US3236679A

US3236679A - Electrostatic spraying

Info

Publication number: US3236679A
Application number: US93368A
Authority: US
Inventors: Lester L Spiller; Edward G Bobalek
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1961-03-06
Filing date: 1961-03-06
Publication date: 1966-02-22
Anticipated expiration: 1983-02-22
Also published as: DE1571125A1; DE1571125B2; NL131144C; FI42052B; CH411637A; GB995257A; NL275608A; DK119649B

Description

Feb. 22, 1966 L. L. SPILLER ETAL ELECTROSTATIC SPRAYING Filed March 6, 1961 ARTICLE HAVING ELECTR/CALLY INSULATIN$ Suki-796E INCLUDING FUNCTION/IL GKOl/PS HDHPTED Fol? CHEMISORPT/ON w/TH ELECTRICAL/y CHHEGED IONS PROJECT ELECTROSTHT/CHLLY CHARGED COflT/NG PflPT/CLES TOWARD THE GEL SUPF'HCE MflINTfl/N THE GEL SURFACE :47 H RELHT/VELY OPPOSITE POLAR/TY WITH RESPECT TO THE ELECTROSTATIC CHHRGE ON THE COflT/NG- PflPT/CLES INVENTORS LESTER L. SPILLER Eowneu 6% BOBHLEK BY 3% ATTORNEYS United States Patent 3,236,679 ELECTROSTATIC SPRAYHNG Lester lL. Spiller, Indianapolis, Ind, and Edward G.

Eobalek, Cleveland, Ohio, assignors to Ranshurg Electra-Coating Corp, Indianapolis, Ind, a corporation of lndiana Filed Mar. 6, 1961, Ser. No. 93,368 19 Claims. (Cl. 117-93A) The present invention relates to the surface pretreatment of insulating materials to facilitate the deposition of electrostatically charged coating particles thereon; to the pretreated insulating material; and to the coated pretreated insulating material.

At the present time, electrostatic spraying is carried out by either mechanically atomizing the paint and then depositing the atomized spray particles in an electrostatic field in order to increase the proportion of the spray particles which are deposited upon the object to be coated, or by atomizing under the influence of an electrostatic field and electrostatically depositing the spray particles. The present invention is applicable to either system.

The difiiculty of electrostatically depositing coating particles on insulating surfaces is well known. When electrostatic atomization is included. in the process, the reduced voltage drop between the atomizing means and the surface to be coated causes less effective atomization. Moreover, the negative charge is built up on the insulating surface (when the atomizer is negatively charged) which tends to repel charged coating particles, thereby causing waste of coating material and lack of uniformity in the coating.

Prior efforts to overcome the problem have included pretreatment with conductive solvents or other liquids, but this impairs coating quality and the effectiveness of the treatment varies quickly with time as the solvent evaporates. Moreover, the insulating surface may be adversely affected by the solvent or other liquid as, for example, the grain of wood may be rasied and plastic surfaces may be softened or dissolved. The invention contemplates stable pretreatment without adversely affecting coating quality and without masking surface appearance such has the attractive natural grain of wood.

The invention is particularly directed to the pretreatment of wood, but other insulating surfaces such as fiberboard, Masonite hardboard and other hardboards, glass and plastics such as polyethylene, nylon and glycolterephthalate polyesters may also be pretreated in the same Way for the same purpose of improving electrostatic spray application. Accordingly, the invention pertains to pretreatment of insulating surfaces to make the same receptive to coating by coating materials applied by electrostatic means, and the pretreatment of wood simply represents a preferred species of the invention.

In accordance with the invention, there is chemisorbed on the electrically insulating surface of the article to be coated, a thin conductive gel layer swollen with polar liquid of high dielectric constant and comprising ionizable organic compound capable of ionizing to form an organic ion of one electrical sign and ions of opposite electrical sign. The gel layer is preferably stable under normal atmospheric conditions.

When the thin, electrically conductive gel layer is chemisorbed on the insulating surface, the surface exhibits an improved capacity to accept electrostatically charged coating particles which are projected toward the surface, the conductive capacity of the gel layer permitting the surface of the article being sprayed to be maintained at a particle attracting potential with respect to the electrostatic charge on the charged coating particles.

A simplified flow diagram illustrating the invention is shown in the accompanying drawing.

