Influence of bis(triethoxysilylpropyl) tetrasulfide amount on the properties of silica-filled epoxidized natural rubber-based composites

2.1 Materials

The investigations were performed using the following materials:

ENR (Epoxyprene 50) containing 50±2% epoxy groups distributed randomly in the polymer chain was used. Epoxyprene 50 was purchased from San-Thap International Co., Ltd. (Bangkok, Thailand). Its characteristics are as follows: glass transition temperature (T_g), -24°C; density, 1.02 g/cm³; Mooney viscosity ML(1+4)100°C, ∼80–100 MU.

Silica, Ultrasil 7000 GR from Evonik Industries (Rheinfelden, Germany), had the following characteristics: specific surface area (N₂), 175 m²/g; specific surface area (CTAB), 160 m²/g; loss on drying, 5.2%; pour density, 270 g/l; and electrical conductivity, ≤1300 μS/cm.

The other ingredients such as zinc oxide (ZnO), stearic acid, TESPT (Si 69), N-tert-butyl-2-benzothiazolesulfenamide (TBBS), and sulfur were also of commercial grades.

2.2 Measurements

The vulcanization characteristics of the studied rubber composites were determined at 150°C on a Moving Die Rheometer (MDR 2000; Alpha Technologies, Heilbronn, Germany) according to ISO 3417:2008. The mechanical properties of the composites studied were determined according to ISO 37:2011. The Shore A hardness of the composites studied were determined according to ISO 7619-1:2010, whereas the wear resistance was determined according to ISO 4649:2010.

The complex dynamic modulus and the heat build-up were determined on a Goodrich flexometer (Moscow, Russia) at an 850 min^-1 deformation rate. The dynamic properties [dynamic storage modulus (E′) and mechanical loss angle tangent (tan δ)] of the studied composites were investigated using a Dynamic Mechanical Thermal Analyzer Mk III system (Rheometric Scientific, Epsom, Surrey, UK). The data were obtained at a frequency of 5 Hz and strain amplitude of 64 μm in the temperature range from -80°C to 80°C, using a heating rate of 3°C/min under a single cantilever bending mode. The dimensions of the investigated samples were as follows: width 10 mm, length 25 mm, and the thickness measured with a micrometer varied between 1 and 2 mm.

2.3 Preparation of rubber composites

The formulations of the compounds based on ENR (Epoxyprene 50) are presented in Table 1.

The rubber compounds were prepared in two stages according to the mixing schedule presented in Table 2. At the first stage, the mixing was performed on a Brabender Plasti-Corder PLE651 (Brabender® GmbH & Co. KG, Duisburg, Germany) fitted with a 300-cm³ cam-type mixer. The silane coupling agent was mixed with the filler before placing the mixture into the mixer chamber. At the second stage, sulfur and the accelerator were added to the mixture compounded on an open two-roll laboratory mill L/D 320×160 and friction 1.27.

The vulcanization process of the ENR-based compounds was carried out in an electrically heated hydraulic press using a special homemade mold at a temperature of 150°C and under a pressure of 10 MPa.

3.1 Curing properties

Table 3 summarizes the main curing properties of the studied rubber compounds that were determined from their cure curves (Figure 1).

Figure 1

Cure curves of the investigated rubber compounds taken at 150°C.

It is known that the minimum torque, determined using a Moving Die Rheometer, gives an idea of the viscosities of the investigated rubber compounds. It can be seen from Table 3 that the value of the minimum torque (ML) of the compound containing no silane coupling agent (S0) is about 12 dN.m. The minimum torque of compounds containing from 1.0 to 3.0 phr TESPT (S1, S2, and S3) does not change with the increase in the amount of the silane coupling agent, and its value is about 15.0 dN.m. The increase in the amount of the silane coupling agent in the compounds, from 4.0 to 7.0 phr TESPT (S4, S5, S6, and S7), leads to a reduction of the minimum torque, respective to the viscosities of the rubber compounds studied. It is obvious that the compound containing the greatest amount of TESPT (S7) has the lowest minimum torque (ML) value. Although the viscosity of the rubber compounds is lower in the presence of greater amounts of the silane coupling agent (S4, S5, S6, and S7), it is obvious that it is commensurate with that of the compound containing no TESPT (S0). This is an indirect evidence that a reaction is taking place between the epoxy group of ENR and the silanol group of silica, which, in turn, improves the dispersion of the filler particles in the rubber matrix, even in the presence of lower amounts of organosilane.

