Monday, January 31, 2011

Real Time IV Measurement? Nah!

This blurb produced a spontaneous eyebrow lift for me. How about you?
"The unique technology represented in PTi's real-time IV Monitoring System is based on precise measurement of melt temperature and capillary piping pressures such that the viscosity and subsequent IV can be accurately calculated. These results are benchmarked against the ASTM D4603 Solution Test Method for measuring IV and possess an accuracy of +/- 0.02 dl/g."
I.V. measurement (I.V. meaning inherent viscosity, not be confused with intrinsic viscosity [*]) involves dissolution of the PET into a solvent at high temperature and then measuring the drain time of the solution through a glass capillary. The whole process takes quite some time. While I don't doubt that this new technique can make viscometric measurements on a PET melt and correlate them to IV, call this Real Time IV Monitoring is factually incorrect. A correlation to another measurement is never the same as the other measurement, and like all correlations, is fraught with potential errors being introduced by uncontrolled variables.

To me it would be far more helpful to look at the raw data that is actually being measured and track/monitor that. There would be a significant learning curve to understanding the data, and PTi has certainly gone through it. When multiple input measurements are being reduced to a single output, there will be situations were increases in one or more variables are offset by decreases in one or more other variables, all of which makes me more leery of the IV output. If you are measuring multiple inputs, why not study all of them? It's like working with just the average of a distribution.

I also find it strange that it is the IV that is being correlated since it has no scientific basis unlike the intrinsic viscosity. Well, maybe not. MFI is an awful test with no scientific basis, and yet is still used to buy and sell billions of pounds of polymer a year, so why should IV be any different?

[*] Don't worry, IUPAC nomenclature is far worse. Inherent viscosity is called logrithmic viscosity (not so bad), but the intrinsic viscosity is call the limiting viscosity number - what a mouthful, and how can a "number" have units?

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