Green Chemistry - ACS Publications - American Chemical Society

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Chapter 2

New Process for Producing Polycarbonate Without Phosgene and Methylene Chloride 1

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Downloaded by NORTH CAROLINA STATE UNIV on September 5, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0626.ch002

Kyosuke Komiya , Shinsuke Fukuoka , Muneaki Aminaka, Kazumi Hasegawa, Hiroshi Hachiya, Hiroshige Okamoto, Tomonari Watanabe , Haruyuki Yoneda , Isaburo Fukawa , and Tetsuro Dozono 2

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Chemisty & Chemical Process Laboratory, Asahi Chemical Industry Co., Ltd., Kojima-Shionasu, Kurashiki-City, Okayama 711, Japan

A new polymerization technology for manufacturing polycarbonates without using phosgene and methylene chloride has been established. The innovative and environmentally benign process is based on the entirely new concept, "Solid-State Polymerization of Amorphous Polymers". Asahi's new process essentially consists of (1) prepolymerization, (2) crystallization, and (3) solid-state polymerization. The solid-state polymerization process is able to cover a very wide range of molecular weight polycarbonates from the lowest disk grade (Mw 15,000) to the ultra high molecular weight grade (Mw>60,000), which is very difficult to produce by conventional processes. The polycarbonates obtained by Asahi's new process are colorless with good transparency; other excellent features include, for example, better heat stability and reworkability than that of polycarbonates produced by the phosgene process. Furthermore, the ultra high molecular weight polycarbonate produced by the new process also has excellent properties, which are missing in the conventionally available polycarbonate, such as excellent solvent resistance and excellent steam resistance. It is well known that phosgene is currently used industrially in large scales in the two most important processes for manufacturing polycarbonates and isocyanates all over the world. Phosgene, however, is notorious for its high toxicity (ACGIH T L V - T W A 0.1 ppm) and corrosiveness. Therefore, environmentally benign processes, which are able to be commercialized and that do not require phosgene have been desired earnestly for a long time.

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Corresponding authors Current address: Division of Chemicals Technology, Yakou, Kawasaki-City, Japan Current address: Department of Research and Development Administration, Hibiya-Mitsui Building 1-2, Yurakucho, 1-chome, Chiyoda-ku, Tokyo, Japan Current address: Fuji Office, Samejima, Fuji-City, Japan

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0097-6156/96/0626-0020$12.00/0 © 1996 American Chemical Society In Green Chemistry; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

Downloaded by NORTH CAROLINA STATE UNIV on September 5, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0626.ch002

2. KOMIYA ET AL.

New Process for Producing Polycarbonate

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Asahi Chemical Industry Co., Ltd. has succeeded in developing alternative and innovative non-phosgene processes for producing isocyanates and polycarbonates in the pilot scales which are commercially viable. In the production of isocyanates, processes for both aromatic isocyanates, such as methylene diphenyl diisocyanate (MDI), and aliphatic isocyanates, such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (BPDI), have already been developed successfully. A part of those processes has already been reported (i), and the others will be reported in the near future. In this paper, the new innovative process for producing polycarbonates is reported. Polycarbonates are well known to be typical amorphous polymers and to have excellent properties such as heat resistance, impact resistance, transparency, and dimensional stability (2-5). Polycarbonates, therefore, have been widely employed in various applications from nursing bottles to precision instruments (CDs, cameras, etc.), or in structural materials (for electrical applications, electronics, automobiles, construction applications, etc.). The global demand of polycarbonates has been growing more than 10% per year. The production capacity of polycarbonate world wide is about 1 million tons per year, and the boom in polycarbonate plant construction continues. Almost all of the polycarbonates, however, have been produced by the "Phosgene Process". "Phosgene Process". In the phosgene process, the polycarbonate is produced by an interfacial condensation polymerization of bisphenol-A with phosgene and NaOH between two solvents, methylene chloride and water (Figure 1). The obtained methylene chloride solution of the crude polycarbonate is washed with water to remove the by-product, NaCl, but the washed aqueous solution contains not only NaCl but also methylene chloride. The methylene chloride in the polycarbonate solution is also removed, but the complete removal is difficult because methylene chloride has a strong affinity to polycarbonate. Polycarbonates produced by the phosgene process, therefore, typically contain chlorine impurities which have a negative effect on polymer properties. Although the most noted problem associated with this process is the use of phosgene, the process has another significant problem, namely the process must use a very large amount (more than about 10 times by weight of the polycarbonate to be produced) of methylene chloride. Methylene chloride is a toxic chemical (IARC group-2B, possible carcinogenic to humans; E P A group-B2, probable human carcinogen) and is also one of the 17 chemicals targeted for emissions reduction by EPA, known as the 33/50 Program (an E P A voluntary pollution prevention initiative). In the phosgene process, the recovery of methylene chloride can be cosdy, due to its low boiling point (40*C) and its high solubility in water (20gfl). "Melt Process". In the "Melt Process", which has been in development for a long time without success (6), a polycarbonate is produced by performing a molten-state ester exchange reaction between bisphenol-A and diphenyl carbonate in the presence of a catalyst, while eliminating phenol (Figure 2). However, in order to attain the desired degree of polymerization by this process, phenol and, subsequendy, diphenyl carbonate need to be distilled from a formed molten polycarbonate of extremely high viscosity, which is very difficult In addition, it is generally necessary to perform the reaction at a temperature as high as 280°C to 310°C under a high vacuum of 1 mm H g or less for a long residence time. Consequently, this process has many disadvantages, such as discoloration of the polymer due to long residence at high temperature and high vacuum and difficulty in producing a polymer with the molecular weight necessary for structural use(4,7).

