Лако-красочные материалы — производство

In contrast to most organic polymers, in silicone resins (also called silicones or polysiloxanes), the backbone is made of silicon and oxy­gen, with organic groups substituted as side chains on this backbone. Thus, the skeleton of silicones is related to silica and silicates, but some of the oxygens are replaced by organic groups. The most com­mon organic groups are alkyl (methyl to amyl), phenyl or even vinyl groups grafted onto the silicone backbone. Thus, silicone resins are placed at a hybrid position between inorganic and organic polymers.

Silicones are now most commonly manufactured using the Muller — Rochow synthesis. Finely ground elementary silicon is reacted with

alkyl or aryl halides in the presence of cuprous catalyst to form mono-, bi — and trifunctional organohalosilanes. The individual components are then separated by fractional distillation for preparation of silicones. Some important organo-halosilanes are shown in Figure 2.69. The most widely used monomers are the methyl and phenyl substituted halosi — lanes, with hydrolyzable functionality ranging from 1 to 4.

Hydrolysis of chlorosilanes forms unstable silanols, which in turn can condense to form polysiloxanes with elimination of water. By varying the ratio of the M, D, T and Q (occasionally used) monomers, the molecular size and structure of the final polysiloxane can be designed to produce a wide range of products with varying MW and chemical structure. The “D” units are the main components producing linear chain polymers. The “M” units terminate chains and thus limit molecular growth, while “T” units cause molecules to branch. Along with linear structures, low MW cyclic structures are often formed to varying degrees. Based on this chemistry, a range of pure silicone resins and some modified silicones are produced for the coating industry. In the following sections, some important types of silicone resins used in surface coatings are discussed.