CHAPTER 3 RESULTS AND DISCUSSION - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/9945/8/08_chapter 3.pdf · matrix phase and the bright region is the filler phase. It is observed

74

CHAPTER 3

RESULTS AND DISCUSSION

This chapter deals with the various process parameters that are

influencing the properties of nanocomposites and optimization of such

parameters in synthesis. Nanocomposites were characterized starting from the

resin state (prior to curing) to the cured state. The formation and structure of

nanocomposites were examined by TEM, XRD and FT-IR methods. The

effect of incorporation of nanoparticle in unsaturated polyester resin on the

physical properties such as gel time, curing and density were studied.

Mechanical property tests such as tensile, flexural, impact and hardness were

carried out. The cause of introduction of nanoparticle on thermal and thermo-

mechanical properties was studied by HDT, DSC, TGA and DMA methods.

In addition, the environmental stress (water, acid and alkali medium) crack

resistance of unsaturated polyester filled with nanoparticles were studied and

discussed in detailed.

3.1 SYNTHESIS AND PROCESS PARAMETERS

Nanocomposites with different wt% of nanoparticles were made.

The shear mixing favours the dispersion of nanoparticles in the resin matrix.

During mixing, the nanoparticles (such as calcium carbonate, silica, alumina

and zinc oxide) in the resin form the gel. This gel formation increases the

viscosity of resin-nanoparticle mixture. The polymerization takes place after

the addition of methyl ethyl ketone peroxide catalyst and cobalt naphthenate

accelerator in the resin-nanoparticle mixture.

75

The process parameters like temperature, pressure, nature of resin,

type of nanoparticle, resin bath, mixing speed and mixing time affect the final

properties of the nanocomposites. To get consistent and reliable results,

mixing time and speed were varied and other process parameters were kept

constant. The effect of mixing time and speed on the tensile strength of

unsaturated polyester with 7 wt% silica is seen in Figure 3.1. It is observed

that the tensile strength value increases with mixing time and speed. As the

mixing time increases, the tensile strength increases irrespective of the mixing

speed adopted. It attains saturation at 45 min of mixing and further increase in

mixing time shows negligible effect on tensile strength.

Figure 3.1 Effect of rpm and mixing time on tensile strength of

unsaturated polyester/7 wt% silica nanocomposites

The SEM picture of distribution of nano silica (3 wt%) particles in

unsaturated polyester matrix is shown in Figure 3.2. The dark region is the

76

matrix phase and the bright region is the filler phase. It is observed that the

dispersion of nanoparticles after 45 min of mixing is better than that after 30

min of mixing in unsaturated polyester resin. Hence, it has been decided to

carry out mixing for 45 min at 2000 rpm for all the unsaturated polyester

nanocomposites.

Figure 3.2 SEM images of unsaturated polyester/3wt% silica

nanocomposites after (a) 30 min and (b) 45 min of mixing

3.2 EFFECT OF FILLERS ON GEL TIME

It has long been recognized that the surface characteristics of

particulate fillers and fiberous reinforcements greatly influence the curing

behaviour of the thermosetting matrix resin systems. It may be due to the fact

that free radicals formed from radical initiator are able to form a charge

transfer complex with inorganic oxides on the surface of the filler, inhibiting

the curing reaction (Sun and Nusay 2000). Incorporation of nanoparticles into

the resin matrix brings down the rate of curing reaction compared to that of

neat UPR.

Gel time is defined as the time interval between the introduction of

catalyst and accelerator into resin matrix and formation of gel. The gel time is

measured after the addition of catalyst and accelerator. The effect of

incorporation of nanoparticles into the resin system on gel time is presented in

Figure 3.3. Gel time of pure unsaturated polyester resin is 50.7 min. The gel

77

time for the resin filled with 5 wt% calcium carbonate is 35.3 min. Similarly,

the gel time for the resin filled with 5 wt% silica, alumina and zinc oxide are

found to be 32.4, 36.3 and 30.5 min respectively. The reinforcement by

nanoparticles generally accelerates cross-linking and hence decreases the gel

time.

Figure 3.3 Gel-time of unsaturated polyester nanocomposites

3. 3 UNSATURATED POLYESTER/CALCIUM CARBONATE

NANOCOMPOSITES

3.3.1 Transmission Electron Microscopy

The physical and chemical properties of calcium carbonate powder

largely depend upon its particle size, crystalline form and aggregation state.

The smaller particles have large specific surface areas, therefore resulting in a

higher surface loading. Transmission electron microscopy (TEM) analysis

was conducted to verify the size of the synthesized calcium carbonate

nanoparticles and the level of nanoparticle dispersion in the UPR matrix.

TEM micrographs of synthesized nano calcium carbonate and UPR/calcium

carbonate nanocomposites are presented in Figure 3.4. It is found that the size

78

of calcium carbonate particles is about 50–60 nm and is spherical in shape

(Figure 3.4(a)). The good dispersion of nano calcium carbonate in the UPR

matrix without serious aggregation is noted in 5 wt% of nano calcium

carbonate containing nanocomposite (Figure 3.4(b)). It shows the

homogeneous dispersion of calcium carbonate in the UPR matrix and

indicated the good compatibility of nano calcium carbonate with UPR.

Figure 3.4 TEM images of (a) nano calcium carbonate particles (b)

UPR/5 wt% nano calcium carbonate (c) UPR/7 wt% nano

calcium carbonate

Large aggregates are found when the content of nano calcium

carbonate is increased to 7 wt%, as shown in Figure 3.4(c). It is also clear that

the vast majority of these agglomerates are still in the nanometer size range.

At higher nano calcium carbonate content, the distance between the

nanoparticles are smaller, thereby increasing the reuniting chances of the

added nanoparticles leading to decreased dispersion of the calcium carbonate

particles and hence increased aggregation (Wang et al 2007).

3.3.2 Fourier Transform Infra-Red Spectroscopy (FT-IR)

The interactions between UPR and nano calcium carbonate were

studied by Fourier transform infra-red spectroscopy. The Fourier transform

infrared spectra of nano calcium carbonate and UPR/calcium carbonate

nanocomposites are shown in Figure 3.5.

79

Figure 3.5 FT-IR spectra of pure UPR, nano calcium carbonate and

UPR/calcium carbonate nanocomposites

(a) Pure UPR (b) Nano calcium carbonate (c) UPR/1% Nano

calcium carbonate (d) UPR/3% Nano calcium carbonate

(e) UPR/5% Nano calcium carbonate (f) UPR/7% Nano calcium

carbonate (g) UPR/9% Nano calcium carbonate

80

The pure UPR shows the characteristic peaks at 1720–1730 cm-1

(C=O stretching vibrations), 1443 cm-1

(C=C stretching vibrations), 1182 and

1263 cm-1

(CH2 wagging and scissoring) and 750 and 705 cm-1

(out of plane

ring bending vibrations). The nano calcium carbonate has absorption bands at

1436, 873 and 707 cm-1

, the absorption peaks at 873 and 707 cm-1

being often

associated with calcite (Wang et al 2006, Shan et al 2007). It is obvious to

note that the FT-IR spectra of nanocomposites show all the major

characteristic peaks of both nano calcium carbonate and UPR and provides the

necessary proof for the existence of nano calcium carbonate in UPR matrix

(Ma et al 2008).

3.3.3 X-ray Diffraction Analysis (XRD)

X-ray diffraction is commonly used for the characterization of the

crystalline structure of the materials. X-ray diffractogram of nano calcium

carbonate, pure UPR and UPR/calcium carbonate nanocomposites are

illustrated in Figure 3.6. X-ray diffraction pattern of pure UPR does not show

any sharp and intense peaks due to amorphous nature. The nano calcium

carbonate shows the intense peaks at 2 around 30º due to the calcite structure

of calcium carbonate. In pure UPR there is no peak around the 2 value of

30º whereas the nano calcium carbonate and UPR/calcium carbonate

nanocomposites show a peak at 30º. This is due to the development of nano

calcium carbonate crystallinity in the amorphous polymer matrix. It is

interesting to note that due to the progressive increasing of loading of nano

calcium carbonate in the nanocomposites, the intensity of the peak at 30º

increased (Campos et al 2007). At the same time the peak intensity of UPR

has broadened without any shifting due to the amorphous nature of UPR.

81

Figure 3.6 XRD patterns of pure UPR, nano calcium carbonate and


3.3.4 Density

The incorporation or loading of nano calcium carbonate into the

UPR matrix affects its density and is shown in Figure 3.7. The density of

unfilled UPR is 1.21 g/cm3. It remains almost same with the addition of nano

calcium carbonate up to 5 wt%. On further addition it decreases to 1.18 g/cm3

for the UPR containing 7 wt% nano calcium carbonate. The decrease in

density may be due to the voids created in the polymer matrix. The

agglomeration of nano calcium carbonate at higher loading level (> 5 wt%)

leads to the formation of voids in the matrix. On the other hand the entrapped

air during mixing finds it difficult to escape out of polyester matrix and

remains as voids (Jawahar and Balasubramanian 2006). The created voids

decreased the viscosity of the matrix. The viscosity is a parameter which is

82

directly proportional to the density. Hence the decrease in density was noted

for the UPR containing 7 and 9 wt% nano calcium carbonate.

Figure 3.7 Effect of calcium carbonate nanoparticles on the density of

UPR

3.3.5 Mechanical Properties

The interface between the filler particles and the matrix has a great

influence on the mechanical properties of composites. The mechanical

properties can therefore, give indirect information about the interfacial

behaviour.

