PVC processing is sensitive to temperature and shear conditions. At extrusion conditions, PVC is a rubber-like solid, not a true melt, forming a network of tie molecules connecting crystallites. The gelation or fusion level is critical for achieving optimal properties like long term strength, ductility, and stiffness.
The goal is a well-gelled product with a complete network, ideally represented by the middle structure in figure 8.
The optimal gelation level is typically 85-95%. This is measured using methods like the DSC method, the DCMT (Dichloromethane solvent) test (EN580), the ASTM D2152 Acetone test, and the C-ring test. The DCMT test is stricter than the ASTM acetone test.
Extruding PVC too cold at low shear can result in only sintered material. Conversely, too high a temperature and high shear can destroy the required network structure. High cylinder intake temperatures can help cure powdering in the vent area.
A too high gelation level in the venting zone can prevent effective extraction of air from the dry blend, leading to air inclusions (voids) in the pipe wall. A screw design allowing for a void-free pipe wall at a lower-than-maximum vacuum is safer.