vol 7, issue 9

Wood components in materials research

17 September 2012
By Tony James
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Model trees and flowers

We need to find a carbon-neutral alternative to plastic, novel ways of using wood could provide the answer

Plastic polymer pellets

Arboform, a meltable wood polymer, could soon be replacing plastic polymer, as pictured here

Plastic bags hanging on hooks

Scientists are aiming to replace plastic bags with bioplastics to encourage sustainable development

There is more to wood than meets the eye as researchers looks to its components for next-generation materials.

Since the dawn of recorded time, wood has been at the heart of man's endeavours primarily as a fuel and construction material.

Wood is an organic material, a natural composite of cellulose fibres held in a matrix of Lignin. Cellulose, a crystalline polymer derived from glucose, is its largest constituent, followed by hemicellulose, a linking of five carbon sugars, all embedded in Lignin, a complex chemical compound.

It is abundant; there are over a trillion trees on this planet that grow in excess of 10 billion tonnes every year. They are also a sustainable, carbon-neutral renewable source, which in the current environmental-conscious era makes it even more important.

Although wood remains important for construction and fuel, there is augmented attention in its constituent parts as alternatives to less sustainable materials. Researchers around the globe are attempting to make use of this versatile material to develop next-generation wood products.

Products such as recyclable food packaging, tyre replacements, antibacterial medical agents, lignin glue'applications and high-strength fabrics are all under development. As scientists and engineers further learn and'develop new techniques to extract various components from wood, or alternatively to modify wood by adding'components to wood, new and more advanced products will appear on the marketplace.

Novel composite materials

Nanocellulose has garnered interest from many quarters as a novel biomaterial. Its potential applications range from the creation of new kinds of commercially useful materials as well as uses in medical technology all the way to the food and pharmaceutical industries.

At the heart of the manufacturing process is nanocellulose powder, the raw material for creating polymer composites which can be used in lightweight structures for the car industry or as membrane and filter material for biomedicinal applications.

Cellulose is a biopolymer consisting of long chains of glucose with unique structural properties whose supply is practically inexhaustible. It is found in the cell walls of plants where it serves to provide a supporting framework - a sort of skeleton. Cellulose is extremely strong in tension and can be chemically modified in many ways, thereby changing its characteristics. It is also biodegradable. In the search for novel polymer materials with certain desirable characteristics, material scientists have developed such substances as high-performance composites in which nanofibres of cellulose are embedded.

Found in the form of lightweight structural material, these composites have similar mechanical properties to steel, while as nanoporous bio-foam they provide an alternative to conventional insulating materials.

Classical cellulose chemistry on the industrial scale is primarily used in the wood pulp, paper and fibre industry. Commercial research is currently focused on isolating and characterising cellulose in the form of nanofibres. So-called nanocellulose consists of fibres or crystals with a diameter of less than 100nm. Material scientists hope to be able to use nanocellulose to create new lightweight materials boasting high mechanical strength - in short, the ideal material for creating lightweight structures.

The cellulose experts in the wood laboratory at Empa, the Swiss material science centre, isolated cellulose nanofibres from wood pulp. These are several micrometres long, but only a few nanometres thick and are closely interlinked. The fibres have an extremely large surface area on which chemical-physical reactions with substances such as water, organic and inorganic chemicals and polymer compounds can occur.

These cellulose nanofibres can be used as stable, extremely reactive raw materials for technical applications while boasting the additional advantages of being biologically produced and biodegradable. Such applications include reinforcing bio-polymers to create very promising, environmentally safe, lightweight construction material for the car industry, as well as membrane or filter materials for applications in packaging and biomedicine.

Nanocellulose isolated from wood pulp is initially in the form of a water-based suspension. If the material dries out, the cellulose fibres stick together forming rough clumps and it loses its outstanding mechanical properties.

For this reason, the Empa researchers sought to develop a process that allowed them to dry nanocellulose without it clumping and becoming rough. To achieve this, the cellulose was treated using a technique that is easily repeatable on a large scale and is also harmless, even being suitable for applications in the food industry. The method prevents the cellulose fibrils from forming clumps and sticking together

The results are worth looking at: after being re-dispersed in water the dried nanocellulose powder boasts the same outstanding properties as undried, unmodified cellulose. This makes the new product an attractive alternative to conventional cellulose suspensions for the synthesis of bio-nanocomposite materials. Suspensions currently in use consist of over 90 per cent water which causes the transport costs to explode and increases the danger of degradation by bacteria or fungi. In addition, aquatic cellulose suspensions are laborious to work with since usually in the course of chemical processing solvents must be exchanged.

Liquid wood

Plastics are habitually the material of choice for modern designers, heavily used in everything from automotive manufacturing to children's toys. But plastic still has a number of negative selling points. It is non-biodegradable and can contain carcinogens and other toxic substances that carry health concerns.

