Water-borne polymeric complex and anti-corrosive composition Number:6,762,238 from the United States Patent and Trademark Office (PTO) owispatent A coating composition which contains a polymeric complex between polyaniline and a polymeric ion. In addition to the said polymeric complex, the composition contains a water-dispersable binder. The composition is useful as a water-borne paint to be applied onto a metal substrate electrophoretically or non-electrophoretically.<H composition, corrosive, Water, borne, pol, 6762238.html, Patent, pto, 6762238

Title: Water-borne polymeric complex and anti-corrosive composition

Abstract: A coating composition which contains a polymeric complex between polyaniline and a polymeric ion. In addition to the said polymeric complex, the composition contains a water-dispersable binder. The composition is useful as a water-borne paint to be applied onto a metal substrate electrophoretically or non-electrophoretically.

Patent Number: 6,762,238 Issued on 07/13/2004 to Yang, et al.

Inventors: Yang; Sze Cheng (Wakefield, RI), Brown; Richard (Wakefield, RI)
Assignee: The Board of Governors for Higher Education, State of Rhode Island and Providence Plantations (Providence, RI)

Appl. No.: 09/856,935

Filed: August 31, 2001

PCT Filed: December 01, 1999

PCT No.: PCT/US99/28307

PCT Pub. No.: WO00/32844

PCT Pub. Date: June 08, 2000

Current U.S. Class: 524/543 ; 524/457; 524/458; 524/460; 524/461; 524/547; 524/548; 524/550; 524/555; 524/556

Current International Class: C09D 133/00 (20060101)

Field of Search: 524/457,458,460,461,543,547,548,550,555,556

References Cited [Referenced By]

U.S. Patent Documents


4442185	April 1984	Skotheim
4731408	March 1988	Jasne
4933106	June 1990	Sakai et al.
4959180	September 1990	Armes et al.
5186860	February 1993	Joyce, Jr. et al.
5187034	February 1993	Otagawa et al.
5188783	February 1993	Pierce
5215682	June 1993	Destryken et al.
5253100	October 1993	Yang et al.
5290483	March 1994	Kulkarni et al.
5312681	May 1994	Mays et al.
5370825	December 1994	Angelopoulos et al.
5382382	January 1995	Asakura et al.
5489400	February 1996	Liu et al.
5520852	May 1996	Ikkala et al.
5532025	July 1996	Kinlen et al.
5556518	September 1996	Kinlen et al.
5585038	December 1996	Kirmanen et al.
5585040	December 1996	Kirmanen et al.
6010645	January 2000	Angelopoulos et al.
6150032	November 2000	Yang et al.
6656388	December 2003	Yang et al.

Foreign Patent Documents


4957514	Aug., 1992	EP
0560721	Sep., 1993	EP
63-2157222	Sep., 1988	JP
01-254764	Oct., 1989	JP
02-069525	Mar., 1990	JP
2-160823	Jun., 1990	JP
05-262981	Oct., 1993	JP
WO 97/03127	Jan., 1997	WO

Other References

Liu et al (Novel Colloidal Polyaniline Fibrils Made by Template Guided Chemical Polymerization), J. Chem Soc. Chem. Comm. Nov. 1991 pp151-153. .
Liu et al. (Novel Template Guided Synthesis . . . ), Mat. Res. Soc. Symp. Proc. Vo. 247 (Jan. 1992), pp601-606. .
Ahlskog et al., "Heat-Induced transition to the conducting state . . . ", Synthetic Metals, vol. 69, Mar. 1, 1995, pp. 213-215. .
Ghosh et al., "Swellability properties of polyelectrolyte complexes . . . ", Synthetic Metals, vol. 60, Sep. 15, 1993, pp. 133-135. .
Liu et al., "Novel Template Guided Synthesis of Polyamiline", vol. 247 of Mat. Res. Soc. Symp. Proc., Jan. 1992, pp. 601-606. .
Sun et al., "Template Guided Synthesis of Conducting Polymers-Molecular Complexes of Polyamilines and Polyelectrolytes", Polymer Reprints, Aug. 1992, pp. 378-380..

Primary Examiner: Cain; Edward J.
Attorney, Agent or Firm: Gauthier & Connors, LLP

Claims

Having described our invention, what we now claim is:

1. An anti-corrosive coating which comprises: a water-borne polymeric complex comprising a strand of a -conjugated polymer selected from the group consisting of polyaniline, polypyrrole, polythiophene, poly(phenylene sulfide), poly(p-phenylene), poly(phenylene vinylene), poly(furylene vinylene), poly(carbazole), poly(thienylene vinylene), polyacetylene, and poly(isothianaphthene); a polymer strand selected from the group consisting of poly(acrylic acid), poly(methacrylic acid), poly(vinylmethylether-co-maleic acid), poly(methylmethacrylate-co-acrylic acid), poly(ethylmethacrylate-co-acrylic acid) and poly(acrylamide-co-acrylic acid), the polymer strand being non-covalently bonded to the -conjugated polymer strand; and a a non-conductive polymer, the non-conductive polymer being complexed with the water-borne polymeric complex, the water-borne polymeric complex having hydrophilic/hydrophobic regions configured to allow the coating to be water soluble prior to application of the coating onto a surface and water insoluble after the coating has been applied to the surface.

2. The coating of claim 1 wherein the non-conductive polymer is selected from the group consisting of thermoset or thermoplastic resins.

3. The coating of claim 2 wherein the resins are selected from the group consisting of epoxy, acrylic, alkyd, vinyl, urethane and olefinic resins.

4. The composition of claim 1 wherein the non-conductive polymer is an epoxy and the coating further comprises: a curing agent selected from the group consisting of capped polyamines, polymercaptans and polyisocyanates.

5. The coating of claim 1 wherein the non-conductive polymer is an epoxy resin and which further comprises: polycarboxylic acids, polyanhydrides, polyphenols and carboxy-functional polyesters.

