Environmental Persistence and Degradation

Plastic can undergo photo degradation, thermal degradation, mechanical degradation, thermo-oxidative degradation and hydrolysis [22]. All these degradation processes of plastics under environmentally relevant conditions are affected by almost all of the types of degradation mechanisms. For HDPE (high-density polyethylene) and nylon, degradation is primarily via UV-B photooxidation followed by thermo-oxidation. Degraded products of micro- or potentially even nano-sized dimensions [108,109] may further be degraded (e.g., biological) where carbon in the matrix is converted to carbon dioxide and incorporated into biomass [22]. However, UV degradation of plastics floating in water is delayed by lower temperatures and oxygen levels relative to on land, making conversion of macroplastics to microplastics much more rapid on beaches than in the water [22,58,110].

The plastic degradation processes carried out in the laboratory focuses on a single mechanism such as photo, thermal or biodegradation. The time scaling of the process from the abandonment of a waste, its arrival and persistence in the aquatic environment is not known. This scenario is accompanied by a physical fragmentation into particles of increasingly smaller sizes and also by a chemical functionalization due to the photooxidation of the macromolecular chains [60] (Table 11.2).

Even though all the degradation mechanisms work together, the studies discuss weight loss, changes in tensile strength, breakdown of molecular structure and identification of specific microbial strains to utilize specific polymer types [61]. The degradation processes

TABLE 11.1 Crystallinity of Polymers

Polymer Type

Crystallinity

Natural rubber

Low

Polyethylene-low density'

45%-60%

Polyethylene-high density

70%-95%

Polypropylene

50%-80%

Polystyrene

Low

Polycarbonate

Low

Polyvinyl chloride

High

Poly'lactic acid

37%

Polyethylene terephthalate

Described as high in and as 30%-40% in [59]

TABLE 11.2 Weathering and Degradation of Microplastics

Variables Influencing the Weathering of MP

Characteristics That Affect Degradation

a. Polymer type

b. Polymer properties such as density and crystallinity

c. Embedded additives in the polymer structure

d. Environmental exposure conditions

e. Type of weathering processes that can occur even without UV light [49]

a. Complex structure: They are not easily biodegradable.

b. Crystallinity: The crystalline region is more ordered with tightly structured polymer chains. Crystallinity affects physical properties (like density and permeability), which in turn affects their hydration and swelling behavior. This affects availability of sorption sites for microorganisms (Table 11.1).

c. Presence of stabilizers: Stabilizers (antioxidants and antimicrobial agents) act to prolong the life of plastics.

d. Biological ingredients act to decompose the plastic in shorter time frames [21].

are defined for the degradation mechanism under investigation (e.g., thermal degradation) and the result is generated. In contrast, particle formation rates are often not investigated. This is important because polymers such as PE do not readily depolymerize and generally decompose into smaller fragments and further disintegrate into increasingly smaller fragments, eventually forming nanoplastics [62].

For the purpose of studying breakup events to describe the distribution of fragment sizes, the kinetic models are introduced. These models simulate the transport and fate of buoyant and nonbuoyant plastic debris (MPs) in freshwater systems. These models work on the rate equations, which assume that each particle is exposed to an average environment, mass is the unit used to characterize a particle and the size distribution is spatially uniform [63,64]. The models generally focus on chain scission in the polymer backbone through (a) random chain scission (all bonds break with equal probability) characterized by oxidative reactions; (b) scission at the chain midpoint dominated by mechanical degradation; (c) chain-end scission, a monomer-yielding depolymerization reaction found in thermal and photodecomposition processes and (d) in terms of in homogeneity (different bonds have different breaking probability and dispersed throughout the system) [65-67]. The estimation of degradation half-lives has also been measured for strongly hydrolysable polymers through the use of exponential decay equations [61,68,69], though the studies need further investigation in case of chemically resistant plastics [69].

Henceforth, the environmental degradation processes encapsulate MP fragmentation into increasingly smaller particles including nanoplastics, chemical transformation of the plastic fragments, degradation of the plastic fragments into nonpolymer organic molecules and the transformation/degradation of these nonpolymer molecules into other compounds [61].

 
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