by George Marsh

The preservation of our environment requires that we

stop developing materials that will, like many plastics,

last indefinitely. Yet nature’s way, to accept rapid

degradation because there is continual renewal, is not

an option. Industry, especially the automotive sector,

which is an enormous user of bulk materials, would

like a halfway house of reasonably long-lived

materials that nevertheless degrade back into the

environment when they are no longer needed.

Reinforced plastics based on natural, mainly plant-

derived substances show promise of providing this

and may turn out to be one of the material

revolutions of this century.

The automotive industry is in the driving seat of

‘green’ composites because it is here that the need is

greatest. Faced with pressures to produce fuel-

efficient, low-polluting vehicles, the industry has

used fiber reinforced plastic composites to make its

products lighter. But producing the composites is

energy intensive and polluting, while the durability of

conventional composites, often seen as an advantage,

is also their Achilles’ heel. Glass, carbon, and aramid

fiber reinforced polyester, epoxy, and other similar

resins are difficult to recycle and hard to dispose of.

They do not degrade naturally and could linger for

generations.

Use of thermoplastics offers some relief, as these resins

can be thermally recycled to produce new products. But for a

more sustainable future and to meet growing regulatory

pressures – of which the most pressing is the European

Union’s end-of-life of vehicles (ELV) directive requiring that,

by 2015, all new vehicles should be 95% recyclable – a more

complete solution is needed. From present indications, that

could turn out to be ‘green’ composites based on fibers and

resins derived from plants.

Natural fibers come insideA logical starting point is to take recyclable thermoplastic

resins (polypropylene or PP, polyolefin, polyethylene,

polyurethane, and polyamide are some of those already used

in vehicles), and combine them with biodegradable plant-

based fibers. Natural fibers have the potential to reduce

vehicle weight (up to 40% compared with glass fiber, which

Next step forautomotive materials

April 200336 ISSN:1369 7021 © Elsevier Science Ltd 2003

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accounts for the majority of automotive composites), while

satisfying increasingly stringent environmental criteria.

Much less energy is used in growing, harvesting, and

preparing natural fibers than in producing glass fiber. The

energy of plant fibers has been estimated as some 4 GJ/t,

compared with around 30 GJ/t for glass fiber, which has to

be drawn from a melt at several hundred degrees Celsius,

using raw materials obtained through energy-intensive

mining.

Production of glass (or carbon, aramid, etc.) fibers releases

CO2 into the atmosphere, along with NOx and SOx gases

and dust, which can be a health hazard. Dust and fragments

are generated when recycling conventional plastic

composites by grinding them down, and remain an issue

during disposal either to landfill or by incineration. In

contrast, the use of natural fibers can minimize harmful

pollutants, and their eventual breakdown is environmentally

benign. The environmental impacts that remain can be

reduced by choosing crops and farming methods that

economize on fuel, fertilizer, and pesticide, together with

efficient extraction and treatment systems. Natural fibers

emit less CO2 when they break down than is absorbed

during plant growth. They are nonirritating and nonabrasive,

and do not blunt manufacturing tools or processing

equipment. Fiber-producing crops (Fig. 1) are easy to grow

and could take up marginally used agricultural capacity in

developed countries.

Nor are the benefits of natural fibers just environmental.

Potential physical advantages are illustrated by some work

carried out by the UK’s Loughborough University on hemp

fiber reinforcement of phenolic resin1. Phenolics are used in

transport applications requiring fire resistance. By introducing

a two-layer nonwoven hemp mat into the resin, researchers

at the university’s Institute of Polymer Technology and

Materials Engineering more than doubled panel flexural

strength (from 11 MPa to 25 MPa) and improved stiffness by

23%. Impact resistance of unreinforced phenolic, which tends

to be brittle, was markedly improved by the hemp

reinforcement since the fibers help dissipate impact forces

into the matrix. A ductility index improvement from 3.77 to

2.58 also emphasized the rise in toughness. The introduction

of the hemp mat also reduced the number and sizes of voids

formed in the composite during the cure of the

thermosetting resin because the naturally hydrophilic fibers

absorb moisture produced by the cure reaction.