6. The coating of claim 4 wherein the epoxy is a cationic epoxy resin.

7. The coating of claim 4 wherein the polymeric complex is cross-linked to the epoxy resin.

8. A method of forming an anti-corrosive coating which comprises: dissolving a strand of polymeric ion selected from the group consisting of poly(acrylic acid), poly(methacrylic acid), poly(vinylmethylether-co-maleic acid), poly(methylmethacrylate-co-acrylic acid), poly(ethylmethacrylate-co-acrylic acid) and poly(acrylamide-co-acrylic acid in a medium comprised of water; adding a plurality of monomers selected from the group consisting of aniline, pyrrole, thiophene, phenylene sulfide, p-phenylene, phenylene vinylene, furylene vinylene, carbazole, thienylene vinylene, acetylene, and isothianaphthene to the medium; adsorbing the monomers onto the strand of the polymeric ion to form a polymeric adduct; folding the polymeric adduct to form a particle, the particle having an interior and an exterior, at least a portion of the interior of the particle being hydrophobic and at least a portion of the exterior of the particle being hydrophilic, the exterior of the particle interfacing with the medium; subjecting the particle to an oxidizing environment to form a polymeric complex, the polymeric complex comprising a strand of a -conjugated polymer selected from the group consisting of polyaniline, polypyrrole, polythiophene, poly(phenylene sulfide), poly(p-phenylene), poly(phenylene vinylene), poly(furylene vinylene), poly(carbazole), poly(thienylene vinylene), polyacetylene, and poly(isothianaphthene) bonded to the polymer strand; bonding the polymeric complex to a non-conductive polymer wherein the polymeric complex is complexed with the non-conductive polymer to form the coating, the polymeric complex having hydrophilic/hydrophobic regions that allow the coating to be water soluble prior to application of the coating onto a surface and water insoluble after the coating has been applied to the surface.

9. The method of claim 8 wherein the non-conductive polymer is selected from the group consisting of thermoset or thermoplastic resins.

10. The method of claim 9 wherein the resins are selected from the group consisting of epoxy, acrylic, alkyd, vinyl, urethane and olefinic resins.

11. The method of claim 8 wherein the non-conductive polymer is an epoxy and the method further comprises: adding a curing agent to the coating selected from the group consisting of capped polyamines, polymercaptans and polyisocyanates to the coating.

12. The method of claim 8 wherein the non-conductive polymer is an epoxy resin and which further comprises: adding a curing agent selected from the group consisting of polycarboxylic acids, polyanhydrides, polyphenols and carboxy-functional polyesters to the coating.

13. The method of claim 10 wherein the epoxy is a cationic epoxy resin.

14. The method of claim 10 wherein the polymeric complex is cross-linked to the epoxy resin.

15. A method of forming a protective coating on a metal surface comprising: forming a protective coating on a metal surface by dispersing a water-borne polymeric complex comprising a strand of a -conjugated polymer selected from the group consisting of polyaniline, polypyrrole, polythiophene, poly(phenylene sulfide), poly(p-phenylene), poly(phenylene vinylene), poly(furylene vinylene), poly(carbazole), poly(thienylene vinylene), polyacetylene, and poly(isothianaphthene) and a polymer strand selected from the group consisting of poly(acrylic acid), poly(methacrylic acid), poly(vinylmethylether-co-maleic acid), poly(methylmethacrylate-co-acrylic acid), poly(ethylmethacrylate-co-acrylic acid) and poly(acrylamide-co-acrylic acid), the polymer strand being non-covalently bonded to the -conjugated polymer strand in an aqueous medium; binding a cationic epoxy resin to the water-borne polymeric complex to form a cathodically charged complexed solution; and electrophoretically coating a metal with the cathodically charged complexed solution to form the protective coating, the water-borne polymeric complex having hydrophobic/hydrophilic regions configured to render the protective coating water insoluble.

16. The method of claim 15 wherein the metal is aluminum.

17. The method of claim 16 wherein the metal is steel.

18. The method of claim 15 wherein the polymeric complex is present in the protective coating in a range of between about greater than 1% to 6% by weight based upon the total weight of the protective coating.

19. The method of claim 15 which further comprises: providing a net positive charge on the protective coating by controlling the ratio of polymeric complex to cationic epoxy resin in the cathodically charged solution.

20. An anti-corrosive coating which comprises: a water-borne polymeric complex comprising a strand of a -conjugated polymer selected from the group consisting of polyaniline, polypyrrole, polythiophene, poly(phenylene sulfide), poly(p-phenylene), poly(phenylene vinylene), poly(furylene vinylene), poly(carbazole), poly(thienylene vinylene), polyacetylene, and poly(isothianaphthene); a polymer strand, the polymer strand being non-covalently bonded to the -conjugated polymer strand; and a non-conductive polymer, the non-conductive polymer being complexed with the water-borne polymeric complex, the water-borne polymeric complex having hydrophilic/hydrophobic regions configured to allow the coating to be water soluble prior to application of the coating onto a surface and water insoluble after the coating has been applied to the surface.

21. The composition according to claim 20 wherein the polymer strand comprises anionic and cationic functional groups.

22. The composition according to claim 21 wherein the polymer strand is selected from the group consisting of poly(acrylic acid), poly(methacrylic acid), poly(vinylmethylether-co-maleic acid), poly(methylmethacrylate-co-acrylic acid), poly(ethylmethacrylate-co-acrylic acid) and poly(acrylamide-co-acrylic acid).

23. The composition of claim 22 wherein the -conjugated polymer is polyaniline and the polymer strand is poly(methylmethacrylate-co-acrylic acid).

24. The composition of claim 23 wherein the cationic groups are methacrylate segments and the anionic groups are acrylic acid segments.

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

A coating composition which contains a polymeric complex between polyaniline and a polymeric ion. In addition to the said polymeric complex, the compositon contains a water-dispersable binder. The composition is useful as a water-borne paint to be applied onto a metal substrate electrophoretically or non-electrophoretically.

2. Description of Relevant Art

Conductive polymers (.pi.-conjugated polymers) are potentially useful as a polymeric coating materials to impart special electrical, optical and electroactive properties to coated surfaces. When used as a coating on metals it can impart protection against corrosion of the metals (Wessling DE4334628, Kinlen U.S. Pat. No. 5,532,025). The electrically conductive form of the conducting polymers can also be coated on non-conductive surfaces to render the surfaces electrically conductive. Examples of the .pi.-conjugated polymers are polyaniline, polypyrrole, polyacetylene, polythiophene etc.

