CHAPTER 5 EFFECT OF MICROCRYSTALLINE CELLULOSE ON...
Transcript of CHAPTER 5 EFFECT OF MICROCRYSTALLINE CELLULOSE ON...
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CHAPTER 5
EFFECT OF MICROCRYSTALLINE CELLULOSE ON
PERFORMANCE OF POLYURETHANE GREEN
COMPOSITES
A series of polyurethane (PU) green composites were fabricated with varying
amounts viz., 0, 2.5, 5, 7.5 and 10 wt %, of microcrystalline cellulose (MCC) powder.
The obtained PU/MCC biocomposites were evaluated for physico-mechanical
properties, such as density, void content, tensile behaviors and surface hardness, in
order to analyze the effect of MCC content on the performance of the biocomposites.
Incorporation of MCC into PU matrix yielded a significant improvement in tensile
strength and tensile modulus. This result indicates that a strong matrix-filler interaction
was developed during the polymerization process between the hydroxyl groups of the
cellulose crystals and the isocyanate component. Chemical resistance and solvent
sorption studies has been studied. The water uptake behavior was examined in different
environmental conditions, such as in water, acid medium (5% HCl), saline medium (5%
NaCl) and boiling water. Moisture sorption was also examined for PU/MCC composites
at different relative humidity‟s (RHs). The thermal characteristics of the biocomposites
have been studied by thermoanalytical techniques namely, DSC, TGA and DMA
analysis. Microstructural parameters of the PU/MCC green composites have been
evaluated using WAXS and the results are compared with solvent sorption studies. The
contact angle of the composites has been evaluated in order to support the moisture
sorption and water uptake behaviours. The morphological features of cryofractured
PU/MCC composites were analyzed using scanning electron microscopy (SEM).
5.1 Introduction
Polyurethanes (PUs) are a class of polymers that offer great versatility and a
wide range of properties, depending on the components used in the formulation. The
preparation of polymers from renewable sources such as vegetable oil-based materials is
currently receiving increasing attention because of economic and environmental
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concerns [1]. The excellent mechanical properties of PUs, such as high tensile strength
and toughness, are due to, it having a two-phase morphology because of the
incompatibility of the hard and soft components. On the other hand, the use of fillers or
reinforcements in polymer materials is a common practice that allows further tailoring
of mechanical performances. The composite properties depend on the composition of
the main constituents as well as the interfacial adhesion between matrix and filler, these
properties can be different than those of the bulk polymer matrix [2-4]. The importance
in the development of PUs using renewable sources raises from concerns about raw
material processing and development of alternative synthetic routes which are less
hazardous to the environment. In recent years the study of PU and its composites have
resulted in their applications in automotive parts, coatings, sealants, adhesives and other
infrastructure uses. Now-a-days PU biocomposites are in use, because of increasing
demand for light weight, durable and cost effectiveness products, especially in the
automotive market.
PU composites have good wear resistance. Incorporation of glass fiber can
improve the tensile properties, chemical resistance and exhibit good insulating materials
[5]. Naresh has studied thermoplastic polyurethane (TPU)/glass fiber composites in the
form of alternating multilayer sandwich samples. This processing method improved the
tensile modulus, impact properties and yielded higher stiffness with better strength than
pristine TPU [6]. The thermal and electrochemical properties of PU-carbon black
composites have been studied by Furtado et al [7]. The effect of fillers on thermal and
mechanical properties of filled PU has been studied by Benli et al [8]. Chin et al have
reported the effect of sodium chloride filler on thermal ageing and hygroscopic ageing
of PU systems [9, 10]. In our laboratory we have established the structure-property
relationships for various particulate fillers filled PU green composites [11-14].
Marcovich et al [15] investigated PU composites containing cellulose
microcrystals and they noticed an improvement in mechanical properties of the
composites. Recently Xiaodong et al [16] investigated on processing of water borne PU-
cellulose nanocrystal composites processed by casting and evaporation. Cellulose is one
of the most abundant materials in nature since it represents the main structural
component of plants and is also produced, on a much smaller scale, by some sea
animals [17]. Furthermore, attributes such as low cost/toxicity, low density, high
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stiffness, renewable nature and biodegradability applications of cellulose filler
constitute major incentives for exploring new applications of cellulose as reinforcing
fillers in polymer matrix [18-20]. The effect of natural fillers on mechanical properties
of PU composites were reported elsewhere [21-23].
Even though there is lot of literature available on biocomposites, there is no
literature available on addition of microcrystalline cellulose (MCC) in PU matrices.
Based on the above, the present research work was addressed to evaluate the effect of
the incorporation of MCC as filler into a reactive PU polymer for imparting better
properties to the PUs. The mechanical, thermal, morphological and moisture sorption
behaviours of the fabricated PU/MCC biocomposites were investigated. The study was
further extended to water uptake behaviors and the effect of relative humidity to widen
the application window of PU/MCC composites.
5.2 Synthesis of PU/MCC green composites
Castor oil (0.001 mole) was initially dissolved in 50 ml of MEK and placed in a
three-necked round bottomed flask. TDI (0.0015 mole) was added followed by the
addition of a calculated amount of MCC filler and 2-3 drops of DBTL as catalyst, the
contents of the flask were stirred continuously for about 2 h under an oxygen free
nitrogen gas purge at 60-70°C. The solution was degassed under vacuum and poured
into a cleaned glass moulds and allowed to stand for 12 h at room temperature. The
mould was then kept in a preheated circulating hot air oven at 70°C for 8 h. The
toughened PU/MCC composite sheet thus formed was cooled slowly and removed from
the mould. The above procedure was repeated with different MCC filler contents, viz.,
2.5, 5, 7.5 and 10 wt % as well as without MCC.
5.3 Results and Discussion
5.3.1 Fourier transform infrared spectroscopy
Figure 5.1 shows the FTIR spectra of PU/MCC green composites. It can be
noticed that, the absence of absorption band at 2270 cm−1
(-NCO group) clearly
indicates that there was no unreacted -NCO groups [24-26]. The FTIR absorption bands
of PU/MCC biocomposites are tabulated in Table 5.1. The IR spectra showed
absorption band at 3361-3363 cm-1
due to -N–H stretching [27-31]. The shift in the
absorption band shows the formation of hydrogen bond between filler and matrix. Two
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peaks were, infact, observed at 863-868 cm-1
and 724 cm-1
, attributed to 1,4–substituted
phenyl ring. An adsorption peak at 1600 cm-1
, corresponds to >C=O stretching was
observed. The absorption characteristics of –C=C– aromatic ring was observed in the
frequency range 1455-1463 cm-1
and aromatic C-H stretching was observed at 3009-
3010 cm-1
, the adsorption band at 1728-1747 cm-1
corresponds to ester group. Aromatic
C-H bending was observed at 767 cm-1
. The absorption band at 1528-1539 cm-1
(Figure
5.1) shows the presence of urethane linkage.
