Plastic Recycling

Overview of Plastic Recycling

Plastic recycling deals with returning plastic materials to useful applications after their initial use instead of disposing of them as waste. It links material science (structure and properties of polymers) with environmental chemistry and chemical engineering.

In this chapter, the focus is on:

• Why recycling plastics is challenging and important.
• How plastics are identified and sorted.
• Main recycling routes: mechanical, chemical, and thermal.
• Typical products from recycled plastics and their limitations.
• Environmental and economic aspects specific to plastic recycling.

General aspects of polymer structure, plastic types, and processing are covered elsewhere; here we build on that knowledge to explain recycling-specific issues.

Why Plastic Recycling Is Needed and Difficult

Motivation for Plastic Recycling

Plastics are widely used because they are light, versatile, and often inexpensive. This leads to:

• High consumption, especially of short‑lived products (e.g. packaging).
• Large volumes of plastic waste.
• Persistence in the environment because most commodity plastics are not biodegradable.
• Use of non‑renewable resources (primarily fossil hydrocarbons).

Recycling aims to:

• Reduce the demand for virgin polymer and fossil raw materials.
• Decrease the volume of waste sent to landfills or incinerators.
• Lower greenhouse gas emissions compared with producing new plastic.
• Limit environmental pollution, especially macro‑ and microplastics.

Specific Challenges

Plastic recycling is more complex than recycling metals or glass. Key reasons:

• Many different polymer types (e.g. polyethylene (PE), polypropylene (PP), poly(ethylene terephthalate) (PET), polystyrene (PS), poly(vinyl chloride) (PVC), engineering plastics).
• Additives such as plasticizers, stabilizers, pigments, flame retardants, and fillers change properties and complicate recycling.
• Multilayer and composite products (e.g. films combining several polymers, fiber‑reinforced plastics, metal–plastic composites) are difficult to separate.
• Contamination with food residues, other materials, or dirt.
• Degradation of polymer chains during use and reprocessing (e.g. chain scission, oxidation), which generally lowers material quality.

A central concept in plastic recycling is downcycling: often the recycled product has lower performance or value than the original material, restricting its use.

Types and Sources of Plastic Waste

Post‑Consumer vs Post‑Industrial

• Post‑consumer waste
• Arises after use by households or end‑users.
• Examples: bottles, packaging films, disposable cutlery, household items.
• Typically mixed, contaminated, and highly variable in composition.
• Post‑industrial waste
• Produced as scrap during manufacturing (e.g. sprues, off‑cuts, off‑spec parts).
• Usually relatively clean and homogeneous.
• Technically much easier to recycle; often regranulated and reused internally.

Recycling systems often distinguish these waste streams because they require different collection, sorting, and processing steps.

Main Plastic Types in Household Waste

Consumer packaging is dominated by:

• PE (films, bags, bottles, closures).
• PP (containers, caps, rigid packaging).
• PET (beverage bottles, some trays).
• PS and expandable polystyrene (EPS) (yoghurt cups, foam packaging).
• Smaller fractions of PVC and other engineering plastics.

Efficient recycling usually requires separation of these types, because blends of incompatible polymers have poor mechanical properties.

Identification and Sorting of Plastics

Sorting is a central step in recycling: quality and purity of the sorted fractions strongly influence the quality of the recyclate and the available recycling routes.

Collection Systems

• Source separation: Households or businesses separate plastic packaging from other waste (e.g. separate bins or bags).
• Mixed waste collection: Plastics are later separated from residual waste in mechanical sorting plants.
• Deposit–return systems: Specific items (e.g. PET bottles) are collected with high purity and low contamination via deposit systems. These are ideal feedstocks for high‑quality recycling.

Manual and Mechanical Pre‑Sorting

In sorting facilities:

• Large non‑plastic items and contaminants (metal parts, glass, stones) are removed.
• 2D (films, paper) and 3D (bottles, containers) items may be separated.
• Manual sorting is used where automation is difficult or to improve purity.

