Degradation of Polyvinyl Chloride

Another reaction mechanism that occurs in some chain-growth polymers is solvolysis. In this type of reaction, a species reacts with a C-X bond, where X represents a halogen, and breaks it. Specifically, this becomes important when describing the degradation of polyvinyl chloride. Acidic species act to remove the chlorine atom, forming hydrochloric acid. 

The vinyl chloride monomer polymerizes via addition polymerization to form polyvinyl chloride. The final polymer has the chemical composition shown in Fig. 22.1. The polymer exhibits limited crystallinity, though this property is not often considered as important in defining its performance. It tends to be atactic or regionally syndiotactic, surrounded by extended atactic runs. When exposed to temperatures above 100 °C, polyvinyl chloride decomposes, creating free radicals that further attack the polymer chain, as we shall discuss in more detail later. For this reason, the degradation of polyvinyl chloride is autocatalytic... 

Figure 22.4 Radical chain mechanism for the degradation of polyvinyl chloride a) initiation, b propagation and c) termination...

The Roles of Hydrogen Chloride in the Thermal and Photochemical Degradation of Polyvinyl Chloride... 

Ejlertsson J, Svensson BH (1995) A review of the possible degradation of polyvinyl chloride (PVC) plastics and its components phthalic acids esters and vinyl chloride under anaerobic conditions prevailing in landfills. Dept, of Water and Environmental Studies, Linkoping University, Linkoping, p 20... 

M. Bisi, C. Nicolella, E. Palazzi, M. Rovatti and G. Ferraiolo HCl formation via pyrolytic degradation of polyvinyl chloride (PVC) an empirical approach to kinetic modeling, Chem. Engng. Technol, 17, 67-72 (1994). 

G. Montaudo and C. Puglisi, Evolution of aromatics in the thermal degradation of polyvinyl chloride a mechanistic study.. Polymer Degradation and Stability, 262, 229-244, (1991). 

Initially, degradation of polyvinyl chloride leads to a discoloration of the polymer as double bonds form along the backbone of the polymer. The delocalized electrons, along regions of seven or more adjacent double bonds, create colors such as tan, yellow, and pink. The deeper the color is, the greater the extent of degradation. Severe d radation lowers the tensile and impact strength of the polymer. 

Thermal Degradation of Polyvinyl Chloride, D. Braun, http //old.iupac.org/publications/pac/1971/pdf/2602x0173.pdf. The International Union of Pure and AppUed Chemistry, 2010. 

This material does not absorb u.v. radiation at all and so is not degraded by sunlight. The structure of polyvinyl chloride is quite similar ... 

Thermal degradation and degradation kinetics of polyvinyl chloride (PVC) plastisols, plasticised by polymeric plasticiser, were examined using dynamic and... 

The paper consists of a series of slides illustrating the mechanisms responsible for degradation in polyvinyl chloride (PVC), and the compositions of heat stabilisers... 

Acid acceptors are used when acids may be involved in the degradation of the polymer. Calcium stearate is often used with polypropylene to neutralize any mineral acids, such as hydrochloric acid, that might result from the reaction of water with catalyst residues. Various mixed metal laurates are used as stabilizers in polyvinyl chloride. Efforts are underway to replace the cadmium in some of these mixtures (see Chap. 4). Epoxidized soybean oil has been used as the acid acceptor, along with dioctyltinbis(thiogly-colate), to improve the heat stability of polyvinyl chloride.27... 

On the other hand, GC, because of the physical separation it effects, furnishes both a qualitative and a quantitative analysis of polymer-plasticiser mixtures with almost the same ease as the analysis of plasticisers alone, and this is discussed further in Chapter 5.1. It suffices to submit the sample, prepared as for the pyrolysis of plastics, to a controlled pyrolysis in order to disengage the vaporised plasticisers. The polymer is partially degraded, but its pyrolysis products were in all the cases studied by Guiochon and Hennicker [82] much lighter than the plasticisers and in no way prevented their separation and identification. Figure 4.18 shows the separation obtained of four plasticisers (a) dibutyl succinate, (b) tributyl phosphate, (c) dimethyl sebacate and (d) diethyl phthalate and the pyrolysis products of polyvinyl chloride. The latter are eluted during the first minute of operation. 

In addition to their biodegradability, compatibility, and compostability, PHAs were reported to possess gas-barrier properties almost similar to those of polyvinyl chloride and polyethylene terephthalate. These combinations of excellent physico-chemical properties coupled with the current concerns over environmental pollution and waste degradation drive their increasing commercial exploitation in different niche applications spanning from biomedical, packaging, automotive, infrastructure, aerospace to military applications. ... 

Fusion Temperature n In vinyl dispersions, the temperature at which Fusion occurs. The optimum fusion temperature is that at which thermal degradation has not occurred and maximum physical properties are obtained in the final product (Wickson EJ (ed) (1993) Handbook of polyvinyl chloride formulating. Wiley, New York). 

Perhaps the most well known example of the use of additives to prevent thermal degradation is in the thermal stabilisation of polyvinyl chloride. The free radicals produced in this case are chlorine, leading to the formation of hydrochloric acid. The stabilisers must stop these reactions which lead to acid corrosion of the processing equipment. 

Polyvinyl fluoride is more resistant to thermal degradation than polyvinyl chloride thus when PVC thermally degrades in the temperature range between 250 and 350 °C, nearly 100% of its chlorine is cleaved as HCl, while at even higher temperatures (372 to 480 °C) and the same loading time, only 20 to 60% of the fluorine is cleaved as HF from polyvinyl fluoride [32]. 

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