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.
Highly closed surface, Excellent lay-flat
properties
-
PEK (Polyethylene coated Kraft)
Very smooth surface, High level of surface
refinement (closed)
Excellent mechanical properties.
Very smooth surface (lower silicone
consumption), ideal for ‘no-label look’ labels
Excellent mechanical properties (and
transparent)
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.
