Recycling

Research

17 pages, 11496 KiB

Open AccessArticle

Secondary Treatment Facilitating the Mechanical Recycling of Film-Coated Waste Automobile Bumpers

by Tetsuo Takayama, Toshiyuki Niiyama, Tadao Tanabe and Jeongsoo Yu

Recycling 2025, 10(2), 74; https://doi.org/10.3390/recycling10020074 - 14 Apr 2025

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Concerns have been raised regarding the mechanical recycling rates of automotive parts, which appear to be low in comparison to those of other sectors. Addressing this issue demands the promotion of the mechanical recycling of waste automobile bumpers. This study investigates primer treatment [...] Read more.

Concerns have been raised regarding the mechanical recycling rates of automotive parts, which appear to be low in comparison to those of other sectors. Addressing this issue demands the promotion of the mechanical recycling of waste automobile bumpers. This study investigates primer treatment effects on the mechanical properties of injection-molded waste bumpers. The primer treatment effects vary depending on the bumper coating. The flexural strength of coated bumpers is greater: 27.6 MPa vs. 29.2 MPa. The flexural modulus is also greater: 1667 MPa vs. 1761 MPa. By contrast, the notched Charpy impact strength is less: 25.4 kJ/m² vs. 23.4 kJ/m². The flexural strength and flexural modulus of the bumpers lacking a coating are also lower, respectively, at 25.0 MPa vs. 22.9 MPa and 1523 MPa vs. 1314 MPa. However, the notched Charpy impact strength is greater: 40.0 kJ/m² vs. 73.6 kJ/m². These findings suggest that primer treatment can control the mechanical properties of injection-molded parts produced from waste automobile bumpers, which is an important achievement for promoting the mechanical recycling of waste automobile bumpers. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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21 pages, 11384 KiB

Open AccessArticle

Optimization of the Recycling Process for Aligned Short Carbon Fiber TuFF Composites

by Uday Kiran Balaga, Aydin Gunes, Tekin Ozdemir, Chris Blackwell, Mark Davis, Steven Sauerbrunn, Lukas Fuessel, Joseph M. Deitzel and Dirk Heider

Recycling 2025, 10(2), 55; https://doi.org/10.3390/recycling10020055 - 1 Apr 2025

Viewed by 310

Abstract

Recycling of carbon fibers enables a sustainable feedstock for industrial applications of high-performance composite materials. This allows light weighting with recycled carbon fibers due to their superior mechanical properties while reducing the high embodied energy and cost of virgin carbon fiber composites. This [...] Read more.

Recycling of carbon fibers enables a sustainable feedstock for industrial applications of high-performance composite materials. This allows light weighting with recycled carbon fibers due to their superior mechanical properties while reducing the high embodied energy and cost of virgin carbon fiber composites. This study optimizes a pyrolysis cycle for fiber recovery of an aerospace-grade thermoset prepreg and a cleaning (oxidation) step to minimize fiber degradation and left-over resin residue, enabling dispersion and alignment of the recycled, discontinuous fibers in the Tailorable Universal Feedstock for Forming alignment process. The study balances the influence of the optimized thermal cycle (pyrolysis + oxidation step) on recycled carbon fiber strength retention with the ability to disperse at the filament level to create aligned, recycled carbon fiber composite samples with high fiber volume fraction. The optimized thermal cycle for efficient fiber recovery applied a pyrolysis step at 500 °C for 4 h in an inert gas environment and an additional oxidation step at the same temperature for 100 min. This resulted in ~20% strength degradation of the fiber compared to the virgin fiber. The processed recycled composite achieved 44% fiber volume fraction with full modulus translation (~128 GPa) compared to the virgin continuous composite with strength translation (~870 MPa), reaching ~50%. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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20 pages, 1068 KiB

Open AccessArticle

FTIR-Based Microplastic Classification: A Comprehensive Study on Normalization and ML Techniques

by Octavio Villegas-Camacho, Iván Francisco-Valencia, Roberto Alejo-Eleuterio, Everardo Efrén Granda-Gutiérrez, Sonia Martínez-Gallegos and Daniel Villanueva-Vásquez

Recycling 2025, 10(2), 46; https://doi.org/10.3390/recycling10020046 - 18 Mar 2025

Viewed by 602

Abstract

This study examines the potential of machine learning (ML) and deep learning (DL) techniques for classifying microplastics using Fourier-transform infrared (FTIR) spectroscopy. Six commonly used industrial plastics (PET, HDPE, PVC, LDPE, PP, and PS) were analyzed. A significant contribution of this research is [...] Read more.

