Process for Preparing a Latex Based on Polypyrrole and Use of This Latex to Form a Conducting Film

The invention relates to a process for preparing a latex based on polypyrrole, from which it is possible to obtain adherent homogeneous films in a wide range of thickness and having high conductivity.

It has already been proposed, in British Patent Application 2,181,367, to prepare conducting films based on pyrrole by polymerizing pyrrole in an aqueous medium in the presence of a polyvinyl alcohol and a substance capable of providing the property of conductivity, such as a ferric compound or a persulphate.

Similarly, it has been proposed--in the document Journal of Colloid and Interface Science, Vol. 118, No. 2, Aug. 1987, p. 410-416--to prepare a water-stable latex based on polypyrrole by employing a technique of polymerization of pyrrole involving ferric chloride as an initiator and a partially hydrolized polyvinyl acetate as a stabiliser.

However, these processes lead to polymers having low conductivity. The films obtained with these polymers are, in general, poorly homogeneous or adhere only weakly to metals, glasses and metallized glasses.

The object of the present invention is to provide a process which no longer has these drawbacks.

To this end, the invention relates to a process for preparing a latex based on polymers of pyrrole or of a pyrrole derivative in an aqueous reaction medium comprising a ferric salt, a polyvinyl alcohol or a derivative of this alcohol and a codispersant agent.

Codispersant agent is understood to mean organic compounds capable of giving rise to anions. Among these compounds, organic compounds which react with the ferric salt present in the reaction medium, or which have already reacted with a ferric salt before introduction into the reaction medium, are preferred. In general, alkyl or aryl sulphates or sulphonates, acids such as benzoic acid, ammonium salts or alkali metal salts such as ammonium or sodium laurate, trichloroacetate, phenylphosphonate or (glycerol 2-phosphate) are employed.

Compounds chosen from the following are usually employed:

benzoic, para- and meta-hydroxybenzoic, trifluoromethanesulphonic, benzenesulphonic, para-toluene sulphonic and dodecylbenzenesulphonic acids;

salts such as sodium salts of trifluoromethanesulphonic, lauric, trichloracetic, phenylphosphonic, glycerol-2-phosphonic, octylsulphonic, pentadecylsulphonic, hexadecyl-sulphonic, polyvinylsulphonic, polystyrenesulphonic, para-toluenesulphonic, dodecylbenzenesulphonic, para-hydroxybenzenesulphonic, 4-dimenthylaminobenzenesulphonic, 1,3-benzene-disulphonic and dodecyl sulphuric acids.

Preferably, compounds containing a dodecyl sulphuric sulphate group are employed. It is especially preferably to employ sodium dodecyl sulphate.

In the reaction medium, a polyvinyl alcohol or a derivative of this alcohol such as, in particular, an acetate is employed also, as a dispersant agent. A hydrolysed polyvinyl acetate is usually employed, and preferably a hydrolysed polyvinyl acetate having a degree of hydrolysis of between 72 and 96% and an average molecular weight of between 50,000 and 200,000. Good results have been obtained with a hydrolysed polyvinyl acetate having a degree of hydrolysis of between 86 and 90% and an average molecular weight of between 100,000 and 140,000.

In the reaction medium, a ferric salt is employed also, as an oxidizing and doping agent, causing polymerization. In general, an organic or inorganic ferric salt is employed. Usually, a chloride, sulphate or nitrate is employed as an inorganic ferric salt, and compounds of the following type are employed as an organic ferric salt: iron trichloracetate, iron phenylphosphonate, iron (glycerol 2-phosphate), iron pentadecylsulphonate, iron hexadecylsulphonate, iron polyvinylsulphonate, iron polystyrenesulphonate, iron dodecylbenzenesulphonate, iron tosylate, iron trifluoromethanesulphonate and iron dodecyl sulphate. Preferably, ferric chloride is employed.

Finally, the reaction medium contains a pyrrole derivative, which is understood to mean pyrrole, N-methylpyrrole, pyrroles substituted at the 3-position and mixtures of these compounds. Preferably, unsubstituted pyrrole is employed.

The quantity of codispersant agent employed in the process according to the invention can vary according to its chemical nature.

When sodium dodecyl sulphate is used, 0.01 to 7.5 moles per mole of pyrrole monomer, usually 0.05 to 3 moles and preferably 0.1 to 2 moles, are generally employed. Good results have been obtained with concentrations of between 0.3 and 1 moles of sodium dodecyl sulphate per mole of pyrrole monomer.

The quantity of polyvinyl alcohol, or of the derivative of this alcohol, employed can vary within wide limits, and depends on its solubility in water. When a hydrolysed polyvinyl acetate is used, 0.1 to 8 g per g of pyrrole monomer, and usually 0.2 to 5 g are generally employed. Good results have been obtained with concentrations of between 0.3 and 2 g of hydrolysed polyvinyl acetate per g of pyrrole monomer.

