Ask Dr. Dave

Ask Dr. Dave

Adhesive and sealant troubleshooting tips from Dr. Dave Dunn.

Question: I am using an azo initiator to cure a clear acrylic adhesive despite the fact that nitrogen bubbles are given off when the initiator decomposes. However, I am getting too many bubbles and the adhesive is expanding a lot. Can you offer any advice on how to remedy this problem?

Answer: I suspect you are using a low-viscosity adhesive based primarily on mono methacrylates. The trick to minimizing gas bubble formation is to either use a thickened adhesive or to add some dimethacrylates or trimethacrylates. What you are trying to do is stop the gas bubbles from traveling through the adhesive, which causes it to foam. Adding 5-10% of a multifunctional methacrylate will cause the adhesive to gel at a very low conversion of monomer to polymer and usually results in an almost-clear cured adhesive.

Question: What are the best methacrylate and acrylate monomers available for a range of acrylic adhesives?

Answer: This is a difficult question to answer in this short column. Acrylate monomers generally cure much faster than methacrylates but tend to be somewhat more brittle and are more likely to be skin and eye irritants. Adhesives formulators commonly use methacrylate monomers rather than acrylates, one of the major reasons being that adhesion is normally better with these monomers. Methacrylates are available in monofunctional, difunctional or trifunctional varieties. Monofunctional monomers are used for fast-curing adhesives and can be fairly flexible, particularly when formulated with rubber additives. Di- and trifunctional monomers are added to confer heat and solvent resistance, and are the primary monomers used in so-called anaerobic threadlockers and sealants.

Your selection of monomer also depends upon the materials being bonded; certain monomers are suitable for metals, and others are designed primarily for bonding plastics and composites. An important development in recent years is the marketing of so-called “oligomer resins,” which are short-chain polymers that contain reactive groups. These are commonly urethane acrylate or urethane methacrylate oligomers and can act as reactive plasticizers, toughening agents, or cure enhancers. Many of these have mitigated the health and safety concerns often associated with acrylate monomers. I would encourage you to consider modern acrylate monomers for their enhanced properties.

Any views or opinions expressed in this column are those of the author and do not represent those of Adhesives & Sealants Industry, its staff, Editorial Advisory Board or BNP Media.


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