Polyolefins in the Circular Economy: Advances and Recyclability

Abstract

Plastics are widely used across various sectors, with polyolefins such as polypropylene (PP) and polyethylene (PE) making up a significant portion of global consumption. These materials are produced through olefin polymerization using diverse technologies, and their properties are tailored to meet the needs of different industries. Despite their benefits, plastics are often criticized for their environmental impact due to improper waste management, leading to littering on land and in oceans. Therefore, an intelligent and sustainable waste management system is essential to transform plastic waste into valuable resources and prevent pollution.This review article focuses on the production of value-added products through various recycling techniques, particularly the chemical recycling of polyolefins. Current technologies lack the integration of advanced tools such as artificial intelligence (AI) and machine learning (ML), which are necessary for modernizing these processes. Implementing AI and ML into existing technologies presents a significant challenge but is crucial for advancing recycling methods. The objective of this review is to highlight current polyolefin production technologies, address environmental concerns from plastic waste, and explore recycling techniques and novel processes to ensure a circular economy framework for a sustainable plastic value chain.

Keywords

Polyolefins - Technology - Recycling - Chemical and mechanical recycling - Sustainability - Circularity

Significance

This study emphasized that plastic could be a eco-friendly material if the circular economy framework is appropriately implemented for a sustainable plastic value chain. This review highlights technologies for the production of polyolefins and recycling techniques of polyolefin waste with a special focus on chemical recycling.

Introduction

Polyolefins are a class of thermoplastics mainly represented by polyethylene and polypropylene. The global demand for polyolefins is highest among thermoplastics due to the wide range of rigid and flexible applications in addition to excellent properties, including high strength, chemical resistance, and low cost. The market size of polyolefin was valued at USD 256 billion in 2023 and is expected to grow to USD 385 billion by 2031 with a CAGR of 5.2% for various industrial applications [1]. A number of polyolefin technologies are presently employed for the production of high-performance products for different end-use applications. The polyolefins are “one of the greatest innovations of the millennium” and have certainly proved their reputation to be true. Various applications of polyolefins in different sectors like infrastructure, health, transportation, energy transition, defense, and agriculture are depicted in [Figure 1a] [2], [3]. Further advancements in science and technology are pursued for recycling plastic waste into monomers to make polyolefin materials and various other reusable products for all applications beneficial to modern society. This article emphasizes that plastics can be eco-friendly if the circular economy framework is appropriately followed (refuse, redesign/reduce, reuse, repair, refurbish, remanufacture, repurpose) to create a sustainable plastic value chain. The present review covers the current status of polyolefin technology including mechanical and chemical recycling technology with future directions. This article also suggests that current technologies do not incorporate AI and ML, and they need to be updated with advanced tools and methodologies. No article presents a review of the combination of polyolefin technologies and recycling techniques together and its circular economy aspects. This review article aligns with 12 UN’s Sustainable Development Goals, that is, 1, 2, 3, 6, 7, 8, 9, 11, 12, 13, 15, and 17.

Figure 1 (a) Polyolefin application in different sections. (b) Polyolefin technology and products. (c) Polyethylene technology.

Polyolefin technology and products

Various polyolefin technologies are used for the production of high-performance products. The technologies are based on slurry, solution, and gas phase processes. The polymerization process involves the addition reaction of olefin in the presence of a transition metal-based catalyst to produce polyolefins. The presence of a comonomer with a monomer results in the production of copolymers. The polyolefin process is illustrated in [Figure 1b] [3].

The current trend for the advancement of technology includes the development of a high-throughput polymerization process as well as product grades with enhanced properties. Novel proprietary catalyst technologies are also developed to produce polypropylene and polyethylene grades by polymerizing olefin with and without comonomers. Different processing techniques such as injection and blow molding are used to convert resins into end products. Further, the design of high-performance products is achieved by tailoring molecular structures at the production level and incorporating fillers and additives at the processing stage [3].

Polyethylene

Polyethylene grades are the world’s largest synthetic polymers produced commercially by a different class of catalyst systems. The innovation in catalyst technologies opened up a realm for new energy-efficient polymerization processes [3]. Polyethylene synthesis and various process technologies are shown in [Figure 1c]. Slurry and gas phase processes have evolved as better polymerization processes for ethylene. They become economically attractive as the slurry process is operated at low temperature and pressure, whereas the solvent recovery step is eliminated in the gas phase polymerization process. The solution polymerization process acquired new importance because of shorter residence times and the ability to use advanced homogenous metallocene catalysts for specialty polyethylene-based grades [3]. Polyethylene grades are typically classified by their characteristics such as density and melt flow index as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE). LDPE is produced by autoclave or tubular reactor technology having applications in high-clarity film, flexible food packaging, etc. LLDPE is typically made in a gas phase process and its application includes industrial packaging, storage boxes, etc. HDPE can be produced by slurry and gas phase processes for applications such as detergent and milk bottles, thin-walled containers, etc [2] [3] [4] [5]. The representative grades of PE are given in [Table 1].

Table 1 Grades and applications of polypropylene and polyethylene [2].

Grades	MFI range (g/10 min)	Applications
Homo PP grades
Woven sacks and monofilament	2.6–3.4	Raffia tapes used in making woven fabrics and high-tenacity tapes
Films	8–10	Textile overwraps, general purpose packaging, High-tenacity tapes
Extrusion coating	38	Extrusion coating, lamination
Fiber and film	20–38	Staple fiber and multifilament, fiber yarn, and nonwoven
Extrusion and blow molding	2.0	Sheet, strappings, tubing, blow molded container
Injection molding	5–25	Rigid packaging, furniture, and houseware, thin-wall container, multicavity molding, syringe
Impact copolymer PP grades
Extrusion and blow molding	1.5	Sheet blow molded container
Extrusion coating	25	Extrusion coating
Injection molding	3–65	Furniture industrial products, houseware, general purpose molding, batteries and automotive, appliances automotive, compounding
LLDPE grades
Film grades	1–4.5	Mono- and multilayer films for liner bags, industrial consumer packaging, shrink film, shipping sacks, agricultural film, blown films for co-extrusion, stretch wrap, cling film for food or hygiene packaging, bubble wrap
Roto molding	4.5	Overhead water tank, storage tank for agriculture, loft tank, litter bin, toy items
Drip lateral	1.0	Drip irrigation tubing for online laterals/flat and round emitting pipes

Polypropylene

PP grades are the world’s second-largest synthetic polymer produced mainly by Ziegler-Natta catalyst systems. Polypropylene resins are produced in large, continuous reactor systems operating in slurry, bulk, or gas phase. Generally, PP resin is pelletized on compounding extruders where additives such as antioxidants and processing aids are incorporated. PP is categorized as homo, random, and impact depending on their molecular structures and properties. Polypropylene (PP) synthesis and different processes are shown in [Figure 2] [4], [6], [7]. Different grades of polypropylene with respect to melt flow index (MFI) and its applications are given in [Table 1] [2] [3] [4].

