Raw Materials

In the early part of the century, most plastics were derived from cellulose in plants and from the distillation of coal. The first polymerized plastic combined champhor (drawn from plants) and cellulose nitrate (derived from cotton wadding used in civil war cannons). A reaction took place and generated a rigid and flammable cellulose nitrate plastic. Cellulose nitrate was used in clothing, combs, buttons, guitar picks, film for pictures, ping pong and billiard balls. Cellulose nitrate's tendency to explode forced its replacement with cellulose acetate during the 1920's.

Another process which combined benzene with ethylene produced polystyrene. Polystyrene saw early use in toys, radio dial lenses, and consumer products. These two process involved several steps, produced waste, and the quality of the materials was difficult to control.

Today, most plastic materials are derived from ethylene, acetylene, methane, butane, propane, and naphtha. These are all by-products from the distillation and refining of gasoline. A flow chard in Appendix C illustrates how these six by-products are the starting monomers for most plastic materials.

Crude Oil - Plastic's Raw Material

Figure 2-17 illustrates ten by-products from a barrel of crude oil. These by-products are refined into grades of fuels based on the size of the molecule. The raw materials (called feed stocks) for plastics and other petroleum chemicals are the light extracts or small molecule gases. Fourteen percent ofevery barrel of crude oil goes toward the production of plastics! The feed stocks are separated at a refinery by a process called distillation. The distillation of crude oil is similar to the process of producing pure water from steam.

The water molecule of two hydrogen and one oxygen atoms is liquid at room temperature. Boiling the water transforms it into steam. A lid over the boiling water collects, cools, and condenses the steam and returns it to its liquid state. The collected water is then in pure form. Sediments and other contaminants are left in the original container. The water is now distilled water and the process is called distillation.

The distillation process used to divided crude oil into feed stocks works on the same principle. Crude oil is make up of billions of different sized molecules. Depending on the molecular distribution, the oil can be black to amber with a range of viscosity. To distill crude oil, it must be transformed from a liquid to a gas and then condensed into feed stock of different molecule size. A distillation tower, illustrated in Figure 2-18, separates crude oil into its different molecular parts.

Distillation towers are referred to as fractionation towers because their function is to divide cure oil into separate fractions of the same molecular size. The base of the tower crude oil is heated to 1600 degree F. At this temperature, 90% of the oil transforms from a liquid to a gas. The gas then passes to the main tower and begins to rise to the top. The 10% liquid that does not turn to gas is drawn off at the bottom and used as coke fuel source.

As the gas rises in the tower, it is channeled through tubular openings in multiple trays. Each of the tubular opening is partially capped. As the gas collects on the cap, the largest molecules condense back to a liquid. The opening around the bubble cap allows the shorter gas molecules to pass and move up to the next tray. Each tray gets continually cooler as it moves further from the base. Large molecules condense and to liquid at lower levels. The smaller molecules remain in gas form; the smaller molecules rise further up the tower.

At each level, as the gas condenses to liquid, it collects at the base of the tray and is drawn off for further refining. At the very top of the distillation tower the small gas molecules escape. These are called lights extracts. They consist of various amounts of acetylene, ethylene, and ethane gas. These gases are the raw material for many plastic materials. It is common to polymerize light extracts into polyethylene plastic.

Polyethylene Plastic

Polyethylene is a good model for our study of plastic materials because its polymerization is quite simple. Also, consumers are familiar with its abundant uses in film wrap, containers and trash bags. White plastic milk bottles are made from high density polyethylene blow molded into the bottle shape. The best way to understand polyethylene is to visualize its micro structure.

If we could expand a polyethylene milk bottle millions of times and walk into its micro structure, it would be like walking into a dense forest of long vines. The vines would be molecules. In some places the vines would stack into thick, round bundles. These bundles would be the crystalline areas of the micro structure. The entangled vines surrounding the crystals would be the amorphous areas in the micro structure.

Continuing our journey, a single molecule (vine) sits in an amorphous area. This molecule and others around us appear like beaded strings. On close observation, the beads appear more like fuzzy balls. Large fuzzy balls connected to each other would be carbon atoms; small hydrogen fuzzy balls are connected to two sides of each carbon.. A two dimensional drawing of these structures is represented in Figures 2-19. How does polyethylene get to be so complex? The answer lies in the polymerization of polyethylene from the ethylene.

Polymerization Of Polyethylene Plastic

The polymerization of polyethylene plastic begins with formulating a large supply of its monomer ethylene. The most abundant supply can be found from the petroleum industry. The light extract taken from the top of the distillation tower is acetylene, ethylene, and ethane. Using a chemical process called cracking, the two end hydrogen atoms from ethane are removed to produce a molecule of ethylene. The two removed hydrogen molecules are then added to acetylene to form a second ethylene molecule. In this way the three different molecules are now all ethylene.

