How To by Hernon: Guide to Chemical Exposure
Guide to Chemical Exposure By Hernon Manufacturing
Adhesives will be exposed to chemicals throughout their working lives. The likely chemicals vary from industry to industry but common exposures include water, oil, gasoline, kerosene, jet fuel, alcohol and other solvents. Adhesives are available that will resist any or all of these liquids but remember that every adhesive has its trade offs. While one adhesive may show excellent resistance against gasoline, it may have less ability to endure through thermal cycling or may show reduced shear strength when compared to another adhesive offers moderate resistance to gasoline.
When degradation due to chemical exposure occurs, the adhesive bond can be effected in two primary ways. The chemical may chemically interact with the adhesive on the molecular level changing the bonding agent into a new compound and altering physical properties along the way. Or the chemical may permeate the adhesive material, without interacting chemically, diluting and liquefying the adhesive. Both processes are a potential risk but an adhesives expert will be able to point out which risk is greater and what adhesives are resistant against each type of chemical threat. Potting solutions, for example, have a small molecular make-up and are themselves designed to permeate materials to fill gaps and crevices. Therefore, in their cured state, they will be less vulnerable to permeation.
Epoxies, as a class, are without a doubt the most likely solution for a customer seeking chemically resistant adhesives. While in their uncured form, epoxy resins exhibit high susceptibility to thermal and chemical degradation, once a hardening agent is introduced and the epoxy starts curing, the polymer chains that make up the resin interlock. This “cross-linking” forms a firm substance with much better chemical, thermal and sometimes electrical resistance. Keep in mind that this curing process produces heat which can be significant and may effect the substrates you are bonding, sensitive components or other compounds nearby. If unchecked and sufficiently intense, the heat produced by the exothermic (heat producing) curing reaction may even cause thermal degradation to the epoxy being cured.