While the invention is directly concerned with the capacity of articles having insulating surfaces to accept electrostatically charged coating particles, the production of articles having an organic resinous coating thereupon is also of importance. Accordingly, the invention includes articles having an electrically insulating surface and an organic resinous coating adhered to this surface through an electrically continuous conductive layer which comprises ionizable organic compound which ionizes to form an organic ion of one electrical sign chemically bonded to functional groups contained in the article which is coated, and associated ions of opposite electrical sign.

While any increase in conductivity provided by the gel layer tends to improve electrostatic spray application, it is preferred that the gel layer have a resistivity of not more than 10 ohms per square, measured as hereinafter defined; and, preferably, the resistivity of the gel layer is less than 10 ohms per square, e.g., about 10' ohms per square and this is especially true when the article being sprayed is of unusual length between ground connections so that there are very few conductive paths available for electrostatic charge dissipation.

The ions of opposite electrical sign of the aforesaid ionizable organic compounds are preferably monovalent and inorganic and are most preferably strongly electronegative or strongly electropositive, such as hydrogen ions and chloride ions.

The conductivity of the gel layer increases slowly with increasing thickness until, at a thickness of about 1000 A., conductivity increases rapidly with increasing thickness. Accordingly, it is preferred that the gel layer have a thickness of at least about 1000 A. On the other hand, gel layers thicker than 25,000 A. are wasteful of material, since greater thickness does not significantly increase conductivity. Moreover, thicker gel layers are possibly damaging to the water resistance or other desired characteristics of the paint film applied thereon. Accordingly, gel layers thicker than 25,000 A. would not normally be used.

Chemisorption is a known surface phenomenon in which functional groups in a surface become chemically associated with ions forming part of a material applied to the surface. This chemical association can occur through hydrogen bonding, .or by ionic bonding, or by Van der Waal forces, or by dipole association forces, or by a combination of mechanisms, as is known. Ionic bonds are preferred.

Irrespective of chemisorption mechanism, the ionizable organic compound which is applied in liquid form as a solution or dispersion in a suitable solvent appears to form an oriented monolayer which is chemisorbed onto the interface of the insulating material adjacent the applied liquid so that the solution spreads over the insulating surface to form the thin electrically conductive layer which is needed. Indeed, in the thin layers contemplated, spreading of the solution over the insulating surface to form a continuous thin layer evidences the existence of chemisorption, for unless chemisorption of an oriented monolayer takes place, the required degree of spreading may not occur.

The existence of a gel structure of some thickness greater than the monolayer and superimposed upon the monolayer is essential to the achievement of adequate conductivity, The term gel denotes a heterogeneous structure involving an ordered arrangement of at least two coexisting phases. One of the phases of the gel possesses considerable rigidity. In addition to this first phase, there must be a second phase which is fiuid. In the present invention, it is clear that a gel structure is necessary. In every instance of successful operation in accordance with the invention, a gel structure has, in fact, been formed. When the gel layers formed in the invention are photographed using a phase contrast microscope at 1000 magnifications, the gel layers are seen to be physically continuous and to be constituted by a unif-ormly distributed polyphase domain structure.

The existence of the gel structure can also be established by the capacity of the structure to be swollen by the addition of the sorbed component and by the capacity of the structure to lose volume and apparent regularity of distribution of the rigid or discontinuous component of the structure upon loss of fluid. Application of excessive heat to the gel layer or storage of the gel layer in a confined space in the presence of unusually strong desiccants such as phosphorus pentoxide causes the gel layers to lose volume, physical continuity and regularity of domain structure. The loss in physical continuity and regularity of domain structure is easily seen when the dehydrated gel is viewed at 1000 magnifications under a phase contrast microscope. In many instances, a crystalline structure is assumed upon removal of sorbed fluid, Moreover, when the gel structure is destroyed, the non-gelatinous residue is ineffective in accordance with the invention.

In the invention, the gel is swollen with stably bound polar liquid of high dielectric constant, usually water. Other polar liquids of high dielectric constant such as alcohols, illustrated by methanol, ethanol, n-propanol and isopropanol, are also suitable. It will be understood that it is normally impossible to exclude water from a surface coating or surfaces being coated.

It is preferred, in accordance with the invention, that the ionizable organic compound be responsible for the formation of the required gel structure. Nevertheless, ionizable organic compounds which do not form a thin, stable gel when used alone and which, therefore, cannot be used alone in the invention, can be used in accordance with the invention in conjunction with supplementary gel-forming agents such as finely divided particles of silica or methyl cellulose, since this combination forms a stable gel structure comprising the ionizable organic compound which is elfective to provide the conductivity needed for electrostatic coating of an insulating surface. Also, it is not essential that the gel layer be distinct from any gel structure constituting the insulating surface. Thus, wood is in part a gel structure and the ionizable organic compound need only be chemisorbed into the surface of the wood.

