Free Radical Photoinitiators - Type II

Free radical photoinitiators react differently when exposed to UV light. Depending on their reactivity, they are classified as Norrish Type I or Norrish Type II photoinitiators.

Norish Type I initiators are typically compounds containing a benzoyl group which, when exposed to UV light, cleaves to produce two highly reactive free radicals, at least one of which reacts with the monomer to initiate polymerization. Type I photoinitiators are irreversibly incorporated into the polymer matrix.

Norrish Type 2/II photoinitiators require hydrogen donors to react when exposed to UV light, and the most common of these hydrogen donors are amines (amine synergists). Upon UV irradiation, the Type 2 photoinitiator/II photoinitiator abstracts a hydrogen atom from the synergist used to form two very reactive alkylamino radicals, which subsequently initiate polymerization. Type 2/II photoinitiators are generally not incorporated during the reaction, but synergists are incorporated.

TINTOLL is one of the leading photoinitiator suppliers, which is also one of the cornerstone businesses of TINTOLL. Our product portfolio covers free radical and cationic photoinitiators (photo-acid generators) as well as amine synergists. UV-curable materials are widely used in graphic arts, industrial coatings, adhesives, printed circuit boards, and 3D printing. TINTOLL provides application support, product development and custom solutions, and collaborates with customers to develop next-generation photoinitiators.

What is the type II Photoinitiator Mechanism?
Type II photoinitiators are a class of photoinitiators that require a co-initiator to generate reactive intermediates. These initiators absorb light in the UV-Vis spectral range, typically 250-450 nm.

The photoinitiator reaches an excited state after absorbing light and reacts with another molecule to produce an active species. Coinitiators are usually amines.

Type II photoinitiators abstract hydrogen from the co-initiator to generate secondary radicals.

They are generally slower than the type 1 photoinitiator because they are based on bimolecular reactions.

The two most common types of photoinitiators are free radicals and cationics.