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.
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 :
1. To develop an integrated process for the production of lactic acid by fermentation of glucidic substrate coupled to a continuous separation technique (goals to reach : cheap and abundant substrate, low cost of neutralizing agent, continuous mode, minimum conversion yield : 95% residual sugar : less than 1g/l).
2. To improve the lactic acid purification by continuous esterification and distillation (goals to reach : no solvent use, no polluting wastes, continuous mode, minimum yield : 95% high quality product : more than the Heat-Stable specifications).
3. To characterize a new way to synthetize lactide from lactic esters (goals to reach : no solvent use, continuous mode, minimum yield including recirculation : 90% products quality sufficient to polymerize, low cost and non-toxic catalyst).
4. To study deeply the polymerization of PLA-polymers, their physico-chemical properties and their processing behaviours (goals to reach : no solvent use, semi-continuous mode : extruding type, control of molecular weights and of polydispersity, low cost and non-toxic catalyst).
5. To approach completely the problem of filling, blending and alloying of new developed PLA-polymers (goals to reach : blending of PLA with other aliphatic polyesters and filling with starch in order to get different physio-chemical properties, no toxicity of the products, use of existing equipment).
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.
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.
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.
Funding SchemeCSC - Cost-sharing contracts
83435 Bad Reichenhall