Polymer Recycling

How much plastic is wasted/recycled?

6300 Mt waste,80% landfill & environment,10% incinerated

Attractive properties of Plastics

Lightweight, durable, chemical resistance, ease of processing, customisable

Thermoplastics

80% of plastics produced, reversible, recyclable, melted when heated.

Examples of Thermoplastics

PE, PP, PVC

Thermosets

Undegoes a chemical change when heated, cannot be re-melted and reformed.

Examples of Thermosets

Polyurethane, epoxy resons, polyesters

Flory-Huggins Theory

Entropy of mixing is too small, plastics don't mix

Benefits of a Circular Economy

Maintain at highest value, design in reduction of waste,use fewer materials with less energy to produce them

Open-Loop Recycling

Recycled plastics used for a different product

Closed-Loop Recycling

Recycled plastics are used to produce the same product

Melt Temperature

Reversible Phase transition where polymers become disordered liquids

Glass Transition

Reversible transition in amorphous polymers from brittle/glassy state to viscous/rubbery state

Crystallinity

Polymers exhibiting long range order despite high Mw, Chain folded structures , Can't be 100% crystalline

Low Density Polyethylene (LDPE)

Free radical polymerisation, High pressure 200 degrees, Peroxides, Highly branched, broad Mw dist.

High Density Polyethylene (HDPE)

Metal catalysed polymerisation, Low pressure,Linear architecture, Medium Mw dist.

Isotactic v Atactic

Iso = ordered

Syndiotactic

Syndio = swapping

Atactic

Random

Ziegler-Natta Polymerisation of a-olefins

Heterogeneous Catalyst, Chain polymerisation where the active centre is a polymer-catalyst bond.

Processing Thermoplastics

Solid-Liquid-Solid transition, Above Tg and Tm to physically change dimensions into a product

Extrusion polymer processing

Polymer forced out through a shaping die

injection moulding polymer processing

Plastic formed into a mould

Blow moulding polymer processing

Air is blown into mould

Primary Plastic Recycling

Mechanical Recycling, Bottle to bottle - closed loop

Secondary Plastic Recycling

Mechanical Recycling, Recycle to lower value plastic

Tertiary Plastic Recycling

Chemical Recycling, Depolymerisation

Quaternary Plastic Recycling

Energy Recovery, Pyrolysis

Why is mechanical recycling preferred?

Lowest carbon footprint with the minimal overall environmental impact

Mechanical Recycling Sorting

Remove other items, Separate rigid from flexible, Separate coloured from transparent, Separate plastic types

Regranulation

Melt processing, melt filtration

Why are thermoplastics difficult to process?

Different temperatures that are very high with high mechanical shear, reprocessed at highest temperature, degrades lower melting components.

Effects of Degradation from Reprocessing

Changes molecular structure, changes morphology (%crystallinity) and material properties

Radical Issues

High temp and mechanical shear produces scission, branching or crosslinking

Why is contamination bad for reprocessing?

Catalyses degradation of PET and other polymers, Reduce mechanical properties, Immiscible blends

How to mitigate degradation?

Primary antioxidants, secondary antioxidants, Formulate with virgin products, Open-loop recycling

Examples of mechanical recycling

HDPE Milk bottles, PVC window frames

Challenges in recycling

Highly mixed, high operational costs, presence of additives

Benefits of Thermosets

Smaller volume of plastics but higher performance

Crosslinked Topology

High crosslink density and complexity prevents recycling but aids performance and mechanical strength/properties

Crosslinking via radical polymerisation

Unsaturated polyesters using peroxide initiators to make a matrix for glass fibre composites

Dynamic Polymer Networks

Covalent Adaptable Networks can be crosslinked and delinked

Dissociative CANs

Crosslinked at low temperature, Thermoplastic at high temperatures, Breaks before rebinding, Loss of network integrity

Associative CANs

Crosslinked at low and high temperatures, malleable at high temperature, Fixed cross-link density

Polyesters

Unsaturated prepolymers and chain growth polymerisation

Polyurethanes

Step growth polymerisation, Diverse mechanical properties

Chemical Recycling

Basic polymer structure is changed, chemical changes are made through breaking bonds using heat or chemicals

Homogenous Feedstock for Chemical Recycling

Pure Waste streams to make pure monomers

Thermal Depolymerisation

Specific temperature where depolymerisation occurs

Solvolysis

Dissolving polymers at high t and p

Pyrolysis

Works on highly mixed streams and contaminated waste, Needs further purification, Energy intense

Catalytic Cracking

Reduced energy usage, narrow weight dist and more economical, Still requires purification

Hydro-pyrolysis

Hydrogen necessary, produces lower aromatic content, improve fuel made, high energy and hydrogen cost

Gasification

Handle any mixed streams, very high energy requirements

Composites

Notoriously hard to recycle

Carbon Fibre

High embodied energy, Can be recycled but there is waste of the plastic matrix

Why is recycling hampered

Low cost of alternative virgin materials

Cement Co-processing

Reuse of raw materials plus energy recovery, reduction of CO2 footprint of cement manufacturing, open-loop recycling

Pressolysis

Degradation of materials through pressure, zero-emission reclamation and reuse for plastics,polymers and composites. Superheated steam and swings of compression and decompression

Bioplastics

Bio-based, biodegradable or both

Composting of bioplastics

Breakdown to CO2 H2O and biomas

Home composting

Variable uncontrolled environment, low temp, microorganisms limited

Industrial Compositing

Controlled environment, temperature high 60 C, high humidity, aeration high flux

Biobased + Durable

Biobased and not biodegradable, PET, PTT, PE

Biobased + Biodegradable

Bioplastics, PLA, PHA, PBS

Fossil-based and Biodegradable

PBAT, PCL

How much plastic is produced?

400 million tonnes annually, 2 million tonnes of bioplastics, Mostly packaging and fast moving consumer goods

Biobased and durable polymers

Similar methods to fossil fuels except for PEF

Biobased and Biodegradable

Slightly different version of molecules, PLA, Bio-PBS, PHA

Fossil-based and Biodegradable

Agricultural mulch films for PBAT,

Bio-PE

ethylene from ethanol via biomass, Recycling is the same and so are the properties

PLA

Similar to polypropylene but biodegradable from starch feed, compostable, requires sorting

Could everything be synthesised from Biomass

Yes but: 20 GT per year of agricultural waste, extra processing steps, higher CO2 emissions, higher costs

LCA for Bio v Fossil

Initial manufacture emissions and end of life emissions,Certain molecules are more CO2 intensive

Types of biodegradation

Abiotic, biotic

Mechanistic Routes

Biological oxidation, Hydrolysis

Biodegradation

Depends on polymer and specifc environment, structure dependent

Advantages of Bioplastics

Materials, biobased, many monomers, Hydrolysable, Compostable

Challenges of Bioplastics

Cost more, more effort, requires specific conditions for degradation, Compositing releases CO2, Not necessarily have a lower environmental footprint than fossil fuel plastics