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There are two UV curable silicone release systems on the market. Both are solventless and produce release coatings without the use of heat, but differ by their underlying chemistries. One is based on silicone acrylates and cures via a free radical mechanism, while the other release system uses epoxy silicones and cures in the presence of cationic initiators. Both methods can be used to produce release coatings with special release forces, which can be adjusted by the degree of modification of the silicone backbone. The illustration in Figure 2 shows the correlation between the release force and the ratio of modified to unmodified units in the silicone backbone. Epoxy silicones have a lower release force than silicone acryfates with a similar degree of modification due to their lower polarity. Polar organic residues linked to the silicone backbone have a higher interaction with the adhesive component. Radical and cationic curable systems have their advantages and disadvantages. The radical polymerization of the acrylate groups is much faster compared to the cationic polymerization. Therefore, full cure is reached more quickly with silicone acrylates. Figure 3 illustrates the curing speed behaviour for radical and cationic curable systems. The silicones used in this study have the same degree of functionality. The main disadvantage of the radical curable system is the need to inert the coating unit with nitrogen because the presence of oxygen will lead to the termination of the polymerization. Therefore, the laminate must be completely cured before it leaves the inerted UV unit. This requires some technical efforts to meet this necessity for this curable system. However, inerted UV units are state of the art for more than a decade. Their reliability has been proven both in production and for consistent quality of free radical cured silicone coatings. Cationic curing can take place without inerting the coating unit. However, this system undergoes post-curing, i.e. the reaction continues after the release coating leaves the UV curing unit. Furthermore, the cationic UV catalyst can suffer poisioning effects. Thus, alkaline components within the curable system can terminate the polymerization of epoxy silicones. As a consequence, papers, such as clay coated or glassine papers can often not be used for epoxy silicone materials. The use of filmic substrates is also often limited as they may contain ingredients which can poison the cationic catalyst. In addition, the curing of the system is negatively affected by high atmospheric humidity. As a consequence, the cure speed of cationic curing silicones can be very different on different substrates and at different curing conditions. Thus, the degree of curing in the moment of rewinding the web may not have reached a sufficient degree. The silicone continues to post-cure on the rewind reel which may have a negative impact on the release level and silicone transfer. Since the properties of the liner change with time, in-line processes are difficult to realise with cationic curing silicones. For the UV curing of silicone release coatings, there are two different curing mechanisms and based on the properties of these mechanisms, different photoinitiators must be used. Cationic initiators are much more expensive than the radical initiators. On the other hand epoxy silicones are easier to manufacture than silicone acrylates and thus can be offered at lower cost. An advantage to the radical curable system is the much longer shelf life with the photoinitiator included. Radical curable systems can be stored for weeks and even months whereas epoxy silicones can be stored only for a few days. We will describe specific actions that can be employed to reduce various disadvantages in each system.
