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AIR3-CT94-2285
Development of Innovative Biodegradable Polylactic Acid Polymers, Based on Agricultural Raw Materials, for New Industrial Applications |
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Proposal No: | AIR3-CT94-2285 |
| Date Prepared: | October 1997, September 1999 | |
| Source: | Consolidated report December 1997
Second Annual Progress Report |
Summary
The activities were divided in two parts:
For the first task, culture media were optimised for fermentation with high cell density bioreactor. Two media were developed for optimisation of, respectively, biomass production and lactic acid production. It was found that expensive yeast extract could be advantageously substituted by corn steep liquor or bacterial extract. Consequently, medium cost could be significantly reduced. It was demonstrated that Bacillus coagulans had low nutritional requirements in comparison with the other lactic acid bacteria. Extraction of ammonium lactate with microfiltration membrane was carried out using experimental design and response surface methodology. The microfiltration parameters were optimised to attain high permeate flow rate and high stability at steady state. Optimal fermentation with high cell density bioreactor was carried out after the study of dilution rate and cell bleeding rate. Performances of ammonium lactate production were improved in comparison with batch process. A high productivity process was maintained for 200 hours at 45g per litre per hour as compared with 48 hours at 1.2g per litre per hour for batch process. Finally, modelling of fermentation and microfiltration steps was successfully carried out. The model developed could be used as powerful tool to simulate and optimise lactic acid production with industrial strains and membrane bioreactors.
For calcium lactate production crystallisation of calcium lactate was investigated in batch and MSMPR (continuous) crystallises. The Batch mode allowed complete characterisation of the key aspects of the process. Super-saturating appeared to be the main controlling factor for crystallisation. It influenced crystal shape, crystal size, gel formation and crystallisation time. The MSMPR experiments led to a description of the crystal growth rate using size-dependant growth models.
Introduction
The first task of the project was to conceive an innovative process by the integration of production and separation of lactic acid. In the third year studies included:
Activities
Improvement of an integrated process for the production and separation of lactic acid Various aspects of ammonium lactate production were studied and optimised using a high cell density bioreactor coupling fermentation and a microfiltration module. Culture media were optimised for fermentation with expensive yeast extract advantageously substituted by corn steep liquor or bacterial extract. It was demonstrated that Bacillus coagulans had low nutritional requirements in comparison with the other lactic acid bacteria. The microfiltration parameters were optimised to attain high permeate flow rate and high stability at steady state. Consequently, performances of ammonium lactate production were improved in comparison with batch and continuous stirred tank, reactor (CSTR) fermentation. A high productivity process was maintained for 200 hours at 45g per litre per hour as compared with 48 hours at 1.2g per litre per hour for batch process. Yields were increased to O.94g/g and the total amount of lactic acid produced during this process (200 hours, 4.2 litres) was determined at 58500g versus 57g for a batch process (48hours, 1 litre).
Batch and continuous crystallisation of calcium lactate were investigated in batch and MSMPR crystallises. The Batch mode allowed the complete characterisation of the key mechanisms of the process. Super-saturating appeared to be the main controlling factor of the crystallisation. It had great influence on crystal shape, crystal size, gel formation and crystallisation time. MSMPR experiments led to a description of the crystal growth rate using size-dependant growth models.
Improvement of lactic acid purification by continuous esterification and distillation The second task aimed to improve a lactic acid purification process based on continuous esterification and distillation. The first year of this project was essentially devoted to the study of lactic acid concentration and partially to its oligomerization. The second year was focused on the study of the esterification reaction by ethanol and of the continuous distillation of the ester formed. During the second year of this project competition between direct esterification and transesterification of lactic acid oligomers was studied, reaction constants were estimated and the philosophy of the whole process re-thought in order to improve the esterification kinetics. Beside this work, equilibrium curves, volatilities and thermodynamic data were determined, by both theoretical and experimental approaches. Following these results, the third year was spent in scaling-up this process, in order to size a pilot unit for the continuous production of ethyl-acetate, complying with the specifications of Task 3 for lactide synthesis. Several simulations were done with the aid of ASPEN soft in order to optimise the design and select the right packing with regard to mass and energy balances. At the end of this third year of work, and as a conclusion of Task 2 of this project, it can be concluded that the objective has been reached, such that a process can be extrapolate to pilot scale for the production of high quality ethyl-acetate on a continuous mode
Development of a new way to synthesise lactide from lactic ester In the third year efforts within this task were concentrated essentially on the purification stage of the crude product in order to obtain, at the end of the process, a polymer grade lactide. In addition to this main activity, other process parameters were fine-tuned or improved in order to obtain better product quality (higher stereo-specificity) and improved yields.
