Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF POLYANILINE- SALTS

Abstract

An improved process for the preparation of polyaniline-salt: The present method involves a process for the polymerization of aniline into polyaniline salts using cheaper protonic acids such as hydrochloric, sulfuric, nitric, phosphoric and 5-sulfosalicylic acid wherein the polyaniline-salt is in carrier organic solvent such as chloroform, dichloromethane, toluene and the solution is optically transparent. This solution can be used directly for blending with other insulating polymers using conventional method.

Full Text

The present invention relates to a process for the preparation of polyaniline-salts. The present invention particularly relates to a process for the preparation of polyaniline-salts using protonic acids such as hydrochloric, sulfuric, nitric., phosphoric and 5-sulfosalicylic acid. The present invention more particularly relates to an emulsion-polymerization process for preparing an electrically conductive polyaniline-salt wherein the polyaniline-salt is in organic carrier solvent and the solution is optically transparent. A lot of research work in the area of electrically conductive polymers is being carried out at the moment all over the world. These polymers make it possible to replace metallic conductors and semi-conductors in many applications such as batteries, transducers, switches, solar cells, circuit boards, heating elements and in electrostatic discharge (BSD) and electromagnetic interference shielding (EMI) applications. The advantages of electrically conductive polymers compared to metals are, for instance, their low weight, good mechanical properties, corrosion resistance and cheaper synthesis and processing methods. Examplifying kinds of inherently electrically conductive polymers are polyacetylene, poly-p-phenylene, polypyrrole, polythiophene and polyaniline. An advantage with the inherently electrically conductive polymers is that their electrical conductivity is easily varied as a function of the doping time, which is especially seen in the case of low conductivities. It is difficult to obtain low conductivities for filled electrically conductive plastics. Polyaniline has emerged as one of the promising conducting polymers and can be used in a variety of applications, such as paint, antistatic protection, electromagnetic protection, electro-optic devices such as liquid crystal devices (LCDs) and photocells, transducers, circuit boards, etc. However, processing of polyaniline into useful products or devices as described above has been problematic because of its insolubility in common solvents. Synthesis of polyaniline is commonly performed by the method of chemical oxidative polymerization based upon the aqueous solution polymerization system, (see Cao et al., Polymer, 30:2305, 1989). Typically, polyaniline is produced as solid emeraldine salt from chemical oxidative polymerization in the presence of protonic acids such as Hcl and H2SO4 The polyaniline obtained in such way is normally insoluble, which hinders the application of the polyaniline. Smith et al., U.S. Pat. No. 5,470,505, disclosed that the emeraldine salt prepared by standard methods of oxidative polymerization of aniline monomer in the presence of a protonic acid can be dissolved in an acid, particularly strong acid such as concentrated H2SO4, CH3SO3H, CISO3H, CF3SO3H and HNO3 (70% or fuming). The emeraldine salt (polyaniline) dissolved in one of these acid solutions is then processed into desired articles in the applications. Abe et al., U.S. Pat. No. 5,728,321, disclosed a solution of polyaniline (dissolved in an' aprotic polar solvent, such as N-methyl-2-pyrolidone) in doped state can be obtained by a method using a specific protonic acid, such as hydrofluoroboric acid, hydrofluorophosporic acid, perchloric acid, or any other organic acids having acid dissociation constant pKa values of less than 4.8, as dopants in the oxidative polymerization of aniline monomer. Also, the polyaniline obtained according to the above method, which is insoluble in an organic solvent, can be dissolved in an aprotic polar solvent in an undoped state. The undoping of doped polyaniline in order to permit the polyaniline to be soluble in organic solvent is burdensome and increases the production cost. Traditional methods of preparation of polyaniline in a processable form, including the prior arts mentioned above, have to go through the processes of recovering, filtering, washing, and drying of the reaction product to obtain the