Describe how the properties of plastics have implications for their disposal

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Organic Chemistry – Polymers (Cambridge IGCSE 0620 Section 11.8)

Learning Objective

Describe how the physical and chemical properties of the main plastics determine which disposal method is most suitable and what environmental impacts may arise.

Key Definitions

  • Monomer: a small molecule that can join with other identical (or similar) molecules to form a polymer.
  • Polymer: a large macromolecule made up of many repeating monomer units linked together.
  • Addition polymerisation (chain‑growth): monomers add to a growing radical chain without loss of a small molecule. Most common plastics (PE, PP, PVC, PS) are produced this way.
  • Condensation polymerisation (step‑growth): each step joins two functional groups and releases a small molecule (usually water or methanol). PET, nylon‑6,6 and many biodegradable polymers are produced by this route.

Polymerisation Mechanisms (AO2 – exam level)

Addition (radical) polymerisation – three stages

  1. Initiation: a free‑radical initiator (e.g., a peroxide) or heat creates a reactive radical on a monomer.
  2. Propagation: the radical adds to successive monomer units, lengthening the chain (‑CH₂‑CH₂‑)ₙ.
  3. Termination: two radical chains combine or a radical is quenched, stopping growth.

Typical conditions: 150–250 °C, inert atmosphere, peroxide or UV initiator.

Condensation (step‑growth) polymerisation

  • Monomers contain two complementary functional groups (e.g., –COOH and –OH). They react stepwise, forming an ester or amide bond and releasing a small molecule (water or methanol).
  • Removal of the by‑product drives the reaction forward; the process is usually carried out at 200–280 °C under reduced pressure.
  • Resulting polymers are called **condensation polymers** (PET, nylon‑6,6, PLA, PHA).

Common Plastics – type, monomer, polymerisation, physical data

Plastic (polymer name) Monomer(s) Polymerisation type Melting / transition point (°C) Density (g cm⁻³) Key property for disposal
Polyethylene (PE) – LDPE / HDPE Ethene CH₂=CH₂ Addition (radical) LDPE ≈ 105, HDPE ≈ 130 0.91 – 0.96 Very low density, excellent chemical resistance, flexible (LDPE) or rigid (HDPE)
Polypropylene (PP) Propene CH₂=CHCH₃ Addition (radical) ≈ 160 ≈ 0.90 Higher melting point than PE, stiff, good fatigue resistance
Polyvinyl chloride (PVC) Vinyl chloride CH₂=CHCl Addition (radical) ≈ 80 °C (decomposes before true melting) 1.30 – 1.45 Hard, flame‑retardant, high chlorine content
Polystyrene (PS) Styrene C₆H₅CH=CH₂ Addition (radical) ≈ 240 1.04 – 1.06 Transparent, brittle, low impact resistance
Polyethylene terephthalate (PET) – condensation polymer Ethylene glycol HO‑CH₂‑CH₂‑OH + terephthalic acid HOOC‑C₆H₄‑COOH Condensation (step‑growth) Glass‑transition ≈ 80 °C; crystallisation/melting ≈ 260 °C 1.38 – 1.40 Clear, strong, good gas barrier, widely recycled
Nylon‑6,6 – condensation polymer Hexamethylenediamine H₂N‑(CH₂)₆‑NH₂ + adipic acid HOOC‑(CH₂)₄‑COOH Condensation (step‑growth) ≈ 260 ≈ 1.15 High tensile strength, semi‑crystalline
Polylactic acid (PLA) – biodegradable, condensation polymer Lactic acid CH₃‑CH(OH)‑COOH (via lactide) Condensation (ring‑opening polymerisation) ≈ 150‑180 ≈ 1.25 Biodegradable under industrial composting (≈ 58 °C, high moisture)

How Specific Properties Influence Disposal

  • Chemical resistance – Plastics that do not react with acids, bases or common solvents (PE, PP, PVC) persist in the environment and are hard to break down chemically. They are therefore most often dealt with by mechanical recycling or energy recovery.
  • Thermal stability / melting point – A high melting point enables melting and re‑extrusion (mechanical recycling) but also means more energy is needed for incineration; incomplete combustion can generate toxic gases (e.g., HCl from PVC).
  • Hydrophobicity – Water‑repellent surfaces hinder microbial attachment, leading to very slow natural biodegradation and long‑term landfill persistence.
  • Density – Low‑density plastics (PE, PP) float in water and become marine litter; high‑density plastics (PVC, PET) sink and accumulate in sediments.
  • Rigidity & shape – Rigid items (PET bottles, HDPE containers) are easy to compact for landfill but must be shredded before mechanical recycling; flexible films (LDPE) are harder to collect and sort.
  • Additives (plasticisers, stabilisers, flame‑retardants) – Can leach from landfills or be released during incineration, creating secondary pollution (e.g., phthalates, dioxins).
  • Biodegradability – PLA and other bio‑based polymers hydrolyse under controlled temperature and moisture; they are unsuitable for conventional landfill or recycling streams because they can contaminate them.

