How can I make a biodegradable 3D printing filament completely from scratch?

[Ama-fessional Molder]

There is a lot of complex chemistry involved in creating plastic. You can make several different types of "bioplastic" from household ingredients but if you want a melt-processable polymer then I would suggest speaking to a chemical engineer. I haven't been well versed in the creation of bioplastics, however I do know the process is just as complex as traditional petroleum based plastics if you intend to create something practical - so that's what I will detail an example of:

For "normal" plastics, the process begins with something called fluid catalytic cracking. This breaks the petroleum hydrocarbons out of crude oil, giving you things like ethane gas (among others, which are used for other plastics).

Ethane gas is used in the creation of polyethylene. Basically, you link more and more "mono"mer units together using catalysts to form "poly"mer chains. For PE: Ethane (gas) -> butane (liquid) -> paraffin (wax) -> polyethylene (solid). Interesting note to everyone here who, probably, doesn't print with regular old PE, degradation of polyethylene literally reverts it back into paraffin wax. If you have ever been to a public zoo in the U.S., and maybe other places around the world, there are little injection molding units that crap out some animal figurine or another. They use PE in these and since the plastic sits there all day long it degrades, and that's why they have a distinct wax texture - because it kind of is wax.

Ethane is also a base for pvc, teflon, and polystyrene as well.

In any case, the polymerization process involved in PLA, or any other plastic, depends on the type of plastic and desired properties. You need to carefully control this phase in order to achieve chain lengths that result in usable material. It is possible to overdo this process and create useless material, or not take it far enough before stopping the reaction which also results in useless material.

This link should point you in the right direction:

http://www.chemistry.illinois.edu/re.../06_Porter.pdf

Once you have the polymerization process figured out, then you need a way to form it into granules of a size suitable for processing in an extruder. You will also need to research additives, as raw plastic polymers are usually too sticky and degrade far too easily to process reliably. Things like plasticizers, ozone and UV inhibitors, flame retardants, and stabilizers to prevent degradation during repeated melt cycles.