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)
Property/Test
Optimal Range
Result of Low Gelation
Result of Over-shear
Gelation/Fusion Level
85-95% (DSC)
Low Fusion/Sintered
Destroyed Network
Impact & Strength
Max.
Low
Low
DCMT Test
Passes (No loose powder)
Fails (Loose powder)
N/A
ASTM Acetone Test
Passes
Can pass
N/A
FAQ Section
What is the ideal state of PVC after extrusion for optimal pipe properties?
The ideal state is a well-gelled product with a complete network structure where tie molecules connect the crystallites. This is achieved through controlled heat and shear during extrusion and results in maximum impact and strength.
How are undesirable processing outcomes like sintering or overheating identified?
Sintering (too low gelation) results in low impact and strength and can be identified by tests like the DCMT test showing loose powder. Overheating or over-shearing also leads to low impact and strength because the network is destroyed. Process monitoring (temperature, shear) and mechanical tests help identify these issues.
%20(1).jpg)
Nick Belshof
