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CHAPTER 6
EFFECT OF ALKALI TREATMENT OF COIR FIBERS ON
THE MECHANICAL PROPERTIES OF COIR-POLYESTER
COMPOSITES
6.1 INTRODUCTION
The improvement of mechanical properties of coir-polyester by
glass hybridization was discussed in the previous chapter. In order to improve
the mechanical properties of coir-polyester composites to that level, the alkali
treatment of coir fibers and its effect on the mechanical properties of non-
woven and woven coir-polyester composites were discussed in this chapter.
6.2 NEED OF ALKALI TREATMENT OF FIBERS
The treated fiber resulted significant increase in tensile strength,
flexural strength and impact strength in natural fiber-reinforced polymer
composites (Prasad et al (1983)). Alkali treatment of fibers increases the
strength of natural fiber composites (Calado et al (2000)). A strong sodium
hydroxide treatment removed lignin, hemi-cellulose and other alkali soluble
compounds from the surface of the fibers to increase the numbers of reactive
hydroxyl groups on the fiber surface available for chemical bonding.
Moreover, the alkali treatment made the fiber surface clean by the removal of
waxes, hemicellulose, pectin and part of lignin. The removal of these
substances enhanced the surface roughness. Therefore, the mechanical
interlocking at the interface could be improved (Rout et al (2001)).
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Natural fibers can be considered as composites of hollow cellulose
fibrils held together by a lignin and hemicellulose matrix. The cell wall in a
fiber is not a homogenous membrane. Each fiber has a complex, layered
structure consisting of a thin primary wall which is the first layer deposited
during cell growth encircling a secondary wall. The secondary wall is made
up of three layers and the thick middle layer determines the mechanical
properties of the fiber. The middle layer consists of a series of helically
wound cellular microfibrils formed from long chain cellulose molecules. The
angle between the fiber axis and the microfibrils is called the microfibrillar
angle. The characteristic value of microfibrillar angle varies from one fiber to
another. Such microfibrils have typically a diameter of about 10–30 nm and
are made up of 30-100 cellulose molecules in extended chain conformation
and provide mechanical strength to the fiber.
The amorphous matrix phase in a cell wall is very complex and
consists of hemicellulose, lignin, and in some cases pectin. The hemicellulose
molecules are hydrogen bonded to cellulose and act as cementing matrix
between the cellulose micro fibrils, forming the cellulose-hemi cellulose
network, which is thought to be the main structural component of the fiber
cell. The hydrophobic lignin network affects the properties of other network
in a way that it acts as a coupling agent and increases the stiffness of the
cellulose/hemi cellulose composite.
The effect of fiber treatment parameters on tensile, flexural and
impact strength of coir-polyester composites were studied in this work. The
green husk coir fibers were treated with different levels of soaking time and
concentration of alkali solution. As a result of alkali treatment, the surface
modifications are done on the fiber surface and studied using scanning
electron micrographs. The coir-polyester composites were fabricated using
hand lay-up process and the mechanical properties like tensile, flexural and
92
impact strength were evaluated as per ASTM standards. The effect of soaking
time and concentration of NaOH solution were studied based on evaluated
mechanical properties to find out optimum fiber treatment parameters.
6.3 MATERIALS AND MANUFACTURING PROCESS
The green husk coir fibers were mechanically extracted from the
green husk of coconut after soaking the husk in water. The green husk fiber
bales were soaked in water for 3-7 days to remove the colouring matter and to
make the fibers soft.
6.3.1 Fiber treatment
Green husk coir fibers were chemically treated in order to remove
lignin-containing materials such as pectin, waxy substances and natural oils
covering the external surface of the fiber cell wall. This reveals the fibrils and
gives a rough surface topography to the fiber. Sodium hydroxide (NaOH) is
the most commonly used chemical for bleaching and/or cleaning the surface
of plant fibers. It also changes the fine structure of the native cellulose-I to
cellulose-II by a process known as mercerisation. The reaction of sodium
hydroxide with cellulose as follows.
