Innovative Developments for UV Silicone Release Coatings

There are two UV-curable silicone release systems on the market. One is based on silicone acrylates and cures by way of a free-radical mechanism; the other release system uses epoxy silicones and cures in the presence of cationic initiators.

Figure 1. UV-Curable Silicones.

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 in their underlying chemistries. One is based on silicone acrylates and cures by way of a free-radical mechanism; 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. 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 acrylates 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.

Figure 2. Correlation between the Release Force and the Ratio of Modified to Unmodified Units in the Silicone Backbone.

Radical- and cationic-curable systems each have advantages and disadvantages. The radical polymerization of the acrylate groups is much faster than the cationic polymerization. Therefore, full cure is reached more quickly with silicone acrylates. Figure 3 illustrates the curing speed behavior 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 effort. However, inerted UV units have been 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.

Figure 3. Curing Speed Behavior for Radical and Cationic Curable Systems. Real time IR Measured at 1,407 cm-1 (Double Bond) and 1,068 cm-1 (Ether).

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 poisoning 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, often cannot be used for epoxy silicone materials. The use of filmic substrates is also often limited, as they may contain ingredients that 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 realize with cationic curing silicones.

There are two different curing mechanisms for the UV curing of silicone release coatings. Based on the properties of these mechanisms, different photoinitiators must be used. Cationic initiators are 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 a 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.

Specific actions that can be employed to reduce various disadvantages in each system will be discussed.

Figure 4. Iodonium Salt with Hydroxy Groups.

Improvement of the Cationic Curable System

Modification of the Photoinitiator^1-2

The main disadvantage of the cationic curable system is the slow polymerization of the epoxy functional groups compared to acrylate groups. In particular, the solubility of the photoinitiator within the system influences the speed of curing.

Iodonium salts are very effective photoinitiators and are suitable for the curing of epoxy silicones. The problem is finding commercially available iodonium salts that are soluble in epoxy silicones. Presently, there are various salts available on the market that vary by the nature of the substituent at the phenyl group and the type of counter ion.

These structural features influence the compatibility of the iodonium salt within the cationic-curable system. For example, iodonium salts with hydroxyl groups have an increased tendency to crystallize (see Figure 4). This leads to a limited compatibility of such materials in epoxy silicones.

Figure 5. Modification of Iodonium Salt with Hydroxy Groups.

The high tendency of iodonium salts to crystallize is desirable during the manufacturing of the photoinitiator. This makes the purification process much easier and the iodonium salts able to be produced in high yields, which facilitates easy and inexpensive production of the material. Our laboratory has developed a process to modify iodonium salts with hydroxyl groups. This modification is simple and inexpensive and has two distinct advantages: it reduces the tendency for crystallization and improves the salts' compatibility with epoxy siloxanes.

Figure 5 shows two reaction schemes that fulfill these requirements. Formula A shows the reaction of an iodonium salt with an isocyanate. The advantages of this process are that the isocyanate is relatively inexpensive and that the reaction to complete conversion to the targeted product is fast. The same is true for the reaction illustrated in Formula B. Here, a hydroxyl group is converted into a "siliconophil" residue with trimethylchlorosilane.

Both of the photoinitiators illustrated in Figure 5 are high-viscosity liquids at room temperature. They are completely compatible with white spirit, whereas the compound containing the hydroxyl functional group is crystalline with a melting point of 91

Figure 6. Commercially Available Iodonium Salts.

The salts in Figure 6 are solid or highly viscous materials. They are therefore sold as a blend in organic solvents such as glycidether or propylencarbonate. These solvents have the disadvantage of not taking part in the curing process and thus remaining migrateable in the cured silicone coating. Other possible solvents are volatile and may add an odor to both the production process and the release liner.

We evaluated alternative solvents with low VOC and reduced environmental hazards. We identified hydroxy functional siloxanes to be an efficient solvent and co-reactant to iodonium-based photo-catalysts. They are high-molecular-weight, low-viscosity materials that efficiently dilute the photo-catalyst and blend perfectly with the epoxy functional siloxanes. Such a photoinitiator is available now.

Modification of the Epoxy Silicone³

Modifying the silicone is another way to improve the compatibility between the epoxy silicone and photoinitiator. Patent EP 334 068 describes how the solubility of iodonium salts is enhanced when a certain fraction of the epoxy functional groups has been esterified with benzoic acid.4 However, this procedure leads to the possibility of polymerization and the loss of the comparatively expensive epoxy functional groups.

Modification with alcohol groups is a cheaper and safer method to improve the polarity of the silicone. The alcohol functional groups participate in the polymerization and cause a chain transfer reaction. This also has the positive effect of making the iodonium salts more soluble in the silicone polymer. However, it is important that the alcohol-functional groups are evenly distributed within the siloxane backbone. The same effect cannot be observed if a mixture of OH-functional siloxane and epoxy siloxane is used.

