WO1994025533A1

WO1994025533A1 - Conductive surface coatings

Info

Publication number: WO1994025533A1
Application number: PCT/US1994/004005
Authority: WO
Inventors: Mark Mitchnick; Mamoun Muhammed
Original assignee: Mark Mitchnick
Priority date: 1993-04-28
Filing date: 1994-04-06
Publication date: 1994-11-10
Also published as: EP0698065A4; EP0698065A1; AU6665394A

Abstract

Disclosed are conductive compositions for application to a surface, which include zinc oxide particles having a substantially rod shape.

Description

CONDUCTIVE SURFACE COATINGS

The invention relates to surface coatings which have conductive properties when applied to a surface.

Background of the Invention

U.S. Patent 4,971,727 discusses an electrically conductive coating which is useful as a primer or a surface paint on plastics, and suggests using conductive zinc white in the coating. U.S. Patent 5,071,692 discusses a laminated glazing for coating glass or plastic which contain indium tin oxide.

Prior art disclosures of metal oxides include preparations of spherical zinc or titanium oxide particles (U.S. Patents 5,032,390, EPO 433 086 Al, 4,606,869, 3,397,257, 4,543,341, 4,808,398, 2,898,191, 4,9233,518, and 4,721,610), crystalline metal oxides (U.S. Patents 5,093,099, 5,091,765, 4,261,965, 2,900,244, and 4,722,763), including crystalline whisker-shaped zinc oxide (U.S. Patent 5,066,475), and needle-shaped zinc oxide particles (U.S. Patent

5,102,650). U.S. Patent 5,093,099 relates to a process for preparing flaky fine particles of zinc oxide for external use and having an average particle diameter of 0.1-1 micron, an average thickness of 0.01-0.2 micron, and an aspect ratio of at least three. U.S. Patent 5,066,475 relates to whiskers of zinc oxide having a crystal structure which includes a central body and four needle crystal projections radially extending therefrom, and is useful in reinforcing materials. U.S. Patent 5,012,650 relates to needle-like electrically conductive zinc oxide filler which is useful for its low specific volume resistance and electrical conductivity. It is an object of the invention to provide a composition for coating a surface which contains zinc oxide particles of dimensions which allow the particles to maintain good inter-particle contact and to confer conductivity to the surface coating. Zinc oxide particles of the invention, because they are rod- shaped, may assume a side-by-side arrangement or a criss-cross-packing arrangement, once the surface is coated with the composition, such that there are relatively few gaps between the particles for loss of conductivity. It is also an object of the invention to provide a conductive coating comprising zinc oxide particles which have a high ratio of surface area to volume or weight. It is another object of the invention to provide a composition for coating a surface which possesses even spreadability over the surface. It is another object of the invention to provide a composition containing zinc oxide particles for coating a surface which possesses a smooth texture and in which the particles are easily admixed with and dispersed within a spreadable vehicle. Further, it is an object of the invention to provide a conductive surface coating which is transparent or any color which is desired.

Summary of the Invention The invention is based on the recognition that substantially rod-shaped doped zinc oxide particles are useful in a composition for application to a surface to confer conductive and antistatic properties on the surface. The composition is thus capable of forming an antistatic layer on the surface to which it is applied. As used herein, "substantially rod shape" refers to an elongated spherical shape, e.g., having an aspect ratio (i.e., length/diameter) of at least two, or a flattened rod-shape, such as the shape of a green bean; "doped" refers to a mixture of rod-shaped zinc oxide particles and a dopant, the dopant comprising non-rod-shaped metal particles or rod-shaped metal particles other than zinc oxide, of metals described herein, wherein the dopant comprises preferably from 0.01-10% of the composition.

In preferred embodiments, the composition is applicable to any inanimate surface, including natural and synthetic surfaces such as plastic, glass, metal, wood, etc., and the composition further comprises an agent for spreading on the surface. Examples of such agents are those found in paints, glazings, stains, etc., e.g., silicone oils, mineral oils, vegetable oils, latex, or water. The composition containing the substantially rod shaped particles is evenly spreadable upon the surface due to the tendency of the rod shaped particles to lie side-by-side on the surface and fill in irregularities of the surface. Other preferred embodiments include the following. The rod shaped particles may have a substantially spherical cross-section with an aspect ratio of at least two and preferably three. "Substantially spherical cross-section" refers to a spherical or flattened spherical cross-section. The zinc oxide particles of the invention may comprise from 0.1% to as much as 50% of the composition by weight, depending upon the desired thickness and color of the composition. More preferably, the zinc oxide rods comprise between 1% and 30% of the composition; most preferably, between 5% and 20%.

The invention also features a method of making a conductive coating, the method including the step of combining an agent for spreading on a surface with zinc oxide substantially rod-shaped particles, wherein the zinc-oxide particle is formed by bringing into contact in aqueous solution zinc ions, ammonium ions, and a carbonate source to form a precipitate, separating and optionally calcining the resultant precipitate to zinc oxide, wherein the metastable precipitate is formed by controlling the morphology and size of the particles by maintaining, during the precipitation: a temperature between 10 to 40°C; a pH between 5 and 10; and wherein at least one of zinc ion and carbonate source is progressively made available to the solution at a precipitation limiting rate.

The invention also features a method of applying a conductive coating to a surface, comprising the step of applying a conductive coating comprising a surface spreadable agent and zinc oxide particles having a substantially rod shape to a surface.

