Abiotic thermo-oxidative degradation of high density polyethylene: Effect of manganese stearate concentration

An investigation into the stability and degradation of plastics in aquatic environments using a large-scale field-deployment study

The fragmentation of plastic debris is a key pathway to the formation of microplastic pollution. These disintegration processes depend on the materials' physical and chemical characteristics, but insight into these interrelationships is still limited, especially under natural conditions. Five plastics of known polymer/additive compositions and processing histories were deployed in aquatic environments and recovered after six and twelve months. The polymer types used were linear low density polyethylene (LLDPE), oxo-degradable LLDPE (oxoLLDPE), poly(ethylene terephthalate) (PET), polyamide-6 (PA6), and poly(lactic acid) (PLA). Four geographically distinct locations across Aotearoa/New Zealand were chosen: three marine sites and a wastewater treatment plant (WWTP). Accelerated UV-weathering under controlled laboratory conditions was also carried out to evaluate artificial ageing as a model for plastic degradation in the natural environment. The samples' physical characteristics and surface microstructures were studied for each deployment location and exposure time. The strongest effects were found for oxoLLDPE upon artificial ageing, with increased crystallinity, intense surface cracking, and substantial deterioration of its mechanical properties. However, no changes to the same extent were found after recovery of the deployed material. In the deployment environments, the chemical nature of the plastics was the most relevant factor determining their behaviours. Few significant differences between the four aquatic locations were identified, except for PA6, where indications for biological surface degradation were found only in seawater, not the WWTP. In some cases, artificial ageing reasonably mimicked the changes which some plastic properties underwent in aquatic environments, but generally, it was no reliable model for natural degradation processes. The findings from this study have implications for the understanding of the initial phases of plastic degradation in aquatic environments, eventually leading to microplastics formation. They can also guide the interpretation of accelerated laboratory ageing for the fate of aquatic plastic pollution, and for the testing of aged plastic samples.

Leaching and transformation of chemical additives from weathered plastic deployed in the marine environment

Plastic pollution causes detrimental environmental impacts, which are increasingly attributed to chemical additives. However, the behaviour of plastic additives in the marine environment is poorly understood. We used a marine deployment experiment to examine the impact of weathering on the extractables profile, analysed by liquid chromatography-mass spectrometry, of four plastics at two locations over nine months in Aotearoa/New Zealand. The concentration of additives in polyethylene and oxo-degradable polyethylene were strongly influenced by artificial weathering, with deployment location and time less influential. By comparison, polyamide 6 and polyethylene terephthalate were comparatively inert with minimal change in response to artificial weathering or deployment time. Non-target analysis revealed extensive differentiation between non-aged and aged polyethylene after deployment, concordant with the targeted analysis. These observations highlight the need to consider the impact of leaching and weathering on plastic composition when quantifying the potential impact and risk of plastic pollution within receiving environments.

Accessing the intrinsic factors of carbonyl index of microplastics: Physical and spectral properties, baseline correction, calculation methods, and their interdependence

Carbonyl index (CI) is a measure of the degradation status of microplastics. While many studies address environmental factors of microplastic degradation, intrinsic factors like physical properties, spectral properties, baseline correction, and CI calculation methods are less explored. This research focused on these aspects using surface seawater samples. We found that color and shape have limited dependence on particle size or signal-to-noise ratio (SNR). Baseline correction can significantly alter CI values, with the direction of the shift depending on the methods used. Additionally, most CI values before and after baseline correction and those calculated using different methods tend to be strongly correlated. Using the selected CI calculation methods, we found that CI values varied significantly by shape and color. CI's relation to the similarity between the sample and its pristine form suggests an alternative degradation measure. Our findings emphasize the need for standardized CI calculation methods.

Challenges of Raman spectra to estimate carbonyl index of microplastics: A case study with environmental samples from sea surface

This study investigates the feasibility of using the carbonyl index (CI) derived from Raman spectra as an indicator of plastic degradation and its relationship with the CI calculated from Fourier transform infrared (FTIR) spectra, using microplastic samples of polyethylene (PE) from surface seawater. Multiple methods were used to calculate the CI values of FTIR spectra, while proposed methods were used to calculate the corresponding CI values of Raman spectra. Some significant relations between FTIR CI and Raman CI were observed. However, small R2 values suggest weak functional relationships, which can be attributed to the low signal-to-noise ratio (SNR) of Raman spectra. These results highlight the challenges of establishing a functional relationship between FTIR CI and Raman CI, including challenges such as the uniformity of Raman spectra, determining optimal Raman measurement parameters, selecting appropriate peaks for Raman CI calculation, deciding on spectral processing methods, and addressing the interdependence of these issues.

