What are sustainable plastics? A review of interrelated problems and solutions to help avoid unintended consequences

Plastics are affordable and versatile, but there is a growing awareness that they are unsustainable in a number of ways, including concerns about climate, health and biodiversity. A number of solutions are being explored that could enable a more sustainable plastics system. So far, most research has focused on isolated technical solutions that address only one specific sustainability challenge posed by plastics, such as end-of-life management or feedstock alternatives to fossil fuels. Some interventions might mitigate one problem but contribute to another at a different stage of the plastics life cycle. This study is based on a literature review and adopts qualitative system dynamics to analyse the unsustainability of plastics with a holistic, integrative approach. The review shows that there is still no agreed definition of sustainable plastics, so the authors propose one based on inputs from the literature. The paper provides an overview of the impacts of potential solutions on the plastics system, highlighting how some interventions could end up having unintended consequences, perhaps even overshadowing the benefits. The results highlight the need for improved communication transparency between plastics stakeholders and a more vertically integrated, harmonised value chain to effectively implement a sustainability transition in the plastics system.


Background
Plastics are ubiquitous materials, used across a wide variety of sectors and applications due to their low costs and versatile technical properties.Over the past years, increasing attention has been directed towards the sustainability issues related to plastics.Most plastics are produced using fossil fuels, and their footprint has been estimated at 1.7 Gt CO 2 -eq. of greenhouse gas (GHG) emissions per year (Geyer et al 2017, van den Oever et al 2017, Zheng and Suh 2019).Millions of tonnes of plastics are released in the environment every year, posing threats to ecosystems and humans (Boucher and Friot 2017, Jambeck et al 2018, Williams and Rangel-Buitrago 2022).When collected properly, plastics are often disposed in landfills or incinerated, and only 9% of the 6300 Mt cumulative plastics waste generated from 1950 to 2015 has been recycled (Geyer et al 2017, Di Bartolo et al 2021).Most plastics that go through a recycling process are downcycled to a product of a lower value (Dijkstra et al 2020, Bauer et al 2022, Williams and Rangel-Buitrago 2022), which also means that most plastics do not undergo multiple recycling cycles, since this would be uneconomical.Recycled plastics gain a bit longer lifespan than non-recycled plastics, but to date recycling technologies do not yet bring about a circular plastic system (Shamsuyeva and Endres 2021, Williams and Rangel-Buitrago 2022).
On the other hand, plastics also foster sustainability in some applications: for instance, food packaging is essential to our modern lifestyle and significantly reduces food spoilage (Chakori et al 2021(Chakori et al , 2022)).Alternative materials may not be as good at preserving food, may be more expensive, and not necessarily more sustainable (Systemiq 2022, Williams andRangel-Buitrago 2022).Plastics also contribute to renewable energy production: plastic wind turbine blades are light and can turn fast; plastic is used in PV panels, for instance to protect solar cells from environmental factors (Fan 2021, Ali Elamin and Osman Khairy Ahmed 2022, Majewski et al 2022).Vehicles incorporate many lightweight plastic components that can reduce fuel consumption compared to heavier materials which could be used in car and aircraft components (Klemeš et al 2021, Schirmeister andMülhaupt 2022).Plastics are widely used in medical applications because they ensure hygiene, and personal protective equipment played an important role during the COVID-19 pandemic to prevent infection of healthcare workers and others (Ebner andIacovidou 2021, Parashar andHait 2021).For people with disabilities, plastic objects are sometimes essential (Hewitt 2022, Sarchese 2022).Demand and production of plastics has increased over the past decades and is expected to continue to grow (Aurisano et al 2021, Meys et al 2021, Williams and Rangel-Buitrago 2022).While it is often argued that plastics consumption should be significantly reduced (e.g.Bucknall 2020, Cavaliere et al 2020, Hardy et al 2022), the diversity of uses for plastics makes it difficult to imagine our modern society no longer relying on them: hence the need to explore a variety of solutions and interventions.

Scope of the study
Different solutions to the sustainability challenges posed by plastics are being discussed in the literature, ranging from bioplastics, recycling technologies, carbon capture and utilisation, banning single-use plastics, to behavioural changes, etc (e.g.Siltaloppi and Jähi 2021, Damayanti et al 2022, Nikiema and Asiedu 2022, Tejaswini et al 2022).Despite increasing research efforts to find new sustainable solutions for plastics, there is still no common definition of 'sustainable plastic' .Most research to date has focused on technical solutions that only address one aspect of the problem, such as plastic debris, microplastics, or downcycling (Nielsen et al 2020, Hafsa et al 2022, Olatayo et al 2022, Systemiq 2022, Villarrubia-Gómez et al 2022).Even review papers on plastics often focus on sub-systems of the plastics system, e.g. they may provide an overview of recycling technologies or bio-based polymers, or they may look only at environmental impacts (e.g.Hatti-Kaul et al 2020, Meys et al 2020, Macleod et al 2021, Schwarz et al 2021, Wiesinger et al 2021, Carney Almroth et al 2022, Golkaram et al 2022, Lomwongsopon and Varrone 2022, Bachmann et al 2023, Gao et al 2023).Recent studies explored the links between plastics and the Sustainable Development Goals and provided insights into the socio-economic impacts (Nizzetto and Sinha 2020, de Sousa 2021, Kumar et al 2021, Walker 2021).While these papers generally still focus on the environmental aspects of (micro)plastics and consider socio-economic elements affected by or influencing plastic pollution, they differ from most of the other literature in their efforts to look at plastics in a more holistic and integrative way.Yalçin et al (2023) analyse the political debate in Europe on the circular plastics economy and demonstrate the existence of three main narratives, perpetuated by different stakeholder groups and prioritising either the technical, economic, or societal perspective.The authors conclude that the three narratives, which are currently quite polarised, may not be mutually exclusive and aim to stimulate pluralistic and multidisciplinary discussions (Yalçın et al 2023).Pathak et al (2023) argue that research on plastics typically lacks a transdisciplinary perspective, and they urge the adoption of an approach that combines natural and engineering sciences with social sciences to focus on mitigating harm throughout the plastics life cycle.
This study aims to complement existing literature by adopting a holistic and integrative approach to sustainability and plastics, moving beyond environmental impacts.Utilising qualitative system dynamics models, we seek to address the gaps in the current understanding of sustainable plastics.While the sustainability problems of plastics and their potential technological and behavioural solutions are often analysed in isolation, such technologies and practices co-develop.They are embedded in complex sociotechnical systems (Geels 2002, 2004, Farla et al 2012, Köhler et al 2019, Villarrubia-Gómez et al 2022, Pathak et al 2023) with many interdependencies that sometimes make it difficult to predict how the system will react to a given intervention (Braun 2002, Meadows 2008, de Gooyert et al 2016).A sociotechnical system comprises the different activities and actors involved in the production, including the diffusion and use of a given main technology, such as plastics in this case (Geels 2002, 2004, Farla et al 2012, Köhler et al 2019, Pathak et al 2023).The widespread adoption of each solution to the plastics sustainability issues would have different impacts and consequences on the chemical industry (new investments of different types and sizes might be required) and on communities and societies (e.g.large-scale production of bioplastics might impact land use).Taking effective measures against the sustainability issues of the plastics system requires large-scale applications, but not all emerging technologies may have the potential to still be sustainable if brought to a large scale (Jehanno et al 2022, Schirmeister and Mülhaupt 2022, Systemiq 2022).It is therefore necessary to assess the potential impacts of proposed solutions from a holistic, whole-plastics-system point of view, to avoid causing undesirable consequences, which potentially even overshadow the benefits.This paper differs from previous studies in its holistic, comprehensive, multidisciplinary approach to the issue of plastics and sustainability.More specifically, the research questions that guide this study are (RQ1) ≪How are 'sustainable plastics' defined in literature?≫ and(RQ2) ≪What problems and what solutions are mentioned in literature when discussing plastics and sustainability and how are they interrelated with each other in a socio-technical system?≫.

Theoretical background
The research object of this study is the (un)sustainability of the socio-technical system of plastics, which includes the different activities, technologies, and stakeholders involved in design, production, distribution, consumption, and end-of-life management of plastics, i.e. the entire life cycle of plastic materials.The plastics system involves different sectors and industries.
The concept of sustainability is generally understood as a synonym of sustainable development (Jeronen 2013, Meadowcroft 2022), for which the most common definition is 'The ability of fulfilling the needs of current generations without compromising the needs of future generations, while ensuring a balance between economic growth, environmental care and social well-being.' (World Commission on Environment and Development 1987).To be more precise, sustainability reflects the goal which needs to be pursued, while sustainable development refers to the many processes and pathways to achieve it (Bergman et al 2018 as cited in Janoušková et al 2019).In this paper, the expression sustainability problem is used to indicate any environmental, economic, or social challenge posed by plastics, for instance: plastic debris, microplastics or fossil fuels consumption.The term sustainability solution, in turn, refers to any technical, political or behavioural intervention that is considered to alleviate at least one sustainability problem of plastics, e.g.alternative raw materials, recycling targets, or avoiding single use plastics by consumers.
Given the growing demand and production of plastics, effective action to address the unsustainability of the plastics system requires large-scale interventions.The collection of radical and structural changes that these measures will bring is referred to in this paper as the sustainability transition of plastics.A sustainability transition is understood as a fundamental transformation towards sustainable modes of production and consumption (Markard et al 2012).In a transition both the technical as well as the social/cultural dimensions of a system change drastically, therefore a transition differs from incremental processes, which are primarily characterised by gradual technical change (through successive generations of technologies) with relatively little alteration of the societal embedding of these technologies (Elzen and Wieczorek 2005).
In this paper we adopt a holistic approach towards sustainability, with the word 'holistic' meaning that we take a view of the whole plastics system and try to look at the interactions of plastics with all three pillars of sustainability (environmental, economic and social).This approach requires contributions from different disciplines and perspectives (both technical and socio-economical) from different stages of the plastics life cycle.

