Polylactic acid

Overview

Polylactic acid is a thermoplastic suitable for light weight applications and plastic extrusion by 3D printers (reprap wiki). Polylactic acid is actually a polyester and is bio-degradable. Lactic acid can be formed by fermentation of sugars by microorganisms or synthetically from petroleum. Lactic acid is polymerized via sequential condensation reactions with a catalyst and in 130-160 C. Polylactic acid is currently in first generation large scale production using corn starch as a feedstock.

Biological materials and hardware

Initial feedstocks will be converted to input lactic acid by microorganismal fermentation; a feedstock with a high fermentable sugar content should be chosen and an appropriate bacterial strain selected. A fermentation chamber will need to be developed and prototyped. Further research is needed into methods of purifying lactic acid. Lactic acid will be polymerized in a reaction chamber to which must also be developed and prototyped. Quality control is needed for high purity lactic acid necessary for polymerization.

L (+) lactic acid fermentation and its product polymerization by Narayanan et al reviews the production of lactic acid and its use as a plastic monomer. The synthetic route of lactic acid is four steps that involve fixating an activator cyanide group to an acetylaldehyde to form lactonitrile, hydrolysis of lactonitrile with sulfuric acid to yield lactic acid and ammonium salt. For purification via reactive distillation lactic acid is esterified with methanol to methyl lactate and water, methyl lactate is distilled, and hydrolyzed to lactic acid with the addition of water. The production of lactic acid from biological sources is through the fermentation of high energy carbohydrates to lactic acid by Lactic Acid Bacteria. Lactic acid is neutralized and precipitated with calcium hydroxide. Calcium lactate is collected and hydrolyzed with water. For purification lactic acid is esterified with methanol to methyl lactate and removed via distillation, before hydrolysis with water.

"The choice of an organism primarily depends on the carbohydrate to be fermented. Lactobacillus delbreuckii subspecies delbreuckii are able to ferment sucrose. Lactobacillus delbreuckii subspecies bulgaricus is able to use lactose. Lactobacillus helveticus is able to use both lactose and galactose. Lactobacillus amylophylus and Lactobacillus amylovirus are able to ferment starch. Lactobacillus lactis can ferment glucose, sucrose and galactose. Lactobacillus pentosus have been used to ferment sulfite waste liquor." Lactobacillus also have complex nutrition requirements. Rhizopus oryzae are also stereoselective LAB as well as yeasts such as Saccharomyces cerevisiae and Kluyveromyces lactis and have been investigated for their usefulness.

Feedstock starting materials

A variety of feedstocks can be used as a source of sugar for fermenting microorganisms including high starch plants, high sugar plants, or dairy. Feedstocks should be chosen based upon a high yield of sugars suitable for Lactic Acid Bacteria to use as a reducing agent in fermentation.

Fermentation

Organisms are chosen with the metabolic capabilities to utilize sugars in the feedstock. Homofermentative species that produce lactic acid as a sole product should be used for maximum yield and to reduce complicating substrates.There are two main hexose fermentation pathways that are used to classify LAB genera. Under conditions of excess glucose and limited oxygen, homolactic LAB catabolize one mole of glucose in the Embden-Meyerhof-Parnas (EMP) pathway to yield two moles of pyruvate. Intracellular redox balance is maintained through the oxidation of NADH, concomitant with pyruvate reduction to lactic acid. This process yields two moles ATP per mole of glucose consumed. Representative homolactic LAB genera include Lactococcus, Enterococcus, Streptococcus,Pediococcus, and group I lactobacilli. Fermentation is inhibited by endproducts and advances in fermentation technique includes methods to remove lactic acid as it forms allowing continuous operation of bioreactors and higher conversion rates.

Bacterial strain selection and care

Strains could either be isolated from a fermented product or ordered from a supplier. A relatively simple procedure for isolating and identifying LAB from an enriched media can be found here[1]. Alternatively a strain may also be found by contacting relevant research laboratories. IN industry Lactobacillus delbrueckii, L. amylophilus, L. bulgaricus and L. leichmanii. Mutant fungal strains of Aspergillus niger are also reportedly used. A wide variety of carbohydrate sources, e.g. molasses, corn syrup, whey, dextrose and cane or beet sugar, can be used.” A study utilizing sorghum as a feedstock for ethanol and lactic acid fermentation used Rhizopus oryzae NRRL 395 [2] (see Zhan et al.).

The pH of the media will be lowered by the production of lactic acid and maintaining pH in the range of 5-6 is essential to culture health and maximum yield. Older more developed techniques were to add calcium stearate or other salts, which neutralized the pH and precipitated the salt form of lactic acid, however an equal amount of waste is produced alongside the purified lactic acid. Constant removal of waste products and maintenance of optimal growing conditions is the study of cutting edge polylactic acid producers.

Purification of lactic acid

Isomers are molecules of the same chemical formula that exhibit chirality or a difference in positioning on a carbon attached to four different chemical groups. The chemical formula of lactic acid is CH3CHOHCOOH Isomer composition is important to crystallization with heterogenous mixtures forming amorphous configurations. Purification is aided by single isomer production in the fermentation route. Advances in purification have involved semipermeable membrane sieves and more recently electrophoresis technology. “This process uses a desalting ED unit to remove the multivalent cations and concentrate the lactate salt, followed by a ‘watersplitting’ ED unit with bipolar membranes to produce concentrated lactic acid and alkali for recycle.” Alternatively other salts could be used that produce a waste that is easier to handle and recover.

Reactive distillation of an ester of methanol and lactic acid over a 20 stage distillation column yields high purity ester that can be hydrolyzed back to the acid form. http://www.aidic.it/escape20/webpapers/34Edreder.pdf

Polymerization

The preferred route of creating polylactic acid uses the dilactide. Direct polymerization involves a condensation reaction that is near equilibrium and a method to remove water. Predominant procedures is to use lactide in a Ring Opening Polymerization (ROP) over catalysts including tin, aluminium and zinc. Initiators used include butyl lithium (electron donor?) (Achmad et al). A small scale experiment uses a microspatula of Tin(II) chloride to catalyze 5 ml of lactic acid under high heat conditions. Cobalt chloride paper can be used to test for water vapor indicating the reaction is proceeding.

Alternatively, the reaction will proceed under a vacuum and temperatures 150-300 C using distillation equipment to remove formed water. This configuration can be used with or without a catalyst and is an active area of research.

Synthesis of polylactic acid by direct polycondensation under vacuum without catalysts, solvents and initiators by Achmad et al details a procedure. The authors recommend PLA be pursued using processing plants capable of fermenting feedstock, purifying lactic acid, and condensing the product as is proposed for the OSE product ecology. Streptococcus bovis is a LAB suggested for use but the species is also linked to pathogenicity. The process used by the researchers used three phases for treating the lactic acid and polymerizing its monomer: distillation, oligomerization, and polymerization. The reaction was carried out in 4 l sealable flasks, on magnetic stirrers and heaters, with temperature and pressure probes, and connected to a pressure regulator. During distillation sample temperature is brought to 150 C over 90 minutes and maintained for 60 minutes and PLA concentration increases from 90% to 100% as measured by acid-base titrations. Oligomerization phase was a reduction in pressure to 10 mmHg and temperature was raised to 200 C. The polymerization step was maintenance of the reduced pressure and temperature for 89 hours. The condensate was also separated with gel filtration chromatography and measured with a RI detector. Fourier Transform Infrared Spectroscopy was used to analyze molecules functional groups.