K-Value : importance in uPVC Profile Formulation.

PVC compound contains 85-90% S-PVC for profiles/pipes.





K value is a measure of degree of polymerization.
Higher the K value, higher is the viscosity & more difficult the PVC to process.
The melt viscosity at processing temperature is α [Mole. Wt]^3.4 Or, Viscosity = k*[K Value]^3.4, where k is proportionality constant. For S-PVC, the supplieras test report mentions that the K value is 67 +/-1.

That means the K value varies from 66 to 68. Applying the rounding off factor, K 65.6 , K 66 & K 68.4 , K 68, it means that in reality, K 67 resin can have K value anything between 65.6 and 68.4
What is the effect on viscosity?
For K 65.6, it will be k*(65.6)^3.4 = 1504780 k units For K 68.4, it will be k*(68.4)^3.4 = 1734566 k units Thus for the K 67 resin the variation in viscosity at processing temperature can be 15.27% Processing parameters & formulation need to be adjusted to cope with change in viscosity of PVC.

 Hence pipes or profiles will have variation/face rejection. For molding, K 57 processing is harsher

What can be done?

If K value of incoming PVC batch is measured in QC lab & compound lubrication adjusted accordingly, the operator will get the same processability & need not adjust the parameters

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