solid polyaniline due to the insolubility of the polyaniline formed in the reaction mixture, and need additional processes, such as transforming the emeraldine salt into emeraldine base and dissolving the solid polyaniline or emeraldine base in a solvent, to obtain the desired solution of polyaniline. To improve the processability, emulsion polymerization processes for preparing a polyaniline salt of a protonic acid have been reported. (Cao et al., U.S. Pat. No. 5,232,631, Example 6B, 1993; Cao and Jan-Erik, WO94/03528, 1994 I; Cao and Jan-Erik, U.S. Pat. No. 5,324,453, 1994 II; see also, Osterholm et al., P. Synthetic Metals 55:1034-9, 1993). In these disclosures aniline, a protonic acid, and an oxidant were combined with a mixture of polar liquid, typically water and a non-polar or weakly polar liquid, e.g. xylene, chloroform, toluene, decahydronaphthalene and 1,2,4-trichlorobenzene, all of which are either sparingly soluble or insoluble in water. Smith et al (Polymer 35, 2902, (1994)) reported the polymerization of aniline in an emulsion of water and a non polar or weakly polar organic solvent. This polymerization was carried out in the presence of functionalized protonic acid such as dodecylbenzenesulfonic acid which simultaneously acted as a surfactant and protonating agent for the resulting polyaniline. This polyaniline is having good solubility in non polar solvents. Protonic acid primary dopants are described as acting as surfactants in that they are purportedly compatible with organic solvents and enable intimate mixing of the polyaniline in bulk polymers (Cao et al, Synthetic Metals 48:91-97, 1992; Cao et al, U.S. Pat. No. 5,232,631, 1993 which are incorporated by reference). Thus, any surfactant aspect of the primary dopants was thought to contribute to the processability rather than the conductivity of the polyaniline. Heeger's group (Synthetic Metals 48, 91 !992); Synthetic Metals 3514 (1993) reported that emeraldine base doped with a functionalized protonic acid, for example, camphorsulfonic acid and dodecylbenzene sulfonic acid, can be dissolved in a non-polar or moderate polar organic solvent. This three component system has good solubility in common organic solvents and is compatible with many of the classical polymers. Polyaniline salt has been categorized as a interactable material which is neither soluble nor fusible under normal conditions. Several strategies were worked out to introduce solubility and processability in polyaniline. They are : • Dedoping of polyaniline salt to polyaniline base. Dissolving polyaniline base in aprotic solvent and redoping to polyaniline salt. This procedure is burdensome and increases the production cost. • Dissolving the polyaniline salt in concentrated acid. However, they are highly corrosive because the use of concentrated acid. • Preparation of substituted polyaniline; preparation of polyaniline copolymers which are not the homopolymer of polyaniline salts. The conductivity of the substitute polyaniline and copolymer may be much lower than that of the polyaniline. • Preparation of polyaniline salt using functionalized protonic acids both by aqueous and emulsion polymerization process - functionalized protonic acids may be costly. The present method involves a process for the polymerization of aniline into polyaniline salts using cheaper protonic acids such as hydrochloric, sulfuric, nitric, phosphoric and 5-sulfosalicylic acid wherein the polyaniline-salt is in carrier organic solvent such as chloroform, dichloromethane, toluene and the solution is optically transparent. This solution can be used directly for blending with other insulating polymers using conventional methods. Polyaniline salts in organic carrier solvent were prepared directly in one step by polymerizing aniline with cheaper organic: and inorganic acids. In addition to polyaniline-sulfate salt, using this process, other polyaniline salts may be prepared using other acids such as hydrochloric, nitric, phosphoric and 5-sulfosalicylic acid. Polyaniline-salt with higher conductivity (0.1 S/cm) can be prepared when compared with that of polyaniline-sulfate salt (0.01 S/cm). The following drawbacks as reported earlier are not involved in the present process : (i) use of costlier functionalized protonicacid, (ii) use of concentrated acid for dissolving the polyaniline salt which may involve corrosion and handling problems and (iii) conversion of polyaniline salt into polyaniline base, dissolving the polyaniline base in solvents then adding insulating polymer and converting