Reading guide
The next section introduces the methodology adopted in this study.Sections 3 and 4 present the outputs of the study: section 3 addresses RQ1 and provides a definition for 'sustainable plastics' , while section 4 addresses RQ2 discussing different problems and solutions for the sustainability of plastics, and how they are interrelated.The relevance of the results are discussed in section 5, and the key contributions are highlighted in section 6. Section 7 reflects on limitations of this study and section 8 provides suggestions for further research.

Method
The approach adopted in this research involves 3 steps: literature collection, text coding, and qualitative system dynamics modelling.

Literature collection
This research encompasses an examination of scientific reviews, overview articles and grey literature, including reports from international organisations, NGOs, and consultancy firms in the field.The intention of this study is to provide a broad overview, where other studies typically focus on specific aspects of (un)sustainable plastics (e.g.Hatti-Kaul et al 2020, Schwarz et al 2021, Wiesinger et al 2021, Lomwongsopon and Varrone 2022, Gao et al 2023).For this reason, we are looking for a wide range of material, such as review papers and grey literature, discussing the current status and possible future scenarios of plastics in general, on a global level: here we adopt an integrative literature review approach (Torraco 2005, Snyder 2019).This is a flexible type of literature review (sometimes referred to as a narrative or semi-systematic literature review) that allows for the comprehensive collection of diverse knowledge from multiple disciplines (Torraco 2005, Snyder 2019).The scientific reviews and overview articles were retrieved from Web of Science using the keywords 'sustainable plastic * ' and 'circular plastic * ' .Some subject categories were excluded, as they were clearly not relevant for the scope of the review, such as 'Immunology' or 'Virology' .The number of papers dealing with sustainable and circular plastics has increased exponentially in recent years, but only papers published since 2000 were included in this research.The initial search query yielded 332 papers before excluding articles.Papers were excluded if they had a detailed focus on the technical attributes of specific technologies (e.g.level; it was not possible to access 5 of the retrieved papers.In addition to the papers retrieved from Web of Science, a further 6 documents were identified through suggestions from experts in the field and were considered relevant to the scope of the project and therefore included in the study.A summary of the filtering steps is shown in table 1, while a complete list of the documents examined in this study can be found in appendix A. Although broad keywords were chosen for this literature review, many of the papers initially retrieved were excluded from this study, in most cases because of their narrow technical focus: while this type of research is important and much needed, it is not the intention of this study to delve into the technical details of a potential solution.This outcome confirms what is mentioned in section 1.2, namely that most research today looks at a single aspect of the unsustainability of plastics in isolation.

Coding
Text coding is a systematic approach to looking for themes, similarities and differences between sources.The selected papers were coded following an open and axial coding approach, similar to what was done in Eker and Zimmermann (2016) and Gürsan and de Gooyert (2021).The documents were coded manually in Atlas.ti, with no aid from recently introduced AI tools.First, the papers were coded to identify arguments relative to the three pillars of sustainability identified by the United Nations Brundtland Commission (1987), i.e. environmental, economic, and social sustainability.In line with the research questions, the codes retrieved from the texts were categorised according to the following themes: Definition of 'sustainable plastics' (RQ1); Sustainability problems (RQ2); Sustainability solutions (RQ2); Interrelations among problems and solutions (RQ2).Within each of these theme categories, the codes have been further disaggregated by topic, e.g.'biobased plastics' or 'consumer behaviour' .See an example in appendix B. An overview of the main coding categories can be found in figure 1, while the full list of codes can be found in the supplementary material.The code categories do not fully compartmentalise the codes list but provide an overview of the key themes that emerged from the textual analysis.

Qualitative system dynamics model
A system dynamics approach is often applied when exploring complex issues with many interrelated elements, both technological and social (Forrester 1971, Vennix 1996, Sterman 2000, Chakori et al 2021, 2022, Gürsan and de Gooyert 2021, Janipour et al 2021, 2022).System dynamics (SD) helps in dissecting the foundational structure of a system, elucidating the underlying drivers of persistent behaviours over time.
The term 'structure' encompasses the relationships among system components, unveiling their interactions and contributions to the overall behaviour of the system.We regard this as an appropriate method because the topic of sustainable plastics is so complex with many facets that are all interrelated.It is important to address this complexity and the system dynamics approach allows us to do so.In system dynamics, a typical distinction is made between stocks and flows.Stocks represent quantities that accumulate or decrease over time through inflows and outflows.This distinction is helpful because accumulations are also important in the context of sustainable plastics (e.g.greenhouse gases accumulating in the atmosphere, plastics accumulating in the environment) and helps with clearly distinguishing how changing inflows and outflows help move these stocks towards sustainable levels.System dynamics diagrams are instrumental in identifying feedback mechanisms, i.e. closed circles of causal relationships within a system.Feedback loops come in two forms: reinforcing loops, which intensify or amplify behaviour, and balancing loops, which dampen or stabilise it.Understanding these feedback mechanisms helps understanding how complex systems evolve over time, and supports the identification of potential ways to intervene (Sterman 2000).It is worth noting that feedback mechanisms and delays can give rise to resilient structures within the system, potentially leading to unintended consequences when interventions and policies are implemented (Meadows 2008, de Gooyert et al 2016).While SD models can be based on either qualitative or quantitative relationships, here we rely on qualitative SD modelling: indeed, the exploratory nature of this study benefits more from the richness and variety of qualitative data, whereas quantitative data, by their very nature, have to simplify things to such an extent that they would not reflect the complex nature of the issue of sustainable plastics (Coyle 2000, Glaser and Strauss 2017, Denzin and Lincoln 2018, de Gooyert 2019, de Gooyert et al 2019, Flick 2022).When SD modelling does not include quantitative data and simulations, it is sometimes referred to as system thinking (Forrester 1994).However, we prefer the term qualitative system dynamics, as the term systems thinking is also used to refer to approaches that have little to do with system dynamics (e.g.Politis et al 2017, D'Amato and Korhonen 2021).
The coding process generated aggregated variables, which were interconnected through causal links to form qualitative system dynamics models (Eker and Zimmermann 2016, Gürsan and de Gooyert 2021).While many causal relationships were either explicitly stated or implied in the examined documents, there were instances where probable causal links were incorporated, particularly when bridging variables from different sources or when the links were intuitively apparent.The methodology involved iterative repetition of three steps for each document: selecting, coding, and model building.After coding an article, the newly obtained codes were introduced as variables into existing models or used to create new models.However, it was not always necessary to include all new codes as variables immediately: some variables were merged with existing ones, especially when topics were similar, while others were temporarily set aside for subsequent iterations of analysis.This strategic approach was adopted in cases where waiting for additional information was deemed preferable, such as when the causal link was ambiguous.

