This test method is useful for quality-control tests on PVC compounds having a wide range of melt viscosities. Measurements are made at shear rates close to 1 s⁻¹.

In addition to the properties mentioned in Test Method D 1238, this technique is sensitive to plasticizer content, polymer molecular weight, polymer stability (both thermally and rheologically), shear instability, and general composition. The sensitivity of the material to temperature necessitates slightly tighter controls than those stated in Test Method D 1238.

The sensitivity of this test method makes it useful for correlating with processing conditions and as an aid in predicting changes in processing. However, as a one-point measure of flow relative to shear rate, its one drawback is that the same PVC melt flow values can be obtained for materials having different processibility; the chance of this happening is minimized, however, if the compounds are similar in composition.

Correlations with a wide range of processing conditions have supported the conclusions that little or no change in composition occurs during the test. Thus, this test is able to detect and follow profound changes which occur during extrusion, injection molding, milling, or mixing. These changes are due to three types of measured instability in polymers:

5.4.1 Thermal instability due to temperature effect.

5.4.2 Shear instability due to breaking of polymer bonds.

5.4.3 Rheological instability due to nonuniform distributions of widely different viscosity or molecular weight elements.

5.4.4 Thus, implications with respect to PVC molecular structural changes can be detected and predicted.