Science to enable
sustainable plastics
A white paper from the 8th Chemical Sciences
and Society Summit (CS3)
London, UK • June 2020

Contents
About the Chemical Sciences andSociety Summit (CS3)

4

Glossary of terms used in this report5

Summary and recommendations

6

1. The impact of plastics

12

2. New sustainable plastics

20

3. Recyclability of plastics

28

4. Degradation of plastics

36

Appendix
Glossary of acronyms
Structures of common polymers
and their recycling symbols
Further reading

43
44

CS3 participants

46

Cite this report: rsc.li/progressive-plastics-report44
45

About the Chemical Sciences and Society Summit (CS3)

About the Chemical Sciences andSociety Summit (CS3)
The CS3 brings together leading researchers to discuss how the chemical
sciences can help to tackle some of the most daunting challenges that our world
faces. Previous summits have tackled topics as diverse as water resources,
human health, and sustainability.
This White Paper, Science to enable sustainable plastics, summarizes thepresentations, discussions and outcomes of the 8th CS3 meeting, held in
London, UK, on 10–13 November 2019. More than 30 scientists from four
participating countries gathered to discuss four major themes in sustainable
plastics: their impact on the environment; new sustainable plastics; therecyclability of plastics; and the degradation of plastics.
Their goals were to assess the current status of sustainable plastics, identify themost pressing research challenges in this area, and make recommendations
about how the field should progress.
The CS3 initiative is a collaboration between major international chemicalsocieties. For this report, the collaboration involved the Chinese Chemical
Society (CCS), the German Chemical Society (GDCh), the Chemical Societyof Japan (CSJ), and the Royal Society of Chemistry (RSC). The meeting was
supported by the National Science Foundation of China (NSFC), the German
Research Foundation (DFG), the Japan Science and Technology Agency (JST),and the UK Engineering and Physical Sciences Research Council (EPSRC).
CS3 summits are held every two years, and rotate among participating nations.

04

Additive: Most plastics contain various additives that
help to maintain their long term performance. Typical
additives include antioxidants, antistatic agents,
colourants, curing agents, flame retardants, thermal
stabilizers, UV stablizers, plasticizers and lubricants.
Bio-based or bio-derived plastic: A polymer that is
produced from biological resources, including chemicals
derived from plants and algae. For example, polylactide
is produced from sugar, which is harvested from plants
like sugar cane.
Biodegradable plastic: A plastic that undergoes
accelerated degradation by organisms and biomolecules
such as enzymes, forming small molecules that are
metabolized by natural organisms. Biodegradable
plastics should break down to natural materials that canbe returned to the environment without pollution ordeleterious effects.
Biopolymer: A naturally occurring polymer, such as
cellulose or starch.

Environmentally degradable plastics: Plastics that canbe completely broken down and assimilated into thenatural environment over time without any negative
toxicity or environmental effects.
Mechanical recycling: A recycling process in which waste
plastic is reformed into new plastic products throughmelting and extrusion. Mechanical recycling often leads
to poorer material properties and a reduction in value
compared to the original plastics.
Open-loop recycling (also known as cascaded
recycling): A recycling process whereby waste material
is transformed into new product and waste. This process
is often associated with lower-grade applications forplastics. For example, polyolefins are recycled byopen-loop processes to produce additives for tarmac.

About the Chemical Sciences and Society Summit (CS3)

Glossary of terms used in this report

Upcycling: A recycling process to create a product ofhigher quality or value than the original. For example,
plastic pipes could be chemically modified to improvetheir heat resistance, or confer antibacterial properties.

Chemical recycling: A process whereby the polymer isdegraded into monomers and chemical raw materials
that can be reused to make new polymer and basic
chemicals. After chemical recycling, the end-productpolymer and basic chemicals are chemically identicalto that produced initially.
Circular economy: A system in which resources are
kept circulating for as long as possible, throughefficient material use, reuse and recycling loops.
It is an alternative to the linear economy in whichmaterials are made, used and disposed.
Closed-loop recycling: A recycling system in whichplastic is repeatedly recycled and reused withoutcompromising the quality of the material. Such
recycling currently applies to the chemical recyclingof polyethylene terephthalate (PET).
Compostable plastic: A sub-class of biodegradable
plastics that are decomposed in compost, either at lower
temperatures in home composting systems or at higher
temperatures in industrial composting facilities.
Degradable polymer or plastic: A term used to refer to apolymer or plastic containing chemistries that allow thechain to be disassembled either through recycling or
through biodegradation.
Disassembly: The breaking of polymer chains into
shorter sub-units and monomers so as to aid recycling.
Chain disassembly may also be triggered at an
appropriate time in degradable polymers.

05

Summary and recommendations

Summary and recommendations
Plastics have helped to build the modern world. They keep our food fresh andsafe; they are used to build our cities, homes and even the mattresses we sleep
on; they power the green revolution, producing light-weight electric vehicles andsolar cells; they are essential components of mobile phones and computers; andthey enable medical advances, from masks, contact lenses and heart diaphragmpumps to artificial tissues (see box: What are plastics?).
Plastics are essential to create a more sustainable society, and to ensure that
future technologies develop rapidly and cost effectively. Plastic packaging
reduces food waste by prolonging its shelf life, and has an important role toplay in detecting food quality. Recent developments in plastic composites meanthat plastic can form 50% of the primary structure of aeroplanes, resulting insignificant greenhouse gas emissions savings.
Future technologies central to reducing our reliance on fossil fuels will also
require plastics. In electric vehicles, for example, it is possible to replace evenmore metal components than in petrol cars, and to use light-weight plastics
in energy recovery devices, cooling pipes, pumps, fans and casings. The useof plastics in electric vehicles is already growing rapidly. Wind turbine blades
require plastic composites and adhesives, while batteries rely on plastics intheir housing and may even apply them as electrolytes and other components.
Plastics are also widely used in home insulation, reducing energy usage, andthey play critical roles in the construction sector as pipes and conduits, cladding,seals, adhesives and gaskets. In future, plastic composites could replace metals
in load-bearing structures and will likely be important in intelligent buildings ascomponents of detection and monitoring systems. Plastics are essential as theactive layer in water purification systems and deliver efficiencies in agriculture,such as reducing water usage and increasing productivity. Future technology
sectors such as robotics, drones, electronics, personalized healthcare anddiagnostics each rely on the development of better plastic materials.
Despite these benefits, the use of plastics is also causing major environmental
challenges. Plastic manufacturing consumes significant quantities of
petrochemicals: in Europe, for example, it accounts for roughly 4%–6% of all
oil and gas use, according to industry group Plastics Europe. Since plastics areinterwoven with the petrochemical industry, they are subject to its fluctuations,geo-politics and contributions to CO2 emissions.
Discarded plastic pollutes the natural world, with microplastics and nanoplastics
being detected in many ecosystems (see box: Sizing up plastic pollution).
The majority of plastic waste is generated and emitted on land, but researchon plastic pollution initially focused mainly on the marine environment, where

What are plastics?
Plastics are primarily comprised of polymers, along with various additives(such as stabilizers, flame retardants, and plasticizers) that affect thephysical properties of the material.
Polymers are long-chain molecules built from smaller repeating units calledmonomers. Some polymers contain only one type of monomer building
block; others, known as co-polymers, may contain two or more different
types of monomer.
Polymer chains assemble across multiple length scales (see Figure 4.2:
Structure of plastics at different scales p40) and detailed understandingof how the properties of plastics, including their disassembly, relate to theirdifferent structures is a key challenge in polymer science.

06

Possible risks associated with plastic particles cannot be generalized because
microplastics and nanoplastics comprise a very heterogeneous group of particles
that vary in polymer composition, additive content, size, shape, ageing state,
and consequently in their physicochemical properties. However, the ubiquitouscontamination of the environment with microplastics and nanoplastics, along withthe possible associated risks to ecosystems and ultimately to human health, hasrecently attracted a great deal of public and scientific attention.

Summary and recommendations

plastic particles are reported to occur from tropical to pristine polar areas,
and from beaches to deep-sea sediments. Later, river and lake systems were
examined, where plastic particles were found even in remote mountain lakes.
Plastic particles have more recently been found in the atmosphere and in
terrestrial ecosystems, especially in urban and agricultural soils. The ingestionof plastic particles together with food has already been investigated in a varietyof organisms from aquatic and terrestrial habitats, and the resulting effects onorganisms and human health are still under discussion.

