WO2002074841A1

WO2002074841A1 - Substrates having conductive surfaces

Info

Publication number: WO2002074841A1
Application number: PCT/SG2001/000063
Authority: WO
Inventors: Hailin Ge
Original assignee: Hw Electrochem Technology Pte Ltd
Priority date: 2001-03-16
Filing date: 2001-03-16
Publication date: 2002-09-26

Abstract

A process for the formation of surface conductivity on a substrate comprising treating the substrate with a mixture of a monomer, an oxidant and preferably a performance modifier which can be a solvent which helps the monomer, oligomer and the resulting polymer to diffuse into the structure of the substrate, before substantial reaction between them occurs. Then the monomer and oxidant are allowed to react in contact with and on the surface of the substrate to give the substrate an intrinsically conductive polymeric surface. The monomer for producing the intrinsically conductive polymer layer can, for example, be pyrrolle, and aniline or thiophene, and the substrate is preferably a natural or synthetic polymeric material. The surface of the treated substrate preferable has a conductivity of from 1 to 1011 φ/square and more preferably from 10?2 to 108¿ φ/square.

Description

SUBSTRATES HAVING CONDUCTIVE SURFACES

This invention relates to imparting conductivity to at least the surface of a non-conductive substrate and, in particular, a natural or synthetic plastics material.

There are many instances where it is desirable that a basically non- conductive material be given conductive properties so as to avoid static or electrostatic problems. Particular examples are packing foams and plastic trays or tubes for integrated circuit chips and other electronic components where static discharge could, during their storage or transport, destroy them. It is, therefore, normal practice to mount them in a conductive foam or on a conductive tray to avoid this problem.

One conventional form of conductive foam material consists of a plastics material into which have been incorporated small conductive metal or carbon particles.

Another process involves forming a thin metal plating onto the surface of the polymer, e.g. placing the plastics material into a metal ion solution such as divalent copper with a reducing agent such that the copper becomes reduced and deposited on the polymer surface. Such processes, known as electroless plating, whilst providing conducting surface to the polymer, in many ways have a number of disadvantages.

One preferred manner of providing a polymer with a conducting surface is to polymerise a cyclic monomer, such as pyrrole, on the surface of a base polymer and react it with an oxidant such as ferric trichloride or ammonium persulphate to form an intrinsically conductive polymer such as polypyrrole, which renders the resulting polymeric surface conducting. U.S. Patent No. 4 604 247 is an example of this method of forming an intrinsically electrically conductive polymer.

Intrinsically electrically conductive polymers such as this can be used to coat a non-conducting substrate in various ways. For example a solution of the monomer can be coated onto the substrate polymer and then the thus coated substrate immersed in a solution of the oxidant. There are a number of disadvantages with this in that, for example, the monomer may, itself, diffuse into the oxidant solution and so this is liable to render the process non- economic. In addition it is difficult to dispose of the solutions which are left and this can lead to environmental problems.

Another possible procedure is to polymerise the polymer by means of the oxidant and then to dissolve the resulting polymer in a solvent which is sprayed onto the surface of the substrate to render the surface conductive following evaporation of the solvent. Disadvantages of this process are that the resulting polymer, e.g. polypyrrole, is generally difficult to dissolve and it's films can only be adhered with difficulty to substrates.

A further process involves applying the oxidant to the substrate and then exposing that to a vapour or a solution of the monomer so as to cause polymerisation and formation of the conductive surface film. A disadvantage, however, of this procedure is that the oxidants are generally inorganic materials and so there are considerable difficulties in maintaining the oxidant on what is a generally a hydrophobic polymer and so adhesion of the resulting conductive surface polymeric layer is a problem.

As can be seen there are disadvantages in the known ways of producing such conductive polymeric surface coatings. It is, therefore, an object of the present invention to provide an improved procedure for producing a conductive surface coating on non-conductive substrates. According to the invention there is provided a process for the formation of surface conductivity on a substrate comprising treating the substrate with a mixture of a monomer and an oxidant before substantial reaction between them occurs, and allowing the monomer and oxidant to react in contact with and on the surface of the substrate to give the substrate an intrinsically conductive polymeric surface.