Rod-shaped particles of the invention may have a length of between 3 nanometers and 10,500 nanometers; preferably between 100 and 6,000 nanometers; more preferably between 100 and 600 nanometers, inclusive. The rod-shaped particles have a diameter of between 1 nanometer and 3,500 nanometers; preferably between 10 and 2,000 nanometers; more preferably between 33 and 200 nanometers, inclusive. In order to be useful in compositions of the invention, these rod-shaped zinc oxide particles must have an aspect ratio of at least two. Zinc oxide compositions of the invention may be formulated so as to be transparent enough to be useful in, e.g., paints in which a natural or transparent quality is desired. A transparent quality is obtained by including in the composition small rod-shaped particles, i.e., having a length of less than 300 nm and a diameter of less than 100 nm. Larger particles, i.e., having a length longer than 300 nm and a diameter greater than 100 nm, are useful for compositions in which the opacity of the zinc oxide is an asset, e.g., white sunscreens, or white or colored particles. Zinc oxide particles which are not rod-shaped tend to aggregate and clump upon spreading of the composition over a surface. This, in turn, dramatically reduces the conductive properties of a liquid containing such particles. For example, zinc oxide spheres have a geometric shape which allows for single points of contact between particles. Thus, high concentrations of spheres are required to ensure continuous contact between particles. It is preferable that the conductivity of a composition does not substantially diminish upon spreading of the composition on the surface. This may be achieved by including substantially rod-shaped zinc oxide particles in a surface application. Rod-shaped particles possess a geometric shape which allows for inter-particle contacts in a variety of arrangements of the rods. "Evenly applicable", as used herein, is intended to mean that the composition is applicable to the surface so that conductivity is even across the surface. "Surface spreadable agent" refers to an organic or inorganic chemical which possesses sufficient fluid properties to allow it to be spread upon an inanimate surface.

Compositions of the invention include zinc oxide particles having improved weight efficiency in that their rod shape allows them to assume relative particle orientations so as to maximize their conductive properties. For example, less zinc oxide (by weight) is needed in the rod-shaped form than in the spherical form to give equal or better conductivity. This is true because conductivity depends on particle/particle contact, which is largely a particle surface phenomenon, i.e., relatively less particle thickness is required relative to particle surface area. Thus, compositions of the invention provide a large surface area and require relatively less zinc oxide by weight for equivalent or better conductivity.

Another advantage of compositions of the invention is that the rod shape of the particles promotes side- by-side and end-to-end arrangements of the rods rather than the stacking or clumping tendency of crystals or spheres. Thus, compositions of the invention provide a relatively even layer of zinc oxide, with consequent uniform conductivity at the surface. As a result of this superior particle orientation, compositions of the present invention will more evenly cover a surface, e.g., will fill in surface irregularities. Rod shaped zinc oxide particles do not tend to agglomerate and thus will disperse evenly within the composition. A composition of the invention thus will spread easily over the surface to which it is applied.

As used herein, "conductive" and "antistatic" refer to the ability to transmit electricity. Due to the improved inter-particle contact of zinc oxide rod- shaped particles, relatively less zinc oxide rod particles (wt/wt polymer) are required to give good conductivity. An undoped zinc oxide rod preparation with a particle size of about 75 nm length/1450 nm diameter will possess a resistivity of less than approximately 1 x 10¹³ohm.cm.

Conductive coatings of the invention have a resistivity of at least 100 ohm.cm, preferably at least 1000 ohm.cm, more preferably at least 10,000 ohm.cm, and most preferably at least 50,000 - 1,000,000 ohm.cm. High resistivities (e.g, greater than 10,000 ohm.cm) are preferred for certain uses. The resistivity of the zinc oxide rod particle preparation may be from 10^" 10⁸ ohm.cm, preferably 10° - 10⁶ ohm.cm, most preferably 10¹ - 10⁵ ohm.cm.

Conductive coatings of the invention are useful where static electricity creates a problem. Accumulated static electricity causes problems such as interference with functioning electric or magnetic components or devices, such as in a computer or in the process of photographic exposure and development. In addition, static electricity causes dust to be attached to or to lie on the surface of a plastic or glass surface, and also may provide a strong electric shock to the user. In extreme cases, discharge occurs, causing combustion or ignition of inflammable substances. Static also can cause serious defects in photographic film. A conductive coating of the invention may be manufactured as a substantially transparent glazing which may be used as a coating, e.g., for plastic, glass, metal, or any type of inanimate surface, e.g., for computer casings, computer screens, and audio and visual recording materials. These and other properties of the invention will be understood by those skilled in the art from the description herein and from the claims.

Description Drawings

Fig 1 is a flow diagram of a first process for producing the zinc oxide particles described herein; Fig 2 is a scanning electron micrograph of spherically shaped zinc oxide particles;

Fig 3 is a scanning electron micrograph of rod shaped zinc oxide particles;

Fig 4 is a scanning electron micrograph of fiber shaped zinc oxide particles;

Fig 5 is a flow diagram of a second process for producing zinc oxide particles as described herein.

Preferred Embodiments Described below are ultrafine zinc oxide, carbonates and oxalates with defined particle morphologies, and techniques for the production of such defined-morphology-particles which avoid the deficiencies of the prior art calcination or decomposition steps, and which are amenable to simultaneous or sequential co-precipitant doping.

Zinc oxide particles in the form of rods in the size range 30 to 500 and even up to 10,500 nm in length, preferably 50-6,000 nm, more preferably 100- 500 nm in length; a diameter of between 1 and 3,500 nm, preferably between 10 and 2,000 nm, more preferably between 100 and 150 nm and 10 to 150 nm in diameter, are most useful in the invention. The rods will generally have a circular cross section and comprise X- ray amorphous material.

