6.1
Addition Polymerisation
Addition PolymerFormed from alkene monomers via chain-growth polymerisation. No small molecule is lost. All atoms of monomer are incorporated in polymer chain.
| Monomer | Polymer | Uses |
|---|---|---|
| Ethene CH₂=CH₂ | Polyethylene (PE) —[CH₂–CH₂]ₙ— | Bags, bottles, pipes |
| Propene CH₂=CHCH₃ | Polypropylene (PP) | Car parts, fibres, packaging |
| Chloroethene CH₂=CHCl | PVC —[CH₂–CHCl]ₙ— | Pipes, electrical insulation, window frames |
| Styrene C₆H⁵CH=CH₂ | Polystyrene (PS) | Packaging, insulation, cups |
| Tetrafluoroethene CF₂=CF₂ | PTFE (Teflon) —[CF₂–CF₂]ₙ— | Non-stick cookware, medical implants |
| Acrylonitrile CH₂=CHCN | PAN (acrylic) | Fibres (acrylic wool substitute) |
6.2
Condensation Polymerisation
Condensation PolymerFormed by repeated condensation reactions between bifunctional monomers. A small molecule (H₂O or HCl) is eliminated with each bond formed.
Polyamides (Nylons)
Formed from: diamine + dicarboxylic acid (or diacid chloride)
Nylon-6,6: H₂N(CH₂)₆NH₂ + HOOC(CH₂)₄COOH → —[NH(CH₂)₆NHCO(CH₂)₄CO]ₙ— + H₂O
hexane-1,6-diamine + adipic acid → nylon-6,6
Linkage: amide bond –CO–NH–
Nylon-6: caprolactam (ring-opening) — one monomer only
Uses: clothing, parachutes, ropes, gears, toothbrush bristles.
Polyesters
Formed from: diol + dicarboxylic acid
PET (polyethylene terephthalate):
HO-CH₂CH₂-OH + HOOC-C₆H₄-COOH → —[O-CH₂CH₂-O-CO-C₆H₄-CO]ₙ— + H₂O
ethane-1,2-diol + terephthalic acid → PET (Dacron/Terylene)
Linkage: ester bond –CO–O–
Kevlar (polyaramid): diamino + dicarboxylic aromatic monomers; very strong
Uses: PET — bottles, clothing fibres, food packaging; Kevlar — bulletproof vests, helmets.
6.3
Natural Polymers
| Polymer | Monomer(s) | Linkage | Function/Uses |
|---|---|---|---|
| Proteins | Amino acids (20 types) | Peptide bond (–CO–NH–) | Enzymes, structural (hair, skin), hormones |
| Starch | α-glucose | Glycosidic bond (–O–) | Energy storage in plants; food |
| Cellulose | β-glucose | Glycosidic bond | Plant cell wall; paper, cotton |
| DNA/RNA | Nucleotides | Phosphodiester bond | Genetic information |
| Natural rubber | Isoprene (2-methylbuta-1,3-diene) | C=C addition (cis form) | Tyres, gloves (before vulcanisation) |
6.4
Vulcanisation of Rubber
Process and Effect
Natural rubber: cis-poly(isoprene) — chains are tangled but not cross-linked.
Properties: sticky when warm, brittle when cold, weak.
Vulcanisation: heating with sulfur (1–3% for soft rubber; 30% for hard ebonite)
Sulfur forms S–S cross-links between polymer chains at C=C positions.
Result after vulcanisation:
• Stronger, harder, more elastic
• Resistant to swelling in solvents and oils
• Stable over wider temperature range
• Uses: car tyres (with carbon black filler), conveyor belts, hoses
6.5
Biodegradability and Recycling
| Property | Addition Polymers (PE, PP, PVC) | Condensation Polymers (nylon, PET) |
|---|---|---|
| Biodegradability | Non-biodegradable (C–C backbone resists enzymes) | PET: non-biodegradable; nylon: very slow |
| Recycling | Mechanical recycling (HDPE=2, LDPE=4, PP=5, PVC=3, PS=6) | Chemical recycling (depolymerisation back to monomers) |
| Incineration | PE/PP: CO₂ + H₂O; PVC: HCl released (toxic) | CO₂ + H₂O + NOx |
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Bioplastics
PHA (polyhydroxyalkanoates) and PLA (polylactic acid) are produced from biomass and are biodegradable. PLA from lactic acid (fermentation of sugars) is used in packaging and medical sutures. Compostable under industrial conditions.
