Plastics by the Numbers

THE MAJOR PLASTIC RESINS, THEIR USES AND RECYCLED LIVES

Background

Plastics are everywhere in our daily lives: in the furniture we sit on, carpets we walk on, utensils we eat with and clothes we wear.

Plastics are made from long chains of polymers. Polymer comes from the Greek polys (many) and meros (parts). Basically, all polymers are giant molecules made up of repeating units, called monomers. The units may be identical or not. Chemists have their own language for designating different monomers. A single polymer is often made up of thousands of monomers linked together like the cars of a train.

Example: An ethylene molecule (C2H4) is made from two carbon atoms and four hydrogen atoms. The resin polyethylene is formed by linking many monomers of ethylene molecules to form the polymer.

Not all polymers are manufactured. Natural polymers are found in foods such as proteins, starches and carbohydrates. DNA and RNA proteins in our bodies are natural polymers. Polymer molecules give structure, function and protection to all living things. Likewise, synthetic polymers are used to create manufactured products with these attributes.

Some plastic polymers are very hard and rigid (bowling balls, football helmets), while others are soft and flexible (foam mattresses). Some polymers are resistant to heat (adhesives used on the space shuttle), while others can be readily melted (milk jugs). Some polymers can be molded into useful objects over and over, while others may resist being reformed. These properties depend on the structure of the polymer.

Chemists can produce plastic polymers to meet specific needs by controlling the various factors that contribute to a polymer's properties. These factors include the size of the polymer, the structure of the polymer (is it linear or does it have branches) and whether or not additives, such as pigments, are present. The same resin can have different properties if one version of it is "foamed" by adding gases.

When plastics are recycled, their properties can be manipulated chemically to create a new plastic object that may differ functionally from the original object. An example would be recycling 2-liter PET soda bottles into fiberfill insulation for winter jackets.

Purpose

1. To describe some of the major plastic resins and their uses

2. To compare and contrast the properties of different plastic resins

3. To list products that different resins can become through recycling

Procedure

1. Read Table 1. Major Plastic Resins and Their Uses. This resin chart describes six common resins and the variety of uses each has. The uses depend on the properties of each resin listed (e.g., lightweight, flexible, incredibly hard, rigid, durable). The code numbers are used by recyclers to identify different resins.

2. Find a sample of as many of the six resins as you can. (There is a "7" which is used kind of like "none of the above". It is not common.) Since this is an extra credit assignment, each resin you use will add to your grade. It is important that the plastic identification code be visible on each object, for ease in identifying the resin from which it is made.

3. Fill in Table 2. Properties of Plastic Resins as you examine your plastic samples.

4. Cut out the recycling symbol on each sample and attach it to Table 2.

5. What is the short name (HDPE, etc) of each sample?

6. Is it clear? Does it have color?

7. If you fold it, does it bend or does it break?

8. Can you stretch it?

9. Cut a small piece, about 1 cm by 1 cm, of each sample and put them into a small container of tap water and note which float (F) and which sink (S). You may have to push them under at first. Add some salt to the water until you have a saturated solution (some solid salt remains undissolved on the bottom.) Mark if each sinks or floats.

10. Boil a small amount of water. Remove the water from the heat. Once the bubbling stops, add the pieces of plastic. Note whether they sink or float. Also note if any are deformed by the heat.

11. Test each sample with acetone (nail polish remover). Put a drop or two on each piece and note if the plastic starts to dissolve. Follow the safety instructions that came with the nail polish remover.

12. Indicate where your samples came from (shampoo bottle, margarine lid, etc).

13. Indicate one product each of your samples could be recycled into.

Table 1. MAJOR PLASTIC RESINS AND THEIR USES
Resin Code Resin Name Uses Recycled Products
Polyethylene Terephthalate (PET or PETE) Plastic soft drink bottles, mouthwash bottles, peanut butter and salad dressing containers Liquid soap bottles, strapping, fiberfill for winter coats, surfboards, paint brushes, fuzz on tennis balls, soft drink bottles, film, egg cartons, skis, carpets, boats
High Density Polyethylene (HDPE) Milk, water and juice containers, grocery bags, toys, liquid detergent bottles Flower pots, drain pipes, signs, stadium seats, trash cans, recycling bins, traffic-barrier cones, golf bag liners, detergent bottles, toys
Polyvinyl Chloride (V) Clear food packaging, shampoo bottles Floor mats. pipes, hose, mud flaps
Low Density Polyethylene (LDPE) Bread bags, frozen food bags, grocery bags Garbage can liners, grocery bags, multipurpose bags
Polypropylene (PP) Ketchup bottles, yogurt containers and margarine tubs, medicine bottles Manhole steps, paint buckets, videocassette storage cases, ice scrapers, fast-food trays, lawn mower wheels, automobile battery parts
Polystyrene (PS) Videocassette cases, compact disc jackets, coffee cups, knives, spoons, and forks, cafeteria trays, grocery store meat trays and fast-food sandwich containers License plate holders, golf course and septic tank drainage systems, desk top accessories, hanging files, food service trays, flower pots, trash cans, videocassettes

Question. Two plastics that are targeted for recycling from household waste are polyethylene terephthalate (PETE) and high density polyethylene (HDPE). One of the problems of recycling such materials is separating them. Suppose you have been hired to set up a process for separating large quantities (many tons) of waste plastic that is a mixture of PETE and HDPE. Describe how you might perform this separation.


Teacher Notes (Remove this section before giving to students.)

1. Subject and category: Petroleum - Laboratory Activity

2. This lab is adapted from material provided by the American Plastics Council. The original lesson was on the web at one time in "pdf" format. It wasn't there anymore when I checked last.

3. I use this as a take-home lab to be done for extra credit over Spring Break. Points are given for each polymer and for answering the Question. (Many students don't even see the question.)

4. The acetone test is ambiguous. Finger nail polish remover should affect PVC but doesn't always. You could leave it out. You may have safety concerns about the acetone.

5. If I left something out, you can e-mail me at teacher@aloha.net. Bob Brennan, Castle High School, Kaneohe, Hawaii.


Comments? Questions?
Bob Brennan, Castle High School, Kaneohe, Hawaii.