Sintering vs. Gelation vs. Overheating in PVC

The quality and performance of PVC pipes are highly dependent on achieving the correct material structure during the extrusion process. This structure is a network of tie molecules connecting crystallites, which ideally resembles a well-gelled product.

Processing Outcomes

Different processing conditions can lead to undesirable structures.

• Sintering: If extruded too cold at low shear, the PVC dry blend is only sintered. This means particles are fused together insufficiently, resulting in weak material properties like low impact resistance and strength.
• Gelation (Optimal): The desired state is a well-gelled melt where there is an entanglement of tie molecules connecting crystallites. This network structure, depicted in the middle of Figure 8, is crucial for optimal long-term strength, ductility, stiffness, and other properties of PVC. Optimal gelation levels are typically between 85-95%.
• Overheating/Over-shearing: At the other extreme, processing at too high a temperature and high shear can destroy the desired network. This over-sheared PVC also results in low impact resistance and strength.

Measuring Gelation

The level of gelation or fusion can be measured using various tests. Common methods include the DSC method and the DCMT (Dichloromethane solvent, EN580) test, which reveals loose powder on a chamfered surface at low fusion levels (see Photo 3). Other tests include the ASTM D2152 Acetone test, which is considered easier to pass than the DCMT test. A fracture toughness test on a C-ring with a sharp inside notch is another indicator related to brittleness and crack propagation, providing insight into resistance to point loads.

Technical Specifications (Example measure)

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