Chemistry • Year 12 • Module 7 • Lesson 23
Polymers: Properties, Applications & Environmental Impact
Apply structural reasoning to real polymer selection problems, interpret Australian plastic recycling data, and evaluate real-world environmental trade-offs.
1. Interpret Australian plastic recycling rate data
The bar chart below shows estimated recycling rates (as a percentage of material placed on market) for the seven major plastic categories in Australia, based on 2021–22 data reported by the Australian Packaging Covenant Organisation (APCO). 9 marks
1.1 Identify the two plastic categories with the highest recycling rates and state one structural property of each that makes it easier to process in conventional recycling. 4 marks
1.2 PVC (code 3) has a recycling rate of only ~4%. Using your understanding of polymer structure and the lesson content, suggest two reasons why PVC is difficult to recycle compared with PET or HDPE. 2 marks
1.3 LDPE bags are collected separately through REDcycle-style collection points rather than kerbside bins. Suggest one structural or logistical reason why LDPE recycling rates (code 4, ~13%) remain lower than PET (code 1, ~38%), even though LDPE is a thermoplastic. 1 mark
1.4 The data show that collectively, codes 3, 6, and 7 account for only a small fraction of Australian plastic recycling. Predict what happens to most of this material and identify one environmental consequence. 2 marks
2. Cause-and-effect chain — from polymer structure to ocean microplastics
The cause boxes (left) are given. Write the effect in each empty right-hand box, then write the overall outcome at the bottom. Use your own words — one sentence per box is enough. 6 marks
| Most synthetic polymer backbones consist of non-polar C–C bonds. | → | |
| Plastic waste enters the environment and is exposed to UV radiation and mechanical abrasion (wave action). | → | |
| Microplastic fragments (<5 mm) disperse widely through ocean currents. | → | |
| Overall outcome (so…): | ||
3. Compare thermoplastics and thermosets
Complete the comparison table below. One row has been started as an example. 8 marks
| Feature | Thermoplastic (e.g. PET, HDPE) | Thermoset (e.g. epoxy, Bakelite) |
|---|---|---|
| Cross-linking | None or minimal; chains held by intermolecular forces only | |
| Behaviour on heating | ||
| Recyclability | ||
| Typical hardness / flexibility | ||
| Named application |
4. Predict and justify
A materials engineer needs to select a polymer for a new range of reusable coffee cups that must: (i) resist temperatures up to 100 °C without deforming; (ii) be transparent so customers can see the contents; (iii) be conventionally recyclable at the end of its useful life. 4 marks
Candidate polymers: Bakelite (phenol-formaldehyde thermoset) • Polypropylene (PP, code 5) • Polycarbonate (PC, code 7) • Polystyrene (PS, code 6)
4.1 Eliminate any polymers that fail criterion (i) or (iii), giving a structural or practical reason for each elimination. 2 marks
4.2 From the remaining candidate(s), select the most suitable polymer and justify your choice with reference to specific structural properties. 2 marks
5. Case study — NSW single-use plastic ban (2022)
Read the passage and answer the question below. 5 marks
In November 2022 the NSW Government banned a range of single-use plastic items including lightweight plastic bags, polystyrene food containers, plastic cutlery, straws, and cotton bud sticks under the Plastic Reduction and Circular Economy Act 2021. The ban built on APCO's Australian Packaging Covenant commitment and followed modelling by CSIRO researchers showing that single-use polystyrene items were among the most common plastic debris items identified in Clean Up Australia surveys of waterways. Polystyrene (code 6) has a recycling rate of approximately 1% nationally. Post-ban sales data from NSW suggest a 40–50% reduction in polystyrene packaging being placed on the market in the first year.
Q5. Using the information above and your understanding of polymer chemistry, evaluate the extent to which the NSW single-use plastics ban addresses the problem of polymer pollution in Australian waterways. In your response, explain why polystyrene was a priority target based on its structural chemistry, and identify one limitation of the ban as a complete solution to polymer pollution. 5 marks
Q1 — Recycling rate data
1.1 PET (code 1, ~38%) and HDPE (code 2, ~30%). PET is a thermoplastic condensation polymer with ester linkages that can be hydrolysed or re-melted; it has a relatively narrow, well-defined melting range and a long history of established collection infrastructure (drink bottles have a high return rate). HDPE has linear unbranched chains that pack tightly, giving a well-defined melt point (~135 °C) and making it easy to sort by density in water-float separation. Award 2 marks: 1 for each correctly identified polymer + property. (Property does not need to be identical to sample answer; any structurally valid reason accepted.)
