Photo Acid Generators [WPAG series]

Photo acid generators or cationic photoinitiators are chemical compounds which release, through decomposition, cationic species, proton, upon irradiation from light source. They have been employed extensively in industry fields of UV printing, coating, and photo lithography. Several types of photo acid initiators are available.



Mechanism of photolysis

For diazodisulfone


For triarylsulfonium salt


For diaryliodonium salt


In the diazodisulfone type, upon irradiated with light, denitrification takes place and a Brønsted acid is generated via Wolff rearrangement.

In the sulfonium (and iodonium) salt type, the C-S (C-I) bond undergoes radical cleavage by light irradiation to generate Brønsted acid.



Exposure: Only the portion Irradiated by light photo-decomposes the acid generator and generates acid.

Heating: The generated acid remove the protecting group (acetal moiety) of the polymer to generate an alkali-soluble Functional group.

Developing: When developed with an alkaline developer, a positive pattern is formed.


As a cationic UV curing systems, the ring opening polymerization of the epoxy and oxetane has been known.
And it is also known addition polymerization of vinyl ether. Cationic UV Curing system has following strong points.

  • No disturbance of polymerization by oxygen.
  • UV curing of epoxy can allows to reduce the curing shrinkage, compared with radical UV curing of acrylates.

Cationic UV curing

Cationic UV curing technology has used in industrial products such as Release coating, Can coatings, 3D printing, Sealants and adhesives.


By utilizing sensitizers, the following applications are possible:

  • Realization of deep-part UV curing
    Suitable for thick film curing because it utilizes long-wavelength UV light.
  • Utilization of light source with narrow emission line widths, such as LED
    Selection of a suitable sensitizer according to wavelength will promote acid generation.
  • Adhesion of colored transparent plastics through which UV hardly penetrates
  • Curing of filler blended mixture system

In systems in which photo decomposition of is inhibited, high curing property is still able to be realized.

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Author: M. Takahashi

For research use or further manufacturing use only. Not for use in diagnostic procedures.

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