TECHNOLOGY
Japanese take new routes to polyester Ethylene oxide replaces glycol in two new processes; one process also takes new approach to purification A dozen years ago Japan built its first polyester fiber plants—using British technology. Since that time, few Japanese industries have shown a more decisive transformation from licensee to innovator. Toray Industries and Teijin, Ltd., the earliest and still the largest polyester fiber makers in Japan, commercialized in 1963 the direct esterification of terephthalic acid (TA) — the first new approach to polyester synthesis since the fiber was introduced. These companies were also in the forefront in developing continuous polymerization and direct spinning techniques. Now another fresh approach is shaping up in Japan. Both Teijin and Toyobo Co., a leading cotton spinner and synthetic fiber maker, will commercialize continuous processes in which TA reacts with ethylene oxide to make bis(/?-hydroxyethyl)terephthalate, the monomer (bis-HET), that yields polyester by a condensation reaction. The upshot of using oxide,
instead of the ethylene glycol now used in bis-HET production, is a faster reaction rate and the ability to build single-train units of larger scale. In another major departure from current practice, Teijin will purify the product rather than the raw material. It will use crude TA for the synthesis, then purify the product bis-HET to fiber grade. The process will go commercial in a 300 metric-ton-per-day of (bis-HET) unit at Teijin's Iwakuni plant during the latter half of next year. Toyobo, which in 1972 will supply a 50 ton-a-day polymer plant by its new bis-HET process, will use fiber-grade TA as feed. Appealing. On paper the ethylene oxide route to bis-HET is appealingly simple: an addition of oxide to TA, yielding bis-HET with no by-product. In fact, given ethylene oxide's reactive nature, controlling the reaction is a problem, and ordinarily there are byproducts in plenty. Most of them are generated by side reactions among
Polyester fiber producers now purify raw material for reaction with glycol to make polyester intermediate
Purification
Purification
ethylene oxide molecules and between these compounds and the acid. The oxide route has been tried before. Nippon Soda Co., for example, announced a process in 1966 to combine ethylene oxide with fiber-grade TA, but it has never been commercialized. Teijin became Nippon Soda's largest single stockholder at about the same time, but says its own new process is completely different from the Nippon Soda version. Teijin's synthesis has been piloted in a 6 metric-ton-a-month unit since 1968. A big factor in controlling the reaction, research and development planning manager Kazuo Ogata tells C&EN's Mike McAbee, is rapid removal of heat of reaction. Choice of catalyst (Teijin uses a nonmetal catalyst) and operating conditions, as well as some points of reactor design, are other factors in raising bis-HET yield. Unreacted TA and ethylene oxide, amounting to 5 or 10r/c of the product stream, recycle to the reactor.
New Japanese processes use oxide for faster reaction, and one purifies product rather than feed
Purificat
Methanol
Fiber-grade TA
Fiber-grade TA
Methanol
Ethylene glycol Note: HerculesWitten process forms DMT directly from pxylene. with no TA step
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C & E N NOV. 23, 1970
Polyester
Ethylene glycol
Purification Condensation
Polyester
Laser photometer finds broad use
Per-pass yield of bis-HET, however, hasn't been disclosed. Toyobo has operated its own oxide route for the past six years, in two successive pilot plants. "The key points in reaction control," managing director Saichi Morimoto says, "were to find a good catalyst and to set the right operating conditions—especially the oxide-to-acid ratio." Toray Industries, meanwhile, is looking at the same approach Teijin has chosen: replacing glycol with oxide and purifying product rather than raw material. Toray is taking a liquid/gas approach, putting TA into solution before reaction. One line of research is to find a solvent that can serve both for the synthesis and for recrystallization of bis-HET in the purification step. Prime factor. Toray won't comment on the status of its own development work, but the planning department's Mitsuo Ishikawa emphasizes that bringing down the cost of purifying bis-HET is the prime factor. Bringing down by-product formation is secondary. "The kind of by-products you get is more important than their quantity," he points out, "because some are more easily separated than others from bis-HET, and this affects purification cost." Whatever route is used, purity is a crucial point in bis-HET production since trace impurities can lower fiber strength and affect its color. Yet "crude" TA (about 99.97^ pure as produced) is a difficult material to refine further—it's not easily dissolved, and rather than melt, it sublimes near its decomposition temperature. Recrystallization is a way out but requires severe operating conditions. Dimethyl terephthalate ( D M T ) , on the other hand, can be purified with relative ease by a combination of distillation and recrystallization. Accordingly, the methyl ester rather than TA itself was the original choice as raw material for bis-HET production, and it's still widely used. It reacts with ethylene glycol in a transesterification that forms both bis-HET and methanol. The techniques developed later to purify TA to fiber grade permit direct esterification with glycol, eliminating the methanol cycle from bis-HET production. But TA purification remains a big part of polymer cost.
