Polyethylene terephthalate (PET) is the most widely recycled plastic globally, used in beverage bottles, food containers, and textile fibers. Turning post-consumer PET into clean, dry flakes is a critical step before extrusion or pelletizing — and it directly determines the quality of recycled PET (rPET). This guide covers the complete PET flakes washing and drying process, including equipment, parameters, and best practices.
1. Pre-Washing & Sorting
Before washing, PET bottles must be sorted and prepared to remove contaminants that can affect quality.
Steps:
- Manual or Automatic Sorting – Remove non-PET plastics (PVC, PP, HDPE), metals, and hazardous materials.
- Label Removal – Mechanical label removers or hot air systems strip shrink sleeves and paper labels.
- Caps & Rings Separation – These are often PP or HDPE and need to be removed for material purity.
- Pre-Shredding – Bottles are reduced to smaller pieces (30–50 mm) before moving to hot washing.
Tip: Optical sorters with NIR (Near-Infrared) technology can increase sorting purity to over 98%.
2. Hot Washing Process
The hot wash is the main cleaning stage that removes adhesives, sugars, oils, and other contaminants.
Equipment:
- Hot Washing Tank (stainless steel, insulated)
- Caustic Soda Dosing System
- Friction Washers (for high-shear cleaning)
Parameters:
| Parameter | Suggested | Purpose |
|---|---|---|
| Temperature | 85–95 °C | Helps dissolve glue & labels |
| NaOH Concentration | 1–2% | Breaks down adhesives |
| Detergent Additive | 0.3–0.5% | Improves dirt removal |
| Residence Time | 15–20 min | Ensures thorough cleaning |
Tip: Maintain pH above 10 to keep glue in a soluble state and avoid redeposition.
3. Rinsing & Separation
After hot washing, thorough rinsing removes caustic residue and floating contaminants.
Key Steps:
- Sink–Float Separation Tanks – Separate PET (sinks) from caps and labels (float).
- Cold Rinse Tanks – Use clean water at room temperature for final washing.
- Water Recycling System – Filters and reuses up to 80% of process water.
Tip: Keep conductivity <200 μS/cm in final rinse water to ensure chemical-free flakes.
4. Friction Cleaning
A friction washer uses a high-speed rotor to scrub PET flakes while spraying clean water.
- Removes paper fibers, residual glue, and fine particles.
- Improves whiteness and intrinsic viscosity (IV) retention.
5. Mechanical Dewatering
Before thermal drying, remove as much water as possible to save energy.
Equipment:
- Centrifugal Dewaterer (2,000–3,000 rpm) reduces moisture to 2–3%.
- Screw Press (optional) for additional water removal.
6. Thermal Drying
PET must be dried to <0.3% moisture before extrusion to prevent hydrolytic degradation.
Drying Methods:
| Method | Final Moisture | Energy | Notes |
|---|---|---|---|
| Hot Air Dryer | 0.2–0.3% | Moderate | Simple, cost-effective |
| Infrared Dryer | 0.1–0.2% | Low | Fast, good for IV retention |
| Vacuum Dryer | 0.1% | Higher | For high-grade food-contact rPET |
Tip: Drying temperature should be 160–180 °C for 4–6 hours in a desiccant system for pelletizing-grade flakes.
7. Quality Control Checks
Before packaging, check:
- Moisture Content – Use a moisture analyzer.
- Flake Size – Typically 10–12 mm.
- Contamination Level – Black speck count, label specks <50 ppm.
- Color Sorting – Optional step for food-grade rPET.
8. Common Issues & Solutions
| Problem | Cause | Solution |
|---|---|---|
| Yellowing | Overheating during drying | Lower drying temp or shorten residence time |
| Residual Glue | Low NaOH concentration | Increase to 2% and maintain temperature |
| High Moisture | Insufficient drying | Extend drying time or improve dewatering |
9. Recommended PET Washing & Drying Line Layout
The typical PET washing and drying line consists of the following sequential stages to ensure efficient cleaning, contaminant removal, and preparation for recycling or reprocessing:
1. Bale Breaker
Breaks down compressed PET bales into smaller, manageable flakes or pieces for easier processing.
2. Pre-Sorter
Removes large contaminants and non-PET materials before fine processing, improving overall line efficiency.
3. Label Remover
Detaches labels and adhesives from PET flakes, ensuring higher purity of the recycled material.
4. Crusher (or Grinder)
Further reduces PET flakes into uniform sizes to facilitate washing and drying.
5. Hot Washer
Uses hot water, often with detergents or caustic solutions, to remove dirt, oils, and organic residues.
6. Sink-Float Tank
Separates PET flakes from lighter contaminants (e.g., caps, foils, plastics of different densities) via density-based flotation.
7. Friction Washer
Uses mechanical agitation and friction to dislodge stubborn contaminants such as glue or dirt attached to flakes.
8. Rinse Tank
Rinses PET flakes with clean water to remove residual cleaning agents and loosened contaminants.
9. Centrifugal Dewaterer
Removes excess water from PET flakes by centrifugal force, reducing moisture content before drying.
10. Thermal Dryer
Dries PET flakes thoroughly to meet moisture specifications required for downstream processing.
11. Quality Control (QC) & Packaging
Conducts final inspection for contaminants, moisture, and size consistency, then packages flakes for storage or sale.
FAQ
Q: Can I skip hot washing to save costs?
A: Not recommended. Skipping hot washing leaves glue and organic residues, lowering flake value and making them unsuitable for food-grade applications.
Q: What’s the target IV after washing?
A: Around 0.78–0.84 for bottle-grade PET flakes. Excessive washing time or temperature can reduce IV.
Q: How much water is used in PET washing?
A: A closed-loop system can bring usage down to 1–1.5 m³ per ton of flakes.
Clean, dry PET flakes are the foundation of high-quality rPET. By optimizing each stage — from sorting to drying — recyclers can maximize yield, meet food-grade standards, and increase market value.