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Polymers
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Chemical Recycling of Polymers
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Chemical Recycling of Polymers

A special issue of Polymers (ISSN 2073-4360). This special issue belongs to the section "Circular and Green Polymer Science".

Deadline for manuscript submissions: 5 October 2024 | Viewed by 25169

Special Issue Editors

Dr. Paul McKeown

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Guest Editor
Department of Chemistry, University of Bath, Bath BA2 7AY, UK
Interests: metal catalysis; polymerisation; bio-based plastics; plastics chemical recycling
Dr. Simoní Da Ros

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Guest Editor
Warwick Manufacturing Group (WMG), University of Warwick, Coventry CV4 7AL, UK
Interests: life cycle assessment; polymer science; polymer degradation; chemical recycling; chemical reactors modeling; heterogeneous catalysis; parameter estimation and optimal design of experiments
Dr. Luis Roman

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Guest Editor
Division of Chemical and Energy Engineering, London South Bank University, 103 Borough Road, London SE1 0AA, UK
Interests: lithium-ion batteries; renewable energy; green and sustainable chemistry and engineering; chemical recycling; polymers; applied thermodynamics

Special Issue Information

Dear Colleagues,

The plastic economy requires a shift towards a more circular approach, in which the value of the material is recovered rather than lost at the end of its useful life. While mechanical recycling options are available, these are generally associated with some loss of material properties and down-cycling. Compostable and biodegradable plastics are also becoming more common, but these represent processes involving a complete loss of chemical and energetic value. Therefore, the development of chemical or tertiary recycling technologies is expected to play a key role in the transition to a circular approach of plastic use and waste management. This Special Issue will focus on methods of chemical recycling of plastics which aim to produce monomers, for the production of new plastics, or value-added chemicals. Papers concerning chemical recycling methods including, but not limited to, hydrolysis, alcoholysis and pyrolysis are welcome.

Dr. Paul McKeown
Dr. Simoní Da Ros
Dr. Luis Roman
Guest Editors

Keywords

  • chemical recycling
  • degradation
  • depolymerisation
  • hydrolysis
  • alcoholysis
  • pyrolysis
  • circular economy

Published Papers (8 papers)

