INTRODUCTION
The choice of fibres depends on their sensitivities and their mechanical,
thermals and physical properties. Among the used fibres let`s mention:
Fibres of ananas, abaca, sisal, esparto, kenaf, the textile palm.
The literature has show that the fibre extraction influences their length distribution,
their mechanical properties and their components (Sreekala
et al., 1998; Malainine et al., 2003).
The lignocellulosic fibres are constituted by micro fibrils and an amorphous
matrix made up of lignin and hemicellulose (Murali and Mohana,
2007).
Surface modifications is usually applied to impart bonding and adhesion affinity
to matrices, such as the thermal treatment (Bledzki and Gassan,
1999), alkaline treatment (Cao et al., 2006)
and enzymatic treatment (Akin et al., 2001).
The alkaline treatment can clean the fibre surfaces, increase its roughness
modify its composition and stop the process of the moisture absorption as shown
by Mohd et al. (2007). This treatment requires
a great quantity of energy, water and chemical product. It improves interfacial
adhesion (Zafeiropoulos et al., 2007), but it
can be expensive. The use of the enzymes and more precisely the pectinase can
contribute to reduce the environmental impact and the expenses of development
(Calafell and Garriga, 2004). It is a biological catalyst
and an enzymatic preparation with several strongly effective elements for the
depolymerization of vegetable pectins.
The researchers used enzymes for extracting fibre hemp, flax and cotton (Akin
et al., 2001; Calafell and Garriga, 2004;
Evans et al., 2002). After cleaning with the acid
pectinase, the cotton fibres have an almost intact cellulose structure. Compared
to alkali process they note a less significant loss of weight and a better resistance
(Rosenbohm et al., 2003).
Early research studies on natural fibres reinforced composites. Herrera-Franco
and Valadez-Gonzalez (2005) studied the mechanical behaviour of short natural
fibre reinforced HDPE. It resorts that the fibre-matrix interaction depends
on the surface properties of the fibre, witch increases the area of contact,
exposes further the cellulose microfibrils and improves fibre wettability and
impregnation.
This study is composed of three phases, the first interests the understanding
of fibre mechanical properties, the second proposes a preliminary study
of their interfacial features and the third deals with the effect of the
treatment on the behaviour of the fibre and epoxy resin/doum palm fibre
composite.
MATERIALS AND METHODS
The tested fibres have been extracted from the doum palm tree. That tree
belongs to the family of the monocotyledons, like the graminy and musaces.
The leaf is composed of folioles on which grow the leafstalks. The obtained
fibre length varies according to the tree type, its form, age.
Mechanical extraction: The mechanical extraction of natural fibres has
been made with raspadols which, by a combined action of scratching and hyping,
scratches the pulp and frees the fibre (Ghali et al.,
2006). Once extracted and washed, the fibres undergo an operation of drying
and combing. The first step can be accomplished naturally by exposing the product
to sunlight or artificially using an oven. Therefore the obtained fibres are
stuck against each other and a combing operation is required.
The mechanical extraction needs much water and causes a change in the
fibre characteristics because of mechanical stress which it undergoes.
Enzymatic treatment: In this study, fibres have been extracted from
leafstalk of the doum palms, by using the pectinase polygalacturonase as enzyme.
The enzyme activity is 3 to 9 units mg–1 at 25°C, witch can release
a micro mole of galacturonic acid per minute (Akin et
al., 2001). Some fibres have been weighted using an accurate balance
then immersed in a pectinase bath the concentration of which is 10 ml L–1
during 48 h under the ambient temperature. Once this is done fibres have been
rinsed with tepid distilled water. Then brushed to separate them and left in
the air to dry before pulling the whole set in an oven during 3 h at 90°C.
Finally the product has been stored in a bag.
Chemical treatment: This treatment consists on dissolving lignin
and hemicelluloses using aqueous solution in order to recover cellulose
fibres. The process is less harmful and does not attack the fibres mechanically.
It is based on the basic hydrolysis of the leaf components.
In order to quantify the effect of the concentration, palms fibres have been
plunged in a NaOH solution of variable concentration from 1 N to 3 N. The mixture
is put in a drying oven during 2 h at 90°C (Cao et al.,
2006).
