synthesis and characterization of silk fibre reinforced...

175

CHAPTER 6

SYNTHESIS AND CHARACTERIZATION OF SILK FIBRE REINFORCED

CEPU COMPOSITES

This chapter gives information on synthesis and characterization of GA based CEPU and its composites with silk fibre. Silk fibre (SF) used in this study belongs to the species Bombyx mori.

The chain extended polyurethane (CEPU) was synthesized by the reaction of

castor oil with different diisocyanates [toluene-2, 4-diisocyanate (TDI) and hexamethylene diisocyanate (HDI)] and chain extender such as glutaric acid. Biocompsoites have been fabricated by incorporating the short silk fibre into both TDI and HDI based CEPUs. The effect of incorporation of silk fibre into TDI and HDI based neat CEPU on the physico- mechanical properties such as density, surface hardness, tensile strength percentage elongation at break and modulus have been investigated. The dynamic mechanical properties and the thermal stability of neat PUs and the silk fibre incorporated CEPU composites have been evaluated. The TDI based neat CEPU has shown higher mechanical properties compared to HDI based CEPU. The incorporation of 10 % silk fibre into TDI and HDI based CEPU resulted in an enhancement of tensile strength by 1.8 and 2.2 folds respectively. The incorporation of silk fibre into biobased chain extended CEPU increased the glass transition temperature (Tg) of the resultant biocomposites. The morphology of tensile fractured neat PUs and their biocomposites with silk fibre was studied using scanning electron microscope (SEM). The effects of biological fluids and pH on swelling behavior of CEPU and its composites with silk fibre were investigated. 6.1 Introduction

Polyurethane (CEPU) materials are of commercial interest in many

applications because of their excellent properties such as abrasion resistance,

chemical resistance and toughness combined with good low-temperature

176

flexibility [1]. The PU materials are constituted by soft (polyol) and hard segments

(polyfunctional isocyanate and chain extender/crosslinker). There are various ways of

combining a wide variety of polyols and diisocyanates to produce tailored PU

products.

Recently, the utilization of the renewable resources in the synthesis of

polymers and fabrication of composites received considerable attention because of

their potential for substitution of petrochemical based raw materials. Many

researchers have reported the use of natural polymers having more than two hydroxyl

groups per molecule, either as polyol or as crosslinker in the preparation of CEPU, by

allowing them to react efficiently with the diisocyanates [2-8]. The results of these

investigations have shown that natural plant components, act as hard segments in

these CEPUs.

The increased green house gas emission and the anticipated depletion of

petroleum reserves in the near future directed research to develop environmentally

friendly composites based on renewable resources like natural fibres. The advantages

of natural fibres over traditional reinforcing fibres such as glass and carbon fibres are

low cost, low density, high toughness, acceptable specific strength, enhanced energy

recovery, recyclability, biodegradability, etc. [9]. Therefore, natural fibres can serve

as reinforcement by improving the strength and stiffness and also reducing the weight

of the resulting biocomposite materials. The properties of natural fibres vary with

their sources and treatments [10–12]. Natural fibres are largely divided into two

categories depending on their origin: plant-based and animal based. In general, plant-

based natural fibres are lignocellulosic in nature and are composed of cellulose,

hemicellulose and lignin, whereas animal-based fibres are of proteins.

Silk fibre is polar in nature due to the presence of various functional groups

such as –COOH, -CH2OH, -NH2, -NHCO on its surface. Silk fibres are biodegradable

and highly crystalline with well-aligned structure. It has been known that they also

have higher tensile strength than glass fibre or synthetic organic fibres, good elasticity

177

and excellent resilience [13]. Silk fibre is normally stable up to 140 oC and the

thermal decomposition temperature is greater than 150 oC.

Silk fibres possess many advantageous properties, light weight, high tensile strength and tensile modulus and good resistance toward chemicals. Silk fibres have been used for centuries. Bombyx mori silk represents a unique family of structural proteins associated with good bio and hemo biocompatibility with unusually high mechanical strength in fibre form revealing that it is one of the best materials [14-15]. Qiang Lv, et al [16] have reported the composite scaffold composed of polylactic acid (PLA) and silk fibre.

