[John Chapman (OP)]

If plastics are made from oil, why are they not biodegradable? Plastics, I think the older ones at least, are made from oil derivatives. Oil is biodegradable so why isn’t plastic? Bacteria, fungi, aerobes, anaerobes, other micro-organisms, digestion by worms, etc, don’t touch them. Why? What makes something biodegradable?
 

[Chemistry4me]

The properties of the polymers will be different to their individual monomers. Sorry, poor explanation, I know [:I]

[Bored chemist]

It's an odd question. The stuff is made from oil, but it isn't oil.
It's like asking  "If paper is made from trees, how come it isn't green?".

The bacteria etc have enzymes that have evolved to help them digest things like wood and oil, but plastics havent been around for long so the bugs haven't had time to evolve the right set of enzymes.

[John Chapman (OP)]

Thank you for being polite, Bored Chemist. I suspect that "It's an odd question" is a euphemism for "you must be thick as poo" which is, of course, nearer the truth!  []

Rather than your 'paper not being green' metaphor I was thinking it's more like finding out that, despite wheat and butter being biodegradable, it turns out that cakes are not! The problem is partly that I don't know how plastics are made. I sort of assumed that industry distilled out all the fractions of oil and than added certain ones back together, stirred 'til light and fluffy and, voila, polyethylene!

I think that maybe I've been caught out asking a silly question again.  []
 

[glovesforfoxes]

bored chemist is right. the enzymes required to break down the bonds between the monomers in the synthetic plastics haven't been seen in nature yet, which is why it is hard to break them down. the enzymes, if you did not already know, have a highly specialised and specific "active site" which is the area on an enzyme which fits to the substrate (which is the corresponding shape to the active site, like a jigsaw puzzle). the oil can be broken down as the shape of it fits into the active site so the reaction to break it down can be catalysed by the enzyme. this cannot happen in plastic, as it doesn't fit into the active site of any current enzymes.

[John Chapman (OP)]

Thanks glovesforfoxes

So is plastic not merely the sum of the parts? I suppose the difference between plastic and cake is that cake is a mixture whereas plastics are new compounds. Finished cake still has butter in it so 'lipo-phages' can still digest it. But presumably there is no trace of the original oil-based components left in plastics?

Btw, great username.
 

[Bored chemist]

The analogy police might shoot me for this but...
The ingredients in a cake are biodegradable. But it takes more than just mixing them to make a cake- you need to cook it. During that process changes take place in the mixture and those changes can affect the flavour etc of the cake. Incidentally, the brown bits of cakes and toast etc are less digestible than the unbrowned bits so cake is (marginally) less biodegradable than the ingredients. At least that's what I have been told- it doesn't get a chance to biodgrade while I'm here.

[glovesforfoxes]

well if you're going to be picky then plastic is the sum of it's parts - but the synthetic polymers from refined oil are differently constructed and have different components than oil, so it's a much different product. remember that the properties of molecules are much different from their original constituents - take NaCl and Na and Cl individually for example.

there is a massive difference in the way microorganisms would treat organic and mineral oil. mineral oil has been in inhospitable conditions, unable to be accessed by anything as it is so far underground. therefore it would not be an evolutionary pressure to develop enzymes to break down mineral oil, compared to biological oil which is much, much more easily available and still provides a good amount of energy. so i would say it is the case that mineral oil, and the manmade products resulting from it, are not generally biodegradable. i expect that the plastics that are more biodegradable mimic the structure of organic oils, so they are possible to break down. butter, by the way, is made by milk, which is something that bacteria tend to like ^^

an interesting thing to do would be to find it if anyone is developing bacteria that can break down the plastics, maybe through genetic engineering, in the hopes of helping the environment. i might research it later. thanks by the way john ^^ it's the only moniker that's lasted me a year or more, i really like it too.

[Bored chemist]

"there is a massive difference in the way microorganisms would treat organic and mineral oil."
Petroleum is both organic and biological in origin.

"i would say it is the case that mineral oil, and the manmade products resulting from it, are not generally biodegradable. "
Crude oil does get to the surface so there are bugs that destroy it; they have had plenty of time to evolve. It is (slowly) biodegradable.

"there is a massive difference in the way microorganisms would treat organic and mineral oil. "
How do they know?

The factors that stop most plastics biodegrading are the near total insolubillity in water and the fact that they are new on the scene.
An easy (and one of the earliest) way to make plastics biodegradeable is to mix them with something like cornstarch which degrades well. This means that the plastic left behind is full of holes and rips to tiny pieces.

[glovesforfoxes]

i apologise for the inaccuracies - i forgot about surface oil.