Effect of oxirane groups on curing behavior and thermal stability of vinylester resins

The effects of oxirane groups in vinyl ester (VE) resin and reactive diluen t on curing characteristics and thermal behavior of cured resins are descri bed. Stoichiometric (0.5:1, sample A) as well as nonstoichiometric (0.5:0.8 5, sample B) ratios of the diglycidyl ether of bisphenol-A (DGEBA) and meth acrylic acid (MA) were used for the synthesis of VE resins. Resin sample B had more residual epoxy groups because of the stoichiometric imbalance of t he reactants. VE resins thus obtained were diluted with methyl methacrylate (MMA; 1:1, w/w), and controlled quantities of epoxy groups were introduced by partial replacement of MMA with glycidyl methacrylate (GMA), keeping th e overall ratio of resin and reactive diluent constant. Increase of GMA con tent in resin A or B resulted in a decrease in gel time, indicating that th e curing reaction is facilitated by the presence of epoxy groups. An increa se in initiator content also reduced the gel time. In the differential scan ning calorimetry (DSC) scans, a sharp curing exotherm was observed in the t emperature range 107 +/- 3-150 +/- 1 degreesC. The onset temperature (T-ons et) and peak exotherm temperature (T-exo) decreased with increase in GMA co ntent. Heat of curing (DeltaH)also increased with increase in GMA content. A broad exotherm was observed after the initial sharp exotherm that was att ributed to the etherification reaction. Cured VE resins were stable up to 2 50-260 degreesC, and started losing weight above this temperature. Rapid de composition was observed in the temperature range 400-500 degreesC. (C) 200 1 John Wiley & Sons, Inc.