4 PROCESSING TECHNIQUES
In principle, the production techniques for natural fibre composites can be similar to those for glass fibres. Exceptions to this are techniques used where continuous fibres are used like pultrusion (a yarn has to be made first) or where fibres are chopped like in spray-up or SMC-prepreg preparation. Four examples of techniques are discussed below.
4.1 RTM, vacuum injection
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 fibres are impregnated. Tailored lay-ups and high fibre volume contents are possible. Therefore, the technique enables the manufacture of very large products with high mechanical properties. A difference compared to glass is the springy character of the natural fibres. To enable proper fibre placement and high fibre volume contents, a preforming step may be required. Preforming is pressing the mats with a small amount of binder (like H2O) into a more compact shape.
Dense mats of flax can be difficult to impregnate. Better resin flow can then be obtained by using the thicker leaf fibres like sisal.
4.2 SMC
An important difference with glass SMC (sheet moulding compound) is the production of the prepreg. Normally prepregs are made by chopping the glass strands and dropping them on a film of resin-filler compound. This preparation will not work for natural fibres since the chopping is very difficult. Other techniques are being developed. An appropriate method to get a layer of fibres with an anisotropic orientation which is loose enough to provide sufficient fibre flow during the moulding process depends on the type of fibre and on the way in which the raw material is being supplied.
Figure 2: Catamaran boat hull made by RTM in polyester and flax fibres
4.3 Vacuum pressing (1)
4.3 Vacuum pressing (2)
This resembles the vacuum injection, but it is a bit quicker and less sophisticated. After fibre placement a basket with resin is poured in the middle of the product, a rigid top mould is put on and at the edge of the set of moulds vacuum is applied. Successful replacement of glass fibres by natural fibres has meanwhile been achieved within the series production of a component of a caravan's coach, see Figure 3. Mechanical properties are comparable while a weight saving of 10 percent is achieved. Although these benefits are small, natural fibres could take the place of glass since the raw material cost is lower.
4.4 Sandwich technology
Today, composite laminates in glass polyester are produced in a continuous way up to a width of 3 m and with infinite length. Bonded on two sides of a foam block they build stiff sandwich panels that are used a lot in trucks, trailers and building construction. They provide thermal insulation and can fulfil a primary structural function. Small scale prototyping has proved that substitution of glass by natural fibres is feasible. A bit less insulating, but still very well suitable for wall and roof construction are sandwiches made of natural fibre composite skins and bamboo pillars as the sandwich core. An optimal combination of two different mechanical tour de force made by nature. This concept is now under development.
Figure 3: Caravan coach component, manufactured by vacuum pressing with flax mats, UP and a PU-foam core
Figure 4: Structural sandwich panel made of flax-PP skins and PS foam block
Figure 4: Structural sandwich panel made of bamboo
Compared to corrugated iron the 'vegetable sandwich' is not only more elegant, it is more durable, it insulates far better, and it uses renewable and local resources. Furthermore, the zinc-coat on the steel pollutes, and when the zinc has gone rust will appear. Finally, in hot climates, a steel roof gives no insulation, and the heat under such a roof can be unbearable.
5 OPPORTUNITIES FOR LOW-INVESTMENT PRODUCTION
Production of glass fibres, followed by weave, mat and prepreg manufacture, are based on machinery and high investments. It takes place in the industrialised countries, so for most countries it is an imported product to be paid with hard dollars. The production of natural fibres however, can be carried out by manpower and traditional know-how. In those countries, such as in South East Asia where natural fibres can be grown quickly and at low cost, the material resources are local. Importing of non-domestic materials, like glass fibres, at high prices in foreign currency can be avoided.
Production techniques like vacuum injection, hand lay-up, and vacuum pressing are appropriate for a cheap and easy manufacture of parts with in principle infinite dimensions.
An example of 'manufacture on the spot' is shown in Figure 5. The large vessels are latrines made with jute fibres and MF latex resin. Only MF powder was brought to the place of manufacture, water was added and the jute was impregnated by hand.
Figure 5: Columbian development project; manufacture of water tanks and latrines with jute and MF resin
6 KEY WORD IN SISAL APPLICATIONS: LOCAL USE
The main economic advantage of natural fibres may be found in their local availability. Automotive applications of natural fibre composites have proven themselves very well, especially in the German automotive industries, but for the moment mainly with the fibres that are grown in Northern parts of Europe being flax and hemp.
Some sisal is used in some technologies where fast impregnation is required, like the Polyurethane Reaction Injection Moulding (RIM) techniques used for interior parts like door upholstery. Sisal has a less dense character than flax, thus providing a good resin flow. A 50 percent-50 percent hybrid mat of flax and sisal is an often used semi-finished material.
But it is doubtful that in these industries the 'tropical' fibres can compete with the 'temperate climate' fibres, since raw material prices are comparable, while transport cost and lack of control over availability and quality remain disadvantages.
Therefore, exporting sisal as a resource for composite components in a "value-added" form would be advantageous. This might be in the form of semi-finished materials like pultrusion profiles or prepregs or as finished components. The processing will be cheaper than in the European countries, which enables a better competition.
Better opportunities for fibres like sisal are to be found in local use, such as in automotive industries in countries like Mexico or Brazil or in the use in other local composite industries based on relatively expensive glass fibres. If the right technologies are introduced a very effective use of locally available materials can be found in all kinds of every-day structural applications like house construction or boat building.