This study examines the potential of machine learning (ML) and deep learning (DL) techniques for classifying microplastics using Fourier-transform infrared (FTIR) spectroscopy. Six commonly used industrial plastics (PET, HDPE, PVC, LDPE, PP, and PS) were analyzed. A significant contribution of this research is the use of broader and more varied spectral ranges than those typically reported in the state of the art. Furthermore, the impact of different normalization techniques (Min-Max, Max-Abs, Sum of Squares, and Z-Score) on classification accuracy was evaluated. The study assessed the performance of ML algorithms, such as k-nearest neighbors (k-NN), support vector machines (SVM), naive Bayes (NB), random forest (RF), and artificial neural networks architectures (including convolutional neural networks (CNNs) and multilayer perceptrons (MLPs)). Models were trained and validated using the FTIR-PLASTIC-c4 dataset with a 10-fold cross-validation approach to ensure robustness. The results showed that Z-score normalization significantly improved stability and generalization across most models, with CNN, MLP, and RF achieving near-perfect values in accuracy, precision, recall, and F1-score. In contrast, the sum of squares normalization was less effective, particularly for CNNs, due to its sensitivity to scale and data distribution. Notably, naive Bayes consistently underperformed because of its limitations in analyzing complex spectral data. The findings highlight the effectiveness of FTIR spectra with broad and variable ranges for the automated classification of microplastics using ML techniques, along with appropriate normalization methods. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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17 pages, 1971 KiB

Open AccessArticle

Evaluating Plastic Waste Management Strategies: Logistic Regression Insights on Pyrolysis vs. Recycling

by Dimitrios-Aristotelis Koumpakis, Christos Vlachokostas, Apostolos Tsakirakis and Savvas Petridis

Recycling 2025, 10(2), 33; https://doi.org/10.3390/recycling10020033 - 1 Mar 2025

Viewed by 1109

Abstract

The global plastic production total has risen to more than 400 million tons per year; this number is mainly driven by industrial appliances. In the EU, where the annual production is about 30 million tons, only 32% of plastic waste is recycled. Therefore, [...] Read more.

The global plastic production total has risen to more than 400 million tons per year; this number is mainly driven by industrial appliances. In the EU, where the annual production is about 30 million tons, only 32% of plastic waste is recycled. Therefore, a need for a robust and efficient waste management strategy has emerged. This study will introduce a novel logistic regression-based decision-making framework that focuses on the environment and the economy while also considering energy intensity and logistics. These factors reflect the use of Life Cycle Assessment (LCA) in this study, which is an approach that determines the selection of waste management strategies across different European countries. This study introduces a model with 94% sensitivity and 97% overall accuracy in order to compare pyrolysis and plastic waste recycling management methods. One of the main findings is the fact that pyrolysis demonstrated a maximum conversion efficiency of 88%; in comparison, the conversion efficiency for recycling was approximately 58%. Pyrolysis also generates by-products, such as syngas and pyrolytic oil, which are valuable. To conclude, this study is a tool for policymakers and industry leaders, so that they can make sustainable waste management decisions with data-driven and evidence-based reasoning. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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19 pages, 2055 KiB

Open AccessArticle

Towards a Circular Solution for Healthcare Plastic Waste: Understanding the Legal, Operational, and Technological Landscape

by Bharghav Ganesh, Sayyed Shoaib-ul-Hasan, Iliass Temsamani and Niloufar Salehi

Recycling 2025, 10(1), 27; https://doi.org/10.3390/recycling10010027 - 14 Feb 2025

Viewed by 943

Abstract

Plastic waste poses a critical challenge in the healthcare sector due to its predominant reliance on a linear “make-use-dispose” model, where plastics are typically incinerated or landfilled. This study examines Swedish healthcare waste management practices, encompassing Swedish and EU regulatory frameworks, hospital protocols, [...] Read more.

