Recycling of Plastic: Sorting, Washing, Shredding, Identification and Classification, Extruding
Plastic recycling is the process of recovering discarded plastic materials, separating them according to their composition, removing contaminants, reducing their size, and processing them into usable secondary raw materials or new plastic products. Because different polymers have different chemical structures, melting temperatures, densities, additives and processing characteristics, efficient recycling requires proper sorting, washing, shredding, identification, classification and extrusion.
At the MSc level, plastic recycling can be understood as a combination of polymer science, materials separation, chemical analysis and polymer processing. The quality of recycled plastic depends strongly on the purity of the feedstock and on preventing the mixing of incompatible polymers.
1. Sorting of Plastic Waste
Sorting is the first major processing step after collection. The objective is to separate plastic waste into relatively homogeneous streams based on polymer type, colour, form and contamination level.
Efficient sorting is essential because many polymers are not mutually compatible during melt processing. Even a small quantity of an incompatible polymer can adversely affect the mechanical properties, appearance and processing behaviour of a recycled material.
Methods of Sorting
1. Manual Sorting
In manual sorting, workers identify and separate plastic articles according to their appearance, product type, colour and recycling code. It is commonly used as a preliminary step before automated separation.
- Removal of non-plastic materials.
- Separation of large plastic articles.
- Removal of visibly contaminated materials.
- Preliminary separation according to polymer type and colour.
2. Sorting According to Recycling Codes
Many plastic products carry a resin identification code. These codes provide useful information about the polymer family, although the presence of a code should not be considered sufficient proof of polymer identity in every case.
| Code | Polymer | Common Examples |
|---|---|---|
| 1 | PET or PETE | Beverage bottles, food containers and polyester fibres |
| 2 | HDPE | Milk bottles, detergent containers, pipes and containers |
| 3 | PVC | Pipes, cable insulation and sheets |
| 4 | LDPE | Films, bags and flexible packaging |
| 5 | PP | Food containers, caps, automotive components and laboratory articles |
| 6 | PS | Disposable containers, packaging materials and expanded polystyrene |
| 7 | Other | Other polymers and polymer mixtures not included in codes 1–6 |
3. Automated Sorting
Modern recycling facilities can use mechanical and sensor-based technologies for automated sorting. These methods increase throughput and improve separation efficiency.
- Magnetic separation: removes ferrous metal contaminants.
- Eddy-current separation: separates non-ferrous metals.
- Air classification: separates materials according to aerodynamic behaviour.
- Near-infrared (NIR) sorting: identifies polymers according to their spectral response.
- Colour sorting: separates plastics according to colour.
- Density-based separation: separates polymers according to differences in density.
2. Washing of Plastic Waste
After preliminary sorting, plastic waste is washed to remove dirt and other contaminants. Washing is particularly important for post-consumer plastics, which may contain soil, food residues, oils, labels, adhesives and other foreign substances.
Objectives of Washing
- Removal of dust and soil.
- Removal of food and organic residues.
- Removal of oils and grease.
- Reduction of labels and adhesive residues.
- Removal of soluble contaminants.
- Improvement of the quality of recycled polymer.
Washing Process
Washing may involve a combination of cold-water washing, hot-water washing, detergent-assisted washing, friction washing and rinsing. The selected method depends on the type and level of contamination.
Pre-washing → Hot or cold washing → Friction washing → Rinsing → Separation of water → Drying
Hot Washing
Hot washing can improve the removal of oils, grease, adhesives and other persistent contaminants. In some recycling systems, alkaline cleaning agents may be used under controlled conditions.
The temperature and chemical conditions must be selected carefully because excessive heat or aggressive chemicals can alter the polymer or accelerate degradation.
Friction Washing
Friction washers use mechanical action to remove contaminants from the surface of plastic flakes. Water assists in carrying away loosened dirt and residues.
3. Shredding of Plastic
Shredding is the mechanical size-reduction step in which large plastic articles are converted into smaller pieces or flakes. The process increases the surface area and makes subsequent washing, separation and melting more efficient.
