Polycarbonates
Saat ve Tarih 02:54, 10/9/2007
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Polycarbonates are a particular group of thermoplastic polymers. They are easily worked, molded, and thermoformed; as such, these plastics are very widely used in the modern chemical industry. Their interesting features (temperature resistance, impact resistance and optical properties) position them between commodity plastics and engineering plastics.
Chemistry
Polycarbonates got their name because they are polymers having functional groups linked together by carbonate groups (-O-(C=O)-O-) in a long molecular chain. Also carbon monoxide was used as a C1-synthon on an industrial scale to produce diphenyl carbonate, being later trans-esterified with a diphenolic derivative affording poly (aromatic carbonate) s. Taking into consideration the C1-synthon we can divide polycarbonates into poly (aromatic carbonate) s and poly (aliphatic carbonate) s. The second one, poly (aliphatic carbonate) s are a product of the reaction of carbon dioxide with epoxides, which owing to the thermodynamical stability of carbon dioxide requires the use of catalyst. The working systems are based on porphyrins, alkoxides, carboxylates, salens and beta-diiminates as organic, chelating ligands and aluminium, zinc, cobalt and chromium as the metal centres. Poly (aliphatic carbonate) s display promising characteristics, have a better biodegradability than the aromatic ones and could be employed to develop other specialty polymers. The type of polycarbonate plastic is one made from bisphenol A, in which groups from bisphenol A are linked together by carbonate groups in a polymer chain. This polycarbonate is characterized as a very durable material, and can be laminated to make bullet-proof "glas", though “bullet-resistant” would be more accurate. Although polycarbonate has high impact-resistance, it has low scratch-resistance and so a hard coating is applied to polycarbonate eye-wear lenses. The characteristics of polycarbonate are quite like those of polymethyl methacrylate (PMMA; acrylic), but polycarbonate is stronger and more expensive. This polyme is highly transparent to visible light and has better light transmission characteristics than many kinds of glass. CR-39 is a specific polycarbonate material — although it is usually referred to as CR-39 plastic — with good optical and mechanical properties, frequently used for eyeglass lenses. |
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