Quantifying the fragmentation of polypropylene upon exposure to accelerated weathering

Abstract

Polymers are omnipresent in our everyday lives. For specific applications, their properties can be extensively modified by various types of additives, e.g., processing stabilizers, antioxidants, UV-stabilizers, flame retardants, and plasticizers. While several additives are nowadays considered to be toxic or persistent in the environment, quantitative data characterizing plastic fragmentation and microplastic formation have not yet been discussed in detail. Here, we present a long-term, laboratory-controlled accelerated weathering study on polypropylene (PP) particles with and without processing stabilizers. We were able to identify the stabilizers as Irgafos® 168, and Irganox® 1010. For both PP sample sets, we monitored the degradation using a combination of various analytical methods, such as gel permeation chromatography, particle size distributions, scanning electron microscopy, solid-state 13C magic-angle spinning NMR and liquid-state 1H, 13C, 31P NMR spectroscopy, differential scanning calorimetry and matrix-assisted laser desorption ionization time of flight mass spectrometry. The stabilizers prevent degradation by simulated solar radiation for about 350 h. Then, degradation sets in rapidly, leading to an exponential decrease in molecular weight and particle size, accompanied by an increase in crystallinity and the formation of oxygen-containing functional groups. After 3200 h, representing approximately 2 years of outdoor weathering, both PP samples exhibit comparable characteristics and sizes, regardless if a stabilizer was initially present. During degradation, an extremely large number of 100,000 daughter particles (4 µm) are formed and released from one MP particle of 192 µm diameter. Their physical properties and chemical composition have largely changed, resulting in a very low molecular weight and a hydrophilic character. These particles no longer resemble pristine PP. We thus expect them to be more prone to biodegradation compared to the starting material.

Implications of a New Test Facility for Fragmentation Investigations on Virgin (Micro)plastics

“…Since fragments (daughter fragments) remained on the parent particle surface after the experimental duration, a rough approximation of the average fragment size and its surface area is possible. Particles in the micron size range adhering to the surface, even after cleaning processes, were reported by Meides et al (2022) as well. Thus, a fragment size distribution can be derived.…”

“…Since biodegradation and hydrolysis are comparably slow processes and thermo-oxidation is negligible in a nearshore environment due to the water’s cooling effect, most studies dealing with fragmentation or degradation focus on abiotic decomposition by photooxidation and direct photolysis. However, a mechanical influence is always necessary to form microplastics. − This partial focus results from the complex task of modeling a natural environment while the aforementioned processes, mechanical abrasion and UV-irradiation, remain independently adjustable. Especially the simulation of a shore environment poses a difficult task since, among other influences, wave breaking, sediment, UV radiation, and saltwater have to be considered in a closed abstract model simultaneously .…”

The complexity of micro-and nanoplastic research in the genusDaphnia– A systematic review of study variability and meta-analysis of immobilization rates

“…Although both processes, milling and UV-weathering, lead to the fragmentation of particles (i.e., after the process mean particle size is smaller), the change in NMP properties resulting from them is quite distinct: While milling, like sonification, mostly leads to changes in the particles’ surface structure and shape, but not to chemical changes[158,160], UV-weathering causes additional chemical changes (including changes in surface charge and composition of functional groups) and embrittlement due to photo-oxidation[15,159,161,162]. While the size of NMP particles and the molecular weight of polymer chains decrease with increasing exposure to UV radiation, the amount of carboxylic acids, peroxides and ketones on the surface increases, a pattern that seems to be generalizable across different polymer types (LDPE[159]; PP[163]; PS[15]).…”

Microplastics Derived from Food Packaging Waste—Their Origin and Health Risks

“…MPs are analyzed through several stages, such as separation, identification, visualization, and quantification. Techniques used to characterize MPs are mainly microscopic (optical microscopy, fluorescence microscopy, Scanning Electron Microscopy—SEM, Transmission Electron Microscopy—TEM, and Atomic Force Microscopy—AFM) and spectroscopic (Fourier Transform Infrared Spectroscopy—FT-IR, Raman Spectroscopy—RS, Nuclear Magnetic Resonance—NMR) methods [ 270 , 271 , 272 , 273 , 274 , 275 , 276 , 277 , 278 , 279 , 280 , 281 , 282 , 283 , 284 , 285 ] ( Figure 7 ). They are mostly used to identify the polymeric composition of MPs, analysis of the shape, color, and size of the particles, as well as their quantity in test samples.…”