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A four-polymer reference table comparing contamination behaviour, melt flow and processing characteristics, and recyclability for PET, HDPE, LDPE/LLDPE, and PP — the technical parameters that determine mechanical recycling processing decisions.
| Polymer Type | Contamination Behavior | Melt Flow & Processing | Recyclability |
| PET (Polyethylene Terephthalate) | Sensitive: Highly affected by moisture and glue; requires intensive drying to prevent brittleness. | High Melt Point: Requires precise temperature control (~250–280°C) to avoid degradation. | Excellent: Widely recycled into new bottles and polyester fiber. |
| HDPE (High-Density Polyethylene) | Robust: Resistant to many chemicals; typically contaminated with labels or oils (in detergent bottles). | Stable: Predictable melt flow (190–230°C); very "forgiving" during extrusion. | High: Primary use for crates, pallets, and non-food grade bottles. |
| LDPE / LLDPE (Low-Density Poly) | Absorbent: Flexible films trap moisture and organic dirt easily; hard to wash and dry. | Viscous: Flows easily but prone to "necking"; requires densification (agglomeration) before extrusion. | Good: Often recycled into bin liners, construction sheets, or irrigation pipes. |
| PP (Polypropylene) | Low Density: Floats easily in water (good for separation), but absorbs odors from food waste. | Fast Flow: High melt flow index (MFI); requires cooling tanks to solidify the strands quickly. | High: Used for battery casings, furniture, and automotive parts. |
Beyond definitions
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How to read this table
- Contamination Behaviour describes how each polymer responds to common contaminants (moisture, food residue, labels, oils) encountered in post-consumer waste.
- Melt Flow and Processing covers the key extruder settings and behaviours an operator must manage for each polymer.
- Recyclability is a relative grade — all four polymers are commercially recyclable in India, but the quality ceiling and buyer range differ.
About this table
Each polymer type behaves differently during mechanical recycling. Understanding contamination sensitivity, melt temperature behaviour, and recyclability grade helps a plant operator configure their washing line, set extruder temperatures, and predict output quality for each feedstock stream. This table covers the four most common polymers in Indian plastic waste: PET, HDPE, LDPE/LLDPE, and PP.
PET (Polyethylene Terephthalate) is highly sensitive to moisture: even 0.5% moisture content during extrusion causes hydrolytic chain degradation, producing a brittle, low-viscosity output. PET must be dried to below 0.02% moisture (200 ppm) before extrusion — industrial dryers operating at 160–180°C for 4–6 hours are standard. Its high melt point (250–280°C) requires careful temperature control to avoid thermal degradation. Despite these demands, PET is the most recyclable polymer — widely accepted for new beverage bottles and polyester fibre.
HDPE is the most forgiving polymer for mechanical recycling. It tolerates label and oil contamination better than other polymers, has a predictable melt flow at 190–230°C, and extrudes cleanly without the drying requirement of PET. LDPE and LLDPE (flexible film polymers) are the most challenging to process mechanically: they trap moisture in the film layers, are difficult to wash thoroughly, and are prone to wrapping around rotating machine components instead of being cleanly cut. Densification (agglomeration) before extrusion is often required to increase bulk density and improve extruder feeding. PP has a high melt flow index (MFI), meaning it flows very easily when molten — which requires fast-cooling water tanks downstream of the die face to solidify extrudate strands quickly before pelletising.
Key insights
- PET requires drying to below 200 ppm moisture before extrusion — skipping this step produces brittle, low-quality output that buyers will reject or heavily discount.
- HDPE is the most forgiving polymer to process — contamination tolerance is high and extrusion is predictable, making it the easiest starting polymer for a first mechanical recycling plant.
- LDPE film requires densification (agglomeration) before extrusion because its low bulk density causes feeding problems in standard extruders designed for granulated or flaked feedstock.
- PP's high melt flow index means it flows readily through the die but needs fast-cooling water tanks immediately after the die to prevent pellet deformation.
Methodology & sources
Technical characteristics described are based on standard polymer science literature and mechanical recycling plant operating experience. Melt temperature ranges reflect typical processing windows; actual settings depend on resin grade, contamination level, and extruder design. This table was created as a course reference; verify with equipment suppliers for plant-specific process parameters.
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