Silica-Styrene-Butadiene Rubber Filled Hybrid Composites: Experimental Characterization and Modeling

Silica�Styrene�Butadiene Rubber FilledHybrid Composites: ExperimentalCharacterization and Modeling

VINAY KUMAR SINGH AND PRAKASH CHANDRA GOPE*

Mechanical Engineering Department, College of Technology,G. B. Pant University of Agriculture & Technology, Pantnagar-263145

U.S. Nagar, Uttarakhand, India

ABSTRACT: The mechanical behavior of epoxy matrix composites filled with nanosized silicaparticles and styrene�butadiene rubber is discussed. Scanning electron microscopy analysis andtensile test carried out at different crosshead speeds on the silica�styrene�butadiene rubber epoxynanocomposites indicated the absence of particle aggregation and a reinforcing effect in terms ofincreased elastic modulus, yield, and ultimate strength. The results of wear test in pin-on-disc modeand hardness test on Rockwell R scale showed that the nanosized silica particles could improve thewear resistance of the epoxy matrix even though the content of the filler is at a relatively low level(1.0�2.0wt%). This makes it possible to develop novel type of epoxy-based material with improvedwear resistance for various applications. About 40�70% increase in impact strength has been noticeddue to addition of 0.5�1.5wt% of styrene�butadiene rubber in 1.0wt% silica-filled epoxy compos-ite. A good correlation between mechanical properties and weight contents of the filler materials,hardness and ultimate strength, and hardness and wear rate has been observed.

KEY WORDS: silica particle, styrene�butadiene rubber, mechanical behavior, impact behavior,hardness testing.

INTRODUCTION

COMPOSITE MATERIALS ARE engineering materials made from two or more constit-uent materials with significantly different physical or chemical properties, which

remain separate and distinct on a macroscopic level within the finished structure. Mostcommercially produced composites use a polymer matrix material often called a resinsolution. There are many different polymers available depending upon the starting rawingredients. The reinforcement or filler materials are fibers or inorganic particles of micronsize to nanosize. Fillers are particles added to the material to lower the consumption ofmore expensive material and to improve the mechanical properties of the resultingmaterial. Organic�inorganic hybrid composite materials are very important for theirextraordinary properties such as mechanical, thermal, electrical, and magnetic as

*Author to whom correspondence should be addressed. E-mail: [email protected]

Figures 1, 6 and 8 appear in color online: http://jrp.sagepub.com

Journal of REINFORCED PLASTICS AND COMPOSITES, Vol. 29, No. 16/2010 2450

0731-6844/10/16 2450�19 $10.00/0 DOI: 10.1177/0731684409355722� The Author(s), 2010. Reprints and permissions:http://www.sagepub.co.uk/journalsPermissions.nav

compared to the pure organic polymers that arise from the synergism between the proper-ties of the respective components. Inorganic particulate-filled epoxy matrix compositeshave been extensively studied during the last two decades due to their increasing applica-tions in coatings, electronic packaging and dental restoratives [1,2]. The particles in thesecomposites are generally of micrometer size. Nowadays, use of nanoparticles as fillers inepoxy matrix composite is attracting a great deal of attention from material scientists,technologists, and industrialists [3,4].

Epoxy resins being a good solvent are widely used in industrial applications because oftheir high mechanical and adhesion characteristics and chemical resistance together withtheir curability in a wide range of temperatures without the emission of volatile bypro-ducts. The properties of epoxy-based organic�inorganic filled composites can be finelytuned by an appropriate choice of the structures of both epoxy pre-polymer and hardenerand type and amount of inorganic filler. Several approaches have been proposed for theincorporation of inorganic structures into organic polymers on a nanoscale: formation ofinterpenetrating networks, incorporation of metals and metals complexes in polymersby coordination interactions, intercalation of 2D layered materials or 3Dframeworks (zeolites, molecular sieves, etc.), and incorporation of inorganic particlesand clusters.

As already said, the use of inorganic nanoparticles can be particularly interesting,thanks to their easy applicability to the common processing techniques used forepoxy-based conventional composites. In fact, micrometer-sized inorganic particles arecurrently widely used for the reinforcement of epoxy matrices to lower shrinkage oncuring and thermal expansion coefficients, to improve thermal conductivity, and tomeet mechanical requirements. The final properties of the composite material areaffected by several factors, such as intrinsic characteristics of each component, the content,the shape and the dimension of fillers, and the nature of the interface [5]. Strong interfacesbetween matrix and filler are needed to achieve high performances, taking into accountthat the load applied on the composites is mainly transferred to the fillers via the interface.To enhance the properties, smaller size and a larger amount of fillers are required,and in this respect the use of submicron particles can lead to a significant improvementof the mechanical properties of the composite materials. In the last decade, a lot ofwork has been carried out in the field of preparation of submicron inorganic particles [6]leading to the possibility of preparing composites reinforced with nanofillers.

