Polymethyl Methacrylate Slabs Research Articles

An experimental study of droplet extinction in the absence of external convection

The flammability of combustible materials in a spacecraft is important for fire safety applications because the conditions in spacecraft environments differ from those on earth. Experimental testing in space is difficult and expensive. However, reducing buoyancy by decreasing ambient pressure is a possible approach to simulate on-earth the burning behavior inside spacecraft environments. The objective of this work is to determine that possibility by studying the effect of pressure on concurrent flame spread, and by comparison with microgravity data, observe up to what point low-pressure can be used to replicate flame spread characteristics observed in microgravity. Specifically, this work studies the effect of pressure and microgravity on upward/concurrent flame spread over 10 mm thick polymethyl methacrylate (PMMA) slabs. Experiments in normal gravity were conducted over pressures ranging between 100 and 40 kPa and a forced flow velocity of 200 mm/s. Microgravity experiments were conducted during NASA's Spacecraft Fire Experiment (Saffire II), on board the Cygnus spacecraft at 100 kPa with an air flow velocity of 200 mm/s. Results show that reductions of pressure slow down the flame spread over the PMMA surface approaching that in microgravity. The data is correlated in terms of a non-dimensional mixed convection analysis that describes the convective heat transferred from the flame to the solid, and the primary mechanism controlling the spread of the flame. The extrapolation of the correlation to low pressures predicts well the flame spread rate obtained in microgravity in the Saffire II experiments. Similar results were obtained by the authors with similar experiments with a thin composite cotton/fiberglass fabric (published elsewhere). Both results suggest that reduced pressure can be used to approximately replicate flame behavior of untested gravity conditions for the burning of thick and thin solids. This work could provide guidance for potential ground-based testing for fire safety design in spacecraft and space habitats.

On the role of oxygen in the horizontal flame spread of polypropylene

AbstractThe horizontal flame‐spreading behavior of slabs of uninhibited polypropylene (PP) and commercial polymethylmethacrylate (PMMA) is described, and the flame spread and mass vaporization rates are discussed. The quantities of peroxide and carbonyl moieties formed on the surface of PP beneath the flame front, determined by an iodometric procedure and by reaction with 2,4‐dinitrophenylhydrazine, are presented. A statistical analysis was performed to ascertain the significance of these quantities formed in four combustion environments, each differing in oxygen concentration. Flame and surface temperatures of the PP samples at each oxygen concentration as well as flame and surface temperatures of samples burned in 23% oxygen with an added external radiant heat source are presented. The data support the concept that surface oxidative reactions occur in the combustion of PP, that the quantity of oxidative products is related to the concentration of oxygen in the combustion environment, and the possible importance of these reactions in producing heat to propagate the flame is indicated. Comparison of PP and PMMA show that the two are quite dissimilar in flame‐spreading behavior.

On the temperatures of regressing PMMA surfaces

By experimentally establishing bounds on the surgface temperature of polymethylmethacrylate slabs burnings steadily in oxygen-nitrogen mixtures, it is demonstrated that at least for regression rates up to 2 × 10 5 cm/sec, the dependence of regression rate on surface temperature agrees with a theoretical deduction that employs kinetic data for bulk degredation in an asymptotic analyste for large activation energy. If therefore appears that the chemical mechanisms for condensed-phase depolymerization are idntical in bulk degredation and in oxidative flow.

Laminar combustion of polymethylmethacrylate in O 2/N 2 mixtures

The results of experiments on the combustion of polymethylmethacrylate (PMMA, Plexiglas) slabs in laminar flows of O2/N2 mixtures are reported. The Reynolds Number, based on streamwise distance from the leading edge of the slab, ranged from 1.5×103 to 6×104, the pressure from atmospheric to 1/15 atm, and the mole fraction of oxygen from 0.2 to unity. Measurements were made of surface regression rates, of flame standoff distances, of temperatures of the regressing surface, and of conditions for flame extinction. Regression rates agree reasonably well with theoretical results obtained from the flame-surface approximation for the boundary layer on a semi-infinite flat plate. Standoff distances agree with the same type of model, if account is taken of the axial pressure gradient in the test section under hot-flow conditions, and of the reduction in flame temperature produced by dissociation and by radiative heat loss from the flame. The observed relationship between regression rate and surface temperature differs from that obtained earlier in hot-plate linear-pyrolysis experiments, and agrees with theoretical results that employ measured bulk degradation kinetics to calculate surface pyrolysis rates. Since these earlier calculations, based on the assumption of end initiation and unzipping, were believed to be accurate only for a degree of polymerization (DP) less than 2×103, and since the DP of our PMMA samples was 1 to 6×104, it appears that the earlier model may remain a useful approximation for a higher DP than expected. Observed extinction conditions are discussed in the light of some recent developments in extinction theory. Under the assumption that the gas-phase combustion process which occurs in the vicinity of extinction has an over-all order of 2, an extinction criterion is defined in terms of a critical value for the ratio of the square of the test-section pressure to the free-stream oxidizer mass flux. This critical value is influenced by the mole fraction of oxygen primarily through its dependence on flame temperature. The experimental results are consistent with an activation energy of roughly 20 kcal/mole, and a preexponential factor of the order of 1014 cm3/mole sec for the over-all combustion of gaseous PMMA degradation products with O2 in a diffusion flame near the extinction limir.