It appears to be an essential feature of the invention that the oxidant be mixed with the monomer prior to significant polymerisation of the monomer and that the polymerisation and implantation proceed in the presence of the reducing oxidant. This can be achieved, for example, by mixing the two components in solution and then immediately applying the solution to the surface of the substrate and allowing the monomer to polymerise on the substrate and the oxidant to reduce. In such a process the two can be mixed and then spayed or coated onto the substrate. Alternatively the substrate can be dipped into the mixed solution or a surface layer of the solution applied to the substrate. In an alternative process, however, the two components, namely the monomer and the oxidant, can be sprayed onto the surface of the substrate either simultaneously or by spraying one immediately after the other and before there is any opportunity for significant polymerisation of the monomer to occur.

A performance modifier may be added to the mixture of oxidant and monomer prior to significant polymerisation. Such a performance modifier will be chosen so as to provide the conductive polymer layer with various properties. Such a performance modifier can, for example, be a solvent which helps the monomer, oligomer and the resulting polymer to diffuse into the structure of the substrate. Examples of such solvents are an alcohol, ketone, THF, DMSO, NMP or methylene chloride. The performance modifier can also be a material which provides counter ions such as p-toluene sulphonate, dodecyl sulphate or dodecyl sulphonate, or a reaction speed controller such as an acid, e.g. sulphuric acid.

By following the invention one finds that one can provide conductive surfaces on a wide variety of substrates which both adhere well to the substrate and at the same time have good electrical conductivity.

Whilst not wishing to be bound by theory we believe that polymerisation occurs in stages, usually from the monomer to an oligomer and then to the polymer with increasing molecular weight. In the process, polymerisation occurs on the surface of the substrate with assistance of the performance modifier which is preferably present. The oxidant becomes reduced and the reduction product is usually left on the surface of the substrate and can then be eliminated by rinsing at the end of the reaction. In this way the intrinsically conductive polymer becomes integrated or implanted either chemically and/or physically into the surface layer of the substrate. At the same time, however, the intrinsically conductive polymer which is formed by the polymerisation is able to adhere well to the substrate and this is not in any way blocked or hindered by any previous deposition on the surface of the substrate. Also by polymerising the monomer in situ on the surface of the substrate one does not form a separate polymer layer which then needs to be adhered to the substrate.

The products made according to the invention have a wide variety of uses. They can, for example, be used in the packaging for microchips, circuit boards and other electronic components where a conductive layer is required to ensure that there cannot be static charges which would damage the component. They can also be used to make a Faraday cage to minimise electric field interference by making a container, and earthing the conductive layers to provide and overall screen in which electronic equipment is placed. Other uses for such materials include conductive flooring for electronic component production plants where, again, electrostatic discharges need to be avoided, wafer storage and shipping, personal electrostatic protection devices, work stations in semiconductor or electronic factories, conductive paint, cable shielding material, and in the formation of conductive textiles.

Particular uses to which the products according to the invention can be put will depend upon the conductivity of the resulting polymer surface layer and the conductivity achieved by a surface film prepared according to the invention can vary from 1 to 10¹¹ Ω/square and more preferably from 10² to 10⁸ Ω/square. Substrates with an intrinsically conductive layer according to the invention will often have conductivities in the range of 10³ to 10⁵ Ω/square and so be very useful in all applications noted above.

