Polymer Disposal and Recycling Revision Notes | A Level AQA Chemistry

1. The Polymer Problem: Why Disposal Matters

Polymers are everywhere — packaging, clothing, medical devices, car parts. Their strength and chemical resistance make them extraordinarily useful. Those same properties make them extraordinarily difficult to get rid of.

Most synthetic polymers are non-biodegradable. Microorganisms cannot break down the long carbon chains of addition polymers such as poly(ethene) or poly(propene), so they persist in the environment for hundreds of years. This creates a genuine disposal challenge that sits at the heart of the AQA specification.

The core question you need to be able to answer is: given a particular polymer and a particular context, which disposal method is most appropriate, and what are the trade-offs? That requires understanding each method at a chemical level, not just a surface level.

2. Disposal Method 1: Landfill

Landfill is the simplest disposal route — waste polymers are buried in the ground. For decades it was the default option in the UK, and it remains common for mixed or contaminated plastic waste that cannot be sorted economically.

The key advantage of landfill is that it is cheap and requires no sorting or processing of the waste stream. For polymers that are heavily contaminated or made from mixed materials that are difficult to separate, landfill may be the only economically viable option.

The fundamental chemical problem with landfilling polymers is that the C–C backbone of addition polymers is highly stable. There are no functional groups that soil microorganisms can attack enzymatically, so the polymer sits there. Conventional condensation polymers such as PET are marginally more susceptible to hydrolysis, but degradation timescales in landfill are still measured in decades under typical landfill conditions.

3. Disposal Method 2: Combustion (Burning)

Since polymers are largely hydrocarbon-based, they have a high calorific value — they release significant energy when burned. Combustion with energy recovery (sometimes called waste-to-energy incineration) exploits this by burning polymer waste to generate heat or electricity.

Complete combustion of a hydrocarbon polymer produces carbon dioxide and water. For poly(ethene), the repeat unit is $-CH_2-CH_2-$, and complete combustion follows the same chemistry as burning any alkane:

$-[CH_2-CH_2]_n- + 3n\,O_2(g) \rightarrow 2n\,CO_2(g) + 2n\,H_2O(g)$

This is where the chemistry gets more complicated. Not all polymers are pure hydrocarbons. Poly(chloroethene) — PVC — contains chlorine. When PVC burns, hydrogen chloride gas ($HCl$) is produced, which is toxic and corrosive. Incomplete combustion of any polymer produces carbon monoxide ($CO$), a poisonous gas, and particulate carbon (soot).

Modern waste-to-energy plants use scrubbers and filters to remove acidic gases and particulates from flue gases before they are released. This makes combustion considerably cleaner than uncontrolled burning, but the infrastructure is expensive and the $CO_2$ output remains a concern in the context of climate change.

4. Disposal Method 3: Mechanical Recycling

Mechanical recycling is what most people picture when they think of plastic recycling — sorting, cleaning, shredding, melting, and re-moulding polymer waste into new products. It is the most widely practised form of polymer recycling in the UK.

Worked example: The Mechanical Recycling Process

Trace the stages by which a used poly(ethylene terephthalate) (PET) drinks bottle becomes a recycled product.

1. Polymer waste is collected and sorted by polymer type, typically using infrared spectroscopy or density separation. — Different polymers have different properties and cannot be melted together — mixing them produces a weak, brittle material. Sorting is therefore essential for quality recycling.
2. Sorted polymer is washed to remove food residues, labels, and adhesives, then shredded into small flakes. — Contamination degrades the quality of the recycled polymer. Even small amounts of a different polymer type can significantly weaken the final product.
3. The clean flakes are melted and extruded or moulded into pellets, which are then used to manufacture new products. — Thermoplastic polymers (including PET, HDPE, and PVC) soften on heating and can be re-moulded. Thermosetting polymers cannot be recycled this way because their cross-linked structure does not melt.
4. Recycled polymer typically has lower molecular mass and more chain defects than virgin polymer, so it is often used in lower-specification products (e.g. fleece fabric, drainage pipes). — Each recycling cycle degrades the polymer chains slightly. This is called 'downcycling' — the recycled material is used for a less demanding application than the original.

Answer: Mechanical recycling converts waste polymer into usable material without breaking down the polymer chains chemically, but quality typically decreases with each recycling cycle.

A critical limitation of mechanical recycling is that it only works for thermoplastic polymers. Thermosetting polymers — those with extensive cross-linking between chains — do not soften on heating. They cannot be re-moulded and must be disposed of by other means.

5. Disposal Method 4: Feedstock Recycling (Chemical Recycling)

Feedstock recycling takes a fundamentally different approach. Rather than re-moulding the polymer as it is, the polymer chains are broken down chemically into smaller molecules — monomers or other useful hydrocarbons — which can then be used as raw materials for new chemical synthesis.

Temperature and moisture are the most important variables to control because biodegradable polymers typically require both elevated temperature and sufficient humidity to degrade rapidly, as in industrial composting conditions around 60 C.

For condensation polymers such as polyesters, hydrolysis is an alternative route. PET, for example, can be hydrolysed under acidic or alkaline conditions to regenerate the original monomers — ethane-1,2-diol and terephthalic acid — which can then be repolymerised to make new, high-quality PET. This is genuinely closed-loop recycling.

Feedstock recycling is particularly valuable for polymers that cannot be mechanically recycled — heavily contaminated waste, multi-layer packaging, and thermosetting plastics. It represents a more complete solution to the polymer disposal problem, though the economics and infrastructure are still developing.

6. Disposal Method 5: Biodegradable and Compostable Polymers

An alternative approach to disposal is to design the polymer so that it breaks down naturally. Biodegradable polymers contain functional groups — typically ester or amide linkages — that can be hydrolysed by microbial enzymes in soil or water.

Poly(lactic acid) (PLA) is a commercially important example. It is made from lactic acid (a renewable feedstock derived from fermented plant starch) and degrades in industrial composting conditions to carbon dioxide and water. The ester linkages in PLA's backbone are susceptible to hydrolysis in a way that the C–C backbone of poly(ethene) is not.

A common misconception is that 'biodegradable' automatically means environmentally benign. Many biodegradable polymers require industrial composting facilities operating at 60 °C or above to degrade within a useful timeframe. In a landfill or in the ocean, they may persist almost as long as conventional plastics.

7. Comparing Disposal Methods: The Full Picture

No single disposal method is ideal for all polymers in all contexts. The best choice depends on the polymer type, the degree of contamination, the available infrastructure, and the environmental priorities of the situation. This is exactly the kind of multi-factor evaluation that AQA 'evaluate' questions demand.

When you encounter an 'evaluate' question on this topic, the examiner is looking for a balanced argument: state the advantage, then the limitation, then reach a reasoned conclusion. Avoid absolute statements — no method is entirely good or entirely bad.

8. Exam Technique: Answering Disposal and Recycling Questions

AQA questions on polymer disposal typically use 'evaluate', 'compare', or 'suggest' as command words. Each demands a different response structure, and confusing them is one of the most common ways students lose marks.

For 'evaluate' questions, you must present evidence on both sides and reach a conclusion. For 'compare' questions, you must explicitly state both similarities and differences — writing only about one polymer or one method will not score full marks. For 'suggest' questions, there may be more than one acceptable answer, so apply your chemical knowledge to the specific context given.

The key distinction is structural: addition polymers with only C-C backbones are much less susceptible to enzymatic hydrolysis than condensation polymers containing ester or amide linkages.