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Catalyst used in Polyethylene Terepthalate
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  Catalysts for polyethylene terephthalate synthesis Three different compounds of Sb are applied as catalysts for the polycondensation reaction, but the most widely used one is antimony trioxide (Sb2O3). The advantage of this compound is its low price compared to the other two compounds, antimony triacetate and antimony glycoxide. However, the major drawback of this compound is its slow solubility in the reaction mixture. Antimony triacetate comprises 30 % of the applied Sb while the application of antimony glycoxide is limited to certain niche products due to its high prices. Germanium is mainly used nowadays for Japanese markets where Sb application is prohibited. It is applied together with stabilizers based on phosphorus due to its tendency to support oxidative degradation. The applied compound is germanium dioxide. The main drawback of germanium compounds is their high prices. Titanium alkoxides are very efficient catalysts for PET synthesis but their application is hindered by the low quality of the produced polymer. Titanium compounds are mostly used with combination of blue toner like cobalt compounds to mask the yellow colour of the produced polymer. To reduce side reactions, phosphorus based stabilizers are used. Another disadvantage of titanium compounds is their precipitation by hydrolysis. Chelate compounds like citric acid or tartaric acid or 2-hydroxy-ketones are added as precipitation stabilizers. Another approach to prevent precipitation is the impregnation of titanium in finely dispersed charcoal or silica. Catalyst particle size must be maintained below 500 nm to yield a clear polymer. Aluminum compounds were patented by ICI as catalyst in the 1950s. However, severe side reactions limited its widespread. DuPont patented zeolites as polycondensation catalyst. Zeolites are micro-porous crystalline solids with well-defined structures that have been used as finely distributed  additives for polyester. Generally they contain silicon, aluminum and oxygen in their framework and cations, water and/or other molecules within their pores. Zeolites with 1.5- 2.5 wt % water content are applied. However, it has been found that aluminum cations dissolved in the polycondensation reaction mass to be the main catalytic active component. Colour and thermal stability are improved by addition of small amount of cobalt as a blue toner and phosphorus compounds as stabilizers. Beside titanium and aluminium compounds several elements and their combinations like samarium, tin, iron, molybdenum and tungsten, magnesium/potassium/phosphorus and zirconium/silicon/cobalt have been patented as polycondensation catalysts.

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