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Polyvinylchloride (PVC) Additives - Heat Stabilizers |
Heat stabilizers used in manufacturing of Polyvinylchloride: Degradation of Polyvinylchloride occurs when a labile chloride group eliminates as HCl, resulting a chain reaction that generates more HCl and long chain polyenes which is responsible for color change of the product. The process is usually accelerated by HCl, HCL salts, oxygen etc. The role of a heat stabilizer includes removing labile chloride groups (by substituting with stable ligands), preventing the oxidation, absorbing HCl, deactivating the free radicals, terminating the polyene generation reaction and eliminating long chain conjugated polyene sequences by addition to the double bonds. Further, a good heat stabilizer is expected to be highly reactive even with very low levels of addition, and provide good color, long tem stability, odorless and nontoxic. The suitability of a heat stabilizer is usually needs to be tested by formulators, as not one heat stabilizer would work on every Polyvinylchloride formulations. |
Polyvinylchloride is inherently thermally unstable, which made it a less attractive polymer until early 1900s. However, the discoveries made in field of heat stabilizers and lubricants during the period of 1920- 1950, made polyvinylchloride being the second largest volume plastic in the world. Thus, unlike most of the other plastics, polyvinylchloride must have additives in order to process useful articles. Out of these additives, the presence of a heat stabilizer is a must. Other PVC additives: Plasticizers, lubricants, UV light stabilizers, impact modifiers, process aid, fillers and pigments. |
Currently, there are four main categories of heat stabilizers used in PVC industry: Lead based stabilizers: e.g. Lead salts, tribasic lead sulfate, dibasic lead phthalate, etc. Lead based heat stabilizers provide excellent long term heat stability and electrical properties, and are usually the cheapest, however, due to toxicity issues; usage is limited only to some countries depending on their legislation. Further, their inherent opacity limits their use in compounds where high clarity is a must. Organo-tin based stabilizers: e.g. mono/dialkyltin compounds. Usually highly alkyl-substitutes organo-tin compounds are the most toxic, thus trialkyl-substituted tin compounds exhibit the highest toxicity compared to di-alkyl and mono-alkyl substituted ones. Decreasing toxicity is achieved by keeping the trisubstituted fraction be minimal and attaching long chain alkyl groups. Usually organo-tin heat stabilizers provide accelerated fusion as well as enhance clarity. Thus, widely used in clear products. Usually organo tin compounds are the most expensive heat stabilizers; and, they are highly effective in heat stability. Further, organo-tin stabilizers exhibit a very good compatibility with Polyvinylchloride. Mixed metal stabilizers: derivatives of cadmium, barium and zinc: Usually mixtures of metallic-stearates give excellent heat stability, and also provide good light stability. Some metallic-derivatives may be toxic, however, not in all cases. Development of more environmental friendly mixed metal heat stabilizers is continuing, especially to replace lead based compounds in rigid PVC applications, and to achieve enhanced heat stability. Organic stabilizers: The heat stability effectiveness of most of the organic stabilizers is very low compared to the inorganic derivatives. Thus, usually organic-stabilizers are used as a co-stabilizer, where metal-derived stabilizers play the major role. |