Lead compound heat stabilizers

Stabilizers. Heat stabilizers (qv) are included in PVC compounds to counteract the internal generation of hydrogen chloride as well as the external degradative effect of heat. Due to environmental considerations, there is a trend toward decreasing and even avoiding the use of stabilizers based on heavy metals, eg, lead. 

The possibility of obtaining the desired compounds is due to the higher stabilization of the phosphonium intermediate. Heating of the compounds 74 leads to the corresponding 1,3-dienephosphonates 75 [82],... 

Heat stabilizers - [HEAT STABILIZERS] (Vol 12) -m electric insulation [INSULATION, ELECTRIC] (Vol 14) -hydrazine derivatives as [HYDRAZINE AND ITS DERIVATIVES] (Vol 13) -lead compounds [LEAD COMPOUNDS - LEAD SALTS] (Vol 15) -metal soaps [DRIERS AND METALLIC SOAPS] (Vol 8) -for PVC foam [FOAMED PLASTICS] (Vol 11)... 

The inclusion of heat stabilizers is essential to protect the system against thermal decomposition at elevated temperatures during processing. For this purpose, tin carboxylate esters or liquid calcium-zinc stabilizers are preferred. Thio-tin compounds are very effective as heat stabilizers but must be regarded with caution, bearing in mind that they can lead to unpleasant and unacceptable residual odours. Secondary stabilizers that can be used include epox-idized soya bean oil. 

SAFETY PROFILE When heated to decomposition it emits toxic fumes of Pb. Used in paints, electrode position process in the automotive industr as a heating stabilizer. See also LEAD COMPOUNDS. 

Inorganic additives for rubber compounds also include materials that enhance performance under various accelerated stress conditions. Zinc oxide is an effective heat stabilizer for some types of elastomers. Iron oxide, lead compounds, barium salts, and specially treated clays, such as kaolinite, add performance margin in wet aging conditions. 

HALS was based on the discovery that the 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical (TEMPO) (1)), which already was known as an effective radical scavenger [46,47], was a very effective UV stabilizer too [48,49]. However, due to its physical and chemical properties TEMPO itself did not led to practical use. TEMPO is colored and will impart color to the to be stabilized polymer, it is thermally unstable and volatile [49]. Furthermore, it reacts with phenolic antioxidants present in many polymers leading to a reduction of processing and/or long-term heat stability. The discovery that compounds in which the /V-oxyl functionality was replaced by a N—H functionality also showed good UV stabilization activity was the key finding that led to the development of HALS stabilizers [49]. 

Lead Compounds—These were among the earliest stabilizers. Although quite effective as heat stabilizers, they are toxic, impart opacity to PVC compounds, and cause black coloration with time because of poor sulfide stain resistance. The primary use for lead compounds is in electric applications such as wire coatings. The principal lead compounds for this use are tribasic lead sulfate, basic lead sulfate silicate, basic lead carbonate, and basic lead phthalate. Use levels usually range from 3 to 8 parts-per-hundred parts of PVC (phr). 

PVC, a polymer that is widely used because of its low gas permeability and high fire retardancy, is sensitive to light and heat. The degradation leads to dehydrochlorination and color formation [Chapiro, 1962 Bradley, 1984 Thomas et al., 1986]. To process the resin it is necessary to add heat stabilizers. The most common additives include metal organic compounds, acid acceptors and alkalis, dioctyl phthalate, and dioctyl sebacate [Bradley, 1984]. PVC is only slightly crystalline. However, it has been used to produce heat-shrink products [Bradley, 1984]. 

Stabilizers are almost invariably added to PVC to improve its heat and light stability. The species found effective in stabilizing PVC are those that are able to absorb or neutralize HCl, react with free radicals, react with double bonds, or neutralize other species that might accelerate degradation. Lead compounds, such as basic lead carbonate and tribasic lead sulfate, and metal soaps of barium, cadmium, lead, zinc, and calcium are used as stabilizers. Obviously, they can react with HCl. Epoxy plasticizers aid in stabilizing the resin. Another group of stabilizers are the organotin compounds, which find application because of their resistance to sulfur and because they can yield crystal-clear compounds. 

Other lead stabilizers are of much more specific applications. For example, dibasic lead phthalate, which is an excellent heat stabilizer, is used in heat-resistant insulation compounds (e.g., in 150°C wire), in high-fidelity gramophone records, in PVC coatings for steel, and in expanded PVC formulation. 

Heat stabilizers may be classified as either primary or secondary. Primary heat stabilizers inhibit dehydrochlorination in chlorine-containing polymers and react with any liberated hydrogen chloride to delay further degradation. Mixed metal salts are primary heat stabilizers and form metal chlorides with hydrogen chloride. However, mixed metal salts have a destabilizing effect which results in discolouration of the polymer. This effect is counteracted by the introduction of secondary heat stabilizers such as organophosphites and epoxy compounds. Organotin and lead primary heat stabilizers can be used alone. 

Lead heat stabilizers are used for wire and cable applications as primary additives. They may be based on organic compounds including stearates and phthalates or inorganic salts such as sulphates, phosphates and carbonates. Although knowledge of the toxicity of lead has initiated the search for its replacement, no suitable, cost-effective material has yet been identified. 

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