A Unified Classification of Waste Plastic Chemical Recycling: Pyrolysis, Gasification, Depolymerization, and Dissolution

Abstract

Waste plastic valorization has become a critical challenge in achieving global circular economy goals and mitigating climate change. However, the field suffers from fundamental inconsistencies in terminology and classification that impede scientific communication, technology development, and policy formulation. This study critically analyzes existing classification frameworks for waste plastic valorization technologies and identifies major sources of confusion: (i) ambiguous classification of dissolution processes across physical and chemical recycling categories; (ii) conflation of dissolution with chemical depolymerization despite their distinct mechanisms and products; (iii) inconsistent definitions of pyrolysis and gasification that blur the distinction between fundamentally different thermochemical processes; (iv) arbitrary temperature‐based subcategorization of pyrolysis lacking scientific basis; and (v) misleading designation of energy recovery as “thermal recycling.” I propose a coherent classification scheme based on process‐type logic: dissolution is repositioned within chemical recycling based on its operational characteristics, chemical recycling and feedstock recycling are explicitly differentiated, and energy recovery is divided into direct (combustion of waste plastic) and indirect (combustion of thermochemically‐derived fuels) pathways. This unified framework aims to establish a common foundation for education, research, and evidence‐based policymaking in sustainable waste plastic management.

Inconsistent terminology hampers communication in plastic recycling. This perspective compares definitions in standards and authoritative sources and proposes a unified classification: dissolution is placed within chemical recycling, chemical and feedstock recycling are distinguished, pyrolysis and gasification are defined by process, and energy recovery is split into direct and indirect pathways.

1. Introduction

Environmentally sound and efficient waste plastic valorization has been a major global concern for decades, with its importance further amplified by accelerating climate change. Despite extensive research efforts, practitioners, researchers, and policymakers continue to face difficulties in communication and interpretation when engaging with academic literature or participating in scientific discourse. These challenges largely stem from the absence of a unified framework for classifying waste plastic valorization approaches and associated technologies. This lack of standardization has persisted over time and remains unresolved. Two primary factors contribute to this issue. First, waste plastic valorization technologies have evolved differently across countries, leading to fragmented and inconsistent classification practices. Second, the rapid emergence of new technologies has outpaced existing classification systems, leaving several approaches inadequately defined or ambiguously positioned. Such inconsistencies in terminology and classification have significant implications. They hinder coherent education, complicate the contextualization of research findings, and limit the development of effective and harmonized policy frameworks. From a green chemistry perspective, these inconsistencies are not merely semantic but fundamentally limit the ability to evaluate and compare technologies on a scientifically rigorous basis. The application of key green chemistry principles, such as atom economy, energy efficiency, and the promotion of circular material flows, requires clear and consistent definitions of the processes involved. Inconsistent terminology does not in itself preclude a comparison of pyrolysis and gasification, nor does it alter the methodological requirements of life cycle assessment, which depend on the functional unit, system boundaries, and allocation choices [1]. It does, however, make such comparisons more difficult to set up and to interpret. When the same process is described by different names, or when different processes share one name, it becomes harder to identify which technologies and outputs a study actually covers, to collate literature data, and to calculate recycling rates consistently [2]. In this context, establishing a unified and conceptually consistent classification framework is not only a matter of clarity in communication but also a prerequisite for the practical implementation of green chemistry principles in waste plastic valorization. In particular, it can help clarify the roles and relationships of emerging recycling technologies, especially within the rapidly evolving field of chemical recycling, thereby enabling more meaningful comparisons and fostering technological progress. In response to this need, this study proposes a conceptual framework for the classification of waste plastic valorization technologies, with a particular focus on resolving terminological and conceptual ambiguities in chemical recycling. Since the classification and definition of waste plastic valorization technologies rely on a clear understanding of each approach, this paper also aims to provide an overview of their principles, characteristics, and recycling pathways. The proposed framework is intended to serve as a foundational reference for future refinement, facilitate clearer communication among stakeholders, and support the development of research and policy in this field. Ultimately, this framework aims to redefine how waste plastic valorization technologies are interpreted, compared, and assessed within the context of sustainable chemistry.

2. Core Terminology in Waste Plastic Valorization

From the 1990s to the present, two key concepts have underpinned all discussions of waste plastic valorization:

• Recovery: Any operation whose principal result is that waste serves a useful purpose by replacing other materials that would otherwise have been used to fulfill a particular function (Art. 3(15) of the Waste Framework Directive (WFD) [3]). Recovery comprises material recovery and energy recovery.