Figure 2 Polypropylene technology.

Polyolefin linkages with sustainable development goals (2030)

Polyolefins are an important class of material essential for accomplishing different aspects of the UN 17 Sustainable Goals 2030 as shown in [Figure 3]. Polyolefins are important for low-cost housing development and cheaper household appliances, which can play a very crucial role in reducing world poverty. Polyolefins are being used in sustainable and cheaper food packaging to support the zero hunger goal. These materials are being used extensively in the medical and health sectors for human well-being. The use of PP-based masks saved human lives in the recent COVID-19 pandemic. Polyolefins are being used to make also IV bottles, syringes, gowns, shoe covers, and many others. Polyolefins are also used in various products like water filters and other sanitary products connected with clean water and sanitation goals. PE and PP grades are used in battery cases and separators for energy production. The composites of polyolefins enhance performance characteristics and are lightweight materials used in electric vehicles contributing to economic growth. PP and PE materials are also used in building and infrastructure sectors. Polyolefins are used in agriculture domains for mulch film, greenhouses, pipes, drip lateral tubes, sprinklers, etc. impacting life on land [3], [8]. Plastics are not inherently bad. Proper plastic waste management and recycling are key to reducing plastic pollution from the environment. Plastic waste can generate valuable chemicals and other new low-cost products using various technology contributing to sustainability and circularity domain.

Figure 3 Correlation of polyolefins with UN sustainable development goals.

Plastic waste recycling methodology and applicability conditions

The methodology for plastic waste recycling offers a process to estimate the additional amount of plastic waste recycled through both chemical and mechanical recycling activities, which includes the following [9]:

• Installation of new recycling facilities.

• Capacity additions or technology improvement of existing recycling facilities.

• Incentivizing or facilitating an increase in recycling through the collection of plastic waste.

Recycling activities may include collection, sorting, washing, drying, crushing, melting, pelletization, and making of new products and/or recycling of plastic waste that otherwise would have been managed in a way that would not allow for a second life of the material.

The key components of the methodology include:

• Scope and applicability

• Baseline scenario

• Activities

• Quantification of emission reductions

• Monitoring and reporting

• Co-benefits

Applicability conditions

Plastic waste recycling methodology is applicable when the following conditions are fulfilled [9].

• Demonstrate additional recycling: Show a measurable increase in plastic waste recycling compared to a baseline. Project activities should involve mechanical and/or chemical recycling processes.

• Meet quality standards: Ensure that waste plastic is free from hazardous materials and that the recycled plastic meets specific quality standards suitable for high-value applications.

• Address environmental and social impacts: Identify, assess, and mitigate potential environmental and social impacts associated with recycling activities.

• Comply with regulations: Adhere to all relevant local, national, and international laws and regulations. Recycling projects can generate waste recycling credits (WRC), which can be traded or sold, offering financial incentives to support waste reduction initiatives.

Circularity through plastic waste recycling

The amount of plastic that is produced in the world every year has increased explosively since 1950, that is, 2 MMT to 400 MMT in 2023 [10] [11] [12] [13]. Global plastics use is projected to continue rising over the coming decades to reach more than 1.2 bn metric tons by 2060 [11], [14] [15] [16]. Global plastic waste generation has more than doubled from 2000 to 2022 to approximately 400 MMT. Plastic waste is projected to increase substantially in the coming decades, rising from 353 MMT in 2019 to 1014 MMT in 2060 [11]. Approximately 57 MMT/annum of plastic waste is being generated globally. India has emerged as the largest contributor and produces approximately 10.2 MMT of plastic waste each year, which is nearly 18% of the global total. Nigeria ranks as the second-largest producer of plastic pollution, generating 3.9 MMT annually whereas the USA produces approximately 0.05% of the global total [14]. Very few parts of globally produced plastics are being recycled after their use, which is currently 9%. The rest of 19% is incinerated, 50% goes to landfills, and 22% of the waste goes into uncontrolled dumpsites or aquatic environments [11], [13]. The share of recycling as a waste-management practice is projected to rise to 17% in 2060, which is up from 9% in 2019 [11]. Recycling polymers is the way to minimize environmental issues caused by polymeric waste accumulation produced after its complete end-use applications from various sectors. Recycling polymeric waste materials helps conserve feedstocks because most of the polymer materials are made from fossil-based resources. Recycling of polymers is mainly categorized into two parts: mechanical and chemical recycling [17].

Mechanical recycling

Mechanical recycling refers to the re-extrusion of plastic waste to convert it into pellets or products, without significantly changing the chemical structure of the original material. In other words, the polymer chains are not chemically disrupted in the recycling process. Mechanical recycling is also known as physical recycling [12], [17]. Mechanical recycling of polymer contains several steps as explained in [Figure 4a] that involves shorting, washing/drying, shredding, and extrusion to make granulates, fakes, or pellets of the right size and quality, which may then be utilized in the right industrial processes [18] [19] [20]. To create a newer product, the pellets are further heated and extruded [21]. Re-extruded material can also be used with fresh material for superior properties. Since thermoplastic polymers may be re-melted, processed, and molded into final products, only such materials are kept in this procedure [22]. The properties like elongation at break and impact strength of mechanically recycled thermoplastic polymers may change and do not remain the same because of thermal degradation from heat, mechanical stress/friction, etc [18]. Recycled polymer properties are subject to a number of extrusion cycles or thermal passes of polymers like polypropylene/polyethylene in the extrusion technique. The MFI increases as the number of extrusion cycles increases for polypropylene due to a reduction in molecular weight as shown in [Figure 4b]. The change in molecular weight has a strong influence on the rheological and mechanical behavior of thermoplastic polymers. Molecular weight and mechanical properties like tensile strength and elongation at break decrease as the extrusion cycle increases [18], [23].

Figure 4 (a) Steps in mechanical recycling of polymer. (b) MFI of polypropylene reprocessed multiple times. (c) Chemical recycling routes.

