Lecture 9 - Recycling of materials

Question 1

What are the 2 main strategies to reduce plastic waste?

Answer 1

1. Recycling
2. Usage of biodegradable plastics

Question 2

What is the main problem associated with recycling plastic waste?

Answer 2

Difficulty in recycling
- Contamination in recycling streams
- Multi-layered packaging : hard to separate layers and hard to recycle due to adhesives (so use chemical recycling or ensure adhesives is water-soluble)

Question 3

What are some challenges with using biodegradable plastics as a strategy to reduce plastic waste? [2]

Answer 3

1) Biodegradable plastics like PLA and PHA are chemically similar to PET (polyesters). Due to poor consumer sorting, PLA and PHA often end up in PET recycling bins, contaminating the stream. Unlike PET, which melts at ~260°C, **PLA and PHA degrade at much lower temperatures (~60–80°C), forming unwanted residues **and reducing recycled PET quality.

2) Biodegradable plastics may also take very long to degrade naturally in the environment

Question 4

All bioplastics are biodegradable. True or false?

Answer 4

False.
Bioplastics are plastics formed from renewable sources (can be regenerated fast enough, from starch fermented by microbes), but not all of them are biodegradable (e.g. biobased PE, PET)

Question 5

What are some challenges with using biodegradable plastics as a strategy to reduce plastic waste?

Answer 5

1) Biodegradable plastics like PLA and PHA are chemically similar to PET (polyesters). Due to poor consumer sorting, PLA and PHA often end up in PET recycling bins, contaminating the stream. Unlike PET, which melts at ~260°C, PLA and PHA degrade at much lower temperatures (~60–80°C), forming unwanted residues and reducing recycled PET quality.

2) Biodegradable plastics may also take very long to degrade naturally in the environment

Question 6

What is the current best strategy to reduce plastic waste?

Answer 6

Mechanical recycling of plastics

Question 7

Plastic recycling technologies - mechanical recycling

What are the 2 types of recycling under mechanical recycling, and the quality of polymers produced?

Answer 7

1. Primary recycling (industrial recycling) : recycling of plastic materials that are waste products from industrial processes, and have yet to reach consumers (so they are typically clean) → high quality polymers formed ;; can be used for food packaging
   2.Post consumer recycling (PCR) : Recycle of post-consumer plastics → poorer polymer quality ;; downcycled (used for parts on furniture etc and cannot be used for food packaging)

Question 8

Plastic recycling technologies - mechanical recycling

What is the main method used for dry recycling, and which part of the separation process can it be used at?

Answer 8

Waste (plastic) materials are put into air classifiers / ballistic separators to separate lighter from heavier materials.
- Placed at the beginning of process : separate plastic from other materials
- Placed at end of process : separate* end-of-life plastics* (cannot be further reprocessed) from main plastic streams (i.e. removing small bits of degraded plastics from PET/HDPE/… recycling lines)

Question 9

Plastic recycling technologies - mechanical recycling

What are the 4 different wet processes in mechanical recycling?

Answer 9

1. Eddy current separator
2. Float and sink
3. Jigging
4. Hydrocycloning

Question 10

Plastic recycling technologies - mechanical recycling

How does sink and float separation work? Which part of the sorting process is it usually placed at?

Answer 10

Usage of water to separate high density plastics from low density plastics (polyolefins - PE, PP).
- Placed at the beginning of the process, effective for first separation but needs to be combined with other separation techniques.

Question 11

Plastic recycling technologies - mechanical recycling

What is the purpose of an eddy current separator and how does it work? Is it placed at the beginning or end of the separation process?

Answer 11

It utilises a high speed magnetic rotor to generate electric current (eddy current), which removes nonferrous (non magnetic) metals such as Cu and Al from waste plastic, glass and paper. (Electric field in metals repel the electric field generated, causing them to “fly” further and get separated)
- Purpose : to separate non ferrous metals from waste streams
- Typically placed at the beginning of the sorting process

Question 12

Plastic recycling technologies - mechanical recycling

How does hydrocyclocloning work and what kinds of materials is it good at separating?

Answer 12

Input materials (liquid slurry) enters the hydrocyclone tangentically (at an angle), causing them to spin and generate centrifugal force.

Heavier materials flow out in the underflow, while lighter materials flow out in the overflow (upwards)
- Good for separating plastics (overflow) from metals (underflow)

Question 13

Plastic recycling technologies - mechanical recycling

Hydrocyclocloning involves moving parts in the machine. True or False?

Answer 13

False.

Question 14

Plastic recycling technologies - mechanical recycling

How does jigging work and its purpose?