Figure 5.1. FTIR spectra of PU/MCC green composites
5.3.2 Physico-mechanical behaviors
5.3.2.1 Density
The density of the PU and MCC were 1.018 and 1.50 g/cc respectively. The
measured density of PU composites are tabulated in Table 5.2. The density of the
composites lies in the range 1.028 – 1.088 g/cc. After incorporating MCC, a slight
increase in density of the biocomposites have been noticed, as expected, because of the
increase in high density MCC filler content in PU matrix. Furthermore addition of MCC
filler induces intermolecular attraction between –OH groups of MCC filler and urethane
groups of PU matrix, which in turn increases the density of the composites. The
theoretical density was calculated for composites using the weight additivity principle.
The theoretically calculated density values lies in the range 1.030-0.143 g/cc and these
values were slightly higher than that of corresponding experimentally obtained values.
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This may be due to the formation of void at the interface of matrix and filler in the
composites.
Table 5.1. Important band assignments of FTIR spectra of PU/MCC green
composites
Group Expected
peaks
(cm-1
)
Observed characteristic IR bands (cm-1
) for
varying amounts of MCC filled PUs (wt %)
0 2.5 5 7.5 10
C=O 1630-
1690
1600 1600 1600 1600 1600
N-H stretching
with hydrogen
bonding
3200-
3400
3361 3360 3362 3363 3363
Aromatic C-H
stretching
3000-
3100
3010 3009 3010 3010 3010
C=C aromatic
ring
1450 1456 1455 1463 1463 1455
1,4– substituted
phenyl ring
860, 762 863, 724 865, 724 865, 724 868, 724 867, 724
O
||
C O (ester)
1750-
1700
1742 1728 1731 1747 1747
O
||
-NH C NH-
(urethane peak)
1528 1537 1538 1538 1538 1539
Aromatic C-H
bending
680-860 809 767 767 767 767
5.3.2.2 Void content
The calculated void content of the PU/MCC green composites is given in Table
5.2, (as per equation (2), in chapter 2). The void formation in the composite is due the
poor filler and polymer interaction. The void content in the PU/MCC green composites
increases as the percentage of the MCC concentration increases and it lies in the range
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0.75 - 4.82 %. This is due to higher filler loading, filler-filler interaction increases which
lead to aggregation of filler and also poor wettability of filler at higher dosage.
5.3.2.3 Resilience
The measured resilience values are tabulated in Table 5.2. The effect of filler
content on resilience of PU/MCC green composites is insignificant and it lies in the
range 14-15.
5.3.2.4 Surface hardness
Surface hardness is a property measured laterally, whereas the modulus is
measured longitudinally. The measured values of shore A hardness is given in Table
5.2. These surface hardness values will reflect directly on the dimensional stability of
the PU/MCC green composites. The surface hardness values gradually increases with
increase in MCC content in PU and it lies in the range 68-72 shore A.
5.3.2.5 Mechanical behaviors
The measured tensile behaviors of the PU/MCC green composites are tabulated
in Table 5.2. The tensile strength lies in the range 4.73 – 7.67 MPa. The tensile strength
of the PU/MCC composites increased up to 5 wt. % of MCC loading, which can be
attributed to effective transfer of the stress and load onto the MCC filler. A further
increase in MCC content resulted in agglomeration of filler and poor
adhesion/interaction between MCC and PU resulting in micro-cracks developing at the
interfaces under load, which led to failure.
The tensile modulus of pristine PU was 2.72 MPa and a significant improvement
in tensile modulus (5.09 MPa) was noticed up to 5 wt. % of MCC loading; on further
increase in filler content, the tensile modulus decreased. A slight reduction in
percentage elongation at break for MCC filled PU composites was noticed. This was
probably due to better interaction of filler with the PU. The restrictions imposed by the
filler on the molecular mobility of PU chains would reduce the percentage elongation at
break; this is a common observation with almost all filled composites [21-22]. A
significant improvement in tensile strength and tensile modulus at lower filler dosage
can also be attributed to the hydrogen bond formation between filler and polymer. The
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hydroxyl groups of MCC may also react with the diisocyanates and form PU [15]
(Figure 5.2 (a)). Figure 5.2(b) shows a schematic representation of the hydrogen bond
formation between urethane groups of PU and –OH groups of MCC. On increase in
Figure 5.2. Schematic representation of (a) urethane group formation between TDI
and MCC and (b) hydrogen bond formation between PU and MCC
Table 5.2. Physico-mechanical properties of PU/MCC biocomposites
PU/MCC
(wt/wt, %)
Theo.
density
(g/cc)
Expt.
density
(g/cc)
Tensile
strength
(MPa)
Elongat
ion @
break
(%)
Tensile
modulus
(MPa)
Void
content
(%)
Surface
hardness
(Shore A)
Resili
ence
100/0 - 1.028 4.73 157.3 2.72 - 68 14
97.5/2.5 1.030 1.024 5.43 153.4 4.59 0.75 69 14
95/5 1.059 1.032 7.67 127.5 5.09 2.54 71 13
92.5/7.5 1.105 1.059 6.64 155.9 4.90 4.16 72 14
90/10 1.143 1.088 4.20 142.9 3.76 4.82 72 15
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filler loading in to PU matrix, the filler suffer less wettability, disruption of inter-
urethane hydrogen bonds resulting in reduction in chain mobility, and increased
structural heterogeneities by aggregation of the filler. Hence, tensile properties of the
PU composites were lowered as the filler loading was increased above 5 wt. %. The
results obtained are in agreement with the results reported elsewhere for PU/guar-gum
composites [23].
5.3.3 Chemical resistance
The PU/MCC specimens exposed to the 5% different chemical reagents such as
KOH, H2SO4, H2O2, KMnO4 and acetic acid at room temperature for 7 days and were
evaluated for the percentage change in weight and the results are given in Table 5.3.
From the table it was noticed that there was no significant chemical influence on change
in weight, color and thickness of PU/MCC green composites. From this study it was
noticed that, PU composites are chemically resistive to alkali, acid and reducing agents.
However, all samples are highly sensitive to oxidizing agent and were degraded in 5%
KMnO4 solution.
