STUDY OF MICROPLASTIC FORMATION AND SURFACE CHANGES OF PET UNDER CENTRAL ASIAN CLIMATIC CONDITIONS

ABSTRACT

Therefore, in this study, polyethylene terephthalate (PET) samples were subjected to artificial ultraviolet (UV) radiation or a combination of UV and water exposure for six months to simulate natural environmental conditions. Three differently colored PET types with varying manufacturing histories were included in the study. Surface changes induced by weathering were analyzed using scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy. The results showed that the three types of secondary PET exhibited different surface alterations and displayed signs of weathering over distinct time intervals. Significant differences were observed between samples exposed only to UV radiation and those subjected to natural environmental conditions. After three months of UV exposure in a climatic chamber, the surfaces began to show alterations, which progressively developed into networks of fine cracks. When subjected to mild mechanical stress, fragments started detaching from the degraded surface layer.

АННОТАЦИЯ

В данном исследовании образцы полиэтилентерефталата (ПЭТ) подвергались искусственному ультрафиолетовому (УФ) излучению или комбинации УФ-воздействия и контакта с водой в течение шести месяцев для имитации природных условий окружающей среды. В исследование были включены три типа ПЭТ разного цвета с различной историей производства. Поверхностные изменения, вызванные атмосферным воздействием, анализировали с использованием сканирующей электронной микроскопии (СЭМ) и инфракрасной спектроскопии Фурье (FTIR). Результаты показали, что три типа вторичного ПЭТ проявляли различную степень поверхностных изменений и признаки старения в разные временные периоды. Были выявлены значительные различия между образцами, подвергавшимися только УФ-излучению, и теми, которые находились в условиях, близких к природным. Через три месяца УФ-воздействия в климатической камере на поверхности начали появляться изменения, которые постепенно развивались в сеть мелких трещин. При лёгком механическом воздействии из разрушенного поверхностного слоя начинали отделяться фрагменты.

Keywords: secondary PET, physicochemical analysis, IR spectroscopy, UV radiation analysis, degree of crystallinity.

Ключевые слова: вторичный ПЭТ, физико-химический анализ, ИК-спектроскопия, анализ УФ-излучения, степень кристалличности.

INTRODUCTION

Microplastic particles (MPs, defined as plastic fragments up to 5 mm in size) are a well-documented source of environmental pollution [1,2]. Microplastics are classified as either primary or secondary based on their origin. Primary microplastics are manufactured directly at a small scale and can be controlled at the source, e.g., by minimizing environmental release during pellet transport [3] or banning their use entirely [4]

Secondary microplastics, on the other hand, are produced through the fragmentation of larger plastic items, often as a result of weathering. Such degradation can occur during the product’s usage or after entering the environment. Secondary microplastics are the most abundant form in nature [5]. In the environment, plastics are exposed to multiple weathering processes, and the relative importance of each depends on the location and the physical–chemical properties of the material [6].

Weathering is directly linked to polymer degradation. For many common polymers, photo-oxidative degradation is the dominant form. Hydrolysis is another degradation pathway, though many polymers are stable under normal conditions. However, hydrolysis can be accelerated when combined with other factors, such as UV radiation [7,8]. While mechanical forces are not a direct degradation pathway, they play a critical role in the fragmentation of plastics into microplastics [9]. Various degradation pathways of polymers have been studied since the 1960s, and many aspects of these processes are well documented [10]. Nevertheless, the effect of weathering on microplastic formation requires further elucidation. Many studies report that the majority of microplastics found in the environment originate from the embrittlement and fragmentation of weathered plastics. Although this concept is widely cited in the literature, detailed information on the actual mechanisms and kinetics of microplastic formation remains limited [8]. In addition, while the role of mechanical stress in microplastic formation is often emphasized, systematic studies on the form or intensity of stress are scarce. One proposed model suggests that microplastics are generated via surface ablation, in which the weathered surface layer detaches into small fragments [11]. Numerous studies have reported the release of micro- and nanoplastics from plastics following UV exposure. In addition to particle detachment, surface alterations and cracking have also been observed [12,13]. However, the connectivity between fragments formed along cracks has been little studied. A rare example is the work of Julienne et al., who investigated the weathering of polyethylene films, the initiation and propagation of cracks, and subsequent fragment formation. Although microplastics are often treated as a single pollutant type, their size, shape, polymer type, and other characteristics influence their behavior. These factors also play a role in their formation. Chemical composition, physical structure, and form of polymers are expected to significantly influence this process.

