20 Juni 2008

Greener, stronger natural composites with coats of cellulose from bacteria

First use of bacteria to deposit sticky coatings of nanosized cellulose particles on the surfaces of plant fibers

Researchers from Germany and the United Kingdom report the first use of bacteria to deposit sticky coatings of nanosized cellulose particles on the surfaces of plant fibers, a process that may expand the use of natural fibers in renewable plastic composites used as strong, lightweight materials for cars, airplanes, and other products. The coated fibers provide strength and will make biocomposites more durable without affecting their biodegradability. They are more suitable for recycling (or compositing) than commonly used petroleum-based composites, the researchers say. Their study is scheduled for the June issue of ACS’ Biomacromolecules and is available for download.

Oil equals waste
biopact_bio-nano_composites-1.jpgThe enormous success of composites in our world is due to their specific mechanical properties, based on a strong interaction between the different components and their durability. However, it is usually impossible (or at least very difficult) to separate the different components again and, therefore, to recycle the composites, which generates severe end-of-life disposal problems.

Landfill, through which 98% of composite waste is disposed of will become prohibitively costly through the new European waste legislation in most European Union (EU) member states. The EU end-of-life vehicles directive, applying to all passenger cars and light commercial motor vehicles, will allow only an incineration quota of 5% for disused cars by 2015. Another EU legislation, the Waste Electrical and Electronic Equipment (WEEE) Directive, also affects composite and polymer manufacturers by forcing them to provide for recycling of their products.

As a result of these new legislations, both manufacturers and end-users will need to move away from traditional materials and will require new strategies for environmentally and economically viable materials. Truly green biodegradable composites made entirely from renewable agricultural resources could offer a unique alternative to address these issues for materials used in low load bearing applications.

Renewable composites
A broad range of renewable or partially renewable polymers, such as cellulose acetate butyrate (CAB), polylactic acid (PLA), or Dupont’s Sorona, is now commercially available. Alternative fillers such as natural fibers have already been explored for certain applications. Advantages of natural fibers are their low cost, low density, abundance, renewability, and (potentially their) biodegradability. They also display high specific stiffness and strength as well as acoustic and thermal insulation properties due to their hollow and cellular nature.

Their drawbacks of these biocomposites arise mainly due to their inconsistency in their dimensions and mechanical properties, their water sensitivity, and their low compatibility with many hydrophobic polymeric matrices. Bad or no adhesion at the interface between the two components will lead to a composite with poor mechanical properties because the stress transfer to the reinforcing phase through the matrix phase will not be effective.

To improve the interaction between natural fibers and the matrix, it is necessary to modify the natural fibers or the biobased polymers to compatibilize them, which is required for the design of truly green composites that can compete with conventional composite materials, such as glass fiber reinforced polypropylene. Chemical modifications such as silanization of natural fibers or anhydride grafting of biobased polymers have been studied and found to lead to increased composite properties. However, these modifications affect the green image of the final composites.

Microbes and microfibrils
bm-2008-00169g_0006-1-1.gifIn the new study, Alexander Bismarck and colleagues describe a new, clean way to overcome the problem, by using a bacterium – Acetobacter xylinum – to deposit cellulose microfibrils on the fibers, as a green reinforcing agent for the design of nanocomposites. They put sisal and hemp fibers in bacterial fermenters, and let the microorganisms produce the nanosized cellulose fibrils while they grow on the larger fibers.

Cellulose microfibrils can be extracted from wood or many other plant-based materials, but pulping and bleaching processes are not environmentally friendly. Cellulose whiskers can also be extracted from tunicate, a sea animal. Bacterial or microbial cellulose is produced by certain bacteria belonging to the genera Acetobacter, Agrobacterium, Alcaligenes, Pseudomonas, Rhizobium, or Sarcina, the most efficient producer of bacterial cellulose being Acetobacter xylinum.

This microbe, an obligate aerobe, produces extracellular cellulose microfibrils to provide a firm matrix that floats and, therefore, allows the embedded bacteria to stay in close contact with the atmosphere. The produced cellulose pellicles play a great role in promoting colonisation of the cells on the substrate and provide protection against competitors. Cellulose pellicles were also observed to protect Acetobacter xylinum cells from UV light: The elastic modulus of single bacterial cellulose fibril is much higher than those of natural fibers and is in the same order as that of glass fibers. This makes bacterial cellulose a very promising green nanoreinforcement. Moreover, the very good mechanical properties obtained for some cellulose-reinforced renewable nanocomposites prompt the scientists to assume that the interfacial adhesion between bacterial cellulose and renewable polymers should be good.

Inspired by nature, creating very complex hierarchical structures by assembly of molecules of different sizes where high mechanical resistance is needed, such as in plant cell walls, animal shells, and bones, the scientists thus propose an alternative way of modifying natural fiber surface.

They produced a hierarchical structure by cultivating cellulose-producing bacteria in presence of natural fibers, which resulted in significant coverage of the fiber surfaces by bacterial cellulose. This green modification is aimed at improving the interfacial adhesion to biobased polymers and might lead to truly green fiber reinforced hierarchical nanocomposites with enhanced properties and much better durability.

The researchers coated hemp and sisal fibers with the nano-sized particles of bacterial cellulose through a special fermentation process. The coated sisal fibers showed much better adhesion properties than the original fibers without losing their mechanical properties, ideal properties for their use in composites, the researchers say. The modified hemp fibers also had improved adhesion properties but showed a loss of strength.

Land use efficiency
Research into renewable composites is speeding ahead, and opening a new debate on which way we best use a given supply of biomass. There are many bioconversion options, from the production of energy in solid, liquid or gaseous form, to the production of biochar for CO2-sequestration in soils, or to the production of renewable materials like biocomposites and bioplastics.

Recent research shows that growing biomass for the production of bulk chemicals (for the manufacture of biopolymers and bioplastics) as alternatives to oil based polymers, may in some cases be a more efficient way to reduce carbon emissions than using that land for the production of biomass for energy. And as researchers succeed in making natural composites and plastics more durable so they can compete with oil based rivals, this debate gets ever more relevant.

References:
Marion Pommet, Julasak Juntaro, Jerry Y.Y. Heng, Athanasios Mantalaris, Adam F. Lee, Karen Wilson, Gerhard Kalinka, Milo S. P. Shaffer, and Alexander Bismarck, “Surface Modification of Natural Fibers Using Bacteria: Depositing Bacterial Cellulose onto Natural Fibers To Create Hierarchical Fiber Reinforced Nanocomposites”, Biomacromolecules, 9 (6), 1643–1651, 2008, DOI: 10.1021/bm800169g. (Abstract)

Source: Biopact, 2008-06-16.

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