TABLE OF CONTENTS Silicones constitute a unique class of polymers due to their semi-organic structures. They are distinguished from other organic polymers in having a silicon-to-carbon backbone structure instead of the normal carbon-to-carbon structure. Because of this unique structure, silicones possess much higher thermal resistance than other polymers and, at the same time, possess and maintain excellent electrical properties at high temperatures and under humid conditions. A series of ultra-pure silicones were developed for the electronics industry in the early 1960s and today find extensive applications as circuit board conformal coatings, semiconductor junction coatings, encapsulants, and adhesives.
Unlike other polymers, silicones are not entirely organic. Chemically, silicones consist of alternate Si and O atoms linked to organic sidegroups such as aliphatic or aromatic hydrocarbons and, as such, are more accurately classified as semi-organic. The -Si-O-Si-O- backbone structure is referred to as siloxane. The general structure for a linear silicone polymer is represented in Fig. 2.33.
In this structure, the group R 1 may be the same as or different from R 2. These appendages may represent any one or combination of organic groups such as:
methyl(CH 3)
phenyl(C 6H 5)
allyl (-CH 2CH=CH 2)
vinyl (-CH=CH 2)
trifluoropropyl (-CH 2-CH 2CF 3)
Silicone polymers are classified by the nature of these side groups according to ASTM D1418 (Standard Practice for Rubber and Rubber Lattices-Nomenclature) as follows:
| Q | Siloxane |
| MQ |
