chapter 3 experimental details -...
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CHAPTER 3
EXPERIMENTAL DETAILS
3.1 INTRODUCTION
The natural fibre composites are to be prepared in view of
achieving the desired properties. In general, the factors that affect the
properties of natural fibre composites are listed below.
1. Type of matrix and fibre
2. Method of fabrication
3. Length, orientation and quantity of fibre
4. Interfacial adhesion between fibre and matrix
5. Stress transfer at the fibre-matrix interface
In this chapter, the matrix material used for the preparation of
composites is discussed in detail. The physical, chemical and mechanical
properties of liquid and cured state of resin are reported in the section 3.2.
The fibre extraction from the Sansevieria cylindrica leaves is described in the
section 3.3. Experiments related to the microstructural, physico-chemical and
mechanical characterisation of raw SCFs are described in the section 3.4. The
chemical treatments performed on SCFs and the experiments related to
characterisation of treated SCFs are separately discussed in the sections 3.5
and 3.6, respectively.
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The composites fabrication using untreated and chemically treated
SCFs is discussed in section 3.7. The experiments related to characterisation
of untreated and treated SCFP composites are presented in section 3.8 and
3.9, respectively. The above process sequences are illustrated in the
Figure 3.1.
Figure 3.1 Scheme of the present investigation
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3.2 MATERIALS
The matrix used in this research was unsaturated polyester resin.
Methyl Ethyl Ketone Peroxide [MEKP] and cobalt naphthenate were used as
curing catalyst and accelerator. The molecular structures of the polyester
matrix, catalyst and accelerator are illustrated in Figures 3.2 (a), (b) and (c),
respectively.
HO C R C
O O
O CH
CH3
CH2 OC
O
CH CH C
O
O CH
CH3
CH2 OH
(a)
HO O C O OH
CH3
C2H5
(b)
O-
O
-O
O
Co++
(c)
Figure 3.2 Molecular structures of (a) polyester matrix, (b) catalyst and
(c) accelerator
Pure resin mixed with accelerator and catalyst was cast in the
mould of size 300 mm × 150 mm × 3 mm. The cast plate as shown in Figure
3.3, was used to prepare test specimens as per ASTM standards for
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conducting tensile, flexural, Impact and hardness tests. The tensile and
flexural tests were performed using INSTRON universal testing machine
4301, Impact testing was performed using KARL FRANK GMPH 53568 and
hardness test using Rockwell hardness tester. The above tests were performed
at Composites Technology Centre, Indian Institute of Technology-Madras,
Chennai. The properties of liquid resin and the plate fabricated out of pure
resin are illustrated in the Table 3.1. The tensile fractograph of cured pure
polyester resin is shown in the Figure 3.4.
Figure 3.3 Fabricated cured pure resin
Table 3.1 Typical properties of the unsaturated polyester resin
Liquid resinAppearance Yellow viscous liquidViscosity@25 ºC 200 – 300 cpsSpecific gravity@25 ºC 1.11±0.02Volatile content 40±2 wt%Acid value 25±5 mg of KOH/g
Cured resinTensile strength 33±1.5 MPaTensile modulus 2.2±0.3 GPaElongation at break 1.5±0.14 %Flexural strength 40.6±3.21 MPaFlexural modulus 1.53±0.28 GPaShear strength 4.1±0.64 MPaImpact strength 0.4±0.05 J/cm2
Barcol hardness 40±3
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Figure 3.4 Tensile fractograph of cured pure polyester resin
The reinforcement used in this investigation was SCFs. It is
extracted from Sansevieria cylindrica leaves by “mechanical decortication”
method that is described in section 3.3. The chemicals used for the treatment
of SCFs are sodium hydroxide, potassium permanganate, benzoyl peroxide,
acetone, stearic acid and ethyl alcohol. The matrix, catalyst, accelerator and
chemicals used for the modification of SCFs were supplied by Raja Traders,
Tirunelveli, Tamil Nadu, India.
