In new plants direct esterification is preferred method of PETP production. The advantages of direct esterification are,
1) The higher reaction rate
2) The lower weight of T A compared with DMT (storage cost)
3) The use of water instead of methanol as condensation agent
4) No transesterification catalyst required
5) Higher molecular masses are obtained
Temperatures of around 220 – 260°C are generally maintained to overcome the lower solubility of TA The water formed is continuously removed by distillation.
Esterification is followed by gradual pressure release, the temperature is simultaneously increased and excess ethylene glycol is distilled off The formation of PETP takes place in the polycondensation stage similar to DMT process.
Various types of reactors such as rotating disc contactors, wiped film reactors, partially filled screw extruders have been developed as finishers for polycondensation reaction. Consideration of limitations of mass heat transfer is very important in polycondensation reaction leading to the formation of PETP.
The ester interchange reaction is operated at 150 – 210°C at atmospheric pressure. The methanol and ethylene glycol emerging from the reactor are passed through rectifying (distillation) column and EG is fed back to the reactor. After a conversion of 90 – 95%, the reaction mixture is passed on to the polycondensation stage.
In the polycondensation stage, the reaction temperature is raised to 265 – 285°C to keep the reaction mixture molten and polymerization fast For PETP production, a dual catalyst system in which one component is specially active for ester interchange and the other for polymerization is used The production of high molecular mass polymer requires the complete removal of ethylene glycol, and therefore a vacuum is applied. Especially in the final stage of the polycondensation reaction, a very high vacuum is required since the reaction system becomes highly viscous.
Melt polycondensation of PETP is not generally carried out beyond particular extent of polymerization since the degradation reactions dominate the process and the product quality may suffer from undesirable byproducts. To attain higher molecular masses, the products may be subjected to solid-state polymerization.
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The production of high molecular mass polyesters differs from that of polyamides. In the case of nylons, the chemical equilibrium favors the polyamide under polymerization conditions. With polyester formation, however, the equilibrium is much less favorable In order to drive the reaction in the forward direction, the condensation product must be removed continuously, usually by application of vacuum For polyester reactors, a high vacuum, a high temperature, and a high interfacial area with sufficient surface renewal are required
Generally thermoplastic polyesters are produced in two stages In the first stage a polyester precondensate is produced by transesterification of dicarboxylic diesters or esterification of dicarboxylic acid HOOC – X – COOH with exess dihydric alcohol HO – Y – OH. The precondensate then reacts in a second step with the elimination of dihydric alcohol to form high molecular mass polyesters (> 10000).
There are two major routes to synthesize PETP industrially. The objective in each case is to obtain an intermediate product – Bis(hydroxyethyl)terephthalate (BHET).
Two major routes to synthesize BHET are,
1) Ester interchange of dimethyl terephthalate (DMT)
2) Direct esterification of terephthalic acid (T A)
The second stage of polymerization of BHET can be represented as,
The direct esterification of terephthalic acid (T A) and ethylene glycol was generally not preferred earlier because of the difficulties in the purification of TA due to its low solubility and high melting point. However, with improvements in technology, the direct esterification method has been gaining importance. The process is claimed to give polyesters with superior quality.[ad#aryshi1]
