Melt transesterification process

Researchers at GE and Bayer independently developed commercially feasible synthetic processes for BPA-PC in the 1950s and began commercial production in the early 1960s. Bayer was awarded the U.S. patent for PC produced via the interfacial polymerization process and GE the U.S. patent for the melt transesterification process. However, until recently, the major part of BPA-PC was produced via the interfacial process. Further information on the history of PC development can be found in previously published reviews. 

Polycarbonates of numerous bisphenols have been extensively studied. However, most commercial polycarbonates are derived from bisphenol A. At first, both direct-reaction and melt-transesterification processes were employed (Figure 4). In direct-reaction processes, phosgene reacts directly with bisphenol A to produce a polymer in a solution. In transesterification, phosgene is first reacted with phenol to produce diphenyl carbonate, which in turn reacts with bisphenol A to regenerate phenol for recycle and molten, solvent-free polymer. Transesterification is reported to be the least expensive route. It was phased out, however, because of its unsuitability to produce a wide range of products. 

In the melt transesterification process, bisphenol A and diphenyl carbonate are heated to high temperatures... 

PCs of numerous bisphenols have been extensively studied. However, most commercial PCs are derived from bisphenol A (BPA) and is depicted in Fig. 1.9. Both solution and solvent free, melt-transesterification processes are used to manufacturer polycarbonates. 

The historical direct reaction route, which utilised phosgenation of a solution of BPA in pyridine, proved inefficient commercially because of the need for massive pyridine recycle. Calcium hydroxide was used as an HCl scavenger for a period of time. In the historical transesterification process, BPA and diphenyl carbonate are heated in the melt in the presence of a catalyst, driving off by-product phenol, which is recycled to diphenyl carbonate. Using a series of reactors providing higher heat and vacuum, the product polymer was eventually produced as a neat melt. 

Transesterification. There has been renewed interest in the transesterification process for preparation of polycarbonate because of the desire to transition technology to environmentally friendly processes. The transesterification process utilizes no solvent during polymerization, producing neat polymer direcdy and thus chlorinated solvents may be entirely eliminated. General Electric operates a polycarbonate plant in Chiba, Japan which produces BPA polycarbonate via this melt process. 

PCT, PETG, PCTG and PCTAs can all be prepared readily via standard melt-phase poly condensation processes [34, 35], The diacid can be delivered via transesterification of the dimethyl esters or via direct esterification of the diacids. Numerous conventional catalyst and catalyst combinations can be employed. The use of a catalyst or catalyst combination is important for the manufacture of polyesters via the melt-phase process and has been well reported in the literature [36-41], Appropriate catalyst systems enable the production of polyesters with high processing rates and high molecular... 

The melt polymerization process involves the base-catalyzed transesterification reaction of BPA with diphenyl carbonate (Fig. 8). A small amount (less than 0.01% molar) of basic catalyst such as Na, Li, K, or tetralkylammonium hydroxide or carbonate is used during the initial stages of the reaction. The reaction is performed under vacuum at 180-300°C. At later stages of the reaction, the temperature and the vacuum are increased (less than ImmHg) to remove phenol and drive the product to high molecular weight. Subsequently, the polymer becomes very viscous, and special devices, such as devolatilizing extruders, are required to ensure complete removal of phenol. 

Poly(ethylene terephthalate) (PET) which is manufactured by a stagewise melt polymerization process consisting of transesterification, prepolymerization and finishing polymerization steps, is one of the fastest growing thermoplastic polyesters used extensively for fibers, films, bottles, injection molded parts and other products [1], Considerable scientific effort has been made to elucidate its properties. Important aspects can be studied using solid-state NMR spectroscopy to determine morphology, orientation and mobility in the bulk material. 

Low molecular mass linear and branched polyester resins are produced in a one-stage process at 125-240 C. The volatile condensation products are removed in vacuo (melt condensation process) or by passing a stream of inert gas through the resin melt (gas stream condensation process). Polycondensation in solution with azeotropic removal of water by solvent distillation (azeotropic process) is of lesser importance. High molecular mass copolyesters are produced in two stages as is used for poly(ethylene terephthalate). A precondensate is first obtained by transesterification of dimethyl terephthalate with an excess of diols. In the second stage, the molecular mass of the precondensate is adjusted to the desired value by polycondensation in special reactors with the maximum possible elimination of water and excess diols in vacuo at ca. 250 C. 

