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

Deadline for manuscript submissions: closed (25 April 2024) | Viewed by 31157

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Dr. Florian Part

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Institute of Waste Management and Circularity, Department of Water-Atmosphere-Environment, BOKU University, Muthgasse 107, 1190 Vienna, Austria
Interests: advances materials; nanomaterials; polymer (nano-)composites; chemical risk assessment; waste management; recycling technologies; life cycle assessment
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Dear Colleagues,

Plastics or synthetic polymers are omnipresent in our daily lives because they are characterized by their advantageous material properties, such as lightweight, plasticity, and durability of thermoplastics and thermosets. In addition, elastomers like rubber with cross-linked longer polymer chains have enabled reversible elasticity and are therefore also used for a large area of applications (from household products to the medical, energy, agriculture, construction, automotive, and avionic sector). No one can imagine life without plastics nowadays, but currently, and especially future generations, are confronted with the fact that improper waste collection and insufficient recycling have led to massive environmental pollution. For this reason, many countries under the leadership of the United Nations have agreed to improve the situation through a range of measures, from sustainable production and consumption to waste prevention, collection, and recycling. With regard to the reduction of greenhouse gases and the Paris Climate Agreement targets, it is noted that this Special Issue does not address topics around waste-to-energy or waste incineration. Since innovative solutions should cover the entire value chain of plastics and the efforts of all scientific disciplines are required, this Special Issue focuses on the following topics:

Recycling-oriented collection systems, especially for polymer composites and post-consumer wastes
Novel technologies for separation by polymer type to improve the quality of recyclates
Approaches towards sustainable value chains and process optimisation to advance feedstocks, molecules, materials/compounds/composites and end-products
Innovations in mechanical recycling and remelting processes
Innovations in chemical recycling
Quality assessment of recycling products and identification of emerging markets
Identification and risk assessment of substances of (very high) concern and emerging pollutants in recyclates and other polymeric recycling products (“trade-offs” by cross-contaminates)
Material flow, socioeconomic, value cycle/chain analysis, or life cycle assessment of possible recycling routes in the circular economy

Dr. Florian Part
Guest Editor

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Keywords

polymer composites
packaging materials
plastics from electronic equipment and WEE
car tire recycling
compounding
marker-based and/or automated sorting
re-granulation
thermochemical recycling
end-of-waste criteria of recycled plastic
life cycle and socioeconomic assessment of recycling routes

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Published Papers (11 papers)

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Research

22 pages, 5171 KiB

Open AccessArticle

Flash Pyrolysis of Waste Tires in an Entrained Flow Reactor—An Experimental Study

by Balan Ramani, Arqam Anjum, Eddy Bramer, Wilma Dierkes, Anke Blume and Gerrit Brem

Polymers 2024, 16(12), 1746; https://doi.org/10.3390/polym16121746 - 20 Jun 2024

Viewed by 1556

Abstract

In this study, a flash pyrolysis process is developed using an entrained flow reactor for recycling of waste tires. The flash pyrolysis system is tested for process stability and reproducibility of the products under similar operating conditions when operated continuously. The study is performed with two different feedstock materials, i.e., passenger car (PCT) and truck tire (TT) granulates, to understand the influence of feedstock on the yield and properties of the pyrolysis products. The different pyrolytic products i.e., pyrolytic carbon black (pCB), oil, and pyro-gas, are analyzed, and their key properties are discussed. The potential applications for the obtained pyrolytic products are discussed. Finally, a mass and energy balance analysis has been performed for the developed pyrolysis process. The study provides insight into the governing mechanisms of the flash pyrolysis process for waste tires, which is useful to optimize the process depending on the desired applications for the pyrolysis products, and also to scale up the pyrolysis process. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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20 pages, 10529 KiB

Open AccessArticle

Highly Efficient and Eco-Friendly Thermal-Neutron-Shielding Materials Based on Recycled High-Density Polyethylene and Gadolinium Oxide Composites

by Donruedee Toyen, Ekachai Wimolmala, Kasinee Hemvichian, Pattra Lertsarawut and Kiadtisak Saenboonruang

