Improvement of shear strength of aluminium-polypropylene lap joints by grafting maleic anhydride onto polypropylene

The effect of the amount of maleic anhydride-grafted polypropylene (PP-g-MAH) in polypropylene (PP) on lap shear strength of adhesive-bonded aluminium sheet/PP/aluminium sheet has been investigated after surface pretreatment of the aluminium sheets by sandpaper grinding and by γ-aminopropyltriethoxy silane solution after sandpaper grinding. For pure PP low lap shear strengths, 3.44 and 4.57 MPa, respectively, are obtained and fracture with almost no plastic deformation at the fracture surface is observed. Modification of PP by addition of 5–30 wt% amount of PP-g-MAH gives obvious promotion of the lap shear strength. It is due to the chemical interactions between –OH, Al3+ or amino group –NH2 at the surface of the aluminium sheets and the polar functional anhydride groups and carboxylic groups –COOH on PP-g-MAH at the interface. Fracture occurs in the polymer and shows severe plastic deformation. The maximum values of the lap shear strengths, 10.30 and 10.64 MPa for the two surface pretreatments, respectively, are obtained at amount of 20 wt% PP-g-MAH in PP. With further high amount of PP-g-MAH in PP, the lap shear strength declines due to the formation of two regions, one with high content of PP-g-MAH and another with high content of PP, with lower strength of chain entanglements between them than the interfacial adhesive strength between PP-g-MAH and aluminium sheet. It is shown that a high degree of modification of PP or surface of aluminium sheets could result in excessive interfacial chemical-bonding density at the interface and would have no effect and even a negative effect on adhesive properties.