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Plastics and the Ocean. Группа авторов
Читать онлайн.Название Plastics and the Ocean
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isbn 9781119768418
Автор произведения Группа авторов
Жанр Химия
Издательство John Wiley & Sons Limited
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.
Figure 1.2 Left: Global plastic resin production (2015 data), Right: Percentage use of the production in different application sectors (miscellaneous categories not shown).
Source: Redrawn from data in Geyer et al. (2017)
1.2 Plastics at Present and in the Future
At present, plastic production is a relatively energy‐efficient operation supporting a vast global manufacturing industry providing an array of useful products at a resource cost of only about 4–6% of the annual global petrochemical demand (compared to the ~50% used for transportation). The embodied energy2 EE (MJ/kg) of a material is a useful measure of how “energy‐expensive” a given material might be and is the sum of all energy expenditure associated with producing a unit mass of the material or a functional unit of a product. This energy is not “embodied” in a product in the sense that all such energy can be recovered from the material. Market cost, however, is an unreliable guide to the EE of a material or product. Common plastics generally have a lower EE compared to metal, close to that of glass, but higher than that of wood. Most of this energy is typically derived from fossil fuel, a dwindling non‐renewable resource that should invariably constrain global plastics production. But, a shortage of feedstock is not expected, especially in the US, at least in the foreseeable future; the boom in natural gas in the US (with about 500 trillion ft3 of proven gas reserves) guarantees the availability of low‐cost feedstock for plastics at least the next couple of centuries. Also widely anticipated is the freeing up of about 45% of the demand on global crude oil for gasoline production because of the expected growth in electric vehicles worldwide (CIEL 2021). The petrochemicals sector, including plastics, will then become the major driver for the petroleum industry, accounting for about a third of the future oil demand (IEA 2020).
As shown in Figure 1.3, manufacturing plastic resin requires a regular supply of fossil‐fuel feedstock, a source of processing energy, as well as commons resources such as air or water, a category often either overlooked or incompletely accounted for in calculating the cost of the product, shown on the left side of the figure. In the process, the carbon in the feedstock is sequestered in the plastic resin, while that used as fuel to generate energy for the operation is released as CO2. A suite of externalities that impact air, water, and the generation of solid waste accompany the manufacturing process. The result of this operation are plastic resin pellets. These resin pellets must be transported and further processed thermally to be shaped into useful consumer products that we are familiar with. This also requires additional energy and results in emissions, though to a relatively lesser extent compared to in manufacturing. It is the combination of these externalities, referred to as “embodied impacts,” from manufacturing, use, and disposal, that is a major concern given the already‐apparent signs of man‐made climate change.
Figure 1.3 Schematic representation of the manufacturing process for plastics resin.
Global plastic production will keep increasing in the foreseeable future, especially given the availability of low‐cost feedstock and the growing demand for resin. Even at present, the plant capacity for resin production both in the US and globally exceeds the current demand for resin. But, producers are already investing in additional plant capacity3 anticipating a higher resin demand in future years. By 2050, the consumption of oil used to manufacture plastics is expected to outpace that by automobiles (IEA 2018). As production volumes invariably determine future environmental impacts of the industry, estimating resin production in the medium term is of special interest. An approximate estimate of future demand for plastics might be based on the analysis given in Figure 1.4 that plots the global resin production (MMT) with the world population (in Billions), with the trend therein extrapolated into future decades. Historic data fit a second‐order polynomial model (R 2 > 0.99) and when extrapolated using projected future global population, the plot suggests an annual resin production of about 1040 MMT in 2050 and 2410 MMT in 2100. However, implicit in the extrapolation is