Polyurethane foams, thermal decomposition

Polyurethanes. These polymers can be considered safe for human use. However, exposure to dust, generated in finishing operations, should be avoided. Ventilation, dust masks, and eye protection are recommended in foam fabrication operations. Polyurethane or polyisocyanurate dust may present an explosion risk under certain conditions. Airborne concentrations of 25—30 g/m are required before an explosion occurs. Inhalation of thermal decomposition products of polyurethanes should be avoided because carbon monoxide and hydrogen cyanide are among the many products present. 

Thermal degradation of foams is not different from that of the solid polymer, except in that the foam structure imparts superior thermal insulation properties, so that the decomposition of the foam will be slower than that of the solid polymer. Almost every plastic can be produced with a foam structure, but only a few are commercially significant. Of these flexible and rigid polyurethane (PU) foams, those which have urethane links in the polymer chain are the most important. The thermal decomposition products of PU will depend on its composition that can be chemically complex due to the wide range of starting materials and combinations, which can be used to produce them and their required properties. Basically, these involve the reaction between isocyanates, such as toluene 2,4- and 2,6-diisocyanate (TDI) or diphenylmethane 4,3-diisocyanate (MDI), and polyols. If the requirement is for greater heat stability and reduced brittleness, then MDI is favored over TDI. 

W.D. Wooley, Nitrogen-containing products from the thermal decomposition of flexible polyurethane foams. Brit. Polym. J., 4, 27 -3 (1972). 

Bilbao, R., Mastral, J.F., Ceamanos, 1, and Aldea, M.E., Kinetics of the thermal decomposition of polyurethane foams in nitrogen and air atmospheres, Journal of Analytical and Applied Pyrolysis, 37 (1), 69-82, 1996. 

The overwhelming majority of foams are TPs, but TSs are also foamed with CBAs, although some of them do create problems. Popular TS foams are made from polyurethane, polyester, phenolic, epoxy, and rubber. Thermal decomposition of the blowing agent with certain plastics such as TS polyesters cannot be applied in this system because the heat of polymerization is not high enough to induce decomposition. But chemical reactions simultaneously produce gas and free radicals they typically involve oxidation and reduction of a hydrazine derivative and peroxide. The reactions are catalyzed by metals, which can be used repeatedly. 

A munber of nitrogen derivatives of phosphoric and polyphosphoric acid (ammonium polyphosphate, melamine pyrophosphate) are used for improving the flame retardance of polyurethanes and other polymers. In thermal decomposition these compounds produce ammonia and the corresponding phosphoric acids which catalyze dehydration and other reactions, causing polymer dehydration during combustion. The coke produced in this process is more or less foamed. Ammonium polyphosphate and melamine pyrophosphate are added to compositions of intumes-cent coatings used for fire protection of various structural elements in construction... 

M. Ravey and E. M. Pearce, Elexible polyurethane foam. 1. Thermal decomposition of a polyether-based, water-blown commercial type of flexible polyurethane foam, J. Appl. Polym. Sci., 63, 47-74 (1997). 

Hydrogen cyanide concentrations in the thermal decomposition products from a flexible polyurethane foam were reduced 85% when the foam was treated with 0.1% or 1.0% CuiO by weight and thermally decomposed via a two-phase heating system in the NIST Cup Furnace Smoke Toxicity Apparatus. 

The third route is defined as substractive (lUPAC), in that certain elements of an original structure are selectively removed to create pores. Examples include the formation of porous metal oxides by thermal decomposition of hydroxides, of porous glasses by chemical etching, of activated carbons by controlled pyrolysis, of ceramic foam membranes by burning off a polymer (e.g. polyurethane), of alumina by anodic oxidation of aluminium to give oriented cylindrical pores with a narrow size distribution. 

Fires involving materials treated with anti-ChEs or bulk pesticides stores (CM and OP) may release both the unaltered anti-ChE and thermal decomposition products. Also, certain phosphorus-based fire retardants that arc subjected to heat and flame in a fire may result in the generation of anti-ChEs and other toxic materials. Thus, although fire retardants may slow the rate of fire progression, their involvement in a conflagration may result in an increase in the toxic potency of combustion products (Purser, 1992). For example, polyurethane foams treated with a trimethylol propane polyol base containing phosphoms-based retardants formed a highly neurotoxic combustion product [irimethylolpropane phosphate (TMPP) Petajan et at., 19751. 

Woolley and Fardell conducted comprehensive investigations of flexible and rigid polyurethane foams, including laboratory and full-scale experiments. It was established that a characteristic yellow smoke evolved from the polyurethanes at 200 to 300 °C, especially from the toluylene diisocyanate-based flexible foams. The yellow smoke comprised the total nitrogen content of the original polyurethane and amounted to about 25 per cent m/m of the foam. It was supposed that the thermally released isocyanate polymerized immediately in some way to form a particulate material (the solid part of the yellow smoke). Its apparent decomposition temperature was about 800 C. Mass spectrometry of its decomposition products detected hydrogen cyanide, acetonitrile, pyridine, and benzonitrile. The proportion of HCN increased with temperature. 

Ashida et burnt only 0.3 g of polyurethane foam in an air stream of 150 cm /min and found a maximum of HCN generation at about 550 °C. Jellinek / aj Uy 45 proposed the kinetics and mechanism ofHCN production from polyurethanes and polyimides. They found that HCN which evolved at lower temperatures underwent oxidation and thermal decomposition between 800 and 1000 C, whilst its total amount remained constant due to the additional HCN production from the char residue even above 800 °C. 

Purser, D. A. and Grimshaw, P. The incapacitative effects of exposure to the thermal decomposition products of polyurethane foams. Fire and Materials, 8, No. 1, 10 (1984)... 

Kinetic studies have been made on the thermal decomposition of a poly(oxypropylene)triol-toluene di-isocyanate copolymer foam. Following a diffusion rate-controlled step, the cellular structure collapses to a viscous liquid and degradation then occurs on a random scission basis. Products of degradation of A-monosubstituted and A A-disubstituted polyurethanes have been analysed by direct pyrolysis in the ion source of a mass spectrometer. The mono-substituted polymers depolymerize quantitatively to di-isocyanates and diols, whereas the disubstituted materials decompose selectively to secondary amines, olefins, and carbon dioxide. The behaviour of the monosubstituted polymers has been confirmed in an i.r. study of the degradation of model compounds. A study of the thermal degradation in vacuum of polyurethanes prepared from butanediol, methylene bis(4-phenylisocyanate), and hexanedioic acid-ethylene glycol-propylene glycol polyesters has been reported and reaction mechanisms proposed. ... 

Chao, C.Y.H. Wang, l.H. Comparison of the thermal decomposition behavior of a non-fire retarded and a fire retarded flexible polyurethane foam with phosphorus and brominated additives. 7. Fire Sci. 2001, 19(2), 137. 

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