The .pi.-conjugated polymers are electrically conductive when they are doped by ionic compounds. In the electrically conductive state, the .pi.-conjugated polymer backbone is a polycation. The positive charge on the .pi.-conjugated polymer backbone is the mobile charge that leads to electrical conductivity. The dopants are the counter ions that balance the positive charges. The difficulties in using conventional conducting polymers for coatings are associated with two of their properties; (1) they are unstable in their doped state and (2) they lack processability. The reason for the lack of processability comes from the fact that the conducting polymers are .pi.-conjugated polymers. The delocalized .pi. electronic structure leads to a stiff polymer chain and strong inter-chain attraction. Thus, the conventional conducting polymers cannot be easily dissolved, melted or blended with other polymers.

The lack of material stability comes from the fact that the ionic dopants are easily lost or segregated from the .pi.-conjugated polymers. Examples of the dopants used in the prior art include hydrogen chloride, p-toluene sulfonic acid, 4-dodecylbenzne sulfonic acid, and dinonylnaphthaienedisulphonic acid (Jen et al., U.S. Pat. No. 5,069,820, Dec. 3, 1991; Elsenbaumer, U.S. Pat. No. 5,160,457, Nov. 3, 1992; Cao et al., U.S. Pat. No. 5,232,631, 1993; Kinlen U.S. Pat. No. 5,567,356, Oct. 22, 1996). When these conducting polymers are exposed to heat, water, solvents and/or moisture, these molecular dopants are lost. Once the dopants are lost, the polymer loses its electrical conductivity and its electroactivity. The loss of dopants occurs either during the manufacturing process or during the service life of the coated product. In certain cases, molecular anions with bulky organic groups are used to reduce the rate of loss of the dopant. This only slows down the rate of dopant is loss, it does not eliminate the problem. Even when the dopes are not lost, the electrical conductivity can be lost due to the diffusion of dopants at a microscopic length scale. The detachment of the dopants from the .pi.-conjugated polymer backbone at a microscopic length scale (0.1 .mu.m length) leads to dedoping. A microscopic scale phase segregation between the polymer and the dopant is easily promoted by heat or solvent. The molecular dopants tend to segregate from the vicinity of the polymeric chain of the .pi.-conjugated polymer backbone which results in a loss of the desirable properties.

A problem with the conventional .pi.-conjugated polymers is that they are brittle, hard and solid. In coating applications, the conventional .pi.-conjugated polymers do not adhere to the surface of the substrate. Thus the .pi.-conjugated polymers are blended with an insulating, non-conductive resin to form a mixture that could be adherent to the surface of a substrate. See U.S. Pat. Nos. 5,532,025, 5,543,084 and 5,556,518. When the conducting polymer is imbedded in the matrix of a non-conducting polymer such as an epoxy, polyurethane, polyacrylate or alkyd binders, the rate of dopant loss is reduced in the macroscopic level (e.g 0.1 mm length), but the problem of segregation at a microscopic length scale (e.g. 0.1 .mu.m length) is not eliminated. The electroactive properties will show signs of degradation over a period of several months. For a number of applications, the material stability is not good enough. In addition to the problem with the service life of coatings or blends of these .pi.-conjugated polymers, there are problems with the manufacturing process.

The dopants are easily lost during the manufacturing process either because of heat or because of contact with water or polar solvents. For example, U.S. Pat. No. 5,543,084 discloses a method for electrocoating a blend of epoxy and polyaniline. The conductive polymer PANI-PTSA (polyaniline doped by p-toluenesulfonic acid) was mechanically blended in aqueous solution and then electrophoretically coated on metal. From the disclosure it is evident that the anionic dopant of PANI-PTSA was lost before the .pi.-conjugated polymer was co-deposited with epoxy. A redoping by immersing the coating in camphor sulfonic acid was needed to restore polyaniline to its electrically conductive state. It is expected that the dopants incorporated by redoping will be easily dedoped again by either heat or by exposure to moisture.

Coatings that use undoped polyaniline (emeraldine base) have been disclosed in the literature (McAndrew et al. U.S. Pat. No. 5,441,772, and Epstein et al. U.S. Pat. No. 5,824,371). These .pi.-conjugated polymers without dopant are nonconductive because there is no charge carrier on the polymer backbone. For most applications it is essential to maintain the .pi.-conjugated polymers in the electrically conductive state. Thus it is desirable to have an electrically conductive polymer that is both processable and is stable against the loss of dopants.

An alternative to the above mentioned remedies is to synthesize a molecular complex of the .pi.-conjugated polymer and a polymeric dopant. If the polymeric dopant is strongly bonded to the .pi.-conjugated polymer the dopant will not be easily lost during the manufacturing process and the service life of the conducting polymer. A method was previously disclosed for synthesizing processable conducting polymers with stable dopants (Liu et al. U.S. Pat. No. 5,489,400). In this disclosure, a template-guided chemical polymerization was used to obtain a polymeric complex that contained a stand of polyaniline and a strand of a polyelectrolyte. The reaction product is a non-covalently bonded molecular complex between a conducting polymer and a polyelectrolyte. The molecular complex contains the two linear chains of the component polymers bonded in a side-by-side fashion. The complex is a double-strand synthetic polymer. When polyaniline is the conductive strand, dsPAN designates the double-strand polyaniline. Compared with the double-strand biopolymer, DNA, the synthetic dsPAN is less ordered in structure and is generally not in a helical conformation. Examples of the polyelectrolytes are poly(styrenesulfonic acid) and poly(acrylic acid). Since the two strands of polymers are bonded strongly, these polymeric complexes are stable and do not dedope easily.

