Use of Antioxidant-modified Precipitated Silica in Natural Rubber

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1 Use of Antioxidant-modified Precipitated Silica in Natural Rubber Use of Antioxidant-modified Precipitated Silica in Natural Rubber P.M. Sabura Begum* Department of Applied Chemistry, Cochin University of Science and Technology, Kerala , India Received: 7 June 2010, Accepted: 8 December 2010 SUMMARY Fillers play a dominant role in modifying the properties of the base polymer. Precipitated silica is a promising non-black filler for rubber vulcanisates. As silica is hydrophilic and rubber is hydrophobic, uniform dispersion of silica in rubber is difficult and gives rise to problems such as high initial viscosity. Silica has a number of hydroxyl groups, which result in filler filler particle agglomeration and reagglomeration. Incorporation of silica into rubber is quite difficult compared with the incorporation of carbon black. The main aim of this study was to reduce the hydrophilicity of silica by coating antioxidant onto its surface, and hence achieve easy incorporation of silica into rubber. The preparation of antioxidant-modified precipitated silica and its use as filler in rubber compounds were investigated. The curing and mechanical properties of the compounds were compared with those of conventional compounds containing unmodified silica and antioxidant in the same dosage on natural rubber. The cure and mechanical properties were found to be superior for modified silica composites compared with unmodified silica composites. Keywords: natural rubber; antioxidant; precipitated silica INTRODUCTION Carbon black and silica are the most widely used fillers for rubber reinforcement. Carbon black is a reinforcing filler for hydrocarbon rubbers [1]. As both are hydrophobic substances, mixing and reinforcement problems do not usually arise when these two are mixed. The limitation of using carbon * Corresponding author. address: pmsabura@cusat.ac.in Smithers Rapra Technology, 2011 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4,

2 P.M. Sabura Begum black is that the final products will be black. Precipitated silica, which is of mineral origin, is one of the most promising alternatives to carbon black as far as reinforcement is concerned. Besides an extremely reactive surface, it has a particle size as small as that of carbon black, and is used as reinforcing filler [2]. It can be used partly to replace carbon black in tyres in order to reduce tyre rolling resistance and thereby lower fuel consumption [3]. The presence of silanol groups on the surface of the silica causes the formation of a strong filler filler network via the hydrogen bonds, and this results in poor silica dispersion and distribution [4]. The hydroxyl groups on the surface of the silica control surface acidity. This intrinsic acidity can influence vulcanisation [5]. The higher moisture content increases the dispersion time of silica into the rubber. Absorbed water can decrease cure rate, tensile strength, bound rubber content [6], and also abrasion resistance [7]. Rubber articles under severe service conditions undergo different types of degradation such as ozonisation, oxidation, etc. Although ozone is present in the atmosphere at concentrations normally in the 0 7 pphm range [8], it can severely attack unsaturated rubber products under stress [9]. In recent years there has been a gradually increasing demand for antidegradants to give optimum protection to rubber goods. Derivatives of p-phenylenediamine (PPD) offer excellent protection to rubber vulcanisates as antioxidants, antiozonants, and antiflex cracking agents. p-phenylenediamine antidegradants function as primary antioxidants and are recognised as the most powerful class of chemical antiozonants, antiflex cracking agents and antioxidants. PPDs are extensively used in tyre belting and in moulded and extruded rubber products as antiozonants and antiflex cracking agents. PPDs are also used as polymer stabilisers. To overcome the difficulty of dispersing silica in the rubber matrix, and also to protect the rubber from deterioration owing to heat, light, oxygen, and ozone, antioxidant-modified silica is prepared. This study explains the modification of silica with antioxidant and its use as filler in natural rubber. The mechanical properties are measured and compared with those of vulcanisates containing equivalent amounts of antioxidant and neat silica. EXPERIMENTAL Materials The materials used were natural rubber conforming to ISNR 5 grade Mooney viscosity (M 1 + 4, 100 C), equal to 85 (obtained from the Rubber 216 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011