Material Identification Technologies

Common technologies for separating plastics by type:

• Density‑based separation
• In water tanks, lighter plastics (e.g. PE, PP) float, denser ones (e.g. PET, PVC) sink.
• Works only for certain combinations and is influenced by fillers and contamination.
• Near‑infrared (NIR) spectroscopy
• Plastics have characteristic infrared absorption patterns.
• Sensors identify polymer type on a conveyor belt and trigger air jets to separate items.
• Widely used for packaging; struggles with black or heavily filled plastics because they absorb infrared light.
• X‑ray and other sensors
• X‑ray transmission can distinguish materials with high chlorine content (e.g. PVC) from others.
• Color sensors recognize transparent vs colored items or specific colors (e.g. clear vs green PET).
• Magnetic and eddy current separation
• Used primarily to remove metals, which must be removed before further plastic processing.

Complications in Sorting

• Multilayer packaging: Laminates of, for example, PET/PE, PP/PA (polyamide), or plastic–aluminum composites cannot be easily classified as a single material.
• Black and very dark plastics: Difficult to detect by NIR; they often end up in mixed fractions.
• Labels, closures, and adhesives: Can introduce foreign materials into otherwise pure streams (e.g. PVC sleeves on PET bottles).

The purity of sorted fractions is crucial: even a few percent of an incompatible polymer can significantly degrade the mechanical properties or processability of the recyclate.

Mechanical Recycling of Plastics

Mechanical recycling uses physical and thermal processes to transform plastic waste into new plastic products without deliberately changing the polymer’s chemical structure.

Process Steps

Although details vary, typical steps are:

1. Shredding and size reduction
• Waste plastics are cut or ground into flakes or regrind, which facilitates washing and separation.
2. Washing and cleaning
• Removal of dirt, labels, glues, and contents (e.g. food residues).
• Often involves water, detergents, friction washers, and sometimes hot caustic solutions (e.g. NaOH) for PET bottles.
3. Density separation (where applicable)
• Float–sink separation to separate different plastics or remove heavier contaminants.
4. Drying
• Reduces moisture before melting, especially important for hygroscopic polymers (e.g. PET, PA).
5. Melt processing
• The clean flakes are melted in an extruder.
• Molten plastic can be filtered to remove remaining solid contaminants (e.g. metals, paper, unmelted pieces).
6. Granulation
• The melt is cut into pellets or granules (regranulate), which can be used in standard plastic processing (e.g. injection molding, extrusion).

Closed‑Loop vs Open‑Loop Recycling

• Closed‑loop recycling
• The recycled plastic is turned back into the same product type, ideally with similar quality.
• Example: PET bottle‑to‑bottle recycling using high‑purity input and additional purification steps.
• Often requires strict control over the input stream, advanced decontamination, and quality control.
• Open‑loop recycling (downcycling)
• Recyclate is used for other products with lower performance requirements.
• Example: mixed polyolefin recyclate from packaging used for park benches, pallets, or construction products.
• Mechanical properties and appearance are often inferior to virgin materials.

Quality Issues in Mechanical Recycling

Typical problems specific to mechanical processes:

• Polymer degradation
• Heat and oxygen can cause chain scission (reducing molecular weight) or crosslinking.
• Leads to embrittlement, discoloration, and lower mechanical strength.
• Stabilizers (antioxidants, UV stabilizers) may be added during reprocessing.
• Incompatibility of polymer blends
• Many polymers do not mix on a molecular level; blends can show phase separation and poor adhesion between phases.
• This results in weak, brittle materials.
• Compatibilizers (special additives that improve adhesion between phases) can partially mitigate this.
• Residual contamination
• Remaining metals, paper, or other plastics cause defects, discoloration, or processing problems (e.g. gas bubbles).
• Odors from absorbed organic compounds limit use in sensitive applications (e.g. food packaging).

Because of these issues, recyclates from mixed household waste are often used in non‑food, non‑critical applications where appearance and high mechanical performance are less important.

Chemical Recycling of Plastics

In chemical recycling, polymers are converted into monomers or other small molecules using chemical or thermal processes. The goal is to restore the feedstock to a level comparable with virgin raw materials or to obtain valuable chemicals.

Chemical recycling is particularly attractive for:

• Highly contaminated streams.
• Mixed or complex plastics that are not suitable for high‑quality mechanical recycling.
• Polymers with well‑defined depolymerization chemistry.

Depolymerization to Monomers

Some condensation polymers can be “unmade” under suitable conditions.