Generations of seamen prized thetensile strength of coir, sisal, flax,jute, kapok, and other naturalfibers in ropes and sails.

Such enhancement seems less surprising when one recalls

the generations of seamen who prized the tensile strengths of

coir, sisal, flax, jute, kapok, and other natural fibers in the

ropes and sails of their vessels. Automotive manufacturers,

too, have utilized these properties for years in interior mats,

felts, and textiles. Moreover, natural fiber reinforced plastics

(NFRPs) have been in production vehicles for almost a

decade, Mercedes Benz having set the precedent in 1994 by

using jute reinforced plastic for the interior door panels of its

E-Class vehicles (Fig. 2). Jute, like hemp, grows well in Europe

and is one of several agricultural crops that has a particularly

fibrous bast, or outer sheath to the stems – analogous to tree

bark. Because the long, strong bast fibers lie somewhere

between woodstocks and E-glass (the most commonly used

form of glass fiber) in terms of the mechanical properties,

they can substitute for either.

Glass fiber substitution, especially for car interior items

like door panels, parcel shelves, and headliners where

conventional composites represent over-engineered solutions,

offers a promising way forward. Vehicle manufacturers and

April 2003 37

Fig. 1 Hemp and wheat crops. Usable stem material comprises fibers made up of cellulose cells bound together with pectin and lignin. (Courtesy of The Eden Project.)

their suppliers who have adopted NFRPs have noted that, in

addition to their high strength and stiffness per weight

(Table 1) and environmental virtues, the materials have other

benefits too. These include acoustic insulation, easier health

and safety management, rapid production by compression or

injection molding, and potentially lower cost.

The fibers cannot be used in their natural state, however.

Basic cellulose fibers must be separated out from the pectin

resin that connects them to the woody core of the stem by

dew retting. Hemicellulose, which accounts for much of the

moisture absorption, and lignin, which connects individual

fiber cells, are then removed by hydrothermolysis or alkali

extraction. An alternative to retting, which also removes

some of the hemicellulose and lignin from green harvested

flax, is the ‘Duralin’ method developed by Ceres BV in the

Netherlands. Duralin fibers produced when flax straw is

steamed, dried, and cured are more moisture resistant and

durable than untreated fibers, as well as partially separated.

Another fiber separation method is steam explosion, used

after traditional dew retting. This also expands the fibers,

giving them a bigger surface area for bonding with the

matrix. Separated fibers usually need drying first, however, to

about 2-3% moisture level.

Fibers for higher grade applications require a surface

modification treatment, such as acetylization2, to enhance

adhesion with the thermoplastic. Alternatively, if the resin is

the widely favored PP, fibers can be modified with maleic

anhydride-treated polypropylene molecules (MAPP). Even a

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April 200338

Table 1 Comparison of properties of various natural and synthetic fibres. (Source: Qinetiq.)

Fiber Specific Tensile Specific Tensile Specific Costgravity strength strength modulus modulus ratiog.cm-3 GPa GPa/g.cm-3 GPa GPa/g.cm-3

Spruce 0.60 0.98-1.77 1.63-2.95 10-80 17-133 1pulpSisal 1.20 0.08-0.50 0.07-0.42 3-98 3-82 1Flax 1.20 2.00 1.60 85 71 1.5E-glass 2.60 3.50 1.35 72 28 3Kevlar 49 1.44 3.90 2.71 131 91 18Carbon 1.75 3.00 1.71 235 134 30(standard)

Fig. 2 Flax/polypropylene underbody components have replaced glass fiber reinforced plastic components in vehicles such as the Mercedes Benz A-Class. (Courtesy of Mercedes Benz.)

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tiny percentage of MAPP in water greatly strengthens the

resulting composite. After the fiber is brought together with

the thermoplastic, the resin may need degassing to expel any

air introduced along with the fibers. Consolidated material

can be made into NFRP mats, woven fabric, film, prepregs

(fiber forms pre-impregnated with resin, which are then

partially cured), and other material forms suitable for

fabrication.