into electrically conducting polyaniline blend. The main objective of the present invention is to provide a process for the preparation of polyaniline-salt, wherein, the electrically conductive polyaniline-salt is in organic carrier solvent. The other objective of the present invention is to provide a process for the preparation of polyaniline-salts using cheaper protonic acid such as hydrochloric, sulfuric, nitric, phosphoric and 5-sulfosalicylic acid protonic acid. Yet another objective of the present invention is to provide a process for the preparation of an electrically conductive polyaniline-salt in powder form. Accordingly the present invention provides an improved process for the preparation of polyaniline-salt which comprises polymerizing aromatic amine in the presence of protonic acid selected from hydrochloric, sulfuric, phosphoric and 5-sulfosalicylic acid or mixture thereof and mixture of aqueous hydrocarbon solvents such as herein described in the presence of ionic surfactant and a radical initiator such as herein described, at 25-30°C temperature for at least 24 hrs., separating the polyaniline-salt in solution form by conventional method from the reaction mixture, optionally precipitating by adding non solvent to the above polyaniline salt solution to get the desired polyaniline-salt. The present invention is directed to a process for the preparation of an electrically-conductive polyaniline-salt in non aqueous organic carrier solvent. The present invention is also directed to a process for the preparation of an electrically-conductive polyaniline-salt in the powder form. The protonic acid used may be such as hydrochloric, sulfuric, phosphoric and 5-sulfosalicylic acid and mixture thereof. In an embodiment of the present invention, the aromatic amine used may be aniline or substituted aniline such as 2-methyl aniline and 3-methyl aniline. In an another embodiment of the present invention, the hydrocarbon solvent used may be chlorinated solvent such as chloroform, dichloromethatie, aromatic hydrocarbon such as toluene. In yet another embodiment of the present invention, the anionic surfactant used may be such as sodium lauryl sulfate, clioctyl sodium sulfosucciriate and cationic surfactant used may be cetyltrimethylammonium bromide. In still yet another embodiment of the present invention, the radical initiator used may be benzoyl peroxide. In a feature of the present invention, the separation of polyaniline sulfate in organic solvent may be effected by pouring the reaction mixture into water. In another feature of the invention, the separation of the polyaniline-sulfate salt from the reaction mixture may be carried out by filtration. These embodiments will be apparent from the ensuing detailed description of the present invention. The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention EXAMPLE 1 The following example illustrates the preparation of the polyaniline-5-sulfosalicylic acid salt in weakly polar organic solution by the emulsion-polymerization pathway using sodium lauryl sulfate anionic surfactant. A solution containing 1.44 g of sodium lauryl sulfate dissolved in 40 ml of distilled water is mixed with a solution containing 5.85 g benzoyl peroxide in 60 ml chloroform. The milky-white emulsion thus formed is mechanically stirred at 25°C. 2.3 ml aniline and 5-sulfosalicylic acid (5.1 g) in 100 ml of water, is added dropwise to the mixture over a period of approximately 20 minutes. The reaction is allowed to proceed for 24 hours (reaction time was varied as 12, 16, 24 hrs). The color of the emulsion at this time becomes green. Separated the bottom oily green phase containing the polyaniline and a upper aqueous phase. The upper aqueous phase is removed with a separating funnel and 1500 ml water is added to the green phase, the aqueous phase is removed and the green polyaniline phase is subsequently washed with three 1500 ml portions of water. Sodium sulfate (5 g) is added to the polyaniline phase and filtered through filter paper. The polyaniline phase thus obtained is appeared uniform to the naked eye and the polymer remained solubilized in the organic phase. EXAMPLE 2 The following example illustrates the preparation of the polyaniline-salts in weakly polar organic solution by the emulsion-polymerization pathway using sodium lauryl sulfate sulfate anionic surfactant. A solution containing 1.44 g of sodium lauryl sulfate dissolved in 40 ml of distilled water is mixed with a solution containing 5.85 g benzoyl peroxide in 60 ml chloroform. The milky-white emulsion