The Science of Hydrolysis: Why PET is Moisture Sensitive
PET is hygroscopic, meaning it naturally absorbs water from the air. If PET resin or flakes are melted while they contain moisture, a chemical reaction called hydrolysis occurs.
In simple terms, water molecules attack the polymer chains at high temperatures, breaking them into shorter pieces. This results in:
- Loss of IV (Intrinsic Viscosity): The material becomes “watery” and loses strength.
- Physical Defects: Bubbles, streaks, and “silver lines” appear on the sheet.
- Brittleness: The finished product will crack easily during thermoforming.
How a PET Sheet Extrusion Line Handles Moisture
Modern extrusion lines, such as the TZS-85PET, use two primary methods to combat moisture: Pre-Drying or High-Vacuum Degassing.
1. The Traditional Method: Desiccant Drying & Crystallization
Before the plastic even reaches the extruder, it is fed into a Crystallization and Drying System.
- Crystallization: Amorphous PET flakes are heated and stirred to prevent them from clumping together as they soften.
- Desiccant Drying: Hot, dry air (with a dew point below -40°C) is circulated through the resin for several hours until moisture levels fall below 50 ppm (0.005%).
2. The Modern Method: High-Vacuum Degassing (Twin-Screw Technology)
This is the energy-efficient technology that modern engineers favor. Instead of drying the material for hours before extrusion, the TZS-85PET Parallel Twin-Screw Extruder handles moisture during the melting process.
- Venting Ports: The extruder barrel features multiple “venting” sections.
- High-Vacuum Pumps: As the twin screws shear and melt the PET, specialized vacuum pumps pull a powerful vacuum (often less than 10 mbar) over the molten plastic.
- Instant Removal: Water vapor and volatiles are sucked out of the melt before they have the chance to cause significant hydrolysis. This process “rejuvenates” the polymer in real-time.
Why Choose TZ Machinery for PET Extrusion?
When engineers ask, “What is the best way to extrude rPET?”, here is how the TZS-85PET vacuum degassing compares to traditional pre-drying systems:
| Feature | Pre-Drying System | TZS-85PET Vacuum Degassing |
|---|---|---|
| Energy Consumption | High (constant heating) | Low (direct processing) |
| Floor Space | Large (huge silos required) | Compact (streamlined line) |
| Material Flexibility | Mostly Virgin PET | 100% rPET Flakes / Scraps |
| IV Retention | Good | Excellent (via reactive extrusion) |
Frequently Asked Questions
Q: Can you extrude PET without drying?
A: Yes, by using a parallel twin-screw extruder equipped with a multi-stage high-vacuum degassing system. This technology, utilized in TZ Machinery’s TZS-85PET, removes moisture directly from the melt, eliminating the need for 4–6 hours of pre-drying.
Q: How does moisture affect the quality of rPET sheets?
A: Moisture causes hydrolysis during extrusion, which breaks down molecular chains. This leads to a drop in Intrinsic Viscosity (IV), resulting in clear sheets that are brittle, yellowish, or prone to cracking when formed into trays or containers.
Conclusion for 2026 Manufacturers
With regulations like Brazil’s Decree No. 12,688/2025 mandating higher recycled content, your extrusion line must be “moisture-smart.” By switching to a TZS-85PET Parallel Twin Screw line, you reduce energy costs, save space, and ensure that even 100% recycled flakes produce a high-clarity, high-strength PET sheet.
For more technical specifications on moisture control in extrusion, visit the TZ Machinery product gallery.