Polypropylene is a non-polar polymer, and has a limited affinity to fibre sizing compounds. Maleic anhydride grafted onto the polymer results in polarity, and a more efficient interaction with the fibre size. The bond can be the result of chemical reaction[10], or physisorption with interdiffusion of the polymer chains into the sizing and local dipole–dipole bonding[11]. Hydrogen bonding to the oxygen of the maleic anhydride endgroup is another likely bond type. These bonds are illustrated in Fig. 1. Reaction mechanisms of the grafting of maleic anhydride onto polypropylene have been described by Lin[12]and De Roover et al.[13]. The interfacial bond can also be improved by other means. Thomason and Schoolenberg[14]investigated the influence of different surface coatings on the interfacial shear strength. Most modifications are based on an increase in polarity at the interface