In situ preparation of polyaniline coated fumed and precipitated silica fillers and their composites with nitrile rubber (Investigation on structure-property relationship)
Introduction
Conductive or semi-conductive polymeric composites are used in electronic or electrical applications like electrostatic charge dissipation (ESD), touch control switches, electro magnetic interference (EMI) shielding, pressure sensor [1], [2], [3], [4]. This type of composites may be prepared by mixing suitable metal powder as conducting filler in insulating polymer matrixes. But incorporation of metal powders increases the cost, weight and degradability of the composites [5]. Another way to prepare this type of composites is to usage of intrinsically conducting polymers (ICP) as conducting fillers in insulating polymer matrixes. Among ICPs, polyaniline (PAni) is most stable, can be easily synthesized, have noble properties and potential applications [6], [7], [8]. The main problem for practical application of PAni lies in its inadequate processability [9], [10]. So, instead of using PAni as such, PAni filled composites prove to be a better means of its practical applications because combined attainment of improved processability, fairly good mechanical properties coupled with good conductivity [10], [11]. Some interesting study has been carried out on grafting and coating of PAni on different substrate to prepare micro-spheres and electro-rheological characteristic of their suspensions [12], [13], [14]. The literature dealing with the synthesis of PAni by different methods like solution, emulsion, miniemulsion, micro-emulsion, inverse miniemulsion, and electrochemical [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25] and conduction mechanism [25], [26], [27], [28], [29], [30], [31], [32], [33], [34] is very well studied. But the literature dealing with the utilization of PAni is relatively less. The PAni as a filler does not impart good mechanical strength to the polymer matrix. However, if it is adhered to a strong substrate and that is used as filler in polymer matrix, some improvement in mechanical property is also expected [35]. The coating of PAni on fillers like clay, silica, silicates, carbon black, poly (methyl methacrylate), etc. can produce conductive filler or micro-sphere [12], [13], [14]. But the literature dealing with the coating of PAni on silica fillers and use of them with other polymer matrix for the production of conductive composite is relatively less.
The present paper deals with the in situ synthesis and coating of PAni on fumed silica (particle size 40–100 nm) and precipitated silica (particle size 100–1000 nm) particles. These PAni coated silica fillers were used as conducting filler in nitrile rubber matrix. The composites were prepared by solution mixing process followed by vulcanization. The dielectric properties of the composites under conditions of varying frequency, temperature and pressure have been studied. The morphological study was carried out through SEM analysis. The morphology can be correlated to the change in dielectric properties with filler type. The TGA, FT-IR analysis and pH value provided better understanding of PAni to silica interaction and interaction of PAni coated silica fillers with NBR matrix.
Section snippets
Materials
Aniline, zinc oxide (ZnO), methyl ethyl ketone (MEK), sulphur (Merck Ltd., Mumbai, India) ammonium peroxy disulphate (APS), p-toluene sulphonic acid (PTSA), ammonia solution (S.D. Fine Chem. Ltd., Mumbai, India), methanol (SISCO Research Laboratories Pvt. Ltd., Bombay, India), hydrochloric acid (Ranbaxy, India), NBR (Gujarat Apar Polymer Ltd., India), mercaptobenzothiazole (MBT), zinc diethyl dithio carbamate (ZDC) (ICI India Ltd.), stearic acid. All are GR grade.
Pyrogenic or fumed silica
Effect of fumed silica (FS) to aniline ratio on the conductivity of PAni coated fumed silica
The weight ratio of fumed silica (FS) to aniline was varied from 0.25:1.00 to 1.00:1.00 during synthesis. Initially with the increase in weight percentage of FS, the conductivity of the resultant PAni coated fumed silica (PCFS) is found to increase and attains the maximum value when the weight ratio of FS to aniline is 0.5:1. But further increase in the weight percentage of FS, the conductivity of the PCFS is decreased as shown in Fig. 1. The electrical conduction especially in 50% oxidized
Summery and conclusion
Pyrogenic or fumed silica (FS) particles are finer having higher surface area compared to precipitated silica (PS), consequently amount of PAni coating is more in FS particles. Further acidic groups are present on FS particles compared to basic groups on PS particles. The acidic nature of FS particles causes partial doping of PAni deposited on their surface leading to higher conductivity and agglomerated structure. Higher conductivity and agglomeration tendency of PAni coated FS (PCFS)
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