Plastics and the Ocean. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Химия
Год издания: 0
isbn: 9781119768418
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Schematic illustration of classification of plastics according to inherent biodegradability.

      Plastics manufacturing is too broad a subject to be discussed in any detail here, and the following is a minimal introduction to allow readers who are not familiar with plastics to better appreciate the following chapters. Feedstock for polyolefin production are gaseous chemicals, called monomers, that are derived from oil, natural gas, or coal. Generally, a distilled fraction of oil, such as naphtha, is thermally cracked into these olefin monomers. Hydrocarbons in natural gas can also be converted to olefins; alternatively, biomass‐derived ethanol can also be converted into ethylene for use as a monomer. The ethylene made by any of these processes is polymerized using specialized catalyst systems,such as the Ziegler‐Natta or metallocene catalysts, to obtain olefin polymers with closely controlled molecular weight, chain geometry, and crystallinity. Pressure and temperature are the key variables that determine the structure and properties of the resin formed. The reaction is a catalyzed free‐radical polymerization, but several different reactor technologies, such as autoclaves, tubular reactors, stirred tanks, and fluidized bed reactors, are generally used in the manufacture of polyolefin resins. Mostly the same types of reactors are used in the manufacture of PP as well. Styrene monomer used in the manufacture of PS by free‐radical polymerization is a liquid, allowing emulsion polymerization in the liquid phase. The result of this resin manufacturing process are the virgin plastic pellets used by processors who convert these into useful plastic products.

      An important part of this latter operation is ‘compounding’, where the plastic is melted and intimately mixed with chemical compounds called additives, meant to improve the properties of the plastics to obtain their best performance in the intended product (see Chapter 2). Mixing can be conveniently carried out in a compounding extruder at a temperature high enough to melt the plastic. The compounded plastic is then used to mold products by one of many techniques, the ones popular with thermoplastics being injection molding, extrusion, and blow molding. These approaches do not work well with thermoset plastics that need to be compression molded.

Schematic illustration of a plot of the GWP (kg CO2-e) versus Embodied Energy (GJ) per 1000 kg of common plastics.
Plastic Water (L) Acidification (kg SO2‐e) Eutrophication (kg N‐e) Ozone Depletn. (kg CFC‐11‐e) Smog (kgO3‐e)
HDPE 8143 5.22 0.26 12 × 106 129
LDPE 11553 6.54 0.30 1.3 × 106 148
LLDPE 7383 4.69 0.25 1.2 × 106 125