The Polymer Identification Challenge
Common plastics like PET, HDPE, PVC, PP, LDPE, and PS are visually indistinguishable when mixed in a waste stream. RGB cameras cannot distinguish them because differentiation occurs in the near-infrared spectrum (900–1700 nm), beyond visible light. Cross-contamination causes recycled bale degradation; PVC in PET releases hydrochloric acid during reprocessing, damaging equipment and breaching food-grade standards.
| Challenge Area | Root Cause | Downstream Impact |
|---|---|---|
| Visually identical polymers | PET, PP and LDPE share colour and texture ranges | Mis-sorted bales, reprocessing failure |
| Black plastic identification | Carbon black absorbs all visible light; RGB useless | Black plastics unrecycled globally |
| Surface contamination | Dirt, labels and grease mask surface colour cues | RGB cameras mis-classify dirty items |
| PVC cross-contamination | PVC in PET or PP stream degrades entire bale | HCl release, equipment damage, rejected lot |
| Mixed flake / fragment streams | Shredded fragments lack shape or colour identity | No viable non-spectral classification method |
| Manual sorting throughput | Human sorters limited to 30–40 items per minute | Insufficient for high-volume conveyor rates |
Why RGB Cameras Cannot Solve This
NIR/SWIR imaging captures unique spectral absorption profiles for each polymer across 900–1700 nm. PET shows characteristic peaks at 1150 nm and 1450 nm; PP at 1190 nm and 1380 nm; PVC at 1150 nm and 1680 nm. These signatures remain stable regardless of part colour, cleanliness, or shape, making spectral imaging the only reliable in-line polymer identification method at conveyor speed.
| Detection Method | Limitation for Polymer ID | Verdict |
|---|---|---|
| RGB camera (visible light) | Reads surface colour only — cannot detect polymer chemistry | Not viable for polymer ID |
| Manual visual sorting | Cannot distinguish same-colour polymers; fatigues rapidly | Not viable at scale |
| Density / float-sink | Batch process only; cannot sort on moving conveyor | Not inline capable |
| X-ray fluorescence (XRF) | Detects elements, not polymer bonds; slow per-point scan | Impractical at line speed |
| Raman spectroscopy | Point measurement only; dark/black surfaces cause fluorescence | Limited throughput |
| Human + conveyor system | Accuracy drops below 80% for mixed clear/white polymers at speed | Not reliable for purity targets |
Suggested NIR/SWIR System Architecture
A 640×512-pixel global-shutter SWIR camera above the conveyor paired with high-intensity NIR illumination optimized for 900–1700 nm. Per-pixel spectral classification models trained on polymer absorption signatures assign material identity to each region, producing labelled material maps in under 33 milliseconds. Classification results trigger timed air-jet nozzles or mechanical diverters, routing items to designated polymer bins while rejecting PVC and non-plastic contaminants.
| Identification Capability | Spectral Method | Performance | Condition |
|---|---|---|---|
| PET identification | 1150 nm + 1450 nm absorption peaks | > 95% accuracy | Clear, coloured, contaminated |
| HDPE identification | 1190 nm + 1720 nm absorption peaks | > 95% accuracy | All colours including black |
| PVC detection and rejection | 1150 nm + 1680 nm absorption peaks | > 97% detection | Critical contamination flag |
| PP identification | 1190 nm + 1380 nm absorption peaks | > 95% accuracy | All surface states |
| LDPE / LLDPE identification | 1190 nm + 1380 nm variant peaks | > 93% accuracy | Film and rigid formats |
| PS identification | 1190 nm absorption signature | > 93% accuracy | Clear and coloured |
| Black plastic identification | NIR absorption (RGB blind) | > 92% accuracy | Carbon-black filled grades |
| Non-plastic contaminant | Spectral null or cellulose signature | > 95% rejection | Metal, paper, glass |
Expected Outcomes & ROI
| Outcome Metric | Baseline (RGB / Manual) | Target (NIR/SWIR) | Improvement |
|---|---|---|---|
| Polymer classification accuracy | 60–75% (same-colour confusion) | 95%+ per class | 25–35% accuracy gain |
| Black plastic recovery rate | Near zero (RGB blind) | > 90% identified | New revenue stream |
| PVC contamination in PET bale | 2–5% (undetected) | < 0.3% | > 90% reduction |
| Manual sorter headcount | 3–6 per shift | 0–1 (oversight) | 3–5 FTE redeployed |
| Sorted bale purity (PET) | 85–90% | > 97% | Food-grade rPET eligible |
| Throughput capacity | Limited by human sort rate | Conveyor-speed limited | 3–5× increase |
| Material value per tonne | Mixed / contaminated rate | Premium purity rate | 30–60% value uplift |