3 Use of Antioxidant-modified Precipitated Silica in Natural Rubber Research Institute of India, Kottayam, Kerala, India), conventional zinc oxide, stearic acid, precipitated silica, naphthenic oil, diethylene glycol (DEG), cyclohexylbenzothiazyl sulfenamide (CBS), tetramethylthiuram disulfide (TMTD), sulphur, and antioxidants IPPD [N-isopropyl-N - phenyl-p-phenylenediamine], 6PPD [N-(1,3-dimethylbuty1)-N -pheny1- p-phenylenediamine], and DPPD [N,N -diphenyl-p-phenylenediamine]. Preparation of Antioxidant-modified Precipitated Silica Antioxidant (1 phr) was mixed with precipitated silica (50 phr) in a torque rheometer (Brabender plasticorder) at 50 rpm far above the melting temperature of the antioxidant for 5 min. The antioxidants used in this study to modify silica were IPPD, 6PPD, and DPPD. Preparation of Composites Compounds were prepared as per the formulation given in Table 1. They were prepared by mill mixing on a laboratory-size (16 33 cm) two-roll mill at a friction ratio of 1:1.25 as per ASTM D (2001). After complete mixing of the ingredients, the stock was passed out at a fixed nip gap. The samples were kept overnight for maturation. Table 1. Formulation of composites Ingredients (phr) E-1 E-2 F-1 F-2 G-1 G-2 Natural rubber ZnO Stearic acid Antioxidant-modified 51 (IPPD) 51 (6PPD) 51 (DPPD) precipitated silica Precipitated silica IPPD 1 6PPD 1 DPPD 1 Naphthenic oil DEG CBS TMTD S Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4,

4 P.M. Sabura Begum TESTING Characterisation Surface Area The surface areas of the modified precipitated silica and unmodified precipitated silica were measured using the BET method. Surface area analysis was done using a Tristar 3000 BJH surface analyser (Micromeritics). Measurements were carried out under nitrogen adsorption at liquid nitrogen temperature. Cure Characteristics The cure characteristics of all mixes were determined using a rubber process analyser as per ASTM standard D Subsequently, the rubber compounds were vulcanised up to the optimum cure time at 150 C in an electrically heated hydraulic press. The mouldings were cooled quickly in water at the end of the curing cycle and stored in a cool dark place for 24 h prior to physical testing. The vulcanisates were tested for mechanical properties according to the relevant ASTM standards. Tensile Properties The tensile properties were measured using Shimadzu universal testing machine (model AG-1 50 kn) according to ASDM D 412. Samples were punched out from moulded sheets using a dumb-bell-shaped die. The crosshead speed was maintained at 500 mm/min. Tear Strength The tear strength was determined according to ASTM D-624 using angular specimens punched out from moulded sheets. The test speed was 500 mm/min. Hardness The Shore A hardness of the sample was determined using a Zwick 3114 hardness tester according to ASTM D Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011

5 Use of Antioxidant-modified Precipitated Silica in Natural Rubber Compression Set The compression set (%) was determined according to ASTM D (method B). The compression set was calculated using the following expression: Compression set (%) = t 0 t 1 t 0 t s 100 where t 0 is the initial thickness of the specimen, t 1 is the final thickness of the specimen, and t s is the thickness of the spacer bar. Abrasion Loss The abrasion resistance of the samples was tested using a DIN Cylindrical samples of 15 mm diameter and 20 mm length were kept on a rotating sample holder, and a 10 N load was applied. Initially, a prerun was given for the sample and its weight was taken. The sample was then given a complete run and the final weight was noted. The difference in weight was reported as the abrasion loss. It was expressed as the volume of the testpiece being abraded by its travel through 42 m on a standard abradant surface. The abrasion loss was calculated as follows: V = m ρ where V is the abrasion loss, m is the mass loss, and r is the density of the sample. Heat Build-up A Goodrich flexometer conforming to ASTM D was used for measuring heat build-up [10]. A cylindrical sample of 2.5 cm height and 1.9 cm diameter was used for the test. The oven temperature was maintained at 50 C. The sample preconditioned in the oven for 20 min was subjected to a flexing stroke of 4.45 mm under a load of 10.9 kg. The temperature rise (DT, C) at the end of 20 min was taken as the heat build-up. Flex Cracking Flex cracking was determined using a De Mattia flexing machine according to ASTM D [10]. Standard specimens measuring 15 cm 2.5 cm Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4,