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Exercises
- State the type of polymerisation used to make (a) PVC (b) nylon-6,6 (c) PET. In each case name the monomer(s).(a) PVC: addition polymerisation; monomer: chloroethene (vinyl chloride). (b) Nylon-6,6: condensation; monomers: hexane-1,6-diamine + adipic acid (hexanedioic acid). (c) PET: condensation; monomers: ethane-1,2-diol + terephthalic acid (benzene-1,4-dicarboxylic acid).
- Explain why natural rubber is vulcanised and how this changes its properties.Natural rubber is cis-polyisoprene with no cross-links — chains slide past each other → sticky, weak, degrades in oil. Vulcanisation: heating with S forms S–S cross-links between adjacent polymer chains at C=C sites. Cross-linking prevents chains from sliding → stronger, harder, more elastic, more resistant to heat/solvents/oils. Low %S = soft rubber (car tyres); high %S = ebonite (hard rubber).
- Write the repeat unit of nylon-6,6 and identify the amide bond.Repeat unit: –NH(CH₂)₆NHCO(CH₂)₄CO–. The amide bond is –CO–NH– (formed by condensation between –COOH and –NH₂, releasing H₂O). Nylon-6,6 has two amide bonds per repeat unit (one from each junction of diamine and diacid).
- Why are most synthetic polymers not biodegradable? What problems does this cause?Most synthetic polymers (PE, PP, PVC, PS) have C–C or C–F backbone bonds that are not recognised or cleaved by biological enzymes. They accumulate in landfills and oceans for hundreds of years. Photodegradation produces microplastics (<5 mm), which enter food chains, harm marine organisms, and have been detected in human blood. PVC degradation releases toxic HCl. Environmental persistence = major pollution crisis.
- Distinguish between thermoplastic and thermosetting polymers with examples.Thermoplastic: chains not cross-linked; soften when heated (melt) and can be remoulded repeatedly. Examples: PE, PP, PVC, nylon. Can be recycled. Thermosetting: extensively cross-linked (3D network); permanently hard; do not soften on heating (decompose/char). Examples: Bakelite (phenol-methanal), epoxy resins, vulcanised rubber. Cannot be remoulded; more difficult to recycle.
- What is PLA and why is it considered an improvement over conventional plastics?PLA (polylactic acid): condensation polymer from lactic acid (CH₃CH(OH)COOH). Lactic acid is produced by fermentation of sugars (corn starch, sugarcane). PLA is biodegradable under industrial composting conditions (60°C, humidity, microbes). Carbon-neutral life cycle (CO₂ absorbed during crop growth = CO₂ released in degradation). Used for packaging, disposable cups, medical sutures. Drawbacks: requires industrial composting (not home compost or ocean), competes with food crops for land.
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Quiz
Unit 6: Polymers
25 QsQ1
Addition polymerisation differs from condensation polymerisation because:
Addition: alkenes join without losing atoms. Condensation: bifunctional monomers react, eliminating H₂O (or HCl). Both types can use one or two monomers; strength and biodegradability vary independently.
Q2
The repeat unit of PVC is:
Monomer: CH₂=CHCl (chloroethene). Addition: —CH₂–CHCl—. All C and H/Cl atoms from monomer incorporated in chain.
Q3
Nylon-6,6 is a polyamide formed from:
Hexane-1,6-diamine + adipic acid: H₂N(CH₂)₆NH₂ + HOOC(CH₂)₄COOH. The “6,6” refers to six carbon atoms in each monomer. Nylon-6 comes from caprolactam (one 6C monomer only).
