SMOKE DENSITY

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. 

Fig. 25-2. Double-beam, double-pass transmissometer for measuring smoke density in stacks. A[, chopper wheel A, beam gating wheel A3, aperture D, detector Fj, spectral filter F2, solenoid-activated neutral density filter L, lamp M, half-mirror/beam splitter Rj, solenoid-activated zero calibration reflector R2, retroreflector (alignment bullseye not shown). Design patented. Source Drawing courtesy of Lear Siegler, Inc.

As an example, a foam prepared from III, alumina trihydrate as a filler, benzoyl peroxide as a curing agent, and azobis formamide as a blowing agent, leads to a material with an oxygen index of 48, a long-term stability to at least 150 °C, and a smoke density about one fifth that of a commercial foam [284]. 

Allied to flammability is smoke density suppression especially in confined spaces, e.g., airliners, houses, warehouses. Many aromatic compounds bum with a smoky flame (e.g., styrene), whereas corresponding aliphatic compounds tend to burn with a clean "transparent" flame. This is because air-bome poly-aromatic vapours decompose to give volatile carbon (smoke) in low oxygen environments. 

The carbon black generated by a fire from a rubber source increases the smoke density other products are highly toxic and often corrosive. The halogens, phosphates, borates, and their acids evolved during a fire corrode metals and electrical and electronic equipment. Hence many of the fire retardants described below cannot be used in situations where the toxic gases evolved will create their own hazards. In these cases inorganic hydroxides are used, at filler-type addition levels. Aluminium hydroxide and magnesium hydroxide are used as non-toxic fire retardant systems. 

Smoke density. Optical density measurements on the smoke evolved from burning plastic samples were carried out using an NBS Smoke Chamber. The samples, which measured 75mm x 75mm, with a thickness of 0.6 - 4mm, were burned in the flaming mode in accordance with ASTM E662-79. Specific smoke density (Ds) values reported are the averages of three independent determinations. 

Additive Flammability 01 D (3min) Smoke Density Redn (3min) D (max) Time(min)to D = 00 CO Evolution (ppm) Actual(3min) Redn(3mins) Max. Red" (max). 

The traditional way in which smoke obscuration has been measured is by determining the maximum smoke density (or the specific maximum smoke density) by means of a smoke density chamber developed by the National Bureau of Standards (NBS smoke chamber, ASTM E662). This instrument measures the obscuration inside a static 500 L chamber, after a sample has been exposed, vertically, to a 2.5 W/cm2 radiant source. 

It is noteworthy to restate that there was no correlation in the series of experiments shown in Table II between the maximum smoke density in the NBS smoke chamber, flaming mode, and the obscuration in the full scale tests. 

TSR 15). The data (Tables IV-VI) suggest that this instrument provides a satisfactory method for measuring heat release, even in the horizontal mode. Furthermore, it can differentiate between those materials which are prone to release much heat rapidly and those which perform much better in terms of heat release. The reliability of smoke data is, in principle, lower than that of heat data. In order to establish some criteria, the Tables include SmkFct values at 5 min (in MW/m2), which will be compared with SmkFct and SmkPar values for the same materials tested in the Cone and with values of specific maximum smoke density measured in the NBS smoke chamber. 

Smoke Density. The smoke density was measured in an NBS Smoke Chamber following the procedures of ASTM E 662. 

Lee, T.G., "The Smoke Density Chamber Method for Evaluating the Potential Smoke Generation of Building Materials," 1973, National Bureau of Standards (National Institute of Standards and Technology), Gaithersburg, MD, Technical Note, 757, ASTM E-662). 

Tests for Fire Resistance of Roof Covering Materials, 1983. (similar to ASTM E 108) Tests for Flame Propagation and Smoke Density Values for Electrical and Optical Fiber Cables in Spaces Transporting Environmental Air, 1991. 

These are similar to nephelometers, except that they measure the attenuation of a light beam due to the combined effects of absorption and scattering by the sample. The instrument consists of a light source, a collimator and a photo-detector. The most common application is the measurement of smoke density in chimney stacks. In this case the optical surfaces exposed to the smoke are kept clean by flows of clean air. The density of the smoke is expressed in terms of per cent opacity, per cent transmittance or optical density, where ... 

Figure 3. Relationship of smoke density to level of chemical retention in 3/8-in. Douglas-fir plywood evaluated by the 8-ft tunnel furnace method...

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