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Global warming, growing awareness of environmental and waste-management issues, dwindling fossil resources, and rising oil prices are some of the reasons why “bio” products are being promoted more and more for sustainable development. There are a number of applications (packaging, agriculture, household use, and more) where biodegradable polymers and composites are particularly suitable as sustainable alternatives.
(Published on March - April 2008 – JEC Magazine #39)
ANNE BERGERET, PROFESSOR ECOLES DES MINES D’ALÈS
We now generate huge amounts of plastic waste, especially in the packaging industry and agriculture. Oil-based polymers are becoming more and more expensive, due to rising oil prices that are due in part to dwindling fossil resources. All this has sparked renewed interest in natural materials produced from renewable resources. The development of new materials now focuses on things like biodegradability properties and environmental safety aspects.
Biodegradable polymers obtained from renewable resources can be classified into three families: 1) agro-polymers like polysaccharides that are obtained by biomass fragmentation processes; 2) polymers like polylactic acid (PLA), that are produced by biomass fermentation processes; and 3) polymers obtained from conventional polymerisation of monomers extracted from biomass, like polyhydroxyalkanoate or PHA. There is also a fourth family of biodegradable polymers like polycaprolactones (PCL) or aliphatic and aromatic copolyesters, but these are obtained from non-renewable fossil resources. A wide range of such biopolymers is now commercially available, offering all sorts of properties that enable them to compete with nonbiodegradable polymers in several industrial sectors.
Research activities at the Ales School of Mines
The Ales School of Mines (Alès, France) is an engineering school under the tutelage of the Ministry of Industry. It includes three research centres, including the Materials Research Centre with about 20 people (excluding PhD students) working on advanced polymer materials. The Centre’s objectives are:
For about ten years now, the Centre has been involved in several different research programmes to develop innovative biodegradable polymers and composites from renewable resources and the corresponding characterisation systems. Its main investigations, for which the results are presented here, concern the development of biodegradable polymers from biomass, such as starch and proteins. In recent years, its research activities have focused more specifically on biopolymers derived from micro-organisms or biotechnologies, such as PLA and PHA.
The Centre’s resources include polymer processing equipment for single- and twin-screw extrusion, 95T and 60T injection moulding, calendering, compression moulding, and more; grinding and sieving equipment; and methods to develop specific surface treatments for fibres and fillers. The Centre uses methods like FTIR analysis and calorimetric or thermo-gravimetric measurements to study, among others, the mechanical (static and dynamic tests), physico-chemical, and thermal properties of materials. It also uses accelerated ageing tests or a recently developed in situ ageing test called environmental stress cracking (ESC) to study the ageing resistance of polymers and composites, and standard tests to measure the biodegradability of polymers in terms of degradation rates in compost and biological oxygen demand (BOD). A specific enzymatic test has also been developed to screen biodegradation kinetics before conducting long-term standard tests.
Case study projects
COTONBIOMAT – FP5 – Development of biodegradable protein-based materials from cotton seed derivatives using dry technologies
Agricultural items like mulching films, silage films, bags and plant pots can be considered as interesting applications in which biosourced and biodegradable polymers could replace oil-based polymers (mainly polyolefins), which generate a huge amount of hard-to-recycle waste.
Biodegradable protein-based engineering polymers are challenging alternatives. With a worldwide production of about 33 million metric tons, cotton-seed cakes are currently the most abundant source of plant proteins after soybeans. These products seem to be very attractive for non-food applications such as biodegradable polymers. In most cases, wet processes such as casting are used for these materials; the objective is to be able to use the dry processing technologies (such as extrusion and thermo-moulding) that are currently used for synthetic polymers (Figure 1).
Cotonbiomat was a research project in the E.U. Commission’s Fifth Framework Programme (FP5) that focused on developing this protein-based biopolymer through dry processes. The Materials Research Centre of the Ales School of Mines became involved in this project, which was managed by the International Agronomy Research Centre CIRAD (Montpellier, France) and implemented in co-operation with South American companies and institutions from Brazil and Argentina.
