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Question: I am starting to formulate some acrylic UV adhesives and notice from MSDS sheets that the adhesives seem to be based almost exclusively on methacrylate monomers rather than acrylates. Why is this?
Answer: As you point out, two types of acrylic monomers are used in UV systems-acrylates and methacrylates. A range of monofunctional (one reactive group per molecule) and multifunctional (more than one reactive group per molecule) monomers are available, giving wide flexibility in formulating these systems.
Acrylates are the most widely available acrylic monomers and are used extensively for coatings. They are characterized by high cure speed and tend to provide hard, brittle coatings. Many acrylates often have severe skin irritation properties, which is not a problem where coatings are applied via automatic systems. However, adhesive formulators who often encounter accidental skin contact have been wary of using acrylates, especially when the alternative methacrylates have low skin irritancy. In addition, methacrylates generally feature better adhesion properties and somewhat more flexibility. In general, methacrylates cure more slowly than acrylates.
The use of monomers alone in UV systems tends to lead to relatively fast-curing, highly crosslinked, brittle polymers. The inclusion of oligomers, where the reactive groups are grafted or terminated onto short chain length polymers, allows the incorporation of the properties of other polymers into the formulation. Acrylates have become more common in these applications, because the higher molecular weight oligomers do not cause skin irritancy issues.
For example, the use of urethane acrylates or urethane methacrylates can yield very flexible or very tough formulations, depending on the nature of the urethane. Both aromatic and aliphatic urethanes are readily available in a range of molecular weights and functionalities. Other oligomers include polyester, polyether, and polybutadiene acrylates and methacrylates.