Polyurethane foams ignitability

Anderson, M., Sleight, R., and Torero, J.L., Downward smolder of polyurethane foam Ignition signatures, Fire Safety Journal, 35, 131-148, 2000. 

Considering the relatively small, controlled amount of com-husfihles in an LWR, it is surprising Jiiit fiiL S at nuclear power plants. should be important. The first incident to attract attention was the fire in the San Onofre cable trays (FRPJ). This was followed by spontiiiieous combustion of uncured polyurethane foam in the cable seals at Peach Bottom 1 (1971). The incident at Browns Ferry in 1975 was similar, except that a candle ignited the polyurethane foam. These events showed the effectiveness of fire as an initiator of multiple system failures -... 

Polyether-based foams account for more than 90% of all flexible polyurethane foams. The properties of foams are controlled by the molecular structure of the precursors and the reaction conditions. In general, density decreases as the amount of water increases, which increases the evolution of carbon dioxide. However, the level of water that can be used is limited by the highly exothermic nature of its reaction with isocyanate, which carries with it the risk of self-ignition of the foamed product. If very low density foams are desired, additional blowing agents, such as butane, are incorporated within the mixing head. 

Early flexible polyurethane foams were of the so called standard type and were readily ignited by a small flame. 

POLYURETHANE FOAM SHEETS OR BLOCKS. These are required to resist ignition source 5 (17 gram wood crib) of BS5852 Part 2 except that the flames may penetrate the full depth of the specimen and that the mass loss (due to burning and liquid residues falling from the test rig) shall not exceed 60 grams. 

Figure 7.13 Spread and ignition results for (a) plywood, (b) rigid polyurethane foam, (c) gypsum board and (d) asphalt shingle [18]...

As the reaction propagates, the oxygen inside the porous matrix is completely consumed leaving residual char. Figure 3.11 shows a picture of smoldered polyurethane foam. The foam was ignited at the top of the sample and the reaction was allowed to propagate downward leaving a black char behind. 

The first step of the degradation process, as shown in Figure 3.12, corresponds to the different magnitudes of the net heat flux imposed onto the material. A typical ignition plot is presented in Figure 3.13 for polyurethane foam. If the net heat flux is weak (i.e., for polyurethane foam <6 kW/m2... 

FIGURE 3.13 Ignition characteristics of polyurethane foam as a function of an external heat flux. (Extracted from Anderson, M. et al., Fire Safety /., 35, 131, 2000.)... 

The fire began when pyrotechnics used during a rock concert ignited the polyurethane foam lining of the walls and ceiling of the stage, and spread quickly... 

ASTM D 3675 Surface Flammability of Flexible Cellular Materials Using a Radiant Heat Energy Source This method may be used on cellular elastomeric materials such as flexible polyurethane foam and neoprene foam. It employs a radiant panel heat source consisting of a 300 by 460-nun (12 by 18 in.) panel in front of which an inclined 150 by 460-m (6 by 18 in.) specimen of the material is placed. The orientation of the specimen is such that ignition is forced near its upper edge, and the flame front progresses downward. Factors derived from the rate of progress of the flame front and heat liberated by the material under test are combined to provide a flame spread index. The method was developed to test cellular elastomeric materials which could not be tested by ASTM E 162. 

No differences in flammability characteristics between the 0.1% Cu20-treated and untreated flexible polyurethane foam were observed. These characteristics were examined to assure that the positive effect on toxicity was not contradicted by negative effects on the flammability properties. The flammability characteristics examined were (1) ignitability in three systems (the NIST Cup Furnace Smoke Toxicity method, the Cone Calorimeter, and Lateral Ignition and Flame Spread Test (LIFT)), (2) heat release rates under small-scale (Cone Calorimeter) and medium-scale (furniture calorimeter) conditions, (3) heats of combustion under small-scale (Cone Calorimeter) and medium-scale (furniture calorimeter) conditions, (4) CO/CO2 ratios under small-scale (Cone Calorimeter) and medium-scale (furniture calorimeter) conditions, (5) smoke obscuration (Cone Calorimeter), and (6) rate of flame spread (LIFT). 

One can see that self-ignition temperatnre is higher than flash ignition, which is nnderstandable, as pilot flame makes the ignition occnrs faster. However, a difference between these temperatnres varies significantly between 21 °C for polyurethane foam and lire for PVC film. 

Use of this plasticizer, which is commercially available (Fyrolflex RDP) permits to obtain, non-fogging, flame retarded polyurethane foam. Incorporation of halide (preferably chlorine) in trialkyl plasticizer results in an ignition resistant, UV stable plasticizer. Effect of a plasticizer on heat stability of PVC, flexibility of plasticized material at room and low temperatures is usually balanced by the selection of a required proportion between aryl and alkyl components. Low temperature properties can be improved by alkylphettylation with special attention to isopropyl substituents. Also haze can be eliminated in PVC composition by use of alkylated derivatives (e.g., methylphosphonate). Matty efforts were made by international groups of specialists to analyze reasons for increased toxicity of some phosphates such as tricresyl and triphenyl phosphates. Their extensive findings are reported elsewhere. ... 

Hilado reported a calculation for the decomposition products of cellulose, polystyrene, poly(propylene oxide), and rigid polyurethane foam on the basis of Madorsky s data (Table 2.11). Following these results, he investigated the circumstances under which a specimen would ignite in a standard testing equipment used widely in the United States for building materials. 

The cross sectional area of the equipment was 0.14 m and the linear rate of flow of air during the test procedure was 72 m/min, thus, the cross section was swept by 10 m of air per minute. The minimum amount of the calculated gas mixture for providing ignition was 0.33 m min for wood, 0.111 m /min for polystyrene, 0.272 m /min for poly(propylene oxide), and 0.258 m min for rigid polyurethane foam. 

Polymeric foamed materials are very specific in terms of ignitability and flame spread. It has been shown that differences in the surface area of foamed polymers and cell size have a larger effect on flammability than do density or differences in chemical structure. The chemical stmcture, of course, may dictate the surface area or porosity in the formation of foam. For example, flexible polyurethane foams can be ignited by a smoldering cigarette. A textile material normally used to enclose the foam, as is common in upholstered furniture and mattresses, actually helps ignition if snitable flame retardant-treated textiles are not used. 

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