Chemical recycling—catalytic pyrolysis

Chemical recycling is a process in which polymeric waste can be converted into monomers, oligomers, and other raw materials by changing its chemical structure that can be used as raw materials for the manufacturing of plastics or other products. There are various chemical recycling technologies, such as pyrolysis, gasification, hydro-cracking, and depolymerization, as depicted in [Figure 4c] [18]. Chemical recycling is mainly for films, laminates (single layer/multilayer), or single-use plastic articles. It can also be used for plastic waste, which would otherwise result in incineration or landfill. The chemical recycling route can be categorized into three: pyrolysis/gasification, depolymerization, and dissolution. In chemical recycling, pyrolysis is an interesting technology to convert plastic waste into useful feedstock using thermal catalytic and noncatalytic steps. Depolymerization is an outstanding strategy for recycling waste plastic into constituent monomers in the presence or absence of a catalyst. Dissolution/precipitation is a unique step for single-use plastic (monolayer/multilayer) to remove additives, colors, or other impurities to make the polymer waste similar to virgin polymer [18].

Transition metal catalysts are used for the catalytic pyrolysis of plastic waste into hydrocarbon oil and other products. Advantages of transition or bimetallic catalysts are large surface area, higher thermal stability, and synergy effect between the metals. Transition metal or bimetallic catalyst enhances hydrogen (H₂) fraction many fold compared to the process without a catalyst. Major products formed during cracking/pyrolysis using bimetallic catalysts are carbon nanotubes/nanofibers and H₂ gas as shown in [Table 2] [24].

Table 2 Bimetallic catalysts and zeolite used for polyolefin pyrolysis.

Catalysts	Polyolefin	Outcomes	Reference
Mo–MgO and Fe	Low-density polyethylene (LDPE)	High-quality carbon nanostructure materials	[25]
La2O3 and Ni–Cu	Polypropylene (PP)	Carbon nanotubes and carbon nanofibers	[26]
MgO and Ni/Mo	Polypropylene (PP)	Carbon nanotubes	[27]
Ni and Al	Polypropylene (PP)	Multiwalled carbon nanotubes	[28]
Ni–Al and Zn, Mg, Ca, Ce, Mn	Polypropylene (PP)	Carbon nanotubes	[29]
Ni and Ce	Polyethylene (PE)	H2 gas	[30]
Ni and Fe	Low-density polyethylene (LDPE)	Carbon nanotubes	[34]
MgO and Co/Mo	Low-density polyethylene (LDPE)	Multiwalled carbon nanotubes and H2 gas	[31]
Ni and Fe	Polypropylene (PP)	Carbon nanotubes (CNT)	[32]
Ni and Mn–Al	High-density polyethylene (HDPE)	Carbon nanotubes and H2 gas	[33]
Y-zeolite	Polystyrene (PS) and mixed polyolefins	90% aromatics	[35]
USY-zeolite	Polypropylene (PP), polyethylene (PE)	80% C5–C7 along with gaseous fraction C3–C4	[36]
Co-Y-zeolite	High-density polyethylene (HDPE)	40% of gaseous fraction	[38]
Regenerated ZSM-5	Mixed Plastic (HDPE/PP/PS/polyethylene terephthalate (PET)/polyvinyl chloride (PVC))	60% liquids fraction	[39]
ZSM-5	Mixed plastic (PE/PP/PS/polyethylene terephthalate (PET)/polyvinyl chloride (PVC))	58.4% conversion to gases phase	[40]

Zeolite catalysts are very unique and efficient for the pyrolysis of polyolefins and mixed plastics. Catalytic pyrolysis of polypropylene, polyethylene, and mixed plastic in the presence of various zeolites produces liquid fractions as well as gaseous fractions [36] [37] [38] [39] [40]. The biomass feedstock consists of extractive (0–14%), lignin (16%), hemicellulose (20%), and cellulose (>40%). Co-feeding of biomass feedstock in the pyrolysis process of polyolefins can be used to improve the quantity and quality of the product [24]. The catalytic pyrolysis process for the conversion of polypropylene (PP) resin and polypropylene resin/sugarcane bagasse biomass is studied as a possible route to produce hydrocarbon oil and carbon nanotube in quartz reactor separately. The unique transition metal-based catalyst [Ni/Mo/Mg(OR)₂] was used to produce desired products through a pyrolysis process. The catalyst used for the process was prepared by the sol-gel method. Catalytic pyrolysis of polypropylene (PP) and polypropylene/biomass produced hydrocarbon oil in a temperature range of 350–500 °C and multiwall carbon nanotube obtained at 800 °C in a quartz reactor. GC–MS, SEM, and Raman spectra confirm the formation of C6–C24 hydrocarbon oil and multiwall carbon nanotube [41], [42]. A lot of studies have been carried out in the past on co-feeding of biomass and polyolefins pyrolysis and outcomes are mentioned in [Table 3].

Table 3 Plastic/biomass feed using zeolite-based catalysts and their products.

Catalyst	Biomass/polyolefin	Outcomes	Reference
Pt/mesoporous MFI, Mesoporous MFI, HZSM-5, Al-SBA-16 (silica mesoporous)	Laminaria japonica/PP	60% conversion, 20–40% of monoaromatic hydrocarbons, 5–20% of polyaromatic hydrocarbons	[43]
ZSM-5	Cornstalk/HDPE	90% conversion, 20-30% of aromatic hydrocarbon	[44]
Ni-CaO-C	Pine sawdust/LDPE	90% conversion, 86.74% H2 gas	[45]
Al-SBA 15 (Silica mesoporous), ZSM-5	L. japonica/PP, cornstalk/polystyrene	30% conversion, 35% oxygenated hydrocarbons. 79% of monoaromatic hydrocarbon	[46], [47]
Ni/-Al2O3	Rice husk/PE	80% conversion, 45% H2 gas	[48]

Chemical recycling—hydrothermal liquefaction (HTL)

Hydrothermal liquefaction (HTL) is also one of the important processes in the chemical recycling domain. The hydrothermal liquefaction (HTL) process comprises the shredding/washing and plastic waste dissolution in aqueous solvent in the presence of high temperature and pressure. The obtained mixed gases go to the gasifier section and then enter into the separator via the condenser to get the oil and gaseous parts separately as explained in [Figure 5a]. HTL is one of the best available techniques for recycling from various types of plastic residues and biomass [49].

Figure 5 (a) Block flow diagram of HTL process. (b) Degradation Pathway of polyolefin in HTL process.

The hydrothermal liquefaction process can be utilized for mixed plastic waste recycling for fuel, gas, and other raw material production. Plastic or mixed plastic may be used in the HTL process at subcritical or supercritical conditions [50]. Recycling of polyolefin using the HTL process is studied in detail in the literature. Su et al. (2004) studied the effects of various parameters like reaction time and temperature on the water-to-polyethylene ratio. Hydrocarbon oil and gas are obtained as products along with residue. Hydrocarbon oil yield decreased with increasing reaction time and temperature, resulting in an increase in the gas yield. The polyethylene/water ratio not only affected the yield of the products but also altered their chemical composition. Moriya et al. (1999) compared the hydrothermal cracking of polyethylene with supercritical water to thermal cracking without water. The formation of coke was suppressed, resulting in a higher oil yield and a slower degradation of polymers during the supercritical water cracking process [51] [52] [53] [54] [55] [56]. Jin et al. (2021) [56] pursued their efforts and studied a new low-pressure (∼2 MPa) hydrothermal processing method to convert polyolefin waste, but in these cases, they added 50% or more polypropylene to convert this combined feed into oils with 87% yield at 450 °C and 45 min as shown in [Table 4].

