Surface Modification Of Natural Fibres For Biocomposites Biology Essay

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The use of natural fibres and nanofibres in biocomposites are grown due to the flexibility in process, in addition to the high mechanical properties, and above all, low cost. All these advantages making the natural fibres more accepted to use in composites. The composites are materials that consist of two or more substances that might be different in physical and chemical properties.1-7 Different kind of materials are combined together to develop products with more strong structure and more functional not possible to achieve by any of the substance alone. Composites, the inquire materials are becoming an important part of our daily life materials due to the advantages such as, relatively low weight, high tensile strength, and the flexibility during processing.8-12 They are used as materials in making aircraft structures, in building as insulators, and also in biomedical applications. 13-15

As president Obama said when he visited Argonne National Laboratory in Illinois, 08/03/2013: '' The only way to break the gas price spike cycle is to shift our cars off oil''16. Going back to nature, finding alternative materials is needed. The oil crises made the biocomposites as a magic materials used in very wide range in industry. As any material, the biocomposites is under research and it needs more time to find more applications.

Using natural fibres in composites gain acceptance and even the number of publications and books are increased this means, the new world go toward the nature and why not. But still the use of glass and carbon fibre in composite is more dominate than natural fibre.17 The application of natural fibres in composite is insignificant compared to others mineral and polymers. Yet, natural fibres have many advantages, since they are inexpensive, available, environmentally friendly, and renewable. But the problem with using natural fibres in composites is related to the hydrophilic nature of fibres, the high moisture absorption and also the contact with matrix is quite poor, ending up with low tensile strength.18 Thus fibre-based composites needs a modification of the fibre surface, and it can be done by either chemical or physical processes. The physical aspects of surface modification of natural fibre are discussed in this essay. Before that we need to know a bit more about natural fibres.

Natural Fibres

Most of natural fibres comes from plant. They are sustainable and renewable, the plants produce natural fibre classify to primary and secondary. Jute, hemp and sisal are examples of primary plants, while pineapple and oil palm are example of secondary plants. Table 1 shows the most natural fibres used in commercial composites.

Table 1. Major natural fibres used in commercial composites.19

Fibre source

world production (103 ton)

Sugar cane bagasse

75,000

Bamboo

30,000

Jute

2300

Kenaf

970

Flax

830

Sisal

378

Hemp

214

Coir

100

Ramie

100

Abaca

70

Grass

700

Surface Treatment

The main problem of using natural fibre in composites is the poor contact due to the hydrophilic nature of natural fibre. Nevertheless, it is not always the surface modification of natural fibres needed in composites. Joseph et. al. made a composites with phenol-formaldehyde matrix and fibres from pineapples leafs and banana without surface modification.20 The results shown in table 2 compared with synthetic fibres, the properties of these composites depends mainly on fibre length and loading. On the same study they used phenol-formaldehyde matrix with oil palm fibre and the surface of oil palm fruit was modified and in general the tensile strength improved. The surface of fibre can be modified physically by stretching, refining and calendaring. The physical treatments change the structure and surface properties of fibre and hence improve the mechanical interlocking with the polymer.

Table 2 Mechanical properties of natural fibre compared with other kind of fibres.21

Type of Fibre

Density, kg/m3

Tensile strength, MPa

Young's modules GPa

Carbon

1400

4000

230-240

Cotton

1500

287-597

5.5-12.6

Sisal

1500

511-935

9.4-22

Banana

1300

750

29-32

Pineapple

1560

172

62

Soft wood kraft

1500

1000

40.0

E-glass

2500

2000-3500

70.0

Refining

If we look for instance to the wood fibre and its application in paper industry. The fibres usually are refining meaning, the physical treatment are applied to modify the surface of fibre. It is one of the most important stage in order to obtain high quality paper. The main idea beyond refining is to improve the fibre-fibre bonding and increase the tensile strength or sometimes you need to cut the long fibre in order to achieve good formation. The refining method is modifying the surface of fibre within water by metal bars. Figure 1 illustrate the refining mechanism.

Fig.1 Refining mechanism23

The refining change the morphology of fibre. The effect mainly is cutting long fibre and produce fines, also significant change in the fibre wall and some of removed parts return and attached to the fibre surface.

New Physical methods of surface modification

Corona and plasma treatments are used recently for modification of the surface of fibres.24-25 These methods are physical meaning, you do not need to produce any substances that might hazard the environment like some toxic substance produce by chemical treatment of fibres. Especially the plasma is very friendly to the environment. Sustainability and end of life after use are important considerations to make when developing polymer composites from renewable resources.26 The toxicity and environmental issues of using different chemical treatments for enhancing the properties of these materials. Some chemical materials may be toxic, and therefore, the use of such material may harm the environment.27

Plasma treatment

Plasma is ionized gas with electrons, photons and ions, it have the ability to alter the surface properties of natural fibres by creation a free ions and electrons on the surfaces of natural fibres.28 In this treatment the surfaces of fibres are bombarded with a beam of high energy, the surface roughness and surface chemistry are changed without using any substance might harm the environments. The plasma not only remove the impurity on the surface of fibre but also making the fibres more porous, meaning the mechanical bonding between the fibre and the matrix is increased as shown in figure 2. different kind of surface modifications can be achieved by changing the gas bombarded to the surface. One other important aspect is the time, with plasma you need only few minutes, while on the traditional methods you might need days and can't achieve the same mechanical properties.