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Research

16 pages, 2239 KiB  
Article
Effects of Copper(II) Oxide on the Co-Pyrolysis of Waste Polyester Enameled Wires and Poly(vinyl chloride)
by Xiaolu Wang, Bingyi Li, Zhidong Xia, Wei Zhou, Yufeng Wu, Zhaoxi Zhu and Guangze Zhu
Polymers 2024, 16(1), 27; https://doi.org/10.3390/polym16010027 - 20 Dec 2023
Viewed by 704
Abstract
The emission of chlorinated pollutants is one of the main problems when recovering copper (Cu) via pyrolysis from waste enameled wires. This is mainly attributed to other wastes which possess high poly(vinyl chloride) content, such as electrical wires and cables, which are often [...] Read more.
The emission of chlorinated pollutants is one of the main problems when recovering copper (Cu) via pyrolysis from waste enameled wires. This is mainly attributed to other wastes which possess high poly(vinyl chloride) content, such as electrical wires and cables, which are often recycled together with enameled copper wires. In this research, to control the chlorinated pollutants, copper(II) oxide (CuO) was chosen and demonstrated to be an efficient dechlorinating agent, and CuO did not introduce any impurities that influence the quality of the recovered Cu. The pyrolysis and co-pyrolysis of polyester enameled wires, PVC, and CuO were investigated, and special attention was paid to chlorinated compounds in released pyrolytic products. In particular, the co-pyrolysis of this ternary mixture was studied for the first time, and some new pyrolysis behaviors were discovered. For example, the results of Py-GC/MS analyses showed that the addition of CuO removed about 75% of the chloro-organic products, the main types of which were chloroaromatic compounds rather than the more toxic chloroesters. Moreover, pyrolysis gases were collected and characterized via ion chromatography, and the results showed that the chlorine content in the pyrolysis gases decreased by about 71%. TG analysis indicated that CuO only minimally affected the pyrolysis of polyester paint. However, through the chlorine fixation effect, CuO influenced the dechlorination and dehydrochlorination of PVC, as well as secondary reactions between HCl and pyrolysis products of polyester paint, therefore changing the products and behaviors of co-pyrolysis. Mechanism of reducing chlorine-containing pollutants and reaction mechanism of forming typical pyrolysis products closely correlated to the effects of CuO were also proposed, providing theoretical guidance for the recycling of waste enameled wires. Full article
(This article belongs to the Special Issue Chemical Recycling of Polymers)
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14 pages, 2330 KiB  
Article
Optimization and Kinetic Evaluation for Glycolytic Depolymerization of Post-Consumer PET Waste with Sodium Methoxide
by Saqib Javed, Jonas Fisse and Dieter Vogt
Polymers 2023, 15(3), 687; https://doi.org/10.3390/polym15030687 - 29 Jan 2023
Cited by 9 | Viewed by 2995
Abstract
Glycolysis of post-consumer polyethylene terephthalate (PET) waste is a promising chemical recycling technique, back to the monomer, bis(2-hydroxyethyl) terephthalate (BHET). This work presents sodium methoxide (MeONa) as a low-cost catalyst for this purpose. BHET product was confirmed by gas chromatography-mass spectrometry (GCMS), Nuclear [...] Read more.
Glycolysis of post-consumer polyethylene terephthalate (PET) waste is a promising chemical recycling technique, back to the monomer, bis(2-hydroxyethyl) terephthalate (BHET). This work presents sodium methoxide (MeONa) as a low-cost catalyst for this purpose. BHET product was confirmed by gas chromatography-mass spectrometry (GCMS), Nuclear Magnetic Resonance (NMR) Spectroscopy, melting point, and Differential Scanning Calorimetry (DSC). It was shown, not surprisingly, that PET conversion increases with the glycolysis temperature. At a fixed temperature of 190 °C, the response surface methodology (RSM) based on the Box-Behnken design was applied. Four independent factors, namely the molar ratio of PET: MeONa (50–150), the molar ratio of ethylene glycol to PET (EG: PET) (3–7), the reaction time (2–6 h), and the particle size (0.25–1 mm) were studied. Based on the experimental results, regression models as a function of significant process factors were obtained and evaluated by analysis of variance (ANOVA), to predict the depolymerization performance of MeONa in terms of PET conversion. Coefficient of determination, R2 of 95% indicated the adequacy for predicted model. Afterward, the regression model was validated and optimized within the design space with a prediction of 87% PET conversion at the optimum conditions demonstrating a deviation of less than 5% from predicted response. A van ‘t Hoff plot confirmed the endothermic nature of the depolymerization reaction. The ceiling temperature (TC = 160 °C) was calculated from Gibbs’ free energy. A kinetic study for the depolymerization reaction was performed and the activation energy for MeONa was estimated from the Arrhenius plot (EA = 130 kJ/mol). The catalytic depolymerization efficiency of MeONa was compared under similar conditions with widely studied zinc acetate and cobalt acetate. This study shows that MeONa’s performance, as a glycolysis catalyst is promising; in addition, it is much cheaper and environmentally more benign than heavy metal salts. These findings make a valuable contribution towards the chemical recycling of post-consumer PET waste to meet future recycling demands of a circular economy. Full article
(This article belongs to the Special Issue Chemical Recycling of Polymers)
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7 pages, 2484 KiB  
Communication
Reusable Macroporous Oil Sorbent Films from Plastic Wastes
by Junaid Saleem, Moghal Zubair Khalid Baig, Adriaan Stephanus Luyt, Rana Abdul Shakoor, Said Mansour and Gordon McKay
Polymers 2022, 14(22), 4867; https://doi.org/10.3390/polym14224867 - 11 Nov 2022
Cited by 3 | Viewed by 1519
Abstract
