Kraft lignin fillers have been shown to enhance the mechanical characteristics of different types of polymeric-based sealants; however, the mechanism for reinforcement and the degree of interaction was not well understood. In this study, interactions between a polyurethane-based elastomer and various Kraft lignin fillers as well as kaolin and titanium dioxide filler combinations were investigated in an effort to determine the most useful elastomer-filler combination for use as a construction joint sealant. A relationship that describes and relates the characteristics of polymer-filler interaction requires that the surface-energy properties of both the polymer matrix and the filler be determined. Measurements using the sessile drop method are used to evaluate the critical surface energy of the polymer matrix, whereas the rate of rise of a liquid through a bed of filler provides the means to evaluate the surface energetics of the various fillers. Results obtained from this study show that the tensile modulus of the different elastomer formulations are proportional to the equilibrium work of adhesion, as described by the relationship derived by Lee that relates the degree of interaction of the formulation components and the mechanical properties of the blends.