As previously indicated, the ionizable organic compound is intended to ionize and form an organic ion of one electrical sign which is chemisorbed on the electrically insulating surface which includes functional groups adapted for chemisorption with the electrically charged organic ions which are formed by ionization. Normally, most electrically insulating surfaces, such as wood or other cellulosic materials, contain more than enough functional groups adapted for chemisorption for the purposes of the invention. Most insulating materials, and specifically wood and glass, contain a sufficiently high density of functional groups capable of forming hydrogen bonds or ionic bonds to permit chemisorption to provide the thin layers which are desired. In cellulosic products generally, and in wood specifically, there are naturally occurring hydroxyl groups. Additionally, through inevitable oxidation of the cellulose molecule, carboxyl and peroxide groups are also present. Glass is densely packed with hydroxyl groups. Nylon contains amide and carboxyl groups. When the insulating material does not normally possess a suificient density of functional groups, these can be supplied in known manner, as for example, polyethylene may be flame treated, corona discharge treated in oxidizing atmospheres or acid chromate treated to supply an adequate density of functional groups to permit the solution of ionizable organic compound to spread and form the thin adherent layers which are required.

From the standpoint of the application of coatings utilizing as a binder an organic film-forming resin from organic solvent solution, especially where a portion of the thinner or solvent medium is constituted by polar organic liquids such as methyl ethyl ketone, acetone, alcohols such as ethanol and propanol, or esters such as butyl acetate, it is preferred that the gel layer exhibit such a balance of hydrophobic and hydrophilic surface characteristics that it will easily receive and be wetted by many types of coatings which can vary markedly in wetting and spreading properties. For this purpose, it is preferred to select gel layers which provide a water drop contact angle and a paraffin oil drop contact angle, each of which is less than about 30. As is conventional, the contact angle is measured through the liquid of the drop.

A desirable balance of hydrophobic and hydrophilic surface characteristics of otherwise suitable ionizable organic compounds is also evidenced by the mineral spirits tolerance of the organic compound. Thus, the ionizable organic compound can be dissolved in isopropanol to a concentration of 50% by weight of the compound and the solution can be titrated with mineral spirits until a phase separation is obtained. A phase separation is normally observed by the occurrence of a cloudy appearance in the solution which indicated the presence of a solid precipitate or emulsified fluid suspended in the solution. Preferred ionizable organic compounds for use in accordance with the invention possess only a limited tolerance for mineral spirits, e.g., they will tolerate the addition of not more than 50 cc. of mineral spirits per gram of 50% solvent solution before a phase separation takes place. Preferably, the mineral spirits tolerance per gram of 50% solution is between 10 and 40 cc. of mineral spirits. In performing the mineral spirits tolerance test, a mineral spirits having the following specifications can be used:

Boiling range 318386 F. Flash point (F.TCC) 105. Specific gravity (60/60 F.) 0.791. Aniline point (F.) 130. KB value 38.0, Aromatic content 18%. Parai'lins Naphthenes 27%.

The ionizable organic compound which is used in accordance with the invention may be of various types, including organic acids, bases or salts, and the organic ion formed by ionization may have either a positive or negative electrical charge. The preferred ionizable organic compounds are salts which form gel layers upon deposition, these salts providing positively charged organic ions upon ionization. Thus, the preferred compounds used in the invention are illustrated by quaternary and diquaternary ammonium halides, conveniently and desirably the chlorides having a ratio of carbon to nitrogen of from 10:1 to 30:1. Some high molecular weight compounds of carbon and nitrogen possessing a ratio of carbon to nitrogen up to about 4:1 are known to improve adhesion of organic coatings to various surfaces as taught in United States Patent No. 2,887,405, but these do not form thin conductive layers and are not useful in accordance with the invention. Similarly, many quaternary ammonium compounds are known antistatic agents, but most of these have a ratio of carbon to nitrogen in excess of 30:1 or less than 10:1 and, again, do not form the required thin conductive layers and are. not useful in the invention.

It is particularly preferred to have present in either the quaternary or diquaternary ammonium chloride a.

single substituent having a long carbon chain, desirably a carbon chain containing from 22 carbon atoms. The preferred substituents are alkyl groups which are essentially lacking in carbon to carbon unsaturation, and these are preferably straight chain groups, though branched chain groups may be tolerated, especially when the extent of branching is relatively small. Aromatic hydrocarbon substituents are not preferred, but may be tolerated.

