Polymers heat stabilisation

Some additives are closely tied to specific polymers. Heat stabilisers are targeted exclusively at PVC, and it is sales of PVC rather than overall plastics sales that matter to the heat stabiliser supplier. Increased sales of polypropylene do not compensate at all. Plasticisers are almost as dependent on PVC as 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. 

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. 

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. 

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. 

IR and NMR spectra of basic lead carboxylates used as heat stabilisers for halogenated polymers, e.g. PVC, indicated that these salts were not complexes, or double compounds of lead oxide, as suggested in most textbooks, but unique compounds of interesting structure. Stabilisers studied included monobasic lead phthalate, tribasic lead maleate, tetrabasic lead fumarate, dibasic lead phthalate and dibasic lead stearate. Lead stabilisers probably functioned in these polymers by converting hydrogen chloride to water, interfering with acid catalysis of the elimination reaction. 6 refs. 

P. Gijsman and M. Gitton, Hindered amine stabilisers as long-term heat stabilisers for polypropylene, Polym. Degrad. Stab. 1999, 66, 365-371. 

Stabilisers are intentional additives that help to retard the decomposition of plastics by heat, light (mainly UV), oxidation, and mechanical shear during processing and use. Antioxidants are required for almost all polymers, especially ABS, PE and PS. Heat stabilisers are required during the processing of PVC, because its molecules are very... 

Heat stabilisers - to prevent decomposition of the polymer during processing. 

Zimmer AG [99, 100] claim that PTT can be heat stabilised by contacting the polymer with a melt unstable, organic nitrogen-containing stabilising compound e.g., 1-aminoethanol, triacetonediamine, 4-aminosalicyclic acid, or a melt stable alcoholate such as sorbitol or polyvinyl alcohol. 

Additive products have been improved, and several completely new products have been marketed. Individual polymers such as PVC and polypropylene have undergone changes in relative importance and market share. New health and safety eonsiderations and regulatory pressines have had major effects on the sales of eertain additives, espeeially heat stabilisers, brominated flame retardants and plastieisers. The trend towards reoyeling is afleeting additive selection. 

Heat stabilisers are needed in PVC because unstabilised PVC polymer degrades when it is heated, giving off hydrogen chloride gas, whether during fabrication or in service life. 

The selection of a heat stabiliser requires consideration of the precise processing operation and its temperature range, the effect of the stabiliser on melt rheology, its compatibility with the polymer formulation as a whole, possible plate-out, and its effects on ultimate properties, cost, and health and safety issues. Several heat stabilisers have been criticised on environmental grounds. 

Polyene formation is not necessarily stopped altogether by a heat stabiliser, but it can be restricted to short sequences of a few (say five) carbon-carbon bonds, so that the polymer does not absorb too much light in the visible region of the spectrum, causing colour changes. This also discourages crosslinking and/or chain scission. 

PVC is one of the primary thermoplastic polymers used for the production of biomedical disposable devices owing to its compatibility with a large number of additives (i.e., plasticisers, impact modifiers, heat stabilisers), its wide-ranging mechanical properties which yield rigid to flexible end products and, last but not least, the ability to produce relatively low-cost materials [28]. 

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

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