TECHNOLOGY
PPG Offers Modified Polyester Technology Fusion cooking process with glycol recovery system cuts batch cooking time by half and glycol losses by as much as 75%
Pittsburgh Plate Glass is getting set to license a pair of techniques de signed to slash cooking time and raw material losses in making unsaturated polyester resins. Thus, unsaturated polyesters join in the changes affecting the saturateds, as highlighted in the current round of expansions among fiber makers and their suppliers (C&EN, June 21, page 4 6 ) . A fusion cooking process, together with a glycol recovery system, has shortened batch cooking time by half for many resin compositions in PPG's own operations. Glycol losses have dropped as much as 7 5 % . The six polyester plants operated by the firm's
coatings and resins division in the U.S. and Canada are now using these de velopments. Modification of conven tional production units requires little more than piping changes around ex isting equipment. In the cooking operation, polyester makers carry out the first of two poly merizations that produce a thermoset ting resin. A linear condensation polymer is formed in the kettle by re action of unsaturated dibasic acids or their anhydrides with dihydroxy al cohols. Phthalic anhydride or similar dibasic acids are often added as well, to reduce the amount of unsaturation in the resin. For example, in a typical
Sparging Is Continuous In PPG Modification Water vapor and noncondenmbles
Noncondensables
f^=J Ofyeoi
Water/solvent ι azeotrope I and glycol Water and glycol /y Water and "*" lost glycol to sewer 1 Saturated solvent Inert gas
«*•» Solvent (xylene or toluene)
GLYCOL TRAP
ΖφΙ Wuter4ree glycol
«~ Inert gas Sparge ring
POLYESTER KETTLE ; t \
A conventional system for cooking polyester resins uses recirculating solvent to form a low-boiling azeotrope with by-product water, speeding removal of water from the process. The batch is sparged with inert gas near the end of the cooking period to remove the last traces of water
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C&EN
JULY
12,
1965
PPG's fusion cooking system replaces solvent infec tion with continuous gas sparging to remove by-product water during pofyesteri ft cation. Olyco! vaporized with the water is recovered by fractionation. More effective removal of water cuts cooking time
general-purpose formulation, a mole each of maleic anhydride and phthalic anhydride are polyesterified with a little over two moles of propylene glycol. The viscous polymer that results from cooking is blended with an unsaturated monomer, usually styrene, before shipment. The end-user, preparing to add fillers and mold the resin, carries out the second polymerization (curing) by mixing initiators with the resin-styrene blend. This causes the styrene to cross-link the polyester chains by adding to their unsaturation sites. This complex network of linkages gives unsaturated polyesters their thermosetting character. Solvent Process. Polyester cooking, at best a slow procedure, is hastened to completion by continuously removing by-product water. Conventional practice is to add a solvent, such as toluene or xylene, that forms a lowboiling azeotrope with water. This aids in vaporizing most of the water as it forms. The vapor is condensed and separated by layering. Recovered solvent is recycled. Unfortunately, as much as 15% of the glycol in a batch (for a typical resin, the kettle batch may be 40% glycol by weight) boils off along with the azeotrope. When the vapors condensed, this glycol remains in the waste water and is lost. Another drawback is that the recycle solvent is saturated with water after condensate separation. Returning this water to the kettle tends to retard the esterification and adds to cooking time. For many polyesters, 24 hr. cooking time isn't unusual in conventional processing. Another way to remove water during cooking is to sweep it from the kettle with a stream of inert gas. PPG's fusion cooking technique, developed by J. A. Seiner at the company's Springdale, Pa., technical center, replaces the solvent altogether with a fairly large flow of nitrogen, carbon dioxide, or other nonreactive gas. A gas sparging rate of 20 cu. ft. per minute during the main part of the reaction would be about optimal for a 1000-gal. batch, increasing less than proportionately with batch size, according to Mr. Seiner. PPG has found that high-volume gas sparging removes water more effectively than does either conventional solvent cooking or sparging with vaporized solvent—another possibility
the company investigated. Thus, the fusion cooking method, described in U.S. patents 3,109,831 through 3,109,834, speeds the reaction markedly. The usual batch time of 15 to 16 hr. for cooking a typical phthalate resin, for example, has been cut to about six hours with fusion cooking. Since resin-styrene blending is a faster operation than is cooking, time saved in the kettle is reflected throughout the production schedule. Fusion (solventless) cooking has been used before. It was, in fact, one of the earliest commercial methods for polyester production. The problem was glycol loss: High gas rates would sweep out substantial quantities of these volatile ingredients along with the water being formed. Since low gas rates meant very long cooking times, these early fusion methods were supplanted by solvent cooking. The latter offered more attractive production rates, though glycol losses were still fairly high. Fractionation. PPG gets around the glycol loss obstacle by coupling fusion cooking with a glycol recovery technique. The main departure from conventional solvent recovery is to maintain the unit's vapor condenser at a temperature above the boiling point of w a t e r - u p to about 240° F. It thus becomes, in effect, a partial condenser, allowing water vapor to escape with the inert gas stream but condensing glycol vapors. Glycol trickles back into the batch through the kettle's vapor pipe, flowing countercurrent to vapor rising toward the condenser. The latter is close to batch reaction temperatureabove 400° F. during the final stages of cooking. The vapor strips any remaining water from the recovered glycol. With this recovery system, PPG has been able to operate with a smaller excess of glycol per b a t c h perhaps 5% less glycol in a batch in a kettle. If the system were made part of a new plant design, it would likely include a small fractionation column, followed by a partial condenser, to recover glycol. An existing unit can often be converted by altering piping around the kettle and condenser, PPG says. One of its own plants was modified in only two working days. Its Circleville, Ohio, unit, which doubled PPG's polyester capacity upon completion last year, incorporated the fusion process and glycol recovery method in its design.
Automation Cuts X-Ray Analysis Time Automated diffractometers, now available commercially, are becoming routine What is probably one of the greatest time-savers to come along in instrumental analysis—the automated x-ray diffractometer—is on the verge of becoming a routine tool. Several instrument manufacturers are now making the diffractometers commercially available, after hovering for several years on the edge of the market with experimental models. What's more, an increasing number of crystallographers at university, industrial, and government laboratories are ordering them. During the past year nine such instruments made by Charles Supper Co. (Watertown, Mass.) and automated by Pace Controls Corp. (Needham Heights, Mass.) have been installed to join two already in use, and there is a considerable backlog of orders. Picker X-Ray Corp. has installed five of its new automated units since March. The White Plains, N.Y., company is now delivering at a rate of more than one instrument per month and has a backlog of about a dozen orders. Philips Electronic Instruments (Mount Vernon, N.Y.) will install its first production units this summer and also has a backlog of about a dozen orders. General Electric is promoting a system and says it already has several in the field. Until about last year the only automated x-ray diffractometers operating in this country were one of two imported instruments, the handful of experimental models made by potential commercial builders, and a few units built by individual research workers. If there is one virtue that x-ray crystallographers have had to practice until now, it is patience. Although much of modern structural chemistry is based on their work, and although they have elucidated the structures of hemoglobin, vitamin B 1 2 , and other exceedingly complex compounds, x-ray crystallographers have always been handicapped by the tedium of obtaining data with manual x-ray diffractometers. Many studies have literally taken years. But now it seems that these bottlenecks are about to be broken. All of the new automated instruments cut data collection time by as much as 95%, compared with manual methods. JULY
12, 1 9 6 5
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