1). The FED is expressed as the sum of contributions to toxicity from individual species and normalised to 1g of fuel in 200L fire effluent, as used in BS 6853 (1999). Recent work by Allan et al. The resulting substituted urea can then react with another isocyanate to produce a biuret linkage (Scheme4). The sample is a 7575mm square solid sheet and the standard for smoke measurement states that the results are only valid at the thickness tested (typically 14mm). HCN analysis was performed using infrared (IR) spectroscopy using a short path-length gas cell, which is a questionable method for the quantification of HCN due to its poor IR absorption, high potential for interferences and a poor limit of detection. Allergy 63:p583591, Pitts WM (1995) The global equivalence ratio concept and the formation mechanisms of carbon monoxide in enclosure fires. The results of these experiments and the mechanism of decomposition derived correlated well with work by Rein et al. 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). As fires grow, they become ventilation controlled, and fires in enclosures such as buildings rapidly change from well-ventilated to under-ventilated. These processes occur at around 300C with the precursor chemicals including TDI, MDI, HDI, polyols (both polyether and polyester-polyols) and aromatic amines. Additionally, the self-addition of isocyanates to produce isocyanurates (v in Fig. Both of the materials showed a clear relationship with the HCN yield increasing with . Equation (1985) when a flexible polyurethane foam was first heated at a temperature below its auto-ignition temperature, followed by flaming combustion of the remaining char and residue at a higher temperature. In addition totheir flammability, polyurethanes form carbon monoxide, hydrogen cyanide and other toxic products on decomposition and combustion. The authors proposed that once formed, these compounds could partially polymerise with volatilised TDI in the vapour phase to produce Woolleys yellow smoke. The yield of CO had a wide range during the under-ventilated tests due to inconsistent flaming of the sample with yields from 100250mgg1. 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. The isocyanate precursors used in the production of polyurethane foams usually consist of aromatic diisocyanates such as toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI). April 30, 2021 April 16, 2021 by Shobita Ravichandran. 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. Generalised decomposition mechanism for polyurethane foams both in nitrogen and in air. This resulted in the reported HCN yields for the under-ventilated conditions being lower than expected in all of the tests. In the UK, the rapid rise in fire deaths, in particular those from smoke toxicity, between the late 1950s and the early 1980s has been attributed to the rapid growth in low cost polyurethane foam furniture, with superior comfort and lower cost than the natural fillings that preceded it. Test methods, such as the steady state tube furnace (ISO/TS 19700) and the controlled atmosphere cone calorimeter (ASTM E 1354), facilitate ventilated controlled conditions which give yields of CO and HCN comparable to those observed in under-ventilated post-flashover compartment fires. These substances can include carbon dioxide, carbon monoxide, formaldehyde, aldehydes, and other volatile organic compounds (VOCs), as well as soot and particulates. In China and Japan, there are specific restrictions on the use of materials with high fire toxicity in high risk applications such as tall buildings, while an increasing number of jurisdictions permit the alternative performance based design approaches to fire safety. At 1000C the hydrogen cyanide produced accounted for a range of between 3.8 and 7.3% by weight. TNO Report. Results from these burn tests are used for comparison with the model developed to simulate flame spread, heat fluxes, and smoke development over time (Figure 1). Isocyanates should be considered when assessing the fire toxicity of polyurethane foams, due to their acute irritating effects and chronic effects associated with exposure. These reactions are accelerated in the presence of oxygen, which reduces the temperature of the decomposition steps. The polyester fabric produced 9293mgg1 of CO when burned with very little difference in the flaming or non-flaming conditions. The half-scale ISO 9705 experiments showed a wider range of ventilation conditions up to ~2.0. In the large scale test room, the sample smouldered for 1.5 to 2h, resulting in a HCN yield of 1.03mgg1. Similarly, Busker et al. The chemical additives to the polyurethane are said to give off a distinct chemical odor that lessens after adequate ventilation. Fire and Materials 9:p125134, Levin BC, Paabo M, Bailey CS, Harris SE (1986) Toxicity of the combustion products from a flexible polyurethane foam and a polyester fabric evaluated separately and together by the NBS Toxicity Test Method. It is difficult to draw more general conclusion from this work because the fuel-to-air ratio was not quantified, and the degree of mixing of fresh air and fire effluent, in the exposure chamber, is unknown. Bench-scale methods used for generation of toxic fire