Thus, particularly preferred quaternary halide salts are quaternary ammonium chlorides having a carbon to nitrogen ratio of from 10:1 to 30:1 and having a structural formula selected from the group consisting of:

$113 [R-ITI-CHaPCland w r [R1| lXl| lCHs]*- 2Cl CH3 CH3 in which R is a hydrocarbon substituent containing from 10-22 carbon atoms, preferably an alkyl substituent, and X is a divalent hydrocarbon chain containing from 1 to 10 carbon atoms, preferably a divalent alkylene radical illustrated by the radical:

H H H ire-(L It a It A particularly preferred quaternary ammonium salt for use in the invention is:

Further specific illustrations of ionizable organic compounds which have been successfully used in accordance with the invention are as follows:

Where x and y are 7, 9, 11, 13, 15, or 17 (mainly l1) [GH3(CH:)15N(CHa)3]+C1- NOTE: The terms coco and tallow in the above formulas indicate the hydrocarbon radicals derivable from the corresponding oil or fat.

The corresponding quaternary ammonium hydroxides are useful in conjunction with surfaces containing strongly 6 acidic functional groups such as plasticized chlorinated sulfonated polyethylene.

Ionizable organic compounds which ionize to form negatively charged organic ions may also be used in the invention. These are illustrated by organic hydrocarbonsubstituted acids such as alkyl and aryl esters of acids of phosphorus and sulfur. Thus, phosphoric acid esters, phosphonic acids and diphosphonic acids are useful as are the corresponding acids of sulfur.

To illustrate, ethyl acid phosphate, butyl acid phosphate, amyl acid phosphate, phenyl acid phosphate, ptoluene phosphonic acid, di-ethyl diphosphonate, ethyl acid sulfonate, p-toluene sulfonic acid, and p-phenolsulfonic acid are all effective in accordance with the invention. The corresponding acyl chlorides, such as p-toluenesulfonyl chloride, are equivalent to the acids and may be used as illustrated by p-toluenesulfonyl chloride. P- toluenesulfonic acid is especially effective.

A further suitable acid of phosphorus is illustrated by H(CFgCF3)n'i. -OH

OH 11:42-52 which ionizes to provide positively charged hydrogen ions.

The organic acids of phosphorus and sulfur and the acyl chlorides thereof are particularly useful in connection with insulating surfaces having hydroxyl functionality available for ester formation. Thus, glass is desirably treated with the acids which have been mentioned. Moreover, these acids need not form separate gel layers but, instead, may become incorporated in the surface of the gel structure which is treated, as for example, in the gel structure of wood, gelatin or polyvinyl alcohol. Similarly, the SH groups available in vulcanized rubber and ligno-cellulosic materials have the same general bonding properties toward acids as do hydroxyl groups and these -SH groups can be relied upon to achieve the chemical association which is desired.

When ionization of the ionizable organic compound produces a positively charged organic ion, such as the organic cation of a quaternary ammonium salt, the cation is basic and is adapted for bonding with the carboXyl groups present in cellulosic materials including wood as well as in many other organic materials. Moreover, carboXyl groups or ions derived therefrom are not essential to chemical bonding with basic cations, and any functional group electronegative with respect to the nitrogen base cation is suitable.

Conversely, if the ionizable compound selected is an organic acid which ionizes to form negatively charged organic anions, these can become associated with relatively electro-positive functional groups in the electrically insulating material.

It is desired to emphasize that the swollen gel layers which are formed in the invention are relatively stable. Preferably, the gel should resist for indefinitely long periods the removal of the effective fluid content thereof by evaporation when subjected to terrestrial atmospheres maintained at room temperature and having a partial vapor pressure of water of 10 mm. of mercury.

The ionizable organic compounds which are employed in accordance with the invention may be applied as thin layers to the insulating surface in any convenient manner as by hand wiping, brushing, or spraying. Indeed, it is merely necessary, in connection with the invention, to apply the ionizable organic compound by employing a rag moistened with the same as part of a dusting operation prior to coating. The electrically continuous layers of the invention do not have to be applied to the entire insulating surface to be coated, through uniform application is preferred. Thus, application of the gel layer in the form of a network of lines is also effective, but is not preferred.

