Carboxyl-Terminated Butadiene-Acrylonitrile-Modified Epoxy Resin and Its Graphite Fiber-Reinforced Composite

Abstract

Measurements of dynamic mechanical properties, optical and scanning electron microscopy and small-angle X-ray scattering were carried out to characterize the state of cure, possible phase separation and morphology of both HX-205 and F-185 neat resins and their graphite fiber reinforced composites. HX-205 is a diglycidyl ether bisphenol A (DGEBA) based epoxy resin and F-185 is a rubber-modified epoxy resin containing 86.5 weight % HX-205, 8.1 weight % Hycar 1300 x 9 (a liquid carboxyl-terminated polybutadiene-acrylonitrile, CTBNX) and 5.4 weight % Hycar 1472 (a solid copolymer of butadiene-acrylonitrile having acrylic acid pendant group). The neat resins and the composites were prepared using identical curing cycles. The neat resins as well as the matrix materials in the composites appear to have the same state of cure as characterized by dynamic mechanical properties. The F-185 resin contains CTBN-rich domains with sizes ranging from 50 Å (and possibly smaller) to 20 µm and larger. The F-185 neat resin and the F-185 matrix in the composite both display ductile fracture behavior compared to a brittle fracture of HX-205 neat resin and its composite, indicating a toughening effect of the CTBN inclusions. The morphology of the CTBN domains in the F-185 matrix appear to differ from that in the F-185 neat resin. There are a greater fraction of smaller CTBN domains in the F-185 matrix than in the F-185 neat resin. Because CTBN domains in the size range of the order of several hundred angstroms are less effective in improving fracture toughness (6,8), the fact that there are a greater fraction of smaller CTBN particles in the composite matrix may partially explain the reported observations that some of the composites made with the CTBN-modified DGEBA epoxy resin did not show significant improvement in fracture toughness. This study indicates that, when using multiphase resins to make composites, the neat resin and the matrix of the composite may not have similar morphology even when prepared under the same curing program.