To eliminate the residual aqueous solution traces fibres have been washed
and rinsed several times then bleached with chloride at the ambient temperature
and finally rinsed to eliminate the chlorine residual traces.
| Table 1: |
Nomenclature used for the final composites |
|
Composite preparation: Manufacturing has been done by using compression
molding techniques. Doum palm fibres are dried in an air-circulating oven
at 80°C during 24 h. Then the fibres are cut to the desired length,
20% of the fibre volume has been chosen, the epoxy resin has been mixed
with 1 wt.% of accelerator. The whole mixture is put in glass mold, the
fibres are added to the mixture by keeping the same fibre direction. The
mold is closed at a 0.5 kg cm–2 pressure during 10 h from each
flexural test, five specimens were prepared by cutting rectangular samples
with 80×15×4 mm3. Table 1 presents the abbreviations
of the final composite.
Mechanical properties: The tensile tests on the doum palm fibres
are carried out under standard conditions of relative humidity and temperature
according to the French norm NF G 07-002. For each test the fibres characterization
is made on 10 samples, chosen according to their titles (linear densities).
The speed of adopted test is 5 mm min–1. The length between
grips is 50 mim–1. For reasons of simplification, the fibres
are assumed to be of circular form where the diameter is given by:
The tensile modulus E is given by the tangent at origin of the stress-strain
curve. Flexural tests (three point bending) of composite were realized using
the same machine under the same conditions. Five samples were tested in each
experiment and the average value is reported. The evolution of the stress (MPa)
with respect to the strain (%) is plotted.
RESULTS AND DISCUSSION
Fibre morphology: The effect of modification upon the fibre was examined
using a SEM Microscope (PHILIPS XL-30). Prior to the analysis, the fibres were
immersed in gold, in order to ensure their conductivity. The examination of
the treated and untreated fibre surfaces revealed a dimensional variation of
the doum palm fibre. The diameter of untreated leafstalk fibres is of the order
of 280 μm. One notes that the diameter of microfibrils of cellulose is about
7.64 μm on the sun side and 8 μm in inside of the leafstalk.
|
| Fig. 1: |
Influence of the biological treatment on the leafstalk
fibres morphology |
|
| Fig. 2: |
(a) Biological treatment of the foliole fibres, (b)
Biological treatment of the leafstalk fibres and (c) Alkaline treatment
of the leafstalk fibres |
The structure
of the doum palm fibre is similar to that of the natural fibres (sisal, esparto)
(Bledzki and Gassan, 1999). It represents a natural composite
where, the ultimate fibres of cellulose form the reinforcement and the other
substances form the matrix. Figure 1a shows the untreated
leafstalk fibre with an external depot of lignin on the surface. Figure
1b shows that the biological treatment has reduced the extent of lignin
and improved the surface of the leafstalk fibre. It appears clearly that all
the alkaline treatment decrease the section of leaf stalks doum palm fibres.
For concentrations 1 and 3 N the diameter of the leafstalk fibres are 280 μm
and 209 μm, respectively; however the biological treatment has given fibres
which contain less gummy substances then the alkaline treatment.
The alkaline treatment has eliminated impurities of lignin and of hemicelluloses
in fibres without attacking cellulose microfibrils. This is conform to studies
already made on kenaf fibres (Mohd et al., 2007).
The biological treatment has improved the surface as it gives smooth one.
From Fig. 2, it can be seen a variation of fibre shapes due
to different treatments. Leafstalks and folioles fibres treated by enzymes have
a circular section as fibres of the sisal, palm, banana and bamboo (Murali
and Mohana, 2007), but alkaline treatment yield ellipsoidal fibres.
Mechanical properties of fibres: In Fig. 3a-c,
typical stress-strain curves for treated and untreated leafstalks doum
palm fibres are plotted. For each treatment, we present some tests as
well as then middle curves.
