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  Direct coupling of melt polymerization and solid state processing for petUSPTO Application #: 20070187860Title: Direct coupling of melt polymerization and solid state processing for pet Abstract: Strands of molten polyethylene terephthalate (PET) from a PET polycondensation reactor are solidified, pelletized, and cooled only to a temperature in the range of 50° C. to a temperature near the polymer Tg by contact with water. The still hot pellets are conveyed, optionally followed by drying to remove water, to a PET crystallizer. By avoiding cooling the amorphous pellets to room temperature with water and cool air, significant savings of energy are realized. (end of abstract)
Agent: Dennis V. Carmen Eastman Chemical Company - Kingsport, TN, US Inventor: Richard Gill Bonner USPTO Applicaton #: 20070187860 - Class: 264143000 (USPTO) Related Patent Categories: Plastic And Nonmetallic Article Shaping Or Treating: Processes, With Severing, Removing Material From Preform Mechanically, Or Mechanically Subdividing Workpiece, To Form Particulate Product (e.g., Flakes, Etc.), Subsequent To Extruding Step, From Strands The Patent Description & Claims data below is from USPTO Patent Application 20070187860. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED CASES [0001] This application is a continuation of U.S. application Ser. No. 10/663,856 filed Sep. 16, 2003. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention pertains to the commercial manufacture of polyethylene terephthalate ("PET") polymers. [0004] 2. Background Art [0005] PET has numerous uses, principle among which are for films, fibers, and food containers. Despite the stringent matrix of properties required for such uses, particularly for food packaging, some PET has become a commodity polymer. Commercial production of PET is energy intensive, and therefore even relatively small improvements in energy consumption are of considerable commercial value. [0006] The production of PET (inclusive of copolymers) begins with an esterification step where the dicarboxylic acid component, predominantly terephthalic acid, is slurried in ethylene glycol and heated to produce a mixture of oligomers of a low degree of polymerization. This "esterification" step may be followed by a further "oligomerization" or "prepolymer" step, where a higher degree of polymerization is obtained. The product still has a very low molecular weight at this stage. [0007] The previously described steps are then followed by a polycondensation. The polycondensation is catalyzed by metal compounds such as Sb, Ti, Ge, Sn, etc. Polycondensation occurs at relatively high temperature, generally in the range of 280-300.degree. C., under vacuum, water and ethylene glycol produced by the condensation being removed. The polymer at the end of polycondensation has an inherent viscosity generally in the range of 0.4 to 0.65, corresponding to a molecular weight too low for many applications. [0008] Commercial production of PET polyesters has required a subsequent post-polymerization in the solid state, termed "solid stating." In this stage of the process, the PET granules are heated in inert gas, preferably nitrogen, at temperatures below the melt temperature, i.e. from 210-220.degree. C. in many cases. Solid stating is complicated by the fact that most PET polymers, following extrusion from the melt and pelletizing, are substantially amorphous. In order to prevent the pellets from sintering and agglomerating in the solid stater, the pellets are first crystallized over a period of 30 to 90 minutes at a lower temperature, e.g. 160-190.degree. C., typically in a flow of inert gas or air. It should be noted that "so lid stating" herein refers to the solid state polycondensation per se, and not to the combined processes of crystallization and solid state polycondensation. These procedures are well known to those skilled in the art, as evidenced by U.S. Pat. Nos. 5,597,891 and 6,159,406. [0009] In the conventional PET process, the polymer is extruded directly from the polycondensation reactor into strands. The hot, extruded strands are contacted with cool water prior to chopping into pellets, dried, and stored into silos prior to crystallizing. Conventional pelletizing processes as well as a pelletizing process wherein strands are stretched prior to pelletizing are disclosed in U.S. Pat. No. 5,310,515. Conventional wisdom dictates that at least the surface of the pellets must be cooled to 20.degree. to 30.degree. C. to avoid sinteling during storage. During storage, heat from the hotter interior of the pellets is distributed throughout the pellets. Thus, warm pellets, i.e. pellets whose exterior is significantly higher than 20-30.degree. C. might agglomerate during storage following temperature equilibration. In addition to the decrease in temperature brought about by contact with water, the pellets can be further cooled to the desired temperature with cool air or nitrogen. The pellets are