A mixed flake stream of PE, PP, PS, PVC and PET is placed in water of density 1.00 g/cm³. Which pair floats?

PE and PP, with densities under 1.0 g/cm³

PS and PVC, both of which are denser than water

PET and PS, the two polyesters and styrenics of the stream

PE and PP, with densities under 1.0 g/cm³

PVC and PET, which both float because of their ester or chlorine content

A well-known limitation of near-infrared sorting of plastics is that

items coloured black with carbon black absorb the radiation and cannot be identified

it cannot distinguish any two polymers, since all absorb alike

it works only on liquids and not on solid flakes

it needs the sample to be heated to its melting point first

items coloured black with carbon black absorb the radiation and cannot be identified

A small amount of PVC in a PET recycling stream is especially damaging because

PVC decomposes at PET processing temperatures, releasing HCl that degrades and discolours the PET

PVC melts at a lower temperature and so lowers the density of PET to below that of water

PVC is biodegradable and consumes the PET during processing

PVC has a density far below that of PET, so it cannot be seen by any sensor

In the primary to quaternary classification of recycling, quaternary recycling is

energy recovery by controlled incineration of waste plastic

grinding thermoset waste into powder to use as filler in a new composite

energy recovery by controlled incineration of waste plastic

re-use of clean factory scrap directly in the same product line

depolymerisation of PET to its monomers by chemical reaction

Glycolysis of waste PET, using excess ethylene glycol and a catalyst, mainly yields

BHET (bis(2-hydroxyethyl) terephthalate) and oligomers

dimethyl terephthalate and water as the main products of the reaction

styrene monomer and a carbon-rich char residue after heating

BHET (bis(2-hydroxyethyl) terephthalate) and oligomers

caprolactam, the monomer of nylon-6, and a salt residue

In devulcanisation of scrap rubber the ideal process is to

cleave the S–S and C–S crosslinks selectively, sparing the main chain

break every C–C bond of the main chain so as to recover the monomer in one step

add more sulphur so that the network becomes denser, stiffer and harder to deform

cleave the S–S and C–S crosslinks selectively, sparing the main chain

dissolve the rubber in water to give a stable latex dispersion again

A life-cycle comparison of a PET bottle and a glass bottle for a beverage is to be fair. Which functional unit is appropriate?

delivering 1000 L of beverage to consumers, with all packaging and reuses counted

1 kg of PET against 1 kg of glass

1 bottle of each, whatever its volume and its number of uses

the purchase cost of the raw material needed to make each of the two bottles, whatever they hold

delivering 1000 L of beverage to consumers, with all packaging and reuses counted

Bio-polyethylene made from sugar-cane ethanol is best described as

bio-based but not biodegradable, since its structure is ordinary polyethylene

bio-based and biodegradable, because its carbon comes from plants and so it must be consumed by microbes

bio-based but not biodegradable, since its structure is ordinary polyethylene

fossil-based and biodegradable, because it is made through an ethanol intermediate

neither bio-based nor biodegradable, because it is a polyolefin

Select ALL correct statements about identification and separation of plastics.

Froth flotation can separate polymers of similar density by making their surface wettability different

Electrostatic separation uses triboelectric charging and deflection in a high-voltage field

The resin identification code identifies the polymer, not whether local recycling exists

Froth flotation can separate polymers of similar density by making their surface wettability different

Electrostatic separation uses triboelectric charging and deflection in a high-voltage field

PE and PP can be separated cleanly from each other by float–sink in water

The resin identification code identifies the polymer, not whether local recycling exists

Select ALL correct statements about recycling routes.

Mechanical recycling lowers PET intrinsic viscosity if the flake is not dried, through hydrolysis

Pyrolysis heats plastic without oxygen to give oil, gas and char

Cryogenic grinding uses liquid nitrogen to make rubber brittle before size reduction

Mechanical recycling lowers PET intrinsic viscosity if the flake is not dried, through hydrolysis

Pyrolysis heats plastic without oxygen to give oil, gas and char

Cryogenic grinding uses liquid nitrogen to make rubber brittle before size reduction

Thermosets can be remelted and moulded like thermoplastics after cleaning

Select ALL correct statements about life-cycle assessment.