Q4
Vulcanisation of rubber involves:
S–S cross-links (disulfide bridges) form between polymer chains at double-bond sites when heated with sulfur. Prevents chain slippage → harder, stronger, more elastic rubber.
Q5
PET is a polyester made from:
PET: ethane-1,2-diol (HO-CH₂CH₂-OH) + terephthalic acid (benzene-1,4-dicarboxylic acid). Ester linkages: –CO–O–. PET = polyethylene terephthalate.
Q6
The monomer used to make PTFE (Teflon) is:
PTFE: —[CF₂–CF₂]ₙ— from tetrafluoroethene. The strong C–F bonds make PTFE exceptionally inert, non-stick, and heat-resistant.
Q7
Which natural polymer has a peptide bond (–CO–NH–) as its repeating linkage?
Proteins have peptide bonds between amino acid residues: –NH–CHR–CO– repeat. (Nylon also has amide/peptide bonds but is synthetic.) Cellulose and starch have glycosidic bonds; DNA has phosphodiester bonds.
Q8
The main environmental problem caused by non-biodegradable plastic waste is:
Persistent pollution: PE/PP etc. last hundreds of years. UV breaks them into microplastics (<5 mm) → ingested by marine animals → enter food chain → detected in human tissue. Ocean plastic kills marine life via entanglement and ingestion.
Q9
The linkage in a polyester is:
–CO–O– (ester bond). Formed by condensation of –COOH + –OH → –CO–O– + H₂O.
Q10
Natural rubber (poly-cis-isoprene) without vulcanisation is:
Unvulcanised cis-polyisoprene: chains not cross-linked → sticky when warm (chains slide), brittle when cold (chains become rigid), weak, dissolves in organic solvents. Vulcanisation fixes all these problems.
Q11
Kevlar is used for bulletproof vests because:
Kevlar: aromatic polyamide (polyaramid). Rigid aromatic rings + amide H-bonds between aligned chains → tensile strength ~3.6 GPa (5× steel by weight). Absorbs and disperses kinetic energy of projectiles.
Q12
Starch and cellulose both consist of glucose units but differ because:
α-glucose forms starch (coiled helix → digestible by amylase). β-glucose forms cellulose (straight, H-bonded sheets — microfibrils → structural). Humans lack cellulase enzyme, so cannot digest cellulose.
Q13
The recycling code for PET plastic bottles is:
PET = recycling code 1. Code 2 = HDPE; 3 = PVC; 4 = LDPE; 5 = PP; 6 = PS; 7 = other. PET is one of the most widely recycled plastics (bottles, carpets, fleece).
Q14
PLA (polylactic acid) is described as biodegradable because:
PLA has ester bonds that can be hydrolysed by enzymes (esterases) in microorganisms under industrial composting conditions (high temperature, humidity). It does NOT biodegrade significantly in soil, home compost, or the ocean under normal conditions.
Q15
When PVC is burned, a particular hazard is:
PVC burning: C–Cl bonds release HCl gas (highly toxic, corrosive). Also can produce dioxins (polychlorinated dibenzodioxins) which are persistent organic pollutants. This is why PVC incineration requires careful emission control.
Q16
The monomer of natural rubber is:
Natural rubber = cis-poly(isoprene). Monomer: isoprene CH₂=C(CH₃)–CH=CH₂. Formed in the latex of rubber trees (Hevea brasiliensis). Synthetic rubber uses isoprene or butadiene.
Q17
A thermosetting polymer cannot be remoulded because:
Thermosets (Bakelite, epoxy, vulcanised rubber) have 3D cross-linked networks. Heating breaks the polymer backbone before the cross-links soften → charring/decomposition, not melting. Cannot be recycled by remelting.
Q18
The small molecule eliminated in the formation of nylon-6,6 is:
Condensation of –COOH + H–NH– → –CO–NH– + H₂O. When diacid chloride (–COCl) is used instead of diacid, HCl is eliminated. With adipic acid (–COOH), H₂O is eliminated.