Plastic waste comprises 15% of the total municipal solid waste and can be a rich source for producing value-added materials. Among them, polyethylene (PE) and polypropylene (PP) account for 60% of the total plastic waste, mainly due to their low-end and one-time-use applications. [...] Read more.
Plastic waste comprises 15% of the total municipal solid waste and can be a rich source for producing value-added materials. Among them, polyethylene (PE) and polypropylene (PP) account for 60% of the total plastic waste, mainly due to their low-end and one-time-use applications. Herein, we report reusable oil sorbent films made by upcycling waste PE and PP. The as-prepared oil sorbent had an uptake capacity of 55 g/g. SEM analysis revealed a macroporous structure with a pore size range of 1–10 µm, which facilitates oil sorption. Similarly, the contact angle values reflected the oleophilic nature of the sorbent. Moreover, thermal properties and crystallinity were examined using DSC, while mechanical properties were calculated using tensile testing. Lastly, 95% of the sorbed oil could be easily recovered by squeezing mechanically or manually. Full article
(This article belongs to the Special Issue Chemical Recycling of Polymers)
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13 pages, 2093 KiB  
Article
Methanolysis of Poly(lactic Acid) Using Catalyst Mixtures and the Kinetics of Methyl Lactate Production
by Fabio M. Lamberti, Luis A. Román-Ramírez, Andrew P. Dove and Joseph Wood
Polymers 2022, 14(9), 1763; https://doi.org/10.3390/polym14091763 - 26 Apr 2022
Cited by 3 | Viewed by 2820
Abstract
Polylactic acid (PLA) is a leading bioplastic of which the market share is predicted to increase in the future; its growing production capacity means its end-of-life treatment is becoming increasingly important. One beneficial disposal route for PLA is its chemical recycling via alcoholysis. [...] Read more.
Polylactic acid (PLA) is a leading bioplastic of which the market share is predicted to increase in the future; its growing production capacity means its end-of-life treatment is becoming increasingly important. One beneficial disposal route for PLA is its chemical recycling via alcoholysis. The alcoholysis of PLA leads to the generation of value-added products alkyl lactates; this route also has potential for a circular economy. In this work, PLA was chemically recycled via methanolysis to generate methyl lactate (MeLa). Four commercially available catalysts were investigated: zinc acetate dihydrate (Zn(OAc)2), magnesium acetate tetrahydrate (Mg(OAc)2), 4-(dimethylamino)pyridine (DMAP), and triazabicyclodecene (TBD). Dual catalyst experiments displayed an increase in reactivity when Zn(OAc)2 was paired with TBD or DMAP, or when Mg(OAc)2 was paired with TBD. Zn(OAc)2 coupled with TBD displayed the greatest reactivity. Out of the single catalyst reactions, Zn(OAc)2 exhibited the highest activity: a higher mol% was found to increase reaction rate but plateaued at 4 mol%, and a higher equivalent of methanol was found to increase the reaction rate, but plateaued at 17 equivalents. PLA methanolysis was modelled as a two-step reversible reaction; the activation energies were estimated at: Ea1 = 25.23 kJ∙mol−1, Ea2 = 34.16 kJ∙mol−1 and Ea-2 = 47.93 kJ∙mol−1. Full article
(This article belongs to the Special Issue Chemical Recycling of Polymers)
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12 pages, 6415 KiB  
Article
In-Situ Synthesis of TiO2@GO Nanosheets for Polymers Degradation in a Natural Environment
by Yueqin Shi, Zhanyang Yu, Zhengjun Li, Xiaodong Zhao and Yongjun Yuan
Polymers 2021, 13(13), 2158; https://doi.org/10.3390/polym13132158 - 30 Jun 2021
Cited by 11 | Viewed by 1814
Abstract
Plastic photodegradation naturally takes 300–500 years, and their chemical degradation typically needs additional energy or causes secondary pollution. The main components of global plastic are polymers. Hence, new technologies are urgently required for the effective decomposition of the polymers in natural environments, which [...] Read more.
Plastic photodegradation naturally takes 300–500 years, and their chemical degradation typically needs additional energy or causes secondary pollution. The main components of global plastic are polymers. Hence, new technologies are urgently required for the effective decomposition of the polymers in natural environments, which lays the foundation for this study on future plastic degradation. This study synthesizes the in-situ growth of TiO2 at graphene oxide (GO) matrix to form the TiO2@GO photocatalyst, and studies its application in conjugated polymers’ photodegradation. The photodegradation process could be probed by UV-vis absorption originating from the conjugated backbone of polymers. We have found that the complete decomposition of various polymers in a natural environment by employing the photocatalyst TiO2@GO within 12 days. It is obvious that the TiO2@GO shows a higher photocatalyst activity than the TiO2, due to the higher crystallinity morphology and smaller size of TiO2, and the faster transmission of photogenerated electrons from TiO2 to GO. The stronger fluorescence (FL) intensity of TiO2@GO compared to TiO2 at the terephthalic acid aqueous solution indicates that more hydroxyl radicals (•OH) are produced for TiO2@GO. This further confirms that the GO could effectively decrease the generation of recombination centers, enhance the separation efficiency of photoinduced electrons and holes, and increase the photocatalytic activity of TiO2@GO. This work establishes the underlying basic mechanism of polymers photodegradation, which might open new avenues for simultaneously addressing the white pollution crisis in a natural environment. Full article
(This article belongs to the Special Issue Chemical Recycling of Polymers)
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15 pages, 3471 KiB  
Article
Thermomechanical Devulcanisation of Ethylene Propylene Diene Monomer (EPDM) Rubber and Its Subsequent Reintegration into Virgin Rubber
by Dávid Zoltán Pirityi and Kornél Pölöskei
