FAIR-CT96-3070 Production of biodegradable films and bottles made of polylactic acid polymers (PLA)

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Biopolymers/Gums : FAIR Area 1.2 - Green Chemicals and Polymers Chain : Packaging : Paints/Coatings/Plastics



Final Report

Source: Progress Report for the period to August 2000 (note the project has been extended to August 2001)

Consortium: The project is co-ordinated by Brussels Biotech SA (Belgium) in partnership with . Galactic S.A., Escanaffles (Belgium), Institut Henrijean, Spa (Belgium), Institut fur Kunststoffiechnologie, University of Stuttgart (Germany) and ACE SA, Veka Film Division, Liege (Belgium).

Introduction

Considerable efforts are currently made to improve the quality of our environment, with particular attention given to the processing of refuse of which plastic materials counts for the major part. Within this framework, the use of products derived from a PLA (polylactic acid) polymer, would be a partial solution to this problem. Since, in addition to its biodegradable and biocompatible features, PLA also shows excellent mechanical characteristics.

The development of this polymer also opens new opportunities for diversification within the European agricultural sector, which is currently under recession. Indeed, the development of this biodegradable polymer, synthesised from renewable sources, such as sugar, maize or other fermentable materials, will require the cultivation of hundreds of thousands hectares of agricultural land to produce the required feedstock.

Activities

The process depends on three processes, esterification, LD synthesis and PLA synthesis. At present batch esterification can be followed by batch or continuous distillation with high molecular weight oligomers synthesised in a batch oligomerisation process. Impure lactide synthesis by a continuous process at high temperature is also available, involving a significant meso-LD content, purified by batch recrystallisation technology using a solvent. PLA synthesis is based on pre-polymerisation in a batch reactor and completion of the reaction on an extrusion machine with solvent purification of the polymer.

This project aims to produce a close integration resulting in continuous esterification and distillation with the advantages that it avoids side reactions and chemical inhibition of the esterification kinetics, as well as optimising energy balance by improving heat transfer. This approach also drastically reduces racemisation reactions and other thermal degradation resulting in high conversion yields and high productivity. The process only uses lactic acid, ethyl alcohol and catalyst (no drying or entraining agent) and optimises the mass balance with a reduction in wastes by recycling intermediate fractions.

The approach to LD synthesis, based on intermediate molecular weight oligomers synthesised in a batch oligomerisation process with the possibility to adapt it to a continuous mode also resulting in impure lactide synthesis using a continuous process at high temperature with an apparatus that significantly reduces the meso-LD content , followed by continuous purification by distillation. The advantage is that this process generates impurities of a different nature, which can more easily be eliminated with a conventional purification system such as distillation. In addition it has been demonstrated that the molecular weight of the PLAs can be controlled by adding small quantities of these impurities.

It is also possible to synthesise a lactide with high stereo-specificity (Meso-LD <3%) which in turn allows the purification technique to be selected at will. This allows for the development of a purification process based on distillation and avoids the need to use a conventional purification techniques. These include more cumbersome techniques such as re-crystallisation from solvent which is economically questionable for the high tonnage necessary when producing at the industrial level (100,000 t/year).

The actual synthesis of PLA is based on a one stage continuous polymerisation process in a twin screw extruder. In this novel production technique, the polymerisation time is reduced through the choice of a suitable novel catalyst-cocatalyst system. As a result a one stage continuous polymerisation process in a twin screw extruder is possible without an additional purification step. It is also possible to add plasticiser and stabilisers within an integrated system during the polymerisation process. The end result is the ability to use smaller size equipment and a simpler process

In order to protect know-how with the lactide and PLA synthesis, several patents are being applied for.

Project aims

The project aims to:

• show that this new type of process works on a semi industrial scale
• assemble units able to synthesise around 80t/year of ethyl lactate starting from lactic acid, 30t/year of lactide starting from ethyl lactate and 25t/year of PLA starting from lactide for the formulation of bottles as well as for hygiene and medical films
• producing PLA for the processing industries, with mechanical properties and thermal stability at least similar to that already marketed, at lower cost as a result of the totally original polymerisation process
• carry out a detailed evaluation of the global profitability of the demonstration unit and bring to light process steps where costs can be reduced, including a technical evaluation for a scaling-up to 100,000 t/year and an economic evaluation to determine what is required to reach, at full-scale industrial production scale, a price of 2$/kg
• produce enough PLA for the processing industries to allow them to evaluate its properties, processibility and performance in respect of the suggested applications (bottles or hygiene films) and to make sample products for dissemination.
• involve all parties concerned (disposal of refuse, processing industries, public) to allow fast commercial development of this polymer
• organise a European symposium which will result in contact with a great number of companies interested in this kind of products as well as organise seminars, with a more restricted audience of target groups (bottles producers, non woven producers, etc) focused on a particular type of applications, in order to disseminate the acquired knowledge.

Results

Task 1. Starting with marketable food grade lactic acid, this task aimed at studying an esterification reaction followed by continuous distillation steps in order to produce lactic acid ester to be supplied to Task 2 for lactide synthesis. Technologies used for these steps have been derived from research work conducted in the framework this project, with developments in relation to three key-concepts, i.e. product quality, ecology and economic viability.

