Plastic Waste and Recycling
Explain plastic disposal problems, distinguish physical and chemical recycling, and weigh social, economic and environmental trade-offs.
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A plastic can be useful because it lasts a long time, yet create problems after disposal for the same reason. A recycling decision needs two questions: what happens to the polymer chains, and what are the costs and benefits of the whole scheme?
Durability becomes a disposal problem
Non-biodegradable plastics are not readily broken down by microorganisms under the disposal conditions. They can persist in landfill or as litter. Larger items can entangle animals or be swallowed; weathering can break them into smaller fragments, including microplastics. Fragmentation makes smaller pieces of plastic rather than necessarily breaking the polymer molecules down into harmless substances.
Plastic does not have to be burned to cause harm. Incineration can reduce the volume of waste and recover energy, but burning carbon-containing plastic produces CO2 and may produce other air pollutants depending on the plastic and combustion controls. Review air-pollutant sources and controls to connect the process to its emissions.
Reducing unnecessary use and reusing suitable products can reduce the waste needing treatment. Recycling can recover material, but it needs collection, sorting and a process suited to the waste.
Two ways to recycle
Physical recycling: keep the polymer chains
For the required poly(ethene) example:
- Collect and sort the waste to separate suitable poly(ethene) from other materials.
- Clean it and cut or shred it into small pieces.
- Melt the small pieces and form pellets.
- Use the pellets to make suitable new plastic products.
Poly(ethene) is a thermoplastic: it can soften or melt on heating and be reshaped. Physical recycling changes its form; it does not deliberately turn the chains back into monomers. Mixing incompatible plastics or leaving contamination can weaken the resulting material. Not every plastic can be melted and reshaped by this route.
Chemical recycling: change the molecules
Chemical recycling breaks bonds in the polymer chains. The products depend on the polymer and process:
| Process | What happens to the chains? | Useful products |
|---|---|---|
| Depolymerisation | Suitable polymers are broken back into their monomers. | Monomers recovered as chemical feedstock |
| Cracking plastic waste | Chains break into smaller molecules, often a mixture. | Products suitable for use as fuel |
Cracking does not necessarily recover the original monomer. Recovering ethene from poly(ethene) cannot be assumed just because the chain originally came from ethene. The cracking lesson develops bond breaking and atom conservation.
Recover polyester monomers
The required depolymerisation example is acid-catalysed hydrolysis of a polyester made from a diol and a dicarboxylic acid. Hydrolysis uses water to break the ester linkages. Water is a reactant; the acid is a catalyst.
A diol has two alcohol OH groups. A dicarboxylic acid has two COOH groups. Breaking the polyester’s ester links can recover those original monomers, which can be separated and purified for reuse.
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The equation states the types of products; it is not a molecular equation with specified amounts. You need the ester linkage, water, acid catalyst and recovered monomer types here, rather than a mechanism.
Poly(ethene) has a C–C backbone without ester links, so this polyester-hydrolysis route does not recover ethene from poly(ethene). Having a bond that can be hydrolysed under controlled chemical conditions also does not prove that a plastic will biodegrade quickly in soil or seawater.
Weigh the trade-offs
| Issue | Question to investigate | Why it matters |
|---|---|---|
| Social | Can households understand the sorting rules? Is collection accessible? | Participation and contamination affect how much useful waste is collected. |
| Economic | What do collection, sorting, cleaning and processing cost? Is there demand for the product? | A recovered material needs a useful market; it is not automatically cheaper than new plastic. |
| Environmental | What energy, transport, emissions and waste does the scheme involve? | Recovering material can reduce disposal and demand for new resources, while processing also has impacts. |
Use the data and priorities of the particular situation. Chemical recycling is not automatically better than physical recycling, and a recycling symbol alone does not prove that a local scheme can process an item.
Worked decision: include transport
In an illustrative model, two schemes recover the same amount and quality of plastic. Local scheme A causes 20 kg CO2 emissions during transport and 40 kg during processing. Distant scheme B causes 70 kg during transport and 15 kg during processing. Which has lower combined emissions?
A totals 60 kg CO2 and B totals 85 kg CO2, so A is lower by 25 kg for these stated stages. B has lower processing emissions but higher total emissions. This model does not compare costs, collection access or other impacts, so it cannot establish which scheme is best in every respect.
Apply polyester hydrolysis
Guided practice 1
Recover monomers from a polyester
Guided recycling decision
Connect the ester linkage to hydrolysis
Hints
Hint 1: linkage
Hint 2: reverse the formation
View solution step by step
Name the chemical method
Method
Select depolymerisation rather than physical recycling.Reason
The aim is to change polymer molecules back into monomers, not merely reshape the material.Working
Method: chemical recycling by depolymerisation.Apply the polyester example
Method
Use water with an acid catalyst to hydrolyse the ester linkages.Reason
Hydrolysis reverses the linkage-forming process and recovers the two monomer types.Working
polyester + water → [acid\ catalyst] diol + dicarboxylic\ acid.
Try without prompts
Mind stretcher 1: Choose a route from the chain structureExtension
A plant receives two separately sorted materials: P has repeat unit [-CH₂-CH₂-]ₙ and Q has repeated -C(= O)-O⁻ linkages between carbon-containing sections. A learner recommends water with an acid catalyst to return both to their original monomers. Assess that proposal. For P, suggest a route that keeps the chains as polymers and name the intermediate product in the physical recycling example.
Show answer
Q is a polyester, so acid-catalysed hydrolysis can break its ester links and recover monomers. P is poly(ethene), with no ester links; that treatment does not recover its ethene monomer. Suitable P waste can be cleaned, shredded and melted into pellets for making new products. The chains remain polymers in that physical route.
Mind stretcher 2: One scheme does not win on every measureExtension
In an illustrative comparison of the same sorted waste, scheme A recovers 600 kg of usable plastic for a total cost of 1,200 dollars; scheme B recovers 800 kg for 2,400 dollars. Compare recovered amounts and cost per kilogram. Give one further environmental fact and one social fact you would need before recommending a scheme.
Show answer
A recovers less material but costs 2 dollars per kg; B recovers 200 kg more but costs 3 dollars per kg. Neither wins on both stated measures. Useful environmental evidence could include energy use, transport emissions or remaining waste. Useful social evidence could include collection access, sorting effort or expected participation. A recommendation should state its priorities and use the extra evidence, rather than treating one number as the whole decision.
Mind stretcher 3: “Biodegradable” overclaimExtension
A student writes: “Biodegradable plastics solve plastic pollution completely.” Explain why that claim is too broad.
Show answer
Breakdown depends on conditions (temperature, oxygen, microbes). In the wrong conditions, biodegradables can still persist.
Also, biodegradable plastics do not stop littering or all microplastic problems.
Biodegradable does not mean instant or guaranteed breakdown in all conditions.
Connect the method to the outcome
Physical recycling reshapes suitable plastic while keeping polymer chains. Depolymerisation can recover monomers; cracking produces smaller molecules for uses such as fuel. Judge a recycling scheme using the polymer, the products and social, economic and environmental evidence.
Practise and check
Use the Organic Chemistry topic check to practise and check your understanding.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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