TECHNOLOGY
Toyo Has New Continuous Polyester Processes Company is ready to scale up continuous direct polymerization from TA and high-molecular-weight polyester from DMT Toyo Rayon (Toray) has successfully carried two new continuous polyester fiber polymerization processes through pilot plant development. It now plans to scale them up as well as to sell licenses to a "few technically qualified" firms. The two new processes—contin uous direct polymerization from terephthalic acid (TA), and continuous production of high-molecular-weight polyester from dimethyl terephthalate (DMT)—should give further push to a world-wide swing to continuous poly ester fiber processes (C&EX, Dec. 6, 1965, page 44). Toray \s new systems are outgrowths of two polyester processes previously developed by the firm and used in fullscale plants since 1963. At Mishima,
near λ It. Fuji, Toray has a continuous polymerization plant based on DMT. Its capacity is about 20 metric tons a day. At Ehime, on the island of Shikoku, the firm operates a batch plant based on direct polymerization of TA (C&EN, June 24, 1963, page 52). Capacity is about 30 metric tons a day. Toray, Japan's largest maker of syn thetic fibers, also has conventional batch capacity based on DMT (the company is a licensee of Imperial Chemical Industries, Great Britain). Total capacity, fiber and staple, both continuous and batch, is now about 120 metric tons a day. In developing a continuous direct route from TA, Toray has further simplified its direct TA-based process.
Toyo Rayon Pilots Two New Polyester Processes
56
C&EN
APRIL
4,
1966
The firm is somewhat cryptic in de scribing the process, but the high points are these: Following purifica tion, TA, together with ethylene gly col, is fed continuously into an esterifier to form bis-(2-hydroxyethyl) tere phthalate ( B H T ) , in the presence of a catalyst. BHT is then catalytically polymerized to polyethylene tere phthalate ( P E T ) . The molten PET can be extruded and chipped, later remelted and spun. Or it can be fed directly to spinners. Advantages of the process are sev eral, compared to other systems. In conventional processes based on DMT, it's necessary to form DMT from TA, purify the DMT, then recover methyl alcohol formed on condensation of
DMT with ethylene glycol. Direct processes based on TA eliminate these steps. Compared with batch processes, Toray's continuous system gives savings in power and utilities, produces more uniform and stable polymer with better color, and is easier to automate, the firm says. Continuously produced polymers contain about half as many carboxylic end groups and diethylene glycol units as those made in batch systems, the firm notes. And, if tied in directly with spinning, the process eliminates production, storage, handling, and melting of chips. Cuts Costs. Over-all, Toray figures that the continuous direct process cuts costs about 20r/c over those of conventional batch systems based on DMT and "several per cent" compared with a batch process that uses TA directly. But both continuous and batch direct processes require high-purity "fiber grade" TA. The economics of the process hinge on the price of this raw material. When the price of fiber grade TA equals or is less than that of D M T on a weight basis, then the direct process gives lower costs, Toray says. This, the company adds, is now true in Japan. In Japan, DMT now sells for about 27.8 cents per pound. There is now no market price in Japan for fiber grade TA. Toray is the sole bulk buyer and has a long-term contract with Mitsubishi Chemical for its supply at a price "a little lower" than that of DiMT. Mitsubishi Chemical makes the special high-purity TA, using the Henkel process starting from toluene. Mitsubishi Chemical and Toray worked jointly in developing technology for making the material. But Mitsubishi was the major contributor, Toray says. The trickiest step in the continuous direct process, Toray says, is esterifying TA, whiqh is relatively insoluble in ethylene glycol. The problems are mainly ones of engineering. The firm has applied for patents on two types of reactors.
O n e is a p i p e reactor, t h e
other· a tank. Toray won't give details of either type, nor indicate which is superior, but hints that with the pipe reactor, it's not economical to use a screw conveyor over the whole length. To produce higher-molecular-weight polyester, Toray has modified its original continuous process that starts from DMT. As usual, Toray is sketchy in its description. But the main problems apparently are in handling the
high-molecular-weight material and in preventing thermal degradation. At any rate, the firm says it can make polymer that has an intrinsic viscosity as high as 1.0, compared with 0.7 in conventional batch systems. And it can do this at a cost saving of about 207c Biggest potential outlet for the highmolecular-weight polymer is tire cord, where high-tenacity material is needed. Polyester tire cord is still only in the development stage in Japan, and Toray representatives won't hazard a guess as to when commercial production might start. Toray's original work on continuous polyester processes dates back to 1957. This work first bore fruit commercially in 1962, when Toray licensed its continuous DMT-based process to Germany's Farbwerke Hoechst in exchange for know-how on Hoechst's Trevira WA polyester fiber. Toray's plant came on stream the next year. The continuous DMT-based process is centered on three main reaction steps: ester interchange, prepolymer formation, and final polymerization. Molten polymer can be fed directly to spinners or it can be extruded, chipped, and later spun. If direct spinning is used, this continuous process saves about 10% over conventional batch processes, Toray says. Even with chipping, savings are "several per cent/' the firm estimates. DMT Heated. In the first step, Toray heats DMT and ethylene glycol at atmospheric pressure over a catalyst to form BHT. Ethyl groups are exchanged for methyl groups and methyl alcohol condenses out and is recovered. In the second step, BHT is converted into a prepolymer of polyethylene terephthalate. This step takes place under vacuum. In the third stage, final polymerization takes place under high vacuum, producing final polymer of desired molecular weight. Excess ethylene glycol from both the second and the third reactors is recovered. Methyl alcohol is recovered from t h e first unit.
The reactors are quite complex, Toray says. Two major problems that had to be solved were how to feed material continuously into evacuated reactors and how to maintain proper residence times in the second and third reactors. When the continuous polymerizer feeds product directly into spinners, another tricky problem is adjusting flow rate at the spinner head to match spinner capacity.
Three-Method Analysis Identifies Pollutants 151ST
ACS
NATIONAL
MEETING
Water, Air, a n d W a s t e C h e m i s t r y
A three-pronged analytical approach —gas-liquid chromatography, two-dimensional thin-layer chromatography, and fluorometry—is easing the way to a better understanding of urban air pollution. By combining these techniques, analysts can readily zero in on pollutants undetectable or only laboriously found by other means. Until now, inadequate assay techniques have hindered pollution investigations, according to T. W. Stanley and co-workers at the Robert A. Taft Sanitary Engineering Center in Cincinnati. Such investigations are particularly necessary, Mr. Stanley says, because urban atmospheres, especially where industrial pollution is heavy, contain relatively large amounts of alkylated derivatives of polynuclear aromatic hydrocarbons. Alkylation of the tetracyclic aromatic hydrocarbons usually results in carcinogenic derivatives. The problem has been that compositions of column-chromatographed fractions are more complicated than indicated by absorption spectrometry, the conventional analytical technique. Sometimes, Mr. Stanley says, they are even more complicated than indicated by two-dimensional thin-layer chromatography or gas chromatography. Conventional techniques thus showed up poorly when pitted against the combination used in the Taft study. For example, conventional techniques showed no alkyl derivatives in a particular column chromatograph fraction of the study sample— a benzene extraction of airborne particulates collected from about 100 American cities. T h e fraction, h o w e v e r , was further
separated by gas-liquid chromatography. Bands were collected by holding an alumina thin-layer plate close to the vent. The plates were then examined fluorometrically and phosphorimetrically. Identified. On the basis of the fluorometric and phosphorimetric spectra and the retention times, phenanthrene, anthracene, two alkyl phenanthrenes, and 2-methyl pyrene APRIL
4, 1 9 6 6
C&EN
57
