PROCESSABILITY CHARACTERISTIC OF NATURAL RUBBER HYBRID COMPOSITES: CARBON BLACK-SILICA FILLER SYSTEM

Transcription

1 Prosiding Seminar Nasional Kulit, Karet dan Plastik Ke-5 ISSN : PROCESSABILITY CHARACTERISTIC OF NATURAL RUBBER HYBRID COMPOSITES: CARBON BLACK-SILICA FILLER SYSTEM Ike Setyorini, Indiah Ratna Dewi, Ihda Novia Indrajati* Center for Leather, Rubber and Plastiks * i_novia@kemenperin.go.id ABSTRACT Silica combined with carbon black was successfully applied in natural rubber hybrid composites. The processability of those composites were evaluated through assessment of Mooney viscosity, time to scorch, cure characteristic, reversion tendency and viscous to elastic (V/E) ratio. Types of silica employed in this research were local silica and Zeosil. Silica loading was 0, 5, 10, 15 and 20 phr, and carbon black was set to be fixed. SEM study was conducted to scan the morphology of both silica. The processability parameters had been evaluated using Mooney viscometer and Moving Die Rheometer (MDR). The MDR experiments were carried out in three different temperatures, i.e. 130, 150 and 170ºC. Local silica particles showed smaller size but with agglomeration. Introduction silica markedly increased the Mooney viscosity and minimum torque (ML). Types of silica affect the time to scorch, optimum cure time and maximum cure rate, and V/E ratio. All rubber samples showed reversion tendency at high temperature. Keywords: hybrid composite, carbon black, silica, natural rubber, viscoelasticity Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System 81

2 ISSN : Prosiding Seminar Nasional Kulit, Karet, dan Plastik Ke-5 INTRODUCTION Rubber products have been obtained through several processing steps, i.e. compounding, forming and vulcanization. Compounding is mixing of rubber with various additives. The choice of compounding ingredients as well as the technique greatly affect the next sequence processes, thus properties of rubber product. During processing, rubber compounds are subjected to large shears and heat that will lead to deformation. Therefore, the knowledge of processability characteristic becomes important in order to fulfill the product requirements. Several parameters can be assessed to evaluate the processability of a rubber namely viscosity, elasticity or viscous to elastic (V/E) ratio, time to scorch, cure rate, ultimate state of cure, reversion resistance, green strength, tackiness, etc. Fillers are incorporated into rubber for matrix reinforcement as well as cost reduction (Bindu & Thomas, 2013). Reinforcement is defined as enhancement of strength and strength related properties, abrasion resistance, hardness, modulus (Rattanasom et al., 2007) and wear resistance (Al-Hartomy et al., 2015). Degree of reinforcement given from a filler is not only determined by the of filler-rubber interaction and adhesion (Poikelispää et al., 2015) but also the filler structure network (interaction filler-filler) (Ma et al., 2013). Further, Ma et al.(2013) explains that the interaction between rubber-filler also influences the viscoelasticity and glass transition temperature. Carbon black and silica are the most important reinforcing filler in rubber industries (Sadeghi Ghari & Jalali-Arani, 2016). Carbon black reinforced rubbers exhibit high tensile properties, tear strength, modulus and abrasion resistance, while silica gives improvement in tear strength, abrasion resistance, aging resistance and adhesion properties (Rattanasom et al., 2007). Compare to carbon black, silica also provides lower rolling resistance and wet grip at equal wear resistance in tire application (Hosseini & Razzaghi-Kashani, 2014). The improvement of rubber properties provides by carbon black is obtained from a good interaction between rubber molecule and the surface of carbon black particle. Silica is known by its hydrophilic nature that will generate strong interaction between filler-filler by hydrogen bonding. This leads to poor filler dispersion in rubber matrix (Ahmed et al., 2013). Moreover, active groups existing on the surface particle of silica tends to decelerate the vulcanization rate by adsorbing basic accelerator then deactivating them (Hosseini et al., 2014). Many attempts have been done to overcome this disadvantage. Introduction of silane-based coupling agent and polyethylene glycol (PEG) coating have been applied effectively (Prukkaewkanjana & Amornsakchai, 2015). Based on these advantages and disadvantages, both fillers are often utilized in a combination or dual phase filler system (hybrid filler) to obtain balanced properties. Cabot Corporation has been introduced dual phase filler consisting of carbon black-silica (CSDPF) and became the most successful commercial hybrid filler. This system provides high filler-polymer interaction and low fillerfiller interaction. Thus, overall mechanical properties of the composite are improved (Yu et al., 2016). The carbon black/silica-filled rubber is not a new system. Many studies have been conducted recent years. Rattanasom et al. (2007) evaluates the reinforcement of natural rubber with silica/carbon black hybrid filler. Al-Hartomy et al.(2015) studied the influence of carbon black/silica ratio onto physical and mechanical properties of epoxidized natural rubber. Ahmed et al.(2013) characterizes the natural rubber hybrid composite based on marble sludge and marble sludge/rice husk derived silica. Those studies are focused on cure characteristic, mechanical and dynamical properties, and swelling characteristic. Silica can be obtained from many sources, i.e. mineral containing silica, rice husk ash, etc. Indonesia has potential sources of mineral containing silica. In this study, locally derived silica in combination with carbon black will be applied in natural rubber hybrid composites. The evaluation has been made in the perspective of its performance on the rubber processability characteristic. For comparison, the commercial silica is used within the same loading. The processability of rubber compounds will be characterized through their viscosity, time to scorch, cure characteristic, ratio viscous to elastic (V/E) qualities and tendency of reversion. 82 Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System

