the War effort spurned a rapid expansion of the materials industry in the US with public funding allowing new plastic resin plants to be built to produce vital plastics for the military supply chain.
Postwar years saw the enthusiastic acceptance of plastics by consumers worldwide, thanks mostly to the efforts of industry to promote plastics as a unique “wonder material,” and much was expected of this novel semi‐utopian material that promised a wide range of affordable products. Today, plastics have emerged as the material of choice in a variety of applications ranging from food packaging to spacecraft design. The abundant societal benefits of plastics (Andrady and Neal 2009) are evidenced by the rapid substitution of conventional materials used in packaging, building, transportation, and medicine, with plastics. Plastics have, by now, become indispensable to the modern lifestyle, with their per capita consumption governed generally by the affluence of the country. While the US, Canada, and Japan, for instance, use over 100 kg per capita of plastics annually, India and some countries in Africa or Central Europe, use less than 50 kg per capita (e‐Marketer 2021). To meet this steadily increasing global per capita demand of an average ~46 kg annually, plastic resin production had grown to 359 million metric tons (MMT); 432 MMT inclusive of the polymer used in synthetic textile fibers) in 2019. China accounted for about 30% of the production, and with ~50% of the global resin demand in Asia, the country is well poised to remain as the leading resin manufacturer in the world. The annual global production of plastics in the year 2015 alone, if processed into a thin plastic “cling film,” was estimated to be large enough to wrap the entire earth in plastic wrap (Zalasiewicz et al. 2016).
An estimated (Geyer et al. 2017) 7300 MMT of plastic resin and fiber was manufactured globally from just after WWII until the year 2015. By 2020, this figure rose to 8717 MMT. More than half of this was either PE (~36%) or PP (~21%). In addition, the thermoplastic polyester (e.g., poly(ethylene terephthalate) [PET]) used in beverage bottles, polystyrene (PS) in packaging, and poly(vinyl chloride) (PVC) as a building material, were also produced. Reflecting their high‐volume use, these same 4–5 classes of plastics typically dominate the plastic content in the municipal solid waste stream (MSW), in urban litter, as well as plastic debris in the marine environment. The current discussion is therefore focused on this limited set of plastic types: PE, PP, and PS foam that dominates floating plastic debris in surface waters of the ocean and nylons or polyamide (PA). PET, PS, and PVC, mostly found in the deep sediment. Deep‐sea sediment is the most important sink or repository of waste plastics that enter the ocean every year. While no systematic quantitative assessment is available, there is little doubt that plastics accumulate in the benthic sediment and a recent estimate places it conservatively at about 14 MMT (Barett et al. 2020).
1.1 What Are Plastics?
The term “plastic” is used in common parlance as if it is a single material. But it is, in fact a broad category of materials that include hundreds of different types. Plastics are a sub‐class of an even larger group of materials called the polymers, characterized by their unique long chain‐like molecular architecture, made up of repeating structural units. They tend to be giant molecules with average molecular weights (g/mol) in the range of 105–106 (g/mol). Being a subset of polymers that can be melted and re‐formed into different shapes repeatedly, they are therefore called thermoplastics. The word “plastic” is derived from ‘thermoplastic [See Box 1.1]. Hundreds of chemically distinct types of thermoplastics exist, even though only a few are used in most consumer plastic products.
This is somewhat analogous to the about 95 elemental examples in the group ‘metals’ and their numerous commercially available blends, even though only a few common ones such as copper or aluminum are extensively used. The same is true of plastics, but even within a single type of plastic such as polyethylene (PE)1 several different varieties of resins with different characteristics are available. For instance, the common varieties of PE are low‐density polyethylene (LDPE), high‐density polyethylene (HDPE), medium‐density polyethylene (MDPE), and linear low‐density polyethylene (LLDPE) resins. Each of these varieties includes different grades of that plastic with range of properties despite their identical repeat‐unit chemical structure. For instance, one grade of LDPE (low molecular weight grade) is a soft wax used as a lubricant, while another (ultra‐high molecular weight grade) of PE, is spun into fibers so strong that they are used as an antiballistic material in military hardware. Therefore, in research reports, identifying a material just as a “plastic” or even as “polyethylene” is not particularly informative; details of at least the type, if available the grade, and its basic properties should be mentioned in order to compare data across publications.
Box 1.1 Thermoplastics and Thermosets
All plastics are polymers but not vice versa; plastics or thermoplastics include only those types of polymer that can be melted and re‐formed into different shapes repeatedly. Therefore, polymers such as tire rubber, polyurethane foam, or epoxy resin as well as cellulose or proteins, that do not melt on heating by virtue of their molecular architecture, are not thermoplastics but are thermosets. What is commonly described under “plastic debris” or “microplastics” in marine debris literature, however, often includes some thermosets such as epoxy resin, reinforced polyester (e.g., glass‐reinforced plastic (GRP)) and tire rubber particles. In this chapter, we will use the term “polymer” interchangeably with “plastic” for convenience of discussion.
Figure 1.1 Classification of plastic types commonly found in the marine environment.
Plastics owe their impressive success as a material to their unusual molecular structure that obtains a unique combination of advantages (Singh and Sharma 2008). Very long, chain‐like molecules in polymers result in strong attractive forces between them that allow for the development of unusual strength in the material. If the long‐chain molecules are flexible enough, they can also profusely entangle with each other, resulting in resistance to deformation, contributing to the strength of plastics. Thermoplastics can easily be formed into different shapes at relatively low temperatures to obtain lightweight (low density) products that are strong, transparent, bio‐inert, and gas‐impermeable, thereby making them ideal as packaging materials. Thermosets, especially polymer composites reinforced with fillers or carbon fibers, serve as a durable, high‐strength, and corrosion‐resistant material that allows a new degree of design freedom that is exploited in building design and transport applications. It is this combination of characteristics that impart the versatility of plastics in numerous applications. No wonder we now annually produce enough plastics that exceed the global biomass of human beings. Figure 1.1 shows the classification of common plastics in the marine environment.
Figure 1.2 shows a breakdown of the mix of plastic resins manufactured worldwide along with the main application sectors for different resin types. PE is the resin produced in the highest volume(~50%) followed by PP and PET. The figure shows that over 35% of resins produced are used in packaging where products are expected to have the shortest service life and are particularly likely to end up as urban or beach litter.