3.3 EXTRACTION PROCEDURE FOR SCFs
The SCFs were separated from the Sansevieria cylindrica leaves,
which were collected from farms around the city of Tirunelveli in Tamil
Nadu, India, using a mechanical process called decortication. In this process,
the Sansevieria cylindrica leaves were fed into a fibre-extracting machine
called a mechanical decorticator that is schematically shown in Figure 3.5.
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The machine consists of a pair of feed rollers and a beater. The leaves were
fed into the beater through the feed rollers between a squeezing roller and a
scraper roller. The fibres were extracted, after separating the pulps. The
decorticated fibres were dried in the sunlight for 24 h to remove the moisture
content, and machine combed for separation. The separated SCFs are shown
in Figure 3.6.
Figure 3.5 Schematic diagram of a mechanical decorticator
Figure 3.6 Dried SCFs
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3.4 CHARACTERISATION OF SCFs
The leaves and fibre specimens of Sansevieria cylindrica were used
for microstructural analysis. The physical analysis of SCF was carried out to
find the density, fineness and cross sectional area of the fibre. The chemical
analysis of SCF was also made to predict the chemical composition. The
tensile behaviour of the SCFs is also discussed in this section.
3.4.1 Microstructural Studies of Sansevieria cylindrica Leaf and Fibres
The microstructural analysis was performed using SEM and
Polarised light microscope, in order to understand the internal structure of the
Sansevieria cylindrica leaf and fibre.
3.4.1.1 Preparation of specimens and sectioning
Healthy Sansevieria cylindrica leaf was collected for
microstructural analysis. For anatomical studies, the leaf was cut into small
pieces (10 mm x 10 mm) and immersed in FAA (5 ml formaldehyde + 5 ml
acetic acid + 90 ml 70% ethyl alcohol) for 24 h. Then the specimens were
dehydrated through a graded tertiary-butyl alcohol series and embedded in
paraffin (Sass 1940). Sections of 10 – 12 µm were cut on a rotary microtome,
affixed to a glass slide and stained with a mixture of Toluidine blue,
safranine, fast green and Lugo’s iodine solution (O’Brien et al 1964).
Polarised light micrographs were taken with a Nikon Labphoto 2 microscopic
unit. For normal observations, bright field imaging was used. For the crystal
studies (starch grains and lignified cells) polarised light was employed.
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3.4.1.2 Scanning Electron Microscopy
Transverse sections of Sansevieria cylindrica leaf and fibres were
examined using a SEM, HITACHI Model S3000H. The leaf section and
fibres were given uniform gold coating on all sides except their cross sections
to make the surfaces conductive. The cross sections of the Sansevieria
cylindrica leaf and fibres were examined at different magnifications. In
addition, the longitudinal view of the fibre was also observed.
3.4.2 Physical Properties of SCFs
The goal of this section is to describe the methodology for
measuring the fibre cross-section, its microstructural features and the physical
properties of fibre (i.e., its density and fineness). The cross-sectional area of
the fibre is needed for measuring the tensile strength of the fibre. The
microstructural features will be useful to identify the internal structure of the
fibre.
Optical microscopy of the fibre cross-section revealed that it is
irregular. To measure the cross-sectional area of the fibre, an adjacent piece of
tensile tested fibre (the piece close to the broken portion) was examined using
the SEM. In order to get acceptable cross-sectional area of the fibre, a 40-mm-
long fibre was cut into four sections of 10 mm each, and four micrographs
were taken. The images were post-processed using ImageJ software. A
contour line was drawn to delineate the fibre cross-section (Figure 3.7), and
the area was determined.
The porosity fraction (PF) of the natural fibres is defined as the
ratio of the total surface area of the fibre lumens to the raw surface area of the
fibre cross-section (Beakou et al 2008). The actual load bearing area of the
natural fibre will be less than the calculated cross sectional area, as it
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possesses porosity. The PF of the SCF cross-section was computed using
SEM and ImageJ. The actual load bearing cross sectional area of the SCF can
be arrived using the following equation.
