Testing pressure-sensitive adhesives under standard conditions of temperature and relative humidity is always a preferred means to compare data from lab to lab and from lot to lot, but it is sometimes necessary to evaluate the performance of an adhesive under conditions that it would encounter in the field.
Many adhesives exhibit different properties when subjected to variations in temperature. Some of these adhesives are thermoplastic. They soften when heated and become more brittle when cooled. [In the case of pressure-sensitive adhesives (PSAs), the nature of the bond is nearly always thermoplastic.] If for the sake of this discussion we limit our testing to tack, peel, and shear, we can develop a series of observations that can be applied to most testing situations.
In terms of the relative effect of temperature on the common PSA properties of tack, peel, and shear resistance, as the temperature increases, the adhesive mass softens and “wets” the substrate more rapidly and completely, and the tack and peel properties increase. At some point, however, the adhesive softens to the point that its cohesive strength is less than the substrate adhesion, and the adhesive mass splits, failing cohesively. As the temperature continues to increase, the adhesive mass continues to soften, and the tack and peel values continue to go down.