For many members of the general public, plastics now epitomize a disposable
way of life and are associated with cheap, low-quality and low-value products.
The visible evidence of plastic pollution and as yet unknown impacts of thesematerials are driving a reconsideration of their life cycles, designs and uses.
Technical solutions will be needed to ensure that in future plastics combine
useful properties with better end-of-life options, and chemistry will play a central
role in delivering these.
Developments in chemistry will be key to understand and mitigate the impactof plastics in the environment. Chemistry can help to develop efficient ways torecycle the plastics we use today and, in the longer term, create replacementsthat are made from sustainable starting materials, are more amenable torecycling at end-of-life, and have reduced environmental persistence or impact.
Sustainability across the entire plastics life cycle must be a core design featureof the polymers of the future. It is also clear that a suite of materials will be
required to meet the myriad of applications, just as different plastics are applied
today. This means that underpinning investment is recommended in a rangeof different technologies and options. It is also essential to emphasize that no
single solution is suitable for all scenarios, geographies or products. Different
countries already employ a wide range of waste management practices, withvarying degrees of environmental impacts. As such, the most sustainable option
for a particular location is not necessarily a global solution.
In some scenarios, improved sustainability will arise from deployment ofpolymers built entirely from renewable, biologically-derived feedstock
chemicals, or from wastes like CO2 where the raw materials used to produce thepolymer are carbon neutral. For some applications, durable or longer-lasting
polymers, which can be reused multiple times prior to efficient closed-loop

Sizing up plastic pollution
The size of a piece of plastic is an important factor in determining its
impact on the environment. Commonly accepted size ranges are:Macroplastics: larger than 2.5 centimetres across
Mesoplastics: 5 millimetres to 2.5 centimetres
Microplastics: 1 micrometres to 5 millimetres

Nanoplastics: smaller than 1 micrometre
1 centimetre = 10 millimetres
1 millimetre = 1,000 micrometres
1 micrometre = 1,000 nanometres07Summary and recommendations

recycling, will be the best option. In yet other scenarios, it will be important
to design polymers to incorporate special chemical and physical features tomake them ‘degradable on demand’ – such features will reduce energy use andimprove selectivity for closed-loop recycling. Fundamentally, there is a need todesign polymers for efficient disassembly. Such an approach has the potential toenable closed-loop recycling over multiple cycles and to reduce, or even nullify,
environmental persistence if they escape from waste systems.
We recognize that building this new future for plastics necessitates a majorcollaboration between sciences, engineering, technology, materials design,
humanities, human behaviour, policy, regulation, economics and business. This
report focuses only on the contributions and solutions that could be technicallyfeasible, and on the research challenges specific to chemistry and the chemical
sciences. It deliberately avoids making recommendations on policies orregulations for recycling, waste management systems, use of financial incentives
and taxation (the Further reading appendix p45 includes references to recent
policy briefings and reports from expert working groups that address some ofthese important parallel issues).
In coming up with our recommendations, we emphasize that technology alone
cannot provide all solutions and that parallel advances in waste management,
regulation, economics and behaviour will be needed to deliver the infrastructure andecosystems for a sustainable plastics future. We signal that experts in chemistry are
well placed to provide impartial evidence for policymaking and standardization ofplastics, as well as in environmental monitoring and detection.
We propose four major research challenges and their underpinning research
priorities. These research challenges are interlinked and symbiotic, as such we
do not recommend unbalanced selection or weighting of research in a particular
direction. Our philosophy is that plastics should not be deliberately released or
dumped into the environment and that efficient closed-loop waste management
systems are vital to implement these technological advances and solutions.
Meeting the technical challenges necessitates close integration with a range ofother technical disciplines, as well as in parallel with the broader considerations
outlined above.
To ensure future researchers are able to invent effectively in this space, it isimportant to offer multi-disciplinary training and education of chemists in areas
such as polymer science, materials engineering, process design, eco-toxicology,
molecular biology, environmental sciences, life cycle assessment and data
science. We, as a scientific community, recognize the benefits of, and urgent
need for, outreach, advocacy and public engagement activities to stimulate
a public dialogue about the impacts and solutions of future plastics, and to
examine material selection choices using a life cycle approach, and in the context
of sustainable development goals.

08

a. Develop new analytical methods to study microplastics and nanoplastics fordifferent types and shapes and at different size and time scales. These should
include high-throughput spectroscopy and microscopy techniques that can
rapidly generate reproducible data about the size distribution, structure andproperties of microplastics in the environment.
b. These data should be applied in predictive models that simulate the transportand distribution of microplastics in the environment, which will help tounderstand the location, fate and persistence of plastic waste. In future, thesemodels may also help to predict the environmental impact of new plastics
before they get to market.

Summary and recommendations

Research challenge 1: To understand the impacts of plastics
throughout their life cycles

c. Research is needed to properly understand the factors that influence theformation of microplastics and nanoplastics, across a range of environments
including soils, freshwater, and oceans.
d. We must understand whether there is any toxicity from plastics and theirdegradation products, at all size scales of waste fragments. This will requiredetailed knowledge of the biological pathways involved in the chemical andparticle toxicity of microplastics. It should also consider both direct andindirect ecological effects, such as a reduction in food availability for otherspecies, transportation of pollutants, and formation of biofilms.
e. The environmental impact of plastics is not limited to their end-of-life
degradation. Impacts also arise across their whole life cycle. We need detailed
life cycle and sustainability assessments, both to understand the trade-offs
in environmental impacts and to identify research opportunities focused on
reducing impacts.

Research challenge 2: To develop new sustainable plastics
a. New plastics must be designed for the circular economy and to facilitate
recycling. Researchers should develop polymers that improve the efficiencyof closed-loop recycling. This could allow for the production of materials thatshow outstanding properties but which are able to ‘degrade on demand’,with the degradation products being chemically recycled to polymer ortransformed into other high value products. Central to this concept is todesign polymer structures so they can be disassembled and controllably
broken into smaller sub-units – oligomers and small molecules – which can berecycled and reused. New plastics capable of maintaining desirable material
properties after mechanical recycling should also be targeted, for example
through chain extension or self-healing processes.
b. Research is needed to produce new polymers from non-virgin petrochemicaland bio-based raw materials. Alternative feedstocks should include recycled
chemicals, for example from plastics recycling, (sustainable) biomass,industrial wastes like CO2, and modified biopolymers like cellulose or starch.
These monomers, used to create the polymers, must be scalable, abundant
and truly sustainable. In the longer term, the manufacturing of plastics mayneed to be decoupled from the use of virgin fossil fuels and from the fuelindustry.
c. New processes are needed to manufacture, process and recycle plastics
that adhere to rigorous sustainability metrics. Central to delivery of efficient
processes will be to develop catalytic systems and reactions. Even as theseare being developed in the laboratory, they should demonstrate a reasonable
potential for manufacturing scalability and efficiency.
09

Summary and recommendations

d. Improving plastic sustainability in some sectors will require production ofdurable or longer-lasting polymers. These plastics need to maintain theirproperties and performances over appropriate use time scales but be
designed from the outset with managed end-of-life scenarios. Research
into alternative reinforcing interactions, especially to produce alternatives tothermoset resins and networks, is a priority. Exploration of the use of noncovalent or equilibrium interactions is recommended as a means to ensure
properties are delivered while facilitating recycling and / or reprocessing.
Research should also focus on how to control crystallization and loss of
crystallinity in materials, as crystalline regimes confer useful properties but
slow chain disassembly after use.
e. Efforts to design new plastics must consider their properties across multiple
length scales, including monomer sequences, polymer chain interactions, aswell as nanostructures and microstructures. It is important to develop new
experimental methods and theories to understand the relationships between
the structures and properties of polymers. In future, it may be possible toapply homoplastic composites, whereby a single polymer chemistry performs
multiple functions through control across length scales. For example,
ensuring that in packaging only one type of material is used for the packaging,adhesive, and barrier layer would simplify recycling.
f. Design data, analytical characterization methods and models should be
developed that inform both polymerization and depolymerization processes.
New theory and methodology is needed to accurately predict polymer
performance and disassembly across multiple length and time scales.

Research challenge 3: Closed-loop plastics recycling
a. Chemical technologies are needed to improve labelling, identification andseparation of waste plastics and composites into single-component, purepolymers. These polymers should be reprocessed into new products and / orfully recycled to pure polymer by chemical recycling.
b. More efficient chemical recycling processes are needed to recover valuable
oligomers, monomers and / or small molecules from wastes. Chemical
technologies should be developed which function effectively using polymer
composites and mixtures of plastics. These technologies include newcatalysts, processes and reactions allowing chemical recycling of current large
scale polymers. For example, polyolefins cannot yet be chemically recycledwith sufficient efficiency or chemical selectivity to monomers.
c. To deliver better plastic recyclability, reactions for polymerization anddepolymerization that are closer to equilibrium should be investigated. Thisapproach may bring additional property benefits, such as self-healing plastics
with extended lifetimes as well as reductions in energy demand required forrecycling.
d. In some scenarios, it may be appropriate to process waste plastics so as torecover the energy they contain – by incinerating them to generate electricity,
for example. In these scenarios, any CO2 or other greenhouse gases produced
should be captured and stored in a sustainable way. It may also be possible torecycle waste CO2 into new plastics to achieve ‘carbon recycling’.

10

a. Research must reduce both the short term and long term environmental
impact of plastics. For some applications, this involves the development ofenvironmentally degradable polymers. These are important for applications
such as disposable medical products, personal care items, and certain
packaging and agricultural plastics. Such environmentally degradable
polymers should be a backstop to guard against the leakage of waste plastic
into the environment – they should not be seen as a substitute for recycling,or an excuse to dispose of plastics in the environment. We recommend careful
examination of standards and labelling of polymers so as to ensure there
is proper understanding of terms such as compostable, biodegradable andenvironmentally degradable polymers. Research is necessary to define thecause, extent, and time scale of the polymers’ degradation reactions and tounderstand the impacts of degradation products in the natural environment.