Polymers 2024, 16(8), 1139; https://doi.org/10.3390/polym16081139 - 18 Apr 2024

Viewed by 1284

Abstract

Due to the increasing demands for improved radiation safety and the growing concerns regarding the excessive use of plastics, this work aimed to develop effective and eco-friendly thermal-neutron-shielding materials based on recycled high-density polyethylene (r-HDPE) composites containing varying surface-treated gadolinium oxide (Gd₂O₃) contents (0, 5, 10, 15, and 20 wt%). The results indicate that the overall thermal-neutron-shielding properties of the r-HDPE composites were enhanced with the addition of Gd₂O₃, as evidenced by large reductions in I/I₀, HVL, and TVL, as well as the substantial increases in ∑_t and ∑_t/ρ of the composites. Furthermore, the results reveal that the values for tensile properties initially increased up to 5–15 wt% of Gd₂O₃ and then gradually decreased at higher contents. In addition, the results show that the addition of Gd₂O₃ particles generally increased the density (ρ), the remaining ash at 600 °C, and the degree of crystallinity (%XC) of the composites. This work also determined the effects of gamma irradiation on relevant properties of the composites. The findings indicate that following gamma aging, the tensile modulus slightly increased, while the tensile strength, elongation at break, and hardness (Shore D) showed no significant (p < 0.05) differences, except for the sample containing 5 wt% of Gd₂O₃, which exhibited a noticeable reduction in elongation at break. Furthermore, by comparing the neutron-shielding and mechanical properties of the developed r-HDPE composites with common borated polyethylene (PE) containing 5 wt% and 15 wt% of boron, the results clearly indicate the superior shielding and tensile properties in the r-HDPE composites, implying the great potential of r-HDPE composites to replace virgin plastics as effective and more eco-friendly shielding materials. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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17 pages, 4378 KiB

Open AccessArticle

Searching for the Achilles’ Heel of Urethane Linkage—An Energetic Perspective

by Tamás Horváth, Karina Kecskés, Anikó Jordán Csábrádiné, Emma Szőri-Dorogházi, Béla Viskolcz and Milán Szőri

Polymers 2024, 16(8), 1126; https://doi.org/10.3390/polym16081126 - 17 Apr 2024

Viewed by 1206

Abstract

A sudden increase in polyurethane (PU) production necessitates viable recycling methods for the waste generated. PU is one of the most important plastic materials with a wide range of applications; however, the stability of the urethane linkage is a major issue in chemical recycling. In this work, termination reactions of a model urethane molecule, namely methyl N-phenyl carbamate (MPCate), are investigated using G3MP2B3 composite quantum chemical method. Our main goal was to gain insights into the energetic profile of urethane bond termination and find an applicable chemical recycling method. Hydrogenation, hydrolysis, methanolysis, peroxidation, glycolysis, ammonolysis, reduction with methylamine and termination by dimethyl phosphite were explored in both gas and condensed phases. Out of these chemicals, degradation by H₂, H₂O₂ and CH₃NH₂ revealed promising results with lower activation barriers and exergonic pathways, especially in water solvation. Implementing these effective PU recycling methods can also have significant economic benefits since the obtained products from the reactions are industrially relevant substances. For example, aniline and dimethyl carbonate could be reusable in polymer technologies serving as potential methods for circular economy. As further potential transformations, several ionizations of MPCate were also examined including electron capture and detachment, protonation/deprotonation and reaction with OH⁻. Alkaline digestion against the model urethane MPCate was found to be promising due to the relatively low activation energy. In an ideal case, the transformation of the urethane bond could be an enzymatic process; therefore, potential enzymes, such as lipoxygenase, were also considered for the catalysis of peroxidation, and lipases for methanolysis. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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16 pages, 2242 KiB

Open AccessArticle

Chemical Recycling of PET Using Catalysts from Layered Double Hydroxides: Effect of Synthesis Method and Mg-Fe Biocompatible Metals

by Ana P. Arcanjo, Denisson O. Liborio, Santiago Arias, Florival R. Carvalho, Josivan P. Silva, Bernardo D. Ribeiro, Marcos L. Dias, Aline M. Castro, Roger Fréty, Celmy M. B. M. Barbosa and Jose Geraldo A. Pacheco

