Toughness of SMI-modified ABS alloys and the associated deformation behavior

The mechanical toughness of modified ABS (acrylonitrile-butadiene-styrene) alloys was evaluated using Izod impact, tensile, and compact tension tests. The modified ABS alloys contain 20 wt % of styrene-N-phenylmaleimide (SMI) that is added to enhance the thermal resistance of the ABS. In this study, the effects of matrix composition, rubber/ matrix adhesion, and rubber par ticle structure on the alloy toughness were investigated. Results from the tensile test and Izod impact test ranked the alloys in an order that is dif ferent from that given by K-Ii (stress intensity factor for crack initiatio n), measured from compact tension specimens. This is due to the difference in energy-absorption characteristics for crack initiation and crack growth. The conclusion is supported by optical micrographs on the deformation zone size. The microdeformation behavior of the alloys was examined using trans mission electron microscopy (TEM), which revealed different rubber tougheni ng mechanisms between Izod and tensile specimens. The former contains numer ous extensive crazes, while the latter, only a very few short crazes, excep t in regions within a few micrometers from the fracture surface. The domina nt matrix deformation mechanism for the tensile specimens is believed to be shear deformation. Another interesting observation from the study is rubbe r particle cavitation, commonly observed in tensile specimens and Izod spec imens with solid rubber particles; it did not occur in the Izod specimens c ontaining salami-type rubber particles. This is attributed to the salami st ructure that increased the straining rate for the rubber phase, leading to ductile-brittle transition of the rubber. The transition to brittle deforma tion of the rubber phase prevented rubber particle cavitation. The microsco pic examination indicated that toughening mechanisms by the rubber particle s can be very different among the mechanical tests, which should be taken i nto account for the rubber toughening of polymers. (C) 1999 John Wiley & So ns, Inc.