Summary and recommendations

Research challenge 4: To understand and control plastic
degradation

b. Research is needed to understand the degradation of polymers in the broad
range of environments in which they are being discovered. This should
include how polymer and plastic structures are influenced by humidity,
pH, temperature, light, organisms and enzymes, as well as the interplay
between these factors. It is recommended that such studies are conductedso as to yield better data about how the polymer chemistry, morphology andcrystallinity influences degradation in a range of real environments. The fateand impact of the degradation products must also be fully considered, asshould any additives included in these plastics.
c. Research is needed to develop materials that are both recyclable andenvironmentally degradable. Such materials could then be recycled multiple
times, through cycles of polymerization-depolymerization, without loss ofproperties; but at the end of efficient recycling, they could be degraded to
non-toxic products that can be metabolized by organisms in the environment.
In parallel, it may also be important to develop strategies to understand andimprove the environmental degradability of some of thenon-degradable polymers used today.
d. Environmentally degradable polymers will need to be competitive in
performance and cost. As such, the monomers and polymerization processes
used to make these polymers must be scalable, efficient, economical, andsustainable. There is unlikely to be any single material able to deliver allproperties for all applications, consequently a range of new environmentallydegradable polymers is needed, each tailored towards specific applications
and degradable over different time scales.

111. The impact of plastics
Plastics are an essential part of modern life, and an
indispensable tool in sustainable development. Some of
today’s petrochemical-based plastics also have concerning
environmental impacts, from the greenhouse gases involved
in raw material extraction and manufacturing to waste plastic
pollution in the environment. Chemistry can underpin efforts
to understand and mitigate these impacts by developing
sustainability tools and, where appropriate, circular
economies for plastics.

1. The impact of plastics

Introduction
Plastic production has grown substantially over the past few decades, and shows
little sign of slowing down. The annual production of plastics currently stands at
almost 360 million tonnes per year, surpassing most other human-made materials(see figure 1.1: Popular plastics). By 2015, humans had cumulatively made agrand total of roughly 8,300 million tonnes of plastics. About 6,300 million tonnesof that had already become waste, only 9% of which was recycled (Sci. Adv. 2017,
DOI: 10.1126/sciadv.1700782).
In a world where one million plastic bottles are bought every minute, profligate
consumption is clearly a contributing factor. But so too is the poor managementof waste plastic, which too often results in discarded plastic leaking into
the environment. Researchers have estimated that 31.9 million tonnes ofmismanaged plastic waste enters the environment every year, with 4.8–12.7
million tonnes of that going into the oceans (Science 2018, DOI: 10.1126/science.
aar7734) and significant quantities contaminating terrestrial ecosystems(Environ. Sci. Technol. 2019, DOI: 10.1021/acs.est.9b02900). This pollution posesa threat to wildlife, to our environment, and to ourselves.
Plastics manufacturing requires not only petrochemical feedstocks but alsoenergy. Thus, their manufacture contributes to overall industrial CO2 emissions(see figure 1.2: The carbon footprint of plastics). Manufacturing methods that
apply non-fossil energy sources or reduce energy consumption can have a hugebenefit. Today, the feedstock monomers used to make the most ubiquitous
plastics are co-products of the fossil fuel industry, but in the long term we willneed to reduce our reliance on fossil fuels and develop plastic technologies
that are not predicated on virgin petrochemical feedstocks (see chapter 2:
New sustainable plastics). It is worth emphasizing that plastics have a lower
embedded energy than many alternative materials, such as glass or steel.
Provided that efficient waste management can be developed, there are many
scenarios in which using plastic is a more sustainable choice, particularly in termsof CO2 emissions, than selecting an alternative material. Selecting a material
for a particular application, such as packaging, should involve a balanced andevidence-based life cycle and sustainability assessment. We strongly caution
against substituting plastics for ‘alternatives’ without such analysis.
Figureplastics
Figure 1.1: Popular
Popular plastics

PP

Polypropylene

Others19%
PS

Polystyrene

6%

PET

8%

Polyethylene
terephthalate

PUR

Polyurethane(L)LD-PE
18% (linear)
low density
polyethylene

8%
10%
PVC

Polyvinyl
chloride

14

19%

12%

HD-PE
High-density
polyethylene

In 2018, global plastics production
reached 359 million tonnes. Abouthalf of these plastics were made fromdifferent forms of polyethylene (PE)or polypropylene (PP). (Percentages
show relative demand in Europe).
See Appendix for the structures ofcommon polymers.
Plastics – the Facts 2019, PlasticsEurope (2019)1. The impact of plastics

Figure 1.2:
1.2: The
Figure
The carbon
carbonfootprint
footprintofofplastics
plastics

Annual emissions from the

Plastic
Life Cycle

2.8
BILLION

1.34

tonnes CO2

BILLION

tonnes CO2

860
MILLION

tonnes CO2

2019

There are many strategies to replace fossil-based polymers that could improve
the sustainability of plastics. One approach involves substituting existingfossil-based plastics with their bio-derived equivalents. The polymer moleculesin the bio-derived plastic are identical to those in the fossil-derived version,
but their monomers come from different sources. This contrasts with analternative approach – replacing existing polymers with entirely new materials.
New polymers may be very desirable when plastics are being designed tofacilitate recycling or environmental degradation, but it can lead to lessdesirable consequences, such as incompatibility with current processing or
blending technologies, or produce materials that have inferior physical andchemical properties.

2030

2050

The production, management andincineration of plastics added 860
million tonnes of CO2 (or equivalentgreenhouse gases) to the atmospherein 2019, equal to the emissions fromalmost 200 typical coal-fired power
stations. For comparison, global CO2
emissions exceeded 36 billion tonnes
in 2019. The carbon footprint ofplastics is expected to triple in sizeby 2050.
Plastic & Climate: The Hidden Costs of a Plastic
Planet, Center for International Environmental
Law (2019)

Microplastics aboundThe impact of plastic pollution partly depends on the size of the plastic waste (seebox: Sizing up plastic pollution p07). Large plastic objects, such as polyethylenebags, have been found inside whales’ stomachs. Seabirds have been found to eat
smaller chunks of plastic, while discarded fishing lines ensnare mammals likesea lions. In 2012, the United Nations estimated that plastic waste killed roughly
one million seabirds and more than 100,000 marine mammals each year.
Fragments or fibres of waste plastic that are less than 5mm across are known
as microplastics. These may be produced at that size or they can form whenlarger plastic items break down in the environment. Microplastics have probably
been in the environment since plastics were first mass produced, but the firstreport of their detection in oceans dates from 1972 (Science 1972, DOI: 10.1126/
science.175.4027.1240), and over the past 15 years research has shown thatthese particles are ubiquitous in the environment.
Microplastics are just the right size for plankton, bivalves and other filter feeders,and soil dwelling organisms at the base of the food chain to consume. Larger
animals that feed on these creatures then accumulate microplastics in their own
bodies. For example, a recent survey of 102 sea turtles found that every single
animal had microplastics in its guts (Global Change Biology 2018, DOI: 10.1111/
gcb.14519).
15

1. The impact of plastics

It is not yet known whether these tiny fragments harm the animals that ingest
them. Microplastics have been shown to slow the digestive systems of aquaticworms, for example, so that they put on less weight as they grow. Additives
to plastics, such as phthalate plasticizers, can also bioaccumulate in worms,
potentially hampering their reproduction. Meanwhile, humans are exposed tomicroplastics, for example when they eat seafood, although the consequences ofthis exposure are unknown. The oral uptake of microplastics may have negative
effects on the gut microbiome, which is essential for the absorption of nutrientsand plays an important role in the immune system. One potential risk that has
been intensively discussed but not yet sufficiently investigated and understoodis the transition of microplastics via the digestive tract into cells and tissue,
which can lead to inflammatory processes. A further yet unexamined risk is theinhalation of microplastic particles via the air we breathe. When microplastics
break into fragments smaller than one micrometre, they form nanoplastics
which, because of their smaller sizes, are much more likely to be absorbed byliving creatures. As yet, relatively little is known about the biological impact ofnanoplastics.
Microplastics may even hamper the natural world’s ability to counteract climatechange. Microscopic organisms called phytoplankton use photosynthesis tocollectively capture millions of tonnes of carbon per day from the atmosphere,
and some types of phytoplankton excrete this carbon in fecal pellets that sink tothe bottom of the ocean, acting as a long term carbon storage mechanism. But
when phytoplankton consume microplastics, it changes the composition of
fecal pellets so that they are more likely to float, and release their carbon backinto the atmosphere.

Tracking microplastics
To understand more about the origins and behaviour of microplastics, researchteams collect microplastic samples from water, sediments, soils, air, plants
and animals. These samples are studied using a wide range of techniques,
including optical and electron microscopy, X-ray analysis, and Raman andinfrared spectroscopy. This analysis can reveal what the microplastics are madeof, their physical forms, and how they are generated. Plastics break down into
microplastics via a range of different chemical and physical mechanisms that
include exposure to ultraviolet radiation, oxidation, thermal breakdown, andsimply tumbling around in surf (see chapter 4: Degradation of plastics).
These data can feed into large scale simulations of plastic waste distribution
and behaviour in the environment. Preliminary models of plastic pollution in the
Pacific Ocean, for example, are already starting to predict how plastic waste
moves around (Nat. Commun. 2019, DOI: 10.1038/s41467-019-08316-9).
Complementary research in freshwater and terrestrial systems should continue
in parallel.
To generate more of these data, researchers need to be able to run their analyses
more quickly, perhaps using high-throughput methods that could operate for
instance in river systems, sewage treatment plants or on research vessels at sea.
The predictive models also depend on understanding how long certain plastics
are likely to survive in the environment and how quickly they degrade, whichis intrinsically linked to their chemical and material properties. Microplastics
also leach additives, and may pick up persistent organic pollutants or toxic
metals during their time in the environment, both of which can have additional
environmental impacts. In principle, these factors can also be incorporated into
microplastic models.
Microplastic samples in the marine environment should be collected not only
from the sea surface, but also from a range of depths in the ocean, and in oceanfloor sediments. This approach could help to reveal the fate of microplastics ofvarious densities, and also improve our understanding of the historical fate of16

Microplastic fragments found in theocean include fibres, granules andflakes.
Atsushi Takahara and Atsuhiko Isobe

1. The impact of plastics

microplastics that entered the oceans decades ago (Sci. Adv. 2019, DOI: 10.1126/
sciadv.aax0587). Some researchers are already studying the plastic in preserved
samples of dead animals in the International Environmental Specimen Bank.
This repository contains hundreds of thousands of specimens, including fishand seabirds, and researchers are gearing up to explore this trove to look forevidence of historic microplastic pollution.
Bringing data from many different sources and combining them into useful models
will require a multidisciplinary research effort, involving more collaboration
between many different fields: polymer chemistry, analytical chemistry,
biochemistry, environmental chemistry, physical oceanography and informatics.