Comparative evaluation of the carbonyl index of microplastics around the Japan coast

The carbonyl index (CI) of polyethylene and polypropylene microplastics (MPs) (2950 particles) collected in coastal waters around Japan was investigated. The CI of MPs was calculated by the specified area under band technique. The mean MP CI in all samples (regardless of shape and color) was 0.69 ± 0.34 and 0.70 ± 0.34 for polyethylene and polypropylene, respectively, and there was no significant difference in the color or shape of the MPs. The polyethylene, white, and fragment MPs CI was negatively (p < 0.05) correlated with the major length of the MPs. Large MPs with relatively little deterioration were distributed along the west coast of the Sea of Japan, whereas small MPs were distributed along the east coast. Our findings of this gradual change in the deterioration of MPs, based on geographical distribution, are in accordance with literature CI-size and MP degradation hypotheses.

Biodegradation kinetic modeling of pro-oxidant filled polypropylene composites under thermophilic composting conditions after abiotic treatment

This work aims at modeling and characterizing the kinetics of biodegradation of polypropylene loaded with cobalt stearate as pro-oxidant after abiotic treatment. Eight films of these composites were prepared using different pro-oxidant loadings. These films were treated abiotically using accelerated weathering for 40 h, and biotically using aerobic composting as per ASTM D 5338. The experimental data were analyzed using an eight-parameter Komilis model containing a flat lag phase. The model formulations involved hydrolysis of primary solid carbon and its subsequent mineralization. The first step was rate controlling and it included hydrolysis of slowly (C_s), moderately (C_m), and readily (C_r) hydrolyzable carbon fractions in parallel. The model parameters were evaluated by means of nonlinear regression technique. The surface morphology of the films before and after the biodegradability test supported the biodegradation results. The model parameters and undegraded/hydrolyzable/mineralizable carbon evolutions involved moderately and readily hydrolyzable carbons but with the absence of slowly hydrolyzable carbon. These exhibit degradability in the range of 11.20-36.42% in 45 days. Biodegradability increases with progressive increase in pro-oxidant loading. The rate of degradation reaches maximum (0.322-0.897% per day) at around the 39th-12th day. For all the films, readily hydrolyzable carbon fractions and their hydrolysis rate constants (k_r) are appreciably increased with increasing pro-oxidant loading. All the films show the presence of growth phase because of their high initial readily hydrolyzable carbon fractions. The SEM images after the abiotic and subsequently biotic treatments were progressively rougher. The methods presented here can be used for the design and control of other similar systems.

Effect of pro-oxidant concentration on characteristics of packaging films of cobalt stearate filled polypropylene

In this work, polypropylene (PP) filled with different proportions of CoSt were prepared in a twin-screw extruder by compounding technique. Eight films of these compounds were prepared using compression moulding. The modified PP films were characterized for chemical, physical, thermal, and morphological properties (before and after biodegradation). The biodegradation of the CoSt filled PP films was studied under controlled composting conditions, and the degradation intermediates were evaluated for their ecotoxicological impact. The CoSt present in the PP film was confirmed by Fourier transform infrared spectroscopy. As the addition of CoSt was progressively increased, the tensile strength and thermal stability decreased as shown by UTM and thermogravimetric analysis. The compounding of CoSt in PP reduced its crystallinity as revealed by the differential scanning calorimetry and X-ray diffraction analysis, and this led to enhanced degradation of PP. After biodegradation, SEM results of modified PP films showed rougher morphology than before biodegradation. The maximum biodegradation (19.78%) was shown by the film having 2 phr CoSt. The ecotoxicity tests of the degraded material, namely, microbial test, plant growth test, and earthworm acute-toxicity test demonstrated that the biodegradation intermediates were nontoxic. Hence, CoSt filled PP has high industrial potential to make biodegradable flexible packaging.

Determination of the carbonyl index of polyethylene and polypropylene using specified area under band methodology with ATR-FTIR spectroscopy

The current measurement techniques described in the literature for the determination of the carbonyl index (CI) for polyolefins such as polyethylene and polypropylene were compared and contrasted. These were all found to be inconsistent or inaccurate and were not capable of differentiating significant changes in carbonyl peak evolution throughout accelerated ageing. As a consequence of these findings, a methodology, specified area under band (SAUB) is presented here to more accurately represent the CI as a general means of reporting. The increased precision in the methodology is explained and compared to other methodologies for determining CI. The SAUB method is also shown to be capable of elucidating the differences in relative extent and rates of CI for different polyolefins, exposed to the same conditions over the same time period.