The softening point of a tackifier resin can influence its compatibility with other polymers. The important parameters that determine the softening point of a material are its molecular weight, glass transition temperature, and chemical composition. Tackifier resins are manufactured with different softening points, ranging from liquid to 160°C. The most common method for measuring this property is Ring and Ball Softening Point (RBSP), although the Drop Point method is sometimes cited. The two methods normally produce results within a few degrees centigrade of each other. Generally, the higher the softening point, the greater the molecular weight will be as long as the comparison is between resins with similar molecular composition.
For the same generic chemical type, a low softening point resin will have better compatibility in a specific polymer than a higher softening point. A good example of this is the addition of a pure-monomer, styrene-based resin (Kristalex F85 in Europe/Asia or Kristalex 3085 in the Americas) to an ethylene vinyl acetate (EVA) polymer. The tackifier resin has a lower cloud-point (the lower the value, the more compatible) compared with the same resin having a higher softening point (Kristalex F100 in Europe/Asia or Kristalex 3100 in the Americas.)
The glass transition temperature (Tg) is the point at which a polymeric material changes state from a hard, brittle material to a flexible semi-solid as is actually a more important property determining the performance of a tackifier resin than the softening point.
For example, natural rubber has a Tg of -59°C. Choosing a compatible tackifier resin with a Tg of +59°C in a 50/50 formulation results in a 0°C Tg of the adhesive, the ideal for PSA performance at room temperature.