Mulch films
Plastic pollution is not restricted to oceans and freshwater. In some countries,
farmers are facing a growing threat from plastic pollution in soil, due in part toan agricultural practice known as mulching. Plastic mulch films, comprised ofpolyethylene sheets, are used to cover crop fields soon after planting. They raisesoil temperatures, limit weed growth and prevent moisture loss; as a result,water usage is reduced, while earlier harvesting and higher crop yields arefeasible. As the world’s population grows and food demand increases, plastic
mulch film has a key role to play in agriculture. For example, in 2018, China
used 1.4 million tonnes of plastic mulch film to cover approximately 20 million
hectares of land used to grow crops.
But when farmers remove the film, it often leaves shreds of residual plastic, andbecause polyethylene is not degradable, these plastic residues accumulate in
the soil. The structure of soil becomes depleted, which may limit plant growthand foul machinery. Mulching is important for efficient agriculture, and despite
its problems farmers are loath to lose the benefits of the practice. Alternative
plastics could reduce these environmental impacts, and one strategy could beto develop plastic films that are environmentally degradable. Such a material
would obviate the need for gathering up waste plastic, while ensuring thatany residues left behind would break down to benign compounds in the soil.
Combining these properties in a single plastic is not trivial, but it is a challengethat chemistry is well placed to tackle (see chapter 4: Degradation of plastics).

Plastic mulch film is used to improvethe efficiency of growing crops.

171. The impact of plastics

Conclusion
Plastic pollution has serious impacts on our natural environment. Microplastics
and nanoplastics are of particular concern, and we need systematic research
to properly understand the factors that influence their formation across a
range of environments including soils, freshwater, and oceans. That will require
new analytical methods that researchers can use to study microplastics andnanoplastics at different size scales.
Predictive models that simulate the distribution of microplastics in theenvironment will also help to understand the fate and persistence of plastic
waste. These models could track the location of microplastics, the standing stockof plastic fragments in environmental reservoirs, and the fluxes between them.
Researchers need to understand the environmental toxicity of plastics andtheir degradation products at all size scales, as well as the biological pathways
involved in the chemical and particle toxicity of microplastics and nanoplastics.
Studies should also consider both direct and indirect ecological effects, such asa reduction in food availability for other species, transportation of pollutants,
and formation of biofilms. These potential impacts may be caused not only
by petrochemical-based plastics, but also by bio-based plastics if they arefragmented into small particles in the environment.
The environmental impacts of plastics are not limited to their end-of-life
degradation. Impacts also arise during the manufacture and application ofplastics, through the use of energy and other resources. We need detailed
life cycle and sustainability assessments, both to understand the trade-offs
in environmental impacts and to identify research opportunities focused on
reducing harm.
Overall, it is clear that a ‘business as usual’ approach to plastic waste isunsustainable and that a range of current and future industries requireinnovation in the plastics sector to deliver more sustainable products. Chemistry
can play a crucial role in ensuring that we can continue to reap the benefits ofplastics, but in a much more sustainable way.
Central to this change in sustainability will be to transform our current ‘linear
plastics economy’ into a ‘circular economy’. The linear economy involves
producing, using and then disposing products; in contrast, the circular economy
aims to turn this into a loop, so that unwanted products are reused or recycled,
many times, in processes powered by renewable energy. Moving to a circular
economy could dramatically reduce our use of resources, and help to meet thetargets set out in the 2016 Paris Agreement on climate change. These changes
will also be critical in ensuring we meet UN Sustainable Development Goals.
It is also essential to develop better ways to recycle plastic waste. To enable this,chemists can develop new technologies to disassemble and break down plastic
waste into oligomers and small molecules that can be reused to make plastics
and other high value products. Meanwhile, using renewable raw materials andenergy-efficient manufacturing processes can help to reduce the climate impactof plastics (see chapter 2: New sustainable plastics). At almost every stage ofthe plastic life cycle, the technologies needed to enable a circular economy forplastics will depend on chemistry.

Above and overleaf: Henderson Island– a remote, uninhabited South Pacificisland whose sandy beaches are World
Heritage sites – was announced in2017 to have the highest density ofplastic rubbish on its shores of any
place in the world.
earthsky.org/earth/henderson-island-pacificplastic-pollution-study

Overleaf: Plastic on a beach in
Lanzarote, 2018.

18

19

1. The impact of plastics

2. New sustainable plastics
To develop sustainable plastics, researchers are creating
polymers that not only have excellent properties during theiruseful life, but also offer options for end-of-life management.
A life cycle assessment approach should underpin thesedevelopments, which should improve the efficiency of polymer
production and reduce energy inputs. In some contexts, it
may be beneficial to use waste plastics, biologically-derived
feedstocks or even CO2 as raw materials to make new plastics.
Ideally, renewable feedstocks should be suitable as ‘drop in’substitutes in existing chemical processes.

2. New sustainable plastics

Introduction
The vast majority of plastics on the market today are made from petrochemicals,
which are ultimately derived from oil and gas. Continuing to use these raw
materials will require a reduction in both energy use and carbon emissions
associated with these processes. These reductions need more research into
improved manufacturing and processing methods. In the longer term, it seems
likely the world will shift from a manufacturing system based predominantly on
fossil fuels, towards one that makes products from a range of sources including
recycled chemicals and renewable raw materials, using processes powered byrenewable energy. Chemistry can also help to develop routes to make plastics
that recycle waste CO2 as a co-monomer and lock it into the polymer backbone.
Most common petrochemical-derived polymers such as polyethylene andpolypropylene take many years to break down in the environment. Thesemolecules are carbon chains that typically do not contain chemical groups that
could act as obvious ‘break points’ for chemical or biological degradation, andin many cases are water resistant. The polymer chains can assemble to form
crystalline regions that confer excellent thermal and mechanical properties,
making the material extremely durable in use, but also less amenable todegradation after use (see chapter 4: Degradation of plastics).
Only a handful of polymers have been designed to be easily degradable in theenvironment – for example the polyester polybutylene adipate terephthalate(PBAT), which is used as a compostable packaging material because it has similar
properties to low-density polyethylene (LDPE) but reacts with water to fully
degrade. There are almost no commercial polymers that have been explicitly
designed to make recycling easier. Plastics need to be designed for recycling
and the area of depolymerization chemistry needs fundamental exploration,
theoretical understanding and exploitation. One option is to exploit the benefits
conferred by biologically sourced monomers, which tend to be more oxygen richthan hydrocarbons, and to use these oxygen-bearing chemical groups to provide‘break points’ for degradation. Another option is to exploit non-permanentinteractions between chains to produce polymers that can disassemble,
or crystallites that can be disrupted with a lower energy cost.
Figure 2.1:
usesof plastics
Figure2.1:TheThe
uses
of plastics
Others17%
Household,leisure & sport
Agriculture
Electrical &electronic

4%
3%
6%

40%

Packaging

10%
Automotive

20%
Building & construction

22

Although packaging was the largest
single application for plastics inEurope in 2018, these materials playa vital role in a wide range of othersectors.
Plastics – the Facts 2019, PlasticsEurope (2019)2. New sustainable plastics

Designing new plastics is important for sectors as varied as packaging andconstruction, and specialized plastics are also needed in many other sectors,
including electronics, transportation, furnishing, leisure and agriculture (seefigure 2.1: The uses of plastics). These areas may accelerate uptake of new
sustainable plastics and help new materials secure a foot-hold in the market.

Polymers from renewable resources
Bio-based polymers currently make up a tiny proportion of the plastics market.
Polylactic acid (PLA, also known as polylactide) is one of the most widely usedbio-based synthetic polymers, found in products such as disposable cutlery
and plastic cups. Thanks to the aliphatic ester chemical groups in the polymer
chains, PLA can be completely broken down by industrial composting processes(see figure 2.2: A sustainable cycle). Life cycle assessments suggest that PLA has
lower overall CO2 emissions compared with conventional petrochemical plastics
such as polyethylene and polypropylene (J. Polym. Environ. 2019, DOI: 10.1007/
s10924-019-01525-9).
PLA is most commonly made by a ring-opening polymerization of lactide,a cyclic diester made from lactic acid. This starting material is produced by
the fermentation of plant sugar extracted from crops such as sugar cane.
Work is ongoing to utilize waste lignocellulosic feedstocks, including foodand agricultural wastes, in order to make lactic acid production even moresustainable. Other lactones and cyclic monomers can be polymerized either tomake variants of PLA or other aliphatic polyesters, and these approaches allow
for some control over the polymers’ chemical, physical, thermal and mechanicalproperties (Chem. Rev. 2018, DOI: 10.1021/acs.chemrev.7b00329).
Other bio-based polymers can be produced using monomers such as
terpenes, including pinene derived from pine tree oil (Polym. Chem. 2014, DOI:
10.1039/C3PY01320K; Angew. Chem., Int. Ed. 2016, DOI: 10.1002/anie.201509379),
limonene oxide derived from waste citrus fruit peel, and succinic anhydride(Angew. Chem., Int. Ed. 2018, DOI: 10.1002/anie.201801400).

materiaawls

R

2.2: A sustainable
cycle
FigureFigure
2.2: A sustainable
cycle

n
tio

ymers
ol

io)degra
(B

P

Recy
c

p

essing

tio
ns

g
lin

or re

ro
c

ica
Appl

Plastics that are made from bio-based
feedstocks, and which are recyclable
or biologically degradable, could offer
a more sustainable alternative topetrochemical-based plastics.