Table 4 HTL process for polyethylene and polypropylene.

Polymer	Polymer/water (g/g)	Temperature (°C)	Pressure (MPa)	Conversion (%)	Product (%)			Reference
					Oil	Gas	Residue
HDPE	01:05	425	23	96.7	90.2	6.5	3.3	[51]
PE	01:06	460	23	96.9	91.4	5.5	1	[52]
PP	–	425	23	96	88	8	2	[53]
PE	05:30	400	25	-	87.4		9	[54]
PP	05:30	400	25	100	86.4		–	[55]
PP	40:70	450	1.55	100	87	13	–	[56]

Mukundan et al. (2022) studied the recycling of polypropylene and biomass (Prosopis juliflora) to obtain hydrocarbon oil and gases using the HTL process with and without catalyst in the temperature range of 340–440 °C. The concentration of PP was taken in the range of 25–75%. Nb-based catalysts depicted high efficiency for the deoxygenation of liquid biomass compounds, which resulted in high hydrocarbons with lower oxygen content. Nb/Al₂O₃ catalyst was the most stable up to 10 cycles among all catalysts used for this study [57]. Feuerbach et al. (2024) studied the HTL process of mixed plastics like low-density polyethylene, polypropylene, polystyrene, polyethylene terephthalate, and restaurant food waste. Mixed plastic feedstocks were used in the fraction of 0-50% and the temperature range of 290–370 °C for 30–60 min. Authors reported negative synergistic interactions however literature suggests positive synergistic interactions in hydrothermal co-liquefaction of biomass and plastics [58]. The supercritical water degradation of low-density polyethylene (LDPE) was investigated in a batch reactor at 380–450 °C, 220–420 bar, and a reaction time of 15–240 min. Hydrocarbon oil, gas, and solids are obtained as a product. Alkanes, alkenes, cycloalkanes, aromatics, and alcohols were present in the hydrocarbon oil phase, while the gas phase mainly contained light hydrocarbons (C1–C6). Further high temperatures (425–450 °C) and reaction times from 30 to 240 min, the concentrations of long-chain hydrocarbons (>C20) and diesel (C9–C20) fraction in the oil phase decreased, while the concentration of gasoline (C6–C8) increased [59] [60] [61].

The depolymerization of PE/PP quickly starts at a short reaction time at a higher temperature (≥425 °C) with free radical dissociation, where C–C bonds break. At the same time, long-chain alkanes are converted first into α-alkenes with β-scission reaction and second into shorter-chain alkanes via hydrogen abstraction. During this period, a small amount of 1-alkenes undergoes hydration to form alcohols. Specifically, 1-alkenes, such as 1-pentadecene, are converted into alcohols like n-pentadecanol through a hydration reaction, where water molecules replace the π bond on the alkene. Further increase in reaction time and temperature the 1-alkenes are converted to alicyclic hydrocarbons via cyclization reaction. Small proportion of alkenes could convert to alkanes with the hydrogenation process. With increased reaction time alicyclic unsaturated hydrocarbons are dehydrogenated to aromatic compounds ([Figure 5b]). Further, the yield of gas products obtained in the gasification process increased via the cracking of alkanes and alkenes. Generally, n-alkanes are dominant in the oils and gases converted from PE, whereas alkenes are dominant in the oils and gases converted from PP [59] [60] [61]. Supercritical water plays an important role in the HTL process that causes partial dissolution of the molten polymer phase. The partial dilution of the polymer phase promotes polymer dissociation and unimolecular reactions like β-scission. In the HTL process polyolefins produced maximum hydrocarbon oil yield at supercritical temperatures. HDPE and LDPE produced oil from 425 and 400 °C onwards, respectively, while PP gives the best yield at 400 °C and higher [62]. Abubakar et al., 2024 studied the catalytic hydrothermal process of polypropylene. Base and acid catalysts K₂CO₃ and HZSM-5 were used for this study. Propene monomer was the main product along with 9% butene when the K₂CO₃ catalyst was used but 22% butene and a lower fraction of propene were obtained after the use of the HZSM-5 acid catalyst [63].

Lee et al. (2024) proposed a novel process for producing monomers like propylene and ethylene from mixed polypropylene and polyethylene using low-pressure hydrothermal liquefaction and microwave steam pyrolysis from the chemical recycling domain. A total of 53% olefinic materials (ethylene and propylene) were obtained in this process. They have also studied the cost calculation of ethylene in the proposed process, i.e., 0.89 USD/kg, which is 72.86 % less as compared to flash pyrolysis [64].

Chemical recycling—thermal oxidation and dissolution/precipitation

Mechanical recycling of polyolefins is cheaper but provides another degraded product as an output. Degradation of polyolefins using chemical recycling produces good feedstocks. Chemical recycling roots are developed without disturbing the properties of obtained raw materials. However, these recent technologies are still far behind existing fossil-based technologies, leading to cheaper monomers. This problem could be possibly avoided by other suitable thermal oxidation of polyolefins to oxidized functionalities such as dicarboxylic acids and others. Polyethylene can be converted into high-value dicarboxylic acids, such as succinic and adipic acid, which have a significantly higher value compared to mechanically recycled Polyethylene [65], [66].

Polyethylene is converted into aliphatic oxidized products, such as methyl ketone acid and alkyl acid, at 130 °C, for 16 h, and at 30 bar, as shown in [Figure 6] [66], [67]. Chen et al. (2024) also studied two novel strategies to recycle polyolefin waste into surfactants, alcohols, and aldehydes [68]. Zefirov et al. (2022) studied and reported the chemical recycling of polypropylene through the thermal oxidative route. Polypropylene decomposed at 150 °C and 14 bar, mainly into acetic acid. Methanol, formic acid, and propionic acid were also detected as products. They have also studied the effect of different reaction media and supercritical CO₂ [69]. Orozco et al. (2022) studied the fast-oxidative pyrolysis of high-density polyethylene. Oxidative pyrolysis is an important alternative to produce hydrocarbon oil and gases from waste plastic. It is also a scalable process from an industrial-scale process point of view. The use of FCC catalysts under oxidative conditions enhances the conversion yield of oil and gaseous products. Conventional catalytic pyrolysis produces mainly gasoline fractions, whereas oxidative pyrolysis gives more light olefin fractions. Pyrolysis in the presence of oxygen contributes to the reduction in heavy oil fraction by approximately 50% [70]. Oxidative thermal degradation of PP was also studied at 250 °C under 16.7 kPa oxygen pressure in a fixed bed reactor system. PP converted into 90% volatile which has a number-average chain length of 10 [71].