Table 5.3. Change in weight of PU/MCC green composites after exposure to
different chemical reagents for 7 days
Chemical
reagents
% Change in weight of PU/MCC for 7 days at room tempr. for
various chemical reagents
100/00 97.5/2.5 95/5 92.5/7.5 90/10
5% KOH 3.5 2.7 1.5 2.3 3.6
5% H2SO4 1.8 2.8 1.7 1.4 2.6
5% KMnO4 * * * * *
5% H2O2 4.0 3.2 3.2 3.4 4.2
5% Acetic acid 3.8 3.1 3.3 3.9 4.8
* denotes the degradation of polymer.
5.3.4 Swelling behavior
The effect of MCC content in PU matrix on swelling behavior have been
studied, by exposing the samples to different organic solvents such as ethyl acetate,
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chlorobenzene and dichloromethane and the obtained results are addressed in Table 5.4.
From the table it can be seen that the maximum solvent uptake was noticed for
PU/MCC composites in chlorobenzene and least solvent uptake was in heptane (non
polar). The order of the percentage swelling in different organic solvents is as follows;
chlorobenzene > dichloromethane > ethyl acetate > heptane. The swelling behaviour of
PU/MCC green composites clearly shows that the solvent uptake strongly depends on
the solubility parameter and polarity of the solvents.
Table 5.4. Change in weight of PU/MCC green composites after exposure to
different organic solvents for 7 days
5.3.5 Moisture adsorption studies
A moisture sorption isotherm can be used to predict the sorption property of a
material and provide the data on the polymer interaction with water. The absorbed water
molecules have been considered to have an effect on the properties of PU/MCC. The
moisture uptake by the composites against time when exposed to different RH
conditions are shown in Figure 5.3. As the RH was increased, the weight gain of the
samples also increased. Moisture sorption curves indicates a large increase in the weight
gain as the RH increased. The plots of moisture uptake as a function of relative
humidity for all PU/MCC green composites are shown in Figure 5.4. The moisture
uptake increases with increase in RH and increase in percentage composition of the
MCC. The sigmoid profile obtained can be divided in to three steps, the first region
represents the bound water which is unfreezable and it is not available for chemical
reactions. In the second region, water molecule bind firmly with polymer composites
than in first region, they are usually present in small capillaries. In the third region,
Solvent
% Change in weight for 7 days at room tempr. for various
chemical reagents
100/00 97.5/2.5 95/5 92.5/7.5 90/10
Heptane 6.7 5.2 5.3 4.7 5.8
Ethyl acetate 81.5 73.5 82.7 82.8 79.4
Chlorobenzene 144.4 147.9 140.1 144.4 152.3
Dichloromethane 128.2 135.8 132.6 135.8 132.5
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where RH is very high, all biocomposites have a tendency to absorb more moisture.
Another possibility is that the loading of MCC increases the free volume of the PU
chain, evidently results in higher moisture uptake. Microcrystalline cellulose having –
OH groups which have the tendency to pick up moisture from the surrounding hence,
PU/MCC systems are more sensitive to relative humidity as the MCC content increases
[32].
0 200 400 600 800 1000 1200
99.6
99.8
100.0
100.2
100.4
100.6
100.8
101.0
Time (min)
Wei
gh
t (%
)
10% RH
20% RH
30% RH
40% RH
50% RH
60% RH
70% RH
80% RH
90% RH
100% RH
10%
7.5%
5%2.5%
0%
Figure 5.3. Moisture sorption plots of PU/MCC biocomposites
0 20 40 60 80 100
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Mo
istu
re c
on
ten
t (g
of
wa
ter/g
of
sam
ple
)
Relative Humidity (%)
0%
2.5%
5%
7.5%
10%
Figure 5.4. The plots of moisture uptake as a function of relative humidity for
different PU/MCC biocomposites
5.3.6 Water uptake behaviors
The hydroxyl groups present in MCC have a tendency to absorb moisture or
water and have low wettability for hydrophobic moieties [33]. The absorption of water
by PU/MCC composites continues till all the hydroxyl groups are bound to water
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molecules; the absorbed water causes swelling of filler and it continues till the cell walls
are saturated with water. Moisture existing as free water in the void structure leads to
weakening of the interfacial bonding [34]. The absorbed water may also cause
irreversible changes in the matrix such as chemical degradation and debonding,
cracking, void formation and blistering. In order to study the effect of temperature on
water uptake behaviours the specimens are exposed to water at room temperature and
below room temperature. The percentage water uptake by PU/MCC composites as a
function of time in water at 10 °C (below room temperature), water at 25 °C, salt
solution, and acid medium are shown in Figures 5.5 (a)-(e). The water uptake behavior
of the composites was different for different environments. The water uptake was high
in acidic medium than in salt medium. The lower water uptake behavior of composites
in salt medium serves the application in marine applications. High penetration of water
molecule in acidic medium may be due to more physical interaction with the composites
in acidic media as compared to salt medium. Thus hydrated ions also undergo surface
solvolysis due to the presence of polar groups in composite, hence they exhibit higher
water uptake in acid medium. Lower water uptake in salt medium may be due to
electrostatic repulsive forces acting among electronegative groups present in composite
matrix. H+ ions have high tendency to break water structure as compared to Na
+ ions.
The size of the H+ ion is smaller as compared to Na
+ ion, smaller is the size of the ion
greater is the penetration. The water uptake at 25 °C is more than at 10 °C as expected.
In boiling water the interaction between water molecules and filler in the PU matrix
increases, which leads to higher water uptake (Figure 5.6). Also at higher temperature
free volumes of composites increases due to thermal expansion, this causes an increase
in water uptake behaviors of the green composites. Figure 5.6 shows that the rate of
water uptake behavior in boiling water is strongly depend on the filler content and
higher filler loaded systems attained the equilibrium state rapidly; the equilibrium time
was reduced drastically and beyond 7 hrs it remained constant [35]. That means higher
filler loaded composites shows higher water uptake as compared to lower filler loaded
composites. This result also proves that the water uptake behavior depends on the
temperature and the environment in which the polymer is exposed. The penetration of
the water in to composites increases with increase in MCC content; this process is due
to increase in the number of hydrophilic (–OH) groups in the composites.