The helium cold plasma treatment bombarded to flax fibres with polyester matrix composites done by Marais et.al.29 The investigation done by water penetration measurements and testing mechanical properties. The results showed that the plasma treatment enhances the fibre-matrix adhesion and also improves the tensile strength and stiffness of the composites.

Fig.2 Cross section of natural fibre a- before plasma treatment b- after plasma treatment, the fibre more porous.28

The morphology of untreated fibre and treated one with plasma is shown in fig 3. with untreated fibre the surface is smooth as shown in fig 3a, where the primary wall of fibre consists of fatty acid and fatty alcohol and many others meaning, the outer layer covers the inner-layers. In fig 3b, using plasma treatment we have a very rough surface and not only that, if the time is sufficient it will remove the outer layer as well as shown in fig 3c. it is not recommended to increase the time of treatments i.e if you go beyond 15 min we might lose the attached fibrils and also the migration of hydrophilic extract increased.

Fig 3. Surface morphology of fibre a- before treatment b and c after plasma treatment.28

Martin and his colleagues, they made composite with sisal fibre and High Density Polyethylene matrix.30 The results showed a significant improvement in mechanical properties by modifying the surface of fibre using plasma.

The interfacial adhesion of Jute fibres and High Density Polyethylene matrix was studied by Seki et. al. as a function of plasma power, where the oxygen plasma beam was used for fibre surface modification.31 The power used was 30 watt and 60 watt. The shear strength increased by 32 and 47% respectively compared with untreated composites.

The effect of plasma treatment on the morphology and wettability of jute fibre and polyester matrix was studied by Sinha et.al.32 On the other hand the consequence on the adhesion interfacial also was investigated. The SEM image revealed the rough surface and a degradation of fibre when plasma used. Plasma enhanced the hydrophobicity in fibre. Nevertheless, the flexible strength of the composites made by different kind of treated fibres, the only fibre treated for 10 minutes gives an improved of tensile strength of 14% compared with untreated one.

Siky et. al. they used different kind of oxygen plasma reactor, the radio frequency and low frequency for Jute fibres treatment with polyester matrix.33 The shear strength increased from 11.6 MPa for untreated fibres to 19.8 and 26.3 MPa with low frequency and radio frequency respectively. In general the results show a significant improvement on the mechanical properties of jute fibres treated by oxygen plasma with radio frequency than low frequency.

Corona treatment

Corona is one of the most popular techniques used recently for fibre modification. By this treatment the surface energy of cellulose fibre is changed. Corona electron discharge treatment of fibre and hydrophobic matrix found be a very effective.

The corona electron discharge treatment was used by Pothan et. al. in order to improve the mechanical properties of fibre-epoxy composites.34 Corona treated fibres shows higher polar components of free surface energy when increasing the treatment energy output.

The mechanical property of hemp fibre in composite was studied by Ragoubi et. al. the tensile index and also the stress strain measurements was performed.35 The physical treatment by corona gives a significant increase in mechanical properties (tensile strength). The modification of fibre enhanced the composites properties with 30% of Young modulus.

The Flax and Hemp fibres were modified by corona discharge treatment and the optimum value of corona treatment length was found by Pizzi et. al. in order to achieve the extreme value of tensile strength.36 The composite tensile strength breaking after 5 min of corona treatment. The fibre appears good mechanical properties with relatively short periods of corona treatment and it found also that no needs for more time of treatment, it will not improve the mechanical properties.

Comparison between mechanical and chemical treatments

The wheat straw fibres was treated by mechanical and chemical means and used with polypropylene matrix, and the mechanical, thermal properties were studied by Joseph et. al . The fibres modified by chemical means appears better mechanical and thermal properties than the physical ones.37 Figure 4 shows the SEM images of fibres surface treated by physical and chemical means. The surface of fibres treated by chemical means are smooth and uniform while the ones treated by mechanical means exhibits irregular surfaces.

Fig 4. The morphology of fibres treated by a- mechanical b- chemical means.37

The effect of mechanical and chemical means on the thermogravimetric analysis of the straw fibres is shown in figure 5. The starting degradation temperatures were 217 oC in mechanical treatments and 242 oC in chemical. More onset degradation temperature, more thermal stability of fibres treated.

Fig 5. Thermogravimetric analysis curve of straw fibres treated by different means.37

Summary

The use of natural fibre on biocomposites is growing, they are inexpensive, flexible in processing, appearing high mechanical properties and above all environmentally friendly. The biocomposites will find more application in near future due to the petroleum and oil crises. But still the interfacial adhesion with matrix is poor because of the hydrophilic nature of fibres. Therefor; an extensive research is going in this area to enhance the contacts between fibre and the matrix. The research tells us that improving the surface of fibre leads to improve the mechanical properties of the composites. The physical treatments of the surface of fibre by corona or plasma improve the mechanical properties but still it seems the chemical treatments are more powerful.

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