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Effect of filler addition and strain rate on the compressive strength of silica styrene-butadiene rubber-filled epoxy composites.

INTRODUCTION

Because of their superior strength-to-weight ratios and wear resistance, silica reinforced polymer composites have found widespread use in various fields such as packaging, dental, engineering, transportation, and aerospace. Hard inert silica particles filled in composite materials lead to increased elastic modulus and compressive strength although the tensile strength may be reduced [1]. Particle filled also increases the stiffness of the material because of the crosslink density of the material by providing additional crosslinking sites at the particle matrix interface [2]. This increase is also due to the reduction of the segmental mobility close to the filler particles [3]. Similarly, rubber as additional filler materials in silica-filled epoxy composites improves the elastic and plastic behavior of materials [4]. From an engineering point of view, one important property of silica rubber-filled composites is their higher compressive strength compared with the respective tensile strength. Lower density is another important property where weight to strength ratio plays a vital role. High cost of polymers is also a limiting factor in their use for commercial applications. Low cost, easily available fillers may be used to bring down the cost of component.

The mechanical behavior of silica-filled polymer composites has been the subject of extensive investigation [5-15]. There is a relative dearth of information available for the compressive properties of hybrid silica rubber epoxy composites.

In this work, the authors have investigated the compressive properties of silica styrene-butadiene rubber (SBR)-filled epoxy composites as a function of silica and SBR content and crosshead speed. Based on the experimental data, sixth order polynomial equation for deformation behavior of different composites have been developed and used to compute the strain energy.

EXPERIMENTAL

Epoxy resin (CY 230), hardener (HY 951), silica particle, and SBR with different weight percentage of silica and SBR were used as received without further purification. Commercial epoxy resin CY 230 and hardener HY 951 were supplied by M/s Petro Araldite Pvt. Limited, Chennai, India, and silica particle and SBR were supplied by M/s Insilco Limited, Gajraula, (Degussa Group) and M/s Taj Resins Pvt. Limited, New Delhi, India, respectively. Different mixtures of silica particles (particle content of 1 and 2 wt%), SBR (0.25, 0.5, 1.0, and 1.5 wt%), and epoxy resin were prepared by mechanical stirring at 3000 rpm. The curing curve of epoxy CY 230 is shown in Fig. 1. Based on the curing curve, the solution obtained by mixing silica particle and SBR in resin is kept in the furnace at a temperature of 90[degrees]C [+ or -] 10[degrees]C for 2 h [11]. At each interval of 30 min, the solution is taken out from the electric furnace and remixed by mechanical stirrer at same speed. After 2 h, the whole solution is taken out and allowed to cool to a temperature of 45[degrees]C. When a temperature of 45[degrees]C has been attained, the hardener HY-951 (8 wt%) is mixed immediately [11]. Because of addition of hardener, high-viscous solution is obtained, which is remixed mechanically at high speed by the mechanical stirrer. The viscous solution so obtained is poured into different moulds for sample preparation for compression testing.

[FIGURE 1 OMITTED]

All compression tests are conducted on 100 kN servo hydraulic universal testing machine (ADMET, USA). Circular specimens with aspect ratio (length to diameter) of 2.0 are tested under displacement mode of control of 0.01, 0.1, 1.0, 10.0, and 100.0 mm/s. Silicon grease is used to lubricate two interfaces between the specimen and grips and to eliminate the interfacial friction influence as much as possible. The results are presented and discussed in subsequent sections.

RESULTS AND DISCUSSION

Characterization of the Composite

The silver-coated samples are examined by scanning electron microscopy (SEM) using a LE0435V6 instrument. The accelerated voltage is 20 kV. Figure 2 shows good dispersion of silica particle in the resin matrix for both 1 and 2 wt% of silica. 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 concentration of silica particles can also be seen in the figure. It is estimated from the observations that the average size of the silica nanoparticles is 130 [+ or -] 12 nm.

[FIGURE 2 OMITTED]

Deformation Behavior in Compression

The deformation behaviors under compressive load are shown in Figs. 3-5. Stress-strain curves for the composite specimens is generally nonlinear in nature showing distinctly the drawing/viscoelastic phenomenon as seen in many polymers. Overall, the mean failure strain ranges from 60% for the 1 wt% silica and 0.25 wt% SBR specimens to 20% for 2 wt% silica and 0.25 wt% SBR specimens. Through comparison of the area up to elastic limit, under each stress-strain curve, it is observed that composite containing 1 wt% silica and 0.25 wt% SBR possesses significantly higher resilience, when compared with the other specimens investigated in this work. Composite containing 1.5 wt% of SBR possesses the lowest resilience value (approximately 12% that of the maximum). Similar observations are seen for total area of fracture.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Based on the deformation behavior of different composites, sixth order polynomial equation of following type is fitted to the experimental data.

