Unintended Epoxy Reaction Before Cure

Once the epoxy adhesive is applied to the substrate, it must harden into a thermosetting film having high cohesive and adhesive strength. Several processes occur during the curing or solidification of epoxy resins that can lead to fundamental problems with adhesive systems.

These problems are generally due to internal stresses that develop within the joint as the result of the curing processes, but they can also be due to side-reactions that occur prior to joint assembly.

The general curing reaction and resulting gradual increase in viscosity of a mixed epoxy resin during application can cause problems. The working or pot life of an epoxy adhesive should always be considered in its selection. The working life must be sufficient for the details of the mixing and application process and any time delay during these processes. The viscosity increase in an epoxy resin - curing agent system due to working life issues could result in poor wetting of the substrate surface resulting in suboptimal adhesion.

Several reaction mechanisms can also occur to an epoxy adhesive once it is mixed and applied to a substrate but before the substrates are mated. These unintended mechanisms can result in a weak boundary layer, which will prevent optimal wetting and reduce the strength of the adhesive.

Many epoxy resin - curing agent mixtures are hygroscopic and will adsorb moisture from the ambient humidity. This is primarily due to the properties of the curing agent. One example is a polyamide curing agent. If the mixed adhesive is applied to the substrate and allowed to wait until the substrates are mated, water can be adsorbed onto the surface of the adhesive mainly through the polyamide molecules. Assemblies that are most prone to this effect are those that have a long time period between adhesive application and joint closures and where relative humidity conditions are high. The very thin layer of water that forms on the adhesive surface will result in a weak boundary layer between the adhesive and the matting substrate when the joint is closed. When in service, the boundary layer fails under stress. The failure mode that is noticed in these cases is generally an adhesion failure at the interface.

Another possible preassembly reaction mechanism has been noted with regard to amine cured epoxy resins. ¹ A variability and reduction in the rate of conversion of epoxy groups in DGEBA epoxy resin cured at room temperature with diethylene triamine (DETA) was noticed. This is due to a side-reaction of the amine with air, resulting in bicarbonate formation. In the coatings industry this type of reaction is termed "amine-blush" and it generally is considered to be a cosmetic problem. However, with adhesive systems the reaction can result in drastically reduced adhesive strength when the uncured epoxy-amine is exposed to ambient air for a significant period of time.

Once concludes that when room temperature curing systems are to be used as adhesives, the assemblies should be joined quickly to preclude various reaction mechanisms from taking place that could degrade the strength of the final bond.