4.5.2 Operating Temperature

All adhesives have a certain temperature at which they undergo degradation. This may occur with brief excursions at high temperature or through long-term thermal ageing. Each chemical type of adhesive and its different formulations will have their own response to thermal conditioning. Cohesion may be weakened, or it may become more brittle and less able to absorb shock, impact or temperature-related dimensional changes. The key properties for good high temperature performance are a high glass transition temperature and resistance to oxidation.

Thermoplastic adhesives can provide excellent cohesive bonds at room temperature. As the working temperature approaches the glass transition temperature of the adhesive, the plastic flow results in bond deformation and cohesive degradation.

Thermoset adhesives consist of crosslinked polymeric chains and have no melting point. When thermoset adhesives are exposed to high temperatures, the critical factor is the rate of force reduction due to thermal oxidation and pyrolysis. Thermal ageing can also affect the adhesion interface, causing chemical and physical changes. Thermal oxidation initiates a progressive split of the chain of molecules, resulting in a loss of weight, strength, elongation and toughness within the adhesive or sealant. Pyrolysis is the simple thermal destruction of the molecular chain of the base polymer in the adhesive or sealant formulation. Pyrolysis causes cleavage of the chain and a decrease in the molecular weight of the polymer, which results in reduced cohesive strength and fragility.

To be considered suitable for applications at high temperatures, an adhesive must provide all the usual functions necessary for good adhesion (wettability, low shrinkage during curing, a similar coefficient of thermal expansion to the substrates, etc.) and must have a high melting point and glass transition temperature (Tg), resistance to oxidation degradation and thermally induced chain break resistance.

Many adhesive and sealant applications require both high- and low-temperature performance. The critical factors to consider for wide temperature range applications are the difference between the coefficients of thermal expansion for adhesive and substrate (the closer the better), the elasticity modulus of the adhesive at application temperatures and the thermal conductivity of the adhesive.

The difference in the thermal expansion of the adhesive and substrates is important at low temperatures, especially because the modulus of elasticity of the adhesive generally increases (it becomes stiffer and more brittle) with decreasing temperature to a point where the adhesive can no longer effectively release concentrated stresses.