PET is a hygroscopic material. Therefore, the absorbed moisture causes significant property deterioration, such as a decrease of tensile strength or a reduced useful life time. Prolonged storage can lead to hydrolysis. Previous studies found that moisture causes rapid hydrolytic degradation which results in a major reduction of the polymer molecular weight. A vast amount of customers store their materials in big bags for a certain amount of time.

The following findings show the relationship of absorbed water, exposure time, and relative humidity conditions in a warehouse. The figure below indicates the moisture absorption of post-industrial regrind (PIR) PET materials, stored at various relative humidity conditions as a function of exposure time. After relatively fast moisture absorption at the beginning of the measurements a plateau can be observed after some time due to an equilibrium state. Moreover, an increase of moisture content with increasing relative humidity and vice versa can be observed.



• After a certain time the data approaches a plateau

• The higher the relative humidity, the higher moisture uptake




Procedures to minimize storage time for post-industrial-regrind (PIR) PET materials:

  • Direct edge trim recirculation
  • Immediate grinding and reuse of skeletons/production scrap (within few hours)
  • In-house storage at controlled ambient conditions of PIR


Positive impacts on processing of PET:

  • Shorter drying times
  • Less energy consumption for drying
  • Increased usage of undried material
  • Higher viscosity (iV) in final product

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