Materials in Today's World

Polymer Formation


As we discussed in the polymer lesson, there are two types of polymerization: addition (or chain) polymerization and condensation (or step) polymerization. In addition polymerization, a free radical attaches to a monomer. This results in an unsatisfied bond on the monomer, which is free to attach to another monomer. This process repeats over and over again building a polymer chain. In condensation, two chemical groups react together. Typically, one of the groups has an exposed hydrogen, while the other has an exposed oxygen-hydrogen. When the two compounds join, a monomer is formed with an exposed oxygen-hydrogen or hydrogen and releases a water molecule, H2O.

Polymers are synthesis by polymerization and the polymer properties are modified by the usage of additives. These additives are used to improve mechanical properties, processability, durability, etc. The five additive types discussed in the e-book are fillers, plasticizers, stabilizers, colorants, and flame retardants. Fillers are added to improve tensile strength, abrasive resistance, and toughness, as well as to reduce cost. Plasticizers are added to transform brittle polymers to ductile ones. Stabilizers are added to protect from degradation due to exposure to ultraviolet light. Colorants are added to provide color to the polymer. Flame retardants are added to eliminate or reduce the flammability of polymers.

Fabrication of plastic polymers can utilize one of several molding techniques: blowing, compression, injection, and transfer, or by extrusion or casting. Fibers can be spun or drawn. Films can be formed by extruding, blowing, or calendaring. The following video (3:11) highlights blow molding for the production of plastic bottles. Again, while you are watching this video, please think back to Lesson 1 of this course and the reading in the textbook about different materials used for carbonated beverage containers.

To Watch

Click for the transcript of How Plastic Bottles are Made.

Whether you're buying apple juice or peanut butter, you've probably noticed that fewer products come in glass containers these days. Plastic packaging is becoming more common. Plastic bottles and jars are lighter to carry and leave no shards of glass to clean up if you drop your grocery bag. Many transparent bottles and jars are made from a type of plastic called polyethylene terephthalate or PET. An automated mixer combines PET pellets with flakes of recycled PET. Reprocessed plastic loses some of its physical properties, so the recycled content cannot exceed ten percent. The PET drops from the mixture into a plastic injection machine that heats it to a piping 600 degrees Fahrenheit. The dry raw material melts into thick and gooey liquid plastic. The machine then shoots it at high pressure into a mold. This plastic injection molding process casts pieces of plastic called preforms. Starter shapes and subsequent machines will transform into bottles or jars. The molded preforms harden almost instantly thanks to a built-in cooling system. These preforms are now on their way to becoming single serving juice bottles.

This is another plastic injection molding machine. It uses the same method to make preforms for a different model: one-and-a-half to two-liter bottles. The preforms next stop is a machine called a reheat stretch blow molder. In a matter of seconds, it heats each preform just enough to make the plastic malleable, then inserts a rod to stretch the preform lengthwise while at the same time blowing in air at extremely high pressure. This forces the preform into a bottle shaped bowl. Cold water circulates within the mold to cool and set the plastic almost instantly. This lightning fast machine turns out ten thousand six hundred bottles per hour. No wonder we've had to show it to you in slow motion. A conveyor belt transports the finished bottles to the packaging area. Before blow molding, the preforms for certain models first pass through an oven.

You have now finished the reading for Lesson 10. Please proceed to the next page and watch the video for Lesson 10.