Question: Do you have any advice for using an azo initiator to both heat-cure an acrylic adhesive and maximize its shelf life?

Answer: Most people use azo initiators to avoid using peroxides, which are sensitive to metals, amines, etc. However, there are only three common azo initiators, and their reactivity is normally characterized by their so-called 10-hour half-lives at a specific temperature (i.e., 10 hours is the time at which 50% of the material is decomposed at that temperature). The most common azo initiator has a 10-hour half-life at 65˚C and is usually used at cure temperatures of 85 to 100˚C. A higher-temperature azo initiator is also available with a 10-hour half-life of 88˚C. Increasing the cure speed is usually done by increasing either the cure temperature or the concentration of the initiator. For maximum cure speed, a low-temperature azo initiator is available with a 10-hour half-life of 52˚C. One trick of the trade for maximum speed at lower cure temperatures is to use a small amount of the “52” version in concert with a larger amount of one of the higher-temperature versions. This way, the 52 initiator starts the curing and the heat generated from polymerization decomposes the other azo initiator.

Shelf life is always an issue with azo initiators. Bear in mind that the raw initiators and formulations containing them must always be stored at low temperatures (preferably below 50˚F) and must always be shipped using controlled-temperature shipping. Free-radical stabilizers will help in increasing shelf life, but must be carefully chosen to avoid loss of cure speed. Some experimentation with different types and concentrations will be needed here.

Question: What is the best way to remove an aerobic-cure adhesive that has run into some holes in a casting and cured to a solid?

Answer: Unfortunately, these adhesives are designed to have outstanding resistance to solids and heat, making removal very difficult. The first thing I would try is a long soak in a very aggressive solvent like methylene chloride, but be sure to cover the hole to avoid evaporation of the solvent. If this method is not successful, your only other option may be to burn the adhesive out. Try using a propane torch, or possibly bake the casting in a high-temperature oven.