The formation of the toxicant in question was the result of an unusual reaction of the polyol in the foam, trimethylol propane, with the phosphate fire retardant in the gas phase. An understanding of the relative reaction rates is vital in controlling the production of the polymer and producing the desired physical properties (Herrington & Hock 1998). These polyols will fragment and volatilise as the temperature increases, leaving behind a char (>600C). Babrauskas et al. (2006) has supported the claim that polyether based polyurethanes are less stable than their polyester based counterparts when decomposed in air. The widespread use of flexible polyurethane foams in furniture and other upholstery, where they are usually covered in some kind of fabric has prompted some authors to investigate the effects of covering the foam on the yield of toxic products. Work by Ravey and Pearce (1997) on the decomposition of a polyether based flexible polyurethane foam suggested that up to 360C the decomposition of the foam was achieved by two main mechanisms. However, the presence of Cu2O reduced the HCN generated by the flexible polyurethane foam by 70-90% at low temperatures. Polymer-Plastics Technology and Engineering 45:p95108, Singh H, Jain AK (2009) Ignition, Combustion, Toxicity, and Fire Retardancy of Polyurethane Foams: A Comprehensive Review. The relation of the FED to the material-LC50 is given in equation4. Almost all unwanted fires are diffusion flames, with inefficient mixing of fuel and oxygen(as opposed to the "premixed" flames found in burner/combustion systems). This slight decrease is probably within the limits of experimental error, as it does not follow the general trend shown by most materials. The Model As the toxic product yields of polyurethane foams are directly related to the ventilation conditions, so is the materials LC50 value. NO. The reaction of a urethane with another isocyanate will produce an allophanate (Scheme5). The open cone calorimeter replicates the early well-ventilated stage of flaming where a fire would be too small to produce enough toxicants to cause harm except in very small enclosures. While the smoke chamber experiment is known to give low HCN yields, and both scenarios are well-ventilated, the yield of HCN was almost 4 times as high during flaming combustion if the sample was allowed to smoulder first. This is unusual as this test method is usually well-ventilated and the results are not similar to reports of other authors (such as Levin et al. Higher temperatures resulted in the volatilisation of most of the polyurethane precursors via the formation of lower molecular weight compounds. P.J. Nitrogen dioxide dissolves rapidly in water to form nitric and nitrous acid. In the smoke chamber, the highest reported yield during flaming combustion was 1.02mgg1. Bench-scale methods used for generating toxic effluents from polyurethane foams have met with controversy. It is generally accepted that the thermal decomposition occurring during flaming combustion is best represented by the thermal decomposition of a material in an inert atmosphere. The cribs used in the ISO 9705 tests were constructed from PIR sticks which burned rapidly, albeit with minimal damage to the room. 2007). Thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) showed that the polyether based polyurethane began to decompose at 258C, with a second decomposition stage at 350C (which could be attributed to the fragmentation of the polyether polyol). Top of Page. Experimental data reported a 28% recovery of DAT which supports the proposed decomposition mechanism. Their apparently transient nature results from their very high reactivity with amines and their fairly high reactivity with water (which is almost always present in fire effluent). The significant increased yields at 1000C and 1200C could also be attributed to pyrolysis of the nitrogenous combustion products into HCN due to the low air flow rate. The reaction of an isocyanate functional group with water (Scheme2) results in the formation of an unstable carbamic acid group, which in turn decomposes to release an amine and carbon dioxide. Fire Science Reviews Polystyrene (EPS and XPS) has significant amounts of styrene offgassing early in life. The use of 13C labelling by Chambers et al. Over this period there was a corresponding shift from the main cause of death in fires being attributed to burns to being attributed to inhalation of smoke and toxic gases. The amount of nitrogen recovered from the char (8%) at 600C is of a similar order to the results reported by Purser and Purser (2008a) in the steady state tube furnace suggesting that the amount of nitrogen in the polyurethane foam converted into HCN when the material is allowed to smoulder first before flaming is similar to that of steady under-ventilated flaming. In a 12-square-foot area, urethane was totally burned off, but the rest suffered more minor damage. In a letter to the editor of the journal, Barbrauskas et al. It is likely that the fire toxicity of fire retarded polyurethane materials is largely dependent on the specific fire retardant present. However, when the TDI was unable to enter the pyrolysis zone, the slower, irreversible decomposition to diaminotoluene (DAT) would occur. Nitric oxide (NO) and nitrogen dioxide (NO2) are non-flammable gases present in fire effluents. Oxygen depletion can be lethal if the oxygen concentration falls below tenable levels (~6%). In particular, the ventilation condition has a . The controlled atmosphere cone calorimeter. 