ABSTRACT:The effects of oxirane groups in vinyl ester (VE) resin and reactive diluent on curing characteristics and thermal behavior of cured resins are described. Stoichiometric (0.5:1, sample A) as well as nonstoichiometric (0.5:0.85, sample B) ratios of the diglycidyl ether of bisphenol-A (DGEBA) and methacrylic acid (MA) were used for the synthesis of VE resins. Resin sample B had more residual epoxy groups because of the stoichiometric imbalance of the 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 the overall ratio of resin and reactive diluent constant. Increase of GMA content in resin A or B resulted in a decrease in gel time, indicating that the curing reaction is facilitated by the presence of epoxy groups. An increase in initiator content also reduced the gel time. In the differential scanning calorimetry (DSC) scans, a sharp curing exotherm was observed in the temperature range 107 Ϯ 3-150 Ϯ 1°C. The onset temperature (T onset ) and peak exotherm temperature (T exo ) decreased with increase in GMA content. Heat of curing (⌬H) also increased with increase in GMA content. A broad exotherm was observed after the initial sharp exotherm that was attributed to the etherification reaction. Cured VE resins were stable up to 250 -260°C, and started losing weight above this temperature. Rapid decomposition was observed in the temperature range 400 -500°C.
The paper describes the effect of combination of char forming additives and char forming catalysts on limiting oxygen index (LOI), smoke density and mechanical properties of neat as well as glass fibre reinforced vinyl ester (VE) resins. Three different samples of VE resins were prepared by reacting diglycidyl ether of bisphenol-A and methacrylic acid in stoichiometric (1 : 2, sample A) and non-stoichiometric (1 : 1.7, sample B and 1 : 1, sample C) amounts. These resins were diluted with reactive diluents (50% w/w) styrene (S)/glycidyl methacrylate (GMA) or a combination of GMA with S for fabrication of neat sheets as well as composites. Incorporation of flame retardant additives (combination of ammonium polyphosphate, azobiscarbonamide, tris (2-hydroxyethyl) isocyanurate and hydrated alumina) in the laminates increased the LOI and decreased the smoke density values. The highest LOI was observed in laminates based on resin C containing flame retardant additives and a phosphorus containing amine as a hardener.
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