• Recycling: A subset of recovery in which waste materials are reprocessed into products, materials, or substances, whether for the original or for other purposes. It excludes energy recovery and reprocessing into materials that are to be used as fuels or for backfilling operations (Art. 3(17) of the WFD [3]). Accordingly, the conversion of waste plastic into monomers or chemical feedstocks for material applications constitutes recycling, whereas its conversion into fuels constitutes energy recovery.

Here, “recovery” refers not only to the generation of energy or materials from waste plastic but also to its effective utilization [4].

3. Historical Classification of Waste Plastic Valorization Technologies

In the late 1980s and early 1990s, the concept of waste plastic recycling was introduced in the field of waste plastic valorization [5]. When societal interest in waste plastic recycling grew substantially during the 1990s, three broad categories were established for waste plastic valorization technologies [6, 7]:

• 1. Mechanical (or material) recycling—A process that recovers secondary raw materials (e.g., pellets) through mechanical operations without significantly altering the chemical structure of the polymer. Because this method yields materials, it has also been referred to as material recycling.
• 2. Feedstock (or chemical) recycling—A process that converts waste plastic back into raw materials, such as monomers or chemicals, using chemical processes. Because feedstock recycling involves chemical processes, it has also been termed chemical recycling. In the 1990s, the term “feedstock recycling” was more widely used in the academic literature than “chemical recycling.” Associated technologies have included pyrolysis, gasification, hydrogenation, and solvolysis (the latter now more commonly termed chemical depolymerization (DP)).
• 3. Energy recovery—A method involving the controlled combustion of waste plastic. While primarily a disposal method, it can be categorized under valorization when the heat generated is captured and utilized as an energy source.

Table 1 describes the characteristics of three categories of waste plastic valorization technologies.

TABLE 1.

Classification of waste plastic valorization technologies, including mechanical, feedstock, and energy recovery pathways, along with their key characteristics (based on 1990s frameworks).

Category	Alternative term	Description	Key features and representative technologies
Mechanical recycling	Material recycling	Recovery of secondary materials (e.g., pellets) via mechanical processing without altering polymer structure	Preserves polymer chain integrity; no significant chemical transformation; yields recycled pellets or regranulate
Feedstock recycling	Chemical recycling	Conversion of waste plastic into monomers, chemicals, or other feedstocks via chemical processes	Polymer chain scission; includes pyrolysis, gasification, hydrogenation, and solvolysis
Energy recovery	Waste‐to‐energy	Controlled combustion of waste plastic with energy capture	Focuses on heat and/or power generation

Of these 1990s categories, mechanical recycling and energy recovery remain in current use, although the alternative term “material recycling” has become ambiguous, as it is used both for mechanical recycling and for material recovery in general [8]. By contrast, “feedstock recycling” has largely been superseded by “chemical recycling” in current frameworks, in which the two terms are treated as synonyms [8, 9]. At the same time, technologies that have matured since the 1990s, such as dissolution and the various routes of DP, have no counterpart in Table 1, and new terms such as “physical recycling” have been introduced to accommodate them [9]. Section 4 therefore examines how these terms are currently defined in international standards and authoritative sources.

4. Current Definitions in Standards and Authoritative Sources

Before the classification and terminology of waste plastic valorization are examined and revisions are proposed, the current definitions of key terms in international standards and authoritative sources must first be established. Table 2 summarizes how five terms, namely, chemical recycling, pyrolysis, gasification, physical recycling, and depolymerization, are treated in ISO 15270:2008 of the International Organization for Standardization (ISO) [8], the recommendations of the International Union of Pure and Applied Chemistry (IUPAC) [10, 11], the United States Department of Energy (US DOE) [12], and the assessment of plastic waste recycling by the European Commission's Joint Research Centre (JRC) [9], together with the ambiguities that remain.

TABLE 2.

Treatment of key terms for plastic recycling technologies in current frameworks and the remaining ambiguities.