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0 3 6 9 12 15 18
0.0
0.5
1.0
1.5
2.0
2.5
Wa
ter a
dso
rb
ed
(%
)
t1/2
(h)
Salt solution
Water at 100C
Water at 250C
HCl solution
(a)
0 3 6 9 12 15 18
0
1
2
3
4
5
Salt solution
Water at 100C
Water at 250C
HCl solution
Wa
ter a
dso
rb
ed
(%
)
t1/2
(h)
(b)
0 3 6 9 12 15 18
0.0
1.5
3.0
4.5
6.0
Salt solution
Water at 100C
Water at 250C
HCl solution
Wa
ter a
dso
rb
ed
(%
)
t1/2
(h)
(c)
0 2 4 6 8 10 12 14 16 18
0
2
4
6
Wa
ter a
dso
rb
ed
(%
)
t1/2
(h)
Salt solution
Water at 100C
Water at 250C
HCl solution
(d)
0 2 4 6 8 10 12 14 16 18
0
2
4
6
8
W
ate
r a
dso
rb
ed
(%
)
t1/2
(h)
Salt solution
Water at 100C
Water at 250C
HCl solution
(e)
Figure 5.5. The plots of percentage water absorbed against time in different
environments for PU composites with (a) 0, (b) 2.5, (c) 5, (d) 7.5 and (e) 10 wt% of
MCC
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0 2 4 6 8
0
4
8
12
16
Wa
ter
ab
sorb
ed (
%)
Time (h)
0 %
2.5%
5%
7.5%
10%
Figure 5.6. Plots of water uptake verses time for PU/MCC biocomposites in boiling
water
Table 5.5. Water uptake and diffusivity data of PU/MCC biocomposites
Properties Composition of PU/MCC composites (wt/wt %)
100/0 97.5/2.5 95/5 92.5/7.5 90/10
Diffusivity in 5% NaCl solution
10-12
(m2/s)
4.33 6.76 7.09 8.70 9.11
Equilibrium water content for 5%
NaCl solution (%) 1.09 2.00 2.66 3.07 3.79
Equilibrium time for 5 % NaCl
solution (hrs) 168 167 168 168 168
Diffusivity in water at 10 °C, 10-12
(m2/s)
5.85 5.14 6.45 8.52 9.07
Equilibrium water content for water
at 10 °C (%) 1.93 2.47 3.33 3.42 5.60
Equilibrium time in water at 10 °C
(hrs) 144 144 142 143 144
Diffusivity in water at 25 °C 10-12
(m2/s)
6.13 5.90 7.39 8.87 8.14
Equilibrium water content for water
at 250C (%)
2.18 3.44 4.57 5.60 5.63
Equilibrium time in water at 25 °C
(hrs) 168 166 168 167 168
Diffusivity in 5% HCl solution 10-12
(m2/s)
5.79 6.50 5.76 7.38 7.42
Equilibrium water content for 5%
HCl solution (%) 2.23 4.58 5.79 6.49 7.67
Equilibrium time for 5% HCl
solution (hrs) 168 167 168 169 167
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The diffusivity of water in the different environments was calculated assuming
the water sorption as a one-dimensional Fickian diffusion into the PU/MCC composites
(Table 5.5). The diffusivity values strongly depend on the MCC content present in the
PU matrix, and also on the environment in which the samples were exposed. The time
to reach equilibrium and equilibrium water uptake are also reported in Table 5.6. The
equilibrium water content in acid medium was almost double as compared to saline
medium. From Table 5.6 it is noted that equilibrium water uptake also strongly depends
on filler content in PU matrix.
5.3.7 Thermoanalytical studies
5.3.7.1 Differential scanning calorimeter
The DSC thermograms of the PU/MCC green composites are represented in
Figure 5.7. The obtained Tg values from DSC thermograms are tabulated in Table 5.6.
Table 5.6. Tg values obtained from DSC
thermograms of PU/MCC green composites
-50 0 50 100 150 200
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
Hea
t fl
ow
(a
u)
Temperature (0C)
2.5 %
10 %
5 %
0 %
7.5 %
Figure 5.7. DSC thermgrams
of PU/MCC green composites
From the table it was observed that Tg of composites slightly increases with
increase in MCC content from 0 to 5 wt.% and further increase in filler loading (>5%),
the Tg values decreases this variation is based on the interaction of filler with PU matrix.
Lower the filler loading there exists a good interaction between filler and matrix, this
MCC content (wt. %) Tg (oC)
0 -14.8
2.5 -15.4
5 -15.1
7.5 -18.3
10 -22.5
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intern raise the Tg values. Higher the filler loading, filler exhibit structural
heterogeneities, this intern gives negative affect on the Tg.
5.3.7.2 Thermogravimetric analysis
To understand the thermal stability and thermal degradation patterns of the
PU/MCC composites the TGA scans were recorded. Some representative TGA and its
derivative thermograms are displayed in Figure 5.8.
0 200 400 600 800
0
20
40
60
80
100
Temperature (0
C)
Weig
ht
(%)
(a)
0.0
0.2
0.4
0.6
0.8
1.0
Deriv
.weig
ht
(%/0
C)
0 200 400 600 800
0
20
40
60
80
100
Temperature (0
C)
Weig
ht
(%)
0.0
0.2
0.4
0.6
0.8
Deriv
.weig
ht
(%/0
C)
(b)
0 200 400 600 800
0
20
40
60
80
100
Temperature (0
C)
Weig
ht
(%)
(c)
0.0
0.2
0.4
0.6
0.8
1.0
Deriv
.weig
ht
(%/0
C)
Figure 5.8. Typical TGA and its derivative thermograms of (a) 0, (b) 5 and (c) 10
wt. % of MCC filled PU green composites
TGA thermograms of all PU/MCC biocomposites undergone three step thermal
degradation process, they suffered nearly no weight loss upto 193 oC and were
completely degraded at around 5500C. The temperature range of decomposition,
percentage weight loss and ash content obtained from TGA thermograms of the
composites are tabulated in Table 5.7.
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Table 5.7. Temperature range obtained from derivative TGA curves of PU/MCC
composites
MCC content
in PU (wt %)
Degradatio
n stage
Temperature (ºC ± 2) Weight loss
(%) To Tp Tc
0
1 193 320 340 23. 3
2 340 380 460 50.7
3 460 466 513 25.3
Ash - - - 0.7
2.5
1 211 309 332 21.2
2 332 372 402 52.0
3 402 463 527 22.5
Ash - - - 4.3
5
1 204 315 334 21.5
2 334 372 434 51.5
3 434 463 543 22.5
Ash - - - 4.5
7.5
1 217 315 332 20.2
2 332 374 434 49.0
3 434 463 529 25.3
Ash - - - 5.5
10
1 211 322 340 24.1
2 340 380 440 47.2
3 440 456 553 21.6
Ash - - - 7.1
To – start of the peak of DTG curve (starting temperature), Tp – peak temperature of
DTG curve and Tc – completion of DTG curve peak
The initial weight loss was occurred in the temperature range 193 – 340 oC,
with weight loss of 21.2 – 24.1%, which may be due to thermal degradation of soft
segment and evaporation of moisture content. The major weight loss of 47.2 – 52.0 %
was found in the second step occurred in the temperature range 332 – 460 oC, due to
thermal decomposition of the hard segments of PU and a small amount of MCC
component. The third step thermal degradation occur at 402 – 553 oC, with weight loss
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in the range 21.6 – 25.6 %. In the third step the weight loss is due to complete pyrolysis
of PU/MCC composites. The ash content of the PU/MCC biocomposites lies in the
range 4.3 – 7.1%, whereas, the ash content for pristine PU was 0.7%. The ash content
increased as expected, with increase in MCC content.