Figure 6 Oxidative conversion of polyethylene into dicarboxylic acids.

A new approach has been studied by Samori et al., 2017 for the separation of polyethylene and aluminum from food packaging. They used N, N-dimethylcyclohexylamine solvent, which separated both metal and plastic from each other and allowed >99% for aluminum and approx. 80% for polyethylene [72]. Multilayer film made of polyethylene (PE) and polyethylene terephthalate (PET) was separated and precipitated by using a mixed solvent of dimethylsulfoxide (DMSO) and toluene. Toluene helped to remove the PE layer along with EVA contamination when heated to 110 °C for 4 h. Separated polymers were characterized to understand their identification using different tools [73], [74]. PET separated when DMSO solvent was mixed in PET and heated to 90 °C for 30 min. Antisolvents used in this process are acetone and water separately for precipitation purposes [73]. Samori et al. (2023) also studied the recycling of multilayer packaging waste separation. Multilayer packaging waste containing aluminum and LDPE separated using various solvents like biodiesel, 2-methyl tetrahydrofuran (2-MeTHF), and cyclopentyl methyl ether (CPME) in the temperature range of 70–150 °C and reaction time range 10–300 min [75]. Various solvents and antisolvents used for dissolution precipitation are mentioned in [Table 5].

Table 5 Solvents used for the dissolution process.

Polymer to be separated	Solvent	Antisolvent for precipitation	Reference
LDPE	Xylene	Isopropanol	[76], [77]
LDPE	Biodiesel	Ethanol	[75]
PE	Xylene	n-Hexane	[78]
PE	Toluene	Acetone	[77], [79]
HDPE	Xylene	n-Hexane	[78]
HDPE	Xylene	Isopropanol	[76]
PP	Xylene	n-Hexane	[78]
PET	γ-Valerolactone	–	[74]
Polyvinyl chloride (PVC)	Dichloromethane	Methanol	[78]
Polyethylene terephthalate glycol (PETG)	Mixture of DMF/THF (60/40)%	n-Propanol	[80]
Ethylene vinyl alcohol copolymer (EVOH)	Mixture of DMSO/Water (60/40)%	Cooling to lower temperature (35 °C)	[80]

Limitations and challenges of polyolefin materials and their recycling

Polyolefin materials are solution providers for the current need for products for different growth sectors due to their low cost and wide range of product properties. However, exponential growth in the uses of polyolefin and its non-biodegradable nature has resulted in poor management of end products causing environmental concerns. This challenge of polyolefin waste management is being addressed by developing different recycling processes as well as an efficient management system of collection and recycling. Furthermore, the non-biodegradable nature of polyolefin for packaging applications is being addressed by developing alternate biodegradable and bio-compostable materials for single-use applications [2]. Recycling of plastic waste is a need of current time. However, mechanical and chemical recycling also poses another challenge to microplastic generation during the process. Further, the mixing of plastic waste with other contaminants results in odor, etc. requiring special treatment before waste is subjected to the recycling process. This also requires redesigning or modifying the recycling process and product through a reduction of the number of materials and grades used in the end product as well as simplification of product formulation. New robust technologies like artificial intelligence (AI) and watermarks need to be incorporated into identification/sorting in recycling processes to improve the efficiency and identification of various plastic components [17], [18].

Mechanical recycling has limitations related to its process which weakens the plastic properties over time whereas chemical recycling provides polymers without deterioration in performance. Chemical recycling dominates over mechanical recycling where mixed, contaminated, and other plastic waste is recycled to get top-grade products ([Figure 7a]). Various technologies are now close to commercializing for recycling of plastic into monomers or feedstock [81].

Figure 7 (a) Circularity of plastic waste. (b) 9R concept of plastic waste management.

Multilayer flexible packaging is efficient and wonderful for food packaging. It is made up of different layers of different polymers. Used polymers in multilayer packaging film have different properties like melting points, thermal stability, and glass transition temperature, due to that multilayer packaging films face difficulty in recycling. Multilayer packaging films are not sorted and mechanically recycled with the currently available infrastructure. To overcome these issues, flexible multilayer packaging needs to be designed with polyolefin materials and another barrier layer also should be recyclable [82].

Plastic recycling has multiple challenges like sorting of plastic, purification against various contaminants, and deodorizing of plastics, which created a lot of issues and increased the cost of the recycling process that need to be addressed. Various washing procedures are already in use, but basic insights into the deodorization efficiencies of different washing media/steps are still to be improved. Roosen et al. (2022) reported various methods for deodorizing plastic waste during recycling, which is suitable for polyolefin and PET. The addition of detergent or an organic solvent along with caustic soda and water increased the deodorization efficiency of plastic waste [83]. Anouar et al. (2015) studied the purification of postconsumer polyolefins via the supercritical CO₂ extraction process. The supercritical CO₂ extraction process has great potential to reduce the amounts of contaminants in postconsumer polyolefins without changing their properties. This process is more efficient as compared to the traditional liquid extraction process using methylene chloride [84]. The growing demand for plastic globally enhances the problem of global plastic pollution in land and ocean. To overcome or minimize this issue an increased and improved plastic recycling process is needed. Micro- and nanoplastic generation during the washing of plastic waste is a major challenge, which is creating a lot of issues related to life on the land and in the ocean. The current filtration system can remove efficiently microplastics >40 μm. Microplastics <5 μm were generally not removed by filtration and subsequently discharged to the water body during plastic recycling, which needs to be stopped. Brown et al. (2023) studied and suggested microplastic pollution mitigation by recommending an additional filtration system that removed the majority of microplastics <5 μm [85]. To reduce plastic waste-related issues, we need to adopt the new concept of waste prevention. The ban on single-use plastic forced the scientific society to think about the best circularity path based on the 9R framework, such as refuse, reduce, reuse, repurpose, and recycle. We need to create public awareness of the importance of the 9R concept for plastic waste management as shown in [Figure 7b]. 9R concept teaches that plastic items that are not recycled in principle can be refused to use and looked for alternatives. Plastic wastes accumulated in our environment can be recovered, repaired, refurbished, and recycled to reduce, reuse, and redesign to achieve a circular economy and inhibit the environmental issues related to waste plastic and microplastic [86], [87].