The ionizable organic compounds of the invention, for purposes of application to the insulating surface, are dissolved in a suitable solvent medium which preferably includes a proportion of a lower alcohol such as ethanol or methanol and which preferably also includes a proportion of water. It is not necessary that the solution include a large proportion of water because the solvent medium would normally and naturally include sufficient water to provide the desired water content in the thin layers which are deposited. Moreover, many of the ionizable organic compounds which are contemplated have the capacity to pick up sufficient water from the atmosphere, at least while they are deposited from organic solvent solution. A particularly suitable solvent medium is a mixture of methanol and an aromatic hydrocarbon such as toluene, in weight proportions of 7:2.

The concentration of the dissolved ionizable organic compound in the solvent medium is selected on the basis of convenience. A concentration of from ll% by weight of ionizable organic compound in the solvent solution which is applied to the insulating surface would represent preferred practice. However, the solution may be more or less concentrated and still be useful in accordance with the invention.

It is desired to point out that the thickness of the gel layer formed by application of a solution of the ionizable organic compound will vary depending upon the concentration of the solution and its manner of application, e.g., a greater Weight of material is deposited by dipping than by wiping with a moistened rag. When only thin film solutions are applied, as by wiping, it is preferred that the solution contain at least 1% by Weight of ionizable organic compound.

The preferred ionizable organic compounds specifically referred to hereinbefore provide, when formed into a gel layer on glass, by dipping the glass into a 1% isopropanol solution, a resistivity of from ohms per square to 10 ohms per square, under normal atmospheric conditions. In contrast, if the ionizable organic compound does not form or enter into a gel structure, the resistivity of the deposit formed by dipping glass int-o a 1% isopropanol solution is consistently in excess of 10 ohms per square, e.g., usually about 10 ohms per square or higher, and is not useful in accordance with the invention.

There may also be included with the ionizable organic compound proportions of other materials. In this connection, components in particulate form, or which assume particulate or crystalline form when the solution dries, are desirably present since they tend to increase surface area and to enhance the effectiveness of pretreatments in accordance with the invention. Thus, non-volatile salts which include highly mobile ions such as alkali metal chlorides, and particularly sodium chloride, are beneficial and may be included in the solutions which are applied to the insulating surfaces in small amounts of up to 1%. Finely divided siliceous materials, typified by extremely finely divided silica, are also beneficial from various standpoints, e.g., finely divided silica contributes an increase in surface area, an increased density of functional groups available for chemisorption and independent gel forming capacity.

The insulating surface which is coated with dissolved ionizable organic compound in accordance with the invention is desirably dried to provide a stable pretreated surface capable of receiving electrostatically charged coating particles. Air drying is adequate. Force drying may also be used. In the force drying operation, heated air may be used and at temperatures up to 130 F., force drying is not detrimental. Higher temperatures may also be tolerated; however, the benefit achieved by the pretreatment of the insulating surface decreases slowly with time at these higher temperatures. During the brief periods re quired to merely remove residual solvents and any surface water which may be present, the gel structure is not detrimentally affected. Still higher temperatures, even exceeding the boiling point of water, may be briefly applied,

so long as the elevated temperature treatment is not continued to the point of removing the fluid component of the gel, e.g., usually water, which is essential to the gel structure and electrical conductivity of the layer. As previously indicated, if exposure to heat or other drying condition is excessive, fluid losses from the gel cause the gel structure to be destroyed and the capacity of the layer to rereceive electrostatically charged coating particles is similarly destroyed.

In the pretreatment of wood, it should be kept in mind that the electrical conductivity of wood varies with its moisture content. When wood is used for structural or ornamental purposes as in furniture or when the wood is to be worked, as by sanding, sawing, turning, planing, etc., it is preferred to dry the Wood to a moisture content below about 10% by weight, e.g., about 7% by weight, based on the weight of the bone dry wood. It is at such low moisture content that the electrical insulating character of wood is emphasized together with the importance of the invention. Wood of higher moisture content is also benefited by the invention, but the need for the treatment is reduced because of the greater electrical conductivity possessed by the wood of higher moisture content. Veneered products, including plywood, are particularly troublesome in the absence of the invention because the adhesive layer bonding the veneer to the core insulates the surface of the veener from the main mass of wood emphasizing the tendency for electrostatic charge to accumulate at the surface.

The natural grain of wood provides non-uniform conductivity and the charged coating particles tend to deposit non-uniformly-following the pattern of the grain. Treating the wood in accordance with the invention effectively overcomes this non-uniform electrostatic deposition.

