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Filling and reinforcing with natural fibers.

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Efforts to increase resin performance and reduced raw-material costs are leading researchers and resin producers to evaluate reinforcements and fillers derived from natural fibers and other biomass. Tests show that these perform as well as or better than conventional materials in virgin and recycled polymers.

Commercializing natural fibers and fillers would yield benefits in supply, cost, and environmental impact. They are abundant, and in many cases they can be culled from waste that would otherwise be discarded. They are cheap, usually costing less than synthetic materials. And they are renewable.

Research shows that some natural fibers and fillers have unique properties that enhance polymer performance. Many are easily modified to improve their interaction with resins. Since they aren't as abrasive as materials like glass and talc, the fibers and fillers would reduce wear on machinery and components during processing.

Proponents say natural fibers also lessen the environmental impact of plastics derived from petrochemicals, since they capture carbon dioxide during photosynthesis and thus act as "green offsets."

Capitalizing on the biodiversity of Latin America, research centers are studying a range of indigenous fibers and fillers. Work is focusing on increasing the polymer compatibility of natural fibers and evaluating their mechanical and processing properties. The goal is to create new products and application opportunities.

The potential market for natural fibers and fillers is enormous. A study by the Syndicate of Vegetable Fibers in the state of Bahia, Brazil, for example, says that the automotive industry in that country alone uses up to 40,000 tons of fiber per year, almost 20 kg (44 lb) per vehicle. The amount of natural fibers this represents is not broken out by automakers, but it's probably minor.

Coconut: A Rising Star

Eloisa Mano, a professor at the Institute of Macromolecules at the Federal University of Rio de Janeiro, is working with scientists on a solution to a huge waste problem--finding a market for discarded coconut shells. About 200 million coconut shells are discarded weekly in Rio de Janeiro alone.

Compared with other natural fibers, coconut shells have a long life because of their high lignin content (41%-45%). Ripe, dried coconut provides a hard fiber, while the green fruit yields a cellulose-like fiber. Studies reveal that about 15% of the weight of discarded green coconut shells can be used for fiber reinforcements.

A project led by Marcos Lopes Dias is researching the reinforcement of recycled polyethylene terephthalate with coconut fibers. The hydrophilic surfaces of the fibers give them an affinity with condensation polymers like PET. The properties of injection-molded parts made with the blend were compared with those of composites of recycled PET and glass fiber prepared under the same conditions. The results show that the coconut-reinforced material has a higher modulus of elasticity (Young's modulus).

For this type of composite, lignin is a suitable coupling agent. The polar hydroxyl groups and non-polar hydro-carbonates present in the lignin enhance adhesion between the resin and fiber. The composites where lignin is added also exhibit significantly reduced water absorption and increased flexural resistance.

"Once commercialized, this composite should reduce the amount of solid waste sent to landfills, because it only uses recycled PET and coconut shells," says Marion dos Santos, a researcher on the project.

Market applications for coconut fibers already exist in the rubber industry, where fibers from ripe and green coconuts reinforce shoe soles. New uses are being explored in construction, where coconut fiber-reinforced thermoplastics may replace concrete in some applications. The composite would have a green benefit, as well, since it requires less energy to produce than concrete. Coconut fibers are also said to have exceptional acoustic-dampening properties.

More Research

Agave, the plant from which tequila is extracted, also has value as a thermoplastic reinforcement. Interest in the material relates to environmental concerns: more than 1 million tons of agave bagasse are discarded as a byproduct of tequila production.

The University of Guadalajara in Mexico is researching the compatibility of agave fibers with virgin and recycled PE Without a coupling agent, the fibers agglomerate and produce discontinuities in the matrix. PP grafted with maleic anhydride appears to be a solution, since it favors interaction between the matrix and fiber and acts as a plasticizer. Composites with up to 55% loadings of agave fibers have been formulated, with a major improvement in tensile modulus.

Another Mexican group working with natural-fiber reinforcements is the Center of Applied Chemistry in Saltillo, Coahuila. Researchers want to understand the effects of different fillers and coupling agents on mechanical and processing properties of composites made with wood four, palm fiber, and the native lecheguilla fiber. In a study of PVC, they found that palm fiber improves impact resistance, and if lecheguilla is used with a silane compatibilizer, properties can surpass those of wood composites.

