4.1 INTRODUCTION
Since the 1940s, there has been an ever increasing development in the field of
synthesis and development of new synthetic polymers, composites and biocomposites.
Now a days, the demand for polymers as a complete product or a part thereof is
tremendous and many products require a diverse of polymers (e.g. the automotive
branch, the information and communication branch, the paint industry, the cosmetics
industry, the pharmaceutical industry, in food packaging, and in lightweight metal
replacements). Compared to more classical materials, such as metal, ceramics, or
wood, polymers cover an astonishing wide range of possible product applications,
since their properties can be easily tailored to fit specific needs. By changing the
polymer and its composition, the mechanical, thermal, structural and electrical
properties can be fine-tuned. Even though polymers can easily be prepared and
shaped, the mechanical or physical properties of many engineering plastics need to be
enhanced by the preparation of multi-phase morphologies [1, 2].
The change in the size and shape of a material caused by mechanical action of
an external force or by various physico-chemical processes is known as deformation.
Depending upon the nature of the material, the mechanical deformation can be
categorized as elastic, plastic, elastomeric and anelastic. Elastic deformation exists
only during the application of load and disappears completely on removal of stress,
while plastic deformation remains even after the removal of stress. The elastomeric
materials exhibit linear elasticity and the deformation is independent of time. The
anelastic deformations are fully recoverable, but time dependent. The resistance of
any material involves both the elastic and plastic properties [2].
The micro indentation hardness technique has in recent years found
widespread applications in polymer research [3]. The technique has been increasingly
used in the characterization of the homo polymers, polymer, blends, and copolymers
[4]. A very attractive feature of this technique is its ability for the micro-mechanical
characterization of polymeric materials [5]. In addition, microhardness may be
successfully used to gain information on morphology. Blends of PVA with other
polymers have been mechanically characterized by many researchers [6,7].
89 | P a g e 4.2
MECHANICAL PROPERTIES OF POLYMER
The mechanical properties of polymers are of interest, in particular in all
applications where polymers are used as structural materials. Mechanical behaviour
involves the deformation of material under the influence of applied forces. The
mechanical properties of polymers are one of the features that distinguish them from
small molecules. When we consider the mechanical properties of polymeric materials,
and in particular when we design methods of testing them, the parameters most
generally considered are stress, strain, Young’s modulus and microhardness. In the
present work following studies have been undertaken for the mechanical
characterization of the biocomposite polymer 1. Microhardness of Biocomposite Polymer
2. Tensile Properties of Biocomposite Polymer
4.3 METHODS OF HARDNESS MEASUREMENT
The hardness implies resistance to local surface deformation against
indentation [8]. If we accept the practical conclusion that a hard body is one that is
unyielding to the touch, it is at once evident that steel is harder than rubber. If,
however, we think of hardness as the ability of a body to resist permanent
deformation, a substance such as rubber would appear to be harder than most metals.
This is because the range over which rubber can deform elastically is very much
larger than that of metals. Indeed with rubber-like materials the elastic properties play
a very important part in the assessment of hardness. With metals, however, the
position is different, for although the elastic moduli are large, the range over which
metals deform elastically is relatively small. Consequently, when metals are deformed
or indented (as when we attempt to estimate their hardness) the deformation is
predominantly outside the elastic range and often involves considerable plastic or
permanent deformation. For this reason, the hardness of metals is bound up primarily
with their plastic properties and only to a secondary extent with their elastic
properties. In some cases, however, particularly in dynamic hardness measurements,
the elastic properties of the metals may be as important as their plastic properties [8].
Hardness measurements usually fall into three main categories: scratch
hardness, indentation hardness and rebound or dynamic hardness.
90 | P a g e 4.3.1
Scratch Hardness
Scratch hardness is the oldest form of hardness measurement and was
probably first developed by mineralogists. It depends on the ability of one solid to
scratch another or to be scratched by another solid. The method was first put on a
semiquantitative basis by Mohs [9] who selected ten minerals as standards, beginning
with talc (scratch hardness 1) and ending with diamond (scratch hardness 10).
