During the last few years, advances in increased converting and dispensing speeds to optimize processes and achieve cost efficiencies have been seen. As the label industry grows and evolves, so does the release liner market. There are demands for improved liner performance, especially in terms of release after die-cutting and label dispensing to minimize web breaks.
Converters and label dispensers now need to operate within a broad range of speeds without sacrificing production line efficiency and quality. The emphasis is not as much on how fast the silicone can cure but on release coating systems that have the appropriate release profile for fast line converting and dispensing. In these instances, a lower high speed release force is required for improved converting performance. As the speed of matrix removal is increased, it is important to ensure that the release force to remove the matrix is not too high, as this will lead to an increased frequency of web breaks (figure 1).
Silicone technology and chemistry
To achieve cross-linking of silicone release coatings, the most predominantly used chemistry is curing via a hydrosilylation reaction1-4. The composition of such silicone release coating formulations consists of a vinyl-functional polydimethylsiloxane, a hydrogen functional polydimethylsiloxane and a platinum catalyst (Pt). Other additives that make up the release coating formulation include inhibitors to achieve long bath life at room temperature, process aids such as anti-mist additives to prevent misting at high line speeds or anchorage additives to aid adhesion to more challenging substrates. The cure is very fast and is catalysed with very low levels of platinum catalyst to cure at relatively low temperatures at workable line speeds.
In general, a release coating system with a high cross-link density will yield more rigid hard coatings which exhibit lower release forces at high peel speeds. An example of a silicone release coating network is shown in figure 2.
The release properties
There are several factors affecting release but rheology is the most critical. These rheological properties are controlled by the chemical nature, molecular weight and formulated composition of the adhesive and the silicone release coating. For the silicone chemist the problem that has to be solved is how to achieve these lower release forces at high peel speeds while keeping a balance of fast cure and good anchorage. Another aspect to consider is the ability to design a fast curing system which can be used at low platinum levels to maximize cost savings. The architecture of the vinyl siloxane as shown in figure 3 (the effect of silicone release coating architecture on release coating properties) may dictate the properties of the release coating. High molecular weight vinyl end blocked siloxanes tend to give rising release force profiles with increased peel speed. They show smooth release and generally interaction with adhesives is low. Pendant and branched polymers can give flatter release force profiles. In addition, branched vinyl polymers can provide fast cure at ultra-low platinum concentrations. However fast curing systems with increased crosslinked density tend to be more of a challenge for achieving good adhesion to the substrate. The rapid rise in 100% solids or solventless systems has been mostly linked to avoiding the use, cost and handling of environmentally hazardous solvents.
It is now clear that rheological factors are very important when considering peel force and the release profile is largely dictated by the adhesive component and its relative ability to dissipate and store energy. On the other hand, the magnitude of release and the shape of the release peel speed curve can be modified by selecting certain silicone polymer architecture.
Figure 4 (the effect of vinyl polymer architecture on the release profile) shows examples of two vinyl end blocked siloxanes of different molecular weight/chain length, a multifunctional siloxane and branched siloxanes of different molecular weights. The high molecular weight vinyl endblocked polymer shows the most inclining release profile and the low molecular weight branched polymer has the least inclining profile. The release profile order ranging from steepest to flattest, and correlates well with the expected increased crosslinked density of the cured network when selecting from the endblocked polymer with 130 dimethylsiloxy repeating unites (i.e. 130 DP) moving to the low molecular weight branched siloxane. In other words the higher the crosslinking density the flatter the flat release profile.
New coatings have been developed to meet today’s market demands for ever increasing converting and label dispensing speeds. These new products allow for a flat release profile (low release force at high peel speeds) to assist converters and dispenser in their process operations. Thanks to these designs they are also very fast curing allowing the systems to operate at minimum platinum catalyst levels.
This article was first published in NarrowWebTech, print issue 2-2016. The editorial team of NarrowWebTech publishes still relevant article on www.narrowwebtech.com in order to give our readers the possibility to read detailed technical articles also online.
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