The inventory phase lists inputs and outputs of each stage of the product system

Cradle-to-grave extends from raw-material extraction to final disposal or recycling

The impact assessment converts inventory data into categories such as kg CO₂-equivalent

The inventory phase lists inputs and outputs of each stage of the product system

Cradle-to-grave extends from raw-material extraction to final disposal or recycling

A reusable bag has a lower footprint than a single-use bag from the first use

The impact assessment converts inventory data into categories such as kg CO₂-equivalent

Select ALL correct statements about biodegradable polymers and microplastics.

Tyre-wear particles and fibres shed in laundry are primary microplastics

Compostable plastics should be kept out of the PET recycling stream

Tyre-wear particles and fibres shed in laundry are primary microplastics

PLA degrades quickly in cold seawater, so it can be littered

Compostable plastics should be kept out of the PET recycling stream

Polymer Recycling, Waste Management and Sustainability

This chapter answers Section XE5.9 of XE5 Polymer Science and Engineering, one of the nine optional sections of the Engineering Sciences (XE) paper, any two of which a candidate chooses in the exam. The syllabus text is: “Polymer waste and its impact on environment; Sources, identification and separation techniques; Recycling classification: mechanical and chemical recycling, recycling of thermoplastics, thermosets and rubbers, applications of recycled materials; Life cycle assessment of polymer products (case studies like PET bottles, packaging bags); Recycling, segregation and disposal strategies of biodegradable and bio-compostable polymers, microplastics.” It teaches the problem, how plastics are identified and separated, the four classes of recycling, the special cases of thermosets and rubbers, life-cycle assessment, and biodegradable polymers and microplastics. The numericals are recycled-content mass balance, bag break-even in a life-cycle comparison and a PET bottle footprint from stated factors.

1. Polymer waste, its sources and its environmental impact

Plastic waste has two main origins. Post-industrial (pre-consumer) waste is scrap from processing, such as runners, trimmings and off-specification batches; it is clean and of known grade and is largely re-used in the factory. Post-consumer waste is discarded products: packaging (the largest single end-use), consumer goods, agricultural films, vehicles, electrical and electronic equipment and textiles. It is mixed, dirty, of unknown composition and widely scattered, which makes collection, sorting and cleaning the costly steps of recycling.

The impact follows from durability: most plastics persist for decades or longer. Landfilled plastic occupies volume and may leach additives; litter blocks drains, entangles and is swallowed by animals, and accumulates in rivers and oceans; open burning and incineration of unsuitable waste release CO₂, soot and toxic gases, notably HCl and dioxins from chlorinated polymers such as PVC; and fragmentation produces microplastics. Against this, plastics also save weight and food waste, so the aim is better waste management, not zero plastic. In India the Plastic Waste Management Rules, 2016, as amended, set producer responsibility (EPR) for packaging and restrict identified single-use items.

2. Identification and separation

The resin identification code is a number from 1 to 7 inside a triangle of arrows moulded on a product: 1 PET, 2 HDPE, 3 PVC, 4 LDPE, 5 PP, 6 PS, 7 other. It names the polymer, not whether it can be recycled locally. Separation exploits differences in physical properties.

