Wednesday, June 22, 2005


Possible new polymer

If RepRap could use the aliphatic polyester polylactic acid (PLA) as its primary working polymer that would be good. The advantages would be that it:
  1. Has a low melting point (150 C; glass transition: 60 C),
  2. Is mechanically pretty strong [yield: 70 MPa (nylon: 80 MPa); modulus: 3.2 GPa (nylon: 3 GPa)],
  3. Is biodegradable, and
  4. Can be made from biomass.
Actually, as Vik and I have pointed out before, having a polymer that is biodegradable is a disadvantage, as it means that products made from it don't lock up atmospheric carbon (but all these recycling directives that insist on it are written by politicians, so what do you expect?). But if the polymer comes from biomass, then at least the whole process is carbon-neutral.

In the case of PLA, the polymer can be made by the fermentation of corn (maize), a crop that will grow in poor soils in a very wide range of climatic conditions. This allows the possibility of having the RepRap machine make its own fermenter, and then having a manufacturing process for high-technology goods that could be bootstrapped straight from an agrarian base with very little capital expenditure. Rich World look out: here come the African farmers :-)

A good source of information on PLA is Rafael A. Auras's Powerpoint file available here.

A colleague of mine at Bath is working on the stuff too, and has promised me a sample. So experiments on it will be blogged in the near future...

P.S. - an excellent paper on polylactic acid production by Narayanan et al. can be found here.

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A couple of "thoughts from the bath":

Is this patented?

Can it be subtly modified to prevent biodegrading?

The PowerPoint presentation plays fine in OpenOffice, by the way.

Vik :v)
I don't think biodegradability will be a problem, The papers I have read said that in order for biodegradable plastics to biodegrade, they need to be somewhere life can live. like buried in the ground, in a soggy garbage heap or inside an animal body. I imagine the suns rays and lack of water will keep any repraps from biodegrading prematurly. (just don't bury them)

I don't think worrying about carbon fixation is an issue as long as the polymer is reusable. people will just melt and reuse old parts rather than purchase new raw materials and tossing the old ones and there are much better ways to sequester CO2 than in a bunch of polymers in landfills. I think it would be cooler if you could use old repraps in your compost heap :)
Biodegrading is a problem when it happens unexpectedly. It releases the carbon into the atmosphere, and prevents you from recycling the material - so you have to make some more. Not very environmentally friendly.

But we need not worry too much as both Polymorph (polycaprolactone) and PLA need to be raised to 60C before they will biodegrade. This temperature is generally found in the centre of compost heaps, so careful siting of the RepRap will ensure durability :)

Vik :v)
That's pretty cool!

So I have some corn, and a kitchen. How to I make myself some PLA? :)

Is the process particularly complex, or is it something that could be carried out by a reprap produced machine?
That's something I'm looking into. Certainly the first stage is a fermentation of the corn with a microbe (i.e. like home-brew beer...). But I'm not sure about the polymerisation step. For some plastics that requires hundreds of bars of pressure, funny catalysts, and high temperatures. I'll report back when I find out, and I think I'll set a student project to make some...
Ideally, it would be nice if it could work with a range of materials. so it can be adapted for whatever is plentiful at the time and place it is used. Fortunatly, this should be pretty easy with our setup. just changing the temperature and speed to account for the different viscosity should do it I think.
I'm in touch with a couple of local suppliers who say they can get bulk polycaprolactone (Polymorph) and polylactic acid (PLA). The standard delivery container seems to be the 25kg bag.

Vik :v)
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