Melt Intrinsic Viscosity (IV)

Purpose

This test measures the melt viscosity of the PET sample and calculates the Intrinsic Viscosity value by correlation of the MIV to known SIV values. Standard curves are created for specific resins. It is best to measure a given company's resin using a calibration curve created from a range of IV samples made by that manufacturer. If a calibration curve for a specific resin is not available a "generic" calibration curve can be used to predict the IV result.

Description

Resin samples can be analyzed directly, but preforms and bottles must first be ground to a particle size that can be loaded into the rheometer. All samples are dried overnight to remove water. The test is run by packing the rheometer melt chamber quickly with the dried PET sample under a stream of nitrogen. After a preset heat soak time, the capillary rheometer is started causing the resin to be pushed through a small orifice. The force required to make this happen is recorded. After each sample has been pushed through the orifice, the melt chamber is cleaned and reloaded from the same sample lot. This process is repeated a total of four times with data being obtained from the last three loadings. The average of these three force readings is then used to predict the IV using the calibration chart. The result is also corrected for the residual amount of moisture left in the samples at the time the test is run.

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