Definition of 'sustainable plastics'
This section presents the first output of the analysis, namely whether a definition for 'sustainable plastics' can be extrapolated from the literature (RQ1).44 out of the 55 documents contained a definition of sustainable plastics, although in some cases this definition ).Looking at the code group 'End-of-life fate' , it might seem that biodegradability (mentioned 7 times) is preferred to recyclability (mentioned 2 times).However, this interpretation could be misleading: in fact, other codes in different code groups imply that recyclability of plastics is desirable, such as all codes related to circularity.
In general, the focus of the analysed documents is on the environmental pillar of sustainability, while less importance is given to the social and economic pillars of sustainability: for example, none of the coded documents mentioned the concept of 'ecoableism' , i.e. that environmental policies may sometimes overlook the needs of people with disabilities.In some cases, (disposable) plastic items are essential for people with disabilities and banning them without providing a viable alternative is a form of social injustice (Hewitt 2022, Sarchese 2022).In one document, it is explicitly stated that the plastics system can only be considered sustainable if innovations in this sector do not put pressure on other sectors, as could happen if several sectors compete for the same resources (e.g.biomass) (Kaur et al 2018).Therefore, it is necessary to take a holistic, integrative perspective in pursuing the transition to sustainability of the plastics system (Gall et al 2020, Nielsen et al 2020, Amulya et al 2021, Bening et al 2021, Klemeš et al 2021, Vanapalli et al 2021, Lomwongsopon and Varrone 2022, Tarazona et al 2022).This also means looking more globally at how the plastics system interacts with other sectors of the economy, and allocating resources in such a way that it contributes to global sustainability.Some documents explicitly mention the need for a combination of solutions and interventions to achieve a sustainable plastics system (e.g.'Circular & Bio-based'; 'Sustainable Business Models') (Chen et  As it was not possible to identify a common definition of sustainable plastic, this paper develops one inspired by the understanding of sustainability expressed by Daly (1991) and later elaborated by Sterman (2011b).Daly (1991) identifies 3 fundamental conditions for ensuring sustainability: (1) the rate of consumption of renewable resources cannot exceed their rate of regeneration; (2) pollution and waste cannot be generated faster than they decompose and become harmless; (3) in the long run, nonrenewable resources cannot be consumed at all.These 3 conditions for sustainability have been expressed by Sterman (2011b) in the form of a stock and flow diagram, as shown in figure 2. In fact, the language of system dynamics can clearly express the concepts elaborated by Daly (1991).Any stock that has an outflow that is larger than the inflow will decrease, therefore harvesting more than renewable resources can regenerate is unsustainable because the carrying capacity (del Monte-Luna et al 2004) of the natural resource is not respected and in the long run the stock will be depleted.Because non-renewable resources do not have an inflow, any outflow results in a decrease of the stock which in the long run is unsustainable.For pollution and waste the inflow cannot be larger than the outflow because this would result in an increase of the stock which is unsustainable in the long run.In this paper, we adapt and extend Sterman's stock and flow diagram to provide a definition of sustainable plastics (figure 3) 1 .Different parts of the model are coloured to highlight how the three pillars of sustainability (environmental, economic and social) are included in our definition of sustainable plastics.Human needs lead 1 A positive causal relationship between two variables is indicated by a plus sign on an arrow.This means that when variable A increases (or decreases), variable B also increases (or decreases) in response.Conversely, a negative causal relationship is denoted by a minus sign, signifying that when variable A increases (or decreases), variable B decreases (or increases) in return.When an arrow features a double bar, it signifies a delay, indicating that changes in the first variable take time to manifest in the second variable.A feedback mechanism is defined by a closed loop of causal relationships between variables.There are two types of feedback mechanisms: a reinforcing feedback mechanism, symbolised by an 'R' at the centre of a closed loop, and a balancing feedback mechanism, represented by a 'B' .A balancing feedback mechanism acts as a stabilising force, counteracting changes in one direction by prompting changes in the opposite direction.In contrast, a reinforcing feedback mechanism drives rapid growth or collapse within a system by amplifying changes in one direction, resulting in even more significant changes in the same direction.A box indicates a stock, i.e. a variable that accumulates or depletes through inflows and outflows.to the production and consumption of plastics, which in turn enable human activities to take place and fulfil human needs (e.g.packaging enables food preservation; car parts enable mobility; insulation contributes to comfortable shelter, etc.).Plastics production (like most human activities) drives the exploitation of natural resources.Only renewable natural resources are considered in this model, in line with Daly's statement that the consumption of non-renewable resources is never sustainable in the long run (Meadows et al 1972, Daly 1991, Meadows 2008, Sterman 2011b) (see also code 'Renewable feedstock' in table 2).The rate of exploitation determines the availability of natural resources, which in turn determines their cost.A limited availability of affordable resources could act as a limiting factor for human activities and fulfilment of human needs (table 2, 'Economic sustainability').Implementing a circular economy (light-blue arrow and text in figure 3) can help control the rate at which natural resources are exploited, as it aims to prolong the use of resources to their maximum potential.
The more plastics are produced and consumed, the more will eventually become waste.Any waste that is not fully recycled or fully biodegraded will cause environmental impacts, therefore the inflow of waste generation should never exceed the outflow of waste recycling and biodegradation in order for the system to be environmentally sustainable (Daly 1991, del Monte-Luna et al 2004, Sterman 2011b).Here, 'Environmental impacts' is used as an umbrella term that encompasses all the environmental impacts caused by plastics: greenhouse gas emissions associated with fossil fuels extraction, plastics production and plastics end-of-life, which ultimately contribute to climate change; toxicity hazards associated with plastics use and end-of-life management; dispersion of macro-and microplastics, which affects natural habitats, animals and ultimately humans (both directly and indirectly, for example through food value chains); etc. Environmental impacts can affect the quality and availability of natural resources and even limit their ability to regenerate (e.g.reduction in soil fertility).
Human activities are driven by the global population size (Daly 1991, Raworth 2017).To ensure sustainability, the human activities should find an equilibrium within two critical thresholds: firstly, a minimum level beneath which the basic human needs (e.g.food, shelter) would not be met ('social foundation' in the doughnut economy model proposed by Raworth (2017); see code 'Fulfilled societal needs' , table 2).Secondly, an upper boundary above which natural resources would face excessive exploitation ('ecological ceiling' in Raworth ( 2017)).It is argued by Daly (1991), Raworth (2017), and other notable authors (e.g.Meadows et al 1972) that the current economic system is not sustainable due to the pursuit of continuous economic growth, which causes human activities to breach the safe boundaries identified by the doughnut economy model.In this definition for sustainable plastics, we emphasise the necessity for human activities to lie within a safe range.Product designers should design products that effectively meet human needs while ensuring efficient use of resources and minimising environmental impacts.Where alternative materials or products are more effective and sustainable in meeting the same human needs, they should be used instead of plastics.
As mentioned already, human needs lead to the production and consumption of plastics, which in turn enable human activities to take place and fulfil human needs.For example, plastics have very good barrier properties and are very light, making them often a much better option for packaging applications than paper (lower barrier properties) or glass (higher weight).Plastics support human activities such as the creation of long food value chains, but at the same time the existence of long food value chains creates a demand for plastics.Plastics should be given a socio-economic sustainability credit in some applications, if not also an environmental credit, for instance in the packaging example they prevent or significantly reduce food waste.So while stopping the use of plastics may sometimes seem like a good option from a strictly environmental perspective, it would have negative social and economic consequences unless we also change the way we as a society meet our needs.This is the complexity of the plastics sustainability transition.
Based on the codes retrieved from literature and the stock-and-flow description for sustainable plastics, we propose a list of conditions that plastics should meet in order to be sustainable.
Plastics are fully sustainable if: • Plastics are used when they are the most effective and sustainable way to meet human needs.In other words, plastics should be avoided where possible and desirable, but not where alternatives would be less effective or less sustainable in performing the same function.• Non-renewable resources are not used.
• The use of renewable resources for plastics does not exceed their capacity to regenerate.• The impact of plastics on the environment is not greater than the environment's capacity to restore itself.• Animal, human bodies, and plants are not impacted by plastics more than they can sustain.• The principles of circularity of plastics are implemented to limit the rate at which natural resources are depleted and to extend the use of resources to their maximum potential.• Plastics production minimises indirect effects by following the 'ladder use of resources' principle, preventing disproportionate diversion of hard-toreplace resources from other sectors of the economy and avoiding waterbed effects or carbon leakage 2 .
2 The 'ladder of use of resources' consists of prioritising one application of a given resource over another based on value creation, economic efficiency and environmental impact.The aim is to minimise the environmental and economic impacts of resource use (and, more generally, of economic activities) by choosing the most sustainable and efficient options first (Odegard et al 2012, Muscat et al 2020).The waterbed effect describes a situation where a change or improvement in one area or sector results in an offsetting negative effect in another related area or sector.The waterbed effect is often understood as a synonym of carbon leakage, the phenomenon in which emissions reduction efforts in one jurisdiction or region leads to an increase in emissions in another jurisdiction or region with less stringent emissions regulations (Rosendahl 2019).
Our definition of sustainable plastics aligns with the only other definition we could find in the literature, which is proposed by the OECD (2018): 'Sustainable plastics are plastic materials used in products that provide societal benefits while enhancing human and environmental health and safety across the entire product life cycle' .However, we acknowledge the difficulty in applying such broad criteria to specific instances of plastics, therefore we have developed a more precise set of criteria for practical assessment.
Our definition integrates various insights from different sources, emphasising the interconnectedness of sustainability's three pillars and the notion of circularity, which although related, is distinct.Through a stock-and-flow approach, we offer a graphical representation of sustainable plastics, providing clarity on the conditions necessary for sustainability.As numerous researchers have pointed out, the human mind cannot easily grasp interrelationships and stock-andflow behaviours without support (Sterman 2000, 2011a, Meadows 2008, Cronin et al 2009).

Interrelated plastics problems and solutions
The following paragraphs report the main findings of the analysis of the interrelations between problems and solutions (RQ2).It is relevant to evaluate the potential impacts of proposed solutions from a holistic perspective, considering the entire plastics system.This involves examining how different solutions interact with one another and with both targeted and non-targeted problems.Such an approach helps prevent undesirable consequences that could outweigh the benefits.Relevant themes emerged during the analysis are discussed in this session, namely bioplastics, product design, global plastic value chains, consumers and plastics use, and plastic waste management.Although biodegradable (and compostable) plastics are often cited as possible enablers for the sustainability transition, their large-scale adoption may potentially come with undesired consequences.The model in figure 4 shows possible end-of-life scenarios for biodegradable plastic products.In the event that biodegradable plastics are dispersed in the environment, their ability to decompose reduces the amount of plastic waste in nature: this is a desirable outcome as there are currently several megatons of plastics in the oceans and more are entering the marine environment every year (Jambeck et al 2018, Kaandorp et al 2023) (see stock 'Plastics accumulated in the environment' in figure 4).However, biodegradable plastics generally require specific environmental conditions to decompose: therefore, they may not be able to degrade in a wide range of environments and may only be able to degrade under industrial conditions (e.g.high temperature) (Skoczinski et al 2021, Schirmeister and Mülhaupt 2022).In fact, due to the different environmental conditions that plastics may face, the degradation of these materials could be rather slow and accompanied by the formation of microplastics (Mastrolia et al 2022, Schirmeister and Mülhaupt 2022, Nguyen et al 2023).This is not always clear to consumers, who might assume that these materials do not harm the environment, and the amount of plastic dispersed in nature may even increase due to this misconception (see reinforcing loop 'R-Rebound effect' in figure 4) (Skoczinski et al 2021, Mastrolia et al 2022, Schirmeister and Mülhaupt 2022, Nguyen et al 2023), leading to an exceeding of the carrying capacity of the environment (del Monte-Luna et al 2004).Also, as mentioned in the beginning of this section, plastics that decompose generate greenhouse gas emissions (Schirmeister and Mülhaupt 2022).While it could be argued that emissions from bio-based materials might be considered neutral as part of the atmospheric carbon cycle (de Kleijne et al 2022), not all biodegradable plastics are bio-based, and a higher concentration of greenhouse gases in the atmosphere could still cause problems in the short term.

