Thermal degradation of rubber

Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

Thermal degradation of rubber vulcanizates involves scission and cross-linking reactions. Tobolsky [9] did a kinetic study of rubber degradation and derived the following simple Maxwellian relation between the stress and the rate constant, k, based on the theory of rubber elasticity ... 

Imaging chemiluminescence technique. Chemiluminescence analysis is suitable for studying the early stages of the thermal oxidation of rubbers. A weak emission of light formed by chemical reactions appears during the oxidative degradation of hydrocarbons. This technique can be used to depth profile the oxidation of rubbers. MR... 

As in purely thermal degradation, thermal oxidation of rubber is accompanied by formation of low-molecular-weight products in yields too high to be accounted for by random attack on the... 

In this Section, an experimental approach for constructing isothermal TTT cure diagrams has been described, TTT diagrams of representative epoxy systems including high Tg and rubber-modified epoxy resins have been discussed, and perturbations to the TTT cure diagram due to thermal degradation and rubber modification have been illustrated. 

M. Lopez-Manchado, L. Torre and L. M. Kenny, Kinetic analysis of the thermal degradation of PP-EPDM blends. Rubber Chemistry and Technology, 23, 73-84 (2002). 

Thermal degradation of plastics and rubber proceeds through a radical mechanism, which may involve three different decomposition pathways ... 

During the last 25 years a variety of processes have been developed for the thermal degradation of tyres in order to recover valuable components from rubber wastes.120-125 Stirred tanks, rotary kilns, fixed beds, fluidized beds and tray systems are examples of reactor types used for the thermal degradation of tyres. Several of these processes are now being used on a pilot plant and industrial scale. Basically, three fractions are derived from the thermal decomposition of tyres gases, liquid oils and solid residues. In the past, the influence of the reaction conditions and the reactor type were studied in order to maximize... 

Thermal degradation of plastic and rubber wastes in inert atmospheres has been extensively studied in the past. It is widely accepted that it takes place through radical mechanisms, two main pathways having been proposed depolymerization by end-chain cracking and random chain scission. In the first case, high concentrations of the starting monomer are obtained, but this mechanism is predominant only in the thermal degradation of a few polymers, such as PS and... 

This section describes the different processes that have been patented for the catalytic conversion of plastic mixtures without any previous thermal treatment. In many cases, the authors claim that the process is also successful in the degradation of rubber wastes or plastic and rubber mixtures. 

Of interest in studies of polymer degradation are two standard tests, the ASTM D 3850 (1994) test method for rapid thermal degradation of solid electrical insulating materials by thermogravimetric method, and the ASTM D 6370 (1999) standard test method for rubber compositional analysis by thermogravimetry. 

B. Dodson, I. C. McNeill Thermal Degradation of Chlorinated Rubber, Evidence for an Alternative Cyclic Structure for Chlorinated Rubber," J. Polvm. Sci. Polym. Chem. Ed. 12 (1974) 2305-2315. 

Thermal degradation of SBR rubber gives rise to monomers (1,3-butadiene and styrene) as well as to some toluene which turns into carbon dioxide, carbon, monoxide and water, at high temperatures. 

Comparative studies on the thermal degradation of polyisoprene, natural rubber (cis-polyisoprene) and gutta percha (t r tns-polyisoprene) all of which have the same chemical composition have shown that they differ in their thermal degradation characteristics. Studies under vacuum at 290-380 "C have shown that the decomposition of natural rubber (NR) is initiated at comparatively low temperatures at a considerable rate, whereas its decomposition rate at higher temperatures (above 330 "C) is to some extent slower than that of gutta percha and polyisoprene (Figure 2.1). The half-life temperature for synthetic polyisoprene is 320 C [1, 2]. 

Figure 2.1 Dependence of the degree of thermal degradation of natural rubber (cis-polyisoprene) (1), gutta percha (tr jns-polyisoprene) (2), and polyisoprene (3) on temperature. Source Author s own files...

Figure 2.4 illustrates normalised mass spectra of the evolved gases from the thermal degradation of various types of rubber at 340 °C. Figure 2.5 shows the ion-temperature... 

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