For crude lactide purification, a continuous distillation with a side extraction point resulted in significantly decreased levels of impurities. Nevertheless, this process did reach the extreme specifications required if lactide is to be used for ring opening polymerisation. Therefore a further purification, melt crystallisation, was added to the process. This technology eliminated residual solid and liquid impurities by selective crystallisation and hence gave a lactide with a purity higher than 99.8%. With lactide obtained in this way, polymerisation tests have been performed and PLA having at least the same molecular weight as PLA obtained starting from Boerhinger lactide have been recovered. Finally, to supply enough polymer-grade lactide to the University of Stuttgart for their research needs, it was found necessary to purify the crude product by re-crystallisation in a solvent. Indeed, the problems encountered with the purification step and the size of the pilot plant did not allow IKT to be supplied in time. An outside company was therefore used to produce re-crystallised product for IKT.
Polymerisation, chemical characterisation and processing behaviour of the PLAs Polylactic acids (PLA) are receiving an ever-increasing industrial attention for use as commodity resins, able to replace petrochemical-based polymers. Serious efforts are being made all over the world to develop an economically viable PLA manufacturing process. Compared to the state of the art in lactides polymerisation, the discovery that some Lewis bases can enhance the bulk polymerisation rate of lactides promoted by tin salt has been a real breakthrough. In order to reach the best balance between chain propagation and the commonly observed de-polymerisation reactions, the experimental conditions have been optimised in terms of temperature, catalyst and co-catalyst concentrations and thermal stability of PLA melt. The laboratory experiments have been up-scaled to a low capacity closely intermeshed co-rotating twin-screw extruder for PLA production. This successful up-scaling has proved the efficiency of the novel continuous one-stage polymerisation.
In order to increase the range of the PLA performances, block and random co-polymerisation of lactides and caprolactone has also been studied and seems to be promising for the future development of the one-stage reactive extrusion process. Finally, in order to check the purity of the monomer produced, polymerisation in glass ampoules has been studied and the PLA properties measured and compared to that of Cargill PLA. Conclusions of this study are encouraging and lead to optimistic forecasts.
Polymerisation of L-LA; physico-chemical characterisation and processing behaviour of the PLAs The polymerisation process, involving a closely intermeshing co-rotating twin screw extruder, has been analysed in terms of the effects that the various process parameters have on the properties of the resulting polymer. Keeping most process properties constant, it was possible to vary key parameters one by one. These included screw speed of the twin screw extruder, mass flow rate of the polymerised material and exit pressure at the extruder head. With this procedure it was possible to determine the influence of each of the parameters on the molecular properties of the resulting polymer especially on the conversion, which in turn is mainly responsible for the physical and technological properties of the resulting PLA-polymer.
Filling, blending and alloying of PLA-polymers PLA-polymers, pure and plasticised with 20 weight percent of polyethyleneglycol have been filled with different types of starch as a renewable filler. Starch types of different sizes, different size distribution and different shapes have been used, followed by analysis of the mechanical properties of the resulting filled polymer-systems. During this set of experiments it has been shown that the particle size distribution has a higher influence on the resulting properties than the mean diameter of the filler particles.
Feasibility study on the packaging applications of PLA After designing the prototype container, using the information of the mechanical data of the new developed and marketable PLA, with state of the art FEM/CAD technical, an optimised tool construction for the prototype container with a suitable lid was realised. The injection moulding process was optimised as far as possible. Unfortunately, the shortage of delivered film material limited production to some samples of 20 x 20 cm. Problems arose from brittleness of the material and the damages to the surface. With the extruded PLA film material it was possible to work out sealing behaviour by using cold/hot tacking impulse sealing and ultrasonic sealing. With the extruded film material the most important gliding properties could be investigated for static and dynamic friction. Further investigations looked at water permeability, water absorption, oxygen transmission, migration and chemical resistance.
SUMMARY
A combination of increased environmental concern, legislation such as the recent EU Packaging Directive and the desire to find natural alternatives to petrochemical based products, has created a potential large market for lactic acid. This would be used as an intermediate for the production of biodegradable polymers, as is already occurring in the United States. This represents a unique opportunity for large scale industrial transformation of massive amounts of glucidic substrates from agro food by products (whey in particular) by fermentation. The purpose of this project is to evaluate the technology available and develop the complete chain from fermentation to final product.
INTRODUCTION
This research is aimed at the development of PLA (polylactic acid) polymers industrial applications. It aims to develop an integrated fermentation process coupled to a continuous separation technique. The goals set are to find a cheap and abundant substrate and a low cost neutralising agent, used in a continuous process with a minimum conversion yield of over 95% with a residual sugar concentration of less than one gram per litre. The lactic acid purification process, based on continuous esterification and distillation, is expected to yield a high quality product which can be used to synthesise lactide from lactic esters (using a low cost and non toxic catalyst), the properties and processing behaviour of which will be studied in depth, in relation to problems of filling, blending and alloying of new PLA polymers, including products filled with starch.
OBJECTIVE
The expected rapidly expanding market for lactic acid as an intermediate for the production of biodegradable polymers represents a unique opportunity for a large scale industrial transformation of massive amounts of glucidic substrates from agro-food by-products (whey in particular). In this context, this project intends :
Since 1973, a considerable amount of research has been devoted to the development of PLA polymers for biomedical applications. The technology for some biodegradable polymers for industrial/consumer applications capable of effectively complete (in performance and price) with mineral oil based polymers does not exist. The proposed research will enable to produce polymers for industrial applications made from renewable raw materials.