2), also results in cross-linking in the polymer. The yields of toxic products followed the expected trend of being higher in the under-ventilated conditions. Additionally, the self-addition of isocyanates to produce isocyanurates (v in Fig. This char can decompose further, leaving behind a residue at >800C, to produce simple organic fragments and some polycyclic aromatic hydrocarbons (PAHs). Others, such as the NF X 70100, and the ISO/TS 19700 SSTF use the furnace temperature setting to ensure a consistent radiant heat flux. What does polyurethane foam give off when burned? The reported yields were extremely low for both CO and HCN, as the NBS smoke chamber apparatus is a well-ventilated fire scenario reported to give low HCN yields (Table6). Fire and Materials 31:p495521, Bott B, Firth JG, Jones TA (1969) Evolution of toxic gases from heated plastics. 9). Insulation, like all building products, has an 'embodied' carbon footprint resulting from energy use during the manufacturing process. Journal of Applied Polymer Science 111:p11151143, Stec AA, Hull TR (2011) Assessment of the fire toxicity of building insulation materials. In a compartment fire, the reactions of under-ventilated flaming occur in both the flame zone and in the hot upper layer. (such as polyurethane foam) burn slower (have a lower (HRR) than higher-density materials (cotton padding) of similar makeup. (2007) suggests that their yields are not heavily dependent on the ventilation conditions and that the yields would likely only increase by a small amount during under-ventilated flaming. 1999). HCN yields reported in under-ventilated conditions vary depending on the composition of the material; with flexible foams producing less than rigid foams and polyisocyanurates producing the most overall. The authors acknowledged that they did not include isocyanates in their calculations. Fire Res 1:p1121. Science 187:p742744, Wisnewski AV, Lemus R, Karol MH, Redlich CA (1999) Isocyanate-conjugated human lung epithelial cell proteins: A link between exposure and asthma? The main asphyxiants, carbon monoxide and hydrogen cyanide have been widely studied and are the best understood (ISO 13571 2007). The relationship between equivalence ratio and yields of CO and other products has been studied in detail for a wide range of materialsduring flaming combustion using two small-scale apparatus designed specifically for this purposethe ASTM E2058 fire propagation apparatus (Tewarson 2002) and the ISO/TS 19700 tube furnace apparatus (ISO/TS 19700 2013), in conjunction with a series of large-scale experiments used for validation (Gottuk & Lattimer 2002; Blomqvist & Lonnermark 2001; Purser & Purser 2008a). At 300C, free isocyanates and alcohols were produced from the decomposition of these biscarbamates. 2008), where inhaled isocyanates rapidly form conjugates with epithelial lung cell proteins (Wisnewski et al. Further fragmentation of these molecules led to the production of HCN, acetonitrile, acrylonitrile and a range of olefinic fragments. Both may be present in fire effluent, for example from PVC or halogenated flame retardants, and since the damage caused by the acidity (the concentration of H+ ions) is independent of the specificanion (Cl or Br), the discussion on HCl is also applicable to HBr. This suggests that the nitrogen in the char will more readily form HCN, even when the flaming is well-ventilated. The test method that produced toxic product yields associated with under-ventilated flaming was the NBS cup furnace toxicity method, which yielded 180210mgg1 of CO and 1620mgg1. The products generated in the flame zone then pass through the heated furnace tube, maintaining a high temperature, as in the upper layer of a compartment fire. It has been suggested that the reproducibility problems arise from the single point measurement (the tip of the probe may be in the centre of the plume, below it, or if mixing is more efficient, the upper layer may be recirculated through the flame), or the timing of the effluent sampling may cause instabilities (for example an initial proposal to sample after 8min was replaced by a proposal to sample when the smoke density reached its maximum). Chambers et al. 