Particularly attractive monomers for producing the intrinsically conductive polymer layer are those which form polypyrrole, polyaniline and their analogues. In that connection see for example Mol. Cryst. Liq. Cryst., 1982, Vol. 83, pp. 253-264,, "Preparation and Characterization of Neutral and Oxidized Polypyrrole Films", G.B. Street, T.C. Clarke, M. Krounbi, K. Kanazawa, V. Lee, P. Pfluger, J.C. Scott and G. Weiser, and references therein (polypyrrole); see "Aqueous Chemistry and Electrochemistry of Polyacetylene and 'Polyaniline': Application to Rechargeable Batteries", pp.248 and 249," A. J. Karwczyk, R.J.Mammone, S.L. Mu, N.L. D. Somaasiri and W. Wu (polyaniline). These polymers exhibit good environmental stability and are potentially inexpensive to produce. Suitable monomers, therefore, are for example: pyrrole and its derivatives, aniline and its derivatives and thiophene and its derivatives. Examples of derivatives of pyrrole and aniline are N-methylpyrrole, 3-methylpyrrole, 3,5-dimethylpyrrole, 2,2'-bippyrrole, N- methylaniline, 2-methylaniline, 3-methylaniline and N-phenyl-1 ,4- diaminobenzene. The oxidant is used with the monomer to form an intrinsically conductive polymer layer. Such oxidants, for example, include chemical compounds that can be reduced by monomers. Such materials are exemplified by, but not limited to, FeCI_3ι Fe₂(SO₄)₃, (NH₄)₂S₂O₈, Na₂S₂O₈, K₂Cr₂O_7, HNO_3, HCIO₄, H₂O₂, quinone, K₃(Fe(CN)₆), H₃PO₄,12MoO₃, H₃PO₄12WO₃, CrO₃, (NH₄)Ce(NO₃)₆, Ce(SO₄)₂, CuCI₂, and AgNO₃. Particularly preferred compounds are FeCI₃, ferric sulphate, persulphates such as ammonium persulphate and sodium persulphate, dichromates, nitric acid, perchloric acid, peroxides, and quinone. Particularly preferred are ferric chloride and ammonium persulphate.

The substrate which is given the intrinsically conductive surface layer according to the invention can be chosen from a very wide range of materials. Examples of substrates which can be used include polyurethane, polyvinyl chloride, acrylics, polystyrene, polypropylene, polyethylene, acrylonitrile- butadiene-styrene, nylon, polycarbonate, polyesters, polyethersulphone, polyphenylene sulphide, acetals, alkyds, polysulphone, cellulose, epoxies, cotton and wood. The most preferred substrates are, however, natural or synthetic plastics materials.

It is preferred that the monomer and oxidant be mixed in solution in a suitable solvent such as water or alcohol. In such a case it is preferred that the concentration of the oxidant be from 0.001 M to saturated and more preferably from 0.01 to 1.0M. Also the concentration of the monomer is preferably from 0.01 to 20% by volume and more preferably from 0.1 to 2.0% by volume. If the performance modifier is present this is preferably present from 0.01 % to 80% by volume. This performance modifier can, in some circumstances be the solvent for both the oxidant and monomer,

The reaction time to produce the conductive polymeric surface film on the substrate can vary over a wide range, e.g. from 3 seconds to 48 hours. Preferably, however, the reaction conditions are chosen so that the reaction is complete within a time of 1 minute to 12 hours.

The reaction temperature can vary widely from -40 to 100°C. More preferably it is from -10 to 40 °C, and room temperature is most preferred.

The invention will now be described with reference to the following examples, in which surface resistivity was tested according to EOS/ESD S11.11-93, and static decay time (or dissipation time) was measured according to FTMS10C,4046.

Example 1

54ml of 0.25M FeCI₃ was prepared as reagent A (oxidant). 60ml 1 % pyrrole was used as reagent B (monomer). 6ml of isopropanol was used as reagent C (performance modifier). The reagents A, B, and C were mixed to form a reaction solution. Immediately, a 7cm x 7cm x 1 cm polyurethane foam was dipped in the solution. The foam was pressed to allow the solution to diffuse into the pores. The mixture was allowed to react for 30 minutes. The foam was then taken out of the solution and squeezed to release remaining solution. The thus treated foam was submerged in water a few times to remove the residuals which could include reduced oxidant, excess polymer, unevaporated solvent and performance modifier.

The resulting foam was dark brown to black in colour. The surface conductivity was 2.9 x 10⁴ Ω/square and static decay time was less than 0.01s. Example 2

350ml 21 % ammonium persulphate was prepared as reagent A (oxidant). 350ml of 7.2% aniline in 0.5M sulphuric acid was used a reagent B (monomer). Reagents A and B were mixed to form a reaction solution. Immediately, a piece of 9cm x 15cm x 1cm polyurethane foam was dipped into the solution, and pressed allow the solution to diffuse into the pores. Reaction was allowed to continue for 2h. The foam was squeezed to release the solution and then submerged in water a few times to remove residuals. The resulting foam was green in colour. The surface resistivity was 8.5 x 10⁴ Ω/square and static decay time was 0.2s.