Bioplastics
If biodegradable plastic products are collected together with other non-biodegradable plastic materials, the sorting system may fail to separate them and this may in turn compromise the quality of the batch of recycled material (Schirmeister and Mülhaupt 2022).Particularly, seawater-degradable plastics might start degrading when exposed to humidity during a mechanical recycling process, which is currently the dominant recycling process, and during service life (Schirmeister and Mülhaupt 2022).Research is ongoing to try and achieve 'biodegradation-on-demand' without biodegradation occurring too slowly when products are dispersed in the environment (Schirmeister and Mülhaupt 2022).Additionally, the more monomers and polymers are mixed during the recycling process, and the more their technical properties differ, the lower the quality of the recycled material (Kaur et al 2018).Low-quality recycled material will end up being downcycled and eventually incinerated (if not landfilled or dispersed).The less (good-quality) recycled material is available, the greater the demand for virgin material will be (Bucknall 2020, Johansen et al 2022).Low-quality recycled plastics hinder the circularity of the system and increase its carbon footprint (Kaur et al 2018, Schirmeister and Mülhaupt 2022).
Biodegradable plastics could also be collected with organic waste, however many composting plants are reluctant to accept plastics, as they could compromise the quality of the compost obtained due to the higher risk of contaminations (Kaur et al 2018, Schirmeister and Mülhaupt 2022).Compost of inferior quality may not be suitable for agricultural uses for safety reasons and farmers may have to increase their use of fertilisers, leading to increased emissions (Schirmeister and Mülhaupt 2022).
More explanatory labels could help reduce confusion between biobased and biodegradable plastics, as well as clarify which environmental conditions allow a product to biodegrade (Tolinski 2012, Skoczinski et al 2021, Williams and Rangel-Buitrago 2022).A harmonised system of standards and certifications could thus reduce consumer errors in waste sorting, as well as harmonised regulations governing the waste management system (Mwanza and Mbohwa While biodegradable plastics are being promoted for several applications (e.g.packaging), some authors suggest that biodegradability is only desirable for products that are difficult to recover from the environment, such as plastic applications in agriculture (Kaur et al 2018, Schirmeister and Mülhaupt 2022, Systemiq 2022).In fact, biodegradation of plastic products that are difficult to recover from the environment could not only prevent the accumulation of macroplastic waste, but also the generation of microplastics.Biodegradable plastics and microplastics generation are fundamentally different and should not be confused: (complete) biodegradation means that plastics are broken down into their basic components, while microplastics (and nanoplastics) are small pieces of plastic that retain the same plastic polymer structure.Although most research on microplastics has so far focused on plastics in the oceans, some estimates suggest that terrestrial microplastics may be much larger than those in the sea and that plastic products in agriculture are one of their main sources (Bucknall 2020, Williams and Rangel-Buitrago 2022): more on microplastics can be read in section 4.4.3.
In conclusion, while biodegradable plastics can help address some of the sustainability challenges posed by plastics, they are not a panacea.Current waste management system infrastructures and regulations may not be ready to handle large volumes of biodegradable plastics, which could hinder the potential benefits of these materials.

Bio-based plastics
Bio-based plastics are one of the most commonly mentioned solutions when discussing the unsustainability of plastics, but also one of the most controversial for more than one reason.
Bio-based plastics are not made from fossil oil, so their production does not encourage the extraction of fossil fuels for feedstock ( ).The reduced demand for oil could affect its price, which could influence how attractive it is to produce traditional plastics (balancing feedback loop 'B-Oil price fluctuation' in figure 5) (Kaur et al 2018, Williams and Rangel-Buitrago 2022).However, the price for fossil fuels is not influenced by demand only, but by many other elements, e.g.geopolitical conflicts.The fluctuating price of fossil fuels, which generally remain cheaper than alternative feedstocks, is often cited in the literature as one of the barriers to the uptake of alternative plastics.While bio-based materials do not rely on fossil feedstocks, they do not necessarily reduce the overall amount of GHGs associated with plastics production: as most bio-based plastics today are derived from first generation biomass (i.e.edible biomass, which is generally cultivated), the increased demand for these products As more attention is paid to the need to reduce the consumption of fossil fuels, biomass looks attractive for an increasing number of applications (e.g.biochemicals, bioenergy, aviation biofuels, and so on): a recent study have found that the projected demand for biomass needed in different sectors will actually exceed the amount of biomass that will be reasonably available (Material Economics 2021).Hence the need to coordinate Bio-based materials could help to address some of the sustainability challenges posed by plastics, but only under certain conditions (e.g. if the biomass feedstock is sustainably sourced).More transparent communication may be needed to avoid overstating the potential benefits of both bio-based and biodegradable materials.

Plastic product design 4.2.1. Sustainable design: different paradigms
The concept of 'sustainable design' is mentioned in several of the documents.It is often stated that many of the challenges faced in the end-of-life of plastics can be addressed during product design (Piontek 2019, Hafsa et al 2022, Hardy et al 2022, Systemiq 2022).Alternative sustainable design paradigms are cited in the literature: design for recyclability, design for durability/reuse, design for biodegradability, design for alternative feedstock materials.As illustrated in figure 6, these sustainable design paradigms are, to a certain extent, incompatible with each other.
For example, a circular plastics system would require higher recycling rates, which could be achieved if products had fewer or no additives (e.g.flame retardants, colours) and if they did not consist of multiple materials mixed together, namely if products had a simpler design (Bucknall 2020, Johansen et al 2022, Schirmeister and Mülhaupt 2022, Williams and Rangel-Buitrago 2022).On the other hand, additives and blends ensure that the final product is fit for purpose, but also its durability.In fact, 'reuse' is one of the key principles of the circular economy (Hafsa et al 2022, Hardy et al 2022, Systemiq 2022).If products are less durable, they are also disposed of more quickly, which drives the demand for new products.
As mentioned in section 4.1.1,high biodegradability is desirable if plastic products are dispersed in the environment.At the same time, the concept of biodegradability is opposed to that of circularity: if all plastics were biodegradable, there would be no waste available for recycling and the plastics system would continue to depend on virgin materials (Kaur et al 2018, Schirmeister and Mülhaupt 2022).
A greater reliance on alternative feedstocks (e.g.waste, biomass or captured carbon) would decrease the need to produce fossil-based virgin plastics and, consequently, the demand for fossil fuels.When the demand for fossil fuels decreases, their price might also decrease and recycled materials or alternative feedstocks may become less attractive compared to fossil plastics.Fluctuations in fossil fuel prices can hamper innovation in the plastics sector (Kaur et al 2018, Siltaloppi and Jähi 2021, Syberg et al 2021, Williams and Rangel-Buitrago 2022).However, the price for fossil fuels is not influenced by demand only, but by more elements too, as was also mentioned in section 4.1.2.Alternative feedstocks are also subject to market trends, and might also be affected by price volatility.One way to address this risk is to diversify the range of options for recycled materials and renewable raw materials to build system resilience in the new plastics economy (Kaur et al 2018).Other interventions (e.g.price manipulation, taxes, incentives) may also help to counterbalance the balancing feedback loop shown in the model.
To conclude, on the one hand, different sustainable design options could interfere with each other, but on the other hand, different solutions could coexist if adopted in different applications: this would, however, require a major coordination effort.2008) defines as a 'fix that fails': an intervention that shows a benefit to the targeted issue in the short-term, but creates an unintended negative effect in the long-term, necessitating additional interventions to address the problem (feedback loop 'R-Recyclates quality (Fix that fails)' in figure 7).In fact, the lower the quality of the recyclates, the more additives may be added, ultimately risking making the plastic difficult to recycle again and potentially toxic when incinerated or dispersed.

