Adhesive performance is dependent on the compatibility of the tackifier resin and the polymer. The simplest way to determine compatibility is to observe the development of tack and adhesion properties when the two are combined. This traditional trial-and-error method of determining compatibility works, but must be used with care in order to avoid hidden incompatibilities that can lead to loss of properties over time. The use of solubility parameters, molecular weight properties, and dynamic mechanical analysis techniques are more sophisticated and much more accurate in determining compatibility.
An incompatible resin may also be added to a polymer system to impact other desirable properties, but it will not produce tack in a polymer system. The fine-tuning of adhesive systems is often made by the addition of a resin having limited compatibility. This may, for example, produce a lower tack level with higher cohesive strength characteristics than would be found using a fully compatible resin. An example of this type of resin formulation is discussed in the
hot melt pressure sensitive section.
There are many factors affecting resin compatibility, but the main parameters are:
Chemical structure: For hydrocarbon resins the structure of the aromatic or aliphatic monomer base materials and the degree of hydrogenation of the product are critical determinants of the compatibility with a given elastomer. The
cloud point method can be used to determine the aliphatic/aromatic balance and degree of hydrogenation of a material and is very useful for selecting the appropriate tackifier for a specific polymer.
Molecular weight and distribution: The
molecular weight and distribution of the resin is a critical determinant of its compatibility with the polymeric system. Molecular weight can be indirectly measured by the
softening point of the resin. In general, the higher the softening point, the greater the molecular weight will be. Softening point also has an influence on the degree of compatibility a tackifier resin has with any polymer type.
Dynamic mechanical analysis (DMA) is a powerful tool to measure the rheology and identify compatibility characteristics of resin/polymer systems.