The application of an organic resinous coating over the pretreated insulating surface does not necessarily destroy the electrical continuity of the layer of ionizable organic compound. Even baking of the organic resinous coating at conventional temperatures does not normally destroy the electrical continuity of the ionizable organic compound beneath the baked organic resinous coating. Thus, the presence of the electrically conductive layers of the invention, in many instances, facilitates electrostatic deposition of subsequent coatings even when the first coating of organic resinous material is itself electrically resistive.

The invention is particularly useful in connection with the electrostatic deposition of charged coating particles on surfaces having a resistivity of 10 ohms per square or more, and the presence of the gel layer desirably lowers the resistance of the surface so that it does not exceed 10 ohms per square.

The term resistivity as herein used is intended to define the resistance in ohms per square, as per square centimeter, measured at the surface, either before or after treatment as the case may be. Moreover, when we speak of surface resistivity, it will be understood to mean the resistance between two electrodes contacting the surface, without regard to the distribution of current within the article as well as along the surface. Conveniently, the resistivity is measured by a Keithley Model 200A electrometer employing a Keithley Model 2008 decade shunt and a DC. power supply having maximum voltage output of 35 volts. Electrode contact with the surface is made through two flexible aluminum foil strips backed with resilient material, as flexible isocyanate foam, in turn mounted on a rigid insulated support, as Lucite. The foil strips are each 1 centimeter in width and 10 centimeters long, mounted so as to be parallel and with their adjacent edges being 1 centimeter apart. When these foil electrodes are pressed against the surface when the resistivity is to be measured, they measure the resistance of 10 onecentimeter squares of the surface arranged in parallel so that the direct reading obtained may be multiplied by a factor of 10 to obtain surface resistance in ohms per 9 square. When current and voltage readings have been made the resistance between the electrodes is calculated in accordance with the formula:

in which R is the unknown resistance, E is the electrometer voltages reading, E is the voltage of the input power supply and I is the current flow through the unknown resistance. When the resistance thus derived is multiplied by 10, the resistivity in ohms per square has been determined.

The resistivity of the path to ground from any point on a surface to be painted must be low enough to permit effective electrostatic coating, low enough to prevent the build-up at any point on the surface being coated of an undesirably high potential with respect to ground. In the case of electrostatic painting systems employing no appreciable mechanical forces for atomization, as relatively low speed bell or disc atomizers, an article surface voltage of approximately 25 RV. above ground will result in very little paint deposition on that portion of the article. When a fairly large article is supported in a manner now normal in electrostatic coating, as for example, if a plywood panel is supported on the conveyor by one or two grounding hangers, the resistivity is sumcient to prevent effective electrostatic coating of areas of the surface substantially remote from the hangers, as a surface voltage of 25 kv. above ground or more rapidly builds up at such remote areas and prevents effective application of further charged paint particles.

Even lower surface voltages, as kv. or more, provide an undesirable situation in that an appreciable amount of paint is repelled and wasted. Moreover, in the case of hand spray guns using a rotating atomizing device, a surface voltage build-up of 15 kv. or more results in discomfort to the operator because of an undesirably large amount of paint being attracted to and deposited on him as the result of the repelling effect of the article surface voltage. Accordingly, while ground circuit resistance may vary as the result of the shape and size of an article and the method of grounding it, the effective resistance from any point on the surface to ground should preferably be low enough to prevent a build-up at such point to 15 kv. or more, and in any event, low enough to prevent a buildup surface voltage to kv. or more at any point to be coated.

The voltages mentioned above result from an undesirably high instantaneous eifective resistance of the leakage resistance path to ground from a point on the surface being painted. The leakage resistance to ground from a portion of the surface being painted, the instantaneous effective resistance, may for example consist of surface resistance, volume resistivity, air leakage resistance, and charging resistance of a capacitive system. The instantaneous effective resistance to ground from a given portion of an article varies as the article shape and size, and can be intentionally varied by the location of the grounding connection, or the use of a plurality of such connections. Where the article is of a character having sufficient resistivity to prevent effective electrostatic coating, the pretreatment of the surface can be supplemented, if desired, by the use of additional grounding connections. In any event, for satisfactory operation of electrostatic painting devices of the character now in general commercial use, the instantaneous effective resistance to ground from any point on the surface being painted must be maintained below about 2 X10 ohms.