Additives: contrasting narratives
While it is argued in the literature that more restrictions should be placed on what can and cannot be added to plastics, there is currently a large knowledge gap about the environmental and health impacts of the wide range of additives currently in use (Cavaliere et

Global plastics value chains 4.3.1. Logistics and costs in the recycling value chain
The model in figure 8 shows the structure of the recycling value chain, highlighting all the steps where losses occur and where interventions would be needed to improve the circularity of plastics.All plastics in use will eventually become waste, and different approaches can be taken to the collection of waste, such as the 'one bin' and 'multiple bins' models (Bing et al 2013, Vanapalli et al 2021, Johansen et al 2022, Systemiq 2022).Literature reports that relying on a single waste collection stream increases the overall (plastic) waste collection rate, but also increases the amount of contamination, which makes it more difficult to recycle plastics (unless more washing and pretreatment processes are included in the value chain) The model in figure 8 shows that the recycling value chain is articulated in several steps.One paper argues that although some companies make ambitious claims about recycling and recycled content targets, they may not be able to meet their commitments because they do not control a large part of the critical steps in the recycling value chain (Kahlert and Bening 2022).Greater vertical integration could improve the traceability of plastic waste through all steps and facilitate coordination between the stakeholders involved in the waste management system, which in turn could facilitate the implementation of major changes (e.g.infrastructure investments) aimed at improving the circularity of plastics (Bening et al 2021, Shamsuyeva and Endres 2021, Johansen et al 2022, Kahlert and Bening 2022).

Plastics export to developing countries
Demand for plastics has been growing over the last few decades, both because of the correlation with global population growth and because of the increasing use of plastics in various economic activities (Nicholson et al 2021, Siltaloppi and Jähi 2021, Babaremu et al 2022, Wiah et al 2022, Williams and Rangel-Buitrago 2022).The more plastics are consumed, the more will eventually become waste.
To date, the waste management systems (WMS) of most countries are quite inefficient, which means that recycling rates for plastics are still low: even in Europe, which is considered the most advanced region in terms of sustainable waste management, a large proportion of plastic waste is still incinerated or landfilled (Kaur et   .Unless these ambitious targets are accompanied by fundamental improvements in the efficiency of the WMS (e.g. higher collection and sorting rates, lower material losses during recycling processes, and so on), some countries may find it difficult to meet them (Kahlert and Bening 2022).
An easy way out is waste exports: Europe used to export millions of tonnes of plastic waste to China until 2018, when the country banned the import of plastic waste from abroad and European countries started exporting waste elsewhere, such as to Malaysia and Turkey (Bucknall 2020, Syberg et al 2021, Systemiq 2022).While waste exports ease the pressure on waste management systems in European countries, they also result in more plastics not being recycled, which essentially nullifies the potential benefits of the recycling targets (Bucknall 2020, Nielsen et al 2020, Klemeš et al 2021, Barrowclough and Birkbeck 2022, Williams and Rangel-Buitrago 2022).Countries where waste management is cheaper usually do not have the best and least impactful facilities, and in some cases do not have a formal waste management system at all, so most plastics end up either being incinerated or landfilled in poorly controlled conditions, which can cause both environmental and health hazards (Mwanza and Mbohwa Demand for plastics in developing countries also tends to increase as incomes rise (Barrowclough and Birkbeck 2022).The combination of rising incomes and globalisation is leading to the spread of Western business models to developing countries (Barrowclough and Birkbeck 2022).If this phenomenon is not accompanied by the development of more efficient waste management infrastructures, The model in figure 9 highlights the need to combine both a global and a local perspective when trying to improve the general sustainability of plastics end-of-life management.Local recycling targets alone are not sufficient to achieve a circular plastics system unless they are combined with improved waste management practices and export bans.The waste management practices that may work in a Western country may not be directly transferable in the context of a low-income country, which may benefit more from a tailored and equitable solution: the need for a just transition is mentioned in a few of the coded papers (Chen et   Another relevant factor influencing consumer behaviour is the trust they place in sustainability claims.Due to the increasing awareness of 'gre- enwashing phenomena' , people tend to have less trust in products that claim to be environmentally friendly, which may discourage them from reducing or replacing traditional plastics (Tolinski 2012, Johansen et al 2022, Williams and Rangel-Buitrago 2022).Examples of greenwashing in plastics include misleading or vague recycling labels-which do not always make it clear whether a product is (potentially) recyclable or (partially) made from recycled material-and bioplastics labels-which may indicate biodegradable, compostable or (partially) biobased materials (Tolinski 2012, Bucknall 2020, Bening et al 2021, Shamsuyeva and Endres 2021).A harmonised labelling and certification system, where labels provide clear information on the composition of the material (e.g. percentage of recycled or biobased content) and the recommended collection point (e.g.plastic or organic bin), would increase consumer confidence in sustainability claims (Tolinski 2012, Bucknall 2020, Skoczinski et al 2021).The more vague and ambiguous labels are, the more they can be perceived as misleading and the more mistakes are made in waste collection.
Improving the efficiency of waste collection and sorting is crucial to enable the circularity of the plastics system (Johansen et al 2022, Schirmeister and Mülhaupt 2022).Landfilling is increasingly discouraged, yet this shift does not inherently result in higher recycling rates (Bucknall 2020, Systemiq 2022).Instead, it may lead to the export of waste to lower-income countries.As detailed in section 4.3.2,while certain non-European nations have imposed bans on plastic waste imports, these regulations do not entirely prevent European countries from exporting plastic waste (Bucknall 2020, Barrowclough and Birkbeck 2022, Williams and Rangel-Buitrago 2022).On a positive note, existing policies have already reduced the export of plastic waste from Europe, from 3.1 to 1.7 million tonnes between 2016 and 2019 (Systemiq 2022).Nonetheless, it is essential to acknowledge that export/import bans alone are not a guaranteed pathway to higher plastic recycling rates; they may potentially lead to increased emissions from incineration (Systemiq 2022).

Fit-for-use and shifting demand to alternatives
As mentioned in the previous section, the growing awareness of sustainability issues of plastics is not sufficient to bring about behavioural change of consumers.In fact, plastics are cheap and versatile and their technical properties can meet a range of societal needs (from food packaging to home insulation, hygienic medical devices, etc) ( 11.
Alternative solutions should prove to be more sustainable than traditional plastic products.Some publications have pointed out that research and studies on sustainability are often conducted with different methodologies or approaches, so they are often difficult to compare and their results may not always be generalisable    As illustrated by figure 12, successfully addressing the issue of microplastics requires to intervene on all microplastics sources.
To reduce the release of microplastics, alternative materials could be preferred in some applications, e.g.cotton could be preferred to synthetic fibres in textile applications.While this would reduce the dispersal of microplastics, it could lead to various types of environmental impacts, such as those associated with the increased cultivation of cotton (figure 13) (Nielsen et al 2020).This can lead to a 'waterbed effect' where a new environmental impact is induced while trying to mitigate another (see footnote 2).

Plastic waste management 4.5.1. Reducing incineration in favour of recycling: energy demand considerations
Incineration of plastics is generally considered less desirable than recycling but preferable to landfilling (Bucknall 2020, Johansen et al 2022, Systemiq 2022).About 40% of plastic waste in Europe is incinerated and some authors believe that this share will increase, as landfilling will remain an option for a limited range of plastic waste (e.g. in the automotive and construction sectors) (Johansen et al 2022, Systemiq 2022).Incineration is useful for disposing of hazardous waste (e.g. in the healthcare sector), but generates greenhouse gas (GHG) emissions (Kaur et al 2018, Systemiq 2022, Williams and Rangel-Buitrago 2022).
Incineration combined with energy production is generally considered more acceptable, although this end-of-life fate is inherently non-circular (reinforcing loop 'R-Incineration lock-in' in figure 14) (Bucknall 2020, Schirmeister andMülhaupt 2022).Since most plastics today are of fossil origin, their calorific value is high, which means that incinerated plastic waste can generate much more energy than other municipal solid waste (MSW) (Bucknall 2020, Schirmeister and Mülhaupt 2022).If all or most of the plastic were to be recycled, incineration units would become much less efficient at producing energy.At the same time, higher recycling rates would also increase the demand for energy, especially if new recycling technologies were adopted on a large scale: today, most recyclable This could lead to a gap between demand and energy production, which would have to be filled with different energy sources.In the short term, this gap may have to be filled by fossil fuels: although it would be more desirable to fill it with renewables, this would require new installed capacity, which may entail a higher initial investment (Schirmeister and Mülhaupt 2022).
One paper states that energy recovery from incineration is actually a rather inefficient process that does not allow the full energy content of the plastic waste to be recovered (Aurisano et al 2021).If recycling is chosen instead, the energy saved from avoided fossil fuel extraction and plastic production is higher, making recycling more energy efficient than incineration (Aurisano et al 2021).This statement is not supported by the rest of the coded literature, as many sources state that recycling is To date, CCU and CCS technologies have not been widely adopted due to their high energy intensity, cost, regulatory and socio-technical barriers (Janipour et al 2021(Janipour et al , 2022)).Until now, waste incineration has not been included in the EU ETS (Emissions Trading System), but the European Parliament has agreed to include it from 2026 (Zanni 2022): this measure will help to incentivise recycling over incineration if combined with decarbonisation of the energy system, as recycling processes (especially chemical recycling processes) also generate emissions.To date, only a few emissions trading schemes in the world cover waste treatment processes (e.g.those in South Korea and New Zealand), while most others focus on power generation units other than waste incinerators, transport, and industry (California Air Resources Board 2018, International Carbon Action Partnership 2022).Once again, the model in figure 14 shows how the plastics sustainability transition and the energy transition (i.e.increased reliance on renewable energy sources) are closely linked.

