Factors influencing the properties of epoxy resins for composite applications

The aim of the work reported here was to determine the influence of an amine curing agent, and postcure cycle on the mechanical and thermal properties of diglycidyl ether of bisphenol A (DGEBA) epoxy resin. The results of this initial study were then used as the basis for selecting material to obtain optimum toughness in epoxy/glass fibre systems. These basic materials were further used to make comparisons with the properties of modified resin systems which contained commercial elastomers. Differential Scanning Calorimetry (DSC), Dynamic Mechanical Thermal Analysis (DMTA), Fourier Transform Infrared Spectroscopy (FTIR), flexural and interlaminar shear tests, Instrumented Falling Weight Impact (IFWI), visual observation, Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) were all used to investigate various properties and the structures which gave rise to them. The properties of cured products were found to be affected by the amounts of curing agent, curing times and temperatures, and the structure of the elastomers. Not surprisingly the maximum thermal and mechanical properties tended to be found in the stoichiometric (standard) mix systems. However, postcuring at higher than room temperature, which was used as the basic curing temperature, led to more conversion. This effect improved the thermal and mechanical properties of both the unmodified and modified resin systems. The maximum flexural strength of 104 MPa of the unreinforced resins was found in the stoichiometric mix ratio after postcure at 150°C for 4 hr. However, the maximum flexural modulus and glass transition temperature (Tg) were found after postcuring at the same temperature for 48 hr. This was believed to be due to increased crosslinking, but unfortunately the longer curing time led to degradation of the resins. In the systems modified with -20 phr of polyetheramine elastomers, the one modified with the lowest molecular weight (2000) was found to have the highest flexural strength (85.8 MPa) and modulus (2.5 GPa). The impact properties of all the composites with modified resin matrices were found to be higher than the unmodified resin matrix composites. The best impact properties were, however, obtained with the elastomer modifier with a molecular weight of 4000. The impact energy at maximum force increased from 11.9 to 16.4 J, and energy at failure increased from 18.7 to 21.6 J. This increase in impact properties was due to the increase in areas of phase separated elastomer particles over similar systems with lower molecular weight modifier.