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Cova Scientific By Cova Scientific • March 2, 2016

UV Epoxy Resin "Dark Cure"

UV epoxy dark cure

Unlike typical free radical curing adhesives & sealants, cationic UV epoxy resins can be formulated to cure between opaque substrates. This "dark cure" process still requires light to activate the adhesive. However, due to the unique cationic polymerization, the cure will complete in the absence of light.

This article works to explain the basics of a UV epoxy "dark cure," and highlight some of the resulting assembly benefits.



Why Can't Typical UV Resins Dark Cure?

As we've already explained, typical UV resins require 100% light exposure in order to start and finish polymerization. This is mainly because typical UV products cure via free radical polymerization which occurs incredibly fast:

  1. UV light activates photoinitiators
  2. Photoinitiators create highly reactive radical species
  3. Radicals almost instantaneously attack monomers (or contaminants like airborne oxygen)
  4. Attacked monomers also become radicals, attacking (bonding to) other monomers, which also become radicals, and this process continues ultimately propagating the polymer product

Again, what's important to note is that this reaction happens almost instantaneously - usually in a matter of seconds. Of course the speed of free radical systems is why they are so useful in assembly processes (and why they can substantially reduce manufacturing expenses). But, this speed also implies that there is very little middle ground: as soon as the system is activated, it is basically already done.

As a result the radical adhesives must be arranged exactly as it is ultimately intended prior to UV light exposure. And if the ultimate arrangement is between two opaque substrates, than the application can not use a free radical system.

typical uv cure


Why Are UV Epoxy Resins Able To Dark Cure?

UV epoxies cure via cationic polymerization which is still very fast (60 second range), but slightly slower than radical polymerization. This is because cationic epoxies utilize acids instead of radicals.

  1. UV light activates photoinitiators
  2. Photoinitiators create a Brönsted or Lewis acid
  3. The acid will quickly attack monomers
  4. Attacked monomers become positively charged cations, which will attack (bond to) other monomers, which also become cations, and this process continues ultimately propagating the polymer product

However, even more important than the reaction speed is the fact that these acids can exist for up to several days. Radicals in comparison normally only last a few seconds because of how highly reactive they are.

As a result, cationic epoxies can be formulated to delay cure. Specific chemicals can temporarily tie up these acids allowing for an open time. In this way, cationic epoxies can be delayed for up to 2 or 3 minutes!

Liquid UV epoxy resins can therefore be activated by UV light, and, as long as the resin is situated within this open time, the reaction will complete between opaque substrates.

uv epoxy dark cure

This process is somewhat similar, but also different from UV epoxy shadow curing. Both mechanisms are only possible because the photo-generated acids are less reactive than free radical species.

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