Lactic acid or 2-hydroxypropionic acid was discovered by the Swedish Scientist Scheel in 1780 being first isolated from sour milk. In 1881 a process was developed for producing it by fermenting startch and sugars with bacteria, allowing it to be produced industrially. A similar process has been carried out for thousands of years using dairy products, fermenting them with bacteria to produce youghurt. Keen RepRap researchers may wish to experiment with yoghurt production.
When made using fermentation or bacterial action, as opposed to making it totally through chemical synthesis as is often done today, the mixture is allowed to ferment until the solution reaches a 10% concentration.
At this point, calcium carbonate (chalk or limestone) is added, which reacts with the acid to form carbon dioxide and calcium lactate. At concentrations of less than 10% the lactate remains in solution and can be separated by filtration.
The solution is then purified with activated carbon, concentrated by evaporation, and finally the lactate is converted into dissolved acid by adding sulphuric acid. Calcium sulphate is produced as a by-product.
If further purification is needed, the acid is turned into a methyl ester, distilled, and converted back into free acid once more. It is not known if RepRap-grade PLA will need further purification.
- Chembytes e-zine "Cleaner synthesis"
- lactic acid
-- Main.VikOlliver - 19 Jan 2006