NFRPs are already well established in Europe, which is

ahead of North America in the development and adoption of

biocomposites. Alain Coquet, product marketing manager for

Visteon Automotive Systems, a supplier that compression

molds thousands of NFRP components every year for Ford,

Citroën, and other OEMs, estimates this lead to be eight

years. “The European [NFRP] market is largely flax-driven,”

explains Coquet. “It’s an increasingly industrial product and

there are growers who can deliver fibers of consistent quality

in the volumes we want. The crop is very ‘green’, grown with

minimal use of chemicals or pesticides, and produces good

fibers. Flax/PP is recyclable, and we can use 100% ground

recyclate in new injection molded components. Costs of

finished components compare with those of glass fiber

reinforced plastic equivalents.”

Visteon has, with partner Technilin, developed its own

flax/PP material based on a low-cost fiber. Meeting a ‘very

high specification’ from Opel, including critical safety

requirements, the R-Flax® material can be used for interior

items such as door panels, where its aesthetic qualities can

even add to consumer appeal. Resistant to scratching and

ultraviolet degradation, R-Flax requires no finishing

treatment and is available in six basic molded-in colors and

up to 150 shades. Visteon expects that the material,

validated for two years and now ready for production, will

capture a significant share of the market for stylish interior

components.

Going structuralBut Coquet, along with many motor industry colleagues,

anticipates that NFRPs will not be limited to nonstructural

roles in vehicle interiors for long. He believes that these

materials, which are already comparable to para-aramids for

strength and can potentially reduce the weight of automotive

composites by 40%, must have structural applications as

well. Despite current major improvements in glass fiber, he is

confident that NFRPs, which are early in their evolutionary

cycle and have great scope for further development, will one

day offer equal mechanical properties.

So far, the disadvantages of natural fiber composites have

prevented this. As far back as 1935, researchers hoping to

replace steel in automobile bodies with paper, wood chips, or

other natural fiber reinforced phenolic resin materials found

that these composites were not strong enough. The German

Trabant car utilizing such materials ultimately proved

unsuccessful. The impact strength of NFRPs is particularly

poor, as is their fire resistance. Unmodified fibers are easily

damaged and weakened during handling or processing.

Composite quality can be marred by poor fiber-matrix

coupling because naturally hydrophilic fibers do not bond

well with thermoplastics and other resins. Low thermal

tolerance rules out certain manufacturing processes normally

used with composites. Fibers degrade too readily, something

that can occur during compounding and molding as well as in

service. When they break down, the material may smell

unpleasant. Rotting is accelerated by the fibers’ tendency to

attract moisture, which causes them to swell.

Agricultural and commercial barriers to establishing a

viable supply chain also have to be overcome. In particular,

price, fiber characteristics, and quality may vary substantially,

depending on cultivation conditions and agricultural policies.

Motor industry experts anticipatethat natural fiber reinforcedplastics will not be limited tononstructural roles in vehicleinteriors for long.

Coquet, however, says that all these issues can be

addressed. For example, a process adapted from the textile

industry and used by Visteon to ‘white’ or degrease the fibers

claims to avoid the problems of moisture uptake, odor, and

fiber wetting. The company’s use of a needling system to

create the mat, rather than stitching or weaving, enhances

the material’s stiffness. Furthermore, Coquet and other

industry insiders hold great hopes for current research

initiatives aimed at improving fiber processing characteristics

and durability.

Visteon is a partner in one of these, the collaborative

Biomat project funded by the UK’s Department for

April 2003 39

Environment, Food, and Rural Affairs (DEFRA), which aims to

enhance the performance of plant fibers for use in injection

moldable thermoplastic composites. The project is led by

Robert West at Qinetiq, previously the UK’s Defence

Evaluation and Research Agency (DERA), one of Europe’s

largest science and technology solutions providers. West is

positive that NFRPs will make the transition from ‘low-grade’

nonstructural applications into fully structural components.