Issue	Treatment in current frameworks	Ambiguity
Chemical recycling	ISO 15270:2008 describes plastics recovery as comprising material recovery, by means of mechanical recycling, chemical or feedstock recycling, and biological or organic recycling, and energy recovery. It defines chemical (feedstock) recycling as the conversion of plastics waste into monomers or new raw materials by changing its chemical structure through cracking, gasification, or depolymerization.	Ambiguous terminology (“organic recycling”); the relationship between feedstock recycling and biological recycling is not addressed. The term “cracking” is used instead of “pyrolysis,” and the two terms are used interchangeably in the literature.
Pyrolysis	IUPAC defines pyrolysis as thermolysis associated with exposure to high temperature, i.e., primarily in terms of thermally induced bond cleavage, noting that the term generally refers to reaction in an inert environment	Pyrolysis is defined in terms of thermolysis, and IUPAC itself uses “pyrolysis,” “thermolysis,” and “thermal degradation” interchangeably.
Gasification	US DOE describes gasification as a process in which carbonaceous feedstocks are converted by heat into syngas through partial oxidation or reforming with a gasifying agent such as air, oxygen, or steam.	The IUPAC Gold Book contains no entry for gasification, and ISO 15270:2008 lists gasification without defining it.
Physical recycling	The JRC treats physical recycling as a variant of mechanical recycling in which the polymer backbone is recovered in its entirety via dissolution in a solvent.	Many publications use “physical recycling” as a synonym for mechanical recycling and “solvent‐based recycling” is used inconsistently.
Depolymerization	IUPAC defines depolymerization as “the process of converting a polymer into a monomer or a mixture of monomers.”	No differentiation among chemical, thermal, and biological depolymerization routes.

ISO 15270:2008 divides material recovery into mechanical, chemical or feedstock, and biological or organic recycling, and identifies cracking, gasification, and depolymerization as chemical recycling routes; however, it neither states the criterion underlying this classification nor clarifies how the associated technologies are related. The IUPAC definition of pyrolysis shows an interchangeable use of terms, and its definition of depolymerization likewise does not differentiate among routes. Notably, no definition of gasification can be found in the IUPAC Gold Book or ISO 15270:2008. The JRC treats physical recycling as a variant of mechanical recycling based on solvent dissolution, whereas many publications use the same term as a synonym for mechanical recycling. ISO 15270 is currently being revised into separate parts, of which only Part 5 (organic/biological recycling) has been published [13]; until the remaining parts are finalized, the ambiguities identified in Table 2 persist in scientific usage and are examined in the following sections.

5. Pyrolysis, Gasification, and Incineration

Thermal treatment is the most widely applied method for waste plastic management, encompassing both recycling and recovery pathways. It can be categorized into two main types: thermochemical conversion processes (including pyrolysis and gasification) and incineration (with or without energy recovery). Thermochemical conversion processes, key to feedstock recycling, involve heat‐induced chemical transformations. During these processes, chemical bonds are broken and reformed, resulting in the formation of new chemical species. Among these processes, pyrolysis is one of the oldest and most widely studied thermochemical routes for waste plastic valorization. Pyrolysis is generally defined as thermal degradation occurring in an inert environment, a definition often used interchangeably with “thermolysis” [10]. However, many studies [14, 15, 16] define pyrolysis as a process in which feedstock is thermally degraded in the absence of oxygen, which creates an unrecognized duality and can cause confusion. These two definitions above are based on the reactional perspective and the process perspective, respectively. From the reactional perspective, pyrolysis denotes the thermal degradation or thermal decomposition of organic molecules, whereas from the process perspective, it denotes a thermal operation carried out in the complete absence of oxygen. These perspectives are not interchangeable: the reactional definition describes what happens at the molecular level, while the process definition describes the operating environment in which the reaction occurs. This distinction is critically important for the pyrolysis of oxygen‐containing feedstocks such as polyethylene terephthalate or biomass‐derived materials. Such feedstocks can release small amounts of internal oxygen during thermal decomposition of feedstocks, and trace oxygen may also be present in the reactor at the start of heating. In both cases, the feedstock can undergo thermal decomposition without reaction with oxygen. In that situation, pyrolysis from the reactional perspective occurs, but in this case, the process‐level definition of pyrolysis as “thermal operation in the absence of oxygen” does not apply. This ambiguity highlights the need to reestablish a clearer definition of pyrolysis. In this paper, pyrolysis is defined from the process perspective as a thermal process occurring in the complete absence of “externally supplied oxygen.” By defining pyrolysis as above, the ambiguity in the widely accepted definition of pyrolysis as thermal degradation “in the absence of oxygen” can be resolved. Oxygen chemically bound in the feedstock (e.g., in polyethylene terephthalate (PET) or biomass) does not affect this classification because it is released as part of the feedstock's own decomposition products rather than supplied as a reactant. Meanwhile, gasification, which is critically important in chemical recycling alongside pyrolysis, is defined here from the process perspective following the description by the US DOE [12]. As a result, the new definition of pyrolysis proposed in this paper facilitates a meaningful comparison between pyrolysis and gasification. To prevent further conflation between reaction‐level and process‐level descriptions, the terms “thermal degradation,” “thermal decomposition,” or “cracking” should be used in place of pyrolysis when referring to the reactional perspective. Table 3 summarizes the distinctions between the reactional and process perspectives of pyrolysis and gasification.

