Natural-fiber feedstock is an attractive alternative to glass fiber for reinforcing thermoplastic composites.
By John Busch
Composite Products Inc.
Edited by Kenneth Korane
Fibers and fillers are commonly used to boost the strength of virgin plastic. Most engineers are familiar with glass-fiber-reinforced thermoplastic composites, which in many applications have replaced die-cast aluminum, stamped steel, and thermoset composites by virtue of lower material and processing costs, improved functionality, and lighter weight. Now a new material, natural fiber, has emerged to compete with glass as the reinforcement for a composite matrix.
Natural reinforcing fibers come from several plant species including flax, kenaf, hemp, and jute. The plants have a fibrous "bast" outer layer surrounding a lightweight, porous "shive" or core. The bast fibers, when isolated, have been traditionally used to make ropes or cordage.
This new option for long-fiber-thermoplastic (LFT) reinforced composites offers designers an attractive alternative when weighing cost versus performance. Naturalfiber-reinforced composites are not as strong as those with glass fibers, but they cost less. In a multitude of applications they can economically improve performance when conventional unreinforced thermoplastics have reached their limits.
A little history
In a crude sense, bricks made of mud and straw are natural-fiber composites that can be traced to the dawn of civilization. In a polymer or plastic matrix, natural-fiber composites date to the early 1900s when cotton phenolics were widely used as insulating materials.
The most recent "new wave" of natural-fiber composites began, arguably, in 1994, when Mercedes-Benz introduced jute-based door panels in the E-Class vehicles. German automakers continue to lead in the use of these materials. For example, DaimlerChrysler's Global Natural Fibre Initiative benefits Third World nations by developing products made from indigenous agricultural materials.
The use of natural fibers as a filler or reinforcement in plastics is increasing at a healthy rate. According to industry consultant Kline & Co., Little Falls, N.J., the North American market for these materials is expected to grow from $150 million in 2000 to $1.4 billion in 2005.
Of the 400 million lb of natural fiber used in North America in 2000, about 95% was wood flour and fiber used as fillers in thermoplastic decking, building materials, furniture and automotive components. About 3.5% of the total, or 13 to 15 million pounds, was long agricultural fibers.
Currently, the vast majority of natural fiber materials go into extruded profiles for products such as synthetic lumber. A recent report from the U.K. Ministry of Agriculture Fisheries and Food estimates that a typical vehicle could use 33 to 66 lb of natural fiber for interior components. The report lists several advantages of natural-fiber composites for automotive components, including weight reductions of 10 to 30%, good impact performance with minimal splitting, and the ability to form complex-shaped parts in a single machine pass.
Recently, DaimlerChrysler introduced the first exterior component made with natural fibers. Researchers have developed a flax/polypropylene underbody covering for the Mercedes-Benz A-Class. Testing of prototype panels has shown the flax composite has strength and durability comparable to the glass composites they would replace. Flax is a renewable and recyclable resource, but large-scale tests are still needed to show the long-term durability and reliability of flax reinforced composites in interior and exterior applications.
PROS AND CONS
The leading driver for substituting natural fibers for glass is lower material cost. Glass fibers typically sell for around $0.80/lb and have a density of 2.5 gm/cc. Natural bast fibers sell for $0.10 to $0.20/lb and have a density of 1.2 gm/cc. Thus, compared with glass fiber, a dollar's worth of natural fiber volumetrically represents between 8 and 16 times more material.
Renewability and recyclability are also important considerations. Natural fibers are grown as agricultural products and do not consume the energy required to melt and process glass. For disposal, even if burned the plant fibers leave little residue and return no more carbon dioxide to the atmosphere than they removed when grown. Recyclability is especially important in Europe, where the End of Life Vehicles Directive sets recycling targets of 85% (by weight) by 2006 and 95% by 2015. As a result, Europe uses relatively more naturalfiber composites than does North America.
The main physical limitation of bast fibers when compared to glass is heat sensitivity. All plant fibers contain natural sugars and cannot survive temperatures above 400F. Processing temperatures must remain below this range, limiting their use to reinforcement in polypropylene, styrenics, and other low-melting-point resins.
Natural fibers also do not have the strength of glass. Generally, tensile and flexural strength and impact resistance are about half that of comparable glass-filled materials. However, this is a broad generalization. Actual comparisons vary widely depending on materials and processing.
A part made with natural fibers will not be as strong as a glass-fiber composite, but in many applications strength is more than adequate. It is often possible to make the part thicker using natural-fiber composites, compensating for the loss of intrinsic strength while maintaining cost and weight savings.
Another limitation, at present, is that the properties of natural fibers can vary from one batch to the next. Growing conditions affect fiber properties. For instance, attack by mold and variations in moisture content can influence performance of the finished part. There are no generally agreed upon standards for properties or performance of these fiber feedstocks.
Several large suppliers have recently entered this market, which should add a level of confidence in the future of the technology. However, a lot of work still needs to be done to assure consistency and reliability of natural-fiber feedstocks.
Several considerations come into play when developing the material mix and manufacturing process for natural-fiber composite parts. Material selection involves trade-offs between cost and performance. Stronger, stiffer, or tougher materials are generally more expensive than the weaker or flimsier alternatives.
