Novel Novolac-Phthalonitrile and Siloxane-Phthalonitrile Resins cured with low melting Novolac Oligomers for Flame Retardant Structural Thermosets
Hardrict, Shauntrece Nicole
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Novel Novolac-Phthalonitrile and Siloxane-Phthalonitrile Resins cured with low melting Novolac Oligomers for Flame Retardant Structural Thermosets Shauntrece Nicole Hardrict Department of Chemistry Virginia Polytechnic Institute and State University ABSTRACT The chemical modification of low molecular weight novolac oligomers and siloxane/silane-containing monomers has led to novel phthalonitrile derivatives with low glass transition temperatures, ranging from -25 to 75 ÂºC. Multi-functional, low molecular weight phenol-formaldehyde novolac resins were blended with these novel phthalonitrile derivatives to achieve low viscosity resin blends. Moderate temperatures and rapid curing cycles were employed (200 ÂºC, 1 h and 225 ÂºC, 4h) to produce networks with high glass transition temperatures (> 250 ÂºC). A decrease in the sharp band at 2230 cm-1, attributed to the nitrile functionality of the phthalonitrile resin, was monitored in FTIR studies and indicated the progress of the reactions. Ninety percent conversion was achieved within ~ 30 min. Thermal analysis of siloxane-phthalonitrile/novolac networks cured for 1h at 200 ÂºC and 4h at 225 ÂºC did not exhibit glass transition temperatures below 250 ÂºC. In dynamic TGA studies, 5% weight loss temperatures up to 418 ÂºC were observed, and the materials exhibited 50 to 56 % char at 800 ÂºC in nitrogen. Networks prepared from a resin blend containing 50 weight% of a phthalonitrile derivative of a 260 g mol-1 novolac oligomer, 50 weight% of the 260 g mol-1 novolac oligomer, and 1.5 mol % triphenylphosphine (based on novolac) (NOV/NOV/TPP) cured at 200 ÂºC for 1h, did not exhibit a Tg below 250 ÂºC via DSC. These networks exhibited a 5% weight loss temperature of 350 ÂºC, and 70 % char at 800 ÂºC in TGA studies under nitrogen. This degree of char formation makes these materials appealing for use in carbon-carbon composites. Post-curing these networks at 200 ÂºC for 1h, and then at 225 ÂºC for 4h, resulted in high thermo-oxidative stability, with a 5% weight loss observed at 447 ÂºC and 50 % char at 800 ÂºC. Blending tetramethyldisiloxane phthalonitrile monomers with 260 g mol-1 novolac oligomers afforded prepolymer resins with low melt viscosities, 560 mPa s at 80 ÂºC. Such viscosities may allow these resins to be processed via vacuum assisted resin transfer molding (VARTM) at low temperatures and heated at elevated temperatures to produce flame resistant three-dimensional networks.
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