Riciclaggio del PVC nelle bottiglie in PET: comprendere le sfide e le soluzioni

In the world of PET recycling, PVC (polyvinyl chloride) is a dreaded contaminant. Even in trace amounts, PVC can wreak havoc during the processing and remanufacturing of post-consumer PET resin, leading to significant quality issues in the final product. For context, PVC contamination as low as 50 parts per million (ppm)—equivalent to just 0.05 kg of PVC in 1,000 kg of PET flakes—can cause severe problems. This minuscule concentration (0.005%) can lead to the breakdown of PET resin, resulting in brittleness, discoloration (yellowing), and the release of hazardous chlorine vapors. These issues undermine two of PET’s most valued properties: clarity E impact strength.

For PET recyclers, controlling PVC contamination is critical, especially when targeting high-end applications like premium polyester fibers, where PVC levels must remain below 50 ppm. To achieve this, recyclers must first understand how PVC enters the PET recycling stream and then implement effective removal strategies.

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Sources of PVC Contamination in Riciclaggio del PET

PVC contamination typically originates from four primary sources:

1. PVC Bottles: Some PVC bottles resemble PET bottles but can be identified by a white “crease” mark when flattened. Trained sorters can manually remove these during the sorting process.
2. PVC Safety Seals: Certain PET bottles, such as mouthwash bottles, feature PVC safety seals that must be removed before granulation.
3. PVC Liners in Bottle Caps: While PVC liners are no longer common in the U.S., they occasionally appear in older or imported bottles.
4. PVC Labels: Wraparound labels made of PVC can also introduce contamination into the PET recycling stream.

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Strategies for PVC Removal in PET Recycling

Removing PVC from PET requires a combination of manual and automated techniques. Here are the most effective methods:

1. Manual Sorting with UV Lighting

Manual sorting remains one of the most reliable methods for PVC removal. Experienced sorters can visually identify PVC bottles, but efficiency can be significantly enhanced with UV lighting. Under UV light, PET plastic emits a blue fluorescent glow, while PVC (due to common additives) often fluoresces green or yellow. This color contrast allows sorters to easily spot and remove PVC bottles. To protect workers from prolonged UV exposure, shifts should be limited to 2 hours.

2. Automatic Sorting Systems

As labor costs rise, automated sorting systems have gained popularity. These systems fall into three main categories:

• Sistemi di smistamento ottico: Use sensors to detect differences in color or material properties.
• Tecnologie di trasmissione: Analyze the chemical composition of plastics.
• Sistemi di scansione della superficie: Detect surface characteristics to differentiate materials.

The most reliable automated method involves X-ray detection, which identifies chlorine atoms present in PVC but absent in PET. Air jets then eject the PVC-contaminated materials from the stream. For optimal results, a 2-3 pass routine is recommended to minimize contamination.

3. Thermal Separation

Thermal separation leverages the differing melting points of PVC (200°C) and PET (260°C). In this process, a mixed stream of PET and PVC flakes is passed over a heated conveyor belt or barrel set to 180-200°C. At this temperature, PVC softens and sticks to the belt, while PET remains unaffected and rolls off into a collection bin. A stationary blade then scrapes off the PVC from the belt. This method is cost-effective and requires minimal investment compared to automated systems.

4. Electrostatic Separation

Electrostatic separation exploits the differing electrical properties of PVC and PET. When exposed to a charging chamber, PVC becomes negatively charged, while PET becomes positively charged. The mixed stream is then passed through charged poles, where opposite polarities attract, separating the two plastics. The key to success lies in applying the right amount of energy—enough to charge the plastics but not so much that they attract each other.

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Conclusion: Ensuring High-Quality PET Flakes

PVC contamination is a significant challenge in PET recycling, but with the right strategies, it can be effectively managed. Whether through manual sorting, automated systems, thermal separation, or electrostatic methods, the goal is to achieve PVC levels below 50 ppm, ideally below 30 ppm, to meet the demands of high-end applications.

Testing for PVC content is essential to ensure the quality of your PET flakes. Even small amounts of PVC can render an entire batch unusable, so implementing a robust PVC removal system tailored to your specific feedstock is crucial. By combining multiple techniques and continuously monitoring contamination levels, PET recyclers can produce high-quality flakes that command premium prices in the market.

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Key Takeaways:

• PVC contamination as low as 50 ppm can degrade PET quality, causing brittleness, discoloration, and hazardous chlorine emissions.
• Sources of PVC contamination include PVC bottles, safety seals, bottle cap liners, and labels.
• Effective PVC removal methods include manual sorting with UV lighting, automated sorting systems (e.g., X-ray detection), thermal separation, and electrostatic separation.
• Aim for PVC levels below 50 ppm, with 30 ppm or lower being ideal for high-end applications.
• Testing and quality control are essential to ensure the final PET flakes meet industry standards.

By adopting these strategies, PET recyclers can minimize PVC contamination, enhance product quality, and maximize profitability in a competitive market.