Actual load bearing area = Total cross sectional area x (1- PF) (3.1)
The dimensions of various microscopic features (i.e., the middle
lamellae, primary wall, secondary wall and fibre lumen) of the SCF were
calculated using the SEM and ImageJ.
Figure 3.7 Area calculation using ImageJ from the polarised light
micrograph showing a transverse section of the structural
fibre (40X)
The density measurement of the SCF was carried out using a
pycnometer for solids with toluene as the immersion liquid following the
procedure of Beakou et al (2008). The fibres were dried for 96 h in a
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desiccator containing silica. They were then cut into lengths of 5-15 mm and
introduced into the pycnometer and then placed in the desiccator for 24 h. The
temperature in the room was 18.1°C. The hygrometry was 57% in the room
and only 3% in the desiccator. Before carrying out the hydrostatic weighing
with toluene, the fibres were impregnated in toluene for 2 h to evacuate the
micro bubbles in the fibres. The density of toluene ( t) is 0.866 g/cm3.
The density of the SCF was calculated using the following formula:
SCF = (m2 – m1) t (3.2)
(m3 – m1) – (m4 – m2)
where SCF is the density of the SCF (g/cm3), t is the density of the toluene
(g/cm3), m1 is the mass of the empty pycnometer (kg), m2 is the mass of the
pycnometer filled with chopped fibres (kg), m3 is the mass of the pycnometer
filled with toluene (kg) and m4 is the mass of the pycnometer filled with
chopped fibres and toluene (kg).
The fineness of the SCF was determined in terms of linear density
in accordance with ASTM D 1577-92. The linear density was obtained as Tex
and denier from the weight of 100 single fibres that were each 60 mm in
length.
3.4.3 Chemical Properties of the SCFs
This analysis was performed to determine the content of lignin,
cellulose, hemicelluloses, wax and moisture of the SCF using conventional
chemical, XRD and FTIR analyses.
The determination of the lignin content was carried out according
to the Klason method (Pearl 1967). The samples were crushed and extracted
with dichloromethane before being hydrolysed in a 72% solution of sulphuric
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acid. Lignin was the only insoluble component and separated from the fibre
and quantified. The cellulose content was measured according to Kurshner
and Hoffer’s method (1933). The samples were crushed and extracted with
dichloromethane, and then a mixture of ethanol and 95% nitric acid was
added. The cellulose that corresponds to the insoluble fraction of the samples
was weighed. The determination of hemicelluloses was carried out according
to standard NFT 12-008. The samples were heated in hydrobromic acid. The
hemicelluloses was transformed into furfural, and the latter was extracted by
distillation and measured by spectrophotometry.
The wax content in the SCF was determined following the method
developed by Conrad (1944). The SCF samples were crushed and subjected to
soxlet extraction using ethanol for 6 h. The resulting solution containing
sugar, wax and other alcohol-soluble substances was transferred to a separator
funnel. Chloroform was added to extract the wax from the alcohol solution.
Purified water was then added and separate layers of chloroform and alcohol
were formed. After separation, the chloroform evaporated from the solution
leaving a waxy residue. After extraction, the dried residue and the SCF
samples were weighed, and the wax content was expressed in terms of the
original weight of the SCF samples.
The moisture content in the SCF was determined by drying the
weighed SCF samples in an oven at 104°C for 4 h. The dried samples were
weighed and the moisture content in the SCF was determined using the
following expression:
% of Moisture Sample weight - Sample weightContent in SCF = before drying after drying 100 (3.3)
Sample weight before drying
X-ray spectra (scan range (2 ) = 10 – 80°; = diffraction angle;
scan speed = 5.0 deg/min) of the SCF were obtained with a Rigaku X-ray
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diffractometer D/Max Ultima III with an X-ray tube producing
monochromatic Cu K radiation. The integrated intensities of the Bragg peaks
in the spectrum of the SCF were identified, and their crystallinity indices were
estimated.