Typically, the particles will display a very homogenous size and aspect ratio distribution with micrographs of the rods showing greater than 75% and even up to 90% of the particles having a substantially similar size and aspect ratio. For instance, within a rod population of nominal aspect ratio 3 and diameter 100 nm, it is possible to produce populations in which 75% of the particles fall within an aspect ratio range of 2 - 4 and diameter range 50 to 150 nm which represents outstanding homogeneity in comparison to prior art methods.

Such zinc oxide particles with defined sizes and morphologies, including those produced by the techniques below, show interesting rheological properties. The rheological properties of the particles assure enhanced dispersability within the paint, polymer, etc., substrate. Surface coatings containing such particles will not exhibit the static electricity properties which may be found on surfaces coated with prior art coatings.

Also described herein in detail is a method for the production of ultrafine zinc precipitates in which zinc ions, ammonium ions and a carbonate source are brought into mutual contact to form a precipitate, the resulting precipitate is separated and optionally calcined to zinc oxide, and is characterized in that a metastable precipitate is formed in which the morphology and size of the particles is controlled by maintaining, during precipitation: a temperature between 10 and 40°C, preferably 15 to 30°C and more preferably 20 to 22°C; a pH between 5 and 10; and wherein at least one of the zinc ion and/or carbonate source is progressively made available to the solution at a precipitation limiting rate.

The above-defined method is in contrast to prior art methods in which a high pH solution of all the precipitant ions (with the zinc as zincate) is caused to precipitate by lowering of the pH. The present invention avoids the production of undesirable high pH artifacts such as microprecipitates of ZnOH which can act as seeds for the uncontrolled growth of zinc carbonate complexes, by initially setting the pH of the mother liquor. In this fashion, high quality, substantially uniform metastable complexes can be reproduceably precipitated with sizes and morphologies which possess interesting rheological properties. Zinc oxide particles produced according to the procedure described herein are ultrafine precipitates of controlled size and morphology produced by the above method and defining substantially homogeneous populations of spheres, rods or fibers with a narrow diameter and aspect ratio distribution. The precipitate which forms as a metastable mixed complex of zinc, or zinc and some other cation such as ammonium or a dopant, and hydroxy, bicarbonate, carbonate or oxalate, etc., may be recovered as the salt by conventional techniques or alternatively must be calcined to produce zinc oxide particles. The calcination temperature will depend to some extent on the exact nature of the carbonate moiety but the metastable nature of the precipitate will generally allow the use of comparatively low calcination temperatures in comparison with classic carbonate decomposition. For instance, calcination temperatures as low as 250-340°C, perhaps even 200°C are achievable in comparison to the 400-800°C required in classic carbonate calcinations. Oxalates and other bicarboxylates/metastable precipitates may even require lower calcination temperatures, such as 120°C.

The precipitates formed by the procedures described herein are metastable and therefore prolonged contact with the chemically reactive environment of the mother liquor will tend to cause maturing or ripening. Prior to physical separation of the precipitate, it can be isolated, to some extent, from its chemical environment. For instance, manipulation of the dissolved carbon dioxide concentration can delay ripening in solution, prior to filtration or centrifugal separation. The precipitates are not water sensitive, unlike the prior art zincate precipitation techniques of U.S. Patent 5,132,104 which require washing of the separated precipitate in polar organic solvents such as acetone or ethanol.

Suitable separation techniques can include an initial surface charge neutralization step of coating the surface of the precipitate suspension with a surfactant such as methacorylate followed by spray- drying. Calcination of the resultant particles will tend to volatilise any surfactant residues remaining after the spray drying operation.

Spray roasting, in which the precipitate containing solution is sprayed into a heated chamber at temperatures approaching 270°C can simultaneously affect dewatering and calcination. Filtration, leading to a more densely packed arrangement can also be used, optionally in conjunction with surfactant based redispersion techniques.

The expression 'carbonate source' includes carbonates, bicarbonates, oxalates, malates, succinates and also carbon dioxide introduced into the aqueous solution as a gaseous phase or evolved _in situ through dissolution or decomposition. Ammonium salts are preferred especially when the resultant zinc oxide is intended for applications in which metal ion contamination should be avoided, such as dermal sunscreens and electrostatic applications. Preferred zinc salts to produce the aqueous zinc solution include the nitrate, sulfate and chloride. The solution may be pure water or a mixture of water and another miscible or immiscible solvent such as an alcohol or acetonitrile.

Control of the relative availability of carbonate to zinc ion concentration within the aqueous solution can be simply achieved by the gradual addition of the carbonate source and/or zinc ion, in solid but preferably dissolved form, to the aqueous mother liquor. Alternatively, the reagents can be added in a form which decomposes to release and make available zinc ion or the carbonate source. For example, urea or ammonium carbamate can release carbon dioxide in a retarded fashion to ensure a suitably low reactive concentration. Metal chelators such as the EDTA family can maintain a low reactive zinc ion concentration in the aqueous solution. When carbon dioxide is used as the carbonate source it is convenient to bubble it through the aqueous solution, optionally in conjunction with a solubility regulator such as ammonia.

Control of the pH within the above defined range is advantageously carried out with dilute reagents such as 0.05 to 0.25 M KOH or NaOH, in conjunction with vigorous mixing. The preferred pH control agent is ammonium hydroxide, such as a 5-10% ammonia in distilled water solution. The ratio of ammonium to carbonate source will generally be lower, for instance approaching unity compared with prior art methods, which may lead to enhanced metastability in the precipitated complexes.

