vol 7, issue 9

Wood in warfare

17 September 2012
By Crispin Andrews
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Wooden tank model

Artist Mike Rea sculpts fantasy timber creations such as this tank

Nanacellulose properties

Many of the useful properties of nanocellulose arise from its complex structure of fibres

Nanacellulose properties

A nanocellulose-based aerogel manufactured at the University of Helsinki can hold 10,000 times its own weight

Military testing new tents

Ballistic panels treated with nanocellulose can withstand ten times the pressure of a hurricane

After years of being ignored in favour of metals and plastics, wood is getting a high-tech military makeover.

Wood has been a military material longer than there's been an organised military, and in a high-tech world of metals and synthetics, it's making a comeback.

Since the first time our prehistoric ancestors picked up a log to stake a claim to territory against a rival group or to chase off giant predatory birds, humans have used wood to get what they want. Viking longships and Robin Hood's longbows, the Russian artillery carriages that inspired Tchaikovsky's 1812 overture, Spitfires with their wooden propellers and fairings, and Mosquito bombers were made almost entirely out of wood when other materials were scarce during the Second World War.

Industrial and technological revolutions made wood obsolete as a weapons delivery mechanism and support structure. Militaries still used wood to build bridges, living quarters and other temporary structures, and to package materials. But as far as weapons were concerned, metal, plastic and fibreglass were either lighter and cheaper, or stronger and more durable. Until now, that is.

A tree grows 100ft tall, withstands countless batterings from the elements and yet, when chopped down, it's loaded on the back of a lorry without much effort. Inside trees there's an organic compound called cellulose that gives wood strength and lightness. Cellulose consists of long chains of sugar glucose linked together into a polymer. It's also inside plant stems, leaves, roots and fruit.

Cellulose has long been used to make paper and textiles - cotton, for example, is a pure form of cellulose. But recently, scientists have developed a highly processed form, nanocellulose. Once mass produced, it will give military engineers stronger, lighter, more durable materials to make their boats, tanks, aircraft and weapons.

Nanocellulose is essentially wood fibre broken down to the nanoscale. Combined with other materials into composites, it's as strong as steel and Kevlar, but much lighter. "It's going to be a replacement material," says Professor Harry Brumer from the University of British Columbia's Michael Smith Laboratories, "it could partially replace steel and other metals, also fibreglass."

The US Department of Defence is already thinking lightweight armour and ballistic glass. There's also talk of small military robots, fortified with nanocellulose particles, being deployed on battlefields. "Maybe soldiers' helmets, too," Prof Brumer adds. "Anything where strength to weight ratio is important, so military personnel don't have to lug extra mass around."

Nanocellulose is composed of bundled sugar chains from the cell walls of wood and other plants. Scientists extract it by grinding pulp or using chemicals to break it down into tiny particles. At this scale, the material has fewer defects, and is therefore stronger. Michael Wolcott from Washington State University's International Academy of Wood Science, explains that when applied to biopolymers, the nanocellulose particles drive multiple properties within the composite material, simultaneously. "Carbon fibres are strong but stiff," he say. "The nanocellulose not only helps overcome the deficiencies of the other material but it fires the material's performance."

Wooden military

Doug Gardner from the University of Maine's Advanced Wood Engineering Composites Center (AEWC) adds that adding 10 per cent nanocellulose into a composite material provides a 70 per cent increase in property effectiveness. Gardner is working with the US army to apply this material to vehicle parts, theatre-based building materials and battlefield protection for military personnel.

Recently, Gardner's team discovered that nanocellulose also enhances the impact strength of composite materials. "Motor vehicles, boats, aircraft, there's future potential to make the large parts and the small," he says, "gun stocks, sightings, optics, anything that will make weapons lighter and more durable. One of my students is looking at conductivity by combining nanocellulose with graphite particles. This could be used one day, by the military, for battery storage."

Professor Olli Ikkala from the University of Helsinki recently unveiled a nanocellulose-based aerogel light enough to float on water and strong enough to hold 10,000 times its own weight.

Aerogels are so light they have been called solid smoke. They're also made from silica, carbon and aluminium. Some are only a few times denser than air. Unlike other aerogels that are stiff and can't bend, Professor Ikkala's aerogel has all the properties of the wood fibres it contains. His team covered the aerogel with a one-atom-thick layer of grapheme carbon for steel-like strength and coated the nanocellulose fibres with titanium dioxide, which repels water but absorbs oil.

Sounds perfect for a patrol boat hull or any military craft or weapon currently made of fibreglass, or other synthetic polymers. Scientists in Brazil have already used nanocellulose from bananas to make a plastic that's 30 per cent lighter, but four times as strong as petroleum-based plastic. Car manufacturer Ford think they can knock 250-750lb off the weight of their cars - increasing fuel economy. "We're focusing on replacing automotive plastics," says Alcides Leão, a researcher at Sao Paulo State University. "But in the future, we may be able to replace steel and aluminium parts."

Military personnel don't fight all day and night. At least western forces, fighting offensive wars far away from home against sporadic attacks from outgunned opponents, don't. Military personnel have to eat and sleep like the rest of us, and it's here where they're often most vulnerable.

Tents are light and easy to put up and take down. But while canvas keeps out the heat and the mosquitoes, it isn't much use against a bullet or mortar fragments. A commander can make all the strategic defence plans they like, but in the desert or mountain slopes, facing hidden assailants with long and short-range weapons, something, sometime will get through.