6 P.M. Sabura Begum 0.6 cm with a semicircular groove moulded transversely in the centre of the strip were used. Samples fixed on the machine were subjected to flexing at a frequency of 300 cycles/min. The number of cycles required to produce different levels of cracking was noted. Swelling Studies Swelling studies of the composites were conducted in toluene to find their crosslink densities using the Flory Rehner equation [11]. RESULTS AND DISCUSSION Surface Area Studies Table 2 shows the surface area values of precipitated silica and modified silica. It is found that the surface area is smaller for modified silica compared with the unmodified precipitated silica. This shows that antioxidants are adsorbed onto the surface of silica under the physical force of attraction. Table 2. Surface area of silica Samples Surface area (m 2 /g) Neat precipitated silica 178 Modified silica 127 Cure Characteristics The cure characteristics of the NR compounds with an optimum concentration of 50 phr silica and 1 phr antioxidant are shown in Table 3. Table 3. Cure characteristics of the mixes Mix Minimum torque (dn m) Maximum torque (dn m) Scorch time (min) Optimum cure time (min) Cure rate index (%) E E F F G G Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011

7 Use of Antioxidant-modified Precipitated Silica in Natural Rubber From the table it can be seen that the cure values of compounds filled with antioxidant-modified silica exhibit a higher cure rate and extent of cure than those of compounds with neat precipitated silica. Chemical surface groups on fillers play an important role in their effect on rate of cure with many vulcanised systems. The physical adsorption activity of the filler surface is of greater importance than its chemical nature. The polar nature of the silica surface adsorbs a part of the curatives, and/or silica zinc ion interaction leads to slowing down of the curing reaction [12]. However, in the case of antioxidantmodified silica, the OH groups on the surface are already hydrogen bonded with the NH group of the substituted phenylenediamine antioxidant. Cure graphs of the three types of antioxidant-filled compound are shown in Figures 1a to c. The maximum torque is a measure of crosslink density and stiffness in the rubber [13]. In general, for all the mixes, the torque initially decreases and then increases. The increase in torque is due to crosslinking of the rubber. It is found that the antioxidant-modified silica increases the torque compared with the neat precipitated silica. This increase is due to the presence of silica rubber crosslinking which imposes more restriction on deformation. (a) (b) (c) Figure 1. Cure graphs of compounds filled with antioxidant-modified silica (NR, IPPD, 6PPD, DPPD (philphex), Brabender mixed) and with neat silica (NR, IPPD, 6PPD, DPPD (philphex), neat) Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4,

8 P.M. Sabura Begum Tensile Properties The tensile properties of NR vulcanisates with antioxidant-modified silica and with neat silica are shown in Table 4. Table 4. Tensile properties of NR vulcanisates Vulcanisate Tensile strength (MPa) Tensile modulus at 300% elongation (MPa) Elongation at break (%) Tear strength (N/mm) E E F F G G The tensile strength behaviour of vulcanisates filled with antioxidant-modified silica and with neat silica is similar. However, there is considerable improvement in other properties for antioxidant-modified silica vulcanisates. The antioxidantmodified silica vulcanisates show a considerable improvement in tear strength. This can be attributed to better dispersion and improved filler rubber interaction. The tensile modulus values also show similar behaviour, indicating better reinforcement. The elongation at break of different vulcanisates show that this property is lower for antioxidant-modified silica vulcanisates than for neat silica vulcanisates. Improved tensile strength and reduced elongation at break are considered to be criteria for higher filler reinforcement [14]. The improvement in tensile properties for antioxidant-modified silica vulcanisates is evidence of better dispersion of filler in the rubber matrix. Other Technological Properties Other properties such as hardness, compression set, abrasion loss, and flex resistance were compared for the vulcanisates with antioxidant-modified silica and with neat silica and are given in Table 5. Table 5. Technological properties of vulcanisates Property E-1 E-2 F-1 F-2 G-1 G-2 Shore A hardness Compression set (%) Abrasion loss (cm 3 /h) Flex resistance (k cycles) Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011

9 Use of Antioxidant-modified Precipitated Silica in Natural Rubber Antioxidant-modified silica vulcanisates showed better abrasion resistance. This was due to the strong adhesion of silica particles to rubber chains. Hardness also showed the same improvement. Compression set was found to be comparatively low for antioxidant-modified silica composites, indicating the higher elasticity of antioxidant-modified silica vulcanisates. The number of flex cycles required for crack initiation was noted, and was comparatively high for antioxidant-modified silica vulcanisates, indicating that antioxidant modification improves the distribution of antioxidant and silica in rubber. Reinforcing Index Reinforcing index (RI) values of NR vulcanisates are given in Figure 2. The reinforcing index is calculated using the equation: RI = (N/N 0 ) (100/m filler content ) where N and N 0 are the nominal values of the mechanical property (tensile strength) measurement for the sample filled with and without silica respectively [15]. The values of antioxidant-modified silica vulcanisates are comparable with those of neat silica vulcanisates. This shows that vulcanisates filled with antioxidant-modified silica have a reinforcing capacity equivalent to that of vulcanisates filled with neat silica. Figure 2. Reinforcing index of NR vulcanisates Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4,