Equipment Used
- Plastic shredders
- Granulators
- Knife mills
- Crushers
Shredders generally use rotating cutting elements to break plastic articles into smaller pieces. Granulators can subsequently produce more uniform flakes.
Factors Affecting Shredding
- Type of polymer.
- Thickness and geometry of the plastic article.
- Moisture content.
- Blade or knife condition.
- Desired particle or flake size.
- Feed rate.
4. Identification of Plastics
Identification determines the polymer present in a plastic waste stream. Accurate identification is important because polymers differ in density, melting behaviour, chemical resistance and compatibility during melt processing.
Major Methods of Identification
1. Visual Identification
Colour, transparency, flexibility, surface appearance and product type can provide preliminary information. However, visual examination alone is not sufficiently reliable for detailed polymer identification.
2. Density Testing
Different polymers have different densities. A density-based separation can therefore be used to distinguish certain plastics.
| Polymer | Approximate Density Range (g/cm³) | General Behaviour in Water |
|---|---|---|
| PP | ~0.90–0.91 | Generally floats |
| LDPE | ~0.91–0.93 | Generally floats |
| HDPE | ~0.94–0.97 | Generally floats |
| PS | ~1.04–1.06 | Generally sinks |
| PVC | ~1.3–1.45 | Sinks |
| PET | ~1.3–1.4 | Sinks |
The exact density depends on polymer grade, crystallinity, additives, fillers and processing history. Therefore, density testing is normally used together with other identification methods.
3. Burn or Flame Testing
Historical laboratory identification schemes may use flame behaviour, odour and combustion characteristics. However, this method has significant limitations because additives and polymer mixtures can alter the observations. Some plastics can also release hazardous combustion products.
4. Infrared Spectroscopy
Fourier-transform infrared spectroscopy (FTIR) is one of the most useful analytical techniques for polymer identification. Different polymers produce characteristic infrared absorption bands associated with their molecular structures.
The measured spectrum is compared with reference spectra to identify the polymer. FTIR can distinguish materials such as PET, PE, PP, PS and PVC and can also provide information about surface contamination and degradation.
5. Near-Infrared Spectroscopy
Near-infrared spectroscopy is widely used in automated sorting systems. The spectral response of a plastic item is detected and software assigns it to a particular polymer category.
5. Classification of Plastics
Plastics can be classified in several ways. For recycling, classification according to polymer structure and processing behaviour is particularly important.
Classification According to Thermal Behaviour
| Class | Characteristics | Examples |
|---|---|---|
| Thermoplastics | Soften or melt on heating and can generally be remoulded repeatedly, although processing can cause degradation. | PE, PP, PET, PVC, PS |
| Thermosetting polymers | Form cross-linked structures and do not melt and flow like conventional thermoplastics after curing. | Epoxy, phenolic resins, melamine-formaldehyde |
| Elastomers | Highly elastic polymeric materials; many are cross-linked and are not conventionally recycled by melt extrusion. | Natural rubber, synthetic rubber |
Classification Important for Mechanical Recycling
Mechanical recycling is particularly suitable for thermoplastic polymers because they can be softened and processed again. Common mechanically recycled polymers include:
- Polyethylene terephthalate (PET)
- High-density polyethylene (HDPE)
- Low-density polyethylene (LDPE)
- Polypropylene (PP)
- Polystyrene (PS)
- Polyvinyl chloride (PVC), under controlled processing conditions
6. Drying of Plastic Flakes
After washing, plastic flakes must be sufficiently dried before extrusion. Residual moisture can cause processing problems and, for moisture-sensitive polymers such as PET, can contribute to hydrolytic degradation during melting.
Drying may be performed using mechanical dewatering followed by hot-air drying or other industrial drying systems. The required drying conditions depend on the polymer and its moisture sensitivity.
7. Extruding Recycled Plastic
Extrusion is a continuous melt-processing operation in which cleaned and dried plastic flakes are melted and forced through a die to produce a continuous polymer strand. The strands are subsequently cooled and cut into pellets.
Basic Principle of Extrusion
Plastic flakes are introduced into the hopper of an extruder. A rotating screw conveys the material through a heated barrel. During transport, the polymer is gradually heated, compacted and melted. The molten polymer is then mixed, filtered and pushed through a die.
