Mechanical Recycling: Collection, Sorting & Extrusion Reprocessing Kinetics

Mechanical recycling is the most widely implemented circular economy pathway for plastics, converting post-consumer waste back into reusable pellets. However, sorting efficiency and thermal degradation during reprocessing limit the quality of recycled resins. Recycled resins undergo chain scission and oxidation, lowering their melt flow index and mechanical properties. Indian recycling firms like Ganesha Ecosphere and Srichakra Polyplast optimize collection, automated sorting, and compounding extrusion to supply high-quality PCR resins to FMCG brands.

• Map the steps of mechanical recycling (collection, washing, density separation, compounding extrusion).
• Explain the working principles of automated sorting technologies (Near-Infrared spectroscopy).
• Analyze the thermal degradation kinetics (chain scission vs. crosslinking) of polymers during extrusion.
• Calculate sorting purity and recovery efficiency from sorting mass balance data.
• Reference international plastics recycling standards including ISO 15270.

3.1 Steps in Mechanical Recycling

1. Collection & Baling: Consolidation of post-consumer plastic waste.
2. Sorting: Separating polymers by type (PET, HDPE, PP, LDPE) and color.
3. Shredding & Washing: Size reduction to flakes, followed by hot washing (with NaOH and surfactants) to remove adhesives and labels.
4. Density Separation (Sink-Float): Separating polymers by density in water:
   • Polyolefins (PE, PP, density $< 1.0$ g/cm³) float.
   • PET and PVC (density $> 1.3$ g/cm³) sink.
5. Reprocessing & Compounding: Extruding flakes with vacuum venting and melt filtration to produce pellets.

3.2 Automated Sorting (NIR Spectroscopy)

Near-Infrared (NIR) sensors identify polymers by analyzing the reflected spectrum ($800 - 2500$ nm) corresponding to CH, OH, and NH vibrational overtones. Air jets blow the identified items into collection bins. NIR cannot identify black plastics (carbon black absorbs all NIR light).

3.3 Reprocessing degradation kinetics

Repeated extrusion passes subject the polymer to high temperature and shear stress, driving degradation:

• Chain Scission (e.g., PP): Homolytic cleavage of carbon-carbon bonds, generating shorter chains $\rightarrow$ viscosity drops, MFI increases.
• Crosslinking (e.g., PE): Free radical reactions lead to chain branching and gel formation $\rightarrow$ viscosity rises, MFI decreases. Antioxidants must be added during recycling to prevent this degradation.

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Problem: A recycling sorting facility processes a bale of mixed bottles containing PET and PVC. An automated optical sorter is configured to eject PVC bottles. The data is:

• Input mass feed rate = $1200$ kg/h.
• Mass fraction of PVC in the feed = $15.0\%$.
• Output stream from the sorter (Ejected Bin) = $165$ kg/h, containing $155$ kg/h of PVC and $10$ kg/h of misplaced PET. Calculate:

1. The mass flow rate of PVC entering the sorter.
2. The PVC sorting recovery efficiency (%).
3. The PVC sorting purity (%) of the ejected stream.

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Solution:

1. Calculate incoming PVC mass rate:

$$\dot{m}_{\text{PVC,in}} = 1200 \text{ kg/h} \times 0.15 = \textbf{180.0 kg/h}$$

1. Calculate PVC sorting recovery efficiency:

$$\eta_{recovery} = \frac{\dot{m}_{\text{PVC,ejected}}}{\dot{m}_{\text{PVC,in}}} \times 100\% = \frac{155 \text{ kg/h}}{180.0 \text{ kg/h}} \times 100\% = \textbf{86.11\%}$$

1. Calculate PVC purity of the ejected stream:

$$\text{Purity (\%)} = \frac{\dot{m}_{\text{PVC,ejected}}}{\text{Total Ejected Mass}} \times 100\% = \frac{155 \text{ kg/h}}{165 \text{ kg/h}} \times 100\% = \textbf{93.94\%}$$

Interpretation: The optical sorter successfully recovers 86.11% of the PVC in the stream with a purity of 93.94%. The remaining 13.89% PVC remains in the PET stream, representing a contamination risk since PVC degrades at PET processing temperatures (releasing HCl, which degrades the machinery and PET polymer).

Ganesha Ecosphere Limited (Kanpur) is India's largest recycler of PET bottles, converting post-consumer waste into recycled polyester staple fiber (rPSF) and food-grade rPET pellets. They utilize sink-float density separation to remove PP caps and PE labels from sinking PET bottle flakes.

Srichakra Polyplast (Hyderabad) operates automated sorting lines equipped with Tomra NIR sorters to supply high-purity recycled HDPE and PP to FMCG clients like Unilever and Coca-Cola, satisfying EPR plastic reuse mandates.

• Mechanical Recycling: Physical reprocessing of plastic waste into pellets without modifying chemical structures.
• NIR Sorter: Automated sorting tool utilizing near-infrared spectroscopy to separate polymers.
• Sink-Float: Density-based fluid separation technique separating polyolefins ($< 1.0$ g/cm³) from heavy plastics.
• Chain Scission: Polymer degradation mechanism reducing molecular weight and viscosity.
• Zeta Potential: (Not applicable, latex parameter).

1. ISO 15270 — Plastics — Guidelines for the recovery and recycling of plastics waste
2. ASTM D7611 — Standard Practice for Coding Plastic Manufactured Articles for Recycling
3. IS 14534 — Bureau of Indian Standards (BIS) guidelines for recycling of plastics

1. Explain the molecular degradation reactions that occur during the extrusion of recycled polypropylene. Why does the Melt Flow Index (MFI) increase?
2. Describe the limitations of Near-Infrared (NIR) sorting systems when dealing with dark-colored plastics. What alternative sensor technologies are used?
3. Design a process flow diagram for a PET bottle wash line, including dry cleaning, shredding, hot washing, sink-float, and extrusion pelletizing.

Q1. Which sorting technology uses vibrational overtones of chemical bonds to separate polymers by type?

• C) Near-Infrared (NIR) Spectroscopy

Q2. During mechanical recycling, polyolefins (PP, PE) are separated from PET/PVC in a sink-float tank because:

• C) Polyolefins have a density $< 1.0$ g/cm³ and float in water

Q3. Thermal degradation of Polypropylene during repeated extrusion cycles results in:

• B) Chain scission, leading to decreased molecular weight and increased MFI

Q4. Why is PVC contamination a critical concern during PET recycling?

• B) PVC degrades at PET processing temperatures (280°C), releasing corrosive HCl

Q5. Which Indian company is a major recycler of PET bottles into recycled polyester fiber?

• C) Ganesha Ecosphere Limited