On the slow crack growth process and associated structure–property relationships in polyamide 12 grades

The influence of micro-structural changes within polyamide 12 (PA12) grades on their resistance against crack initiation and subsequent Slow Crack Growth (SCG) was examined. Possible micro-deformation mechanisms and processes guiding slow crack extensions in PA12 are discussed. Special focus was put on the formation of plastic zones as predecessors to microscopic cracks prior to crack initiation via crack freezing analysis. Simultaneously, variations in the pure SCG resistance after a recorded craze-crack transition were determined via cyclic Cracked Round Bar (CRB) tests. In that context, a homologous series of PA12 grades of increasing molecular weight (MW), as well as a systematically developed series of PA12 grades incorporating a pigment and/or an impact modifier, were selected for this study. Results show a good correlation between the crack initiation resistance and the disentanglement resistance of PA12 chains as well as the size of the plastic zone. In that context, an increasing average MW, leads to increasing disentanglement resistances due to hydrogen bond effects. An impact modification promotes the development of notably larger plastic zones. Contrarily, colored grades exhibit a reduced plastic zone size, rendering a lower amount of dissipated energy before physical crack initiation. Pure SCG resistance follows a similar trend and is improved by high average MW as well as the use of impact modifier. Crack growth acceleration, however, is assumed to occur alongside the relatively weak interfaces between polymer matrix and pigments.