The Mohs hardness scale has been widely used by mineralogists and
lapidaries. It is however, not well suited for metals since the intervals are not well
spaced in the higher ranges of hardness and most harder metals in fact have a Mohs
hardness ranging between 4 and 8.
Another type of scratch hardness which is a logical development of the Mohs
scale consists of drawing a diamond stylus, under a definite load, across the surface to
be examined. The hardness is determined by the width or depth of the resulting
scratch; the harder the material the smaller the scratch. This method has some value as
a means of measuring the variation in hardness across a grain boundary. In general,
however, the scratch sclerometer is a difficult instrument to operate.
4.3.2
Static Indentation Hardness
The methods most widely used in determining the hardness of metals are the
static indentation methods. These involve the formation of a permanent indentation in
the surface of the material under examination, the hardness being determined by the
load and the size of the indentation formed. Because of the importance of indentation
methods in hardness measurements a general discussion of the deformation and
indentation of plastic materials has been described earlier in Chapter 2 (Section 2.6).
In the Brinell test [10,11] the indenter consists of a hard steel ball, though in
examining very hard metals the spherical indenter may be made of tungsten carbide or
even of diamond. Another type of indenter which has been widely used is the conical
or pyramidal indenter as used in the Ludwik [12] and Vickers [13] hardness tests,
respectively. These indenters are now usually made of diamond. The hardness
behaviour is different from that observed with spherical indenters. Other types of
indenters have, at various times, been described, but they are not in wide use and do
not involve new principles.
91 | P a g e 4.3.3
Dynamic hardness
Another type of hardness measurement is that involving the dynamic
deformation or indentation of the material specimen. In the most direct method an
indenter is dropped on to the metal surface and the hardness is expressed in terms of
the energy of impact and the size of the resultant indentation. In the Shore rebound
scleroscope [14] the hardness is expressed in terms of the height of rebound of the
indenter. It has been shown that in this case the dynamic hardness may be expressed
quantitatively in terms of the plastic and elastic properties of the metal. Another
method which is, in a sense, a dynamic test is that employed in the pendulum
apparatus developed by Herbert in 1923 [15]. Here an inverted compound pendulum
is supported on a hard steel ball which rests on the metal under examination. The
hardness is measured by the damping produced as the pendulum swings from side to
side. This method is of considerable interest, but it does not lend itself readily to
theoretical treatment [8].
In practice, the following test methods are in use for hardness determination.
4.3.4 Brinell
In this test a steel ball is forced against the flat surface of the specimen. The
standard method uses a 10-mm ball and a force of 29.42 kN [16]. The Brinell
hardness value is equal to the applied force divided by the area of the indentation:
4.1
in which P is the force in newtons; D is the diameter of the ball in millimetres; and d
is the diameter of the impression in millimetres. A 20-power microscope with a
micrometer eyepiece can measure d to 0.05 mm. The minimum radius of a curved
specimen surface is 2.5D. The results of the test on polypropylene, polyoxyethylene
and nylon-6,6 have been interpreted in terms of stress-strain behaviour [17].
4.3.5 Vickers
This test uses a square pyramid of diamond in which the included angles
between non-adjacent faces of the pyramid are 136°. The hardness is given by,
92 | P a g e ∝
1.854
4.2
where P is the force in newtons and d is the mean diagonal length of the impression in
millimetres. The value of HV is expressed in megapascals. The force is usually applied
at a controlled rate, held for 6-30 s, and then removed. The length of the impression is
measured to 1 µm with a microscope equipped with a filar eyepiece [18]. Cylindrical
surfaces require corrections of up to 15% [16].
4.3.6
Knoop
Another commonly used hardness test uses a rhombic-based pyramidal
diamond with included angles of 174° and 130° between opposite edges. The
hardness is given by,
4.3
where P is the force in newtons, d is the principal diagonal length of indentation in
millimetres and C is equal to 14.23 [16]. The Vickers test gives a smaller indentation
than the Knoop test for a given force. The latter is very sensitive to material
anisotropy because of two fold symmetry of the indentation [19].