Q19
Which polymer is used in Lycra/spandex (elastic fibres)?
Lycra/spandex is a polyurethane-based segmented copolymer: alternating hard (rigid) and soft (flexible) segments. The linkage is –O–CO–NH– (urethane bond). Extremely elastic: can stretch >500% and recover.
Q20
What is a copolymer?
A copolymer contains two or more chemically different repeat units (from two or more monomers). Examples: SBR (styrene-butadiene rubber); ABS (acrylonitrile-butadiene-styrene); nylon-6,6 (diamine + diacid, two monomers). Properties can be tuned by varying the monomer ratio.
Q21
Carbon black is added to rubber tyres because:
Carbon black (finely divided carbon, ~50 nm particles) reinforces vulcanised rubber: tensile strength increases 10×, abrasion resistance greatly improved. Without carbon black, tyre tread would wear out quickly. The black colour is a consequence, not the purpose.
Q22
DNA is a natural condensation polymer. Its monomers are:
DNA monomers are deoxyribonucleotides (four types: A, T, G, C). Each nucleotide contains a nitrogenous base + deoxyribose sugar + phosphate group. Phosphodiester bonds link the 3’ and 5’ carbons of adjacent sugars.
Q23
Polypropylene (PP) is made from:
Addition polymerisation: nCH₂=CHCH₃ → —[CH₂–CH(CH₃)]ₙ— (polypropylene). Used for car parts, food containers, fibres.
Q24
The main advantage of chemical recycling over mechanical recycling of polymers is:
Chemical recycling (hydrolysis, pyrolysis, glycolysis) regenerates monomers from mixed or contaminated polymer waste. Mechanical recycling downgrades the material (less strong, discoloured). Chemical recycling is more expensive but produces higher-quality recyclate.
Q25
PTFE (Teflon) is used as a non-stick coating because:
C–F is one of the strongest bonds in organic chemistry (544 kJ/mol). PTFE is chemically inert (resists acids, bases, solvents), has very low surface energy (food does not stick), and is stable up to ~260°C. Coefficient of friction is among the lowest of any material.
Q26
The repeat unit —[NH–CHR–CO]ₙ— represents:
–NH–CHR–CO– contains the amide bond –CO–NH–. This is the repeat unit of a polypeptide (protein) or synthetic polyamide (nylon). R = side chain (amino acid) or specific group (nylon).
Unit 6 Quiz — Polymers (25 Questions)
Select one answer eachQ1
Addition polymerisation involves:
In addition polymerisation, the C=C double bond of each monomer opens and joins to form the polymer. No small molecule is lost.
Q2
The repeat unit of poly(ethene) is:
Ethene (CH₂=CH₂) undergoes addition polymerisation; the repeat unit is –(CH₂–CH₂)–.
Q3
Condensation polymerisation produces a small molecule as a byproduct. This is typically:
Polyester formation loses H₂O; polyamide from diacid chloride loses HCl. Both are condensation polymers.
Q4
Nylon-6,6 is a polyamide made from:
Nylon-6,6: hexamethylenediamine (H₂N(CH₂)₆NH₂) + adipic acid/adipoyl chloride → polyamide with –CO–NH– links.
Q5
The linkage in a polyester is:
Polyesters form –CO–O– ester links between the carboxyl of one monomer and the hydroxyl of another.
Q6
PET (polyethylene terephthalate) is used for drinks bottles because:
PET is a tough, clear polyester — ideal for food packaging. It is not biodegradable (an environmental problem).
Q7
Proteins are natural polymers of:
Proteins are polypeptides: amino acids joined by –CO–NH– (peptide/amide) bonds formed by condensation.
Q8
Hydrolysis of a polyester gives:
Water breaks ester bonds: –CO–O– + H₂O → –COOH + HO–. Gives back diol and diacid monomers.
