately, there are ways to overcome this issue. With certain ink formulations such as whites and clear coatings, the ‘Sellotape’ effect can be minimized or eliminated. These inks and coatings operate on the principle of creating a textured surface on the ink to create distance between, and eliminate the smoothness of, the two contact surfaces. This creation of space between the two contacting surfaces is generally enough to eliminate or greatly reduce the problem.

THE IMPORTANCE OF WHITE

As previously mentioned, most shrink sleeves are reverse printed. In other words, the ink is printed on the inside of the film, so the ink is trapped between the shrink film and the container. This means the white is the last down – the colors are printed first, followed by the opaque white.
Most shrink sleeves have a lot of opaque white coverage, so the last down white is very critical. Coefficient of friction (COF) is a concern as the ability of the ink to slip over the tooling in high speed seamers and automatic application equipment is essential.

Sleeves generally call for high opacity. To achieve this, printers start putting whites down with a very coarse anilox, which becomes a problem. If whites are being put down with a 10 BCM (billion cubic micron) volume anilox (metric=15.5 cm3/m2), then with high shrink applications the ink is actually going to start piling up on itself and will create an effect referred to as ‘tree barking’. What is generally recommended to achieve the desired opacity without laying down too great a volume is to do multiple bumps of white. Some printers will do two or three bumps of white with a 4-8 BCM anilox roller (metric=6.2-12.4cm3/m2) to get the desired opacity.

Figure 4.5 outlines the recommended values for static and dynamic COF for shrink sleeves. The COF figures are critical and should be monitored very carefully. With a little practice, it is possible to develop a quick feel for what is acceptable by simply rubbing the inner surfaces of a sleeve between your fingers. You can easily feel when the two surfaces stick to each other, or when they are actually too slippery and hence hard to control on high-speed seaming equipment. It is highly recommended, however, to use a flatbed COF testing device in your testing process.

UV PRINTING OF SLEEVES

For the narrow-web printing of sleeves, UV is really a fast growing application in the market. However, there are some challenges with UV printing (Figure 4.6) that are worthy of discussion. Typical UV presses have mercury lamps, often referred to as arc lamps that generate a lot of heat. This means that a press with good heat management (i.e., chiller rollers, cool lamps, etc.) is required. Otherwise, the shrink film will effectively start to shrink on the press and create difficulties for print register as well as future steps in the process. Therefore, a challenge exists in using typical narrow-web legacy presses, as the heat needs to be a key consideration on these presses. 

To overcome the problem of UV heat generation, some converters have turned to cool UV systems. These systems make use of reflectors and different types of light output and may require a change to the ink chemistry. A printer getting into shrink sleeves should talk to their ink suppliers about the type of UV system used on the press so that the right ink chemistry can be matched to the UV curing system- otherwise, the inks will not cure properly.

Another approach to managing heat on presses is to use chill rolls. Many converters will run the chill rolls very cold. Indeed, a lot of the press manufacturers will suggest running the chill rolls at 18 degrees Centigrade (65 degrees Fahrenheit), when in fact this is too cold! They suggest running at this temperature because they are afraid that the films will distort if they do not. However, it should be noted that chemical energy slows down in a cooler environment. As it warms up, the rate of chemical reactivity increases. Therefore, when running presses with any UV or radiation curable technologies, look at the chill rolls and run them at a minimum of 80 degrees Fahrenheit, which will not cause the film harm or damage it in any way. Some films, especially PETg, will benefit from running the temperature closer to 100 degrees Fahrenheit.

At these higher temperatures, the film will be less brittle and will be less likely to break on press. Moreover, ink adhesion improves and curing speeds will be faster, too.

Converters prefer strong blacks and opaque whites, leading them to putting down inks with very coarse anilox rolls and, in some cases, getting to a point where the ink is too heavy and mercury light cannot cure through the thick film. Be careful not to fall into this trap, and work with ink suppliers to understand the proper applications. Determining which anilox roll line count and volumes should be used to print these inks will yield the strongest colors, the strongest densities, and the highest opacities. If you exceed these recommendations, curing problems and other issues will arise. When curing problems start, you will see that adhesion of the inks to the shrink films will be inadequate. Different formulas, especially for whites and blacks, can be applied to achieve higher density and opacity. It is advisable to speak with your ink suppliers about getting the right ink, with the right anilox, to maximize performance and speed.

UV LED SLEEVE PRINTING

Though still relatively new, UV LED is an ideal choice for shrink labels. This can be explained using the Wavelength Comparison chart in Figure 4.7. This compares the output of a mercury lamp . which comprises some 98 percent of the presses in the market today . with that of UV LED.

Reviewing the chart in Figure 4.7, we observe that the mercury lamp outputs of light in the short wavelength, or UVC range, is where ozone is generated. The infrared range, on the far right-hand side of the chart, is where heat is generated. Both ozone, an air pollutant, and heat are undesirable by-products of this technology. Excessive heat generated by the mercury lamp on a press is undesirable. While the web temperature should be warm, the heat generated by mercury lights – reaching temperatures of up to 300 degrees Celsius (572 degrees Fahrenheit) – becomes problematic.