Appropriate control of the relative availability of the various ions allows control of the aspect ratio of the resultant precipitant. Generally speaking, within the above defined process conditions, the slower the rate of reactant addition, the greater is the aspect ratio, i.e. the length of the rods or fibers. Conversely, increasing the addition rate will decrease the aspect ratio, however too rapid an addition will lead to the non-homogenous particle size distributions displayed by prior art techniques. Addition rates will vary with the strength and solubility of the reagents, but as a guide, for a 0.5 molar concentration of zinc ion, an addition rate between 0.5 and 2.0 liters/hour for a 0.4 molar equivalent carbonate source has been workable. It should, of course, be recognized that the hydrodynamics of the solution influence the intended morphology. In particular, in contrast to conventional crystal deposition techniques, high aspect ratios will demand effective mixing, even with relatively dilute reagents to avoid localized regions of aberrant reactant concentration.

The actual hydrodynamic conditions employed during precipitation will depend on reactor size, geometry, number of baffles, etc., but generally speaking will be as high as possible without inducing cavitation or other admission of air bubbles into the system. As a guide, a Reynolds number of at least some hundreds, preferably 8000 and above, will be appropriate for many systems.

The process for making zinc oxide rods allows doping of the zinc precipitate complex and any zinc oxide end products through co-precipitation of the zinc precipitate with a dopant such as yttrium, aluminum, gallium, platinum, bismuth, a lanthanide, curium, molybdenum, nickel, cobalt, antimony, chromium or other group III-VII compounds. Doping increases conductivity of the resulting rod composition. Typically 0.01 to 10% of the resultant particles will comprise the dopant oxide.

Co-precipitation can be performed simultaneously with formation of the zinc oxide to produce homogenous particles. The respective concentrations of dopant oxide to zinc oxide in the end product can be controlled through adjustment of their respective reagent concentrations during precipitation.

Alternatively, doping can be performed sequentially by first forming a zinc carbonate core and then precipitating one or more layers of dopant over the core. The end product powders will then have the dopant on the zinc interface with very little solubility in the solid phase.

Preparation of Zinc Oxide Spheres

Zinc oxide spheres are made by carefully controlled agglomeration of spherical particles, prepared as described above. Referring initially to Fig 1, this procedure includes the steps of forming an aqueous solution of a zinc ion, followed by pH and temperature adjustment. A gaseous carbon dioxide stream is introduced to the zinc solution while a pH and temperature feedback loop maintains precise control over the reaction environment. A precipitate comprising a mixed complex of zinc and hydroxide, bicarbonate and carbonate forms as the carbon dioxide is fed in. The metastable precipitate is separated from the mother liquor which is temperature treated to reform the reagents. The precipitate can be low temperature calcined to form an ultrafine ZnO powder of defined particle size and morphology to form a mixed complex of zinc and hydroxides, bicarbonates and carbonate. In this procedure, the reactor comprises a 2 liter cylinder equipped with vertically extending baffles around its circumference. Stirring was achieved with a central impeller which was speed governed to within 1% of the nominal rpm. Reagent addition to the reactor was via glass conduits opening into the reactor adjacent the impeller, thereby assuring instant mixing. Gaseous reagents were added via microporous sintered glass tips again adjacent the impeller. A central microprocessor received input form pH, temperature and ion-selective probes mounted in the reactor and controlled peristaltic reagent input pumps and high precision reagent input valves. Bulk reagent vessels were equilibrated to the intended reaction temperature.

68.14g of 99.81% pure ZnCl (Sigma Chemical Co., St. Louis, MO; Aldrich Chemical Co., Milwaukee, WI) was dissolved in 1.5 liters of distilled water and fed into the reactor. The temperature was reduced to 22°C and maintained within a degree of this temperature, throughout the experiment. Stirring was set to 175 rpm. The pH was controlled with an ammonium solution comprising 8% ammonia in distilled water which was very gradually added to the stirred solution so as to avoid localized pH perturbation. The pH was maintained in this fashion within the range 9.5-10 throughout the experiment.

A carbonate source comprising carbon dioxide gas 0.1% (balance oxygen and nitrogen) was introduced to the solution at approximately 4.0 1/h. The precipitate formed instantly and the carbon dioxide inflow was continued until an appreciable amount of precipitate was dispersed in the reactor. The precipitate was washed in distilled water and dried. The powder was X-ray amorphous. The powder was calcined at 270°C for 3 hours to form a white powder of submicron particles with a narrow size distribution and with a density of around 5.6 g/cm and surface area of 35 m/g.

The powder was prepared for scanning electron microscopy by the gold coating method. As can be seen from the micrograph of Fig. 2, these process conditions produced a sphere morphology with a diameter between 50 and 150 nm.

Referring once again to Fig. 1, process refinements can include the recycling of the ammonia component, heat separated from the mother liquor after removal of the precipitate, back to the pH adjustment step, marked with the letter A in Fig. 1. Additionally or alternatively, the liquid from this ammonia recovery step can be treated to recover the solvent which can also be recycled to provide a virtually closed environmentally friendly system ("B" in Fig. 1).

Preparation of Shorter Zinc Oxide Rods

To obtain rod-shaped particles, as shown in Fig. 3, the apparatus described above was charged with 1.5 1 of distilled water. Stirring, temperature and pH control were also as above.

Aqueous 0.3 M zinc chloride and 0.2 M ammonium bicarbonate solutions were simultaneously added to the reactor via separate conduits at a respective rate of 0.5 1/h and 0.5 1/h. The resultant precipitate was separated as above.

Calcination of the resultant precipitate was at 250°C for 3 hours. Figure 3 shows the resultant zinc oxide particles which display a rod morphology with diameters between 50 to 100 nm and lengths between 100 to 200 nm.