Plastic flakes → Hopper → Conveying → Heating and melting → Mixing → Melt filtration → Die → Cooling → Pelletisation
Main Parts of a Plastic Recycling Extruder
- Hopper: receives the cleaned plastic feed.
- Screw: conveys, compresses, melts and mixes the polymer.
- Barrel: surrounds the screw and contains controlled heating zones.
- Heating system: supplies thermal energy required for melting.
- Cooling system: controls excessive temperature rise.
- Screen or melt filter: removes remaining solid contaminants.
- Die: shapes the molten polymer.
- Cooling unit: solidifies the extruded polymer.
- Pelletiser: cuts the cooled polymer strand into pellets.
Single-Screw and Twin-Screw Extrusion
Single-Screw Extruder
Single-screw extruders are widely used for melting and conveying relatively uniform recycled thermoplastics. They have a comparatively simple construction and are suitable for many conventional recycling applications.
Twin-Screw Extruder
Twin-screw extruders provide enhanced mixing and compounding capability. They are useful when recycled polymers must be blended with additives, stabilisers, fillers or other compatible materials.
8. Melt Filtration During Extrusion
Even after sorting and washing, recycled plastic may contain small particles of paper, aluminium, wood, degraded polymer or other contaminants. A melt filtration system removes many of these impurities while the polymer is in the molten state.
Filtration improves the cleanliness and consistency of the recycled polymer and helps protect downstream processing equipment.
9. Pelletisation
After filtration, the molten polymer exits through the die as continuous strands or another desired shape. The material is cooled and then cut into small pieces called recycled plastic pellets or granules.
These pellets provide a convenient feedstock for subsequent manufacturing processes such as injection moulding, extrusion, blow moulding and fibre production, depending on the polymer and its quality.
10. Factors Affecting the Quality of Recycled Plastic
| Factor | Effect on Recycled Material |
|---|---|
| Polymer purity | Controls compatibility, processing behaviour and final properties. |
| Moisture | Can cause hydrolysis, bubbles and processing defects in moisture-sensitive polymers. |
| Contamination | Can reduce mechanical properties and cause defects in the final product. |
| Thermal history | Excessive heating can cause chain scission, oxidation or other degradation. |
| Residence time | Long residence times at high temperature can increase polymer degradation. |
| Additives | Plasticisers, pigments, stabilisers and fillers can affect processing and final properties. |
| Polymer compatibility | Incompatible polymer mixtures may produce poor interfacial adhesion and weak materials. |
11. Polymer Degradation During Recycling
Recycling does not necessarily leave the polymer chemically unchanged. Repeated processing can expose polymers to heat, oxygen, mechanical shear and moisture. These factors can alter molecular weight and molecular structure.
Depending on the polymer and processing conditions, degradation may involve chain scission, oxidation, cross-linking or other chemical changes. Chain scission generally reduces molecular weight and may affect melt viscosity and mechanical properties.
12. Complete Plastic Recycling Process
13. Advantages of Plastic Recycling
- Reduces the amount of plastic waste sent to disposal facilities.
- Conserves polymer resources and reduces demand for virgin resin in suitable applications.
- Allows useful materials to be recovered from discarded products.
- Can reduce the environmental burden associated with production of virgin plastics.
- Creates secondary raw materials for manufacturing.
- Supports a circular approach to material use.
14. Limitations and Challenges
- Mixed polymers are difficult to separate completely.
- Contamination can reduce the quality of recycled material.
- Repeated thermal processing can alter polymer properties.
- Some plastics contain additives that complicate recycling.
- Multilayer and composite materials can be difficult to recycle mechanically.
- Thermosetting polymers cannot generally be remelted and reshaped like thermoplastics.
- Recycled polymers may require blending, stabilisation or other processing to obtain the desired properties.
15. Conclusion
Plastic recycling is a multistep materials-processing operation rather than simply a melting process. Sorting and identification determine the composition of the feedstock, while washing and shredding prepare the material for efficient processing. After adequate drying, extrusion converts the cleaned plastic into a homogeneous molten stream that can be filtered, cooled and pelletised.