Modification of stress-strain behaviour in aromatic polybenzoxazines using core shell rubbers

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2,2-Bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)propane (BA-a) is blended with a commercial core shell rubber (CSR), Genioperl P52, based on a siloxane core and an acrylic shell, at a range of loadings (1–32wt.%). Scanning electron microscopy and energy-dispersive X-ray analysis reveals an even distribution with good cohesion between the resin and CSR particles. Measurements carried out by dynamic mechanical analysis and thermogravimetric analysis show modest improvements in glass transition temperature (6°C) and significant enhancement of thermal stability (20%) when CSR (32wt.%) was incorporated. Such improvements are linearly related to CSR content. Moderate reductions in modulus (30%) were observed with the highest (32wt.%) loadings of CSR and were also linearly proportional to CSR content. Thermal analysis demonstrated a small inhibitory effect, with activation energy raised by 4% with the blend containing 32wt.% CSR and 3% in the blend containing 8wt.% CSR. It was found that mechanical stirring of the CSR particles into the molten BA-a monomer was the most practical solution for dispersion and effectively broke down CSR agglomerates in the bulk and produced void free samples upon curing, although some minor defects were apparent with higher loadings of core shell rubber. Four batches of dog bone specimens (containing 0, 8, 16 and 32wt.% CSR) were manufactured and underwent tensile testing. An average increase in extension was observed from 0.82mm for the pristine poly(BA-a), to 1.14mm (32wt.% CSR) was achieved. The introduction of CSR has a deleterious impact on tensile strength (24.67MPa, pristine poly(BA-a) compared with 20.48MPa containing 32wt.% CSR; Young's modulus of 5.4GPa for pristine poly(BA-a) compared with 3.1GPa containing 32wt.% CSR). Following tensile tests, scanning electron microscopy reveals rubber cavitation as the principal toughening mechanism.