Plastic POLYmers are made from building block MONOmers. The naturally occurring monomers can be taken from plants, natural gas or oil.
Long chains of monomers are polymers. Ethylene becomes polyethylene, propylene becomes polypropylene, styrene becomes polystyrene, etc. A polymer comprised of only one monomer is known as a homopolymer. A polymer with two monomers is called a copolymer ( ethylene vinyl acetate EVA )and you guessed it - three polymers is called a terpolymer ( acrylonitrile / butadiene / styrene or ABS ).
Bio-based plastics from plants are promoted as better for the environment because of their renewable sources vis-a-vis types of plastic made from non-renewable petroleum-based fossil fuels. Before you ask a resin producer which has moral superiority, ask first where they get their monomer feedstocks.
Plastic can be derived from both plants and conventional feedstocks without being biodegradable. Braskem's so-called green plastics are plant plastics derived from cane sugar and are not biodegradable. Their polypropylenes and polyethylenes are indistinguishable at a molecular level from fossil fuel based polypropylene and polyethylene. What's most important is perception.
In the US, most plastic is made from natural gas. The resin producers simply take what they want from the natural gas pipeline and put the rest back. PVC is a combination of ethylene and chlorine salt. http://www.pvc.org/en/p/how-is-pvc-made
The other choice of feedstock is naptha. Naptha is the creme de la creme of a barrel of oil which cannot be refined into gasoline or motor oil. Naptha is the feedstock of choice in the rest of the world because it is sort of a by product of the refining process. So it is true that some plastics, not all plastics, are made from oil.
These items could all have been derived from oil or natural gas. Display courtesy of Ocean Star Drilling Rig Museum in Galveston, TX.
Here is a decision guide I put together to capture the ramifications of plastics made from different feedstocks:http://info.brentwoodplastics.com/bioplastics-or-conventional-plastics?&t=61295
Feel good press releases such as this one from Subway play into the public's ignorance about where plastics come from:http://www.prnewswire.com/news-releases/subway-restaurant-chain-continues-to-live-green-with-salad-bowls-made-from-95-percent-recycled-materials-147592125.html.
Remember freshman econ about factors of production ? Where plastic is made and what plastic is made from influence the cost of plastic. The major factors are feedstocks and exchange rate. We have blog posts on both of these subjects.
There is a reason globocorps like Dow are making a huge capital investment bet to get lowest cost raw materials from ( fracked ) natural gas. Ever wonder why Andrew Liveris ( Dow CEO ) gets apoplectic at the mention of removing natural gas export restrictions ? He doesn't want to pay the same price as Japan does ( $14 or $15 ) for feedstocks.
Which polymers are morally superior should be a discussion. If it isn't a priori obvious that bio-based polymers are morally superior, the USDA has decided they are, so that settles it. If you want to decide for yourself, here is the best, balanced assessment of bio-based polymers we have found.
How plastics degrade is another discussion also chockablock full of misinformation. Let us know if you would like to see a post on plastics breaking down. There doesn't seem to be much search volume on the subject.
The author in front of Lyondell Chocolate Bayou resin plant in Texas with examples of natural gas derived high density polyethylene ( HDPE ) products.