using UV irradiation Nina Maria Ainali 1,2 *, Dimitra Lambropoulou 1 - PowerPoint PPT Presentation

Investigation of surface alteration of microplastics by using UV irradiation Nina Maria Ainali 1,2 *, Dimitra Lambropoulou 1 and Dimitrios N. Bikiaris 2* 1 Laboratory of Environmental Pollution Control, Department of Chemistry, Aristotle University of Thessaloniki, GR-541 24 Thessaloniki, Greece 2 Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, GR- 541 24 Thessaloniki, Greece. * Corresponding author: nsainali@chem.auth.gr; dbic@chem.auth.gr 1

Abstract Microplastics are formed by the degradation of plastic wastes under the action of physicochemical mechanisms in environment, existing as contaminants of emerging concern in recent years due to their adverse impact on living organisms and the environment. When common polymers are exposed to the environment are adversely affected by solar radiation (primarily ultraviolet (UV) UV-B), which initiates photooxidative degradation leading to polymer chain breakdown, causing though the deterioration of their mechanical properties after an unpredictable time. In the present study, to improve understanding of characteristics and mechanism of microplastics, four of the most widely used polymers covering a wide spectrum of applications, due to their excellent chemical inertness and high processability such as low-density polyethylene (LDPE ), high-density polyethylene (HDPE) , polypropylene (PP) and polystyrene (PS) in the form of thin films were exposed to UV radiation at 254 nm with constant temperature for several times. After exposure (5, 10, 20, 30, 45 and 60 days), the films were removed from the chamber and UV irradiation influence was evaluated by using FTIR (Fourier-Transform Infrared) Spectroscopy, DSC (Differential Scanning Calorimetry), XRD (X-Ray Diffraction), Py- GC/MS (Pyrolysis-Gas Chromatography/Mass Spectroscopy), SEM (Scanning Electron Microscopy), while their mechanical properties were evaluated. Keywords: microplastics; degradation; aging; UV exposure 2

Results and Discussion FTIR 4000 c) 6000 a) b) 600 LDPE LDPE LDPE HDPE HDPE HDPE 5000 500 PP PP PP 3000 4000 400 Formation of H.H.I. (%) ketones , esters C.I. (%) V.I. (%) 3000 300 2000 and acids 2000 200 1000 100 1000 0 Figure 1 a) Carbonyl index, b) vinyl index 0 0 0 5 10 15 20 25 30 35 40 45 50 55 60 and c) hydroxyl/hydroxyperoxide index 0 10 20 30 40 50 60 0 5 10 15 20 25 30 35 40 45 50 55 60 Time of UV exposure (days) of LDPE, HDPE and PP during UV Time of UV exposure (days) Time of UV exposure (days) exposure for different days. PS 0 days PS 0 days PS 5 days 1.0 PS 5 days PS 10 days PS 10 days PS 20 days PS 30 days PS 20 days PS 45 days Absorbance PS 60 days PS 30 days PS 45 days 0.8 Mechanical PS 60 days 1800 1750 1700 1650 1600 1550 Absorbance Wavenumber (cm -1 ) properties 0.6 b) 0.4 a) HDPE LDPE 800 500 Tensile strain at break (%) Tensile strain at break (%) 400 600 0.2 300 400 200 0.0 200 100 4000 3500 3000 2500 2000 1500 1000 500 0 0 Wavenumber (cm -1 ) 0 10 20 30 40 50 0 10 20 30 40 50 60 Time of UV exposure Time of UV exposure Figure 2 FTIR spectra of neat PS during UV radiation for several times. d) PS c) 8 PP 400 7 Tensile strain at break (%) Tensile strain at break (%) 6 5 200 4 Figure 3 Variation of tensile strength at 3 break point for a) HDPE,b) LDPE, c) PP 2 0 and d) PS during several days of UV 1 exposure. 0 5 10 15 20 0 10 20 30 40 50 60 Time of UV exposure Time of UV exposure

Results and Discussion LDPE DSC HDPE 160 • Progressing shift of PE and PP melting point to PP o C) PS lower temperatures, as irradiation time 150 Melting point ( increases. 140 • Increase in PS T g till the 30 rd day of irradiation, 130 followed by a slight decrease. Figure 4 Variation of a) melting point at LDPE, HDPE and PP films, and b)glass 120 transition temperature of PS after UV 110 exposure. o C) 100 Glass transition ( HDPE 60 days 90 100 0 10 20 30 40 50 60 80 Time of UV exposure (days) 60 40 Py-GC/MS 20 0 HDPE 30 days 100 Relative Intensity • As irradiation time prolongs, the relative amount of low molecular 80 60 weight molecules such as butane and pentane increases. 40 • UV 20 degradation created more vulnerable sites for thermal 0 decomposition to be initiated, therefore resulting in the evolution of HDPE 0 days 100 more small sized hydrocarbons . 80 C20 C4 60 C5 C10 C6 40 C7 Table 1 Ratios of PE peak areas of the hydrocarbons with more than 10 carbon atoms, to C9 C8 hydrocarbons up to 9 carbon atoms. 20 0 Sample name Ratio Sample name Ratio 0 5 10 15 20 25 30 35 Retentio time (min) LDPE 0 days 6.98 HDPE 0 days 7.54 LDPE 30 days 4.83 HDPE 30 days 6.13 Figure 5 Gas chromatographs of unirradiated HDPE, and after 30 and 60 LDPE 60 days 1.22 HDPE 60 days 5.42 days of UV exposure.

Results and Discussion Days of UV exposure SEM 60 0 30 LDPE HDPE PP PS Figure 6 SEM micrographs of LDPE, HDPE, PP and PS films after 0, 30 and 60 days of UV exposure, respectively.

Conclusions ✓ UV exposure provokes yellowing and embrittlement of the studied polymers; in PS case the yellowing starts after only 5 days of irradiation. ✓ Deterioration of mechanical properties as irradiation proceeded; PP mechanical weakening started after 5 days of UV exposure. ✓ FTIR spectra displayed significant alterations at vinyl , carbonyl and hydroxyl bands for PE and PP during irradiation, with the relative carbonyl index in PP being more abrupt for the first 30 days of UV irradiation; degree of chain scission and cross-linking reactions could not be estimated from PS spectra since small alterations were noticed. ✓ DSC analysis depicted a gradual drop in melting point for PE and PP, revealing the correlation between crystallinity and UV degradation process. ✓ SEM micrographs outlined cracks and holes at films’ surface, after only the 30 days of UV exposure. ✓ Py-GC/MS indicated that with progressive UV exposure, the relative amount of low molecular weight compounds is boosted; the occurring UV degradation creates more susceptible sites for thermal decomposition to be originated. 6

Acknowledgements This research was financially supported by the Greek Ministry of Development and Investments (General Secretariat for Research and Technology) through the research project “Intergovernmental International Scientific and Technological Innovation- Cooperation. Joint declaration of Science and Technology Cooperation between China and Greece” (Grant no: T7 ΔKI -00220). https://users.auth.gr/dlambro/ https://bikiarislab.wixsite.com/bikiarislab https://www.researchgate.net/profile/Nina_Maria_Ainali 7 nsainali@chem.auth.gr