Polyvinyl chloride heat degradation

Figure 12.4 depicts schematically the experimental setup used in capillary flow studies. The primary application of the discussion that follows is in capillary viscometry, which is useful to die design. The ratio Rr/R should be greater than 10, so that the pressure drop due to the flow in the reservoir can be neglected.1 The reservoir radius cannot be too large, though, because the time required for uniform heating of the solid polymer load would be too long (see Fig. 5.6). Long heating cannot be used for sensitive polymers such as polyvinyl chloride (PVC), which readily degrade thermally.

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... 

Shepherd and Gilbert [41] examined a range of plastic additives on PL-Gel columns (500 + 2 x 100 + 50A) in tetrahydrofuran (1 mLmin ), using both refractometer and UV detection. The degradation of tris(nonylphenyl) phosphate stabilizer in heat-treated polyvinyl chloride was quantitatively measured, as well as its degradation product nonylphenol. The influence of polymer concentration in the analysis was investigated. 

In the past, industrial and academic researchers traditionally focused on developing stable and durable polymeric materials that resisted exposure to natural forces such as heat, sunlight, oxygen (O2), water and microbial attack. The most widespread modern plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) are inexpensive, easily processable, resistant and durable. In fact, most man-made polymers on the market cannot be biologically degraded because their carbon components are not broken down by microbial enzymes. The hydrophobic character of plastics, their low surface area and high molecular weights (MW) are all features which inhibit or decrease enzyme activity and enhance resistance to microbial attack [1]. 

One such factor is the presence of weak links which can seriously affect the heat resistance. One polymer that is particularly affected in this way is polyvinyl chloride where stepwise dehydrochlorination can occur by means of an unzippering reaction initiated from a weak link. In rubbers the weak link, as has been mentioned, is rather more important in causing degradation under the more common oxidative conditions to be dealt with later. 

Certain polymers require heat stabilizers because of degradation at high processing and performance temperatures. This is notably the case for polyvinyl chloride (PVC), which undergoes dehydrochlorination [43,44]. 

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