Q9
Thermosetting polymers differ from thermoplastics because they:
Thermosets (epoxy resins, Bakelite) have 3D cross-linked networks — permanent structure, cannot be remelted.
Q10
LDPE (low-density polyethylene) is produced at high pressure and has:
LDPE made by radical polymerisation at high pressure — branched chains prevent close packing → lower density.
Q11
HDPE (high-density polyethylene) is produced using a Ziegler-Natta catalyst giving:
Ziegler-Natta catalysts control polymerisation → linear HDPE. Closer packing → higher density and melting point.
Q12
The monomer of natural rubber is:
Natural rubber is cis-poly(isoprene). The monomer isoprene (CH₂=C(CH₃)–CH=CH₂) polymerises naturally in Hevea trees.
Q13
Vulcanisation of rubber involves:
Sulfur reacts with C=C bonds between chains → –S–S– cross-links. Prevents chains sliding — increases durability.
Q14
Starch is a natural polymer of:
Starch = amylose (α-1,4) + amylopectin (α-1,4 and α-1,6). Energy storage polymer in plants.
Q15
Cellulose differs from starch in that:
β-1,4 links give cellulose a linear structure. Chains align and form H-bonds → structural material in plant cell walls.
Q16
DNA is a polymer of:
DNA backbone: deoxyribose sugars linked by phosphate groups. Each nucleotide has a nitrogenous base attached.
Q17
The problem of disposing of addition polymers is that they are:
Most addition polymers (LDPE, PVC, PS) lack bonds that enzymes can hydrolyse — persist in the environment.
Q18
Kevlar is an aromatic polyamide used in bullet-proof vests because:
Kevlar chains align with strong H-bonds between –NH– and –C=O groups. Aromatic rings prevent rotation → rigid, strong fibre.
Q19
A plasticiser added to a polymer:
Plasticiser molecules insert between chains, separating them and reducing van der Waals forces → softer, more flexible material.
Q20
Which polymer is formed from chloroethene (vinyl chloride)?
CH₂=CHCl (chloroethene) undergoes addition polymerisation → PVC. Used in pipes, flooring, electrical insulation.
Q21
PTFE (Teflon) is made from:
CF₂=CF₂ polymerises to PTFE –(CF₂–CF₂)–. Very strong C–F bonds and non-polar surface → non-stick, chemically inert.
Q22
Conducting polymers such as polyacetylene conduct electricity because:
Alternating single and double bonds create a conjugated system with delocalised π electrons that can move — electrical conduction.
Q23
Biodegradable polymers such as PLA (polylactic acid) are preferred because:
PLA is a polyester from lactic acid. Ester bonds are hydrolysable by enzymes/water → reduces environmental persistence.
Q24
The glass transition temperature (Tg) of a polymer is:
Below Tg, polymer chains lack enough thermal energy to move past each other → brittle, rigid glass-like state.
Q25
Recycling of PET bottles involves:
PET can be mechanically recycled (re-extruded) or chemically hydrolysed → terephthalic acid + ethylene glycol for reuse.
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Unit Test — 50 marks
Section A
30 marksQ1 [5 marks]
Compare addition and condensation polymerisation under these headings: monomer type, bond formed, small molecule eliminated, example polymer, recyclability. [5]
Addition: alkene (C=C) monomers; C–C bond formed; no elimination; PE/PP/PVC/PS; mechanically recyclable (remeltable). Condensation: bifunctional monomers (diol+diacid, diamine+diacid); ester or amide bond; H₂O (or HCl) eliminated; nylon/PET/polyurethane; chemical recycling (hydrolysis) or mechanical.
Q2 [5 marks]
Draw the repeat unit of (a) nylon-6,6 and (b) PET. Identify the functional group that is the linkage in each. Name the monomers used for each. [5]
(a) Nylon-6,6: —[NH(CH₂)₆NHCO(CH₂)₄CO]ₙ—. Linkage: amide bond –CO–NH–. Monomers: hexane-1,6-diamine + hexanedioic acid (adipic acid). (b) PET: —[O–CH₂CH₂–O–CO–C₆H₄–CO]ₙ—. Linkage: ester bond –CO–O–. Monomers: ethane-1,2-diol + terephthalic acid (benzene-1,4-dicarboxylic acid).