3.3.5.1 Tensile strength

The variations of tensile strength during loading of nano calcium

carbonate are illustrated in Figure 3.8. Pure UPR shows tensile strength of 58

83

MPa. It is seen that tensile strength increases on adding nano calcium

carbonate up to 5 wt% (71 MPa, 22% higher than pure unsaturated polyester

matrix). As the nano calcium carbonate content increases further (> 5 wt%),

the tensile strength decreases. Since the nano sized calcium carbonate

particles are uniformly distributed with in the unsaturated polyester resin

matrix, tiny cracks will occur between nano calcium carbonate particles and

unsaturated polyester resin, when tensile stress is applied. Mean while,

unsaturated polyester resin among particles will yield by plastic deformation,

with increase of both the surface area of nano sized particles and the contact

area between particles and UPR resin (Ansari and Ismail 2009). Hence the

materials will yield more tiny cracks and particle deformation due to the

tensile stress. But when the calcium carbonate content reaches a certain

critical value, there is an increasing disturbance of bonding between

molecular chains due to excessive proximity with the particles. This may

reduce the length of the rotating chain segment, leading to stress

concentration followed by a decreasing tensile strength. One possible

explanation for this strength reduction is as the amount of nano-sized particles

increases, excessive proximity of the particles is likely leading to poor

fracture resistance due to the development of large number of tiny cracks. The

increase in the number of particles makes the dispersion difficult and

agglomeration easier. The agglomerations of nano calcium carbonate results

in inhomogeneous distribution and hence weaken the interaction between the

filler and the matrix. This subsequently reduces the mechanical properties of

the composite system. Agglomeration will cause defect to the composite

because of the presence of void between the particles. This mean that

agglomerates resulting as a weak point in the composite material can lead to

an undesirable material property (Ahmad et al 2008).

84

Figure 3.8 Effect of calcium carbonate nanoparticles on the tensile

strength of UPR

3.3.5.2 Tensile modulus

The variations of tensile modulus by the increasing addition of

nano calcium carbonate into the resin matrix are depicted in Figure 3.9. The

tensile modulus of the unfilled polyester is found to be 2.8 GPa. While the

progressive addition of nano calcium carbonate into polyester matrix results in

significant improvement in modulus. The increase in tensile modulus with

respect to increase in concentration of the nano calcium carbonate in UPR is

nearly linear. A maximum value of 3.6 GPa was observed at a nano calcium

carbonate content of 9 wt% (Figure 3.9). The increment in tensile properties is

due to the better interaction existing between the nanoparticle with polymer

matrix (Mishra et al 2005, Cherian et al 2007).

85


modulus of UPR

3.3.5.3 Tensile elongation

Generally, the incorporation of filler into the resin matrix leads to

the increase in stiffness of the matrix and hence reduces the elongation. The

tensile elongation results of UPR/calcium carbonate nanocomposites are

illustrated in Figure 3.10. It is interesting to note that the elongation at break

was decreased with the addition of nano calcium carbonate. If there is good

adhesion between the filler and the matrix a dramatically decrease of

elongation at break, even with a lower amount of filler loading, can be

expected. On the other hand, at higher loading of filler results the poor

adhesion, so that the elongation at break may decrease more gradually. The

decrease in elongation at break noticed in all the composites (compared with

the pure resin) suggests a good degree of interfacial compatibility between

particles and the resin (Marcovich et al 2001, Wang et al 2007).

86


elongation of UPR

3.3.5.4 Flexural strength

Flexural strength of pristine polyester and unsaturated polyester

filled with nano calcium carbonate are shown in Figure 3.11. Pure unsaturated

polyester shows flexural strength of 98 MPa. The flexural strength of the UPR

filled with nano calcium carbonate particles increases continuously up to 5

wt% (107 MPa) and further addition of nano calcium carbonate particles

decreases the strength. UPR with 7 wt% nano calcium carbonate shows

flexural strength of 105 MPa. The distribution of the particle in the matrix is

an important factor to be considered in this case. The stress distributions

around the particles increase the stress concentration of the propagating crack,

which in turn induces relatively early failure. This is very much seen at the

higher filler content. For higher filler content (> 5 wt%) the chance for

agglomeration is more which induces high stress concentrated zone near the

87

particles. This factor demonstrates the importance of the particle distribution.

The nanoparticle reinforcement enhances the contact surface area to the

matrix and thereby enhancing the stress transfer from matrix to filler, which

results in improved strength. The poor interfacial property owing to their

micron scale filling and high stress concentration paved the way for less load

transfer from the matrix to the filler (Jin and Park 2008).

Figure 3.11 Effect of calcium carbonate nanoparticles on the flexural

strength of UPR

3.3.5.5 Flexural modulus

The effect of nano calcium carbonate on flexural modulus of UPR

resin is presented in Figure 3.12. The pure polyester resin has the flexural

modulus value of 2.7 GPa. The progressive addition of nano calcium

carbonate in UPR matrix showed the significant improvement in modulus.

The maximum value of flexural modulus of 3.7 GPa (37% higher than that of

88

UPR resin) was observed for 9 wt% of nano calcium carbonate. The addition

of the nanoparticles increases the stiffness of the matrix. With a rise in filler

content, the modulus increases mainly due to the contribution of hard particles

(Yang et al 2006, Shi et al 2004, Hemmasi et al 2010).

Figure 3.12 Effect of calcium carbonate nanoparticles on the flexural

modulus of UPR

3.3.5.6 Impact strength

The improvement of mechanical behaviour can be effectively

achieved by the uniform dispersion of nanoparticles into the polymer matrix.

In the structural application, it is extremely tough to define composite

materials that possess a high toughness, stiffness and impact resistance.

However, the impact resistance of polymer nanocomposites is probably one

of the most important and least understood mechanical properties of

89

polymers. The impact behaviour and properties are mainly improved by small

particles with low aspect ratio.

Izod impact test methods of un-notched specimen are high speed

fracture tests measuring the energy required to break the specimens at high

strain rate conditions. The energy measured in this test is the energy required

to create and propagate a crack. Generally, un-notched specimen will have

high impact energy than notched specimen. The same effect is seen in the un-

notched specimen if the particle agglomeration is large. This effect is usually

seen in specimen with large filler content which acts as stress concentration

sites with in the matrix.

The izod impact strength of UPR/calcium carbonate

nanocomposites at various weight ratios are compiled in Figure 3.13. It is

clear that the nano calcium carbonate particles have a remarkable toughening

effect on UPR. The impact strength of pure UPR was 20 Jm-1

and it increased

slightly to 28 Jm-1

for the UPR/calcium carbonate nanocomposites at 5 wt%

of nano calcium carbonate. Predictably, the impact strength increased

gradually with increasing the nano calcium carbonate content of 5 wt%. The

toughness of UPR was improved by a factor of 1.4 with addition of 5 wt%

nano calcium carbonate particles. Although literature studies reveal that,

micro sized calcium carbonate ller had a positive effect on toughness of

UPR, the improvement in izod impact strength was moderate. However, with

the nano-sized calcium carbonate particles the applicability of the bowing

mechanism is questionable, because such small-sized rigid particles may not

be able to resist the propagation of the crack. Obviously, the toughening effect

of nano calcium carbonate particles on UPR could be contributed to a new

mechanism that was indeed observed for a number of nanocomposites.

According to this mechanism, the nano calcium carbonate particles could act

as stress concentration sites, which could promote cavitations at the UPR

90

matrix-particle boundaries during loading. The cavitation could release the

plastic constraints and trigger mass plastic deformation of the matrix, leading

to improved toughness (Wu et al 2004). At higher filler loading (> 5 wt%) the

agglomerations of calcium carbonate results in inhomogeneous distribution

and hence weaken the interaction between the filler and matrix. This

subsequently reduces the impact strength of composite system (Chen et al

2004, Huang et al 2006).

Figure 3.13 Effect of calcium carbonate nanoparticles on the impact

strength of UPR

3.3.5.7 Hardness

The shore D hardness values of the UPR/nano calcium carbonate

systems are shown in Figure 3.14. The hardness of the unfilled polyester

sample is found to be 56. However the mineral filled composites are harder

than unfilled composites. This observation is in agreement with the fact that

91

the hardness is a measure of resistance to penetration. This resistance is about

4, 11, 16, 18 and 20% higher than that of unfilled UPR for UPR containing 1,

3, 5, 7 and 9 wt% loading of nano calcium carbonate.

Figure 3.14 Effect of calcium carbonate nanoparticles on the hardness

of UPR

3.3.6 Fracture Analysis

3.3.6.1 Tensile fracture

The increase in tensile strength of nano calcium carbonate filled

UPR composites were studied by examining the fracture morphology of the

tested specimens. The SEM pictures of tensile fractured surfaces of

UPR/calcium carbonate nanocomposites are given in Figure 3.15. The tensile

fracture surface of pure UPR polymer cured at room temperature is shown in

Figure 3.15(a). It is seen that fracture surface of pure UPR polymer is smooth

indicating that the failure is brittle in nature. This suggests that the resistance

92

for crack propagation is less and has resulted in low strength. The SEM

picture of the tensile fracture surface of the UPR filled with nano calcium

carbonate of 5 and 7 wt% are shown in Figure 3.15(b) and (c) respectively. It

is observed that addition of nano calcium carbonate changes the fracture

surface morphology of UPR polymer. The crack surface becomes rough in

these cases. This fracture surface roughness indicates that the resistance to

crack propagation is high and the crack has not propagated as easily as seen in

pure UPR. The fracture surface roughness also indicates the torturous path of

propagating the crack. This effect results in higher strength to failure

confirming improved strength of nanocomposites. Though, the fracture

roughness is predominant at 7 wt% nano calcium carbonate, the existence of

agglomeration of particles could have decreased the strength of the

nanocomposites (Jiang et al 2007).

Figure 3.15 SEM images of the tensile fracture surface of (a) pure UPR

(b) UPR/5 wt% nano calcium carbonate (c) UPR/7 wt%

nano calcium carbonate

3.3.6.2 Impact fracture

The izod impact fracture surface of the UPR/calcium carbonate

nanocomposites are given in Figure 3.16. The fracture surface of the virgin

polyester specimen (Figure 3.16 (a)) is relatively smooth, indicating that

minimal energy was required to fracture the specimen. On the other hand, the

93

surfaces of the specimens embedded with nanoparticles depict rougher

features such as out-of-plane flaking and thumbnail-type markings that

require additional energy to be formed. As more energy was imparted from

the strain energy of the deforming specimens for the creation of these

features, a corresponding increase in fracture toughness was obtained vis-à-

vis the neat polyester. Good adhesion between the nano calcium carbonate

particle and polymer matrix can be seen from the fact that there is not much

particle pull out and subsequent cavity formation. The Figure 3.16(b)

micrograph showing good interfacial adhesion between UPR and nano

calcium carbonate. It is difficult to differentiate the nano calcium carbonate

particles from the polymer matrix and it can be seen that the fracture paths

passed either through the nano calcium carbonate particles or through the

polymer matrix and not in the interface between these. The SEM picture

shows that there is perfect compatibility between UPR matrix and filler

surface. Though, the fracture roughness is predominant at 7 wt% nano

calcium carbonate (Figure 3.16(c)), the existence of agglomeration of

particles could have decreased the strength of the nanocomposites (Xie et al

2004)

Figure 3.16 SEM image of impact fracture of (a) pure UPR (b) UPR/5

wt% nano calcium carbonate (c) UPR/7 wt% nano calcium

carbonate

94

3.3.7 Thermal Properties

3.3.7.1 Heat deflection temperature

The heat deflection temperature (HDT) may be taken as the

material’s ultimate use point for a short period of time. The heat deflection

temperatures for UPR/calcium carbonate nanocomposites with different nano

calcium carbonate contents are reported in Figure 3.17. Heat deflection

temperature of unfilled polyester is 65 °C. A significant enhancement of the

heat deflection temperature occurs with increasing nano calcium carbonate

content. The maximum increase in heat deflection temperature observed at 9

wt% nano calcium carbonate is 73 °C. This behaviour was expected because

inorganic nanoparticles have high thermal stability (Zhang et al 2003).

