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

Deadline for manuscript submissions: closed (25 November 2021) | Viewed by 62483

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Special Issue Editors

Prof. Dr. Seeram Ramakrishna

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Guest Editor

Department of Mechanical Engineering, National University of Singapore, Singapore 119077, Singapore
Interests: circular economy; sustainability; energy; biomaterials; nanofibers
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Prof. Dr. Yong Liu

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Guest Editor

Beijing Key Laboratory of Advanced Functional Polymer Composites, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
Interests: polymer nanocomposites; fuel cell core components; functional materials; biomedical materials
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Special Issue Information

Dear Colleagues,

It was estimated that the global production of plastics is approximately 250 million tons per year. Their abundance has been found to transport persistent organic pollutants. Some compounds that are used in plastics, such as phthalates, bisphenol A, and polybrominated diphenyl ether, are under close statute and might be very harmful. It is a disaster that human beings must face more and more man-made chemical waste, especially the huge amount of plastics. According to some researchers, by 2050, there could be more waste plastic than fish in the oceans by weight. To reduce the organic waste, researchers have conducted many studies and developed many methods and applications to make degradable polymers or recycle the waste.

Now, this Special Issue titled “Recycling of Plastics” will focus on the research related to the reduction, recycling, and reuse of polymers. Any efforts to reduce the use of plastics and to promote plastic recycling are valuable. All research related to green chemistry, degradable polymers, biomaterials, and waste plastics is welcome. We hope this Special Issue can contribute to the cleanup of our planet.

Prof. Dr. Seeram Ramakrishna
Prof. Dr. Yong Liu
Guest Editors

Manuscript Submission Information

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Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Polymers is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2700 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

green chemistry
recycle
degradable polymers
waste plastics
reuse of polymers
biomaterials

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Published Papers (7 papers)

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20 pages, 958 KiB

Open AccessArticle

The Mechanics of Forming Ideal Polymer–Solvent Combinations for Open-Loop Chemical Recycling of Solvents and Plastics

by Ioannis Tsampanakis and Alvin Orbaek White

Polymers 2022, 14(1), 112; https://doi.org/10.3390/polym14010112 - 29 Dec 2021

Cited by 17 | Viewed by 5558

Abstract

The inherent value and use of hydrocarbons from waste plastics and solvents can be extended through open-loop chemical recycling, as this process converts plastic to a range of non-plastic materials. This process is enhanced by first creating plastic–solvent combinations from multiple sources, which then are streamlined through a single process stream. We report on the relevant mechanics for streamlining industrially relevant polymers such as polystyrene (PS), polypropylene (PP), high-density polyethylene (HDPE), and acrylonitrile butadiene styrene (ABS) into chemical slurries mixed with various organic solvents such as toluene, xylene, and cyclohexane. The miscibility of the polymer feedstock within the solvent was evaluated using the Relative Energy Difference method, and the dissolution process was evaluated using the “Molecular theories in a continuum framework” model. These models were used to design a batch process yielding 1 tonne/h slurry by setting appropriate assumptions including constant viscosity of solvents, disentanglement-controlled dissolution mechanism, and linear increase in the dissolved polymer’s mass fraction over time. Solvent selection was found to be the most critical parameter for the dissolution process. The characteristics of the ideal solvent are high affinity to the desired polymer and low viscosity. This work serves as a universal technical guideline for the open-loop chemical recycling of plastics, avoiding the growth of waste plastic by utilising them as a carbon feedstock towards a circular economy framework. Full article

(This article belongs to the Special Issue Recycling of Plastics)

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29 pages, 4828 KiB

Open AccessArticle

From Street to Road: An Innovative Approach to Explore Discarded Chewing Gum as a Performance-Enhancing Modifier for Road Pavement Applications

