Лако-красочные материалы — производство

Silicone modified resins differ from silicone resin blends in that they are chemically reacted with hydroxyl functional organic resins to obtain copolymerized products. Because of their high cost and long curing cycle at high temperatures, pure silicone resins have limited application in coatings. Copolymerizing with organic resins expands their usability in coatings by improving ease of curing and solvent resistance and providing a wider latitude of mechanical properties with considerable cost effectiveness. The silicone inter­mediates used for such modifications are low MW silicone resins having reactive silanol (-Si-OH) or alkoxysiloxane (-Si-OR) groups. These functional groups react with hydroxyl groups of organic poly­mers to form C-O-Si bonds. The most common organic polymers that are modified with silicones include alkyds, polyesters, acrylics and epoxies. Silicone modification may typically range from 25 % to 50 %, though higher levels up to 80 % are possible for designing special heat-resistant systems. The reaction is accomplished by heating the organic polymers and reactive silicone intermediate in the presence of a catalyst at about 140 °C. Two reactions are possible: the reaction of silanols with hydroxyls of the polymer or self-condensation of silanol groups. Excessive self-condensation may lead to gelation. Better control of the reaction is possible with an alkoxysilane functional group than a silanol group. The most commonly used catalysts for such reactions are organotitanates, such as tetraisopropyltitanate. Some representative structures of silicone intermediates and the reactions involved are shown in Figure 2.72.