Interaction Between Fumed Silica And Epoxidized Natural Rubber Biology Essay

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Nowadays, the safety and the low rolling resistance of tires have been becoming a hot point of the rubber industry since the concept green tires was put forward [1, 2]. Therefore tires were required not only low rolling resistance but also high wet skid resistance. The low rolling resistant materials generally have a low glass transition temperature or low hysteresis loss, such as cis-polybutadiene and natural rubber. However, these rubbers are extremely difficult to satisfy the requirements for running safety on wet road. There were many studies investigating rubbers to improve the wet skid resistant or rolling resistant [3, 4]. However there was not satisfactory rubber that had both properties until in 1980s the ENR was prepared [5].

Because ENR not only has good elastic properties, but also has special properties for example excellent air permeability [6], high oil and solvent resistant [7], high glass temperature, high wet skid resistance [8] and low rolling resistant [9]. ENR becomes a new material that is considered to be used in high performance tires. The ENR had already been used in Sumitomo Rubber Industries in Japan and was confirmed to have significant performance [10].

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Many properties of tires were determined by the dispersion of filler within rubber matrix prior to building and curing them, therefore the interaction between filler and rubber matrix was extremely significant. There is the view point that the epoxy rings of ENR are reactive to the silanols, so the dispersion of silica in ENR is extremely good. There are a lot of papers to research the interaction between polymer and filler while that of ENR and silica are not so much. Related to the interaction between silica and ENR reports, only S. Varughese et al [11] and Haiyan Xu et al [12] had studied the reaction between them. ENR and silica blends were treated by heating during mixing in both reports. In this article, the interaction between ENR and silica were studied without heating during mixing process. It is expected silica reacted to ENR, which could increase the mechanical properties and improve the dispersion of silica.

2 Experimental

2.1 Materials

The ENR used in this study was ENR-40, having 40 mol/% of the double bonds epoxidation, which was prepared by Agricultural Products Processing Research Institute of Chinese Academy of Tropical Agricultural Sciences. Fumed-silica with particle size 0.007μm was provided by Sigma Co., America. Other additives used were industrial grades.

The formulation for the compound samples: ENR 100, Zink oxide 4, Stearic acid 2, accelerator CZ 1.5, accelerator DM 0.5, Sulfur 1.5, variable content of silica.

2.2 Preparation of ENR/silica composites

The compound of ENR and fumed-silica with 0phr, 5phr, 10phr, 20phr and 30phr were prepared in an open two-roll mixing mill at 50~60℃ for 12 minutes. Optimum cure times were obtained from an Alpha Moving Die Rheometer at 150 ℃. Then the vulcanizates were compression cured at 150℃ for the optimum time. So the films of 2 mm were obtained.

3 Characterizations

Mechanical properties. The tensile strength data were tested according to (Chinese standard GB/T528-1998) ASTM D 412 with an Instron3365 machine using C-type dumbbell samples.

Kraus curves.For Kraus equation testing, the volume fraction in the swollen vulcanizates was calculated at first. The volume fraction of swollen ENR/Silica vulcanizates with different amount silica was tested by equilibrium swelling method. Samples with small pieces of the size 1cm*1cm*0.2cm were weighed to the precision of 0.0001g before swelled. Then the samples were immersed in toluene at room temperature for 96h to get to swelling balance. The swollen samples were taken out, excess liquid on the surface was wiped off, and the samples were weighed immediately. The volume fraction in the swollen vulcanizates was calculated by the equation [13] as follows:

. (1)

In Eq.1, ma is the weight before swelled; mb is the weight after swelled; ρr is the density of ENR; ρs is the density of toluene; is the weight fraction of ENR in ENR/silica vulcanizates.

Kraus equation is used to evaluate the interaction between filler and rubber based on the theory swelling is completely restricted by the adhesion in filler-rubber. The following relation is obtained [14]:

. (2)

. (3)

In Eq.2, Vr is the volume fraction of ENR in filled swollen vulcanizate. Vro is the volume fraction of ENR in unfilled swollen vulcanizates. φ is the volume fraction of filler, and c is a parameter depending on the filler. m described how much swelling is restricted for a given volume fraction of filler.

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FT-IR. Reaction between fumed-silica and ENR was characterized by FTIR in the transmission mode and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Silica was performed by transmission mode while blends by attenuated total reflectance mode. Uncured ENR/silica (weight ration 100/20) packed by filter paper was immersed in toluene for 120h to dissolve the ENR unreacted to silica, then it was dried in vacuum oven to constant weight. The uncured ENR/silica blends and filtered ENR/silica blends were characterized by FTIR.

TGA. Stabilization properties of ENR/silica and pure ENR were performed on a Netzsch STA449C/4/G in nitrogen atmosphere from room temperature to 700 ℃ at a heating rate of 10 ℃/ min. The sample only contained ENR and silica was mixed on the open two-roll mixing mill. The weight ratio of ENR to silica was 100 to 20. The samples' weights were about 5mg.