9). Substituted ureas decompose between 235 and 250C and carbodiimides decompose between 250 and 280C. Again, above 600C the compound and any yellow smoke present was decomposed into smaller volatile fragments. While it is evident that the HCN yield increases as a fire becomes more under-ventilated, the link between the nitrogen content of the fuel and the yield of HCN is less clear. A summary of these structures is shown in Fig. The authors noted that the yields of CO during the well-ventilated testing were higher than expected for both materials, and attributed this to the possible presence of gas phase free radical quenchers, such as halogens or phosphorous containing flame retardants, which would reduce the conversion of CO to CO2 (Schnipper & Smith-Hansen 1995). The protocol has been modified as a toxicity test by the mass transport industries, in the aircraft (EN 2826 2011), maritime (Fire Test Procedure Code 2010), and railway tests (CEN/TS 455452 2009). The radiant heat apparatus, smoke chamber and controlled atmosphere cone calorimeter produced much lower CO yields than would be expected for under-ventilated flaming. Isocyanates are a highly reactive family of compounds that are characterised by the RN=C=O functional group (where R can be any aliphatic or aromatic functionality). When tested with the polyester covering the polyurethane, the yield of HCN during flaming combustion was higher than that of just the polyurethane foam on its own. According to Tim Rodrique, the director of the DFS, investigators suspect that the fires were caused by the exothermic reaction that results from the mixing of the two chemicals used to make . At lower temperatures, decomposition differs, depending on the composition and physical properties of the polymer, although clear trends can be identified. During these tests, the PIR was set up as wall panels covered on two faces with aluminium foil. Avar G, Meier WU, Casselmann H, Achten D (2012) Polymer Science: A Comprehensive Reference, Polymer Science: A Comprehensive Reference, 10, p411-441. Stec and Hull (2011) assessed the fire toxicity of building insulation materials using a steady state tube furnace asdescribed in ISO/TS 19700 (2013). EN 2826. Building renovations, demolition, or building disassembly done years later can disturb spray foam insulation. 1986). Conversely, nitric oxide gas at low concentrations(~20 ppm) has been used to aid breathing in the treatment of respiratory disorders (Kavanagh & Pearl 1995). NBSIR 822532. Fire and Materials 19:p6164, Shufen L, Zhi J, Kaijun Y, Shuqin Y, Chow WK (2006) Studies on the Thermal Behavior of Polyurethanes. Elemental analysis of the polymers showed that the CMHR-FPUR contained 8.22% nitrogen by weight and the PIR contained 6.15% nitrogen by weight. MDI is a diaromatic diisocyanate compound that boils at 208C and is primarily used in the production of rigid foams. Terms and Conditions, The authors proposed that once formed, these compounds could partially polymerise with volatilised TDI in the vapour phase to produce Woolleys yellow smoke. In the gas phase, isocyanates, amines and yellow smoke will begin to decompose at >600C into low molecular weight nitrogen containing fragments (such as benzonitrile, aniline and hydrogen cyanide (HCN)). The non-flaming decomposition of polyurethanes in air or nitrogen can be summarised effectively usinga generalised mechanism based on the available literature (Fig. The half-scale ISO 9705 experiments showed a wider range of ventilation conditions up to ~2.0. Taking into consideration the issues with repeatability of large-scale testing, the authors asserted that the similar trend in HCN yields supported the good relationship between the tube-furnace and large-scale results. At this temperature around one third of the compounds mass was lost as volatile products, and the regenerated alcohol products were mainly present in the residue of the sample. The samples tested included both commercial rigid polyurethane foam and polyisocyanurate foam. ISO 13571 (2007) considers the four major hazards from fire which may prevent escape (toxic gases, irritant gases, heat and smoke obscuration). This results in relativelyhigh yields of CO and HCN during under-ventilated flaming and relatively low yields during well-ventilated flaming. These fumes could lead to irritation of the eyes, throat, and lungs, and could cause more severe allergic reactions for individuals with chemical sensitivities. The non-flaming decomposition of non-fire retarded polyurethane foams in air is generally quite well understood and comparable to the inert atmosphere decomposition, in terms of both products and mechanisms. The most widely used fire-test apparatus, stipulated in smoke regulations in most countries of the developed world, is the smoke density chamber as described inISO 56592 2012, andshownschematically in Fig.