Example: PET Depolymerization

PET can be broken down into smaller molecules such as:

• Terephthalic acid (or dimethyl terephthalate)
• Ethylene glycol

Main approaches (simplified):

• Hydrolysis
• PET + water (often at high temperature and sometimes high pressure, with catalysts) → terephthalic acid + ethylene glycol.
• Requires careful energy and process management.
• Glycolysis
• PET + excess ethylene glycol (with catalysts) → oligomers or bis‑(2‑hydroxyethyl) terephthalate.
• These products can be purified and re‑polymerized to PET.
• Methanolysis
• PET + methanol → dimethyl terephthalate + ethylene glycol.
• Similar to original PET manufacturing pathways.

Benefits:

• High potential product purity after purification.
• Possibility of truly closed‑loop recycling (back to PET suitable for food contact).

Challenges:

• Process complexity and cost.
• Handling of additives and labels.
• Need for efficient collection and sorting to supply appropriate feedstock.

Other Polymers

• Polyamides (e.g. nylon) can be hydrolyzed or otherwise depolymerized under specific conditions to yield monomeric or oligomeric products.
• Some polyurethanes can be chemically broken down into polyol components.

Not all common thermoplastics (e.g. PE, PP) can be efficiently depolymerized to monomers under industrially practical conditions; for these, other chemical recycling routes are more relevant.

Solvolysis and Dissolution‑Based Processes

• Solvolysis uses specific solvents or reactive liquids to selectively dissolve or chemically modify a polymer.
• In dissolution–reprecipitation processes:
• One polymer in a mixture is dissolved in a tailored solvent.
• Insoluble components (other plastics, fillers) are filtered off.
• The polymer is then precipitated from the solution, often giving a high‑purity product.
• This approach is particularly suited to relatively pure waste streams (e.g. single‑polymer films) and can produce high‑quality recyclate, but relies on solvent management and recovery.

Feedstock Recycling (Thermochemical Recycling)

Feedstock recycling aims to convert plastics into synthesis gas or liquid hydrocarbons, which can be used as:

• Raw materials for new polymers.
• Fuels.
• Chemical intermediates.

Important processes:

• Pyrolysis
• Thermal degradation of plastics in the absence of oxygen.
• Produces a mixture of gases, oils, waxes, and solid residue (char).
• PE and PP, for example, yield hydrocarbon mixtures that can be refined or cracked to obtain olefins.
• Gasification
• High‑temperature conversion of plastic waste with limited oxygen or steam.
• Produces synthesis gas (mainly CO and H₂).
• The syngas can be used in existing chemical value chains (e.g. synthesis of methanol, Fischer–Tropsch processes).
• Hydrogenolysis and related catalytic processes
• Plastics are broken down in the presence of hydrogen and catalysts, producing more defined hydrocarbon fractions.

Feedstock recycling sits at the interface between chemical recycling and energy recovery. The classification depends on whether the products are used as fuels (energy recovery) or as chemical raw materials (chemical/feedstock recycling).

Thermal Recycling (Energy Recovery)

Thermal recycling refers to using plastic waste as fuel to recover its energy content, for example in:

• Municipal waste incineration plants with energy recovery (heat, electricity).
• Co‑incineration in cement kilns or industrial furnaces.

Characteristics:

• Plastics, especially polyolefins, have high calorific values, comparable to or higher than many fossil fuels.
• Incineration reduces waste volume significantly and destroys organic pollutants and pathogens.

However, from a materials perspective:

• The polymer is not preserved; it is irreversibly destroyed.
• Only the energy value, not the material value, is recovered.

Environmental and technical aspects:

• Emissions (CO₂, NOₓ, HCl from PVC, and others) require advanced flue‑gas cleaning.
• The presence of halogens and additives (e.g. brominated flame retardants) requires careful emission control and ash handling.
• CO₂ emissions are high because most plastics are fossil‑based.

While energy recovery can be an important part of integrated waste management (especially for non‑recyclable plastic residues), it is not recycling in the sense of maintaining the material cycle.

Design for Recycling

Improving recycling does not only depend on waste treatment technologies; it also depends strongly on how plastic products are designed.

