Chemistry • Year 12 • Module 8 • Lesson 16
Polymers: Structure, Properties & Applications
Build HSC Band 5–6 extended-response technique on polymer structure–property reasoning, evaluating polymer suitability, and critiquing claims about plastics and recycling.
1. Data + scenario — selecting a polymer for packaging (Band 5–6)
8 marks
Scenario. A NSW food company is redesigning packaging for a refrigerated product. The packaging must meet the following requirements: rigid enough to protect the product, able to withstand temperatures from 0°C to 60°C without deforming, recyclable in standard kerbside streams, resistant to moisture, and as low in cost as possible. The company is evaluating three options:
| Option | Polymer | Type | Melting / softening point (°C) | Recyclable (kerbside Code)? | Moisture resistance | Relative cost |
|---|---|---|---|---|---|---|
| A | HDPE | Addition (thermoplastic) | ~130 | Yes (Code 2) | Excellent | Low |
| B | Bakelite (thermoset) | Condensation (thermosetting) | Decomposes >300 | No (thermoset) | Good | Moderate |
| C | PET | Condensation (thermoplastic) | ~250 | Yes (Code 1) | Good | Low–moderate |
Q1. Evaluate the three packaging options and recommend the most suitable choice. In your response you must:
- Classify each polymer type (addition or condensation; thermoplastic or thermosetting) and explain the structural reason for its thermal behaviour.
- Use the data table to compare all three options against at least three criteria from the design brief.
- Explain why Option B cannot be kerbside recycled by referring to its polymer structure.
- Reach an evidence-based recommendation, explicitly acknowledging any trade-offs involved.
2. Source critique — evaluate a claim about plastics (Band 5–6)
7 marks
“All plastics are basically the same material — just carbon and hydrogen chains. So if one plastic bag has a recycling symbol on it, any other plastic item can be recycled in the same stream. Also, when plastics float in the ocean they gradually dissolve into harmless molecules, just like salt dissolves in water. The real solution to ocean plastic is simply to wait, since nature will eventually deal with it.”
— Composite of common misconceptions in non-specialist media.
Q2. Critically evaluate the three claims in this passage. For each error, explain the correct chemistry using lesson terminology. Your response must:
- Identify and correct the claim that all plastics are the same material (refer to addition vs condensation polymers, functional groups in backbones, and thermoplastics vs thermosets).
- Identify and correct the claim that any plastic can go in the same recycling stream.
- Identify and correct the claim that ocean plastics dissolve like salt, using the concepts of polymer persistence and microplastic formation.
- Reformulate the final sentence into a scientifically defensible statement about the actual long-term fate of ocean plastics.
Q1 — Marking criteria (8 marks)
- 1 mark — Classifies Option A correctly: HDPE is an addition polymer (from ethene, an alkene monomer, no by-product) and a thermoplastic (no covalent cross-links, softens on heating as intermolecular forces are overcome).
- 1 mark — Classifies Option B correctly: Bakelite is a condensation polymer (formed with loss of water) and a thermosetting polymer (extensive covalent cross-links form permanently during curing; cannot be remelted).
- 1 mark — Classifies Option C correctly: PET is a condensation polymer (diol + diacid, loss of water, ester links) and a thermoplastic (no covalent cross-links, can be remelted; recyclable Code 1).
- 1 mark — Compares at least three criteria from the table with specific data: e.g. temperature tolerance (HDPE ~130°C, PET ~250°C both meet 0–60°C requirement; Bakelite decomposes above 300°C but is not recyclable), recyclability (Options A and C yes; B no), cost (A lowest, C low–moderate, B moderate).
- 1 mark — Explains why Option B cannot be kerbside recycled: thermoset cross-links are permanent covalent bonds; heating causes decomposition rather than softening, so the material cannot be melted and reformed.
- 1 mark — Moisture resistance: all three are adequate; eliminates this as a differentiating criterion.
- 1 mark — Reaches a clear recommendation: HDPE (Option A) is the best overall choice because it meets the temperature requirement (softening point ~130°C, well above 60°C), is kerbside recyclable (Code 2), has excellent moisture resistance, and is the lowest cost. PET (Option C) is also valid but slightly more expensive. Bakelite is not suitable because it cannot be recycled.
- 1 mark — Acknowledges trade-off: PET offers a higher thermal ceiling and is also recyclable, so if future products needed to withstand higher temperatures, PET might be preferred despite slightly higher cost.
Sample response (key points): HDPE is an addition thermoplastic polymer: ethene monomers join across the double bond with no by-product, and the linear chains are held together by van der Waals forces that can be overcome by heating, allowing remoulding. Its softening point (~130°C) is well above the 60°C maximum in the brief, so it will not deform in use. It is accepted as kerbside Code 2, has excellent moisture resistance, and is the lowest cost option. Bakelite is a condensation thermosetting polymer: its permanent covalent cross-links prevent remelting, so it cannot enter any standard kerbside recycling stream and must be discarded. PET is a condensation thermoplastic: ester links form via loss of water from diol and diacid monomers; it is kerbside recyclable (Code 1) and has a higher thermal ceiling (~250°C). On cost, HDPE is lower than PET. The recommendation is HDPE because it meets all design criteria at the lowest cost and is readily recyclable. The trade-off is that PET has a higher thermal ceiling and could be preferred if future formulation requirements raised the temperature specification.
Q2 — Marking criteria (7 marks)
- 1 mark — Overall: the passage contains three distinct scientific errors.
- 1 mark — Error 1 corrected: plastics are chemically diverse. Addition polymers have hydrocarbon backbones (polyethylene), or chloro-substituted (PVC), or fluorinated (PTFE) chains. Condensation polymers have functional group links in the backbone (amide in nylon, ester in PET). Thermoplastics have no covalent cross-links; thermosets do. These structural differences give very different properties.
- 1 mark — Error 2 corrected: different plastics have different chemical structures, melting points and properties. Recycling codes exist precisely because mixing incompatible polymers produces poor-quality recyclate. Thermosets (e.g. Bakelite, Code 7) cannot be melted at all and must be excluded from streams designed for thermoplastics.
- 1 mark — Error 3 corrected: dissolution and fragmentation are fundamentally different processes. Ionic compounds like NaCl dissociate into ions in water. Synthetic polymers do not dissolve; instead, UV radiation, physical stress and wave action break large plastic items into progressively smaller pieces called microplastics (<5 mm). These do not become harmless molecules.
- 1 mark — Microplastic persistence: the carbon backbone of addition polymers (polyethylene, etc.) is chemically stable; micro-organisms lack enzymes to cleave the C–C bonds, so the fragments persist in ecosystems for hundreds of years.
- 1 mark — Scientifically defensible reformulation: “Ocean plastics do not dissolve. They fragment into persistent microplastics that enter food webs and accumulate in sediments. Nature does not deal with synthetic polymer pollution on human timescales; only source reduction, improved waste management and potentially chemical recycling can reduce the burden.”
- 1 mark — Uses correct lesson terminology throughout (addition polymer, condensation polymer, thermoplastic, thermoset, cross-linking, microplastic, polymer persistence) and maintains evaluative structure (identify error → correct with chemistry).