Large-scale concentrated accumulations of plastics have been found in the South Pacific subtropical gyre and the Eastern Pacific Ocean gyre. The amount of plastic waste entering the oceans has emerged as a major concern. Various antioxidants and stabilizers, which are used to prolong the working life of plastics, slow environmental degradation of plastics waste even further. (3,4) The recalcitrance and impermeability of these plastics make them ideal for applications such as food packaging, sterile medical uses, and construction, among others, but also make them particularly long-lived when they are discarded. Polyolefins are particularly durable, due to their chemical and biological inertness, which is a result of their high molecular weight and hydrophobicity, and the absence of functional groups that are susceptible to attack by microbial enzymes, light, water, etc. Their material properties are adjusted during and/or after synthesis to achieve the desired strength, permeability, porosity, opacity, and color. They can be molded into virtually any desired shape through rotation, injection, extrusion, compression, blowing, or thermoforming. Although these plastics are inexpensive, each is a highly engineered material with precise physical properties. The vast majority of plastics produced today, including all of the aforementioned commodity polymers, are made from nonrenewable petrochemicals, so named because they are derived from fossil oil, natural gas, and coal.
Our study highlights the need for better experimental studies under well-defined reaction conditions, standardized reporting of rates, and methods to simulate polymer degradation using. For example, SSDRs for HDPE and polylactic acid (PLA) are surprisingly similar in the marine environment, although PLA degrades approximately 20 times faster than HDPE on land. Using a mean SSDR for HDPE in the marine environment, linear extrapolation leads to estimated half-lives ranging from 58 years (bottles) to 1200 years (pipes). This approach yields a number of interesting insights. SSDRs for high density polyethylene (HDPE) in the marine environment range from practically 0 to approximately 11 μm year –1. SSDR values cover a very wide range, with some of the variability arising due to degradation studies conducted in different natural environments. A metric to harmonize disparate types of measurements, the specific surface degradation rate (SSDR), is implemented and used to extrapolate half-lives. This Perspective summarizes the existing literature on environmental degradation rates and pathways for the major types of thermoplastic polymers. The amount of plastics accumulating in the environment is growing rapidly, yet our understanding of its persistence is very limited. Plastic waste is currently generated at a rate approaching 400 Mt year –1.
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