When carbon dioxide is used as the precipitant, an additional recycling possibility is to collect carbon dioxide from the calcination step for use as the precipitant, as shown with dotted lines on Fig. 1.

Preparation of Longer Zinc Oxide Rods

Longer zinc oxide rods or fibers may be prepared as follows. In the system described for the preparation of zinc oxide rods, but with a stirring speed of 200 to 250 rpm, 0.3 M zinc chloride and 0.1 M carbamate solutions were simultaneously added to the reactor through respective glass conduits at respective addition rates of 0.5 and 0.7 1/h. Carbamate is stable in solution but breaks down via metal catalysis to liberate reactively available carbon dioxide and ammonia in solution.

The resultant precipitate was separated and prepared for SEM as above. These process conditions produced a longer zinc oxide rod or fiber morphology, as shown in the micrograph in Fig. 4. The rods display an homogenous size distribution between 10 to 50 nm in diameter and 50 to 500 nm in length.

Preparation of Zinc Oxide via Oxalate Route

In the reactor conditions described above in preparation of the shorter zinc oxide rods, but with the stirring speed within 150 to 175 rpm, 0.2 M zinc chloride and 0.1 M oxalic acid were simultaneously added to the reactor at respective rates 0.5 and 0.8 1/h. The precipitate was recovered as described in the rod preparation above, but at a calcination temperature of 125°C. Electron microscopy of the resultant powder showed spherical particles with diameters within the range 50 to 150 nm. Rod morphologies can also be deposited using this reagent system.

Preparation of Doped Zinc Oxide Particles

Referring now to Fig. 5, a scheme for the production of doped zinc oxide particles is shown. In this scheme, two separate metal solutions, the first a zinc ion solution (I, at the top, left) and the second a dopant metal ion solution (II, top, right) are prepared and separately pH and temperature adjusted.

In a first process variant leading to an homogenous dopant/zinc precipitate, the respective metal ion solutions are mixed (III) and introduced to the mother liquor together. In this fashion, the resultant precipitate complex comprises an intimate co- precipitate of dopant and zinc, the proportion of each reflecting their respective concentrations in the mixed input stream. As with the earlier described procedures, pH and temperature feedback loops (IV, IV) can be provided to maintain optional reaction conditions during the precipitation, in particular when it is desired to take account of the differing solubilities of zinc and dopant metal ions at different pHs to assist in regulating proportionality of deposition of the respective metals.

In a second process, variant the mixing of metal ion solutions I and II is avoided and the respective solutions are admitted to the reactor sequentially. The resultant precipitate comprises an initially precipitated zinc complex core surrounded by a layer of dopant ion complex. Again, the pH control of the respective metal solutions may take advantage of the differing solubilities of the respective metals at different acidities. In each case, the respective precipitates are separated and calcined in similar fashion to the above described procedures to produce doped zinc oxide particles of defined size and morphology.

Preparation of Zn/Bi Coprecipitate

A first process variant of the preparation of doped zinc oxide particles was used to produce a mixed coprecipitate of metastable Zn and Bi carbonates. The reactor system described above in the preparation of zinc oxide particles, as shown in Figures 1 and 5, was charged with distilled water and the pH initially adjusted to 8 - 11 with dilute ammonia solution. A first solution was prepared by mixing 0.3 M ZnCl₂ and 0.01 M Bi(N0₃)₃ in the ratio 3:1, the ratio being adjusted with reference to the desired composition of the end product oxide. A second solution comprised 0.1 M NH.HCO^.

The first and second solutions were added dropwise to the aqueous system and the pH carefully maintained at the initial value by dropwise addition of the dilute ammonia solution during vigourous agitation. A composite consisting essentially of metastable zinc and bismuth carbonates was coprecipititated and calcined to obtain a very homogenous mixture of ZnO and Bi-O., having the above defined particle size and distribution.

A variant of this process uses a dual dopant oxide solution, in particular with a solution of Bi and Sb to produce a tri etal coprecipitate.

Preparation of ZnO/Al^O^ Coprecipitate

A second process variant of Figure 5 was used to produce a coprecipitate of ZnO and Al^O., suitable for electronic applications. The water charged reactor system described in the above description of the preparation of zinc oxide particles was pH adjusted to between 8 and 10. With reference to Figure 5, a zinc core precipitate was first produced by dropwise addition of a solution II consisting of 0.3 M ZnCl₂ and a separate carbonate source solution comprising 0.1 M NH.HCO.,. The pH was controlled via the feedback loop with small additions of dilute ammonia to the vigourously agitated aqueous solution.

Referring again to Figure 5, solution I comprised 0.1 M A1N0₃ which was subsequently precipitated onto the suspended zinc precipitate core. Calcination of the mixed precipitate provided a uniform powder of biphase aluminium oxide on zinc oxide appropriate for semiconducting roles, for example, conventional compression sintering to form varistors.

This reagent system can also be used in the process variant of the Zn/Bi coprecipitate preparation described above, for example, at a 5:1 ratio of the Zn:Al solutions to form a homogenous coprecipitate.