Figure 3.17 Effect of calcium carbonate nanoparticles on the HDT

of UPR

95

3.3.7.2 Differential scanning calorimetry

The DSC heating scans of the neat resin and resin filled with nano

calcium carbonate particles are presented in Figures 3.18. The peak exotherm

of pure UPR is at 81 C. The addition of nano calcium carbonate continuously

reduces the peak exotherm. In UPR with 7 wt% calcium carbonate it is 63 C.

Figure 3.18 DSC thermogram of UPR and UPR/calcium carbonate

nanocomposites

However, the shift in the cure exotherm to lower temperature owing

to the incorporation of the nanoparticles hints on the influence of the filler on

the curing of the UPR. The decrease in intensity of exotherm peak in calcium

carbonate filled polymer is due to the decrease in concentration of polymer as

nano calcium carbonate content increases.

96

3.3.7.3 Thermogravimetric analysis

The thermal stabilities of the UPR/calcium carbonate

nanocomposites were studied by means of TGA. The TGA thermograms for

the neat UPR resin and UPR/calcium carbonate nanocomposites are shown in

Figure 3.19. The parameters, including the temperature corresponding to 10

wt% loss (T10%), the temperature at the maximum rate of weight loss (Tmax)

and residue at 800 ºC are summarized in Table 3.1. It is observed that the

thermal stability of the UPR was significantly enhanced by the addition of

nano calcium carbonate. The temperature corresponding to 10 wt% loss of

pure UPR is 263oC. It is found that the increasing addition of nano calcium

carbonate into the UPR, the temperature corresponding to the 10 wt% mass

loss was increased within the range of 307-330oC. The Tmax of the neat UPR

system was 360oC, whereas upon addition of nano calcium carbonate to the

UPR matrix, the Tmax of the nanocomposites appeared within the range of

380-402oC. These results can be interpreted with reference to the addition of

nano calcium carbonate to the UPR matrix, which increased the surface

contact area between the nano calcium carbonate particles and the UPR

matrix. The results can be attributed also to the increased cross linking density

of the nanocomposites. Two more facts with regard to the incorporation of

calcium carbonate in the UPR matrix are (a) the composites present better

thermal stability when compared to the pure UPR, independent of the amount

of calcium carbonate added in the composites and (b) the T10% increases

marginally with loading of calcium carbonate nanoparticles due to the

restriction of the long range chain mobility of the UPR phase within the

nanocomposite by the nanoparticles (Jin and Park 2009, Campos et al 2007,

Xie et al 2004). The char content of the nanocomposites at 800oC also

increased with the addition of nano calcium carbonate.

97

Figure 3.19 TGA thermo grams of UPR/calcium carbonate nanocomposites

Table 3.1 TGA data of pure UPR and UPR/calcium carbonate

nanocomposites

CompositionT10%

(ºC)

Tmax

(ºC)

Weight

residue at

800 ºC (%)

Unfilled UPR 263 360 3.0

UPR/1wt% Nano calcium carbonate 307 380 1.6





98

3.3.7.4 Dynamic mechanical analysis

The variation of tan with temperature for pure UPR and UPR/

calcium carbonate nanocomposites is shown in Figure 3.20. Pure UPR shows

the damping peak at 93 °C and this temperature corresponds to its glass

transition temperature. The glass transition temperature of pure UPR

increased with the incorporation of calcium carbonate nanoparticles. The

glass transition temperature were found to be 110, 114, 124, 122 and 121 °C

for nanocomposites containing 1, 3, 5, 7 and 9 wt% calcium carbonate

respectively. The UPR with 5 wt% of nano calcium carbonate shows

maximum glass transition temperature value of 124 °C. In general, the

increase in glass transition temperature is attributed to the good adhesion

between the polymer matrix and the reinforced particles, so that the nano

meter size particles can restrict the segmental motion of cross links under

loading.

The other important factors that can affect glass transition

temperature are degree of particle dispersion and curing condition. The degree

of particle dispersion depends on size, homogeneity, orientation and spacing

between particles where as curing condition includes curing speed and degree

of crosslinking. The improvement in glass transition temperature may arise

from some of these factors (Yasmin and Daniel 2004).

99

Figure 3.20 Temperature dependence of tan of UPR/calcium

carbonate nanocomposites

The variation of storage modulus with temperature for pristine UPR

and UPR/calcium carbonate nanocomposites are presented in Figure 3.21.

Pure UPR shows storage modulus of 6787 MPa. The addition of nano calcium

carbonate increases the storage modulus of UPR. Increase in the storage

modulus is observed up to 5 wt% of UPR/calcium carbonate nano composites.

The storage modulus increases to 8266, 8457, 10497, 9514, and 9236 MPa for

the content of 1, 3, 5, 7, and 9 wt% nano calcium carbonate in UPR

respectively. It confirms the well dispersed nano calcium carbonate particles

stiffen the UPR matrix. However, when the nanoparticle is > 5 wt%, the

stiffening effect is progressively reduced with increasing temperature most

probably due to the agglomeration of calcium carbonate nanoparticles. As the

temperature increased, both pure UPR and its composites showed gradual

drop in storage modulus. The drop in storage modulus is related to material

transition from glassy state to rubbery state (Yasmin and Daniel 2004).

100

Since storage modulus is inversely proportional to brittleness

(Brostow et al 2006) this implies that brittleness goes down in the same

concentration interval and after a minimum at 5 wt% nano calcium carbonate,

it goes up again. It has been demonstrated that high brittleness corresponds to

low impact strength and vice versa; an equation connecting these two

parameters has been derived (Brostow and Hagg Lobland 2010). Thus, impact

strength results showed in Figure 3.13 (maximum at 5% calcium carbonate)

agree with the storage modulus results (Figure 3.21).

The increase in glass transition temperature and storage modulus

may be related to the confinement of polymer chains as a result of

intercalation into the gallery of the particles. This behaviour is explained

based on the mobility of the polymer chains hindered due to interaction

between the nanoparticle and polymer molecules resulting higher glass

transition temperature. However, when the nanoparticle is > 5 wt%, the

stiffening effect is progressively reduced with increasing temperature most

probably due to the agglomeration of calcium carbonate nanoparticles.

Figure 3.21 Temperature dependence of storage modulus of UPR/

calcium carbonate nanocomposites

101

The dynamic mechanical properties of a material are dependent on

temperature and frequency. Generally these measurements are done over a

frequency range at constant temp (or) over a temperature range at constant

frequency. If a material is subjected to constant stress, elastic modulus will

decrease over a period of time. This is due to the fact that the material undergoes

molecular rearrangement in an attempt to minimize localized stresses.

The variation of tan and storage modulus with temperature at three

different frequencies (5, 10 and 20 Hz) for 3 wt% of UPR/calcium carbonate

nanocomposites are shown in Figures 3.22 and 3.23. The tan peak value

maximum is found to be 93, 99 and 121 °C at 5, 10 and 20 Hz frequency

respectively. The storage modulus increases from 6496 MPa, 6786 MPa and

8457 MPa for 5, 10 and 20 Hz respectively. It is observed that as the

frequency increases, the storage modulus increases and tan peak maximum

shifts to higher values (Thomas et al 2008).

Figure 3.22 Variation of tan of UPR/3 wt% calcium carbonate

nanocomposites with respect to frequency

102

Figure 3.23 Variation of storage modulus of UPR/3 wt% calcium

carbonate nanocomposites with respect to frequency

3.4 UNSATURATED POLYESTER/SILICA NANOCOMPOSITES


The TEM image of synthesized silica is presented in Figure 3.24.

Figure 3.24(a) shows the TEM photograph of the produced nano silica

powder. It is observed that the shape of the particles is spherical with average

diameter of about 20–30 nm. The TEM image of the 5 wt% of nano silica in

the UPR matrix is shown in Figure 3.24(b). It can be seen that the silica

nanoparticles are uniformly dispersed in the UPR matrix. The spherical shape

of the nanoparticles is also clearly seen. When the particle loading is 7 wt%,

as shown in Figure 3.24(c), the degree of dispersion of the silica nanoparticles

becomes rather poor and particle agglomerations are present. Therefore, for

nanocomposites with a high concentration of nanoparticles, aggregation can

take place easily.

103

Figure 3.24 TEM images of (a) nano silica particles (b) UPR/5 wt% nano

silica (c) UPR/7 wt% nano silica


FT-IR spectra confirm the presence of silica in the UPR host and

identified the interaction between UPR and silica phases (Figure 3.25).

104

Figure 3.25 FT-IR spectra of pure UPR, nano silica and UPR/silica

nanocomposites

(a) Pure UPR (b) Nano silica (c) UPR/1% Nano silica

(d) UPR/3% Nano silica (e) UPR/5% Nano silica (f) UPR/7%

Nano silica (g) UPR/9% Nano silica

105

In addition to the characteristic peaks of UPR, the Si–O stretching

vibration at 1100 cm-1

and bending vibration at 475 cm-1

(Wu et al 2002) and

the peak at 795 cm-1

assigned to Si–C linkages (Raman et al 2006) are also

present in the spectra of UPR/silica nanocomposites, verifying the successful

incorporation of the silica nanostructure into UPR matrix. The shift of the

peak at 3450 cm-1

was frequently used to study the hydrogen bonding

between the –OH groups in the silica network and other functional groups

from polymer molecular chains. It can be seen that the peak intensity at 3450

cm-1

decreases greatly (but the extent of decrease has no obvious connection

with the nano silica content) (Chen et al 2003). The additional broad peak at

1060 cm-1

may be attributed to Si–O–Si of polysilanol and Si–O–C bond

(Mishra et al 2007).