Sang Lee et al [17] studied, novel short silk fibre Bombyx mori silk reinforced poly (butylene succinate) biocomposites (PBS) fabricated with varying fibre contents by a compression molding method and their mechanical and thermal properties have been studied in terms of tensile and flexural properties, thermal stability, thermal expansion, dynamic mechanical properties and microscopic observations. The results demonstrate that chopped silk fibres play an important role as reinforcement for improving the mechanical properties of PBS in the present system although raw silk fibres are used without any surface modification normally done to enhance the interfacial adhesion between the natural fibre and the matrix.

This work also suggests that the use of animal-based natural silk fibres as

reinforcement in a natural fibre composite system may be potential for effectively improving the properties and performances of biodegradable polymer matrix resins.

Seong et al [18] prepared the composites of acrylonitrile-butadiene rubber (NBR) and two kinds of short silk fibres (Antheraea pernyi and Bombyx mori) without surface-treatment by a Bambury mixer. This fibril exerts a reinforcing effect on the composite, resulting in low elongation, high strength and high modulus of elasticity.

In practice, short fibre composites are considered as an isotropic material. In

this way, their expected behavior is similar to a homogeneous material and the

techniques developed for those can be applied without great difficulties [19-20].

However, for long fibre composites the mechanical properties are orthotropic, i.e.,

178

strongly dependent on direction. Hence, short fibre reinforced composites are finding

ever increasing application in engineering and consumer goods. They can offer a

unique combination of properties or may be used simply because they are more

economical than competing materials.

Freddi et al reported that silk fibre incorporated composites improved

mechanical strength of the polymer matrix [21]. Very limited attempts have been

made to fabricate the biocomposites using animal based fibre such as silk fibre.

The present research is aimed at evaluating the effect of short silk fibre in

biobased chain extend PU. The evaluation is being made on both the neat CEPUs of

TDI and HDI and their corresponding biocomposites with silk fibre. The initial

research has been carried out with the incorporation of small amount of (5 and 10%)

short silk fibre into biobased chain extended PU. The fabricated neat CEPUs and their

biocomposites have been subjected for physico-mechanical, thermal and

morphological studies.

6.2 Synthesis of silk fibre reinforced CEPU composites

The preparation of glutaric acid based chain extended polyurethane involves a

two step procedure. In the first step, pre PU polymer was obtained by the reaction of

two moles of diisocyanates into one mole of castor oil dissolved in methyl ethyl

ketone in a three-necked round bottomed flask equipped with a reflux condenser at 80

°C for one hour under nitrogen gas atmosphere. In the second step, one mole of

glutaric acid was added into the formed pre PU polymer and the reaction was carried

out with continuous stirring at 80 °C for 30 min. The biobased CEPU composition

was poured into the glass moulds coated with silicone releasing agents to cast the neat

PU sheets. To study the effect of different diisocyanates, neat PU sheets of both TDI

and HDI have been obtained.

The biocomposites have been fabricated by incorporating the degummed and

dried short silk fibre (5 and 10 %) into the CEPU composition and refluxed at 80 °C

for one hour. After one hour, the silk fibre containing CEPU composition was poured

into a silicone releasing agent coated glass mould to obtain the biocompsoites. The

silk fibres reinforced CEPU composites were prepared using both TDI and HDI.

179

6.3 Result and Discussion

6.3.1 Physico-mechanical properties

The calculated physico-mechanical properties such as density, tensile strength,

percentage elongation at break, tensile modulus and surface hardness for both TDI and

HDI based CEPUs and their composites with short silk fibre are given in Table 6.1.

6.3.1.1 Density

The calculated density of neat PUs of TDI and HDI is 1.1083 and 1.055

respectively. The density of all the composites with 5 and 10% short silk fibre have

exhibited lower density (0.9384 -0.9860 g/cc) as compared to corresponding neat

CEPUs. The reduction in density of the silk fibre reinforced CEPU composites

compared to neat CEPUs may be due to the low bulk density of the silk fibre.

6.3.1.2 Surface hardness

Surface hardness is a property measured laterally, whereas the modulus is

measured longitudinally. The surface hardness values of both reinforced CEPU/silk

fibre composites and neat CEPU is given in Table 6.1. The observed surface hardness

value of neat TDI and HDI based CEPU is 26 and 30 shore D respectively. The

surface hardness of silk fibre incorporated (5 and 10 %) TDI and HDI based

CEPU/silk composites values lies in the range 39-53 shore D. The incorporation of

short silk fibre into GA based CEPU showed a significant improvement in surface

hardness as compared to neat CEPUs.