Based on this work on aromatic polyesters, Schnell et al. [30] and Fox [31] independently prepared hnear, bigh-melting, bigh-molecular-weight aromatic polycarbonates in 1953—1954 that were derived from 4,4 -dihydroxy-diphenylalkane monomers. These aromatic polycarbonates could be prepared either (1) by a two-phase interfacial method (a modified Schotten-Baumann reaction) or (2) by a melt transesterification (monomers-as-solvent) process using diphenyl carbonate [32]. Versus earher aUphatic polycarbonates, the aromatic polycarbonates were unique in tbat they could be made into water-clear (colorless) transparent structures tbat possessed excellent long-term mechanical properties. 

It is not surprising, given the concurrent research and commercialization efforts that were focused on bringing BPA-based polycarbonates to the market in the late 1950s, that a patent interference arose. Bayer was eventually awarded the first U.S. patents on BPA-based polycarbonates [35] and for the interfacial method of production. GE was issued a patent covering the melt transesterification production process [36]. 

Although there is an extensive volume of work on the polymerization of polycarbonate and polycarbonate copolymers, today there are only two basic commercial condensation processes used to produce the majority of commercial product interfacial and melt transesterification. For a more in-depth review of these two processes as well as others that are not commercially significant, the following resource is recommended [49]. 

From a manufacturing standpoint, the interfacial process is capital-intensive to purify the resin solution, isolate and dry the resin, and recycle solvents and brine. With melt transesterification, because it is a solventless process, the only recycle streams that must be dealt with are those related to the recovery of phenol for reuse in the production of DPC. Hence, there is no need to invest in solvent recovery infrastructure with the melt process, and polymer purification units and dryers can likewise be avoided. However, these investments are somewhat diminished by the investment required for the preparation and purification of DPC. 

From a product standpoint, the major benefit of the melt transesterification approach is that it produces resin with a most probable (thermodynamic) molecular weight distribution. Hence, under normal processing conditions, the anhydrous resin exhibits no tendency to change molecular weight or the melt flow index (MFI) (via redistribution). 

The polymer is exposed to an extensive heat history in the melt process. Early work on transesterification technology was troubled by thermal-oxidative reactions of the polymer, especially in the presence of basic catalysts (8-11). Early polycarbonates prepared by Fox and others via the melt process had noticeable brown colors. More recent work on catalyst systems, more reactive carbonates, and modified processes have improved the process to the point where formation of color and product decomposition can be effectively suppressed. Polymers with color at least as good as interfacially prepared materials can now be prepared commercially. One of the key requirements for the transesterification process is the use of clean starting materials. Methods for the purification of both BPA and diphenyl carbonate have been developed and patented. Activated carbonates that form high molecular weight polycarbonate at equilibrium in solution at or below room temperature have also been reported, although they are chiefly only of academic interest (66,67). 

Li and co-workers [63] studied the crystallisation and melting behaviour of poly(]3-hydroxybutyrate (P-HB)-co-P-hydroxyvalerate (P-HV)) and a blend of poly(P-HB-co-P-HV)/polypropylene carbonate (30/70 w/w) using DSC and FT-IR spectroscopy. Transesterification occurred between poly(P-HB-co-P-HV) and polypropylene carbonate during the melt blending process. During crystallisation from the melt, the crystallisation temperature of the blend decreased by 8 °C compared with that of neat poly(P-HB-co-P-HV) and the melting temperature decreased by 4 °C. This indicated that the presence of polypropylene carbonate reduced the perfection of the poly(P-HB-co-P-HV) crystals, inhibited by the crystallisation of poly(p-HB-co-P-HV) and weakened its crystallisation ability. The equilibrium melting temperatures of... 

The molecular dynamics of similar chemical structure blends of poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) were investigated using the TSDC technique [74]. Transesterification reactions between the neat components developed during the melt-mixing process. When the a-relaxation processes of the reactive blends were analyzed into their elementary modes by means of relaxation map analysis, the activation energies of the a-relaxation process were found not to be significantly affected by the transesterification reaction. However, the polarizability of the blend was considerably decreased as the PEN content increased, due mainly to the increased stiffness of the polymer backbone. 

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