4.3.7
Rockwell
In this test the depth of indentation is read from a dial [16]; no microscope is
required. In the most frequently used procedure, the Rockwell hardness does not
measure total indentation but only the non-recoverable indentation after a heavy load
is applied for 15 s and reduced to a minor load of 98 N for 15 s. Rockwell hardness
data for a variety of polymers are reported by Maxwell and Nielsen [20,21].
4.3.8 Scleroscopy
In this test the rebound of a diamond-tipped weight dropped from a fixed
height is measured [16,20]. Model C (HSc) uses a small hammer (approximately 2.3
g) and a fall of about 251 mm; model D (HSd) uses a hammer of about 36 g
(approximately) and a fall of about 18 mm.
93 | P a g e 4.3.9 Scratch hardness Test
This test measures resistance to scratching by a standardized tool [16]. A
corner of a diamond cube is drawn across the sample surface under a force of 29.4
mN applied to the body diagonal of the cube, creating a V-shaped groove; its width A,
in micrometres, is measured microscopically. The hardness is given by,
4.4
The constant 10000 is arbitrary.
4.4 MICROHARDNESS OF POLYMERS
The microhardness of a polymeric material - resistance to local deformation is a complex property related to mechanical properties such as modulus, strength,
elasticity and plasticity. This relationship to mechanical properties is not usually
straightforward, though there is a tendency for high modulus and strength values to
correlate with higher degrees of microhardness within classes of materials.
Microhardness has no simple, unambiguous definition; it can be measured and
expressed only by carefully standardized tests.
Scratch tests have been used for microhardness measurements of polymeric
materials (Bierbaum Scratch Hardness Test). These tests are related to cuts and
scratches, and, to some extent, to the wear resistance of materials. Scratch tests are not
always related to the resistance to local deformation and they are now being replaced
by the preferred indentation test [22].
In the indentation test, a specified probe or indenter is pressed into the material
under specified conditions, the depth of penetration being a measure of the
microhardness according to the test method used. The duration of a microhardness
measurement must be specified because polymeric materials differ in their
susceptibility to plastic and viscoelastic deformation. An indenter will penetrate at a
decreasing rate during application of the force, and also, the material will recover at a
decreasing rate, reducing the depth of penetration, when the force is removed.
Therefore, length of time that the force is applied for, must be specified. For most
94 | P a g e elastomers, the indentation will disappear when the force is removed. Consequently,
the reading must be observed with the force applied. Since the measurements are
dependent on the elastic modulus and viscoelastic behaviour of the material, there
may be no simple conversion of the results obtained with testers of different ranges or
by different methods. Also, as already mentioned, values from different indentation
methods may not be related to surface microhardness, resistance to scratching, or
abrasion.
Hardness testing, in the past, has been mainly used as a simple, rapid, nondestructive production control test, as an indication of cure of some thermosetting
materials, and as a measure of mechanical properties affected by changes in chemical
composition, microstructure and ageing.
The wide range of microhardness values found in polymer materials makes it
impractical to produce a single tester that would discriminate over the whole range
from soft rubber to rigid plastics. For this reason, the Rockwell Tester provides many
scale ranges and several types of durometers with varying forces applied to indenters
of various contours [23].
Before starting to discuss the mechanics and geometry of indentation, let us
mention one of the very early publications on the microhardness of polymeric
materials which used various testing techniques. Maxwell studied the indentation
microhardness of plastics in an attempt to explain some of the anomalies previously
noted in these measurements and to determine what physical constants of the material
could be responsible for resistance to indentation [20]. Slow-speed Rockwell-type
tests were compared with high-speed rebound-type tests. Maxwell interpreted these
results in terms of the rheological properties of high polymers: in particular, the
elastic modulus, yield point, plastic flow, elastic recovery and delayed elastic
recovery [20]. Furthermore, he demonstrated the time and temperature dependence of
the response of the material to microhardness measurements. This investigation led to
the conclusion that each type of test gives some important data. However, it was also
shown that the values obtained, or the relative rating of materials shown by such tests,
should be used only after careful analysis of the test data from the viewpoint of the
correlation of the test method with the conditions under which the materials in
question should be employed.