Discussion
This study sets out to identify a definition for sustainable plastics (RQ1), the various problems and solutions around the (un)sustainability of plastics and their interrelations (RQ2).
First, this study finds that there is increasing attention for the topic of sustainable plastics, but that there is still not a shared understanding on what 'sustainable plastics' are.This expression does not seem to be linked to any specific technology; it is commonly interpreted as being related to environmental or economic sustainability, often intersecting with the concept of circular economy (e.We believe that all three aspects of sustainability (environmental, economic, social) should be included in the definition of 'sustainable plastics': while completely stopping the use of plastics may seem like a good option from a strictly environmental perspective, it would have negative social and economic consequences unless we also change the way we as a society meet our needs.Our study synthesises insights from various disciplines to provide a holistic understanding of the plastics system and its relationship with sustainability.We achieve this through qualitative system dynamics models.
The first research question3 was addressed by integrating the input retrieved from the analysed literature with the definition of sustainability provided by Daly (1991) as in Sterman (2011b), and the concept of doughnut economy elaborated by Raworth (2017).We presented our own definition for sustainable plastics in section 3, based on figure 3.
The second research question was answered by drawing several qualitative system dynamics models, reported in the results section.The study leads to several findings, which are discussed in the following paragraphs: 1.An integrative, multidisciplinary approach is needed to achieve the plastics sustainability transition, as most solutions may mitigate one aspect of unsustainability but increase another aspect of unsustainability, a 'waterbed effect' .Solutions should not be considered in isolation but always in a broad environmental, social, and economic context.2. Different solutions compete against one another, with the risk of nullifying each other's potential benefits.This unintended consequence may be mitigated by clearly distinguishing different solutions for different applications.3. The sustainability transition would benefit from greater vertical integration of the plastics value chain and more cross-sectoral cooperation because this will allow implementing solutions and avoiding unintended consequences.
Table 3 offers a summary of the main findings identified during the study and suggested interventions to avoid potential undesired consequences.This table is a simplification of the dynamic complexity discussed in the previous sections, but it provides a quick high-level overview of the topics discussed in section 4. It proves challenging to pinpoint a 'silver bullet' for making plastics more sustainable, as each solution exhibits its own set of drawbacks and potential risks.While identifying a specific technology to address plastics' unsustainability may be difficult, the implementation of harmonised standards, certifications, and labels could alleviate various aspects of the current and potential unsustainability of the plastics system, as highlighted in paragraph 5.3 (finding n. 3).In fact, enhanced harmonisation and integration across the plastic value chain would help prevent mismatches between solutions and the systems in which they are deployed (finding n. 1).The implementation of clearer and more stringent regulations would provide increased assurance to different stakeholders investing in new technologies, while transparent labels would strengthen communication with consumers and their confidence in sustainability claims.At the same time, both the context in which plastics are used and the technologies to produce plastics change continuously, which means that also standards and integration is also a process that requires continuous investments and updates.

An integrative approach towards sustainable plastics
Addressing only some elements of (un)sustainability without others could easily lead to interventions that solve a partial problem in one subsystem, but at the same time create or exacerbate other partial problems in other subsystems.By taking an integrative, systemic approach, this study showed how uncoordinated implementation of sustainable solutions could lead to undesirable consequences.Several authors mentioned biodegradable and compostable plastics as a solution for making plastics more sustainable (Siltaloppi and Jähi 2021, Hardy et al 2022, Williams and Rangel-Buitrago 2022), and some also discussed their limitations, such as the lack of clear biodegradability standards and clear product labelling (Tolinski 2012, Hafsa et al 2022, Systemiq 2022).However, fewer articles focused on the fact that large-scale adoption of biodegradable and compostable plastics may require major changes in waste management systems (Kaur et al 2018, Schirmeister and Mülhaupt 2022).In the case of textile microplastics, while a wider use of cotton instead of synthetic fibres would reduce the generation of microplastics from textile washing, there's a possibility that the increased demand for cotton would lead to different types of environmental impacts (e.g.greenhouse gas emissions from fertiliser use, water consumption: see figure 13) (Nielsen et al 2020).Lastly, various stances have been expressed on the subject of incineration, showing that it is a multifaceted topic: some argue that incineration is necessary to complement recycling, e.g. to eliminate hazardous waste (Bucknall 2020), or that incineration may be preferred to recycling when many pre-treatment processes would be required (Bucknall 2020); some argue that incineration may only be acceptable when combined with energy recovery (Schirmeister and Mülhaupt 2022, Systemiq 2022), but on the other hand this is currently a lock-in for the transition to different end-of-life scenarios for plastics and renewable energy generation (Bucknall 2020, Hafsa et al 2022, Schirmeister and Mülhaupt 2022, Systemiq 2022).Several authors have discussed that high recycling rates, and in particular chemical recycling processes, would increase energy demand (Bucknall 2020, Hafsa et al 2022, Schirmeister and Mülhaupt 2022, Waaijers-van der Loop et al 2022).However, they do not always link this statement with the fact that incinerators are currently important for meeting the energy needs of municipalities (Bucknall 2020, Schirmeister and Mülhaupt 2022): the increase in energy demand due to recycling processes and the lower energy production due to the lower calorific value of municipal solid waste leads to an energy gap that will have to be covered by other sources.From these examples we can conclude that it is crucial to adopt a whole-system perspective when assessing different possible interventions to improve the sustainability of plastics, in order to avoid creating more issues while trying to solve one (i.e. a waterbed effect, see footnote 2 ).

No silver bullet: different solutions for different applications
The results highlighted that different sustainable design paradigms can compete and hinder each other, but that these negative synergies can be partially addressed if additional measures are taken.For example, several documents stated that harmonised legislation should limit the use of additives, which would improve recyclability (Bucknall 2020, Cavaliere et al 2020, Syberg et al 2021, Johansen et al 2022, Williams and Rangel-Buitrago 2022).On the other hand, additives are added to ensure that products are 'fit for purpose ' anddurable (Schirmeister andMülhaupt 2022, Williams andRangel-Buitrago 2022).Durability of plastic products is also desirable because one of the '3 Rs' of circularity is 'reuse': the longer the lifetime of a product, the fewer units need to be produced, purchased and eventually disposed of (Hafsa et al 2022, Hardy et al 2022, Schirmeister and Mülhaupt 2022).These contradictions show that different policies and technologies should be adopted for different plastics applications, but this in turn makes it more difficult to provide clear and simple information and guidance to consumers.As an additional example, design for biodegradability has different requirements and implications than both design for durability and design for recycling.In addition, achieving complete biodegradation of plastic products remains a challenge.Considering the use of biodegradable plastics only for specific applications where we can predict the environmental conditions they will ultimately encounter could be a prudent approach to avoid unintended consequences associated with the introduction of biodegradable materials.For example, the collection of plastic mulch and plastic sheets used in agricultural fields at the end-of-life stage is labour-intensive and costly (Kasirajan and Ngouajio 2012, Razza and Cerutti 2017).When left on agricultural fields, plastic mulch produces a large amount of microplastics, which can enter the food chain (Kawecki and Nowack 2019, van Schothorst et al 2021).Agriculture is therefore a sector that can really benefit from the use of biodegradable plastics, although more research is needed to ensure that plastic sheets fully biodegrade in the field without producing microplastics due to incomplete degradation (Steinmetz et al 2016, Sipe et al 2022).On the other hand, (food) packaging that fully biodegrades in the environment may be more difficult to achieve as packaging may be dispersed in different locations.The use of biodegradable materials in food packaging applications may give the false impression that their dispersal in the environment does not cause harm, whereas leakage of packaging into the environment should be prevented.

Stakeholders' collaboration
The plastics sustainability transition would benefit from greater cross-sectoral and cross-border collaboration among plastics stakeholders.For example, as mentioned in some of the literature, waste management systems are currently governed by different sets of rules in each country and, in many cases, in each municipality (Mwanza and Mbohwa 2017, Siltaloppi and Jähi 2021): on the one hand, the literature review showed that clearer communication with customers would reduce their errors in waste management and increase their trust in sustainability claims, as the risk of greenwashing would be reduced through the adoption of harmonised standards (Tolinski 2012, Bucknall 2020, Hafsa et al 2022, Williams and Rangel-Buitrago 2022).On the other hand, the fact that the waste management system is not harmonised may limit the clarity of disposal instructions that can be provided on product labels, and is also something to consider when thinking about alternative designs for (plastic) products: the potential sustainability benefits of a new design may be offset if the product is not collected separately.More generally, greater data sharing across the value chain and clearer allocation of responsibilities (e.g. through Extended Producer Responsibility schemes) would facilitate the conduct of comparable sustainability assessments that would support the development of evidence-based sustainability policies and investments.
Many reasons have so far limited stakeholders' collaboration towards a more sustainable plastics system, such as lack of detailed regulations, lack of economic incentives, vested interests, lobbying, and a lack of clear responsibility allocations (Piontek 2019, Bening et al 2021, Klemeš et al 2021, Siltaloppi and Jähi 2021, Villarrubia-Gómez et al 2022).The literature suggests that two key barriers to increase sustainability are the lack of a clear problem owner and the lack of an overarching policy framework that encompasses the entire life cycle of plastics (Bucknall 2020, Nielsen et al 2020, Aurisano et al 2021, Wiesinger et al 2021, Williams and Rangel-Buitrago 2022).However, there are increasing efforts to develop international sustainable plastics policies.In 2018, the EU launched the Plastics Strategy as part of the Circular Economy Action Plan.The policy is still being fully developed, but will focus in particular on reducing single-use plastics, increasing recycling rates, setting rules for bio-based and biodegradable plastics, and reducing microplastics (European Commission 2018).In 2022, the United Nations Environment Assembly has called for an international legally binding agreement to end plastic pollution (UNEP 2022).We hope that these and similar policies will not be limited to isolated and incremental solutions, e.g.too low or too ambitious recycling targets without major interventions in the structure of the waste management system; bans on waste exports without mechanisms to incentivise recycling over incineration; restrictions on incineration with energy recovery without supporting the increased use of renewables; bans on secondary microplastics without any action on primary microplastics; promotion of biodegradable plastics without ensuring an adequate waste collection stream; etc.With this paper, we hope to support these and future initiatives to take a holistic, integrative approach that encompasses all aspects of sustainability and considers the interactions between different elements of the plastics system and the interactions of the plastics system with the whole global economic system.Furthermore, we would like to reiterate that the use of additives is currently a major knowledge gap for researchers and regulators (Cavaliere et al 2020, Aurisano et al 2021, Syberg et al 2021, Wiesinger et al 2021, De Weerdt et al 2022, Sørensen et al 2023).Most additives are known only by their commercial names and are not available in widely used environmental impact assessment databases (e.g.Ecoinvent 2003) (Geyer et al 2017).Recent research suggests that the environmental impact of additives may be even worse than that of microplastics (Benjaminsen 2023).This is worrying given the widespread use of additives, which in some cases make up a high percentage of the mass of the plastic product (Geyer et al 2017, Kan et al 2023).Therefore, we urge the authorities to implement a more transparent, regulated and harmonised use of additives in plastic products.