Polymers 2023, 15(15), 3274; https://doi.org/10.3390/polym15153274 - 2 Aug 2023

Cited by 7 | Viewed by 2354

Abstract

The chemical recycling of poly(ethylene terephthalate) (PET) residues was performed via glycolysis with ethylene glycol (EG) over Mg-Fe and Mg-Al oxide catalysts derived from layered double hydroxides. Catalysts prepared using the high supersaturation method (h.s.c.) presented a higher surface area and larger particles, but this represented less PET conversion than those prepared by the low supersaturation method (l.s.c.). This difference was attributed to the smaller mass transfer limitations inside the (l.s.c.) catalysts. An artificial neural network model well fitted the PET conversion and bis(2-hydroxyethyl) terephthalate (BHET) yield. The influence of Fe in place of Al resulted in a higher PET conversion of the Mg-Fe-h.s.c. catalyst (~95.8%) than of Mg-Al-h.s.c. (~63%). Mg-Fe catalysts could be reused four to five times with final conversions of up to 97% with reaction conditions of EG: PET = 5:1 and catalyst: PET = 0.5%. These results confirm the Mg-Fe oxides as a biocompatible novel catalyst for the chemical recycling of PET residues to obtain non-toxic BHET for further polymerization, and use in food and beverage packaging. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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17 pages, 6682 KiB

Open AccessArticle

New Route of Tire Rubber Devulcanization Using Silanes

by Rounak Ghosh, Christian Mani, Roland Krafczyk, Rupert Schnell, Alexander Paasche, Auke Talma, Anke Blume and Wilma K. Dierkes

Polymers 2023, 15(13), 2848; https://doi.org/10.3390/polym15132848 - 28 Jun 2023

Cited by 4 | Viewed by 2753

Abstract

The disposal of tires at the end of their lifespan results in societal and environmental issues. To tackle this, recycling and reuse are effective solutions. Among various recycling methods, devulcanization is considered to be a very sustainable option, as it involves the controlled breakdown of crosslinks while maintaining the polymer backbones. The objective of this study is to develop a sustainable devulcanization process for passenger car tire rubber using silanes. In this study, a thermo-mechanical–chemical devulcanization process was conducted to screen six potential devulcanization aids (DAs). Silanes were chosen as they are widely used in tire rubber as coupling agents for silica. The efficiency of the devulcanization was studied by the degree of network breakdown, miscibility of the devulcanized material, and mechanical properties of the de- and revulcanized material. Compared to the parent compound, a 55–60% network breakdown was achieved for the devulcanizate along with 50–55% of tensile strength recovery. In addition to superior devulcanization efficiency, this DA offers a sustainable alternative to the conventional ones, such as di-phenyl-di-sulphide, due to its compliance with safety regulations. The devulcanizate can be utilized in high-performance applications, such as tires and seals, while 100% devulcanizate can be employed in low-strength technical rubber products. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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10 pages, 15877 KiB

Open AccessArticle

Ground Tire Rubber Particles as Substitute for Calcium Carbonate in an EPDM Sealing Compound

by Vanessa Spanheimer, Gamze Gül Jaber and Danka Katrakova-Krüger

Polymers 2023, 15(9), 2174; https://doi.org/10.3390/polym15092174 - 3 May 2023

Cited by 3 | Viewed by 2174

Abstract

Ground tire rubber (GTR) is a product obtained by grinding worn tire treads before retreading them or via the cryogenic or ambient temperature milling of end-of-life tires (ELTs). The aim of this study is to evaluate if calcium carbonate can be substituted by GTR and, if so, to what extent. Different types of ground tire rubber are incorporated in an EPDM (ethylene–propylene–diene–rubber) model compound as partial or complete substitutes of calcium carbonate. The raw compounds and the vulcanizates are characterized to identify the limits. In general, it is apparent that increasing amounts of GTR and larger particles degrade the mechanical properties. The GTR also influences the vulcanization kinetics by reducing the scorch time up to 50% and vulcanization time up to nearly 80%. This is significant for production processes. The compounds with one-third substitution with the smaller-particle-size GTR show mostly similar or even better properties than the reference. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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11 pages, 2741 KiB