95 | P a g e Baer et al later considered the indentation process in which large loads are
placed on a spherical penetrator and the material beneath the indenter becomes
permanently displaced [17]. In addition, he defined the recovery process which occurs
immediately after the load is released, analysing it in terms of the elastic concepts
developed by Hertz [17, 24].
Muller [18] described the application of the microhardness technique using
small loads, employing the Vickers approach. The effect of various factors on the
microhardness of a wide range of polymers by means of the same approach was
reported by Eyerer & Lang [25]. These authors reported that the diagonals of the
impression did not change after the removal of the load.
In the last two decades the importance of microhardness measurement as a
technique capable of detecting a variety of morphological and textural changes in
crystalline polymers have been amply emphasized leading to an extensive research
programme in several laboratories. This is because microindentation hardness is based
on plastic straining and, consequently, is directly correlated to molecular and
supermolecular deformation mechanisms occurring locally at the polymer surface.
These mechanisms critically depend on the specific morphology of the material. The
fact that crystalline polymers are multiphase materials has prompted a new route in
identifying their internal structure and relating it to the resistance against local
deformation (microhardness).
4.5 PARAMETERS AFFECTING MICROHARDNESS TESTING
Various parameters like temperature and loading time [26], etc. are found to
affect the microhardness value of the tested material. Dependence of microhardness
on some important parameters are discussed below:
A. Effect of Load
The variation of microhardness number with the applied load depends on the
shape of indenter [27]. For pyramid and conical indenters, it has been shown that
microhardness number decreases with increasing load. For ball indenters, however it
is the opposite. The difference can be explained in terms of the combination of two
conflicting effects [28]:
96 | P a g e - Geometry change (softer with increasing load),
- Work hardening during the penetration of the indenter (harder with
increasing load).
The overall effect observed with ball, pyramid or conical indenters will then
depend on which of the two mechanisms dominate in a particular set of
circumstances.
B. Shape and Surface Dimensions of the Sample
For reliable measurements of microhardness, the volume of material
recovering the indenter must be free of external stresses and the surface must be
paralleled with the base of the tester and well polished [29]. All microhardness
measurements must be made sufficiently far apart on the material surface in order that
the plastically deformed regions around the indentation do not overlap each other nor
reach an unsupported edge of the specimen [29].
As far as the lateral dimensions are concerned, a rounded area with a radius
about 3 mm around the indentation, free from any surface defect seems to be required
in order to avoid undesirable effects.
C. Time and Load
Vicker's test, consisting of measuring the width of the remaining indentation
after removing the load, should tend to be an indicator of irreversible processes;
during the indentation process reversible deformation processes are present too, but
these are not detected by this experimental method. Polymers relax as soon as the load
is removed with a quasi-time dependent elastic recovery. Nevertheless, it has been
shown by interference microscopy that most of the recovery occurs in the depth of the
indentation and not in the length of the diagonal. Indeed, for very small test loads, the
recovery along diagonals is so small that it has no effect on the test number obtained.
Consequently, it seems that in the case of Vicker's microhardness number the time
after load removal has only a quite small effect which depends on the material.
97 | P a g e 4.6 ANALYSIS OF MICROHARDNESS
The most controversial aspect of indentation hardness testing at low loads is
the question of whether the recovered hardness number is independent of the load,
and the significance of the logarithmic index when the double cone, Vicker's or
Knoop indenters are used. In this connection, Meyer's rule is very important to
describe the dependence of load on microhardness.
4.6.1
Meyer's Rule
Meyer's Hardness
4
Meyer related the load and the size of indentation, for spherical indenter, as follows:
4.5
where L is load (kg), d is the diameter of recovered indentation, a is load for unit
dimension and n is the Meyer's constant, usually known as logarithmic index.
