The characteristic change of plastic film from common used packing bags under UV photodegradation

Since its invention, plastic has brought great changes to our daily life. It is considered as one of the most basic materials for the development of modern society. Most of the plastic produced are used for packaging and disposable products. Due to the high disposable rate and mismanagement, a large amount of plastic debris enters the marine environment. Potentially lasting several hundred years in the environment, the large plastic debris can be continuously broken into microplastics which are smaller than 5 mm under the action of ultraviolet light, heat, wind, tide, wave and bacteria etc. Therefore, the amount of microplastics will keep rising in the ocean. Microplastics is considered as a new emerging pollutant and has become a global concern. Understanding the degradation process of plastic in the environment is important for evaluating the contribution of plastic debris as one microplastics source. In this work, polyethylene film from 6 kinds of packaging bags commonly used in our daily life, i.e., food packing bag, garbage bag, thick film, and express packing bag, were selected to carry out photodegradation experiment under ultraviolet radiation in the laboratory. Properties of plastic films were investigated by tensile tests, infra-red spectrometry and scanning electron microscope at various time intervals. The results showed that the elongation-at-break of six kinds of polyethylene film decreased with increasing of UV exposure time. While Young’s modulus showed a trend of increase-decrease with increasing exposure time in general. The yield strength showed a general increasing-decreasing or decreasing trend with increasing exposure time. PE films from food packing bag and degradable garbage bag became brittle and tended to break into microplasics after 36 h under UV irradiation of 160 W/m². However, the retained elongation-at-break of PE film from express packing bag was up to 76% after 180 h UV irradiation. Chain scission and structure changes occurred during photodegradation process. FT-IR spectra indicated that difference existed in photodegraded process among PE films from six kinds of plastic bags. Ether group existed in FT-IR spectra of PE film from food packing bag, while ketone, ester, carboxylic acid, and vinyl group appeared in the spectra of thick film and express packing bag. Carbonyl index, crystallinity index and vinyl index of films from food packing bag, thick film and express packing bag increased at various degrees with prolongation of exposure time. The microstructure at tensile fracture surface became smoother after photodegradation, showing appearance of small fragments. This indicated that the intermolecular force decreased significantly, which was consistent with the trend of mechanical properties. The plastic packing bags commonly used in our daily life tend to become brittle and fragment into small pieces under ultraviolet radiation and become one source of microplastics in environment. These results can provide basic data and theoretical support for study of environmental behavior processes of plastic debris and source of microplastics, assessment of environmental risk, etc.