23

2. New sustainable plastics

Plastic rulers amongst recyclable
plastics. These rulers have been madefrom recycled materials.

Researchers have developed highly active and selective catalysts for these (co-)
polymerization reactions, but further improvements are needed to translate theseto sustainable industrial processes that produce plastics with more desirable
characteristics such as higher mechanical strength and better barrier properties.
It is important to note that simply because a monomer can be produced fromrenewable resources does not guarantee improved sustainability. For example,
bio-based polyethylene terephthalate (PET) can be produced by making theethylene glycol co-monomer from glucose, but this gives rise to impacts such as
eutrophication and acidification. The other co-monomer for PET, the aromaticdiacid, cannot currently be economically produced from renewables. Alternative
aromatic monomers derived from sugars show promise to produce fully biobased polymers with similar properties to PET, but there are still challenges withmonomer and polymer production. Better catalysts and processes could help,
for example by using high pressure CO2 together with new catalysts to reducepolymerization temperatures and energy consumption.
It must be stressed that the material properties of a polymer are decisive for its
applications. The thermal and mechanical properties of PLA, for example, define
its possible applications, and exclude many others.

Using waste molecules
Better reuse of waste molecules, either from industrial, agricultural or otherchemical processes, will be important in reducing the carbon emissions
associated with plastics. But CO2 can also be used, in conjunction with aco-monomer, as a building block for plastics. In many ways, it is an ideal
feedstock – it is cheap, abundant, and is the waste product from manyindustrial processes.
For example, polycarbonates can be produced by reacting epoxides with CO2.
This process is operating at commercial scale, and life cycle assessments havequantified the reductions to overall greenhouse gas emissions (Green Chem.
2014, DOI: 10.1039/c4gc00513a). The CO2 copolymerization process is criticallydependent upon the catalyst, and recent inventions have uncovered theopportunity to increase performances using mixed-metal catalysts, for example
of magnesium and zinc (Chem. Sci. 2019, DOI: 10.1039/C9SC00385A). As chemists
develop a better understanding of how these metals cooperate, that should
help to develop better catalysts that can make the process even more efficient,
reducing manufacturing costs and prompting wider adoption of these plastics.
24

2. New sustainable plastics

It is also possible to add carbon monoxide to ethene during its polymerization,
which introduces carbonyl groups into the polyethylene chain. These can act asconvenient break points, so that chemical reactions involving enzymes or light
could degrade the polymer molecules in the environment after a few months.
Such materials are already being used in some plastic packaging, but it takes
extremely high pressures to drive the polymerization reaction that produces
them, raising manufacturing costs and making the process very energy intensive.
So chemists are developing catalysts and methods that can make such polymers
under much more moderate conditions, which could offer a more convenientroute to such degradable plastics (Macromolecules 2014, DOI: 10.1021/
ma5012733 and Polym. Chem. 2014, DOI: 10.1039/c3py01637d).
In the longer term, chemistry has a key role to play in generating completely
new and disruptive technologies for plastics, potentially replacing some carbonbased monomers with new materials based on other elements. For example,
sulfur is a large scale waste from the petrochemical industry and finding ways torecycle it into polymers could be useful. Polythioesters exploit sulfur to introduce
convenient break points in the polymer structure that could be targeted by
future recycling processes.
Natural polymers such as cellulose, chitin and starch can serve as renewable andenvironmentally degradable resources, either to make monomers or as materials
in their own right. Cellulose is a major component of natural plant fibres, andthe world’s most abundant natural polymer. It is already used to make a range of
products including cellophane film, textiles and packaging.
But processing cellulose can be challenging. Cellulose molecules are held
together by networks of hydrogen bonds, which means that cellulose does not
melt and does not easily dissolve in common solvents. Chemists are developinga range of strategies to make cellulose processing easier, for example byexploiting ionic liquids to dissolve cheaper low-grade cellulose feedstocks
such as corn husks or wheat straw. This can reduce the cost or number of steps
involved in cellulose processing (ACS Sustainable Chem. Eng. 2017, DOI: 10.1021/
acssuschemeng.7b00488). Finding new ways to fine-tune the physical propertiesof cellulose derivatives may help to open up applications such as low-costpackaging, or to avoid the use of additives such as plasticizers.

Rolls of cellulose-based plastic film.

25

2. New sustainable plastics

Significant research effort is devoted to the efficient transformation of
lignocellulosic wastes into monomers for plastic production. This research effort
benefits from expertise in heterogeneous catalysis, engineering, bio-processing
and synthetic biology. It also allows for the production of polymer-natural
fibre composites and for the improvement of natural materials like paper byproducing polymer composites.

Conclusion
In future a range of recycled and reused chemical raw materials should be used
to make polymers. These raw materials can and should include the products ofchemical recycling of existing plastics. Because polymers are manufactured atlarge scale, the decisions about which monomers to deploy are guided to a large
extent by feedstock availability and price. Techno-economic and sustainability
assessment should be integrated at an appropriate stage of technology
translation and ensure that fundamental research is well targeted.
The infrastructure used to make and process polymers represents a large
capital investment that should not be abandoned in the short term. Thus, thedevelopment of sustainable ‘drop in’ substitutes for the highest-volume plastics(like polyethylene and polypropylene), which could use the same infrastructure,
is a research priority.
When designing polymers, sustainability assessment over the entire materials
life cycle is important. For example, future materials should be designed tominimize toxicity and environment impacts. As chemists find new ways to turnrenewable molecules and recycled wastes into polymers it is important to design
those polymers so that they can ultimately break down to environmentally
non-persistent biochemicals and products. Moreover, any new plastics will need
to be easier and less energy intensive to recycle. Research is needed to improveunderstanding of how polymer structure and microstructure affect their physicalproperties, recycling and degradation chemistries.

The many uses and varieties of plastic.

26

27

2. New sustainable plastics

3. Recyclability of plastics
In the transition from a linear to a circular economy, much
more of the plastic waste we produce must be recycled.
Chemists can help to achieve this goal by finding more efficient
ways to recycle the plastics we use today, and by developing
new plastics that can be more effectively recycled in the
future. This should include a greater emphasis on chemical
recycling technologies that upcycle waste polymers into
valuable chemicals, to be fed back into polymer manufacturing
processes or used in other processes.

3. Recyclability of plastics

Introduction
Much of the plastic in use today can already be recycled in some way. For
technical, economic and logistical reasons, though, far too little plastic waste
actually goes through recycling processes. According to an analysis led by the
Ellen MacArthur Foundation, roughly one-third of the world’s plastic packaging
waste is lost into the environment. In comparison, about 14% is collected forrecycling, and 40% is disposed of in landfill. The remaining 14% is collected andburned, sometimes in energy-from-waste incinerators that can harness thatheat to generate electricity (see figure 3.1: The fate of plastic packaging waste).
These percentages represent a global average, however, and they vary between
countries.
Some of the waste plastic that pollutes the environment has been discardedcarelessly, but there are also problems with the way that waste plastic is handled
after it is collected. This waste often contains a mixture of different plastics andother waste that must be separated before it can be recycled, a process that canbe very labour intensive. Waste management companies often bale up this waste
and ship it to countries with lower labour costs for recycling, but it is all too
common for some of this plastic to be lost during transport.
Most of the plastic that is recycled goes through mechanical and thermal
processing to re-make other plastic products. Thermoplastics such aspolyethylene and polypropylene could, in principle, be recycled by reprocessing
but in practice most mechanical recycling of these materials leads to a significant
compromise in properties and lower-value applications. Thermosetting plastics
and resins, such as some polyurethanes and epoxy-resins, cannot be easily
chemically or mechanically recycled.
Some niche plastic recycling processes operate efficiently – in Europe, forexample, about half of the mass of a plastic milk bottle is made from recycled
high-density polyethylene, material that partly comes from old milk bottles.