TABLE 3.

Conceptual framework for distinguishing pyrolysis and gasification: Reactional versus process perspectives.

Category	Pyrolysis	Gasification
Reactional perspective	Thermal degradation (thermal decomposition or cracking)	Thermal degradation, partial and complete oxidation, and reduction reactions
Process perspective	Thermal process in the complete absence of externally supplied oxygen.	Thermal process with a limited external supply of oxidizing agents (O2 (or air), steam, CO2, or their mixtures)
Oxygen bound in feedstock (e.g., PET, biomass)	Released as part of the decomposition products; does not affect classification	Released as part of the decomposition products; does not affect classification
Objective	Liquid products or chemicals (e.g., light olefins, benzene, toluene, and xylenes (BTX))	Gaseous product (syngas)

Meanwhile, incineration refers to a process in which the complete oxidation of waste materials is pursued. Figure 1 illustrates the differences among these processes based on the external oxygen supply and the resulting product distributions.

FIGURE 1.

Comparison of thermal routes for waste plastic under varying oxygen conditions.

6. Current Waste Plastic Valorization Landscape and Identified Problems

6.1. Technologies in Current Use

The current technical classification framework for waste plastic valorization expands upon previous classification systems. The newly incorporated or further developed technologies are as follows:

• Dissolution: A process that uses solvents to selectively dissolve and separate specific polymers from waste plastic, recovering material of near‐virgin quality without significant chain scission.

• Subdivided depolymerization: Processes that convert polymers back into their constituent monomers, now collectively termed DP. These processes have been further classified into chemical, thermal, and biological DP. Chemical DP corresponds to the earlier term “solvolysis.”

6.2. Representative Classification Frameworks

Two representative classification frameworks currently in use are examined here to illustrate the nature and extent of existing ambiguities. The Nova Institute classification organizes waste plastic valorization routes along a production chain axis, distinguishing mechanical recycling, chemical recycling, and energy recovery [17]. Within the chemical recycling branch, the framework explicitly includes DP (solvolysis and thermal DP) and thermochemical conversion processes (pyrolysis and gasification). Notably, in this framework, dissolution is classified separately as physical recycling, and the products of pyrolysis and gasification may be directed toward either recycling or recovery (i.e., fuels and energy). A second framework (Figure 2) organizes technologies within a circular economy context, distinguishing a linear model (leading to fuels/energy or landfill) from a circular model that encompasses reuse, mechanical recycling, solvent‐based recycling, and chemical/thermochemical recycling [18]. In this diagram, solvent‐based recycling (i.e., dissolution) appears as a distinct pathway positioned between mechanical recycling and chemical recycling. However, it is not explicitly classified as either physical or chemical recycling, mirroring the broader ambiguity observed across the field.

FIGURE 2.

A circular economy perspective on plastic recycling routes. Reproduced from Ref. [18] (K. Ragaert et al., Curr. Opin. Green Sustainable Chem. 2023, 44, 100871), published under a CC BY 4.0 license. Copyright 2023, The Authors.

A comparative analysis of these two frameworks reveals a shared ambiguity—dissolution consistently occupies an intermediate position that is not reliably assigned to either mechanical or chemical recycling. Beyond this specific case, further examination reveals specific problem areas, as discussed below.

