Assessment of plastic waste materials degradation through near infrared spectroscopy

Aging of plastics and microplastics in the environment: a review on influencing factors, quantification methods, challenges, and future perspectives

The ubiquitous presence of fragmented plastic particles needs comprehensive understanding of its fate in the environment. The long-term persistence of microplastics (MPs) in the environment is a significant threat to the ecosystem. Even though various degradation mechanisms (physical, chemical, and biological) of commonly used plastics have been demonstrated, quantifying the degradation of MPs over time to predict the consequence of plastic littering and its persistence in the environment remains a challenge. Different advanced analytical techniques have been used to quantify the degradation of MPs by introducing various parameters such as bond indices, crystallinity, and carbon-oxygen ratio. However, a simple and widely accepted reliable methodology for comparing the environmental factors and their influence on the MP degradation has yet to be developed and validated. This paper reviews a section of relevant literature (n = 38) to synthesize an overview of methods implemented for the quantification of fragmentation and aging of MPs in natural and artificial environment. In addition, the inherent weakness and extrinsic factors affecting the degradation of MPs in the environment is discussed. Finally, it proposes challenges and future scope as guideline for research on MP degradation in the environment.

A critical review and future perspective of plastic waste recycling

Plastic waste is increasing rapidly due to urbanisation and globalization. In recent decades, plastic usage increased, and the upward trend is expected to continue. Only 9% of the 7 billion tonnes of plastic produced were recycled in India until 2022. India generates 1.5 million tonnes of plastic waste (PW) every year and ranks among top ten plastic producer countries. Large amount of waste plastics could harm environment and human health. The current manuscript provides a comprehensive approach for mechanical and chemical recycling methods. The technical facets of mechanical recycling relating to collection, sorting, grading, and general management to create plastic products with additional value have been elaborated in this study. Another sustainable methods aligned with the chemical recycling using pyrolysis, gasification, hydrocracking, IH2 (Integrated Hydropyrolysis 2), and KDV (Katalytische Drucklose Verolung) techniques have also been highlighted with the critical process parameters for the sustainable conversion of plastic waste to valuable products. The review also adheres to less carbon-intensive plastic degrading strategies that take a biomimetic approach using the microorganism based biodegradation. The informative aspects covering the limitations and effectiveness of all PW technologies and its applications towards plastic waste management (PWM) are also emphasized. The existing practices in PW policy guidelines along with its economic and ecological aspects have also been discussed.

Optical sensors and machine learning algorithms in sensor-based material flow characterization for mechanical recycling processes: A systematic literature review

Digital technologies hold enormous potential for improving the performance of future-generation sorting and processing plants; however, this potential remains largely untapped. Improved sensor-based material flow characterization (SBMC) methods could enable new sensor applications such as adaptive plant control, improved sensor-based sorting (SBS), and more far-reaching data utilizations along the value chain. This review aims to expedite research on SBMC by (i) providing a comprehensive overview of existing SBMC publications, (ii) summarizing existing SBMC methods, and (iii) identifying future research potentials in SBMC. By conducting a systematic literature search covering the period 2000 - 2021, we identified 198 peer-reviewed journal articles on SBMC applications based on optical sensors and machine learning algorithms for dry-mechanical recycling of non-hazardous waste. The review shows that SBMC has received increasing attention in recent years, with more than half of the reviewed publications published between 2019 and 2021. While applications were initially focused solely on SBS, the last decade has seen a trend toward new applications, including sensor-based material flow monitoring, quality control, and process monitoring/control. However, SBMC at the material flow and process level remains largely unexplored, and significant potential exists in upscaling investigations from laboratory to plant scale. Future research will benefit from a broader application of deep learning methods, increased use of low-cost sensors and new sensor technologies, and the use of data streams from existing SBS equipment. These advancements could significantly improve the performance of future-generation sorting and processing plants, keep more materials in closed loops, and help paving the way towards circular economy.