With the UV LED wavelength, as represented by the line graph in the middle of the chart on Figure 4.7, we observe a very narrow wavelength of light primarily in the UVA range with a very high intensity output. Because UV LED does not emit light in the UVC or infrared ranges, no ozone or IR heat is generated from this technology. UV light does not utilize mercury, which is a toxin. 

Since the light source of UV LED is quite different from that of the mercury light, UV LED technology requires different ink chemistry. It is important to note that the UV LED light source of UVA is a deep, penetrating light source. With a deeper penetrating light source and with higher intensity, this technology will cure inks that are darker, denser and more opaque at faster press speeds. When inks are cured better, they will adhere better, which will improve their performance in subsequent processing steps and for end user requirements on various applications. A central problem with traditional UV systems is that the UV mercury light does not cure inks very well, not to mention that lamps degrade over time and reflectors get dirty. While still a new technology to the industry, UV LED will be looked to more and more as a solution in the shrink sleeve industry and other applications such as pressure-sensitive labels and flexible packaging. 

With digital UV inkjet printing, a process known as LED pinning may also be used. In LED pinning, the surface of the printed film is given a light bump of LED to just cure the surface of the printed ink film, but the ink layer is not fully cured. The surface is cured, ready to put down the next layers. Like inkjet, the film does not need to go through turn bars and so this approach works nicely. It would not work in a flexo operation because the film needs to go through a lot of turn bars, which means inter-station curing and full curing, just the same as it would be on a conventional press.

In general, LED is ideal for shrink because there are no concerns about managing the heat out of the press. The key points to remember are:

• Optimization of the UV curing process requires consideration of not only the UV light source, but also the ink chemistry being used.

• UV LED sources are equivalent to or better than existing arc lamp solutions, but require ink reformulation.

• The key to success is the relative higher power of UV LED and specialized ink formulation.

• UV LED is ideal for shrink sleeve applications!

ACHIEVING THE BEST RESULTS

Having reviewed the requirements of the inks and ink formulations for shrink sleeve printing, we will now provide some tips and guidelines for obtaining the best results with each of the main printing processes.

UV flexo. In UV flexo printing, cure is paramount. To be successful in UV flexo printing (Figure 4.8), it is necessary to make sure the inks are cured well and that the right anilox roll is used with the right ink. Make sure that the press and the UV system, i.e., the lamps and reflectors, are well maintained. With high opacity whites and dense blacks, make sure the right ink products are used with the right anilox.

Chill roll temperatures have a huge impact on conversion speeds, conversion success and performance of the substrate as well as inks. We recommend discussing how to use optimal converting settings with the press supplier, or alternatively, the ink manufacturer.

Water-based Flexo. From a water-based standpoint, again, proper anilox selection is key. Talk to the ink suppliers and make sure that for the ink sets being used, the recommended anilox line count and volumes are correct. This will maximize press speeds and performance.

With water-based inks (Figure 4.9), it is important to not use heat to dry them. Rather, it is important to move as much air across the web as possible – air velocity is key. Converters often think that the best way to dry water-based inks is by using a lot of heat, which is incorrect. Additional heat is not required; just use air, but ensure high air velocity.

When using water-based inks for shrink sleeves, it is necessary to catalyze the last down product . which is usually a flood coat of white with a catalyst. This is necessary because there is a lot of moisture in the steam tunnel, and if that moisture contacts the inks, they will start to re-solubilize. Therefore, with water-based inks and steam tunnels, it is necessary to catalyse the last down ink.

Another critical factor with water-based inks is the pH level. Make sure to have a good maintenance program relating to pH.

Solvent based inks have already been mentioned, especially in relation to how acetates can damage shrink film. Make sure to buy the right inks for shrink applications. OPS is particularly sensitive to solvent attack. The other point is to ensure correct ink viscosity. Solvent ink suppliers will typically supply inks that are not press ready, as is the common procedure with narrow web. For wide-web presses, it is typical to provide a concentrate, which the printer dilutes to a certain printing viscosity. In this case, it is important to make sure to dilute the inks with the proper solvents and to the correct viscosity.

Web Offset. A common issue with offset printing is contamination to the seaming area from the fountain solution. This can be caused by the antistatic coating that is typically applied on one side of PETg film. Applied by the film manufacturers, this coating is designed to reduce static during the conversion process. While the general recommendation is to print on the non-antistatic side, some converters prefer printing on the antistatic side for reasons of adhesion and printability. Printing on the antistatic side is not recommended for offset application. See Figure 4.10 for an easy way to tell which side of the film is coated with antistatic, if it isn’t labeled.

Also with offset, it is necessary to be careful with the ink-water balance to avoid ending up with scumming and other problems back in the fountain solution. These contaminants may transfer onto the web in the seaming area and create poor seams or even failing seams during the shrink process. There are new fountain solutions that are available that will not solubilize the antistatic and cause contamination in the seam area. This is very important when using offset printing.