The relative thermal stability of the composites was evaluated by comparing the
decomposition temperatures at various percentage weight losses and the oxidation index
(OI) (Table 5.8). T10, T25, T50 and T75 are the temperatures for 10, 25, 50 and 75 %
weight loss; help us to know the thermal stability of the composites. Form the table it
was noticed that a slight improvement in thermal stability of composites after
incorporation of MCC. From Table 5.8, it was observed that OI values lies in the range
0.027 – 0.494 %. OI values increases with increase in filler content. This result indicates
that a slight improvement in flame retardant behaviors of the composites [11, 36]. It was
observed that the thermal degradation patterns were almost identical for all PU/MCC
composites.
Table 5.8. Thermal data obtained from TGA thermograms for PU/MCC composites
Microcellulos
e content in
PU (wt. %)
Temperature at different weight loss (± 3 ºC)
Oxidation
index (OI)
at 10%
weight
loss
at 25%
weight
loss
at 50%
weight
loss
at 75%
weight
loss
Tmax
0 297 334 382 424 513 0.027
2.5 304 350 386 450 527 0.299
5 302 343 382 442 543 0.313
7.5 303 348 383 438 529 0.382
10 305 344 389 452 553 0.494
5.3.7.3 Dynamic mechanical analysis
Dynamic mechanical analysis (DMA) is one of the useful techniques to measure
the damping and modulus property of the materials. The G‟ and tan δ are recorded as a
function of temperature for PU/MCC green composites and are shown in Figures 5.9
and 5.10 respectively. All the samples showed a reduction in storage modulus (G′)
values with increase in temperature. The incorporation of MCC into PU matrix shows
159
an increase in G′ value upto 7.5 wt. % of MCC loading, on further increase in filler
content G′ value decreases.
-60 -40 -20 0 20 40 60 80 100 120
-1.00E+009
0.00E+000
1.00E+009
2.00E+009
3.00E+009
4.00E+009
5.00E+009
6.00E+009
7.00E+009
G'
(GP
a)
Temperature (o
C)
0%
2.5%
5%
7.5%
10%
Figure 5.9. Plots of storage modulus verses temperature of PU/MCC green
composites
-80 -60 -40 -20 0 20 40 60 80 100 120
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
-80 -60 -40 -20 0 20 40 60 80 100 120
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Ta
n
Temperature (o
C)
0%
2.5%
5%
7.5%
10%
Ta
n
Temperature (o
C)
10%
7.5%
5%
2.5%
0%
Figure 5.10. Loss tangent verses temperature of PU/MCC green composites
Table 5.9. Thermal data obtained from DMA thermograms for PU/MCC green
composites
MCC content
(wt %) Tg (
0C)
Tan δ (peak
max.)
0 14.3 0.63
2.5 20.4 0.59
5 25.0 0.65
7.5 20.7 0.57
10 19.6 0.62
160
The Tg values are obtained by the peak temperature of the tan δ curve and the
results of Tg and tan δ values are tabulated in Table 5.9. The dynamic modulus indicates
the inherent stiffness of material under dynamic loading conditions. The mechanical
damping indicates the amount of energy dissipated as heat energy when the material is
subjected to external loading. It is defined as;
Tan δ = G′′ / G′ (1)
where, tan δ is phase angle between stress, G′′ is elastic loss modulus and G′ is the
elastic storage modulus. Although tan δ shows maximum peak for PU with 5 wt. %
MCC, there is no systematic variation in peak height. The tan δ values of the composites
remains unchanged (see Table 5.9) when compared with pristine PU, which indicates
that the addition of MCC retains the damping property of the PU/MCC green
composites. This Tg values are similar to the Tg values obtained by DSC thermograms.
From Table 5.9 it is seen that the incorporation of MCC act as a reinforcing filler (upto
5 wt. %), where –OH groups of MCC will involve in the hydrogen bond formation with
urethane groups of PU and these –OH groups reacts with diisocyanates to form
urethane linkages [37]. MCC filler is found to reinforce the PU by allowing the greater
transfer of stress at the filler-matrix interfaces, this intern increase the mechanical
property of the composites until 5 wt % filler loading and later decreases due to the
formation of void and agglomeration of the filler in PU matrix.
5.3.8 Molecular transport behaviors of aromatic solvents
Sorption and diffusion behaviours of aromatic probe molecules into PU/MCC
composites have been studied. The typical sorption curves obtained for aromatic probe
molecules such as benzene, toluene and p-xylene are presented in Figure 5.11. The
surface of the polymer composite swells immediately in the solvent, swelling in
underlying will take place slowly. Thus a kind of compressive stress appears on the
surface, these stresses are relaxed by further swelling. During initial stages of sorption,
the uptake of aromatic probe molecules occurs linearly with time as the time proceeds,
the mechanism turns to non-linear [38]. All PU/MCC green composites attained
equilibrium almost at the same time. The sorption data follows the order of, benzene >
toluene > p-xylene. Sorption coefficients of different aromatic solvents in PU/MCC
composite membranes are given in Table 5.10.
161
Figure 5.12 shows that as the filler content increases the solvent uptake
decreases up to 5 wt. % MCC content in PU matrix and later the sorption increases with
further increase in filler content and the pattern is same for all aromatic solvents. This is
due to the reduction in chain mobility due to physical interaction between MCC and PU
at lower dosage of filler and at higher filler content there is an increased structural
heterogeneity by aggregation of the filler. The sorption is in accordance with tensile
behaviour of the composites. The lowering of solvent uptake was noticed for filled
samples than pristine polymer till 5 wt. % of MCC, due to the good dispersion of filler
and good physical interaction between filler and matrix [39]. As the filler content
increases the solvent uptake decreases to certain extent and then increases, this is due to
excellent filler-matrix interaction at initial stages of filler loading which hinders the
solvent penetration in to the membrane, as the filler loading further increases, void tend
to occur at the interface, which leads to increase in free volume of the systems and
consequently increase the solvent uptake in to membrane. Similar observation was made
by Stephen et al [40].