Silica is one of the most applied fillers in polymer composites. Epoxy resin reinforcedwith silica particles having submicron dimensions represents one of the most studiedsystems. Already published results [7�17] evidenced that well-dispersed silica nanoparticlescan effectively enhance the comprehensive properties of epoxy-based nanocomposites,which are unique and different from any other current conventional microcompositewith typical filler amounts of less than 5wt%. Various investigators reported that mechan-ical properties of silica particle/polymer nanocomposites are significantly enhanced even atvery low particle volume fraction [14�16].

The reduction of cost of the composite material is the other prime important factor.Rubbers have a low price and attractive structural features such as elasticity.The addition of rubber to the base polymer such as resin enhances some of the mechanicalproperties. In most elastic materials, such as metals, the elastic behavior is caused bybond distortions. When force is applied, bond lengths deviate from the (minimumenergy) equilibrium and strain energy is stored electro-statically. Rubber is oftenassumed to behave in the same way. Also, rubber is available in different forms based

Silica�Styrene�Butadiene Rubber Filled Hybrid Composites 2451

on the origin, natural or synthetic. Styrene rubber (GR-S or Buna-S) is probably themost important type of synthetic rubber, which is produced by copolymerization ofbutadiene; CH2¼CH�CH¼CH2 (about 75wt%) and styrene, C6H5CH�CH2

(about 25wt%).Styrene rubber resembles natural rubber in processing characterization as well as quality

of finished product. It possesses outstanding properties such as high abrasion resistance,high load-bearing capacity, and resilience. Use of styrene�butadiene rubber (SBR) assecondary binding material may enhance the mechanical properties and reduce the overallcost of the material. It has been reported that silica particles disperses uniformly within therubber matrix [17,18] and enhances the mechanical properties of the hybrid compositesignificantly. The interaction between the rubber and the filler has been studied to deter-mine the effects on failure of the compounds by many researchers [19�23]. Upon anapplied load the rubber must bear the total strain; however, the local strain within therubber phase is greater than the global strain attained by the system. This differencebetween the local and the global strains is termed the strain amplification factor. Thisaspect of the material is studied by stressing it at different crosshead speeds. The interac-tion between the rubber and the filler affects the quasi-static properties of the compoundsas well as the fatigue life. Inclusion of filler like silica into rubber and epoxy resin com-pounds serves to increase the modulus of the compound as well as hinder crack propaga-tion. This reduction in the crack propagation occurs because of localized crystallizationoccurring in the compound due to regions of highly constrained polymer. These regionsoccur near the crack tip as well as in regions of high constraint due to the filler particlessuch as silica. Hence, appropriate proportion of silica�rubber mixed into the epoxy resinmay increase the static as well as dynamic and fatigue properties of the hybrid composites.Keeping view of the above, the preset investigation is aimed with the objectives to developa hybrid composite material containing different percentage of nanosized silica particleand SBR in an epoxy resin matrix.

EXPERIMENTAL

Materials

Epoxy resin (CY 230), hardener (HY 951), silica particle, and SBR were used as receivedwithout further purification. Commercial epoxy resin CY 230 and hardener HY 951 weresupplied by M/s Petro Araldite Pvt. Limited, Chennai, India and silica particle and SBRwere supplied by M/s Insilco Limited, Gajraula, (Degussa Group) and M/s Taj Resins Pvt.Limited, New Delhi, India, respectively.

Preparation of Nanocomposites

Different mixtures of silica particles (particle content of 1 and 2wt%), SBR (0.25, 0.5,1.0, and 1.5wt%), and epoxy resin were prepared by mechanical stirring at 3000 rpm. Thesolution obtained by mixing silica particle and SBR in resin was kept in the furnace at atemperature of 90±10�C for 2 h [24]. At an interval of 30min, the solutions were takenout from the furnace and remixed by mechanical stirrer at same speed. The solution of

2452 V.K. SINGH AND P.C. GOPE

SBR is prepared by solvent benzene before mixing it with the epoxy resin. The residue of

the benzene is removed by the process of evaporation.After 2 h the whole solution was taken out and allowed to cool to a temperature of 45�C.

When a temperature of 45�C was attained the hardener HY-951 (8wt%) was mixed imme-

diately [24]. Due to addition of hardener high viscous solution was obtained, which was

again mixed mechanically at high speed by the mechanical stirrer. The viscous solution so

obtained was poured into different moulds for sample preparation for tensile, wear, hard-

ness, and impact testing. All samples were prepared according to the ASTM standards or

machine specifications.