Figure 7. Cost Distribution for the Production of a Laminate with Radical Curable Release Coatings.

Improvement of the Free Radical-Curable System

Improvement of the Oxygen Tolerance of Silicone Acrylates

The main reason customers hesitate to use the radical-curing system is the necessity for inertization of the coating unit with nitrogen even though the cost of the nitrogen represents only 1% of the total cost to produce the laminate (see Figure 7).

We have developed a method that allows relatively high oxygen concentrations during the production process of the laminate.

Figure 8. Free Radical Polymerization Cycle.

The oxygen, found in the silicone and air, scavenges the radicals effectively, which inhibits the polymerization of the acrylates. In order to obtain an acceptable curing quality of acrylates, it is necessary to reduce the oxygen content of the air from 210,000 ppm to less than 50 ppm in the coating unit. We have discovered additives that allow the curing process to take place in the presence of 200 ppm up to 800 ppm oxygen without any significant reduction in the curing speed or product performance.

The class of compounds that is used in this process is trivalent phosphites, already known as stabilizers in the plastic industry. The reaction rate of phosphites with some oxygen donors is high enough to compete with the oxygen addition to carbon-centered radicals. Figure 8 shows the interaction of the phosphite during the curing process. The phosphite moiety is oxidized stoichiometrically to a phosphate ion and its role is to produce the reactive radical that was previously deactivated by an oxygen molecule.

Figure 9. Influence of the Oxygen Concentration on the Subsequent Adhesion.

The degree of crosslinking in an enriched oxygen environment is considerably improved and the formation of hydroperoxides reduced. This results in a much better curing performance of silicone acrylate systems at higher levels of residual oxygen. Phosphites are also capable of enhancing the degree of cure in organic acrylic systems.

Figure 9 shows the influence of the oxygen concentration in the coating unit on the subsequent adhesion. Although 300 ppm of oxygen is present in the system, the phoshite yields >80% subsequent adhesion whereas without a stabilizer curing would not take place. These results show that it is easier to reach good curing conditions using our method. In addition, the production safety increases. Furthermore, using the stabilizer as illustrated in Figure 10 can reduce the aging of the laminate, thus allowing it to act as an "aging stabilizer." These results show that it is easier to reach good curing conditions using our method.

Figure 10. Aging of Different Laminates.

Novel Photoinitiator for Silicone Acrylates⁶

Many photoinitiators are available for use in this process. For silicone acrylates, 2-hydroxy ketones are recommended, as they have been proven to be highly efficient with the UV spectra of standard UV lamps (medium-pressure mercury lamps). Furthermore, they are often liquids, which is advantageous for blending with silicone acrylates. The most common photoinitiator of this class is 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP, CAS No 7573-98-5).

HMPP is a widely available and well-investigated UV photoinitiator. Although HMPP and its resulting radicals blend with the silicone system, they are not hydrophobic molecules. Thus, they may not easily escape the solvent cage and may not be able to move freely within the silicone matrix to react with the acrylic groups. We therefore concentrated our investigations to find a more hydrophobic photoinitiator. This new photoinitiator is now available.

Figure 11. UV Spectra of the Photoinitiators.

Besides the hydrophobic characteristic, the new photoinitiator has a high UV absorption in the range above 230 nm. The UV spectra shows that more of the UV peaks from standard UV lamps, especially the peak at 255 nm, are used (see Figure 11).

Even though the absorption of UV light is important, the yield of radical conversion from the exited state is even more important. Eventually, all these factors determine the cure-speed potential of a photoinitiator. To measure the cure speed, it is best to run a coating trial. To set up such a trial, one can increase machine speed or reduce the UV output to the critical stage, where the degree of cure is not fully developed. The degree of cure can be measured with the subsequent adhesion test. Figure 12 shows that more hydrophobic photoinitiator leads to better curing, especially at higher machine speed.

Figure 12. Efficiency of Different Photoinitiators for Free-Radical UV Curing.

Another advantage of the new photoinitiator is that it does not produce benzaldehyde as a photoreaction byproduct. This almond odor is often present when using HMPP (in combination with either silicone or organic acrylates). Furthermore, benzaldehyde formed inside the reaction chamber causes the quartz plates to cloud. With the new photoinitiator, this problem is eliminated and the odor is reduced.

Conclusion

It is difficult to expect quantum leaps when it comes to the performance of UV silicone release coatings in this industry. However, the results presented above show that there is potential for the use of this environment-friendly technology.

For more information on release coatings, contact Degussa Corp., e-mail info@tego-rc.com or visit http://www.tego-rc.com.