Chemistry Of Plastics
Plastics consist of very long molecules each composed of carbon atoms linked into chains. One type of plastic, known as , is composed of extremely long molecules that each contain over 200,000 carbon atoms. These long, chainlike molecules give plastics unique properties and distinguish plastics from materials, such as metals, that have short, crystalline molecular structures.
Although some plastics are made from plant oils, the majority are made from fossil fuels. Fossil fuels contain hydrocarbons (compounds containing hydrogen and carbon), which provide the building blocks for long polymer molecules. These small building blocks, called monomers, link together to form long carbon chains called polymers. The process of forming these long molecules from hydrocarbons is known as polymerization. The molecules typically form viscous, sticky substances known as resins, which are used to make plastic products.
Ethylene, for example, is a gaseous hydrocarbon. When it is subjected to heat, pressure, and certain catalysts (substances used to enable faster chemical reactions), the ethylene molecules join together into long, repeating carbon chains. These joined molecules form a plastic resin known as .
Joining identical monomers to make carbon chains is called addition polymerization, because the process is similar to stringing many identical beads on a string. Plastics made by addition polymerization include polyethylene, , polyvinyl chloride, and . Joining two or more different monomers of varying lengths is known as condensation polymerization, because water or other by-products are eliminated as the polymer forms. Condensation polymers include (polyamide), polyester, and polyurethane.
The properties of a plastic are determined by the length of the plastic’s molecules and the specific monomer present. For example, elastomers are plastics composed of long, tightly twisted molecules. These coiled molecules allow the plastic to stretch and recoil like a spring. Rubber bands and flexible silicone caulking are examples of elastomers.
The carbon backbone of polymer molecules often bonds with smaller side chains consisting of other elements, including chlorine, fluorine, nitrogen, and silicon. These side chains give plastics some distinguishing characteristics. For example, when chlorine atoms substitute for hydrogen atoms along the carbon chain, the result is polyvinyl chloride, one of the most versatile and widely used plastics in the world. The addition of chlorine makes this plastic harder and more heat resistant.
Different plastics have advantages and disadvantages associated with the unique chemistry of each plastic. For example, longer polymer molecules become more entangled (like spaghetti noodles), which gives plastics containing these longer polymers high tensile strength and high impact resistance. However, plastics made from longer molecules are more difficult to mold.
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