Plastic waste poses a critical challenge in the healthcare sector due to its predominant reliance on a linear “make-use-dispose” model, where plastics are typically incinerated or landfilled. This study examines Swedish healthcare waste management practices, encompassing Swedish and EU regulatory frameworks, hospital protocols, disinfection methods, and recycling processes. A key barrier to recycling healthcare plastic waste (HCPW) is the uncertainty surrounding effective decontamination. To overcome this, the paper proposes a circular solution involving on-site microwave-assisted disinfection and shredding, followed by chemical recycling through pyrolysis. This approach considers operational, legal, and technological landscapes and underscores the need for a multidisciplinary solution to enable the transition. This paper also presents a stakeholder collaboration and value capture matrix, identifying the shared value in collaboration among key stakeholders, including hospitals and healthcare service providers, on-site disinfection machine manufacturers, waste management firms, and chemical recycling companies, to advance recycling and foster a circular economy for HCPW. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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16 pages, 1323 KiB

Open AccessArticle

Recycling of Bulk Polyamide 6 by Dissolution-Precipitation in CaCl₂-EtOH-H₂O Mixtures

by Ruben Goldhahn, Ann-Joelle Minor, Liisa Rihko-Struckmann, Siew-Wan Ohl, Patricia Pfeiffer, Claus-Dieter Ohl and Kai Sundmacher

Recycling 2025, 10(1), 5; https://doi.org/10.3390/recycling10010005 - 3 Jan 2025

Viewed by 1306

Abstract

To address the problems of virgin plastic production from fossil resources and the growing amount of plastic waste, a rapid transition to a circular economy is being pursued. The separation of mixed plastics into pure fractions is of paramount importance for promoting recycling [...] Read more.

To address the problems of virgin plastic production from fossil resources and the growing amount of plastic waste, a rapid transition to a circular economy is being pursued. The separation of mixed plastics into pure fractions is of paramount importance for promoting recycling and preventing downcycling. In this study, experimental parameters were determined for the selective bulk dissolution of polyamide 6 (PA 6) filaments (1.75 mm diameter, 1 cm length) in CaCl₂-EtOH-H₂O mixtures (CEW) at 75 °C. These parameters included the energy supply mode, dissolution time, CEW composition and CEW:PA mass ratio. Compared with energy supply by microwaves, energy supply by ultrasound improved the yield of dissolved and recovered PA 6 after 5 h from 31% to 52%. In total, the yield of PA 6 after 3 h of bulk dissolution increased from 18% to 69% when the energy supply mode was changed from microwave to ultrasound and the H₂O:EtOH molar ratio of CEW was increased from 0.40 to 1.33 while maintaining an optimal CEW:PA mass ratio of 8.5. Additionally, master plot analysis suggested that dissolution under microwave energy supply followed a contracting cylinder model, whereas dissolution under ultrasonic energy supply aligned with a 2D diffusion or third-order kinetic model. Microscopic observations suggested that, in the case of ultrasonic energy supply, oscillating bubbles on the particle surface enhanced the dissolution rate of PA 6 filaments in CEW. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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31 pages, 4905 KiB

Open AccessArticle

Multi-Domain Assessment of Thermomechanical Recycling Based on Bio-Based and Petroleum-Based Additively Manufactured Components

by Niko Nagengast, Nicolas Mandel, Christian Bay, Frank Döpper, Christian Neuber, Hans-Werner Schmidt, Clara Usma-Mansfield and Franz Konstantin Fuss

Recycling 2025, 10(1), 3; https://doi.org/10.3390/recycling10010003 - 2 Jan 2025

Viewed by 865

Abstract

The surge in global population growth and the escalating demand for social and economic prosperity present formidable challenges in the 21st century. However, asserting the sustainability of some ecological impact reduction initiatives, such as recycling, requires a comprehensive evaluation within various domains, including [...] Read more.

The surge in global population growth and the escalating demand for social and economic prosperity present formidable challenges in the 21st century. However, asserting the sustainability of some ecological impact reduction initiatives, such as recycling, requires a comprehensive evaluation within various domains, including performance, ecology, and economics, and contemporary advancements in integrating quantitative assessments of material and manufacturing properties, coupled with mathematical decision-making approaches, contribute to mitigating subjectivity in determining the efficiency of recycling. This paper implements a robust multi-criteria decision-making (MCDM) approach to address the complexities of recycling, validating its implementation and effectiveness through a case study. The focus is set on the application of bio-based polylactic acid (PLA) and petroleum-based polypropylene (PP) additively manufactured (AM) parts produced through Fused Filament Fabrication (an approach to ecology/performance domains). The work introduces a cost analysis focusing on calculating thermomechanical recycling within the economic domain. The well-known Analytical Hierarchical Process (AHP) provides a structured framework for decision-making (the ecological impact domain) with the focus being on application. The assessment or recycling viability, encompassing AHP calculations, preprocessing, and supplementary tools, is provided by developing an open-source software tool for practitioners in the field of material science and manufacturing. The results indicate a preference for industrial-scaled recycling over virgin or lab-recycled manufacturing, particularly for petroleum-based polypropylene. The versatility and simple utilization of the software tool allow seamless integration for diverse use cases involving different materials and processes. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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12 pages, 1378 KiB