[sigma] = [a.sub.6][[epsilon].sup.6] + [a.sub.5][[epsilon].sup.5] + [a.sub.4][[epsilon].sup.4] + [a.sub.3][[epsilon].sup.3] + [a.sub.2][[epsilon].sup.2] + [a.sub.1][[epsilon].sup.1] (1)

where [sigma] is the stress in MPa and [epsilon] is the strain.

The parameters of the Eq. 1 are presented in Table 1 along with correlation coefficient and energy stored per unit volume.
TABLE 1. Fitted parameters for compression stress-strain behavior.

                                   1.0 wt% silica
                              Styrene-butadiene rubber
Parameters             0.25 wt%  0.5 wt%            1.0 wt%   1.5 wt%

[a.sub.6]               -46,027   2.0 X [10.sup.6]   214.765   392,205
[a.sub.5]               125,892  -2.0 X [10.sup.6]  -286,982  -481,642
[a.sub.4]              -119,026            999,666   139,594   216,069
[a.sub.3]                54,124           -167,424    -26511   -39,264
[a.sub.4]                12,228             7437.3     676.9    1523.9
[a.sub.1]                1287.4             805.87     369.8     339.5
[r.sup.2]                 0.991              0.991     0.997     0.998
Energy (kJ/[m.sup.3])    26.162             20.763    13.785    13.327

                                    2.0 wt% silica
                                Styrene-butadiene rubber
Parameters              0.25 w%            0.5 wt%   1.0 wt%   1.5 wt%

[a.sub.6]               -97,612            287,521   210,690  -161,618
[a.sub.5]               237,893           -428,705  -209,302   241,896
[a.sub.4]              -200,683            238,501    60,856  -140,278
[a.sub.3]                80,834             57,937    1960.2    41,085
[a.sub.4]                16,063             5094.8   -3333.9    6155.7
[a.sub.1]                1489.2             176.64    531.89    495.05
[r.sup.2]                 0.996              0.998     0.998     0.999
Energy (kJ/[m.sup.3])    29.274             25.944     20.47    15.684


Figure 6 shows the comparison between stress-strain curves from model prediction by Eq. 1 and experimental data. It is seen that the model curves are quite consistent with the experimental curve with correlation coefficient r > 0.99.

The energy observed per unit volume (U) in deforming the material to maximum strain [[epsilon].sub.max] is the area under the stress-strain curve. This can be obtained from the integration of the following relationship.

[FIGURE 6 OMITTED]

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

The stress [sigma] as described by Eq. 1 is used to integrate the Eg. 2. The energy absorbed per unit volume is presented in Table 1. The area under the stress-strain curve as shown in Table 1 indicates that due to addition of 0.25 wt% of SBR, the stiffness of the material with lower wt% of silica increases. This illustrates that the addition of SBR helps in binding the silica particle and improves the elastic and plastic behavior. Hence, the addition of SBR improves in modulus of elasticity and yield strength and ultimate compressive strength of the material. As the wt% of silica increased to 2.0, it increases the stress concentration around the silica particle, and the low wt% of SBR fails to reduce the segmental mobility close to the filler particle and, hence, the total energy stored decreased, when compared with 1 wt%.

Compressive Strength

The variation of compression strength with wt% of SBR is shown in Figs. 7 and 8. A remarkable difference can be noticed in the value of the compressive strength with different wt% of silica and SBR. It can be noticed that addition of silica improves the ultimate compressive strength of the hybrid composite materials. It is found that ultimate compressive strength of 1 wt% of silica and 0.25 wt% of SBR is about 133 MPa, when compared with 91.2 MPa of pure epoxy composites at 0.1 mm/s. As the silica weight percent is increased to 2, ultimate compressive strength decreases to 120 MPa. This compressive strength is about 1.3-1.5 times of the ultimate compressive strength of the pure epoxy. A variation of 133-112 MPa is seen for hybrid composite material containing 1 wt% of silica and 0.25 wt% of SBR over all the crosshead speeds from [10.sup.-1] mm/s to [10.sup.2] mm/s. Similar observation vation has been noticed for other combination of filler materials. At higher weight percentage of either silica or SBR, the strength decreases. It is seen that beyond 1 wt% of silica and 1 wt% of SBR, the compressive strength is just about the same as that of the unfilled specimen.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

The ultimate compressive strength is compared with ultimate tensile strength in Table 2. It is seen that the ratio of ultimate compressive strength to ultimate tensile strength varies from 4 to 8.5 as rubber weight fraction varies from 0.25 to 1.5 in 1 wt% silica. A slight increase in the ratio of ultimate compressive strength to the tensile strength is seen for 2 wt% of silica-filled composite when SBR is varied from 0.25 to 1.5 wt%.
TABLE 2. Comparison of compressive and tensile ultimate strength.

