Properties of siloxane-modified epoxy resins

A diglycidyl ether of bisphenol A (DGEBA) epoxy resin was chemically modified using a silanol-terminated polymethylphenylsiloxane (PMPS) and two methoxy-terminated PMPS intermediates with high (3.3/1) and low (0.5/1) phenyl/methyl ratios. At room temperature, the methoxy-terminated PMPS with the high phenyl/methyl ratio was miscible with the epoxy resin and formed a one-phase system; however, both the silanol-terminated PMPS and the methoxy-terminated PMPS with low phenyl/methyl ratio were incompatible with the epoxy resin and phase separation occurred. While, at 120°C, resins modified with all three PMPS intermediates formed homogeneous solutions. A range of the tensile and the fracture properties of unmodified and modified epoxy systems was obtained by changing the modification method, the concentration and the type of PMPS modifiers, the type of curing agent, and the cure cycle. The modified epoxy systems have slightly decreased tensile moduli. The chemical modification method is more efficient in the improvement of the fracture toughness than the physical blending method. The fracture toughness increases with an increase in PMPS concentration. The silanol-terminated PMPS has a better toughening effect than the two methoxyl-terminated PMPS modifiers. The polyoxypropylene diamine (POPDA) cured epoxy systems have much higher fracture toughness values than the 1,3-phenylenediamine (MPDA) cured epoxy systems.