Effects of cure on the mechanical properties of bosmaleimide resins
A commercially available bismaleimide (Compimide 353[sup]R) was blended with a toughening reactive rubber (Hycar[sup]R 1300 X 13). The rate of crosslinking as a function of rubber content and cure temperature was determined using sol-gel analysis and differential scanning calorimetry. Fourier transform infra-red was used to monitor the reaction of the maleimide group. The final degree of crosslinking was dependent on the cure temperature, for 0- 33pphr reactive rubber and independent for blends of 66-100pphr reactive rubber. The addition of a cure accelerator, diazobicyclo[ 2,2,2] octane (DABCO[sup]R) to one bismaleimide reactive rubber blend was studied. The rate and degree of crosslinking were found to be high with the addition of DABCO[sup]R. Extractable monomers present in under cured samples were studied by high pressure liquid chromatography. No detectable difference was observed in the reaction rate of the aliphatic component compared to that of the aromatic components. The mechanical properties of these materials were measured. Materials with only 33pphr reactive rubber still exhibited brittle characteristics, materials with 66pphr and 100pphr showed good tensile and fracture properties, the 66pphr blend being the preferred in terms of tensile strength, modulus and elongation. Addition of a cure accelerator (0.4-1.0% w/w) to the 66pphr material increased the fracture toughness two fold. The mechanism of DABCO[sup]R as a cure accelerator is proposed. It is suggested that the crosslinking mechanism changes from a free radical to an anionic mechanism. The morphology of cure accelerated blends changed depending on the amount of DABCO[sup]R used. Attempts were made to estimate the plastic zone size of these materials using several different methods, however, none of these methods betrayed conclusive evidence for the presence of a plastic zone. The theoretical plastic zone size and shapes were determined using the Irwin, Dugdale, von Mises and Tresca models.