Rubber toughening of an amorphous polyamide

Rubber toughening of an amorphous polyamide (Zytel 330 from DuPont), a-PA, using various maleated elastomers via standard notched Izod impact testing was investigated and the results were compared and contrasted with those of nylon 6. These elastomers used include a triblock copolymer, hydrogenated styrene-butadiene-styrene, SEBS, with its maleated version, SEBS-g-MA, an ethylene/propylene copolymer, EPR, with its maleated version, EPR-g-MA, and an ethylene/1-octene copolymer, EOR, with its maleated versions, EOR-g-MA-X% where X is 0.35, 1.6 or 2.5. Comparison of fracture behavior, as characterized by both linear elastic fracture mechanics techniques and the essential work of fracture methodology, between a-PA and nylon 6 toughened with maleated EOR elastomers was also studied as a function of ligament length, rubber content, rubber particle size and test temperature. The morphology (particle size and its distribution) of the rubber phase was studied as a function of the level of maleation and the rubber type. Rubber particle size can be well controlled by varying the proportions between the two elastomer components in blends based on mixtures of SEBS-g-MA/SEBS or EPR-g-MA/EPR; morphology development is more complex in EOR-g-MA/EOR system because of co-existence of immiscibility and kinetic factors. The immiscibility between these EOR elastomers was examined via transmission electron microscopy; the miscibility boundary occurs at ∆ (% MA) = 0.9~1.25%. Bimodality in particle size emerges in some cases. The room temperature Izod impact strength and the ductile-brittle transition temperature (Tdb) of these blends were found to be strongly dependent on the rubber content, rubber particle size, the rubber type and the matrix type. Either a lower limit or an upper limit in particle size or both for effective toughening may appear depending on the rubber type and the matrix type. For some marginally tough blends of a-PA, far end samples were observed to be tougher than gate end specimens. Higher rubber content led to a lower Tdb; Tdb generally decreased and reached a minimum then increased with rubber particle size depending on the rubber type. Fracture behavior of the blends was shown to depend on ligament length, rubber content, rubber particle size, and test temperature. Typically, shorter ligament lengths, higher rubber content, medium particle sizes and higher test temperatures lead to ductile fracture. Linear elastic fracture mechanics (LEFM) technique was found to well characterize brittle samples while the essential work of fracture methodology was shown to quantify ductile specimens well. In many ways, the trends for a-PA are rather similar to those for nylon 6, implying that any role of the matrix crystalline structure must be of second importance in toughening. However, the a-PA appears to be more easily toughened and achieves higher level of toughening when particle morphology is optimized. This may be due, in part, to the lack of crystallinity in a-PA but issues of molecular structure (entanglement density, chain dynamics, viscoelastic effects, etc.) cannot be ruled out.