Effects of Temperature on Adhesives

[GordMay]

Effects of Temperature on Polymer Adhesives:

Adhesive materials are generally based on organic polymers, and this means that their mechanical and viscoelastic properties will be affected by changes in temperature. This effect will largely be dependent on whether the adhesive is a thermoset or a thermoplastic. Because of this high degree of temperature dependency, the thermal, physical, mechanical, and degradation properties of adhesives have to be characterized.

With thermoplastic adhesives, the major temperature effect is one of melting as the adhesive transforms from a solid to liquid state at the melting point, Tm. With thermosetting adhesives, there is no melting point and the major change caused by temperature is aging and oxidative degradation. With both thermoplastics and thermosets, the glass transition temperature, Tg, is an important consideration, since this is the temperature that the polymer will undergo significant change in physical and chemical properties.

Polymers undergo many thermal reactions when heated other than Tm or Tg. These secondary transitions can occur at temperatures far away from the Tg, Tm, or degradation temperatures. They include additional chemical reactions, solids melting, evaporation of low molecular weight constituents, and amorphous reorganizations to a crystalline state. Table 1 describes the main changes that occur in some key mechanical properties due to changes in temperature.

Effects of Temperature Changes on the Mechanical Properties of Adhesives

Strength
As the temperature increases there is a general decrease in the strength of adhesive materials. Different types of adhesives have varying resistance to higher temperatures. The strength of adhesives is not usually adversely affected by low temperature, although other properties such as stiffness and toughness may be affected and influence the performance in a joint.

Stiffness
As the temperature increases, the stiffness or modulus of adhesive materials will decrease. The magnitude of the decrease will depend on the specific type of adhesive. Reductions in stiffness will affect the rigidity of an adhesive joint. Decrease in temperatures tend to make adhesive materials stiffer, which can alter stress distribution in an adhesive joint.

Toughness
At higher temperatures, adhesive materials tend to become tougher, due to increases in their fracture energies. At low temperatures the opposite occurs and the fracture energy of adhesive is reduced and they become more brittle. In rubbery adhesive there can be large changes in toughness, with high toughness values above the Tg. These processes are inhibited at lower temperatures, below the Tg, and toughness is greatly reduced.

Elongation / Strain
As temperature is increased, the tendency is for adhesives to have higher elongation or strain to failure. This is due to the increased mobility of the polymer chains. The opposite occurs at lower temperatures, which restricts the movement of polymer chains in the adhesive and leads to reduced elongation to failure.

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