6.3.1.3 Tensile behavior

The plots of stress verses strain curves for CEPUs and its composites is

presented in Figure 6.1. The silk fibre reinforced CEPU composites have showed

higher tensile strength and tensile modulus as compared to neat CEPUs. The tensile

strength of TDI and HDI based neat CEPUs are 8.6 and 5.50 MPa respectively. The

tensile strength of 10% silk fibre reinforced TDI and HDI based composites are 15.28

and 12.07 MPa respectively.

180

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80

2

4

6

8

10

12

14

16

TGA TGA + 5 % SF TGA + 10 % SF

(a)

Stre

ss (M

Pa)

Strain (mm/mm)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20

2

4

6

8

10

12 (b)

Stre

ss (M

Pa)

Strain (mm/mm)

HGA HGA + 5 % SF HGA + 10 % SF

Figure 6.1. Stress-strain curves of, (a) TDI and (b) HDI based CEPUs and their

composites with short silk fibre

Table 6.1. Physico-mechanical properties of TDI and HDI based CEPUs and their composites

Sample details Sample code

Tensile strength (MPa)

Elongation at break

(%)

Tensile modulus (MPa)

Density (g/cc)

Surface hardness (Shore D)

TGA A 8.6 170 7.47 1.1083 30

TGA+5% SF B 12.07 165 14.64 0.9860 41

TGA+10% SF C 15.28 156 16.79 0.9570 53

HGA D 5.50 200 3.57 1.055 26 HGA + 5% SF E 8.96 187 10.12 0.9780 39 HGA + 10% SF F 12.03 175 14.81 0.9384 48

The calculated tensile modulus of the composites are found to increase with

increase in the silk fibre content. The percentage elongation of silk fibre reinforced CEPU composites are found to reduce with increase in the silk fibre content. In general, TDI based neat CEPU and its composites with silk fibre have showed higher tensile strength, percentage elongation at break and tensile modulus as compared to that of HDI based neat CEPU and its composites with short silk fibre. This can be attributed to the aromatic nature of TDI which imparts higher stiffness and strength to the composites. A similar behavior was noticed by Satheesh Kumar et al in their studies on polyester nonwoven fabric reinforced castor oil based polyurethane composites [22]. The increased tensile strength and surface hardness with the incorporation of silk fibre into biobased CEPU suggests that, silk fibre acts as a reinforcing fibre.

181

6.3.2 Thermo analytical studies

6.3.2.1 Differential scanning calorimeter

The DSC thermograms for TDI and HDI based CEPUs and their

corresponding composites with 5 and 10 % silk fibre is shown in Figure 6.2. The Tg

values obtained from DSC curves is tabulated in Table 6.2. The Tg of TDI and HDI

based CEPUs are -15 and -17 oC respectively. A slight improvement or retain in Tg

after incorporation of silk fibre was noticed. This may be due to some kind of physical

interaction between polar groups of PU with polar groups of silk fibre. The observed

phenomena in the DSC thermograms were attributed to irregular structures at the

interphase [23].

0 50 100 150-1.0

-0.8

-0.6

-0.4

T-10%

T-5%

TGA

H-5%

H-10%

HGA

Hea

t flo

w

Temp (0C) Figure 6.2. DSC curves of neat CEPU and CEPU/silk fibre composites

Table 6.2. Glass transition temperature obtained from DSC thermograms for CEPU/silk composites

Sample Name Sample code Tg1 (oC) Tg2 (oC)

TGA A -15 -

TGA + 5% SF B -13 -

TGA +10% SF C -11 -

HGA D -17 66

HGA + 5% SF E -14 68

HGA + 10% SF F -13 -

182

6.3.2.2 Dynamic mechanical analyser (DMA) Dynamic mechanical analysis (DMA) is a useful technique to measure the

modulus (stiffness) and dampening properties of composite materials. Dynamic

storage modulus (G`) is an important property to assess the load bearing capability of

a composite material. The temperature dependence of storage modulus (G′) of neat

CEPUs of TDI, HDI and their corresponding composites with different weight

percentage of silk fibre is shown in Figure 6.3. All the samples have shown a

reduction in the G′ values with increase in the temperature. The G′ value of neat PUs