Figure3.1:
3.1: TheThe fate
packagingwastewaste
Figurefateofofplastic
plastic
packaging
8% cascaded
recycling

4% process
losses

14% collected
for recycling
2% closed-loop
recycling

98% virgin
feedstock

78 million
tonnes

14% incineration and/or energy recovery
40% landfilled(annual production)
32% leakage

30

Only 14% of plastic packaging
materials were recycled in 2013.
Just 2% of plastic packaging waste
was used to remake similar-quality
products (closed-loop recycling),
while 8% was turned into lower-value
products (cascaded recycling).
The New Plastics Economy: Rethinking the futureof plastics, Ellen MacArthur Foundation (2016)

Rawmaterials

1

Chemicals
industry

Plastics
manufacturers

Renewable
raw materials

5

4

3

End users2

Productreuse

3. Recyclability of plastics

Figure 3.2: A circular economy for plastics

Mechanical recycling

Chemical recycling

Energy recovery and carbon utilization

Turning an unwanted product into a new version of the same product in thisway is called closed-loop recycling. But other kinds of plastic are much moredifficult to recycle, or are turned into products with a much lower value thanthe original plastic, a strategy known as open-loop or cascaded recycling.
Companies involved in plastic manufacturing and waste handling would havea greater incentive to collect and reprocess plastic waste if more value could be
extracted from it. One option is known as chemical recycling, where polymer
molecules are broken down into monomers or other smaller building blocks
that can contribute to the virgin feedstocks used to make new plastics.
Chemical technologies have the potential to improve the efficiency of bothmechanical and chemical recycling processes. They can tackle the plastic
problem in two ways: by finding better ways to deal with the plastics we use
today, and by developing new plastics that can be more effectively recycled in
the future.

Sustainable plastics should rely on
renewable raw materials (1), andbe reused as much as possible (2).
Once reuse is no longer possible,
they should go through mechanicalrecycling (3) to create new plastic
products, or chemical recycling (4) toafford new feedstock chemicals. Oncethese options are exhausted, waste
plastic can be incinerated to generateenergy, with CO2 captured for use as afeedstock chemical (5).
Adapted from: Taking the European Chemical
Industry into the Circular Economy, Accenture(2017).

Researchers can expand the range of plastics that can be chemically recycled,
for example, and help to extract more value from these materials. New chemical
technologies could also help to directly and selectively recycle mixtures ofplastics, avoiding expensive and labour-intensive sorting processes.
Chemical recycling will play a crucial role in extracting more value from waste
plastic, and is essential to creating a circular economy for plastics (see figure 3.2:
A circular economy for plastics).

Limited options
Plastic recycling strategies vary between countries, but these systems rely onsome common basic elements. Waste plastic is collected from homes, businesses
and other collection points, and transported to a waste management facility
where plastics are sorted into different polymer types. This helps to ensurethat each waste stream offers a relatively pure source of one particular polymer
molecule, making them suitable for reprocessing into new products. In theprocessing facility, plastic is shredded and some impurities removed.
In mechanical recycling approaches, the plastic is melted and extruded so thatthe material can be re-formed into new plastic products, including bottles,
clothing fibres, carpets and furniture. However, the properties of plastics thatare produced by mechanical recycling are often inferior to those produced fromvirgin feedstocks. They may not be as strong, or may be discoloured, for example.
As such, this form of recycling is sometimes called ‘downcycling’, because its
products tend to have a lower value than the original plastics.
31

3. Recyclability of plastics

Shredded plastic waste at a recycling
facility.

In cases where plastic waste is not suitable for mechanical recycling, incinerating
the waste and using the heat to generate electricity can be applied, particularly
if the CO2 this generates can be captured sustainably rather than released tothe atmosphere. But even this solution cannot handle all types of plastic – forexample, polyvinyl chloride (PVC) causes problems in energy recovery systems
because it release acids when burned that corrode pipes in the incinerator.
Chemical recycling could be an excellent and broad-ranging alternative
recycling method, but currently there are relatively few commercially viable
processes. This relates in part to the complex composition of some waste
items, and to the use of additives and improvers which complicate recycling.
Fundamentally, the chemistry of disassembly is not so well understood. Over
many decades, chemists have devoted a great deal of effort to developing the‘forward’ reactions that turn monomers into polymers, but comparatively little
research has gone into the ‘back’ reactions that break down these long chains
into shorter molecules. This is an important new field for future developmentof methodologies, theories and processes.

Chemical recycling
Perhaps the most common form of chemical recycling currently available
involves a process called pyrolysis, which is a controlled thermal decompositionof hydrocarbon polymers. This process typically takes place at high temperaturesof 450–700°C, under an inert atmosphere, so that the plastic does not burn.
The current limitation of the process is that in most cases it creates a mixtureof waxes, oligomers, unsaturates, aromatics and alkanes – in other words, itshows poor selectivity in terms of monomer production and is best suited to
production of fuels from waste plastics. Another option is hydrothermal cracking,which breaks down plastic using catalyzed thermal processes and steam. The
temperature of the process is somewhat lower, at 350–550°C, but it also createsa mixture of products, with only a proportion being feasible for recycling
into plastic. Recycling mixed plastic waste in this way may require additional
technologies that can cope with additives.
Both of these processes require a lot of energy and show limited selectivity,
features that prevent wider commercial use. Better catalysts and chemical
processes could help to accelerate these degradation or depolymerization
reactions and so lower their operating temperatures. It is also important to
explore selective production of valuable chemicals and recycling to monomers
for future polymer production.
32

Below top: A handful of pelletised
recycled plastic. This is PET(polyethylene terephthalate), a plastic
commonly used in liquid containers.
It is relatively easily recycled andmay be reused. Below bottom left:
PET recycling symbol on a plastic
bottle. Below bottom right: Half ofthe mass of a plastic milk bottle canbe made from recycled high-density
polyethylene, material that partly
comes from old milk bottles.

3. Recyclability of plastics

Early stage academic research is exploring different means to catalyze
polyethylene recycling. This builds on a long history of research into thethermal or catalytic decomposition of many types of plastics (J. Jpn. Petro.
Int. 2016, DOI: 10.1627/jpi.59.243). Using tandem catalysis, which applies two
different catalyzed processes at once, it is feasible to explore dehydrogenationof polyethylene to introduce alkene groups. These alkenes can be subjected tocross-metathesis chemistry that allows for production of a range of shorter-chain
alkenes and oligomers. These processes have been optimized to occur at around
175°C, and the reaction has been performed with real plastic waste, including
plastic bottles, food packaging and shopping bags (Sci. Adv. 2016, DOI: 10.1126/
sciadv.1501591). But much more research would be necessary to develop alarge scale process using this approach, including significant improvements
to reaction selectivity and to catalyst loading, tolerance and cost. The process
only works for polyethylene waste, so other plastics like polypropylene andpolystyrene must be completely removed before chemical processing. And
the catalyst relies on expensive precious metals, so alternative metal catalysts
should be explored.

33

3. Recyclability of plastics

It may also be possible to develop chemical recycling technologies that break
down waste plastic into molecules suitable for making more valuable products
than the original plastic, a strategy known as ‘upcycling’. For example, plastics
could be broken down into valuable chemical feedstocks, or old PVC pipes could
be chemically modified to improve their heat resistance, or confer antibacterial
properties (RSC Adv. 2019, DOI:10.1039/c9ra05081g).
Chemical analysis plays an important role in identifying, tracing and sorting
different polymers from plastic waste. Techniques such as X-ray spectroscopy
and infrared spectroscopy can differentiate between different polymers, butthese methods struggle with certain plastics. To meet increasingly stringent
requirements for the traceability of wastes, there will need to be more research
into chemical and spectroscopic markers to improve recycling and ensuringaccountability.
Even with improved sorting technologies, there is also a need for technologies
to process and recycle mixed plastic wastes. Catalytic processes could help toselectively depolymerize certain plastics in mixed waste, making separationeasier. For example, PET and polycarbonates react with ethylene glycol at
different rates in the presence of a catalyst. The reaction depolymerizes
polycarbonate at around 130°C, but only goes to work on PET once thetemperature is raised to 180°C. This means the monomers for each polymer
can be extracted separately, providing a purer source of potential feedstock
chemicals with a much higher value than a mixture could offer (Polym. Chem.
2019, DOI: 10.1039/C8PY01284A). Alternatively, compatibilizers and block
polymers may improve the properties of mixed plastic waste products and allow
higher-value recycling.

Paper-plastic composites
The use of paper-plastic composites in disposable items such as coffee cups andcartons has seen significant growth over the past decade, but some of thesematerials pose serious recycling challenges. Paper itself is readily recycled,
although this requires significant input of energy and water. In the European
Union, more than 70% of new paper products are made from recycled paper.
Overall, the world produces about 400 million tonnes of paper per year, andpackaging accounts for roughly half of paper use.
But paper is increasingly teamed with plastics like polyethylene, along withadhesives, to make it stronger, oil resistant and waterproof. To recycle thesemixed paper-plastic composites, paper fibres must be separated from theother components – but current processes are still not very efficient in thisregard, producing a lot of reject material and fouling machinery in the process.
Meanwhile, the European Union’s Single-Use Plastics directive will ban many
single-use plastic products (where alternatives are available) by 2021, which
may inadvertently increase the use of paper-plastic composites.
Chemists and materials scientists should focus on ways to improve therecyclability of these paper-plastic composites. For example, bio-derived
hydrophobic cellulose esters and ethers could be used to make paper water
repellent and may facilitate recovery of paper fibres from these composites
compared to conventional composites. In principle, these next-generation paper
composites could also replace some of the polyethylene used in food packaging.
But they will need to match the performance of conventional plastics – flexible
but strong, oxygen impermeable, resistant to both liquid water and water vapour.
If they are not recyclable, these new composites could be engineered to becompostable. Solving these challenges necessitates more collaboration between
the plastics and paper industries and also for scientific understanding of how todesign composites for improved performances.

