HEAT STABILISER

for instance, requires heat stabilisers to prevent the polymer from degrading during processing, and needs plasticisers to impart flexibility [42], 

Plasticisers are added to polymers in order to increase their flexibility and improve certain processing characteristics [48]. In rigid polymers, adding plasticisers leads to an important increase of processability but to only a marginal increase of flexibility [49]. 

For general background information, on the regulatory situation up to 1999, refer to the regulatory update (331). The main source of information is the website of the European Stabiliser Producers Association (ESPA) (www.stabilisers.org). 

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The most important class of stabilisers are the lead compounds which form lead chloride on reaction with hydrogen chloride evolved during decomposition. As a class the lead compounds give rise to products of varying opacity, are toxic and turn black in the presence of certain sulphur-containing compounds but are good heat stabilisers. 

For this reason tribasic lead sulphate, a good heat stabiliser which gives polymer compounds with better electrical insulation properties than lead carbonate, has increased in popularity in recent years at the expense of white lead. Its weight cost is somewhat higher than that of lead carbonate but less than most other stabilisers. This material is used widely in rigid compounds, in electrical insulation compounds and in general purpose formulations. 

Amongst heat stabilisers are copper salts, phosphoric acid esters,phenyl-3-naphthylamine, mercaptobenzothiazole and mercaptobenzimidazole. Of these, copper salts in conjunction with halides have been found particularly effective, and some automotive specifications require the use of copper for heat stabilisation. Light stabilisers include carbon black and various phenolic materials. 

Stabilisers. Stabilisers prevent deterioration of the polymer due to environmental factors. Antioxidants are added to ABS, polyethylene and polystyrene. Heat stabilisers are required in processing polyvinyl chloride. Stabilisers also prevent deterioration due to ultra-violet radiation. 

Typical additive packages for engineering thermoplastics have been described by Titzschkau [9], such as processing aids for PA, PP, or PET/PBT, three-component additive packages for polyamides and polyesters (nucleating agent, lubricant and process heat stabiliser) and coated copper stabilisers for polyamides. Additive packages or combinations of up to five or more additives are quite common. A typical white window PVC profile formulation comprises an acrylic impact modifier, TiC>2, CaCC>3, calcium stearate, a... 

Heat stabilisers 12 release agents Light stabilisers 3... 

Braun and Richter [923] have described an application of CE in additive analysis, namely quantitative analysis of heat stabilisers in PVC, such as Irgastab 17A and 18 MOK-N, which are metal-based (in the past usually Cd, Ba and Pb, now nontoxic Ca, Zn and Sn). Quantitative metal analysis is of interest for PVC recycling purposes. Various alternative approaches are possible for such quantitative analysis, such as XRF [924], polarog-raphy [925] and AAS [923], The performance of AAS, CE and complexometric titrations in the analysis of the heavy metal content in PVC was compared [923]. For all methods investigated the metals must be separated from the polymer and transferred into an aqueous phase. 

The organic tin compound OTC (heat stabiliser) was extracted from PVC by US extraction and analysed by UV spectrophotometry [55]. 

Table 5.6 UV/VIS analyses of some selected antioxidants, light and heat stabilisers...

Heat stabiliser Organotin mercaptides/sulfides/carboxylates antimony mercaptides metal carboxylates lead stearate/phosphite/phthalate/sulfate S, Sb, Sn, Ba, Ca, Cd, Mg, Sr, Zn P, Pb, S... 

Crompton [21] has reviewed the use of electrochemical methods in the determination of phenolic and amine antioxidants, organic peroxides, organotin heat stabilisers, metallic stearates and some inorganic anions (such as bromide, iodide and thiocyanate) in the 1950s/1960s (Table 8.75). The electrochemical detector is generally operated in tandem with a universal, nonselective detector, so that a more general sample analysis can be obtained than is possible with the electrochemical detector alone. 

Intermaterial competition affects the additive business. For example, if metallocene polyolefins displace PVC markets, then the demand for heat stabilisers and plasticisers might decrease, while the need for products such as fluoropolymer processing aids, antiblocks and slip additives might increase. At present, PVC as the main user of additives is under threat as never before. However, green PVC, containing more environmentally friendly FRs and plasticisers as well as non-heavy-metal stabilisers, is now environmentally acceptable. 

A similar situation pertains in the PVC stabiliser sector, where environmental restrictions have acted as an incentive to PVC reformulation (replacement of the commonly used liquid and solid Ba/Cd and solid Pb stabiliser systems). Producers of heat stabilisers for PVC nowadays strive to create products that are heavy-metal-free and provide strong weatherability and... 

Heat stabilisers for PVC act by HC1 scavenging and include organotins, mixed metal salt blends, and lead compounds. The latter account for nearly 64 % of volume (in 1994), followed by barium/cadmium and organotin compounds. Cadmium-based heat stabilisers are rapidly being replaced due to environmental concerns. Barium/zinc and calcium/zinc compounds show a high growth rate. It is expected that methyltin stabilisers will soon dominate the growing PVC pipe market. 

These compounds are multifunctional additives. They can act as heat stabilisers, radical traps, decompose hydroperoxides, UV absorbers, etc. (iv) UV absorbers. This is the largest class of UV stabilisers. They work on the same principle as sun-screen lotions they contain chromophores that can absorb light in the 280-400 nm region and release the excess energy as heat and not high-energy radiation. They must be stable under processing conditions and should not react with the polymer nor decompose with UV radiation. 

Not all additives increase biological attack. Heat stabilisers for PVC fabrication, e.g., tin and lead organometallic compounds, promote fungal resistance. 

Additives used in the production of PVC are discussed with reference to the increasing pressure from environmental campaigners. In particular heat stabilisers and plasticisers are examined with data from the European Stabiliser Producers Association regarding market trends, materials substitution, and toxicity issues. 

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