For the purpose of transforming lactic acid into lactic ester the following steps have been taken:

• Design, sizing and erection of a pilot scale reactor working in a continuous mode.
• Design, sizing and erection of pilot scale, distillation columns to purify the ester formed by withdrawing the excess of water produced and recycling the excess of alcohol/water azeotrope back to the reactor.

Very encouraging results have been obtained but material balances have to be improved in the following months.

Task 2. During the third year the start-up of the various processing steps were finalised, in order to obtain a product meeting the specifications and also to make the unit reliable. So far the melt crystallisation and conditioning steps have been finalised and duly tested and several 10-kg samples of L-LD of polymer grade have been produced.

As IKT was not in a position to use the product of this step (solidified melt), a whole series of tests have been conducted on the conditioning unit but unfortunately they demonstrated that the technology used to transform the final product into a crystalline powder was inadequate. Indeed, materials produced with this technology are of the colloidal type which prevents standard packaging (bags of 25 kg). Therefore a new technology has been selected and will be tested in due course.

On the other hand, a great number of pinpointed actions linked to system reliability, progressively made the control of the unit more user-friendlier and ensured that tests are reproducible. The results of this optimisation effort is clear, since they enabled the time required to set up the various stages to drop from hours to minutes.

In addition the succession of experiments, although not always successful, resulted in a better understanding of the various processing steps even if these sometimes meant expensive energy and time expenditure. Not counting certain modifications still planned, the unit will soon allow for the first supply of about 50 kg per week of product to IKT in melted form, increasing to a volume of 100 kg per week.

IKT will be supplied with a solidified melt packed in metal drums of 30 kg. Lactide in flakes and hence packed in bags will be supplied when the new technology is operational.

Task 3 PLA is a very brittle and stiff polymer with a glass transition temperature of around 60·C. The mechanical properties of PLA are comparable to those of polystyrene with an elasticity modulus of 3500 MPa, a maximum tensile strength of 50 MPa, and an elongation at break of 4%. To introduce PLA into other applications requiring other mechanical property profiles, especially higher flexibility and higher impact resistance. To do achieve this it is necessary to use plasticizers such as pre-polymerised hydroxy terminated polyethylene glycol blocks (PEG). These blocks were used as starter molecules at which the L-lactide homopolymerised at both ends of the hydroxy terminated blocks, yielding an ABA-Triblockcopolymer. Subsequently, the influence of different amounts of PEG and the influence of different block length of these prepolymers on the thermal behaviour of the resulting blockcopolymers was investigated by means of DSC and TGA.

Task 4 No usable raw material was available, so trials were not started.

Task 5 A report was put together by the co-ordinator, as the first step of the Acquired Knowledge Exploitation and Dissemination task. The report comprises four chapters covering an assessment of the household plastic waste problem including current ways of disposing of such waste, a description of the main oil-based polymers used for packaging, a detailed study of the equivalent biopolymers and an evaluation of the market size for these new materials when used for packaging applications.

Conclusions

Contributing to efforts made to achieve well balanced and sustainable development for human communities is certainly one of man's first and most important priorities. It is also his duty and responsibility to take all necessary steps to ensure that he leaves behind him, for following generations, an environment which is not a hindrance to the well-being of people who will follow him on this planet.

During the last decades, industrialised countries and some developing ones have enjoyed considerable progress in the standard of living of their people mainly with the advent of new materials and services. However, there is a price to pay for such benefits as more or less disordered or mismanaged growth of the economy has led to often outrageous mistakes or even egoistic decisions : indiscriminate tapping of non-renewable resources, fresh water fouling by industry and municipal authorities alike, air pollution, etc. The list of man's assaults on nature seems Infinitely long.

Human activity always produces waste, that part of commodities which is without immediate use or which needs to be disposed of once it has complete its initial purpose. Plastic materials used for packaging are a good example of. this : they are convenient- to.-package goods, mainly food, but once they have fulfilled their part, they retain their properties of durable and stable materials for times often exceeding human life. Huge piles of discarded packaging plastics are to be found near every human settlement and they are there to stay.

Recycling and incineration are only part of the solution to this problem as they are either difficult or too expensive to put to work or producing low-quality outputs or making yet another negative contribution to air and water pollution.

To alleviate this problem, new environment-friendly plastics need to replace the existing ones as a matter of urgency. These new materials should

• be fully biodegradable into nature's elementary components in modern composting plants
• be made from home-grown renewable resources whose production benefits depressed sectors of the economy
• contribute to decrease the country's dependence on imports of foreign crude oil
• have properties similar or better than those of oil-based polymers
• be processable with existing plant and equipment without too much modifications and/or adaptations
• be available at prices comparable to the ones of oil-based polymers
• offer enough potential for the development of new, cost-effective and useful applications

Among the proposed materials of the third generation of biopolymers, only a few meet all or most of above criteria. Poly(lactic acid) PLA is among them and there are enough indications that it will come out as the preferred one when economy of scale factors come into play. Indeed, PLA being a highly versatile biopolymer can be tailored to meet nearly every challenge facing plastic manufacturers.

The product is there. It meets all technical requirements as a quality replacement for oil-based polymers. Now, comes the time when public authorities need to voice their determination to progressively phase out conventional plastics used for packaging purposes by enacting and enforcing legislation that prohibits the use of non-biodegradable, non-compostable polymers. By acting so they will play their rightful part in establishing a durable and sustainable society.