Previous weathering studies have focused on specific plastic types: films or containers [14]. However, differences in experimental conditions, methodology, and research objectives make comparison of results difficult. The aim of this study was to monitor the weathering of a single polymer type—polyethylene terephthalate (PET)—in various forms. Differences in manufacturing processes are expected to result in distinct material properties. Polyester, particularly PET, is among the most widely used polymers worldwide and one of the most prevalent in the environment [15]. Its main applications are in textiles (67%) and blow-molded bottles (24%), with additional use in films. The diversity of PET applications allowed the selection of three product forms: granules, fibers, and films. Weathering was conducted under UV light alone or in combination with natural environmental conditions. The study primarily focused on surface changes, particularly crack propagation, to assess the characteristics of microplastics generated under weathering.

MATERIALS AND METHODS

Post-consumer clear PET beverage bottles collected from Samarqand municipal recycling centers were shredded, washed with 0.8 % Na₂CO₃ solution, rinsed, and dried at 78 °C for 14 h. Flakes were melt-pressed into 1.2 mm thick films at 265 °C for 6 min under 5 MPa in a hydraulic hot press, then quenched between cold plates. The effect of UV-radiation on the properties of SPET of various colors was determined according to ASTM G154 Model K035 - UV (climatic) application (TS EN ISO 9001:2008, Basaksehir/Istanbul, 220V AC (AC (10%), 50/60 Hz (verified UV climatic apparatus (ISO 4892-3 standard, 1-cycle: bulb UVA - 340 nm, 0.76 W/m2/nm, UV light - 8 hours (70 °C), wind - 4 hours (50 °C)), UV - light - 8 hours (70 °C) - total - 20 hours) ((in the laboratory "TUFT AND GRASS"), [16]. UV - C is a subclass of ultraviolet radiation with a wavelength from 200 to 280 nm is missed in the spectrum of sunlight at the earth’s surface because wavelengths below 300 nm are absorbed by the ozone layer of the atmosphere, so the UV-A class (340 nm) was used.

The surface morphology of PET and SPET samples before and after UV exposure was examined using Scanning Electron Microscopy (SEM, model XYZ, accelerating voltage 10–15 kV). Samples were sputter-coated with a thin layer of gold (~5 nm) to improve conductivity and imaging quality.

RESULTS AND DISCUSSION

After exposure according to ASTM G154 (UVA-340 nm, 70 °C UV + 50 °C condensation, 6 months cycle), significant degradation was observed on the surfaces of PET films and SPET particles of various colors. In clear PET films, micro-cracks, surface roughening, erosion, and the initiation of microplastic formation were detected. In colored SPET particles (blue, green, brown), fewer cracks were observed, with only slight surface dulling and localized oxidation zones around pigment regions [17]. This indicates that the amorphous phase of PET is more sensitive to UV radiation.

Table 1.

Changes in the average molecular weight of PET samples exposed to (natural) UV light [18]

Changes in optical properties: the light transmittance of clear PET films decreased by 8–12%, in colored SPET samples, the degree of haze increased by only 3–5%. This confirms that pigments partially absorb UV radiation and act as stabilizing agents. The carbonyl index was highest in clear PET. Exposure of PET bottle samples—transparent, blue, green—over 6 months showed that the bottle color affects the destructive processes occurring in PE]. The most resistant to photo-oxidative degradation was the transparent PET, which contains no added colorant [19]. After one year of aging, its average molecular weight decreased from 19,869 to 17,946 g/mol, corresponding to a 9.8% reduction. The degree of degradation for PET containing other pigments was as follows: transparent – 17.2%, blue – 18.4%, green – 18.8% . These data indicate that the highest degradation occurred in PET used for green bottles. The depth of degradation appears to be influenced by the initial molecular weight and pigment content. Dark pigments, such as brown, absorb UV radiation more effectively, resulting in deeper degradation [20].

