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Cova Scientific By Cova Scientific • September 29, 2015

Cationic UV Cure Epoxy




Traditional epoxy adhesive cure in one of two ways. They either cure when mixed with a curing agent -- these are two-part epoxies. Or they cure with heat -- these are one-part heat curable epoxies. However, epoxies also have the ability to cure in a third way -- UV cure.

Although UV cure epoxy systems are still based on the same basic epoxy resin, the actual polymerization reaction and the properties of the end product are both very different. In this post we break down how UV epoxies work and explore some of the differences to traditional epoxy systems and free-radical curing acrylates.

Note: We now have an updated version of this article that explores cationic UV cure epoxies in much greater depth.

How a UV cure epoxy polymerizes

An epoxy resin is basically a mixture of carbon-based molecules that all end with epoxy functional groups. This functional group is the key to all epoxy polymerization reactions. On the molecular level when an epoxy resin reacts with an amine catalyst to form a polymer (like in a traditional one or two-part system), these epoxy functional groups are actually chemically bonding with the amine functional groups of the amine catalyst.

Although cationic epoxies still rely on the epoxy functional groups to form the final polymer structure, the role they play is a little bit different. Cationic epoxy resins are filled with photo-sensitive chemicals that when exposed to certain radiation wavelengths will initiate a chemical reaction. This reaction ends up changing the epoxy functional groups in such a way that they actually become self-reactive. So if an epoxy resin is simply made up of molecules ending in epoxy functional groups, and this light-activated reaction causes epoxy functional groups to become self reactive, after a cationic epoxy has been exposed to light it won't be long before all these epoxy groups have reacted with each other. As a result, this resin system ends up forming one giant molecule - a polymer.

UV, visible light, and heat cure capabilities

One interesting aspect of cationic epoxies is that they can be cured in a number of different ways. Depending on the photo-sensitive initiator chosen, these epoxies can be cured with either UV or visible light. Curing within the UV range normally results in a faster cure time, usually in the range of minutes. Visible light curable systems on the other hand still cure relatively fast (still within minutes) and involves a much safer process. For obvious reasons a visible light source is much safer than a UV light source in a work environment.

However, most cationic epoxies can also cure with heat. These photo-sensitive chemicals that initiate the cationic reaction tend to also be heat-sensitive as well. As a result, cationic systems are considered to be dual curable.

Get Started Identifying UV Curing Epoxies That Meet Your Application Specifications

How does a cured cationic epoxy differ from a traditional epoxy?

Upsides of a cationic system vs. traditional epoxy

  • Much faster cure speed and cure is on command (minutes vs. hours)
  • Higher cross-link density
  • Lower coefficient of thermal expansion (CTE)
  • Less shrinkage during cure

Downsides of a cationic system vs. traditional epoxy

How does a UV cure epoxy compare to a free-radical polymerizing acrylic system?

Upsides of a cationic system vs. free-radical

  • No oxygen inhibition - free-radical reactions can be inhibited from oxygen in the air
  • Generally increased adhesion
  • Higher cross-link density
  • Lower CTE
  • Less shrinkage during cure
  • Can dual-cure

Downside of a cationic system vs. free-radical

  • Slower compared to free-radical curing systems (minutes vs. seconds)
  • Very sensitive chemistry and cure can be killed by moisture or other contaminants

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