Free radical polymerization is a chemical process in which polymers are formed through the successive binding of free radical components. Most UV curable adhesives and sealants cure via this chemical processes and therefore understanding the mechanics of free radical polymerization is an important step towards selecting the right type of UV curable polymer for your application.
This post is intended to build on a previous article, Free Radical Polymerization Of Polymer Adhesives, and give a more in depth description of this chemical process and its implications.
An Overview Of Free Radical Polymerization
From a high level free radical polymerization occurs in three steps:
- A free radical (a highly reactive molecule with an unpaired electron) is introduced to a chemical system. This free radical could be introduced in a number of different ways which we will explain later. Because free radicals are so reactive, nearly instantly after they are introduced to the system they will react and bind to nearby molecules. This introduction and first reaction is known as initiation, because these first two steps will initiate the subsequent reaction.
- When a free radical reacts and binds with nearby molecules, the result is of course a larger molecule. However, certain chemicals when reacted with a free radical will not only yield a larger molecule, but the resulting species will be contain an unpaired electron making it a free radical as well. This new free radical will then immediately react with nearby molecules continuing the process. This step in the polymerization process is known as propagation because, as long as the system is not disturbed, the original free radical will continue to propagate or grow via chain-growth until the ultimate polymer is formed.
- Of course this process can't continue forever. Eventually a free radical will react with a molecule that does not result in another free radical, ending the chain-growth propagation. This last step in the polymerization process is known as termination.
Free Radical Polymerization Of Adhesive & Sealant Products
Adhesives and sealants can be formulated to utilize this free radical polymerization process.
Free radical adhesives and sealants are normally initiated by either a photoinitiator or a thermally sensitive peroxide.
- Photoiniators are chemicals that will undergo homolytic cleavage when exposed to the correct radiation wavelength, producing a free radical species. Although this wavelength varies from photoinitiator to photoinitiator, wavelengths in the UV spectrum are normally used. These types of chemicals allow the polymerization process to occur with simple exposure to UV light.
- Thermally sensitive peroxides will break down and produce a free radical in the presence of heat. Adhesives and sealants more commonly utilize photoinitiators than peroxides because the UV curing process is easier to control in an assembly process.
To facilitate the propagation process, adhesives and sealants are formulated with monomers and oligomers that will continue the free radical chain-growth such as acrylates. The choice of monomers and oligomers mainly determines the qualities of the ultimate polymer material.
The main benefit of formulating an adhesive to cure via free radical polymerization is speed. UV curing adhesives and sealants are able to polymerize faster than any alternative (normally within seconds). In our Ultimate UV Resin Guide we explain the main assembly benefits of a UV curing process and even give some real life examples.
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One of the major drawbacks of using a free radical curing system is oxygen inhibition. Free radicals can terminate early by reacting with airborne oxygen. This can leave air exposed surfaces tacky and incomplete. One way to avoid oxygen inhibition is to utilize a cationic UV cure epoxy instead which will provide similar UV sensitivity and cure speed.