Disposal Methods Linked to Plastic Properties

Disposal method Plastic types that suit the method (by property) Advantages Environmental concerns
Mechanical recycling (melting & re‑extrusion) PE, PP, PET – moderate melting points, good melt flow, limited additives Reduces demand for virgin polymer; lower energy use than incineration Polymer chain scission reduces strength after several cycles; contamination with other polymers or additives lowers quality
Chemical recycling (depolymerisation) PET (hydrolysis / glycolysis), PS (solvent depolymerisation) Monomers can be recovered for virgin‑polymer synthesis; can handle coloured or mixed waste Requires strong acids, bases or organic solvents; generates hazardous by‑products that need treatment
Incineration with energy recovery PVC, PS, high‑calorific plastics (PP, PE) – high energy content Greatly reduces volume; electricity/heat can be captured Release of toxic gases (HCl from PVC, dioxins from PS); requires efficient flue‑gas cleaning (scrubbers, filters)
Landfilling All plastics, especially mixed or contaminated streams Simple, low immediate cost; no sorting required Very long persistence (centuries); leachate may contain additives; occupies limited landfill space; source of micro‑plastics
Industrial composting (biodegradation) PLA, PHA – biodegradable polymers that hydrolyse under ≈ 58 °C, high moisture Convert to CO₂, water and biomass; reduces landfill load Only works in specialised facilities; conventional plastics do not degrade and can contaminate compost streams

Quick‑Reference: Property → Preferred Disposal

Property Effect on disposal choice
Low melting point (≤ 130 °C) Easy mechanical recycling; low‑energy incineration but may release volatile organics.
High melting point (≥ 240 °C) Requires high‑temperature recycling equipment; incineration gives high calorific value but needs robust emission controls.
High chemical resistance Limits chemical recycling; favours mechanical recycling or energy recovery.
Hydrophilic / biodegradable (PLA, PHA) Suitable for industrial composting; not appropriate for landfill or conventional recycling.
Presence of chlorine (PVC) Incineration produces HCl → mandatory scrubbing; recycling limited by additive stability.
High density (> 1.30 g cm⁻³) Settles in marine sediments; easier to compact for landfill but harder to separate in recycling streams.

Key Points for Students (exam‑relevant)

  • Plastics are either **addition polymers** (most common) or **condensation polymers** (step‑growth). The type of polymerisation determines the main chemical bond (C‑C vs. ester/amide) and influences which recycling route is possible.
  • Physical data – melting point, density, and chemical resistance – allow you to predict the most feasible disposal method and the likely environmental problems.
  • High durability and chemical stability give plastics their useful properties but also make natural degradation extremely slow.
  • Mechanical recycling works best for polymers that melt cleanly and contain few additives; repeated heating degrades polymer chains, reducing strength.
  • Incineration recovers energy but can emit hazardous gases (HCl from PVC, dioxins from PS). Modern plants must have scrubbers and filters.
  • Biodegradable polymers (PLA, PHA) can be composted industrially, but they must be kept separate from conventional plastics to avoid contaminating the compost stream.
  • Additives such as plasticisers, stabilisers and flame‑retardants may leach from landfills or be released during incineration, adding an extra layer of environmental impact.

Suggested Diagram (for teacher use)

Flow‑chart linking major plastic properties (density, melting point, chemical resistance, hydrophobicity, presence of additives) to the most appropriate disposal routes (mechanical recycling, chemical recycling, incineration, landfill, industrial composting).

Summary

The same properties that make plastics valuable – durability, chemical resistance, a range of melting points – also dictate how they can be dealt with after use. By linking polymer structure, physical data and environmental behaviour, students can evaluate the advantages and drawbacks of each disposal option and understand why selecting recyclable or biodegradable polymers, improving collection systems and developing better waste‑treatment technologies are essential for sustainable plastic management.

1 For PET the Cambridge syllabus usually quotes the glass‑transition temperature (~80 °C) when discussing recycling, because the material is processed in its amorphous state. The crystallisation/melting temperature (~260 °C) is given for completeness.

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