effluentsideally should be capable of reproducing individual fire stages or combustion conditions, for input into models of combustion toxicity. The effects range from tears and reflex blinking of the eyes, pain in the nose, throat and chest, breath-holding, coughing, excessive secretionof mucus, to bronchoconstriction and laryngeal spasms (Purser 2008b). National Fire Protection Association, Quincy, MA, pp 5482, Guo X, Wanga L, Zhanga L, Lia S, Hao J (2014) Nitrogenous emissions from the catalytic pyrolysis of waste rigid polyurethane foam. In a 12-square-foot area, urethane was totally burned off, but the rest suffered more minor damage. p 111711128. In addition, asphyxiation can also occur as a result of lowered oxygen concentration, and is affected by the carbon dioxide concentration. The most commonly reported adverse health effects after airborne isocyanate exposure is asthma due to sensitisation (Piiril et al. . An equivalence ratio of 0.5 represents a well-ventilated scenario, typical of an early growing fire, while a ratio of 2 corresponds to the under-ventilated stage responsible for high yields of toxic effluents. At ~2.0 the CMHR-FPUR resulted in 8% and 11% nitrogen recovered as HCN for 650C and 850C respectively. 10. Rigid MDI based foams are primarily used for insulation in the construction industryand can also be found in the transport industry. The authors noted a primary depolymerisation of the foam which would release volatile TDI and leave the polyol precursors in the condensed phase. eff, should be used, rather than an averaged local equivalence ratio, based on the oxygen supply to the chamber, because, in some experiments, substantial secondary flaming occurred outside the test chamber, such that the amount of oxygen available to combustion exceeded the amount that was fed to the enclosed chamber. Polymer Degradation and Stability 93:p20582065, Tewarson A (2002) SFPE Handbook of Fire Protection Engineering, 3rd ed. Hydrogen chloride (HCl) and hydrogen bromide (HBr) are strong acids which dissociate entirely in water. The resulting substituted urea can then react with another isocyanate to pro-duce a biuret linkage (Scheme 4). 2023 BioMed Central Ltd unless otherwise stated. Aromatic diisocyanates ortho- or para- to one another will have an activating effect on each other, thus increasing their reactivity. Conversely, nitric oxide gas at low concentrations(~20 ppm) has been used to aid breathing in the treatment of respiratory disorders (Kavanagh & Pearl 1995). This was observed by Levin et al. Fire Safety Journal 7:p11, Kavanagh BP, Pearl RG (1995) Inhaled nitric oxide in anesthesia and critical care medicine. Thermosets are cross-linked polymer molecules which, on heating, do not melt but will eventually decompose. This is due to the large range of available fire retardants found in polyurethane foams, which suggests that the toxicity will likely follow the general trends in the literature for all materials regarding fire retardants. The sample is spread evenly in a silica boat over a length of 800mm and fed into a tube furnace at a typical rate of 1gmin-1 with flowing airat a rate of 2-10 L min-1. The PIR foam produced similar HCN yields to the CMHR-PUF until 1.5, after which it increased more rapidly to give a yield of 20mgg1 at ~1.75. (2006) and also Garrido and Font (2015). al, 2014). There is some contradiction the literature as to the effect fire retardants have on the overall toxicity of polyurethane foams. Levin BC, Fowell AJ, Birky MM, Paabo M, Stolte A, Malek D (1982) Further development of a test method for the assessment of the acute inhalation toxicity of combustion products. Fire and Materials 5(4):p133141, Christy M, Petrella R, Penkala J (1995) Controlled-atmosphere cone calorimeter. A large majority of the literature indicates that the addition of fire retardants does not increase toxicity of polyurethane foams. The radiant heat flux in the ISO/TS 19700 apparatus has been measured (Stec et al. In general, isocyanate (R-NCO) exposure causes irritation to the skin, mucous membranes, eyes and respiratory tract (NIOSH 1989). 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 results in relativelyhigh yields of CO and HCN during under-ventilated flaming and relatively low yields during well-ventilated flaming. Levin et al. Both types of foam yielded very similar products at temperatures above 600C. The results showed a HCN yield of 15.8mgg1 at 600C. The trimerisation results in a highly stable isocyanurate ring which confer additional thermalstability to polyisocyanurates (Scheme7). An equation has been developed for the estimation of the FED for lethality from the chemical composition of the environment in the physical fire(such as the bench-scale methods described in the following section) model taken from ISO 13344 (1996) and uses gas-LC50 values for lethality to provide referencetoxicity data for the individual gases to calculate toxic potency, based on rats exposed for 30min. Anyone you share the following link with will be able to read this content: Sorry, a shareable link is not currently available for this article. Faster. Overall, the results suggested that the polyether based polyurethane was less thermally stable in the presence of oxygen than the polyester, and both were generally less stable in air than in a nitrogen atmosphere. This amine may then undergo further reaction with other isocyanates present to produce a urea (Scheme3). The chemistry of polyurethane foams and their thermal decomposition are discussed in order to assess the relationship between the chemical and physical composition of the foam and the toxic products generated during their decomposition. CO also combines with myoglobin in the muscle cells, impairing diffusion of oxygen to cardiac and skeletal muscles (Purser 2008b). (2007)), the sample is raised to a fixed furnace temperature, which is further increased in the gas phase during flaming combustion. 