Answer 14

Water is pushed up and down by a piston, and plastics are separated based on their morphological and physical charactersitics. Low density plastic would end up in the overflow

One of the oldest gravity separation methods, and can be used in place of sink and float

Question 15

Plastic recycling technologies - mechanical recycling

Outline the main steps in mechanical recycling, once plastics are all sorted into their separate streams.

Answer 15

1. Waste material (mixture of glass, plastic, paper) undergo wet and dry separation techniques
2. Plastics are separated into their respective recycling bins
3. Plastics are shredded by passing through rotating blades and flakes are sorted by size (to increase surface area for melting)
4. The flakes are washed and dried
5. Flakes undergo reprocessing (melting, extrusion, pelletisation) to form plastic pellets which can be reused

Question 16

Plastic recycling technologies - mechanical recycling

What are the 2 types of reprocessing plastic flakes can undergo after shredding? Explain how they work

Answer 16

1. Extrusion : plastic flakes are fed into an extruder and pushed into a heated cylinder to be melted. At the end of the extruder, a pelletizer cools and cuts the final polymer into pellets and recooled.
2. Agglomeration : low density plastic films are heated by friction and cooled by water to form agglomerates (clump together), causing the bulk density to increase. This forms chips, which are then fed through extruder for further processing

Question 17

Plastic recycling technologies - mechanical recycling

Why do low density plastics have to undergo agglomeration before being fed into an extruder? Why not just directly feed it into an extruder? [2]

Answer 17

1. Low density plastics would take up a lot of space in the extruder but have less plastic, making extrusion process inefficient.
2. Low density plastics don’t flow well in extruders, leading to issues such as uneven melting / overheating etc

Question 18

Plastic recycling technologies - mechanical recycling

What are the issues related with mechanical recycling? [2]

Answer 18

1. Shredding of plastics causes polymer degradation, affecting their mechanical properties (polymer chain can degrade from heat / oxidation)
   2.Remaining impurities from other packaging components can reduce physical-mechanical characteristics of reprocessed plastics

Question 19

Plastic recycling technologies - mechanical recycling

In mechanical recycling of plastics, near infrared (NIR) can be used to. What does NIR separate?

Answer 19

NIR separates polymers with different functional groups, such as separating polyolefins (C=C or rather C-H) such as PP / PE from polyesters (-OCO-) such as PET

Question 20

Plastic recycling technologies - chemical recycling

What is chemical recycling? What is the advantage of chemical recycling?

Answer 20

Chemical recycling involves the use of chemicals or heat (in the presence of catalyst) to hydrolyse bonds.

• Advantage : chemical recycling forms original starting monomers, thus “closing the loop” and enabling them to be reprocessed to produce high-value added chemicals for upcycling

Question 21

Plastic recycling technologies - chemical recycling

What plastic polymers can chemical recycling be utilised for? [3]

Answer 21

Polymers with polar groups
1. PET, polyester (most common)
2. Nylon (polyamide) → but not super common in food packaging
3. Polycarbonates

Question 22

Plastic recycling technologies

Multi-layered packaging is best recycled with mechanical recycling. True or False?

Answer 22

False.
Multi-layered packaging is hard to separate with mechanical recycling as layers are glued together, making it hard to separate. Chemical recycling is more suitable
- Polymers that react with chemical will be collected as a solution, leaving behind Al layers etc

Question 23

Plastic recycling technologies - chemical recycling

Chemical recycling using chemicals can be used for polyolefins such as PE and PP. True or False?

Answer 23

False.
Hydrocarbon bonds in polyolefins are hard to hydrolyse via chemicals, and chemical recycling is only mainly used for polyesters such as PET.

> but they could be chemically recycled via pyrolysis (using heat)

Question 24

Plastic recycling technologies - chemical recycling

It is impossible to do chemical recycling for polyolefins. True or False?

Answer 24

False. Under chemical recycling, thermal recycling methods such as cracking and pyrolysis could be used to break hydrocarbon bonds in polyolefins

Question 25

# **Plastic recycling technologies - chemical recycling** What are the 4 solvolysis methods and explain how they are carried out? Also state their energy consumption levels and environmental impact

Answer 25

1) **Hydrolysis** : use of water to hydrolyse bonds, carried out in neutral, acidic or alkaline conditions. High energy consumption 2)**Methanolysis** : use of methanol to hydrolyse bonds at high temperature and pressure (suitable for various polymers : polyamides, polycarbonates and polyesters). High energy consumption. 3) **Glycolysis** : A glycol (e.g. ethylene glycol, where have 2 -OH groups) is used to hydrolyse bonds ; with moderaate energy and environmental impact. 4) **Aminolysis** : use of ammonium based ionic liquids to hydrolyse bonds. Best energy and environmental parameters.