of TDI and HDI at 30 oC is 1.5 and 0.98 MPa respectively (Table 6.3). The

incorporation of silk fibre into both TDI and HDI based neat CEPUs has exhibited an

increased G′ value. The composites have showed an increased G′ with increase of the

silk fibre content. The incorporation of 10% silk fibre into TDI and HDI based chain

extended PU has enhanced the storage modulus of neat PU by 4.4 and 1.25 folds

respectively. Silk fibres are found to reinforce the PU matrix by allowing a greater

stress transfer at the fibre-matrix interface, thereby increasing the stiffness of the

overall material. The plot of tan δ as a function of temperature for all CEPU

composites is presented in Figure 6.4. With silk fibre loading the temperature curves

shifted towards higher temperature.

-60 -40 -20 0 20 40 60 80 100 1200

5

10

15

20

25

30

F

E

D

C

B

A

G' (

MPa

)

Temperature (0C) Figure 6.3. Variation of storage modulus as a function of temperature for;

(A) TGA, (B) TGA + 5% SF, (C) TGA + 10% SF, (D) HGA, (E) HGA + 5% SF and (F) HGA + 10% SF

183

-60 -40 -20 0 20 40 60 80 100 120-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

F

ED

CB

A

tan

delta

Temperature (0C) Figure 6.4. Plot of Tan δ as a function of temperature for, (A) TGA,

(B) TGA+ 5% SF, (C) TGA + 10 % SF, (D) HGA, (E) HGA + 5% SF and (F) HGA+10% SF

Table 6.3. Storage modulus of, (A) TGA, (B) TGA+ 5% SF, (C) TGA+10% SF, D) HGA, (E) HGA + 5% SF and (F) HGA + 10% SF composites obtained from

DMA curves Samples G’ (MPa) at 30oC

A 1.50

B 4.5

C 6.6

D 0.98

E 0.99

F 1.22

Latare Dwan’Isa et al [24] made a similar observation when silk fibre was

incorporated into soy oil based bio PU. The peak value of tan δ of HDI based neat

CEPU and its composites have found to be lower than that of TDI based neat CEPU

and their corresponding silk fibre composites.

6.3.2.3 Thermogravimetric analyser (TGA)

The thermogravimetric analysis (TGA) is a useful technique to determine the

quantitative degradation based on the weight loss of a composite material as a

function of temperature.

184

0 100 200 300 400 500 600 700

0

20

40

60

80

100

GC

BA

F

E

DW

eigh

t (%

)

Temperature (0C) Figure 6.5. TGA curves for, (A) TGA, (B) TGA+ 5% SF, (C) TGA+10% SF,

(D) HGA, (E) HGA+5% SF, (F) HGA+10% SF and (G) silk fibre

Table 6.4. Data obtained from TGA analysis from CEPU / silk fibre composites

Weight loss (%) at different temperatures

Temperature (oC) Sample code

100 150 200 250 300 350 400 450 500 550 600

Silk fibre 3.0 3.5 4.4 8.5 18.5 38.0 45.0 49.0 67.0 90.0 95.0

A 0.3 1.5 2.0 5.5 20.0 40.0 58.0 70.0 85.0 97.5 99.0

B 0.6 0.6 2.0 4.0 14.0 33.0 57.0 69.5 84.0 90.0 98.5

C 0.1 0.9 3.0 5.5 15.0 33.0 60.0 70.0 84.0 90.0 98.5

D 0.4 0.4 1.2 3.1 10.0 35.5 49.0 63.0 85.0 96.0 99.0

E 0.9 1.5 2.0 4.0 10.0 30.5 42.5 57.5 84.0 91.5 99.0

F 1.2 1.5 2.3 5.0 21.3 33.0 46.3 61.0 84.0 90.0 98.5

Figure 6.5 shows the TGA thermogram of silk fibre, neat CEPUs and silk fibre reinforced CEPU composites. The weight loss as a function of temperature for silk fibre, neat TDI and HDI based chain extended PU and their corresponding composites with 5 and 10 % silk fibre is tabulated in Table 6.4. The silk fibre did not show major weight loss in the temperature range of 100-200 oC as similar to the results reported elsewhere [25]. This initial weight loss may be due to the evaporation of moisture and low volatile impurities. The beginning of the major weight loss of silk fibre ws found to be above 300oC. It was reported in the literature that the chemical change in the silk fibre could be expected after 190oC [26]. Relatively, the HDI based neat CEPU has

185

(A) (B)

(C) (D)

(E) (F)

showed lower weight loss than TDI based neat CEPU. The observed weight loss of all the silk fibre reinforced CEPU composites are likely to be intermediate between neat CEPU and silk fibre. The silk fibre reinforced TDI and HDI CEPU composites exhibited reduced weight loss as compared to their individual components.

