Epoxy-Functional Silicones

Difunctional and multifunctional epoxy silicones 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, and release. Properties of cured silicone-modified epoxies vary from hydrophilic to hydrophobic depending on the epoxy content, degree of substitution, and ring-opening of epoxides to formdiols. The ring-strained epoxycyclohexyl group is more reactive than the epoxypropoxy group and undergoes thermally or chemically induced reactions with nucleophiles including protic surfaces such as cellulosics of polyacrylate resins.

The compatibility of epoxy-functional silicones with conventional epoxies varies. In simple unfilled systems, total solubility is required. For filled systems, it is often desirable to consider systems that are miscible but have only limited solubility since microphase separation can allow a mechanism for stress-relief.

Epoxy silicones with methoxy groups can be used to improve adhesion to substrates such as titanium, glass, or silicon. They also can  improve chemical resistance of coatings by forming siloxane crosslinks upon exposure to moisture.

A UV initiator for cycloaliphatic epoxides is OMBO037 described in the Catalyst Section. Epoxy-functional siloxane copolymers with polyalkyleneoxide functionality provide hydrophilic textile finishes.

These materials, characterized by a combination of cycloaliphatic and siloxane structures, have outstanding weathering characteristics, controlled release and coefficient of friction, and excellent electrical properties. They can be cured either by cationic UV photoinitiators or conventional epoxy hardeners. In cationic UV-cure systems, the cycloaliphatic epoxy silicones combine the properties of reactive diluents with surfactant properties. The release properties can be employed to make parting layers for multilayer films. If high levels of epoxy-functional silicones are used in UV-cure formulations, cationic photoinitiators with hydrophobic substitution are preferred.