epoxidized natural rubber blends: dynamic properties, curing characteristics and swelling studies

Polymer Testing 19 (2000) 879–888

Material Properties

Styrene butadiene rubber/epoxidized natural rubber blends: dynamic properties, curing characteristics and swelling studies H. Ismail *, S. Suzaimah School of Industrial Technology, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia Received 29 June 1999; accepted 11 August 1999

Abstract The dynamic properties, curing characteristics and swelling behaviour of styrene butadiene rubber (SBR) and epoxidized natural rubber (ENR) blends were studied. The incorporation of ENR 50 in the blends improved processability, stiffness, resilience and reduced the damping property. In terms of curing characteristics, the scorch time, t2 and curing time, t90 of the SBR/ENR blends decrease with increasing ENR content. At room temperature (23°C) and at 100°C the swelling degree of the SBR/ENR blends decreases with increasing ENR content.  2000 Elsevier Science Ltd. All rights reserved.

1. Introduction It has been a long-term practice of the rubber technologist to find polymer compounds of desired processing and vulcanisate properties, as well as high performance [1]. To achieve these objectives, rubber–rubber and rubber–plastic blends were studied by various researchers [2–5]. A blend can offer a set of properties that may give it the potential of entering application areas not possible with either of the polymers comprising the blend. Epoxidized natural rubber (ENR) is a rubber that has properties which resemble those of synthetic rubbers rather than natural rubber [6,7]. It can offer unique properties such as good oil resistance, low gas permeability, improved wet grip and rolling resistance, coupled with high strength. Many blends based on ENR and other polymers like PVC [8–10] and chloroprene rubber * Corresponding author. Tel.: +60-04-6577888; fax: +60-04-6573678. E-mail address: [email protected] (H. Ismail)

0142-9418/00/$ - see front matter.  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 9 ) 0 0 0 5 8 - 6

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[11,12] have been reported. Baker et al. [13] studied the usefulness of ENR/BR blends in tyre tread applications. They reported that this blend was the best practical solution for tyre treads, in spite of the cost and doubtful wear resistance of ENR. Our blend system was selected to make use of the excellent properties of ENR. In this study, the dynamic properties, curing characteristics and swelling behaviour of styrene butadiene rubber (SBR) and ENR were investigated. The effect of ENR composition on the dynamic properties at curing temperatures of 140, 150 and 160°C were studied. The effect of ENR composition on the degree of swelling of ENR/SBR blends at room temperature (23°C) and 100°C was also examined.

2. Experimental 2.1. Materials and formulations ENR 50 was supplied by the Rubber Research Institute of Malaysia (RRIM), SBR 1502 and other ingredients such as sulphur, zinc oxide, stearic acid, N-cyclohexyl-1-benzothiazyl sulfenamide (CBS), poly-1,2-dihydro-2,2,4-trimethyl quinoline (Flectol H) and process oil (Dutrex 729) were obtained from Bayer (M) Ltd, while carbon black, N330, was purchased from Malayan Carbon (M) Ltd. All materials were used as supplied and a semi-efficient vulcanization (semiEV) system was employed. The full recipes of the blends are shown in Table 1. 2.2. Mixing procedure Mixing was carried out in an internal mixer, Haake Rheomix, Model CTW 100. SBR and ENR were first premixed, prior to the addition of other ingredients. Mixing was carried out at a temperature of 60°C and at a rotor speed of 60 rpm. The total mixing cycle was 苲7 min. Finally, the carbon black filled blend was sheeted out on a two-roll mill.

Table 1 Formulations of carbon black filled SBR/ENR rubber blends Ingredients

phr

SBR 1502 ENR 50 Carbon black (N330) Process oil Zinc oxide Flectol H CBS Sulphur Stearic acid