The X-ray diffraction patterns of pristine UPR, nano silica and

UPR/silica nanocomposites are shown in Figure 3.26. Since the pristine UPR

is amorphous in nature it gave shallow peak at 2 around 20 º. Due to the

crystalline nature, nano silica comparatively showed an intense peak at 2

around 19.4 º. The XRD patterns of UPR/silica nanocomposite of different

compositions are almost the same as that of silica, indicating that the crystal

structure of silica was not altered by the presence of UPR (Chen et al 2003,

Lai et al 2007).

106

Figure 3.26 XRD patterns of pure UPR, nano silica and UPR/silica

nanocomposites

3.4.4 Density

Density values of UPR/silica nanocomposites are shown in

Figure 3.27. The pristine cured UPR has the density value of 1.21 g/cm3. It

remains almost same with the addition of nano silica content up to 5 wt%. On

further addition it decreases to 1.19 g/cm3 for the UPR containing 7 wt% nano

silica. The agglomeration of nanosized particle at higher loading level can

increase the free volume in the composites. The generated free volume

decreased the viscosity of the matrix and is the reason for decrease in density

(Jawahar and Balasubramanian 2006, Sun 2006).

107

Figure 3.27 Effect of silica nanoparticles on the density of UPR



The tensile strength of UPR/silica nanocomposites is shown in

Figure 3.28. When the nano silica content was below 5 wt%, tensile strength

of UPR/silica nanocomposites increased with the increase in the nano silica

content. However, in composites containing high silica content (> 5 wt%), the

value of tensile strength decreased. The tensile strengths of nanocomposites

were 63, 66, 69, 65 and 61 MPa for 1%, 3%, 5%, 7% and 9 wt% nano silica

respectively. Increase in tensile strength of the UPR/silica nanocomposites

may be due to the uniform distribution of nanoparticle within the matrix. This

can be explained based on the stress within the polymeric matrix, the local

stress can be more easily transferred into the tougher particle with the result

that the matrix appears to be amenable to a larger local plastic deformation

108

and the end result is a higher composite strength when the particles are in

intimate contact with the polymer matrix. However, the voids between the

nanoparticle and polymer matrix and the nanoparticles agglomeration result in

the decrease in the tensile strength. This is justified by the reports that the

physicochemical interaction between the particle and the matrix plays a

significant role in the obtained composites. In other words, the strong

chemical bonding improves the mechanical properties of the composites as

compared with the weak linkage by Van der Waals and hydrogen bonding

(Guo et al 2006, Hunag et al 2006).

Figure 3.28 Effect of silica nanoparticles on the tensile strength of UPR


The effect of nano silica on the tensile modulus of unsaturated

polyester is shown in Figure 3.29. Pristine unsaturated polyester shows a

tensile modulus of 2.8 GPa. Tensile modulus drastically increases as nano

109

silica content is increased and it is 4.8 GPa for 9 wt% nano silica. The

modulus of UPR/9 wt % of silica is ~ 1.71 times higher than that of pure UPR

polymer.

Figure 3.29 Effect of silica nanoparticles on the tensile modulus of UPR


The effect of addition of nano silica in the UPR matrix, on the

elongation at break is given in Figure 3.30. The elongation at break of these

composite systems decreases with increasing filler loading. The insignificant

effect of mineral particle shapes on the elongation at break of the composite

was observed. The elongation is usually inversely proportional to tensile

strength which means that increasing the tensile strength of filled material

usually contributes to a decrease in elongation. The presence of filler reduces

the amount of a tough material available in this system and therefore reduces

110

the elongation at break of the composites. However, the elongation properties

are rather reduced with the addition of fillers and are attributed to changes

in motion, stress concentration and crack initiation and propagation (Ahmad

et al 2008).

Figure 3.30 Effect of silica nanoparticles on the tensile elongation of UPR


The flexural strength of UPR/silica nanocomposites are shown in

Figure 3.31. Flexural strength of pure UPR matrix is 98 MPa. It increases to

the value of 112 MPa for the nanocomposite with 5 wt% of nano silica.

Thereafter it decreases to the value of 109 MPa for the nano silica content of

7 wt%. This result can be attributed to the high intermolecular interaction

between the silica and macromolecular chains in the UPR/silica

nanocomposite up to 5 wt% nano silica content (Jin et al 2008). In the case of

111

excessive silica content (> 5 wt%), it is possible to result in a higher

occurrence of agglomeration under the gravitational interaction between silica

particles and unsaturated polyester resin substrate. The aggregate formation

may be attributed to the particle–particle interaction due to the decrease in the

interparticle distance with increasing particle loading (Goyala et al 2008, Li et

al 2002).

Figure 3.31 Effect of silica nanoparticles on the flexural strength of UPR


The flexural modulus of unsaturated polyester and unsaturated

polyester filled with nano silica is shown in Figure 3.32. Flexural modulus of

unfilled polyester resin is 2.7 GPa. The flexural modulus has increased to 3.2,

and 3.9 GPa for UPR containing 1 and 9 wt% nano silica respectively.

Flexural modulus shows maximum increase of 3.9 GPa (44% higher than pure

unsaturated polyester) for 9 wt% nano silica. The important parameter which

112

affects this property by incorporating nano fillers is the quality of interface in

the composites. ie, the adhesive strength and interfacial stiffness of the

composite medium. These two factors play a crucial role in the stress transfer

from the matrix to the filler and the elastic deformation. This is very much

applicable to the nanoparticle filled polymers, due to high surface area of the

filler which increases the contact area to the matrix ie., the interface (Faruk

and Matuana 2008, Ma et al 2007). The general trends demonstrated by these

plots manifest that the addition of the nanoparticles increases the stiffness of

the matrix.

Figure 3.32 Effect of silica nanoparticles on the flexural modulus of UPR


The relationship between the impact strength and the addition of

silica nanoparticles are shown in Figure 3.33. The impact strength of UPR/

silica nanocomposites was remarkably increased to the maximum when

113

5 wt% of the silica nanoparticles was added. Beyond 5 wt%, the impact

strength of composites decreased with increase in the addition of the

nanoparticles. The maximum impact strength of the composites was found to

be 33 Jm-1

which is about 65% higher than that of the pure matrix (Cao et al

2002). There are two influential factors associated with the impact strength of

materials (i) dispersibility of UPR resin against impact energy and degree of

adsorption of inorganic rigid nano sized particles against impact strength. In

the case of agglomeration within silica particles, stress concentration is likely

to occur. When the impact energy is applied, the material can neither

uniformly disperse the applied force, nor enable silica particles to fully absorb

the stress within the material, leading to possible damage (Katz and Milewski

1978, Huang and Wang 2007, Xu a et al 2007, Wu et al 2002).

Figure 3.33 Effect of silica nanoparticles on the impact strength of UPR

114

3.4.5.7 Hardness

The data on hardness measurements for the UPR/silica

nanocomposites is presented in Figure 3.34. The values indicate that the

mineral filled composites are harder than the unfilled composites. It is in

general known that both hardness and tensile strength enhancements are due

to an increase in the crosslinking density, the hardness can be taken as the

index of crosslinking density at the surface of the composite, while the tensile

strength as the index of the cross linking density through the bulk of the entire

composite. The silica fillers form highly crosslinked network during cure of

UPR. Hence, the hardness of the composites increases.

Figure 3.34 Effect of silica nanoparticles on the hardness of UPR

115



The fracture surface of UPR filled with nano silica is seen in

Figure 3.35 (a and b). Fracture surface of UPR with 5 wt% nano silica is

rougher than that of pure UPR polymer (Figure 3.15 (a)) and this suggests

improvement in strength for 5 wt% nano silica filled UPR composites. At 7

wt% nano silica, the presence of irregular voids is noticed. This indicates that

particles are peeled off from the matrix as crack propagates and has created

void at the positions where silica particles have been existing. This also

suggests that the bonding between matrix and silica particle is poor (Kang

et al 2001).

Figure 3.35 SEM images of the tensile fracture surface of (a) UPR/5 wt

% nano silica (b) UPR/7 wt% nano silica


The fracture morphology of the UPR filled with different

concentrations of nano silica is shown in Figure 3.36. The crack surface has

become rough after the addition of nano silica and increased the impact

116

strength. Figure 3.36(a) shows the UPR/5 wt% nano silica, the failure seems

to have occurred mainly in the matrix, which can be explained by the

improved interfacial adhesion resulting in better impact strength. These highly

dispersed nanoparticles and the strong interfacial interaction between the

grafted nanoparticles and the matrix are believed to favour the pinning effect

in the case of crack propagation. As a result, the composites acquire high

impact resistance. In the case of UPR/7 wt% nano silica composites (Figure

3.36(b)), severe agglomeration of the nanoparticles is observed. This

nanoparticles agglomeration is reducing the impact strength (Jiao et al 2009).

Figure 3.36 SEM images of the impact fracture surface of (a) UPR/5 wt

% nano silica (b) UPR/7 wt% nano silica



The heat deflection temperature of UPR/silica nanocomposites with

different nano silica contents are presented in Figure 3.37. Up to 5 wt% of

nano silica, the heat deflection temperature continuously increases and further

addition of 7 wt% nano silica does not show much increase in heat deflection

temperature. The nano silica dispersion in the matrix leads to such significant

improvement in heat deflection temperature. The reduce rate of increase in

117

heat deflection temperature with silica loading above 5 wt% can be attributed

to the inevitable aggregation of the particles (Gupta 2008).

Figure 3.37 Effect of silica nanoparticles on the HDT of UPR


The addition of silica particles in the UPR resin does not show any

shift in the peak exotherm of UPR resin. This reveals that the silica does not

contribute to the curing reactions but there is decrease in the intensity of

exothermic peak of UPR polymer. The single peak is observed for UPR and

UPR/silica nanocomposites. This shows that curing reactions occurs

uniformly in UPR and UPR filled with nano silica particles. The uniform

curing of nano silica filled UPR suggests that there is a time balance between

the nanoparticles entering the matrix and curing, ultimately leading to good

dispersion of nanoparticles in the matrix (Figure 3.38).