0 10 20 30 40 50 60 70
0
10
20
30
40
50
60
70
80
Qt (
%)
t1/2
(min)
Benzene
Toluene
p-Xylene
Figure 5.11. Plots of Qt as a function of t1/2
for PU/5% MCC composite with
different solvents
The D values for sorption is high for benzene and low for p-xylene. This is
probably due to low molecular volume of benzene when compared to p-xylene. This
result indicates that there may be structural and morphological changes that take place
PU/MCC biocomposite membranes. The value of D for sorption decreases with increase
in filler concentration upto 5 wt. % loading, > 5 wt. % of MCC increases the sorption
capacity of the composite membrane due to low wettability of filler in matrix and
increased filler-filler interaction.
162
0 10 20 30 40 50 60 70
0
20
40
60
80
Qt
(%)
t1/2
(min)
0%
2.5%
5%
7.5%
10%
(a)
0 10 20 30 40 50 60 70
0
10
20
30
40
50
60
70
80
Qt
(%)
t1/2
(min)
(b)
0%
2.5%
5%
7.5%
10%
0 10 20 30 40 50 60 70
0
10
20
30
40
50
60
70
(c)
Qt
(%)
t1/2
(min)
0%
2.5%
5%
7.5%
10%
Figure 5.12. Effect of filler content in PU on the percentage mass uptake for (a)
benzene, (b) toluene and (c) p-xylene
Table 5.10. Sorption (S± 0.3 %), diffusion (D± 0.4 %) and permeation (P ± 0.35
%) coefficients of PU/MCC green composites
MCC
content
in PU
(wt %)
Benzene Toluene p-Xylene
S
(g/g)
D x 107
(cm2/s)
P x 107
(cm2/s)
S
(g/g)
D x 107
(cm2/s)
P x107
(cm2/s)
S
(g/g)
D x 107
(cm2/s)
P x 107
(cm2/s)
0 80.34 2.76 2.21 74.60 2.53 1.88 60.89 1.83 1.11
2.5 78.51 2.24 1.75 75.93 2.08 1.57 57.34 1.52 0.87
5 72.37 2.18 1.57 64.82 1.97 1.27 52.13 1.27 0.66
7.5 75.64 2.37 1.79 72.91 2.03 1.48 55.97 1.63 0.91
10 82.03 2.39 1.96 75.05 2.17 1.62 59.08 1.59 0.93
163
Permeability (P) of the solvent through the composites is P = DS. The
permeation of solvent molecules into polymer composite membranes depends on
diffusivity and sorptivity. The permeability of organic solvent molecule into PU/MCC
composites depends on solubility of polymer in solvents and on diffusivity of solvents
in to polymer composites. The decrease in permeation values are observed from Table
5.10 as increase in filler content upto 5 wt. % and on further loading of filler the
permeation values increases. This variation of permeation depends on the extent of
interaction of filler with polymer matrix.
To know the type of sorption mechanism, n and K values are calculated using
following equation (2);
ln( / ) ln K n ln tM Mt (2)
where, K and n are empirical parameters, Mt and M∞ are the mass uptake at time t and at
equilibrium. n value denotes the transport mode, if n=0.5, suggest Fickian mode and if
n=1, follows non-Fickian mode of transport.
Table 5.11. System parameters (n and K) and penetration velocity (ν) values for
PU/MCC green composites
MCC
content in
PU(wt. %)
Benzene Toluene p-Xylene
n
K (102
g/g
minn)
ν (102
cm/s) n
K (102
g/g
minn)
ν (102
cm/s) n
K (102
g/g
minn)
ν (102
cm/s)
0 0.58 2.48 56.24 0.42 5.34 52.98 0.46 5.75 36.77
2.5 0.59 2.52 55.98 0.42 6.05 50.06 0.44 6.55 34.49
5 0.60 2.59 51.36 0.45 4.81 49.30 0.50 5.32 29.51
7.5 0.55 3.25 52.38 0.47 4.40 54.48 0.44 6.95 28.97
10 0.56 3.32 59.70 0.45 5.07 52.78 0.45 6.78 33.47
The plots of ln(Mt/M∞) verses ln t for different solvents are represented in Figure
5.13. From Table 5.11, n values lies in the range of 0.42-0.60 indicates the mass uptake
by the composites follows Fickian mode of transport and are accurate to ± 0.0015. Table
5.11, shows that there is no systematic variation in K values, and are independent from
164
filler concentration and penetrant size. The variation in n and K values depends on the
restriction posed by the filler on sorption of the solvent in to the membrane.
Penetration velocity is determined by taking the slope of initial portion of the
sorption curve, dwg/dt by using following equation;
(v)=1 dwg
2 A* dt (3)
where, dwg/dt denotes the slope of the percentage weight gain versus time curve, ρ is
the density of the solvent at 25 oC. A* denotes the area of the sample and number 2
accounts for sorption takes place on both sides.
2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8
-2.0
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
ln (
Mt/
Min
f)
ln(t)
0%
2.5%
5%
7.5%
10%
(a)
2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
ln (
Mt/
Min
f)
ln(t)
0%
2.5%
5%
7.5%
10%
(b)
2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
ln (
Mt/
Min
f)
ln(t)
0%
2.5%
5%
7.5%
10%
(c)
Figure 5.13. Plots of ln (Mt/M∞) versus ln t for (a) benzene, (b) toluene and (c) p-
xylene
165
The calculated penetration velocity of the solvents in to PU/MCC green
composite materials are tabulated in Table 5.11. The penetration velocity in benzene is
higher as compared to toluene and p-xylene. This is due to low molecular volume of
benzene [41]. The penetration velocity also depends on filler content. Decrease in
penetration velocity up to 5 wt. % of MCC, further increase in filler addition the
penetration velocity increases.
5.3.9 Wide angle X-ray scattering spectroscopy
The X-ray patterns obtained for all PU/MCC biocomposites are shown in Figure
5.14. From figure it was noticed that a broad and intense peak in 2θ region 20.3-21.0o.
Increase in MCC content, increases the peak height at 2θ region 20.3-21.0o was noticed
up to 5% MCC and further increase in MCC content reduction in peak height was
observed [42-43]. Microcrystalline parameters of PU/MCC composites were calculated
using three different asymmetric distribution functions and the results were given in
Table 5.12. To ascertain the most suitable asymmetric distribution, fitness test was
made using simulation. It is evident from Figure 5.15 that there is a good agreement
between experimental and theoretically calculated x-ray data. In all cases the goodness
of the fit was less than 15%. From Table 5.12, it is evident that the exponential
distribution has less standard deviation (δ) as compared to other distribution functions
and hence, we have used the corresponding results for further interpretation.
