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Processing PET is not as simple as polyolefins, since the hygroscopical moisture in PET leads to an iV loss during extrusion. In turn, this results in a loss of mechanical properties, such as brittleness and low strength of the final product. As a consequence it is important to know the required iV of the final product. The iV loss in the melt phase is primarily influenced by the moisture content of the input material.
During Solid State Polycondensation (SSP), the molecular weight of the polyester is increased. This increase is related to a chemical process where “short” PET molecules are forming longer molecules by separation of ethylene glycol under the influence of temperatures higher than 160–170 °C, but still in the solid phase of the material. Because of the oxidation of PET at temperatures higher than 160–170 °C, which is cracking the molecule chains, SSP has to be done in absence of oxgen, commercially at temperatures higher than 190 °C.