Synthetic polyester yarns have been known and used commercially for several decades, having been first suggested by W. H. Carothers, U.S. Pat. No. 2,071,251 and then by Whinfield and Dickson, U.S. Pat. No. 2,465,319. The polyester polymer that has been used most frequently for commercial purposes is polyethylene terephthalate, made from ethylene glycol and dimethyl terephthalate or terephthalic acid, and these polymeric precursors have been conveniently reacted together commercially by ester interchange or direct esterification, respectively, followed by polymerization, generally in multiple stages, with appropriate provision to remove condensation products, such as water, and also excess ethylene glycol that is preferably recycled with removal of unwanted water and by-products, as appropriate.
Originally, polyester yarns were made by a batch operation, involving several separate processes, first making the polyester polymer, and then melt-spinning the polymer into filaments, and then further processing the filaments into continuous filament yarns (or staple fibers) as described, e.g., by Ludewig in "Polyester Fibers, Chemistry and Technology", first published in German in 1964, English translation by John Wiley & Sons Ltd., 1971, and these various separate processes were themselves sometimes carried out in separate stages, in batches. However, as indicated in the literature, there has always been a desire to economize, and so to couple the various separate stages together. Some manufacturers have operated a wholly continuous process, starting with the polymeric precursors that are reacted together and polymerized to form a polyester polymer melt that is extruded into solid filaments that are processed into continuous (multi-filament) yarns as a wholly continuous process. However, many manufacturers, in various countries, have not changed to a continuous process, because of the problems presented by continuous operations. An objective of the present invention is to solve some of these problems in the production of continuous filaments, especially in the form of spin-oriented feed yarns.
In the early 1970's, there was a radical development in the preparation of filament yarns because of the introduction commercially of high speed spinning, whereby spin-oriented filament feed yarns were prepared by high speed spinning at speeds of the order of 3 km per minute for use as draw-texturing feed yarns (DTFY). This technology has been described in many references, such as by Petrille and by Piazza and Reese in U.S Pat. Nos. 3,771,307 and 3,772,872. The feed yarns have sometimes been referred to as POY (for partially-oriented yarns). Accordingly, during the decades of the 1970's and 1980's, many billions of pounds of continuous filament spin-oriented polyester filaments have been manufactured commercially and used, mostly as feed yarns for draw-texturing, i.e. to prepare textured yarns that are used in fabrics and garments, and, accordingly are dyed to produce attractive colors. Filament yarns are particularly sensitive to any lack of uniformity, that generally shows up as differences in dyeability in the resulting fabrics. Much has been written about the importance of achieving dye-uniformity in fabrics of filament yarns, and accordingly of achieving dye-uniformity in the spin-oriented yarns, and the difficulty of achieving this desirable objective, because of their sensitivity to changes in manufacturing conditions, etc. This concept of controlling dyeability is referred to in the trade by use of "merge numbers", which are assigned by fiber producers to assure customers that any fiber sold under any particular merge number will dye uniformly, i.e. similarly to other fiber sold under that same merge number, within specified limits; in the case of DTFY, it is the draw-textured yarn that is dyed, and the DTFY is merely an intermediate, as indicated by the term "feed yarn" or "feeder yarn", this being well known in the art. Any product that is not "mergible" is generally much less profitable, so an important commercial objective is to increase the yield of mergible product or, in other words, to decrease the proportion that is sub-standard in the sense of not being mergible, i.e. not within the specified limits.
The objective (of achieving a fully continuous operation from the polymeric precursors to the package of filament yarn wound, e.g., on a bobbin) has been made more difficult by the need to achieve dye-uniformity, as explained by Bosley et al, U.S. Pat. No. 4,025,592. Bosley's solution to the problem was to improve dye-uniformity by adding small amounts of diethylene glycol (DEG) so as to achieve a predetermined dyeability referred to as the percent dyeability (of a test sample), being a relative measurement of the K/S factor, as described therein. Bosley described how the downstream dyeability (of textured yarn products prepared from his melt-spun yarns) could be determined periodically so that the amount of DEG added upstream could be adjusted to maintain a substantially uniform dyeability. In Example 1, Bosley described a three vessel polyester continuous polymerization system coupled to two spinning machines, one for low speed spinning to give fully-drawn (150 denier, 34 filament) yarns (of break elongation 31%) by a coupled spinning/drawing process, while the other machine was for high speed spinning to give spin-oriented DTFY (425 denier, 34 filament, break elongation 131%), that was subsequently draw-textured to provide textured yarn (of substantially equivalent 150 denier). Such a technique has been operated commercially for several years to provide spin-oriented feed yarns and/or drawn yarns to give products of controlled dyeability.
However, despite the significant advance achieved by the Bosley process, it has not yet been possible to provide mergible products in every particular case by following Bosley's teaching. Thus, a problem that has existed for several years, and that has defied solution hitherto, has been to provide an improvement over Bosley's technique for controlling dyeability in a continuous process for producing spin-oriented yarns from polymeric precursors. (It should be recognized that a continuous operation, of its very essence, must be operated over a rather long period of several days or weeks, during which period a variety of different products may be required, according to the demands of the market, at any particular time, depending on fashion and other considerations; this is in contrast to a batch system, which can be a custom-made operation.)