快猫短视频

FORGET visions of 21st-century cars built from sleek, synthetic wonder
materials. The car of the future could be moulded from cashew nut oil and hemp.
And that鈥檚 not all. In the boot could be a set of golf clubs built around jute
fibres, nestling next to a tennis racket stiffened with coconut hair. And the
bicycle frames strapped to the flax-based roof rack may derive their strength
from the fibres of any one of 2000 other suitable plants. The truth is, natural
fibres are undergoing a high-tech revolution that could see them replace
synthetic materials in applications as diverse as boat hulls, bath tubs and
archery bows. Among their many virtues are low cost, low weight and ease of
recycling.

The synthetics being challenged by this natural revolution are composite
materials鈥攑lastics reinforced with glass or carbon fibres, for example. As
anyone who goes fishing using a fibreglass rod, or plays golf with
graphite-reinforced 鈥渋rons鈥� will tell you, composites are very strong and much
lighter than the metals and alloys they replace. Using a plastic matrix and
reinforcing fibres gives composites the best properties of each. The fibres are
very strong and stiff but brittle, and would snap easily on their own. By
comparison, the plastic matrix is soft and flexible, and transmits any applied
force to the fibres. Both fibres and plastics are light, and combined they give
composites a very high strength-to-weight ratio.

Greener than glass

For high-performance materials, carbon or aramid鈥攑olyamides such as
Kevlar鈥攁re the fibres of choice. They are very strong and highly
heat-resistant, so they can be used in applications where conditions are
fierce鈥攊n aircraft such as the Eurofighter 2000 for example. But they also
tend to be expensive, so for more general applications the workhorse fibre for
composite materials is glass.

Glass fibres have many benefits. They are cheap, strong and relatively easy
to manufacture. But there are disadvantages too. They are very abrasive, which
makes them dangerous to work with and increases wear in machines used to cut
products made from composites. More importantly, glass fibres could present a
health risk to those working with them. Like asbestos fibres, they are very
small and once breathed in can lodge in the lungs and be absorbed into the
body.

But the biggest problem with glass and other synthetic fibres is how to
dispose of them at the end of their lifetimes. Fibreglass is not biodegradable
and cannot be burned because the fibres are left behind as clinker, a nasty
residue which can ruin a furnace. Nor are glass fibres easy to recycle. They
break easily during the rough and tumble of reprocessing. Of course, they can
always be dumped. But, says Jamie Hague, assistant director of the BioComposites
Centre, part of the University of Wales at Bangor, dumping is becoming costly as
more countries apply landfill taxes.

This is where natural fibres come into their own. 鈥淭hey are abundant,
renewable, cheap and low density,鈥� says Bob Coutts, chief research scientist in
the composites and adhesives division of Australia鈥檚 national research
organisation, the CSIRO, in Melbourne. 鈥淎nd they are biodegradable.鈥� Even if
they are burned at the end of their lives, rather than left to biodegrade,
natural fibres have an environmental advantage. 鈥淎ll plants are carbon dioxide
neutral,鈥� says Hague, who along with his colleagues is researching and promoting
the use of naturally derived materials. 鈥淚f they are burnt at the end of their
life, no additional CO₂ is released into the atmosphere.鈥� What鈥檚 more,
they can be recycled several times and maintain their length and strength. 鈥淭hey
are naturally designed to be incredibly tough,鈥� says Callum Hill, from the
School of Agricultural and Forest Sciences at Bangor.

But are natural fibres as good as their artificial counterparts? Hague
believes there are plenty of situations in which they could do just as good a
job. But there are key differences. Unlike relatively simple synthetic fibres,
natural fibres are much more complicated. They are made up of bundles of
cellulose microfibrils, thin strands of cellulose which are wound together and
surrounded by layers of hemicellulose and lignin. This built-in microstructure
means natural fibres can absorb a lot of energy, so that weight for weight, they
are just as strong as glass ones. But, they are much less dense. So if panels
made of natural fibres are to be as strong as those made of synthetic fibres,
they have to be thicker.

