Polyethylene terephthalate (PET) may be produced from ethylene glycol and either dimethyl terephthalate (DMT) or terephthalic acid (TPA). High purity is required of all raw materials. In either case, the first step of the reaction is the formation of a prepolymer, bis-hydroxyethyl terephthalate (bis-HET). Subsequent polymerization of this material (with the removal of ethylene glycol) forms the polymeric polyethylene terephthalate. The extent of polymerization (apparent from the molecular weight of the polymer) is a function of the polymerization conditions and significantly affects the properties of the resin that is produced. As the polymer grows in length, both molecular weight and viscosity of the reacting mass increase; thus intrinsic viscosity (IV) is frequently used as a measure of polymer molecular weight. When very high molecular weights are desired, as is the case for bottle-grade PET resins, the polymerization may be carried out in stages, with different reaction conditions being utilized in each stage. Until the mid-1960s, DMT had been the preferred feedstock for PET manufacture, partly because the ester could generally be made in purer form than the acid. With the development of high-purity TPA processes, notably by Amoco, the free acid gained acceptance and is now the preferred feedstock. The use of high purity TPA (PTA) eliminates the need to recover or recycle methanol and has the added advantage that esterification to the prepolymer is considerably more rapid than the transesterification reaction, which is the first step when starting from DMT. When starting with terephthalic acid (TPA), the first step in the polymerization sequence is an esterification rather than a transesterification:
 The major breakthrough in the technology of this reaction involved operating at pressures above atmospheric and temperatures greater than the normal boiling point of glycol, to achieve shorter reaction times. Molar glycol to TPA ratios of 1.1:1 to 2:1 are used (presumably some polymerization occurs accounting for a molar ratio of less than 2.0:1). Reaction temperatures range from 258°C to about 263°C. Pressures are below 25 psig, and the water of reaction is removed from the system through a reflux column. Industrial use of these high temperatures and super atmospheric pressures is now almost universal. Whether TPA or DMT is the starting material, the second step in the polymerization sequence, polycondensation of bis-hydroxyethyl terephthalate, is the same.
 When the polymer is to be used for fiber, its molecular weight should be between 14,000 and 20,000. The reaction temperature must be above the melting point of the polymer (260-265°C) and below the temperature at which decomposition occurs too rapidly (300°C), so that temperatures between 275°C and 290°C are favored for polycondensation. The removal of glycol vapors (under vacuum) drives the equilibrium toward polycondensation. The partial pressure of glycol over the polymer melt must be reduced to less than 6 mmHg if useful molecular weights are to be obtained. Because the diffusion rate of by-product ethylene glycol from the molten polymer is rate limiting near the end of the reaction, the ethylene glycol must be separated as quickly as possible. This is accomplished by high vacuum and by mixing the melt so as to continuously expose a large amount of surface. The time of the reaction is at least two hours at 290°C, depending upon the type of reactor used. The properties of PET are set mainly by the degree of polymerization, as indicated by the molecular weight or intrinsic viscosity (IV) of the resin. During the polycondensation of PET, the viscosity of the melt increases continuously as the molecular weight increases. Eventually, however, the increase in viscosity ceases as thermal degradation of the polymer begins. The commercial processes for the production of fiber-grade PET produce polymers with IVs of 0.50 to 0.65. These materials are usually spun directly to fiber, but some may be converted into fiber-grade chips by strand pelletizing or band casting. The barrier resins suitable for biaxially-oriented bottles must have an IV of at least 0.72, if injection-molded preforms are used, or an IV of 1.04 if extruded preforms are used. Polyethylene terephthalate can be polycondensed in melt-stirred autoclaves up to an IV of 0.6; in special heavy-duty reactors, IVs of up to 0.85 can be achieved without significant thermal damage. However, for bottle-grade resins (which require an IV of over 0.72 and a polymer free from residual color and taint), variations of polymerization processes have been developed, which operate in the solid state. The reaction kinetics are determined by the partial pressure of the ethylene glycol and by the temperature. DuPont's NG3™ process is a departure from conventional polyester technology and is built around the novel use of a rotoformer in place of conventional polymerization reactors. Rather than relying on the melt plant to produce an intermediate with a high IV (typically around 0.6-0.65), NG3™ produces a low IV intermediate and then carries out most of the polymerization in the solid state. The novel feature of the technology is the formation of pastilles, a form of polymer solid particle that can be further polymerized in the conventional solid-state polymerization plant. The pastilles are formed from low molecular weight polymer (degree of polymerization 20-30). This particle formation technology imparts a unique crystalline structure to the pellets providing the required strength for further processing in the SSP. This crystal structure is the basis of composition of matter patents that have been issued for the NG3™ intermediate and final products. This crystal structure effectively reduces equipment and time required for crystallizing and conditioning the particles prior to entering the SSP. The NG3™ process utilizes a two-vessel continuous polymerization vessel arrangement. The NG3™ process eliminates the need for extensive overhead condensing and vacuum systems and the associated inherent difficulties in handling high viscosity polymer. Due to the two-vessel system the NG3 process is claimed to offer significant capital and operating cost savings. This new PERP report compares PET production economics via conventional technologies and DuPont's new NG3™ process. In addition, Nexant's market forecast for PET is provided. | 
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