Asahi Kasei’s new polycarbonate (PC) production process is the world’s first to use carbon dioxide as a starting material. Commercial application of the process began in Taiwan in June 2002 at the 50,000 ton/year PC plant of Chimei-Asahi Corporation, a joint venture between Asahi Kasei and Chi Mei Corporation. In recognition of the outstanding advance in ecological performance this technology represents, it was today announced that a Green and Sustainable Chemistry Award will be conferred by Japan’s Minister of Economy, Trade and Industry.
Polycarbonate is an engineering plastic characterized by outstanding heat resistance, impact resistance, dimensional stability, and transparency. It is used in a wide range of fields including automotive, electrical, electronic, office equipment, and sheet applications. Recent demand growth is particularly strong for application in optical discs such as CDs and DVDs.
Annual worldwide PC production is about 2.7 million tons. Except for Chimei-Asahi’s production, all of the world’s PC is made using carbon monoxide as a starting material. The vast majority of PC – about 2.48 million tons/year – is produced using the “phosgene process” where carbon monoxide and chlorine are combined to form phosgene, a toxic gas, as an intermediate process material.
The phosgene process inherently involves a number of environmental and economic shortcomings in addition to the high toxicity of phosgene itself. It necessitates the use of a large quantity of the low-boiling-point solvent methylene chloride, to which exposure must be restricted. Further, a large quantity of waste water containing methylene chloride must be treated. The use of chloride compounds also leads to corrosion of plant and equipment. Finally, chlorinated impurities remain in the product polymer.
Anticipating that the environmental and economic aspects of the phosgene process would become problematic, Asahi Kasei began research on an alternative process for PC about twenty years ago. The newly commercialized technology is the world’s first PC process in which part of the main chain of the PC molecule comes from carbon dioxide.
The new process enables high-yield production of high-quality PC and high-purity ethylene glycol using ethylene oxide, carbon dioxide, and bisphenol-A as starting materials. Ethylene oxide is produced in large volumes throughout the world, and is generally reacted with water to produce ethylene glycol. World-wide, about 14.5 million tons of ethylene glycol are produced annually, mainly as a feedstock for polyethylene terephthalate which is used for PET beverage bottles and polyester fibers. The carbon dioxide used is a by-product of ethylene oxide production, and is ordinarily released into the atmosphere.
Because carbon dioxide is chemically very stable, its incorporation into the molecular backbone of PC had been considered impractical. The successful development of the new PC process represents a triumph of innovative chemistry in that all of the carbon dioxide used is integrated into the product. The reduction in atmospheric release of carbon dioxide amounts to 173 tons per thousand tons of product PC.
While many companies have attempted to develop non-phosgene processes for PC, success has been limited by the marked technological barriers faced both in relation to a production step for a safe monomer to replace phosgene and the polymerization step for product PC. Based on a new concept, the groundbreaking Asahi Kasei process overcomes both of these technological barriers to enable production of a wide range of PC for optical discs and sheets at lower cost than the conventional process. At the same time, the challenges of improving selectivity and energy conservation in the production of ethylene glycol have also been overcome. This new technology not only avoids the problems of the phosgene process, but also achieves high yield, resource conservation, energy conservation, and reduced carbon dioxide emissions in a comprehensive array of ecological advantages.
With lower plant construction costs and lower feedstock costs, it is anticipated that it this new non-phosgene process will be widely adopted for PC production throughout the world.
|