The dsPAN disclosed in this '400 patent is one of three types. The first type is a water-soluble polymeric complex of polyaniline. This type of dsPAN is not suitable for anticorrosion coating applications because a pure dsPAN coating is redissolved in contact with water therefore the coating is lost in rain or humid air. It is conceivable that the water-soluble dsPAN can be incorporated in a polymeric binder that prevents water dissolution of the coating. The hyrophilicity of this type of dsPAN is, however, still a problem for corrosion protection. The coating will absorb moisture or swell in water and thus reduce the adhesion of binder to the metal substrate.

A second type of dsPAN disclosed was an insoluble solid that precipitates from the aqueous reaction medium. This type of dsPAN can only be mixed with the binder by vigorous mechanical mixing (in a manner similar to that used for blending single-strand PANI-PTSA with epoxy described in Example 13 of Kinlen et al. U.S. Pat. No. 5,543,084). Although a blend made in this manner overcomes the problem of dedoping in U.S. Pat. No. 5,543,084, it is still not ideal. The dispersion contains large and brittle particles. The particles are not small enough for optimal polymer-metal interaction even when the precipitated particles are ground with a ball mill. The large particles do not `wet` the metal surface. Another problem is that the mechanically stirred suspension is not a stable dispersion. It is difficult to maintain a uniform and stable suspension for large scale industrial production.

The third type of dsPAN disclosed in the '400 patent is a colloidal suspension of small particles. Although the particle size is suitable for the electroactive polymer to interact with the metal surface to impart protection of the metal, the concentration of the colloidal particles in water is quite low (less than 1 gm of colloidal particles per liter of water). This low concentration is incompatible to the preferred high-solid content coating formulation.

PCT Publication WO 97/03127 discloses a chemically modified dsPAN that is soluble in polar organic solvents and can be applied to metal surfaces as a paint. The coating disclosed protected metals from corrosion. These organic soluble dsPANs overcame the water absorption (swells problem of the water-soluble dsPAN disclosed in Liu U.S. Pat. No. 5,489,400. This type of dsPAN is suitable for use as either a solvent-based paint or as a blend with hydrophobic epoxy oligomer for a thermoset coating. This type of dsPAN is not disperable in water to make a stable mixture with water-borne epoxy. Thus this type of dsPAN is not suitable for a water-borne coating application.

Electrophoretic deposition of resins on metals provides excellent corrosion protection for steel used in automobiles and appliances. Using a conducting polymer as additive in the electrophoretic coating bath should enhance the effectiveness for protecting the metals from corrosion. Electrophoretic coatings containing single-strand conducting polymers are disclosed in WO93/14166 and U.S. Pat. Nos. 5,128,396, 5,543,084, 5,556,518. However, the dopants used in these disclosures are non-polymeric small molecular ions which have the disadvantage of dedoping in the e-coat processing and dedoping of the coated metal due to heat or moisture.

SUMMARY OF THE INVENTION

The present invention comprises a composition that advantageously avoids the problem of dedoping common to all the single-strand conducting polymers. A polymeric complex of polyaniline is dispersed in water as latex-like small particles. The polymeric complex is hydrophobic enough so that the problem of water-absorption and swelling of the painted surface is avoided. The particle sizes of the latex-like suspension are small enough so that the .pi.-conjugated polymers may interact with the metal surface effectively. The percentage of the .pi.-conjugated polymers in the non-conducive binder can be low enough so that the mechanical strength of the coating is essentially the same as the coating without the .pi.-conjugated polymer.

The invention comprises a family of two-component polymeric complexes of .pi.-conjugated polymers that are suitable for water-borne coating applications. The invention also embodies the process of making the polymeric complexes, the use of the polymeric complexes in an anti-corrosion formulations, a family of coating compositions containing the polymeric complexes, the method of applying the coating composition on a surface and the coating compositions per se.

More particularly, the present invention comprises: 1. A water-borne coating composition comprising a polymeric complex between a .pi.-conjugated polymer, a polymeric ion (that serves as a dopant for the .pi.-conjugated polymer), and a non-conductive polymer (that serves as a binder or resin for coating). 2. A water-borne coating composition as in (1) where the .pi.-conjugated polymer and the polymeric dopant are strongly (non-covalently) bonded to form a molecular complex. 3. A water-borne coating composition as in (2), where the polymeric molecular complex has substantial any with the non-conductive polymeric binder so that the water-borne dispersion will not segregate, and the dried coating film is homogeneous in composition. 4. A composition described in (3) that can be electrophoretically deposited on a metal substrate to form a coating that contains a polymeric dopant, and a non-conductive polymeric binder. All three components are electrodeposited onto the metal surface with negligible loss of dopants. 5. A coating composition described in (3) or (4) that has enhanced anticorosion performance compared with a coating composition that does not contain a polymeric complex of the .pi.-conjugated polymer. 6. A coating composition of (3), (4) or (5), in which the non-conductive polymer is a thermoset polymer and the polymeric complex of the .pi.-conjugated polymer serves the dual function of an anticorrosion additive and a crosslinking agent. 7. Optionally, the said electroactive (or conducting) polymeric complex is used as an additive to a commercial electrocoat formulation that contains additional cross-linking agents. 8. An electrocoating process that allows deposition of the coating composition onto a metal surface. 9. An electrocoating process that allows cathodic deposition of the coating composition onto a metal surface. 10. An electrocoating process of (9) that forms a protective coating on the surface of aluninum alloys. 11. An electrocoating process of (9) that forms a protective coating on the surface of steel.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a photograph of a cool control sample; and

FIG. 2 is a photograph of a sample coated with a composition of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one aspect of the invention, a two component dsPAN complex that is water-borne and satisfies the requirement of hydrophobic/hydrophilic balance is disclosed. The two-component polymeric complex should contain two strands of polymers. One strand is a .pi.-conjugated polymer that imparts electroactive and conductive properties to the coating so that it is effective for protecting metals from corrosion by its interaction with the metal surface.