The complex structure of natural fibres also makes it hard to measure their
mechanical properties and spot any potential weak points. 鈥淭here is an enormous
amount of variability in properties of fibres from just one plant, depending on
the part of the plant from which they are taken,鈥� says Laurence Mott of the
BioComposites Centre. Different fibres have different lengths and
cross-sectional areas鈥攁nd also different defects. 鈥淸Natural] fibres are
subject to defects such as microcompressions or pits and cracks,鈥� says Mott.

Along with Steve Shaler of the University of Maine and Leslie Groom of the US
Department of Agriculture Forest Service, Mott has developed a technique for
studying the properties of natural fibres. He is using environmental scanning
electron microscopy and a miniature system for gripping fibres to extract the
same performance data that engineers have come to expect for glass and carbon
fibres. He hopes his work will determine which defects have least effect on the
properties of natural fibres, with a view to developing more high-tech
applications. 鈥淲e want to design damage accumulation models. We could then
design reliable [natural fibre] composites for demanding applications.鈥�

Even so, a variable fibre length means inconsistent properties in the
composites. With fibreglass, fibre lengths can be controlled and they can be
oriented to enhance strength in a particular direction. One way to achieve the
same flexibility using a natural fibre is to regenerate the cellulose and make
it more uniform. The idea is to dissolve the microfibrils in solvent and then
precipitate them under very controlled conditions. 鈥淲e are developing high
performance regenerated fibres from wood pulp,鈥� says Coutts.

Flaxen trim

The stakes are certainly high, particularly where environmental legislation
is the main driving force, as it is in car manufacturing. 鈥淚n Germany, car
manufacturers are aiming to make every component recyclable or biodegradable,鈥�
says Hague. Which doesn鈥檛 leave a lot of scope for fibreglass composites.

Daimler-Benz is a world leader in using natural fibres in its vehicles. Since
1995, for example, the door panels in the Mercedes G-class have been made from
plastics reinforced with flax fibres. A British manufacturer is also working
with flax fibres, while ambary or kenaf, which gives similar fibres to jute, is
being considered in the US, where it grows better than flax. The parts,
including rear window shelves and seat backs, are simple to make. Fibres are
laid out to form a mat and loosely stitched together by a process called
needle-punching. The plastic is injected around and between them to form a
surrounding matrix, and the part is pressed into shape.

快猫短视频s at Daimler-Benz鈥檚 research centre in Ulm, Germany, are now
considering using hemp. Hemp fibres are more rigid and better suited to
processing than flax, and could replace the fibreglass still used in both
exterior and interior parts. If the load-bearing and impact-absorbing components
of cars could be made from a material reinforced with natural fibres, that would
be a significant breakthrough.

Another stumbling block awaits the developers of a fully recyclable or
biodegradable car鈥攖he plastic matrix. Most of the plastics used to make
composite materials are derived from oil and are non-biodegradable. The answer
could lie either in developing a suitable fully biodegradable plastic, or using
a plastic derived from a renewable natural resource. There are several
biodegradable polymers on the market, but they are much more expensive than
conventional plastics. Researchers at the BioComposites Centre are looking for
cheaper sources and processes. 鈥淲e are looking at a range of starting materials
from plant sources, which could then be chemically modified to give the desired
properties. For example, cashew nut shell liquid gives a phenolic-based polymer,
but it鈥檚 not really biodegradable,鈥� says Hague.

With natural materials like this, the composite could be burned after use
because it would be totally CO₂ neutral. The same goes for matrix
materials derived from lignin and tannins. 鈥淭he CSIRO is developing a new bark
tannin adhesive system for particleboard,鈥� says Coutts. 鈥淢atrix materials are
being prepared by liquefaction of wood by Japanese researchers, and workers in
the US are developing plastics from lignin.鈥� ATO-DLO in the Netherlands is
getting in on the act too, with some biodegradable synthetic plastics. Hague
prefers another option. 鈥淥ur best hope is in using modified polysaccharides out
of plants,鈥� he thinks. 鈥淏ut if you modify a substance, it鈥檚 debatable whether it
is biodegradable any more. It depends on the time scale you are talking
补产辞耻迟.鈥�

Another challenge is to improve the interaction between the matrix and the
fibres. Plant fibres tend to be hydrophilic鈥攚ater loving鈥攚hereas the
matrix materials tend to be hydrophobic鈥攚ater hating. This means that the
two don鈥檛 like mixing. Hill is investigating ways of chemically modifying the
natural fibres to make the two components more compatible. The hope is to double
the resin impregnation rate through the fibres. Altering the fibre surface could
also improve the bond between the fibres and the matrix, giving greater
stiffness and strength.

