Читать книгу Plastic and Microplastic in the Environment - Группа авторов - Страница 16

Large plastic debris in freshwater environments gradually degrades into smaller particles when exposed to some factors in the environments (Figure 1.2). In the degradation of plastic, the average molecular weight of polymers is drastically reduced (Andrady 2011). The degradation can categorize as agents causing this process: biodegradation is an action of microorganisms; photodegradation is an action of light and sunlight; thermal degradation is the action of high temperatures; thermooxidative degradation is an oxidative breakdown at medium temperatures; hydrolysis degradation is a reaction with water. In the environment, UV radiation from sunlight is the major factor of plastic degradation, which speeds up the oxidative breakdown of polymers (Andrady et al. 1996). Polymers such as high‐density polyethylene (HDPE), low‐density polyethylene (LDPE), PP, and nylons are degraded mainly by the UV‐B radiation in sunlight as they are exposed to environments. As degradation is initiated, the thermooxidative process can continue without further exposure to UV radiation. The autocatalytic degradation may occur as long as the existence of oxygen in the system (Andrady 2011). During the degradation process, plastic wastes typically discolor, evolve surface features, and become weak and brittle, gradually. Other forces such as waves, wind, and human and animal activities can easily crack the embrittled plastics into small particles. The degradation and fragmentation of plastics is the major process for the formation of secondary MPs in aquatic environments (Kershaw 2015).

Figure 1.2 (a) Degradation and fragmentation of plastic under environmental factors; (b) and (c) the cracks seen at the surface are caused by photochemical degradation

Source: Ter Halle 2016, p. 15 / With permission from American Chemical Society.

Figure 1.3 SEM of typical fibers: (a) polyester‐cotton blend; (b) polyester; (c) acrylic

Source: Napper 2016, p. 03 / with permission from Elsevier.