CHARACTERIZATION OF COMPOSITES

The specimens were silver-coated and examined by scanning electron microscopy (SEM)

using a LEO435V6 instrument. The accelerating voltage was kept 20 kV.The state of dispersion of nanoparticles into the resin matrix plays a significant role on

the improvement of mechanical properties of the composite. Figure 1 shows good disper-

sion of silica particle in the resin matrix. It is seen from the figure that silica particles are

well dispersed in the epoxy resin matrix in a preferred orientation. The absence of any

voids around the particle indicates a good adhesion between silica particle and epoxy

matrix. The concentrations of silica particles are also seen in the figure. From the above

figure it is evidenced that there is no chemical reaction between silica-rubber and epoxy

resin. It is estimated from the observations that the average size of the silica nanoparticles

is 130 nm±12nm.

Figure 1. Scanning electron micrograph for 2 wt% silica reinforced composite material at magnification10,000X.

Silica�Styrene�Butadiene Rubber Filled Hybrid Composites 2453

MECHANICAL PROPERTIES

The tensile test specimen (Figure 2) prepared from each nanocomposite were loaded

in uniaxial tension on 100 kN servo hydraulic universal testing machine (ADMET,

USA Make) at different crosshead speeds of 0.01, 0.1, 1, 10, and 100mm/min. From

the stress�strain curves, the yield strength (0.2% offset), ultimate strength, modulus of

elasticity, and percent elongation were determined. The room temperature and humidity

during testing were 21�C and 78%, respectively.Remarkable differences have been observed in the stress�strain behavior due to addi-

tion of silica and SBR in epoxy resin matrix. From the stress�strain data of hybrid com-

posite following types of relation are modeled:

� ¼ E", for elastic region ð1Þ

¼ �0"n, for plastic region ð2Þ

where E is the modulus of elasticity, r0 and n are strength coefficient and strain strength-

ening exponent. r and e are true stress and strain. The variation of r0 and n with weight

percent of silica and SBR content are shown in Figure 3(a)�(c). A sharp increase in the

value of r0 and n has been observed due to addition of silica particle up to 1wt% there-

after the variation is negligible.Figure 3(b) and (c) shows the variation of r0 and n with weight percent of SBR for

1- and 2wt% of silica, respectively. In both the cases it is observed that exponent n

decreases linearly and r0 exponentially with weight percent of SBR for both 1- and

2wt% silica contents.Regression analyses has been conducted on the basis of least square method and fol-

lowing relation have been obtained to estimate r0 and n for hybrid composite material

from weight percent of filler materials:

�0 ¼ 649:95 expð�2:2385WRÞ, R2 ¼ 0:993 ð3Þ

where WR is the weight percent of SBR and R is the regression coefficient. Equation (3) is

valid for both 1 and 2wt% of silica content. Similarly linear models have been derived for

the prediction of exponent n from the weight percent of SBR as follows:

n ¼ 1:0582� 0:4817WR R2 ¼ 0:994 for 1 wt% of silica ð4Þ

n ¼ 0:9301� 0:3722WR R2 ¼ 0:989 for 2 wt% of silica ð5Þ

2450

Displacement transducers

24 124 80

152

12 24

10 20

Figure 2. Specimen geometry for tensile test (all dimensions are in mm).

2454 V.K. SINGH AND P.C. GOPE

1

10

100

1000(a)

(b)

(c)

2Silica (wt%)

s 0 (

MP

a)s 0

(M

Pa)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

n

so

n

1

10

100

1000

1.4

Rubber (wt%)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

n

s0

n

0 0.5 1 1.5

0.0 0.2 0.4 0.6 0.8 1.0 1.2

1

10

100

1000

1.4

Rubber (wt%)

s 0 (

MP

a)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

n

s0

n

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Figure 3. (a) Effect of silica (wt%) on r0 and n at 0.1 mm/min crosshead speed. (b) Effect of SBR (wt%) on r0

and n for 1.0 wt% of silica at 0.1 mm/min crosshead speed. (c) Effect of SBR (wt%) on r0 and n for 2 wt% ofsilica at 0.1 mm/min crosshead speed.

