Characterization of the interface between cellulosic fibers and a thermoplastic matrix

The applicability of the microbond test to evaluate the interfacial properties between cellulosic fibers and thermoplastics was studied. Acetylation and heat treatment were applied to modify the surface of cellulosic fibers (rayon, cotton, and wood). The apparent diameters and surface free energies of the fibers were estimated by dynamic contact angle (DCA) analysis. Interfacial shear strengths between the cellulosic fibers and the polystyrene matrix were determined using the microbond test method. The test results indicate that acetylation increases the total surface free energy of the wood fibers, whereas heat treatment dramatically decreases the surface free energy of all cellulosic fibers tested. For heat treated and acetylated fibers, the greater the surface free energy, the greater the interfacial shear strength (ISS) regardless of fiber types. For control group fibers, a low ISS exists even though the fibers have high surface free energies because of the formation of a weak boundary layer. The high ISS between the acetylated wood fiber and the polystyrene matrix is attributed to the improved wetting and spreading of the melting polystyrene on the acetylated wood fiber surfaces. As such, the interfacial properties between the cellulosic fibers and polystyrene matrix system can be successfully characterized by dynamic contact angle and microbond test.