A general kinetic model for the photothermal oxidation of polypropylene

The Duration of Dry Events Promotes PVC Film Fragmentation in Intermittent Rivers

The majority of microplastics (MPs) found in the environment originate from plastic fragmentation occurring in the environment and are influenced by environmental factors such as UV irradiation and biotic interactions. However, the effects of river drying on plastic fragmentation remain unknown, despite the global prevalence of watercourses experiencing flow intermittence. This study investigates, through laboratory experiments, the coupled effects of drying duration and UV irradiation on PVC film fragmentation induced by artificial mechanical abrasion. This study shows that PVC film fragmentation increases with drying duration through an increase in the abundance and size of formed MPs as well as mass loss from the initial plastic item, with significant differences for drying durations >50% of the experiment duration. The average abundance of formed MPs in treatments exposed to severe drying duration was almost two times higher than in treatments nonexposed to drying. Based on these results, we developed as a proof of concept an Intermittence-Based Plastic Fragmentation Index that may provide insights into plastic fragmentation occurring in river catchments experiencing large hydrological variability. The present study suggests that flow intermittence occurring in rivers and streams can lead to increasing plastic fragmentation, unraveling new insights into plastic pollution in freshwater systems.

Natural iron-containing minerals catalyze the degradation of polypropylene microplastics: a route to self-remediation learnt from the environment

Virgin and environmentally aged polypropylene (PP) micropowders (V-PP and E-PP, respectively) were used as reference microplastics (MPs) in comparative photo- and thermo-oxidative ageing experiments performed on their mixtures with a natural ferrous sand (NS) and with a metal-free silica sand (QS). The ferrous NS was found to catalyze the photo-oxidative degradation of V-PP after both UV and simulated solar light irradiation. The catalytic activity in the V-PP/NS mixture was highlighted by the comparatively higher fraction of photo-oxidized PP extracted in dichloromethane, and the higher carbonyl index of the bulk polymer extracted with boiling xylene, when compared with the V-PP/QS mixture. Similarly, NS showed a catalytic effect on the thermal degradation (at T = 60 °C) of E-PP. The results obtained indicate that, under suitable environmental conditions (in this case, an iron-containing sediment or soil matrix, combined with simulated solar irradiation), the degradation of some types of MPs could be much faster than anticipated. Given the widespread presence of iron minerals (including the magnetite and iron-rich serpentine found in NS) in both coastal and mainland soils and sediments, a higher than expected resilience of the environment to the contamination by this class of pollutants is anticipated, and possible routes to remediation of polluted natural environments by eco-compatible iron-based minerals are envisaged.

Exploring the release of hazardous volatile organic compounds from face masks and their potential health risk

Hazardous chemicals released from the petroleum-derived face mask can be inhaled by wearers and cause adverse health effects. Here, we first used headspace solid-phase microextraction coupled with GC-MS to comprehensively analyze the volatile organic compounds (VOCs) released from 26 types of face masks. The results showed that total concentrations and peak numbers ranged from 3.28 to 197 μg/mask and 81 to 162, respectively, for different types of mask. Also, light exposure could affect the chemical composition of VOCs, particularly increasing the concentrations of aldehydes, ketones, organic acids and esters. Of these detected VOCs, 142 substances were matched to a reported database of chemicals associated with plastic packaging; 30 substances were identified by the International Agency for Research on Cancer (IARC) as potential carcinogenic to humans; 6 substances were classified in the European Union as persistent, bioaccumulative, and toxic, or very persistent, very bioaccumulative substance. Reactive carbonyls were ubiquitous in masks, especially after exposure to light. The potential risk of VOCs released from the face masks were then accessed by assuming the extreme scenario that all the VOC residues were released into the breathing air within 3 h. The result showed that the average total concentration of VOCs (17 μg/m3) was below the criterion for hygienic air, but seven substances, 2-ethylhexan-1-ol, benzene, isophorone, heptanal, naphthalene, benzyl chloride, and 1,2-dichloropropane exceeded the non-cancer health guidelines for lifetime exposure. This finding suggested that specific regulations should be adopted to improve the chemical safety of face masks.