Silica�Styrene�Butadiene Rubber Filled Hybrid Composites 2455

Tensile Properties

Figure 4(a)�(e) shows the effect of crosshead speed on strengths and percentage elon-gation for different composite material containing 1.0wt% silica with 0.25, 0.50, 1.0, and1.5wt% of SBR. Figure 4(a) indicates that beyond the crosshead speed of 0.1mm/min,there is continuous decrease in strengths whereas there is a little change in elongation.It indicates that there is no loss of ductility. Figure 4(c)�(e) indicates that at small cross-head speed of about 0.01mm/min both yield and ultimate tensile strength seems to besame for the materials with excess of 0.50wt% SBR. It is also seen from Figure 4 thatultimate tensile strength increases where as yield strength decreases with increase in cross-head speed, beyond the crosshead speed of 0.01mm/min. This indicates that there isdecrease in yield strength due to addition of SBR in silica hybrid composite material,and the material is expected to behave less elastically throughout practically its entirestrength range under higher crosshead speed condition, whereas sharp increase in ultimatestrength, the hybrid material containing silica and SBR are expected to behave plasticallythroughout its entire strength range under higher crosshead speed. Hence, it can beconcluded that silica�styrene�butadiene rubber mixed hybrid composite has decreasedelastic region and higher ductility, a higher plastic region as compared to base material.However, for all combination of the silica and SBR, the percentage elongations remainsame, indicating no loss of ductility throughout the entire strength range under differentcrosshead speed conditions.

Figure 5(a) and (b) shows the effect of weight percentage of SBR on the strength andpercentage elongation of the hybrid composite material at 0.1mm/min crosshead speed.Figures indicate that strength decreases continuously as the weight percentage of SBRincreases. At higher weight percentage of SBR i.e., 1.5wt% of rubber the yield strength isapproximately very close to the value of ultimate tensile strength. Similar behavior hasbeen observed with 2wt% silica and different weight percent of SBR. From Figure 5(a)and (b) it can be concluded that with higher weight percentage of SBR, the hybrid com-posite material may behave elastically throughout the strength, but the strength capacitydecreases three times than that of without SBR.

From the results, remarkable differences can be seen on the ultimate tensile strength ofthe hybrid composite material having different weight percent of SBR and silica tested atdifferent crosshead speeds. It can be noticed from the results that for all specimens con-taining 1.0wt% silica, the ultimate tensile strength is highest among the other compositionreported in the work at lower crosshead speeds ( _"5 1 mm/min). About 25% increase inultimate tensile strength due to addition of 1.0wt% of silica has been noticed as comparedto pure epoxy for all the crosshead speeds _"5 1 mm/min. Further addition of silica on theepoxy resin decreases the ultimate tensile strength of the hybrid composite. Similar obser-vations have been noticed for yield strength and modulus of elasticity. About 1.6 timesincrease in modulus of elasticity has been observed due to addition of 1.0wt% of silica at0.1mm/min crosshead speed. Further addition of the silica particle decreases the modulusof elasticity but is higher than the neat epoxy material. About 1.4 times increase inmodulus of elasticity is noticed with 2.0wt% of silica. Yield strength also increases withaddition of silica content in the composite material. About 1.36 and 1.27 times increase inyield strength has been observed on addition of 1.0 and 2.0wt% of silica, respectively atthe lower crosshead speed of 0.1mm/min. At crosshead speed above 1mm/min,yield strength and ultimate tensile strength decreases drastically. About 50% decreasein ultimate tensile strength has been recorded at 10mm/min crosshead speed for

2456 V.K. SINGH AND P.C. GOPE

0

5

10

15

20

25

30

35

40

45

50(a)

(b)

(c)

10.0Cross head speed (mm/min)

s u a

nd s

ys u

and

sy

s u a

nd s

y

0

5

10

15

20

25

30

35

40

s u/s

y an

d %

Elo

ngat

ion

s u/s

y an

d %

Elo

ngat

ion

s u/s

y an

d %

Elo

ngat

ion

su (MPa)sy (MPa)(sy/su)*100

% Elongation

0

5

10

15

20

25

30

35

40

45

Cross head speed (mm/min)

0

5

10

15

20

25

30

35

40

45

50

0

5

10

15

20

25

30

35

Cross head speed (mm/min)

0

10

20

30

40

50

60

70

80

90

100

0.0 0.1

0.1

1.0

1 100.01

0.1 1 100.01

su (MPa)

su (MPa)

sy (MPa)

sy (MPa)

(sy/su)*100

(sy/su)*100

% Elongation

% Elongation

Figure 4. Effect of crosshead speed on mechanical properties (a) 1.0 wt% of silica-filled hybridcomposite. (b) 0.25 wt% SBR and 1.0 wt% of silica-filled hybrid composite. (c) 0.50 wt% SBR and 1.0 wt%of silica-filled hybrid composite. (d) 1.0 wt% SBR and 1 wt% of silica-filled hybrid composite. (e) 1.5 wt% SBRand 1 wt% of silica-filled hybrid composite.