34

3. Recyclability of plastics

Conclusion
There is a widespread perception that recycling is costly and difficult for plastics.
In terms of the fundamental energy requirements of recycling, plastics have
many advantages compared to materials like glass or steel, not least because
typical depolymerization temperatures (ceiling temperatures) range from
100°C–600°C for most polymers. Nonetheless, the science of plastic recycling anddisassembly needs urgent development and the technologies necessary to allow
for efficient recycling should be developed. Polymer science can help to unlock
the value in waste plastics and to improve recycling processes. Chemistry candevelop technologies that make it possible to recycle a broader range of plastics,
with lower energy demands and lower CO2 emissions. One of the key targets is
to develop better catalysts and processes for recycling that are highly selective
in producing monomers for further polymerization. Chemistry is also essential
to quantify and qualify the recyclability of plastics in general, to help guidedecisions by manufacturers, consumers and policymakers.
Companies are eager for more sustainable approaches to manufacturing andrecycling of plastics. We recommend the investigation of chemical recycling
technologies as they have the potential to deliver multiple recycling loops,
without compromising the quality of the product. But the additives to plastics
need careful consideration as they may foul chemical recycling processes, andfuture developments need to work in parallel with material property demands toensure that a greater proportion of plastics are recycled.
Nevertheless, it is almost inevitable that some plastic will find its way into theenvironment. Developing new plastics that are more amenable to environmental
degradation could mitigate the impact of this pollution (see chapter 4:
Degradation of plastics). This research effort would rely on many of the samechemical strategies used to develop plastics that are more recyclable, such as
introducing chemical groups that act as convenient break points for polymer
molecules. As such, environmentally degradable polymers may also facilitate
recycling.

Degradable plastics can be one tool
to mitigate the impacts of plastic
pollution.

35

4. Degradation of plastics
Although some biodegradable and compostable plastics areavailable, there remains some confusion among the public andin industry regarding what these terms mean, and around theconditions and time scales necessary for these degradation
processes. Research is needed to understand exactly how allplastics degrade in different circumstances, and the fate of
their degradation products. This knowledge will help chemists
to design polymers that are durable in life yet degradable
on demand, and to ensure that there are not unintended
environmental consequences resulting from the degradation
products of plastics.

4. Degradation of plastics

Introduction
Most plastics are designed to be durable, so that they maintain their structure
and performance during their useful life. But in application sectors such as
packaging this durability can give rise to pollution, because discarded waste
plastic can persist in the environment for years or even decades. It is also
important to emphasize that for some applications, designing plastics to be
more durable and longer-lived would be a more sustainable option and should
be explored. In some scenarios allowing for plastic articles to be reused multiple
times, for example in refillable packaging, will represent a more sustainableoption than providing multiple disposable packaged articles. Nonetheless,there is a fundamental problem with having materials that are not degradable inapplications where they can make their way into the environment.
Designing a plastic to be durable in life, and degradable at the end of its life, isa major challenge. Biodegradable plastics are available today and have beendeveloped for a number of sectors and contexts. For example, some materials
were initially developed for use in medical implants that slowly break down ina patient’s body. Some plastics are best suited to degradation within industrial
composting facilities, which require the same sort of collection and processing
infrastructure as conventional recyclable polymers. Other polymers are designed
to break down in home composting bins. For such degradable polymers it isvital to understand what conditions are required, how long it takes, and what
breakdown products it leaves behind. It is recognized that that current standards
and test methods cannot predict breakdown in the range of complex naturalenvironments (R. Soc. Open Sci. 2018, DOI: 10.1098/rsos.171792).
As we use increasing amounts of degradable plastics, their breakdown products
will potentially have a greater impact on our world. Research is needed tounderstand the impacts of these degradation products and to ensure they do notchange the environment in unexpected ways, by encouraging different microbes
to thrive in soil as they feed on the plastic, or by altering the pH of soil. It is vitalto anticipate and understand these impacts.
Developing new degradable polymers offers an opportunity to reduce theenvironmental impact of plastic pollution. But that research must be informed
by a deeper understanding of the underlying chemistry of their degradation
processes, and the long term consequences of this strategy. This research canalso help to set standards for degradable plastics that take better account of
their behaviour in real-world situations.

Breaking down plastics
It is important to recognize that bio-derived polymers – those made frommolecules extracted from biological material such as plants – are not necessarily
biologically degradable. Conversely, many biologically degradable polymers
areactually prepared from petrochemical feedstocks. In terms of the preparationof environmentally degradable polymers, there is no need to focus only on
petrochemical or biologically-derived raw materials, since either could be
used effectively.
Bio-derived polymers may offer the opportunity to use more sustainable
feedstocks to make plastics with a lower carbon footprint. Biologically degradable
polymers can help to tackle a different problem, by reducing the persistence ofplastics in the environment. With those two goals in mind, polymers that are bothbio-derived and biologically degradable look particularly attractive.
The leading biologically degradable polymers by production volume arepolylactic acid (PLA) and polybutylene adipic terephthalate (PBAT), with PLA
being produced from biomass and PBAT being predominantly petrochemical.
38

4. Degradation of plastics

Other common bio-based and biodegradable plastics include polybutylene
succinate (PBS) and polyhydroxyalkanonates (PHAs) (see figure 4.1: Global
production of biodegradable and bio-based plastics). In total, about 2.1 million
tonnes of these bio-based or biologically degradable polymers were produced in
2019, making up a very small proportion of the overall polymer market.
Compared with conventional plastics used in packaging, current biodegradable
alternatives need improvements to both their properties and degradation
conditions. Chemistry can help to develop improved mechanical, thermal andrheological properties for these biodegradable plastics, and find ways to mass
produce them more cheaply. Biologically degradable polymers will get cheaperas their scale of production increases.
Another challenge is that different materials require different conditions
for complete degradation, including different temperatures, microbes andhumidities. Some plastics may be degradable in the wider environment, but
the rate of breakdown may vary significantly between soils, rivers, or oceans.
The sheer variety of environmental conditions means that it is difficult to design
polymers that are universally degradable in any circumstances.
Aside from the polymer chain chemistry, the interaction between polymer chains(or hierarchical structure) is a key factor that determines whether a plastic isdegradable. PHAs, for example, form lamellar crystalline regions, roughly 10
nanometres in diameter. These crystalline sheets are separated by amorphous
regions and together they build up crystalline grains called spherulites, which
can be 100 micrometres across and form the building blocks of the larger
structure of a plastic (see Fig 4.2: The structure of plastics p40). Consequently,
the bulk plastic material does not behave like isolated polymer molecules –instead, plastics behave differently at various physical scales, being more or lessresistant to degradation.
Plastic degradation generally happens in different stages. Larger pieces ofplastic erode into smaller pieces, and then longer polymer chains in thosepieces break down to shorter molecules with a lower molecular weight.

Roughly half of the plastics madefrom bio-based materials are
not biologically degradable. Two
prominent biodegradable plastics(PBAT and PBS) are largely made frompetrochemical precursors. Just over
one-third of so called ‘bioplastics’
are both bio-based and biologicallydegradable.
Bioplastics market data 2019, European
Bioplastics (2019)

Figure 4.1: Global production of biodegradable and bio-based plastics

Figure 4.1: Global production of ‘bioplastics’

PE

Other
(bio-based/
non-biodegradable)

11.8%

PET

PBAT

13.4%

PBS

1.1%
9.8%

4.3%
13.9%

PA 11.6%

0.9%

1.2%

PHA

1.4%

PP

9.2%

PTT

Other
(biodegradable)

PLA

21.3%

PBAT polybutylene adipate terephthalate
PBS polybutylene succinate
PLA polylactic acid
PHA polyhydroxyalkanoatesSb
starch blends
PTT polytrimethylene terephthalatePP
polypropylenePApolyamides
PET polyethylene terephthalatePEpolyethylene

Bio-based/non-biodegradable

44.5%

Petrochemical-based/biodegradable

17.7%

Bio-based/biodegradable

Sb

37.8%

39

4. Degradation of plastics

Figure 4.2: Structure of plastics at different scales
Figure 4.2: Structure of plastics at different scales

CrystalAmorphous

−10nm

1

2

100μm

Crystalline
spherulite

These steps depend on physical and chemical processes, such as weatheringand abrasion, water, ultraviolet light, oxygen, heat, various chemicalmechanisms, or biological organisms such as microbes.
Ultimately, the polymers undergo complete biological degradation to form
small molecules, which are naturally processable or readily metabolized. In
some circumstances, this kind of environmental degradation may be the best
way to deal with plastics and to reduce pollution from plastics. Nevertheless,a thorough life cycle assessment is always needed to offer a useful comparisonof the impact of different waste management strategies.
To design new environmentally degradable plastics, chemists can incorporatechemical groups into polymers that enzymes can attack. But researchers mustalso consider how these enzymes will access the polymer molecules and takeaccount of different degrees of crystallinity or surface treatments. Further
research into the chemical mechanisms of polymer degradation should help to
guide that discovery process, as will studies on the residues produced by suchbreakdown processes. Crucially, it is important to study these processes in thenatural environment, as well as the laboratory.
This research should identify the molecular weights and chemical structuresof degradation products, but it should also consider intermediate breakdown
products, including shorter polymer chains and nanoparticles. We also needbiological assays to study what effects these molecules have on living organisms,
using techniques such as in vitro enzyme assays.