Open AccessArticle

Ultra-Dispersed Powders Produced by High-Temperature Shear-Induced Grinding of Worn-Out Tire and Products of Their Interaction with Hot Bitumen

by Vadim Nikol’skii, Tatiana Dudareva, Irina Krasotkina, Irina Gordeeva, Alexandre A. Vetcher and Alexander Botin

Polymers 2022, 14(17), 3627; https://doi.org/10.3390/polym14173627 - 1 Sep 2022

Cited by 4 | Viewed by 1598

Abstract

Structural features of crumb rubber (CR) particles obtained by grinding on rollers and ultra-disperse powder elastomeric modifiers (PEM) obtained by high-temperature shear-induced grinding (HTSG) of CR or co-grinding with butadiene styrene thermoplastic elastomer (SBS) have been studied by electron and optical microscopy methods. Samples of modified bitumen were obtained at different mixing times (1–40 min) in a wide temperature range (120–180 °C). The products of interaction of PEM with hot bitumen precipitated on filters when washed with solvent from modified bitumen (MB) were studied by scanning electron microscopy (SEM). The self-similarity PEM particles and PEM breakdown fragments in bitumen up to the size of 100–200 nm were noted. The rapid (for 1 min) decomposition of PEM particles into fragments is shown, which is due to the specific structure formed as a result of HTSG. It has been suggested that this fragmentation may be caused by bitumen penetrating deep into the porous particle and breaking it, due to differently directed swelling pressure and precede the classical swelling associated with the penetration of solvent between rubber macromolecules, or occur concurrently with it. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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20 pages, 989 KiB

Open AccessArticle

Evaluation of Marker Materials and Spectroscopic Methods for Tracer-Based Sorting of Plastic Wastes

by Christoph Olscher, Aleksander Jandric, Christian Zafiu and Florian Part

Polymers 2022, 14(15), 3074; https://doi.org/10.3390/polym14153074 - 29 Jul 2022

Cited by 15 | Viewed by 4291

Abstract

Plastics are a ubiquitous material with good mechanical, chemical and thermal properties, and are used in all industrial sectors. Large quantities, widespread use, and insufficient management of plastic wastes lead to low recycling rates. The key challenge in recycling plastic waste is achieving a higher degree of homogeneity between the different polymer material streams. Modern waste sorting plants use automated sensor-based sorting systems capable to sort out commodity plastics, while many engineering plastics, such as polyoxymethylene (POM), will end up in mixed waste streams and are therefore not recycled. A novel approach to increasing recycling rates is tracer-based sorting (TBS), which uses a traceable plastic additive or marker that enables or enhances polymer type identification based on the tracer’s unique fingerprint (e.g., fluorescence). With future TBS applications in mind, we have summarized the literature and assessed TBS techniques and spectroscopic detection methods. Furthermore, a comprehensive list of potential tracer substances suitable for thermoplastics was derived from the literature. We also derived a set of criteria to select the most promising tracer candidates (3 out of 80) based on their material properties, toxicity profiles, and detectability that could be applied to enable the circularity of, for example, POM or other thermoplastics. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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15 pages, 3261 KiB

Open AccessArticle

Latent Recycling Potential of Multilayer Films in Austrian Waste Management

by Gerald Koinig, Bettina Rutrecht, Karl Friedrich, Chiara Barretta and Daniel Vollprecht