3 Prosiding Seminar Nasional Kulit, Karet dan Plastik Ke-5 ISSN : MATERIALS AND METHOD Materials Rubber Sheet Smoked (RSS) was supplied by local industry in Indonesia (PTPN). Local silica local were prepared by Balai Besar Keramik, Indonesia. The particle size is ranged from 0.01 to 0.2 μm, density 2.65 g/cm 3 and ph Silica commercial used in this research was Zeosil 175P (Rhodia Silica Qingdao) ph slurry Carbon blacks were grade N-330 and N-660 (Ex.OCI). Other compounding additives include polyethylene glycol (PEG 4000) Ex. Korea, ZnO were purchased from Bratachem, Indonesia. Antioxidant 2,2,4-Trimethyl-1,2-Dihydroquinoline (TMQ) Ex.Kemai, activator Aflux 42M, paraffinic oil was purchased from CV. Indrasari, Semarang, Indonesia, N-Cyclohexylbenzothiazole-2-Sulfenamide (CBS) Ex. Northeast and Sulfur SP-325 Ex.Miwon. Apparatus Two roll mill laboratory scale equipped with cooling water, mini SEM SEC, Moving Die Rheometer Gotech M-3000A and Mooney Viscometer MonTech MV Morphology of Silica The morphology of the silica types was examined by scanning electron microscopy with 500x magnification. Compound Preparation NR and the additives were compounded using two roll mill following composition as described at Table 1. After compounding, the rubber compounds were soaked into water for 5 min to terminate the vulcanization which might be proceeded. The compounds then stored in a conditioned room for 24 h before subsequent processes. Table 1. Compounds formulation (phr) Ingredients TK0 TKL1 TKL2 TKL3 TKL4 TKZ1 TKZ2 TKZ3 TKZ4 RSS ZnO Aflux 42 M N N Silica local Zeosil Paraffinic oil TMQ PPD CBS Sulfur PEG Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System 83

4 ISSN : Prosiding Seminar Nasional Kulit, Karet, dan Plastik Ke-5 Mooney Viscosity Mooney viscosity, ML(1+4), were measured using MonTech Mooney Viscometer MV 3000 at 100ºC. Large rotor was applied with the speed of 2 rpm (0.21 rad/sec). The temperature at upper and lower die were maintained at 100±0.03 ºC. Samples were prepared using sample cutter, then placed in lower die. The term ML (1+4) means 1-minute preheating and 4-minutes viscosity testing. The tests carried out in duplicate. Moving Die Rheometer Experiments Time to scorch (ts 2), optimum curing time (t 90), cure rates, viscous (S ) and elastic (S ) torques, and reversion tendency were assessed using Moving Die Rheometer Gotech M-3000A. The samples were tested at 130, 150 and 170ºC in a fixed angle and frequency of 3 deg and 100 cps, respectively. From rheometer recorded data, V/E ratio can be assessed through dividing the viscous torque (S ) with the elastic torque (S ). The raw data were extracted and analyzed using MS Excel. RESULTS AND DISCUSSION Silica Morphology The morphology of local silica and Zeosil are depicted on Figure 1. Local silica has lower particle size and the distribution is nearly narrow, but agglomerates are found (inside the circle). Zeosil (Figure 1b) even though has larger particle size, but there is no agglomeration occurs. Figure 1. Morphology of silica, a) local, b) Zeosil The agglomerates are assumed as a result from hydroxyl groups on the surface of silica particle (silanol groups) which is hydrophilic in nature. This will lead to a strong particle-particle interaction by hydrogen bonding (Ahmed et al., 2013), thus in turn forming an agglomerate. Agglomerates is known to bring adverse effect to either processing or final properties of the rubber composite. 84 Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System

