Fig.1 Ventilator housing, made from flax based SMC (21 vol% flax)

Natural fibres, as a substitute for glass fibres in composite components, have gained renewed interest the last decade, especially in automotive industries. Fibres like flax, hemp or jute are cheap, have better stiffness per unit weight and have a lower impact on the environment. Although the automotive industry takes the lead in the revival of natural fibres, applications are mainly restricted to upholstery applications where acoustic and thermal insulation, low cost and an environmental image are advantages. Structural applications are rare since existing production techniques are not applicable and availability of semi-finished materials with constant quality is still a problem. This paper describes the results of the Dutch R&D program ‘Biolicht’ where research on material level, manufacturing processes and structural designing with natural fibre composites was carried out. It demonstrated that production techniques for structural applications like SMC or RTM are feasible.

The first composite material known in history was made in ancient Egypt some 3,000 years ago, which was clay reinforced by straw to build walls. With the development of other more durable construction materials like metals, the interest in natural fibres was lost. The rise of composite materials began during the sixties, when glass fibres in combination with tough rigid resins could be produced on large scale. The last decade there is a renewed interest in the natural fibre as a substitute for glass. Reasons for this include a required weight saving, a lower raw material price, ‘caloric recycling’ or of ecological kind like the need for renewable resources.
On the other hand natural fibres have their shortcomings, and these have to be solved in order to be competitive with glass. The nature of the fibre incorporates lower durability and lower strength compared to glass fibres. However, recent developed fibre treatments have improved these properties considerably.
Secondly, existing manufacturing technologies are not directly applicable on natural fibre composites. Required process adjustments or semi finished materials were also subject of the research.

In order to understand how fibres should be treated, an investigation of the fibre properties has to be made.

The vegetable nature is full of examples where cells or group 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. Hereunder a few successful results of evolution are described:

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 and are bonded together by a matrix, which can be lignin or pectin. The pectin surrounds the bundle and bonds it to the stem. The pectin is removed during the retting process. This enables the 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 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 moderate 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, it is cheap, has a reasonable strength and resistant to rot. Jute is mostly used for packaging (sacks and bales).

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. The abaca fibre, which is from the banana plant, is durable and resistant to seawater.

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 and it is a thick and coarse but durable fibre. Applications are ropes, matting and brushes.

* tensile strength strongly depends on type of fibre, being a bundle or a single filament

With the rise of composite materials there is a renewed interest for natural fibres. Their moderate mechanical properties prevent the fibres from using them in high-performance applications (e.g where carbon reinforced composites would be utilised), but for many reasons they can compete with glass fibres. Advantages and disadvantages determine the choice:

Recently the use of natural fibres for composite applications is being investigated intensively in Europe. As a result, many automotive components are now produced in natural composites, mainly based on polyester or PP and fibres like flax, jute or ramie. Until now however, the introduction in this industry is lead by motives of price and marketing (‘processing renewable resources’) rather than technical demands. The range of products is restricted to interior and non-structural components such as door upholstery or rear shelves. Reason for this is the traditional shortcomings of natural fibre composites being a) low impact strength and b) poor moisture resistance.

Recent research results show that there is a large potential in improving those two properties.

Other factors that stimulate the use of these materials are the developments of suitable production techniques and the corresponding semi finished materials like prepregs.

The fields of research mentioned above were subject to the Biolicht project:

This Netherlands research program started 2 years ago and will be finalised during this summer. Main objective is to investigate the feasibility of natural fibre composites for the Dutch trailer, coach-work and bus industry. The project group consists of:

Within Biolicht, different technologies were investigated especially those appropriate for the manufacture of structural components, outer body panels or other parts typical for trucks, trailers and busses. Sheet moulding compound (SMC), resin transfer moulding (RTM) and vacuum injection moulding were the main processes investigated but other techniques such as thermoplastic processing were also considered. Research in to the fibre itself was a necessary first step:

In case of flax, treatments are required to turn the just harvested green flax into fibres suitable for composite processing. Traditionally, the first step is (dew-)retting. It is a rotting process that removes the pectin which connects the fibre bundles with the wood core of the stem. After the retting, hemicellulose and lignin can be removed by hydro-thermolysis or alkali reactions. The hemicellulose is responsible for a great deal of the moisture absorption. The lignin is the connecting cement between the individual fibre cells. Although the lignin builds the fibre bundle, in a composite it will be a weaker link.
A proven treatment that replaces the retting and also removes hemicellulose and lignin to a certain extend is the ‘Duralin’ method of Ceres. Only harvested green-flax is used. The method is a combination of hydro-thermolysis and post-curing. The thus treated fibres are more durable and moisture resistant and they are partially separated (Lit. 4). Another method where the lignin is removed and the bundles are fully split into elementary fibres is developed by IAF-Reutlingen and is called is the steam-explosion method. The traditional dew-retting of the flax is still required in this method.
During harvesting, pre-treatments and processing, the handling plays an important role. Failure spots on the fibres can be induced, which cause a reduction of the tensile strength.

Important for a good adhesion between fibre and matrix is the degree of wetting during the production process. When applying thermosets the viscosity can be low, so the wetting is good. For some lay-ups, the specific strength and stiffness will even be better compared to glass composite. Problems that have been encountered were related to moisture and air.

The fibre moisture can affect the chemical reaction. In order to prevent this, the fibres have to be dried before, preferably down to 2 to 3 %. In standard room conditions, the moisture content is over 10 %. The compatibility of the moisture and the applied resin is important. Problems like foaming resin can be encountered.

Air can be present in the fibres and in the resin. The surface of the natural fibre has a geometry and a chemical condition on which air bubble growth will be initiated. In order to prevent many voids and a poor fibre matrix interface it is necessary to dry the fibres and to degas the resin. A very simple but appropriate degassing method has been developed and proven.

Because of a higher processing viscosity of thermoplastic polymers, a proper wetting of the fibres is difficult. High temperatures can also cause unwanted changes of the fibre surface or even destroy the fibres. Nevertheless, a low price, reasonable processing temperatures and recyclability are the reason for a growing interest in polypropylene. Unmodified PP however, will not have a proper adhesion with the fibres by applying consolidation forces alone. Mechanical properties are hardly improved, the fibres simply act like a filler. Natural fibres will only act as a reinforcement if compatibilisers are used. An interface between fibre and matrix should correct the natural rejection of both materials. An often used compatibiliser is MAPP, a modification of a PP chain with maleic anhydride. A few percentage of MAPP added to the PP, will lead to much higher strength properties of the material. Combining fibres, thermoplastic and compatibilisers can be done in several ways. Possible techniques are:

In principle, the processing techniques of natural fibre composites can be similar to those of glass fibres. However, techniques where continuous fibres are used like pultrusion or where fibres are chopped like spray-up or SMC, require some adjustments in fibre handling. Four examples of techniques are discussed below:

Resin transfer moulding or vacuum injection are clean, closed mould techniques. Dry fibres are put in the mould, then the mould is closed by another mould or by just a bagging film and resin is injected. Either with over-pressure on the injection side or vacuum at the other side the resin is forced to impregnate the fibres. Tailored lay-ups and high fibre volume contents are possible. Therefore, the technique enables the manufacture of very large products with high mechanical properties (see figure 3). Results with natural fibres so far are satisfying. Best results are obtained at higher V_f’s. At lower contents, for example less than 20%, the reinforcement is not what it should be theoretically and fibres act more like a polluter rather than reinforcer. A typical problem of natural fibres is their 'springier' character. To enable proper fibre placement and high fibre volume contents, a preforming step is required. Preforming is pressing the fibrous mats with a small amount of binder (like water) into a more compact shape. Following material properties of samples made by vacuum injection technique were obtained at Delft University: