142

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

143

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

144

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

145

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.

146

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

147

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

148

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

149

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,

150

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

151

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

152

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.

153

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

154

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

155

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

156

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.

157

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

158

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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