Another strand is a polymeric ion that is non-covalently bonded to the .pi.-conjugated polymer at a multiplicity of sites due either to electrostatic attraction, hydrogen bonding or van der Waals force. The strong bonding between the two component of the polymeric complex provides the needed stability against the loss of ionic dopants. The improved dopant stability in the double-strand polymeric complex overcomes the deficiencies of the single-strand conducting polymers in the prior art. An example of a double-stranded polymeric complex is the dsPAN which has polyaniline as the .pi.-conjugated component in the complex, and a polymeric anion as the second strand. Examples of other .pi.-conjugated complexes include polyaniline, polypyrrole, polythiophene, poly(phenylene sulfide), poly(p-phenylene), poly(phenylene vinylene), poly (furylene vinylene), poly(carbazole), poly(thienylene vinylene), polyacetylene, and poly(isothianaphthene) having charges thereon when the polymer is in its electrically conductive form.

Examples of the polymeric amions are poly(acrylic acid), poly(methacrylic acid), poly(vinylmethylether-co-maleic acid), poly(methylmethacrylate-co-acrylic acid), poly(ethylmethacrylate-co-acrylic acid), poly(acrylamide-co-acrylic acid), and other anionic polymers.

In addition to the anionic polymers, polymers containing both the anionic and the cationic functional groups such as a protonable amine and a tertiary amine can be used. The ionic functional groups in the second strand serve at least two functions: (1) the anionic groups are the counter ions to the positive charge carriers on the .pi.-conjugated polymer, and (2) both the anions and the cations on the polymer help disperse dsPAN in water so that it can be used as a war-borne coating material.

Another aspect of this invention is that the double-strand polymeric complex is balanced with respect to its hydrophilic and hydrophobic properties. The complex needs to be sufficiently hydrophilic so that complex can be dispersed in water but not too hydrophilic to absorb water after the coating is applied to metals. An example of a dsPAN that is too hydrophilic to be useful for coating is that of a molecular complex between polyaniline and poly(styrenesulfonic acid) Liu et al. U.S. Pat. No. 5,489,400. This polymeric complex PAN:PSSA is water-soluble and is dissolved in that as a highly solvated random-coil of polymer. This dsPAN satisfies one of the requirements that it be dispersable in water but it does not satisfy another requirement for coating. When PAN:PSSA is coated on a surface without a polymeric binder, it is readily re-dissolved when it is immersed in water. When PAN:PSSA is coated on a surface with a polymeric binder such as epoxy or polyurethane, the coating has the problem of being hygroscopic. It absorbs water and swells which is not a desirable property for coatings applications.

It is known that a proper hydrophobic/hydrophilic balance can be achieved to obtain a latex-like dispersion of the complexes by one or both of the following methods: (1) use a polymer (the second strand of the two-component polymeric complex) that contains a certain number of ionic functional groups (hydrophilic) and a certain number of non-ionic organic hydrophobic functional groups attached to the polymer backbone so that the desired hydrophobic/hydrophilic balance is achieved; or (2) use a synthetic procedure that effects a certain polymeric chain folding (similar to the tertiary structures commonly found in globular proteins) that exposes most of the hydrophilic functional groups at the surface of the polymer particles to provide suspension of the particles in water, but maximizes the hydrophobic content of the interior of the polymer particle.

As an example of the first method, the polymeric complex of polyaniline and poly(methylacrylate-co-acrylic acid) is synthesized with an appropriate choice of a relative number of methylacrylate segments and the acrylic acid segments in the polymer chain. A higher percentage of methylacrylate segments make the polymeric complex more hydrophobic, and a higher percentage of acrylic monomer unit makes the polymeric complex more hydrophilic.

The second method involves the molecular self-assembly of a precursor of the polymeric complex. The self-assembled precursor particles are dispersed in water and have a structure that, after a template guided polymerization, results in a polymeric complex that is water-borne but hydrophobic enough for coatings application. The method employs a two-step synthetic procedure. In the first step, precusor particles are prepared that have a hydrophobic core and a hydrophilic surface. In the second step, the precursor particles are polymerized to form a water-borne polymeric complex of the .pi.-conjugated polymer. This synthetic strategy is illustrated in the synthesis of the water-borne, latex-like dsPAN. The synthesis of a water-borne polymeric complex between polyaniline and a polymeric ion poly(vinylmethylether-co-acrylic acid) PVME-MA.

In the first step, the aniline monomers are added to the polymeric ion PVME-MA dissolved in water. The aniline monomers are adsorbed onto the backbone of PVME-MA to form an adduct which has hyrophobic segments at the sites where the aniline monomers are adsorbed. When a substantial length of the polymeric adduct (PVME-MA):(Aniline).sub.n becomes hydrophobic the polymer chain of the adduct folds into a globular particle with the hydrophobic chains packed in the interior of the globular particle and the hydrophilic groups populate the water/particle interface. It is possible that aggregates of the adducts are present in the aqueous solution at this stage. A solution shows characteristic light scattering phenomenon indicating the formation of particles with 20 to 100 nm radius of gyration. The size of the adduct (PVME-MA):(Aniline).sub.n particles are adjustable by using a mixed solvent of water and alcohol, the acidity of the solution, and by controlling the temperature of the solution.

In the second step, the precursor is polymerized by introducing an oxidizing agent such as hydrogen peroxide or sodium persulfate. During this stage, the aniline monomers adsorbed in the adduct (PVME-MA):(Aniline).sub.n are polymerized into a polyaniline which is strongly, but non-covalently, bonded to the polymer PVME-MA to form a polymeric complex (PVME-MA):polyaniline. The polymeric complex is dipersed in the aqueous solution without precipitating out of the solution (as the single-strand polyaniline would precipitate) due to the hydrophilicity at the surface of the particle. The polymeric complex polyaniline:(PVME-MA) is a latex-like suspension in water. It forms a polymer coating on the surface of metal, plastic, glass etceters. Once the water in the coating is evaporated, the coating is no longer redissolvable by water or common solvents. This coating has a high enough content of the hydrophobic groups so that the coating does not have a problem with swelling in water (which is in combat with the water-soluble PAN:PSSA complex) and thus it is suitable for coating applications.