Formulations

Zinc oxide particles contained in compositions of the invention have a length to diameter ratio of at least two and preferably three and have dimensions within the range of 3 - 10,500 nm length and 1 - 3,500 nm diameter. Within this range, the size of the zinc oxide rod will depend upon the type of surface to which the composition of the invention is to be applied and whether transparent or opaque compositions are desired. Smaller rods are useful for compositions which are transparent, e.g., clear glazings or paints. Rods having a length of less than 300 nm and a diameter of less than 100 nm are optimal smaller rods to confer transparency to the composition. Compositions which are non-transparent or white can also be made, according to the invention, using zinc oxide rods having a length of longer than 300 nm and a diameter of greater than 100 nm. The following examples of compositions of the invention are illustrative of conductive surface coatings, as taught and claimed herein, and are not meant to be limiting. The zinc oxide rods in compositions of the invention may be combined with other metal oxides, e.g., titanium dioxides, as described in U.S. Patent 5,032,390. For example, a mixture of zinc oxide rods and titanium dioxide particles, e.g., of a generally spherical shape may be useful in a composition of the invention. Other components useful in compositions of the invention include thickeners, emulsifiers, fragrances, water-proofing agents, etc.

Zinc oxide rods may be surface modified in order to make them more compatible in a given formulation. For example, the surface of a zinc oxide particle may be treated with silicone-like compounds in order to increase its compatibility with oil-based compositions. For other surface modifications useful in the invention, see "Chemistry of Pigments and Fillers", D.H. Soloman et al., Eds., 1983, Wiley Inter-science, hereby incorporated by reference.

Conductive Paints Any component which is found in conventional paints may be used in a formulation in compositions of the invention.

An opaque paint may be prepared from rod-shaped zinc oxide particles by mixing the larger zinc oxide rods with any type of commercially available paint using enough zinc oxide to impart the desired color. For example, 10% by weight zinc oxide rods of, for example, 450 nm length and 150 nm diameter are combined with the commercial paint.

Zinc oxide rods described herein may be included in a paint or similar polymer, or base having the particles incorporated therein, typically in an amount corresponding to 0.01 to 50 wt%. Higher aspect ratios are generally preferred to enhance interparticle contact and thereby electrical conductivity. The elongate shape and ultrafine size of the present particles leads to good dispersability of the particles within the substrate allowing smaller quantities of the particles to be used in comparison to prior art topical compositions.

Paints for coating plastics are particularly preferred as compositions of the invention. Rough or smooth plastics may be coated with compositions of the invention.

Plastics which are highly crystalline or have low surface polarity, for example, polyacetal resin, polyester resins (polyethylene terephthalate, polybutylene terphthalate, fully aromatic polyester, etc.)., etc., are normally subjected to physical or chemical surface treatments, before coatings are applied thereon, because of the paints' low adhesiveness. Conductive primers or conductive primer surfacer paints which provide proper adhesiveness merely by direct static coating without requiring any surface treatment are known. For example, see U.S. Patent 4,971,727, the contents of which are hereby incorporated by reference. Preferably, a conductive paint of the invention will not degrade the surface and the interior of plastics, will be adhesive if roughening of the plastic surface prior to application of the coating is to be avoided, will be a tough coating with little shrinkage or internal stress at the film forming time, will be quick drying, will be strong to physical impact, and will be highly flexible. A conductive paint of the invention will also be stable to solvents, water (sticking) and heat (including thermal cycling).

A conductive primer or conductive primer surfacer paint may contain the following coating forming components: (A) polyurethane base resin, 50-97 (% by weight), (B) opening ring expansive spiro-ortho-ester base resin, (containing a catalyst) 2.0-40 (% by weight), (C) cellulose derivative, 1.0-9.0 (% by weight), (D) hydroxyl group containing surface active agent, 0.05-1.0 (% by weight), and (E) zinc oxide particles having a substantially rod shape in an amount sufficient to confer a surface resistance value less than 10¹³ ohm.cm.

Component (A) is characterized by high tackiness and high elasticity and not only enhances the paint's adherence to the surface, but elevates the coating's impact resistance. The polyurethane resin, as used herein, is a generic term representing all denatured polyurethane resins, being any resins, so far as they have polyurethane resins as their main component material, whether they are thermoplastic or thermosetting. Thermoplastic polyurethane base resins with mean molecular weights (Mn) ranging from about 2,000 - 10,000, or more preferably from about 4,000 - 7,000 are preferred. Component (B) confers a nonshrinking quality to the coating by undergoing intramolecular irreversible ring opening in the presence of a cationic catalyst, and thus relieves shrinkage of the coating as it forms a film on the surface. It thus has the effects of not only relieving the outside stress, but retrenching the film's residual strain (internal stress) resulting from contraction. This component consists of spiro- orthoester base resins; 2.2-bis [4-(2,3-epoxy-propoxy) phenyl] propane 6-hexanolyd polyaddition product, 8, 10, 19, 20-tetraoxatrispiro (5,2,2,5,2,2) heneicosan-2,14-diene, etc., may be mentioned as representative examples. Of these compounds, preferable are spiro-ortho-ester base resins with degree of spiro-ortho-esterification degree 250-500 g/eq., preferably about 300-400 g/eq., and with epoxy values 0-5.0 g/eq. preferably about 4.65 g/eq.

Component (C) confers film forming (thermal fluidity) and film hardness; it is composed of a cellulose derivative. Of cellulose derivatives, those useful are cellulose esters such as cellulose acetate, cellulose propionate, cellulose butylate, cellulose acetate propionate, cellulose acetate butylate, cellulose nitrate, etc., particularly, with degree of butylation or propylation being 17-55%; cellulose acetate butylate and cellulose acetate propionate which are more highly butylated or propylated are preferable; their hydroxyl group concentration should be 1.0-3.0, preferably about 1.0, in number (per 4 anhydros glucose units), and their viscosity should be 0.01-20.0 sec, preferably about 0.2 sec (standard falling-ball viscosity) .