6.3. Problems With Current Classification of Chemical Recycling, Feedstock Recycling, and Dissolution

Chemical recycling and feedstock recycling have historically been treated as synonymous concepts. However, the recent maturation of the dissolution technology as a viable recycling method has introduced significant confusion into established classification systems. The dissolution process, also called solvent‐based purification [19], is a multistep technique comprising polymer dissolution, filtration, precipitation, and solvent extraction [20]. Crucially, the polymer recovered through this process is of near‐virgin quality, with minimal chain cleavage occurring during treatment. Consequently, dissolution cannot be accommodated within the traditional definition of chemical recycling (or feedstock recycling), which inherently requires the breakdown of polymers into low‐molecular‐weight molecules. Since the dissolution phenomenon proceeds via physical rather than chemical transformation of the polymer chains, many researchers have classified it as physical recycling [21, 22, 23, 24]. However, this designation has itself generated confusion because mechanical recycling (extrusion) also involves changes in the physical state of polymers. This has led to three conflicting approaches in the literature: (i) grouping both dissolution and mechanical recycling under physical recycling [25, 26], (ii) treating mechanical recycling as synonymous with physical recycling, thereby excluding dissolution [27, 28, 29, 30, 31], or (iii) treating mechanical recycling and dissolution as separate categories [32]. Such inconsistency inevitably generates conceptual confusion.

6.4. Problems With Depolymerization and Dissolution

6.4.1. Definition and Scope of Depolymerization

DP is usually defined as the process of converting a polymer into a monomer or a mixture of monomers [11]. Currently, there is no formal criterion specifying the level of monomer recovery necessary for classification as DP [33]. Hence, the term DP is also frequently used for pyrolysis, in which polyolefins undergo nonselective cracking and monomer yields are low. In this paper, the term is restricted to processes in which monomers are the primary target product. Currently, various methods are employed to achieve DP, which can be classified into three main categories:

• Solvent‐based: This method primarily targets waste plastic composed of condensation polymers. It is variously referred to as chemical DP [34], solvolysis [35], chemolysis [36], solvolytic DP [37], or chemolytic DP [38] (for clarity, solvent‐based DP is hereafter referred to as chemical DP).

• Thermal‐based: This approach utilizes pure heat and primarily targets polymers such as polystyrene (PS) [39], polymethyl methacrylate (PMMA) [40], and polytetrafluoroethylene (PTFE) [41]. It is commonly known as thermal DP or pyrolysis DP [42, 43].

• Biological/enzymatic‐based: This method employs microorganisms or enzymes to break down polymers, mainly targeting PET, polyamides (PA), and polyurethane (PU). It is classified as biological DP or enzymatic DP [44, 45].

Figure 3 presents reaction pathways for the three depolymerization methods.

FIGURE 3.

Representative reaction pathways for the three depolymerization categories.

6.4.2. Differentiation of Chemical Depolymerization and Dissolution in Classification

Chemical DP, together with dissolution processes, utilizes solvents to recover products from waste plastic. Thus, both dissolution and chemical DP can be collectively defined under the term “solvent‐based recycling” [46]. However, in many studies [47, 48, 49, 50], the term “solvent‐based recycling” is used exclusively to refer to dissolution processes. Figure 4 illustrates the distinction between these two approaches in detail.

FIGURE 4.

Comparison of PET recycling pathways: Dissolution versus chemical depolymerization.

Notably, the two processes differ significantly in their reaction mechanisms, process configurations, and the types and potential applications of the resulting products.

6.5. Reassessing Conceptual Ambiguities and Terminological Inconsistencies in Pyrolysis and Gasification

6.5.1. Conceptual Ambiguities in Defining Pyrolysis and Gasification

Contrary to the definitions previously discussed, in the literature, pyrolysis is sometimes defined as the thermal conversion of carbonaceous materials under oxygen‐free or oxygen‐limited conditions [51, 52]. Such definitions implicitly treat gasification as a subset of pyrolysis, thereby grouping two fundamentally distinct processes under a single conceptual framework. This conflation can lead to significant confusion in distinguishing their respective reaction mechanisms and product characteristics. A related boundary case is the process described in recent studies as “oxidative” or “autothermal” pyrolysis, in which a small amount of oxygen or air is supplied to the reactor to generate heat in situ [53]. Such processes are operated at temperatures lower than typical gasification temperatures and aim at liquid products or light olefins rather than syngas, so that their objective and dominant reaction mechanism are analogous to those of pyrolysis. They therefore lie at the boundary between pyrolysis and gasification, as defined in this paper. Attaching the modifier “oxidative” to a term defined by the absence of externally supplied oxygen risks blurring the very distinction that this paper seeks to clarify. In my view, this ambiguity arises because the conventional definitions of pyrolysis, gasification, and incineration are based not only on the supply of oxygen but also on other factors, such as the main product and the dominant reaction mechanism, including the major role of reforming reactions in gasification; in the case of gasification, the product is even embedded in the name of the process itself. If these technologies could be defined solely by the degree of external oxidant supply, they could be distinguished as pyrolysis, partial oxidation process, and combustion process, and boundary problems such as this one would be resolved more easily. However, such a redefinition would require a fundamental revision of terms and definitions that are already deeply established, with considerable confusion as a consequence; it is therefore beyond the scope of this paper and is left to future discussion among experts in the field. Therefore, it is recommended that oxidative pyrolysis processes be explicitly identified as boundary case and described according to their operating mode. Specifically, such processes should be described as “autothermal pyrolysis with partial oxidation for in situ heat supply,” with the equivalence ratio also being reported.

