Green nanocomposites

Engineers at Imperial College London and Manchester University are planning to use a gel product from coconut milk to develop a biodegradable nanocomposite.

As well as being a traditional Philippine dessert, the researchers say that the gel — nata de coco — is a bacterial cellulose sugar compound that has the appropriate mechanical properties to reinforce bio-derived polymer nanocomposites for applications in the automotive, construction and packaging industries.

‘With oil prices increasing and renewable resources becoming more of an interest in the general area, our interest is to develop materials with triggered biodegradability for low-load but high-volume applications,’ said Imperial composite engineer Dr Alexander Bismarck.

Green nanocomposites currently being explored, he said, include nanocrystalline cellulose reinforced renewable polymers and regenerated cellulose. In the forthcoming project the researchers will focus on bacterial cellulose nanocomposites.

‘Microcrystalline cellulose is obtained by digesting tree or wood components and extracting only the cellulose components that reinforce wood, while bacterial cellulose is an extracellular matrix produced by acid-producing bacteria,’ he said. ‘The advantage of these bacteria is that they basically use sugar. By eating the sugar in coconut milk the bacteria produce nata de coco, which has extremely high crystallinity and high modulus, which indicates a stiff material — much higher than that of microcrystalline cellulose.

‘This high crystallinity is required to prevent its dissolution in the cellulose solvent during the regeneration phase of the cellulose matrix in the presence of the natural fibres and the nanocellulose. However, we need a good interface between the cellulose matrix and the cellulose reinforcements to create a strong composite.’

Other advantages include the abundant supply of bacterial cellulose. Nata de coco is produced on a massive scale in south-east Asia, and its renewable characteristics would allow the researchers to make an end product that would last as long as necessary and only degrade under composting conditions.

The team recently demonstrated, in a PhD project that formed the basis of this EPSRC-funded proposal, that it is possible to attach bacterial cellulose to natural fibres by culturing the bacteria in the presence of natural fibres.

‘We can culture the bacteria, which produce the bacterial cellulose, and deposit this cellulose around or on to natural fibres such as sisal, hemp and abaca. And we have shown that it is possible to enhance the adhesion between a natural fibre and the matrix using these modified fibres,’ said Bismarck.

‘so we want to continue, to show it is possible to create composites with a much-improved mechanical performance that can be used in automotive applications. For instance, the natural fibres and the matrix together would form a composite that could be used directly as the entire cover for the inner lining of a car door, which is now made of polypropylene,’ he said.

Dr Steve Eichhorn, a specialist in interfacial characterisation using Raman spectroscopy at Manchester, added: ‘We are going to make some reinforced foams, and they could be used in lightweight construction. Because it is biodegradable it is only going to have a short lifespan, but we are hoping to develop that further so we can switch on the biodegradability of the material.’

In addressing the short lifespan issue, the researchers have to overcome the challenge of creating a biodegradable nanocomposite that can deal with heat and moisture.

‘First we have to solve the processing issues. If we use conventional biodegradable polymers, the problem is that they are not very resistant to heat, and they absorb quite a bit of moisture. And of course, if you have a natural fibre in there that also likes moisture, your composite would degrade rather rapidly,’ said Bismarck.

‘If we manage to get away from standard polymers such as polylactic acid (PDLLA) or polycaprolactam, to pure, regenerated cellulose, then that would help overcome the problem because regenerated cellulose does not really take up much moisture.’

According to Bismarck, the physical properties of the composites would depend on the mechanical properties of the natural fibre used as reinforcement.

‘The question is how much bacterial cellulose or nanocellulose will we be able to introduce into the composite, and the crystallinity of the regenerated cellulose matrix itself. So by tuning these parameters you can tune the elasticity, or the modulus, of the entire composite and also the toughness or strength,’ he said.