Cationic curing

Cationic curing is a photoinitiated, acid catalyzed ring opening polymerization of epoxy resins using onium salts.

On exposure to UV light energy and in the presence of a hydrogen donor (typically the resin components of a formulation) cationic photoinitiators produce a long lived acidic species. This species ring opens an epoxy group to form a carbocation which goes on to react with other epoxy groups producing a crosslinked network.

The principle is similar to free radical polymerization, with initiation, propagation and chain transfer steps. However, only the initiation phase is dependent on the use of a UV light source. All other stages of the polymerization reaction are thermally driven. The acid generated remains active after the removal of UV light energy until it is consumed and the entire film is cured. This is known as dark cure.

Polymerization steps:

1. Initiation – Photoinduced reaction

On exposure to UV light energy onium salt photoinitiators produce a strong Brønsted acid which goes on to propagate the polymerization reaction.

2. Propagation - Thermally driven reaction

The strong acid ring opens an epoxy and/or oxetane