Contributions
This study provides two main contributions to the literature.First, this paper enriches the existing literature on transition studies (in particular the literature on sustainability transitions) by providing an additional case study, namely the plastics sustainability transition (Farla et al 2012, Markard et al 2012, 2020, Köhler et al 2019).Indeed, the plastics sustainability transition is still less addressed in the literature compared to other sustainability transitions, such as the energy transition (Markard et al 2012, Stanitsas et al 2019, Giganti and Falcone 2022).A recent research agenda on sustainability transitions also notes that the reorientation and decline of industries (e.g. the transition of the established petrochemical industry, authors' note) has so far received much less attention than the emergence of new industries (Köhler et al 2019).
Second, this paper contributes to the green or sustainable chemistry literature by showing that a narrow technical or techno-economic focus can be problematic for drawing generalisable conclusions about what the best solution to a sustainability problem should be.Understanding what a sustainable system should look like cannot overlook the complex constellation of stakeholders and their interactions.As noted in the introduction, technologies do not operate in a vacuum but are embedded in complex sociotechnical systems (Geels 2002, 2004, Farla et al 2012, Köhler et al 2019, Villarrubia-Gómez et al 2022).Therefore, while detailed technical knowledge of different solutions is needed, it is important to remember that we cannot draw conclusions about the sustainability of the (future) plastics system from this type of knowledge alone (Elzen andWieczorek 2005, Villarrubia-Gómez et al 2022).
In addition to contributing to the literature, this study also makes a practical contribution by providing an overview of the potential drawbacks of different sustainable plastics solutions that might be overlooked if the scope of the research is too narrow.The results of this analysis promote a better understanding of the plastics system and can therefore support different stakeholders in making decisions to advance the sustainability transition of plastics.
While a closer collaboration among international stakeholders is advised, and the plastics sustainability transition should be global, at the same time the results show that not all solutions can be implemented in every country (Markard et al 2012, Truffer 2012, Mwanza and Mbohwa 2017, Hafsa et al 2022).For example, as mentioned in some of the reviewed papers, some solutions might be too expensive to be implemented in less developed countries (Truffer 2012, Mwanza and Mbohwa 2017, Hafsa et al 2022).Thus, while the effort to move towards a sustainable system should be global, it might be implemented differently in different contexts: both global and local perspectives are needed, and these perspectives should be coordinated and not hinder each other where avoidable (Van Den Bergh et al 2011, Markard et al 2012, Truffer 2012).
This paper also provides a methodological contribution, demonstrating how an integrative literature review and qualitative system dynamics modelling can support each other to investigate complex issues such as the sustainability challenges posed by plastics, since the former allows for the collection of a broad range of knowledge and the latter provides a tool to integrate information from different sources in a coherent way.The same approach could be used to analyse other complex and multidisciplinary issues, i.e. a similar procedure could benefit research in other streams of the sustainability transition literature: see for instance Gürsan and de Gooyert (2021) for an additional example.

Limitations
Like any literature review, this study is limited by the choices made in the selection of documents, such as the search string.In this paper, the choice was made to perform an integrative literature review, which employs a more flexible protocol compared to a systematic literature review (Torraco 2005, Snyder 2019, Cho 2022).A simple and broad search string was chosen: a search string that included a list of plastics sustainability issues and solutions would have been very long and would likely have risked omitting some potentially interesting topics.While relying on a different search string could have led to slightly different documents, codes and variables, as the study focused on review and overview articles, the risk of overlooking important topics is reduced.Articles with a narrow geographical scope have generally not been included in order to ensure the generalisability of the conclusions.It is true, however, that most of the documents, often implicitly, adopted a Western perspective on the issue.The coding and modelling procedure is, to a certain extent, subjective.Nevertheless, the authors made an effort to ensure the transparency of the analysis by keeping track of all the codes and the documents from which they were originated.The qualitative system dynamics modelling approach allows for the inclusion of variables and linkages that may be difficult to quantify, if not by making a significant number of assumptions and perhaps simplifying the structure of the model.On the other hand, qualitative models lack the insights that could be gained from simulations.

Outlook and suggestions for future research
During the literature review, it was noted that some topics have been studied in more depth than others.Therefore, we recommend that future research should focus on the following points: 1.The economic feasibility of a transition to sustainable plastics, often understood as a transition to a circular plastics economy.A2 show an example of the codes retrieved from analysed documents.In figure A2, the 'Interrelations' theme folder contains several topic groups (e.g.Bans; Biobased, biodegradable, composting; Circular; …).The topic group 'Bans' is shown in the figure.The numbers in the brackets indicate how many quotes (i.e.portions of text) are linked to a given code, namely how many times the code was applied to quotes in the papers.In figure A2 it can be seen that the code 'Bans: Landfill directive leads to more incineration' was connected to 3 different quotes.A single quote might contain more than one code.

Figure 2 .
Figure 2. Three necessary conditions for sustainability.Reproduced from Sterman (2011b), with permission from Springer Nature.

Figure 4 .
Figure 4. Biodegradable and compostable plastics: is the current system ready to deal with them?
2017, Siltaloppi and Jähi 2021).Although the model in figure 4 seems to suggest that more plastic waste collection streams can improve the quality of each stream, other research suggests that this approach could lead to more consumer errors (Johansen et al 2022): see section 4.3.1.On the other hand, a single plastic waste collection bin increases the collection rate of plastics, but leads to more contamination unless combined with innovative sorting systems or pre-treatment of plastics for recycling (Kaur et al 2018, Johansen et al 2022, Schirmeister and Mülhaupt 2022, Systemiq 2022).
can lead to increased consumption of fertilisers and pesticides to ensure agricultural productivity (feedback loop 'B-Undesired consequences') (Siltaloppi and Jähi 2021, Tarazona et al 2022).On the other hand, the consumption of second-generation biomass (non-edible biomass) could avoid this problem, when the biomass source does not require cultivation (e.g.bio-waste) (Kaur et al 2018, Amulya et al 2021, Systemiq 2022).To date, the manufacturing processes of bio-based plastics are more energy intensive than those of fossil plastics, hence this is another way bio-based plastics might end up causing more impacts than traditional materials (Amulya et al 2021, Mastrolia et al 2022, Schirmeister and Mülhaupt 2022).More sectors of the economy are dependent on biomass, so the increased competition for land could threaten some economic activities (e.g.food production) (Bucknall 2020, Amulya et al 2021, Systemiq 2022, Tarazona et al 2022), lead to overexploitation of natural resources (Siltaloppi and Jähi 2021) and eventually to conflicts due to the geographical distribution of such resources (Amulya et al 2021, Schirmeister and Mülhaupt 2022, Systemiq 2022).
'Additives' is a generic term for all substances added to a plastic polymer to improve the final product's physical and/or aesthetic properties (Aurisano et al 2021, Wiesinger et al 2021, Jehanno et al 2022, Olatayo et al 2022, Pfaendner 2022, Schirmeister and Mülhaupt 2022).As mentioned in section 4.2.1, additives are subject to conflicting narratives regarding their role in the sustainability of plastics: on the one hand, they can, for example, increase the durability of the product or facilitate the manufacturing process (Aurisano et al 2021, Wiesinger et al 2021, Pfaendner 2022).On the other hand, they can cause toxic emissions when the plastic product is incinerated or dispersed in the environment (Cavaliere et al 2020, Aurisano et al 2021, Wiesinger et al 2021, Babaremu et al 2022, Pfaendner 2022, Schirmeister and Mülhaupt 2022).Additives can also make plastics more difficult to recycle, especially with mechanical recycling technologies and if plastic polymers are not fully sorted: they are considered contaminants that reduce the quality and usability of recycled plastics (Bucknall 2020, Vanapalli et al 2021, Wiesinger et al 2021, Johansen et al 2022).However, sometimes more additives are added to recyclates to compensate for their low technical or aesthetic properties (e.g.reduce recyclates' odour) (Pfaendner 2022).This practice could end up being an example of what Meadows ( al 2020, Aurisano et al 2021, Syberg et al 2021, Wiesinger et al 2021, De Weerdt et al 2022).In addition, most research looks at the health and environmental effects of one chemical at a time, but in reality humans and ecosystems may be exposed to multiple chemicals at the same time, and the effects of combined additives may also be different (Aurisano et al 2021).Greater collaboration between the various plastics stakeholders is needed to ensure a better exchange of information, which in turn would provide a basis for further regulation of the use of additives (Aurisano et al 2021, Nicholson et al 2021, Wiesinger et al 2021, Waaijers-van der Loop et al 2022).
2017, Ali et al 2022, Barrowclough and Birkbeck 2022, Hafsa et al 2022, Kahlert and Bening 2022).Unless further legislation is introduced, the export of waste abroad will continue to hinder the implementation of a circular plastics system (Syberg et al 2021, Williams and Rangel-Buitrago 2022).The documents analysed discussed waste exports from European countries (or the Global North in general), but similar trends can also be observed in the USA.North American plastic waste used to be largely exported to China: following the Chinese ban on waste imports, the main importers of US plastic waste are Malaysia, Vietnam and Indonesia (McCormick et al 2019, Bourtsalas et al 2023).Plastic waste that is not exported but treated domestically is generally landfilled or incinerated, while only 8.7% is recycled (comparable to the world average, but much lower than in some European countries) (McCormick et al 2019, Bourtsalas et al 2023).Annual per capita plastic consumption is also reported to be higher in the US than in Europe (Williams and Rangel-Buitrago 2022).