Cell - OH + NaOH Cell – O- Na
+ + H2 O + [Surface impurities] (6.1)
It is worth pointing out that mercerisation de-polymerises the native
cellulose-I molecular structure producing short length crystallites. However,
there seems to be varying interpretations of the term ‘mercerisation’. The
standard definition for mercerisation proposed by ASTM D1695 is “the
process of subjecting a vegetable fiber to the action of a fairly concentrated
aqueous solution of a strong base so as to produce great swelling with
93
resultant changes in the fine structure, dimension, morphology and
mechanical properties”.
For alkali treatment, the three levels of concentration of NaOH
(2%, 5% & 8%) were selected and five levels of soaking time (24 h, 48 h, 72
h, 96 h & 120 h) were selected based on literature (Prasad et al (1983)).
The treated coir fiber reinforced composites were fabricated for the
combinations of different levels of soaking time and concentration of alkali
solution. The structure of coir fiber is shown in Figure 6.1 and the treated coir
fibers are shown in Figure 6.2.
Figure 6.1 Structure of coir fiber
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Figure 6.2 Treated Coir fibers
Figure 6.3 shows the cross section and surface of a coir fiber after
the NaOH treatment.
Figure 6. 3 SEM image of treated fiber
95
Figures 6.4, 6.5 and 6.5 show the comparison of NaOH
concentration for different soaking hours in 2%, 5% and 8% respectively. All
the images were taken with 60 X magnification.
Figure 6.4 SEM images of treated coir fiber in 2 % NaOH Concentration
Figure 6.5 SEM images of treated coir fiber in 5% NaOH Concentration
96
Figure 6.6 SEM images of treated coir fiber in 8% NaOH Concentration
From SEM images it was observed that the NaOH concentration
and soaking time increases, it makes the fiber soft and form internal hollow
structure by removing lumen in fiber. The excess removal of lumen affects
the mechanical properties when the fiber was used as reinforcement in
composites. To find the effect of NaOH concentration and soaking time the
composites were prepared using treated fibers.
6.4 MECHANICAL TESTING
6.4.1 Tensile Testing
Tensile tests were conducted using Shimadzu tensile testing
machine at a cross head speed of 5mm/min as per ASTM D638 - 08. The
length, width and thickness of each sample were approximately 165 mm, 25
mm & 3 mm respectively.
97
The tested mechanical property values for the different levels
soaking time and concentration of NaOH are shown in Table 6.1.
Table 6.1 Tensile strength of treated coir fiber-reinforced polyester
composites
Sl.No.Soaking time
(hours)
Tensile strength (MPa)
Concentration
of NaOH (2%)
Concentration
of NaOH (5%)
Concentration
of NaOH (8%)
1 t-24 18.56 19.34 20.92
2 t-48 19.58 21.55 23.17
3 t-72 21.94 23.56 21.42
4 t-96 22.83 18.34 19.76
5 t-120 17.98 17.19 18.59
6.4.2 Flexural Testing
The rectangular test pieces of 125 × 12.5 × 3 mm dimension for
flexural test were cut from the prepared non woven composites. The specimen
was prepared as a beam and the load was applied in the middle of the
specimen. The tests were carried out at a temperature of 27°C and the relative
humidity of 50%. For statistical purposes, a total of 5 samples were tested.
The tested values for the different combination of fiber treatment processes
are shown in Table 6.2.
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Table 6.2 Flexural strength of treated coir fiber-reinforced polyester
composites
Sl.No.Soaking time
(hours)
Flexural strength (MPa)
Concentration
of NaOH (2%)
Concentration
of NaOH (5%)
Concentration
of NaOH (8%)
1 t-24 24.19 26.77 18.72
2 t-48 32.22 24.42 44.45
3 t-72 21.86 39.85 21.91
4 t-96 48.08 24.86 21.19
5 t-120 29.24 24.31 20.42
6.4.3 Impact Testing
The impact strength of the samples was measured using ATS
FAAR Impact tester as per ASTM D256 – 06a1 standards. The test specimen
was supported as a vertical cantilever beam and broken by a single swing of a
pendulum. The pendulum strikes the face of the sample and total of 5 samples
were tested and the mean value of the absorbed energy was taken. The tested
result of impact strength values are given in Table 6.3.