A slow process
Any new environmentally degradable plastics should be completely assimilated
into the environment within a defined time, and without generating ecotoxic
degradation products. Defining this time scale will depend upon application.
Today’s hydrocarbon plastics generally break down far too slowly, anddegradation often stops once the plastic has reached the size of microparticles
or nanoparticles, which may themselves cause environmental problems (seechapter 1: The impact of plastics).
Current European Union regulations state that a plastic is considered
compostable if, after 12 weeks of composting, no more than 10% of the plastic’s
mass is caught by a sieve with holes 2 millimetres wide. That means the material
that passes through the sieve may contain microscopic fragments of plastic
particles. If this residue is applied to agricultural land, which is common inmost countries (Sci. Adv. 2018, DOI: 10.1126/sciadv.aap8060), it is important forscientists to establish whether soil microbes can complete the breakdown ofthese particles. This may also allow some materials that are never likely to
fully degrade to be confused with fully degradable structures, for example
oxo-degradable polyethylene.
40

CrystalAmorphous

Polymer
chain

Plastics have different structuralfeatures at different scales – theseparts of the material do not degradein the same ways, or at the same rate.
Plastics often contain tough crystalline
grains called spherulites that can bea few hundred micrometres wideand strengthen the material. Withinthese spherulites, some polymer
chains are highly organized into
crystalline regions that may be tens ofnanometres thick; these are separated
by amorphous regions where thepolymer chains are more tangled andare generally easier to break down.
Adapted from a figure by Keiji Numata.

4. Degradation of plastics

PLA is a fully hydrolyzable polymer and its degradation product, lactic acid, isa natural metabolite and can be bio-processed to CO2 and water. Nonetheless,understanding the time scale for complete degradation is important and defining
acceptable rates for degradation will depend on applications. For example, PLA
has been shown to break down over the course of months in a field, but after one
year visible flakes of material may remain and take much longer to fully degrade.
This underscores the need for more environmental testing to understand thetimes taken and the fate of breakdown products, work that will require chemists
to collaborate more closely with biologists and ecologists. It also requires amolecular perspective that looks at the bonding and structure of the polymers,
and the mechanisms involved in disassembling them.
Designing environmental degradability into plastics does not mean thatenvironmental degradation should be the default option for dealing with thatwaste. Indeed, the degradation mechanisms increase the end-of-life options
available to society. For example, PLA is an aliphatic polyester that can easily be
depolymerized, so it can be managed through closed-loop chemical recycling(ChemSusChem 2019, DOI: 10.1002/cssc.201902755).
Since enzymes are usually less effective at attacking crystallites, it may bepossible to promote environmental degradation by using polymers that haveamorphous regions by design yet retain the required physical and mechanicalproperties. Blending different polymers together, or creating composites withother materials, could help to maintain the strength of the amorphous plastics
during their use. For example, branched aliphatic polyesters tend to form more
amorphous materials that degrade more rapidly than crystalline variants. Earlystage research suggests that blending linear and branched polyesters may help
to fine tune the physical properties of the finished plastic so that it still performs
well during use.
Polyhydroxyalkanoates (PHAs) are another promising class of polymer thatare both bio-based and biologically degradable. The start of PHA production
involves bacterial production of the polymers – as much as 90% of the weight ofdry cells can be polymer. There are already some PHAs in commercial production
and alternative chemical routes to the materials are also feasible in the future.
Aliphatic polycarbonates also have potential as degradable plastics and may
show a lower carbon footprint than conventional petrochemical polymers,
because they can be made by reacting epoxides with CO2 (see chapter 2: Newsustainable plastics). The properties and manufacturing processes of thesepolycarbonates still need optimization, including improvements to their thermal
and mechanical strength. For example, it is been shown that CO2-derived
polypropylene carbonate (PPC) film has a very low permeability to water vapour,and may be useful as an agricultural mulch film (see chapter 1: The impact ofplastics). In 2018, a field trial in China using PPC mulch film showed that somecrops, including potato and quinoa, were just as successful as when conventional
polyethylene film was applied. The PPC film was less successful for cotton,
largely because the film degraded too quickly on the field, and further research
to strengthen the film is ongoing.
Recyclable mail wraps and degradable
food packaging.

41

4. Degradation of plastics

In academic laboratories, a wide range of other degradable polymers arebeing explored, with the majority of research focused on aliphatic polyesters,
polyamides, polycarbonates and polyacetals. Another area for investigation is
the use of modified amino acids to build synthetic polymers that mimic natural
materials such as spider silk, and to make fabrics and foams.

Conclusion
Despite the immense potential for these innovative new polymers, it is difficult
for them to compete with conventional petrochemical polymers that have been
in mass production for 60 years. So one of the most immediate challenges forchemists is to find cost reductions and performance enhancements for existing
biologically degradable polymers, to help them gain market share. This will
require new routes to monomers, new catalysts to stitch them together, andnew ways to enhance the physical properties of these plastics – for example,
by blending polymers or increasing cross-linking between polymer chains. It
is also essential to stimulate more fundamental research and innovation in
the development of degradable polymers. This exploratory science needs tobegin quickly because new materials technology pipelines require significant
optimization of technology and processes to deliver new products.
As degradable plastics become more common, we need much better
understanding and information regarding the different types of degradationand the products formed by degradation. Developing the emerging field ofdegradation in the environment will require the input of chemistry, as willsetting standards and legislation in this sector. As part of this work, effective
communication of the results to the general public will be vital and chemists
must engage in outreach activities to improve public awareness of how plastic
waste is currently managed, and the different ways it can be sustainably
managed in the future.
Decisions about whether researchers should design plastics to be degraded inchemical recycling facilities, industrial composting units, or in the environment,
must be guided by life cycle assessments and a full awareness of the fate
and activity of breakdown products. It is also important to note that adding
degradable linkages and chemistries into polymers will likely facilitate recyclingof those materials, thus offering two benefits. Ultimately, chemists should be
aiming to design polymers for ‘degradation on demand’, so that waste plastic canbe handled in the most sustainable way possible.

42Appendix

Appendix

Glossary of acronyms
PA polyamide

PBAT polybutylene adipate terephthalate

Illustrated with Nylon 6

PBS polybutylene succinate

PCL poly ε -caprolactone

PE polyethylenen
PHA polyhydroxyalkanoate

PLA polylactic acid/polylactide

PP polypropylene

PPC polypropylene carbonate

PPT polytrimethylene carbonate

PS polystyrene

PVC polyvinyl chloride

PET polyethylene terephthalate

O
O

O

O

n

Structures of common polymers and their recycling symbols
PET
O

HDPE

O
O

O

High-density PE

n

LDPE

PVC

PP

PS

OTHERS
Other plastics, including acrylic,
acrylonitrile butadiene styrene,
nylon, polycarbonate, andpolylactic acid.

Low-density PE

44

United Nations Environment Programme ‘Biodegradable Plastics & Marine Litter:
Misconceptions, Concerns and Impacts on Marine Environments’ 2015. Available
at wedocs.unep.org/handle/20.500.11822/7468

Appendix

Further reading

Suschem ‘Plastics Strategic Research and Innovation Agenda in a Circular
Economy’ 2018. Available at suschem.org/publications
PlasticsEurope ‘Plastics – the Facts’ 2019. Available at plasticseurope.org/en/
resources/publications/1804-plastics-facts-2019
House of Commons Library, Briefing Paper number 08515 ‘Plastic Waste’ 2020.
Available at commonslibrary.parliament.uk/research-briefings/cbp-8515/
The National Academies of Sciences, Engineering and Medicine ‘Closing the Loopon the Plastics Dilemma: Proceedings of a Workshop in Brief’ 2020. Available atdoi.org/10.17226/25647

45

CS3 participants

CS3 participants
China
Zhibo Li (Chair)
Zheng Huang
Hua LuXianhong Wang
Changrong Yan
Jun Zhang
Zhigang Shuai
Lidong Han

Qingdao University of Science and Technology
Shanghai Institute of Organic Chemistry, CAS
Peking University
Changchun Institute of Applied Chemistry, CAS
Chinese Academy of Agricultural Sciences
Institute of Chemistry, CAS
Chinese Chemical Society (CCS)
Chinese Chemical Society (CCS)

Germany
Andreas Greiner (Chair)Markus Biesalski
Christian Laforsch
Stefan Mecking
Brigitte Voit
Markus Behnke
Hans-Georg Weinig

University of Bayreuth
Technical University of Darmstadt
University of Bayreuth
University of Konstanz
Leibniz Institute of Polymer Research Dresden(IPF)
German Research Foundation (DFG)
German Chemical Society (GDCh)Japan
Atsushi Takahara (Chair)
Hirofumi Hinata
Keiji Numata
Kotaro SatohToshiaki Yoshioka
Satoshi Miyashita
Ryoji Nakamura
Mitsuo Sawamoto

Kyushu University
Ehime University
RIKEN Center for Sustainable Resource Science
Tokyo Institute of Technology
Tohoku University
Japan Science and Technology Agency (JST)Japan Science and Technology Agency (JST)Chemical Society of Japan (CSJ)

United Kingdom
Charlotte Williams (Chair)
Matthew Davidson
Andrew Dove
Tamara Galloway
Jennifer Garden
Steve Howdle
John Hand
Jennifer Channell
Sarah Thomas
Stuart Govan
Rebecca Quine

46

University of Oxford
University of Bath
University of Birmingham
University of Exeter
University of Edinburgh
University of Nottingham
Engineering and Physical Sciences ResearchCouncil (EPSRC)
Engineering and Physical Sciences ResearchCouncil (EPSRC)
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