It is desired to emphasize that the very thin gel layers which are applied in accordance with the invention are light-permeable and they do not mask the natural surface of the insulating articles to which they are applied. Thus, the natural grain of wood is not obscured when the surface of the Wood is pretreated in accordance with the invention and then overcoated with a clear varnish or W lacquer. The gel layers may, however, be pigmented or dyed if desired.

The invention is illustrated in Examples III to V, VII, and IX to XIV which follow, these being contrasted with the control Examples I, II, VI and VIII to demonstrate the usefulness of the invention.

Example I A 10" x 6 panel of maple veneer thick and having a moisture content of 7% based on the bone dry weight of the wood was grounded with two conductive grounds, one at the mid-point of each 6" dimension, and electrostatically sprayed with nitrocellulose lacquer using electrost atic atomization with a potential of kv. applied to the atomizing bell for a period of time in which a similar metal object would have been satisfactorily coated. The product showed accumulations of deposited lacquer on the surfaces of the veneer adjacent the ground connections. The bulk of the veneer surface facing the atomizing bell was substantially uncoated.

Example II Example I was repeated using a similar panel of maple veneer having a moisture content of 10% on the same basis. Atomization was somewhat improved and the veneer was coated across its entire surface. However, the coating was not uniform in thickness but instead exhibited a pattern of slightly thick deposits which followed the grain pattern of the wood. Of considerable significance is the fact that the coating Was substantially thicker adjacent the ground connections.

Example 111 Example I was repeated using a similar panel of maple veneer having a surface layer applied by wiping with a cloth moist with a 2% by weight solution of dodecyl trimethyl ammonium chloride dissolved in a 7:2 volume ratio mixture of methanol and toluene, the wiped veneer being air dried over calcium chloride desiccant at room temperature. A uniform coating was produced extending over the veneer surface facing the atomizing hell with the exception of a slightly increased thickness of coating adjacent the ground connections.

Example IV Example III was repeated with the exception that the 2% solution of dodecyl trimethyl ammonium chloride was sprayed onto the veneer until a uniform wetness was observable to the eye and the treated veneer was dried by force drying using heated air at F. for 1 hour. A uniform coating resulted from the electrostatic spray.

Example V Example II was repeated using a similar panel of maple veneer wiped to form a film of dodecyl trimethyl ammonium chloride as in Example III. The electrostatical- 1y spnayed product was uniformly coated.

Example VI A section of glass was electrostatically sprayed as in Example I with the same results, e.g., small accumulations of coating material localized in the areas of the ground connections and no overall coating on the glass surface facing the atomizing bell.

Example VII Example VI was repeated after wiping as in Example III to provide a film of dodecyl trimethyl ammonium chloride on the surface of the glass. A uniform coating extending over the glass surface facing the atomizing bell was produced.

Example VIII A 10" x 6" section of film composed of ethylene glycolterephthalate homopolyester having a melting point of about 250 C. and of high molecular weight evidenced by capacity to be cold drawn was electrostatically sprayed as in Example I without any pretreatment and similar unsatisfactory results obtained.

Example IX Example VIII was repeated after wiping the polyester film as in Example III to provide a thin layer of dodecyl trimethyl ammonium chloride on the surface of the ethylene glycol-terephthal'ate homopolyester film. A uniform coating was produced.

Example X The dodecyl trimethyl ammonium chloride used in the preceding Examples III to V, VII and IX, is replaced, one by one, by each of the remaining compounds previously listed. In each instance, uniform coatings are produced on the previously insulating surfaces which refused to adequately accept electrostatic deposits in the absence of surface pretreatment in accordance with the invention.

Example XI The 2% solution of the ionizable organic compounds of the previous Examples III to V, VII, IX and X, is replaced with a 10% solution and these examples are otherwise repeated. Coating proceeds easily in all instances.

Example XII The 2% solution of dodecyl trimethyl ammonium chloride is modified by the addition to the solution of 0.1% by weight of sodium chloride and 1% by weight of sodium chloride based on the weight of the solution and Examples III to V, VII and IXXI are repeated. Satisfactory coating is obtained in all instances.

Example XIII Example III was repeated using a 2% by weight solution of p-toluene phosphonic acid dissolved in a 7:2 volume ratio mixture of methanol and toluene. Results equivalent to that reported in Example III were obtained.

Example XIV Example XIII was repeated using a square of glass in place of the panel of maple veneer. A uniform coating extending over the glass surface facing the atomizing bell was produced.