Heating billions of ethylene molecules to 475-530o F, with .05% oxygen, the chemical polymerization reaction begins. The reaction breaks the weak double bond between the carbon atoms of the ethylene molecules. Once broken, the bond links head to head with an adjacent ethylene molecule. The ethylene molecules begin to link-up forming long chains. Polymerization produces a polyethylene molecule with covalent bonded hydrogen and carbon atoms. Figure 2-20 illustrates a segment of two adjacent polyethylene molecules. One has a side branch. The chain is held together with covalent bonds. Van der Waal's forces act between chains. The molecules can measure 10,000 to as high as 1,000,000 linked carbons in length. The pressure developed during the reaction measures between 30,000 and 40,000 pound per square inch (PSI). Figure 2-21 illustrates the polymerization of ethylene to polyethylene.

The length of the chains is controlled by chemicals that terminate the reaction. These chemicals are called end groups. The more end groups that are added to the mixtures the shorter the average length of the chains. The shape of the chains can be linear, branched, or circular and these formations are controlled by the type of monomer, exposure time and pressure of polymerization. The higher the pressure, and longer the polymerization, the more linear the molecules.

Polyethylene plastics is produced in four different grades: low, medium, high, and ultra high molecular weight densities. The polyethylene grades are determined by the average length and shape of its molecule. Molecules vary in lengths from low density that averages 19,000 carbons long, to ultra high density with one million carbons atoms long. Medium density polyethylene's molecule is approximately 100,000 carbons long and is 5 time longer than a comparable plastic with low density. Medium density polyethylene finds wide application in containers, packaging, and the familiar fifty gallon trash barrel. High density polyethylene molecules average 200,000 carbon atoms long. High density yields rigid results and has wide application as one gallon plastic milk containers. An ultra high molecular weight (length) linear polyethylene is used to make the plastic suppermarket carryout bags. Its molecules average a million carbons in length.

Viscoelastic Behavior

Plastics material have been described as viscoelastic materials having both liquid and solid properties. While they appear like solid materials at room temperature, the amorphous micro structure tends to relieve internal stress. The amorphous areas can be defined as a high viscosity, slow liquid. The viscoelastic behavior of plastic can be observed in a plastic lid used with coffee containers. In a cool room (72o F), the lid behaves like a solid. Exposed to sun light the lid begins to distort. The warm sunlight partially expands the amorphous micro structure and then the molecules begin to move rapidly. The molecule's movement produces a decrease in lid's viscosity (viscosity is a measurement of a liquids resistance to flow). The high viscosity area appear solid, but is actually a slow liquid.

Viscoelaticitiy is easily detectable in candle wax. As children most of us have softened wax by warming it in our hands or melted it with a candle flame. This was possible because candle wax averages 50 carbons in length. Compared this with a coffee can lid made from polyethylene plastic with a molecule size of over 50,000 carbons linked together. The molecules of wax and polyethylene are the same, except polyethylene is much longer. Thus, with a slight increase in temperature, the wax softens and flows. The difference in molecular size also means that these two materials have different microstructures. To understand this property, we must discuss how any micro structure is affected by heat.

Recall the atomic structure of two segments of polyethylene molecules in Figure 2-21? The covalent bonding and Van der Waal's force are shown enlarged millions of times. Again recall that the micro structure is string like, with one side branch. The molecules are in motion. Their motion depends on the space between molecules. The distance between molecules is dependent upon temperature. An increase in temperature increases the space between molecules. The more space between molecules the greater the viscous flow properties of the plastics. The closer the molecules are, the greater the elastic properties of the plastic. The amorphous micro structures are high in viscous properties. Crystalline micro structures are incapable of viscous behavior so they act as elastic solids.

All plastic materials exhibit some degree of temperature sensitive viscoeleatic behavior. All plastics appear solid, yet there is always some measurable movement. The plastic lid's ability to appear taut but to remain flexible ehough to open is an observable form of viscoelastic behavior. Other plastics with longer molecules and a higher percentage of crystallinity do not demonstrate observable viscoelastic behavior. However, these plastics will exhibit a measurable change over a longer period.

The principle of viscoelasticity is fundamentally the most important principle in our understanding of thermoplastic materials. Recall our metophor for plastic's molecular structure? A forest of vines. The movement of the molecular structure, in the amorphous area, is similar to vines floating in a light breeze. The breeze represents temperature. The vines represent molecules. An increase in temperature increases the molecules' motion. If the heat is high enough, the Van der Waal's force between molecules will be eliminated and the plastic will melt. The motion of the molecule coupled with the elimination of the holding force causes the molecule chains to move. Thus shape of the plastic product changes. If the temperature is high enough, the crystalline areas unravel and become amorphous structures. When this happens, the plastics in soften or melt. If the temperature is high enough to overcome the Van der Waal's force, the plastic will inevitably melt.

The ability to use changes in temperature to alter the holding power of Van der Waal force allows plastic materials to be easily shaped, molded and manufactured. Once the temperature is lowered, the molecule returns to their crystalline and amorphous micro structure and the plastic materials become relatively stable.

By contrast, metals and ceramics remain solid with a slight change in temperature. They do not have viscous properties. They are elastic solids. These materials, unlike plastics, remain stable and have predictable physical properties. In contrast, the micro structure of plastic materials are in a continuous state of change. The rate of change is different for each plastic. Increases in temperature and pressures accelerate the rate of change.To select the best materials product designers can exploit plastic's versatility if they understand how temperatures and micor structures affect their materials.