Meanwhile, pyrolysis is frequently categorized into low‐ and high‐temperature regimes. However, the temperature ranges associated with these categories vary considerably across studies, leading to inconsistency and ambiguity [54, 55, 56]. While it may be appropriate to describe a process as being conducted at temperatures lower than typical operating conditions, referring to it as “low‐temperature pyrolysis,” as if a universally accepted definition exists is potentially misleading. In some cases, arbitrary temperature thresholds (e.g., around 500 °C) are used to distinguish between low‐ and high‐temperature pyrolysis, accompanied by generalized claims that low‐temperature pyrolysis predominantly produces liquid oil, whereas high‐temperature pyrolysis mainly yields syngas [57, 58]. However, such interpretations are the result of overlooking the fact that the product distribution of pyrolysis is strongly dependent on factors such as feedstock properties and catalyst presence. As a result, temperature alone cannot serve as a reliable basis for classification of pyrolysis. For instance, the pyrolysis of polystyrene at approximately 800 °C can still predominantly yield liquid products [59]. Taken together, these definitions either oversimplify the inherently complex behavior of pyrolysis or reflect a conceptual misunderstanding that effectively reduces gasification to a subset of pyrolysis.

6.5.2. Ambiguity in Pyrolysis/Gasification Terminology

A recurrent issue in the literature is the use of the compound term “pyrolysis/gasification process” (or “pyro‐gasification”), which is frequently employed in publications and is often described as comprising two reaction zones: pyrolysis and gasification. However, upon closer examination, several such systems typically operate either as a pyrolysis process or as a gasification process rather than as a true combination of both [60, 61, 62]. The origin of this usage can be traced to some two‐stage processes developed in the 1990s, such as the thermoselect process, which incorporated a pyrolysis unit directly coupled with a gasification unit [63, 64]. In this configuration, the pyrolysis unit serves a preparatory role by converting solid waste into fluid tar, thereby facilitating subsequent gasification. From the perspective of organic conversion, this system is more accurately described as a pyrolysis‐assisted gasification process. Accordingly, the compound term “pyrolysis/gasification” is appropriate only when a pyrolysis stage and a gasification stage, in which an externally supplied gasifying agent (O₂ or air, steam, or CO₂) reacts with the pyrolysis products, are both present as distinct reactors or reaction zones.

6.6. Problems With Energy Recovery Terminology

Two distinct issues arise in relation to energy recovery terminology. First, some publications describe incineration or energy recovery as “thermal recycling” [65, 66, 67, 68, 69]. However, this terminology is fundamentally misleading as the heat generated from the combustion of waste plastic is recovered and utilized but is not reintroduced into the plastic production cycle. From this perspective, energy recovery may be more appropriately classified as a recovery operation rather than a recycling process. Although it is conceivable that carbon dioxide generated from incineration could, in principle, serve as a feedstock for monomer production—thereby constituting a form of chemical recycling—such pathways remain largely at the research stage. Second, the current category of energy recovery is defined solely with reference to the direct incineration of solid waste plastic. However, the combustion of fuel oil and gases derived from pyrolysis or gasification to recover heat or electricity, a pathway that has been studied and developed for many years, also constitutes energy recovery. This indirect form of energy recovery offers significant advantages over direct incineration: impurity precursors in the waste plastic can be removed during the upstream conversion step, yielding cleaner fuels with reduced environmental emissions. Furthermore, the liquid and gaseous fuels produced through pyrolysis or gasification can be combusted in a variety of devices (boilers, gas engines, fuel cells, etc.), offering greater flexibility and the potential for higher energy efficiency. These advantages justify treating indirect energy recovery as a distinct subcategory within the framework.