Example 3

0.5m FeCI₃ in ethanol was used as reagent A (oxidant). 0.75% pyrrole in methanol was used as reagent B (monomer). Tetrahydrofuran was used as reagent C (performance enhancer). 10 Parts of reagent A, 7 parts by volume of reagent B and 3 parts of reagent C were mixed to form a reaction solution. 1.5ml of this solution was sprayed onto a 100mm x 100mm x 0.2mm PVC film and allowed to react for 5 minutes. The film was rinsed to remove residuals. The resulting film was transparent.

A strong sticky tape was applied on the film and then removed from it.

No implanted intrinsically conductive surface polymer layer was stripped. The surface resistively was 5.4 x 10³ Ω/square and static decay time was less than 0.01 s.

Claims

1. A process for the formation of surface conductivity on a substrate comprising treating the substrate with a mixture of a monomer and an oxidant before substantial reaction between them occurs, and allowing the monomer and oxidant to react in contact with and on the surface of the substrate to give the substrate an intrinsically conductive polymeric surface.

2. A process as claimed in Claim 1 in which the monomer and oxidant are mixed in solution, the mixture is immediately applied to the surface of the substrate and the monomer allowed to polymerise on the substrate and the oxidant to reduce.

3. A process as claimed in Claim 1 in which the monomer and the oxidant are sprayed onto the surface of the substrate either simultaneously or one immediately after the other and before there is any opportunity for significant polymerisation of the monomer to occur.

4. A process as claimed in any preceding claim in which a performance modifier is additionally added to the mixture of oxidant and monomer prior to significant polymerisation.

5. A process as claimed in Claim 4 in which the performance modifier is a solvent which helps the monomer, oligomer and the resulting polymer to diffuse into the structure of the substrate such as an alcohol, ketone, THF, DMSO, NMP or methylene chloride, a material which provides counter ions such as p-toluene sulphonate, dodecyl sulphate or dodecyl sulphonate, or a reaction speed controller such as an acid, e.g. sulphuric acid.

6. A process as claimed in any preceding claim in which the monomer for producing the intrinsically conductive polymer layer is pyrrole, aniline, thiophene, N-methylpyrrole, 3-methylpyrrole, 3,5-dimethylpyrrole, 2,2'- bippyrrole, N-methylaniline, 2-methylaniline, 3-methylaniline or N-phenyl-1 ,4- diaminobenzene.

7. A process as claimed in any preceding claim in which the oxidant is a compound which can change its valence, such as FeCI₃, Fe₂(SO )₃, (NH₄)₂S₂O₈, Na₂S₂O₈, K₂Cr₂O₇, HNO₃, HCIO₄, H₂O₂, quinone, K₃(Fe(CN)₆), H₃PO_4,12MoO₃, H₃PO₄12WO₃, CrO₃, (NH₄)Ce(NO₃)₆, Ce(SO₄)₂, CuCI₂, or AgNO₃.

8. A process as claimed in any preceding claim in which the substrate is polyurethane, polyvinyl chloride, acrylics, polystyrene, polypropylene, polyethylene, acrylonitrile-butadiene-styrene, nylon, polycarbonate, polyesters, polyethersulphone, polyphenylene sulphide, acetals, alkyds, polysulphone, cellulose, epoxies, cotton or wood.

9. A process as claimed in any preceding claim in which the monomer and oxidant and optionally the performance modifier are mixed in solution and the concentration of the oxidant is from 0.001 M to saturated and more preferably from 0.01 to 1.0M, the concentration of the monomer is preferably from 0.01 to 20% by volume and more preferably from 0.1 to 2.0% by volume and the concentration of the performance modifier, if present, is from 0.01% to 80% by volume.

10. A process as claimed in any preceding claim in which the conductivity of tthhee rreesstulting surface is from 1 to 10¹¹ Ω/square and more preferably from 10² to 10⁸ Ω/square.