Zinc-based heat stabilizers

Organophosphoms compounds, primarily phosphonic acids, are used as sequestrants, scale inhibitors, deflocculants, or ion-control agents in oil wells, cooling-tower waters, and boiler-feed waters. Organophosphates are also used as plasticizers and flame retardants in plastics and elastomers, which accounted for 22% of PCl consumed. Phosphites, in conjunction with Hquid mixed metals, such as calcium—zinc and barium—cadmium heat stabilizers, function as antioxidants and stabilizer adjutants. In 1992, such phosphoms-based chemicals amounted to slightly more than 6% of all such plastic additives and represented 8500 t of phosphoms. Because PVC production is expected to increase, the use of phosphoms additive should increase 3% aimually through 1999. 

Coordination complexes of zinc, cobalt, and platinum show a stability similar to complexes involving copper and iron. They are stable to concentrated, nonoxidizing acids and bases. The fact that such complexes sublimate at 550 to 600°C without decomposing points to their extreme heat stability [7],... 

The mechanism of stabilisation of PVC by calcium/zinc stabilisers is briefly discussed and the development of highly efficient, solid and liquid calcium/zinc stabilisers based on novel types of calcium, zinc, calcium-zinc or calcium-zinc-phosphite intermediates for PVC is reported. These new stabilisers are capable of providing PVC compounds having equivalent or improved static heat stability when compared with commercial barium-zinc stabilisers even without the addition of high cost costabilisers. Data in graph form on PVC compounds containing these stabilisers are included. 33 refs. 

Polyvinyl chloride window profile formulations containing organic-based and calcium-zinc stabilisers were produced and the performance of the stabilised PVC compounds compared. Properties evaluated included rheological properties, heat stability, colour, gloss,... 

Metal Soaps—Barium and cadmium stabilizers are less toxic than lead and have lower cost on the basis of use levels, better sulfide stain resistance, and good clarity. They are a major class of heat stabilizers. Calcium and zinc soaps are also included in this classification because of their similar performance. The soaps used are based on a wide variety of anions such as octoates (2-ethyIhexanoic acid derived), benzoates, laurates, stearates, and substituted phenolates. 

In the IJS 2()0 million US market, the main heat stabilizers include lead compounds, organotin compounds, and mixed metal/salt blends based on chemicals such as barium, cadmium, and zinc, and significant changes are expected. Over US 75 million of the business is in lead- and cadmium-containing stabilizers, but these are under increasing scrutiny on health and safety grounds. 

However, these effects can be prevented or reduced by antioxidants, i.e. organic compounds which help protect the plastics imder hostile conditions. Other additives called heat stabilizers help stop plastics, particularly PVC, from decomposing dimng processing. They are often compoimds based on epoxies, or on calcium, zinc, tin and other metals. 

Synergetic mixtures presenting no health hazard are worthy of mention. They are zinc-based compounds mixed mainly with calcium soaps. Since zinc-containing stabilizers function only at very low concentration in PVC (0.1-0.4% by weight), it is necessary to ensure a good dispersion of the stabilizer mixture in the polymer matrix to obtain the best stabilizing effect. Too high a concentration of zinc stabilizer decreases the heat stability of PVC appreciably [55]. To avoid this, epoxide compounds should be used as HCl acceptors in conjunction with the stabilizer mixture. 

At the same chlorine content, CPE evolves considerably less hydrogen chloride dining combustion tests. PVC/CPE combinations should be considered for apph-cations requiring low corrosive gas evolution. The most common compounding technique is to use ultrafine calcium carbonate as filler for maximum hydrogen chloride absorption. This may be necessary in replacing lead stabilizers (whose reaction products with hydrogen chloride are typically very heat-stable) with zinc-based equivalents. 

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