Separation techniques
Technique	Principle	Use and limit
Float–sink (density)	Density against a liquid: water 1.0 g/cm³	PE and PP (about 0.90 to 0.97) float; PS (about 1.05), PVC and PET (both about 1.3 to 1.4) sink. PVC and PET overlap
Near-infrared (NIR) sorting	Each polymer has a characteristic NIR absorption spectrum	Fast, automatic, air-jet ejection; fails on black plastics coloured with carbon black
Froth flotation	Wetting agents make some surfaces hydrophilic, others stay hydrophobic and attach to air bubbles	Separates polymers of overlapping density, for example PVC from PET
Electrostatic	Triboelectric charging by friction, then deflection in a high-voltage field	Dry process, for dry flakes of similar density; moisture hurts

⚠️ Why PVC must be removed from PET

PET is processed near 270 to 280 °C, where even a trace of PVC releases HCl; the acid degrades PET and darkens it, and PVC and PET have almost equal density, so float–sink fails. Detecting chlorine or using flotation, electrostatic or NIR sorting is needed.

3. Classes of recycling: primary to quaternary, mechanical and chemical

Recycling classification
Class	What happens	Example
Primary	In-plant re-use of clean scrap into products of equal quality	Regrind of runners fed back to the moulding machine
Secondary	Mechanical reprocessing of used plastic, often to a lower-value product	Washed HDPE bottle flake made into pipes or crates
Tertiary	Chemical recycling to monomers, chemicals or fuels	PET glycolysis to BHET; pyrolysis oil
Quaternary	Energy recovery by controlled incineration	Waste-to-energy plant; cement kiln fuel

Mechanical recycling (secondary) follows sort, shred, wash, dry, melt-filter, extrude and pelletise. Each cycle exposes the polymer to heat and shear, so chains undergo scission (and PET also suffers hydrolysis from moisture, lowering intrinsic viscosity, so flake is dried thoroughly and may be upgraded by solid-state polycondensation); incompatible polymers and dirt form weak phases. Properties therefore fall with each cycle, and mixed streams give low-grade products. It is cheapest and best for clean, single-polymer thermoplastics such as PET, HDPE and PP.

Chemical recycling breaks the polymer down. Pyrolysis heats plastic in the absence of oxygen (a few hundred °C) to give oil or wax, gas and char, suited to polyolefins and polystyrene (styrene-rich oil) but not PVC (HCl). Gasification (partial oxidation) gives syngas. For condensation polymers the reverse of polymerisation is possible, depolymerisation: glycolysis of PET with ethylene glycol gives BHET (bis(2-hydroxyethyl) terephthalate); methanolysis with methanol gives dimethyl terephthalate and ethylene glycol; hydrolysis with water gives terephthalic acid and ethylene glycol (nylon-6 can similarly be hydrolysed to caprolactam). The monomers can be purified and repolymerised to virgin-quality polymer, so these routes tolerate colour and some contamination, but cost more energy and capital than mechanical recycling. Polyolefins have C–C backbones and do not depolymerise cleanly, so they go to pyrolysis.

4. Thermosets, rubbers and applications of recycled material

Thermosets are crosslinked and cannot be melted or dissolved, so ordinary mechanical recycling fails. The options are grinding into powder or fibre for use as filler in new composites, pressing or panels; chemical routes such as glycolysis of polyurethane to recover polyol, solvolysis or pyrolysis of fibre composites to recover glass or carbon fibre; and energy recovery, for instance as cement-kiln fuel. Prevention (longer life, repair, design for disassembly) matters most.

Vulcanised rubber, mostly tyres, is also crosslinked. Crumb rubber is made by ambient grinding or cryogenic grinding, where liquid nitrogen cools the rubber below its glass transition so it shatters into fine, clean particles; steel and fabric are removed. Crumb goes into rubberised asphalt, sports and playground surfaces, mats and moulded goods. Devulcanisation selectively cleaves the sulphur crosslinks (S–S and C–S bonds) by thermomechanical, chemical, microwave, ultrasonic or biological means so the rubber can flow and be revulcanised, often blended with virgin rubber; some main-chain C–C scission also occurs, so properties fall short of virgin. Tyre pyrolysis yields oil, gas, recovered carbon black and steel, and tyres also serve as fuel in cement kilns.