by Nader Nciri, Namho Kim and Namjun Cho

Polymers 2021, 13(12), 1963; https://doi.org/10.3390/polym13121963 - 14 Jun 2021

Cited by 11 | Viewed by 4158

Abstract

To uncover the potential benefits of discarded chewing gum (DCG) as a performance-enhancing modifier for road pavement applications, its influence on the asphalt binder’s attributes was profoundly examined. The base AP-5 asphalt along with its specimens dosed with various fractions of DCG (e.g., 3, 6, and 9 wt%) were analyzed by Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thin-layer chromatography-flame ionization detection (TLC-FID), scanning electron microscopy (SEM), atomic force microscopy (AFM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). Brookfield viscometer, ring and ball softening point, needle penetration, and dynamic shear rheometer (DSR) tests were adopted to inspect the physical and rheological changes of asphalt cement after DCG incorporation. FT-IR disclosed that the asphalt-gum interaction was not chemical but physical in nature, whilst XRD demonstrated the existence of talc filler in DCG, which may confer the bituminous mixes with exceptional engineering properties. Iatroscan analysis evinced that the gum treatment particularly altered the aromatic and resin fractions; meanwhile, the content of saturates and asphaltenes remained relatively unchanged. SEM divulged that the DCG has a complete dissolution within the bitumen matrix, which becomes rougher due to higher dose administration. AFM revealed that the steady gum introduction amplified the size of bee-like structures, shrunk their peri-phase domains, and wiped out the para-phase domains entirely. TGA/DTGA/DSC data highlighted that the high-temperature-stable additive slightly affected the thermal properties of blends. DSR and empirical rheological tests showed that the waste gum made the bitumen less vulnerable to heat and tender, thereby boosting its resistance against fatigue cracking at intermediate service temperatures. On top of that, DCG widened the thermal window of bitumen performance grade (PG), and preserved its viscosity at standard temperatures, leading to maintaining an appropriate workability for asphalt mix. In brief, the use of discarded chewing gum as an asphalt modifier is feasible and could mitigate plastic pollution and provide durable roadways by delivering superior performance. Full article

(This article belongs to the Special Issue Recycling of Plastics)

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21 pages, 69472 KiB

Open AccessArticle

Analysis of the Storage Stability Property of Carbon Nanotube/Recycled Polyethylene-Modified Asphalt Using Molecular Dynamics Simulations

by Caihua Yu, Kui Hu, Qilin Yang, Dandan Wang, Wengang Zhang, Guixiang Chen and Chileshe Kapyelata

Polymers 2021, 13(10), 1658; https://doi.org/10.3390/polym13101658 - 20 May 2021

Cited by 55 | Viewed by 4315

Abstract

Carbon nanotubes (CNTs) can improve the storage properties of modified asphalt by enhancing the interfacial adhesion of recycled polyethylene (RPE) and base asphalt. In this study, the interaction of CNT/RPE asphalt was investigated using molecular dynamics simulation. The base asphalt was examined using a 12-component molecular model and verified by assessing the following properties: its four-component content, elemental contents, radial distribution function (RDF) and glass transition temperature. Then, the adhesion properties at the interface of the CNT/RPE-modified asphalt molecules were studied by measuring binding energy. The molecular structural stability of CNTs at the interface between RPE and asphalt molecules was analyzed through the relative concentration distribution. The motion of molecules in the modified asphalt was studied in terms of the mean square displacement (MSD) and diffusion coefficient. The results showed that CNTs improved the binding energy between RPE and base asphalt. CNTs not only weakened the repulsion of RPE with asphaltenes and resins, but also promoted the interaction of RPE with light components, which facilitated the compatibility of RPE with the base asphalt. The change in the interaction affected the molecular motion, and the molecular diffusion coefficient in the CNT/RPE-modified asphalt system was significantly smaller than that of RPE-modified asphalt. Moreover, the distribution of the asphaltene component was promoted by CNTs, resulting in the enhancement of the storage stability of RPE-modified asphalt. The property indexes indicated that the storage stability was significantly improved by CNTs, and better viscoelastic properties were also observed. Our research provides a foundation for the application of RPE in pavement engineering. Full article

(This article belongs to the Special Issue Recycling of Plastics)

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9 pages, 1668 KiB

Open AccessArticle

Waste-to-Fuels: Pyrolysis of Low-Density Polyethylene Waste in the Presence of H-ZSM-11

by Nahyeon Lee, Junghee Joo, Kun-Yi Andrew Lin and Jechan Lee

Polymers 2021, 13(8), 1198; https://doi.org/10.3390/polym13081198 - 7 Apr 2021

Cited by 39 | Viewed by 6177

Abstract

Herein, the pyrolysis of low-density polyethylene (LDPE) scrap in the presence of a H-ZSM-11 zeolite was conducted as an effort to valorize plastic waste to fuel-range chemicals. The LDPE-derived pyrolytic gas was composed of low-molecular-weight aliphatic hydrocarbons (e.g., methane, ethane, propane, ethylene, and propylene) and hydrogen. An increase in pyrolysis temperature led to increasing the gaseous hydrocarbon yields for the pyrolysis of LDPE. Using the H-ZSM-11 catalyst in the pyrolysis of LDPE greatly enhanced the content of propylene in the pyrolytic gas because of promoted dehydrogenation of propane formed during the pyrolysis. Apart from the light aliphatic hydrocarbons, jet fuel-, diesel-, and motor oil-range hydrocarbons were found in the pyrolytic liquid for the non-catalytic and catalytic pyrolysis. The change in pyrolysis temperature for the catalytic pyrolysis affected the hydrocarbon compositions of the pyrolytic liquid more materially than for the non-catalytic pyrolysis. This study experimentally showed that H-ZSM-11 can be effective at producing fuel-range hydrocarbons from LDPE waste through pyrolysis. The results would contribute to the development of waste valorization process via plastic upcycling. Full article