The logarithmic index can be considered as capacity of work hardening and
may be determined by the slope of the line by plotting log d versus log L. The value
of ‘n' varies from nearly 2.0 to 3.0 depending on the condition of the material. For
fully soften state, 'n' has higher value and decreases with the degree of cold work
imparted to the material.
4.6.2
Theory of Microhardness
Basically, the Vicker's hardness method consists in applying a pressure by the
tip of a square base pyramidal indenter on the smooth surface of the testing specimen.
Morley [30] expressed the Vicker's hardness number by the relation
2 2
98 | P a g e 1.854
/
4.6
where k is a constant.
According to the theory proposed by Schultz and Hanemann [31], the
magnitude of L bears a definite relation with the size of diagonal, d, of the impression
and is given by
4.7
where a and n are constants.
4.8
If n is smaller than two (n < 2), Hv will have a lower value at higher load. When n is
equal to 2 (n = 2), Hv will be constant and independent of load.
4.7 STRESS, STRAIN, AND YOUNG’S MODULUS
Mechanical properties are probably the most important ones to be considered
in many polymer applications. Thereby, the mechanical behaviour varies from stiff to
brittle to extremely flexible [32]. When we consider the mechanical properties of
polymeric materials, and in particular when we design methods of testing them, the
parameters most generally considered are stress, strain, and Young’s modulus. Stress
is defined as force applied per unit crosssection area, and has the basic dimension of
Nm-2 in SI units. These units are alternatively combined into the derived unit of
Pascals (abbreviated Pa). In practice they are extremely small, so that real materials
need to be tested with a very large number of Pa in order to obtain realistic
measurements of their properties. As a result more practical unit of MPa (i.e. 106Pa)
are employed instead.
99 | P a g e Strain is dimensionless quantity, defined as increases in length of the specimen
per unit original length. It represents the response of the material to the stress applied
to it.
The ratio of stress to strain is known as Young’s modulus. This parameter also
has dimensions of force per unit area, but is a characteristic of the material, not
merely a value imposed on it by a specific set of test conditions, as in case for stress
itself. Material for which the evaluation of Young’s modulus is particularly
appropriate are those which most closely approximate to an elastic solid [33-35].
4.8 EXPERIMENTAL
4.8.1
Microhardness
Microhardness measurements were carried out on the prepared pure PVA
polymer film and biocomposite film. The specimen was put on the stage of
microscope in such a way that the surface to be indented was perfectly horizontal and
to avoid any displacement of the specimen during the indentation. The indentations
were then carried out by mhp 160 microhardness tester with a Vickers diamond
pyramidal indenter. The applied load was varied from 10 to 80 g. For each value of
load, at least 5 indentations were made and average hardness number was computed.
Usually, the value of Hv was within ±5% of the average value. The value of Hv was
calculated for each specimen from the relation1.854
4.9
where L is the load (kg) and d is the diagonal of indentation (mm).
Different biocomposite specimens having different concentration of PVA and
Palm leaf powder (PL) were tested by the above procedure. All the measurements
were carried out at room temperature.
4.8.2
Tensile Test
The tensile test of the biocomposites was carried out on the Instron Universal
Testing Machine. The dumb-bell shaped samples were stretched at a speed of 5
mm/min. The thickness of each sample was about 1.00 mm, width 6 mm and length
100 | P a g e 20 mm. The final mechanical properties were evaluated from at least four different
measurements. Tests were performed at room temperature.
4.9 RESULT AND DISCUSSION
4.9.1
Microhardness
Microhardness of a biocomposite is greatly dependent on the chemical and
morphological nature of the material and reinforcing material, therefore, by a proper
selection of the components of the material, the hardness of the material, may be
desirably altered. The variation in Microhardness for various samples is shown in fig
4.1 with load ranging from 10 to 80 g.