GE Plastics South America, mean while, is working with researchers at the State University of Campinas (Unicamp) in Brazil to develop composites of polyamide 6 and the Amazonian fiber curaua. Marco Aurelio de Paoli, director of the research at Unicamp, says there is large-scale production of these fibers in the state of Parana. The composite, used in automotive applications, takes filler loadings of up to 20%, and "properties obtained in the parts are the same as those from glass fiber."

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Piacava, a palm fiber with high mechanical resistance that grows in the northern Brazil's state of Bahia, is also being evaluated. In a project led by professors Luis Ernesto Roca Bruno and Evandro Sena Freire at the State University of Santa Cruz, scientists are experimenting with the fiber in recycled thermoplastics. The results reportedly show promising thermal and acoustic performance. To improve the interface between matrix and fiber, an ionomer of ethylene is used as a coupling agent.

Availability Determines Potential

Analia Vasquez of the Research Institute of Material Science and Technology (Intema), Mar del Plata, Argentina, works as a research leader on a project to produce sisal-reinforced compounds. The fiber doesn't have the best mechanical properties compared with other natural fibers, but it's abundant.

"Finding markets for natural fibers like sisal and jute is a big concern because in most Latin American countries where they grow, they have been replaced by polypropylene fibers," Vasquez says.

Her team produced masterbatches with fiber levels of 70% in a polypropylene matrix. "But in finished parts, the loading should not be greater than 30% or 40%," she advises. "Otherwise, interfacial adhesion and fiber distribution deteriorate."

Vasquez says that a potential market for these PP masterbatches in Latin America is wood replacement in construction and furniture.

Further afield, Intema partnered with Fiat in a European project, called "Ecofina," that used natural fiber-reinforced thermoplastics in automotive parts like interior panels, spoilers, and spare-tire mountings.

Vasquez believes that the success of natural fibers as reinforcements will be determined as much by their abundance as their performance. This view substantially increases the pool of materials available. She says that some agricultural lignocellulosic waste like peanut, rice, and cotton shells could be used as reinforcements. It would be necessary, however, to chemically treat or size them for different polymers, as is done with glass fibers.

Some of the work at Intema deals with reinforcement of biodegradable resins. The use of biobased fibers to reinforce these polymers has obvious green advantages, since conventional reinforcements diminish somewhat the full environmental benefit of a biodegradable resin.

Vasquez says that the hydrophilic character of a biodegradable resin determines the degree of compatibility between the base resin and the natural fiber. In considering widely available natural polymers like starch, "one challenge is to assure their RH (relative humidity) stability, mainly in food-contact applications." Another is to improve their ductility.

Because of the high cost of biodegradable resins, reinforcement with sisal-based cellulose nanofibers, with a Young's modulus of up to 130 GPa, is under study. "Nanofibers have a high aspect ratio--they are 100-200 nm in length and 1-20 nm in diameter--and can be used in loadings of up to 5 wt% without affecting the transparency or viscosity of the base material," she says. "With additions of only 5%, resistance would be the same as that of a 30% conventional fiber loading."

Studies in coupling agents for sisal-PP blends at Simon Bolfvar University (USB) in Venezuela show that composites using PP grafted with diethyl maleate have increased impact resistance. The use of an acetylation treatment on the fibers improves the adhesive characteristics of their surfaces and increases aspect ratio. This in turn upgrades tensile properties.

Sea-Based Resource

Other USB studies find that seashells could be sources of inorganic mineral fillers to reinforce polyolefins and reduce their cost. The main component of seashells is calcium carbonate, synthesized by mollusks from metabolized carbonate and calcium in the environment. Compared with other fillers, seashells have important advantages in their purity and abundance.

Using PP and high-density polyethylene as matrices, mixtures of crushed, cleaned, and dried seashell powder at loadings of up to 22 parts per hundred were studied. An analysis of thermal properties of injection-molding compounds showed that with a higher filler loading, thermal stability of the composite increased, while viscosity was almost unchanged. The Young's moduli of the compounds increased proportionally with filler content. HDPE compounds, by contrast, showed higher sensitivity to filler loading. In both compounds, the fillers augment crystallization.
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Title Annotation:LATIN AMERICA
Author:Florez, Laura
Publication:Plastics Engineering
Geographic Code:1USA
Date:Apr 1, 2008
Words:1541
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