Typical uses of recycled materials
Recycled material	Typical applications
PET	Polyester fibre and fleece, strapping, sheet; bottle-to-bottle where food-contact approval exists
HDPE, PP	Pipes, crates, bins, automotive parts, non-food bottles
Mixed plastics	Plastic lumber, pavers, shredded plastic in bituminous roads
Crumb rubber	Rubberised asphalt, sports surfaces, mats

5. Life-cycle assessment of polymer products

Life-cycle assessment (LCA), standardised in ISO 14040 and 14044, has four phases:
(1) goal and scope, including the functional unit and system boundary;
(2) life-cycle inventory (LCI), listing all inputs (materials, energy, water) and outputs (emissions, waste) of each stage;
(3) impact assessment (LCIA), converting the inventory into categories such as global warming (kg CO₂-equivalent), acidification and resource use;
(4) interpretation, with sensitivity checks. The functional unit is the quantified function that all compared systems must deliver, such as “1000 L of beverage delivered to the consumer” or “carrying 10 kg of shopping home”. Comparing per kilogram of material is wrong, because a lighter pack that costs more per kilogram can still be better per function.

System boundaries are named by their ends: cradle-to-gate (raw material to factory gate), cradle-to-grave (extraction, production, transport, use, and final disposal or recycling) and cradle-to-cradle (end of life feeds a new product). For PET bottles the important levers are bottle mass (light-weighting), recycled content, transport distance and end-of-life route, and a glass bottle is better only if reused enough times to repay its higher production and transport burden. For packaging bags, a single-use bag has a small footprint per bag, so a reusable bag wins only above a break-even number of uses, and a paper bag is not automatically better. Results change with the allocation method and the recycling credit, which must be stated.

🧠 Worked example: bag break-even and bottle footprint

Functional unit: carrying shopping home on one trip. A single-use bag is 0.04 kg CO₂e per trip; a reusable bag costs 2.4 kg CO₂e to make and replaces one single-use bag per trip, with use-phase and disposal ignored. Break-even uses = 2.4 / 0.04 = 60 trips. Bottles: 1000 L in 0.5 L bottles of 20 g is 2000 bottles = 40 kg PET; at 2.3 kg CO₂e per kg (virgin) the footprint is 92 kg CO₂e. With 25% recycled PET at 1.0 kg CO₂e per kg, the factor is 0.75 × 2.3 + 0.25 × 1.0 = 1.975 and the total is 40 × 1.975 = 79 kg CO₂e. These factors are illustrative, supplied in a question.

6. Biodegradable and compostable polymers, and microplastics

Biodegradable means broken down by microorganisms into CO₂, water and biomass (and methane without oxygen); the rate depends on the environment, not on the polymer alone. Bio-based (made from renewable feedstock) and biodegradable are different: bio-polyethylene is bio-based but not biodegradable, whereas PBAT is fossil-based but biodegradable. Compostable adds that the material biodegrades under composting conditions within a set time and leaves no toxic residue, as certified under standards such as EN 13432 and ASTM D6400, with biodegradation measured as CO₂ evolved (for example in ISO 14855). PLA needs the warm (around 58 °C), controlled conditions of an industrial composter and degrades very slowly in soil or sea, while some PHAs also degrade in soil and marine settings. Oxo-degradable plastics only fragment through pro-oxidant additives and are not biodegradable.

Segregation and disposal. Compostable plastics should go to industrial composting or anaerobic digestion where such a facility accepts them, not into the PET or polyolefin recycling stream, where PLA is a contaminant that spoils the recyclate; they should not be littered, since they will not rapidly degrade outside the right conditions. Clear labelling and certification marks, colour coding and separate collection allow this. Where no facility exists, a compostable bag is just waste.

