Thermal degradation polyurethanes

Renman L., Sangd C., Skarping G. (1986) Determination of isocyanate and aromatic amine emission from thermally degraded polyurethanes in foundries. Am. Ind. Hyg. Assoc. J., 47, 621-628. 

The thermal degradation of mixtures of the common automotive plastics polypropylene, ABS, PVC, and polyurethane can produce low molecular weight chemicals (57). Composition of the blend affected reaction rates. Sequential thermolysis and gasification of commingled plastics found in other waste streams to produce a syngas containing primarily carbon monoxide and hydrogen has been reported (58). 

Literature Survey on Thermal Degradation, Thermal Oxidation, and Thermal Analysis of High Polymers. Ill , PLASTEC Note 20 (1969) 29) N T. Baldanza, Literature Search Injection Molding Processing Parameters , PLASTEC Note 21 (1969) 30) A.H. Landrock, Polyurethane... 

Polyurethane materials are extremely versatile in that it is possible to produce a large variety of structures which range in properties from linear and flexible to crosslinked and rigid. The crosslinked PURs are thermosets, which are insoluble and infusible and therefore cannot be reprocessed by extrusion without suffering extensive thermal degradation. At present, the main sources of recyclable waste are flexible PUR foams and automobile waste. Waste and scraps of these materials may consist of 15-25% by weight of total PUR foam production. 

Even though poly(ortho esters) contain hydrolytically labile Linkages, they are highly hydrophobic materiads and for this reason are very stable and can be stored without careful exclusion of moisture. However, the ortho ester linkage in the polymer is inherently thermally unstable and at elevated temperatures is believed to dissociate into an alcohol and a ketene acetal (33). A possible mechanism for the thermal degradation is shown below. This thermal degradation is similar to that observed with polyurethanes (34). 

Karlsson D, Spanne M, Dalenea M, Skarping G (2000) Airborne thermal degradation products of polyurethane coatings in car repair shops. J Environ Monit 2 462 169... 

Table I. Calculated Activation Energies (E s) for Thermal Degradation of Polyurethanes (PU s) and Lignin...

For the polyurethane composition considered here the maximum (adiabatic) temperature jump is equal to 25 K. This means that at ordinary working temperatures of the mold, i.e., 40 - 80°C, the maximum increase in the material temperature does not reach an unacceptable level of close to 200°C, where thermal degradation of the polymer can begin. We can restrict the maximum temperature growth Tmax by using the mathematical model based on Eqs. (4.10) - (4.13) and then constructing Tmax - vs - to curves for various mold temperatures. These provides a means of choosing the optimum process parameters when the temperature must not exceed Tmax. 

The mechanism is believed to be that a hydrogen atom next to the ether linkage is attacked. This radical reacts with oxygen from the air to form a peroxide radical, which in turn takes another hydrogen atom from the backbone of the chain to form a hydroperoxide. This hydroperoxide breaks down into two more radicals (see Figure 7.12). When polyether polyurethanes are heated in an atmosphere of nitrogen, this thermal degradation does not take place and the material retains its properties. 

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. 

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. 

Determination of 4,4/-methylenedianiline (5a) in hydrolyzed human urine may serve as a biomarker for exposure to methylenebis(4-phenyl isocyanate) produced on thermal degradation of polyurethane foam. The urine is hydrolyzed with H2SO4 and 4,4 -methylenedianiline (5a) is derivatized to the corresponding pentafluoropropionamide. 

Le Bras, M. Bourbigot, S. Delobel, R. Camino, G. Eling, B. Lindsay, C. Reels, T. Thermal degradation of polyurethane and polyurethane/ expandable graphite coatings. Polym. Degrad. Stab. 2001, 74, 493-499. 

S Hirose, K Kobashigawa, Y Izuta, H Hatakeyama. Thermal degradation of polyurethanes containing lignin smdied by TG-FTIR. Polym International 47 247-256, 1998. 

---