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Natural Fiber Composites: Bast, Leaf and Seed Fibers

February 22, 2017

In the future, natural fiber reinforced composites will gain an even larger presence in the composite materials industry especially as the demand for environmentally friendly products grows.

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Woven fabric like the one pictured above are environmentally friendly and offer many of the same benefits
as more conventional fibers used in composites.

Fiber reinforced composites have been used since ancient times, when Egyptians mixed straw and clay to use as building material for their homes. Many composites exist in nature as well. For example, wood is a natural composite composed of cellulose fibers in a lignin matrix. Cellulose fibers have also been used more recently to reinforce phenolics, urea, and melamine resins.

Three Types of Natural Fibers

There are three basic types of natural fibers currently used in natural fiber composites. Bast fibers are exemplified by flax, hemp, and jute and usually consist of a wooden core surrounded by a stem. The stem is composed of fiber bundles, which are made of individual fibers made of cellulose and hemicellulose bonded in a matrix of either lignin or pectin. Leaf fibers are usually sisal (agave), banana, or palm and are generally coarser than bast fibers. They are used in ropes and coarse textiles. Finally, seed fibers include cotton, coir, and kapok. Cotton is by far the most common and is used mostly in textiles. However, coir is coconut husk and is usually used in ropes, matting, and brushes instead of the textiles more common for cotton and kapok.

Benefits of Natural Fibers

Natural fibers offer many of the same benefits as more conventional fiber reinforced composites, but also boasts increased sustainability and ecofriendliness. Known as NFPCs, natural fiber reinforced polymer composites have seen a significant increase in interest in the last decade. Adding tough and lightweight natural fibers into a polymer matrix can produce a high specific stiffness and strength.

Potential Issues to Consider

However, the use of natural fibers in modern composites is not free of problems. The content of natural fibers (namely cellulose, lignin, pectin, and waxy substances) allows moisture absorption from the surroundings. Because of that, the bindings between the fiber and polymer become weak. Additionally, the fibers and matrix are difficult to couple because both the fibers and matrices can have varied chemical structures. This weak bonding causes the stress transfer between phases to be ineffectual if not carefully engineered. Therefore, special fiber treatments (which generally focus on removing the hydroxyl functional groups of natural fibers) are necessary to achieve the high strengths of synthetic composites.

In the future, natural fiber reinforced composites will gain an even larger presence in the composite materials industry especially as the demand for environmentally friendly products grows. As research produces natural fiber composites with continually higher strength to weight ratios, they will be able to be used in more and more demanding applications, therefore opening up even more markets for them to grow into.

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