Discussion on PVC Processing and Gelation (Plasticization)

Impact of Gelation Degree on Product Performance

The performance of PVC products is closely related to the degree of gelation during processing. Insufficient gelation results in brittle products with inadequate mechanical properties, while excessive gelation may cause yellow lines and also lead to poor mechanical performance. Therefore, gelation degree is one of the most critical parameters in PVC processing.

When the gelation degree reaches approximately 60%, the product exhibits maximum tensile strength. At around 65% gelation, the product achieves maximum impact strength. At about 70% gelation, the product shows maximum elongation at break.

For PVC materials used in potable water pipe applications, a gelation degree between 60% and 65% is considered optimal because within this range, the product can achieve both high tensile strength and high impact strength.

Influence of Temperature on Gelation Degree

Polymer materials do not form a melt below about 80°C and remain in a glassy state—hard, brittle, and unsuitable for processing. As the temperature increases to around 160°C, PVC enters a high-elastic state but still lacks flowability; the material only softens and becomes more viscoelastic.

True melt flow suitable for PVC processing occurs within the temperature range of 160–200°C. However, when the processing temperature exceeds 200°C, the long-term thermal stability of any stabilizer becomes insufficient, causing the material to degrade. Therefore, the processing temperature must be controlled within 160–200°C to achieve proper gelation. Within this range, temperatures around 170–180°C generally provide optimal gelation.

Methods to Improve Gelation Degree

Gelation degree can be improved by increasing the barrel and screw temperatures. After the screw speed becomes stable, increasing the feeding rate can further enhance gelation. Under conditions where the extruder is operating within its rated capacity, increasing the screw speed can also improve gelation.

Allowing the dry blend to undergo a proper aging period of 12–48 hours is another effective method. The aging process helps eliminate static electricity and reduce contamination, increases bulk density, enhances gelation degree, promotes uniform dispersion of low-molecular-weight additives to avoid extrusion instability, and by lowering the temperature of the calibrator to some extent, also contributes to better gelation.

Methods to Evaluate Gelation Degree

Gelation degree can be evaluated by monitoring the main motor load. For example, on a 65/132 extrusion line, a main motor current of 46–52 A is generally suitable; for low-calcium formulations, 45–50 A is recommended. This requires proper matching of screw speed (16–22 r/min), adequate feeding, and appropriate temperature settings.

The gelation degree can also be assessed by observing the material through the vacuum vent of the extruder. When more than 60% of the screw channel is filled with melt and the powder in the screw flight appears crumb-like with the bottom flattened under compression, the gelation is considered adequate.

During start-up, the gelation degree can be judged by observing the viscoelasticity of the melt at the die exit.

Another method is to evaluate the melt pressure and melt temperature at the confluence core, though the accuracy may be affected by faulty instruments or sensors clogged with degraded material.