The use of bimodal blends of vinyl ester monomers to improve resin processing and toughen polymer properties

Vinyl ester (VE) monomers with bimodal molecular weight distributions were prepared by reacting methacrylic acid with blends of monodisperse epoxy resins ranging in molecular weight from 350-7000 g/mol. Monodisperse vinyl ester monomers were prepared from epoxy resins of a single molecular weight. The extent of vinyl ester formation was found to be near complete and side reactions, such as etherification, did not occur to a significant extent. The viscosities of these vinyl ester resins were measured as a function of styrene content. It was found that resin viscosity, eta, increased exponentially and predictably as both the styrene content (S) decreased and as the number average molecular weight (M-n) of the vinyl ester monomers increased: eta similar to exp(M-n)/exp(S). Cure kinetics studies showed that the vinyl ester reactivity ratio decreased to 0.1 from 0.6 for bimodal blends relative to monodisperse resins while the styrene reactivity ratio increased from 0.4 to 0.6. Thus, the microgels in bimodal blends were smaller than in monodisperse resins. Emissions studies proved that decreasing the styrene content reduced the VOC emission rate and total emissions. Higher VE molecular weights decreased the overall emissions due to a reduction in monomer mobility. T-g decreased from 143 to 125 degrees C as M-n of the VE monomers increased from 540 to 920 g/mol; yet, T-g of these bimodal blends were still equal to or greater than that of commercial VE resins (similar to 125 degrees C). The fracture toughness of bimodal blends increased from similar to 100 to similar to 330 J/m(2) as VE M-n increased from 540 to 920 g/mol because of matrix toughening. The fracture properties did not improve as the styrene content increased from 35 to 45 wt% because of corresponding changes in the morphology. Yet, there were numerous low VOC bimodal formulations with fracture properties in excess of the low VOC Dow Derakane 441-400 (110 J/m(2)) and even the industry standard Derakane 411-350 (240 J/m(2)). (c) 2005 Elsevier Ltd. All rights reserved.