FTIR spectra of the SCF were recorded using a Perkin Elmer
Spectrum RXI FTIR spectrometer in a KBr matrix with a scan rate of 32
scans per minute and a resolution of 2 cm-1 in the wave number region from
400–4000 cm-1. The SCF samples were chopped into small particles using
scissors and ground to a fine powder using a mortar and pestle. This powder
was mixed with KBr and pelletised by pressurisation to record the FTIR
spectra under standard conditions.
3.4.4 Tensile Behaviour of the SCFs
The dried SCFs were tested in dry conditions under tensile loading
at gauge lengths (GL) of 10, 20, 30 and 40 mm in an INSTRON universal
testing machine of type 5500 R (Figure 3.8) according to the ASTM D 3822-
01 standard. The GL was varied to determine its effect on the tensile
properties. Pneumatic grips were used to clamp the fibre with a pressure of
0.4 MPa. The load was measured using 1.0 kN capacity load cell. The
displacement of the fibre was measured by a short-stroke transducer with a
resolution of approximately 0.1 µm. The tensile tests were conducted with a
cross head speed of 0.1 mm/min. The average strain rates were on the order of
0.6 s-1 and 0.15 s-1 for the gauge lengths of 10 mm and 40 mm, respectively.
Due to the variability of the natural fibres, 20 samples were tested at each GL,
and the average value was reported. All testing was conducted at ambient
temperature (~21°C) and a relative humidity of about 65%.
The compliance of the loading and gripping systems was
determined by obtaining the force versus displacement behaviour of the fibre
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at various GLs following the methodology used by Chawla et al (2005). The
total cross–head displacement during fibre testing, t, is expressed using the
following expression.
t/ F) = (l/EA) + c (3.4)
where F is the applied force, l is the GL, E is the Young’s modulus of the
fibre, A is the cross-sectional area of the fibre and c is the machine
compliance. A plot of t / F versus GL (l) yields a straight line of Slope
1/(EA) and intercept c, which is the compliance of the load train.
Figure 3.8 INSTRON universal testing machine used for tensile testing
3.5 SURFACE TREATMENTS ON SCFs
The wax and hemicelluloses contents present over the surfaces of
SCFs reduce the adhesion between matrix and fibre. The surfaces of SCFs
were chemically treated to improve interfacial bonding with a polyester
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matrix. The raw SCFs were subjected to different surface treatments using
alkali, benzoyl peroxide, potassium permanganate and stearic acid.
3.5.1 Alkali-treated SCF (ASCF)
The SCFs were soaked in a stainless steel vessel containing 10%
sodium hydroxide solution for 1 h. The fibres were washed thoroughly with
water to remove the excess of sodium hydroxide on the fibres. Final washing
was done with water containing a little acetic acid. Fibres were dried in an air
oven at 70 oC for 3 h (Sherely Annie Paul et al 2008).
3.5.2 Benzoyl-peroxide–treated SCF (BSCF)
The ASCFs were placed in 6% benzoyl peroxide diluted acetone
for half an hour. The fibres were then removed out and dried in air for 24 h
(Augustine Paul et al 1997).
3.5.3 Potassium-permanganate–treated SCF (PSCF)
The ASCFs were soaked in 0.5% potassium permanganate diluted
acetone for half an hour. The fibres were then taken out and dried in air for 24
h (Sherely Annie Paul et al 2008).
3.5.4 Stearic-acid–treated SCF (SSCF)
In all, 1% stearic acid was dissolved in ethyl alcohol. The solution
was then added dropwise to the ASCFs placed in a stainless steel vessel with
continuous stirring. These fibres were then dried in an air oven at 80 oC for 45
min (Augustine Paul et al 1997).