Rubber (wt%)  Silica  Compressive     Tensile   [[sigma].sub.uc]/
              (wt%)   ultimate        ultimate  [[sigma].sub.u]
                      strength (MPa)  strength
                                      (MPa)

0.25             1.0          133.10     31.04               4.29

0.50             1.0          108.80     23.12               4.71

1.00             1.0           92.60     15.33               6.04

1.50             1.0           84.42     10.15               8.32

0.25             2.0          119.59     23.59               5.07

0.50             2.0           94.76     20.12               4.71

1.00             2.0           83.93     11.98               7.01

1.50             2.0           77.85      8.16               9.54


The rigidity and compressive strength of silica particle are much higher compared with that of epoxy. So, a higher compressive strength is expected due to addition of silica to epoxy. A similar trend is observed in the current material under investigation. However, on increasing the weight fraction of silica from 1 to 2, decreasing compressive strength is noticed. About 10% decrease in compressive strength is recorded, when the silica fraction is increased from 1 to 2 wt%. The decrease in compressive strength for higher weight fraction of silica may be due to existence of more stress concentration zone near the silica particles.

On the other hand, SBR decreases the compressive strength of the material. Because of addition of SBR, substantial elastic and then plastic deformation takes place in the matrix, and the silica particles are not able to resist the load. This type of behavior is confirmed from the increased value of yield strength of the silica SBR-filled hybrid composite material. The average compressive yield strength of unfilled and 1.0 wt%-filled epoxy composite is 72.0 and 43.0 MPa, respectively. The compressive yield strength of 1.0 wt% silica and 0.25 wt% SBR-filled composite varies from 91.0 to 96.0 MPa. About 1.2-1.3 times increase in the value of compressive yield strength is noticed due to addition of small weight fraction of SBR in silica-filled composite, when compared with unfilled material and about 2-2.3 times, when compared with silica-filled epoxy composites. The aforementioned results indicate that the elastic and viscoplastic nature of rubber helps in the improvement of the elastic properties of the material. It is described that the matrix strength, the interfacial bond strength between filler material and the matrix, and the level of porosity play important role in the improvement of the compressive strength [1]. In this case, the increase in the elastic properties may be due to reduction of the porosity of the composite and improvement in strength of the bonding between filler material and matrix because of the addition of SBR as secondary filler material.

Compressive Modulus

The influence of silica and SBR content on compressive modulus has been presented in Fig. 9. Figure 9 reveals that compressive modulus of composite material containing 2 wt% silica is higher than 1 wt% over all crosshead speed. As the SBR content increases, the compressive modulus decreases. The compressive modulus is found to be maximum for the hybrid composite containing 2.0 wt% of silica and 0.25 wt% of SBR material (4.779 GPa) followed by 1.0 wt% silica (4.456 GPa) and gradually decreases with increasing SBR content. The compressive modulus for 1 wt% of silica and 1.5 wt% of SBR materials is 0.214 GPa, which is the lowest among the all composites presented in this article. The compressive modulus of neat epoxy resin at 0.10 mm/s crosshead speed is 0.360 GPa. In this investigation, it is found that the compressive modulus of silica and butadiene rubber-reinforced epoxy composites is higher than the neat epoxy resin except for low silica and higher styrene-butadiene-filled composites. The effect of filler content of SBR on the compressive strength is shown in Fig. 10. An inverse linear relationship exists between the both. Keeping in mind the importance of compressive modulus in design and analysis, the following equation is optimized.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[E.sub.C] = 0.567([W.sub.R][).sup.-0.919] (3)

where [E.sub.C] is the compressive modulus in GPa, and [W.sub.R] is the weight fraction of SBR. The correlation coefficient is found to be 0.81. The variance of the slope (exponent) and intercept are 0.106 and 0.045, respectively. The Student's test result confirmed the relationship with probability p < 0.01.

Effect of Crosshead Speed

The effects of crosshead speed on compressive strength and compressive modulus are shown in Figs. 11 and 12. Composite containing both silica and SBR shows little crosshead speed dependency in the range of 10-100 mm/s. However, an increase in yield strength is observed for composites with low weight percent of filler material at crosshead speeds less than 10 mm/s. About 82% increase in yield strength for 1.0 wt% silica and 0.25 wt% of SBR and about 62% increase for 0.25 wt% of styrene-butadiene and 2.0 wt% of silica is observed as crosshead speed increased from 0.1 to 100 mm/s. With other combinations of filler contents, the compressive yield strength also increases. For all composites, it is seen that the compressive strength and the ratio of compressive yield strength to compressive strength increases with crosshead speed. This indicates that the material is expected to behave more elastically and less plastically over all the crosshead speeds. Figure 11 also indicates that the percentage reduction in height remains constant over all the crosshead speeds. Hence, it can be concluded that the nanosized silica and SBR-filled epoxy resin shows increased elastic region.