118

Figure 3.38 DSC thermogram of UPR and UPR/silica nanocomposites


The TGA curves of UPR/silica nanocomposites with various silica

contents are shown in Figure 3.39. From these curves, the 10% weight loss

temperature (T10%) and maximum weight loss temperature (Tmax, which is

defined as the peak in the derivative TGA curve) and weight residue obtained

at 800 ºC are listed in Table 3.2. The thermal stability of the nanocomposites

was increased with increased nano silica content. The interaction between

UPR and nano silica will obviously immobilize the polymer chains and in the

process restrict the movement of free radicals formed during the initiation of

degradation. The nano silica probably initiated some cross linking which

immobilized the polymer chains, giving rise to more restricted free radicals

movement and higher thermal stability. The weight residue of the UPR/silica

119

nanocomposites is higher than that of pristine UPR. Therefore, UPR/silica

nanocomposites have good thermal stability.

Figure 3.39 TGA thermo grams of UPR/silica nanocomposites

Table 3.2 TGA data of pure UPR and UPR/silica nanocomposites

CompositionT10%

(ºC)

Tmax

(ºC)

Weight

residue at

800 ºC (%)


UPR/1wt% Nano silica 307 406 5.9


UPR/5wt% Nano silica 336 408 5. 9



120

The result also shows that the thermal resistance of UPR is

enhanced with nano silica, probably due to the thermal insulation effect of

nano silica (Huang et al 2006, Lai et al 2007, Jeon et al 2007).


The dynamic storage modulus and the tan for pristine polyester

and silica nanocomposites are measured as the function of temperature and

the plots are shown in Figure 3.40. Figure 3.40 shows the mechanical loss

spectra (tan ) of unsaturated polyester/silica nanocomposites with different

filler contents. The maximum increase in glass transition temperature is

observed for unsaturated polyester with 5 wt% of nano silica (124 °C). On

further addition of nano silica (> 5 wt%), glass transition temperature

continuously decreases. For the unsaturated polyester with 9 wt% nano silica,

glass transition temperature is 114 °C. The increase in the glass transition

temperature of nanocomposites in comparison with pure polyester could be

attributed to the strong interfacial interaction between the nanoparticle and

matrix and the decreases in the free volume to limit the motion of molecular

chains. However, when the nano silica content was increased, agglomeration

takes place correspondingly, the free volume of matrix surrounding these

agglomerates enlarged, resulting in easy chain motion and lower glass

transition temperature was observed (Wu et al 2002, Xiong et al 2006,

Zhoua et al 2002).

121

Figure 3.40 Temperature dependence of tan of UPR/silica

nanocomposites

The storage modulus of unsaturated polyester/silica

nanocomposites depends on the temperature (as shown in Figure 3.41). It is

apparent that the storage modulus of the nanocomposites is higher than those

of the neat polyester throughout the whole temperature range. A maximum

increase is observed in polyester matrix with 5 wt% of nano silica of 10385

MPa, which is 53% higher than that of pure unsaturated polyester. At higher

nano silica contents (>5 wt%), the storage modulus continuously decreases as

nano silica content increases. Unsaturated polyester/9 wt% nano silica shows

a storage modulus of 8168 MPa, which is due to the poor dispersion of silica

nanoparticles in the matrix (Yu et al 2006) and also due to aggregation of

silica nanoparticles in the matrix (Lu et al 2006, Rong et al 2004).

122

Figure 3.41 Temperature dependence of storage modulus of UPR/silica

nanocomposites

The variation of tan and storage modulus against temperature of

UPR/3 wt% silica nanocomposite with respect to different frequencies are

illustrated in Figures 3.42 and 3.43. As the frequency increases, both the

values of tan and storage modulus increase. The glass transition temperature

varies from (103, 110 and 119 °C) and storage modulus from (6194, 6258 and

9428 MPa) at frequencies of 5, 10 and 20 Hz respectively. It is observed that

both glass transition temperature and storage modulus increase as frequency

increases.

123

Figure 3.42 Variation of tan of UPR/3 wt% silica nanocomposites with

respect to frequency

Figure 3.43 Variation of storage modulus of UPR/3 wt% silica


124

3.5 UNSATURATED POLYESTER/ALUMINA

NANOCOMPOSITES


A typical TEM micrograph of synthesized alumina particles is

shown in Figure 3.44(a). Alumina particles are spherical and most of them

were dispersed in the primary particle form with a diameter of about 60-70

nm. Figures 3.44 (b) and 3.44(c) are TEM micrographs of UPR/alumina

nanocomposites with nano alumina 5 and 7 wt% respectively. For 5 wt%

alumina nanocomposite, a good dispersion is achieved. Most of the alumina

particles are uniformly distributed as nanosized particles in the UPR matrix.

The particles appear to be agglomeration free and the individual particles can

be identified very clearly.

However, more aggregates are found in the UPR/alumina

nanocomposite with 7 wt% of alumina nanoparticles, which suggests a poor

dispersion. The poor dispersion may be due to the interaction between

particles leading to agglomeration. This is reasonable, considering that at high

nano alumina concentration, the inter particle distance is small and hence,

occulation of these nanoparticles can occur (Chan et al 2002).

Figure 3.44 TEM image of (a) alumina nanoparticles (b) UPR/5 wt%

nano alumina and (c) UPR/7 wt% nano alumina

125


The structure of nanocomposites was studied using FT-IR

spectroscopy and the results are shown in Figure 3.45.

Figure 3.45 FT-IR spectra of pure UPR, nano alumina and

UPR/alumina nanocomposites

(a) Pure UPR (b) Nano alumina (c) UPR/1% Nano alumina (d)

UPR/3% Nano alumina (e) UPR/5% Nano alumina

(f) UPR/7% Nano alumina (g) UPR/9% Nano alumina

126

Pure UPR shows the characteristic peaks at 2985 cm-1

(C-H stretching vibration), 1720–1730 cm-1

(C=O stretching vibrations),

1443 cm-1

(C=C stretching vibrations), 1182 and 1263 cm-1

(CH2 wagging

and scissoring) and 750 and 705 cm-1

(out of plane ring bending vibrations).

The nano alumina shows the characteristic peak at 1411, 1635, 2385 and

3348 cm-1

(shows the existence of water molecules that were entrapped in the

nano alumina) (Li et al 2007). The composites show a characteristic peaks at

2940 cm-1

(C-H stretching vibrations), 1725 cm-1

(C=O stretching vibrations),

1635 cm-1

, 1443 (C=C stretching vibrations), 1182 and 1263 cm-1

(CH2 wagging and scissoring) and 750 and 705 cm-1

(out of plane ring

bending vibrations). The differences noted in the regions 1500–1000 and

3000 cm-1

among the spectra of pure UPR, pure nano alumina and

UPR/alumina nancomposites indicate polymer-alumina interactions.


Figure 3.46 shows the X-ray diffraction patterns for the pure UPR,

nano alumina and UPR/alumina nanocomposites. The X-ray diffraction

patterns of nanocomposites show that with the addition of more and more

alumina in the UPR matrix, the intensity of the peak due to alumina

(2 around 66) increases. The intensity of alumina peak in the X-ray

diffraction pattern of nanocomposites shows that the amount of alumina

increases with increase of the level of addition (Rui et al 2004).

127

Figure 3.46 XRD patterns of pure UPR, nano alumina and


3.5.4 Density

The pristine cured UPR has the density value of 1.21 g/cm3. It

remains almost same with the addition of nano alumina content up to 5 wt%

(Figure 3.47). The UPR containing 7 wt% nano alumina content showed a

density value of 1.17 g/cm3.

128

Figure 3.47 Effect of alumina nanoparticles on the density of UPR



The tensile strength of UPR/alumina nanocomposites containing

different filler content is given in Figure 3.48. Pure UPR shows tensile

strength of 58 MPa. Tensile strength of nano alumina filled UPR are 61, 64,

66, 62 and 60 MPa for 1, 3, 5, 7 and 9 wt% nano alumina loaded UPR

respectively. Unsaturated polyester with 5 wt% of nano alumina shows

maximum increase in strength (14% increase over than pristine UPR) and on

further addition of nano alumina (> 5 wt%) the tensile strength decreases.

Tensile strength of nanocomposites is enhanced when the interfacial adhesion

is improved. This result can be ascribed to better stress transfer at the

interface between matrix and nano alumina. The improvement of interfacial

adhesion can prevent dewetting at the UPR/nano alumina interface during

tensile deformation. Therefore, well adhering nano alumina can bear part of

129

the load applied to the matrix and contribute to the tensile strength of the

nanocomposites (Shang et al 1994, Zhang et al 2003, Chen et al 2004). For

higher loading of nano alumina in the resin matrix, nanosized particle

agglomeration is easier. Since the agglomerated particles generate defects in

the material, stress concentration is likely to occur within the resin or

agglomerated particles will generate slippage within the material due to

external force, resulting in decreased tensile properties (Huang et al 2006,

Xu a et al 2007).

Figure 3.48 Effect of alumina nanoparticle content on the tensile

strength of UPR


Figure 3.49 shows the effect of nano alumina content on tensile

modulus of the UPR matrix. It is observed that modulus of nanocomposites

130

increases continuously with increasing nano alumina content over pure UPR.

Tensile modulus of pure UPR is 2.8 GPa.


modulus of UPR

On the addition of nano alumina, tensile modulus increases to 3.0,

3.3, 3.5, 3.7 and 3.9 GPa for 1%, 3%, 5%, 7% and 9 wt% of nano alumina

respectively. An improvement in modulus of 1.39 times was observed for the

addition of 9 wt% of nano alumina. It is well known that the filler particles

reduce the molecular mobility of polymer chains, resulting in a less flexible

material with a higher tensile modulus (Joseph et al 2011).


The tensile elongation of UPR/alumina nanocomposites is shown in

Figure 3.50. If there is good adhesion between filler and the matrix, decrease

of the elongation at break, would be observed.

131


elongation of UPR

In the system under study, tensile elongation decreased while

comparing with that of the neat resin and such a decrease has been slightly

enhanced further by increasing the filler content. It is well known that

incorporation of filler particles usually decreases elongation of filled polymer

composite due to the intrinsic stiffness of the inorganic filler (Tian et al 2005).