thus formed is mechanically stirred at 25°C. 2.3 ml aniline and acid (hydrochloric acid -17.5 ml; sulfuric acid - 9.0 ml; phosphoric acid - 5.5 ml; nitric acid - 12.6 ml and 5-sulfosalicylic acid - 5.1 g) in 100 ml of water, is added dropwise to the mixture over a period of approximately 20 minutes. The reaction is allowed to proceed for 24 hours. The color of the emulsion at this time becomes green. Separated the bottom oily green phase containing the polyaniline and a upper aqueous phase. The upper aqueous phase is removed with a separating funnel and 1500 ml water is added to the green phase, the aqueous phase is removed and the green polyaniline phase is subsequently washed with three 1500 ml portions of water. Sodium sulfate (5 g) is added to the polyaniline phase and filtered through filter paper. The polyaniline phase thus obtained is appeared uniform to the naked eye and the polymer remained solubilized in the organic phase. The isolated polyaniline-salt samples are analyzed by electronic absorption spectral technique using Hitachi U 2000 spectro photo meter. Polyaniline-sulfate salt in organic solvent according to Examples 1 are recorded and three peaks are observed at around 360-380, 530-540 and 825-850 nm which corresponds to polyaniline-salt system. EXAMPLE 3 The following example illustrates the preparation of the polyaniline-salt in the powder form by the emulsion-polymerization pathway. The organic layer obtained in Example 1 and 2 which contains polyaniline-salt in organic solvent is poured into 500 ml of acetone. Polyaniline- sulfate salt is thus precipitated out from the organic solvent. The precipitate is then recovered by filtration and the solid is washed with 2000 ml of distilled water followed by 250 ml of acetone. The powder is dried at 100°C, till the constant mass is reached. The polyaniline-sulfate salts in the dry powder form are compressed into pellets using a 16 mm diameter Macro-Micro KBR die and a 12-ton laboratory hydraulic press. The powder is placed in the die and a pressure of 2000 1bs is applied. Each pellet thus formed is measured to determine its diameter and thickness. The pellets are in the shape of disks. In measuring the conductivity a pellet is coated with silver paint on both the sides having the same cross sectional area and the resistance is measured using an ohmmeter. Lead resistance is 0.03 Ohms for the pellets. Conductivity is calculated using the following formula: Conductivity = (Thickness)/(resistance.times.area) = d/(RA) The conductivity of the polyaniline-5-sulfosalicylic acid prepared by Example 3 with time 16, 24 and 36 hrs are found to be 0.4, 0.6 and 0.01 S/cm respectively. The conductivity of the polyaniline-salt prepared by Example 3 with different acids such as hydrochloric, sulfuric, nitric, phosphoric and 5-sulfosalicylic acid are found to be 0.1, 0.1, 0.2, 0.005 and 0.6 S/cm respectively. Thermal analyses are performed by the simultaneous differential thermal analysis and thermogravimetric analysis technique using the Metier Toledo Star system, and accordingly the samples of Example 3 are evaluated. Polyaniline-sulfate samples are found to be stable up to 200.degree. C. EXAMPLE 4 The following example illustrates the preparation of the polyaniline-sulfuric acid salt in weakly polar organic solution by the emulsion-polymerization pathway using dioctyl sodium sulfosuccinate anionic surfactant. A solutfon contaning 30g of dioctyl sulfosuccinate dissolved in 40 ml of distilled water is mixed with a solution containing 5.85 g benz;oyl peroxide in 60 ml chloroform. The milky-white emulsion thus formed is mechanically stirred at 25°C. 2.3 ml aniline and sulfuric acid (6 ml) in 100 ml of water, is added dropwise to the mixture over a period of approximately 20 minutes. The reaction is allowed to proceed for 24 hours. The color of the emulsion at this time becomes green. Separated the bottom oily green phase containing the polyaniline and a upper aqueous phase. The upper aqueous phase is removed with a separating runnel and 1500 ml water is added to the green phase, the aqueous phase is removed and the green polyaniline phase is subsequently washed with three 1500 ml portions of water. Sodium sulfete (5 g) is added to the polyaniline phase and filtered through filter paper. The polyaniline phase thus obtained is appeared uniform to the naked eye and the polymer remained solubilized in the organic phase. EXAMPLE 5 The following example illustrates the preparation of the polyaniline-sulfuric acid salt in weakly polar organic solution