Digital. Most of the digital inks will need a primer, and many film suppliers have pre-primed films for the digital applications, especially for HP presses. Or, the converter can buy their own primer and prime it in line. If the printer wishes to undertake their own priming, then they need to note that the seam area must be left clean and free of primer.

Primers and inks must be kept out of the seam areas. The seaming solution is a solvent, requiring raw film to raw film contact in order for the chemical weld to work.

TYPICAL PRESS-SIDE /LABORATORY INK TESTS

Ink manufacturers highly recommend that printers undertake some typical test procedures (Figure 4.11) before putting sleeve products in the market. These should include having dyne solutions to test the dyne of the substrate, and understanding from the film supplier where the optimum dyne should be, and if corona treating is required. Once you know where the dyne level of the film should be, then treat it to the optimum level.

It is also important to do some simple cure tests on the printed film. Do some tape adhesion tests; in this industry, the standard is 610 tape, though 810 and 600 tapes are also used. Look at different tapes to make sure that the film is based on requirements, and that the tests are being passed. Some ink suppliers also advise the use of solvent rubs to determine cure – it is recommended to liaise with the ink supplier to determine the correct process.Resistance tests. Bicycle crinkle tests are very important, as are block tests, or chemical resistance testing for the dairy and household markets. COF tests have already been mentioned and are also important to do. Finally, make sure to do some actual shrinking in the lab to make sure inks do not fail these same tests after being shrunk.

ACHIEVING SPECIAL EFFECTS

Apart from the conventional printing processes and techniques that have been discussed, the industry is starting to see printing being done on the outside surface of the sleeve to add a variety of special effects such as gloss, matte, and tactile varnishes that appeal to our sense of touch. Some examples include: pearlescence, iridescence, bright metallics, fluorescence, and different security features.

Some of these effects are best implemented in the lower shrink areas of the container. If cold foiling is to be applied (Figure 4.12) in a high shrink area, then it will probably be necessary to do a screened plate and vignette in the higher shrink areas. That way, when it does shrink, it shrinks together and it will look better. Otherwise, the foil will bunch up on itself and become dull and grey.

The same applies to metallics:

• Keep them out of the higher shrink areas.

• If they are going to be put in the high shrink areas, it is necessary to screen back the pruning plate and, as is done with the graphics, distort it back. This way, when it shrinks 70%, it will not pile. Good foil effect silver, which costs a lot of money, will look like a standard grey ink if it piles, which just wastes the foil and metallic effects.

With any of the specialty effects, make sure to talk to the suppliers as there are tricks to being successful in incorporating these types of effects. For example, if applying cold foil to a sleeve, it can be done with surface printing or it can be done with reverse printing. Reverse printing requires the use of a different cold foil than does surface printing.

Heat activated adhesives. In the article about shrink sleeve substrates we mentioned the phenomenon of container expansion in the tunnel and subsequent contraction after cooling, with the result being that the shrink film is loose and a cylindrical bottle will spin in the user's hand when it is being opened. To overcome this, some converters apply a heat-activated checker board or dash pattern adhesive to the film, which keeps the shrink sleeve from turning on the container, aids orientation during the shrinking process, locks the sleeve to avoid orientation while on the shelf, and even prevents the sleeve from falling off.

There are a variety of adhesive patterns that are used, depending on the overall sleeve and container dimensions and the degree of adhesion required. See Figure 4.13.

INK MIGRATION

If sleeves are being produced for beverages or food, then it is necessary to be aware that the printed sleeve can affect the odor and taste of the product inside the container. This is not a problem with glass or metal containers, but with plastic containers it is necessary to use low migration inks.

Figure 4.14 outlines the common principles concerning food packaging and labeling, while Figure 4.15 outlines the rules governing ink migration. 

It does not matter what kind of ink chemistry is used as they all have the opportunity to migrate. Ink suppliers have developed low migration chemistries that can be used for food applications, and there are many shrink applications in this segment of the market.

SUMMARY GUIDELINES

Based on the information provided in this article, Figure 4.16 provides summary advice to label and package printers that are considering the move into shrink sleeve printing.

A foundational understanding of ink chemistry and its components will better equip you when making ink decisions for the shrink sleeve labels you produce. Selecting the proper ink for your application, and partnering with your ink suppliers throughout the process for guidance and support, will place you in good stead to produce the perfect shrink sleeve label.

INGREDIENT
SOLVENT
WATER-BASED
UV CURING
OIL / OFFSET

PIGMENTS
Yes
Yes
Yes
Yes

RESINS
Nitro
Acrylic
Oligomers
Phenolic
Alkyd

DILUENTS
Solvents
Water/amine
Monomers
Mineral / Vegetable Oil

SOLVENTS
> 30%
< 5%
~0
Zero

ADDITIVES
Wax
Antifoam
Silicones
Plasticizers
Wax
Antifoam
Wax
Photo Initiator
Stabilizers
Wax
Stabilizers
Fillers