What are Plastic Water Bottles and Bags made of?
(Student Activities #2 and #3, Appendix D)
Plastics have unique characteristics that make them useful and helpful in everyday life. They are light-weight, flexible and durable. They may be molded into various forms, bottles, bags, containers, cell phone cases, even clothing. Plastics are polymers. Polymers, (poly means many), are composed of monomers, (mono means one), single molecules that combine chemically into long chains of many monomers. (Molecules are particular combinations of atoms that create matter, like H 2O, which is the molecule for water from 2 atoms of hydrogen and 1 atom of oxygen.) How are polymers created? The monomers are combined with heat and a catalyst (helper) chemical and the result is a polymer; there are many different polymers though, depending on the monomers. (You don't recognize the monomers in the polymers!) The bonds between the molecules have changed and reorganized into strong chains, which makes them so durable. There are polymers that occur naturally, such as cellulose and cotton in plants, and even DNA, and some scientists are starting to try using the plant polymers in plastics. Why would this be good? They would probably be able to decompose, and, they are renewable resources. The monomers used to create most polymers come from petroleum products. 4 Petroleum products are nonrenewable resources. Even though plastics currently use less than 4% of the world's oil, the consumption of oil continues to increase for gasoline, and the demand for plastics continues to rise as well. 4 Which is a better use for the oil?
No one would consider drinking a petroleum product, yet after they are transformed into plastic, by altering the chemical bonds, we use the plastic to hold liquids that we drink. Also, additional chemicals, plasticizers and colorants are added to give the plastics we love to use additional attributes such as flexibility, texture, color, and resistance to melting. Some plastics, called thermosets, stay rigid and don't melt when they are heated. Thermosets are created by forming covalent cross-linked bonds in the polymer chain. (A covalent bond exists between atoms when they share an electron.) These covalent bonds are naturally going to be strong, and, super-hard to break. Thermoset plastics are products like CD, TV, and cell phone cases. Other plastics, called elastomers, don't have as many bonds as the thermosets, but also are resistant to reshaping through heat. These are used primarily for tires. The majority of plastics manufactured are thermoplastics. 5
Water bottles are made from Polyethylene terephthalate (PET or PETE) which is an example of a thermoplastic created from two monomers: terephthalic acid and ethylene glycol, both of which come from crude oil. They are represented chemically as C 8H 6O 4, (terephthalic acid) + C 2H 6O 2, (ethylene glycol), which gives you the polyethylene teraphthate, (C 1 0H 8O 4) n + H 2O,(water). 7 The n, signifies that it repeats n number of times. The atoms and bonds are rearranged, and it keeps repeating. It's still the same Carbon and Hydrogen elements that were in the monomers. It's amazing what a little rearranging can do! So you see, the monomer that comes from oil is really still there. About 30% of manufactured PET is used for water and soda bottles and the rest is used to make polyester thread, for clothing and carpets. 5
The caps on the water bottles come from a different plastic, polypropylene, a more rigid thermoplastic. Polypropylene comes from the monomer, propylene, represented by, C 3H 6. It becomes the polymer, polypropylene, represented as: (C 3H 6) n , where the n stands for n number of times the monomer is repeated in the polymer. 7
We have to wonder if these plastics are safe to use for drinking bottles since they come from crude oil, as well as additional chemicals and pigments added to enhance the characteristics of the plastics. For the most part, these plastics have been considered safe to use. One plastic, Polycarbonate, was found to leach a hormone-like chemical, bisphenol A, into the liquids when the liquid or the bottle was heated. These bottles have been banned in many states, including Delaware. The Food and Drug Administration has tested the PET bottles over time and concluded and reported that they are safe and don't leach any harmful chemicals. In fact, rumors circulated on the internet warning people not to use PET bottles that were subjected to heat, such as those left in a hot car are untrue. This makes sense since the chemicals in the plastic need about 700 degrees centigrade to melt, and don't contain the problem chemical in polycarbonate bottles. 6 It has to make you a little wary, since the polycarbonate bottles were originally thought to be safe!
Plastic bags come from another thermoplastic, Low-density Polyethylene, (LDPE). It is a very flexible plastic, used primarily for bags. It is also made from the monomer ethylene, C 2 H 4, which also comes from crude oil. 7 This monomer becomes the polymer, LDPE, as, (CH 2H 4) n where, again, the n stands for n number of times the monomer is repeated in the polymer. 7
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