Plastics

Forty years ago, anything made of plastic was considered "cheap." That’s certainly not true today when plastics are used in thousands of products ranging from computers, automobile parts and important medical equipment to toys, cookware, sports equipment, and even clothes. And the plastics industry continues to grow rapidly. Just where do plastics come from?

What are plastics?

It may surprise students to learn that gutta-percha, shellac, and the horns of animals—all naturally occurring substances—were used as plastic material before the first synthetic plastics were produced. Gutta-percha is derived from the sap of certain trees, and shellac is made from the secretions of a tiny scale insect. Before horn can be used, however, it must be "plasticized," or softened, by being boiled in water or soaked in an alkaline solution.

The first synthetic plastic was made from the plant material cellulose. In 1869, John Wesley Hyatt, an American printer and inventor, found that cellulose nitrate could be used as an inexpensive substitute for ivory. The mixture could be plasticized with the addition of camphor. Celluloid, as this new material was called, became the only plastic of commercial importance for 30 years. It was used for eyeglass frames, combs, billiard balls, shirt collars, buttons, dentures, and photographic film.

In 1951, two young research chemists for Phillips Petroleum Company in Bartlesville, Okla., made discoveries that revolutionized the plastics world. Today, the plastics they discovered—polypropylene and polyethylene—are used to produce the vast majority of the thousands of plastics products all over the world. (Read more about their discoveries in "Serendipity, Science & Discoveries" in this publication.)

The source for today’s wide variety of plastics? Petroleum.

Petroleum to Plastics

The technological road from oil field to finished plastic product has numerous fascinating side trips. Here’s the route taken in the petroleum-to-plastics process:

1. Petroleum is drilled and transported to a refinery.

2. Crude oil and natural gas are refined into ethane, propane, hundreds of other petrochemical products and, of course, fuel for your car.

3. Ethane and propane are "cracked" into ethylene and propylene, using high-temperature furnaces.

4. Catalyst is combined with ethylene or propylene in a reactor, resulting in "fluff," a powdered material (polymer) resembling laundry detergent.

5. Fluff is combined with additives in a continuous blender.

6. Polymer is fed to an extruder where it is melted.

7. Melted plastic is cooled then fed to a pelletizer that cuts the product into small pellets.

8. Pellets are shipped to customers.

9. Customers manufacture plastic products by using processes such as extrusion, injection molding, blow molding, etc.

Plastics Processing Methods

Extrusion Molding -- the main process used to form plastics. A heated plastic compound is forced continuously through a forming die made in the desired shape (like squeezing toothpaste from a tube, it produces a long, usually narrow, continuous product). The formed plastic cools under blown air or in a water bath and hardens on a moving belt. Rods, tubes, pipes, SlinkysŪ, and sheet and thin film (such as food wraps) are extruded then coiled or cut to desired lengths.

Plastic fibers also are made by an extrusion process. Liquid resin is squeezed through thousands of tiny holes called spinnerets to produce the fine threads from which plastic fabrics are woven.

Injection Molding -- is the second most widely used process to form plastics. The plastic compound, heated to a semifluid state, is squirted into a mold under great pressure and hardens quickly. The mold then opens and the part is released. This process can be repeated as many times as necessary and is particularly suited to mass production methods. Injection molding is used for a wide variety of plastic products, from small cups and toys to large objects weighing 30 pounds or more.

Blow Molding -- pressure is used to form hollow objects, such as the soda pop bottle or two-gallon milk bottle, in a direct or indirect method. In the direct blow-molding method, a partially shaped, heated plastic form is inserted into a mold. Air is blown into the form, forcing it to expand to the shape of the mold. In the indirect method, a plastic sheet or special shape is heated then clamped between a die and a cover. Air is forced between the plastic and the cover and presses the material into the shape of the die.

Classroom Activity

Plastics:  Imagine Life Without Them

Suppose you could step inside a time machine and go back 60 or 100 years. You may easily convince yourself a day without cars, telephones, and television—maybe even computers—might be kind of fun. Have you thought about the little things, though? Little things are often what are missed the most.

Imagine a day without touching something made of plastic!

People were cleaning their teeth before there was a choice of red, blue or purple plastic toothbrushes, but would you really want one made from metal or wood? Milk and shampoo used to be sold in glass bottles, and that wasn’t a problem unless you dropped one and it broke. What a mess!

The word plastic comes from the Greek word plastikos, meaning "able to be molded." That characteristic, or what scientists call "property," makes plastic perfect for things like action figures and dolls. What other properties does plastic have that make it a good choice for particular products? Try these experiments, then use what you learn to list 10 or even 20 good candidates for plastic.

Materials

Place the two stacks of books about 9" apart. Lay the aluminum strip across the books, making a level bridge. Lay the string parallel to the strip. Next place the canned good in the middle of the strip. Use the string to measure the deflection (bend) in the strip and write down your observation.

Repeat the process with the wood and plastic, then discuss what you have observed about the strength and stiffness of each material. (Which material would you choose for a toothbrush handle? How about the toothbrush bristles?)

Next, soak the tennis ball in water until it is thoroughly wet. Place it first in the plastic sack and then the paper one, letting it sit in each for 5 minutes before shaking each bag vigorously. Which bag stays dry? Does the water cause the paper bag to sag and even break? What can you conclude about how each material repels or absorbs water? Which bag would you choose to carry your carton of ice cream?

The information, or data, you’ve gathered from your experiments is just what a scientist uses to decide what material to use in an engineering design!

(Activity provided by the engineers of Phillips Petroleum Company.)

Sources

Gideon, Joyce Kirkpatrick. The Plastics Pioneers:Phillips 66 Company. Phillips Petroleum Company. 28 December 1990.

"Life of the Past." Investigating the Earth. Houghton Mifflin Company. Boston, Massachusetts. 1993.

"Plastics: Manufacture." Compton’s Living Encyclopedia. Online. America Online. 22 April 1997.

Webster’s New Universal Unabridged Dictionary. New World Dictionaries/Simon & Schuster. New York, New York. 1993.