Question 26

# **Plastic recycling technologies - chemical recycling** What are the 3 main types of chemical recycling methods?

Answer 26

1. Solvolysis (usage of solvent to hydrolyse bonds) - mainlyfor polymers with polar functional groups like polyesters, polyamides 2. Catalytic depolymerisation (including enzymatic depolymerisation) 4. Thermal recycling - suitable for polyolefins too

Question 27

# **Plastic recycling technologies - chemical recycling** What are the 3 categories of catalytic depolymerisation, and how does it work?

Answer 27

1. **Enzymatic catalysis** : microbial and fungal species aid in degradation of various polymers (hard to enzymatically degrade polyolefins) 2. **Hydrogenolysis** : hydrogen used to cleave bonds between a carbon atom and another atom (e.g. C-C, C-O, C-N), in the presence of a catalyst. After fragmentation, H atoms are added to resulting fragments. ***Carried out in the presence of hydrogen and Milstein type Ru-PNN complexes*** 3. **Hydrosilylation** : addition of H-Si to unsaturated bonds using a borane / iridium catalyst. Addition of H-Si makes polymers more reactive to catalyst action, causing depolymerisaiton

Question 28

# **Plastic recycling technologies - chemical recycling** What are kinds of polymers does hydrosilyation depolymerise (specific names)? [3]. What is the drawback of this method of enzymatic catalysis?

Answer 28

- PET, PS, PVC → hydrosilyation involves electrophilic addition of H-Si across unsaturated bonds (works well for unsaturated polymers like polyolefins, polyamides, polyesters) - Drawback : high cost and low yield

Question 29

# **Plastic recycling technologies - chemical recycling** What are the 2 main methods of thermal degradation under chemical recycling? State how they work.

Answer 29

1. Pyrolysis : processes that causes plastics to decompose at high temperatures (500°C) in the absence of oxygen. Liquid / wax mixtures rich in hydrocarbons are collected, leaving aluminium / laminate layer behind 2. Hydrocracking : heavier chemical fractions in petroleum (e.g. naptha) or plastic polymers are chemically broken down to produce monomers such as ethylene and propylene, which can polymerise to form polymers such as PE/PP

Question 30

# **Plastic recycling technologies - chemical recycling** The liquid / wax mixtures collected from pyrolysis is suitable for reprocessing into plastics directly. True or False?

Answer 30

False. All polymers (polyesters, polyolefins etc) can degrade in pyrolysis to form a mixture with different monomers, thus they cannot be reprocessed directly (need pure monomers to be reprocessed). Thus, it is an ideal raw material for a refinery instead to separate out the various monomers

Question 31

# **Plastic recycling technologies - chemical recycling** What conditions are needed for hydrocracking? [2]

Answer 31

1. Requires bifunctional catalyst with acidic function to enhance cracking activity 2. High surface area support with materials such as zeolite

Question 32

# **Plastic recycling technologies - recycling of other materials** Outline the process of paper recycling. [5]

Answer 32

1. Paper is separated based on type : paper vs cardboard 2. Paper is de-inked 3. Metal parts such as staples, and other parts such as glues are removed through chemical wash 4. Bleach may be added to whiten slurry 5. Cellulose fibres are mixed with new pulp and reprocessed into paper.

Question 33

# **Plastic recycling technologies - recycling of other materials** What is a problem with paper recycling? What is a solution to overcome this?

Answer 33

With each subsequent round of recycling, cellulose fibres shorten and making it less useful to be recycled into new paper (poorer mechanical properties) - To overcome this, cellulase enzyme is used to break down cellulose into glucose monomers, which are fed to microbes for fermentation, to generate biofuels such as ethanol or other materials

Question 34

# **Plastic recycling technologies - recycling of other materials** All glass materials can be recycled. True or False?

Answer 34

False, glass materials such as windows have other materials incorporated into it, making it unsuitable for recycling. Only glass bottles can be separated now.

Question 35

# **Plastic recycling technologies - recycling of other materials** Outline the process of glass recycling [5]

Answer 35

1. Glass is collected and sorted based on colours (e.g. green glass can only be recycled into green glass etc) **2. Labels are removed** 3. Glass is washed 3. Glass is shredded to increase surface area 4. Glass is melted, poured into mould and cooled down

Question 36

# **Plastic recycling technologies - recycling of other materials** Outline the steps in aluminium recycling [5]

Answer 36

1. Aluminium is melted at 730°C (a layer of Al₂O₃ may form during melting, and is scrapped off the ground) 2. Molten metal flows down into a furnace (holding area) 3. Furnace is tipped and molten metal is poured into a mould, forming alumiunium blocks (ingots), and transported to another recycling facility 4. Ingots undergo **rolling** and flattened into aluminium sheets to relax bonds in aluminium 5. Alumiunium sheet is converted into a diverse range of packaging items (cans, aerospace materials, household items)