Structure-property relations in an injection-moulded, rubber-toughened, semicrystalline polyoxymethylene

Abstract

A relationship has been identified between the injection-moulded structure of a rubber-toughened polyoxymethylene (POM), and its mechanical properties. The material used was a commercially available POM (Dupont ST100) which contained 20% to 30% polyurethane rubber within a 50% to 60% crystalline matrix. These percentages were invarient through the thickness. A strong sensitivity towards the development of a core-skin morphology was discovered; microscopy and microhardness techniques revealed the skin depth to be 1200 μm. The skin layer was found to consist of individual sheets, 2 to 4 μm thick, that were stacked parallel to the plaque face. By contrast, the core contained spherulites, of 100 to 300 μm diameter, that surrounded oriented discrete 2 to 4 μm thick rubber rods. Morphological differences between the core and skin were reflected in their respective mechanical properties. Tensile response in the skin was ductile, with elongations reaching 300%, while the core exhibited more brittle behaviour (only 25% elongation). In both regions the yield strength was 45 MPa, a value expectedly reduced from the homopolymer (69 MPa) due to the presence of the rubber phase. Fatigue crack propagation response in the skin of the blend was found to be superior to that of the neat resin; however, the core behaviour was a function of orientation. A combination of inferior FCP response and the noticeable presence of a preferred plane of fracture, highlighted the significant weakness of the core material when loaded in a direction transverse to the injection moulding direction.