Chemistry • Year 12 • Module 7 • Lesson 23
Polymers: Properties, Applications & Environmental Impact
Develop HSC Band 5–6 extended-response technique: synthesise structural chemistry, real data, and environmental context into evidence-based judgements.
1. Data + scenario — evaluating polymer choices for CSIRO packaging research (Band 5–6)
8 marks Band 5–6
Scenario. CSIRO researchers are evaluating packaging materials for single-serve food containers to be used at outdoor events and collected for composting. The team is comparing conventional expanded polystyrene (EPS, code 6) with two biopolymer alternatives: polylactic acid (PLA) and polyhydroxyalkanoate (PHA). The table below summarises key properties measured across three criteria.
| Property | EPS (polystyrene) | PLA | PHA |
|---|---|---|---|
| Thermal resistance | Softens ~95°C | Softens ~55–60°C | Softens ~170°C |
| Biodegradability | Non-biodegradable; fragments to microplastics | Industrial composting (>60°C); does not degrade in landfill or ocean | Biodegrades under a range of conditions including marine environments |
| Cost of raw material (relative) | Low (1×) | Moderate (4–6×) | High (8–15×) |
Data adapted from CSIRO Bioplastics Research Bulletin, 2023 (illustrative values).
Q1. Evaluate the three packaging materials for use as single-serve outdoor food containers to be collected for composting. In your response you must:
- Define biodegradability and explain why EPS fails this criterion at a molecular level.
- Compare PLA and PHA on at least three criteria drawn from the data table or lesson content.
- Use a named Australian context (e.g. CSIRO research, APCO packaging covenant, or Clean Up Australia findings).
- Identify a limitation of PLA that makes it unsuitable despite being biodegradable.
- Reach an evidence-based judgement about which material is best suited to the described application.
2. Source critique — media claim about polymer recycling (Band 5–6)
7 marks Band 5–6
Source. Extract from a social media post shared widely in Australia, December 2023:
"Great news! Scientists have now developed bioplastics that solve the plastic pollution crisis. PLA bioplastic cups are the future — they break down naturally in the environment within months, so we no longer need to worry about recycling codes or ocean microplastics. Switching to bioplastics is all we need to do. The plastics problem is solved."
Q2. This source contains at least two distinct scientific flaws. For each flaw:
- Identify the specific claim that is incorrect or misleading.
- Explain the correct chemistry or scientific understanding, using precise lesson vocabulary.
- Describe how the flaw could be detected or tested experimentally or with published evidence.
Conclude with a brief statement evaluating whether switching to PLA bioplastics alone is sufficient to resolve global polymer pollution.
Q1 — Marking criteria (8 marks)
Mark 1 (definition): Biodegradability is the ability of a material to be broken down by living organisms (microorganisms) into simpler substances such as CO⊂2;, water, and biomass.
Mark 2 (EPS molecular explanation): EPS (polystyrene) has a C–C backbone with non-polar bonds; microorganisms lack the enzymes to cleave these bonds efficiently. It also contains a bulky, non-polar phenyl side group. As a result, polystyrene does not biodegrade; UV and mechanical stress only break it into progressively smaller microplastic fragments.
Marks 3–5 (comparison, 1 per criterion): Any three of: Thermal resistance — PLA softens at ~55–60°C (unsuitable for hot food); PHA remains stable to ~170°C (suitable). Biodegradability — PLA only degrades under industrial composting; PHA degrades under a range of conditions including marine environments. Cost — PLA is 4–6× the cost of EPS; PHA is 8–15×, potentially limiting large-scale adoption. Contamination risk — PLA that escapes composting stream contaminates PET recycling; PHA degrades if it escapes.
Mark 6 (Australian context): Any valid reference: CSIRO bioplastics research / APCO commitment to reduce problematic plastics / Clean Up Australia showing PS as among the most common waterway debris items / Qenos recycling initiatives / NSW single-use plastics ban 2022.
Mark 7 (PLA limitation): PLA requires certified industrial composting infrastructure (>60°C). If PLA cups are disposed of in general waste, landfill, or littered in the environment, they persist similarly to conventional plastics. Unreliable collection at outdoor events means contamination of other waste streams is likely.
Mark 8 (evidence-based judgement): PHA is the best choice for this specific application because: it meets thermal requirements for food contact, biodegrades even if containers escape the composting stream (reducing microplastic risk), and outdoor event scenarios commonly have imperfect waste separation. The higher cost is a limitation but may be justified for high-profile events with sustainability commitments. A judgement that selects PLA with a valid conditional (e.g. only if certified composting collection is guaranteed) also earns this mark if justified.
Q2 — Marking criteria (7 marks)
Flaw 1 — "breaks down naturally in the environment within months" (3 marks):
Identification (1 mark): The claim that PLA breaks down naturally in the environment is incorrect.
Correct chemistry (1 mark): PLA is compostable only under industrial conditions (temperatures above ~60°C and controlled humidity for 3–6 months). In natural environments (soil, ocean, rivers, home compost), PLA does not degrade at meaningful rates — studies detect PLA fragments persisting for years. The ester linkages in PLA require specific hydrolysis conditions not present in most natural settings.
Detection / evidence (1 mark): This could be tested by placing PLA samples in simulated natural environments (seawater, soil at ambient temperature, home compost) vs a certified industrial compost chamber and measuring mass loss and fragment count over 12 months. Published CSIRO and European Bioplastics research already demonstrates negligible degradation at ambient temperatures.
Flaw 2 — "we no longer need to worry about recycling codes or ocean microplastics" (3 marks):
Identification (1 mark): The claim that bioplastics eliminate concern for recycling codes and microplastics is misleading and false.
Correct chemistry (1 mark): (a) PLA mixed into conventional PET recycling streams (both code 1) contaminates the recycled PET, degrading its properties and reducing market value — so recycling infrastructure and code literacy remain essential. (b) The existing stock of 12+ billion tonnes of synthetic polymers already in landfill, waterways, and the ocean is not removed by switching new products to PLA; microplastic accumulation from legacy plastics continues.
Detection / evidence (1 mark): Contamination effect on PET recyclate can be measured by adding known concentrations of PLA to PET melt and measuring tensile strength and clarity degradation. Existing ocean microplastic surveys (e.g. CSIRO sampling of the Great Pacific Garbage Patch and Australian coastlines) already quantify the legacy plastic stock.
Concluding evaluation (1 mark): Switching to PLA alone is insufficient to solve global polymer pollution: it does not address the existing legacy plastic load; it requires industrial composting infrastructure not universally available; it creates new contamination risks for PET recycling; and the claim that it removes the need for recycling codes is directly false. A systemic response requiring redesigned infrastructure, better collection, and life-cycle assessment for all materials is needed.