1.2 Any two of: (a) PVC degrades on heating to release HCl gas, which is toxic and corrosive to recycling equipment; (b) PVC is often heavily plasticised with variable additives (e.g. phthalates), making it chemically inconsistent and difficult to reprocess into a uniform product; (c) PVC has a very similar density to other plastics, making it difficult to separate by conventional float-sink sorting.
1.3 LDPE bags are thin-film and flexible, causing them to jam sorting machines in standard MRF (materials recovery facility) equipment. They must be collected separately through REDcycle-style programmes and cannot easily be mixed with rigid plastics. Their low mass per item also reduces economic viability.
1.4 Most code 3, 6, and 7 plastics are sent to landfill (some may be incinerated for energy recovery in some states). Environmental consequence: landfill accumulation contributes to long-term soil and groundwater contamination; if items escape to waterways they photodegrade into microplastics that accumulate in marine food chains.
Q2 — Cause-and-effect chain
Effect 1: Non-polar C–C bonds are chemically inert; bacteria and fungi lack enzymes capable of cleaving them efficiently, so synthetic polymers resist microbial decomposition (are non-biodegradable).
Effect 2: UV radiation breaks some polymer chains and mechanical abrasion shatters brittle fragments into progressively smaller pieces, producing microplastics (fragments <5 mm) without fully degrading the carbon backbone.
Effect 3: Microplastics are ingested by filter-feeders, fish, seabirds, and other marine organisms; they bioaccumulate up trophic levels and have been detected in human food, blood, and placental tissue.
Overall outcome: Because synthetic polymers are non-biodegradable and fragment into persistent microplastics, the current model of single-use plastic production is incompatible with healthy marine and terrestrial ecosystems unless effective collection and recycling prevents environmental release. (Any evidence-based summary accepted.)
Q3 — Thermoplastics vs thermosets
Cross-linking thermoset: extensive covalent cross-links form a 3D permanent network. | Behaviour on heating — thermoplastic: softens and melts, can be re-moulded; thermoset: decomposes/chars, no melt. | Recyclability — thermoplastic: generally recyclable by re-melting; thermoset: not conventionally recyclable. | Hardness — thermoplastic: variable, often flexible; thermoset: hard and rigid. | Application — thermoplastic: any reasonable example from lesson (PET bottles, HDPE pipes); thermoset: epoxy adhesive, Bakelite switches, cookware handles. Award 1 mark per completed row (max 8 for 4 rows × 2 columns; cross-linking row already given).
Q4 — Predict and justify
4.1 Eliminate Bakelite: it is a thermoset, cannot be recycled by melting, fails criterion (iii). Eliminate PS: PS is a thermoplastic but has essentially zero recycling infrastructure in Australia (code 6, ~1% rate), failing criterion (iii) in practice; also PS is brittle and may fracture under thermal cycling. (Polycarbonate / code 7 could also be flagged as rarely recycled.)
4.2 Polypropylene (PP, code 5) is the strongest remaining candidate: it is a thermoplastic (recyclable), has a melting point of ~160–168 °C so it withstands 100 °C without deforming, and can be made in semi-transparent to translucent grades. Polycarbonate (PC) has even better transparency and higher temperature resistance but is code 7 (rarely recycled), making PP the better balance. Award marks for any selection supported by structural reasoning consistent with the lesson.
Q5 — NSW plastics ban case study
The ban addresses a specific, high-volume source of problematic plastic: polystyrene was targeted because its bulky, non-polar phenyl side groups prevent tight chain packing, making it brittle and prone to fragmentation into very small microplastic pieces that are nearly impossible to retrieve from waterways [1]. Its ~1% national recycling rate means the vast majority was entering landfill or the environment [1]. A 40–50% reduction in PS placed on market within one year shows a significant reduction in new pollution input [1]. However, the ban does not address the enormous existing stock of plastic already in waterways, landfill, and soils, including millions of tonnes of older PE, PVC, and PET waste [1]. One limitation: banning single-use items does not solve the structural problem of non-biodegradable polymer design — replacement materials (e.g. cardboard) also carry an environmental cost that requires full life-cycle assessment to evaluate [1]. Award up to 5 marks for: PS structural fragmentation chemistry; low recycling context; evidence of impact; limitation of ban; any valid evaluative statement.