Lecture 9 - Recycling of materials Flashcards
What are the 2 main strategies to reduce plastic waste?
- Recycling
- Usage of biodegradable plastics
What are the problems associated with recycling plastic waste?
Difficulty in recycling
What are some challenges with using biodegradable plastics as a strategy to reduce plastic waste?
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
All bioplastics are biodegradable. True or false?
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)
What are some challenges with using biodegradable plastics as a strategy to reduce plastic waste?
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
What is the current best strategy to reduce plastic waste?
Mechanical recycling of plastics
Plastic recycling technologies - mechanical recycling
What are the 2 types of recycling under mechanical recycling, and the quality of polymers produced?
-
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)
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?
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 from main plastic streams (i.e. removing degraded plastics from PET/HDPE/… recycling lines)
Plastic recycling technologies - mechanical recycling
What are the 4 different wet processes in mechanical recycling?
- Eddy current separator
- Float and sink
- Jigging
- Hydrocycloning
Plastic recycling technologies - mechanical recycling
How does sink and float separation work? Which part of the sorting process is it usually placed at?
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.
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?
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.
- Purpose : to separate non ferrous metals from waste streams
- Typically placed at the beginning of the sorting process
Plastic recycling technologies - mechanical recycling
How does hydrocyclocloning work and what kinds of materials is it good at separating?
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)
Plastic recycling technologies - mechanical recycling
Hydrocyclocloning involves moving parts in the machine. True or False?
False.
Plastic recycling technologies - mechanical recycling
How does jigging work and its purpose?
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
Plastic recycling technologies - mechanical recycling
Outline the main steps in mechanical recycling, once plastics are all sorted into their separate streams.
- Waste material (mixture of glass, plastic, paper) undergo wet and dry separation techniques
- Plastics are separated into their respective recycling bins
- Plastics are shredded by passing through rotating blades and flakes are sorted by size
- The flakes are washed and dried
- Flakes undergo reprocessing to form plastic pellets which can be reused
Plastic recycling technologies - mechanical recycling
What are the 2 types of reprocessing plastic flakes can undergo after shredding? Explain how they work
- 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.
- Agglomeration : low density plastic films are heated by fruiction 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
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]
- Low density plastics would take up a lot of space in the extruder but have less plastic, making extrusion process inefficient.
- Low density plastics don’t flow well in extruders, leading to issues such as uneven melting / overheating etc
Plastic recycling technologies - mechanical recycling
What are the issues related with mechanical recycling? [2]
- Shredding of plastics causes polymer degradation, affecting their mechanical properties (polymer chain can degrade from heat / oxidation)
- Remaining impurities from other packaging components can reduce physical-mechanical characteristics of reprocessed plastics
Plastic recycling technologies - mechanical recycling
In mechanical recycling of plastics, near infrared (NIR) can be used to. What does NIR separate?
NIR separates polymers with different functional groups, such as polyolefins (C=C or rather C-H) such as PP / PE and polyesters (-OCO-) such as PET
Plastic recycling technologies - chemical recycling
What is chemical recycling? What is the advantage of chemical recycling?
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
Plastic recycling technologies - chemical recycling
What plastic polymers can chemical recycling be utilised for? [3]
Polymers with polar groups
1. PET, polyester (most common)
2. Nylon (polyamide) → but not super common in food packaging
3. Polycarbonates
Plastic recycling technologies
Multi-layered packaging is best recycled with mechanical recycling. True or False?
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
Plastic recycling technologies - chemical recycling
Chemical recycling can be used for polyolefins such as PE and PP. True or False?
False.
Hydrocarbon bonds in polyolefins are hard to hydrolyse via chemicals, and chemical recycling is only mainly used for polyesters such as PET
Plastic recycling technologies - chemical recycling
It is impossible to do chemical recycling for polyolefins. True or False?
False. Under chemical recycling, thermal recycling methods such as cracking and pyrolysis could be used to break hydrocarbon bonds in polyolefins
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
-
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 - Glycolysis : A glycol (e.g. ethylene glycol, where have 2 -OH groups) is used to hydrolyse bonds ; with moderaate energy and environmental impact
- Aminolysis : use of ammonium based ionic liquids to hydrolyse bonds. Best energy and environmental parameters
Plastic recycling technologies - chemical recycling
What are the 4 main types of chemical recycling methods?
- Solvolysis (usage of solvent to hydrolyse bonds) - mainlyfor polymers with polar functional groups like polyesters, polyamides
- Catalytic depolymerisation (including enzymatic depolymerisation)
- Thermal recycling
- suitable for polyolefins too
Plastic recycling technologies - chemical recycling
What are the 3 categories of catalytic depolymerisation, and how does it work?
- Enzymatic catalysis : microbial and fungal species aid in degradation of various polymers (hard to enzymatically degrade polyolefins)
- 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
- 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
Plastic recycling technologies - chemical recycling
What are kinds of polymers does hydrosilyation depolymerise? [3]. What is the drawback of this method of enzymatic catalysis?
- PET, PS, PVC
- Drawback : high cost and low yield
Plastic recycling technologies - chemical recycling
What are the 2 main methods of thermal degradation under chemical recycling? State how they work.
- 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
- Hydrocracking : heavier chemical fractions in petroleum (e.g. naptha) are chemically broken down to produce monomers such as ethylene and propylene, which can polymerise to form polymers such as PE/PP
Plastic recycling technologies - chemical recycling
The liquid / wax mixtures collected from pyrolysis is suitable for reprocessing into plastics directly. True or False?
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.
Plastic recycling technologies - chemical recycling
What conditions are needed for hydrocracking? [2]
- Requires bifunctional catalyst with acidic function to enhance cracking activity
- High surface area support with materials such as zeolite
Plastic recycling technologies - recycling of other materials
Outline the process of paper recycling. [5]
- Paper is separated based on type : paper vs cardboard
- Paper is de-inked
- Metal parts such as staples, and other parts such as glues are removed through chemical wash
- Bleach may be added to whiten slurry
- Cellulose fibres are mixed with new pulp and reprocessed into paper.
Plastic recycling technologies - recycling of other materials
What is a problem with paper recycling?
What is a solution to overcome this?
With each subsequent round of recycling, cellulose fibres shorten and making it less useful to be recycled into new paper.
- 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
Plastic recycling technologies - recycling of other materials
All glass materials can be recycled. True or False?
False, glass materials such as windows have other materials incorporated into it, making it unsuitable for recycling.
Only glass bottles can be separated now.
Plastic recycling technologies - recycling of other materials
Outline the process of glass recycling [5]
- Glass is collected and sorted based on colours (e.g. green glass can only be recycled into green glass etc)
2. Labels are removed - Glass is washed
- Glass is shredded to increase surface area
- Glass is melted, poured into mould and cooled down
Plastic recycling technologies - recycling of other materials
Outline the steps in aluminium recycling [5]
- Aluminium is melted at 730°C (a layer of Al2O3 may form during melting, and is scrapped off the ground)
- Molten metal flows down into a furnace (holding area)
- Furnace is tipped and molten metal is poured into a mould, forming alumiunium blocks (ingots), and transported to another recycling facility
- Ingots undergo rolling and flattened into aluminium sheets to relax bonds in aluminium
- Alumiunium sheet is converted into a diverse range of packaging items (cans, aerospace materials, household items)
