Chemical Knowledge 08:Explanation of PVC Product Manufacturing Process

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Update time: 26-03-16    Views: 9

Explanation of PVC Product Manufacturing Process

1. Effect of Gelation Degree on Product Properties

• The performance of PVC products is closely related to their gelation degree.
• Poor gelation results in brittle products with unsatisfactory mechanical properties.

Excessive gelation causes yellow lines in the products and also leads to unqualified mechanical properties.

Therefore, gelation degree is critical during the processing of PVC products.

1. Tensile strength is maximized at a gelation degree of 60%.
2. Impact strength is maximized at a gelation degree of 65%.
3. Elongation at break is maximized at a gelation degree of 70%.

For PVC materials used in water supply pipes, the optimal gelation degree is 60–65%, as this range balances both tensile strength and impact strength.

2. Effect of Temperature on Gelation Degree

• At temperatures below 80°C, polymer materials do not melt and remain in a glassy state — hard, brittle, and unprocessable.
• As temperature rises to 160°C, the material enters a high-elastic state, becoming softer with increased viscoelasticity, but still cannot flow.
• For PVC melt processing with proper flowability, the temperature must be within 160–200°C.
• However, PVC will decompose under prolonged heating above 200°C, regardless of the stabilizer used.

Thus, the processing temperature for controlling gelation must be maintained at 160–200°C.Within this range, gelation is optimal when the temperature is set to 170–180°C.

3. Methods to Improve Gelation Degree

1. Increase the temperature of the barrel and screw.
2. With normal screw speed, raise the feeding speed of the feeder.
3. Increase the extruder speed within its rated range and sufficient feeding capacity.
4. Provide a proper maturing period (12–48 hours) for the dry blend.

Functions of the Maturing Period:

1. Eliminate static electricity and reduce contamination.
2. Increase apparent density.
3. Improve gelation degree.
4. Ensure uniform dispersion of low-molecular-weight polymers and prevent unstable extrusion.
5. Lower the temperature of the adapter to partially improve gelation.

4. How to Evaluate Gelation Degree

1. By main motor current

Taking the 65/132 production line as an example:

Standard: 46–52 A

Our company (low-calcium products): 45–50 A

Premise: screw speed 16–22 r/min, full feeding matching screw speed, temperature settings matching screw speed and current.

2. By visual inspection at the main machine vacuum vent

The material fills more than 60% of the screw flight channel; powder in the flight groove has a bean-curd dregs-like texture, and the material at the bottom of the groove is flattened.

3. By viscoelasticity of the melt at the die opening

This method is especially suitable for startup conditions.

4. By melt pressure and temperature at the adapter

Limitation: inaccurate results may occur if instruments fail or sensors are blocked by degraded material.