As can be seen from Fig. 3a-c all grades of doum palm
fibres exhibited essentially two zones: The first is linear where the
stress is proportional to the strain and the second is non linear and
presents a non elastic behavior. The fibre section of the doum palm is
a parameter which influences the mechanical properties. The alkaline and
the enzymatic treatments have reduced the fibre diameter. The near observation
of the stress evolution shows a small drop followed by resumption for
treated leafstalks fibres in the zone between 50 and 120 MPa.
|
| Fig. 3: |
Typical stress-strain curves for treated and untreated
leafstalks doum palm fibres. (a) Untreated, (b) Alcaline treatment
1 N and (c) Enzymatic treatment |
| Table 2: |
Tensile properties for natural fibres |
|
This for
treated leafstalks fibres with an aqueous concentration of 1, 1.5 N and
also those that incurred an enzymatic treatment.
This phenomenon results from the microfibrils break that generates a decrease
of the stress until a final breaking of the fibre. For untreated fibres, gummy
substances in the fibre ensure the cohesion between microfibrils of cellulose.
Mechanical properties of the doum palm fibres well as that of esparto and of
sisal are shown in Table 2 (Ghali et
al., 2006; Murali and Mohana, 2007). The treatments
influence the mechanical properties such as the tensile modulus, the tensile
strain and the tensile strength.
Even if the treatment has reduced the fibre diameter of the tensile modulus,
the tensile strain and the tensile strength have been improved for a concentration
of aqueous solution equal to 1.5 N. The enzymatic treatment has eliminated
supplemental impurities of the fibre, but has not significantly improved
the mechanical properties. If the concentration of aqueous solution exceeds
3 N, properties of leafstalks fibres will start to degrade.
The comparison between the mechanical properties only is not sufficient
as fibres present neither the same densities nor the same linear densities.
Then, it is worth better, to well characterize fibres, to compare specific
ratios of specific tensile strength and specific tensile modulus for different
natural fibres. The best ratio is the best the fibres are as they have better
resistance for low densities. The specific tensile strength and modulus of various
fibres are also listed in Table 2. The enzymatically treated
fibres have specific tensile strength ratios greater than that of untreated
doum palm fibres or alkali treated. These ratios near of those of sisal and
the palm (Murali and Mohana, 2007) and superior to those
of the esparto (Ghali et al., 2006) show a good
compromise between the resistance and the density.
Mechanical properties of composite: Flexural properties of the
composite achieved as well as others presented at the literature are shown
in Table 3. One can observe that flexural strength and
flexural modulus are improved after the treatment.
The treated composite results in better properties. Alkaline treatments results
in freeing the hydrogen bonds making them more reactive and giving some porous
fibres. It generates the increase of the void content in fibres, the improvement
the wettability and the fibre/matrix contact. All these factors provide better
mechanical properties compared to the ER/PF. The proprieties of the doum palm
fibre/epoxy resin are comparable to resole-epoxy resin hemp and ramie fibre
but superior to resole-epoxy resin flax fibre (Maffezzolia
et al., 2004; Van-de et al., 2003)
show that flax fibre reinforced can be a good reinforcement for epoxy composites.
Nevertheless, strength properties of the composite remain low if no treatment
is performed to enhance the adhesion (Van-De et al.,
2003). The ER/3PF has some mechanical properties lower than those of ER/1.5PF.
It is justified by tensile tests realized on the fibres alone because these
are degraded at a concentration equal to 3 N, but ER/3PF remains always better
that ER/PF.
CONCLUSION
The fibres extracted from the leaves of the doum palm were underwent
an alkali and an enzymatic treatment. The morphological study was carried
out starting from the images obtained by SEM witch show the effect of
the type of the treatment on the fibre diameter and surface quality. The
biological treatment eliminates the residual impurities, gives smooth
and circular fibres. The alkaline treatment yields less clean, more porous
and oval surfaces. The tensile tests on doum palm fibres reveal that fibre
mechanical properties reach their optima for a concentration of 1.5 N
beyond of which they degrade gradually till their minima. The biological
treatment of the leafstalks fibres does not improve the mechanical properties,
but it gives the best specific tensile strength ratio. A better resistant
fibre with low densities is provided. This is an attractive factor for
the manufacturing of light weight materials. The present study showed
the usefulness of doum palm fibre as a good reinforcing agent for composite
fabrication. Flexural properties depend on the concentration of aqueous
solution. It is seen that the alkali treated composites showed superior
flexural properties than untreated composites. The ER/1.5PF has the best
flexural strength and flexural modulus.
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