This procedure is used to determine the intrinsic viscosity (IV) of the material. IV is a measure of the Molecular Weight (MW) of PET. The properties of PET are better at higher MW (higher IV) and worse at lower MW (lower IV). Comparing the SIV results from resin and molded parts can provide an understanding of the drying and process conditions.
The test procedure used is ASTM D4603. The solvent system used is 60% phenol/ 40% tetrachloroethane. The PET concentration is 0.50% and samples are analyzed at 30°C. The test can be run on resin, film, preforms and bottles. Corrections are made for samples if they contain >1.% additives or colorants. It is important to know the level of additives present when submitting a sample for analysis. PTI uses Rheotek equipment to measure IV . SIV data reported to the customer will be measured at a single concentration to obtain the inherent viscosity. The intrinsic viscosity is mathematically calculated from the inherent viscosity using the Billmeyer Equation.