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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

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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

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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

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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

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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.

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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)

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(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

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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)

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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

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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

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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.

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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

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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.