Silica�Styrene�Butadiene Rubber Filled Hybrid Composites 2457

composite material with 1 and 2wt% of silica whereas up to 40 and 20% increase in

yield strength and modulus of elasticity has been observed at the same crosshead speed.

It is seen that at higher crosshead speed, composite with 2wt% silica seems to be superior

to 1wt% silica.The variation of Ef/Eo withWR is shown in Figure 6(a) and (b). From Figure 6, it is seen

that a non-linear relation exist between modulus of elasticity and weight percentage of

filler materials. Keeping in view the importance of modulus of elasticity in design and

analysis, an attempt has been made to model an empirical relation of the following type to

interpret the filler�polymer interaction.

Ef

Eo¼ a ðWRÞ

b, for 1:0 wt% of silica ð6Þ

Ef

Eo¼ c ðWRÞ

d, for 2:0 wt% of silica ð7Þ

(d)

(e)

s u a

nd s

ys u

and

sy

s u/s

y a

nd %

Elo

ngat

ion

s u/s

y a

nd %

Elo

ngat

ion

10.00.0 0.1 1.0

0.1 1 100.010

5

10

15

20

25

Cross head speed (mm/min)

0

10

20

30

40

50

60

70

80

90

100

0

2

4

6

8

10

12

14

16

Cross head speed (mm/min)

0

20

40

60

80

100

120

su (MPa) sy (MPa)

(sy/su)*100 % Elongation

su (MPa) sy (MPa)

(sy/su)*100 % Elongation

Figure 4. Continued.

2458 V.K. SINGH AND P.C. GOPE

where E is the modulus of elasticity in MPa and subscription ‘f’ and ‘o’ denotes filled andunfilled polymer sample, respectively. The regression constants a, b, c, and d at particularcrosshead speed are presented in Table 1. WR denotes the weight percentage of rubber.

Similar variation of modulus of elasticity with weight fraction of filler material is shownby Bondyopadhyay et al. [17] with a=0.22 and b=2.43. In the present case, the highervalue of a as shown in Table 1 indicates greater reinforcement in the rubber matrix andhence, improvement of tensile strength and modulus of elasticity.

WEAR RESISTANCE

All the wear tests were conducted by using pin-on-disc wear testing machine (CameronPlint Wear & Friction Machine, Sr. No. TE 97/705) fitted with Muyford super lathe, deadweight tester, transducer with calibration of 0.892mv/v, microscope, and chart recorder.Cylindrical pins are made from the cast composites with a conical hole having an includedangle of 60�, diameter 5mm on the end face (Figure 7). Reduction in the end face conical

0

5

10

15

20

25

30

35

40

45

50(a)

(b)

1.5

Rubber (wt%)

0

10

20

30

40

50

60

70

0

5

10

15

20

25

30

35

40

1.50

Rubber (wt%)

0

10

20

30

40

50

60

70

s u a

nd s

ys u

and

sy

su (MPa) sy (MPa)

su (MPa) sy (MPa)

(su/sy)*100 % Elongation

(su/sy)*100 % Elongation

s u/s

y a

nd %

Elo

ngat

ion

s u/s

y a

nd %

Elo

ngat

ion

0 0.25 0.5 0.75 1 1.25

0.00 0.25 0.50 0.75 1.00 1.25

Figure 5. (a) Effect of rubber on mechanical properties of 1 wt% silica-filled hybrid composites at0.1 mm/min crosshead speed. (b) Effect of rubber on mechanical properties of 2 wt% silica-filled hybridcomposites at 0.1 mm/min crosshead speed.

Silica�Styrene�Butadiene Rubber Filled Hybrid Composites 2459

hole diameter of the pin gives volume of wear. This reduction in diameter is measured by

means of microscope with magnification of 10X. The wear rate is calculated from the

following relation:

Wear volume ¼

�2 tan �

D2 D2�D1ð Þ

4 �D3

2�D3

1ð Þ12

n o

total timemm3=min , ð8Þ

0.0

0.4

0.8

1.2

1.6

2.0

2.4(a)

(b)

Rubber (wt%)

Ef/

Eo

0.01 mm/min crosshead speed

0.1 mm/min crosshead speed

1.0 mm/min crosshead speed

10 mm/min crosshead speed

0.0

0.4

0.8

1.2

1.6

2.0

2.4

1.50

Rubber (wt%)

Ef/

Eo

0.01 mm/min crosshead speed

0.1 mm/min crosshead speed

1.0 mm/min crosshead speed

10 mm/min crosshead speed

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.25 0.50 0.75 1.00 1.25

Figure 6. (a) Variation of Ef/Eo with SBR (wt%) (1 wt% of silica). (b) Variation of Ef/Eo with SBR (wt%)(2 wt% of silica).

