High Temperature Bonding Sealing
High Temperature bonding & sealing
High temperature bonding & sealing is a tricky task. For an adhesive bond to be useful, it must not only withstand the mechanical forces that act on it, but it must also resist the elements to which it is exposed during service. One of the most degrading elements for organic adhesives is heat. To synthesize high temperature polymer systems for structural adhesives is a never ending challenge.
Effects of Elevated Service Temperature
All polymeric materials are degraded to some extent by exposure to elevated temperature. Not only do elevated temperatures lower short-term physical properties, but properties will also likely degrade with prolonged thermal aging. Thus, several important questions need to be asked for an adhesive if high service temperatures are expected.
- What is the maximum temperature that the bond will be exposed to in service?
- What is the average temperature to which the bond will be exposed?
Ideally, one would like to have a definition of the entire temperature – time relationship representing the adhesive’s expected service history. This data would include time at various temperatures, number of temperature cycles, and rates of temperature change.
Creep and Lack of Cohesive Strength
Certain polymers have excellent resistance to high temperatures over short durations (e.g., several minutes or hours). The short-term effect of elevated temperature is primarily one of increasing the molecular mobility of the adhesive. Thus, depending on the adhesive, the bond could actually show increased toughness but lower shear strength. Certain polymers with lower glass transition temperatures will show softness and a high degree of creep at elevated temperatures.
However, prolonged exposure to elevated temperatures may cause several reactions to occur in the adhesive. These mechanisms can weaken the bond both cohesively and adhesively. The main reactions that affect the bulk adhesive material are:
These reactions generally result in brittleness and loss of cohesive strength. Thermal aging can also affect adhesion by causing changes at the interface. These changes include:
- Internal stress on the interface due to shrinkage of the polymer
- Chemical reactions with the substrate, and
- Reduced peel or cleavage strength because of brittleness
If heating brings a non-cross-linked adhesive above its glass transition temperature, the molecules will become so flexible that their cohesive strength will drastically decrease. In this flexible, mobile condition, the adhesive is susceptible to creep and greater chemical or moisture penetration occurs. Generally with a cross-linked adhesive, prolonged heating at an excessively elevated temperature will have the following effects:
- Split polymer molecules (chain scission) causing lower molecular weight, degraded cohesive strength, and low molecular weight byproducts.
- Continued cross-linking resulting in bond embrittlement and shrinkage.
- Evaporation of plasticizer resulting in bond embrittlement.
- Oxidation (if oxygen or a metal oxide interface is present) resulting in lower cohesive strength and weak boundary layers.
Most organic adhesives degrade rapidly at service temperatures greater than 150°C. However, several polymeric materials have been found to withstand up to 250-300°C continuously and even higher temperatures for a short-term basis. To use these materials one must generally pay a premium in adhesive cost and also be able to provide long, high temperature cures, often with pressure. Long-term temperature resistance, greater than 250-300°C, can only be accomplished with inorganic or ceramic-based adhesives.