All of the chemical treatments and their reactions are schematised
in Figure 3.9.
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3.6 CHARACTERISATION OF CHEMICALLY TREATED SCFs
In view of predicting the enhancement in the surface morphology,
physico-chemical and mechanical characteristics of the chemically treated
fibres, it becomes necessary to analyse the fibres after chemical treatment and
compare the same with the properties of raw fibres. In this section the studies
on experiments related to treated SCFs are present.
3.6.1 Surface Morphology of the Chemically Treated SCFs
The effects of modification upon the fibre surface were examined
using a SEM, JEOL model 6390 for the untreated SCFs (USCFs), ASCFs,
BSCFs, PSCFs and SSCFs samples. Prior to the analysis, the samples were
coated with platinum (layer thickness ~ 30 nm) to avoid sample charging
under the electron beam. The secondary electrons were used for imaging. The
surface morphology of untreated and treated SCFs was examined at different
magnifications. In addition, the transverse sections of ASCFs, BSCFs, PSCFs
and SSCFs were also observed in order to calculate their cross-sectional areas.
3.6.2 Physical, Chemical and Mechanical Characterisation of the
Chemically Treated SCFs
The physical, chemical and mechanical characterisations of the
chemically treated SCFs were performed by following the experiments
explained in the sections 3.4.2, 3.4.3 and 3.4.4 for raw SCFs. In addition
under physical analysis of the chemically treated SCFs, the percentage weight
loss of treated SCFs due to chemical treatments was evaluated using the
following expression.
% weight loss of treated SCFs = ((Wx – Wy)/Wx) x 100 (3.5)
where Wx is the weight of a dry USCF sample (g) and Wy is the weight of a
dry SCF sample after chemical treatment (g).
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3.7 PREPARATION OF SCFP COMPOSITES
The compression moulding method was adopted for the fabrication
of composites. The cleaned and dried SCFs were chopped into different
lengths of 10 mm, 20 mm, 30 mm, 40 mm and 50 mm. A known weight of
SCFs of definite length was randomly spread between two mild steel plates.
Extreme care was taken to obtain a uniform distribution of fibres. A load of
40 metric tons was applied on the mild steel plates by hydraulic compression
to form a single sheet. This compressed sheet was placed in a mould with a
size of 300 mm x 150 mm x 3 mm. Then, 97.5% of unsaturated polyester
resin was mixed with 2% MEKP (catalyst) and 0.5% cobalt naphthenate
(accelerator). The prepared matrix solution was degassed before pouring. The
degassed matrix solution was applied on the compressed sheet by using a
brush, and air bubbles were removed carefully with a grooved roller. The
mould was closed, and hydraulic pressure was applied until complete closure.
The closed mould was kept under pressure for 24 h. The composites were
fabricated in the form of a flat plate with a size of 300 mm x 150 mm x 3 mm.
Composite plates were prepared by varying fibre weights of 10%, 20%, 30%,
40% and 50%, in each fibre length mentioned above. A short SCFP composite
plate is shown in Figure 3.10.
3.8 CHARACTERISATION OF SCFP COMPOSITES
The raw SCFP composite plates were used to prepare the test
samples as per ASTM standards. Then the samples were used to evaluate
mechanical properties like tensile, flexural, impact and hardness. The
fractured surfaces of composites were analysed using SEM. XRD analysis
was performed to identify the chemical functional group existing in SCFP
composites. Water absorption characteristics of raw SCFP composites were
also studied.