It is considered that the difference between the maximum and minimum torque (ΔM) gives an idea of the cross-link density. It can be seen from Table 3 that the ΔM value of the compound containing no TESPT (S0) is the lowest one – about 35.0 dN.m. The ΔM values of compounds containing from 1.0 to 5.0 phr TESPT (S1–S5) do not change substantially with the increase of the silane coupling agent amount and vary in the range from 39.5 to 42.0 dN.m. In this case, the values of ΔM are about 20% higher than that of the compound containing no TESPT (S0).

In terms of the scorch time (T_s2), a clearly expressed trend is observed – T_s2 decreases with increasing amount of silane coupling agent in the compounds. Therefore, the T_s2 of the TESPT-free compound (S0) is around 10 min, and that of the compound containing the greatest amount of TESPT (S7) is around 6 min 30 s. This could be explained with the assumption that only part of the silanol groups of silica interacts with the epoxy groups of ENR, and the remaining free silanol and epoxy groups interact with the functional groups of the accelerator, causing a prolonged scorch time [8].

From Table 3, it can be seen that the cure time (T₉₀) of the rubber compounds does not change substantially according to the absence or presence of various amounts of TESPT and is in the order of 20–23 min.

3.2 Mechanical properties

The mechanical properties of the investigated ENR composites are summarized in Table 4.

It can be seen from Table 4 that the vulcanizates of the mixture containing no TESPT (S0) have the lowest value of modulus 100 (M₁₀₀) – about 7.8 MPa. The M₁₀₀ values of the vulcanizates based on mixtures containing from 1.0 to 5.0 phr TESPT (S1–S5) increased with increasing amount of the silane coupling agent. In this case, the values of M₁₀₀ are around 20–60% higher than that of the vulcanizates containing no organosilane (S0). An insignificant reduction in the values is observed at a concentration of >5.0 phr TESPT (S6, S7). It is apparent from Table 4 that modulus 300 is not reached for all studied vulcanizates.

In terms of tensile strength (σ), vulcanizates of all the tested rubber compounds have commensurate values, and no trend is observed in σ alteration of the vulcanizates with the increase of the amount of silane coupling agent in the composition of their compounds.

The vulcanizates of the mixtures containing no TESPT (S0) have the highest value of elongation at break (ε₁) – 225%. The vulcanizates of mixtures containing different amounts of TESPT have lower values of ε₁, varying from 150% to 190%, without a clearly expressed trend of ε₁ alteration with the increasing amount of organosilanes in the mixtures.

It can be seen from Table 4 that the residual elongation (ε₂) of the vulcanizates of the investigated rubber compounds is not affected by the amount of the silane coupling agent, and its value is the same for all vulcanizates (5%). The Shore A hardness of all vulcanizates varies within the range of 86–88 relative units.

The abrasion values of vulcanizates of rubber compounds studied vary in the range between 130 and 160 mm³, not showing a strict trend of dependence on the amount of the silane coupling agent in the mixtures. However, it could be noted that the vulcanizates of rubber compounds containing 5.0 and 6.0 phr TESPT (S5 and S6) have the lowest values of abrasion.

The data about the mechanical properties of the vulcanizates of rubber compounds studied indicate that the best complex of properties (modulus 100, tensile strength, elongation at break, and abrasion) is observed in composites containing 5.0 phr TESPT (S5). The data obtained also indicate that although the interaction between the epoxy groups of ENR and the silanol groups of silica through the hydrogen bonds improves the dispersion of filler in the rubber matrix, to obtain vulcanizates with good mechanical properties, the presence of some amount of silane coupling agent is necessary.

3.3 Dynamic mechanical properties

The filler dispersion in the rubber mixtures has an important effect on the dynamic properties of the vulcanizates and particularly on the heat build-up that corresponds to the rolling resistance of tires. Figure 2 presents the complex dynamic modulus (E*) of the composites studied as a function of the dynamic deformation determined on a Goodrich flexometer.

Figure 2

Dynamic mechanical properties: complex dynamic modulus (E*) of the studied composites as a function of dynamic deformation.

