A mass loss model for char forming polymers is developed from mechanistic pyrolysis kinetics and solved analytically. Under conditions of flaming combustion the coupled rate equations for thermal degradation products and reactants reduce to a single rate law for the residual mass. Exact results are obtained for the mass loss history which include an equilibrium char yield whose value depends only on the relative rates of gas and char formation at a particular temperature. Reaction rate constants for thermolysis of chemical bonds, gas production, and char formation are determinable from parametric fits of the mechanistic charring model to thermogravimetric data. Predictions of the nonisothermal mass loss during constant heating rate experiments are in agreement with experimental data over the expected range of validity.
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