“The search for durable, ecologically-sound materials is

prominent,” he says, “and we hope to see early adoption of a

technology that could support the automotive industry in its

efforts to meet green goals.”

During the four-year program, which officially commenced

in December last year, researchers will investigate a class of

molecules developed by Qinetiq that appear to be superior to

MAPP and other fiber-matrix coupling agents. They will

explore promising compatibilizers based on novel silane

chemistries. Silanes could improve durability, it is suggested,

by promoting direct C-Si bonding rather than the usual, more

hydrolyzable C-O-Si bonds. Researchers will also experiment

with ultrasonic means to separate fibers from pectin and

lignin resins.

Other Biomat partners include the BioComposites Centre

at the University of Wales, Bangor; injection molder Birkbys

Plastics Ltd; design specialists Engenuity Ltd and Premier

Engineering Solutions Ltd; hemp grower Hemcore Ltd and

flax grower BioFiber Ltd; and AEI Compounds Ltd. Project

teams will assess various forms of flax and hemp fiber, as

well as coppiced willow processed by the BioComposites

Centre. The latter organization will, with AEI Compounds,

evaluate material properties and develop processes, while

Qinetiq and Birkbys Plastics will manage the injection

molding studies. Engenuity and Premier Engineering Solutions

will contribute design studies and stress analyses. Visteon

will be heavily involved, particularly during the later stages of

the program.

Task groups will explore ‘gentle’ processes, such as rubber

milling, for consolidating fibers into matrix resins without

damaging them. Methods for blending the material phases

ready for injection molding, including roll mill, co-kneader,

and twin-screw contra-rotating compounders, will be

compared. Fiber qualities at every stage from cultivation

through fiber extraction and treatment to component

manufacture will be evaluated. An important deliverable will

be an integrated set of mechanical property, fire resistance,

water uptake, and durability parameters that will enable

users to have high confidence in the behavior of these

materials. Towards the end of the project, knowledge gained

will be utilized in the manufacture of a large demonstration

structural component, which will then be subjected to

running trials in a Ford production car (Fig. 3).

Aspects of NFRP processing are a major research focus

elsewhere. For example, the Centre of Lightweight Structures

at the Technische Universiteit Delft has sought to adapt glass

fiber reinforced plastic processing techniques for use with

natural fibers3,4. One consideration is how to subdue ‘springy’

natural fibers when constructing the preforms for subsequent

use in fabrication by resin transfer molding, vacuum infusion,

vacuum pressing, and similar processes. New binders have

been developed for this purpose. Because of their tendency to

stick together, natural fibers are harder to chop and scatter

onto resin film than glass fibers when preparing prepreg

materials so, once again, existing methods have to be

modified. Researchers at the center have compared

properties of natural fiber sheet molding compound (SMC)

with widely used glass-based SMCs (Table 2). Results are

encouraging when long fibers are used, but impact strength

remains a point of vulnerability. The research has been

carried out under the Dutch Biolicht R&D program, which has

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April 200340

Fig.3 The Model U Ford hybrid-electric car makes extensive use of recyclable composites.Corn-based materials are used in the interior roof fabric and floor matting, while soy andcorn-derived resins replace carbon black in the tires. The synthetic polyester used to coverseats and door panels can also be recycled back to an identical polyester. (Courtesy of Ford Motor Company.)

also resulted in experimental fabrication of semistructural

parts such as a ventilator housing made from SMC containing

21% by volume of flax.

Much research is focused on interfacial properties. In the

recent FLAXComp project financed by the Flemish

Government in Belgium, a combined treatment of fibers with

alkali and diluted resin improved adhesion between fibers and

epoxy thermoset (in this case) to the extent that interlaminar

shear strength was doubled5. This resulted in 250% and

500% improvements in composite strength and modulus,

respectively, in the transverse direction, while longitudinally

strength increased by 40% and the modulus by 60%. Isabel

Van de Weyenberg, principal researcher in the Composite

Materials Group at the Katholieke Universiteit Leuven,

concluded that natural fibers, despite their limitations, have a

bright future in composites if present research momentum is

maintained.

Mark Hughes from the BioComposites Centre is also

confident about the structural possibilities of NFRPs,

especially those reinforced with long fibers. “Their extremely

low weight, with high specific strength and stiffness, will win

out,” he says. “Their Young’s modulus values can compare

with those of glass, strength is adequate for many

applications, and their low conductivity can be an advantage.

We have shown over the last several years that properties

acceptable for semistructural applications, where perhaps

impact strength is not so important, can be delivered

already. Natural fibers offer their own specific technical

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April 2003 41

Table 2 Comparison of properties of sheet molding compounds produced from glass (two different volume fractions) and natural fiber (two different fiberdiameters). (Source: Centre of Lightweight Structures.)

SMC Glass SMC Glass SMC Flax SMC Flax SMC20% wt. cont. 40% wt. cont. 21% wt. cont. 21% wt. cont.(Vf = 15%) (Vf = 31%) (Vf = 22%) (Vf = 22%)

fibers 6.25 mm fibers 25 mm

E-modulus (GPa) 8.5 10.5 7 11

Tensile strength (MPa) 95 130 40 80

Flexural modulus (GPa) 10 13.5 7 13

Flexural strength (MPa) 125 240 83 144

Impact strength (KJ/m2) 50 85 11 22

Fig. 4 Half fringe photoelasticity (HFP), a form of quantitative birefringence analysis, is one technique used by the BioComposites Centre at the University of Wales, Bangor, tonondestructively investigate effects of fiber damage on the interfacial behavior of fiber-reinforced composites. (a) Shows the localized birefringence pattern, under plane polarized light,seen in the epoxy matrix of a strained single filament hemp fiber composite where fiber fracture has occurred. (b) Shows the birefringence pattern observed in a similar composite during afragmentation test. (Courtesy of BioComposites Centre, University of Wales.)

properties; they are not simply a cheap alternative to

glass.”

Admitting that low fracture toughness remains a

weakness, Hughes says the BioComposites Centre is involved

in the drive to develop physical, chemical, and morphological

modifications of fibers to improve their synergy with matrix

resins6. Surface chemistries can, he asserts, be manipulated

to add binding or other functionalities. The center

collaborates with Warwick University’s Manufacturing Group

in the promotion of sustainable materials. Both are members

of the Sustainable Composites Network, set up two years ago

to bring together all parties in the supply/use chain, from

growers through processors and research bodies to vehicle

manufacturers, in a common forum. Among the avenues

being explored within the network are the possibilities for

plant-derived resins.

Bioresins tooIn the move towards biocomposites, the greatest attention

has been paid to fibers since these contribute most of a

composite’s stiffness and strength. But to meet

environmental aims fully, matrix resins will need to be

bioderived too. Significant developments are taking place in

this arena too.

Well ahead of the field is agricultural machinery giant John

Deere and Co., who last year introduced a side panel based

on a new bioresin for the Deere 50-Series hay baler. The

factory’s entire line of hay balers now includes styling panels

and cab roofs made with HarvestForm™ – a durable

composite that comprises soybean and corn-based polymer

resins (Fig. 5). Deere claims that the corn/soy combination

brings strength, flexibility, corrosion resistance, and

endurance to the panels, which weigh 25% less than steel.

HarvestForm utilizes a polyurethane-type resin developed

by Urethane Soy Systems Corporation (USSC). As Tom Kurth,

USCC’s president, explains, “SoyOyl™ is made from soybean

oil, which is a natural replacement for petroleum oil. End

products made with these oils have virtually the same

characteristics, and are equal in performance. The biggest

difference is that SoyOyl products can be produced for less

than standard petroleum-based products.”

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April 200342

Fig. 6 These modern door inner trim panels are molded using mats of 60% natural fiber in a Baypreg® polyurethane resin. (Courtesy of Bayer Polymers.)