The polyaniline:(PVME-MA) complex synthesized in this manner has the advantages over the single-strand polyaniline because it is water-dispersable and is resistant to dedoping by water or solvent. The polymeric complex with proper hydrophobic/hydrophilic balance is also better suited for coating applications comparing to the water-soluble polymeric complexes disclosed in U.S. Pat. No. 5,489,400 became it reduces the water-swelling problem in the dried coating. The composition of this invention is different from that of a copending international application Yang et al, WO 97/03127 (Electroactive polymer coatings for corrosion control). WO 97/03127 discloses an organic solvent soluble dsPAN while the present invention discloses a water-borne coating composition.

A uniform and stable water-borne resin composition.

Improved uniformity of coating due to molecular association between dsPAN and a non-conductive water-borne resin.

Another aspect of the present invention is a water-borne coating composition that contains a conductive polymeric complex and a non-conductive resin. The resin can be a thermoplastic or a thermoset polymer. The non-conductive polymeric resin serves as a binder for the conductive polymer and it is preferable that the polymeric resin is chosen from a commercial available thermoplastic or thermoset coating material. The polymeric complex of the .pi.-conjugated polymer has the functional groups that leads to a substantial extent of molecular association between the .pi.-conjugated polymeric complex and the non-conductive resin. This intermolecular association is advantageous in two respects: (1) the coating is uniform at a microscopic or nanometer scale with the advantage of stable unsegregated paint or coating bath and a reliable performance for the coated products, and (2) when used as an electrophoretic coating material, the molecular association insures that the resin and the .pi.-conjugated polymeric complex are not separated during the electrocoat process.

The intermolecular association between the .pi.-conjugated polymeric complex and the non-conductive resin comes foam the electrostatic attraction, hydrogen bonding, the hydrophobic interactions, and chemical bond formation. Some of these intermolecular interactions can be demonstrated in an aqueous paint formulation containing a cationic epoxy as the non-conductive resin and the polyaniline:(PVME-MA), or the polyaniline:PAA complex as the eletroactive, .pi.-conjugated polymer. A brief review of the structural features of the cationic resin and the O-conjugated polymeric complex is useful for understanding the intermolecular interactions.

A commercial cationic epoxy resin is a multi-functional epoxide oligomer having chemically-bound tertiary or quaternary ammonium functional groups in addition to the epoxy and hydroxy groups present in other types of epoxy resins. Low-molecular weight epoxy resins with molecular weights between 1,000 and 1,500 are reacted with a secondary amine, forming hydroxyl groups alongside terminal tertiary amino groups. The amine functional groups are subsequently neutralized with a weak organic carboxylic acid such as lactic acid or acetic acid to produce a polymer which is in the form of a water soluble salt. The water-borne resin is then a polycation in an acidic solution.

The .pi.-conjugated polymeric complexes such as the polyaniline:PVME-MA or the polyaniline:PAA complexes two strands of polymers. One of the strands is a polycarboxilic acid. For example, a PVME-MA with a degree of polymerization of about 1000 has about 500 carboxylic functional groups attached to the polymer backbone. A substantial fraction of the carboxylic acid functional groups are ionized if due pH of othe solution is higher than 3 or 4. At this pH the number of positive charges on the polyaniline chain is less than that of the PVME-MA chain. The net charge of the dsPAN is negative. Thus the .pi.-conjugated polymeric complex is a polyanion.

When PVME-MA and the cationic epoxy are mixed in a solution, the electrostatic attraction between the polyanions and the polycations leads to the formation of polymeric complexes. When the electrical charges on the polyanions and the polycations are matched exactly, the polymeric complexes are precipitated from the solution. This phenomenon can be observed in a titration of the PVME-MA solution with a cationic epoxy resin. The precipitation of the charge balanced polyanion/polycation complex is an indication of strong association between the polycation and the polyanion. For a practical formulation for use as water-borne coating, the cationic epoxy resin and the .pi.-conjugated polymeric complex are mixed in a manner that avoids going to the charge-balance point and thus prevents the precipitation. For applications as a cathodically deposited coating, the overall charge of the molecularly associated particles will be positive so that the particle will be electrophoretically driven to the cathode.

In addition to the electrostatic association, there exists multiple hydrogen bonding between the carboxylic acid groups of the polyaniline:PVME-MA or polyaniline:PAA and the nitrogen atoms in the cationic epoxy resin. A type of cationic epoxy resin contains segments of bisphenol-A which provides some hydrophobic interaction with the .pi.-conjugated polymeric complex so that the hydrophobic interactions nay also contribute to the intermolecular association.

The following example demonstrates the existence of the intermolecular association. An aqueous solution of polyaniline:PVME-MA (or polyniline:PAA) mixed with a cationic epoxy was used as an electrophoretic coating solution 20 parts (by weight) of cationic epoxy was mixed with 1 part of polyaniline:PVME-MA. The resulting intermolecularly associated complex of dsPAN and the cationic epoxy in a pH 4.5-5.5 solution is not electrically neutral as evident from the fact the solution is stable without precipitation. Two identical aluminum A6061 coupons (1".times.2") were immersed in the resin/dsPAN suspension, and are connected to a DC power supply. An electrophoretic process was carried out with applied voltages ranging from 50 V to 200 V for 90 seconds of coating. When the two electrodes were removed from the coating bath, it was found that the anode had no polymer deposition while the cathode had a uniform layer of polymeric coating. Both coupons were put in an oven (180 C.) for 20 minute. The polymer deposited on the cathode was cured to form a green-colored epoxy coating. The cationic epoxy used was a bisphenol-A diglycidyl ether oligomer which is transparent and had no color. The green-color evidenced the presence of dsPAN in the coating. The UV-visible absorption spectra of the epoxy coating were measured with a reflectance spectrometer. The spectrum obtained matches with that of polyaniline in the electrically conductive state of emeraldine salt. In a control experiment, a solution of dsPAN (pH 4.5) without cationic epoxy was used as an electophoretic bath. The electrophoretic coating resulted in a thin layer of green color on the anode and no deposition on the cathode.

The experiments described in the preceding paragraph provide further support for the existence of the intermolecular association between the dsPAN and the cationic resin. The dsPAN has net negative charge so when it is the only polymeric component in the solution, it electrophoretically migrates to the anode. When dsPAN and the cationic epoxy are mixed in a solution, the intermolecular association between dsPAN and the cationic epoxy takes place. With a proper ratio of mixing, the net charges of the dsPAN:epoxy complex can be made to be positive so that the complex is deposited as one unit onto the cathode.

