PVC original whiteness standard
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After checking the standard for general-purpose polyvinyl chloride resin by suspension method (GB/T 5761-2006), I found that it does not stipulate the original whiteness of PVC. What do experts think of this issue?
Raw whiteness makes no sense! The 160 degree heat aging whiteness characterizes the thermal stability of the resin. However, this characterization is not scientific enough, because the resin not only decomposes thermally during processing, but also decomposes under the high shear force of the twin screws. Therefore, the torque curve on the rheometer can more intuitively reflect the thermal stability performance of the resin. However, I have also tested the original whiteness of the resin, and it is generally above 90.
What you said above makes sense. When you think about PVC processing, the conditions must exceed 160 degrees for 10 minutes. Even with the protection of heat stabilizer, it seems that the whiteness will definitely decrease. Therefore, it is more meaningful to examine the aging whiteness. I have encountered PVC with an original whiteness of 94 and a thermally aged whiteness of 78, while another type of PVC has an original whiteness of about 92 and an aged whiteness of about 80. What is the reason for this difference?
There is also a Congo red test to characterize thermal stability. I wonder which one is closer to the actual processing?
As mentioned on the second floor page, Congo Red tests the static thermal stability of PVC, which is subject to shearing force during actual processing. Therefore, the dynamic thermal stability test of the torque rheology test is closer to the actual processing situation, or the double-roller mill can also reflect the actual processing situation.