Ask Dr. Dave: May 2012
Question: We use two-component epoxies in our metal bonding operations. Although the adhesives work well, we find it frustrating to have to handle two components and make sure that they are mixed thoroughly and in the correct proportions. Can you recommend a one-component adhesive that could do the same job?
Answer: Reactive adhesives come in two forms: two-component or one-component. Two-component adhesives are mainly epoxies, reactive acrylics or polyurethanes. Customers use two-component adhesives because they cure at room temperature and can fill large gaps. Like you, however, they often use them by necessity rather than by choice because of the reasons you mentioned.
The only true one-component adhesives are cyanoacrylates and anaerobic methacrylates. These adhesives rely on surface moisture and metal surfaces, respectively, to initiate the curing. All other one-component adhesives require an external chemical or energy source to initiate curing. These one-component adhesives include hot melts, which must be heated; reactive hot-melt urethanes, which require heat plus atmospheric moisture for curing; polyurethanes, which require atmospheric moisture for curing; and heat-cured epoxies. Last but not least, UV- or visible-light-cured adhesives allow very fast curing (“on command”) but do require one substrate to be transparent to the radiation.
Cyanoacrylates or anaerobics are obvious possibilities for you, but they tend to be expensive if you are bonding large areas. These adhesives are also limited to bond gaps of around 20 mils unless you use a surface primer. Before you make a change, you need to consider the durability requirements of your adhesively bonded structure, such as temperature resistance, oil or solvent resistance, and possible exposure to atmospheric conditions, such as moisture.
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.