The controllable association between dsPAN and the cationic epoxy is advantageous. It makes the coating bath more homogeneous and the suspension is more stable than the mixtures of the single-strand polyaniline particles with the same epoxy. The molecularly associated deposition on the electrode is less likely to be phase-segregated during the heat curing process. The result is a more homogeneous and better coating.

With an appropriate catalyst, it is also possible to chemically link the epoxy functional group and the carboxylic functional groups of the two components.

The polymeric complex may be used as a crossed agent for thermoset resins.

Another aspect of the present invention is that the polymeric complex of the .pi.-conjugated polymer may be used as a cross-linking agent for thermoset polymers. For example, the PAN:PAA and PAN:PVME-MA complexes may serve as a cross-linking agent when epoxy is used as a thermal set resin. The epoxide and the hydroxyl functional groups in the epoxy resin are reactive with the carboxylic functional groups in the polymeric complexes PAN:PAA or PAN:PVME-MA. This reaction may be promoted by the presence of an acid or base catalyst or by heat.

The ability for reaction with the binder resins to function as a cross-linker is unique to the double-strand conducing polymers. The single-strand polyaniline or other conducting polymers are not able to contribute to the cross-linking of the binder resin even if the dopants are chosen to contain one or two carboxylic functional groups. If the small molecular dopants are reacted with the resin, the dopant will most likely to be detached from polyaniline or other .pi.-conjugated polymer. It will change the conducting polymer from the doped conductive state to an undoped insulating state and thus lose its electroactivity.

The cross-linking reaction is mostly completed when the .pi.-conjugated polymer containing an epoxy resin is cured at an elevated temperature (60 C. to 180 C.). An obvious advantage is that it is not absolutely necessary to have another cross-linker present in the formulation. A less obvious, but equally valuable advantage is that the cured coating contains a more homogeneous concentration of the .pi.-conjugated polymers and the .pi.-conjugated polymers will not be mobile during the service life of the coating. This may be advantageous over the single-strand polyaniline which is only blended with the resin polymer without covalent bonding to anchor the .pi.-conjugated polymer. Under heat or aging, the .pi.-conjugated polymers may migrate and segregate into domains within the coating. Such possibilities will make the performance of the coating decline with time. The .pi.-conjugated polymer is strongly bonded with the polymeric second strand which, in turn, is covalently anchored to the 3-dimensional network of the thermal set resin after curing. With a molecularly dispersed mixture of the dsPAN in epoxy, and the covalent anchoring of the polymeric complex in the resin, the properties and the performances of the coating is expected to be more stable than the polymeric blends of the single-strand polyaniline.

It is found that when the electroactive polymer is used with more than 3% weight of the epoxy resin, the curing is complete and the cured resin mixture is a tack-free and hard coating. When the coating formulation is slightly more acidic than the normal e-coat bath, a complete curing of the epoxy is possible with as low as 1% of the .pi.-conjugated polymers. It is believed that the acidic form (which has mobile radical cations on the .pi.-conjugated backbone) is capable of catalyzing the cross-linking reaction between the dsPAN and the epoxide. The catalyst may also promote the polymerization of the epoxides.

Effective for anticorrosion without the need for high dsPAN content.

A surprising discovery of this invention is that a very low percentage of dsPAN in a binder resin is effective for an improved protection against corrosion. Samples of epoxy coatings on aluminum and steel with dsPAN content ranging from 6% to 1% of PAN:PVME-MA or PAN:PAA were tested against the conventional electrophoretic coatings. Both the ASTM B-117 salt-fog spray tests and electrochemical impedance spectroscopy tests showed that the dsPAN containing epoxy coatings are more effective than the traditional electrocoats.

The dsPAN performs better in an environment with wider pH range. The dsPAN coating is more resistant to deprotonation than the single-strand PAN.

The dsPAN is advantageous over the single-strand PAN in another aspect. The single-strand PAN loses its electrical conductivity when it is in contact with water of pH 5 or higher. In fact a pH titration of the single strand polyaniline showed that the electrical conductivity begins to decrease at pH 3 to 4. The single-strand polyaniline changes its color from green to blue when the pH value is raised above pH 5. The double-strand polyaniline is much more resistant to deprotonation. For example a water suspension of PAN:PAA and PAN:PVME-MA remains in the green, conductive state until pH 8.5 to 9. The polyanionic second strand in the double-strand polymer is responsible for providing a microscopic environment that shifts the pH value for the conductor-to-insulator transition. ("Double-strand polyaniline as a molecular quasi-memory to chemical stimuli," G. P. Kota, L. Sun, H. Liu, S. C. Yang, Mat. Res. Sac. Symp. Proc., Vol. 488, pp359-364 (1998).)

When the coatings containing polyaniline and resin binders are tested in neutral pH aqueous salt-fog spray (ASTM B-117) or immersion over a period of time, the loss of corrosion resistance and the loss of electrochemical impedance were found to be coincident with the time when the color of the polyaniline component is changed from green to blue, signifying that the conductive green colored state (emeraldine salt) of polyaniline is more effective anticorrosion ingredient than the insulating blue colored state (emeraldine base). Furthermore, when a conducting polymer is deprotonated with amonia before coating on metal, the coating does not have a measurable advantage in anticorrosion than the coating without the .pi.-conjugated polymer. This again indicates that the conductive state is much more effective than the noncoductive state of polyaniline for anticorrosion. It was found that the coating (with a mixture of .pi.-conjugated polymer and the nonconductive resin) does not need to be an electrical conductor. It is only required that the .pi.-conjugated polymer is in its electrically conductive state. Even when the conductive polymers do not form a continuous network in the coating to render macroscopic conductivity, the .pi.-conjugated polymers in the coating resin are still effective for anticorrosion. When the .pi.-conjugated polymers are deprotonated to change to its emeraldine base form the effectiveness for anticorrosion is lost.

