Effect of Modified Liquid Rubber on Increasing Toughness of Epoxy Resins

Epoxy resins are considered as one of the most important classes of thermosetting polymers for many industrial applications, but unfortunately they are characterized by a relatively low toughness. In this respect, many efforts have been made to improve the toughness of cured epoxy resins by the introduction of rigid particles, reactive rubbers, interpenetrating polymer networks and thermoplastics within the matrix. In this work, hydroxyl-terminated polybutadiene (HTPB) as a modifier, firstly, was crosslinked by variable content of divinylbenzene (DVB) in the presence of epoxy resin (ER) and then the modified matrix was cured with 1-methylimidazole as a curing agent. Infrared spectra showed the existence of a chemical reaction between modified HTPB (MHTPB) and the ER. Most of the tensile properties attained a peak at an approximately 20 phr (part per hundred rubber) DVB content, where the toughening reached its maximum. For both notched and un-notched specimens, a two-fold increase in izod impact strength was obtained by the addition of just 20 phr DVB compared to the neat resin. On the addition of DVB, the Izod impact strength varied from 0.54 to 0.71 kJ/m2 for notched specimens and from 1.43 to 6.66 kJ/m2 for un-notched specimens. Whereas, KIC varied from 1.35 to 2.59 MPa.m1/2 with increasing DVB content. By SEM analysis the average diameter was found to be about 5 urn (corresponding to maximum of toughness) for modified rubber particles. The overall results have shown that it is possible to obtain an excellent impact strength and good mechanical properties with the use of MHTPB as a toughening agent for the epoxy resins.