DMA. Dynamic mechanical properties of pure ENR and ENR/silica vulcanizates were performed on a German Netzsch DMA242. The test was carried on tension condition; temperature range from -120 ℃ to 100 ℃ at a heating rate of 5 ℃/ min; and at frequency of 3.33 HZ.

SEM. The dispersion of silica in ENR/silica and NR/silica blends was analyzed on a scanning electron microscope (SEM), model Hitachi S-800. The samples were fractured after immersed in liquid nitrogen for a while, and the fracture surfaces were obtained to test.

4 Results and Discussion

4.1 Influence of silica content on cure parameters and mechanical properties of ENR/silica composites

Table 1 showed the values of scorch time, optimum cure time, minimum torque and maximum torque of ENR/Silica composites. As showed in Table 1, with increasing amount of silica, both the optimum cure time and scorch time of ENR/silica composites increased. The scorch time and optimum cure time had some relationship with the fillers properties, for example, particle size, specific surface area, moisture content [15]. That was definitely true that while silica had small particle size, high surface area and low moisture content, then longer scorch time and optimum cure time obtained. Besides, that was also attributed to the reaction of silica and accelerator. Because silica with high surface area could adsorb accelerator, which reduced the accelerator reactivity, thus retarded the cure process.

Table 1 Cure Parameters of ENR/silica Composite with Different Silica Content

Silica content [phr]

Scorch Time

ts1

[min]

Optimum Cure Timet90

[min]

Minimum Torque

[dN·m]

Maximum Torque

[dN·m]

Max Torque -Min Torque

[dN·m]

0

4.2

7.1

0.25

6.61

6.36

5

7.0

9.5

0.45

7.16

6.71

10

9.3

12.5

0.60

7.77

7.17

20

10.5

15.3

0.69

8.63

7.94

30

11.4

17.5

1.10

10.88

9.78

The maximum torque was increasing with the silica amount increasing, which indicated that the crosslink densities were enhanced [16]. That was attributed to the interaction between silica and ENR, which made macromolecular chains of ENR matrix controlled by the increasing silica amount. The macromolecular chains of pure gum could move freely, when silica added, this situation was broken. The filler would be filled into the free space of the macromolecular, thus the free space was induced and the macromolecular would need bigger force to move, so the maximum torque increased.

Fig.1 Modulus at 100% and 300% of ENR/silica composites with different silica content

The mechanical properties were performed and the results were displayed. In Fig. 1, the modulus of 100% and 300% was enhanced with the silica amount increasing, which was attributed to the non-deformation properties of silica. There was also something with the crosslink density. Fig.2 expressed that the tensile strength of the composites increased first with the silica increasing and arrived to maximum at 10phr, then decreased with the silica more than 10phr. The elongation showed the same trend except the pure gum. Compared with the pure gum, the elongation of the composites filled with silica was smaller. That was attributed to the strong interaction between silica and ENR.

Fig.2 Elongation at break and tensile strength of ENR/silica composites with different silica content

4.2 FTIR

Fig.3 shows the FTIR spectrum of silica, ENR40 and ENR40/silica blend. As displayed in Fig.3, the strong absorption peak of silica at 1099 cm-1 was attributed to the asymmetrical stretching vibration of Si-O-Si. The broad peak at 3428 cm-1 was attributed to -OH of the silica's surface. Other peaks at 950 cm-1 and 807 cm-1 were assigned to the stretching and the deformation of Si-O respectively. ENR/silica blend's spectrum not only appeared the characteristic peaks of silica, also the peaks of ENR, such as the stretching vibration of methyl and methylene at 2961 cm-1, 2926 cm-1, and 2856 cm-1, the bending stretching vibration of methyl and methylene at 1450 cm-1 and 1377 cm-1. Compared to silica, the asymmetrical stretching vibration of Si-O-Si and the hydroxyl of silica were moved to 1088 cm-1 and 3288 cm-1 respectively in ENR/silica blend; while the characteristic epoxy group of pure ENR at 830 cm-1 was moved to 834 cm-1 in ENR/silica. That indicated there was reaction between ENR and silica, and the reaction equation was showed in scheme 1, which conformed the assumption of Manna et al [17] during ENR and silica mixing process.

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Scheme 1 Reaction between ENR and silica

Fig.4 displayed the FTIR spectrum of filtered ENR/silica blends. After filtered, the ENR/silica blends still had the ENR's characteristic peaks obviously. That revealed that ENR had intensely reaction on silica, so the solvent can't dissolve the reacted ENR on silica.