Table 6.3 Impact strength of treated coir fiber-reinforced polyester
composites
Sl.No.Soaking time
(hours)
Impact strength (kJ/m2)
Concentration
of NaOH (2%)
Concentration
of NaOH (5%)
Concentration
of NaOH (8%)
1 t-24 48.02 55.24 58.75
2 t-48 53.78 58.45 54.23
3 t-72 56.86 54.97 52.78
4 t-96 54.25 52.41 49.43
5 t-120 49.23 50.05 48.26
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The comparison of mechanical properties of untreated and treated
coir fiber reinforced polyester composites are given in Table 6.4.The
untreated green husk coir fiber reinforced composites exhibited the average
tensile, flexural and impact strength of 16.2 MPa,38.5 MPa and 41.2 kJ/m2
respectively.
Table 6.4 Comparison of mechanical properties and optimum parameters
Sl.
No.Properties
Coir-polyester composites Optimum parameters
UntreatedTreatedIncrease in
strength (%)
Concentration
of NaOH (%)
Soaking
Time
(Hours)
1Tensile
strength(MPa)16.2 23.6 31 5 72
2Flexural
strength(MPa)38.5 49.1 22 2 96
3Impact
strength(kJ/m2)
41.2 58.8 30 8 24
6.5 RESULTS AND DISCUSSION
6.5.1 Effect of soaking time on the mechanical properties
The effect of soaking time for different concentration of alkali
solution for tensile, flexural and impact properties are shown in Figures 6.7,
6.8 and 6.9 respectively.
100
16
18
20
22
24
26
t-24 t-48 t-72 t-96 t-120
Soaking time(Hours)
Ten
sile s
tren
gth
(MP
a)
2% NaOH concentrat ion 5% NaOH concentrat ion 8% NaOH concentrat ion
Figure 6.7 Tensile strength values for different levels of soaking time
15
20
25
30
35
40
45
50
t-24 t-48 t-72 t-96 t-120
Soaking time(Hours)
Fle
xu
ral
str
en
gth
(MP
a)
2% NaOH concentrat ion 5% NaOH concentration 8% NaOH concentration
Figure 6.8. Flexural strength values for different levels of soaking time
45
47
49
51
53
55
57
59
t-24 t-48 t-72 t-96 t-120
Soaking time(Hours)
2% NaOH concentrat ion 5% NaOH concentrat ion 8% NaOH concentrat ion
Figure 6.9 Impact strength values for different levels of soaking time
Imp
ac
t s
tren
gth
(k
J/m
2)
101
(i) For the maximum value of tensile strength, 96 hours
treatment in 2% aqueous solution gives 22.83 MPa, 72 hours
treatment in 5% aqueous solution gives 23.56 MPa and 48
hours treatment in 8% aqueous solution gives 23.17 MPa.
(ii) For the maximum value of flexural strength, 96 hours
treatment in 2% aqueous solution gives 48.08 MPa, 72 hours
treatment in 5% aqueous solution gives 39.85 MPa and 48
hours treatment in 8% aqueous solution gives 44.45 MPa.
(iii) For the maximum value of Impact strength, 72 hours
treatment in 2% aqueous solution gives 56.86 MPa, 48 hours
treatment in 5% aqueous solution gives 58.45 MPa and 24
hours treatment in 8% aqueous solution gives 58.75 MPa.
6.5.2 Effect of concentration of NaOH on the mechanical properties
The effect of concentration of NaOH solution for different levels of
soaking time for tensile, flexural and impact properties are shown in Figures
6.10, 6.11 and 6.12 respectively. The NaOH concentration is also playing
significant role on the improved value of mechanical properties. In general,
high concentration of alkali solution and shorter soaking time provides better
impact strength whereas the low and medium concentration of alkali solution
for longer shorter time provides improved flexural and tensile properties
respectively.