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

Analysis of Variance and Factor Effects

Design of experiment is a powerful analysis tool for modeling and analyzing the influence of control factors on performance output. The plan of the experiment is developed for assessing the influence of the weight fraction of silica, SBR, and crosshead speed. The experimental results are analyzed with analysis of variance (ANOVA), which is used for identifying the factors significantly affecting the performance measures. The results of the ANOVA with the compressive modulus and compressive strength are shown in Tables 3 and 4, respectively. The sixth column of Tables 3 and 4 shows the probability values (p value), which is derived from the cumulative distribution function (CDF) of F. Output vector contains p values for the null hypotheses on the N (silica and SBR) main effects. The vector p shows the p values for the two filler materials silica and SBR (Table 3), 0.556 and 0.048, respectively. These values indicate that SBR affect the modulus of elasticity as p = 0.048 (meaning is that less than 48 samples of 1000 would have equal means). The critical F value at the 0.01 level of significance with 1, 3 degree of freedom is 34.1. This analysis is carried out for a significance level of [varies] = 0.01, i.e., for a confidence level of 99%. Tables 3 and 4 show the p values, that is, the realized significance levels, associated with the F tests for each source of variation. The sources with F value more than the standard F (corresponding to degree of freedom and confidence level) value are considered to have a statistically significant contribution to the performance measures.
TABLE 3. ANOVA of modulus of elasticity for 1 and 2 wt% silica and
0.25 and 1 wt% rubber.

Source  SS     DF  MS     F       Prob > F

Silica  0.021   1  0.021   0.700     0.556
SBR     5.128   1  5.128  174.35     0.048
Error   0.029   1  0.029
Total   5.178   3

[F.sub.0.01,1,3] = 34.1

TABLE 4. ANOVA for compressive strength.

Source  SS      DF  MS       F         Prob > F

Silica  189.75   1  189.751  2702.038    0.0123
SBR     603.44   1  603.439  8592.940    0.0069
Error     0.07   1    0.070
Total   793.26   3

SS = sum of squares; DF = degree of freedom; MS = mean square
[F.sub.0.01,1,3] = 34.1.

Significance test at 99% confidence level. [F.sub.0.01,1,3] = 34.1.


On the examination of the percentage contribution (p%) of the different factors for modulus of elasticity and compressive strength, it can be seen that SBR as filler material has the highest contribution, when compared with silica filler material, thus wt% of SBR is an important factor to be taken into consideration while casting the composite material.

CONCLUSIONS

Addition of nanosize silica particle and SBR in small weight fraction to epoxy resin led to the increase in compressive strength and compressive modulus of the composite material. It is seen that the ratio of ultimate compressive strength to ultimate tensile strength varies from 4 to 8.5 as rubber weight fraction varied from 0.25 to 1.5 in 1.0 wt%-filled epoxy resin. About four times increase in compressive modulus, when compared with neat epoxy is found with addition of 0.25 wt% of SBR to silica-filled epoxy resin. Effect of crosshead speed on the compressive strength of the material is significant up to 0.5 weight fraction of SBR, thereafter, the effect is insignificant. The ratio of compressive yield strength to compressive strength increases with crosshead speed. This indicates that the material is expected to behave more elastically and less plastically over all the crosshead speeds. The sixth order polynomial model based on least square regression analysis well describes the material behavior even at higher crosshead speed. The material parameters deduced in the article can be used directly in the code development for simulation computations.

ACKNOWLEDGMENTS

The authors thank Prof. Abhishek Yadav, Electrical Engineering Department, College of Technology, Pantnagar, for his valuable comments during the work and assistance with the computational work. They also thank Dr. M.P. Singh, Dean, College of Technology, Pantnagar, for his support and permission for publishing the results.

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Correspondence to: Prakash Chandra Gope; e-mail: pcgope@rediffmail.com

DOI 10.1002/pen.21815

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C] 2011 Society of Plastics Engineers

Prakash Chandra Gope, Vinay Kumar Singh

Mechanical Engineering Department, College of Technology, G. B. Pant University of Agriculture & Technology, Pantnagar 263145, U.S. Nagar, Uttarakhand, India
COPYRIGHT 2011 Society of Plastics Engineers, Inc.
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Author:Gope, Prakash Chandra; Singh, Vinay Kumar
Publication:Polymer Engineering and Science
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Date:Jun 1, 2011
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