Polylactic acid

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• Article Title: 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.

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.

Status Brief

A process for the production of polylactic acid is currently in the research and development phase. A process has been proposed using purified L(+) lactic acid and a vacuum capable reactor on Polylactic_acid/Manufacturing_Instructions. The project may move forward in Talk:Bioplastics collaboration with Dr Joshua Pearce at Michigan Technological University, whose lab has the capacity to test and characterize tested products. A protocol for production of lactic acid from microorganisms on a defined media and agricultural feedstock will be the focus of development. Rapid development of the protocols and developing the necessary collaborations on MTU's campus will take place over the next 11 months with a goal of project implementation in June-July 2013.

Documentation Brief

A review of publicly available information is located at Polylactic_acid/Research_Development. Review of the polymerization step is considered complete, while extensive research on production and purification of lactic acid substrate is still needed. Further research into preparing the product for use in extrusion is still needed.

Current Challenge

Current challenges include the design of an OSE reactor, process for local production of lactic acid, and import replacement of catalysts. A subject matter expert must review the process and offer feedback and critiques.

Background

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. Please see the page on silage, which could be a source of abundant lactic acid, while also serving as an excellent mechanism for processing biomass into animal food and biogas feedstock.

Lactic acid producing microorganisms

Organisms are chosen with the metabolic capabilities to utilize sugars in the feedstock. Homofermentative species that produce optically pure L(+)-lactic acid as a sole product should be used for maximum yield and to reduce complicating products. With the recent interest in lactic acid as a platform for bioplastics a great deal of research has gone into identifying optimal strains and defining their growth conditions and capabilities. Lactobacillus (bacteria) and Rhizopus oryzae (fungus) have been been identified as platforms for commercial lactic acid production. However, recently (2009) thermophilic Bacillus coagulans (2-6) has been identified as an even more promising microorganism for fermentative lactic acid acid production. Its optimal growth temperature of 50 C is higher than other fermentative species preventing contamination even with unsterilized feedstocks, it produces optically pure L(+) and lacks a D-LA dehydrogenase, and achieves very high conversion efficiency of 97.5%.

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.

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.).

Culturing and growing microorganisms

A review of lactic acid producing microorganisms must be completed and species selected for testing. Well defined food cultures are also possible source that could provide an additional service. Tests of monoculture versus mixed culture will eventually be conducted. Lactobacilli are optimal for silage and inoculation schemes could be useful.

Lactic acid microorganisms can be grown in the OSE fermentor or as silage. Operation of a fermentor for lactic acid production requires control of pH and removal of product.

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.

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. High molecular weight polymers are desirable and more difficult to produce. Vacuum dehydration with catalysts are now producing high molecular weight species.

SnCl2 or tin octenoate is a proposed first generation catalysts with an appropriate initiator. The reaction will be driven by added heat and removal of water by a vacuum and liquid condensor. A reactor chamber configured for vacuum will most likely be necessary.

Quality and value adding

PLA can be recycled with hydrolysis back to monomers.

Plasticizers such as OLLA and polyethylene glycol can improve plastic characteristics.

Process Design

Demonstrate small scale catalyst and processing from purchased commodity chemicals

1. Configure fluidized bed reactor for solid state polymerization under vacuum, no or limited gas feed from below. Alternatively could conduct experiments in glassware.

2. Obtain lactic acid from chemical supplier along with SnCl2 and p-toluenesulfonic acid. Possibly other protonic acids for substitution.

3. Follow melt/solid polycondensation protocol to obtain high molecular weight PLLA.

4. Experiment with plasticizers particularly oligomeric lactic acid and polyethylene glycol which are compatible with OSE product ecologies.

5. Demonstrate thermomoldable PLLA with a range of characteristics from small scale production.

Material production on-site Lactic acid monomer 1. Obtaining feedstocks and chosen polylactic acid producing microorganism.
2. Small scale glassware test runs of conditions.
3. Deciding upon protocol for fermentor.
4. Construction of fermentor and subsystems. Construction or purchasing of lactic acid purification equipment (maybe test with small scale runs).
5. Production run of fermentor.
6. Purification of monomer.
7. Demonstrate polymerization of locally produced resource.

Catalyst and reagents

1. Tin may be available from waste streams or be refinable in the area. Tin chloride is currently used in finishing mirrors as part of that product ecology.
2. Production of higher level organic molecules such as p-toluenesulfonic acid will require further chemical engineering projects.

System Engineering Breakdown Diagram

Design Rationale

The OSE polylactic acid process is designed to conform to OSE standards and utilize open source information to maximize efficiency on a small localized scale. High purity starting substrates will be used to minimize downstream purification machinery. It is an easier task and necessary to demonstrate the ability to produce the end product of the process using commodity materials. Over time resource substitutability and expansion of OS capabilities will allow the production of the materials. A first step is to demonstrate and disseminate the background and process information, and test the feasibility of the end goal.

Catalysts assisted by optimal conditions have high reactivity and result in a high quality product. The current approach proposes to build the machinery necessary for high quality polymerization reactions and demonstrate their use as modular pieces in a bioplastic enterprise. First demonstrations are proposed to use commercial chemical monomers while monomer production and purification is pursued from local feedstocks.

L(+) lactic acid in the form of should be obtained in 90%+. Tin(II) chloride dihydrate and p-toluenesulfonic acid 99%+ should be obtained. Necessary drying, solvent, and vacuum equipment must be obtained. The prototype 1 fluid bed reactor can be configured for a vacuum environment and used to control the reaction. If product is obtained it should be tested with industry measurements and FeF dogfooding. Addition of plasticizers and comonomers can be tested.

Production of substrates monomeric lactic acid will be from silage and fermentors unless one process demonstrates clear advantages. Research and development will need to be done on tacticity effects on PLA before a homolactic or heterolactic microorganism is chosen. For preliminary fermentations a homolactic species will be used.

Purification of monomeric lactic acid is proposed to involve dialysis of calcium lactate stream that incrementally introduces semipermeable membranes (first based on size then charge bias). Reactive distillation will be used as second purifying step where methanol esterifies with lactic acid to produce methyl lactate. Methyl lactate can be distilled in the bottom of a fraction column and then hydrolyzed to methanol and lactic acid. This two-step purification will have to be tested for its polymerization quality.