Improvement of fracture toughness and glass transition temperature of DGEBA-based epoxy systems using toughening and crosslinking modifiers

Several toughening and crosslinking modifiers were tested in two epoxy resin systems based on the diglycidyl ether of bisphenol A (DGEBA) with the objective to improve the critical stress intensity factor KIC and the glass transition temperature (Tg) simultaneously. An amine hardener (isophorone diamine (IPD)) and a homopolymerization initiator (1-ethyl-3‐methylimidazolium acetate (EMIM Ac)) were used as curing agents. The highest effect on the KIC value of the resin system DGEBA/IPD (KIC = 0.72 MPa1/2; Tg = 164°C) was achieved with the dendric polymer Boltorn P501 (10 wt%), but it decreased the Tg (KIC = 1.39 MPa1/2; Tg = 136°C). A high toughening effect with a low decrease of Tg was achieved with a combination of a self‐organized block copolymer (Nanostrength M22N) and silica nanoparticles (Nanopox F400) (KIC =1.15 MPa1/2; Tg =157°C). The KIC value of the resin system DGEBA/EMIM Ac was improved from 0.44 to 0.66 MPa1/2. An improvement of both, the thermal and mechanical properties was established for a combination of a poly(tetrahydrofuran) as toughening modifier (PolyTHF2000) with the post‐crosslinking modifier diethylphosphite (DEP) in the resin system DGEBA/IPD (KIC = 0.86 MPa1/2; Tg = 180°C). A system with chemical linkages between both modifiers was investigated for comparison but yielded inferior results.