RESULTS
The overall activities have been divided into a series of specific tasks, as follows:
Task 1: An innovative process, integrating production and separation of lactic acid. This included study and characterisation of new lactic bacteria strains capable of using different substrates for lactic acid production, as well as optimisation of culture medium in batch mode in order to define key parameters for a continuous process with cell recycling. Batch and continuous cultures were used to study a new lactic bacteria strain with various substrates; further work was performed on the optimisation of culture medium in batch and continuous mode. Additional knowledge was obtained in relation to its diauxic character, induction pathway of fructose degradation and inhibition by glucose and sucrose. Different parameters were evaluated in term of kinetics analysis. These related to specific growth rate, yield and productivity. It was found that yeast extract is an important growth factor and that there is a possibility of its being substituted by cell hydrolysates (which are less expensive than yeast extract). A very simple on line method to directly measure the lactic acid concentration in the culture medium was developed. In addition, studies were undertaken to evaluate parameters affecting microfiltration performance, i.e. temperature, viscosity and fouling.
Task 2: The aims of this task were to improve both the esterification of lactic acid and the distillation of the ester formed. Investigations started with the study of the concentration of the acid by evaporation of both free and part of the intramolecular water in order to increase the yield of the subsequent esterification step, as the reaction is strongly affected by the water content. Work started with a bench top study on batch mode to determine qualitatively and quantitatively the physico chemical changes occurring during this treatment required to control a continuous concentration process with a constant content at the system outlet. A dynamic study of the behaviour of lactic acid in a pilot falling film evaporator working in continuous mode provided information on heat transfer coefficients and entrainment in condensates under various conditions. For the oligomerisation step, batch and thin film approaches were compared and preliminary work was undertaken on the esterification of partially oligomerised lactic acid with ethanol.
Task 3: For the development of lactides, alcohol extraction from lactic ester to obtain an oligomer was first investigated in a batch reactor in order to determine the optimum parameters of the process in terms of catalyst, pressure and temperature. As a result, a wide range of lactic ester oligomers can now be synthesised. These were used in cyclisation/ depolymerisation experiments in order to determine the best oligomer for lactide synthesis. On the basis of identified chemical and process constraints, a reactor was designed to carry out the cyclisation/depolymerisation reaction. The first experiment conducted showed the efficiency of this apparatus and enabled it to be adapted to cater for process problems such as crude recovery, feed transfer and reactor cleaning. Subsequently, two physical parameters feed composition and temperature influence have been studied and show great impact on the conversion and selectivity of the crude synthesis product.
Task 4: Polymerisation and polymer characterisation were studied with the aim of developing an economically viable PLA polymer manufacturing process. Various organometallic compounds have been investigated for their ability to promote the ring opening polymerisation of lactides and co-polymerisation of lactides with other lactones in bulk. The results indicated benefits of tin and aluminium based catalysts, reflecting the fast polymerisation caused by the former and the exceptional thermal stability of PLA including the latter. Critical kinetic parameters such as catalyst concentration, polymerisation time and temperature, addition of transfer agents, monomer composition and purity were investigated in detail together with the efficiency of various stabilising agents so as to produce polyesters which could be melt processed without substantial degradation. Compared with the current state-of-the-art in lactide and lactone polymerisation, significant advances have been made, enabling the production of melt stable PLA polymers in a one stage reactive extrusion process.
Task 5: Studies of the modification of the brittle PLA polymers were conducted using up to 10% of Poly caprolactone (PCL) as an impact modifier. Two different types of PCL were tested. Because of the poor interaction between PLA matrix and PCL dispersed phase, block co-polymers of PLA and PCL were tested as coupling agents. A study was carried out to determine the behaviour of Triblend systems containing PLA as a matrix, poly(ethylenglycol) (PEG) as a plasticiser and native corn starch as a filler. The glass transition point of the PLA is lowered under possible usage temperatures and crystallisation of PLA is enhanced by the addition of PEG (up to 50% by weight). This plasticised diblend (semi crystalline polymer) could be filled with native starch. The objective is to lower the price of PLA significantly without losing mechanical properties.
Task 6: An initial information search, including data on physical and mechanical properties available from the literature, and EU regulations covering materials used in the manufacture of plastics and coatings intended to come into contact with foodstuffs, was carried out.
PARTICIPANTS
Coordinated by Brussels Biotech SA (B) together with the following partners: Centre National de la Recherche Scient. Delegation Regionale Nord Est Nancy (F), Buck Werke GmbH & Co BAD Technologiezentrum Fronau Reichenhall (D), Universite de Liege (B) and Universitat Stuttgart, Institut fur Kunststofftechnologie (D).
© Copyright 2006 Policy Statements
Updated
by CPL Press:
03/07/2007
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