Plastics Are Made From Crude Oil

Plastics are made by combining small molecules that are found in crude oil to form giant molecules. The process is called polymerization. For example, Figure 2-5 shows how small molecules of acetylene, ethane, and ethylene can be combned to form the larger molecules of pentane, gasoline, paraffin and polyethylene.

The first three gas molecules shown in Figure 2-5, are found in large quantities entrapped in crude oil. The three molecules are also found in natural gas deposits. (Actually, natural gas is methane.) The larger molecules of pentane and gasoline are liquid and also are make up a significant part of crude oil. Notice they are just longer molecules of hydrogen and carbon. By increasing the size of the gasoline molecule by four carbons and eight hydrogen a solid (paraffin) is produced. A much greater leap in physical properties is produced by extending the size of the paraffin molecule by 1,000 times to produce a polyethylene plastic. Polyethylene duplicates both the feel and burning qualities of paraffin.

The small molecules used to make the large plastic molecules are called monomers. For example, ethylene gas is the monomer for polyethylene plastic. A monomer is the generic term meaning single (mono) unit (mer) and describes any molecules used as the starting unit to form a plastic. Plastics in their pure form are called polymers, - - poly means "many" and mer means "unit"; thus polymers imply material that combine many units. All plastic materials are made by "polymerization," the process of linking small molecules (called monomers) together to form giant molecules (called polymers).

Polymerization

There are many types of polymerization, however, additions and condensation are the two most important. "Addition polymerization" implies simple adding molecules together. The process deals with complex chemical reactions and is highly explosive. Polymerization must be closely controlled in high pressure containers that are instantly cooled or heated. The process is chemically controlled with a variety of complex neutralizers and catalysts. Most thermoplastic materials are made by addition polymerization.

"Condensation polymerization" consists of elminating a small amount of water during the chemical reaction. Most thermosets and many thermoplastics are made by this process. In this chapter we will only discuss addition polymerization and leave condensation for a later chapter. First, polymerization process combine monomers to form polymers.

Monomers

The monomer used in the polymerization process determines the type of plastic. As shown in Figure 2-6, polyethylene plastic starts with the monomer ethylene gas. Other examples are polyvinyl chloride, produced from vinyl chloride monomer, polystyrene, produced from styrene monomer, and polyacrylonitrile, produced from acrylonitrile monomer. The polymerization process can be made even more complex resulting in copolymers and tretrapolymers.

Copolymers are made by using two different monomers during polymerization. For example, the monomers styrene (solvent) and butadiene (natural rubber) are polymerized to produce the copolymer polybutalstyrene. Adding the monomer acrylonitrile (solvent) with butadiene and styrene the tretrapolymers ABS is made. ABS is a versatile plastic used to house instruments; form pipes, luggage, computer keys, and numerous consumer products. Once monomer are polymerized, the plastic is a copolymer.

Polymers

A polymer is the generic term meaning many ("poly") units ("mer"). Polymers can result in paints, a form of rubber, and various oils, solvents, plus other applications. Plastics are just one type of polymer. The pure form of a plastics generally referred to as a polymer. The polymer cannot be processed alone it must be modified with additives. Once modified with additives the polymer becomes a material useful for process or product applications, plastic as we know it.

Plastics

Plastic materials are generally divided into the two classifications: thermoplastics and thermosets. The most recent handbook, Plastic Materials and technolology by Irvin Rubin, lists over fifty different plastic materials. These materials are modified with additives to enhance its processability and product stability into thousands of different grades. For our purposes we will focus on those thermoplastic and thermoset materials because they are used extensively in commodity plastics. These are the materials most often used in packaging, household appliances, computer instruments, textiles, and medical products.

Table 2-4 illustrates four commodity thermoplastics: polyethylene, polystyrene, polyvinyl chloride, and acrylonitrile - butadiene - styrene (ABS), their chemical structure, and typical applications. A compete list of the most important thermoplastics plastics can be found in Appendix A. Thermosets are listed in Appendix B.

Degree of Polymerization

The "degree" of polymerization refers to the average length or size (weight) of a plastic's molecule chain. There are two averages calculated. One number represents the average for length; another indicates the average size or weight of the molecule chain. Knowing the length is important since any increase in length significantly increases the materials properties. An increase in weight of a plastic's molecule will also increase the material's properties.

The number is derived by scientifically counting (a very complex process) the number of carbon atoms that are linked together in each molecular chain and then dividing that number by the number of chains. Sometime it is more useful to add the atomic weights of each molecular chain and divided it by the number of chains. In linear molecules the chains are like ropes so the number average (chain length) is the most important calculation. In branched molecules the chain forms like the limbs on trees so the weight average (chain size) is the most important calculation. Understanding the physical size of the molecules in a plastics materials allow the designer to assess and compare the properties of different grade of plastics. To understand the different grade plastics we must study their micro structures