Microplastics are plastic particles smaller than 5 mm (the usual upper limit); nanoplastics are smaller still (below about 1 µm). Primary microplastics are made small: cosmetic microbeads, industrial abrasives and pre-production pellets, or shed in use, such as tyre-wear particles and synthetic-textile fibres released in washing. Secondary microplastics come from fragmentation of larger litter by sunlight (photo-oxidation), abrasion and waves. They persist, spread through water, soil and air, are ingested by organisms and enter food chains, adsorb hydrophobic pollutants and may carry additives and microbes. Controls include bans on microbeads, filters on washing machines, better capture in wastewater treatment and less littering.

⚠️ Size and origin are not the same

Size alone defines a microplastic (under 5 mm); “primary” and “secondary” describe origin. A fibre shed by a polyester shirt is primary; a fragment of a weathered bottle is secondary.

Key takeaways

Post-consumer plastic is mixed and dirty, which makes sorting the costly step; burning chlorinated plastics gives HCl and dioxins; the Indian rules use EPR.
Resin codes are 1 PET, 2 HDPE, 3 PVC, 4 LDPE, 5 PP, 6 PS, 7 other; PE and PP float in water, PS, PVC and PET sink, and PVC/PET need NIR, flotation or electrostatic separation.
Primary is in-plant re-use, secondary mechanical, tertiary chemical (pyrolysis, glycolysis, methanolysis, hydrolysis) and quaternary energy recovery.
Thermosets and vulcanised rubber cannot melt: use them as filler, devulcanise, make crumb, pyrolyse or burn for energy.
LCA has four ISO phases; compare on a functional unit, not per kilogram, and state the boundary (cradle-to-grave) and allocation.
Bio-based is not biodegradable; compostable needs certified composting, kept out of PET recycling; microplastics are below 5 mm, primary or secondary.

