Effect of bis (3-triethoxy silylpropyl) tetrasulphide on the mechanical properties of flyash filled styrene butadiene rubber

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1 Journal of Scientific & Industrial Research Vol. 63, March 2004, pp Effect of bis (3-triethoxy silylpropyl) tetrasulphide on the mechanical properties of flyash filled styrene butadiene rubber Nabil A N Alkadasi, Bhimrao D Sarmade*, U R Kapadi and D G Hundiwale** Department of Polymer Chemistry, School of Chemical Sciences, North Maharashtra University, P O Box: 80, Jalgaon *Division of Polymer Chemistry, National Chemical Laboratory, Pune Received: 29 September 2003; accepted: 04 December 2003 Flyash does not contribute to properties of composites and hence there are severe limitations on its use as a filler. The paper reports determination and discussion on mechanical properties of SBR composites filled with flyash (treated and untreated). The flyash was treated with a silane coupling agent (TESPT) [Bis (3-triethoxysilylpropyl) ] tetrasulphide. The treatment resulted in enhancement of mechanical properties of composites of the rubber. In this work, properties of composites filled with treated and untreated flyash (separately) are compared. The properties under consideration were tensile strength, modulus at 100 and 400 per cent elongation, and hardness. Tensile strength was improved by 20 per cent while modulus at 100 per cent was improved by 30 per cent. Similarly Young s modulus also was improved by 22 per cent. Keywords: Styrene butadiene rubber, Bis (3-triethoxy-silyl) propyltetrasulphide, Flyash, Composites, Mechanical properties IPC: Int Cl. 7 : C 01 B 21/00 Introduction Silane coupling agents such as, mercaptosilane, tetrathiosilane, and azosilane usually increase the modulus of the composite. A silane-coupling agent contains functional groups that can react with the rubber and the surface of flyash resulting in increased reinforcing effects. The introduction of bis (3- triethoxysilylpropyl) tetrasulphide (TESPT), has widened the application of filler in reinforcement of rubbers. Rolling resistance and traction properties of tires have been shown to improve by the introduction of TESPT coupling agent 1. The chemical structure of TESPT is shown below. EtO OEt EtO EtO Si (CH 2 ) 3 S 4 (CH 2 ) 3 Si OEt OEt TESPT It has also been highlighted that the utilization of the surface hydroxy groups of filler provides some advantage for using coupling agent for tire industry 1,2. The present study aims at investigating the effect of TESPT on of stress -strain behavior of styrene butadiene (SBR) rubber, i.e. studying mechanical properties of flyash- filled SBR vulcanizes. Flyash-filled SBR vulcanizates prepared by the conventional method, i.e. roll milling and compression **Author for correspondence Alkadasinabil@Maktoob.com

2 288 J SCI IND RES VOL 63 MARCH 2004 Table 3 Characteristics of styrene - butadiene rubber Trade name Techlen SBR 1502 Manufacture Volatile matter Polymerization system Ash content Organic acid IPCL Baroda, India 0.2 per cent Cold Emulsion polymerization. 0.3 per cent 6.6 per cent Mooney viscosity 50 Specific gravity 0.94 molding, with and without the treatment of coupling agent are used for comparison, keeping the recipe same. Silane coupling agent (TESPT) was selected as it is used to improve the reinforcing capability of fillers with silanol group on their surface (e.g. silicas, silicates and clay), and also as an integral part of curing systems to improve cross-linking network properties. The main constituent of flyash is silica hence TESPT was selected. Since, silica particles tend to agglomerate due to hydrogen bonding of silanol groups, present on the surfaces of silica particles. This effect demands higher mixing time. Since the surfaces of the silica particles are acidic, vulcanization accelerators which are basic rendered inefficient which hampers curing process. The above said problem can be over using come silane-coupling agent like TESPT. Experimental Procedure Materials The coupling agent: Bis(3-triethoxy silyl propyl) tetrasulphide (TESPT) was imported from (DEGUSSA- HULS) West Germany. Physical characterization of coupling agents is given in Table 1. Flyash was procured from Thermal Power Station, Deepnagar, Bhusawal (India) (Table 2). Other chemicals used were manufactured by Bayer India Ltd. (Table 3). Particle Size Analysis Surface area is a major parameter in connection with filler-matrix interaction for reinforcing purposes. The finer the particle size the higher is the surface area and higher the reinforcement. The details regarding particle size distribution of the flyash used in the study are given in Figure 1. The data was used to find out the mean particle size, which was found to be 2 µm. The analysis is carried out on Shimadzu SALD-2001 instrument provided by Shimadzu (Asia Pacific) Pvt Ltd Singapore. Treatment on Flyash by Coupling Agent 1.0 per cent solution of coupling agent in ethyl alcohol was used for 100g filler (flyash) 6-12, i.e., 1 g of the coupling agent was used per 100 g of flyash. The filler (flyash) was mixed with the solution of coupling agent in ethanol with stirring to ensure uniform distribution of the coupling agent, mixing was continued for 30 min. The treated filler (flyash) was then dried at 100 o C in an oven for about 5 h to allow complete evaporation of ethanol. Preparation of Composites