Gardener's AEWC colleagues have developed ballistic panels, strong enough to withstand bullets and mortar fragments. These panels have a wood core that's surrounded by synthetic resins and fibres. The panels fit inside tent frames, and at 70lb are light enough for two people to carry. Four soldiers can fit one panel in half an hour, with no tools or training. "The panels are built to withstand 10 times the pressure of a hurricane," says AEWC director Habib Dagher. "Flexible straps give slightly, preventing the tent frame from being destroyed during the few milliseconds that an average blast lasts."

Wood-plastic of the future

The US Army Corps of Engineers has awarded the AEWC $2.5m to develop these blast resistant panels. In tests, the system withstood blasts from 75ft, 33ft, and 21ft with no failures in the structure, the anchoring or the panels. The US army has field prototypes in Iraq and Afghanistan.

"The wood is strong and lightweight and the surrounding synthetic materials give it tensile reinforcement," says AEWC research engineer Josh Clapp. Clapp explains that under sudden abrupt force, wood bends and eventually ruptures. "We don't want splinters flying everywhere after a blast," he adds, "they're dangerous projectiles."

Wood-plastic composites were first introduced into the decking market in the early 1990s. They're also used to make furniture, fencing and window frames.

Manufacturers mix ground wood particles and heated thermoplastic resin. They bind the strands, particles, fibers, or veneers of wood, together with adhesives. Typically, they use the same hardwoods and softwoods used to manufacture lumber, but sometimes also sawmill scraps, or polar, a common but not a structural species.

Composites outperform pure wood, requiring minimal maintenance and are resistant to rotting and insect attack. "The military are always interested in improving the quality of modern materials and looking at more sustainable and renewable markets," Gardener says. He adds that putting nanocellulose with all its properties into AEWC's ballistic panels would further enhance their blast-resistance properties.

Dutch company Lignostone makes bullet-proof composite products for the military. Their engineers combine a hard compressed beechwood with Kevlar, glass epoxy or carbon to make doors and window frames, flexible separation walls, secure accommodation units and desks. Specially prepared and compressed, this material is several times harder than hickory.

Military forces, particularly occupying armies, have to deal with civilian unrest as well as enemy combatants. Too heavy-handed an approach with conventional weapons will soon turn the former into the latter. Wooden batons, night sticks and even wooden bullets are part of most militaries' non-lethal weapons arsenal. Soldiers are trained to use them, although listening to reports of military atrocities against civilians, you have to wonder whether this is just a PR stunt. When training, military personnel use wooden hand weapons. So do martial artists. Traditionalists still prefer wooden gun stocks to metal or plastic.

Taking a bullet

The cellular structure of wood also makes it a useful military material, or to be more precise, what scientists can learn from its cellular structure. Back in the 1980s, Professor George Jeronimidis from Reading University was intrigued by the way wood could absorb energy from an impact. He says: "You drive a nail into wood and the wood takes the nail. Do this with harder, stronger materials like plastic or glass and the material cracks or shatters."

Jeronimidis explains that wood's structure collapses into itself to adapt to the incoming object. Whether that object is a nail, a bullet or shrapnel; the wood absorbs the impact.

Funded by the MOD and the Post Office, Jeronimidis developed synthetic materials with the same cellular structure and similar properties. "The MOD were looking for blast resistant materials and bullet proof protection for their soldiers," he says, "they wanted something light that would stop a threat, something high performance with a lower mass."

It was also a time when the IRA was sending letter bombs to targets on mainland Britain. The Post Office wanted a container that could contain an explosion. Jeronimidis and his team made them blast resistant containers with a wood-like structure out of glass fibre, carbon fibre and Kevlar. "The container redirected the blast upwards and the walls captured the fragments," Jeronimidis says. "It's the same reason nuclear waste carriers sometimes contain wooden impact absorbers."

Nanocellulose's time

Jeronidis's technology worked, but was too expensive and time-consuming to mass produce, but he believes that advancements in manufacturing technology means his design could re-appear, today. "Focused application and manufacturing costs would no longer be prohibitive," he says.

It's the same with nanocellulose. Scientists have known about this material since the early 1980s. But according to Swedish research and development company Innventia, at first it took 30,000kWh/tonne to delaminate the fibres - too much energy for profitable commercialisation. Innventia claim to have reduced this by 98 per cent, using fibre pre-treatment methods.

Other companies and research institutes are doing similar things, but according to Michael Wolcott nanocellulose development is still in the early stages and it could be a while before its deployed commercially on a large scale. "These things are driven by process economics, there has to be high enough added value to the product," Prof Brumer adds. "At the moment we're not sure about scale. Nanocellulose has been clearly proven to enhance composite properties in laboratory-scale materials, but the next challenge is to obtain the material at industrially-relevant amounts at economical costs.

"Like all these things it's driven by economics,' he adds. "In two to three years there should be a viable commercial market."

The military, like all government departments, have sustainability targets and have to reduce reliance on fossil fuels. There's no need to burn fossil fuels to produce nanocellulose. It's also the most abundant natural polymer on the planet. If Ikkala is right, it could in the not too distant future replace petroleum, a key ingredient in everything from plastic to tire rubber.

Defence departments and contractors around the world, take note.

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