Practice questions (16)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

On the resin identification code, the number 5 inside the triangle of arrows stands for
1. polystyrene
2. low-density polyethylene
3. polyvinyl chloride
4. polypropylene
Show answer
Answer: D — polypropylene
The code runs 1 PET, 2 HDPE, 3 PVC, 4 LDPE, 5 PP, 6 PS, 7 other. Polystyrene is 6, LDPE is 4 and PVC is 3.
A mixed flake stream of PE, PP, PS, PVC and PET is placed in water of density 1.00 g/cm³. Which pair floats?
1. PS and PVC, both of which are denser than water
2. PET and PS, the two polyesters and styrenics of the stream
3. PE and PP, with densities under 1.0 g/cm³
4. PVC and PET, which both float because of their ester or chlorine content
Show answer
Answer: C — PE and PP, with densities under 1.0 g/cm³
PE (about 0.91 to 0.97) and PP (about 0.90) are less dense than water and float; PS (about 1.05), PVC and PET (about 1.3 to 1.4) sink. A denser salt solution is needed to float PS.
A well-known limitation of near-infrared sorting of plastics is that
1. it cannot distinguish any two polymers, since all absorb alike
2. it works only on liquids and not on solid flakes
3. it needs the sample to be heated to its melting point first
4. items coloured black with carbon black absorb the radiation and cannot be identified
Show answer
Answer: D — items coloured black with carbon black absorb the radiation and cannot be identified
Carbon black absorbs near-infrared strongly, so black plastics return no usable spectrum. NIR does separate polymers by characteristic absorption, works on dry solid items at speed and needs no heating.
A small amount of PVC in a PET recycling stream is especially damaging because
1. PVC decomposes at PET processing temperatures, releasing HCl that degrades and discolours the PET
2. PVC melts at a lower temperature and so lowers the density of PET to below that of water
3. PVC is biodegradable and consumes the PET during processing
4. PVC has a density far below that of PET, so it cannot be seen by any sensor
Show answer
Answer: A — PVC decomposes at PET processing temperatures, releasing HCl that degrades and discolours the PET
PVC degrades well below PET melt-processing temperature and releases hydrogen chloride, which attacks PET. Its density is close to PET, which is why float–sink cannot remove it, but it can be detected by NIR or X-ray based sorters.
In the primary to quaternary classification of recycling, quaternary recycling is
1. grinding thermoset waste into powder to use as filler in a new composite
2. energy recovery by controlled incineration of waste plastic
3. re-use of clean factory scrap directly in the same product line
4. depolymerisation of PET to its monomers by chemical reaction
Show answer
Answer: B — energy recovery by controlled incineration of waste plastic
Quaternary recycling is energy recovery. In-plant re-use of scrap is primary, mechanical reprocessing is secondary, and chemical recovery of monomers or fuels is tertiary.
Glycolysis of waste PET, using excess ethylene glycol and a catalyst, mainly yields
1. dimethyl terephthalate and water as the main products of the reaction
2. styrene monomer and a carbon-rich char residue after heating
3. BHET (bis(2-hydroxyethyl) terephthalate) and oligomers
4. caprolactam, the monomer of nylon-6, and a salt residue
Show answer
Answer: C — BHET (bis(2-hydroxyethyl) terephthalate) and oligomers
Glycolysis breaks the ester links with ethylene glycol to give BHET and oligomers. Dimethyl terephthalate comes from methanolysis, styrene-rich oil from polystyrene pyrolysis, and caprolactam, the monomer of nylon-6, from depolymerising nylon-6.
In devulcanisation of scrap rubber the ideal process is to
1. break every C–C bond of the main chain so as to recover the monomer in one step
2. add more sulphur so that the network becomes denser, stiffer and harder to deform
3. cleave the S–S and C–S crosslinks selectively, sparing the main chain
4. dissolve the rubber in water to give a stable latex dispersion again
Show answer
Answer: C — cleave the S–S and C–S crosslinks selectively, sparing the main chain
Devulcanisation aims to break the crosslinks so the rubber can flow and be revulcanised, while preserving the main chain. Full chain scission gives a degraded, low-grade product, and in practice some main-chain scission does occur, which limits the properties.
A life-cycle comparison of a PET bottle and a glass bottle for a beverage is to be fair. Which functional unit is appropriate?
1. 1 kg of PET against 1 kg of glass
2. 1 bottle of each, whatever its volume and its number of uses
3. the purchase cost of the raw material needed to make each of the two bottles, whatever they hold
4. delivering 1000 L of beverage to consumers, with all packaging and reuses counted
Show answer
Answer: D — delivering 1000 L of beverage to consumers, with all packaging and reuses counted
The functional unit must express the service delivered, here beverage delivered, so that packs of different mass, volume and reuse are compared on equal terms. Per kilogram or per bottle ignores that glass is heavier but can be refilled.
Bio-polyethylene made from sugar-cane ethanol is best described as
1. bio-based and biodegradable, because its carbon comes from plants and so it must be consumed by microbes
2. bio-based but not biodegradable, since its structure is ordinary polyethylene