Application of Hyperspectral Imaging for identification of aging state of Styrene-Butadiene-Styrene

In the context of the circular economy, the sorting process during recycling of polymers is essential as regards the efficiency of the process itself and the quality of the so-obtained recycled materials. In this paper, the application of Hyperspectral Imaging (HSI) is proposed for this purpose, providing additional insight into the state of aging and the polymer quality. The underlying study comprises HSI in the wavelength range of 1115-1678 nm considering artificially aged Styrene-Butadiene-Styrene (SBS), where aging is performed for 2, 5, 10, 15 and 20 days in a forced-draft oven at a temperature of 105 °C. The obtained HSI spectra are normalized using the Standard Normal Variate (SNV) method, with the normalized spectra as well as their first and second derivative entering the modeling attempt for SBS aging. For the latter, different partial least squares regression (PLSR) models are evaluated, where the original spectra achieved a correlation of R²=0.94 and a root mean squared error of prediction (RMSEP) of 1.83 days, showing the suitability of HSI for the proper identification of the state of aging of SBS and its potential use for other polymers.

Multispectral Wavebands Selection for the Detection of Potential Foreign Materials in Fresh-Cut Vegetables

Ensuring the quality of fresh-cut vegetables is the greatest challenge for the food industry and is equally as important to consumers (and their health). Several investigations have proven the necessity of advanced technology for detecting foreign materials (FMs) in fresh-cut vegetables. In this study, the possibility of using near infrared spectral analysis as a potential technique was investigated to identify various types of FMs in seven common fresh-cut vegetables by selecting important wavebands. Various waveband selection methods, such as the weighted regression coefficient (WRC), variable importance in projection (VIP), sequential feature selection (SFS), successive projection algorithm (SPA), and interval PLS (iPLS), were used to investigate the optimal multispectral wavebands to classify the FMs and vegetables. The application of selected wavebands was further tested using NIR imaging, and the results showed good potentiality by identifying 99 out of 107 FMs. The results indicate the high applicability of the multispectral NIR imaging technique to detect FMs in fresh-cut vegetables for industrial application.

Influence of long-term natural degradation processes on near-infrared spectra and sorting of post-consumer plastics

The large-amount production and application of plastics since the 1950s has led to different environmental problems, and the production amount is still increasing. In 2015, 79 wt% of all plastic waste was accumulated in landfills or the natural environment. Due to their negative influence to the environment, the problems of landfilling and marine litter need urgent treatments. Accordingly, measures like excavation of landfill sites and ocean clean-ups were conducted to reduce their environmental influences and move further towards a closed loop of material cycles. For a possible recycling, the valuable material fractions need to be separated from other materials. Besides, to ensure a high-quality recycling and enable the different recycling processes of plastics in different degradation levels, it is necessary to separate degraded and non-degraded plastics. In this study, the possibility to classify and sort landfill and marine litter plastics is investigated. For this purpose, waste plastics from different origins (lightweight packaging (LWP) waste, landfill, and marine litter) were collected and analyzed with the state-of-the-art technology in sorting plants: near-infrared spectroscopy. With self-developed programs, the classification possibility and performance was determined. The classification accuracy of degraded plastics (from landfill and marine litter) is improved from > 75% to > 97% through adjusting the sorting recipe. Besides, the long-term degraded plastics under natural environment were able to be separated from LWP waste: the same kind of materials can be classified according to their origin (LWP or after long-term degradation), which makes a quality control possible and enables an extra treatment for degraded plastics.

Towards Higher Quality of Recycled Plastics: Limitations from the Material’s Perspective

The increasing consumption of plastics and plastic products results in correspondingly substantial volumes of waste, which poses considerable environmental burdens. With the ongoing environmental actions, the application of circular economy on this waste stream is becoming inevitable. In this paper, the topics of plastics recycling, circular economy on plastics, and challenges to plastic waste recycling are critically reviewed. In the first part of this paper, the development of research on plastic recycling was viewed from 1950 until 2020 using the scientific database Web of Science, and 682 related studies were found and used to assess the changing research priorities along that timeline. The following sections discuss the potentials and requirements to enhance the quality of the produced recycled plastic, in connection with the factors that currently limit it. In conclusion, the quality of recycled plastic is generally determined by the homogeneity of the recovered plastic feed. There are various strategies which could be implemented to overcome the hindrances identified in the paper and to improve the quality of the recycled plastic, such as working on enhanced product designs for minimised waste heterogeneity and controlling the materials’ degree of contamination by applying advanced sorting.