16
18
20
22
24
26
2% 5% 8%
Concentration of NaOH(%)
Ten
sile s
tren
gth
(MP
a)
t -24 t-48 t-72 t-96 t-120
Figure 6.10 Tensile strength values for different levels of NaOH
concentration
102
15
20
25
30
35
40
45
50
2% 5% 8%
Concentration of NaOH(%)
Fle
xu
ral
str
en
gth
(MP
a)
t -24 t-48 t-72 t-96 t-120
Figure 6.11 Flexural strength values for different levels of NaOH
concentration
45
47
49
51
53
55
57
59
2% 5% 8%
Concentration of NaOH(%)
t -24 t-48 t-72 t-96 t-120
Figure 6.12 Impact strength values for different levels of NaOH
concentration
6.5.3 SEM analysis of fractured surfaces
The SEM images of untreated green husk fiber reinforced polyester
composites after tensile, flexural and impact testing are shown in
Figures 6.13, 6.14 & 6.15 respectively.
Imp
ac
t str
en
gth
(k
J/m
2)
103
Figure 6.13 SEM image of untreated fiber coir composites after tensile
testing
Figure 6.14 SEM image of untreated fiber coir composites after flexural
testing
Figure 6.15 SEM image of untreated fiber coir composites after impact
testing
104
The increased mechanical properties of treated coir fiber-
reinforced polyester composites after tensile, flexural and impact fracture are
studied using the SEM images shown in Figures 6.16 to 6.18 respectively.
The treatment process removes lignin, hemi cellulose and other
soluble compounds on the surface of the fiber and makes the fiber soft to
adhere easily with the polyester resin matrix. The reasons for the improved
mechanical properties are the removal of impurities on the fibers during alkali
treatment. The level of interfacial adhesion is increased by the use of treated
fibers in composites. It can be expected that due to alkaline treatment, hemi-
cellulose and lignin are removed, the inter fibril region is likely to be less
dense and less rigid, and that makes the fibrils able to rearrange themselves
along the direction of loading (Report of Delft University (2003)).
Figure 6.16 SEM image of treated fiber coir composites after tensile
testing
105
Figure 6.17 SEM image of treated fiber coir composites after flexural
testing
Figure 6.18 SEM image of treated fiber coir composites after impact
testing
Figure 6.19 shows the SEM image of treated woven coir
composites after tensile, flexural and impact fractures.The woven coir mat
was prepared using treated coir fibers.The treatment parameters for optimum
value of tensile,flexural and impact strength of non-woven treated coir fiber
reinforced polyester composite were used.
106
A significant increase of mechanical properties were obtained in
treated coir fiber-reinforced woven composites.The comparison of
tensile,flexural and impact strength values of treated and untreated coir fiber-
polyester composites are given in Table 6.5.
Figure 6.19 SEM images of treated woven coir composites after fracture
Table 6.5 Comparison of mechanical properties in untreated and
treated coir fiber-reinforced woven composites
Sl.
No.Properties
Woven Coir-Polyester
compositesOptimum parameters
Untreated Treated
Increase in
strength
(%)
Concentration
of NaOH
(%)
Soaking
Time
(Hours)
1Tensile
strength(MPa)19.9 33.3 40 5 72
2Flexural
strength(MPa)31.3 54.2 42 2 96
3Impact
strength(kJ/m2)
49.9 62.2 20 8 24
107
6.6 SUMMARY
Alkali treatment of coir fiber in 5% aqueous solution for 72
hours results in a 31% increase in tensile strength, 2% aqueous
solution for 96 hours results in a 22% increase in flexural
strength and 8% aqueous solution for 24 hours results in a
30% increase in impact strength in non-woven treated coir
fiber reinforced polyester composites
The 40 % increase of tensile strength,42 % increase of flexural
strength and 20 % increase of impact strength were achieved
in treated woven fiber reinforced polyester composites.
The mechanical properties are greatly improved by the fiber
treatment process and the treatment parameters, soaking time
and concentration of NaOH solution are also playing major
role in increasing the tensile, flexural and impact properties of
green husk coir fiber reinforced composites.