A. Microhardness of pure PVA film
Fig 4.1 illustrates the variation of Vickers hardness number Hv with load
ranging from 10 to 80 g. It is evident that microhardness increases with increasing
load. Initially, the microhardness increases with load at a faster rate but later it attains
a saturation value of 1.2829 kg/m2 at 50 g. Finally, Hv becomes almost independent
of load above 50 g.
B. Microhardness of PVA incorporated Palm leaf
For composite film with 5wt% PL+PVA the hardness number increases upto
the load value of 50 g then it becomes constant. For specimen with 10wt%PL+PVA
the Hv increases linearly in the entire load region as shown in fig 4.1 and similar result
is obtained for composite specimen with 15wt%PL+PVA and 20wt%PL+PVA. The
increase in Hv with increase in load in all pure PVA and all the PVA: Palm leaf
composite specimens is explained on the basis of strain hardening phenomenon in
polymeric materials. There are spectrums of micromodes in polymers and each
micromode is sensitive to characteristic temperature and stress condition. When
sufficient number of micro modes become active at certain stress condition then large
scale plastic deformation begins.
The variation of Hv with load is also explained on the basis of frictional force
given by Amonton and Chery [35]. According to this theory the microhardness may
be correlated with the frictional force. The coefficient of friction decreases with
increasing load and the frictional force is found to increase linearly with increasing
101 | P a g e load. Hence, the variation of Hv with load is curvilinear. The microhardness initially
increases with increasing load. On applying load the composite is subjected to some
strain hardening. Finally, when Hv value tends to become constant the polymer is
completely strain hardened. The rate of strain hardening is greater at low load and
decreases at higher load [35].
The rate of strain hardening in different samples is related to weight
percentage ratio of two materials in composite which governs the degree of cross
linking in the composite. Hence the different saturation values are observed for
different samples.
Pure PVA
10% PL+PVA
20% PL+PVA
Microhardness No. Hv (kg/mm2)
5
5% PL+PVA
15% PL+PVA
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
10
20
30
40
50
60
70
80
Load (in gm)
Fig 4.1 Variation of Hv with load for pure PVA and bio-composite of PVA
incorporated Palm Leaf.
The effect of addition of palm leaf powder in to the biocomposite on their
microhardness has been studied by varying the concentration of palm leaf powder
from 5wt% to 20wt% concentration in to the biocomposite. The result is shown in
Fig.4.1, which indicate that the value of Hv increases with increase in palm leaf
content. The value of Hv increases with increasing the applied load. It is also greater
for higher weight percentage of palm leaf powder.
102 | P a g e The microhardness of all the biocomposites are found to be higher than that of
pure PVA specimen. Thus incorporation of Palm leaf in otherwise soft PVA in all
weight proportions leads to the hardened biocomposites. The increases in Hv with
increase in weight percentage of palm leaf powder increases the crystallinity in the
biocomposite system, which is also confirmed from the XRD and DSC studies
described in Chapter 3. The stable and smooth texture of the surface of biocomposites
are detected from the AFM studies discussed in Chapter 3 also contributes to the
strengthened material with stiff and hardened surface as depicted with increases in
microhardness. Hence Palm leaf powder is compatible with PVA in developing
structural network in biocomposite having mechanically stable and smooth surface.
4.9.2
Strain Hardening Index
The dependence of microhardness on load can be studied with the help of
Meyer's relation:
Taking logarithms of both sides of the equation, we have
log L = log a + n log d
4.10
where L is load, d is the length of diagonal measured in division, a is a constant
representing load for unit dimension and n is the logarithm index number which is
the measure of strain hardening.
