QUESTION: We are formulating acrylic adhesives (both reactive acrylics and anaerobics), but are having problems with the sourcing of monomers. Some suppliers can’t seem to give us monomers that produce shelf-stable products, even though they purport to have added extra free-radical stabilizers. Can you offer some advice?
ANSWER: These types of adhesives can be quite complicated to formulate. Without knowing the specifics of your formulation, I can only suggest the following: in addition to the polymerizable monomers and other additives, two major factors influence the reactivity of these adhesives: initiators/accelerators and stabilizers.
Initiators and accelerators are usually hydroperoxides and aromatic amines. When the applied adhesives come in contact with metallic surfaces like iron or copper, these initiation systems generate free radicals, which polymerize the monomers to give cured products. Stabilizers, on the other hand, come in two types. The first type of stabilizer, atmospheric oxygen, is a strong free-radical stabilizer. As a result, these adhesives are often packaged in thin containers made of low-density polyethylene that allow the air to permeate the package.
Secondly, extra stabilizers such as quinones are added to give additional shelf stability. In your case, it seems that the extra stabilizers are not giving the desired stability. I would suggest that you look at the strong possibility that the monomers contain traces of iron or copper, perhaps residues from your supplier’s manufacturing equipment. Traces of these metals are the real enemy of the anaerobic adhesives formulator, and you should ask your suppliers to remove them or complex them with a metal chelator. You can also add a metal chelator to your formulation, but be careful about not adding too much and affecting your cure speed.
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