Natural Fibers: The New Fashion In Automotive Plastics

For centuries they have been made into baskets, clothing, sacks, ropes, and rugs. They have even been smoked. Now plant-derived natural fibers of kenaf, hemp, flax, jute, and sisal are making their way into components of cars.

In the last decade, natural-fiber composites of thermoplastics and thermosets have been embraced by European car makers for door panels, seat backs, headliners, package trays, dashboards, and trunk liners. Now the trend has reached North America. Natural fibers have benefited from the perception that they are "green" or eco-friendly. What is proving more important is their ability to provide stiffness enhancement and sound damping at lower cost and density than glass fibers and mineral fillers.

Technology for using natural-fiber composites in interior trim is being cultivated by Tier I and II automotive suppliers, typically in partnership with producers of natural-fiber-based mat materials. Most developmental work is focused on polypropylene-based composites produced by compression molding or thermoforming extruded sheet or commingled mats of PP and plant fibers. However, at least one SRIM-type polyurethane application is commercial.

While still newly sprouted, North American interest in natural fibers is spreading like a garden weed. Even Owens Corning, the leader in glass-fiber reinforcements, is exploring joint-venture opportunities with producers of natural fibers to broaden its composites repertoire in automotive. Here are other signs of spreading activity:

In January, Kafus Bio-Composites signed a joint-development agreement with Visteon Automotive Systems, Dearborn, Mich., to develop natural-fiber automotive composites in North America. Kafus is a subsidiary of Kafus Industries of Vancouver, B.C., the largest North American grower and processor of kenaf, a relative of cotton. Last month, Kafus Bio-Composites was due to start up a new natural-fiber composites production facility in Elkhart, Ind. It will produce nonwoven mats of kenaf and PP, similar to those Kafus already makes in Europe.

Besides Visteon, the new Kafus plant will supply Tier I processor Findlay Industries of Findlay, Ohio. Findlay has been a pioneer in natural-fiber automotive composites on this continent since 1994, when it became the exclusive North American licensee for the LoPreFin process of Germany's R&S Stanztechnik. R&S has been active in introducing natural-fiber composites to the European auto industry. Rob Hayward, Findlay's manager of manufacturing and engineering, says Findlay received a 4-star dynamic side-impact rating for door quarter panels made of a LoPreFin PP/PET/natural-fiber composite. The panels appear on the '99 Saab 9S and are believed to be the first natural-fiber composite to meet this stringent specification.
The success of the Saab door panel reportedly led General Motors to evaluate the same material as a full door-panel substrate for its 2003 model-year smaller passenger cars. LoPreFin is also featured in the package trays and door-panel inserts of GM's 1999 Saturn LS and Opel Vectra. It is also used for the structural headliners of the '99 Mack Vision truck. The headliners are strong enough to hang speakers, dome lights, and curtain rails that are not attached to the roof steel.

Automotive composites giant Cambridge Industries, Madison Heights, Mich., is making flax/PP composites for Freightliner Century COE C-2 heavy trucks. Last month, Cambridge announced that it will also use flax/PP composites for rear-shelf trim panels of the 2000-model Chevrolet Impala. The company's Canandaigua, N.Y., plant makes its own needlepunched nonwoven mats from baled flax straw and PP fibers. The mats are then molded, covered, and trimmed in one step, eliminating adhesives and secondary operations. Says Cambridge CEO Larry Kazanowski, "We expect to see additional applications for flax/PP during the next five years for more rear shelves, door-trim panels, trim bolsters, specialty headliners, luggage trim, and load floors."

Composite Products Inc. (CPI), Winona, Minn., makes composites of PP and flax or kenaf. They're being field tested for door panels, instrument panels, package shelves, and cab back panels for SUVs or pick-ups.

Gaining on wood & glass

Use of natural fibers in plastics has been limited mostly to wood flour. But the long fibers derived from stalks (so-called "bast" fibers) and leaves of other plants are said to provide better mechanical reinforcement in plastics than shorter wood fibers Delphi Interior Systems Div., Troy, Mich. "Natural-fiber materials have better strength-to-weight ratio to replace glass-filled and some unfilled plastics in interior trim," he explains. Staff project engineer David Reed of General Motors Corp.'s Materials & Fastening Engineering Technical Center in Warren, Mich., adds: "Design capabilities, piece costs, tool costs, and part performance are being hotly contested between wood and other natural-fiber composites."