Open AccessArticle

Effects of an Inertization System on Waste Plastic Pyrolysis in a Fixed Bed Reactor

by Saša Papuga, Stefano Vecchio Ciprioti, Milica Djurdjevic and Aleksandra Kolundzija

Recycling 2025, 10(1), 2; https://doi.org/10.3390/recycling10010002 - 1 Jan 2025

Viewed by 962

Abstract

This paper reports the results of a study on the significance of the inertization system configuration of a laboratory-scale fixed bed batch reactor with regard to the yield of pyrolysis oil and reactor conversion. Two typical reactor inertization systems were investigated depending on [...] Read more.

This paper reports the results of a study on the significance of the inertization system configuration of a laboratory-scale fixed bed batch reactor with regard to the yield of pyrolysis oil and reactor conversion. Two typical reactor inertization systems were investigated depending on whether the carrier gas (nitrogen in this study) was added from the top or from the bottom of the reactor. Polypropylene (PP) packaging waste (100 g) was used as a model sample. A factorial experimental design was adopted for one categorical parameter, the arrangement of parts of the reactor inertization system. All experiments were conducted at 475 °C, with a carrier gas flow rate of 0.1 L/min and a reaction time of 90 min. Statistical analysis and processing of the results showed that the configuration of the inertization system had a remarkable impact on the pyrolysis oil and gas yield, while its impact on the overall reactor conversion was negligible. When applying the two observed methods of reactor inertization, the average yields of pyrolysis oil and gas differed by 1.7% and 1.8%, respectively. All of the applied statistical treatments had a significance level of 0.05, i.e., there was only a 5% chance of incorrectly rejecting the hypothesis of equality of arithmetic means of pyrolysis yields when the two different methods of reactor inertization were applied. The explanation of this behavior is attributed to the temperature change inside the reactor, which shows that this particular fixed bed reactor suffers from local overheating in its middle part. Local overheating of the middle part of the reactor is more pronounced in the case of inerting the reactor from the bottom, which leads to greater excessive cracking of volatile products compared to the mode of inerting the reactor from the top part and thus greater formation of non-condensable gases, i.e., a reduction in the yield of pyrolytic oil. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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12 pages, 1684 KiB

Open AccessArticle

Effect of Joining Mechanism on the Mechanical Recycling of Polymer–Metal Composite Parts

by Sandra Boekhoff, Harald Zetzener and Arno Kwade

Recycling 2024, 9(6), 106; https://doi.org/10.3390/recycling9060106 - 4 Nov 2024

Viewed by 1651

Abstract

In order to be able to recycle composite components made of polymer and metal, which are used in the automotive industry, the joints must be broken. The success of the separation is influenced by the stress and also by the joining mechanism between [...] Read more.

In order to be able to recycle composite components made of polymer and metal, which are used in the automotive industry, the joints must be broken. The success of the separation is influenced by the stress and also by the joining mechanism between the polymer and the metal. Here, force-fit and form-fit connected components are produced and crushed in a rotor impact mill with two different rotors. The results show that the crushing results differ significantly for the different rotors and for the various joining processes. In short, the hammer-type rotor provides much finer and better-separated fragments and the force-fit joints enable a better separation of metal and polymers. The additional cooling of the samples also changes the result in a way, where deep cooling significantly improves the separation of the metal and the polymer. Different types of polymers also led to a different separation result with both rotors. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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18 pages, 5460 KiB

Open AccessArticle

GC/MS Screening of Substances Released from Post-Consumer Recycled HDPE Pellets into 95% Ethanol: Reproducibility and Variation between Production Batches

by Melanie Brandt, Jens-Peter Vietzke, Dennis Bankmann and Heiner Gers-Barlag

Recycling 2024, 9(5), 101; https://doi.org/10.3390/recycling9050101 - 16 Oct 2024

Viewed by 2277

Abstract

The use of post-consumer recycled (PCR) plastic materials in sensitive packaging applications, such as for cosmetic products and detergents, requires a clear understanding of the identities and quantities of chemical substances, which they may release into packed products. With many potential sources of [...] Read more.

