4.5.4 Chemical Exposure

There is no single adhesive that is ideal for all chemical environments. Some families of adhesives, such as epoxies, are know to have broad resistance to many types of chemicals. PUs, silicones, acrylics and UV curing adhesives can provide acceptable chemical resistance against a certain range of chemicals, although they cannot withstand as many chemical environments as epoxies. The chemistry of an adhesive can vary substantially, even within a family. Two adhesives from the same family may have different additives and curing reactions that affect their chemical resistance.

There are two properties of adhesive joints that protect them from exposure to chemical or solvent environments. The high degree of crosslinking of the chain, found in such thermoset adhesives as epoxies, phenolics, PUs and modified acrylics, tends to resist various chemicals when exposed to temperatures below the glass transition temperature (Tg). Therefore, adhesives with higher Tg can often resist higher temperatures and tolerate more chemicals. Another important property of good chemical resistance is the area of bond exposure to the chemical. The thinner the bond line, the less direct contact with the chemical itself, which protects the adhesive from exposure. This is especially true if the substrates are neither porous nor permeable to the chemical environments in question.

In the families of adhesives and their possible applications, it is necessary to consider chemical and thermal resistance together. The temperature of the chemical medium in which an adhesive joint is immersed is a significant factor in the ageing properties of adhesives. As the temperature increases, the fluid absorption by the adhesive increases and the rate of degradation increases. In general, chlorinated solvents, ketones and high boiling solvents, such as dimethylformamide and dimethyl sulfoxide, are considered harsh environments.