The results of flexural strength of the composite system as a

function of nano alumina filler loading are presented in Figure 3.51. Flexural

strength of pure UPR matrix is 98 MPa. The graph shows an increasing trend

as the filler loading increases up to 5 wt% (109 MPa). However, slight

decrease in flexural strength is observed at 7 wt% alumina (105 MPa). High

aspect ratio provides high surface area, hence results in more contact area

132

between the filler and the matrix. Therefore, by presumably good adhesion

and bonding existing between the filler and matrix, positive reinforcement

effect occurs in alumina filled UPR which might increase the strength of the

composites. The effective bonding between inorganic fillers and matrix

components typically improved the flexural strength of polymer composites.

The agglomerations of alumina results in inhomogeneous distribution and

hence weaken the interaction between the filler and matrix. This subsequently

reduces the flexural strength of alumina composite system (Ahmad et al

2008).

Figure 3.51 Effect of alumina nanoparticle content on the flexural

strength of UPR


The flexural modulus of particulate composites can be higher than

that of the pure polymer matrix. It is found that flexural modulus increases

133

with increasing nano alumina content and is illustrated in Figure 3.52. That

means the interaction between nanoparticles of alumina and UPR matrix is so

strong that the nanoparticles are able to carry the applied load. Flexural

modulus of unfilled polyester resin is 2.7 GPa. The flexural modulus has

increased to 2.8 and 3.5 GPa for UPR with 1 and 9 wt% nano alumina

respectively. Flexural modulus shows maximum increase of 3.5 GPa, 29%

higher than that of UPR resin for 9 wt% of nano alumina (Jawahar and

Balasubramanian 2006, Jin et al 2008, Ahmad et al 2008, Yang et al 2006,

Wang et al 2007).

Figure 3.52 Effect of alumina nanoparticle content on the flexural

modulus of UPR


The relationship of nano alumina content and impact strength of

UPR/alumina nanocomposites is shown in Figure 3.53. Pure UPR shows

134

impact strength of 20 Jm-1

. When the nano alumina content is 5 wt% the

impact strength of nanocomposites increases (32 Jm-1

) to a maximum and

then decreases with further addition of nano alumina. UPR with 7 wt% nano

alumina shows impact strength of 29 Jm-1

.

Figure 3.53 Effect of alumina nanoparticle content on the impact

strength of UPR

This variation of impact strength can be attributed to two things.

First when nano alumina content is < 5 wt% there is seldom agglomerated

nano alumina in the matrix. The presence of fine particles dispersed within

the matrix makes plastic deformation easier. So, during the fracture of a

composite in which the nanoparticle is well dispersed, the stress will have to

be bigger to start the micro crack in the UPR matrix and the impact energy

will largely be absorbed by the exhibited plastic deformation, which occurs

more easily around the nanoparticles. Hence, the good nano alumina

dispersion resulting in less agglomeration leads to a better impact strength of

135

the nanocomposites. The second reason for the variation in the impact

strength is that when nano alumina content is > 5 wt%, it easily agglomerates

into large agglomerated particles, which will become the site of stress

concentration and can act as a micro crack initiator. So, a larger aggregate is a

weak point that lowers the stress required for the composite to fracture and

hence the impact strength of the nanocomposites would be decreased

(Mareri et al 1988, Zhang et al 2003, Cao et al 2002, Qi et al 2006).

3.5.5.7 Hardness

The durometer hardness values of UPR/alumina nanocomposites at

different composition of nano alumina content are shown in Figure 3.54. The

values indicate that the mineral filled composites are harder than unfilled

composites.

Figure 3.54 Effect of alumina nanoparticle content on the hardness of

UPR

136

The resistance to the penetration of UPR increased linearly when

there is progressive addition of nano alumina in the UPR matrix.



The SEM pictures of tensile fractured surfaces of UPR/alumina

nanocomposites are given in Figure 3.55. The possible origins of crack

initiation in a composite material are air bubble or voids, resin rich area,

foreign matter such as dust particles, particle size and poor particle matrix

adhesion (Moloney et al 1987). The fractured surface of the unfilled resin

(Figure 3.15(a)) shows a brittle failure. At low levels of nano alumina in UPR,

good adhesion between the particle and polymer matrix can be seen from the

fact that there is not much particle pull out and subsequent cavity formation

(Figure 3.55(a)). Another possible mode of failure (Figure 3.55(b)) is noted in

nanocomposites wherein agglomeration of the nano alumina particles is seen.

Since the nano alumina particles are randomly oriented, large numbers of

them are subjected to tensile stresses acting perpendicular to the plane and

crack propagation occurs parallel to the plane. It is clearly seen in UPR/7 wt%

nano alumina sample (Figure 3.55(b)) have experienced a high level of

debonding and particle pull out. This accounts for the lower strength and

higher modulus values (Kar et al 2008).

Figure 3.55 SEM images of the tensile fracture of (a) UPR/5 wt% nano

alumina and (b) UPR/7 wt% nano alumina

137


The izod impact fracture surface of the UPR/alumina

nanocomposites is shown in Figure 3.56. The fracture surface of the virgin

polyester specimen (Figure 3.16(a)) is relatively smooth. This indicates that

the resistance to crack propagation is less and leads to brittle failure. The

addition of nano alumina (< 5 wt%), the crack surface becomes rough, which

indicates that the crack propagation in the matrix is difficult due to the

presence of particles. The nano alumina particles have guided the crack to

propagate in a torturous path and lead to high strength of composites. UPR

with 5 wt% nano alumina shows the scratches at the fracture surface

(as shown in Figure 3.56(a)). The scratches are due to the particle peel off

from the material. During this process the particle could have scratched the

surface and tried to resist the propagating crack. This offers some resistance

to the crack propagation and has increased the strength of the

nanocomposites. Though the fracture surface of 7 wt% nano alumina filled

UPR composites is rough, the impact strength decreased. The agglomeration,

voids, high stress concentrations, etc., have decreased the impact strength for

7 wt% (Figure 3.56(b)) nano alumina filled nanocomposites (Xie et al 2004,

Joshi et al 2010).

Figure 3.56 SEM images of the impact fracture surface of (a) UPR/5 wt%

nano alumina (b) UPR/7 wt% nano alumina

138



The heat deflection temperature of UPR composites with nano

alumina is shown in Figure 3.57. The heat deflection temperature value

increases with increase of nano alumina up to 5 wt% and for much higher

nano alumina content heat deflection temperature not showing any increases.

Figure 3.57 Effect of alumina nanoparticles on the HDT of UPR


The addition of nano alumina particles decreases the exothermic

peak from 81 C for pure UPR to 55 C for UPR containing 7 wt% nano

alumina (Figure 3.58).

139

Figure 3.58 DSC thermogram of UPR and UPR/alumina nanocomposites


The TGA curves of pure UPR and its nanocomposites with

different compositions of nano alumina are shown in Figure 3.59. The onset

temperature, degradation temperatures at 10% weight loss and end

temperature obtained from the TGA data of pure UPR and UPR/alumina

nanocomposites are given in Table 3.3, which indicates that the thermal

stability of the pure UPR was enhanced by the incorporation of alumina

particles. For pure UPR, the maximum degradation temperature is 360 °C,

while for the composites it increases to 376, 379, 381, 383 and 385 ºC for

1, 3, 5, 7 and 9 wt% of nano alumina in UPR/alumina nanocomposites

respectively. In all cases, single step degradation occurs. Therefore, the

incorporation of the nano alumina resulted in pronounced improvement in

140

thermal stability. This can be attributed to the homogeneous distribution of

nano alumina particles as well as the tortuous path in the composites that

hinders diffusion of the volatile decomposition products in the composites

compared to that in pure UPR (Yasmin and Daniel 2004).

Figure 3.59 TGA thermo grams of UPR/alumina nanocomposites

Table 3.3 TGA data of pure UPR and UPR/alumina nanocomposites

CompositionOnset

Temp(ºC)

T10%

(ºC)

End

Temp(ºC)

Tmax

(ºC)

Weight

residue at

800 ºC

(%)

Unfilled UPR 165 263 422 360 3.0

UPR/1wt% Nano alumina 168 279 425 376 4.9





141


The tan for pristine polyester and alumina nanocomposites are

measured as the function of temperature (Figure 3.60). On addition of nano

alumina particle in the matrix the tan peak shifts to higher temperature,

which suggests that there is an increase in glass transition temperature. The

glass transition temperature increased from 93°C to 120 °C after addition of 5

wt% nano alumina. On further addition of nano alumina (> 5 wt%) glass

transition temperature decreases continuously. It decreases to 117 and 111°C

for 7 and 9 wt% of nano alumina respectively. Since the glass transition

process is related to the molecular motion, the glass transition temperature is

considered to be affected by molecular packing, chain rigidity and linearity.

The increase in glass transition temperature may be attributed to maximizing

the adhesion between polymer and nano alumina particles. Because of the

nanometer size which restricts segmental motion near the organic–inorganic

interface, which is a typical effect for the inclusion of nano alumina in the

polymer system (Sh et al 2005, Lu et al 2006). The existence of

agglomeration of particles in the matrix (> 5 wt%) possibly decreases the

glass transition temperature at higher nano alumina content. The

agglomerated zone is a weak zone due to weak interface bonding between

matrix and particles, which causes low glass transition temperature values.

The effect of nano alumina on storage modulus of unsaturated

polyester is shown in Figure 3.61. The storage modulus of the UPR/alumina

nanocomposites increases up to 5 wt% nano alumina content in unsaturated

polyester matrix. Pure polyester shows storage modulus of 6787 MPa.

Unsaturated polyester with 5 wt% of nano alumina shows maximum value of

10233 MPa, which is 51% higher than pure unsaturated polyester matrix. It

confirms that well dispersed alumina nanoparticles stiffen the polyester

matrix (Xie et al 2004).

142

Figure 3.60 Temperature dependence of tan of UPR/alumina

nanocomposites

Figure 3.61 Temperature dependence of storage modulus of UPR/

alumina nanocomposites

143

However, when the nanoparticle content is increased (> 5 wt%) the stiffening

effect is progressively reduced with increasing temperature most probably due

to agglomerisation of alumina nanoparticles (Jin et al 2009). For unsaturated

polyester with 9 wt% nano alumina, storage modulus is 8106 MPa. However,

the rate of decrease of storage modulus in nano alumina filled unsaturated

polyester is low.

