Timothy F. Scott

Photo-DSC cure kinetics of vinyl ester resins. I. Influence of temperature

Abstract Isothermal DSC was employed to monitor the photopolymerization kinetics of a commercial vinyl ester resin (VER) photoinitiated by the camphorquinone/amine photoinitiator system. The maximum rate of photopolymerization was found to increase tenfold as the isothermal cure temperature was varied from −30 to 90xa0°C due to faster propagation and greater initiator efficiency counterbalanced by a faster termination rate. After photopolymerization, the samples were temperature ramped in the DSC to monitor the dark polymerization reaction. The reduced polymerization rate increased with the scan temperature due to an increase in propagation rate. The summation of the isothermal and residual polymerization heats indicated that the final degree of cure after postcure was independent of the isothermal cure temperature. The onset temperatures for the recommencement of cure during the dark temperature scanning experiments were found to correspond closely with their respective isothermal cure temperatures as expected due to vitrification during the isothermal cure stage.

Kinetics and network structure of thermally cured vinyl ester resins

Abstract Bisphenol-A diglycidyl ether dimethacrylate was blended with styrene at varying concentrations and this model vinyl ester resin (VER) was compared with two commercial VERs. The VERs were characterized using gravimetry, FTIR spectroscopy, NMR spectroscopy, differential scanning calorimetry (DSC) and DMTA. NMR spectroscopy differentiated between a novolac epoxy-based multimethacrylate oligomer and the two bisphenol-A epoxy-based dimethacrylate oligomers. Reaction kinetics were studied using scanning and isothermal DSC and isothermal FTIR spectroscopy using benzoyl peroxide as the thermal initiator. The presence of oxygen was found to inhibit significantly the polymerization. Increased initiator concentration raised the rate of isothermal polymerization, but did not affect the final conversion while increased styrene concentration reduced the polymerization rate constant and increased the total conversion. This was interpreted in terms of the variations in the termination rate and the stability of the styryl radical on the cure rate and the effect of vitrification on the extent of cure. From measurements of the dynamic mechanical properties as a function of temperature, the breadth of the glass transition tan δ curve and the magnitude of the rubbery modulus was found to increase while the tan δ maximum decreased with increased crosslink density. The T g , as measured by DSC, and the temperature of the tan δ maximum, as measured by DMTA, were not significantly affected by the styrene content in the resin per se , but were dependent on the combined effects of composition and crosslink density of the network.

Photo-DSC cure kinetics of vinyl ester resins II: influence of diluent concentration

Abstract The photopolymerization kinetics of two commercial vinyl ester resins (VERs) and a model VER photoinitiated by the camphorquinone/amine photoinitiator system were monitored using isothermal DSC. A decrease in styrene concentration in model VERs was found to raise the rate of photopolymerization. In contrast, when the styrene was replaced by a monomethacrylate diluent, the photopolymerization rate passed through a maximum near 70xa0wt% diluent monomer. This difference in the variation of the rate of polymerization with decreased monomer concentration was attributed to the competition of the effects of the higher reactivity of the methacrylyl radical relative to the styryl radical and the lower termination rate for divinyl-rich systems (both of which tend to raise the maximum polymerization rate) and the effects of the reduction in the initiation efficiency and decrease in k p due to increased fraction of pendant double bonds (which lower the polymerization rate) when the concentration of diluent monomer was reduced. Subsequent dark polymerization was observed during a temperature ramp and the onset of polymerization was independent of resin composition due to vitrification effects during the isothermal photocuring stage. The kinetics during the dark polymerization stage was discussed in terms of the radical concentration and the propagation rate constant. Increases in the concentration of either diluent monomer raised the extent of isothermal cure during the isothermal polymerization because vitrification was delayed by the lower crosslink density and the plasticizing effect of the diluent. Higher levels of diluent also raised the maximally attainable conversion due to reduced topological restrictions for reaction in networks of lower crosslink density.