Circularity and sustainability aspect of polyolefin

Polyolefins are the best example in the chemical recycling domain, which convert used plastic into monomer and that monomer is used as a raw material to produce again polypropylene and polyethylene and their functional polymers using various polymerization techniques as shown in [Figure 8]. Pyrolysis of low-density polyethylene (LDPE) in the presence of β-zeolite and silica-encapsulated platinum nanoparticles resulted in naphtha, which is the main feedstock for ethylene production. Various other catalysts like zirconia nanoparticles on silica pores (ZrO₂–SiO₂) and ZSM-5 zeolite, with an atomic Si/Al ratio of 21 converted polyethylene and polypropylene into light alkanes [88] [89] [90]. Pyrolysis oil obtained from pyrolysis of polyolefin waste is a useful component of chemical recycling. Further conversion of pyrolysis oil into low-carbon olefin is a necessary step toward circularity. Pyrolysis oil from polyolefin waste can be used in steam crackers to produce ethylene, propylene, and other chemicals. These monomers as a feedstock can be repolymerized into virgin polymers [90]. Wang et al. (2022) reported depolymerization of polyethylene waste to its constituent monomer ethylene, which is further converted into propylene using tandem catalytic conversion via ethenolysis. This propylene monomer from polyethylene can be repolymerized into polypropylene using a conventional process [91].

Figure 8 Circularity and sustainability of Polyolefin.

Conclusion and future directions

Polyolefins are produced by catalytic polymerization of olefins in slurry, solution, and gas phases. The advancements in technology have resulted in high-throughput production of different grades of polypropylene and polyethylene. These grades are used for various end-use applications in different current and emerging growth sectors like packaging, agriculture, transportation, defense, and energy transition. This has resulted in a continuous increase in the demand for polyolefin products touching market potential of USD ̴ 385 billion by 2031. The factors such as high growth, wide range use, and low cost of polyolefin material in the last few decades have led to a significant accumulation of waste polyolefin. The prolific use and non-biodegradable nature of polyolefin in the packaging sector also added another dimension of environmental concerns due to poor waste management systems of collection and microplastic generation on land as well as in water bodies. These challenges are currently being addressed in the framework of the circular economy.

Chemical recycling is currently getting more attention as an approach to converting waste into useful products and chemicals to minimize the use of current oil-based resources to meet the growing demand for polyolefin. In chemical recycling technologies hydrothermal liquefaction (HTL), thermal oxidation and dissolution/precipitation processes are being developed. HTL process can play an essential role in the recycling of polyolefins in the future due to its lower energy consumption and less complexity involvement. The dissolution/precipitation process is very useful for single and multilayer recycling and can play a pivotal role in the chemical recycling domain for giving new life to plastic waste. The pyrolysis process is very popular and plays an important role in chemical and other monomer synthesis from plastic waste. These chemicals and monomers are further used as raw materials to produce various polymers such as polypropylene, polyethylene, and other functional polyolefins using different polymerization techniques. A combination of chemical and mechanical recycling is an example of circularity that diverts plastic waste from being sent to landfills or incineration and back into use. Reduce, refuse, reuse, refurbish, rethink, repair, remanufacture, repurpose, and recycle are key steps for a robust plastic waste management system that follows sustainability and circularity. These 9Rs are essential in reducing and managing plastic waste generation, enhancing resource efficiency, and minimizing the negative impact of plastic waste on the environment.

Future advancements in polyolefin production and recycling technologies will be based on digital tools like AI/machine learning (ML)/deep learning (DL) and flexible production units. In the case of recycling, AI-powered tools can monitor and neutralize odors during the recycling process, which can reduce the risk of toxic diseases. Future directions should also be more centric toward closed-loop recycling systems to create a circular economy by continuously recycling waste plastics into premium quality products. While other methods, such as mechanical and chemical recycling, offer valuable benefits and complement the overall recycling ecosystem, closed-loop recycling systems enhance sustainability, efficiency, and economic savings, reduce pollution, and help to meet regulatory standards.

Abbreviations

AI: Artificial Inteligence

Al: Aluminum

Al2O3: Aluminum oxide

Ca: Calcium

CaO: Calcium oxide

Ce: Cerium

Co: Cobalt

Dl: Deep learning

EVA: Ethylene vinyl acetate

EVOH: Ethylene vinyl alcohol copolymer

Fe: Iron

HTL: Hydrothermal liquefaction

HDPE: High-density polyethylene

HP: Homopolymer

ICP: Impact copolymer

La₂O₃ : Lanthanum (III) oxide

LDPE: Low-density polyethylene

LLDPE: Linear low-density polyethylene

MFI: Melt flow index

MgO: Magnesium oxide

ML: Machine Learning

Mn: Manganese

Mo: Molybdenum

Nb: Niobium

Ni: Nickel

PP: Polypropylene

PE: Polyethylene

PET: Polyethylene terephthalate

PETG: Polyethylene terephthalate glycol

PVC: Polyvinyl chloride

PS: Polystyrene

RCP: Random copolymer

SiO₂ : Silicon dioxide

Zn: Zinc

ZrO₂ : Zirconium dioxide

Contributors’ Statement

VK Gupta: Manuscript concept and idea, critical revision of the manuscript; Saurabh Tiwari: Data collection/interpretation, Drafting the manuscript and corrections; Vivek Tembhare: Data Collection.

Conflict of Interest

The authors declare that they have no conflict of interest.

Acknowledgment

The authors would like to acknowledge Reliance Industries Limited (RIL) for providing the resources and support for this work.

