Impurity Removal in Recycled Polyester Filament Yarn: From Raw Material Pretreatment to Melt Filtration

As demand for sustainable textiles continues to grow, quality control in recycled polyester filament yarn production has become increasingly important. Compared with recycled polyester staple fiber, filament yarn generally requires stricter impurity control. Even small amounts of contaminants can cause spinneret blockage, filament breaks, fuzz, and fabric defects such as black spots and color specks, affecting both production efficiency and final product quality.

Therefore, impurity removal must be carried out throughout the production process, from raw material pretreatment to melt spinning. The main approaches include bottle flake pretreatment, advanced melt filtration, and chemical recycling and purification in certain production routes.

Ⅰ. Bottle Flake Pretreatment: Removing Impurities at the Source

Pretreatment mainly removes non-PET plastics, metals, labels, adhesives, oil, sand, and other contaminants from waste PET bottle flakes.

1. Material Sorting and Label Removal

After unpacking, waste bottles can be sorted using near-infrared (NIR) material identification, X-ray detection, eddy-current separation, and AI-based optical sorting. These technologies help remove PVC, PP, metal contaminants, and unsuitable bottle materials. Mechanical label removal is then used to reduce label and adhesive residues.

2. Crushing and Washing

The bottles are typically crushed into flakes measuring approximately 5–12 mm. The flakes then undergo pre-washing, hot washing, and multiple rinsing stages. Hot washing may use an alkaline solution at approximately 75–85°C to help remove oil, sugar residues, and adhesive contaminants.

3. Density Separation and Secondary Sorting

Density-based separation is used to distinguish different materials. Under typical water-based conditions, PET flakes sink, while lighter materials such as PP and PE generally float, allowing them to be separated.

Air separation, vibrating screens, secondary optical sorting, and manual inspection may then be used to remove dust, fine particles, discolored flakes, and remaining contaminants.

4. Dewatering and Drying

After washing, the flakes undergo centrifugal dewatering and drying to reduce moisture content and minimize the risk of hydrolytic degradation during high-temperature processing. Some production processes can achieve a moisture level below 50 ppm, depending on the raw material condition, equipment, and process requirements.

Ⅱ. Melt Spinning: Multi-Stage Melt Filtration

Although pretreatment removes most visible contaminants, fine solid particles, carbonized residues, and gels may remain. Advanced melt filtration is therefore an essential step in maintaining recycled polyester filament yarn quality.

1. Extrusion and Melting

The pretreated PET flakes or recycled PET chips are fed into an extruder and melted under controlled temperature conditions. A typical PET melt-processing temperature is approximately 260–280°C, although the actual settings depend on the raw material and production process.

2. Multi-Stage Continuous Melt Filtration

Production lines commonly use plate-type or candle-type filters equipped with filtration media of different grades to progressively capture contaminants.

Depending on the product requirements, filtration ratings of approximately 20–40 μm may be used for filament yarn production, while some fine-denier filament processes may require filtration at around the 10 μm level.

Differential pressure monitoring helps identify filter blockage. Depending on the system design, filters can be switched or replaced as needed to maintain stable melt flow and reduce the risk of spinneret blockage, fuzz, and filament breaks.

3. Solid-State Polycondensation and Final Filtration

For production routes that use solid-state polycondensation (SSP), PET chips can be treated under vacuum or an inert atmosphere to increase intrinsic viscosity and improve subsequent spinning performance. This process can also help reduce certain volatile, low-molecular-weight compounds.

Some production lines include an additional filtration component before the melt enters the spinneret, providing a final barrier against residual particles. It is important to note that SSP primarily improves polymer properties and cannot replace melt filtration.

Ⅲ. Chemical Recycling: Further Purification Through Depolymerization

In addition to mechanical recycling, some recycled polyester filament yarn production processes use chemical recycling. For example, waste PET can be depolymerized through processes such as glycolysis. The resulting monomers or oligomers are then purified through methods such as distillation, crystallization, or adsorption before being repolymerized into PET.

This approach can remove certain contaminants and produce high-purity recycled raw materials. However, chemical recycling generally involves more complex processing, higher equipment and energy requirements, and greater production costs than conventional mechanical recycling.

Ⅳ. How Impurity Removal Affects Filament Yarn Quality

The effectiveness of impurity removal directly affects production stability and final product quality.

• Residual PVC and other incompatible materials: These may decompose at high temperatures, contaminating the melt and causing discoloration or color variations.
• Metal and hard particles: These can block filtration media or spinneret holes, increasing the risk of fuzz and filament breaks.
• Carbonized residues and gels: These may create black spots or color specks on yarns and fabrics, reducing appearance quality.
• Insufficient moisture control: Moisture can cause PET hydrolysis, affecting molecular weight, melt viscosity, and spinning stability.
• Discolored contaminants: These may lead to uneven filament color and make downstream dyeing and quality control more difficult.

Ⅴ. Conclusion

Impurity removal in recycled polyester filament yarn production is a systematic process that requires effective control from raw material sorting and washing to drying and melt filtration.

Pretreatment removes most foreign materials, while advanced melt filtration further controls fine particles, carbonized residues, and gels. By improving raw material quality, selecting appropriate filtration equipment, and carefully controlling key process parameters, manufacturers can reduce spinning defects, improve production stability, and achieve more consistent product quality.

Effective impurity control is therefore an important foundation for producing high-quality recycled polyester filament yarn and supporting the wider adoption of sustainable textile materials.