A non-isothermal healing model for strength and toughness of fusion bonded joints of amorphous thermoplastics

A study to investigate the influence of processing on the fusion bonding of graphite (AS4) poly(etheretherketone) (PEEK) thermoplastic composites (BASF commingled PEEK/graphite NCS woven fabric) using a polyetherimide (PEI) film at the interface is presented. Fundamental to all fusion bonding processes is the intermolecular diffusion between surfaces in intimate contact. A model based on the healing theory of amorphous polymers has been proposed to predict strength and toughness as a function of non-isothermal process history. This model considers two different microscopic failure mechanisms of a healed interface. For the first time, using non-isothermal data and proper data reduction procedures, it is possible to differentiate between these two mechanisms, which are otherwise indistinguishable from isothermal data. Temperature dependent reptation times representative of the kinetics of chain diffusion in the polymer have been evaluated for both mechanisms over a large range of process temperatures using fracture tests conducted on lap shear specimens manufactured using a hot press. Three alternate and independent techniques to estimate the reptation time in PEI indicate that the model based on the average interpenetration distance is most representative of the physical system. Lap shear strength predictions based on this formulation have been generated for various non-isothermal conditions measured in the hot press and are within 20% of the experimental data. The model was used to show that in isothermal processes, maximum strength and toughness can be achieved in less than 1 s for temperatures exceeding 290°C. Application of the model to a highly non-isothermal technique such as resistance welding using amorphous film technology is also presented. Model predictions show that asymptotic strength may be achieved in relatively short process times with appropriate welding conditions.