Q3 [5 marks]
Explain how vulcanisation changes the properties of natural rubber and why these changes make vulcanised rubber more useful industrially. [5]
Natural cis-polyisoprene: no cross-links, chains slide → sticky/warm, brittle/cold, poor oil resistance. Vulcanisation: heating with S, S forms S–S bridges at C=C positions between chains. Cross-links prevent chain slippage. Result: harder, stronger, elastic (recovers shape after deformation), stable temperature range (-50 to +100°C), resistant to oils/solvents. Industrial uses: car tyres (20% S+carbon black), hoses, conveyor belts, seals.
Q4 [5 marks]
Describe THREE environmental problems caused by polymer waste and for each suggest a solution. [5]
1. Ocean plastic accumulation (non-biodegradable PE/PP bottles, bags): → ban single-use plastics; deposit return schemes; improved waste collection. 2. Microplastic pollution from photodegradation: enters food chains, detected in human blood. → Reduce plastic production; develop biodegradable alternatives (PLA, PHA). 3. PVC incineration releasing HCl and dioxins: toxic air pollutants, persistent. → Sort PVC from waste stream before incineration; specialist high-temperature incinerators with scrubbers; phase out PVC for food contact.
Q5 [5 marks]
A student claims that bioplastics (PLA) solve the plastic pollution crisis. Evaluate this claim, stating advantages and disadvantages. [5]
Advantages: PLA is made from renewable resources (corn starch, sugarcane) → reduces fossil fuel use; biodegradable (under industrial composting conditions, 60°C, weeks to months); carbon-neutral lifecycle (CO₂ fixed during crop growth = CO₂ released in degradation); suitable for medical sutures (biodegrades in body); replaces single-use plastics. Disadvantages: requires industrial composting (NOT home compost, NOT marine environment) — infrastructure limited; competes with food crops for arable land (food vs fuel debate); production still energy-intensive; degrades slowly in ambient conditions; cannot be recycled with conventional plastics (contaminates stream); more expensive than petrochemical plastics. Conclusion: PLA is an improvement but does not solve the crisis alone — requires combined approach (reduction, infrastructure investment, behaviour change).
Q6 [5 marks]
Compare natural rubber with synthetic rubber (SBR — styrene-butadiene rubber). Include: monomer(s), structure, properties, and industrial relevance. [5]
Natural rubber: monomer isoprene (2-methylbuta-1,3-diene); cis-polyisoprene; from Hevea trees; good elasticity, low hysteresis; vulnerable to oil/ozone degradation; ~45% of world rubber supply. SBR: copolymer of styrene (~25%) and butadiene (~75%) by emulsion polymerisation; random copolymer; similar elasticity; better abrasion/heat/oil resistance than NR; cheaper and more consistent supply. Both vulcanised for use in tyres (SBR ~70% of global tyre production). NR preferred for truck tyres (high-speed flexing generates heat — NR has lower heat build-up); SBR preferred for car tyres (better wet grip, abrasion).
Section B
20 marksQ7 [10 marks]
(a) Describe four different types of polymer (addition, condensation, natural, thermosetting) with a named example of each. For each, state the monomer(s), linkage, and one use. [6] (b) Explain what cross-linking is and how it affects the physical properties of a polymer, with reference to vulcanised rubber and a thermosetting polymer (Bakelite). [4]
(a) Addition: PE from ethene (C–C chain, no linkage group); use: packaging. Condensation: nylon-6,6 from diamine+diacid (amide bond –CO–NH–); use: clothing. Natural: protein from amino acids (peptide bond –CO–NH–); use: enzymes. Thermosetting: Bakelite from phenol+methanal (methylene bridges; 3D cross-linked network); use: electrical fittings, handles. [1.5 each, 6 total]
(b) Cross-linking: covalent bonds connecting adjacent polymer chains, forming a network. In vulcanised rubber: S–S bridges connect cis-polyisoprene chains at C=C positions. Prevents chains sliding past each other → elastic (recovers shape), stronger, heat-resistant. Soft rubber (2–3%S); hard rubber/ebonite (30%S). In Bakelite: phenol and methanal react to form branched chains + methylene (–CH₂–) cross-links in all three dimensions. Creates a rigid, infusible, extremely hard 3D network. Cannot be melted or remoulded — thermosetting.