Dry technologies require proteins with a thermoplastic behaviour, i.e. viscous flow at high temperature. In many cases, the glass transition temperature of proteins is very close to their thermal degradation temperature. To widen the processing window, proteins are mixed with small molecules likely to lower their glass transition temperature by plasticization. Due to the hydrophilic nature of many amino acids, polyols (such as glycerol or sorbitol) are commonly used to plasticize proteins.
We investigated the influence of a number of parameters, including plasticizer type and content, storage conditions, presence of cottonseed shells, presence of lipids, and processing conditions. The results showed that the presence of plasticizers tends to reduce Young’s modulus and tensile strength and to increase elongation at break.
This effect is stronger when the plasticizer content and the number of hydroxyl groups from the plasticizer are increased. Storage conditions also have a strong influence on mechanical properties, water being a good plasticizer for proteins. The presence of shells tends to reduce the mechanical performance of films. At very low contents (
BIOCOMPOSITES – ADEME/AGRICE – Development of a multilayered starch-based biocomposite for packaging applications
Current packaging materials consist of a variety of oil-derived polymers (mainly polyolefins such as polyethylene, polypropylene and polystyrene), metals, glass, paper, or various combinations thereof. Food-grade packaging has to meet specific optimum requirements, especially for storage and interaction with food. Hence, the development of new bio-based food packaging materials can be considered as a tremendous challenge, both for academia and industry. The Materials Research Centre of the Ales School of Mines and the Vitembal Company (Remoulins, France) joined forces to develop an innovative multi-layered biodegradable composite to replace the expanded polystyrene (EPS) trays commonly used for food packaging, especially for fish, meat and vegetables.
Starch was considered as a suitable alternative to produce the required foamed structure. Expanded starch was processed through a standard extrusion technology (temperature varying from 30 to 160°C from feeder to die) to obtain sheets that were then thermoformed to shape the final trays (Figure 3).
The expansion process was induced by water and nucleating agents. Talc and a citric acid-based component were used respectively as physical and chemical nucleating agents. The main drawbacks of starch are its high water sensitivity and low mechanical properties. Therefore, natural fibres were incorporated into starch, and two external biodegradable and bio-based polyester films (120 μm) were calendered on both sides of the foamed starch sheet so as to limit its water absorption and to enhance its overall mechanical properties (Figure 4). The efficiency of different natural fibres with various cellulose contents (from 30 to 99.5%) was compared: hemp, cotton linter, cellulose and wheat straw (Table 1).
Surprisingly, similar E/ρ values (E: bending modulus; ρ: density) (Figure 5a) were measured for the cellulosereinforced biocomposite and for EPS, the other biocomposites having slightly lower properties. Moreover, the new biocomposite has better impact properties (Figure 5b) than EPS. But the main focus was water sensitivity, which remains ten times lower for the biocomposite than for EPS because of the intrinsic foamed microstructure.
Expanded starch is composed of about 80% open cells with the following diameters: about 1000 μm for pure expanded starch and 580-780 μm for natural fibre-reinforced starch foams. Industrial EPS trays are a two-layer system with an open-cell layer (75-85% open cells) in contact with food to optimize exudate absorption and a closed-cell layer (85-95% closed cells) that acts as a diffusion barrier. Moreover, EPS cells are smaller (about 300 μm) (Figure 6). Various degradation tests were investigated. The bio-composite’s weight variation with composting time was measured (ISO 14855 composting test) (Figure 7). About 50% degradation was recorded after 4 months due to fungus growth (Aspergillus, Hyphomycetes) (Figure 8). The oxygen consumption of microorganisms (ISO 14432 BOD) in contact with the biocomposite shows a degradation of about 30% after one month.
Today’s studies focus on optimising the cell morphology, but also on using new natural fibres such as sugar cane and coconut fibres, where specific surface treatments are applied to lower the water sensitivity of the multilayer biocomposite.