Adhesive and coating systems that use ultraviolet (UV) or visible light to begin the curing are intrinsically very attractive systems to the manufacturing engineer, allowing assembly of components and then “curing on command” by exposing to the radiation. The adhesives can be cured using free radicals for acrylic adhesives or cations for cationic monomers.
Two types of acrylic monomers are used in these systems: acrylates and methacrylates. A wide range of mono-functional (one reactive group per molecule) and multi-functional (more than one reactive group per molecule) monomers are available, giving wide flexibility in formulating these systems. In recent years, visible light photoinitiators have become widely available, and systems using LED light are now possible instead of high-intensity UV systems. The major issue with this type of curing is surface inhibition by atmospheric oxygen, often necessitating nitrogen blanketing, particularly for coating applications.
In principle, cationic systems should be the most versatile UV systems because of the wide range of monomers and oligomers that are theoretically available. In practice, there are a limited number of commercially available monomers and oligomers, namely some cycloaliphatic epoxies, vinyl ethers and oxetanes, and even fewer photoinitiators.
Cationic systems are very fast curing and often cure to completion after irradiation is ceased. Although oxygen is not a problem, the major chemical limitations to photocationic systems are inhibition by atmospheric moisture and by small quantities of basic or alkaline materials. The vast majority of UV systems are the acrylic versions.
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