3. fossil-based and biodegradable, because it is made through an ethanol intermediate
4. neither bio-based nor biodegradable, because it is a polyolefin
Show answer
Answer: B — bio-based but not biodegradable, since its structure is ordinary polyethylene
Origin of the carbon (bio-based) and the ability to biodegrade are separate: bio-polyethylene has the same structure as fossil polyethylene and is not biodegradable. PBAT shows the opposite combination: fossil-based but biodegradable.
Select ALL correct statements about identification and separation of plastics.
1. Froth flotation can separate polymers of similar density by making their surface wettability different
2. Electrostatic separation uses triboelectric charging and deflection in a high-voltage field
3. PE and PP can be separated cleanly from each other by float–sink in water
4. The resin identification code identifies the polymer, not whether local recycling exists
Show answer
Answer: A — Froth flotation can separate polymers of similar density by making their surface wettability different; B — Electrostatic separation uses triboelectric charging and deflection in a high-voltage field; D — The resin identification code identifies the polymer, not whether local recycling exists
Flotation, electrostatic separation and the meaning of the code are correct. PE and PP both float in water, so water cannot separate them.
Select ALL correct statements about recycling routes.
1. Mechanical recycling lowers PET intrinsic viscosity if the flake is not dried, through hydrolysis
2. Pyrolysis heats plastic without oxygen to give oil, gas and char
3. Cryogenic grinding uses liquid nitrogen to make rubber brittle before size reduction
4. Thermosets can be remelted and moulded like thermoplastics after cleaning
Show answer
Answer: A — Mechanical recycling lowers PET intrinsic viscosity if the flake is not dried, through hydrolysis; B — Pyrolysis heats plastic without oxygen to give oil, gas and char; C — Cryogenic grinding uses liquid nitrogen to make rubber brittle before size reduction
Hydrolytic degradation of PET, pyrolysis and cryogenic grinding are stated correctly. Crosslinked thermosets cannot be remelted.
Select ALL correct statements about life-cycle assessment.
1. The inventory phase lists inputs and outputs of each stage of the product system
2. Cradle-to-grave extends from raw-material extraction to final disposal or recycling
3. A reusable bag has a lower footprint than a single-use bag from the first use
4. The impact assessment converts inventory data into categories such as kg CO₂-equivalent
Show answer
Answer: A — The inventory phase lists inputs and outputs of each stage of the product system; B — Cradle-to-grave extends from raw-material extraction to final disposal or recycling; D — The impact assessment converts inventory data into categories such as kg CO₂-equivalent
Inventory, boundary and impact assessment are described correctly. A reusable bag carries a larger production burden and wins only after its break-even number of uses.
Select ALL correct statements about biodegradable polymers and microplastics.
1. Microplastics are plastic particles smaller than 5 mm
2. Tyre-wear particles and fibres shed in laundry are primary microplastics
3. PLA degrades quickly in cold seawater, so it can be littered
4. Compostable plastics should be kept out of the PET recycling stream
Show answer
Answer: A — Microplastics are plastic particles smaller than 5 mm; B — Tyre-wear particles and fibres shed in laundry are primary microplastics; D — Compostable plastics should be kept out of the PET recycling stream
The size limit, the primary sources and the segregation rule are right. PLA needs industrial composting conditions and degrades very slowly in cold seawater.
A plant must make 2160 kg of bottle resin containing 25% recycled PET by mass. Recycled pellets are made from collected bales with an overall bale-to-pellet yield of 72% (sorting and washing, then reprocessing). The bale mass required is ____ kg (integer).
Numerical answer — type the value.
Show answer
Answer: 750
Recycled pellets needed = 0.25 × 2160 = 540 kg. Bales = 540 / 0.72 = 750 kg. Multiplying by 0.72 instead of dividing gives 389 kg, the usual slip.
In a comparison where the functional unit is carrying shopping home on one trip, a single-use bag has a footprint of 0.045 kg CO₂e per trip and a reusable bag has a footprint of 1.8 kg CO₂e, all of it from production (use and disposal add nothing). Each trip replaces one single-use bag. The number of trips at which the two footprints are equal is ____ (integer).
Numerical answer — type the value.
Show answer
Answer: 40
Break-even n satisfies 0.045 n = 1.8, so n = 1.8 / 0.045 = 40 trips. Beyond 40 uses the reusable bag has the lower total.
The functional unit is 1000 L of beverage in 1.0 L PET bottles of 28 g each. Take 2.2 kg CO₂e per kg for virgin PET and 0.9 kg CO₂e per kg for recycled PET, and let 30% of the PET mass be recycled. Considering only the PET resin, the footprint per functional unit is ____ kg CO₂e (one decimal place).
Numerical answer — type the value.
Show answer
Answer: 50.7
1000 bottles × 0.028 kg = 28 kg PET. Mass-weighted factor = 0.70 × 2.2 + 0.30 × 0.9 = 1.54 + 0.27 = 1.81 kg CO₂e per kg. Footprint = 28 × 1.81 = 50.68, so 50.7 kg CO₂e. All-virgin PET would give 61.6.