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СКАЧАТЬ (Andrady and Rajapakse 2019). However, some lubricants are nonylphenol based, which are known as endocrine disruptors (Boehme et al. 2010).

      2.2.7 Light Stabilizers

      Plastics are also susceptible to degradation via photo‐oxidation, which is the result of the combined action of light and oxygen, that follows a similar oxidation cycle as in thermal oxidation that was previously discussed (see also Chapter 8). Light stabilizers interfere with the physical and chemical processes of light‐induced polymer degradation. The most important light stabilizer classes are benzophenones, benzotriazoles, organic nickel compounds, and sterically hindered amines (HALS; Jia et al. 2007). UV absorbers, such as benzophenones and benzotriazoles, are extensively used to stabilize thick sections of polyolefins, poly(ethylene terephthalate) (PET), polyurethane (PU), poly(vinyl acetate) (PVA), natural rubber, and epoxy formulations. Organic nickel compounds quench or deactivate the excited states of chromophores arresting oxidation. HALS is a particular potent free‐radical quencher that is effective at very low concentrations (≈0.1%).

      The protection of plastics from the effects of light can also be achieved through the addition of carbon black (CB) and other pigments such as titanium dioxide (Accorsi et al. 2001) that essentially shield the plastic from UV radiation. Light stabilizers significantly control the weathering of plastics exposed to sunlight as well as fragmentation via loss of MW from photo‐oxidation of the polymer. Typical loadings in plastics are relatively low, with <1%, and migration and toxicity have not received special attention.

      2.2.8 Colorants

Colorants are chemical compounds that not only impart color to plastic materials but can also affect other properties, such as weather resistance, light stability, and transparency of the plastic. Colorants fall into two distinct classes: dyes and pigments; main distinction being dyes are soluble in the plastic matrix while pigments are insoluble. Most commonly used are the azo dyes that make up >50% of all dyes listed in the Color Index (Ambrogi et al. 2017) and used in textiles, paper, leather, rubber, or even foodstuffs (Ambrogi et al. 2017). Since organic dyes dissolve in the polymer, they do not scatter but only absorb light. Therefore, even at high concentrations of the dye, the plastic tends to be transparent or translucent. Some dyes such as aromatic amines, are known carcinogens and phthalocyanines have detectable estrogenic activity (Yang et al. 2011).

      Pigments remain discrete particles that are well dispersed in the polymer matrix (Bolgar et al. 2016). Scattering and absorption of light by the pigment particles makes the plastic partly opaque (Andrady and Rajapakse 2019). Pigments are classified as either organic or inorganic. Organic pigments include benzimidazoles, quinacridones, and mono‐azos and provide the most brilliant opaque colors available (Ambrogi et al. 2017). Inorganic pigments are based on metals and can be divided into three classes: white pigments (TiO₂), carbon black (CB), and special effect pigment (Huckle and Lalor 1955). TiO₂ is the most widely used pigment in the plastic industry due to its high refractive index and ability to provide a high degree of opacity and whiteness. In addition, TiO₂ is known for its excellent durability and general nontoxicity. Black pigmentation in plastics is typically based on CB, the second most used pigments in volume by the plastic industry. CB also has dual functionality in that it can also act as a reinforcing filler, conductive filler, and light stabilizer improving the weatherability and stability of the plastic (Huang 2002). Special pigments impart vibrant colors to plastic materials and include fluorescent pigments, pearlescent pigments (mica coated with TiO₂), and metallic pigments (aluminum bronzes, copper, copper‐zinc alloys, and zinc).

      2.2.9 Fillers and Reinforcements

      Fillers are relatively cheap, solid substances that are added to plastic formulations in high percentages to adjust volume, weight, and mechanical performance (Zweifel et al. 2001). Inert fillers often do not compromise the functional properties of the plastic; they are cheaper than resin and can significantly reduce the cost of the formulation. Fillers can also serve as reinforcing agents that improve the mechanical performance and durability of the plastic. They are used as powders, fibers, or nanotubes.

      Filler consumption globally in 1999 was 66% calcium carbonate, 6% talc, 6% clays, 3% wollastonite (CaSiO₃), and the remaining 19% included silica, glass, asbestos, alumina, rutile, CB, and carbon nanotubes (CNTs; Civancik‐Uslu et al. 2018; Zweifel et al. 2001). Reinforcements are generally strong fibers including glass, carbon, or aramide fibers (Alam et al. 2019; Hansen et al. 2013). Fillers and reinforcements are used virtually in all polymers, but the largest fraction (i.e. over 90%) is used primarily in rubbers, PVC, and polyolefins. The improved mechanical properties in a filled plastic are derived from the interface layer between the polymer and the filler. The stronger the interfacial interactions, the better the mechanical performance of the composite (i.e. polymer/filler mix). The efficacy of the filler is also dependent on adequate dispersion in the polymer matrix. Good dispersion is achieved through extrusion, dispersing agents, or by surface treatment of the filler to improve compatibility with the polymer matrix.

      Fillers can be microscopic (1 μm) or macroscopic (<100 μm) in size and have a very low propensity to leach out of the plastic. However, degradation and wear of a filled plastic can release fibers into surrounding environments either during use or after disposal (Froggett et al. 2014). In addition, the use of nano‐particulate fillers, such as CNTs or silica nanoparticles (silica‐NPs), has gained popularity over the past 20 years due to the superior material properties of nanocomposites when compared to conventional composites. Currently, commercial availability is low as these materials are relatively new. Nonetheless, production volumes are increasing with market sizes projected to grow significantly (Hendren et al. 2011).

2.3 Sources, Transport, and Fate of Additives in the Ocean

Despite the growing concerns of plastic waste accumulation in the ocean, the environmental transport and fate of plastic additives are not well understood (Tian et al. 2020). Additives in plastics can be released from products into the air, water, and soil in all phases of the product life cycle and can be transported to the marine environment by numerous, interconnected, pathways (Figure 2.2).

Additives are well known to leach from plastics in the marine environment (De Frond et al. 2019; Koelmans et al. 2014; Paluselli et al. 2019; Pereao et al. 2020; Sun et al. 2019; Teuten et al. 2009). However, in addition to leaching, there are other sources of the same chemicals present in the ocean, some of which may even be more important, including direct industrial releases, wastewater effluents, atmospheric deposition, runoff, and river transport resulting from all human activities, including tire wear and application of sewage sludge in agriculture, resuspension from sediments, among other routes (Figure 2.2). For example, 14,742 metric tons of styrene were released into the environment from U.S. industries in 2019, independent from its leaching from plastic products, as reported to the U.S. Environmental Protection Agency’s Toxic Release Inventories (Table 2.4). Studies have shown that effluent from wastewater treatment plants and runoff, especially near plastic product manufacturing, as well as atmospheric deposition, are major sources of already leached plastic additives to aquatic environments (Kim et al. 2021; Liu et al. 2020; Peng et al. 2007; Staples et al. 1997; Zhang et al. 2013, 2018c).

      Many discussions on movement of plastic additives within the ocean focus on the plastic itself as the main carrier but it is not the only significant transport mechanism (Zarfl and Matthies 2010). Koelmans et al. (2016) states that the fraction of hydrophobic organic compounds (HOCs), including organic additives, held by plastic is negligible compared to that held by other СКАЧАТЬ