6.3.3 Morphological behavior

For studying the interfacial bonding between the matrix and the reinforcement the composite is tested by scanning electron microscopic (SEM). Figures 6.6 (A) to (F) shows the SEM photomicrographs for tensile fractured neat TDI and HDI based CEPUs and their corresponding composites with 5 and 10 % silk fibre.

Figure 6.6. SEM images of (A) TGA, (B) TGA+ 5% SF, (C) TGA+10% SF, (D) HGA, (E) HGA+5% SF and (F) HGA+10% SF

186

The neat CEPUs (Figures 6.6 (A) - (B)) have exhibited a layer like structure with two phases. The two phases is due to soft and hard segments of PU. Hydrogen bonding between >NH and >C=O groups of urethanes result in the phase separation between hard and soft segments of PU. The silk fibre reinforced CEPU composites (Figures 6.6 (C) - (F)) demonstrated a good interfacial bond between the silk fibre and CEPU. The good dispersion of fibres in matrix followed by proper wetting of fibres and a good fibre-matrix adhesion are expected to enhance the mechanical properties of a composite. Silk fibre was found to the well dispersed in PU matrix. Wetting of fibres can be seen in Figures 6.6 (C) – (F). It can be observed that the fibres have been pulled out from the CEPU matrix during the course of crack propagation.

6.3.4 Swelling behavior of CEPU/silk fibre composites

The aim of the present study is to evaluate the swelling behavior of TDI and

HDI based CEPU and their corresponding composites with 5 and 10% silk fibre in

different media such as physiological fluids (5 % urea, glucose, synthetic urine and

saline water), different pHs (4, 7.0 and 9.2) and different concentration of salt

solutions (0.5-5%).

The swelling property of these composites directly measures its utility in

environmental applications as an adsorbent and as biomaterial in medical,

pharmaceutical and biological applications. The swelling behavior of PU materials

may vary by varying the specific environmental parameters such as temperature, pH,

solvent quality, concentration, light intensity and wavelength. In view of the

importance of CEPU material for biomedical applications, it is very important to

study and understand its swelling behavior in various physiological media.

6.3.4.1 Physiological fluids preparation

To study the swelling behavior of samples in different physiological fluids

viz., saline water (0.9 g NaCl/100 ml), synthetic urine (0 .8 g of NaCl + 0.1 g of

MgSO4 +0.2 g of urea + 0.06 g of CaCl2 in 100 ml water), urea (5g/100 ml) and

D-glucose (5 g/100 ml) were prepared. The buffer solutions of pH 4, 7.0 and 9.2 were

prepared using pH tablets at 22 oC. Salt solutions with different concentrations viz.

0.1, 0.5, 1.0 and 5% solutions were prepared using distilled water.

187

6.3.4.2 Measurements

The neat CEPU and CEPU/silk fibre composites were cut circularly (diameter

= 1.0 cm) using a sharp edged steel die. Pre weighed (0.5-1g) samples were immersed

in 100 ml swelling media (body fluid, pH and salt solution) at room temperature. At

specified intervals of time, specimens were removed from the containers, solution

adhering to the surface of the specimens were removed using soft tissue paper and

weighed immediately using an analytical balance having ± 0.1 mg accuracy. The

weighing continued until the specimens attain the equilibrium values.

The percentage of swelling ratio (S) and percentage equilibrium contents

(EWC %) were calculated using the following equations;

S= 1000

0 xW

WWt

− (1)

where, W0 and Wt are the weights of sample in the dry state and weight at time t

respectively.

6.3.4.3 Effect of biological fluids on swelling behavior

The swelling behavior of TDI and HDI based CEPUs and their corresponding

composites with 5 and 10 % silk fibre in different physiological fluids are shown in

Figure 6.7. The swelling ratio of CEPUs and their composites with silk fibre in

different physiological fluids are given in Table 6.5.