The tan curve and storage modulus of UPR/alumina

nanocomposites at different frequencies are shown in Figure 3.62 and 3.63. It

shows that as the frequency increases the value of tan and storage modulus

increases.

144

Figure 3.62 Variation of tan of UPR/3 wt% alumina nanocomposites

with respect to frequency

Figure 3.63 Variation of storage modulus of UPR/3 wt% alumina


145

3.6 UNSATURATED POLYESTER/ZINC OXIDE

NANOCOMPOSITES


Transmission electron microscopy (TEM) analysis was conducted

to verify the size of the synthesized zinc oxide nanoparticles and the level of

nanoparticle dispersion in the UPR matrix. The TEM image of nano zinc

oxide is shown in Figure 3.64(a). It is observed that the shape of the particles

is spherical with an average diameter of about 40-50 nm. The TEM image of

UPR matrix having 3 wt% of nano zinc oxide is shown in Figure 3.64(b). It

can be seen that the zinc oxide nanoparticles are uniformly dispersed in the

UPR matrix. The spherical shape of the nanoparticles is also clearly seen.

When the nanoparticle loading is 5 wt% in UPR matrix (Figure 3.64(c)), the

degree of dispersion of the zinc oxide nanoparticles becomes rather poor and

particle agglomerations are present. Therefore, for nanocomposites with a

high concentration of nanoparticles, aggregation can take place easily. It is

also clear that the vast majority of these agglomerates are still in the

nanometer size range.

Figure 3.64 TEM images of (a) nano zinc oxide (b) UPR/3 wt% nano

zinc oxide and (c) UPR/ 5 wt% nano zinc oxide

146


The chemical structures of nano zinc oxide and UPR/zinc oxide

nanocomposites are determined by FT-IR measurement and are depicted in

Figure 3.65.

Figure 3.65 FT-IR spectra of pure UPR, nano zinc oxide and UPR/zinc

oxide nanocomposites

(a) Pure UPR (b) Nano zinc oxide (c) UPR/1% Nano zinc oxide

(d) UPR/3% Nano zinc oxide (e) UPR/5% Nano zinc oxide

(f) UPR/7% Nano zinc oxide (g) UPR/9% Nano zinc oxide

147

The characteristic peak at 450 cm-1

in FT-IR spectra of the

UPR/zinc oxide nanocomposites indicates the presence of zinc oxide

(Siqingaowa et al 2006). The appearance of peak at 3410 cm-1

indicates the

presence of –OH group (Tang et al 2006). While the band located at 500–700

cm-1

is attributed to the stretching vibration of Zn-O bond (Chen et al 2008).

Since the characteristic peak of UPR and zinc oxide are both present in the

FT-IR spectra of UPR/zinc oxide nanocomposites, it indicates that successful

synthesis of UPR/zinc oxide nanocomposites in this work.


XRD analysis was used to investigate the crystalline structure of

pure UPR, nano zinc oxide and its nanocomposites and their XRD patterns

are illustrated in Figure 3.66. XRD patterns of nano zinc oxide particles

shows peaks at scattering angles (2 ) of 31.4, 34.0, 35.8, 47.2, 56.3, 62.5,

67.6, and 68.8 corresponds to the reflection from 100, 002, 101,102, 110, 103,

200, and 112 crystal planes, respectively (Gu et al 2004).

The XRD of cured UPR resins shows no characteristic peaks

indicating the amorphous character of the cured unsaturated polyester resin

matrix. However the XRD patterns of UPR/zinc oxide nanocomposites show

peaks at around 31, 34, 36, 47, 56, 62, 67 and 68º, which can be ascribed to

the nano zinc oxide dispersed in UPR/zinc oxide nanocomposites. It is worth

to observe that when the amount of nano zinc oxide increases in the

nanocomposites, the intensity of the peaks due to nano zinc oxide (2 around

36) also increases (Park et al 2007).

148

Figure 3.66 XRD patterns of pure UPR, nano zinc oxide and UPR/zinc


3.6.4 Density

The effect of nano zinc oxide on density of UPR/zinc oxide

nanocomposite is illustrated in Figure 3.67. The pristine cured UPR has the

density value of 1.21 g/cm3. It remains almost same with the addition of nano

zinc oxide up to 3 wt%. On further addition, the density decreases and

UPR/zinc oxide nanocomposite containing 5 wt% of nano zinc oxide showed

a density value of 1.17 g/cm3. The nano particles due to their small size have

high surface energy and they possess very strong Vander Waals forces.

Loading of nano zinc oxide beyond 3 wt% in UPR favours agglomeration and

leads to reduction in the interaction between the nano zinc oxide particle and

the UPR.

149

From TEM image (Figure 3.64 (c)) entrapment of air is noted at

higher loading of nano zinc oxide in UPR. This phenomenon may

considerably decrease the viscosity of the system and will lead to decrease in

density of the system (Jawahar and Balasubramanian 2006).

Figure 3.67 Effect of zinc oxide nanoparticle content on the density

of UPR



The tensile strengths of pristine UPR and UPR/zinc oxide

nanocomposites having different loading of nano zinc oxide are presented in

Figure 3.68. Pure UPR shows tensile strength of 58 MPa. The tensile strength

increases with increase in nano zinc oxide content up to 3 wt% (63 MPa). An

improvement in tensile strength is due to the homogeneously dispersed

nanosized zinc oxide particles in UPR matrix (Yinghong et al 2002).

150

However, a further increase in the amount of nano zinc oxide particles results

in a decrease in the tensile strength. The decrease in tensile strength may be

due to the decrease in viscosity of the polyester resin at high level of zinc

oxide loading (> 3 wt%). The entrapped air present in the agglomerated nano

zinc oxide particles in the UPR matrix remains in the system as micro pores

and will sufficiently reduce the tensile strength of the system (Jawahar and

Balasubramanian 2006, Jiao et al 2009).

Figure 3.68 Effect of zinc oxide nanoparticles on the tensile strength

of UPR


The effect of the different content of nano zinc oxide on the tensile

modulus of UPR/zinc oxide nanocomposites is shown Figure 3.69. The tensile

modulus of nanocomposites increased to a maximum when nano zinc oxide

content was 9 wt% (3.4 GPa) (Zhang et al 2003).

151

Figure 3.69 Effect of zinc oxide nanoparticle on the tensile modulus

of UPR


The results of tensile elongation against different nano zinc oxide

content in UPR/zinc oxide nanocomposites are shown in Figure 3.70.

Elongation at break decreases continuously with increasing nano zinc oxide

content. This may be due to the addition of zinc oxide particles into the UPR

matrix restrict the movement of UPR chains (Mohsen et al 2009). It also

attributes to the fact that ductility decreases when stiffness is increased by

reinforcement (Ahmadi et al 2004).

152

Figure 3.70 Effect of zinc oxide nanoparticle on the tensile elongation

of UPR


The flexural strength of UPR/zinc oxide nanocomposites are shown

in Figure 3.71. The nanoparticle reinforcement enhances the contact surface

area to the matrix and thereby enhancing the stress transfer from matrix to

filler, which results in improved strength. Flexural strength of pure UPR

matrix is 98 MPa. The flexural strengths of the UPR/zinc oxide

nanocomposites increase up to 3 wt% of nano zinc oxide (104 MPa) loading

in the UPR matrix. The better interfacial bonding may occur between nano

zinc oxide particles and UPR resin, which absorb some energy and the

flexural strengths of the nanocomposites increase. In the case of higher

loading of nano zinc oxide, agglomeration occurs. When there is

agglomeration of particles the stress transfer from the nano particles to the

153

UPR molecular chains is hindered. This leads to stress concentration within

the materials. So the bending fracture strength of materials has a declining

trend (Kim et al 1997, Huang et al 2006).

Figure 3.71 Effect of zinc oxide nanoparticle on the flexural strength

of UPR


The flexural modulus values for the UPR/zinc oxide

nanocomposites are shown in Figure 3.72. Flexural modulus of cured pristine

unsaturated polyester resin is 2.7 GPa. Progressive increase of nano zinc

oxide content in UPR progressively increases the flexural modulus and at 9

wt% loading of zinc oxide the nanocomposite shows a flexural modulus of

3.4 GPa. According to percolation theory, if the distance between particles is

small enough, these zones join together and form a percolation network,

which increases the flexural modulus (Tian et al 2005).

154

Figure 3.72 Effect of zinc oxide nanoparticle on the flexural modulus

of UPR


The specific surface area of nanoparticles is large and this leads to a

large contact area between the filler and the matrix. The Figure 3.73 shows

the effect of nano zinc oxide content on the impact properties of UPR/zinc

oxide nanocomposites. Pure cured UPR shows impact strength of 20 Jm-1

. Up

to a loading of 3 wt% of nano zinc oxide in UPR matrix, a slight increase in

impact strength is noted and which perhaps can be related to the interfacial

microstructure between the matrix and fillers (Li et al 2007). There is a better

adhesion between the zinc oxide nanoparticles and the matrix and so the

particles could not create the void, leading to high toughness in the

nanocomposites. However on further addition (> 3 wt%) particle

agglomeration and the increase in intermolecular distance of UPR

considerably decrease the interactions of the macromolecular chains at higher

155

loading of nano zinc oxide particle in UPR matrix. These factors may help in

rapidly creating cracks and hence the toughness of the composites is reduced

(Wang et al 2007). By the addition of nanoparticles, the impact strength is

decreased by 5% when the concentration of nano zinc oxide is just 5 wt%.

Impact strength continuously decreases with further addition of nano zinc

oxide. The decrement is ~ 20% in the presence of 9 wt% nano zinc oxide.

This decrement is due to the brittle failure of nanocomposites. Impact strength

of nanocomposites is generally reported to decrease modestly, even for high

level of exfoliation (Pomogailo and Kestelman 2005, Kar et al 2008).

Figure 3.73 Effect of zinc oxide nanoparticle on the impact strength

of UPR

3.6.5.7 Hardness

Addition of nano zinc oxide exhibits significant improvement in

hardness. Hardness increases with increase in filler amount and is shown in

156

Figure 3.74. The increase in hardness is mainly due to the uniform

distribution of nanoparticles and decrease in interparticle distance with

increasing particle loading in the matrix results in increase of resistance to

indentation of UPR matrix. If the nanoparticles are much closer to each other

in the matrix and hence, nanoparticles will resists more strongly the

penetration of the indentation in the matrix (Goyala et al 2008).

