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Flame Retardant Mechanism of Silica Gel/Silica



Takashi Kashiwagi, Jeffrey W. Gilman, Kathryn M. Butler, Richard H. Harris Jr., John R. Shields, A Asano


Various types of silica, silica gel, fumed silicas and fused silica were added to polypropylene and polyethylene oxide to determine their flame retardant effectiveness and mechanisms. Polypropylene was chosen as a non-char-forming thermoplastic and polyethylene oxide was chosen as a polar char-forming (slight) thermoplastic. Flammability properties were measured in the cone calorimeter and the mass loss rate was measured in our radiative gasification device in nitrogen to exclude any gas phase oxidation reactions. The addition off low density, large surface area silicas, such as fumed silicas and silica gel to polypropylene and polyethylene oxide significantly reduced the beat release rate and mass loss rate. However, the addition of fused silica did not reduce the flammability properties as much as other silicas. The mechanism of reduction in flammability properties is based on the physical processes in the condensed phase instead of chemical reactions. The balance between the density and the surface area of the additive and polymer melt viscosity determines whether the additive accumulates near the sample surface or sinks through the polymer melt layer. Fumed silicas and silica gel used in this study accumulated near the surface to act as a thermal insulation layer and also to reduce the polymer concentration near the surface. However, fused silica used in this study mainly sank through the polymer melt layer and did not accumulate near the surface. The heat release and the mass loss rate of polypropylene decreased nearly proportionally with an increase in mass loading level of silicca gel up to 20% mass fraction. Polyethylene oxide samples with fumed silicas and silica gel formed physically strong char/silica surface layers. This layer acted not only as thermal insulation to protect virgin polymer but also acted as a barrier against the migration of the thermal degradation products to the surface.
Fire and Materials
No. 6


silica, polypropylene, flame retardants, polyethylene oxide


Kashiwagi, T. , Gilman, J. , Butler, K. , Harris, R. , Shields, J. and Asano, A. (2000), Flame Retardant Mechanism of Silica Gel/Silica, Fire and Materials, [online], (Accessed April 12, 2024)
Created November 1, 2000, Updated February 19, 2017