3 ALKADASI et al.: MECHANICAL PROPERTIES OF FLYASH FILLED STYRENE BUTADIENE RUBBER 289 The compounding of the rubber was carried out on laboratory scale two roll mill.the rubber was first masticated for 5 min. Additives were added sequentially, as shown in Table 4. After the addition of all of the additives the compounding was continued for 30 min to get the additives mixed homogeneously. This compounded matter was then vulcanized in compression molding machine at 150 o C for 30 min in chrome plated mould having cavity dimensions cm. Scanning Electron Microscopy (SEM) SEM photographs were taken using Leica Cambridge (Stereoscan 440) Scanning Electron Microscope (Cambridge, UK). Polymer specimens were coated with gold (50 µm thick) in an automatic sputter coater (Polaron equipment Ltd., Scanning Electron Microscope Coating unit E 5000, UK). Acceleration Potential of 20 K V was employed during the present investigation. Photograph of representative areas of the sample were taken at different magnifications. Table 1 Physical characterization of coupling agents (Si-69) Chemical name Bis (3 triethoxy silyl) propyl tetrasulphide Typical purity 95 per cent Molecular weight Specific gravity 1.07 Refractive index Flash point Boiling point 91 o C 250 o C Density g/cc Viscosity 11-12C P ph 7-9 Table 2 Constituents of flyash Compounds Percentage Silica (SiO 2 ) Alumina (Al 2 O 3 ) Magnesium oxide (MgO) Potassium oxide (K 2 O) Calcium oxide (CaO) Sodium oxide (Na 2 O) 00.15

4 290 J SCI IND RES VOL 63 MARCH 2004 Figure 1 Graph of particle size distribution of flyash Component Table 4 Compounding recipe Proportion (phr) SBR 100 Stearic acid 2.0 Zinc oxide 3.0 Antioxidant * 1.0 Accelerator (1) ** 0.50 Accelerator (II) *** 0.50 Sulphur 1.50 Treated Filler Curing time Curing temp Variable 30 min C * Antioxidant: N (1,3-dimethyl butyl)-n-phenyl-p-phenylene diamine. ** Accelerator (I): Tetramethyl thiuram disulphide (TMTD) *** Accelerator (II): Zinc diethyl dithiocarbamate (ZDC) Measurement of Mechanical Properties Mechanical properties such as, tensile strength, modulus at 100 and 400 per cent elongation were determined by subjecting dumbbell shaped specimens (in conformation with ASTM D 412) to a universal testing machine (R & D Equipment, Mumbai,India). The sheets from which specimen were cut had been conditioned for 24 h prior to subjecting to universal testing machine at a 100 kg load cell and, at a crosshead speed of 50 cm / min. Hardness was measured on Durometer (Blue-steel, India) on shore A scale. Results and Discussion Comparison between treated and untreated flyash-filled SBR was done. The adhesion between untreated flyash and SBR was found to be very poor due to low surface activity of the filler towards the matrix. This

5 ALKADASI et al.: MECHANICAL PROPERTIES OF FLYASH FILLED STYRENE BUTADIENE RUBBER 291 resulted in weaker interaction, which reflected in poorer mechanical properties of the vulcanizate. On the other hand, treated flyash showed good interaction with SBR due to increased surface activity, resulting in better mechanical properties of the vulcanizes. Modulus at 100 and 400 per cent Elongations Figure 2 and 3 illustrate the dependence of modulus at 100 per cent and at 400 per cent elongation with volume fraction of flyash (treated and untreated) in SBR composites. It is seen that in both the cases modulus increases initially, attains a maximum value for particular value of concentration of fillers and then it decreases. The peak values of moduli at 100 per cent elongation lie at 1.0 and 0.75 MPa for treated and untreated flyash composites, while that for 400 per cent elongation lie at 1.7 and 1.19 MPa for treated and untreated flyash, respectively. The modulus at 400 per cent elongation of treated flyash is about 1.43 times larger than that of untreated flyash. The initial rate of increment in the property with increasing volume fraction of the filler was similar in both the cases. However, after volume 0.30 fraction, the rate of increment for composites filled with treated flyash was slightly higher.

