Elevated temperatures have long been recognised as the number one enemy of organic adhesives, degrading their efficiency in a number of ways. There are two main reactions that cause a breakdown of bond strength at elevated temperature – thermal oxidation and pyrolysis.
Thermal oxidation starts a progressive chain scission of the molecules that make up the adhesive, resulting in loss of weight, strength, elongation and toughness within the body of the adhesive.
The materials or substrates bonded will have an impact on this rate of oxidation – some adhesive interfaces with certain metals will increase the rate of oxidation through a chemical process. This explains, for example, why aluminium will display higher levels of thermal stability than stainless steel when used with most structural adhesives.The addition or incorporation of anti-oxidants within the adhesive system will help to limit or slow down the rates of oxidation, providing increased performance at elevated temperature.
Pyrolysis is the thermal destruction of the molecular chain of the base polymer within the adhesive. The molecular chains undergo scission and molecular weight decreases, cohesive strength is reduced and embrittlement occurs.
Resistance to pyrolysis is determined by the inherent heat resistance of the polymer. Typically, epoxy adhesives using an aliphatic polyamine curing agent are only suitable for use in environments where the temperature will not exceed 65°C. However, the use of aromatic diamine curing agents will improve this significantly. The addition of high temperature resistant co-reactants and anhydride curing agents will further increase the thermal stability of the epoxy system. This can achieve temperature resistance of >150°C with potential short-term resistance of up to 230°C.
All of our products are suitably heat resistant for their end use, however Aerok 7205, 7001p and 7001LZ, have been tested in extreme conditions to ensure they are fit for the altitude and temperature they will be exposed to within the Aerospace industry.