Polymers 2022, 14(8), 1553; https://doi.org/10.3390/polym14081553 - 11 Apr 2022

Cited by 7 | Viewed by 2942

Abstract

This work presents a hand sorting trial of Austrian plastic packaging, which showed that according to an extrapolation of the 170,000 t separately collected waste collected in Austria, 30 wt% are flexible 2D plastic packaging. Further, the applications for these materials have been catalogued. The composition of these films was evaluated via Fourier-Transformed Infrared Spectroscopy, which showed that 31% of all films were made of polyethene, 39% of polypropylene, 11% of polyethene–polyethene terephthalate composite, and 8% of a polyethene–polypropylene composite, further resulting in the calculation that of all flexible packaging, 20 wt% are multilayer films. These findings were used to calculate the latent potential for raising the current recycling quota of 25.7% to the mandated rate of 55% in 2030. To this end, scenarios depicting different approaches to sorting and recycling small films were evaluated. It was calculated that through improving the sorting of films the recycling rate could be increased to 35.5%. This approach allows for the recycling of monolayer films by avoiding contamination with foreign materials introduced by multilayer films that impede the recyclates’ mechanical properties. The evaluation showed that sorting multilayer films of this fraction could raise the recycling quota further to 38.9%. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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14 pages, 1625 KiB

Open AccessArticle

Environmental Consequences of Rubber Crumb Application: Soil and Water Pollution

by Jan Fořt, Klára Kobetičová, Martin Böhm, Jan Podlesný, Veronika Jelínková, Martina Vachtlová, Filip Bureš and Robert Černý

Polymers 2022, 14(7), 1416; https://doi.org/10.3390/polym14071416 - 30 Mar 2022

Cited by 20 | Viewed by 4944

Abstract

End-of-life tires are utilized for various purposes, including sports pitches and playground surfaces. However, several substances used at the manufacture of tires can be a source of concerns related to human health or environment’s adverse effects. In this context, it is necessary to map whether this approach has the desired effect in a broader relation. While the negative effects on human health were investigated thoroughly and legislation is currently being revisited, the impact on aquatic or soil organisms has not been sufficiently studied. The present study deals with the exposure of freshwater and soil organisms to rubber crumb using the analysis of heavy metal and polycyclic aromatic hydrocarbon concentrations. The obtained results refer to substantial concerns related to freshwater contamination specifically, since the increased concentrations of zinc (7 mg·L⁻¹) and polycyclic aromatic hydrocarbons (58 mg·kg⁻¹) inhibit the growth of freshwater organisms, Desmodesmus subspicatus, and Lemna minor in particular. The performed test with soil organisms points to substantial concerns associated with the mortality of earthworms as well. The acquired knowledge can be perceived as a roadmap to a consistent approach in the implementation of the circular economy, which brings with it a number of so far insufficiently described problems. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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13 pages, 2449 KiB

Open AccessArticle

Low-Temperature Mechano-Chemical Rubber Reclamation Using Terpinene as a Swelling Agent to Enhance Bond-Breaking Selectivity

by Lei Guo, Donghui Ren, Wenchao Wang, Kuanfa Hao, Xiurui Guo, Fumin Liu, Yuan Xu, Miaomiao Liu and Haichao Liu

Polymers 2021, 13(24), 4272; https://doi.org/10.3390/polym13244272 - 7 Dec 2021

Cited by 7 | Viewed by 3120

Abstract

Common swelling agents used in the mechano-chemical rubber devulcanization process usually require high temperatures to achieve satisfactory swelling effects, which results in severe production of pollutants and reduces the selectivity of bond scissions. This work presents an environmentally friendly swelling agent, terpinene, which can swell the rubber crosslink structures at low temperatures. Both a rubber swelling experiment and a rubber reclaiming experiment with a mechano-chemical devulcanization method are conducted to explore the swelling effects of terpinene. After soaking in terpinene at 60 °C for 90 min, the length elongation of the rubber sample reaches 1.55, which is much higher than that in naphthenic oil and is comparable to that in toluene. When adding 3 phr of terpinene for every 100 phr of waste rubber during the reclaiming process, the bond scissions exhibit high selectivity. After revulcanization, the reclaimed rubbers have a tensile strength of 17 MPa and a breaking elongation of 400%. Consequently, the application of terpinene as the swelling agent in the LTMD method can greatly improve the properties of reclaimed rubbers, thereby enhancing the dual value for the economy and environment. Full article

(This article belongs to the Special Issue Recycling of Plastic and Rubber Wastes)

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