Sources FAO Report, Assessed on 22 February 2016
In principle, the production techniques for natural fibre composites can be similar to those for glass fibres. Exceptions to this are techniques used where continuous fibres are used like pultrusion (a yarn has to be made first) or where fibres are chopped like in spray-up or SMC-prepreg preparation. Four examples of techniques are discussed below.
4.1 RTM, vacuum injection
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 fibres are impregnated. Tailored lay-ups and high fibre volume contents are possible. Therefore, the technique enables the manufacture of very large products with high mechanical properties. A difference compared to glass is the springy character of the natural fibres. To enable proper fibre placement and high fibre volume contents, a preforming step may be required. Preforming is pressing the mats with a small amount of binder (like H2O) into a more compact shape.
Dense mats of flax can be difficult to impregnate. Better resin flow can then be obtained by using the thicker leaf fibres like sisal.
4.2 SMC
An important difference with glass SMC (sheet moulding compound) is the production of the prepreg. Normally prepregs are made by chopping the glass strands and dropping them on a film of resin-filler compound. This preparation will not work for natural fibres since the chopping is very difficult. Other techniques are being developed. An appropriate method to get a layer of fibres with an anisotropic orientation which is loose enough to provide sufficient fibre flow during the moulding process depends on the type of fibre and on the way in which the raw material is being supplied.
Figure 2: Catamaran boat hull made by RTM in polyester and flax fibres
4.3 Vacuum pressing (1)
4.3 Vacuum pressing (2)
This resembles the vacuum injection, but it is a bit quicker and less sophisticated. After fibre placement a basket with resin is poured in the middle of the product, a rigid top mould is put on and at the edge of the set of moulds vacuum is applied. Successful replacement of glass fibres by natural fibres has meanwhile been achieved within the series production of a component of a caravan's coach, see Figure 3. Mechanical properties are comparable while a weight saving of 10 percent is achieved. Although these benefits are small, natural fibres could take the place of glass since the raw material cost is lower.
4.4 Sandwich technology
Today, composite laminates in glass polyester are produced in a continuous way up to a width of 3 m and with infinite length. Bonded on two sides of a foam block they build stiff sandwich panels that are used a lot in trucks, trailers and building construction. They provide thermal insulation and can fulfil a primary structural function. Small scale prototyping has proved that substitution of glass by natural fibres is feasible. A bit less insulating, but still very well suitable for wall and roof construction are sandwiches made of natural fibre composite skins and bamboo pillars as the sandwich core. An optimal combination of two different mechanical tour de force made by nature. This concept is now under development.
Figure 3: Caravan coach component, manufactured by vacuum pressing with flax mats, UP and a PU-foam core
Figure 4: Structural sandwich panel made of flax-PP skins and PS foam block
Figure 4: Structural sandwich panel made of bamboo
Compared to corrugated iron the 'vegetable sandwich' is not only more elegant, it is more durable, it insulates far better, and it uses renewable and local resources. Furthermore, the zinc-coat on the steel pollutes, and when the zinc has gone rust will appear. Finally, in hot climates, a steel roof gives no insulation, and the heat under such a roof can be unbearable.
5 OPPORTUNITIES FOR LOW-INVESTMENT PRODUCTION
Production of glass fibres, followed by weave, mat and prepreg manufacture, are based on machinery and high investments. It takes place in the industrialised countries, so for most countries it is an imported product to be paid with hard dollars. The production of natural fibres however, can be carried out by manpower and traditional know-how. In those countries, such as in South East Asia where natural fibres can be grown quickly and at low cost, the material resources are local. Importing of non-domestic materials, like glass fibres, at high prices in foreign currency can be avoided.
Production techniques like vacuum injection, hand lay-up, and vacuum pressing are appropriate for a cheap and easy manufacture of parts with in principle infinite dimensions.
An example of 'manufacture on the spot' is shown in Figure 5. The large vessels are latrines made with jute fibres and MF latex resin. Only MF powder was brought to the place of manufacture, water was added and the jute was impregnated by hand.
Figure 5: Columbian development project; manufacture of water tanks and latrines with jute and MF resin
6 KEY WORD IN SISAL APPLICATIONS: LOCAL USE
The main economic advantage of natural fibres may be found in their local availability. Automotive applications of natural fibre composites have proven themselves very well, especially in the German automotive industries, but for the moment mainly with the fibres that are grown in Northern parts of Europe being flax and hemp.
Some sisal is used in some technologies where fast impregnation is required, like the Polyurethane Reaction Injection Moulding (RIM) techniques used for interior parts like door upholstery. Sisal has a less dense character than flax, thus providing a good resin flow. A 50 percent-50 percent hybrid mat of flax and sisal is an often used semi-finished material.
But it is doubtful that in these industries the 'tropical' fibres can compete with the 'temperate climate' fibres, since raw material prices are comparable, while transport cost and lack of control over availability and quality remain disadvantages.
Therefore, exporting sisal as a resource for composite components in a "value-added" form would be advantageous. This might be in the form of semi-finished materials like pultrusion profiles or prepregs or as finished components. The processing will be cheaper than in the European countries, which enables a better competition.
Better opportunities for fibres like sisal are to be found in local use, such as in automotive industries in countries like Mexico or Brazil or in the use in other local composite industries based on relatively expensive glass fibres. If the right technologies are introduced a very effective use of locally available materials can be found in all kinds of every-day structural applications like house construction or boat building.
Sources FAO Report, Assessed on 22 February 2016
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