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Next step for automotive materials
Next step for
automotive materials 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 plantderived 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 fuelefficient, 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 inside A 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 plantbased fibers. Natural fibers have the potential to reduce vehicle weight (up to 40% compared with glass fiber, which
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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.
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.)
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 the tensile strength of coir, sisal, flax, jute, kapok, and other natural fibers 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
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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.)
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
Table 1 Comparison of properties of various natural and synthetic fibres. (Source: Qinetiq.)
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Fiber
Specific gravity g.cm-3
Tensile strength GPa
Specific strength GPa/g.cm-3
Tensile modulus GPa
Specific modulus GPa/g.cm-3
Cost ratio
Spruce pulp Sisal Flax E-glass Kevlar 49 Carbon (standard)
0.60
0.98-1.77
1.63-2.95
10-80
17-133
1
1.20 1.20 2.60 1.44 1.75
0.08-0.50 2.00 3.50 3.90 3.00
0.07-0.42 1.60 1.35 2.71 1.71
3-98 85 72 131 235
3-82 71 28 91 134
1 1.5 3 18 30
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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 structural But 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 anticipate that natural fiber reinforced plastics will not be limited to nonstructural roles in vehicle interiors 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
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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
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 and corn-derived resins replace carbon black in the tires. The synthetic polyester used to cover seats and door panels can also be recycled back to an identical polyester. (Courtesy of Ford Motor Company.)
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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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Table 2 Comparison of properties of sheet molding compounds produced from glass (two different volume fractions) and natural fiber (two different fiber diameters). (Source: Centre of Lightweight Structures.)
SMC
Glass SMC 20% wt. cont. (Vf = 15%)
Glass SMC 40% wt. cont. (Vf = 31%)
Flax SMC 21% wt. cont. (Vf = 22%) fibers 6.25 mm
Flax SMC 21% wt. cont. (Vf = 22%) 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
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
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, to nondestructively 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 a fragmentation test. (Courtesy of BioComposites Centre, University of Wales.)
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manufacturers, in a common forum. Among the avenues being explored within the network are the possibilities for plant-derived resins.
Bioresins too
Fig.5 Side panels on John Deere hay balers incorporate polyurethane resins derived from corn and soy beans. (Courtesy of John Deere and Co.)
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
In 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.”
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.)
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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 hurricaneresistant 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 friendly thing you can do for a car that burns gasoline is to make lighter bodies” (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 soybased. 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 lowerweight 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
REFERENCES
FURTHER READING
1. Richardson, M., and Zhang, Z., Nonwoven Hemp Reinforced Composites. Reinforced Plastics, (April 2001)
i.
2. Hill, C. A. S., et al., Industrial Crops and Products (2000) 8 (1), 53
ii. The Textile Consultancy. The use of natural fibres in nonwoven structures for applications as automotive component substrates, MAFF, UK, 2000
3. Centre of Lightweight Structures, www.clc.tno.nl
Sebe, G., et al., RTM hemp fibre-reinforced composite automotive components. Presented at: Automotive Components Workshop, Brands Hatch, UK, (1998)
4. Pott, G. T., et al., Upgraded natural fibres for polymer composites. In: Euromat 97 (1997) 2, 107 5. Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Belgium, www.mtm.kuleuven.ac.be 6. Hughes, M., et al., Composite Interfaces (2000), 7 (1), 13 7. Technical Report CCM 01-01, www-test.ccm.udel.edu/research/acres
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automotive materials 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 plantderived 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 fuelefficient, 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 inside A 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 plantbased fibers. Natural fibers have the potential to reduce vehicle weight (up to 40% compared with glass fiber, which
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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.
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.)
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 the tensile strength of coir, sisal, flax, jute, kapok, and other natural fibers 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
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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.)
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
Table 1 Comparison of properties of various natural and synthetic fibres. (Source: Qinetiq.)
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Fiber
Specific gravity g.cm-3
Tensile strength GPa
Specific strength GPa/g.cm-3
Tensile modulus GPa
Specific modulus GPa/g.cm-3
Cost ratio
Spruce pulp Sisal Flax E-glass Kevlar 49 Carbon (standard)
0.60
0.98-1.77
1.63-2.95
10-80
17-133
1
1.20 1.20 2.60 1.44 1.75
0.08-0.50 2.00 3.50 3.90 3.00
0.07-0.42 1.60 1.35 2.71 1.71
3-98 85 72 131 235
3-82 71 28 91 134
1 1.5 3 18 30
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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 structural But 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 anticipate that natural fiber reinforced plastics will not be limited to nonstructural roles in vehicle interiors 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
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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
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 and corn-derived resins replace carbon black in the tires. The synthetic polyester used to cover seats and door panels can also be recycled back to an identical polyester. (Courtesy of Ford Motor Company.)
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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
APPLICATIONS FEATURE
Table 2 Comparison of properties of sheet molding compounds produced from glass (two different volume fractions) and natural fiber (two different fiber diameters). (Source: Centre of Lightweight Structures.)
SMC
Glass SMC 20% wt. cont. (Vf = 15%)
Glass SMC 40% wt. cont. (Vf = 31%)
Flax SMC 21% wt. cont. (Vf = 22%) fibers 6.25 mm
Flax SMC 21% wt. cont. (Vf = 22%) 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
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
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, to nondestructively 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 a fragmentation test. (Courtesy of BioComposites Centre, University of Wales.)
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manufacturers, in a common forum. Among the avenues being explored within the network are the possibilities for plant-derived resins.
Bioresins too
Fig.5 Side panels on John Deere hay balers incorporate polyurethane resins derived from corn and soy beans. (Courtesy of John Deere and Co.)
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
In 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.”
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.)
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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 hurricaneresistant 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 friendly thing you can do for a car that burns gasoline is to make lighter bodies” (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 soybased. 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 lowerweight 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
REFERENCES
FURTHER READING
1. Richardson, M., and Zhang, Z., Nonwoven Hemp Reinforced Composites. Reinforced Plastics, (April 2001)
i.
2. Hill, C. A. S., et al., Industrial Crops and Products (2000) 8 (1), 53
ii. The Textile Consultancy. The use of natural fibres in nonwoven structures for applications as automotive component substrates, MAFF, UK, 2000
3. Centre of Lightweight Structures, www.clc.tno.nl
Sebe, G., et al., RTM hemp fibre-reinforced composite automotive components. Presented at: Automotive Components Workshop, Brands Hatch, UK, (1998)
4. Pott, G. T., et al., Upgraded natural fibres for polymer composites. In: Euromat 97 (1997) 2, 107 5. Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Belgium, www.mtm.kuleuven.ac.be 6. Hughes, M., et al., Composite Interfaces (2000), 7 (1), 13 7. Technical Report CCM 01-01, www-test.ccm.udel.edu/research/acres
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