Figure 9 .
Figure 9. Plastic exports to developing countries.

4. 4 .
Consumers and plastics use 4.4.1.Challenges to clear communication and a behavioural transition Consumer behaviour also plays a role in the sustainability transition of plastics.Public awareness of sustainability issues of plastics has increased in recent years, particularly due to the accumulation of plastics in the environment and the increasing focus on microplastics (Tolinski 2012, Mwanza and Mbohwa 2017, Dijkstra et al 2020).It has been observed that the more people know about the plastics system and the inefficiency of the current recycling system, the more concerned they are about the use of plastics (figure 10) (Cavaliere et al 2020).Health concerns increase if plastics are used in direct contact with food (Cavaliere et al 2020).Concerns may push them to change their behaviour, e.g. by reducing plastic consumption and choosing different materials whenever possible (Piontek 2019, Cavaliere et al 2020, Hafsa et al 2022, Muposhi et al 2022).They will be more committed to making an effort if they perceive that people around them are also committed, e.g.other citizens, but also public authorities (through proenvironmental policies) and companies (through voluntary initiatives) (Cavaliere et al 2020, Dijkstra et al 2020).A behavioural transition is expected to reduce the amount of plastic leaking into the environment every year: most of the inflows are post-consumer waste, especially food packaging (Forrest et al 2019, Piontek 2019, Hafsa et al 2022).However, the variable 'Versatility of plastics' could significantly reduce the strength of this balancing feedback loop, as problem awareness alone may not lead to large changes in consumer behaviour: see section 4.4.2 for more details.
(Klemeš et al 2021, Siltaloppi and Jähi 2021, Jehanno et al 2022, Johansen et al 2022, Schirmeister and Mülhaupt 2022, Waaijers-van der Loop et al 2022).If sustainability measurement tools were harmonised, it would be easier to recognise their validity globally and this would improve fact-based decision-making by authorities and companies to

Figure 11 .
Figure 11.Fit-for-use and shifting demand to alternatives.

Figure 14 .
Figure 14.Reducing incineration in favour of recycling: energy demand considerations.
energy intensive (Shamsuyeva and Endres 2021, Hafsa et al 2022, Jehanno et al 2022, Sadhukhan and Sekar 2022, Schirmeister and Mülhaupt 2022).However, this apparent contradiction could be resolved if one considers that chemical recycling processes are still emerging (Nielsen et al 2020, Bening et al 2021, Jehanno et al 2022, Sarkar et al 2022): if scaled up and optimised, the energy demand for chemical recycling processes could be reduced, making this option more attractive than the inefficient combination of incineration with energy recovery and virgin product manufacturing.Greenhouse gas emissions from incinerators could also be mitigated by carbon capture and storage (CCS) or carbon capture and utilisation (CCU) facilities.However, these units are also energy intensive (Bucknall 2020, Johansen et al 2022, Systemiq 2022)-making incineration even less energy efficient.Incineration produces different types of GHG emissions, and only CO 2 emissions can be captured by CCU and CCS technologies: other GHGs have a much greater impact on climate change than CO 2 , but receive much less attention (de Richter et al 2017, Aurisano et al 2021, Harmsen et al 2023).

Figure A2 .
Figure A2.Quotes linked to the code 'bans: landfill directive leads to more incineration' .

Table 1 .
Filtering steps used in the literature.

Table 2 .
Definition for 'sustainable plastics'-codes from literature.
Compostable plastics primarily break down in industrial or domestic composting facilities rather than in natural environments (ASTM Standards as in Song et al 2009, European Commission 2022).They are subject to more rigorous and specific definitions compared to biodegradable plastics (European Commission 2022).
4.1.1.Biodegradable and compostable plastics: is the current system ready to deal with them?oxygen, water and light (van den Oever et al 2017, Di Bartolo et al 2021).Contrary to what many consumers might think, biodegradable plastics are not necessarily bio-based, but can also be fossilbased (van Groenestijn et al 2019, Di Bartolo et al 2021, Zwicker et al 2021).While the terms 'biodegradable' and 'compostable' are often used interchangeably, compostable materials are a subset of biodegradable materials.
Amulya et al 2021, Skoczinski et al 2021, Syberg et al 2021, Hardy et al 2022, Mastrolia et al 2022, Williams and Rangel-Buitrago 2022 (Johansen et al 2022, Pfaendner 2022, Schirmeister and Mülhaupt 2022, Waaijers-van der Loop et al 2022).On the other hand, separate bins would reduce contamination, but could also reduce the overall plastics collection rate due to increased complexity and burden on consumers (Mwanza and Mbohwa 2017, Shamsuyeva and Endres 2021, Siltaloppi and Jähi 2021, Kahlert and Bening 2022, Pfaendner 2022).For instance, in a 'multiple bins' model, consumers might be confused about where to throw multimaterial products (e.g.milk cartons), and bio-based or biodegradable plastics (Bucknall 2020, Hafsa et al 2022, Mastrolia et al 2022, Williams and Rangel-Buitrago 2022).Advanced sorting techniques may eventually increase recycling rates (Kaur et al 2018, Siltaloppi and Jähi 2021, Kahlert and Bening 2022, Systemiq 2022, Tejaswini et al 2022).Material losses are expected at all these stages, and plastics collected for recycling but not ultimately recycled are usually incinerated (Kaur et al 2018, Gall et al 2020, Skoczinski et al 2021, Kahlert and Bening 2022, Stegmann et al 2022).Even when plastics are recycled, they may not end up being used if their quality is too low or their cost is too high compared to virgin plastics (Forrest et al 2019, Bening et al 2021, Shamsuyeva and Endres 2021, Babaremu et al 2022, De Weerdt et al 2022, Jehanno et al 2022, Kahlert and Bening 2022).Demand for recyclates could be stimulated, for example, by targets for recycled content rather than just for recycling, and by reducing the cost of recycling activities (Piontek 2019, Syberg et al 2021, De Weerdt et al 2022, Hafsa et al 2022): on the latter point, there is no consensus on the best strategy for reducing costs.Some argue that economies of scale would reduce costs, particularly for chemical recycling technologies that have yet to be scaled up (Bening et al 2021, Hafsa et al 2022, Schirmeister and Mülhaupt 2022).Others argue that transport costs are sometimes the largest contributor to overall costs, so a more capillary distribution of recycling facilities across the territory could be of greatest benefit (Bing et al 2013, Bening et al 2021, Babaremu et al 2022).The best solution may differ depending on the country or geographical area considered (Bing et al 2013).
al 2018, Klemeš et al 2021, Nicholson et al 2021, Shamsuyeva and Endres 2021, Hafsa et al 2022, Paquibut et al 2022, Systemiq 2022, Tejaswini et al 2022, Wiah et al 2022).The waste management systems in many countries are not able to cope with the growing amount of plastic waste (Forrest et al 2019, Shamsuyeva and Endres 2021, Wiah et al 2022, Nguyen et al 2023).At the same time, growing public awareness of plastics issues is leading Mwanza and Mbohwa 2017, Forrest et al 2019, Hardy et al 2022, Schirmeister and Mülhaupt 2022).Demand for plastics will only shift to alternatives (i.e.alternative products or plastics made from alternative feedstocks) if they are able to meet the same societal needs (Dijkstra et al 2020, Hafsa et al 2022, Waaijers-van der Loop et al 2022).This is expressed by the variable 'Interchangeability' in figure

Table 3 .
Summary of findings.
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