Effect of low-profile additives on the durability of adhesively bonded Sheet Moulding Compound (SMC)

Abstract The mechanical bonding properties and the failure behaviour of adhesively bonded SMC substrates were investigated under the aspect of the influence of low-profile additives on the durability of joints. For this purpose, three SMC materials with different amounts of polyvinyl acetate (PVAc) and saturated polyester were bonded with a structural and semi-structural adhesive and tested under cleavage peel test in reference condition and after thermal and hygrothermal ageing. In addition characterisations of the individual materials and their ageing effects were made. In principle, destructive testing of bonded SMC substrates showed a dependence on the used adhesive and the appearance of a type of fibre bridging. Depending on the PVAc content, different fibre tearing rates occurred. Thermal ageing led to post-crosslinking of the materials and increasing stiffness. Moreover, strong fibre tear-out appeared. Hygrothermal ageing in the SMC results in physical reversible plasticizing effects and chemical ageing by hydrolysis of the low-profile additives, presumed by performed DMTA tests. A weakened interface and disbonding failure phenomena occurred with increased PVAc content of semi-structural bonded specimens. Thus, in terms of adhesive bonding of SMC structures, a focus should be put on the type and quantity of low-profile additives used. The reduction of PVAc content led to the required quality of the joint with regard to the durability.