In Green Chemistry; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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GREEN CHEMISTRY

COCh

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Downloaded by NORTH CAROLINA STATE UNIV on September 5, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0626.ch002

Phosgene

Solvent H2O CH2CI2

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1. Use of dangerous phosgene 2. Use of a very large amout of

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Figure 1. Phosgene process of polycarbonate, and its problems.

In Green Chemistry; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

KOMIYA ET AL.

New Process for Producing Polycarbonate

• * CH,'

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DPC

Downloaded by NORTH CAROLINA STATE UNIV on September 5, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0626.ch002

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Prepolymerization ^ (Molten-State)

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Discoloration of the polymer

Figure 2. Melt process of polycarbonate, and its problems.

In Green Chemistry; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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GREEN CHEMISTRY

Downloaded by NORTH CAROLINA STATE UNIV on September 5, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0626.ch002

^

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Asahi's new process • Non-phosgene • Non-methylene chloride

Figure 3. Solid-state polymerization process of polycarbonate

In Green Chemistry; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

2. KOMTYA ET AL.

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New Process for Producing Polycarbonate

Raw materials of Asahi's New Process. In Asahi's new process, an aromatic dihydroxy compound, such as bisphenol-A, and a diaryl carbonate, such as diphenyl carbonate, are also used as raw materials. Although diphenyl carbonate is conventionally produced by the reaction of phenol with phosgene, Asahi Chemical has succeeded in developing an innovative process for producing diphenyl carbonate from dimethyl carbonate without phosgene (8,9), a process thought to be difficult to develop. Furthermore, a process for producing dimethyl carbonate from an alkylene carbonate and methanol have been developed (10). As the alkylene carbonate is produced from the alkylene oxide and C0 , die carbonate group source of the polycarbonate obtained by Asahi's process is C0 . Details of these processes, producing diphenyl carbonate and dimethyl carbonate, will be reported in other papers. 2

Downloaded by NORTH CAROLINA STATE UNIV on September 5, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0626.ch002

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Asahi's Solid-state Polymerization Process A new polymerization technology for manufacturing polycarbonate has been established without using phosgene and methylene chloride (11,12). The new process, called "Solid-state Polymerization Process", consists of three steps, namely, prepolymerization, crystallization, and solid-state polymerization (Figure 3). In the prepolymerization step, the amorphous prepolymer is obtained by molten-state prepolymerization of bisphenol-A and diphenyl carbonate. The amorphous prepolymer is converted to the crystallized prepolymer in the crystallization step, and finally, in the solid-state polymerization step, the polycarbonate of the desired molecular weight is obtained. Although solid-state polymerizations of polyamides and polyesters (which are crystalline polymers), have been known since 1939 and 1962 (13,14), until now, it has been considered impossible to produce polycarbonate by solid-state polymerization, because polycarbonates are amorphous polymers and become molten at the temperatures necessary to effect polymerization. The key technology in solid-state polymerization of polycarbonate is the crystallization of the amorphous prepolymer. It has been found that the low molecular weight amorphous prepolymer is easily crystallized, and the obtained crystallized prepolymer retains its solid-state when it is heated to the temperatures necessary for polymerization. Process Description by Step Prepolymerization Step. A clear amorphous prepolymer is obtained by performing a molten-state prepolymerization between bisphenol-A and diphenyl carbonate, while eliminating phenol (Figure 4). In this step, obtaining a low molecular weight (Mw 2,000-20,000) prepolymer with low melt viscosity is sufficient, therefore, the prepolymerization can be easily carried out at a relative low temperature (< 250°C). Discoloration of the prepolymer does not occur because of low temperature and short residence time compared to the melt process which must be performed at a temperature as high as 280°C to 310°C. Crystallization Step. The crystallization step gives a crystallized prepolymer with high melting point that is sufficient for performing the solid state polymerization (Figure 5). It has been found that the clear, amorphous, low molecular weight prepolymer can be easily converted to a white, opaque, crystallized prepolymer by treating it with a suitable solvent, or by heating it at a temperature higher than the glass transition temperature (Tg) of the amorphous prepolymer.

In Green Chemistry; Anastas, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

Downloaded by NORTH CAROLINA STATE UNIV on September 5, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0626.ch002

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GREEN CHEMISTRY

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