Among the four different biological fluids, the samples in 5 % urea solution

had shown the highest swelling ratio whereas the samples in saline solution showed

lowest swelling ratio. The higher swelling ratio in urea and water may be due to the

polar-polar interaction between the solvent and the PU. Generally, HDI based CEPU

showed higher swelling ratio than TDI based CEPU. The incorporation of 5 and 10 %

silk fibre into CEPU was found to increase the swelling ratio of the resultant

composites. The composites with 10% silk fibre exhibited higher swelling ratio than

5% silk fibre composites.

188

0 2000 4000 6000 80000

10

20

30

40

50

TGA HGA TGA (5% SF) HGA (5% SF) TGA (10% SF) HGA (10% SF)

Swel

ling

ratio

(g/g

)

Time (min)

5% Urea

0 2000 4000 6000 80000

10

20

30

40

50


Time (min)

Swel

ling

ratio

(g/g

)

D-gulcose

0 2000 4000 6000 80000

10

20

30

40


Time (min)

Swel

ling

ratio

(g/g

)

Synthetic urine

0 2000 4000 6000 80000

10

20

30


Time (min)

Swel

ling

ratio

(g/g

)

Saline water

Figure 6.7. Effect of physiological solutions on swelling behavior of

CEPU/ silk fibre composites

Table 6.5. Percentage swelling of CEPU/silk fibre composites in physiological fluids

Swelling ratio (S) g/g

Physiological fluid TGA TGA+

5% SF TGA+

10% SF HGA HGA +

5% SF HGA + 10% SF

5% Urea 28 34 46 31 41 55 Glucose 23 33 40 28 38 48 Synthetic urine 18 27 31 28 32 41 Saline 15 21 26 19 25 33

As silk fibre percentage increased from 5 to 10 %, the swelling behaviors of

the composites increased from 15 to 49 g/g and from 19 to 55 g/g for TDI and HDI based CEPU/ silk composites respectively. The order of swelling behavior of CEPU composites is as follows; HGA + 10 % SF > TGA+10 % SF > HGA + 5% SF > TGA + 5% SF > HGA > TGA in all the physiological fluids under investigation.

189

6.3.4.4 Effect of pH The swelling behavior of TDI and HDI based CEPUs and their corresponding

composites with 5 and 10% silk fibre in different pH solutions is presented in Figure 6.8. The swelling ratio of CEPUs and their corresponding composites with silk fibre (5 and 10%) in different pH solutions are given in Table 6.6. All samples showed higher swelling ratio at pH 7.2 (neutral) compared to acidic (4.0) and alkaline (10.0) pH solutions. Compared to TDI based CEPU, HDI based CEPU has shown higher swelling ratio in the pH solutions. In general, the incorporation of silk fibre into CEPU was found to enhance the swelling ratio of the resultant composites. In particular, HDI based CEPU composites exhibited higher swelling ratio compared to TDI based CEPU composites.

0 1000 2000 3000 4000 5000 6000 70000

10

20

30

40pH - 4


Time (min)

Swel

ling

ratio

(g/g

)

0 1000 2000 3000 4000 5000 6000 70000

10

20

30

40

50pH - 7

Time (min)

Swel

ling

ratio

(g/g

)


0 1000 2000 3000 4000 5000 6000 70000

10

20

30

40

50


pH - 9.2

Time (min)

Swel

ling

ratio

(g/g

)

Figure 6.8. Effect of pH media on the swelling behavior of CEPU/ silk fibre

composites

190

Table 6.6. Swelling ratio (S) of neat CEPUs and CEPU / silk fibre composites in different pH Swelling ratio (S) g/g

pH TGA

TGA+ 5% SF

TGA+ 10% SF

HGA HGA + 5% SF

HGA + 10% SF

4 24 30 39 28 35 43

7 31 39 48 36 43 53

9.2 27 34 42 32 39 47 6.3.4.5 Effect of concentration of salt solutions on swelling behavior

The effect of different concentrations (0.1-5%) of aqueous NaCl solutions on

the swelling behavior of CEPU and its composites with silk fibre is shown in Figure

6.9. The calculated equilibrium swelling ratio is given in Table 6.7.

It was noticed that the swelling ratio of all the samples decreases with increase

in the concentration of NaCl. HDI based CEPU showed higher swelling ratio as

compared to TDI based CEPU. This could be due to the flexible nature of HDI based

CEPU having tendency to show higher sorption behavior. The incorporation of 5 and

10 % silk fibre into HDI and TDI based CEPUs showed the higher swelling ratio

compared to neat CEPU. It was also noticed that, swelling ratio of the composites

increases with increases in the content of silk fibre. This may be due to the presence

of voids in the composites.

