Modification and compatibility of epoxy resin with hydroxyl‐terminated or amine‐terminated polyurethanes

Abstract Phenolic hydroxyl‐terminated (HTPU) and aromatic amine‐terminated (ATPU) PU modifiers were prepared by reacting two different macroglycols (PTMG, polytetramethylene glycol, M n = 2000, and PBA, Polybutylene adpate, M n = 2000) with 4,4′‐diphenylmethane diisocyanate (MDI), then further coupling with two different coupling agents, bisphenol A or 4,4′‐diaminodiphenyl sulfone (DDS). These four types of PU prepolymers were used to modify the epoxy resin with 4,4′‐diamino‐diphenyl sulfone as a curing agent. From the experimental results, it was shown that the values of fracture energy, G IC , for PU‐modified epoxy were dependent on the macroglycols and the coupling agents. Scanning electron microscopy (SEM) revealed that the ether type (PTMG) of PU‐modified epoxy showed the presence of an aggregated separated phase, which varied between 0.5 μm and 4 μm in the ATPU (PTMG) and between 1 μm and 1.5 μm in HTPU (PTMG) modified system. On the contrary, the ester type (PBA) PU‐modified epoxy resin showed a homogeneous morphology and consequently a much smaller effect on toughening for its good compatibility with the epoxy network. In addition, it was found that the hydroxyl‐terminated bisphenol A as a coupling agent improved fracture toughness more than the amine‐terminated DDS because of effective molecular weight buildup by a chain extension reaction. The glass transition temperature ( T g ) of modified epoxy resin as measured by dynamic mechanical analysis (DMA) was lower in PTMG‐based PU than in a PBA‐based PU series with the same weight of modifier.