7. Proposed Revised Framework and Terminology

7.1. Recommended Terminological Revisions

Based on the analysis presented above, the following main terminological revisions are proposed:

• 1.Mechanical/physical recycling → Mechanical recycling
• 2.Solvent‐based recycling (or physical recycling) → Dissolution process (solvent‐based recycling can include chemical DP and dissolution process)
• 3.Pyrolysis/gasification process → Pyrolysis process or gasification process (when distinct pyrolysis and gasification stages both play independent roles), or “pyrolysis‐assisted gasification” (when the pyrolysis stage serves only an auxiliary role)
• 4.Low‐temperature pyrolysis/high‐temperature pyrolysis → Pyrolysis (without temperature‐based subclassification)
• 5.Thermal recycling → Energy recovery

Table 4 summarizes the proposed classification framework, including correspondence between proposed terms and former designations.

TABLE 4.

Summary of proposed terminological revisions with corresponding rationale.

Former designation	Proposed revised term	Rationale
Mechanical/physical recycling	Mechanical recycling	Eliminates ambiguity with dissolution; aligns with process‐based classification logic
Solvent‐based recycling (for dissolution only)	Dissolution process	Distinguishes from chemical depolymerization; solvent‐based recycling becomes an umbrella term
Low‐temperature/high‐temperature pyrolysis	Pyrolysis (no temperature subclassification)	Temperature alone is insufficient and misleading as a classification criterion
Pyrolysis/gasification process	Pyrolysis process or Gasification process (or “pyrolysis/gasification” only when distinct pyrolysis and gasification stages are both present)	Prevents conflation of two distinct thermochemical processes
Thermal recycling	Energy recovery	Combustion products are not reintroduced into the plastics production cycle

7.2. Proposed New Classification Scheme

A revised classification scheme for waste plastic valorization is proposed in Figure 5. This classification method is based on respecting historical backgrounds and existing classification systems while minimizing the diversification into overly complex categorization.

FIGURE 5.

Proposed revised classification scheme for waste plastic valorization technologies.

The key features of the proposed scheme are as follows:

• Dissolution is reclassified under chemical recycling: Although dissolution does not involve chain scission, it relies on chemical solvents and standard chemical unit operations, such as dissolution, filtration, precipitation, and solvent recovery. A potential objection is that, in the absence of molecular transformation, dissolution should not be classified as chemical recycling; indeed, current frameworks classify it as physical recycling on the basis of the molecular‐transformation criterion. In the proposed framework, however, the nature of the unit operation is adopted as the primary criterion for three reasons. First, this criterion is consistent with the original logic of the terminology. When the categories were established in the 1990s, “mechanical” and “chemical” recycling were named after the nature of the operations employed (mechanical operations vs. chemical processes), whereas the molecular‐transformation criterion was adopted later, in frameworks such as ISO 15270:2008 [8] and the JRC report [9]. Second, the molecular criterion alone cannot separate dissolution from mechanical recycling because both retain the polymer backbone. A further distinction is therefore needed, and the resulting term, “physical recycling,” has been applied inconsistently—as a variant of mechanical recycling and as a synonym for mechanical recycling. Classifying both mechanical recycling and dissolution as physical recycling renders the term overly broad, whereas treating physical recycling as synonymous with mechanical recycling leaves no appropriate position for dissolution. A criterion based on the unit operation assigns dissolution unambiguously. Third, the unit operation determines the factors that matter for plant design, safety, cost, and technology assessment, such as solvent handling and recovery, separation steps, and equipment. In these respects, dissolution resembles solvent‐based chemical DP far more than it resembles mechanical recycling. It should be noted that this process‐based classification is proposed as a scientific and engineering framework and is not intended to replace legal definitions, in which “chemical recycling” generally implies a change in chemical structure.

• DP is subdivided into three distinct categories: Chemical DP (solvolysis), thermal depolymerization, and biological depolymerization. Chemical DP employs solvents to break condensation polymer linkages. Thermal DP relies solely on heat to induce chain scission. Biological DP utilizes microorganisms or enzymes to selectively degrade polymer backbones. Although these three pathways differ substantially in their reaction mechanisms, process conditions, and target feedstocks, they share a common defining feature: the primary product is a monomer or monomer mixture intended for reintroduction into the polymerization cycle.