by the emulsion-polymerization pathway using cetyltrimethylammonium bromide cationic surfactant. A solution containing 2.0 g of cetyltrimethylammonium bromide dissolved in 40 ml of distilled water is mixed with a solution containing 5.85 g benzoyl peroxide in 60 ml chloroform. The milky-white emulsion thus formed is mechanically stirred at 25°C. 2.3 ml aniline and sulfiiric acid (6 ml) in 100 ml of water, is added dropwise to the mixture over a period of approximately 20 minutes. The reaction is allowed to proceed for 24 hours. The color of the emulsion at this time becomes green. Separated the bottom oily green phase containing the polyaniline and a upper aqueous phase. The upper aqueous phase is removed with a separating funnel and 1500 ml water is added to the green phase, the aqueous phase is removed and the green polyaniline phase is subsequently washed with three 1500 ml portions of water. Sodium sulfate (5 g) is added to the polyaniline phase and filtered through filter paper. The polyaniline phase thus obtained is appeared uniform to the naked eye and the polymer remained solubilized in the organic phase. EXAMPLE 6 The following example illustrates the preparation of the poly(2-methyl aniline)-sulfuric acid salt in weakly polar organic solution by the emulsion-polymerization pathway. A solution containing 1.44 g of sodium lauryl sulfate dissolved in 40 ml of distilled water is mixed with a solution containing 5.85 g benzoyl peroxide in 60 ml chloroform. The milky-white emulsion thus formed is mechanically stirred at 25°C. 2.7 ml 2-methyl aniline and sulfiiric acid (6 ml) in 100 ml of water, is added dropwise to the mixture over a period of approximately 20 minutes. The reaction is allowed to proceed for 24 hours. The color of the emulsion at this time becomes green. Separated the bottom oily green phase containing the poly(2-methyl aniline) and a upper aqueous phase. The upper aqueous phase is removed with a separating funnel and 1500 ml water is added to the green phase, the aqueous phase is removed and the green poly(2-methyl aniline) phase is subsequently washed with three 1500 ml portions of water. Sodium sulfate (5 g) is added to the poly(2-methyl aniline) phase and filtered through filter paper. The poly(2-methyl aniline) phase thus obtained is appeared uniform to the naked eye and the polymer remained solubilized in the organic phase. The main advantages of the present invention are : (i) the preparation of polyaniline-salts, wherein, polyaniline salt is electrically conductive and the salt is in organic carrier solvent, and (2) the preparation of an electrically-conductive polyaniline-salt using cheaper protonic acid such as hydrochloric, sulfuric, nitric, phosphoric and 5-sulfosalicylic acid .. In view of the above, it will be seen that several advantages of the invention are achieved and other advantageous results attained. As various changes could be made in the above methods and compositions without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense. We Claim: 1. An improved process for the preparation of polyaniline-salt which comprises polymerizing aromatic amine in the presence of protonic acid selected from hydrochloric, sulfuric, phosphoric and 5-sulfosalicylic acid or mixture thereof and mixture of aqueous hydrocarbon solvents such as herein described in the presence of ionic surfactant and a radical initiator such as herein described, at 25-30°C temperature for at least 24 hrs., separating the polyaniline-salt in solution form by conventional method from the reaction mixture, optionally precipitating by adding non solvent to the above polyaniline salt solution to get the desired polyaniline-salt. 2. An improved process as claimed in claim 1, wherein the aromatic amine used is aniline or substituted anilines selected from 2-methyl aniline and 3-methyl aniline. 3. An improved process as claimed in claims 1 - 2, wherein the hydrocarbon solvent used is chlorinated solvent is selected from chloroform, dichloromethane or aromatic hydrocarbon such as toluene. 4. An improved process as claimed in claims 1 - 3 wherein the anionic surfactant used is selected from sodium lauryl sulfate, dioctyl sodium sulfosuccinate and cationic surfactant used is cetyltrimethyl (ammonium bromide. 5. An improved process as claimed in claims 1-4, wherein the radical initiator used is benzoyl peroxide. 6. An improved process for the preparation of polyaniline-slat substantially as herein described with reference to the examples.

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