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.
Figure 1.10 A plot of the GWP (kg CO2‐e) versus Embodied Energy (GJ) per 1000 kg of common plastics. Ranges of values for non‐plastic materials are indicated by shaded rectangles, for comparison.
The energy expended and the greenhouse gases such as CO2 emitted (expressed as the global warming potential, GWP (kgCO2‐e)) in producing common plastics, estimated in 2011 by Franklin Associates (for the American Chemical Council), is given in Figure 1.10 and compared with that for copper, alumina ceramic, paperboard, and thermoplastic rigid foams. Similar data for other plastics such as composites, metals such as nickel or aluminum, and for materials such as wood, clay, and stone lie outside the scale of this diagram. These estimates depend on several variables, including the feedstock used, the mix of energy employed in the process, the reaction engineering employed, and the specific grade of the resin produced. Estimates will therefore vary from location to location and even temporally. Still, the figure provides a general appreciation for the magnitude of energy and emissions associated with manufacturing different materials. Most of this energy is expended in extracting and purifying the feedstock rather than in polymerizing the monomer.
Individual plastics cannot be “ranked” for environmental desirability based on the figure, as many other externalities are associated with manufacturing resins. Acidic emissions that potentially acidify the oceans, nitrogen release that can result in eutrophication, ozone‐depleting gases that affect the stratospheric ozone layer, and ground‐level smog‐forming emissions are some of these. A detailed life cycle (LCA) analysis based on reliable inventory data is needed to evaluate the impact of these on the environment. Estimated impacts were recently reported for three classes of polyethylenes (Table 1.4). Again, the impacts will vary with the location as well as the process employed in the manufacture.
Table 1.4 Estimated environmental impacts of plastic manufacture (per 1000 kg) of plastic.
| 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 |
Based on American Chemical Council/Franklin Associates.
1.6 Polymers: A Basic Introduction
Structurally, all polymers have very long chain‐like molecules but their chemical formulae are relatively simple because often, the same structural unit repeats throughout the long molecular chain. For instance, polyethylene (PE), the plastic manufactured in the highest volume globally, has a long structural formula, a part of which may looks like the following:
It is merely a repetition of (‐CH2‐CH2‐) units placed end to end. Its structural formula is therefore, conveniently written as (‐CH2‐CH2) n , where n is the number of repeat units in the chain molecule, that can run into hundreds or thousands. As each repeat unit has a molecular weight of 28 (g/mol), that of the entire molecule is (28 × n) g/mol. Regardless of the length of the chain molecule, chemically, it is still a polyethylene. Since all PE molecules will not have identical chain lengths but different values of n, there is no unique molecular weight for polyethylene or for any other polymer (in contrast with simple organic molecule that have fixed molecular weights). Typically, a sample of a polymer is a mixture of structurally similar chains of different lengths and one can only refer to an average molecular weight for the entire distribution of molecules in the sample. Generally, two types of such averages, namely number‐average (M n in g/mol) and weight‐average (M W in g/mol), are used to express the molecular weights of plastics.
where N i is the number of chain molecules having a molecular weight, M i , and N is the total number of molecules in the sample (N = ∑N