Evaluation of natural ageing responses on Burmese amber durability by FTIR spectroscopy with PLSR and ANN models

Amber ageing is an inevitable process, which is very important in precious organic gemstone relics protection. In order to explore the mechanism of amber ageing and estimate the durability of Burmese amber, this research investigates the changing spectral features of Burmese ageing amber via Fourier Transform Infrared Spectroscopy (FTIR) and solid ¹³C Nuclear Magnetic Resonance spectroscopy (NMR) and develops the regression models for its micro-hardness by micro-FTIR spectra. The Partial Least Squares Regression (PLSR) and Artificial Neural Networks (ANN) methods as well as Competitive Adaptive Reweighted Sampling (CARS) algorithm for wavelength variables selection have been applied to predict and assess the Vickers hardness of amber samples with different ageing degrees. As a result, the FTIR and the solid ¹³C NMR spectra reveal that the contents of CO groups (of esters) increase substantially, and which of the other oxygenic groups (CO (of acids), COC, COCC) increase modestly in amber ageing. When comparing with the results of four different models (PLSR, ANN, CARS-PLSR and CARS-ANN), the CARS-PLSR model obtained the optimal results as follows: the squared correlation coefficient of calibration(R²cal) is 0.9230 and the root mean square error of calibration (RMSEC) is 1.2977 HV; the squared correlation coefficient of prediction (R²pre) is 0.7762 and the root mean square error of prediction (RMSEP) is 2.2208 HV. The overall results sufficiently demonstrate that FTIR spectroscopy technique coupled with appropriate chemometrics methods are very promising tools to estimate and predict the hardness property of Burmese ageing amber.

Weathering and fragmentation of plastic debris in the ocean environment

Fragmentation of plastic macro-debris into secondary microplastics [MPs] is primarily the result of their extensive oxidation under exposure to solar UV radiation. The heterogeneity in the marine zones with respect to their oxidative potential for plastics, introduces a marked zonal bias in their ability to carry out weathering and fragmentation. Comparing the oxidative environments of the beach zone and the upper pelagic zone with floating plastics, it is argued that the latter tends to preclude photooxidative fragmentation. Abundant MPs found in seawater are therefore more likely to have originated on beaches or land and subsequently transferred to the water, as opposed to being generated by weathering of floating plastic stock. Laboratory-accelerated weathering of plastics in seawater obtains efficient micro-fragmentation and in some instances photo- dissolution of the plastic debris, but these results cannot be reliably extrapolated to natural weathering conditions in the ocean environment.

New mechanism for autoxidation of polyolefins: kinetic Monte Carlo modelling of the role of short-chain branches, molecular oxygen and unsaturated moieties

Using quantum-chemical calculations and kinetic modelling, we reveal the dominant reaction pathway in polymer autoxidation, (i) whether oxygen promotes or hamper degradation and (ii) how defects in the polymer backbone affect degradation.

Oxidation of ultralene and paraffin due to radiation damage after exposure to soft X-rays probed by FTIR microspectroscopy and X-ray fluorescence

Radiation damage upon soft X-ray exposure is an important issue to be considered in soft X-ray microscopy. The work presented here is part of a more extended study on the topic and focuses on the effects of soft X-rays on paraffin, a common embedding medium for soft-tissues, and on ultralene and Si₃N₄ windows as sample supports. Our studies suggest that the sample environment indeed plays an important role in the radiation damage process and therefore should be carefully taken into account for the analysis and interpretation of new data. The radiation damage effects were followed over time using a combination of Fourier transform infrared (FTIR) microspectroscopy and X-ray fluorescence (XRF), and it was demonstrated that, for higher doses, an oxidation of both embedding medium and ultralene substrate takes place after the irradiated sample is exposed to air. This oxidation is reflected in a clear increase of C=O and O-H infrared bands and on the XRF oxygen maps, correlated with a decrease of the aliphatic infrared signal. The results also show that the oxidation process may affect quantitative evaluation of light element concentrations.