Table 1. Coefficients a, b, c, and d at different strain and 1 wt% of silica particles.

S.N Crosshead speed (mm/min) a b R2 c d R2

1 0.01 0.195 0.712 0.874 0.310 0.770 0.9452 0.10 0.677 0.704 0.965 0.633 0.369 0.7933 1.00 0.574 0.667 0.901 0.759 0.317 0.9924 10.0 1.042 0.493 0.936 0.938 0.468 0.964

R: regression coefficient.

2460 V.K. SINGH AND P.C. GOPE

where D1 is the initial groove diameter (mm), D2 is the final groove diameter (mm), D is the

outer diameter of the specimen (mm), and 2� is the groove angle, 60�.Thus the average wear volume obtained from both pin ends are shown in Figure 8. All

the tests were conducted at pressure 0.15N/mm2 and 1500 rpm.Figure 8(a) shows that addition of silica particle is more effective in reduction of wear

rate under 0.15N/mm2 of applied load. It is seen from Figure 8(a) there is about 85% of

wear reduction due to addition of silica particle, which is about 1/5th times of pure epoxy.

It is also evident from Figure 8(a), that beyond 1wt% of silica, there is no effective

decrease in wear rate. Hence, it can be concluded that particle content of about 1.0wt%

seems to be most effective in wear reduction.Figure 8(b) shows the effect of SBR on the wear characteristics of the silica-filled

composite material. Figure 8(b) shows that wear rate increases due to the addition of

35 m

m

100

mm

12 mm

53 mm

8 mm

(a)

(b)

(c)

Slide

Force transducer

Pin

Disc

End loading byhydraulic cylinder

5 mm dia. × 60° Inc.

Figure 7. (a) Configuration of the disc. (b) Configuration of the pin. (c) Principle of operation of the carriage.

Silica�Styrene�Butadiene Rubber Filled Hybrid Composites 2461

SBR both for 1.0 and 2.0wt% of silica composite. The rate of increase of wear rate isincreased when the SBR weight percentage is less than 0.5 as compared to the higher

weight percentage of SBR.From the above discussion, it can be concluded that addition of the silica particle

reduces the wear rate of unfilled epoxy significantly. The reduction of wear rate could

be up to five times. The addition of SBR has insignificant effect on the wear rate. However,it slightly increases the wear rate as compared to silica-filled epoxy composite.

The wear mechanism of particulate-filled epoxy composite under the pin-on-disccondition has been discussed by Durand et al [25]. The mechanism includes formation

of cracks, development from cracks to waves and productions of debris.From the present observations it can be explained that with the addition of silica particle

into the epoxy matrix, during the wear testing the formation and propagation of cracks are

hindered to a certain degree by the silica particles on or near the surface layer. This resultedin the formation of fine material waves and the formation of finer debris. Furthermore, the

silica particles reduce the wear rate when the applied load is less than the critical one. Both ofthese contributed to the reduction of the wear rate of the epoxy matrix.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0(a)

(b)

2.0

Silica (wt%)

Wea

r ra

te (

mm

3 /m

in)

0.77

0.78

0.79

0.80

0.81

0.82

0.83

0.84

0.85

1.500

Rubber (wt%)

Wea

r ra

te (

mm

3 /m

in)

Wear rate at 2% Si Wear rate at 1% Si

0.0 0.4 0.8 1.2 1.6

0.000 0.250 0.500 0.750 1.000 1.250

Figure 8. (a) Effect of silica (wt%) on wear rate at 1500 rpm speed and 0.15 MPa pressure. (b) Effect of SBR(wt%) on wear rate at 1500 rpm speed and 0.15 MPa pressure.

2462 V.K. SINGH AND P.C. GOPE

As it is seen from the SEM micrographs, the filler materials (i.e., nanosilica particle) areuniformly distributed over the epoxy matrix. Hence, the mean distance between neighbor-ing particles in case of 2wt% silica particles is less than 1wt% silica. Under lower particlecontents, the dispersion of the particle in the epoxy matrix is good enough so that wearrate decreased with the increase of the particle content. So, in the present investigation it isseen that the wear rate with 2wt% silica content is less as compared to 1wt% silica.

Addition of SBR increases the wear rate, but it is insignificant as compared to the effectof silica particle. The increase in wear rate due to the addition of SBR may be dueto increase in bonding strength between silica particle, SBR and epoxy matrix. Due toincrease in bonding strength, formation of fine material waves and fine debris is less ascompared to silica-filled epoxy composite, due to which wear rate slightly increases due toaddition of SBR.

