Exercises: Sustainability, recycling and critical analysis

The mechanical recycling of a thermoplastic can include crushing, washing and reshaping by melting. a) Justify why PET can be reprocessed by this route whereas a highly crosslinked thermosetting resin, such as Bakelite, cannot be melted and moulded again in the same way. b) Explain two processes that limit the indefinite repetition of cycles in thermoplastics, relating them to chain length and the presence of contaminants. Distinguish reprocessing by melting from crushing a thermoset to use it as a filler material.

A specific poly(lactic acid) (PLA) container has been tested for industrial composting under controlled conditions of humidity, oxygen and microbial activity, at temperatures of approximately $55$–$60\,^{\circ}\mathrm{C}$. a) Explain why the result of that test does not show that any PLA object degrades rapidly in the sea, where temperature, thickness, microorganisms and exposure time change. b) Distinguish fragmentation into small particles from biodegradation and mineralisation. Justify why the certification of a product under specific conditions cannot be automatically extended to a whole family of resins.

A circular economy proposal adopts the priority "prevent waste, reuse and, then, recycle". A batch of recovered PVC contains mixtures of pigments, plasticisers and other additives of varied origin. a) Explain how this mixture can make it difficult to use in an application that requires constant composition, strength and safety. b) Compare recycling that allows a product of equivalent quality to be made with using the material in a lower-performance application. Argue why reducing unnecessary materials and designing parts that are easy to separate can avoid problems that subsequent recycling does not solve.

A plant processes $1000\,\mathrm{kg}$ of raw PET packaging waste. In sorting, it removes $150\,\mathrm{kg}$ of unsuitable materials; in washing, it loses $50\,\mathrm{kg}$ of labels, adhesives and sludge; and in extrusion, it loses $40\,\mathrm{kg}$ through purges, residual moisture and other materials removed. The flow is shown in the diagram:

1. a) Calculate the final mass of recycled PET pellets and the overall mass yield relative to the input. b) Explain why this yield is not sufficient to evaluate sustainability: identify two additional pieces of data on energy, water, emissions or polymer quality. All losses refer to absolute masses of the same load; neglect other inputs and accumulations.

A study compares bags for carrying a shop of $15\,\mathrm{kg}$. This functional unit requires one high-density polyethylene bag of $10\,\mathrm{g}$ or two paper bags of $40\,\mathrm{g}$ each. Manufacture emits $1.80\,\mathrm{kg}$ of CO₂ equivalent per kilogram of polyethylene and $1.10\,\mathrm{kg}$ per kilogram of paper. The process water consumed is, respectively, $5.0$ and $50.0\,\mathrm{L}$ per kilogram of material. a) Calculate the manufacturing emissions and water consumption per functional unit for both options. b) State what comparison these results allow and what information is missing to conclude which has the lower impact over its whole life cycle, including reuse, transport and end of life.

The glycolysis of PET is a chemical recycling route that uses ethane-1.2-diol and produces bis(2-hydroxyethyl) terephthalate, BHET. In the long-chain limit, the formal balance per unit is:

The molar mass of the PET unit is $192.2\,\mathrm{g}\times\mathrm{mol}^{-1}$ and that of BHET is $254.2\,\mathrm{g}\times\mathrm{mol}^{-1}$. a) $961.0\,\mathrm{kg}$ of pure PET are treated with excess diol and $1143.9\,\mathrm{kg}$ of purified BHET are isolated. Calculate the yield relative to the stoichiometric maximum. b) Explain why the mass of the product can exceed that of the PET fed in without violating the conservation of matter. Compare a purification opportunity and an energy cost of this route with mechanical recycling.

Two containers that each deliver $1\,\mathrm{L}$ of water per use are compared. A disposable PET bottle generates $110\,\mathrm{g}$ of CO₂ equivalent per use, including manufacture and end of life. A glass bottle generates $480\,\mathrm{g}$ in its manufacture and $40\,\mathrm{g}$ in its final recycling; each use adds $22\,\mathrm{g}$ for washing and $18\,\mathrm{g}$ for additional transport. a) Formulate the cumulative emissions of both systems after $n$ uses and calculate the smallest integer number of uses for which glass emits strictly less. State how many reuses after the first use this result represents. b) An independent loss rate of 12 % per cycle gives an approximate mean life of 8.3 uses, without setting a maximum for each bottle. Explain why the mean, the dispersion and the actual losses must be considered before generalising the environmental advantage.

A polyethylene bag with pro-oxidant additives is advertised as "100 % eco-friendly, oxo-biodegradable and zero waste". Tests show that, under UV radiation and oxygen, it fragments rapidly, but they provide no measurements of complete conversion of the carbon into CO₂ or of effects on organisms. a) Explain how oxidation and chain scission can produce small fragments without demonstrating biological mineralisation. b) Evaluate which advertising claims remain unjustified by these data and propose measurable evidence to test them: test conditions and duration, carbon balance and quantity of residual particles. Relate your proposals to the risk of microplastic formation and dispersal. As regulatory information for this analysis, Article 5 of Directive (EU) 2019/904 establishes the prohibition of placing on the market products made from oxo-degradable plastic. Relate this measure to the difference between fragmentation and biodegradation.

Three students compare EPS trays with a mycelium and straw biocomposite. The process team reports a wet load of $200\,\mathrm{kg}$ per batch of 1000 trays; during drying, 40 % of that mass is removed. Pasteurisation consumes $180\,\mathrm{MJ}$, not including drying. The environmental team estimates a transport of that wet load of $120\,\mathrm{km}$, with emissions of $0.12\,\mathrm{kg}$ of CO₂ equivalent per tonne and kilometre; for EPS it has only a manufacturing factor of $2.8\,\mathrm{kg}$ of CO₂ equivalent per kilogram. The ecotoxicology team does not yet have data on additives or leachates. a) Calculate the dry mass, the pasteurisation energy per kilogram of dry mass and the transport emissions of the batch. b) Write a joint conclusion that distinguishes calculable results from uncertainties. State why the equivalent mass of EPS, the drying energy and other data are missing for comparing the complete footprints.

Topic challenge. To what extent does your daily life depend on synthetic polymers? Become an environmental auditor in your own home and design a plan for transitioning to more sustainable materials.

1. Household audit: Select 5 everyday objects (packaging, textiles or kitchenware). Identify the polymer they are made of (resin identification code or label: PET, HDPE, nylon, etc.) and estimate their mass using kitchen scales.

2. Macromolecular chemistry: Classify them according to their polymerisation (addition or condensation). Draw the condensed structural formulae of the monomers and the repeating unit of at least two of them: one formed by addition and the other by condensation.

3. Structure and properties: Justify how intermolecular forces (e.g. dispersion forces or hydrogen bonds) influence their flexibility, heat resistance and suitability for mechanical recycling.

4. Critical analysis: Consult the Repak or EPA website for the recycling rate of plastic packaging in Ireland, compare it with the figure for Spain (approx. $51.5\,\%$), and write a short report proposing viable alternatives (reuse, biopolymers) to replace the highest-impact plastic in your sample.

Assessment criteria:

• Rigour in the chemical formulation of monomers and the assignment of the polymerisation mechanism.

• Coherent argumentation linking molecular structure, macroscopic properties and recyclability.

• Analytical quality of the report based on the household data collected and the comparison with official sources.