100–0 0–100 40 5 5 1 1.1 1.6 2.0

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2.3. Measurement of curing characteristics and dynamic properties The MDR 2000 moving die rheometer (MDR), a rotorless curemeter, has gained much acceptance by the rubber industry since its introduction in 1988. In many cases, this equipment is replacing the oscillating disk rheometer (ODR) as described in ASTM Standard Test Method D 2084. The dynamic properties of the blends before, during and after cure were studied at 140, 150 and 160°C. A unique signal processing system and Fourier transform software separates the complex torque into elastic torque (S⬘) and viscous torque (S⬙). The tan d value is derived by dividing S⬙ by S⬘. In addition to the dynamic properties, the MDR gives digital outputs of curing characteristics such as scorch times, cure times, cure rates and torque values. 2.4. Measurement of swelling behaviour Determination of the swelling percentage of blends was carried out according to ASTM D 471. Cured test pieces of blends of dimension 30×5×2 mm were weighed using an electrical balance and this was considered to be the original weight [14] (Wi). The test pieces were immersed in ASTM#3 oil (which is similar to IRM 903) at room temperature and 100°C for 72 h and 26 h, respectively. The test pieces were then removed from the oil, wiped with tissue paper to remove excess oil from the surface and weighed (Wt). The swelling percentage of the blends was then calculated as follows: Swelling percentage⫽

Wt−Wi ⫻100 Wi

The sample was then reimmersed in the oil and the process was repeated for 72 h at room temperature and 26 h at 100°C. The swollen weight of the blends was measured continuously at certain periods (t) in order to plot a graph. 3. Results and discussion The dynamic properties of an uncured blend from the MDR 2000 rheometer can be used to predict the blend’s process performance. ML (the minimum value for S⬘) is commonly considered as representative of the uncured blend’s elastic modulus [15]. Fig. 1 shows that at three different curing temperatures, the ML value of the SBR/ENR blend initially increases at the blend ratio of 75/25 but starts to decrease with further increases of ENR in the blend. This indicates that the incorporation of the ENR has improved the processability of the blend. It can be seen also that ML at 160°C⬍ML at 150°C⬍ML at 140°C. At the higher curing temperature, viz 160°C, the lowest ML value indicates the better processability at this temperature. Its lower viscosity caused easier flow of the blend as a result of additional thermal energy through higher kinetic energy and consequently increased mobility of the polymer chain. Curemeter MH (S⬘ maximum torque) generally correlates with durometer hardness and/or modulus. Fig. 2 shows that at three different temperatures, MH increases with increasing ENR composition in SBR/ENR blends. This figure shows that incorporation of ENR has increased the stiffness of the blend.

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Fig. 1. The effect of ENR 50 composition (% R) on ML for various SBR/ENR 50 blends at different curing temperatures.

Fig. 2. The relationship between MH and ENR 50 composition (% R) for various SBR/ENR 50 blends at different curing temperatures.

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Values for tan d are derived by dividing S⬙ by S⬘, where S⬙ (viscous torque) relates to the damping characteristics of a rubber compound and S⬘ is the elastic modulus. Fig. 3 shows the reduction of tan d at MH with increasing ENR composition in the blend at three different temperatures. The incorporation of ENR reduces the blend hysteresis and improves its resilience. This might be due to the ENR being a more resilient rubber with a lower glass transition temperature than SBR. The effect of the SBR/ENR blend ratio on S⬙ at MH is shown in Fig. 4. It can be seen that S⬙ at MH initially increases at a SBR/ENR blend ratio of 75:25 (for 160 and 140°C) and 50:50 (for 150°C), but starts to decrease as the ENR increases in the blend. As viscous torque (S⬙) relates to the damping characteristics or loss modulus, this result shows that ENR has a beneficial effect by reducing the damping characteristics of the blend, especially when it comprises more than 25% of the SBR/ENR blend. Fig. 5 shows the variation of scorch time, t2, of the SBR/ENR blend with blend ratio. It can be seen that t2 decreases with increasing ENR composition in the blend. Owing to the activation of an adjacent double bond by the epoxide group, the scorch time for ENR 50 is shorter than that of SBR. The reduction of t2 with increasing ENR composition in the blend is more significant, especially at lower curing temperature, i.e. 140°C. At 160°C, the dependence of t2 on the blend ratio is less due to enough thermal energy being available to overcome the activation energy for vulcanization. It can be seen in Fig. 5 that the observed scorch time of the blend deviates negatively from the calculated value based on the interpolation between the scorch time of the two component rubbers. The difference in scorch time between observed and calculated values is given by Z: Z⫽t2 (observed)⫺t2 (interpolated) A plot of Z versus the blend ratio of SBR/ENR for three curing temperatures is shown in Fig. 6. The negative deviation of scorch time, t2, from the interpolated value is attributed to the induction effect of ENR 50 on SBR molecules that causes an overall increase in the rate of crosslinking of the blend. According to Poh and Tan [16] this phenomenon might be due to more activated precursors to crosslinks being formed as a result of the activation of the double bond by the epoxide group. The availability of the activated precursor will accelerate vulcanization not only between ENR 50 molecules, but also induces faster crosslinking between ENR 50 and SBR molecules. It can be seen also from Fig. 6 that the deviation of t2 from the interpolated value is greatest at 140°C, compared to 150 and 160°C. This indicates that the induction effect of ENR 50 is more significant at a lower temperature of vulcanization. As the temperature increases, the Z value decreases as a result of the decreasing effect of the activated double bond in ENR 50; that is, enough thermal energy is available to overcome the activation energy of vulcanization and consequently the induction effect of ENR 50 becomes less important. Fig. 7 shows the variation of cure time, t90, of the SBR/ENR blend with blend ratio. As for scorch time, t2, the t90 value also decreases with increasing ENR 50 content in the blend. Again, at a similar blend ratio, the value for t90 is lowest at 160°C, followed by 150 and 140°C. The relationship between percentage swelling and time of SBR/ENR blend at room temperature (23°C) is shown in Fig. 8. It can be seen that the degree of swelling increases with increasing time. However, at a similar swelling time, the degree of swelling decreases with increasing ENR

