Two common types of UV curing (light curing) adhesives are those based on acylate and epoxy. Both are cured using UV or blue light between 365 nm and 405 nm. The difference between them lies in their chemistry. Both the curing process and final properties differ.

Do you want to learn more about the basics of light curing adhesives first? Click here to go to our post about this subject.

UV acrylic: Free radical curing

Acylic based light curing adhesives cure using free radicals. As the adhesive is illuminated by certain light waves, the light energy breaks apart the photoinitiators in it to create free radicals. Free radicals are extremely reactive. Instantly, they will start chemical reactions– initiation. This chain reaction continues very quickly into the next step called propagation (polymerization and cross-linking). As soon as the light source is removed, the reaction stops immediately. The reaction also stops as two radicals meet and react with each other – termination.

Because radicals are so reactive, the reaction can be difficult to control. Oxygen can for example inhibit the curing reaction. The radicals, that exist when the adhesives is illuminated, can instead of reacting with the components in the adhesive react with oxygen molecules from the surrounding air. By doing so, the reaction is terminated too early. As a result, we get shorter polymer chains than expected which gives us a sticky-feeling surface with worse mechanical properties. Learn more about what you can do to prevent oxygen inhibition here.

Example of an acrylic based UV adhesive: Epo-Tek OG603

UV epoxy: Cationic curing

A cationic curing adhesive contains a different type of photoinitiator than within radical curing formulations. The initiator is activated by light or heat and forms a “super acid” which is a cation. The cation has a proton (H+) which is used to initiate a ring opening polymerization reaction (ROP) of the epoxy. The reaction is slower than radical curing since the cation is less reactive than a radical.

Unlike radical curing UV adhesives, the curing reaction continues even after the light is removed. The properties of the adhesive continue to develop up to 24 hours after removal. There is no termination step in the reaction as for radicals. Instead the polymerization continues until there is no more material left to react with, although it is with a slower pace.

Example of an epoxy based UV adhesive: Epo-Tek OG116-31

Comparison