Toughening of epoxy resins with a liquid-crystalline epoxy resin

An epoxy resin based on diglycidyl ether of bisphenol A(epoxy resin E-51)was toughened with a liquid-crystalline epoxy resin, 4-expoxypropoxy phenyl-4-epoxypropoxy benzoate (PHBHQ) which presented a Schlieren texture in the temperature range of 131 similar to 94degreesC during cooling. A mixed aromatic diamine, 4, 4'-diaminodiphenylmethane(DDM) :4, 4'-diaminodiphenylsulfone (DDS) : 4,4'-diaminodiphenylester(DDE) = 5 : 2 : 1 weight ratio, was used as the curing agent, and the curing conditions were adopted as 100degreesC/l h, 120degreesC/2 h, 150degreesC/2 h and 180degreesC/2 h. The influence of PHBHQ on the properties of final resin systems was investigated. Compared with other toughing agents such as rubbers and thermoplastic resins, PHBHQ can improve the toughness of epoxy resin E-51 greatly without affecting the modulus and thermal resistance. The impact strength of epoxy resin E-51 modified with 50 wt% PHBHQ reached 40.2 kJ/m(2) which was higher than that of pure epoxy resin E-51 system with 23.4 kJ/m(2). The thermal property of toughing epoxy resin systems was tested with dynamic mechanical thermal analysis (DMTA) The results showed that the glass transition temperature increased about 30degreesC with only 9 wt% PHBHQ. Wide-angle X-ray Diffraction (WAXD) was used to characterize the crystalline structure. Two diffraction angles were observed at the 9degrees and 23degrees, which showed that the nematic molecular arrangement has been kept during the crosslinking of the toughing system when the reaction was carried out in the thermal stability range of liquid-crystalline phase. A two-phase structure was observed on the fracture surface by scanning electronic microscopy (SEM) The oriented mesogenic domain which was dispersed in the continuous epoxy matrix can initiate, branch and terminate the cracks and endow toughening epoxy networks with excellent overall properties.