2012). The toxic effect of carbon monoxide is characterised by a lowered oxygen-delivery capacity of the blood, even when the partial pressure of oxygen and the rate of blood flow are normal. The average CO yield expected from under-ventilated flaming is ~200mgg1 for polyurethane materials. However, while the char produced when the polymer was heated at 370C contained only 20% of the total nitrogen from the polymer, 40% of that (8% of the total nitrogen in the polymer) was recovered as HCN when the char was burned at 600C. Polyurethane is widely used, with its two major applications, soft furnishings and insulation, having low thermal inertia, and hence enhanced flammability. Self-addition reaction of two isocyanates to produce a uretidione, Self-addition reaction of three isocyanates to produce a isocyanurate ring, Reaction of two isocyanates to produce a carbodiimide. (2013) further supported the presence of two separate decomposition mechanisms for flexible foams. Free of VOCs and HAPs. In some bench-scale apparatus the heat flux is constant, and often insufficient to sustain flaming at such low oxygen concentrations; further, an unknown quantity of fresh air bypasses the fire plume, so the ventilation condition, and hence, remains undefined. Woolley et al. . National Fire Protection Association, Quincy, pp 283, Purser DA (2007) The application of exposure concentration and dose to evaluation of effects of irritants as components of fire hazard. Both authors read and approved the manuscript. The most notable and abundant of these was hydrogen cyanide which increased in yield from 700 to 1000C. Refer to MSDS. Using polyurethane indoors can expose you to fumes with a number of potential side effects. The results indicated that the formation of the precursor, TDI, was much faster and preferable to depolymerisation when the volatile compounds could escape. The use of 13C labelling by Chambers et al. 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. Stec and Hull (2011) assessed the fire toxicity of building insulation materials using a steady state tube furnace asdescribed in ISO/TS 19700 (2013). This steric hindrance can be offset by increasing the temperature of the reaction or by performing the reaction in the presence of a catalyst (Vilar 2002). Anal Bioanal Chem 387:p219236, Herrington R, Hock K (1998) Flexible Polyurethane Foams, 2nd edn. Polyurethane. The authors suggested that the presence of oxygen does not affect the mechanisms by which CO and HCN are produced. National Fire Protection Association, Quincy MA, USA, pp 296, Purser DA, Purser JA (2008a) HCN yields and fate of fuel nitrogen for materials under different combustion conditions in the ISO 19700 tube furnace. The first is by combination with the ferric ion in mitochondrial cytochrome oxidase, preventing electron transport in the cytochrome system and inhibiting the use of oxygen by the cells. Nitrogen dioxide dissolves rapidly in water to form nitric and nitrous acid. Some of these methods attempt to address the transition through the fire stages by monitoring the formation of toxic gases as a function of time, as the oxygen concentration falls, and the fire condition changes from well-ventilated to under-ventilated. Sensory and upper respiratory tract irritation stimulates the trigeminal and vagus nerve receptors in the eyes, nose, throat and upper respiratory tract causing discomfort, then severe pain. Taking this into consideration, the reported yields of isocyanates, aminoisocyanates and amines are still relevant, as the results of Blomqvist et al. At higher temperatures the decomposition of the foams produced increasing amounts of HCN from 600 to 900C, followed by a sharp rise between 9001000C. The revised protocol is based on continuous sampling of the fire effluent. Most fuel nitrogen is released as N2, but in well-ventilated combustion conditions a proportion is released as oxides of nitrogen (mainly NO) and in under-ventilated combustion conditions a proportion is released as HCN (Purser & Purser 2008a). The toxic product generation during flaming combustion of polyurethane foams is reviewed, in order to relate the yields of toxic products and the overall fire toxicity to the fire conditions. 13) (UK Fire Statistics 2013). Damage was most severe to the exterior. The production of HCN and other low molecular weight nitrogenous compounds from the high temperature decomposition of polyurethanes has been reported in the literature in recent years.
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