natural fibre composites in structural components

7/28/2019 Natural Fibre Composites in Structural Components

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Natural Fibre Composites in Structural Components:Alternative Applications for Sisal?

W. D. (Rik) Brouwer [30]The Netherlands

1 INTRODUCTION

The use of composite materials dates from centuries ago, and it all started with natural fibres. In ancient Egyptsome 3 000 years ago, clay was reinforced by straw to build walls. Later on, the natural fibre lost much of itsinterest. 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 largescale. 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 theecological 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 improvedthese properties considerably. To understand how fibres should be treated, a closer look into the fibre isrequired.

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 withhigh strength and stiffness per weight, and it is the building material of long fibrous cells. These cells can befound in the stem, the leaves or the seeds of plants. Hereunder a few successful results of evolution aredescribed.

Table 1: Properties of glass and natural fibres

PropertiesFibre

E-glass flax hemp jute ramie coir sisal abaca cottonDensity g/cm3 2.55 1.4 1.48 1.46 1.5 1.25 1.33 1.5 1.51

Tensile strength* 10E6 N/m2 2400 800 - 1500 550 - 900 400 - 800 500 220 600- 700 980 400

E-modulus (GPa) 73 60 - 80 70 10 - 30 44 6 38 12

Specific (E/density) 29 26 - 46 47 7 - 21 29 5 29 8

Elongation at failure (%) 3 1.2 - 1.6 1.6 1.8 2 15 - 25 2 - 3 3 - 10

Moisture absorption (%) - 7 8 12 12 -17 10 11 8 - 25

price/Kg ($), raw (mat/fabric) 1.3(1.7/3.8)

- 1.5(2/4)

0.6 - 1.8(2/4)

0.351.5/0.9 - 2

1.5 - 2.5 0.25 -0.5 0.6 - 0.7 1.5 - 2.5 1.5 - 2.2

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

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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 andhemicellulose, bonded together by a matrix, which can be lignin or pectin. The pectin surrounds the bundle thusholding them on to the stem. The pectin is removed during the retting process. This enables separation of thebundles 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 thematerial is the lignin between the individual cells. Especially in the case of flax, a much stronger composite isobtained if the bundles are pre-treated in a way that the cells are separated, by removing the lignin between thecells. 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 fineyarns for textile (linen). Other bast fibres are grown in warmer climates. The most common is jute, which ischeap, 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 anumber 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 thetotal production of leaf fibres, sisal is the most important. It is obtained from the agave plant. The stiffness isrelatively 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 permeabilitywhen 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 inless demanding applications such as stuffing of upholstery. Coir is an exception to this. Coir is the fibre of thecoconut 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 mechanicalproperties restrain the fibres from using them in high-tech applications, but for many reasons they can competewith 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 theenvironment.

+ Producible with low investment at low cost, which makes the material an interesting product for low-wagecountries.

+ 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 politics3 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 basedon polyester or PP and fibres like flax, hemp or sisal. The adoption of natural fibre composites in this industry islead by motives of a) price b) weight reduction and c) marketing ('processing renewable resources') rather thantechnical demands. The range of products is restricted to interior and non-structural components like door upholstery or rear shelves. (Figure 1)

Figure 1: Interior parts of the Mercedes A-200 made by Natural Mat Thermoplastic

The use of natural fibres in automotive industries has grown rapidly over the last 5 years, see Table 2:

Table 2: The use of natural fibres in automotive industries

1996 1999 2000 (forecast)

Germany 4 000 14 400Rest of EU 300 6 900

Total: 4 300 21 300 24 000

source: nova Institute

In 1999, natural fibres used in the automotive industries comprised 75 percent flax, 10 percent jute, 8 percenthemp, 5 percent kenaf and 2½ percent sisal. There are prospects for 5 to 10 kg natural fibre to be used per car,thus requiring 80 000 to 160 000 tons in western Europe.

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The use of natural fibres in automobiles has largely been restricted to upholstery applications because of thetraditional shortcomings of natural fibre composites, low impact strength and poor moisture resistance. Recentresearch results show that there is a large potential in improving those two properties. This potential can befound in either in pre-treatments of the fibres or in improving the chemistry while impregnating the fibres withthe matrix material.

3.1 Pre-treatments

Treatment is required to turn just-harvested plants into fibres suitable for composite processing. For example incase of flax, the first step is retting. It is a controlled rotting process to get rid of the pectin that connects thefibre bundles with the wood core of the stem.