Kevin Kieltyka, manager of natural-fibers development at Visteon, says that his company is looking to replace wood-filled PP with natural fibers. "We get better low-temperature impact and better side-impact results, as shown in commercial door panels in Europe," he says. Visteon aims to use post-industrial and post-consumer recycled PP in these composites, Kieltyka notes. Visteon is working mostly with kenaf and hemp blends and PP or TPO resins. Recycled nylon 66 may also be used. Door inserts, package trays, and trunk liners are being developed for 2000-model Ford cars. Entire door-panel substrates are also in the works.

Whereas wood-fiber composites compete largely with talc- or mica-filled PP in auto parts, natural long-fiber composites are aiming at glass-fiber composites. Lingering health concerns about inhaling glass fibers is contributing to the trend in Europe.

Natural fibers do not confer nearly as much impact resistance as glass fiber, except at very low temperatures. Also, natural fibers are limited to processing temperatures up to about 350 F. Disagreeable odors are emitted by some natural fibers if proper care is not taken in processing.

However, natural fibers have specific gravities of 1.25-1.50 g/cc, versus 2.6 g/cc for glass. That reportedly helps give them a higher strength-to-weight ratio for reinforcing plastics.

"Natural-fiber mats are being made now that give equivalent, if not higher, reinforcement than glass mat," says John Stoll, v.p. of advanced technology for Astechnologies, Inc., Roswell, Ga. "For a bit more thickness, you can make a part reinforced with natural fiber that's as good as one made with glass." In a headliner, he says, you typically need 10-15% more natural fiber than you would glass.

Efforts are afoot to boost properties of natural-fiber composites through improved resin-fiber coupling. "Impact strength can be significantly improved through improved bonding. One potential solution being evaluated is use of maleated PP," says Delphi's Shah.

Leaf and stalk

Leading contenders for wood and glass replacement are bast fibers from flax, hemp, and kenaf. Jute is another candidate. These fibers are carded or air laid and then usually needlepunched into mat form. Also being explored are leaf fibers from sisal as well as abaca and banana leaves. The leaves are stripped of their pulp mass, leaving thousands of monofilaments that are dried and baled for processing into mats.

David Agneta, president of Kafus Bio-Composites, says there is no one perfect fiber for all applications. "By using some blend of fibers, you can achieve the optimal physical properties for a particular application," he notes. For example, he says fibers from kenaf, hemp, and jute are very fine and are easily dispersed. They are more supple and have higher elongation and tensile and flexural strengths than some other natural fibers. This makes them well suited to compression molding of intricate details, tight radii, complex curves, and/or deep draws. In contrast, sisal and banana-leaf fibers tend to be coarser and stiffer and have more memory, making them suitable for flat parts such as headliners but less desirable for complex three-dimensional molding.

Processors are also learning that natural fibers can vary in quality, depending on where they come from and how they are processed. For example, flax fiber from Canada reportedly has a lower aspect ratio (and thus potentially less reinforcing effect) than flax from Sweden.

Low cost is one of the main attractions of natural-fiber reinforcements. According to Dr. John Busch, president of the industry consulting group IBIS Associates, Inc., Wellesley, Mass., base prices for natural fibers range from essentially zero for flax, an agricultural byproduct, to between 3¢ and 25¢/lb for jute and kenaf, respectively, which are grown expressly for their fibers. Refined fibers sell for 15-40¢/lb (around 25¢/lb average) versus 50-75¢/lb for glass fibers.

When natural fibers are converted into mats, the price rises to $1.00-1.50/lb. With higher-volume consumption, Busch expects compression molding mat to eventually sell for less than $1/lb.

New sources

Facilitating the drive toward natural fibers is the emergence of North American suppliers of mats ready for use in molding or forming. "Europe has had natural-fiber mat facilities. Now with Kafus bringing on line the first full-scale commercial facility in North America, it will be a different story," says Visteon's Kieltyka.

At its new Elkhart plant, Kafus Bio-Composites will make its patented kenaf-based Flexform nonwoven mat, plus LoPreFin kenaf-based composites, as part of a strategic alliance with R&S Stanztechnik. Kafus already makes both products in Europe.

Kafus's Flexform mat is 0.25-0.50 in. thick and typically consists of 50% natural fibers (predominantly kenaf) needlepunched together with 50% PP fibers. LoPreFin nonwoven mat is a trilayer structure with a natural-fiber core and needlepunched outer layers of PP fibers with PET fiber woven in.

Here are some of the other players in natural-fiber composites. Most of them have active development programs with auto-parts makers:

Astechnologies installed three mat-making lines in August at its new plant in Brussels, Belgium, and is now putting similar equipment in its Jasper, Ga., site. It uses hemp, kenaf, flax, sisal, and abaca. "We can make mat that is 100% natural fiber or blend it with PP fiber in a 50/50 ratio," says John Stoll.