Characterizing microplastic size and morphology of photodegraded polymers placed in simulated moving water conditions

Aquatic plastic debris experiences environmental stressors that lead to breakdown into smaller micro-sized plastic particles. This work quantified microplastic formation with the environmental stressors of UV irradiation followed by mechanical strain induced by movement of water with an emphasis on connecting our results to changes in the materials chemical/physical properties. Polypropylene, polyethylene, and polyethylene terephthalate thin films and polypropylene injection-molded sheets were irradiated with 254 nm UV light, placed into aquatic microcosms, collected through sieving, and counted under a microscope. Results showed increasingly more particles in smaller size classes, the smallest being 74-177 μm. Mechanical strain from the turbulent water caused 2.3-3× more microplastics to be formed for the thinnest (∼25 μm) film and 1.4-2× more for thicker films and sheets. The most common morphology of microplastics was fibers, particularly in thicker polypropylene samples, which was attributed to absorbance of the photons and the changes observed in the crystallinity and glass transition as measured with differential scanning calorimetry (DSC). When irradiated for 24, 48, or 72 h, longer irradiation resulted in more microplastics formed by polypropylene films, which correlated with changes in the glass transition temperature as measured by DSC and the extent of oxidation as measured with FTIR. Irradiation at 300 nm produced fewer microplastics due to slower kinetics of phototransformations. Overall, this work evaluates the impact of combined photodegradation and water motion toward microplastic particles formed. It provides quantitative evidence that mechanical strain of water movement exacerbates photo-induced formation of microplastics and shows that the existence of fibers in natural systems can be the result of photodegradation.

The Fate of the Peroxyl Radical in Autoxidation: How Does Polymer Degradation Really Occur?

Bolland and Gee's basic autoxidation scheme (BAS) for lipids and rubbers has long been accepted as a general scheme for the autoxidation of all polymers. This scheme describes a chain process of initiation, propagation, and termination to describe the degradation of polymers in the presence of O₂. Central to this scheme is the conjecture that propagation of damage to the next polymer chain occurs via hydrogen atom transfer with a peroxyl radical. However, this reaction is strongly thermodynamically disfavored for all but unsaturated polymers, where the product allylic radical is resonance-stabilized. Paradoxically, there is no denying that the autocatalytic degradation and oxidation of saturated polymers still occurs. Critical analysis of the literature, described herein, has begun to unravel this mystery. One possibility is that the BAS still holds for saturated polymers but only at unsaturated defect sites, where H transfer is thermodynamically favorable. Another is that peroxyl termination rather than H transfer is dominant. If this were the case, tertiary peroxyl radicals (formed at quaternary centers or quaternary branching defects) may terminate to form alkoxy radicals, which can much more readily undergo chain transfer. This process would lead to the creation of hydroxy groups on the degraded polymer. On the other hand, primary and secondary peroxyl radicals would terminate to form nonradical products and halt further degradation. As a result, under this scenario the degree of branching and substitution would have a major effect on polymer stability. Herein we survey studies of polymer degradation products and of the effect of polymer structure on stability and show that indeed peroxyl termination is competitive with peroxyl transfer and possibly dominant under some conditions. It is also feasible that oxygen may not be the only reactive atmospheric species involved in catalyzing polymer degradation. Herein we outline plausible mechanisms involving ozone, hydroperoxyl radical, and hydroxyl radical that have all been suggested in the literature and can account for the experimentally observed formation of hydroperoxides without invoking peroxyl transfer. We also show that oxygen itself has even been reported to slow the degradation of poly(methyl methacrylate)s, which might be expected if peroxyl radicals are unreactive toward hydrogen transfer. Discrepancies between the rate of oxidation and the rate of degradation have been observed for polyolefins and also support the counterintuitive notion that oxygen stabilizes these polymers against degradation. We show that together these studies support alternative mechanisms for polymer degradation. A thorough assessment of kinetic studies reported in the literature indicates that they are limited by their propensity to use models based on the BAS, disregarding the chemical differences intrinsic to each class of polymer. Thus, we propose that further work must be done to fully grasp the complex mechanism of polymer degradation under ambient conditions. Nonetheless, our analysis of the literature points to measures that can be used to enhance or prevent polymer degradation and indicates that we should focus beyond just the role of oxygen toward the specific chemical nature and environment of the polymer at hand.