The use of composite materials dates from centuries ago, and it all started with natural fibres. In ancient Egypt some 3 000 years ago, clay was reinforced by straw to build walls. Later on, the natural fibre lost much of its interest. Other more durable construction materials like metals were introduced. During the sixties, the rise of composite materials began when glass fibres in combination with tough rigid resins could be produced on large scale. During the last decade there has been a renewed interest in the natural fibre as a substitute for glass, motivated by potential advantages of weight saving, lower raw material price, and 'thermal recycling' or the ecological advantages of using resources which are renewable. On the other hand natural fibres have their shortcomings, and these have to be solved in order to be competitive with glass. Natural fibres have lower durability and lower strength than glass fibres. However, recently developed fibre treatments have improved these properties considerably. To understand how fibres should be treated, a closer look into the fibre is required.
2 NATURAL FIBRES IN COMPOSITES
The vegetable world is full of examples where cells or groups of cells are 'designed' for strength and stiffness. A sparing use of resources has resulted in optimisation of the cell functions. Cellulose is a natural polymer with high strength and stiffness per weight, and it is the building material of long fibrous cells. These cells can be found in the stem, the leaves or the seeds of plants.
* tensile strength strongly depends on type of fibre, being a bundle or a single filament
2.1 Bast fibres (flax, hemp, jute, kenaf, ramie (china grass))
In general, the bast consists of a wood core surrounded by a stem. Within the stem there are a number of fibre bundles, each containing individual fibre cells or filaments. The filaments are made of cellulose and hemicellulose, bonded together by a matrix, which can be lignin or pectin. The pectin surrounds the bundle thus holding them on to the stem. The pectin is removed during the retting process. This enables separation of the bundles from the rest of the stem (scutching).
After fibre bundles are impregnated with a resin during the processing of a composite, the weakest part in the material is the lignin between the individual cells. Especially in the case of flax, a much stronger composite is obtained if the bundles are pre-treated in a way that the cells are separated, by removing the lignin between the cells. Boiling in alkali is one of the methods to separate the individual cells.
Flax delivers strong and stiff fibres and it can be grown in temperate climates. The fibres can be spun to fine yarns for textile (linen). Other bast fibres are grown in warmer climates. The most common is jute, which is cheap, and has a reasonable strength and resistance to rot. Jute is mainly used for packaging (sacks and bales).
As far as composite applications are concerned, flax and hemp are two fibres that have replaced glass in a number of components, especially in the German automotive industries.
2.2 Leaf fibres (sisal, abaca (banana), palm)
In general the leaf fibres are coarser than the bast fibres. Applications are ropes, and coarse textiles. Within the total production of leaf fibres, sisal is the most important. It is obtained from the agave plant. The stiffness is relatively high and it is often applied as binder twines.
As far as composites is concerned, sisal is often applied with flax in hybrid mats, to provide good permeability when the mat has to be impregnated with a resin. In some interior applications sisal is prefered because of its low level of smell compared to fibres like flax. Especially manufacturing processes at increased temperatures (NMT) fibres like flax can cause smell.
2.3 Seed fibres (cotton, coir, kapok)
Cotton is the most common seed fibre and is used for textile all over the world. Other seed fibres are applied in less demanding applications such as stuffing of upholstery. Coir is an exception to this. Coir is the fibre of the coconut husk, it is a thick and coarse but durable fibre. Applications are ropes, matting and brushes.
With the rise of composite materials there is a renewed interest for natural fibres. Their moderate mechanical properties restrain the fibres from using them in high-tech applications, but for many reasons they can compete with glass fibres. Advantages and disadvantages determine the choice:
2.4 Advantages of natural fibres:
+ Low specific weight, which results in a higher specific strength and stiffness than glass.
This is a benefit especially in parts designed for bending stiffness.
+ It is a renewable resource, the production requires little energy, CO2 is used while oxygen is given back to the environment.
+ Producible with low investment at low cost, which makes the material an interesting product for low-wage countries.
+ Friendly processing, no wear of tooling, no skin irritation
+ Thermal recycling is possible, where glass causes problems in combustion furnaces.
+ Good thermal and acoustic insulating properties
2.5 Disadvantages of natural fibres:
- Lower strength properties, particularly its impact strength
- Variable quality, depending on unpredictable influences such as weather.
- Moisture absorption, which causes swelling of the fibres
- Restricted maximum processing temperature.
- Lower durability, fibre treatments can improve this considerably.
- Poor fire resistance
- Price can fluctuate by harvest results or agricultural politics
3 RECENT DEVELOPMENTS IN NATURAL FIBRE COMPOSITES
The use of natural fibres for technical composite applications has recently been the subject of intensive research in Europe. Many automotive components are already produced in natural composites, mainly based on polyester or PP and fibres like flax, hemp or sisal. The adoption of natural fibre composites in this industry is lead by motives of a) price b) weight reduction and c) marketing ('processing renewable resources') rather than technical demands. The range of products is restricted to interior and non-structural components like door upholstery or rear shelves.
