Combustion of Polymer Vapors

TRPcai values calculated from Ty Ti) values and from T values are significantly different. 

Combustion is a process where polymer vapors react chemically with oxygen from air in the reaction zone of the flame, generating heat and products of complete and incomplete combustion. The fire intensity and thermal and nonthermal hazards in fires depend on the release rates of polymer vapors. 

Design/Test Conditions ASTM E906 [37] ASTM E2058 [31] ASTM El 354 [36] ASTM E662 [49]  

Airflow Co-flow Co-flow/natural Natural None 

External heaters Sflicone carbide Tungsten-quartz Electrical coils Electrical coils 

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Being organic substances, polymers provide an excellent fuel source for the propagation of fire. The combustion of polymers occurs via a free-radical mechanism the heat from the fire vaporizes and ionizes the constituents of the gas forming a cloud of free radicals. The spread of combustion occurs via the well-known free radical mechanisms of propagation chain branching and termination. There are a number of ways to interrupt the chain of events involved in fire propagation ... 

For the direct quantification of the A//g values in ASTM E2058 [31], polymer samples (100 mm or 100 mm in diameter and 25 mm thick) arc exposed to external heat flux in a co-flowing air with oxygen concentration reduced to below 10% to maintain nonfiaming combustion condition [21-23]. By measuring the release rate of polymer vapors under the nonflaming steady-state combustion condition, the A7/g value of a polymer is calculated from Eq. (11.1). In the calculations, g" is the applied external heat flux, q is zero, and g" is the external heat flux value where release rate of polymer vapors is zero. Table 11.4 lists A//g and g" values for selected polymers determined in this fashion in the ASTM E2058 apparatus [21-23]. 

The release rates of polymer vapors, heat, and products are measured at various external heat flux values in normal air as well in air with variable oxygen concentration and flow rate in the standard test apparatuses [31,36, 37]. An illustration of the combustion test is shown in Figure 11.6. This test was performed in... 

Between abont 400 and 900 s in Figure 11.1, PP was burning at the steady state as a solid polymer with formation and burning away of very small polymer melt bubbles at the surface with no accumulation of the polymer melt at the surface. The flames were completely lifted off the surface due to the high release rate of polymer vapors resulting in 0. From Eq. (11.8) and properties of PP, the predicted steady-state 2ch value for the combustion of solid PP is 780 kW/m, which agrees well with the experimental value. 

The q l values have been measured in small and large-scale fires [55-57]. The q l values can also be determined indirectly by using the measured steady-state release rate of polymer vapors in the combustion of solid polymers along with the known values of A77g and g" for the polymers in Eq. (11.1) [21, 22, 55-61]. Examples are shown in Figures 11.10 and 11.11. In Figure 11.10, q values for... 

Figure 11.15. Relationship between the generation rate of polymer vapors in combustion in normal air at 50 kW/m and FPl determined for fire propagation in 40% oxygen concentration. Data were measured in our laboratory in the ASTM E2058 Fire Propagation Apparatus [21, 22, 40, 80-86].

Maximum possible mass stoichiometric yield of product j (g/g) Total mass of oxygen available for combustion per unit total mass of polymer vapors burned normalized by the mass stoichiometric oxygen-to-fuel ratio Flame heating parameter (kW /m )... 

Additives functioning in the vapor phase exhibit the highest efficiency because they interfere with the combustion chemistry. Total load levels as low as 3 wt% are sufficient for certain flammability performance requirements. The halogen-containing systems that function in the vapor phase are characterized by higher heat releases than systems operating in the condensed phase. The steps involved in the combustion of polymers as described by Troitzsch (10), shown in Fig. 4.20, are summarized below. 

An analysis of the percentage of phosphorus in the residue after combustion of polymer materials shows that phosphorus can remain in the condensed phase, almost completely or partially, or that it can completely vaporize [76,77]. The latter phenomenon depends not only on the chemical nature and initial concentration of the phosphorus flame retardant but also to a high degree on the nature of polymer substrate and the structure of the material. The relationship between the amount... 

The amount and physical character of the char from rigid urethane foams is found to be affected by the retardant (20—23) (see Foams Urethane polymers). The presence of a phosphoms-containing flame retardant causes a rigid urethane foam to form a more coherent char, possibly serving as a physical barrier to the combustion process. There is evidence that a substantial fraction of the phosphoms may be retained in the char. Chars from phenohc resins (qv) were shown to be much better barriers to pyrolysate vapors and air when ammonium phosphate was present in the original resin (24). This barrier action may at least partly explain the inhibition of glowing combustion of char by phosphoms compounds. 

Many of the hazards from the polymer industry arise from the monomers used as raw materials. Many monomers are reactive and flammable, with a tendency to form explosive vapor mixtures with air. All have a certain degree of toxicity vinyl chloride is a known human carcinogen. The combustion of many polymers may result in the evolution of toxic gases, such as hydrogen cyanide... 

And it is also very important to give the material flame retardancy as well as high breakdown voltage. The mechanism of burning and combustion of silicone elastomers is very different from that of other synthetic polymers [1-4], Normally the synthetic polymers produce inflammable gases and water vapor in burning. Silicone elastomers also produce inflammable gases such as cyclo-siloxanes. However, silicone makes a three-dimensional structure by the oxidation reaction of siloxane side chain Furthermore, after the combustion it makes the ash silica. This makes silicones different from other polymers. 

H(OCH2CH2) Cl. Any of a group of polymers, usually colorless liquids with very low vapor pressure at room temperature. Mw from 100 to 600. Miscible with water, d for a low molecular weight polymer is 1.18 (20C), for a high molecular weight polymer 1.14 (10C). The former sets to a glass at -90C, the latter sets to a wax I ike solid at 20C. Combustible. 

Flame Retardation. Polymer combustion, a highly complex process, is composed of a vapor phase, in which the reactions responsible for the formation and propagation of the flame take place, and a condensed phase, in which fuel for the gas reactions is produced. Flame retardancy, therefore, can be improved by appropriately. pa modifying either one or both of these phases ( ). The approaches aimed at reducing the flammability of polymer systems can be grouped into the following three categories ... 

Improvements in the reduction of flammability of polymers with low clay contents and better processability have been reported, in addition to reductions in the concentration of toxic vapors produced in the combustion stage [116-120]. In connection to their flame-retardant properties, exfoliated nanocomposites based on PP [121, 122, 115, 123], PS [115, 123, 124], poly(ethylene-vinyl acetate) [125, 126], styrene-butadiene rubber [127], PMMA [128], polyesters... 

Direct dryers may use air, inert gas, superheated vapor, or products of combustion as the heating medium. Combustion gases are seldom used in polymer drying because of possible product contamination. Inert gas eliminates the explosion and fire hazard and may be desirable to prevent oxidation of polymers prior to the introduction of stabilizers. Use of snperheated vapor as a heat carrier is highly desirable when solvent is vaporized in the dryer and has to be recovered. 

Hazardous Ingredients < 0.1% Acrylic add Hazardous Decomp. Prods. Decomp, or combustion of the dry polymer may generate irritating vapors, CO, CO2, acrylate monomers, hydrocarbons... 

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