6 292 J SCI IND RES VOL 63 MARCH 2004 Figure 2 Modulus at 100 per cent as a function of volume fraction of treated and untreated flyash SBR composites Figure 3 Modulus at 400 per cent as a function of volume fraction of treated and untreated flyash SBR composites Tensile Strength The dependence of the tensile strength on volume fraction of flyash is shown in Figure 4. It is seen that on increasing the volume fraction of (both treated and untreated) flyash the tensile strength increases up to a certain value since the filler has reinforcing ability. Both the treated, as well as untreated flyash filler showed this ability. After attaining the maximum (corresponding to 0.45 volume fraction) in case of treated flyash, the decline started. This decline is because of dewetting effect, which has resulted from inadequate matrix material to hold filler particle. The peak values of tensile strength of the composites correspond to 1.95 and 1.60 MPa for treated and

7 ALKADASI et al.: MECHANICAL PROPERTIES OF FLYASH FILLED STYRENE BUTADIENE RUBBER 293 untreated flyash, respectively. The tensile strength of treated flyash SBR composites is higher than that of untreated flyash-sbr composites. Thus, treatment improved the extent of reinforcement substantially. Hardness Dependence of the hardness on concentration of treated and untreated flyash in SBR is shown in Figure 5. It is seen that, hardness of the treated and untreated flyash-sbr composite linearly increases on increasing the concentrations of filler. Thus it is clear from Figure 5, that the treated flyash rubber composites have higher hardness values than the untreated flyash rubber composites. The increase in hardness of the treated flyash SBR composites may be attributed to the additional curing characteristics associated with the coupling agent 10. Young s Modulus Young s modulus as a function of volume fraction of filler for treated and untreated flyash filled SBR composites is represented in Figure 6. The peak value for treated flyash composites is obtained to be 2.15 MPa at 0.49 volume fraction and that for untreated the value is 1.70 MPa at 0.40 volume fraction, i.e. It shows 1.27 times higher Young s modulus than that of untreated flyash composites. Plate 1 SEM of untreated flyash -75µ Figure 4 Tensile strength as a function of the volume fraction of treated Plate and 2 untreated SEM of flyash treated flyash SBR composites -75 µ

8 294 J SCI IND RES VOL 63 MARCH 2004 SEM of Composites SEM photomicrographs of treated and untreated flyash filler are shown in Plate 1 and 2. It is evident from these photographs that treated flyash exhibits uniform, spherical shape and fine discrete particulate nature while untreated flyash shows tendency to form agglomerates. Thus TESPT influences the orientation of flyash particles and is responsible for higher strength. SEM of fractured surfaces of (treated and untreated) flyash filled vulcanizates are shown in Plate 3 and 4. The fractured surface of the composites filled with untreated flyash showed non-adhesive appearance and formation of agglomerates while that filled with treated flyash showed a very uniform distribution,regular, adhesive appearance of the fractured surface indicating further enhancement in polymer filler attachment. Figure 5 Hardness as a function of volume fraction of treated and untreated flyash SBR composites Figure 6 Young s modulus as a function of volume fraction of treated and untreated flyash SBR composites

9 ALKADASI et al.: MECHANICAL PROPERTIES OF FLYASH FILLED STYRENE BUTADIENE RUBBER 295 Plate 3 SEM of untreated flyash filled SBR at volume fraction (0.55) Plate 4 SEM of treated flyash filled SBR at volume fraction (0.55) Conclusion Higher values of moduli are obtained in the case of treated flyash composites. They show superior values of tensile strength as compared to untreated ones, indicating the active involvement of coupling agent in the composites. The hardness of treated flyash filled composite is slightly higher due to dual roll of TESPT, i.e., coupling agent as well as co-curing agent. Scanning electron microscopic (SEM) photomicrographs also show enhanced polymer-filler interaction in the presence of silanes. References 1 Hashim A S & Azahari B, Rubber Chem Technol, 71 (1997) Wolff, Rubber Chem Technol, 69 (1993) Plueddemann E P, Silane coupling agents (Plenum Presses, NewYork) Hundiwale D G, Kapadi U R, Desai M C & Sachin B, J Appl Polym Sci, 85 (2002) Alkadasi N A N, Hundiwale D G & Kapadi U R, J Appl Polym Sci, (accepted). 6 Pal P K & De S K, Rubber Chem Technol, 55 (1983) Debnath S, De S K & Khastgir D, J Appl Polym Sci, 37 (1989)

10 296 J SCI IND RES VOL 63 MARCH Chakravrty P P S N & De S K, J Appl Polym Sci, 28 (1983) Kadivec I, Zoran S & Marjetka S, Polym Int, 33 (4) (1994) Hozman H D & Ishak ZA M, J Appl Polym Sci, 69 (1998) Bajaj P & Jha R, Polym Eng Sci, 29 (8) (1989) Pickwell R J, Rubber Chem Technol, 56 (1982) **Author for correspondence Alkadasinabil@Maktoob.com