5 Prosiding Seminar Nasional Kulit, Karet dan Plastik Ke-5 ISSN : Figure 2. Mooney torque plot vs time a) local silica, b) Zeosil Compound Viscosity Mooney viscosity has been proven as powerful tool in the rubber viscosity measurement for both raw and compound states. Figure 2 illustrate the typical Mooney torque versus time plot. This figure provides valuable information related to initial peak viscosity and Mooney viscosity in term ML(1+4). Initial viscosity is characterized by first peak of Mooney curve. From Figure 2 it is clearly seen that Introduction of silica (both local and Zeosil) into rubber matrix had markedly increase either the initial viscosity or Mooney viscosity. The viscosity rises almost doubled with introduction of 5 phr silica. Malac (2011) stated that initial viscosity is a function of green strength and can be a measurement of storage shelf life. Accordingly, combination of silica and carbon black had improved the composites green strength. It is needed for composites to have a high green strength because during processing they are subjected to a series of strain and heat that may cause deformation. Progressive loading of local silica had caused variation of the viscosity, both initial and ML(1+4). This is probably due to poor filler dispersion in rubber matrix caused by agglomeration, as mention earlier. Otherwise, Zeosil had given different response to local silica (Figure 2b). The ML(1+4) also exhibits similar trend to initial viscosity. Consideration should be taken to composites with higher viscosity respect to processing because it means slowly to flow. Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System 85

6 ISSN : Prosiding Seminar Nasional Kulit, Karet, dan Plastik Ke-5 Time to Scorch and Curing Characteristic Curing curve illustrates the rubber compound behavior during curing process. Figure 3 and Figure 4 depict the curing curve of CB/local silica-filled NR and CB/Zeosil-filled NR, respectively, on various temperatures. It can be figured out that these curves are divided into three regions, i.e. induction period (1), curing (2) and over curing (3). Induction period is characterized by the minimum elastic torque and time to scorch (ts 2). During this period, compounds should have a proper flow characteristic in order to fulfill the mold cavities within a short period before vulcanization takes place. Induction period dominates primarily by physical interaction. The chain entanglement as well as interaction between rubber molecule chains and the surface of filler particles, results in physical crosslinks. The minimum elastic torque (ML) represents the extent of this crosslink, when no vulcanization occurred. Thus it can be a measure of viscosity of uncured rubber (Ahmadi & Shojaei, 2013). Combination of carbon black and silica (local or Zeosil) provides higher physical crosslink as indicated by higher ML (see Table 2), but along with progressive loading of silica ML remains unchanged. Temperature decreases ML, but further increasing temperature do not give significant changes. It is probably caused by the mobility of the rubber molecule. At high temperature, molecules have sufficient energy that increase their mobility, so that resulted in a lower physical interaction between molecule chains and filler particles. Therefore, less physical crosslinks are made. Scorch time (ts 2) gives a sign of vulcanization onset. Rubber compounds are safe to be worked out below this time. Therefore, scorch time illustrates the processing safety. Figure 5a-b shows the scorch time of rubber samples as a function of silica loading and temperature. At low temperature (130ºC), combination of local silica and carbon black reduces scorch time. It tends to increase upon increasing silica loading. Nevertheless, at higher temperature scorch time seems to be independent to silica loading. Zeosil provides quite higher scorch time at low temperature and increasing loading of Zeosil had cause it to rise. At higher temperatures the trend is similar to local silica. With this regard, Zeosil provide better processing safety at low temperature, while at high temperature is comparable. 86 Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System

7 Prosiding Seminar Nasional Kulit, Karet dan Plastik Ke-5 ISSN : (c) Figure 3. Curing curve of CB/local silica filled NR at a) 130ºC, b) 150ºC, c) 170ºC (c) Figure 4. Curing curve of CB/Zeosil filled NR at a) 130ºC, b) 150ºC, c) 170ºC Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System 87

8 ISSN : Prosiding Seminar Nasional Kulit, Karet, dan Plastik Ke-5 Table 2. Rheometer torques of rubber samples Sample T (ºC) MH (kg.cm) ML (kg.cm) ΔM (kg.cm) S /S (MH) (kg.cm) S /S (ML) (kg.cm) TK TKL TKL TKL TKL TKZ TKZ TKZ TKZ TK TKL TKL TKL TKL TKZ TKZ TKZ TKZ TK TKL TKL TKL TKL TKZ TKZ TKZ TKZ Figure 5. Scorch time (ts2) as a function of silica loading a) local silica b) Zeosil 88 Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System

