We live in a world dependent upon polymers generated from petrochemicals which are able to be molded using heat and pressure.
We call these polymers "plastics" although clay and glass are also plastic. It is unlikely someone would mistake a glass pop bottle for a "plastic" pop bottle though and so the term gets used for a wide variety of common substances.
There are six principal types of polymeric substances used to make the plastic objects in our world.
A polymer is generated from monomers linked together to form a chain. An analogy is a beaded necklace with one bead plugging into the next. Put enough together and you get a long chain.
In the case of polymers, the beads are molecules - monomers - such as ethylene and vinyl chloride. Polyethylene Terephthalate is actually composed of two monomers in an alternating sequence joined by ester linkages.
The number of monomers in a chain is somewhere between 5,000 and 1,000,000 or more depending the polymer's use. Polymers are really just long chains of carbon atoms joined together.
The big six are:
(1) PETE or PET (polyethylene terephthalate) used in two liter pop bottles and mouthwash bottles. It is transparent with high impact strength and is impervious to acids and atmospheric gases. These latter properties make it ideal for dealing with the acid and carbon dioxide in soda pop.
(2) HDPE (high density polyethylene) is used in milk jugs and trash bags. It is soft, flexible, and opaque but mechanically tough.
(3) PVC (polyvinyl chloride) is used in cooking oil bottles and for packaging around meat. It is a rigid and transparent with high impact strength. More importantly, it is impervious to oils and other organic materials so it can be used to transport petroleum or natural gas.
(4) LDPE (low density polyethylene) is used in grocery and produce bags, food wraps, and bread bags. It is softer than its "high density" cousin but not as mechanically tough. This is why grocery bag handles stretch so much when overloaded. The handles actually convert from low density to high density polyethylene in the process.
(5) PP (polypropylene) is used in shampoo bottles, yogurt containers, straws and diapers. It can also be found in fleece vests and carpeting. It is opaque with a high melting point and high tensile strength. It has the lowest density of the major commercial polymers.
(6) PS (polystyrene) is used to make hot beverage cups, take-out boxes, egg cartons and anything else made of Styrofoam. It is easily fabricated and when blown into Styrofoam, it has tremendous insulating properties but it is also readily soluble in organic solvents.
The numbers associated with each polymeric compound appears in the little recycling triangle somewhere on the surface of the plastic object. These numbers aren't the toxicity as one of my students thought. They are simply an indication of the type of polymer present.
Some of these polymers are easier to recycle than others. But they must all be sorted according to the different type of polymer involved in order for recycling to make any sense. Mixing different types means the recycled product is virtually un-useable.
Recycling plastics would be ideal but as a society we are far from 100 per cent efficient. Indeed, for PET, it is estimated only 30 per cent is recycled. The rest ends up in landfills or the environment.
However, there is exciting news in the bio-degradation world regarding PET. Japanese researchers have reported in Science that they have found a microbial organism - which they have named Ideonella sakaiensis 201-F6 - capable of eating the material.
The novel bacterium uses the polymer as a major food source. It degrades the polymer initially into a dimer composed of one ethylene glycol unit and one terephthalic acid unit. If this mixture is isolated it could, in fact, be used to generate new PET bottles and other objects.
The organism continues the degradation step chewing on the dimer to generate the original monomers themselves and then breaking the terephthalic acid down to carbon dioxide and water. In other words, it uses PET in the same way we metabolize sugar but through an entirely different mechanism.
The scientists found the bacterium by screening 250 PET debris-contaminated samples of soil, sediment, and wastewater along with the activated sludge for a PET bottle recycling site. Careful isolation of the organism and cultivation on low-crystallinity PET eventually led to pure cultures of the bacterium.
The biological degradation process is not quick. The microbes only consume the polymer at a rate of 0.13 milligrams per square centimeter per day. However, now that scientists know where to look and what enzymes are involved, it may be possible to tweak the process through genetic engineering.
We may eventually have an effective way to convert polymers back to their starting materials and address the accumulation of plastics in our environment.