Figure 2 shows that the vulcanizates of the rubber compound containing no TESPT (S0) have the lowest values of the complex dynamic modulus (E*) for all dynamic deformations studied. The E* values of vulcanizates of compounds from S1 to S7, containing various amounts of the silane coupling agent (TESPT), are higher by 5–15% than the E* values of the vulcanizates of the mixture without TESPT (S0). However, no clearly expressed trend is observed in the alteration of E* values of the vulcanizates of mixtures S1 to S7 with the increase of organosilane amount. It is apparent from Figure 2 that the decrease in the complex dynamic modulus (E*) of the vulcanizates of the rubber mixture without TESPT (S0), with the increase of dynamic deformation (Payne effect), is higher than that of the vulcanizates containing the silane coupling agent. The filler-filler interaction in the absence of the silane coupling agent is stronger. At lower values of the dynamic deformation (up to 4%), the filler-filler structure is preserved and the E* values remain relatively high. However, with increasing dynamic deformation, the filler-filler structure collapses and the E* values decrease drastically. The phenomenon is accompanied by a higher heat build-up.

Figure 3 presents the dependence of the heat build-up on the dynamic deformation for the vulcanizates based on the studied compounds, as determined by using a Goodrich flexometer. It can be seen from Figure 3 that the heat build-up data of the investigated vulcanizates are in complete compliance with their complex dynamic modulus. The heat build-up increases with the increase of dynamic deformation. The values of the heat build-up of the vulcanizates of compounds S1 to S7, containing TESPT in different amounts, are commensurate with each other throughout the tested interval. Upon dynamic deformations up to 16%, the heat build-up value of the vulcanizates from the mixture containing no TESPT (S0) is commensurate with those of the vulcanizates containing the silane coupling agent. However, it is obvious that upon dynamic deformations >16%, the heat build-up of the vulcanizates of the mixture containing no TESPT (S0) is greater than that of the vulcanizates from compounds containing organosilane. Thus, upon the highest tested deformation (20%), the value of heat build-up for the vulcanizate of the mixture containing no TESPT (S0) is about 30% higher than those of the vulcanizates from all compounds containing the silane coupling agent.

Figure 3

Dynamic mechanical properties: heat build-up of the studied composites as a function of dynamic deformation.

The properties measured by dynamic mechanical thermal analysis (DMTA) were the dynamic storage modulus (E′) and the mechanical loss angle tangent (tan δ). Figure 4 presents the temperature dependence of the storage modulus (E′) of the composites based on the studied ENR composites. Figure 4 shows that all vulcanizates studied are in a glassy state in the temperature range from -80°C to 0°C. In this temperature range, there are no essential differences observed in the values of dynamic storage modulus (E′) according to the quantity of silane coupling agent used in the mixtures, as well as to the temperature change. The decrease of storage modulus with temperature increase, i.e., the transition from a glassy to a rubbery state, occurs at a temperature of about -5°C. In the temperature range from 0°C to 80°C, when the tested samples are in a rubbery state, the values of the storage modulus of vulcanizates of all rubber mixtures studied are completely commensurate and do not depend on the amount of the silane coupling agent in the compounds.

Figure 4

Dynamic mechanical properties: dependence of the storage modulus (E′) on the temperature of the investigated composites.

The mechanical loss angle tangent (tan δ), being the ratio between the dynamic loss modulus (E″) and dynamic storage modulus (E′) (tan δ=E″/E′), illustrates the macromolecules’ mobility as well as the phase transitions in the polymers. Noteworthy is the temperature dependence of tan δ, as tan δ at 60°C is considered to correspond to the tire rolling resistance, whereas tan δ at 0°C corresponds to the wet grip. Figure 5 presents the dependence of the mechanical loss angle tangent (tan δ) on the temperature of the investigated composites.

Figure 5

Dynamic mechanical properties: dependence of the mechanical loss angle tangent (tan δ) on the temperature of the investigated composites.

Figure 5 shows that in the whole temperature range, the values of tan δ of the vulcanizates of all rubber compounds tested are entirely commensurate between themselves in spite of the differences in the silane coupling agent amount in their composition. It is known that the tan δ peak corresponds to the glass transition temperature of the investigated vulcanizates. It can be seen from Figure 5 that the glass transition temperature (T_g) of all vulcanizates does not change essentially according to the amount of organosilane in the compounds, and is in the order of +5°C to +10°C.

The data obtained show that the best complex of dynamic properties [complex dynamic modulus (E*), heat build-up, dynamic storage modulus (E′), and tan δ] is observed with the composites containing 5 phr TESPT. The results obtained indicate that although the interaction between the epoxy groups of ENR and the silanol groups of silica through hydrogen bonds improves the filler dispersion in the rubber matrix, to obtain vulcanizates with good dynamic properties, the presence of some quantity of silane coupling agents is necessary.