A comparative study of accelerated weathering of epoxy resins based on DGEBA and HDGEBA

The durability of epoxy resins is an important factor in applications of composites to timber engineering. We carried out a comparative study of the effect of accelerated weathering on epoxy resins based on the glycidyl ether of bisphenol A (DGEBA) or hydrogenated diglycidyl ether of bisphenol A (HDGEBA) cured with 2,2,4-trimethylene-1,6-hexadiamine, using various characterization tools, including mechanical and thermal testing and SEM to evaluate the effect of exposure to UV light, moisture and elevated temperature cycles on the properties of the resins. Mechanical testing showed that after 6 months accelerated weathering reduced by 50% the tensile strength of DGEBA-TMDA while HDGEBA-TMDA epoxies lost slightly less strength over the same exposure period. Elongation at break increased after 6 months of accelerated weathering from 5.1 +/- 0.1% to 7.5 +/- 0.3% for DGEBA-TMDA epoxy specimens, and from 0.9 +/- 0.3% to 2.0 +/- 0.1% for HDGEBA-TMDA epoxy specimens. FTIR of the epoxies showed increases in the absorbance of hydroxyl and carbonyl groups for both epoxies on accelerated weathering, but the hydroxyl and carbonyl indexes reached values 90 and 40% higher respectively for DGEBA than HDGEBA-based epoxy resins. While susceptibility to accelerated weathering on the molecular scale was clearly greater for the DGEBA-based epoxy on the molecular scale, this did not translate into a significant deterioration in mechanical properties relative to HDGEBA over the time scale of the accelerated weathering experiments.