In order for a material to function as an adhesive it must have a balance of elastic and viscous properties. It must possess sufficient viscous flow properties to bring the adhesive and the substrate into intimate enough contact for adhesive bonds to form. Once these bonds form, the material must have elastic strength to hold the system together. In an adhesive system, the base polymer or elastomer provides most of the elastic properties while the tackifier provides the viscous component. Once combined, the system exhibits the viscoelastic behavior that characterizes the adhesive. The study of the viscoelastic behavior of materials under variable conditions of stress and temperature is the science of rheology. As a tackifier modifies the rheological properties of an elastomer (e.g., a styrenic block copolymer, natural rubber, or ethylene vinyl acetate) the properties of tack; the ability to stick; and adhesion, the binding force of the adhesive, begin to develop.
While the viscoelastic properties of an adhesive are critical to its function, they are not alone in determining the ultimate level of tack and adhesion developed by a given system. These properties are strongly influenced by the chemistry of the adhesive, the surface properties of the substrate, and the interactions between them. Since there is a wide diversity of elastomers and tackifiers available to the formulator, a basic understanding of tackifier resin selection criteria and familiarity with the various families of tackifiers is critical.