Component (D) may be a fluorine or silicon base hydroxyl group containing reactive surface active agent having film surface adjusting ability and reactivity which provides film surface adjustment and layer sticking to finish coating. Examples include perfluoro alkyl and oranosilexane, both of which are introduced into the compound via the hydroxyl group, as a silicon base. Part of such hydroxyl groups are exposed from the coating surface, thereby providing the finish coating layer with proper sticking property. Component (E) is the zinc oxide rod-shaped particles described herein. As discussed above, the rods may be combined with other conductive materials, if desired, for example, conductive carbon, conductive titanium, conductive antimony trichloride, graphite, etc. The above components A, B, C, D, and E are integrally combined to form a coating which gives high coating performance, coating efficiency and sticking on the surface to be coated.

As with nonconductive paint compositions, improvements in the conductive coatings of the invention may be made by adding, as required, pigments, fillers or other various additives thereinto. The coating forming components and a pigment or other additive are mixed together to provide a conductive primer or conductive primer surfacer paint.

A conductive paint, or any other type of conductive coating described herein may be applied by conventional methods, including, static coating with spray gun, disc, etc. Drying and hardening of the paint may be effected by cross-linking the coating components by taking advantage of the heat at the baking-hardening time, using it in combination with a thermosetting type finish coating paint like baking paint, etc. It is, of course, possible, however, to subject the paint, after applied, to a heat treatment and then apply normal temperature setting type finish coating paint. It is also practicable to apply a normal temperature setting process in combination with a normal temperature setting finish coating paint, using the aforementioned components within their compounding ranges. The coating, after applied, should be set by normal temperature drying and hot air drying for an arbitrary time period; in the case of hot air drying, appropriate conditions are at 40-140° C for 5-20 min. Appropriate dried coating thickness is 15-30 μm; the standard may be set at 22±2 μm.

A conductive primer or conductive primer surfacer paint is prepared as follows. The following components are combined: (A) Polyurethane base resin (manufactured by Dainihon Ink Chemical Co.) 47.2%, (B) Spiro-ortho- ester base resin (catalyst: di-n-butyl-tin- dilaurate) (manufactured by Toa Synthetic Chemical Co.) 10.2%, (C) Cellulose acetate butylate (manufactured by Eastman Kodak Co.) 3.8%, (D) Hydroxyl group containing silicon base reactive survace active agent (manufactured by Dainihon Ink Chemical Co.) 2.1%, zinc oxide particles as described above of 150 nm length, and 50 nm diameter, 44.8%, and i-butanol 1.9%. Each component is diluted with an appropriate diluting agent to adjust its concentration (e.g., methyl ethyl ketone, methyl isobutyl, i-propanol, i-butanol, ethyl acetate, butyl acetate, etc.). The paint solution is coated to dry film thickness of 22±2 μm on a plate of polyacetal, polyethylene terephthalate, polybutylene terephthalate or fully aromatic polyester (composing monomer units. Thereafter, the coating is hot-air dried at its temperature of 80-140° C for 20-30 min, thereby thermosetting the coating forming components. Then a melamine alkyd paint ("amilack" manufactured by Kansai Paint Co., Ltd.) which is available on the market generally for automobile outer boards is prepared with a thinner. This paint is coated on the plate by static coating to a dry film thickness of 30-40 μm and, after 10 min. setting, hot-air-dried at 140° C for 30 min., thereby effecting thermosetting.

Conductive Glazing

Conductive glazings are useful to coat, e.g, plastic and glass surfaces, such as windshields or rear windows of motor vehicles or for coating glass or plastic surfaces on buildings. Described below are conductive glazings which are mechanically resistant, heated laminated glazings which are applied to a support such as glass or polyurethane plastic. Supports which are useful for the glazings according to the invention include single glass sheets or else of laminated glasses formed by glass sheets connected to one another by a plastic insert layer. The supports can also be of plastic such as polycarbonate, acrylic polymers, vinyl polychloride, polystyrene, or cellulose esters. When the support is of glass, it can be formed by a soda-lime silica glass conventionally used for automobile glazings and for buildings. It can be a clear glass, i.e., not tinted, exhibiting a significant light transmission, for example greater than 90% at a thickness of 4 mm. It can also involve a glass tinted through its entire thickness and able to provide increased summer comfort for the passengers of the vehicle or room equipped with such glass, because of its small energy transmission factor. As tinted glass, so-called "TSA" glass containing Fe₂0₃ can be used in proportions by weight on the order of 0.55 to 0.62%, FeO for about 0.11 to 0.16%, which leads to an Fe²+/Fe ratio on the order of 0.19 to 0.25, with CoO of less than 12 ppm and even preferably less than 10 ppm. As a result, for example, for a glass thickness of 3.85 mm light transmission (T_L) is raised close to 78%, and energy transmission factor (T„) is relatively low and close to 60%, which leads to a T_r T_E ratio on the order of 1.30. As the tinted glass, in particular when the regulation calls for only a 70% light transmission, a glass which is a little more tinted than the "TSA" but exhibits a light transmission which is a little weaker than "TSA", namely a "TSA²⁺," can also be used.

The electroconductive coating is formed using a layer of coating containing the zinc oxide rod shaped particles described herein. Undoped or doped zinc oxide rod compositions, as described above, are useful. The electroconductive coating can also consist of several thin layers forming a stack essentially comprising multilayers of conductive metal such as silver, inserted between two dielectric layers, such as zinc oxide. These non-zinc oxide layers can be obtained by different methods of forming thin layers, for example vacuum method (heat evaporation, cathode sputtering, magneton . . .), pyrolysis (from compounds in solution or suspension form, in powder form) or tempering. The zinc oxide glazing exhibits good electrical properties and is mechanically stable. Advantageously, a zinc oxide layer having a thickness on the order of 180 nm may give it a resistance per square meter on the order of 10 ohms. It may also have a thickness of at least 330 nm, which may give it a resistance per square meter which is equal or less than 5 ohms. Preferably, the ITO layer may have a thickness of 350-380 nm which may make it possible for it to exhibit a resistance per square meter on the order of 4.5-4.0 ohms.