Fig.3 FTIR spectrum of silica, ENR-40, and ENR-40/silica blend

Fig.4 FTIR spectrum of filtered ENR-40/silica

4.3 Kraus Equation

There were reports [18] that used Kraus equation to analyze the interaction between filler and rubber. The interaction of silica and ENR determined by Kraus equation was showed in Eq.2. Vr and Vro calculated according to Eq.1 were showed in Table 2. Then the Kraus curve was obtained with the value of Vr. As showed in Fig.5, the plot Vro/Vr versus φ/(1-φ) describes how much swelling was restricted for a given volume fraction of filler. Vro calculated according to Flory-Rehner theory, was described the interaction between polymer and solvent.

Table 2 Volume fraction of ENR/silica with different content of silica

Silica content /phr

0

5

10

20

30

Vr

0.183

0.189

0.190

0.205

0.225

As pointed by Kraus[14], if the filler swells just as much as the rubber matrix, the vulcanizates will be uniform and the value of Vro/Vr will be equal to the gum. The ratio of Vro/Vr was independent of the amount of filler. Nevertheless, most filler couldn't be swelled. So if the polymer chains were attached to the filler surface and the movements of the matrix were restricted, the rubber swelling less than pure gum, the ratio Vro/Vr will decrease as the amount of filler increase. Obviously, if the filler had no effect on the matrix, the ratio increased with increasing amount of filler. Taking the pure gum ratio Vro/Vr =1 as standard, if Vro/Vr >1, filler and rubber doesn't have reaction, on the other hand Vro/Vr <1, there is reaction between filler and rubber. As showed in Fig.5, the ratio Vro/Vr decreased with the volume fraction of filler increasing and Vro/Vr <1. That indicated that polymer chains were attached to the filler, less swelling restricted in ENR, silica had effective interaction on ENR.

Fig.5 Kraus curve of ENR/silica with different silica content

4.4 SEM

The morphology of the ENR/silica (weight ratio 100/20) and NR/silica (weight ratio 100/20) were characterized by SEM. The morphology displayed the dispersion of silica, in which the interface of NR/silica was clear while ENR/silica was ambiguous. Compared with NR/silica, the dispersion of silica was more uniform in ENR/silica. As showed in Fig.6 c), there were silica aggregations rising high, which indicated the non-uniform dispersion. The difference between ENR/silica and NR/silica was because the ENR had the polar agent which made it have better consistency with silica. Besides, it was also attributed to the effective interaction between silica and ENR.

a) NR/silica b) ENR/silica

c) NR/silica d) ENR/silica

Fig.6 SEM of ENR/silica and NR/silica

4.5 TGA

The Thermal stabilization was also a standard to evaluate the interaction between filler and rubber matrix [19]. It was said that higher thermal stability was not only attributed to the formation of coupling bond, but also due to the crosslink density of the composites because the macromolecular chains were restricted by filler. The temperature at 5% weight loss was used as the decomposition onset temperature. The decomposition onset temperature of ENR was 361.1 ℃, while ENR/silica was 363.3 ℃, which was a little higher than pure ENR. There was evident that the degradation temperature of ENR-grafted silica was lower than that of pure ENR because of the oxirane ring opening during grafting reaction. However, the test in this article the degradation temperature of ENR/silica was a little higher than pure ENR, which might be attributed to the effective interaction between ENR and silica.

Fig.7 TGA of ENR and ENR/silica

4.6 Dynamic Mechanical Analysis

It is known that polymers filled with filler have a lot of changes not only in mechanical properties, such as tensile strength and modulus, but also in dynamic mechanical properties, such as loss factor and dynamic modulus [20]. DMA is an important method to characterize the interaction between filler and rubber [21].

Fig.8 showed the storage modulus (E') dependence on temperature for ENR and ENR/silica vulcanizates. It was obvious that the storage modulus of ENR/silica vulcanizate was higher than pure ENR vulcanizte. That was because the addition of filler could enhance the stiffness of ENR. From Fig.9 we can see that the glass transition temperature of ENR/silica was moved to the higher temperature. That was due to the effective interaction between ENR and silica. The macromolecular chains were reacted to the silica which reduced the mobility. Therefore, the macromolecular chains in ENR/silica would need higher temperature to get the same mobility compared to pure ENR.

Fig.8 E' of ENR and ENR/silica vulcaniztes with temperature

Fig.9 tanδ of ENR and ENR/silica vulcaniztes with temperature

5 Conclusions

The main objective of this study was to verify the chemical interaction between ENR and silica. FTIR, Kraus curves, SEM, TGA and DMA resulting showed that there was definitely chemical interaction between ENR and silica. Kraus curves revealed there was effective interaction between ENR and silica; FTIR confirmed that the interaction was chemical reaction for the characteristic peaks' moving. Furthermore, TGA and DMA verified the reaction because the macromolecular chains were restricted by silica. SEM displayed that silica dispersed better in ENR than in NR, which also attributed to the better interaction between ENR and silica.