stored, and then subsequently reheated to the desired crystallization temperature. These steps of heating, cooling, and reheating entail a significant energy penalty in an already energy intensive process. SUMMARY OF THE INVENTION [0010] In the present invention, PET pellets from the polycondensation reactor are cooled only to a temperature below the glass transition temperature of the particular polymer or copolymer, and at or above 50.degree. C., and held within this temperature range up to entry into the crystallizer. Despite the higher temperature of the feed pellets, agglomeration does not occur. BRIEF DESCRIPTION OF THE DRAWINGS [0011] FIG. 1 illustrates the prior art process of PET production from polycondensation through solid stating. [0012] FIG. 2 illustrates one embodiment of a subject invention PET process from polycondensation through solid stating. [0013] FIG. 3 illustrates yet another embodiment for the subject invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) [0014] The esterification, oligomerization, and other process steps up to and including polycondensation may be performed conventionally or by any process where pellets are produced from a polymerization melt. The improvement provided by the subject invention takes place during and/or following pelletization, and through the crystallization stage. [0015] The PET polymers are conventional, and are polymers prepared from terephthalic acid and ethylene glycol. While dimethylterephthalate may in principle be used as well as terephthalic acid, use of the latter is preferred. In addition, the PET polymers may contain up to 20 mol percent, preferably up to 10 mol percent, and more preferably no more than 5 mol percent of dicarboxylic acids other than terephthalic acid, and the same mol percentages of glycols (diols) other than ethylene glycol. [0016] Examples of other suitable dicarboxylic acids which may be used with terephthalic acid are isophthalic acid, phthalic acid, naphthalene dicarboxylic acids, cyclohexane dicarboxylic acids, aliphatic dicarboxylic acids, and the like. This list is illustrative, and not limiting. In some cases, the presence of minor amounts of tri- or tetracarboxylic acids may be useful for generating branched or partially crosslinked polyesters. Isophthalic acid and naphthalene dicarboxylic acids are the preferred dicarboxylic acid when mixtures of acids are employed. [0017] Examples of diols other than ethylene glycol which may be employed include, but are not limited to, 1,2-propane diol (propylene glycol), 1,3-propane diol (trimethylene glycol), diethylene glycol, triethylene glycol, dipropylene glycol, 1,4-butane diol, 1,6-hexanediol, cyclohexane diol, neopentyl glycol, and cyclohexanedimethanol. Preferred glycols other than ethylene glycol include diethylene glycol, and most preferredly, cyclohexanedimethanol ("CHDM"), the latter generally used as a mixture of isomers. In addition, polyols such as pentaerythritol, glycerine, and trimethylolpropane may be used in most minor quantities when branched or partially crosslinked polyesters are desired. Most preferably, only difunctional carboxylic acids and difunctional hydroxyl-functional compounds (glycols) are employed. The subject invention process is also applicable to other polyesters wherein pellets formed from the melt are amorphous. [0018] In the description which follows, reference to equipment such as extruders, pelletizers, mechanical dryers, crystallizers, and to the process steps performed therein, are conventional unless indicated otherwise. Pelletizers are available commercially from firms such as Reiter Automatic Apparate-Maschinenbau GmbH, Germany, and Gala Industries, Eagle Rock, Va. Pelletizers, for example, are described in U.S. Pat. Nos. 4,123,207; 4,500,271; 4,728,276; 5,059,103; 5,310,515; 5,403,176; and 6,551,087; while a variety of mechanical dryers are disclosed in U.S. Pat. Nos. 4,447,325; 4,565,015; 5,638,606; 6,138,375; and 6,237,244. All foregoing patents are incorporated herein by reference. [0019] A conventional PET process is shown in FIG. 1. In FIG. 1, the PET polymer 1 is polycondensed in the melt at about 285.degree. C. in polycondensation reactor 2. The polymer is pumped through outlet 3 to extrusion die 4 through which the molten polymer, still very hot, exits as a plurality of strands 5. Below the die may be a grooved plate 6, the extruded strands following the grooves. Cool water 7 is directed over the strands and the plate, cooling the strands rapidly, e.g. to a surface temperature in the range of 75.degree. to 150.degree. C., following which the strands enter a pelletizer 8, which chops the strands into pellets 9 several mm in length. The still warm pellets fall into a moving stream of cool water, generally at 20.degree. C. to 30.degree. C., in conduit 10, which conveys them to a mechanical separator 19, i.e. a screen, and by air supplied through line 13 or by mechanical means, into dryer 12. Continue reading... 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