Charring Polymers Research Articles

Combustion of polyvinyl chloride studied by a low-pressure moving-wire technique

The low-pressure moving-wire technique (MWT) provides a means of achieving steady-state polymer combustion. For charring polymers, the steady state is not controlled by the rate of char combustion, as is the case for the opposed-flow diffusion flame technique. At low pressure, the polymer flame is expanded to a size that is convenient for microprobe sampling. It is then possible to identify the flammable gases evolving under actual combustion conditions, thereby avoiding the common extrapolation from nonflame pyrolysis studies. As a demonstration of the technique, we made measurements of the structure of a polyvinyl chloride (PVC) flame at reduced pressure (0.10 bar). The material evolving from the burning polymer was estimated to be ≅0.4% benzene and ≅14% pentene isomers (and/or butenal). Butene isomers (and/or propenal) and lower-molecular-weight species accounted for the remainder. The total mass flux from the burning surface was 0.17 kg m −2 sec −1. The surface temperature at ignition was ≅660°K.

Diffusion controlled combustion of polymers

A theoretical and experimental study of polymer combustion in an opposed flow diffusion flame (OFDF) is presented. An algebraic formula is derived, expressing the burning rate as a function of the fluid mechanic and thermodynamic variables. A polymer sample feed system has been developed which continuously positions the burning polymer surface within ±0.01 mm of a given set point, allowing accurate regression rate and detailed solid and gas phase flame structure measurements. Regression rate measurements of twelve commercial polymers as a function of oxygen concentration and oxidizer flowrate are reported. From these measurements and the theory, values of the Spalding transfer number, B , are derived and can serve as a useful flammability index for these materials. The OFDF technique also provides a quantitative method for evaluating the effectiveness of flame retardants. Solid and gas phase temperature profiles for charring and non-charring polymers under various oxygen concentrations and oxidizer flow conditions indicate markedly different chemical reaction mechanisms for charring and non-charring polymers.

Thermogravimetry of thermally stable aromatic and heterocyclic polymers

Thermally stable polymers were studied using thermogravimetry (TG) to 1400°C in helium. The heterocyclics were a polybenzimidazole and a polyimide. The aromatics consisted of two phenol formaldehydes, a phenylphenol phenol formal dehyde, and two polyphenylenes. Computer code analysis provided rate parameters for an Arrhenius type correlation. The rate parameters represented well the TG run. Overall pyrolysis mechanisms were examined in terms of rate parameters and computer code curves, which normalized out the polymer char yield and reinforcing fiber content. Standardization was emphasized to provide quality TG data and accurate parameters relatively free from procedural variables of polymer synthesis, degree of cure, sample preparation, thermobalance operation, data analysis, and pyrolysis beyond 1000°C.