After the retting, hemicellulose and lignin can be removed by hydro-thermolysis or alkali reactions. Thehemicellulose is responsible for a great deal of the moisture absorption. The lignin is the connecting cementbetween the individual fibre cells. Although the lignin builds the bundle, in a composite it will be the weakestlink.

During harvesting, pre-treatments and processing, the handling plays an important role. Failure spots on thefibres can be induced, which cause a reduction of the tensile strength.

3.2 Matrix impregnation

3.2.1 Thermosets

The degree of wetting during the production process is important for a good adhesion between fibre and matrix.When applying thermosets the viscosity can be low, which eases the wetting. For some lay-ups, the specificstrength and stiffness will even be better compared to glass composite. Problems that can be encountered arerelated 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 percent. In standard room conditions, the moisture content is often over 10 percent.

Air is always present in the fibres and in the resin. The surface of the natural fibre has a geometry and achemical condition on which air bubble growth will be initiated, especially in vacuum processes like vacuuminjection. In order to prevent many voids and a poor fibre matrix interface during vacuum injection it isnecessary to dry the fibres and to degas the resin.

3.2.2 Thermoplastics

Because of a higher processing viscosity of thermoplastic polymers, a proper wetting of fibres is difficult. Hightemperatures can also cause unwanted changes of the fibre surface or even destroy the fibres. Nevertheless, alow price, reasonable processing temperatures and recyclability are the reason for a growing interest inpolypropylene. Unmodified PP however, will not have a proper adhesion with the fibres by applyingconsolidation forces alone. Mechanical properties are hardly improved, the fibres simply act like a filler. Naturalfibres will only act as a reinforcement if compatibilisers are used. An interface between fibre and matrix shouldcorrect the natural rejection of both materials. An often used compatibiliser is MAPP, a modification of a PP

chain with maleic anhydride. A small amount of MAPP added to the PP, will lead to much higher strengthproperties of the material.

Another promising development of thermoplastic prepregs is by means of latex emulsions or dispersions. Thewetting is perfect and quick. Appropriate polymers or combinations of polymers are being investigated.

3.2.3 Biological plastics



Besides synthetic polymers, materials 'developed by nature' can be used, such as modified starch, cellulose-esters or polylactide. The nature of these materials result in a good adhesion with the fibres. These materialsare interesting if products must be fully bio-degradable. A higher price and moisture sensitivity aredisadvantages.

3.2.4 Elastomers

For some applications, like tanks or vessels, the structure is not subject to bending loads, but tensile loads only(iso-tensoid winded tanks). In that case the matrix can be flexible, like for example natural rubber. This has thedvantage that the tank is foldable when it is empty.

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 madefirst) or where fibres are chopped like in spray-up or SMC-prepreg preparation. Four examples of techniquesare discussed below.

4.1 RTM, vacuum injection

Resin transfer moulding or vacuum injection are clean, closed mould techniques. Dry fibres are put in themould, then the mould is closed by another mould or by just a bagging film and resin is injected. Either withover-pressure on the injection side or vacuum at the other side the fibres are impregnated. Tailored lay-ups andhigh fibre volume contents are possible. Therefore, the technique enables the manufacture of very largeproducts with high mechanical properties. A difference compared to glass is the springy character of the naturalfibres. 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. Normallyprepregs are made by chopping the glass strands and dropping them on a film of resin-filler compound. Thispreparation will not work for natural fibres since the chopping is very difficult. Other techniques are beingdeveloped. An appropriate method to get a layer of fibres with an anisotropic orientation which is loose enoughto provide sufficient fibre flow during the moulding process depends on the type of fibre and on the way inwhich 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 abasket 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 beenachieved within the series production of a component of a caravan's coach, see Figure 3. Mechanicalproperties 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

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Therefore, exporting sisal as a resource for composite components in a "value-added" form would beadvantageous. This might be in the form of semi-finished materials like pultrusion profiles or prepregs or asfinished 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 incountries like Mexico or Brazil or in the use in other local composite industries based on relatively expensiveglass fibres. If the right technologies are introduced a very effective use of locally available materials can befound in all kinds of every-day structural applications like house construction or boat building.

[30] Delft University, Buizerdlaan 39, Nootdorp 2633BH, The Netherlands. Phone: ++31 152 782 463; E-mail:

[email protected]

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natural fibre composites in structural components

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