Flame Retardant Properties of Phenolic Thermoset Amended with Siloxane and Montmorillonite Clay
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Significant growth of phenolic resin systems has occurred within the last decade due to their importance in aircraft interiors and some segments of mass transportation. While phenolic matrix composites are inherently fire retardant, better properties are required in some applications (such as submarines), where safety tolerances can be very demanding. The direction of this research was to determine if pendant dimethylsiloxanes or montmorillonite clay (nano-layered aluminosilicates) would enhance the fire retardant properties of a phenolic system crosslinked with 1,3-bisoxazoline. Additionally, we sought to determine whether their effect on the modulus and other mechanical properties was complementary.
The first investigation dealt with the covalent addition of an epoxy-siloxane pendant to a novolak phenolic backbone, followed by crosslinking. Some recent patents have suggested that siloxanes are effective fire retardants for phenolic systems, although whether these commercial siloxanes are covalently attached, or part of a blend, is unclear. We attempted to produce only covalently attached siloxanes, with IR and NMR evidence for the reaction, and a stoichiometric excess of phenolic -OH to assure relatively complete reaction. Varying siloxane composition in these plaques did not improve the peak heat release rate (PHRR) within the ±10% standard deviation observed, nor was the mass loss affected. Values were found to be about 180 kW/m2 and 66%, respectively (at 75 kW/m2 heat flux test conditions.) The flexural properties generally decreased; for instance, the flexural modulus dropped from 734 Kpsi for the neat phenolic to 500 Kpsi for the 8% siloxane-modified phenolic. Lower modulus was to be expected given the flexibility of the incorporated siloxane bonds.
The second investigation dealt with incorporation of nano-sized clay platelets into the phenolic using two approaches, solvent and extrusion, each followed by crosslinking with 1,3-bisoxazoline. Only one formulation of quaternary ammonium ion-exchanged clay produced an homogenous blend. This was co-precipitated with the phenolic in a toluene-ethanol solvent, evaporated and subsequently crosslinked. The solvent-processed blends showed a 5% increase in the PHRR, while the glass transition temperature (T g) dropped from 264°C to 154°C. Both results are explained by occluded solvent in the crosslinked plaques, despite extended processing times designed to remove the solvent before crosslinking.
Extruded blends showed some reproducible improvement in the PHRR, but not significant within the limitations of a standard deviation of ± 10%. The flexural properties were not improved for the clay-modified phenolics. However, X-ray diffraction and transmission electron microscopy images showed evidence of intercalation of the polymer into clay galleries, which is an important beginning in understanding eventually how to exfoliate the clay into these systems.
The production of materials with both clay and siloxane was also attempted. The siloxane modification seemed to change the rate at which crosslinking can occur in the presence of clay, and plaques could not be poured before the material set up, preventing either mechanical or mass loss calorimetry testing.
CitationDekar, A. (2000). Flame retardant properties of phenolic thermoset amended with siloxane and montmorillonite clay (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.
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