Fig.5 Side panels on John Deere hay balers incorporate polyurethane resins derived fromcorn and soy beans. (Courtesy of John Deere and Co.)

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April 2003 43

REFERENCES

1. Richardson, M., and Zhang, Z., Nonwoven Hemp Reinforced Composites.Reinforced Plastics, (April 2001)

2. Hill, C. A. S., et al., Industrial Crops and Products (2000) 88 (1), 53

3. Centre of Lightweight Structures, www.clc.tno.nl

4. Pott, G. T., et al., Upgraded natural fibres for polymer composites. In: Euromat97 (1997) 22, 107

5. Department of Metallurgy and Materials Engineering, Katholieke UniversiteitLeuven, Belgium, www.mtm.kuleuven.ac.be

6. Hughes, M., et al., Composite Interfaces (2000), 77 (1), 13

7. Technical Report CCM 01-01, www-test.ccm.udel.edu/research/acres

FURTHER READING

i. Sebe, G., et al., RTM hemp fibre-reinforced composite automotive components.Presented at: Automotive Components Workshop, Brands Hatch, UK, (1998)

ii. The Textile Consultancy. The use of natural fibres in nonwoven structures forapplications as automotive component substrates, MAFF, UK, 2000

Deere tested the new material extensively to prove

durability and performance. A prototype panel fabricated by

Contemporary Products, Inc. of Milwaukee, weighing 11 kg

and measuring 2.4 x 0.9 m, was structurally comparable to a

metal version (albeit thicker) and could be produced by resin

transfer molding at considerably lower cost than a standard

pressed and machined metal equivalent. Although current

matrix products contain preservatives, future bioresins could

be engineered to degrade in the presence of certain triggers,

to meet 21st century requirements for controlled

degradability.

The creation of low- and high-performance polyurethanes

from soy has benefited from research carried out at the

University of Delaware under the Affordable Composites

from Renewable Sources (ACRES) program7. The ACRES

program, a multidisciplinary effort encompassing genetic

engineering and composites manufacturing science under the

direction of Richard Wool, is pursuing chemical techniques to

enhance the structures of soy-based liquid molding.

Moreover, although the composite panels produced by Deere

currently utilize glass fiber reinforcement rather than natural

fiber, ACRES researchers have produced full biocomposites

incorporating natural fibers such as flax, hemp, and even

chicken feathers.

In late 2001, the US Department of Energy awarded an

$11 million grant (over four years) to the ACRES program

under the umbrella of the Affordable Resins and Adhesives

from Optimized Soybean Varieties (ARA) program. The ARA

mission is to promote the widespread use of composites,

resins, and adhesives made from renewable resources.

Researchers are developing low-cost resins and adhesives

from soy; studying the structure functionality of soy oil and

proteins at molecular and genomic levels; and working to

identify key structures and DNA markers that can be used to

develop suitable soybean varieties both in terms of

performance and processing. Results are expected to benefit

several sectors ranging from automotive to hurricane-

resistant housing. As well as the University of Delaware,

research partners include Kansas State University, the

National Germplasm Resources Laboratory (US Department

of Agriculture), Sandia National Laboratory, and the United

Soybean Board. Industrial partners include Ashland Inc., Cara

Plastics, Inc., and North Central Kansas Processors.

“The most environmentally friendlything you can do for a car thatburns gasoline is to make lighterbodies” (Henry Ford)

In presentations that Wool, ACRES’ director, gives to

interested parties, he shows a 1938 photograph of Henry

Ford demonstrating the resilience of a fiberglass car body by

taking an axe to it. The resin used in the composite was soy-

based. Ford, believing that “the most environmentally

friendly thing you can do for a car that burns gasoline is to

make lighter bodies”, had hoped to shift from steel to lower-

weight materials. He had even targeted biocomposites, but

progress was halted by World War II.

Now, at last, Ford’s dream of fully recyclable vehicle

structures constructed from biodegradable plant-derived

materials could be coming true. MT