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Figure 3.10 Short SCFP composite plate
3.8.1 Mechanical Tests
Test specimens were cut from the composite plates as per the
ASTM standard. Tensile testing was carried out in a FIE universal testing
machine UTE–40 with a 400-kN capacity with a gauge length of 100 mm and
a cross head speed of 1 mm/min, as per ASTM D 638-01 (2002). The three-
point flexural properties were determined by an INSTRON universal testing
machine 4301 with a 5-metric ton capacity, a test span of 50 mm and a cross
head speed of 1 mm/min, according to ASTM D 790–00 (2002). The Izod
impact test was done on unnotched specimens with a KARL FRANK GMBH
53568 impact testing machine with a pendellange of 390 mm. The impact test
was carried out with an impact speed of 3.46 m/s and an incident energy of
2.75 J according to ASTM D 6110-97 (2002). The hardness of the composites
was measured using a Rockwell hardness testing machine, according to
ASTM D 785-98 (2002). A minimum of six samples were tested in each case,
and the average value is reported. All testing was conducted at ambient
temperature (~ 21 C) and a relative humidity of about 65%.
3.8.2 Scanning Electron Microscopy (SEM)
Fractography of the failure surface of composite samples were
examined using a SEM, JEOL model 6390. The fractured portions of the
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samples were cut, and the SEM micrographs were taken. A uniform coating
of platinum was given to the samples in order to make the surface conducting,
and then they were examined under the above microscope. The secondary
electrons were used for imaging. The surface of the fractured specimens under
tensile and impact tests were examined at different magnifications.
3.8.3 X-ray Diffraction (XRD) Analysis
XRD analysis was performed on the short SCFP composite
possessing the critical fibre length and optimum fibre weight percent. X-ray
spectra (scan range ( ) = 10-80 ; = diffraction angle; scan speed = 5.0
deg/min) of the short SCFP composites were obtained with a Rigaku X-ray
diffractometer D/Max Ultima III with an X-ray tube producing
monochromatic Cu K radiation. The integrated intensities of the Bragg peaks
in the spectrum of the short SCFP composites were identified, and their
crystallinity indices were estimated.
3.8.4 Water Absorption Test
In order to measure the water absorption characteristics of the
composites, rectangular specimens were prepared with dimensions of 39 mm
x 10 mm x 3 mm. The specimens were dried in an oven at 105 °C, cooled in a
desiccator using silica gel and immediately weighed. A Denver Instron
balance was used for weight measurement. The dried and weighed specimens
were immersed in distilled water according to ASTM D 570-99 (2002). The
water absorption tests were carried out by immersing the specimens for 2 h
and 24 h in hot and cold water, respectively. After immersion, the excess
water on the surface of the specimens was wiped up using a piece of soft cloth
and the final weights of the specimens were taken. The water absorption in
percentage was calculated using the following equation (3.6).
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Water absorption (%) = Final weight – Original weight x 100 (3.6)
Original weight
3.9 CHARCTERISATION OF CHEMICALLY TREATED SCFP
COMPOSITES
Separate composite plates were prepared using each type of
chemically treated SCFs such as ASCFs, BSCFs, PSCFs and SSCFs, with
critical fibre length and optimum fibre weight percent. The composites were
fabricated by compression moulding method, which was explained in section
3.7. The experiments related to tensile, flexural, impact, hardness and water
absorption characteristics of chemically treated SCFP composites were
explained in section 3.8. The tensile, flexural, impact, hardness and water
absorption characteristics of these composites were analysed and compared
with untreated SCFP (USCFP) composites. Based on the mechanical
properties, the optimum chemical treatment for short SCFP composite was
identified and reported. SEM micrographs were utilised to describe the failure
modes of composites.
3.10 SUMMARY
SCFs were extracted from Sansevieria cylindrica leaves using
mechanical decortication method. The fibres were subjected to chemical
treatments in order to improve the interfacial bonding between fibre and
matrix. The experimental procedures related to microstructural, physico-
chemical and mechanical characterisation of raw and chemically treated SCFs
were studied. Raw SCFP composites were prepared by compression moulding
method by varying the fibre length and fibre weight percentage. Treated
SCFP composites were prepared from each type of chemically treated SCFs
separately at critical fibre length and optimum fibre weight percentage. The
experiments relevant to mechanical properties of raw and chemically treated
SCFP composites were also studied in this chapter.