Teijin's Kazuo Ogata The et cetera is important
Bis-HET, like DMT an ester of TA, is in theory easier to purify than is TA itself. In practice there are snags. Bis-HET is corrosive and hard to pump. It tends to polymerize during distillation. Teijin wrote off distillation but found a practical alternative. Combination. "We use a combination of methods to purify bis-HET, including recrystallization, et cetera," says Dr. Ogata. "The et cetera," he adds, "is very important." It's also undisclosed. (Teijin has filed patent applications in Japan covering some 100 points related to this process, as well as series of applications in major countries abroad. Some Japanese patents have issued, but none so far covering the purification step.) Compared to direct esterification of fiber-grade TA, Teijin says, its new route will result in lower polymer cost in a full-scale plant. The faster reaction rate makes feasible single-train units in the 150 to 300 metric-ton-a-day range, compared to the 20 to 40 tona-day range the company has found practical for a DMT-based bis-HET unit. Also, in Japan, ethylene oxide is a cheaper feedstock than is ethylene glycol. Teijin claims roughly equal savings in synthesis and purification steps. Toyobo, whose process starts with fiber-grade TA, expects a lc/c saving in polymer cost, compared to the DMT route to bis-HET that it now uses. The fact that DMT and fiber-grade TA routes to bis-HET continue to flourish side by side points up the number of factors other than the process itself that go into choosing a route to polyester fiber. One such point in choosing an ethylene oxide-based process is that bis-HET during polymerization produces ethylene glycol as byproduct. There is no problem if the company operates glycol-based processes as well, or if there is a large merchant market for glycol, but a new liability otherwise.
A new laser photometer has been combined with application development to expand the use of light scattering as a detection and measuring technique. The instrument is the Differential I, and it automatically measures light scattered by microparticles suspended in liquid. Dr. Philip J. Wyatt of Science Spectrum, Santa Barbara, Calif., the instrument's manufacturer, points out that classes of particles have unique light-scattering characteristics which permit deduction of many inherent structural features. The modular instrument incorporates an argon-ion laser, tunable to five wave lengths. The output is planepolarized, enabling studies of light scattering as a function of wave length and polarization. The intensity—1 milliwatt at 5145 A.—is an order of magnitude greater than that of a mercury source, Dr. Wyatt points out. The instrument costs $9750, including laser and recorder. In operation, the monochromatic, highly collimated laser beam illuminates a liquid sample contained in a special cuvette. A photodetector automatically scans the scattered light over preselected ranges between 10° and 170°, a recorder plotting intensity versus angle. Dr. Wyatt stresses the ease of operation compared to manually run instruments with galvanometer indication. Applications. Among uses for the instrument in the physical sciences are determination of colloidal sizes, size distributions, and dielectric structure. The unit is also useful in investigating nucleation and condensation phenomena, and in determining molecular weights, refractive indexes, and polymer structures. In microbiological studies, the instrument can be used to deduce parameters such as average cell size, size distribution, and cell wall thickness. Also, changes in particles induced by heat, chemicals, or impurities are easily monitored. One new application made possible by the instrument's sensitivity and light beam collimation, according to Dr. Wyatt, is determination of the sensitivity of bacteria to antibiotics. Penicillin and its derivatives, for example, cause cell walls of sensitive bacteria to deteriorate. Differential light scattering patterns of bacterial suspensions are extremely sensitive to changes in shape, size, structure, or size distribution, Dr. Wyatt points out. Light scattering can determine sensitivity in minutes, he says, compared to the 24 to 48 hours required to grow bacteria in conventional test methods. NOV. 23, 1970 C&EN 43