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Fig. 3. The relationship between tan d at MH and ENR 50 composition (% R) for various SBR/ENR 50 blends at different curing temperatures.

Fig. 4. The relationship between S⬙ at MH and ENR 50 composition (% R) for various SBR/ENR 50 blends at different curing temperatures.

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Fig. 5. The relationship between scorch time, t2, and ENR 50 composition (% R) for various SBR/ENR 50 blends at different curing temperatures.

Fig. 6. Variation of the difference in scorch time between the observed value and interpolated value (Z) of the various SBR/ENR 50 blends at different curing temperatures.

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Fig. 7. The relationship between cure time, t90, and ENR 50 composition (% R) for various SBR/ENR 50 blends at different curing temperatures.

composition in the blend. As the natural rubber is epoxidized, its chemical and physical properties change according to the extent to which the mole % of modification is introduced. For instance, the glass transition temperature, Tg, is raised, room temperature resilience is reduced, the rubber becomes increasingly more oil resistant and impervious to gases, polymer viscosity is increased and the polymer becomes more polar as the degree of epoxidation is increased [17]. Gelling [18] reported that as the fraction of the epoxide group increases, the swelling resistance in oil increases and the oil resistance of ENR is comparable to some of the speciality synthetic elastomers such as NBR. Fig. 9 shows the effect of blend ratio on the degree of swelling at 100°C for SBR/ENR blends. It can be seen that the trend is similar to that at room temperature (23°C). Blends with larger ENR content exhibit better swelling resistance. However, the swelling equilibrium at 100°C was achieved faster. The temperature increment from room temperature to 100°C also resulted in the reduction of the time required to achieve equilibrium swelling, this being due to the fact that increasing the swelling temperature increases the mobility of the polymer chains [19]. Consequently, oil penetration becomes easier and swelling equilibrium at 100°C is faster. 4. Conclusion From this study, the following conclusions can be drawn: 1. At three different curing temperatures, ML (the minimum value for S⬘) and MH (S⬘ maximum

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Fig. 8. The relationship between percentage of swelling and time of SBR/ENR 50 blends (cured at 150°C) at room temperature.

Fig. 9. The relationship between percentage of swelling and time of SBR/ENR 50 blends (cured at 150°C) at a temperature of 100°C.

torque) indicate that the incorporation of ENR improves the processability and stiffness of the SBR/ENR blends. 2. The incorporation of ENR also reduces the SBR/ENR blend hysteresis and improves its resilience. 3. For curing characteristics, a negative deviation of scorch time and cure time from the interpolated value of the blends was observed. This observation was attributed to the induction effect of the ENR 50 in the SBR/ENR blends, producing more activated precursors to crosslinks, thus enhancing interphase crosslinking. 4. The crosslinking degree of SBR/ENR blends decreases with increasing ENR content. Increasing the temperature results in a reduction in the time required to reach an equilibrium amount of swelling.

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