10 P.M. Sabura Begum Crosslink Density The crosslink density of vulcanisates with antioxidant-modified silica and with neat silica is shown in Figure 3. It can be seen that the antioxidant (IPPD)-modified silica has a better value. The increased crosslink density of vulcanisates filled with antioxidant-modified silica indicates a better interaction between the rubber and silica particles. Figure 3. Crosslink density of vulcanisates Heat Build-up Heat build-up values of vulcanisates with antioxidant-modified silica and with neat silica are shown in Table 6. Table 6. Heat build-up values of vulcanisates Vulcanisates Heat build-up DT ( C) E E The heat build-up value for a vulcanisate filled with antioxidant-modified silica is lower than for a vulcanisate filled with neat silica. Friction between 224 Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011

11 Use of Antioxidant-modified Precipitated Silica in Natural Rubber the silica particles is reduced by the antioxidant, which acts as a lubricant. Thus, it reduces the heat developed by the frictional strain. Incorporation of Silica The mixing sequence of IPPD-modified silica and of neat silica and IPPD in natural rubber is shown in Figure 4. During mixing it is observed that the Figure 4. Mixing sequence of IPPD-modified silica and neat silica + IPPD in natural rubber Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4,

12 P.M. Sabura Begum Figure 4. Continued Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011

13 Use of Antioxidant-modified Precipitated Silica in Natural Rubber IPPD-modified silica is incorporated easily into the rubber matrix, in a shorter time compared with the neat silica. This may be due to the lower hydrophilic nature of the modified silica. Nature of Ash of NR Compounds Figure 5 shows the nature of ash of compounds containing antioxidantmodified silica and with neat silica. The nature of ash indicates the uniform distribution of IPPD-modified silica in the rubber matrix compared with neat silica and antioxidant. CONCLUSIONS Modification of silica with antioxidants gives improved mechanical properties such as tensile strength, tear strength, modulus, etc. Flex crack resistance, abrasion resistance and hardness are also found to be increased. (a) (b) Figure 5. Nature of ash of compounds containing antioxidant-modified silica and with neat silica. (a) with modified silica; (b) with neat silica Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4,

14 P.M. Sabura Begum Lower heat build-up and lower compression set are also observed for vulcanisates with modified silica compared with neat silica. Modified silica is easily incorporated, and the filler distribution is found to be more uniform compared with neat silica. REFERENCES 1. Grosch K.A., Rubber Chem.Technol., 69 (1996) Nunes R.C.R., Fonsecs J.L.C., and Pereira M.R., Polymer Testing, 19 (2000) Cochet P., Bassiquant I., and Bomal Y., Presented at a meeting of the ACS Rubber Division, Cleveland, Ohio, October (1995). 4. Pakpum Phewphong, Pongdhorn Saeoui, and Chakrit Sirisinha, Polymer Testing, 27 (2008) Medalia A.I. and Kraus G., in Science and Technology of Rubber, ed. by Mark J.E., Erman B., and Eirich R.F. Academic Press, New York, NY, Ch. 8, p. 387 (1994). 6. Wolff S., Wang M.J., and Tan E.H., Kauts. Gummi Kunsts., 47(2) (1994) wagner M.P., Rubber Chem.Technol., 49 (1976) Davies K.M. and Lloyd D.G., in Developments in Polymer Stabilization 4, ed. by Scott G. Applied Science Publishers, London, UK, p. 124 (1981). 9. Sulekha P.B., Rani Joseph., Madhusoodanan K.N., and Thomas K.T., Polymer Degradation and Stability, 77 (2002) Annual Book of ASTM Standards (2000). 11. Flory P.J. and Rehner J., J. Chem. Phys., 11 (1943) Bandyopadhyay S., De P.P., Tripathy D.K., and De S.K., Rubber Chem. Technol., 69 (1996) Chakraborty S.K. and Setua D.K., Rubb. Chem. Technol., 55 (1982) Boonstra B.B., Reinforcement by fillers, in Rubber Technology and Manufacture, 2nd edition, ed. by Blow C.M. Butterworth Scientific, Stoneham, MA, Ch. 7, p. 269 (1982). 15. Shinzo Kohjiya and Yuko Ikeda, Rubb. Chem. Technol., 73 (2000) Progress in Rubber, Plastics and Recycling Technology, Vol. 27, No. 4, 2011