• References

• 1 SNS Insider Strategy and Stats. Polyolefin Market Report Scope & Overview 2023 Online: www.snsinsider.com/reports/polyolefin-market-2704 (last accessed September 27, 2024)
• 2 Gupta VK, Sapre AV. J. Mod. Polym. Chem. Mater. 2023; 2: 5
• 3 Trivedi PM, Gupta VK. J. Polym. Res. 2021; 28: 45
• 4 Gupta VK, Thakare YN, Desai BK. Olefins to polyolefins: Science and technology trends. In Advances in Petroleum Engineering II: Petrochemical. Studium Press; USA: 2015
• 5 Gupta VK, Bhajiwala HM, Dhamaniya S, Kalita A, Joshi R. US Patent 10344103 2019
• 6 Kumawat J, Gupta VK, Vanka K. ChemCatChem 2016; 8: 1809-1818
• 7 Kumawat J, Gupta VK, Vanka K. J. Phys. Chem. C 2017; 122: 285-296
• 8 The United Nations. The 17 Goals of Sustainable Development – The 2030 Agenda for Sustainable Development 2024 Online: www.sdgs.un.org/goals (last accessed September 27, 2024)
• 9 Verra SP. PWRM0002 Plastic Waste Recycling Methodology, v1.1 2022 Online: https://verra.org/methodologies/pwrm0002-plastic-waste-recycling-methodology-v1-1/ (last accessed August 12, 2024)
• 10 Statista. Distribution of global plastic materials production (2022) by region. Online: www.statista.com/statistics/281126/global-plastics-production (last accessed September 25, 2024)
• 11 OECD Library. Global Plastics Outlook: Policy Scenarios to 2060, OECD Publishing, Paris (June 2022). Online https://www.oecd-ilibrary.org/environment/global-plastics-outlook_aa1edf33-en (last accessed September 20, 2024
• 12 Kolluru S, Thakur A, Tamakuwala D, Vishnu VK, Ramakrishna S, Chandran S. Polym. Bull. 2024; 81: 1-42
• 13 OECD Library. Plastic pollution is growing relentlessly as waste management and recycling fall short (February 2022). Online www.oecd.org/en/about/news/press-releases/2022/02/plastic-pollution-is-growing-relentlessly-as-waste-management-and-recycling-fall-short.html (last accessed September 15, 2024)
• 14 Chemical Industry Digest. India emerges as leading producer of plastic waste (2024). Online www.chemindigest.com/india-emerges-as-leading-producer-of-plastic-waste-study-reveals (last accessed September 27, 2024)
• 15 Lau W, Shiran Y, Bailey R, Cook E, Stuchtey MR, Velis CA, Godfrey L, Boucher J, Murphy MB, Thompson RC, Jankowska E, Castillo AC, Pilditch TD, Dixon B, Koerselman L, Kosior E, Favoino E, Gutberlet J, Baulch S, Atreya ME, Fischer D, He KK, Petit MM, Sumaila UR, Neil E, Bernhofen MV, Lawrence K, Palardy JE. Science 2020; 369: 1455-1461
• 16 Lebreton L, Andrady A. Palgrave. Commun. 2019; 5: 1-11
• 17 Hamad K, Kaseem M, Deri F. Polym. Degrad. Stab. 2013; 98: 2801-2812
• 18 Kim R, Laurens D, Kevin VJ. Waste Manage. 2017; 69: 24-58
• 19 Goodship V. Plastic recycling. Sci. Prog. 2007; 90: 245-268
• 20 Pegoretti A. Adv. Ind. Eng. Polym. Res. 2021; 4: 105-115
• 21 Pietroluongo M, Padovano E, Frache A, Badini C. Sustainable Mater. Technol. 2020; 23: e00143
• 22 Akesson D, Kuzhanthaivelu G, Bohlen M. J. Polym. Environ. 2021; 29: 985-991
• 23 Zoe O, Shaver M. Macromol. Rapid Commun. 2021; 42: 2000415
• 24 Fadillah G, Fatimah I, Sahroni I, Musawwa MM, Mahila TM. I, Muraza O. Catalysis 2021; 11: 837
• 25 Aboul-Enein A, Awadallah A. Chem. Eng. J. 2018; 354: 802-816
• 26 Aboul-Enein A, Awadallah A. Polym. Degrad. Stab. 2019; 167: 157-169
• 27 Bajad G, Tiwari S, Vijayakumar R. Mater. Sci. Eng., B 2015; 194: 68-77
• 28 Shen Y, Gong W, Zheng B, Gao L. Appl. Catal. B 2016; 181: 769-778
• 29 Nahil M, Wu C, Williams P. Fuel Process. Technol. 2015; 130: 46-53
• 30 Wu S, Kuo J, Wey M. J. Anal. Appl. Pyrolysis 2019; 10464
• 31 Aboul-Enein A, Awadallah A. Mater. Chem. Phys. 2019; 238: 121879
• 32 Wang J, Shen B, Lan M, Kang D, Wu C. Catal. Today 2019; 351: 50-57
• 33 Wu C, Nahil M, Miskolczi N, Huang J, Williams PT. Environ. Sci. Technol. 2014; 48: 819-826
• 34 Yao D, Wu C, Yang H, Zhang Y, Nahil MA, Chen Y, Williams PT. Energy Convers. Manage. 2017; 148: 692-700
• 35 Onwudili J, Muhammad C, Williams PJ. Energy Ins. 2019; 92: 1337-1347
• 36 Kassargy C, Awad S, Burnens G, Kahine K, Tazerout M. Fuel 2018; 224: 764-773
• 37 Miandad R, Barakat M, Rehan M, Aburiazaiza AS, Ismail IM. I, Nizami AS. Waste Manage. 2017; 69: 66-78
• 38 Akubo K, Nahil MA, Williams PT. J. Energy Inst. 2019; 92: 195-202
• 39 Lopez A, Marco DI, Caballero BM, Adrados A, Laresgoiti MF. Waste Manage. 2011; 31: 1852-1858
• 40 Lopez A, Marco DI, Caballero BM, Laresgoiti MF, Adrados A, Aranzabal A. Appl. Catal., B 2011; 104: 211-219
• 41 Gupta VK, Tiwari S. Indian Patent, 380369 2021
• 42 Gupta VK, Tiwari S. Indian patent application No. 202421013784 2024
• 43 Kim YM, Lee HW, Choi SJ, Jeon JK, Park SH, Jung SC, Kim SC, Park YK. Int. J. Hydrogen Energy 2017; 42: 18434-18441
• 44 Zhang B, Zhong Z, Ding K, Song Z. Fuel 2015; 139: 622-628
• 45 Chai Y, Gao N, Wang M, Wu C. Chem. Eng. J. 2020; 382: 122947
• 46 Muneer B, Zeeshan M, Qaisar S, Razzaq M, Iftikhar H. J. Cleaner Prod. 2019; 237: 117762
• 47 Lee H, Choi S, Park S, Jeon JK, Jung SC, Kim S, Park YK. Nanoscale Res. Lett. 2014; 9: 1-8