9 Prosiding Seminar Nasional Kulit, Karet dan Plastik Ke-5 ISSN : Curing phase, the second region of curing curve, is defined as the period where chemical crosslinks are formed. Accelerator reacts with the activator forming active accelerator complex, in which is subsequently reacted with molecular sulfur to produce active polysulfide species. This species is then reacted with the unsaturated sites of rubber molecule chains and is resulted in crosslink precursors. The crosslink precursors are then react to additional unsaturated site of rubber or another crosslink precursors to form polysulfide crosslink (Allahbakhsh et al., 2013; Wu et al., 2013). Optimum polysulfide crosslinks areoften characterized by optimum curing time, t 90. Figure 6a-b depicts the t 90 as function of silica loading for local and Zeosil, respectively. Zeosil exhibits typical long curing time of silica-filled rubber observed by higher optimum curing (t 90) at low temperature. The silanol (-SiOH) groups existed on the surface of silica particleis contributed to this slow cure. Silanols interact with amine groups present in the accelerator. Moreover, it also reacts with zinc ion from ZnO in the formulation, competes with the activating function of zinc ion(hewitt, 2007). Thus, the availability of these substituents are reduced and in turn reduces the curing efficiency. The introduction of polyethylene glycol (PEG) is believed to overcome this defect by forming protective layer surrounding silica s particle surface. But, its effect on Zeosil is less pronounced since t 90 increases upon progressive silica loading. The maximum crosslinks formed during this phase is represented by the maximum elastic torque (MH). In general, combination of carbon black/silica provides higher MH than control (carbon black only). MH tends to increase upon silica loading. The chemical crosslink form during curing phase is often assessed through the torques differences (MH-ML). Accordingly, the crosslink density is increase upon silica loading for both local silica and Zeosil. The temperature changes do not affect these two parameters significantly. It is interesting that combination of carbon black and silica (both types) provide higher crosslink density but with higher cure time. Consideration should be taken for application in industry according this finding. The last region of curing curve is the over cure phase. This phase characterizes the behavior of the crosslinks against prolong heating. Figure 3 and Figure 4evidently show that all rubber samples exhibit reversion phenomenon at higher temperature (150 and 170ºC). Prolong heating caused the crosslink to breakdown because of desulfurization reaction of polysulfidic bonds (Ahmadi & Shojaei, 2013). Thus, the crosslink densities will be reduced. Reversion is more severe with increasing of temperature. This is typical characteristic of natural rubber because of its high double bonds content. Figure 6. Optimum cure time (t90) as a function of silica loading Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System 89

10 ISSN : Prosiding Seminar Nasional Kulit, Karet, dan Plastik Ke-5 Figure 7. Cure rate maximum as a function of silica loading a) local silica, b) Zeosil Viscous to elastic (V/E) ratio Flow of a material is related to its viscosity. Viscosity of rubber compounds are rather complex because of its viscoelasticity. At one time, rubber compound possesses both viscous and elastic element. Those elements are often expressed as the viscous to elastic (V/E) ratio. Two compounds may have similar viscosity, but with very different V/E ratio. Figure 8and Figure 9 depicts the V/E ratio of the rubber samples as a function of silica loading. At low temperature (130ºC), viscous elements show their domination on carbon black-filled rubber at induction period. Incorporating silica into rubber matrix leads to decrease the viscous elements quality at the same period. Increasing silica loading tends to shift the V/E ratio approach to 1, means both elements are existed within nearly comparable. This confirms the higher ML of CB/silica-filled rubber. Higher physical crosslinks had cause elastic element to increase. When the temperature is raised, this domination is changed. Viscous elements have been found to increase. This will improve the rubber flow during processing. The length of dominating element is evidently affected by the silica loading. This greatly correlated to the scorch time. Extra care should be taken to treat the rubber compounds with higher viscosity in which viscous elements are dominating. One should consider this property in order to obtain the perfect final product shape and dimensional as desired. 90 Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System

11 Prosiding Seminar Nasional Kulit, Karet dan Plastik Ke-5 ISSN : (c) Figure 8. Plot S /S vs time of CB/local silica-filled NR a) 130ºC b) 150ºC c) 170ºC Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System 91

