Cationic curing

Cationic curing is a photoinitiated, acid catalysed ring opening polymerisation 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 polymerisation, with initiation, propagation and chain transfer steps. However, only the initiation phase is dependant on the use of a UV light source. All other stages of the polymerisation 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.

Formulations typically consist of 3 materials:

• Photoinitator onium salts supplied under the SpeedCure name.
• Cycloaliphatic epoxide resins supplied under the UviCure name.
• Oxetane resins supplied under the UviCure name.

Polymerisation steps:

1. Initiation – Photoinduced reaction

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

2. Propagation – Thermally driven reaction

The strong acid ring opens an epoxy and/or oxetane

Cationic curing technology offers several benefits to help you achieve high performance properties for your project