• 48 Xu D, Xiong Y, Ye J, Su Y, Dong Q, Zhang S. Chem. Eng. J. 2020; 392: 123728
• 49 Byrappa K, Yoshimura M. Handbook of Hydrothermal Technology: A Technology for Crystal Growth and Materials Processing. William Andrew: Applied Science Publishers; 2001
• 50 Georgina C, Laredoa JR, Edith MR. Cleaner Chem. Eng. 2023; 5: 100094
• 51 Moriya T, Enomoto H. Polym. Degrad. Stab. 1999; 65: 373-386
• 52 Su X, Zhao Y, Zhang R. Fuel Process. Technol. 2004; 85: 1249-1258
• 53 Chen W, Jin K, Wang N. ACS Sustainable Chem. Eng. 2019; 7: 3749-3768
• 54 Zhao P, Yuan Z, Zhang J, Song X, Wang C, Guob Q, Ragauskas AJ. Sustainable Energy Fuels 2021; 5: 575-583
• 55 Lachos PD, Lu T, Chen WT. Sustainable Green Chemistry in Polymer Research. Volume 1, Biocatalysis and Biobased Materials. American Chemical Society; Washington, DC: 2023. pp 101-116
• 56 Jin K, Vozka P, Gentilcore C, Kilaz G, Wang NL. Fuel 2021; 294: 120505
• 57 Mukundan S, Wagner JL, Annamalai PK, Ravindran DS, Krishnapillai GK, Beltramini J. Fuel Process. Technol. 2022; 238: 107523
• 58 Feuerbach S, Toor SS, Costa PA, Paradela F, Marques PA. A. S, Castello D. Energies 2024; 17: 2098
• 59 Colnik M, Kotnik P, Knez Z, Skerget M. J. Supercrit. Fluids 2020; 169: 105136
• 60 Zhang H, Su X, Sun D, Zhang R, Bi J. J. Fuel Chem. Technol. 2007; 35: 487-491
• 61 Chen W, Jin K, Wang N. ACS Sustainable Chem. Eng. 2019; 7: 3749-3758
• 62 Boel MJ, Wang H, Ahmad AL, Megido L, LaFuente JM. G, Shiju NR. React. Chem. Eng. 2024; 9: 1014-1031
• 63 Abubakar F, Alfayez I, Suleymano H, McGregor J. Catal. Today 2024; 439: 114807
• 64 Lee S, Lee H, Lee J, Cho H. Available at SSRN: https://ssrn.com/abstract=4815465 2024
• 65 Sullivan KP, Werner AZ, Ramirez KJ, Ellis LD, Bussard JR, Black BA, Brandner DG, Bratti F, Buss BL, Dong X, Haugen SJ, Ingraham MA, Konev MO, Michener WE, Miscall J, Pardo I, Woodworth SP, Guss AM, Roman-Leshkov Y, Stahl SS, Beckham GT. Science 2022; 378: 207-211
• 66 Smak TJ, de Peinder P, Van der Waal JC, Altink R, Vollmer I, Weckhuysen BM. ChemSusChem 2024; 17: 7
• 67 Sewon O, Erin E. Chem. Soc. Rev. 2024; 53: 7309
• 68 Chen S, Hu YH. ChemSusChem 2024; 17: e202301449
• 69 Zefirov VV, Elmanovich IV, Stakhanov AI, Pavlov AA, Stakhanova SV, Kharitonova EP, Gallyamov MO. Polymer 2022; 14: 744
• 70 Orozco S, Lopez G, Suarez MA, Artetxe M, Alvarez J, Bilbao J, Olazar M. ACS Sustainable Chem. Eng. 2022; 10: 15791-15801
• 71 Hayashi J, Nakahara T, Kusakabe K, Morooka S. Fuel Process. Technol. 1998; 55: 265-275
• 72 Samorì C, Cespi D, Blair P, Galletti P, Malferrari D, Passarini F, Vassura I, Tagliavini E. Green Chem. 2017; 19: 1714-1720
• 73 Cecon V, Curtzwiler G, Vorst K. Macromol. Mater. Eng. 2022; 307: 2200346
• 74 Sanchez-Rivera K, Munguía-Lopez A, Zhou P, Cecon V, Yu J, Nelson K, Miller D, Grey S, Xu Z, Bar Ziv E, Vorst K, Curtzwiler G, Van Lehn R, Zavala V, Huber G. Resour. Conserv. Recycl. 2023; 197: 107086
• 75 Samorì C, Pitacco W, Vagnoni M, Catelli E, Colloricchio T, Gualandi C, Mantovani L, Mezzi A, Sciutto G, Galletti P. Resour. Conserv. Recycl. 2022; 190: 106832 Available at SSRN https://ssrn.com/abstract=4222936
• 76 Pappa G, Boukouvalas C, Giannaris C, Ntaras N, Zografos VV, Magoulas K, Lygeros AI, Tassios DP. Resour. Conserv. Recycl. 2001; 34: 33-44
• 77 Georgiopoulou I, Pappa GD, Vouyiouka SN, Magoulas K. Resour. Conserv. Recycl. 2021; 165: 105268
• 78 Achilias D, Giannoulis A, Papageorgiou G. Polym. Bull. 2009; 63: 449-465
• 79 Walker TW, Frelka N, Shen Z, Chew AK, Banick J, Grey S, Kim MS, Dumesic JA, Van Lehn RC, Huber GW. Sci. Adv. 2020; 6: 7599
• 80 Sanchez-Rivera KL, Zhou P, Kim MS, Gonzalez Chavez LD, Grey S, Nelson K, Wang SC, Hermans I, Zavala VM, Van Lehn RC, Huber GW. ChemSusChem 2021; 14: 4317-4329
• 81 Lange J.-P. ACS Sustainable Chem. Eng. 2021; 47: 15722-15738
• 82 Bauer AS, Tacker M, Uysal-Unalan I, Cruz RM. S, Varzakas T, Krauter V. Foods 2021; 10: 2702
• 83 Roosen M, Harinck L, Ugduler S, De Somer T, Hucks AG, Bele T, Buettner A, Ragaert K, Van Geem K, Dumoulin A, De Meester S. Sci. Total Environ. 2022; 812: 152467
• 84 Anouar S, Guinot C, Ruiz J, Charton F, Dole P, Joly C, Yvan C. J. Supercrit. Fluids 2015; 98: 25-32
• 85 Brown E, MacDonald A, Allen S, Allen D. J. Hazard. Mater. Adv. 2023; 10: 100309
• 86 Nandi S, Mahish SS, Das SK, Datta M, Nath D. Polym. Plast. Technol. Mater. 2023; 62: 1663-1683
• 87 United Nations. Economic and Social Council, ECE/CES/2023, 3
• 88 Tan Y, Cheng Y, Xu J, Wang H. J. Giant. 2024; 100307
• 89 Wang H, Tsang ES. C. Cell Rep. Phys. Sci. 2024; 5: 102075
• 90 Zou L, Xu R, Wang H, Wang Z, Sun Y, Li M. Natl. Sci. Rev. 2023; 10: nwad207
• 91 Wang NM, Strong G, DaSilva V, Gao L, Huacuja R, Konstantinov IA, Rosen MS, Nett AJ, Ewart S, Geyer R, Scott SL, Guironnet D. J. Am. Chem. Soc. 2022; 12: 18526-18531

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Eingereicht: 29. September 2024

Angenommen nach Revision: 28. Februar 2025

Artikel online veröffentlicht:
17. April 2025