• Chemical recycling and feedstock recycling must henceforth be distinguished: Once dissolution is included within chemical recycling, the historical synonymy between chemical recycling and feedstock recycling is no longer tenable. In the proposed framework, the two terms are, therefore, defined by two distinct criteria. Chemical recycling is defined by the nature of the process: “a process that recovers polymers, monomers, or other chemicals from waste plastic through chemical processes,” where chemical processes refer to the use of chemical reagents, solvents, or thermochemical reactions, as distinct from biocatalytic processes using enzymes or microorganisms. Feedstock recycling is defined by the nature of the product: “a process that converts the polymers in waste plastic back into monomers or other basic chemicals.” Each process can thus be assigned to each category by a separate, explicit criterion. Maintaining both terms conveys information that neither provides alone. Dissolution is chemical recycling but not feedstock recycling, because it recovers intact polymers. Conversely, biological DP is feedstock recycling but not chemical recycling because it recovers monomers through biocatalytic rather than chemical processes. Chemical DP, thermal DP, and thermochemical conversion (pyrolysis, gasification, and hydrogenation) belong to both categories, whereas organic recycling (composting and anaerobic digestion) belongs to neither category.

• Energy recovery is subdivided into two categories: (a) Direct energy recovery—direct incineration of solid waste plastic—and (b) indirect energy recovery—combustion of fuel oil or gas derived from upstream pyrolysis or gasification processes. This distinction is justified by the substantially different environmental profiles and technical characteristics of the two pathways.

Unlike previous frameworks, the proposed scheme eliminates intermediate or ambiguous categories and classifies each process by explicit criteria.

To make the proposed framework operational, Table 5 specifies the three criteria used for classification, namely, (1) whether chemical processes are used, (2) whether monomers or basic chemicals are recovered, and (3) whether the product is used as a material or as a fuel, and demonstrates their application to representative technologies discussed in this paper.

TABLE 5.

Application of the proposed classification criteria to representative technologies.

Technology (example)	Criterion 1: Chemical recycling (uses chemical processes)	Criterion 2: Feedstock recycling (recovers monomers or basic chemicals)	Criterion 3: Final use of the product	Classification
Polymer dissolution	Yes	No	Material	Chemical recycling
PET glycolysis (chemical DP)	Yes	Yes	Material	Chemical and feedstock recycling
PS/PMMA thermal DP	Yes	Yes	Material	Chemical and feedstock recycling
Polyolefin pyrolysis to petrochemical feedstock	Yes	Yes	Material	Chemical and feedstock recycling
Steam gasification to syngas for chemical synthesis	Yes	Yes	Material	Chemical and feedstock recycling
Enzymatic PET depolymerization (biological DP)	No	Yes	Material	Feedstock recycling

8. Conclusions

This paper has systematically identified and examined recurring sources of terminological and classificatory ambiguity in the waste plastic valorization literature and has proposed a revised framework and updated terminology to address them. The principal contributions are as follows:

• 1.The reclassification of dissolution under chemical recycling.
• 2.The explicit differentiation of chemical recycling from feedstock recycling.
• 3.The rejection of temperature‐based pyrolysis subclassification.
• 4.The explicit differentiation between pyrolysis and gasification.
• 5.The elimination of the misleading term “thermal recycling” in favor of energy recovery.
• 6.The disambiguation of energy recovery into direct and indirect forms.

The adoption of a unified, internally consistent classification framework and terminology is a prerequisite for productive academic discourse, coherent research programs, and evidence‐based policymaking in waste plastic valorization. The proposals presented in this article aim to establish such a common foundation, fostering more diverse and in‐depth discussions on valorization technologies—particularly chemical recycling. Furthermore, this unified framework provides a rigorous basis for international policies, such as the Global Plastics Treaty, by ensuring consistent criteria for circularity targets. Clear differentiation of processes enables accurate benchmarking of energy efficiency, thereby guiding industrial investments toward truly sustainable technologies.

Funding

This study was supported by the Korea Ministry of Climate, Energy and Environment (RS‐2026‐25502834).

Disclaimer

The opinions expressed in this publication are the view of the author(s) and do not necessarily reflect the opinions or views of ChemSusChem, the Publisher, ChemistryEurope, or the affiliated editors.

Conflicts of Interest

The author declares no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.