The use of post-consumer recycled (PCR) plastic materials in sensitive packaging applications, such as for cosmetic products and detergents, requires a clear understanding of the identities and quantities of chemical substances, which they may release into packed products. With many potential sources of and thus different types of potentially releasable substances, a reliable non-targeted screening method is required to assess these materials. Such a method should be readily applicable in industrial practice and provide a realistic estimation of substance release. This investigation focused on the use of gas chromatography/coupled mass spectrometry (GC/MS) to analyze substances, which recycled HDPE (rHDPE) plastic pellets release into 95% ethanol under accelerated testing conditions. The results of the repeated testing of reference samples clearly demonstrated the good reproducibility of the described methodology, with standard deviations of repeated determinations of the total released substance amounts of 6.8–8.1%. The application to several production batches of three commercial rHDPE grades additionally demonstrated that the batch-to-batch variation of substances which rHDPE materials release can be confined to less than 10% of variation of the total detectable substance amount. The described methodology is therefore seen as a pragmatic, repeatable assessment of recycled HDPE plastic batches with a view to substance release. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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20 pages, 1980 KiB

Open AccessArticle

Mechanical Recycling of PET Multi-Layer Post-Consumer Packaging: Effects of Impurity Content

by Giusy Santomasi, Francesco Todaro, Andrea Petrella, Michele Notarnicola and Eggo Ulphard Thoden van Velzen

Recycling 2024, 9(5), 93; https://doi.org/10.3390/recycling9050093 - 8 Oct 2024

Cited by 2 | Viewed by 3785

Abstract

The recycling of PET trays is highly challenging. The aim of this paper was to investigate the issues related to the mechanical recycling process and, the correlation between feedstock composition and the quality of the produced rPET. Four feedstocks with different degrees of [...] Read more.

The recycling of PET trays is highly challenging. The aim of this paper was to investigate the issues related to the mechanical recycling process and, the correlation between feedstock composition and the quality of the produced rPET. Four feedstocks with different degrees of impurity were mechanically recycled at a laboratory pilot scale. The optical and thermal properties of the rPET products were examined to determine the quality and to seek relations with the starting level of impurities. The final products of the PET trays’ mechanical recycling were found to be affected by the presence of impurities (organics) and multi-material (non-PET) elements in the feedstocks. The rPET products crystallised faster for contaminated feedstocks showed lower molecular mass and higher yellow index values due to thermal degradation. Yellowing is a crucial parameter in assessing the thermal degradation of rPET. Injection moulded samples corresponding to higher contamination levels, reported values of Yellow Index equal to 179 and 177 compared to 15 of mono-PET sample. The intrinsic viscosity decreased from 0.60 dL/g to just above 0.30 dL/g, and losses were more significant for soiled or multi-material feedstocks. A method of improving the final quality would involve the purification of the starting feedstock from impurities. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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17 pages, 567 KiB

Open AccessArticle

Closing the Loop between Plastic Waste Management and Energy Cogeneration: An Innovative Design for a Flexible Pyrolysis Small-Scale Unit

by Dimitrios-Aristotelis Koumpakis, Alexandra V. Michailidou, Christos Vlachokostas and Christos Mertzanakis

Recycling 2024, 9(5), 92; https://doi.org/10.3390/recycling9050092 - 5 Oct 2024

Cited by 1 | Viewed by 3213

Abstract

This study proposes a simplified unit that can be employed in an industrial facility for the utilization of its own abundant plastic waste, primarily from discarded packaging, to achieve full or partial energy autonomy. By converting this waste into synthetic pyrolysis oil equivalent [...] Read more.

This study proposes a simplified unit that can be employed in an industrial facility for the utilization of its own abundant plastic waste, primarily from discarded packaging, to achieve full or partial energy autonomy. By converting this waste into synthetic pyrolysis oil equivalent to 91,500 L, the industry can power a combined heat and power generation unit. The proposed unit was designed with a focus on maintaining high temperatures efficiently while minimizing oxygen exposure to protect the integrity of hydrocarbons until they transform into new compounds. Pyrolysis stands as a foundational procedure, paving the way for subsequent thermochemical transformations such as combustion and gasification. This study delves into the factors affecting pyrolysis and presents analytically the mathematical formulations and relevant calculations in order to effectively design and apply a real-life system. On this basis, fuels from plastic waste can be produced, suitable for utilization in typical equipment meant to produce heat, estimated for six months’ operation and 800 MWh of electricity. This study enhances the transition towards a more circular and resource-efficient economy with technologies that unlock the latent energy contained within the discarded matter. Additionally, it demonstrates the feasibility of a moderate investment in a co-generation system for industries utilizing 568 tonnes of plastic waste per year. The design and accurate calculations of this study highlight the theoretical potential of this technology, promoting environmental sustainability and resource conservation. Full article

(This article belongs to the Special Issue Challenges and Opportunities in Plastic Waste Management)

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