Is froth flotation a potential scheme for microplastics removal? Analysis on flotation kinetics and surface characteristics

Increasing microplastics (MPs) cause significant threats to the ecosystem and society. The tremendous advances concerning the sources, occurrence, chemical behavior, toxicology, and ecological effects contribute to the emerging MPs removal. Based on the intrinsic hydrophobicity of MPs, froth flotation can remove MPs from water environments via bubble attachment on hydrophobic surfaces. This study comprehensively investigated plastic, aqueous, and operating variables in the flotation removal of polyethylene terephthalate (PET) and polystyrene (PS) MPs, assisted by numerous bench-scale experiments and a first-order model with rectangular distribution of floatability. Froth flotation performed better to remove MPs with higher density, larger size, and lower concentration. K⁺ (0-50 mM), Na⁺ (0-150 mM), and Ca²⁺ (0-10 mM) did not affect the flotation recovery of MPs. MPs particles could be thoroughly removed by froth flotation when humic acid (HA) and Al³⁺ concentrations were less than 30 mg/L and 0.05 mM, respectively. 100% of MPs could be removed at a rapid flotation rate under aeration volume of 5.4 mL/min and frother dosage of 28 mg/L. Non-covalent interactions and near-surface water film might favor the adhesion of hydrophilic species and obstruct the flotation removal of MPs. The froth flotation-based MPs removal had potential application in multiple flow systems due to its simplicity and continuity.

The degree and source of plastic recyclates contamination with polycyclic aromatic hydrocarbons

In this research, the degree and source of recyclates contamination with polycyclic aromatic hydrocarbons (PAH) was studied in eight different polyolefin recyclate samples; four originating from post-consumer packaging waste and four originating from a mixed source (post-industrial, post-commercial, and post-consumer). The aim was to assess the applicability of these recyclates in the different products' categories. Furthermore, the impact of previous contamination with PAH was excluded by analysing pure plastics before and after undergoing simulated recycling processes. Polythene recyclates originating from post-consumer plastic packaging waste had lower concentrations of the 16-US-EPA PAH (922.15 ± 420.75 μg kg⁻¹) in comparison to the ones of a mixed origin (2155.43 ± 991.85 μg kg⁻¹), r = −0.35, p > 0.05. The degree of recyclates contamination with PAH was always within the REACH limits for consumer products (<1.0 mg kg⁻¹). On the other hand, only polythene recyclate sample originating from post-commercial waste did not comply with the REACH limits for children articles (0.5 mg kg⁻¹). Hence, the source of plastic waste defines the quality of recyclates. All in all, the results indicated that the contamination of polyolefin recyclates with PAH is attributed to the material's previous contamination, or the sorption of plastics to organic compounds from the surrounding environment. Exposing plastics containing PAH additives to heat during extrusion could result in further accumulation of PAH in plastics.

The degree and source of recyclates contamination with polycyclic aromatic hydrocarbons was studied in 8 polyolefin recyclate samples; 4 from post-consumer packaging waste and 4 from a mixed source (post-industrial, post-commercial, and post-consumer).

Assessment of Supercritical CO2 Extraction as a Method for Plastic Waste Decontamination

Due to the lack of advanced methods to clean plastic waste from organic contaminants, this study aimed at evaluating supercritical extraction as a decontamination method. Oil-adhesive high-density polyethylene (HD-PE) oil containers were subjected to supercritical extraction using supercritical carbon dioxide. The extraction was conducted at 300 bar, applying various temperatures (i.e., 70, 80 and 90 °C). The study assessed the impact of temperature on the decontamination efficiency. The variation in the samples’ quality was first analyzed using near infrared (NIR) spectroscopy. An analysis of the content of polycyclic aromatic hydrocarbons (PAHs) was followed. Samples treated at 70 and 80 °C showed higher extraction efficiencies, in spite of the lower extraction temperatures. The NIR analysis showed that the plastic specimens did not experience degradation by the supercritical decontamination method. Moreover, the NIR spectra of the extracted oil showed the presence of a wide range of compounds, some of which are hazardous. This has been confirmed by a GC-MS analysis of the extracted oil. Based on the provided assessment, the quality of the decontaminated HD-PE plastic samples—from a contamination point of view—is enhanced in comparison to untreated samples. The level of PAHs contamination decreased to be within the allowed limits defined by the REACH regulation, and also met the specifications of the German Product Safety Committee. This study proved the effectiveness of the supercritical extraction using CO₂ in extracting organic contaminants from plastics, while maintaining their quality.