For these thicknesses, the zinc oxide layer may be transparent, if desired, by using zinc oxide rods of less than 300 nm length and less than 100 nm diameter. Thus, color pigments may be added to the glazing to attain the desired color. Optionally, the zinc oxide layer may be white, if rods of longer than 300 nm and greater than 100 nm diameter are used. Depending on the glazings desired, the thickness, the color of the glass support, the associated plastic materials and/or the thickness of the zinc oxide layer can be varied. To improve the electrical conduction of the thin electroconductive zinc oxide layers, they can be subjected, after deposition, to a heat treatment, methods of which are described herein and are well known in the art. Heat treatment of the layer can be performed, under a normal or reduced pressure, in an atmosphere which preferably is neutral or reducing, for example, under atmosphere of H₂ or of N₂ or even under an H₂+H₂ mixture. This treatment can again be performed under vacuum. This heat treatment is performed at a temperature lower than that of the deposition of the electroconductive layer, namely at a temperature lower than 400°C. Preferably, the treatment temperature is between 250° and 350°C. Thus, an automobile glazing usable in the invention can be made with a "TSA" glass support of a thickness of 3 mm, covered by a zinc oxide layer of a thickness on the order of 350-380 nm or on the order of 180 nm. With the layer thickness of 180 nm, the "TSA" glass thickness can be slightly larger to reach up to 3.5 or 4 mm. If it is desired to use "TSA²+" glass in place of "TSA" glass, while having light transmission coefficients compatible with the regulations, a "TSA +" glass support of 3 mm of thickness and a layer of ITO having a thickness of 180 nm can be selected. Of course, glazings with glass thicknesses less than those set forth above are, a fortiori, possible since this makes it possible to increase the light transmission.

Conductivity

Acceptable conductivities of conductive coatings within the invention are as follows. Conductive coatings of the invention will have a resistivity of at least 100 ohm.cm, preferably at least 1000 ohm.cm, more preferably at least 10,000 ohm.cm, and most preferably at least 50,000 - 1,000,000 ohm.cm. High resistivities (e.g, greater than 10,000 ohm.cm) are preferred for certain uses. The resistivity of the zinc oxide rod particle preparation may be from 10^{" 1} - 10⁸ ohm.cm, preferably 10° - 10⁶ ohm.cm, most preferably 10¹ - 10⁵ ohm.cm. Methods of testing for conductivity are well known within the art.

Other Embodiments

Other examples of bases to which zinc oxide rods may be added include resins, wood stains, sealers, caulking, roof shingles, automobile clear coats, etc. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Other embodiments of the invention are found within the following claims.

Claims

1. A conductive coating composition for application to an inanimate surface, comprising zinc oxide particles having a substantially rod shape in combination with a dopant.

2. The composition of claim 1 wherein said surface is one of plastic, glass, metal, wood or plaster.

3. The composition of claim 2 wherein said surface is wood, plaster, plastic or metal and said agent comprises a latex or oil-based agent for spreading on said surface.

4. The composition of claim 2 wherein said surface is plastic and said agent comprises a polyurethane for spreading on said surface.

5. The composition of claim 1, 2, 3 or 4 wherein said rod shaped particles have a substantially spherical cross-section with an aspect ratio of at least two.

6. The composition of claim 5 wherein said particles have a length of between 3 and 10,500 nanometers, inclusive, and a diameter of between 1 and 3,500 nanometers, inclusive.

7. The composition of claim 6, said particles having a length of less than 300 nm and a diameter of less than 100 nm.

8. The composition of claim 7, said particles having a length of less than 200 nm and a diameter of less than 65 nm.

9. The composition of claim 6, said particles- having a length longer than 300 nm and a diameter greater than 100 nm.

10. The composition of claim 9, said particles having a length longer than 450 nm, and a diameter greater than 150 nm.

11. The composition of claim 6 wherein said zinc oxide particles comprise 0.1 - 50% of said composition by weight.

12. The composition of claim 1 further comprising a surface coating.

13. The composition of claim 12 comprising a silicone oil, mineral oil, latex, or polyurethane.

14. The composition of claim 1 wherein said dopant comprises 0.01 - 10% by weight of the composition.

15. The composition of claim 1 wherein said dopant comprises about 0.2% by weight of the composition.

16. The composition of claim 14 wherein said dopant is selected from the group comprising titanium, yttrium, aluminum, gallium, platinum, bismuth, a lanthanide, curium, molybdenum, nickel, cobalt, antimony, and chromium.

17. A method of making a conductive coating, said method comprising the step of: combining an agent for spreading on a surface with a doped zinc oxide substantially rod-shaped particle preparation, wherein said preparation is formed by bringing into contact in an aqueous solution zinc ions, ammonium ions, and a carbonate source, separating and optionally calcining the resultant metastable precipitate to zinc oxide, wherein a metastable precipitate is formed by controlling the morphology and size of the particles by maintaining, during the precipitation: a temperature between 10 to 40°C; a pH between 5 and 10; and wherein at least one of the zinc ion and carbonate source is progressively made available to the solution at a precipitation limiting rate.

18. A method of applying a conductive coating to a surface, comprising the step of: applying a conductive coating comprising a spreadable agent and doped zinc oxide particles having a substantially rod shape to a surface.