(This article belongs to the Special Issue Recycling of Plastics)

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19 pages, 28017 KiB

Open AccessArticle

Improving Rheological and Mechanical Properties of Various Virgin and Recycled Polypropylenes by Blending with Long-Chain Branched Polypropylene

by Sascha Stanic, Thomas Koch, Klaus Schmid, Simone Knaus and Vasiliki-Maria Archodoulaki

Polymers 2021, 13(7), 1137; https://doi.org/10.3390/polym13071137 - 2 Apr 2021

Cited by 15 | Viewed by 4949

Abstract

Blends of two long-chain branched polypropylenes (LCB-PP) and five linear polypropylenes (L-PP) were prepared in a single screw extruder at 240 °C. The two LCB-PPs were self-created via reactive extrusion at 180 °C by using dimyristyl peroxydicarbonate (PODIC C126) and dilauroyl peroxide (LP) as peroxides. For blending two virgin and three recycled PPs like coffee caps, yoghurt cups and buckets with different melt flow rate (MFR) values were used. The influence of using blends was assessed by investigating the rheological (dynamic and extensional rheology) and mechanical properties (tensile test and impact tensile test). The dynamic rheology indicated that the molecular weight as well as the molecular weight distribution could be increased or broadened. Also the melt strength behavior could be improved by using the two peroxide modified LCB-PP blends on the basis of PODIC C126 or PEROXAN LP (dilauroyl peroxide). In addition, the mechanical properties were consistently enhanced or at least kept constant compared to the original material. In particular, the impact tensile strength but also the elongation at break could be increased considerably. This study showed that the blending of LCB-PP can increase the investigated properties and represents a promising option, especially when using recycled PP, which demonstrates a real “up-cycling” process. Full article

(This article belongs to the Special Issue Recycling of Plastics)

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37 pages, 3437 KiB

Open AccessReview

Strategic Possibility Routes of Recycled PET

by Damayanti and Ho-Shing Wu

Polymers 2021, 13(9), 1475; https://doi.org/10.3390/polym13091475 - 2 May 2021

Cited by 159 | Viewed by 25378

Abstract

The polyethylene terephthalate (PET) application has many challenges and potential due to its sustainability. The conventional PET degradation was developed for several technologies to get higher yield products of ethylene glycol, bis(2-hydroxyethyl terephthalate) and terephthalic acid. The chemical recycling of PET is reviewed, such as pyrolysis, hydrolysis, methanolysis, glycolysis, ionic-liquid, phase-transfer catalysis and combination of glycolysis–hydrolysis, glycolysis–methanolysis and methanolysis–hydrolysis. Furthermore, the reaction kinetics and reaction conditions were investigated both theoretically and experimentally. The recycling of PET is to solve environmental problems and find another source of raw material for petrochemical products and energy. Full article

(This article belongs to the Special Issue Recycling of Plastics)

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16 pages, 4489 KiB

Open AccessReview

Realization of Circular Economy of 3D Printed Plastics: A Review

by Caihan Zhu, Tianya Li, Mohamedazeem M. Mohideen, Ping Hu, Ramesh Gupta, Seeram Ramakrishna and Yong Liu

Polymers 2021, 13(5), 744; https://doi.org/10.3390/polym13050744 - 27 Feb 2021

Cited by 65 | Viewed by 10003

Abstract

3D printing technology is a versatile technology. The waste of 3D printed plastic products is a matter of concern because of its impact on the circular economy. In this paper, we discuss the current status and problems of 3D printing, different methods of 3D printing, and applications of 3D printing. This paper focuses on the recycling and degradation of different 3D printing materials. The degradation, although it can be done without pollution, has restrictions on the type of material and time. Degradation using ionic liquids can yield pure monomers but is only applicable to esters. The reprocessing recycling methods can re-utilize the excellent properties of 3D printed materials many times but are limited by the number of repetitions of 3D printed materials. Although each has its drawbacks, the great potential of the recycling of 3D printed waste plastics is successfully demonstrated with examples. Various recycling approaches provide the additional possibility of utilizing 3D printing waste to achieve more efficient circular application. Full article

(This article belongs to the Special Issue Recycling of Plastics)

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