High Temperature Adhesives & Heat Resistant Adhesives
- Temperature Resistance
- Differential Thermal Expansion
- Thermal Cycle – Thermal Shock
Heat Resistant Adhesives
Why is it that some adhesives are listed as high temperature resistant when they resist only 150 °C while others are classified as high temperature resistant and resist 300 °C or more? The classifications are based on what typical temperature resistance is for that technology. For example, a standard anaerobic threadlocker resists 150 °C so a threadlocker that resists 230 °C is listed as high-temperature resistant threadlocker. Standard ethyl based cyanoacrylates resist 82 °C where some specialty grades can withstand 250 °C.
Organic adhesives such as cyanoacrylate, epoxy, acrylic, anaerobic, and UV curable acrylates will break down around 250, and 300 °C. Where inorganic based adhesives such as ceramic threadlockers can resist up to 650 °C and silicone based can exceed 350 °C.
Differential Thermal Expansion
The ability of the adhesive itself to resist breakdown due to high temperature is only one facet of a complex set of conditions. Adhesives can soften as temperature increases. This can be an added benefit as they are able to absorb stresses related to differential coefficients of expansion and contraction. However, when designing with adhesives for use in high-temperature applications, it is best to test the joint strength at the temperature required. This is often listed as Hot Strength.
The service temperature listed for the adhesive is a guide. The factors affecting the actual service temperature of anyone bonded joint include: adhesion to the substrates being bonded, the bond area, the speed of the temperature change, the amount of load or stress on the joint, the gap filled with the adhesive, the coefficients of expansion of the two substrates, the length at high temperature. Most adhesives can resist higher temperatures than what is listed as the service temperature for short periods of time providing adhesion to the substrate is high, and the stress is low.
Thermal Cycle / Thermal Shock
Many applications require the joint to be repeatedly heated and cooled. Adhesives designed to resist thermal cycle have higher degrees of flexibility to absorb the stresses related to the two components expansion and contraction. When two components, joined by mechanical means, experience thermal cycling micro cracking or stress fractures can form in the substrates. Adhesives that absorb stress can eliminate the breakdown of the substrates.
The rate at which the temperature change occurs can be a large factor in how successful the bond is at resisting failure. Thermal shock is a very fast change in temperature.
For further help and advice on heat resistant adhesives, please contact Permabond.