The hydrophilicity of cellulose comes from its many hydroxyl groups, which
bond to water molecules making the fibres swell by up to 30 per cent of their
dry volume. Other chemical groups can be substituted for the hydroxyls to give
different properties to the fibre or the composite. Cellulose is already widely
modified by acetylation with acetic anhydride in the textiles industry. 鈥淭here鈥檚
room for improvement in a number of properties through acetylation. It makes the
fibres more water-resistant, improves strength when wet, and reduces their
tendency to rot,鈥� says Hill.

Growth industry

Other reactions can be used as stepping stones to greater modification. If
carboxylic acid is introduced to the fibre, for example, it will react with
adipic acid and hexamethylene diamine to give a natural fibre attached to a
nylon matrix via covalent bonds. 鈥淭here are lots of chemical games to play
here,鈥� says Hill. 鈥淲e can also graft on a double bond to the natural fibre, and
react this with other compounds and polymers.鈥�

But Hill stresses that care must be taken to avoid weakening the natural
fibre composites. He suggests a co-polymerisation using butadiene and styrene to
create 鈥渞ubbery鈥� and 鈥済lassy鈥� areas respectively. This would combine high
strength with flexibility and resistance to impacts. 鈥淗opefully the rubbery area
would dissipate a lot of energy by molecular rearrangement,鈥� he says.

Tailor-made composites with these properties could prove valuable in car
parts, marine transport and a host of specialist applications such as sports
equipment. The BioComposites Centre has a spin-off company, J B Plant Fibres,
producing natural fibre materials for a variety of uses. 鈥淚t gives
industrialists the chance to use these materials,鈥� says Hague.

But don鈥檛 expect the vegetable-based, biodegradable car to appear overnight.
Until natural fibres are accepted by the major manufacturing industries, they
won鈥檛 be replacing fibreglass in widespread use. But their potential is clear.
鈥淲e鈥檙e right at the beginning of a new technology,鈥� says Hill.

* * *

Wood for the trees

NATURAL fibres aren鈥檛 just replacing synthetic materials. Composites
reinforced with jute may take the place of wood in packaging and building
applications in south and southeast Asia. An international project funded by the
Common Fund for Commodities, a funding body of the United Nations, is developing
the materials for use in tea chests made in India, and for internal boarding in
houses.

鈥淲e are looking at producing a pre-form pack for making the tea chests using
a composite made from jute fabric and felt and a polymer matrix,鈥� says Neil
Hancox of the British company AEA Technology in Harwell, Oxfordshire, which is
coordinating the technical side of the project with ATO-DLO of the Netherlands.
鈥淚ndia and southeast Asia in general are running out of suitable timber for
plywood to build these chests,鈥� he says. Most of the tea chests, which are used
for packaging tea leaves for export, are built by local carpenters, usually on
site at the plantation. The challenge is to produce a material that the
carpenters can handle in the same way as wood.

鈥淭he primary benefit in terms of the environment would be forest
conservation,鈥� says P. V. Narayanan, director of the Indian Institute of
Packaging. 鈥淎nd wherever jute is used directly, it has the added advantage of
being biodegradable.鈥� Jute may be biodegradable, but the polymer matrix or resin
used in composites is not. 鈥淭here is a wide variety of biodegradable matrix
materials that could be used,鈥� says Hancox, 鈥渋ncluding casein, cellulose
diacetate, and even starch-based resins.鈥�

A starch-based resin would certainly be cheap and easily
biodegradable鈥攖oo easily, perhaps. 鈥淲e made some samples with good
properties,鈥� says Hancox, 鈥渂ut when we soaked them in water they came to
pieces.鈥� He thinks a tea chest could be made using a sandwich-like material with
a cellulose diacetate outer matrix to keep water out. For now though, Hancox鈥檚
team is focused on matrices based on a cheap urea-formaldehyde resin and other
plastics.