Vazo catalyst

Table II. Half Life of Vazo Catalyst Temperature °C...

The time to tQ is the time for the wood-monomer mass to reach oven or curing temperature at T5. During the period of constant temperature, the induction period, the inhibitor is being removed by reaction with the free radicals. Once the inhibitor is eliminated from the monomer and wood, the temperature rises to a maximum which corresponds to the peak of the exothermic polymerization reaction. Polymerization continues to completion although at a decreased rate and the temperature returns to that of the curing chamber. The time to the peak temperature depends upon the amount of catalyst present, the type of monomer, the type of crosslinker, and the ratio of the mass of monomer to that of the wood. The wood mass acts as a heat sink. Figure 4 illustrates the effect of increased Vazo catalyst on the decrease in time to the peak temperature, and the increase in the peak temperature(10)... 

Figure 4. Temperature-time curves. Effects of varying concentrations of Vazo catalyst on the polymerization exotherm of basswood-MMA...

The gel or Trommsdorff effect (11) is the striking autoacceleration of the vinyl polymerization reaction as the viscosity of the monomer-polymer solution increases. Chain termination involving the recombination of two free radicals becomes diffusion controlled and this results in a decrease in the rate of termination. The concentration of active free radicals therefore increases proportionally. To sum up the gel effect the rate of Vazo catalyst initiation increases with temperature the rate of propagation or polymerization increases with the viscosity and the rate of termination of the growing polymer chains decreases with the viscosity. This of course also results in an increase in the molecular weight of linear polymers, but this has no practical significance when crosslinking is part of the reaction. 

The rapid decomposition of vazo catalyst with an increase in temperature (Table I) can be used to advantage in the bulk vinyl polymerizations in wood. A moderate temperature of 60 °C can be... 

Table II. Effect of Vazo Catalyst on Polymerization of Basswood-MM A Composite...

Figure 5. Time to exothermic peak (tp) vs. percent vazo catalyst added to basswood-MMA composite. Exothermic peak temperature and percent monomer conversion are shown on the curve.(Reproduced with permiS sion from Ref. 31. Copyright 1972, Springer-Verlag.)...

Data are presented in Table III for the polymerization of basswood-MM A-trimethylol propane trimethacrylate (TMPTMA) impregnated samples containing 0.25% vazo catalyst. The melting point of the three-dimensional polymer is raised by adding TMPTMA to... 

Sample Monomer Vazo Catalyst (%) Moisture Content (%) Polymer Loading (%) (mm) T rc )... 

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