In order for the process to show a good yield, the quantity of ferric salt to be employed is large. When ferric chloride is used, 0.3 to 3 moles of ferric chloride are generally employed per mole of pyrrole. Good results have been obtained with concentrations of between 2 and 2.7 moles of ferric chloride per mole of pyrrole.

The process according to the invention is preferably performed in an aqueous medium, but the quantity of water needed can vary within wide limits, and depends chiefly on the other components.

The process according to the invention can advantageously be carried out according to the following stages:

in a first stage, water, the ferric salt, the polyvinyl alcohol or the derivative of this alcohol and the codispersant agent are introduced into the reactor to form the reaction medium;

in a second stage, pyrrole or the pyrrole derivative is added with stirring to this reaction medium in the presence of water.

The latex finally obtained after these two stages is concentrated by centrifugation or ultrafiltration or by any known method. It is then washed with water. The concentrated latex is washed and then redispersed in an aqueous solution of polyvinyl alcohol or of a derivative of this alcohol. This aqueous solution usually contains from 0.1 to 50 g of polyvinyl alcohol or of the derivative of this alcohol per 100 ml of water, and preferably from 0.5 to 10 g. In general, the chemical compound already used in the first stage is employed.

The temperature at which the process is carried out is generally between 0.degree. and 50.degree. C., and preferably between 5.degree. and 40.degree. C., when working at atmospheric pressure.

The pressure at which the process is carried out is not in itself critical. It is generally between 0.1 and 10 bar, and is preferably equal to atmospheric pressure.

The process according to the invention can be carried out in any apparatus or any reactor permitting the combination of the working conditions described above.

The present invention also relates to the use of the latex obtained to form a conducting film. In effect, the process according to the invention makes it possible to obtain stable latices, composed of conducting polymers, capable of forming adherent, homogeneous and highly conductive films in a wide range of thickness.

The films obtained are homogeneous and, as a result, do not laminate spontaneously. It is hence possible to obtain very thin layers such as, in particular, submicronic layers, films of 0.2 to several tens of microns, but also layers of several hundred microns.

The conducting polymers obtained can, in particular, be used as active or passive electrode materials, a binding agent in electrodes, electromagnetic screening, components of batteries, accumulators or capacitors, electrochemical and electrochromic devices, paints for electromagnetic absorption and also, depending on the thickness of the films obtained, for display, for data recording and as materials for electronic components and circuits.

The latices obtained according to the invention can be deposited on any support such as, in particular, glass, metals, metallized glasses, glass fibres, textiles and plastics.

The invention is illustrated by the examples which follow.

150 ml of water, ferric chloride (FeCl.sub.3.6 H.sub.2 O) and polyvinyl acetate, 88% hydrolysed and of average moleecular weight 120,000, are introduced into a 500-ml round-bottomed flask, the quantities of these products are noted in Table 1.

To the reaction medium thereby obtained, pyrrole, dissolved in 50 ml of water, is added dropwise and with stirring.

The flask is maintained for 2 hours at 20.degree. C. The product obtained is spread on metallized glass (ITO glass) and on nickel, and then dried at room temperature overnight.

After this drying, the film obtained is washed with ethanol one or more times until the ethanol is clear.

The characteristics of the films obtained are collated in Table 1: the films obtained are poorly homogeneous and have low adhesion, and their conductivity is low.

150 ml of water, 29.7 g of ferric chloride FeCl.sub.3.6 H.sub.2 O and 1 g of polyvinyl acetate, 88% hydrolysed and of average molecular weight 120,000, are introduced into a 500-ml round-bottomed flask, and 3 g of pyrrole, dissolved in 50 ml of water, are then added dropwise and with stirring.

The flask is maintained for 2 hours at 20.degree. C.

The product obtained is centrifuged for 30 minutes at 15,000 rpm and washed twice with water.

The product obtained is spread on metallized glass (ITO glass) and on nickel and then dried at room temperature overnight.

The characteristics of the films obtained are collated in Table 2.

The films obtained show low conductivity.

150 ml of water, 29.7 g of ferric chloride FeCl.sub.3.6 H.sub.2 O, 4 g of polyvinyl acetate, 88% hydrolysed and of average molecular weight 120,000, and 6.3 g of sodium dodecyl sulphate are introduced into a 500-ml round-bottomed flask.

3 g of pyrrole, dissolved in 50 ml of water, are then added dropwise and with stirring.

The flask is maintained for 2 hours at 20.degree. C.

The latex obtained is centrifuged for 30 minutes at 15,000 rpm and washed twice with water.

It is then dispersed in a solution of 2 g of 88% hydrolysed polyvinyl acetate in 100 ml of water for Example 7, and 2 g of 88% hydrolysed polyvinyl acetate in 300 ml of water for Example 8.

The latex obtained is spread on metallized glass (ITO glass) and on nickel, and then dried at room temperature overnight.

The characteristics of the films obtained are collated in Table 3: the films obtained have high conductivity and good adhesion.