Polyurethanes decomposition

Samples are normally heated to just above softening point and then allowed to cool down before a test is carried out. With castable polyurethanes, decomposition takes place when the sample is softened, so either the annealing cycle must not be used or the temperatures must not be taken up as high. 

Since polyurethanes are frequently used in household objects, their thermal degradation and products generated during burning were studied frequently [3-5]. Among these can be included studies on polyester-urethanes [6], polyether-urethanes [7], phenol-formaldehyde urethane [8], studies on the influence of fire retardants on polyurethane decomposition [9, 10], generation of isocyanates during decomposition [11], and other studies [12-17]. Some reports on thermal decomposition of polyurethanes are summarized in Table 14.1.1. 

Polyurethane chemolysis by reaction with ammonia or various amines has been described in the literature. Sheratte92 has proposed a process based on polyurethane decomposition by various agents. Several examples were provided for polyurethane degradation with ethanolamine (120 °C), ammonia and ammonium hydroxide (180 °C), diethylene triamine (200 °C) and other basic reagents. In all cases the process involves, simultaneously or subsequently, reaction with propylene oxide, which allows the different amines obtained to be quantitatively converted into polyols according to the reaction shown in Scheme 2.6. The polyols derived from this process were used in the reformulation of new polyurethanes by polymerization with the corresponding isocyanate, and were suitable for application in rigid foams. 

Similarly, a recent patent combines aminolysis and hydrolysis reactions for achieving polyurethane decomposition.98 Thus, scrap polyurethane is reacted with a mixture of diethanolamine and aqueous sodium hydroxide. The simultaneous attack of these agents on the polymeric chains allows the reaction time to be appreciably shortened. The reaction product, obtained as an emulsion, is subjected to a second treatment with propylene oxide in order to transform the amines and ureas present in the mixture into polyols, giving a final product which is substantially free of any hydrogen-containing nitrogen atoms. The polyols produced have been found to be particularly suitable for the preparation of fresh polyurethane polymer which can be used as an elastomer or flexible foam. 

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

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. 

Most thermoplastic elastomers are stable materials and decompose only slowly under normal processing conditions. If decomposition does occur, the products are usuaHy not particularly ha2ardous and should not present a problem if good ventilation is provided. Extra caution should be exercised when processing polyurethanes, especiaHy those containing polycaprolactone segments. In these cases the decomposition products may include isocyanates and caprolactam, both of which are potential carcinogens. 

Figure 3 C-nuclear magnetic resonance spectra of polyurethane type II macroazo initiator before (a) and after (b) decomposition of azo group [20].

Sheratte55 reported the decomposition of polyurethane foams by an initial reaction with ammonia or an amine such as diethylene triamine (at 200°C) or ethanolamine (at 120°C) and reacting the resulting product containing a mixture of polyols, ureas, and amines with an alkylene oxide such as ethylene or propylene oxide at temperatures in the range of 120-140°C to convert the amines to polyols. The polyols obtained could be converted to new rigid foams by reaction with the appropriate diisocyanates. 

These high energy species are extremely reactive, with themselves and with nucleophiles, and can generate runaway exotherms. With water, rapid evolution of carbon dioxide results. Some instances are reported [1], A compound of this class was resposible for the worst chemical industry accident to date. Di-isocyanates are extensively employed, with polyols, to generate polyurethane polymers. The polymerisation temperature should be held below 180°C or decomposition may occur which, in the case of foams, may induce later autoignition. 

The isosorbide polyurethanes based on the aromatic diisocyanates P(I-TDI) and P(I-MDI), possess more rigid structures with both polymers forming brittle films and brittle compression moldings. Their glass transition temneratures are above their decomposition temperature of 260°C. The thermostability of isosorbide polyurethanes correspond to that of conventional polyurethanes with similar structure based on 1,4-cyclohexanedimethanol for which degradation temperature of 260°C has been determined. 

