Silicone-based additives have been used as fire retardants for thermoplastics and present the advantages of improving the processing and impact resistance of the polymers. In this study, we used three different siliconebased additives as modifiers of a vinyl ester resin. The additives were fine powders made up of about 50 wt % polydimethylsiloxane and 50 wt % silica. The differences among them were the functional groups inserted in the polymer chains and the size and size distribution of the particles. The additives were dispersed in resin containing 35 wt % styrene. To cure the mixture, a conventional catalyst and initiator were used, and the reaction was carried out in three ways, which differed in the curing temperature, postcuring temperature, time, and addition of dimethylaniline (DMA) as a promoter of the polyaddition reaction. Dynamic mechanical analysis showed that the phase behavior of the resulting composites depended strongly on the curing conditions. The flexural modulus of composites containing 5 wt % additive was lower than that for the cured resin. The impact resistance of the composites also depended on the curing conditions but not on the composition or size of the particle of the additive. The fracture morphologies of specimens subjected to impact resistance tests were different for samples cured in the presence or in the absence of DMA, which suggested that it influenced the mechanism of network formation.
Silicone-based additives have been used as fire retardants for thermoplastics, presenting the advantages of improving processing and impact resistance of the polymers. In this work we used three different silicone-based additives as modifiers of a thermoset based on a vinyl ester resin. The additives are fine powders made up of about 50 wt % ultra high molecular weight polydimethylsiloxane and 50 wt % silica. The differences between them are the functional groups inserted on the additives and the size and size distribution of the particles. The additives were dispersed in resin containing 35 wt % of styrene. For curing the mixture a conventional catalyst and initiator were used and the reaction was carried out in two ways, differing in the curing temperature, the post curing temperature, and the time, and in the addition of dimethylaniline (DMA) as a promoter of the polyaddition reaction. The samples were characterized by thermogravimetric analyses and swelling experiments. The fire retardances of the samples were evaluated by the determination of the flash-ignition, self-ignition, and pyrolysis temperatures (ASTM D1919 -91a), and of the oxygen index (ASTM D-2863-91). The results obtained showed that the silicone-based additives and the methods used in the preparation of the modified resin influence the flash-ignition, self-ignition, and pyrolysis temperatures, but not the oxygen index. Samples cured by different methods present different network characteristics, which influence their thermal decomposition. The volatile species produced by thermal decomposition may be a combination of inert and active species. The network structure may influence only the inert fraction of the volatiles, not the combustibles. These volatile inert species (smoke-black, water vapor, carbon dioxide, etc.) probably dilute the combustibles in the solid and in the gaseous phase, increasing the flash-ignition temperature of the samples.
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