(b) Cross-linking: covalent bonds connecting adjacent polymer chains, forming a network. In vulcanised rubber: S–S bridges connect cis-polyisoprene chains at C=C positions. Prevents chains sliding past each other → elastic (recovers shape), stronger, heat-resistant. Soft rubber (2–3%S); hard rubber/ebonite (30%S). In Bakelite: phenol and methanal react to form branched chains + methylene (–CH₂–) cross-links in all three dimensions. Creates a rigid, infusible, extremely hard 3D network. Cannot be melted or remoulded — thermosetting.
Q8 [10 marks]
(a) Discuss the use of polymers in modern society, giving five specific beneficial applications across different sectors. [4] (b) Evaluate the approach of mechanical recycling versus chemical recycling for polymer waste management, discussing efficiency, quality of recyclate, cost, and applicability to different polymer types. [6]
(a) 1. Medical: PLA sutures biodegrade in body (no removal surgery needed); PTFE vascular grafts (inert). 2. Aerospace/defence: Kevlar in helmets and body armour (high tensile strength:weight ratio). 3. Construction: PVC window frames (weatherproof, low maintenance, better insulator than Al); HDPE pipes (corrosion-resistant, flexible). 4. Food: PET bottles and cling film (impermeable, food-safe); nylon packaging films. 5. Electronics: PTFE as insulation in cables; ABS plastic in computer housings and 3D printing.
(b) Mechanical recycling: shred → melt → re-extrude. Advantages: established infrastructure, low cost, lower energy than virgin production. Disadvantages: each cycle degrades polymer (shorter chains, discolouration — “downcycling”); requires clean, sorted feedstock; limited cycles; mixed/contaminated waste cannot be processed. Applicability: PET, HDPE, PP (codes 1,2,5) most commonly mechanically recycled; PVC and PS rarely collected. Chemical recycling: depolymerisation (hydrolysis of PET/nylon back to monomers; pyrolysis of PE/PP to fuels/monomers). Advantages: handles mixed/contaminated streams; produces virgin-quality monomers; can address plastics mechanical recycling cannot. Disadvantages: currently expensive (>5× mechanical); limited industrial scale; energy-intensive; pyrolysis products contain contaminants. Best approach: tiered system — reduce → reuse → mechanical recycle (clean) → chemical recycle (mixed/contaminated) → energy recovery (incineration with heat recovery) as last resort.
(b) Mechanical recycling: shred → melt → re-extrude. Advantages: established infrastructure, low cost, lower energy than virgin production. Disadvantages: each cycle degrades polymer (shorter chains, discolouration — “downcycling”); requires clean, sorted feedstock; limited cycles; mixed/contaminated waste cannot be processed. Applicability: PET, HDPE, PP (codes 1,2,5) most commonly mechanically recycled; PVC and PS rarely collected. Chemical recycling: depolymerisation (hydrolysis of PET/nylon back to monomers; pyrolysis of PE/PP to fuels/monomers). Advantages: handles mixed/contaminated streams; produces virgin-quality monomers; can address plastics mechanical recycling cannot. Disadvantages: currently expensive (>5× mechanical); limited industrial scale; energy-intensive; pyrolysis products contain contaminants. Best approach: tiered system — reduce → reuse → mechanical recycle (clean) → chemical recycle (mixed/contaminated) → energy recovery (incineration with heat recovery) as last resort.