1 INTRODUCTION
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. Hereunder a few successful results of evolution are described.
Table 1: Properties of glass and natural fibres
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:
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. (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
The use of natural fibres in automobiles has largely been restricted to upholstery applications because of the traditional shortcomings of natural fibre composites, low impact strength and poor moisture resistance. Recent research results show that there is a large potential in improving those two properties. This potential can be found in either in pre-treatments of the fibres or in improving the chemistry while impregnating the fibres with the matrix material.
3.1 Pre-treatments
Treatment is required to turn just-harvested plants into fibres suitable for composite processing. For example in case of flax, the first step is retting. It is a controlled rotting process to get rid of the pectin that 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 bundle, in a composite it will be the weakest link.
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.
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 specific strength and stiffness will even be better compared to glass composite. Problems that can be encountered are 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 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 a chemical condition on which air bubble growth will be initiated, especially in vacuum processes like vacuum injection. In order to prevent many voids and a poor fibre matrix interface during vacuum injection it is necessary 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. 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 small amount of MAPP added to the PP, will lead to much higher strength properties of the material.
Another promising development of thermoplastic prepregs is by means of latex emulsions or dispersions. The wetting 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 materials are interesting if products must be fully bio-degradable. A higher price and moisture sensitivity are disadvantages.
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 the dvantage that the tank is foldable when it is empty.
Sources FAO Report, Assessed on 22 February 2016
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. Hereunder a few successful results of evolution are described.
Table 1: Properties of glass and natural fibres
Properties
|
Fibre
| ||||||||
E-glass
|
flax
|
hemp
|
jute
|
ramie
|
coir
|
sisal
|
abaca
|
cotton
| |
| Density 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) |
0.35
1.5/0.9 - 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 filament2.1 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.2.5 Disadvantages of natural fibres:
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
- 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. (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
|
14 400
| |
| Rest of EU |
300
|
6 900
| |
| Total: |
4 300
|
21 300
|
24 000
|
source: nova InstituteIn 1999, natural fibres used in the automotive industries comprised 75 percent flax, 10 percent jute, 8 percent hemp, 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.
The use of natural fibres in automobiles has largely been restricted to upholstery applications because of the traditional shortcomings of natural fibre composites, low impact strength and poor moisture resistance. Recent research results show that there is a large potential in improving those two properties. This potential can be found in either in pre-treatments of the fibres or in improving the chemistry while impregnating the fibres with the matrix material.
3.1 Pre-treatments
Treatment is required to turn just-harvested plants into fibres suitable for composite processing. For example in case of flax, the first step is retting. It is a controlled rotting process to get rid of the pectin that 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 bundle, in a composite it will be the weakest link.
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.
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 specific strength and stiffness will even be better compared to glass composite. Problems that can be encountered are 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 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 a chemical condition on which air bubble growth will be initiated, especially in vacuum processes like vacuum injection. In order to prevent many voids and a poor fibre matrix interface during vacuum injection it is necessary 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. 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 small amount of MAPP added to the PP, will lead to much higher strength properties of the material.
Another promising development of thermoplastic prepregs is by means of latex emulsions or dispersions. The wetting 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 materials are interesting if products must be fully bio-degradable. A higher price and moisture sensitivity are disadvantages.
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 the dvantage that the tank is foldable when it is empty.
Sources FAO Report, Assessed on 22 February 2016
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