Cargill Ltd. makes Durafibre from the bast of the flax plant. The cleaned, baled fibers can be melt compounded or made into mats (as at Cambridge Industries). Cargill also makes fine-particle-size Durafill from the shive (inner core) of flax. Durafill is suitable for compounding into thermoset BMC or SMC, as well as thermoplastics for extrusion or molding. CPI is partial to the shive product: "While bast fiber provides a bit better tensile strength, shive has lower density," says CPI application design engineer Charles Weber.

Dexter Corp.'s Nonwoven Materials Div. makes sisal-based nonwoven mats in roll form for compression molding or thermoforming. A wet-form nonwoven process similar to papermaking is used for these mats, which also contain some wood pulp, polyester fiber, and a bit of EVA binder.

Georgia Composites' developmental product is made of partially consolidated recycled PP reinforced with sisal. It is produced on a double-belt laminating press. "We have thermoformed headliners that match the performance of glass," says general manager David Holty. He adds that the sisal/PP composite can be used to boost the performance of "Woodstock"-type materials (50/50 PP and wood flour). When the two are compression molded together in a laminate, the sisal/PP significantly increases tensile modulus and strength for applications such as door-panel inserts and trunk liners, Holty claims.

Global Resource Technologies has compounded virgin and recycled PP and HDPE with kenaf, jute, hemp, sisal, flax, coconut fiber, and wood pulp since 1996. Global does its own fiber processing, has molding capabilities to 1200 tons, and supplies compounded pellets for extrusion, injection, or compression molding. Technical manager Collin Felton says these chopped-fiber compounds can incorporate well over 50% natural fiber and achieve flexural moduli over 1 million psi, compared with 200,000 psi provided by nonwoven mats.

Kenex Hemp Ltd. has installed a mat-making line, which will start up in late fall. It will produce mats from hemp or blends of hemp with flax or jute and virgin or recycled PE or PP.

Pinnacle Technology teamed up with the U.S. Dept. of Agriculture's Forest Products Laboratory to develop and commercialize an "agro-plastic" manufacturing process using abundant wheat straw. The product is ground to a fine powder with an aspect ratio near 1. President Donna Johnson says the company is raising capital for a demonstration plant and will license its technology. Pinnacle has compounded 50% wheat straw with PP or HDPE for injection molding and extrusion. Wheat straw reportedly provides much lower density and significantly higher tensile and flexural strengths, flex modulus, and HDT than mineral-filled polyolefins.

How they are processed

Several approaches are being used to make natural-fiber composites. In the case of nonwoven mats combining PP fibers and natural fibers in a needlepunched sandwich, the mat is heated in an oven until soft and then transferred to a cold press where it is compression molded. R&S says its low-pressure compression molding process produces kenaf composite parts in less than half the time it takes to injection mold other materials.

Delphi's Shah says, "Natural fibers cannot be used for a Osurface look.' They must be laminated to something." So his team is exploring two key technologies: One is low-pressure compression molding in which a preheated natural-fiber/PP mat is in-mold laminated with a TPO coverstock. Shah is not revealing details of the second low-pressure process, which is still in development, other than to hint that it involves lower capital-equipment costs. Both technologies are being considered for applications like instrument-panel topper pads or door-trim panels.

CPI offers to license its patented process for in-line compounding and compression molding of glass- or natural-fiber composites. CPI is working with PP composites of flax from Durafibre or kenaf from Kafus. "We take the raw fibers and resin and compound them to create a bulk molding preform, which is then placed into a mold," says Weber. CPI has data showing that its flax-reinforced PP can outperform mineral-filled PP in tensile strength and notched Izod impact.

C.A. Lawton Co. recently began offering a somewhat similar Extrusion Compression Molding process that can compound and mold natural fibers in-line.

Natural fibers are also being used in polyurethane composites. The first commercial example is the inner door panel for the 1999 S-Class Mercedes-Benz, made in Germany of 35% Baypreg F semi-rigid PUR elastomer from Bayer and 65% of a blend of flax, hemp, and sisal. The 2-mm-thick door panel is made by the new NafpurTec process from Bayer's Hennecke Machinery Unit, whereby a robot places natural-fiber mat in an open mold and second robot pours PUR over it before the mold is closed. The process will be used to make a sunroof cover for a European 2000-model car.

Krauss-Maffei's new NFI process and Cannon's InterWet technology both introduce natural fibers directly into the mix head, eliminating the need for a mat.

Still another approach is being used in headliners for the German '99 Opel Zefira minivan. A rigid PUR foam core is sandwiched between two sheets of Dexter's sisal mat. Then the sandwich is compression molded with a PUR adhesive to bond the layers.