The outstanding properties of epoxy resins—toughness, rigidity, elevated temperature performance, chemical resistance, and adhesive properties—have enabled their acceptance in a wide range of critical electronic, optical, and aerospace applications. Silicon-based materials play key roles in extending the range of physical properties of conventional epoxy resin systems enabling acceptance in applications with challenging requirements such as ULSI encapsulation, die-attach adhesives, optical component mounting, aerospace nanocomposites, UV-cure controlled release, and controlled coefficient of friction coatings. The main categories of silicon-based materials used in conjunction with epoxy resins are:
- Silane Coupling Agents. These find applications as adhesion promoters for composites, coatings, and adhesives.
- Difunctional and Multifunctional Epoxy-Terminated Silicones. These include lower molecular weight siloxanes with discrete structures and higher molecular weight silicones with either pendant or terminal epoxy functionalization. Depending on specific structures and formulations they selectively impart a wide range of properties associated with silicones—low-stress, low temperature properties, dielectric properties, release, and optical stability.
- Silicone Resin Modifiers—Monofunctional Silicones. Siloxane and silicone diluents can reduce viscosity of epoxy systems as well as lower surface tension allowing ease of handling, facilitating higher filler loading and infiltration of prepregs and fine structural components.
- Cycloaliphatic Silanes and Silicones. These materials, characterized by a combination of cycloaliphatic and siloxane structures, have outstanding weathering characteristics, controlled release and coefficient of friction along with excellent electrical properties. They can be cured either by cationic UV photoinitiators or conventional epoxy hardeners.
- Silane and Silicone Hardeners. These materials, include siloxane and silicone with diamine, polyamine, and dianhydride functionalities.
- Organosilane-Modified Silica Nanoparticles. A range of silica structures from 20 nm to 1 micron have been modified with silanes to reduce hydroxyl content allowing improved dispersion. Other versions have monolayers with isolated amine functionality, providing controlled interactions with resins.
With proper selection, formulators can achieve a balance of performance properties, thereby taking advantage of the unique properties of epoxies and silicones.
General Comparison of Silicone- & Bisphenol-Based Epoxies
|Silicone Epoxies||Bisphenol Epoxies|
|Electrical properties||Dielectric constant|
Chloride, ionic impurities (ppm)
|Optical properties||Color, Gardner|
|Thermal properties||Low temperature (Tg)|
Thermal stress relaxation (piezo)
-2 to -3
-6 to -7