It is also based on petroleum, a non-renewable resource that will soon be harder to come by. Increases in the price of crude oil leads to parallel rises in the price of plastics. But there is a new chemical invention that could do away with these long-standing concerns and at its heart is wood.

Initially developed several years ago at the Fraunhofer Institute for Chemical Technology ICT, liquid wood - or to give it its trade name, Arboform - is a meltable wood polymer, which is injection-moulded to produce different parts.

One of the first applications of the liquid wood was in replacing traditional wooden figures in traditional German nativity dioramas. The objects in the nativity'scene are usually carved in wood, but they may be made of modelling paste, pewter, plastic or even beeswax. With the development of liquid wood it was possible for the figures to be manufactured cheaply by'injection moulding using a natural wood product.

But how can that be possible, given that wood neither melts nor dissolves in any common solvent. Lignin is the most commonly occurring natural polymer after cellulose. In wood it holds the plant fibres together, forming a rigid but elastic composite material. In the paper industry, the two substances are separated using a chemical process.

"The cellulose industry separates wood into its three main components - lignin, cellulose and hemicellulose," former ICT team leader Emilia Regina Inone-Kauffmann says. "The lignin is not needed in papermaking, however. Our colleagues mix that lignin with fine natural fibres made of wood, hemp or flax and natural additives such as wax. From this, they produce plastic granulate that can be melted and injection-moulded. The mixture is then extruded to produce a granulate."

Arboform can be processed in an injection-moulding machine like any normal thermoplastic, and used to manufacture aesthetically pleasing products such as nativity figures, loudspeaker units, car instrument panels, chessmen for board games, or the stock of hunting rifles.

Bolstering Arboform's environmental credentials, Norbert Eisenreich, team member of the project at Fraunhofer Institute for Chemical Technology ICT, also discovered that the substance was highly recyclable. "To find that out, we produced components, broke them up into small pieces, and re-processed the broken pieces - ten times in all. We did not detect any change in the material properties of the low-sulphur bio-plastic, so that means it can be recycled," Inone-Kauffmann adds.

The product has been commercialised by Tecnaro, in Eisenbach, Germany. The company even has a golf tee in its product range. Less experienced golf players often hook the tee out of the ground together with the golf ball. The Arboform tee is so pliable that it doesn't damage the sharp blades of the lawnmower. Because the tees are made from a biodegradable material, they decompose on the compost heap together with the grass cuttings.

With its extrusion process for liquid wood, the company is venturing into unexplored territory. The method allows plastics to be processed in a continuous stream like pasta dough which vastly increases the speed of production. "In a few months' time, we hope to start production of artists' crayons and cosmetic pencils," Jurgen Pfitzer, who founded the company together with colleague Helmut N'gele, says. "At present, top-quality artists' crayons are still manufactured from imported cedar wood, while lip and eyebrow pencils are generally made using conventional petroleum-based plastics."

Plastic packaging from wood

Plastics are also a vital in the food packaging industry, vital for increasing the shelf life of food and reducing food waste. However, most of today's plastics are petroleum-based, but scientists are now trying to create a climate-friendly alternative to plastics from renewable resources; bioplastic and microfibrillated cellulose (MFC).

Cellulose can be broken down into what is called microfibrillated cellulose. MFC consists of plant fibres that are only 100 nanometres in diameter, but can be extremely long, making them highly suitable as a reinforcement material for biodegradable plastics. MFC membranes have also been shown to be impermeable to gases such as oxygen and can therefore be used to protect foodstuffs.

In Norway, Borregaard is one of the main suppliers involved in the project, as the company produces the fibrils that will make bioplastic impermeable to oxygen. They have been working with SINTEF, the Scandinavian research organisation, investigating barrier properties of food packaging, making use of nanotechnology to improve the shelf life of foods by limiting their exposure to oxygen.

"Bioplastics can make a contribution to sustainable development. Our aim is to develop materials and packaging that will add as little as possible to our environmental footprint, and ideally, will be climate-neutral," Age Larsen, the project manager and senior scientist at SINTEF, says. "In any case, as the oil runs down we are going to need alternative raw materials.

"If we can demonstrate that there good alternatives to petroleum-based products, I can imagine that this could help to bring in such measures a bit faster."

Although modern plastics are recyclable they often end up a step or two lower down the quality ladder than the raw material. "This is why we believe that MFC fibrils in combination with bioplastics will help us produce high-quality, environmentally friendly packaging for products such as bottles, jars and plastic foil," Larsen adds.

Hans Henrik Ovrebo is in charge of technology development at Borregaard, and he is certain that microfibrillated cellulose is a very promising raw material. "This project will give us valuable insight into the properties of this special type of plant fibre, and how it can be used in plastic products that are impermeable to gases," he says. "We think that in the long run, MFC could be used in several new products. At the moment, we are in the process of reconfiguring one of our pilot plants in Sarpsborg to produce this type of cellulose." *

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