0 5 10 15 20 25 30 35 40 45
0
1000
2000
3000
4000
5000
6000
7000
Inte
nsi
ty (
au
)
2 (0)
0%
2.5%
5%
7.5%
10%
Figure 5.14. XRD patterns of 0, 2.5, 5, 7.5 and 10 wt% of MCC filled PU
biocomposites
166
From X-ray patterns it can be studied that, as the filler content increases upto 5
% in PU matrix the surface weighted crystallite size (Ds) corresponding to main peak
increases, with further increment in filler concentration decreases the Ds values. This
variation in Ds is due to the formation of hydrogen bond between urethane groups of PU
and hydroxyl groups of MCC and also the -OH groups of MCC involve in the formation
of PU with diisocyanate (TDI). The number of unit cell (<N>) measured in the direction
perpendicular to plane, increased up to 5 wt % MCC and decreased with further
increase in filler content. The values of <N> and lattice strain (g) are the governing
factor for broadening of x-ray patterns. The variation in these microcrystalline
parameters indicates that there is variation in the behaviours of PU/MCC green
composites due to change in chemical structure and morphology with compositions.
Table 5.12. Microstructural parameters for PU/MCC green composites obtained
by (a) Exponential distribution function
MCC
content in
PU (wt %)
2θ (o) <N> δ (%) g (%) Ds (nm) α* d (nm)
0 20.6 2.87 0.060 0.5 0.54 0.85 0.437
2.5 20.6 3.09 0.058 0.5 0.57 0.88 0.431
5 21.0 3.30 0.034 0.5 0.61 0.91 0.430
7.5 20.8 3.41 0.050 0.5 0.60 0.92 0.426
10 20.3 3.17 0.057 0.5 0.56 0.89 0.423
(b) Reinhold distribution function
MCC
content in
PU (wt %)
2θ (o) Ds (nm) <N> δ (%) g (%) α* d (nm)
0 20.6 0.54 2.87 0.060 0.5 0.85 0.437
2.5 20.6 0.58 3.1 0.059 0.5 0.89 0.431
5 21.0 0.61 3.31 0.034 1.0 0.82 0.430
7.5 20.8 0.60 3.41 0.052 1.0 0.85 0.426
10 20.3 0.57 3.19 0.059 0.5 0.89 0.423
167
(c) Lognormal distribution func tion
MCC content
in PU (wt %)
2θ (o) Ds (nm) δ (%) g (%) d (nm)
0 20.6 0.50 0.09 3.0 0.437
2.5 20.6 0.52 0.09 2.5 0.431
5 21.0 0.55 0.08 3.0 0.430
7.5 20.8 0.54 0.09 2.0 0.426
10 20.3 0.52 0.09 3.0 0.423
Figure 5.15. Experimental (++) and simulated (--) patterns using exponential
distribution function for (a) 2.5, (b) 5, (c) 7.5 and (d) 10 wt. % of MCC loaded PU
composites
According to Hosemann‟s model, the change in crystal size is attributed to the
interplay between the strain present in the polymer network and also the number of unit
cell coherently contributing to the X-ray reflection. This concept has been quantified in
terms of α*, called enthalpy. The calculated average values of α* (0.85-0.92) is low,
which is in agreement with Hosemann‟s observation on other polymers. The α* values
represents the amount of energy needed for the formation of the polymer network. The
phase stabilization in PU/MCC composites has been quantified in terms of parameters
α*.
Inte
nsi
ty i
n A
.U
Inte
nsi
ty i
n A
.U
Inte
nsi
ty i
n A
.U
Inte
nsi
ty i
n A
.U
(Sin Ѳ)/λ in A-1 (Sin Ѳ)/λ in A-1
(Sin Ѳ)/λ in A-1 (Sin Ѳ)/λ in A-1
168
0 2 4 6 8 10
0.54
0.56
0.58
0.60
0.62
Microcrystalline cellulose (wt %)
Crystallite size
Tensile strength
Cry
sta
llit
e s
ize
(a)
2
4
6
8
T
en
sile
str
en
gth
MP
a
0 2 4 6 8 10
0.54
0.56
0.58
0.60
0.62
2
4
6
8
Crystallite size
Tensile strength
Cry
sta
llit
e s
ize
Microcrystalline cellulose (wt %)
Ten
sile
str
en
gth
MP
a
(b)
0 2 4 6 8 10
0.50
0.52
0.54
0.56
C
ry
sta
llit
e s
ize
Crystallite size
Tensile strength
2
4
6
8
Microcrystalline cellulose (wt %)
T
en
sile
str
en
gth
MP
a
(c)
Figure 5.16. Tensile strength and crystallite size obtained by (a) Exponential, (b) Reinhold
and (c) Lognormal distribution models as a function of MCC content in PU/MCC
biocomposites
0 2 4 6 8 10
0.53
0.54
0.55
0.56
0.57
0.58
0.59
0.60
0.61
0.62
Microcrystalline cellulose (wt %)
crystallite size
sorption coefficient
Cry
sta
llit
e s
ize
(a)
72
74
76
78
80
82
So
rp
tio
n c
oeff
icie
nt
0 2 4 6 8 10
0.54
0.56
0.58
0.60
0.62
crystallite size
sorption coefficient
So
rp
tio
n c
oeff
icie
nt
Microcrystalline cellulose (wt %)
Cry
sta
llit
e s
ize
72
74
76
78
80
82
(b)
0 2 4 6 8 10
0.50
0.51
0.52
0.53
0.54
0.55
0.56
Microcrystalline cellulose (wt %)
Cry
sta
llit
e s
ize
72
74
76
78
80
82
crystallite size
sorption coefficient
So
rp
tio
n c
oeff
icie
nt
(c)
Figure 5.17. Sorption coefficient and crystallite size obtained by (a) Exponential, (b)
Reinhold and (c) Lognormal distribution models as a function of MCC content in
PU/MCC biocomposites
169
The comparison of tensile strength obtained for all the PU composites with the
crystallite size resulted from all the distribution functions is shown in Figures 5.16 (a) -
(c). From the figures it is evident that these two are in accordance to each other. The
benzene uptake and crystallite area as a function of MCC content has been plotted in
Figure 5.17(a)-(c). From these figures it was noticed that sorption values also depend on
crystallite size, as the crystallite size increases the benzene uptake decreases. The other
solvents are also follows the same trend.
