The raw materials to make PET are monoethylene glycol (MEG) and purified terephthalic acid (PTA) or dimethyl terephthalate (DMT). PTA is the preferred feedstock for process economic reasons although DMT may be favoured in polyester film applications due its adhesion addition quality. The main process steps are raw material preparation, esterification/transesterification, pre-polycondensation and polycondensation.
The PTA is mixed with the MEG and catalyst solution in a paste and fed into the esterification system. The esterification takes place under atmospheric pressure by splitting off the water.
When DMT is used, the DMT melt and catalyst are pumped with MEG into the transesterification system, where the reaction takes place under atmospheric pressure by splitting off the methanol. The materials split off are rectified and recovered.
The product from the esterification/transesterification step is then sent to the pre-polycondensation unit where the reaction takes place under a vacuum. The pre-polycondensation product is fed to the final polycondensation reactor which operates under increased temperature and vacuum. The polyester melt is processed either into fibres/filaments or sent to the solid state polycondensation (SSP) unit to make bottle grade chips.
Researchers have been looking for ways to remove the SSP stage. For example, Lurgi Zimmer has developed a direct process for making the PET bottle preforms without the SSP step. It is based on an integrated process that produces a high viscosity melt from which the chips can be fed directly to the preform unit.
DuPont, in alliance with Fluor Daniel, has developed the NG3 process which is claimed to reduce the number of steps from six to four and to lower capital costs by 40% and overall manufacturing costs by 10-15%. Designed to produce PET resins for the bottle resin market, the process employs a pre-polymerisation step that allows the melt phase to operate under positive pressure, eliminating the need for a vacuum system. The particle formation steps simultaneously form and crystallise the low molecular weight intermediate pellets. The approach used eliminates the finisher in the conventional melt process and one crystallisation stage in the SSP step.
Eastman has developed the IntegRex process which integrates the PX-to-PTA and PTA-to-PET processes. Costs are saved by eliminating steps, such as the hydrogenation in the PTA process and solid stating step, as well as in-process storage stages. The process was commercialized in 2007 in a plant in South Carolina that was claimed to have three times the capacity in half the footprint of conventional PET technology.
Synetix has developed a new titanium catalyst that can replace antimony-based catalysts and works in both the esterification and polycondensation processes. In batch systems, the Synetix catalyst is claimed to effectively increase plant capacity by 15%.
Polyethylene terephthalate
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Polyethylene terephthalate
Uses and Outlook
Polyethylene terephthalate (PET) exists both as an amorphous (transparent) and a semi-crystalline (opaque and white) thermoplastic, and can be made into either a resin or a film. The semi-crystalline PET has good strength, ductility, stiffness and hardness while the amorphous PET has better ductility. PET also has good processability and can be recycled for other applications such as polyester fibres.
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