Acid-base interactions and covalent bonding at a fiber-matrix interface: contribution to the work of adhesion and measured adhesion strength

The work of adhesion, W-A, and the practical adhesion in terms of the inter facial shear strength, tau, in some polymer-fiber systems were determined t o establish a correlation between these quantities. An attempt was made to analyze the contributions of various interfacial interactions (van der Waal s forces, acid-base interaction, covalent bonding) to the 'fundamental' and 'practical' adhesion. The surface free energies of the fibers were altered using different coupling agents. To characterize the strength of an adhesi on contact, the ultimate adhesion strength, tau(ult), was determined for th e onset of contact failure. The adhesion of non-polar polymers occurs throu gh van der Waals interaction only; therefore, fiber sizing does not affect the adhesion strength. For polar polymers, such as poly(acrylonitrile butad iene styrene) and polystyrene, adhesion is sensitive to fiber treatments: s uppression of the acid-base interaction by using an electron-donor sizing a gent gamma-aminopropyl- triethoxysilane results in a decrease of both 'fund amental' and 'practical' adhesion. In the case of epoxy resins? the main co ntribution to the work of adhesion is made by covalent bonds. Since the pro cess of their formation is irreversible, the work of adhesion determined fr om micromechanical tests seems to be more reliable than indirect estimation s, such as from wetting and inverse gas chromatography techniques. Fiber tr eatment by sizing agents results in considerable changes in the intensity o f adhesional interaction with the epoxy matrix. A correlation between the w ork of adhesion, the ultimate interfacial shear strength, and the strength of macro-composites has been found.