Figure 1. SEM images of SPET samples ((transparent, blue, green)

SEM images of unaged, transparent PET films revealed smooth, homogeneous surfaces without visible cracks or defects. Similarly, colored SPET flakes (transparent, blue, green) showed relatively smooth surfaces, with only minor surface irregularities likely resulting from the mechanical shredding and melt-pressing process. After 6 months of UV exposure (natural sunlight or laboratory UVA-340 nm, 20-hour cycles), significant surface degradation was observed in SEM images of transparent PET. Micro-cracks, erosion patterns, and small fissures were clearly visible, indicating the onset of photodegradation. The edges of the micro-cracks showed slight roughening, which is consistent with chain scission in the amorphous regions. These surface defects are expected to facilitate the detachment of microplastic particles. Colored SPET samples exhibited less severe surface degradation. Blue, green flakes showed localized roughening around pigment particles but fewer micro-cracks than transparent PET. Green SPET surfaces showed some pigment-associated micro-erosion, suggesting that dark pigments absorb UV radiation, promoting localized degradation. SEM analysis confirmed that the formation of micro-cracks and surface fissures on PET and SPET surfaces under UV exposure can lead to the release of microplastic fragments. Transparent PET, being most sensitive to UVA radiation, showed the highest potential for microplastic generation, while colored SPET flakes, particularly brown and blue, exhibited comparatively lower susceptibility. SEM imaging demonstrated that UV radiation induces significant surface degradation in PET and SPET materials. The extent of surface damage strongly depends on color and pigment content. These observations correlate well with molecular weight reduction and mechanical property deterioration measured by intrinsic viscosity and tensile tests, confirming the role of photodegradation in microplastic formation [21-23].

Figure 2. Infrared (IR) spectrum analysisof SPET samples: (a) transparent, b) blue, c) green)

Infrared (IR) spectrum analysis of the SPET samples revealed that the green specimen exhibited the highest degree of photodegradation, followed by blue, and then transparent. The enhanced intensity of the hydroxyl band (~3510 cm⁻¹) and the carbonyl band (~1710 cm⁻¹) suggests elevated oxidation and hydrolysis in the colored samples. Furthermore, the broadening and decrease in the ester C–O–C stretching band (~1240 cm⁻¹) indicate that chain scission and ester bond cleavage are more pronounced in the colored SPET, particularly in the green SPET [24-26].

CONCLUSION

This study investigated the formation of microplastics and the surface degradation behavior of post-consumer PET materials under Central Asian climatic conditions using controlled UV exposure and combined environmental weathering. The results demonstrate that PET degradation is strongly influenced by polymer structure, degree of crystallinity, and the presence of pigments. Transparent PET, which contains no color additives and exhibits a higher amorphous fraction, showed the most pronounced susceptibility to UVA-induced photo-oxidation. This was evidenced by significant increases in surface micro-cracks, extensive erosion patterns, and the highest reduction in molecular weight over six months of exposure.

Colored SPET samples (blue, green) showed comparatively reduced surface degradation, suggesting that pigment additives partially absorb or scatter UV radiation, thereby limiting photo-oxidative reactions. However, infrared spectral analysis revealed that green PET experienced the highest degree of chemical oxidation, indicated by intensified hydroxyl (~3510 cm⁻¹) and carbonyl (~1710 cm⁻¹) bands and more substantial reduction of ester C–O–C (~1240 cm⁻¹) peaks. These findings confirm that pigments can both protect and locally intensify degradation depending on their optical properties and interaction with polymer chains.

SEM analysis established that prolonged UV exposure initiates micro-crack formation, which evolves into complex crack networks capable of producing microplastic fragments. The morphology of the detached fragments—elongated and fiber-like with median dimensions of 15×2×1.5 µm—supports the mechanism of surface-layer ablation proposed in degradation models. Quantitative estimations show that a fully degraded surface layer can release 1.1–7.45 million microplastic particles per cm² for films and 0.36–2.15 million particles per cm² for granules, highlighting the environmental significance of weathered PET waste.

Changes in intrinsic viscosity and molecular weight further confirmed progressive chain scission, with the highest degradation (18.8%) observed in green PET. This demonstrates that both material history and pigmentation influence degradation kinetics under Central Asian environmental conditions.

Overall, the findings of this study provide clear evidence that PET waste, especially in film form, undergoes substantial physical and chemical degradation when exposed to UV radiation and moisture cycles typical of regional climate. These weathering processes directly facilitate microplastic formation. The results emphasize the urgent need for improved PET waste management strategies, the development of UV-resistant formulations, and region-specific environmental risk assessments for polymer degradation [25-40].

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