Figure 3.74 Effect of zinc oxide nanoparticle on the hardness of UPR



The tensile fracture surface of the UPR nanocomposites containing

3 and 5 wt% of nano zinc oxide are illustrated in Figure 3.75(a) and 3.75(b)

respectively. The better interfacial bonding between polymer matrix and

fillers is shown in Figure 3.75(a). Hence, on applying tensile stress, the

157

particle being very strong does not break leading to crack propagation through

the interface. Even though the particle pullout happens in this case it is at a

higher tensile strength because of better bonding in these composites. Figure

3.75(b) shows the SEM picture of UPR/5 wt% nano zinc oxide; indicate

agglomeration of nano zinc oxide resulting in poor interfacial bonding. This is

in accordance with the mechanical properties of composites.

Figure 3.75 SEM images of tensile fracture surface of (a) UPR/3 wt%

nano zinc oxide (b) UPR/5 wt% nano zinc oxide


The SEM images of the fractured surface of nanocomposites are

depicted in Figure 3.76 (a) and (b). The well dispersed nano particles in UPR

matrix toughen the system and there is good interfacial bonding due to the

chemical interaction between the deposited layers and the UPR matrix.

Though the fracture surface of the UPR nanocomposites containing 5 wt%

nano zinc oxide is rough (Figure 3.76 (b)), the impact strength decreased. The

agglomeration, voids, high stress concentrations, etc., have played a definite

role in decreasing the impact strength at higher loading of nano zinc oxide

(> 5 wt%) in UPR matrix. The agglomerated particles serve as weak points

and lower the stress required for the composite to fracture.

158

Figure 3.76 SEM images of impact fracture surface of (a) UPR/3 wt%

nano zinc oxide (b) UPR/5 wt% nano zinc oxide



The increase in HDT is observed for nano zinc oxide in UPR/zinc

oxide nanocomposites up to 9 wt% (Figure 3.77).

Figure 3.77 Effect of zinc oxide nanoparticles on the HDT of UPR

159

The nanoparticle dispersion in the matrix leads to such significant

improvement in heat deflection temperature. The stiffness of the nanoparticles

contributes to the presence of immobilized or partially immobilized polymer

phases (Gupta et al 2008).


The differential scanning calorimetric traces of UPR/zinc oxide

nanocomposites are shown in Figure 3.78. It is seen that for UPR/zinc oxide

nanocomposites, there is no change in exothermic peak temperature. However

the addition of nano zinc oxide alters the intensity of exothermic peak.

Figure 3.78 DSC thermogram of UPR and UPR/zinc oxide nanocomposites

160


TGA curves of pristine UPR and UPR containing different amount

of nano zinc oxide are shown in Figure 3.79.

Figure 3.79 TGA thermo grams of UPR/zinc oxide nanocomposites

The parameters derived from the TGA curves are listed in Table

3.4. From Table 3.4 it is obvious that the thermal stability of the

nanocomposites increased with increasing nano zinc oxide loading in UPR.

The temperature at which the pristine UPR looses 10% of the weight is noted

at 263 ºC. This parameter is shifted to 296 ºC for UPR/zinc oxide

nanocomposite containing 9 wt% of nano zinc oxide. Similarly, the

progressive addition of nano zinc oxide in to UPR resin increases the

temperature at which the degradation rate is maximum. Zinc oxide possesses

high thermal stability and hinders the evaporation of the small molecules

161

generated during the thermal decomposition of the cured UPR and limits the

continuous decomposition of the cured UPR matrix (Xie et al 2004, Liang et

al 2003). This effect is clearly seen at the higher loading of nano zinc oxide

since the char residue increases when the amount of nano zinc oxide in cured

UPR matrix increases above 5 wt%.

Table 3.4 TGA data of pure UPR and UPR/zinc oxide nanocomposites

CompositionT10%

(ºC)

Tmax

(ºC)

Weight residue

at 800ºC

(%)


UPR/1wt% Nano ZnO 263 373 1.9






The temperature dependence of tan for the UPR/zinc oxide

nanocomposites is shown in Figure 3.80. The glass transition temperature of

unfilled polyester is 93 °C and is increased with nano zinc oxide content up to

3 wt% (120 °C). Progressive incorporation of nano zinc oxide in UPR

decreases the glass transition temperature and the values are 113, 111, 101 °C

for 5, 7 and 9 wt% of nano zinc oxide respectively. The addition of rigid

fillers in the matrix made it difficult to move polymer chain and therefore

damping decreased and glass transition temperature was shifted to higher

temperature. The damping is lowered when filler particles are better dispersed

and bonded strongly with the matrix. The increases in glass transition

temperature are thought to result from constrained chain mobility by well

dispersed fillers. However, in weak interface system no significant changes in

162

damping and glass transition temperatures are lowered due to severe

agglomeration of fillers (Kang et al 2001).

Figure 3.80 Temperature dependence of Tan of UPR/zinc oxide

nanocomposites

Storage modulus of pristine UPR and UPR/zinc oxide

nanocomposites as a function of temperature are depicted in Figure 3.81. Pure

polyester shows storage modulus of 6787 MPa. Addition of 3 wt% of nano

zinc oxide into UPR leads to an increase in storage modulus (9917 MPa). At

higher nano zinc oxide loading (9 wt%), storage modulus has decreased to

8047 MPa. The nanocomposites with optimum quantities of the nano fillers

will favour the good interaction between the nano fillers and the matrix and is

responsible for the increase in storage modulus (Thomas et al 2008). The poor

interfacial bonding and crosslinking could have decreased the glass transition

163

temperature and storage modulus of nanocomposites containing higher wt%

of nano zinc oxide.

Figure 3.81 Temperature dependence of storage modulus of UPR/zinc


The data on the storage and loss modulus for pristine UPR and

UPR/zinc oxide nanocomposites obtained at different frequencies are shown

in Figure 3.82 and 3.83. The nanocomposites had higher storage modulus and

tan than pristine polyester at higher frequencies. But they had lower storage

modulus and tan at low frequencies (Huang et al 2007).

164

Figure 3.82 Variation of tan of UPR/3 wt% zinc oxide nanocomposites

with respect to frequency

Figure 3.83 Variation of storage modulus of UPR/3 wt% zinc oxide


165

3.7 EFFECT OF ENVIRONMENTAL STRESSES

The choice of a plastic for different environmental conditions, such

as high temperature, weather, freezing thawing and various chemical

atmospheres, is one of the most important and complex tasks facing designers

and materials engineers. A more economic practice might well be the proper

selection of fillers, processing aids, resins and surface modifiers. To utilize

particulate composites to their fullest potential, the performance characteristic

of the materials during their entire service life must be known, because

virtually all properties of plastics are affected by a rise in temperature or

severe weather conditions or chemical atmosphere, but in different ways and

to different extents. Therefore, an independent approach in each case is

required from the stand point of understanding the behaviour of plastics and

their applications to materials selection. If there is sufficient adhesion

between the filler and the matrix, the influence will be susceptible to attack by

water (or) any environment resulting in loss of strength. The resistance of the

matrix-filler interface is ultimately responsible for the properties of polymers.

Hence, the present investigation was carried out on the effect of

environmental stresses on the mechanical properties of UPR/calcium

carbonate, UPR/silica, UPR/alumina and UPR/zinc oxide nanocomposites.

The tensile, flexural, impact and hardness properties of the above said system

are examined in water, acid and alkali media before and after exposure. The

results obtained on the above investigation are critically analyzed and

discussed.

3.7.1 Water Resistance

Water alters the properties of the polymeric matrix (Springe 1981,

Halpin 1983) and a growing body of evidence indicates that water has a

distinct influence upon the matrix inter phase (Bascom 1974, Drzal 1986).

166

The absorbed water may hydrolyze the interfacial bond. The absorbed water

which has diffused into the matrix may also act as the plasticizer (Bajaj et al

1992) by concentrating near hydrophilic sites in the polymer, or in microvoids

or cracks, or debond the particle-matrix interface which would decrease

the tensile strength (Springer 1988 ). It is seen in the Figures 3.84, 3.88, 3.92

and 3.96.

To explore the effect of the observed improvement in the adhesive

bonding between nanoparticle and polyester resin the flexural properties were

recorded after immersion in water. The flexural strength decreased drastically

(Figures 3.85, 3.89, 3.93 and 3.97). The impact (Figures 3.86, 3.90, 3.94,

3.98) and hardness properties were also decreased following exposure to

water (Figures 3.87, 3.91, 3.95 and 3.99).

3.7.2 Acid Resistance

In all the nanocomposite systems the tensile and flexural strengths,

after exposure are lower than that of before exposure as shown in Figures

3.84, 3.85, 3.88, 3.89, 3.92, 3.93, 3.96 and 3.97. This may be due to the

interaction of the acid with the inorganic nanoparticles leading to dissolution

and removal from the matrix, as the nanoparticles are susceptible to attack by

the acid. Impact strength and hardness of all nanocomposites were also

decreased considerably in acid.

3.7.3 Alkali Resistance

All the mechanical properties (tensile strength, flexural strength,

impact strength and hardness) deteriorated drastically (Figure 3.84 to 3.99).

This may be attributed to the hydrolysis of the ester linkages in the polymer

matrix, leading to degradation in the polymer chains and decreased molecular

weight, together with the hydrolysis of the interfacial bonds.

167

Figure 3.84 Effect of water, acid and alkali medium on the tensile

strength of UPR/calcium carbonate nanocomposites

Figure 3.85 Effect of water, acid and alkali medium on the flexural


168

Figure 3.86 Effect of water, acid and alkali medium on the impact


Figure 3.87 Effect of water, acid and alkali medium on the hardness of


169


strength of UPR/ silica nanocomposites


strength of UPR/silica nanocomposites

170


strength of UPR/silica nanocomposites


UPR/silica nanocomposites

171


strength of UPR/alumina nanocomposites



172





173


strength of UPR/zinc oxide nanocomposites



174




UPR/zinc oxide nanocomposites

Documents

CHAPTER 3 RESULTS AND DISCUSSION - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/9945/8/08_chapter 3.pdf · matrix phase and the bright region is the filler phase. It is observed