Figure 8(a) shows that wear rate of pure epoxy (CY-230) cured with hardener (HY-951)is 4.605mm3/min, which is about four times of the wear rate obtained in all hybrid com-posite containing silica and SBR. The wear rate for silica-filled composite is about0.825mm3/min and for silica filled with SBR epoxy composite wear rate varies between0.778 and 0.810mm3/min for different weight percentage of SBR.

HARDNESS

All hardness tests are conducted on a Rockwell hardness testing machine supplied byP.S.I. Pvt Limited, New Delhi on R scale. The effect of weight percentage of silica andSBR on the hardness values of hybrid composites are shown in Figure 9(a) and (b).

It is found that hardness of neat epoxy resin (CY-230 and 8wt% of HY-951) is 85 HRR.The hardness of the fabricated composite made of epoxy resin and 0.5, 1.0, and 2wt% ofthe silica are 99.56, 120.17, and 120.98 HRR, respectively. Figure 9(a) indicates that thehardness increases with the silica content reflecting the reinforcement formed in the hybridcomposite. It is also seen from Figure 9(a) that, beyond 1wt% of silica, the improvementin the hardness value of the hybrid composite is insignificant. This shows that the optimumvalue of silica is about 1.0wt%, which increases about 40% of hardness as compared tobase material.

In the contrast, the addition of SBR to silica-filled composite decreases the hardness. Itis seen that addition of 0.5wt% of SBR to 1.0 and 2.0wt% of silica-filled composite yieldsthe hardness value of 82.27 and 83.85 HRR, respectively, which is just equal to the hard-ness of pure epoxy resin composites.

Hence, it can be concluded that use of SBR as additional filler material in silica-filledresin composite decreases the hardness value. An amount of 0.5wt% of SBR may be usedin the silica-filled composite to get the reasonable mechanical properties including hard-ness. Beyond the value of 0.5�1.0wt%, it decreases the hardness drastically.

The variation of the hardness with the ultimate tensile strength of hybrid composite isshown in Figure 10(a). Figure 10(a) illustrates that the hardness value follows a linearrelation with ultimate strength of the silica�styrene�butadiene rubber hybrid compositematerial.

An attempt has been made to correlate hardness with the ultimate strength. Followingcorrelation has been obtained from the least square analysis:

�u ¼�

10

� �� HRRð Þ, ð9Þ

Silica�Styrene�Butadiene Rubber Filled Hybrid Composites 2463

with correlation coefficient of 0.866 with 50% reliability, where ru is in MPa and hardness

HRR in Rockwell ‘R’ scale.From Table 2 it is seen that the ratio of experimental value to the predicted value of

ultimate strength lies between 0.50 and 2.27.Figure 10(b) is drawn to show the effectiveness of the present model. The figure shows

that most of the data except one falls within the scatter band of 1. This illustrates the

effectiveness of the proposed model to estimate the ultimate strength from the hardness.

The correlation coefficient value of 0.886 indicates a good correlation between hardness

and ultimate strength and a direct relation between them. The students’ t test results is

found to be 4.582 (t7,0.01=3.0), which confirm the correlation between the factors hard-

ness and ultimate strength with probability p� 0.01.To assess a possible correlation between surface hardness and wear resistance of

silica�styrene�butadiene rubber-filled composites material regression and correlation

tests were done. Figure 10(c) shows the correlation between both. The R2-value of 0.7135

indicates a correlation and an inverse relation between them and students’ t-test confirmed

80

90

100

110

120

130

Silica (wt%)

Har

dnes

s (H

RR

)

0

20

40

60

80

100

120

140(b)

(a)

Rubber (wt%)

Har

dnes

s (H

RR

)

1% Silica 2% Silica

0 0.4 0.8 1.2 1.6 2

0 0.2 0.60.4 0.8 1 1.2 1.4 1.6

Figure 9. (a) Effect of silica (wt%) on hardness. (b) Effect of SBR (wt%) on hardness.

2464 V.K. SINGH AND P.C. GOPE

y = 0.2617x + 0.8347

R2 = 0.6898

0

5

10

15

20

25

30

35

40

45

50(a)

(b)

(c)

Hardness (HRR)

Ulti

mat

e st

reng

th (

MP

a)

0.0

0.5

1.0

1.5

2.0

2.5

Experimental ultimate strength

Exp

erim

enta

l/Pre

dict

ed u

ltim

ate

stre

ngth

0 20 60 80 100 120 14040

0 10 20 30 40 50 60

y = –0.0004x + 0.8457

R2 = 0.7135

0.77

0.78

0.79

0.80

0.81

0.82

0.83

0.84

0.85

Hardness (HRR)

Wea

r ra

te (

mm

3 /m

in)

0 25 50 75 100 125 150

Figure 10. (a) Variation of hardness with ultimate tensile strength. (b) Effectiveness of predicted ultimatestrength by model. (c) Variation of hardness with wear resistance.

