Thermal and mechanical properties of epoxidized natural rubber modified epoxy matrices

Two sets (A and B) of bisphenol A-diglycidyl ether (DGEBA) based epoxy resi n formulations were modified with epoxidized natural rubber (ENR 50) and it s liquid version (LENR 50), and cured with amino propoxylate initiator/acce lerator at ambient temperatures. The ENR 50 loading range was 1.6-5.9wt%. B oth sets could be loaded up to 12wt% with LENR 50. Significant improvements in tensile toughness and impact toughness could only be observed for set A formulations. At the maximum LENR 50 loading achieved, the improvement in tensile toughness is 250% in comparison with that of the neat formulation; that for impact toughness is 125%. Differential scanning calorimetry reveal s multiple transitions, characteristic of these systems. Scanning electron micrographs of fractured surfaces show uniform rubber dispersions in the su bmicrometre size range. At the loading levels used, LENR 50 particle disper sions fall within the range of 0.33-0.47 mu m in size; those of ENR 50 are 0.48-0.67 mu m in average size. Improvements in toughness are similar for b oth versions of epoxidized natural rubber. For both LENR 50 and ENR 50 modi fied epoxy systems, the extremes of 0.80 (set A) and 1.95 (set B) in glycid yl ether/reactive hydrogen molar ratios considered show distinct failure me chanisms, that of ductile failure with yielding in the former and brittle f ailure in the latter, irrespective of reactive diluent content. (C) 1999 So ciety of Chemical Industry.