The results clearly demonstrate that the swelling ratio of the network

decreases with increasing concentration of salt and followed the order of swelling

behavior that is; 5 < 1 < 0.5 < 0.1. It is well known that the water absorbency

decreased with an increase in the ionic strength of the salt solutions. This result may

be attributed to the reduction in the osmotic pressure difference between the

composites and the external salt solution with increasing ionic strength. These results

are all consistent with the decrease of water absorbency noticed for polyacrylamide

composite and the corresponding interpretation reported elsewhere [27].

191

0 2000 4000 6000 8000 100000

10

20

30

40

0.1 wt% 0.5 wt% 1.0 wt% 5.0 wt%

Time ( min )

Swel

ling

ratio

(g/g

)

TGA- 5% SF

0 2000 4000 6000 8000 10000

10

20

30

40

50

60

0.1 wt % 0.5 wt % 1.0 wt % 5.0 wt %

HGA-5% SF

Time (min)

Swel

ling

ratio

(g/g

)

0 2000 4000 6000 8000 10000

10

20

30

40

50

0.1 wt% 0.5 wt% 1.0 wt% 5.0 wt%

Time (min )

Swel

ling

ratio

(g/g

)

TGA-10 % SF

0 2000 4000 6000 8000 100000

10

20

30

40

50

60

Time (min)

Swel

ling

ratio

(g/g

)

HGA-10% SF

0.1 Wt% 0.5 Wt% 1.0 Wt% 5.0 Wt%

Figure 6.9. Effect of concentration of salt solution on swelling behavior of CEPU/silk fibre composites

0 2000 4000 6000 8000 10000

10

20

30

40

50

Time (min)

Swel

ling

ratio

(g/g

)

HGA

0.1 wt % 0.5 wt % 1.0 wt % 5.0 wt %

0 2000 4000 6000 8000 10000

10

20

30

40 TGA

0.1 wt % 0.5 wt % 1.0 wt % 5.0 wt %

Time (min)

Swel

ling

ratio

(g/g

)

192

Table 6.7. Swelling ratio of neat CEPU and CEPU/silk fibre composites in different concentrations of NaCl solution

Swelling ratio (%) Concentration

of NaCl (g/lt) TGA

TGA+ 5% SF

TGA+ 10% SF

HGA HGA + 5% SF

HGA + 10% SF

5 22 25 29 23 28 32

1 25.8 27 32 28 34 39

0.5 32.6 35 40 36 43 51

0.1 38.5 41 48 47 52 57

6.4 Conclusions

In the present research investigation short silk fibre reinforced CEPU

composites have been fabricated and characterized. The main objective of the current

research study is the investigation of the effect of short silk fibre content on the

performance of CEPU based biomaterials. The incorporation of a little amount of

short silk fibre is found to enhance the mechanical properties of neat CEPU sheets.

This is due to the incorporation of high strength and high modulus fibre into soft

CEPU matrix and also due to some kind of physical interaction between silk fibre and

CEPU matrix. A significant enhancement in the mechanical properties was noticed

after incorporation of even a little amount of short silk fibre into neat CEPU. DSC and

DMA studies confirmed that, the thermal transition temperature curves of neat CEPU

has shifted to slightly higher temperature side after incorporation of the silk fibre. The

observed thermal stability of the fabricated biocomposites is found to be intermediate

between that of silk fibre and neat PU. The two phase morphology of neat CEPU

(both TDI and HDI based) was confirmed from the SEM studies. The SEM studies

also revealed that the composites have better interfacial bonding which contributes to

the improved mechanical properties.

Swelling studies of the biocomposites revealed that CEPU/silk fibre

composites showed variation in their swelling capacity in different media. HDI based

CEPU/silk fibre composites showed higher swelling capacity than TDI based

composites. Among all physiological fluids, higher swelling ratio was noticed in 5%

193

urea. Composites also exhibited higher swelling behavior in pH 7 than other pHs. As

the concentration of salt solution increases the swelling ratio dramatically reduces.

The swelling ratio of the composites strongly depends on the concentration of salt

solution. This may be due to the reduction in osmotic pressure difference between

the composite and the salt solution with increasing ionic strength.