For instance, toughness or impact strength is often of paramount importance with thermoplastic composites. Ingredients of the composite — fibers, thermoplastic resins, and various additives — partly determine impact strength. But the manufacturing process that combines these constituents and converts them into a finished part also contributes to the overall strength of the finished structure. Based on experience with general manufacturing of composite parts, here are some factors to consider to gain the greatest impact strength at the least cost.
Processing: Most composite-manufacturing techniques have three inherent drawbacks. First, they require a "middleman" producer to prepare the feedstock, which can double the price of raw materials. Second, middleman producers limit the number and variety of formulations available to the molder. Finally, manufacturing precompounded feedstock, whether sheets or pellets, exposes the polymer and fibers to heat and mechanical stress during both material compounding and part production. This double exposure harms material properties.
The best way to retain virgin resin properties, fiber length, and uniform fiber distribution throughout the finished part is to preheat the fibers and gently mix the composite feedstock immediately before molding, without an interval for cooling. This manufacturing strategy can be generally described as In-line Compounding and Molding.
Polymer selection: When selecting the polymer for a composite, one important consideration is resin viscosity at process temperature, commonly measured by the melt-flow index (MFI) of the polymer. MFI is directly related to the shear force within the extruder, but is not the only factor determining the amount of shear. Resins with a high MFI subject the fiber to lower shear and minimize fiber breakage. Manufacturers can also change resin viscosity by increasing the extruder temperature, but natural fibers limit this option.
Very-high MFI resins, on the other hand, are more expensive and do not maximize impact strength or stiffness in the finished composite. Choosing the best resin, therefore, is a matter of making trade-offs. In practice, moderately high MFI resins represent the best trade-off within the temperature limits of the fiber.
The polymer also has a dramatic effect on impact strength and other mechanical properties of the finished composite. Unfortunately, there are no steadfast rules for predicting these relationships. Tougher resins may be more viscous, cost more, or may not impart as much rigidity to the composite. Some resins offer superb impact strength, but require additional processing, most notably drying. Some resins cannot be painted. Others will not survive exposure to solvents, seawater, or sunlight. Selecting the best polymer is, to say the least, a complicated affair.
Additives: The optimal composite formulation always involves more than fiber and polymer. There are a host of additives, including colorants, that impart unique and desirable properties to the composite mixture. For instance, when mixing fibers with polypropylene, it is beneficial to add approximately 2% maleated polypropylene coupling agent. This enhances dispersion and mixing of the fibers in the melt and improves impact strength. Other common additives include UV and heat stabilizers, flame-retardants, biocides, and pigments. These do not typically have any effect on the retention of fiber length, except for titanium dioxide, a white pigment that can cause fibers to break.
Additional considerations: The proportion of ingredients has a direct and obvious effect on properties. More fiber correlates with higher strength and stiffness. Reducing fiber content lowers the composite density and increases ductility or elongation at failure. With natural fibers, typical fiber loading is 40%.
Finally, part design has an enormous and direct effect on the final results. A poorly designed part will not provide adequate performance regardless of the material.
In-line compounding and continuous molding
Composite Products Inc. (CPI) recently completed a two-year series of pilot studies to determine the feasibility of natural-fiber composites as an economic and technical alternative to long-glass-fiber thermoplastic composites. These studies used both bast and shive feedstocks and built a useful base of knowledge for improving the design and manufacturing of natural-fiber composites.
During the course of these studies the greatest processing success was with bast fibers about 3⁄4 in. in length, provided by one supplier located in the Midwestern U.S. Parts ranged from 2 to 10 lb, formulated with polypropylene resin and loaded to a level of up to 40% fiber by weight.
From experience with material selection, process conditions, and the properties of a finished part, CPI has developed some simple insights to guide production practices with natural-fiber composites and minimize the shearing force that the LFT composite sees:
- Use low-viscosity, high-MFI resins.
- Keep compounding screw RPM as low as possible.
- Minimize or eliminate back pressure.
- Premelt and homogenize the resin and additives and preheat the reinforcement prior to mixing the constituents.
In addition, the LFT part's design must be such that a high degree of shear is not introduced during molding. However, focusing on elements discussed above constitutes a good start toward building high strength, cost-effective natural-fiber/thermoplastic composites.
One process that limits mechanical and thermal stresses during manufacturing is direct, in-line compounding. CPI's in-line compounding system delivers resin and additives by gravimetric feeders to two extruders. In the first extruder, high shear and mixing action of the rotating screw thoroughly melts and homogenizes the ingredients. Molten resin and additives are conveyed from this first extruder, through transition tooling, to the rotating screw of a second extruder.
Here, a second gravimetric feeder delivers fiber reinforcement. The initial flights of the extruder's screw preheat the fiber prior to mixing with the molten output of the first extruder. Preheating keeps the fiber from chilling and solidifying the resin when these two material streams meet. The proprietary design of the second screw provides gentle mixing of the fiber/resin melt. This minimizes damage to the reinforcing fibers and maintains the initial fiber length in the compounded thermoplastic composite.
The continuously compounded material exits the second extruder and is directed to an accumulator. The temperature-controlled accumulator is an open-ended hydraulic cylinder containing a piston. It alternately accumulates and discharges molten composite material, transferring the material (manually or automatically) into a matched metal mold. The accumulator effectively couples the continuous process of compounding with the discontinuous or cyclic process of molding.