The plot for log L versus log d for Pure PVA Polymer and various
biocomposite samples are shown in fig 4.2. It can be observed that for all the
specimens, the strain hardening index has two values: one for the low load region
ranging from 10 to 40 g and the other for the high load region ranging from 40 to 80
g. The strain hardening index (n) is increasing at higher load region. The different
value of n for pure PVA and palm leaf reinforced biocomposite in two regions are
shown in Table 4.1. The variation in strain hardening index shows, the material
becomes tough at higher load region due to formation of crystalline region inside the
specimen. The increase in strain hardening index in low load region observed due to
increase in tensile strength and microhardness of material, while at higher load region
103 | P a g e strain hardening index is decreasing with increase in concentration of palm leaf
powder, this change in index is observed because the material is becoming hard as
well brittle. The increase in crystalline region is also confirmed by DSC and XRD
characterization as discussed in section 3.4. In the case of microhardness this
crystalline region causes the saturation region at higher loads.
2.65
Pure PVA
5%PL+PVA
15%PL+PVA
20%PL+PVA
10%PL+PVA
2.6
2.55
Log L (Load)
2.5
2.45
2.4
2.35
2.3
2.25
2.2
2.15
1
1.2
1.4
1.6
1.8
2
Load d (Digonal Size)
Fig. 4.2: Variation of Log L with Log d for with varying percentage of Palm
Leaf.
104 | P a g e Table 4.1: Value of Strain Hardening Index Number n for PVA Palm leaf
biocomposite.
Value of Slope
S.No.
Sample
Low Load region
High Load Region
1
Pure PVA
0.078
0.477
2
5%PL+PVA
0.100
0.483
3
10%PL+PVA
0.120
0.392
4
15%PL+PVA
0.133
0.318
5
20%PL+PVA
0.159
0.348
4.9.3
Young’s Modulus, Tensile Strength and Elongation at Break
The mechanical properties of the pure PVA and palm leaf reinforced
biocomposite, such as Young’s Modulus, tensile strength and elongation at break are
shown in fig 4.3. The tensile strength and Young’s modulus of the films increases
significantly from 7.40 MPa to 34.42 MPa and from about 435.64 MPa to 2926.05
MPa with increasing palm leaf content from 0 to 20%, whereas the elongation at
break decreases from 9.55% to 2%. The increase in Tensile strength shows that the
developed biocomposites have improved exhibiting greater strength with good
mechanical properties. The increases in Young’s Modulus and tensile strength of
biocomposites leads to the development of material having good elastic properties
with great strength. However, the decrease in % elongation at break with increase in
the wt% of Palm leaf suggests the decrease in the elastic region of deformation
leading to relatively brittle material. The increases in Young’s Modulus and tensile
strength can be attributed to the increase in the crystallinity of biocomposites with
increase in proportion of Palm leaf which is confirmed from XRD and DSC studies,
described in Chapter 3. However, the increases in crystallinity leads to the
embrittlement of the material as the % elongation at break decreases, although, this
decreases in elongation tends to stabilized beyond the 10 wt% of palm leaf specimens.
105 | P a g e Fig. 4.3:
Variation in Young’ss Modulus,, Tensile Sttrength andd Elongatio
on with
varying peercentage oof Palm Leaf.
106 | P a g e 4.10 CONCLUSION
The present study reveals that hardened biocomposite of PVA and Palm leaf powder
can be developed. The presence of good compatibility between PVA and palm leaf powder
imparts hardening and brittleness to the biocomposite as detected from microhardness
measurements and their tensile testing. It is clear from both the mechanical studies that pure
PVA is soft in comparison to developed biocomposite. The microhardness of developed
biocomposite increases with increasing concentration of palm leaf powder. It is observed that
for all the biocomposite, the strain hardening index has two different values at lower and
higher load region. The strain hardening index is increasing in low load region due to
improved mechanical properties, while it is decreasing at higher load due to hardening and
brittleness of developed material.
Similar results have been found in tensile test of the developed biocomposite, for pure
PVA the tensile strength and Young modulus both having low value but due to softness of
pure PVA it has higher elongation. For developed biocomposite, as we are increase
concentration of palm leaf powder the tensile strength and Young modulus both are
increasing but due to increases in hardness and brittleness, elongation decreases with higher
concentration of palm leaf powder.
107 | P a g e 4.11 References:
[1]
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