12 ISSN : Prosiding Seminar Nasional Kulit, Karet, dan Plastik Ke-5 (c) Figure 9. Plot S /S vs time of CB/Zeosil-filled NR a) 130ºC b) 150ºC c) 170ºC CONCLUSION Local silica and Zeosil (commercial) has been successfully applied in natural rubber combined with carbon black. The performance of both silicas are evaluated through assessment of their processability characteristic involving compound viscosity, time to scorch, cure characteristic and viscous to elastic (V/E) ratio. Morphology of local silica showed smaller particle size but agglomeration was found, while Zeosil exhibit larger size but without agglomeration. Introduction of both types of silica had markedly increased the initial and Mooney viscosity. Minimum torque (ML) increased with the addition of silica and almost independent with progressive silica loading. Local silica tended to reduce the scorch time, while Zeosil was in contrast. Progressive silica loading increased the scorch time. Local silica did not affect the optimum cure time (t 90), but Zeosil tended to increase along with increasing its loading. Maximum crosslink (MH) was higher in compound with carbon black and silica, and the crosslink density as well. All rubber samples exhibited reversion phenomenon at high temperature (150 and 170ºC). Rubber compounds containing silica were rather dominated by viscous element than that of elastic element. When temperature raised the domination was shifted. ACKNOWLEDGEMENT The authors gratefully acknowledged for the support from Balai Besar Keramik for local silica supply. REFERENCES Ahmadi, M., & Shojaei, A. (2013). Cure kinetic and network structure of NR/SBR composites reinforced by multiwalled carbon nanotube and carbon blacks. Thermochimica Acta, 566, Ahmed, K., Sirajuddin, S., & Zahid, N. (2013). Characteristics of natural rubber hybrid composites based on marble sludge / carbon black and marble sludge / rice husk derived silica. Journal of Industrial and Engineering Chemistry, 19(4), Al-Hartomy, O. A., Al-Ghamdi, A. A., Farha Al Said, S. A., Dishovsky, N., Mihaylov, M., & Ivanov, M. (2015). Influence of carbon black/silica ratio on the physical and mechanical properties of composites based on epoxidized natural rubber. Journal of Composite Materials, 50(3), Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System

13 Prosiding Seminar Nasional Kulit, Karet dan Plastik Ke-5 ISSN : Allahbakhsh, A., Mazinani, S., Kalaee, M. R., & Sharif, F. (2013). Cure kinetics and chemorheology of EPDM/graphene oxide nanocomposites. Thermochimica Acta, 563, Bindu, P., & Thomas, S. (2013). Viscoelastic behavior and reinforcement mechanism in rubber nanocomposites in the vicinity of spherical nanoparticles. Journal of Physical Chemistry B, 117(41), Hewitt, N. (2007). COMPOUNDING PRECIPITATED SILICA IN NITRILE. In Compounding Precipitated Silica in Elastomers (pp ). Norwich: Elsevier. Hosseini, S. M., & Razzaghi-Kashani, M. (2014). Vulcanization kinetics of nano-silica filled styrene butadiene rubber. Polymer (United Kingdom), 55(24), Ma, J.-H., Zhang, L.-Q., & Wu, Y.-P. (2013). Characterization of Filler-rubber Interaction, Filler Network Structure, and Their Effects on Viscoelasticity for Styrene-butadiene Rubber Filled with Different Fillers. Journal of Macromolecular Science, Part B, 52(January 2013), Malac, J. (2011). Mooney Viscosity, Mooney elasticity and processability of raw natural rubber. Journal of Material Science and Engineering with Advanced Technology, 3(1), Poikelispää, M., Das, A., Dierkes, W., & Vuorinen, J. (2015). The effect of coupling agents on silicate-based nanofillers/carbon black dual filler systems on the properties of a natural rubber/butadiene rubber compound. Journal of Elastomers and Plastics, 47(8). Prukkaewkanjana, K., & Amornsakchai, T. (2015). An anomalous reinforcement of ordinarily weak synthetic rubber. Journal of Polymer Research, 22(8), Rattanasom, N., Saowapark, T., & Deeprasertkul, C. (2007). Reinforcement of natural rubber with silica/carbon black hybrid filler. Polymer Testing, 26(3), Sadeghi Ghari, H., & Jalali-Arani, A. (2016). Nanocomposites based on natural rubber, organoclay and nano-calcium carbonate: Study on the structure, cure behavior, static and dynamic-mechanical properties. Applied Clay Science, 119, Wu, J., Xing, W., Huang, G., Li, H., Tang, M., Wu, S., & Liu, Y. (2013). Vulcanization kinetics of graphene/natural rubber nanocomposites. Polymer, 54(13), Yu, P., He, H., Jia, Y., Tian, S., Chen, J., Jia, D., & Luo, Y. (2016). A comprehensive study on lignin as a green alternative of silica in natural rubber composites. Polymer Testing, 54, Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System 93

14 ISSN : Prosiding Seminar Nasional Kulit, Karet, dan Plastik Ke-5 94 Processability Characteristic of Natural Rubber Hybrid Composites: Carbon Black-Silica Filler System