Important principles:

• Material simplification
• Use as few different polymers as possible per product or package.
• Prefer single‑material constructions where feasible.
• Avoiding problematic additives
• Some colorants (especially carbon black), certain flame retardants, and other additives hinder recycling or create environmental concerns.
• Where possible, use additives that are compatible with recycling and well‑characterized.
• Recyclable combinations
• Design closures, labels, and seals that are compatible with the main polymer or can be easily removed during washing or sorting.
• Avoid multilayer structures combining incompatible polymers when high‑barrier properties can be achieved by alternative designs.
• Clear labeling
• Use standardized markings for polymer type and, if relevant, for recyclability.
• Facilitate correct separation by consumers and automated systems.
• Consideration of multiple lifecycles
• Anticipate that materials may go through several reprocessing steps and may require stabilization.

This approach is often summarized under the term “design for recycling” or broader “design for circularity”, and aims to support a functioning circular economy for plastics.

Environmental and Economic Aspects Specific to Plastic Recycling

Environmental Benefits and Trade‑Offs

Potential environmental benefits of effective plastic recycling:

• Reduced consumption of fossil resources for polymer production.
• Lower greenhouse gas emissions compared with producing virgin plastics (depending on process and energy mix).
• Less plastic in landfills and the environment, lowering the risk of microplastic generation and ecosystem impacts.

However, trade‑offs exist:

• Collection, sorting, washing, and reprocessing require energy and water.
• Some recycling technologies (especially chemical processes) can be energy‑intensive.
• Transport of waste and recyclates adds to environmental burdens.

The net benefit depends strongly on:

• The type and purity of input waste.
• The efficiency of the recycling process.
• The quality and lifetime of the products made from recyclate.
• The energy source (fossil vs renewable) for processing.

Economic Factors

Key economic influences:

• Market value of recyclate vs virgin polymer
• Virgin polymer prices fluctuate with oil and gas prices.
• High virgin prices make recycling more competitive; low prices can discourage it.
• Quality and consistency of recyclate
• Processors require predictable material properties.
• Inconsistent quality reduces willingness to use recyclates and pushes them into low‑value applications.
• Scale and infrastructure
• Sorting and recycling plants require significant investment.
• Economies of scale are important for economic viability.
• Regulatory framework
• Deposit systems, recycling targets, landfill bans, and taxes on non‑recycled plastics change the economic balance.
• Mandatory recycled content in certain products (e.g. beverage bottles) creates stable demand and supports high‑quality recycling.

Together, these factors determine which recycling routes are implemented at large scale and how quickly new technologies (e.g. advanced chemical recycling methods) can be introduced.

Typical Applications of Recycled Plastics

Because of quality and regulatory constraints, recycled plastics are often used in specific product categories:

• Recycled PET (rPET)
• Fibers for textiles, carpets, and nonwovens.
• Straps, sheets, and some packaging.
• With high‑purity feedstock and advanced decontamination: beverage bottles (closed‑loop).
• Recycled polyolefins (rPE, rPP)
• Garbage bags, construction films, pipes (non‑pressure, non‑potable applications).
• Pallets, crates, and garden furniture.
• Technical parts with moderate property requirements.
• Mixed plastic recyclates
• Thick‑walled products, boards, and profiles for outdoor applications.
• Products where surface quality and precise mechanical properties are less critical.

The specific use depends on:

• Polymer type and degree of contamination.
• Color (mixed streams often produce dark or gray recyclate).
• Regulatory requirements (especially for food contact and products with high safety relevance).

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Summary

Plastic recycling encompasses several routes:

• Mechanical recycling preserves the polymer structure, is widely applied to relatively clean and homogeneous waste streams, and often results in downcycling.
• Chemical recycling breaks polymers down to monomers or other feedstocks and can, in principle, produce materials comparable to virgin products, but is technologically and economically demanding.
• Thermal recycling (energy recovery) recovers energy but not materials and is mainly relevant for non‑recyclable residues.

Efficient plastic recycling depends on:

• Effective collection and sorting systems.
• Control of contamination and polymer degradation.
• Product design that facilitates recycling.
• Economic and regulatory conditions that support the use of recyclate.

Understanding these aspects is essential for evaluating plastics as materials in a sustainable, circular economy context.