Type of Document Master's Thesis Author Hardrict, Shauntrece Nicole URN etd-01012004-153124 Title Novel Novolac-Phthalonitrile and Siloxane-Phthalonitrile Resins cured with low melting Novolac Oligomers for Flame Retardant Structural Thermosets Degree Master of Science Department Chemistry Advisory Committee
Advisor Name Title Riffle, Judy S. Committee Chair Esker, Alan R. Committee Member McGrath, James E. Committee Member Keywords
- phenolic networks
- flame retardance
- composite matrix
Date of Defense 2003-12-18 Availability unrestricted AbstractNovel 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
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 ºC. 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 ºC, 1 h and 225 ºC, 4h) to produce networks with high glass transition temperatures (> 250 ºC). 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 ºC and 4h at 225 ºC did not exhibit glass transition temperatures below 250 ºC. In dynamic TGA studies, 5% weight loss temperatures up to 418 ºC were observed, and the materials exhibited 50 to 56 % char at 800 ºC 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 ºC for 1h, did not exhibit a Tg below 250 ºC via DSC. These networks exhibited a 5% weight loss temperature of 350 ºC, and 70 % char at 800 ºC 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 ºC for 1h, and then at 225 ºC for 4h, resulted in high thermo-oxidative stability, with a 5% weight loss observed at 447 ºC and 50 % char at 800 ºC.
Blending tetramethyldisiloxane phthalonitrile monomers with 260 g mol-1 novolac oligomers afforded prepolymer resins with low melt viscosities, 560 mPa s at 80 ºC. 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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