The length of the polymer chain on PET (click here to find more about PET plastic) determines the weight of the material and the physical properties that make PET useful as a material packaging. Intrinsic Viscosity (IV) is a measure of the molecular weight of the polymer and reflects the melting point, crystallinity and tensile strength of the material. The value of IV as part of the specification to determine the grade of PET to be used.
High IV values in PET are used in beverage bottles, carbonated beverage bottles, cosmetic bottles. Low IV values can be used in fiber and textile industries. PET waste with high IV values can be recycled for use as a fiber material or strapping band.
PET is highly hygroscopic and absorbs moisture from the surrounding air. A small amount of water vapor will hydrolyze PET in the melting phase, thereby reducing the molecular weight. Hydrolysis can occur due to moisture and this can often be seen as an IV reduction (Intrinsic Viscosity) of the product.
Each time PET is recycled, the value of IV will decrease. So that the virgin resin PET bottle that has been recycled and formed into a PET bottle again, will not have the same IV as before. That’s why sometimes a few percent of virgin resin is added when recycling PET to increase the value of IV.