Polymers 2021, 13(7), 1116; https://doi.org/10.3390/polym13071116 - 01 Apr 2021
Cited by 16 | Viewed by 3897
Abstract
Rubber waste remains a challenge for material science because its covalently cross-linked structure hinders the establishment of the circular economy of rubber. Devulcanisation may provide a solution, as it converts rubber vulcanisates back into their original, uncured state. Devulcanised rubber may be revulcanised [...] Read more.
Rubber waste remains a challenge for material science because its covalently cross-linked structure hinders the establishment of the circular economy of rubber. Devulcanisation may provide a solution, as it converts rubber vulcanisates back into their original, uncured state. Devulcanised rubber may be revulcanised or incorporated into virgin rubber, thus waste is utilized and the use of primary resources is reduced at the same time. In this paper, we treated sulphur-cured EPDM (ethylene propylene diene monomer) rubber on a two-roll mill at various temperatures and frictions. We determined the effectiveness of devulcanisation via Horikx’s analysis, which suggested that low devulcanisation temperatures would result in a 50% decrease in cross-link density with minimal polymer degradation. The devulcanisate was recycled via two methods: (a) revulcanisation with extra curing agents, and (b) mixing it with various amounts of the original rubber mixture, preparing rubber samples with 25, 50, 75, and 100 wt% recycled content. Tensile tests revealed that the samples’ elastic properties were severely compromised at 75 and 100 wt% devulcanisate contents. However, tensile strength decreased only by 15% and 20% for revulcanisates containing 25% and 50% recycled rubber, respectively. Full article
(This article belongs to the Special Issue Chemical Recycling of Polymers)
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18 pages, 6593 KiB  
Article
Recyclable High-Performance Epoxy-Anhydride Resins with DMP-30 as the Catalyst of Transesterification Reactions
by Wenzhe Zhao, Le An and Shujuan Wang
Polymers 2021, 13(2), 296; https://doi.org/10.3390/polym13020296 - 18 Jan 2021
Cited by 39 | Viewed by 6328
Abstract
Epoxy-anhydride resins are widely used in engineering fields due to their excellent performance. However, the insolubility and infusibility make the recycling of epoxy resins challenging. The development of degradable epoxy resins with stable covalent networks provides an efficient solution to the recycling of [...] Read more.
Epoxy-anhydride resins are widely used in engineering fields due to their excellent performance. However, the insolubility and infusibility make the recycling of epoxy resins challenging. The development of degradable epoxy resins with stable covalent networks provides an efficient solution to the recycling of thermosets. In this paper, 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) is incorporated into the epoxy-glutaric anhydride (GA) system to prepare high-performance epoxy resins that can be recycled below 200 °C at ordinary pressure via ethylene glycol (EG) participated transesterification. The tertiary amine groups in DMP-30 can catalyze the curing reaction of epoxy and anhydride, as well as the transesterification between ester bonds and alcoholic hydroxyl groups. Compared with early recyclable anhydride-cured epoxy resins, the preparation and recycling of diglycidyl ether of bisphenol A (DGEBA)/GA/DMP-30 systems do not need any special catalysts such as TBD, Zn(Ac)2, etc., which are usually expensive, toxic, and have poor compatibility with other compounds. The resulting resins have glass transition temperatures and strengths similar to those of conventional epoxy resins. The influences of GA content, DMP-30 content, and temperature on the dissolution rate were studied. The decomposed epoxy oligomer (DEO) is further used as a reaction ingredient to prepare new resins. It is found that the DEO can improve the toughness of epoxy resins significantly. This work provides a simple method to prepare readily recyclable epoxy resins, which is of low-cost and easy to implement. Full article
(This article belongs to the Special Issue Chemical Recycling of Polymers)
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13 pages, 3239 KiB  
Article
Reprocessable, Reworkable, and Mechanochromic Polyhexahydrotriazine Thermoset with Multiple Stimulus Responsiveness
by Li Chen, Siyao Zhu, Innocent Toendepi, Qiuran Jiang, Yi Wei, Yiping Qiu and Wanshuang Liu
Polymers 2020, 12(10), 2375; https://doi.org/10.3390/polym12102375 - 15 Oct 2020
Cited by 13 | Viewed by 3419
Abstract
Developing recyclable, reworkable, and intelligent thermosetting polymers, as a long-standing challenge, is highly desirable for modern manufacturing industries. Herein, we report a polyhexahydrotriazine thermoset (PHT) prepared by a one-pot polycondensation between 4-aminophenyl disulfide and paraformaldehyde. The PHT has a glass transition temperature of [...] Read more.
Developing recyclable, reworkable, and intelligent thermosetting polymers, as a long-standing challenge, is highly desirable for modern manufacturing industries. Herein, we report a polyhexahydrotriazine thermoset (PHT) prepared by a one-pot polycondensation between 4-aminophenyl disulfide and paraformaldehyde. The PHT has a glass transition temperature of 135 °C and good solvent resistance. The incorporation of dual stimuli-responsive groups (disulfide bond and hexahydrotriazine ring) endows the PHT with re-processability, re-workability, and damage monitoring function. The PHT can be repeatedly reprocessed by hot pressing, and a near 100% recovery of flexural strength is achieved. The PHT can also degrade in inorganic acid or organic thiol solutions at room temperature. The thermally reworkable test demonstrates that, after heating the PHT at 200 °C for 1 h, the residuals can be easily wiped off. Finally, the PHT exhibits a reversible mechanochromic behavior when damaged. Full article
(This article belongs to the Special Issue Chemical Recycling of Polymers)
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