Toughenability of Nylon-6 with CaCO3 filler particles: new findings and general principles

The mechanical behavior of Nylon-6 blends modified by two types of CaCO3 particles of 0.7 and 3.5 µm diameter with particle volume fractions ranging from 0.05 to 0.28 was studied between -30 and 60°C in slow tension, and at 20°C in bending impact. Additional experiments were also carried out at 20°C to determine the plane stress fracture toughness of the blends in Single-Edge-Cracked-Plate configurations; all fracture behavior was followed extensively by SEM fractography. Experiments demonstrated that the particles are attached to the matrix only through a differential thermal–contraction–pressure and particle separation preceded plastic response in all instances. As a consequence of the above ease in debonding, the yield strengths of the blends drop systematically with increasing particle concentration. In slow tension all blends showed a well defined plastic stretching response following necking, but the stable post-necking stretch was severely limited by an overabundance of large particle clusters which acted as super-critical flaws to initiate premature termination of stretching. The present findings show that in these blends with their high plastic resistances, critical flaw sizes that trigger brittle response are in the range of 8–12 µm, well under the sizes of many of the particle clusters encountered in the blends. In contrast with the attractively tough response of the rubber modified Nylon-6 blends of Murato lu et al. [Polymer 36 (1995) 921; Polymer 36 (1995) 4771] all present blends showed only disappointing brittle behavior under Izod impact conditions. This was traced to the development of substantial levels of triaxial tensile stresses arising from only partial separation of rigid particles from the matrix in the early phases of impact response. Based on the new findings a number of general principles on toughenability with both compliant and rigid particle modification are presented and supported by simple micro mechanical models. Author Keywords: Nylon-6/CaCO3 blends; Toughenability of brittle polymers; Toughenability with stiff-particle