Converting plastic back to bio-degradable substances [closed]

I have read that every plastic ever made still exists. Plastic must be made of some substances that are biodegradable. Even if they aren't, why can't we change their form? Why can't we change plastic into something else?

• You're contradicting yourself and apparently never heard of recycling. Oct 9, 2017 at 19:19
• Some plastics decompose, albeit slowly, e.g. urethanes from photodegradation. Some decompose rapidly, e.g. celluloid. Polyvinyl alcohol is a water-soluble, biodegradable plastic. Question your source of information! Oct 10, 2017 at 0:32

Some definitions: plastic is a synthetic (man-made) organic polymer usually composed of a large number of simple monomer units chemically united (generally chemically) and usually used for its structural properties. For instance, the polypropylene fibers which make up a polypropylene rope would commonly be called a plastic but the polyvinyl alcohol used as a water thickener would normally not.

Biodegradation is any sort of chemical decomposition by environmental causes. These causes may include sunlight, moisture (water), and microbes. Heat, erosion, and oxidation are generally not included.

1. If you have read that every plastic ever made still exists, then you need to be aware that your gullibility is extreme and the source for that claim should not be relied upon. Surely, you've seen plastic burn. Surely, you don't believe that the burned plastic magically reassembles itself. Hence it is obvious that not all plastic ever made still exists.

2. We can change plastic. Because most plastic items contain many other chemical compounds, (often colorants, antioxidants, plasticizers, & fillers) it can be quite expensive to decompose them into useful materials.

3. Chemistry isn't magic. Any industrial chemistry involves people, equipment, transportation, and energy. That is, it takes money. Most plastic is made from natural gas (the polymer, that is), which is cheap and fairly pure (consisting of mostly a half-dozen chemical compounds). There are tens of thousands, perhaps hundreds of thousands, maybe millions, of different plastics that have been made. Because of all the other stuff mixed into the polymers, and because the polymers can be very different, chemically, it is usually cheaper and easier to make a plastic starting from natural gas than starting from pre-existing plastic. Just the mix of materials separated from the polymers would be terribly expensive to dispose of.

4. The forces of nature will eventually bury some plastic, break up some plastic, and decompose some plastic. Nothing lasts forever. Very little of the plastic existing today will be present in 10,000 years. Erosion, aging, and biodegradation are the 3 categories of this degradation, along with mechanical forces. Much plastic exists in the oceans as microscopic bits. Some bacteria are able to eat some plastics. All plastics burn. Eventually, all of the polymers present in the plastic here on Earth will either be converted to CO2 (either in the air or captured in the ground (eventually as carbonate) OR subducted with the tectonic plates and thermally decomposed.

Plastics are usually organic polymers. During the process of polymerization covalent bonds are formed. This reaction is exothermic, meaning it sets free energy. So, in order to reverse the process of polymerization and break these bonds again, it would take a lot of energy.

It is fair to assume the energy needed will also break other bonds in the polymer, forming multiple products at once. I guess some kind of inseparable tar will be formed if oxygen is excluded. When oxygen is included the polymers will probably just burn, also forming multiple products at once - and CO2.

• That being said, some polymers can be depolymerised. Indeed various byproducts are created, but you can get big part of monomer back. Oct 9, 2017 at 23:12
• I wasn't really aware of that. I guess my answer is not fully correct then. Hmmm. Oct 9, 2017 at 23:18

The initial statement is incorrect. Plastics contain carbon-carbon bonds and everything with carbon-carbon bonds can be burnt to $\ce{CO2}$ given sufficient burning temperature. Some plastics are indeed burnt so it is impossible for all plastics still to be around.

However, most plastics can be reasonably approximated by a long, saturated carbon chain much like crude oil except with less other organics mixed in. As is the case with crude oil, there are very few organisms capable of digesting plastic in any form so its bio-lifetime is very long. However, given enough plastics and evolutionary pressure, microorganisms that can digest it will come along. I believe there have been some found already but I cannot point to any sources right now.

• I am aware that this answer is basically a shorter rehash of alphonse’s.
– Jan
Oct 10, 2017 at 16:22