UV curable acrylates are one of the fundemental types of specialty polymer adhesives. As you might have guessed, these one-part UV cuable systems polymerize in a very different fashion than one or two-part epoxy or urethane systems. UV acrylate adhesives cure via free radical polymerization. This blog post is dedicated to explaining this particular adhesive cure mechanism, as well as the benefits and drawbacks associated with it in an adhesive system.
Other cure mechanisms:
How Free Radical Polymerization Occurs In UV Acrylate Adhesives
There are two fundemental steps that should be understood about this cure mechanism, intiation and chain-growth.
- Initiation: Basically, the reaction is initiated by the introduction of free radicals to the liquid system. Free radicals are chemical species (atoms, molecules, ions, ect.) that have an unpaired electron and are highly reactive as a result. Once a critical number of these species are introduced to a liquid acrylate adhesive system, a chain-growth reaction will occur amongst the individual molecules that will result in the polymerization of the material. In the case of UV acrylates, these free radicals can easily be introduced via photoinitiators - chemicals that decompose into free radicals with exposure to light. So when the system is hit with UV light, the photo-initiators decompose into free radicals and the chain-growth reaction begins.
- Chain-Growth: As highly reactive species, free radicals will immediately begin to react with the molecules around them as soon as they are introduced to a new environment. Liquid acrylate adhesives are purposely designed so that when these free radicals react, the resulting molecules also contain unpaired electrons. In other words, the free radical reaction actually produces another, larger, free radical species. As you might be able to guess, this free radical species will have the same sort of reaction with nearby molecules - producing even larger free radical species. In this way, the molecules in a liquid acrylate system are able continually react and grow bigger through each reaction. Basically, they add another link to the chain through each reaction, and the chain is always able to stay reactive. Ultimately all the molecules in the liquid system are added to the chain, resulting in the solid polymer material.
One of the implications of this chain-growth polymerization is that the polymer will branch out from the point of original initiation. As a result, some UV cure acrylates are able to polymerize in small dark sections (sections in which photo-initiators will not decompose) as long as the chain growth reaction is initiated nearby and is able to grow into these areas. We should stress that this method only works with relatively small dark sections.
Benefits And Drawbacks Free Radical Adhesives
- Fast and on command cure: By far the biggest benefit to free radical curing systems is that they can be cured within seconds on command with light exposure. Additionally, they have an unlimited worklife, as long as you don't expose them to light.
- Oxygen inhibition: Oxygen will stop the free radical chain growth and as a result air exposure can result in a tacky, unfinished surface to acrylate adhesives.
- Cannot use fillers: Because filler materials such as electrically and thermally conductive fillers will block out light, these materials are generally not used in UV curing systems.