Natural Fibers for the Reinforcement of Composite Materials

A composite material is generally defined by the assembly of two phases: the reinforcement (usually in the form of fibers) and the matrix. The matrix preserves the geometric arrangement of the fibers, and ensures load transfer between them via the interfaces. The matrix (in other words, the binder) may belong to the polymer, metal or ceramic family. Under the term "natural fibers" we find organic fibers of plant (cellulose) and animal (protein) origin, and mineral fibers such as asbestos (which will not be presented in this article).

In this article, therefore, only organic and renewable fibers and associated composite materials - materials using a polymer as matrix - are discussed.

The use of natural fibers as reinforcement in composite materials is justified by :

• develop a local resource in less industrialized countries;

• develop materials and technologies that reduce environmental impact;

• use renewable resources.

The aim of this article is not to make an inventory of all the fibers available, but to present some interesting examples, illustrate their reinforcement capabilities and take a step back from their uses. There are many varieties of natural fibers, but only a few have outstanding mechanical properties that justify their use as reinforcements in composite materials. These fibers have a structural role in nature. For example, a flax stalk is reinforced by fibers placed on the outside, which act as support tissues; they offer interesting mechanical performances, given their function and the high slenderness of these plants.

Plant fibers are widely used, as they are the most readily available. Their complex structure can be likened to that of composite materials reinforced with cellulose fibrils arranged in a helix. To understand the origin of their behavior, it is necessary to know their microstructure and composition. These topics will be presented in simplified form below. The notion of fibers in a plant has a botanical meaning, which describes a single elongated, thick-walled plant cell, so there are different natures of cells. The plant fibers described in this article correspond to those that have long been used for textile applications or rope-making, and have a function in nature generally of support and rarely of conduction.

Although rarely used industrially in composites, animal bristles have a very high elongation at break and a high capacity to absorb mechanical energy compared to synthetic fibers.

The processing technologies that can be used to produce parts from biocomposites (polymer + natural fibers) are identical to...