Temperature Settings and Gelation Effect in PVC Extrusion
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Achieving Optimal PVC Properties Through Temperature Control
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.
Impact of Temperature and Gelation
Fusion Level Measurement
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.
Processing Temperature
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.
Venting and Air Extraction
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.
FAQ Section
Why is controlling gelation important for PVC pipe quality?
Controlling gelation is crucial because it ensures the formation of a strong network of tie molecules connecting crystallites in the pipe wall. This network is essential for achieving optimal long-term properties like strength, ductility, stiffness, and resistance to crack propagation.
How does temperature relate to achieving the correct gelation level?
Temperature, combined with shear, provides the energy needed to fuse the PVC particles and develop the tie-molecule network. Processing at too low temperatures results in insufficient fusion (sintering), while too high temperatures risk degrading the polymer or destroying the network.
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Humberto Goldoni