Silica�Styrene�Butadiene Rubber Filled Hybrid Composites 2465

the correlation between the factors with p� 0.01 as value of ‘t’ obtained is 4.574 and thetabulated value at 99% confidence level with eight degree of freedom is t8,0.01=2.9.

IMPACT STRENGTH

For the impact testing, Izod test was performed on the notched specimens as recom-mended by ASTM standard for the rigid plastics. Specimen for this test had dimensions of10� 10� 75mm3 (Figure 11). Three specimens of each type were tested in accordance withthe same standard. The results obtained from the test are depicted in Figure 12(a) and (b).The impact strength of pure epoxy resin (CY-230+8wt% HY-951) is 6.25Nm. Figure12(a) shows that composite containing silica particle exhibits the lowest impact strengthamongst all the hybrid composite studied. Addition of 0.5wt% of silica lowered 15% ofthe impact strength as compared to pure epoxy. Decrease of about 30�35% has beenobserved due to addition of 2wt% of silica.

On the other hand, rubber particles are known for their resistance to dynamicloading situations. In the present investigation, an improvement in the impact strengthhas been observed due to addition of SBR. A remarkable increase of impact strength from4.55 to 6.4Nm (about 40% increase) has been noticed due to addition of only 0.5wt% ofSBR in 1wt% silica-filled epoxy composites. A gradual increase in impact strength isobserved beyond 0.5wt% of SBR, Figure 12(b). It is found that there is an increaseof 60�70% in impact strength due to addition of 1.5wt% of SBR in silica-filledepoxy composite.

From the present observations it can be concluded that the silica particle lowers theimpact resistance whereas SBR improves the impact resistance when added as filler mate-rial. Addition of the silica particle as filler material probably causes localized stress con-centration due to angularities in the regions of particle corners and would, thereforefacilitate failure under impact conditions. On the other hand, SBR improves the resistanceof the material under impact loading by improving the bond between silica and resin. Anoptimum filler contents with respect to impact strength is found to be 1.0wt% of silica and1.5wt% of SBR. However, keeping view of other mechanical properties an averageamount of 0.5wt% of rubber is significant and recommended as optimum filler amount.

Table 2. Effectiveness of relation between ultimate strength (MPa) and hardness (HRR) forsilica�styrene�butadiene rubber filled hybrid composite material.

Composition ru (MPa)

Silica(wt%)

Rubber(wt%) HRR Experimental

Predicted fromhardness

Experimental/Predictedfrom hardness

1.0 0.00 120.17 45.40 37.73 1.201.0 0.25 112.06 31.04 35.19 0.881.0 0.50 82.27 23.12 25.83 0.901.0 1.00 80.26 15.33 25.20 0.611.0 1.50 14.22 10.15 04.47 2.272.0 0.00 120.98 35.79 37.99 0.942.0 0.25 113.77 23.59 35.72 0.662.0 0.50 83.85 20.12 26.33 0.762.0 1.00 77.06 11.98 24.20 0.50

2466 V.K. SINGH AND P.C. GOPE

CONCLUSIONS

Epoxy nanocomposites reinforced with nanosized silica particles and SBR were

prepared. Such nanocomposites were experimentally characterized by means of micros-

copy, tensile testing, hardness, wear, and impact testing. Remarkable improvements in the

mechanical properties have been noticed due to addition of silica and butadiene rubber in

4.0

4.5

5.0

5.5

6.0

6.5(a)

(b)

2.00

Silica (wt%)

Impa

ct r

esis

tanc

e (N

m)

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

1.5Rubber (wt%)

Impa

ct s

tren

gth

(Nm

)

1wt% 2wt%

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

0.0 0.3 0.5 0.8 1.0 1.3

Figure 12. (a) Effect of silica (wt%) on impact resistance. (b) Effect of SBR (wt%) on impact strength of hybridcomposites containing 1 and 2 wt% silica.

2 45°

Q

75 mm

28 mm

22 mm

10 m

m

10 mm

Included angleHammerstrikeshere

Strikingdistanceof pendulum

Figure 11. Specimen geometry for Izod impact test.

Silica�Styrene�Butadiene Rubber Filled Hybrid Composites 2467

epoxy resin. The wear rate of the composites was reduced drastically by adding a smallamount (i.e., 1.0�2.0wt%) of 130±12 nm sized silica filler. Regression models weredeveloped to simulate the mechanical behavior of such materials from the volume contentof the filler materials.

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2468 V.K. SINGH AND P.C. GOPE