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Cationic curing, a polymerization process initiated by cationic photoinitiators enables the development of strong, heat-resistant materials with low shrinkage and high resistance to environmental factors.

Technology basics

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

Upon 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.

Formulations typically consist of 3 materials:

  • Photoinitiator onium salts, available under the SpeedCure name.
  • Cycloaliphatic epoxide resins, available under the UviCure name.
  • Oxetane resins, available under the UviCure name.

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.


chemical-equation-cationic.png

2. Propagation - Thermally driven reaction

The strong acid ring opens an epoxy and/or oxetane

 

chemical-equation-2.png

Benefits of cationic curing:

  • Very low shrinkage
  • No oxygen inhibition
  • Excellent adhesion
  • Improved coating properties
  • Dark cure reaction gives high conversion

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Cationic photoinitiators

Cationic photoinitiators are materials that release positive ions when exposed to light. These photoinitiators can be used in UV-curing applications, where they initiate a polymerization reaction when exposed to UV light. This reaction causes the material to harden and solidify, making it useful for applications such as coatings, adhesives, and 3D printing. 

Name CAS number Properties

SpeedCure 937



 
71786-70-4 & 68609-97-2
  • Benzene free
  • Good reactivity
  • Low yellowing
  • Cost efficient sensitization 
 

SpeedCure 938

61358-25-6
  • Benzene free
  • Good reactivity
  • Produces long lived acid species
  • Low yellowing
  • Cost efficient sensitization

 

SpeedCure 939

178233-72-2
  • Benzene free
  • Very high reactivity in silicone epoxy systems
  • Produces long lived acid species
  • Low yellowing

 

 

 

 

Name CAS number Properties

SpeedCure 976

89452-37-9 & 108-32-7
  • Very high reactivity
  • Produces long lived acid species
  • Low yellowing
  • Expensive to sensitize

 

SpeedCure 992

74227-35-3 & 108-32-7
  • High reactivity
  • Produces long lived acid species
  • Low yellowing
  • Expensive to sensitize

 

Cationic resins

Cationic resins have positively charged groups, or cations, along its backbone. Cycloaliphatic epoxy resins are key components of epoxy coatings, valued for their high adhesion, low shrinkage, and chemical resistance. Specialist oxetane resins are used in conjunction with
cycloaliphatic epoxy resins to improve speed of cure, viscosity control,
hardness, and adhesion to difficult substrates.

Name

Structure

Properties

UVICURE S105

Molecule UviCure S105
  • Low shrinkage
  • Viscosity (ca. 300 mPa.s @25° C)
  • Hard polymers
  • High crosslink density and high Tg

UVICURE 128

UviCure S128.jpg

  • Low shrinkage
  • Viscosity (ca. 600 mPa.s @25° C)
  • Increased flexibility over UviCure S105
Name
Structure
Properties

UVICURE S130

UviCure S130.jpg
  • Improved adhesion
  • Improved flexibility
  • Increases reactivity

UVICURE140

UviCure S140.jpg

  • Improved adhesion
  • Improved flexibility

 

 

Name
Structure
Properties

UVICURE S150

UviCure S150.jpg
  • Improved adhesion

  • Improved flexibility

  • Increased toughness

  • Increased thermal stability

UVICURE S160

UviCure S160.jpg
  • Improved adhesion

  • Improved hardness

  • Increased chemical resistance

  • Increased thermal stability

UVICURE S170

UviCure S170.jpg
  • Hybrid oxetane/methacrylate functionality

  • Reduced phase separation in hybrid systems

  • Adhesion promotion in free radical systems

 

Have questions about cationic curing?

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