194

6.5. References

1. G. Oertel, Polyurethane Handbook, Carl Hanser Verlag, (1985).

2. A. Campanella, L.M. Bonnaillie and R.P. Wool, J. Appl. Polym. Sci., 112 (4)

(2009) 2567 – 2578.

3. R. Tanaka, S. Hirose and H. Hatakeyama, Bioresource Techn., 99 (9) (2008)

3810-3816.

4. N.B. Shelke, M. Sairam, S.B. Halligudi and T.M. Aminabhavi, J. Appl. Polym.

Sci., 103 (2) (2007) 779-788.

5. M.N. Satheesh Kumar and Siddaramaiah, J. Appl. Polym. Sci., 106 (2007)

3521–3528.

6. S. Roopa and Siddaramaiah, J. Reinforced Plast. and Compo., 26 (7) (2007)

681-686.

7. L. Bao, Y. Lan and S. Zhang, Iranian Polym. J., 15 (9) (2006) 737-746.

8. .H. Bao, Y.J. Lan and S.F. Zhang, J. Polym. Res., 13 (6) (2006) 507-514.

9. S. Misra, A.K. Mohanty, L.T. Drzal, M. Misra, S. Parija, and S.K. Nayak, Comp.

Sci. Technol., 63 (2003) 1377–85.

10. L.Y. Mwaikambo and M.P. Ansell, Macromol. Chem., 272 (1999) 108–16.

11. A.K. Mohanty, D. Hokens, M. Misra and L.T. Drzal, Bio-composites from bio-

fibres and biodegradable polymers. In: Proceed. (in CD) of 16th Ann. Tech.

Conf., Am. Soc. Comp., September 9–12, Blackburg, VA; (2001).

12. K. Oksman, M. Skrifvars and S-F. Selin, Comp. Sci. Technol., 63 (2003)

1317–24.

13. J. Perez-Rigueiro, C. Viney, J. Llorca and M. Elices, J. Appl. Polym. Sci., 70

(1998) 2439–47.

14. D.L. Kalpan, and K. Mcgrath, In protein - based materials (Brikhauser, Boston,

1997) 105-124.

15. K.Y. Lee, S.J. Kong, W.H. Park, W.S. Ha and I.C. Kwon, J. Biomat. Sci.

Polym., Ed., 9 (9) (1998) 905-914.

16. Qiang Lv, Kun Hu, Qingling Feng, Fuzhai Cui and Chuanbao Cao, Comp. Sci.

and Techn., 67 (14) (2007) 3023-3030.

17. Sang Muk Lee, Cho Donghwan, Won Ho Park, Seung Goo Lee, Seong Ok Han

and T. Drzal Lawrence, Comp. Sci. and Tech., 65 (2005) 647-657.

18. Seong Ok Han, Sang Muk Lee, Won Ho Park and D. Cho, J. App. Polym. Sci.,

100 (6) (2006) 4972 – 4980.

195

19. D. Zhao and J. Botsis, Intl. J. Fract., 82 (1996) 153–74.

20. S. Choi and K. Takahashi, J. Mater. Sci., 31(3) (1996) 731–40.

21. G.Freddi, M. Romano, M. Massafra and M. Tsukada. J. Appl. Polym. Sci.,

56(12) (1995) 1537–45.

22. M.N. Satheesh Kumar, K.S. Manjula and Siddaramaiah, J. Appl. Polym. Sci., 105

(2007) 3153–3161.

23. H. Hespe, E. Meisert, W. Eisele, L. Morbitzer and W. Goyert, Kolloid-Z, 250

(1972) 797.

24. J.P. Latere Dwan’isa, A.K. Mohanty, M. Misra and L.T. Drzal, J. Mater. Sci., 39

(2004) 2081 – 2087.

25. S.M. Lee, D. Cho, W.H. Park, S.G. Lee, S.O. Han and L.T. Drzal, Compos. Sci.

and Tech., 65 (2005) 647–657.

26. H. Zhang, J. Magoshi, M. Becker, J.Y. Chen and R. Matsunaga, J. Appl. Polym.

Sci., 86 (2002) 1817–20.

27. J.P. Zhang, R.F. Liu, A. Li and A.Q. Wang, Polym. Adv. Technol., 17 (2006) 12–9.

synthesis and characterization of silk fibre reinforced...

Documents