TGA analysis shows that polymer degradation starts at about 235°C which corresponds to the temperature of decomposition of the cellobiose monomer (m.p. 239°C with decom.). Torsion Braid analysis and differential scanning calorimetry measurements show that this polymer is very rigid and does not exhibit any transition in the range of -100 to +250 C, e.g. the polymer decomposition occurs below any transition temperature. This result is expected since both of the monomers, cellobiose and MDI, have rigid molecules and because cellobiose units of the polymer form intermolecular hydrogen bondings. Cellobiose polyurethanes based on aliphatic diisocyanates, e.g. HMDI, are expected to be more flexible. 

As already mentioned, polyurethanes decompose on heating into isocyanates and hydroxy compounds, the decomposition temperature depending on... 

Similar arguments explaining the phase separation were employed by Chou et al. [44]. The dynamics of phase separation was observed using an optical microscope during the course of polyurethane-unsaturated polyester IPN formation at different temperature. Chou et al. suggested that an interconnected phase formed through the spinodal decomposition mechanism developed quickly and was followed by the coalescence of the periodic phase to form a droplet/matrix type of morphology. The secondary phase separation occurred within both the droplet and the matrix phases. Chou et al. did not explain, however, why secondary phase separation occurred. 

There has also been a preliminary study of the thermal decomposition of polyurethanes using a carbon-14 labelled isocyanate (76). Changes in intrinsic viscosity were correlated with loss of carbon-14 from the polymer. The results tended to confirm earlier conclusions that scissions occurred at the urethane bonds. 

Old polyurethane on rims may be removed either on a lathe, by solvent attack, or by freezing in liquid nitrogen. For more complex shapes, the reinforcing may be recovered by the above methods as well as by pyrolysis in a specially designed chamber, where the material is heated in the absence of air to above the decomposition temperature of the polyurethane. The fumes are then burned using special after-burners. 

Hydrolysis can be defined as the decomposition of a compound by reaction with water, the water taking part in the reaction. The effect is enhanced by the presence of either acids or alkalis. The chemistry of polyurethanes leads to the probability of hydrolytic attack. The mechanism is illustrated in Figure 2.41. 

Arc resistance is the tracking of an electrical arc over the surface of a polymer. The arc will initially track through the air, but, depending on the composition of the polyurethane surface, decomposition of the polymer can take place and a more conducting track formed. The polyurethane will decompose to carbon that will readily carry the current. Figure 7.15 illustrates the diagrammatic method of how to determine the tracking resistance of polyurethanes. 

All safety guards, where fitted by the manufacturer, must be kept in use. The cutting equipment must be sharp and the clearances suitable for polyurethane. Poor machining practices can cause localized heating, and the decomposition fumes are dangerous. 

TG-MS is ideally suited to reveal differences in pressure behaviour during thermal decomposition of materials. This has been illustrated by Mol [144] in TG-MS analysis of toluene diisocyanate (TDI) and methylene bis-4-phenol isocyanate (MDI)-based polyurethanes, where the observed greater increase in pressure for the TDI polyurethane than for the MDI derivative indicates a higher loss of low molecular weight fragments. This is not possible to deduct from the TG curves alone. Such indications are of great... 

Spindler and Frechet1391 prepared hyperbranched polyurethanes by step-growth polymerization (Scheme 6.8) of protected, or blocked , isocyanate AB2 monomers. The method is dependent on the thermal dissociation of a carbamate unit into the corresponding isocyanate and alcohol moieties.140,411 Decomposition temperatures range from ca. 250°C for alkyl carbamates to ca. 120°C for aryl carbamates.1401... 

Scheme 6.8. Synthesis of polyurethanes based on the thermal decomposition of a blocked fervisocyanate monomer.

Kumar and Ramakrishnan demonstrated that the thermal decomposition (107 °C) of 3,5-dihydroxybenzoyl azide gave rise to the labile 3,5-dihydroxyphenylisocyanate, which afforded (at 110 °C in dry DMSO with a catalytic amount of dibutyltin dilaurate) the polyurethane in 95 % yield. The polydispersity is greater than two, which is indicative of the poor mobility of the monomers the polymers were soluble in aqueous base, confirming the presence of free phenolic termini. 

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). 

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