Limiting Oxygen combusting plastics

Elame-spread and smoke-density values, and the less often reported fuel-contributed semiquantitive results of the ASTM E84 test and the limited oxygen index (LOI) laboratory test, are more often used to compare fire performance of ceUular plastics. AH building codes requite that ceUular plastics be protected by inner or outer sheathings or be housed in systems aH with a specified minimum total fire resistance. Absolute incombustibHity cannot be attained in practice and often is not requited. The system approach to protecting the more combustible materials affords adequate safety in the buildings by aHowing the occupant sufficient time to evacuate before combustion of the protected ceUular plastic. 

There can also be a flammability limit associated with dust clouds. The flammability limits of combustible dusts are reported as the minimum explosion concentrations. The upper explosion limits for dust clouds have not been determined due to experimental difficulties. In the fourteenth edition of the Fire Protection Handbook [National Fire Protection Association (NFPA), Boston, MA, 1975], numerous results from the U.S. Bureau of Mines reports are listed. These results were obtained with dusts 74 fim or smaller. It should be noted that variations in minimum explosive concentrations will occur with change in particle diameter, i.e., the minimum explosive concentration is lowered as the diameter of the particle decreases. Other conditions which affect this limit are sample purity, oxygen concentration, strength of ignition source, turbulence, and uniformity of the dispersion. The NFPA tabulation is most extensive and includes data for dusts from agricultural materials, carbonaceous matter, chemicals, drugs, dyes, metals, pesticides, and various plastic resins and molding compounds. Except for metal... 

The limiting oxygen index (LOI) test is used to measure the minimum concentration of oxygen necessary for candle-like burning for 3 minutes or more. A higher LOI indicates that more oxygen is needed to support combustion. The presence of flame retardants increases the LOI. Unmodified PP has a LOI of 17. The key fire properties of some plastics are compared with PP in Table 20. Suitable flame retarded grades of PP can have a LOI of 28. 

The PVC has some unique characteristic properties that differentiate it from other common plastics. It possesses good combustion resistance due to the presence of chlorine in polymer chain. When burnt, PVC generates hydrogen chloride gas that retards combustion reactions by limiting oxygen supply to the PVC surface (Siunmers, 2005). 

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