Degradation of Plasticised PVC

In many international museum collections, degradation of plasticised PVC materials, in the form of clothing and footwear, furniture, electrical insulation, medical equipment, housewares, vinyl records and cassette tapes, toys and packaging materials used to store objects, has been detected as early as five years after acquisition. 

Degradation of plasticised PVC materials in museums is frequently observed as migration of the plasticiser from the bulk phase to surfaces. From there, plasticiser evaporates at a rate dependent on its vapour pressure. This process may be detected as a tacky feel to the plastic, increasing brittleness and subsequent discolouration of the PVC polymer itself. The mechanism by which 

The rate and extent of deterioration of plasticised PVC and the migration and loss of plasticiser, particularly phthalates, are related. DEHP inhibits the degradation of the PVC polymer, therefore when it either migrates to surfaces or is absorbed by other materials, PVC materials become discoloured, tacky 

In addition, as esters, phthalate plasticisers are susceptible to hydrolysis when exposed to strongly acidic or alkaline environments. Acid hydrolysis causes the formation of crystalline phthalic acid, volatile 2-ethyl hex-l-ene and 2-ethylhexanol. Acidic environments may develop when the PVC polymer degrades to form hydrogen chloride. Oxygen attack on alkyl groups in the DEHP molecule also results in the formation of phthalic acid. 

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Booth, G. H. Robb, J. A. (1968). Bacterial degradation of plasticised PVC - Effect of some physical properties. Journal of Applied Chemistry, 18, 194. 

Tirpak, G. (1970). Microbial degradation of plasticised PVC. Society of Plastic Engineering... 

In the wall thickness fluctuations up to 5 % may occur. As a result of the uneven temperature in the molten polymer during rotation, and also by the not always exactly reproducible rate of cooling, deviations in the dimensions of the finished product may amount to 5 %. Requirements are, that the materials can be molten completely, that the melt is sufficiently low-viscous, and that the molten polymer does not degrade too rapidly. Besides plasticised PVC, HDPE and LDPE are often used, as well as copolymers of PE such as EVA (ethylene - vinyl acetate copolymerj.Because the shear stresses in this process are extremely low, a narrow molar mass distribution is to be recommended, as discussed in 5.4. Cycle times vary between 3 and 40 minutes, dependent on the wall thickness. Cycle times can be reduced considerably by using machines with multiple moulds, since the cycle time... 

The influence of dicarboxylic acid ester plasticisers on the thermal degradation of PVC significantly depends on the physical state of the PVC-plasticiser system. If PVC retains the structure formed in the stage of suspension polymerisation, the additive produces inhibition of the process of thermal dehydrochlorination. In the case of true diluted PVC solutions in ester plasticisers, the polymer exhibits accelerated degradation, in accordance with a high value of the solvent basicity. 7 refs. 

Poly(vinyl halides) - The photocatalysed oxidation of PVC has been undertaken in the presence of titanium dioxide and zinc oxide pigments and the extent of dehydrochlorination measured. Acetic and formic acids were major products along with carbon dioxide. Copper(II) dialkyldithiocarbamate complexes are also sensitisers. Photodegradable PVC has also been developed by grafting with benzophenone chromophores. Plasticised PVC also degrades and discolours on irradiation but this is due mainly to the plasticiser. ... 

Since the properties and degradation reactions of plastics are usually influenced by their formulations and manufacturing processes, this chapter will first discuss those areas. Four plastics have been identified as being more vulnerable to degradation than others in museum collections cellulose nitrate, cellulose acetate, plasticised PVC and polyurethane foam. The most frequently seen... 

This is rather contra-intuitive. Museum conservators and designers are usually advised either to improve ventilation or to include adsorbent materials to remove volatile degradation products from the air space surrounding plastic objects during storage. This would accelerate the loss of plasticiser and thereby reduce the longevity of both new and deteriorated PVC objects. 

Polydiene rubber is a common low voltage electrical insulation material, used because of its low electrical conductivity and elasticity. But, it can degrade rapidly by peroxidation, even if there are antioxidant stabilisers in the system, which can lead to electrical shortages and fires. Polydiene rubbers are recently being replaced by saturated ethylene-propylene (EP) rubbers or plasticised PVC. 

Azodicarbonamide (AZC) is by far the most widely used CBA. It can foam PVC, including plasticised PVC, as well as the polyolefins, the styrenics, polyamides, PPO and some thermoplastic elastomers. AZC decomposes exothermically at around 210 °C to give nitrogen gas, along with other degradation products such as mea, cyanuric acid and cyamelide as solid by-products. Another possible residue is semicarbazide, traces of which have been found in foamed gaskets made of PVC blown with azodicarbonamide (see Chapter 7). 

PVC (Polyvinyl chloride) - is a very flexible substrate often used in the production of imitation leathercloth and flooring. It suffers badly from UV degradation and plasticiser migration. Both properties can be upgraded with a suitable coating. 

FTIR studies of the surface of degrading PE shows the development of species identified as polysaccharides and protein by photo-acoustic FTIR [26]. These are both characteristic of the growth of microorganisms [26,29]. Ikram and co-workers [35] have shown that in normal soils at 25°C, NR gloves showed 54% loss of thickness after 4 weeks and 94% mass loss after 48 weeks. On the other hand nitrile and neoprene rubbers showed insignificant loss in this time and plasticised PVC showed a smaller mass loss (11.6%) due entirely to biodegradation of the plasticiser. Ikram went on to show (see Table 3) that the rate of mass loss is strongly dependant upon the nutritional quality of the soil [36]. 

For samples that meet the solubility requirements of the SEC approach, analyses were also reported for additives in polymers such as PVC and PS [28,29]. Direct SEC analysis of PVC additives such as plasticisers and thermal stabilisers in dissolution mode has been described [28,30,31 ]. In the analysis of a dissolved PS sample using a SEC column of narrow pore size, the group of additives was separated on a normal-phase column after elution of the polymer peak [21]. Column-loading capacity of HPSEC for the analysis of additives, their degradation products and any other low-MW compounds present in plastics has been evaluated for PS/HMBT, PVC/TNPP and PVC/TETO (glyceryl tri[l-14C] epoxyoleate) [31]. It was shown that HPSEC can be used to separate low-MW compounds from relatively large amounts of polymers without serious loss of resolution of the additives the technique has also been used for the group analysis of chlorohydrin transformation products of the TETO model compound [32]. 

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