MIR spectral characterization of plastic to enable discrimination in an industrial recycling context: III. Anticipating impacts of ageing on identification

Ageing of polymers entails important structural changes and degrades their functional properties, particularly their aspect. Since sorting is a primordial step in recycling to achieve acceptable mechanical properties, the use of promising technologies such as MIR-HSI (Mid-Infrared Hyperspectral Imagery), which could overcome black plastics sorting issue, has to take into account the influence of ageing on identification. As ageing strongly impacts spectra, it can create confusion between materials, especially in an automatized scheme. Based on laboratory FTIR-ATR (Fourier-Transform Infrared Attenuated Total Reflection), this work investigates spectral evolutions of natural and accelerated photodegradation of Waste of Electric and Electrical Equipment plastics (WEEE) as PE, PP, HIPS, ABS and PC to help identifying a polymer despite its ageing degree. Oxidation marks were described and retrieved within a stock of about one hundred of real waste samples, then differentiated from other sources of spectral alteration as formulation. Laboratory ageing data were found to be consistent and often more extreme than real waste samples values. Generally, styrenics showed stronger spectral alteration than polyolefins despite their respective aspects. No significant spectral alteration of PC was obtained here or observed in the waste stock. As an important oxidation marker, the carbonyl peak was also found to often enable fast identification through its wavenumber. If well taken in account, ageing should not induce confusion with other polymers, even formulated, as characteristic signals are different. Finally, the different industrial sub-ranges within MIR are not affected at the same degree, possibly influencing a technological choice for industrial sorting.

Decontamination and recycling of agrochemical plastic packaging waste

Agrochemical containers shall undergo decontamination before being considered for recycling. This study provides an assessment on the feasibility of the triple-rinsing decontamination procedure, while evaluating the appropriateness of the material's quality for recycling. To achieve the objectives of the study, (1) the effectiveness of the decontamination procedure was investigated; (2) containers´ long storage times and changes on the polymer´s structure were assessed; and (3) the quality of the recycled material was tested. Results showed that the triple-rinsing procedure was ineffective for the container´s complete decontamination, yet a further washing step - performed during the simulation of the recycling process - allowed for an improved degree of decontamination for recycling. Photo-oxidation imposed significant changes on the chemical structure of the polymer, where the active ingredient could be detected by FTIR, even after the application of rinsing and extraction. The chemical structure of the bulk material has not changed, indicating that the pesticide mobilization was only confined to the surface. The mechanical tests showed material quality appropriateness, where tensile strength values were within the suggested ranges, providing a possibility for further utilization of this material when appropriate decontamination is applied.

Acrylonitrile Butadiene Styrene and Polypropylene Blend with Enhanced Thermal and Mechanical Properties for Fused Filament Fabrication

Acrylonitrile butadiene styrene (ABS) is the oldest fused filament fabrication (FFF) material that shows low stability to thermal aging due to hydrogen abstraction of the butadiene monomer. A novel blend of ABS, polypropylene (PP), and polyethylene graft maleic anhydride (PE-g-MAH) is presented for FFF. ANOVA was used to analyze the effects of three variables (bed temperature, printing temperature, and aging interval) on tensile properties of the specimens made on a custom-built pellet printer. The compression and flexure properties were also investigated for the highest thermal combinations. The blend showed high thermal stability with enhanced strength despite three days of aging, as well as high bed and printing temperatures. Fourier-transform infrared spectroscopy (FTIR) provided significant chemical interactions. Differential scanning calorimetry (DSC) confirmed the thermal stability with enhanced enthalpy of glass transition and melting. Thermogravimetric analysis (TGA) also revealed high temperatures for onset and 50% mass degradation. Signs of chemical grafting and physical interlocking in scanning electron microscopy (SEM) also explained the thermo-mechanical stability of the blend.