The dsPAN, being more resistant to deprotonation, is applicable for a wider range of environments. The traditional single-strand polyaniline is effective only in a slightly acidic environment (pH<5). The dsPAN containing resin is effective for pH neutral environment (pH 5-7) and the sea water (pH 8).

dsPAN is compatible with commercial water-borne coating formulations.

The double-strand conductive polymers can be used as an additive to the commercially available water-borne paints or electrocoat baths. The water-borne paint formulation with the added conducting polymer is a stable suspension if a proper procedure (described in the examples) for mixing the additives is followed. The new formulation can be electrocoated by a normal electrocoat procedure. Samples of electrocoated metals cured at 180.degree. C. resulted in hard coatings. The coated samples were tested and were found to have improved anticorrosion properties compared to the control samples.

Curing with conventional curing agents:

The conventional chemically modified or capped polyamines, polymercaptans or polyisocyanates be used as curing agents. These curing agents need to be chemically modified or capped to prevent reaction with epoxy in the electrophoretic coating bath. The cross-linking is completed at elevated temperature during baking.

Alternatively, a high-temperature curing agent can be used. Examples of high-temperature curing agents are polycarboxylic acids, polyanhydrides, polyphenols and carboxy-functional polyesters. These curing agents, in absence of a catalyst, do not react with the epoxy at room temperature but reacts at an elevated temperature during baking.

The coating may be applied onto metal surfaces with or without the conventional surface treatment.

The traditional processes for coating aluminum alloys include surface treatments that use chromate surface conversion. The coatings of the preset invention can be applied onto the surface of a bare aluminum alloy without surface pretreatment. The present coating on aluminum samples without surface pretreatment showed good resistance to corrosion. The present coating formulation is useful as a coating that replaces the toxic chromates in coatings applications.

The coating may be applied either by painting or electrodeoposition.

The water-borne coating composition containing the double-strand conducting polymer may be applied to a metal surface by painting, immersion, or electrodeposition.

EXAMPLE 1

Synthesis of Polyaniline: Poly(acrylic acid) Complex with r=N.sub.AN /N.sub.--COOH =1, [Polyaniline:poly(acrylic acid), r=1

(Here, we use the symbol ":" to indicate the non-covalent bonding between two polymers. The value of r specifies the ratio N.sub.AN /N.sub.--COOH)

Step 1: Adsorption of Aniline onto Poly(acrylic acid) to Prepare [Poly(acrylic Acid):(Aniline).sub.n ]:

A complex [poly(acrylic acid):(Aniline).sub.n ] is prepared by adsorbing (or binding) the aniline monomer onto the poly(acrylic acid) in a water/methanol solution. The adsorbed aniline molecules are later polymerized into polyaniline in Step 3.

Mix 10 ml of methanol with 7.208 gm of poly(acrylic acid) aqueous solution (containing 25% of PAA, Polysciences, MW=90,000). Water is added to increase the volume of the solution to 100 ml. This solution is rigorously stirred with a magnetic stirrer for 15 minutes. This solution contains 0.025 moles of poly(acrylic acid).

Slowly add 2.328 g of freshly distilled aniline to the poly(acrylic acid) solution under rigorous stirring. An additional 10 ml of methanol is added. Stirring is continued for 30 minutes. The total amount of aniline equals 0.025 mole. The mixture has a pH value of about 5.

The following observations are consistent with the formation of a molecular complex between the aniline molecules and the poly(acrylic acid). The viscosity of the solution is significantly increased upon the addition of aniline. The measured increase in intrinsic viscosity is much more than that expected from a simple mixtre of aniline and poly(acrylic acid). For a simple mixture with no binding between aniline and the complexes, the intrinsic viscosity should be about equal to the sum of the two components in pH 5 solution. The high viscosity is consistent with the binding of aniline onto the poly(acrylic acid) chain. When aniline is adsorbed onto poly(acrylic acid), the polymer chain is more extended than that of the original poly(acrylic acid) random coil, and thus the viscosity is much higher. The aniline molecules may bind to poly(acrylic acid) by hydrogen bonding, or the anilinium ions may be strongly attracted by the electrostatic force from the ionized portion of the poly(acrylic acid). The latter electrostatic is known as "counter ion condensation" for polyelectrolyte (Reference: G. Manning, J. Chemical Physics, 89, 3772 (1988), Accounts of Chemical Research, 12, 443 (1979)). The non-covalent binding between aniline monomers and the poly(acrylic acid) is represented by a colon ":" in the symbol for the adddet poly(acrylic acid):(An).sub.n.

Step 2: Formation of an Emulsified Poly(acrylic acid):(An).sub.n Adduct

100 ml of 2 M HCl is added to the poly(acrylic acid):aniline solution. The solution turns milky white immediately due to the scattering of the ambient light by a macro-emulsion of the polymeric complex. When the solution is continuously stirred vigorously, the intensity of light scattering decreases and the color of the scattered light gradually changes from milky white to nearly transparent with a tint of turbidity. When this faintly turbid solution is examined by illumination with a focused beam of white light (or sun light) and viewed at an angle against a dark background, the scattered light has a blue tint.

The solution initially turns to milky white macro emulsion because the acid added to the solution decreases the degree of ionization of the poly(acrylic acid):(An).sub.n adduct formed in Step 1. The unionized adduct becomes more hydrophobic and folds into particles that contain an interior hydrophobic core that is rich in aniline adsorbed to the poly(acrylic acid). The exterior surface of the particles may be more hydrophilic with some ionized carboxylate groups in contact with the surrounding water molecules. The emulsified particle in this case is likely to be an aggregate of the polymeric adduct poly(acrylic acid):(An).sub.n which is hydrophobic if the aniline molecules remain bounded to the poly(acrylic acid) when the hydrochloric acid is added. Immediately after the addition of the hydrochloric acid, the size of the aggregated particle is large, but the aggregates rearrange into smaller particles in the methanol solution.

The change in the light scattering is consistent with an initial formation of macro-emulsion that scatters visible light of all colors, and, the subsequent transformation into micro emulsion with smaller particle size that scatters only the shorter wavelength region of the visible light. The presence of methanol or other polar organic solvents helps to break the