5.3.10 Contact angle measurement
The hydrophilic behaviours of the PU/MCC green composites can be
investigated by contact angle measurement. The photographs of contact angle made by
water on the surface of the PU/MCC green composites is represented in Figures 5.18
(a)-(e). The contact angle values along with surface energy are given in Table 5.14.
Figure 5.18. Digital photographs of shape of water droplets on the surface of (a) 0,
(b) 2.5, (c) 5, (d) 7.5 and (e) 10 wt. % of MCC filled PU green composites
Table 5.13. Contact angle and surface energy values at 25 ºC for PU/MCC
biocomposites
MCC content
in PU (wt. %)
Water (0) Surface energy
(mJ/m2)
0 92.4 69.8
2.5 87.3 76.2
5 84.5 79.7
7.5 79.7 85.7
10 75.4 91.1
The contact angle varies from 92.4 to 75.4o and surface energy from 69.8 to 91.1
mJ/m2
with increase in MCC content from 0 to 10 wt % respectively. The contact angle,
170
values slightly decreases with filler content and the surface energy values increases, this
is due to the presence of MCC filler in PU. The filler which is added to the PU is
hydrophilic in nature hence, decreases the contact angle values with increase in filler
loading.
5.3.11 Morphological behaviours
In order to understand the interfacial bonding between matrix and filler the
cryofractured PU/MCC green composites were characterized by scanning electron
microscopy (SEM). Figures 5.19 (a) – (e) shows the SEM photomicrographs of PU and
PU/MCC biocomposites. Figure 5.19 (a) shows that the fracture surface of the pristine
PU matrix, which is completely featureless. The finer hard segment is homogeneously
distributed in the soft component.
Figure 5.19. SEM photomicrographs of (a) 0, (b) 2.5, (c) 5, (d) 7.5 and (e) 10 wt. %
of MCC loaded PU green composites
In contrast, Figures 5.19 (b) – (e) shows fracture surfaces of the PU/MCC
samples; their appearance was qualitatively different. From the figure, it was noticed
that the minor phase (MCC) was dispersed in the major continuous PU phase. Figure
5.19 (b) indicate the good interaction between PU and MCC. The higher the filler
concentration, greater the density of crack deflection sites, producing smaller and denser
ripples [Figs. 5.19 (c)–(e)]. The PU/MCC composites with lower MCC content
171
demonstrated a good interfacial bond between PU and filler. The good dispersion of
MCC in PU matrix followed by proper wetting and a good filler-matrix adhesion are
expected to enhance the mechanical properties of a composite (Figs. 5.19 (b) – (c)).
SEM images indicate that at higher MCC loading (Fig. 5.19 (e)) agglomeration of the
filler in PU matrix and micro void formation were noticed.
5.3.12 Biodegradation studies
The specimens exposed to A. niger in potata dextrore broth showed change in
surface appearance. The growth of these fungi is seen on the surface of the composites
(Figure 5.20). The weight loss of the PU/MCC biocomposites depends on the
percentage composition of the MCC content in the PU matrix (Table 5.14) [44].
Figure 5.20. Growth of Aspergilllus niger on the surface of (a) 0, (b) 2.5, (c) 5, (d)
7.5 and (e) 10 wt. % of MCC content in PU/MCC biocomposite
Table 5.14. Change in weight loss occurred during biodegradation
MCC content in
PU (wt. %)
Weight loss
(%)
0 2.47
2.5 5.29
5 6.72
7.5 7.94
10 9.12
Gong and Zhang also made similar observation for PEG and MDI based PUs in
presence of mixed microorganisms [45]. The weight loss in PU/MCC biocomposites
172
was found to be more than amount of MCC present in the corresponding composite.
This disclosed the fact that there was degradation of PU matrix along with MCC.
5.4 Conclusions
The toughened sheets of PU/MCC biocomposites were fabricated with varying
amounts (0 - 10 wt. %) of MCC content. The tensile results clearly indicate that MCC
significantly improves the tensile strength and tensile modulus of the PU composites at
lower dosage (i.e., 2.5 – 5 wt. %) of filler loadings. This can be attributed to the strong
filler–matrix interaction. The microcrystalline cellulose become physically bonded to
the matrix through the hydrogen bond formation i.e., –OH groups of the MCC filler and
the urethane groups of PU networks. This interaction leads to an increase in the
mechanical behaviours of the composites. FTIR results reveal the hydrogen bond
formation between MCC and PU. There is no significant change in weight when
PU/MCC composites are exposed to different chemical environments except KMnO4.
The order of the percentage swelling in different organic solvents is as follows;
chlorobenzene > dichloromethane > ethyl acetate > heptane. The phenomenon of
solvent uptake behavior will strongly depend on the solubility parameter and polarity of
the solvents. The water uptake behaviour of the composites has been studied in different
chemical environments. The water uptake sequence of the composites followed the
following order, i.e., acid medium > water at 250C > salt medium. The water uptake in
boiling water was greater than that in all the other media. The water uptake and RHs
behaviour strongly depends on the nature of the medium and; weight fraction of MCC.
DSC results reveals that Tg values increases upto 5% and further increase in filler
loading (5%), the Tg values decreases. The TGA thermograms indicate a slight
improvement in thermal stability of the PU/MCC composites as compared to pristine
PU. The TGA curve also reveals that all PU/MCC green composites under goes three
steps thermal degradation processes. From DMA analysis it is concluded that the
variation in Tg values are similar to the Tg values obtained by DSC thermograms. The
sorption coefficient follows the order of, benzene > toluene > p-xylene. It is observed
that as the filler loading increases the solvent uptake by the PU/MCC green composites
at equilibrium decreases upto 5 wt. % and with further increase in filler content there is
increase in equilibrium sorption, this depends on the interaction between filler and
matrix. From WAXS studies it was observed that, as the filler content increases upto 5
173
% in PU matrix the surface weighted crystallite size (Ds) corresponding to main peak
increases, on further increment in filler content reduces the Ds values. The sorption
values also depend on crystallite size, as Ds increases the sorption decreases. The filler
which is added to the PU is hydrophilic in nature hence, increases the contact angle
values with decrease in filler loading, and increases the surface energy. The SEM
images of the PU/MCC green composites indicate a good dispersion of the cellulose
crystals and good physical interaction between filler and PU matrix. At higher dosage of
filler content agglomeration of filler in PU matrix was observed. The variation in weight
loss due to biodegradation depends on MCC content in the PU matrix.
174
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