The nitrocellulose lacquer used in the foregoing examples is as follows:

Percent /2 second nitrocellulose 40-42 Dioctylphthalate Castor oil 20 Dammar resin 10 Castor oil alkyd containing 20 pounds of oil per 100 pounds of an alkyd resin of equimolecular proportions of phthalic anhydride and glycerin 23 The plasticized resin system referred to above was dissolved in a solvent mixture consisting of 60% butyl acetate and 40% xylene to a concentration of 20% resin solids. The lacquer was clear and had a viscosity of 40 seconds measured at room temperature using a #4 Ford Cup.

The invention is defined in the claims which follow.

We claim:

1. A method of electrostatically coating an article having an electrically insulating surface including functional groups adapted for chemisorption with electrically charged ions comprising depositing upon said article a thin electrically continuous conductive gel surface layer containing polar liquid of high dielectric constant and stable under normal atmospheric conditions, said gel layer consisting essentially of ionizable organic compound capable of ionizing to form a positively charged organic ion capable of chemisorption with said functional groups in said surface, and strongly electronegative monovalent ions, and having a thickness of at least about 1000 A. and a resistivity of not more than 10 ohms per square,

said ionizable organic compound being responsible for the formation of the required gel structure, and projecting eleetrostatically charged coating particles toward said surface while dissipating the electrostatic charge on said surface to maintain said surface at a relatively opposite polarity with respect to the electrostatic charge on said coating particles.

2. A method as recited in claim 1 in which said gel layer has a thickness of up to about 25,000 A.

3. A method as recited in claim 1 in which said gel layer is a physically continuous layer as viewed under a phase contrast microscope at 1000 magnifications.

4. A method as recited in claim 1 in which said polar liquid of high dielectric constant comprises water and said gel layer is resistant to destruction of its gel character upon prolonged exposure to an atmosphere maintained at room temperature and at a water vapor pressure of 10 mm. of mercury.

5. A method as recited in claim 1 in which said gel layer possesses a water drop contact angle and a parafiin oil drop contact angle each of which are less than 30 measured through the liquid of the drop.

6. A method as recited in claim 1 in which a 50% solution of said compound in isopropanol will tolerate the addition of not more than 50 cc. of mineral spirits per gram of said 50% solution before a phase separation takes place.

7. A method as recited in claim 1 in which said gel layer has a resistivity low enough to prevent the buildup at any point of a surface voltage with respect to ground of 15 kv.

8. A method as recited in claim 1 in which said ionizable organic compound is a quaternary ammonium halide having a ratio of carbon to nitrogen of from 10:1 to 30: 1.

9. A method as recited in claim 1 in which said article is a cellulosic article.

10. A method as recited in claim 9 in which said cellulosic article is wood having a moisture content of up to 10% by weight, based on the bone dry weight of the wood.

11. A method as recited in claim 9 in which said wood comprises an adhesively secured surface layer of veneer.

12. A method of electrostatically coating an article having an electrically insulating surface including functional groups adapted for chemisorption With electrically charged ions comprising depositing upon said article a thin electrically continuous conductive gel surface layer containing polar liquid of high dielectric constant and stable under normal atmospheric conditions, said gel layer consisting essentially of ionizable organic compound capable of ionizing to form a charged organic ion of one electrical sign capable of chemisorption with said functional groups in said surface, and monovalent ions of opposite electrical sign, and having a thickness of at least about 1000 A. and a resistivity of not more than 10 ohms per square, said ionizable organic compound being responsible for the formation of the required gel structure, and projecting electrostatically charged coating particles toward said surface while dissipating the electrostatic charge on said surface to maintain said surface at a relatively opposite polarity with respect to the electrostatic charge on said coating particles.

13. A method as recited in claim 12 in which said charged organic ion is a positively charged organic ion and said ions of opposite electrical sign are strongly electronegative.

14. A method as recited in claim 13 in which said ionizable organic compound is an organic acid of an element selected from the group consisting of sulfur and phosphorus.

15. A method as recited in claim 1 in which said organic acid is a phosphonic acid.

16. A method as recited in claim 15 in which said organic acid is p-toluene phosphonic acid.

17. A method of electrostatically coating an article 13 having an electrically insulating surface having a resistivity of at least 10 ohms per square, said surface including functional groups adapted for chemisorption with electrically charged ions, comprising depositing upon said article a thin electrically continuous conductive gel surface layer swollen with polar liquid of high dielectric constant and stable under normal atmospheric conditions, said gel layer being physically continuous as viewed under a phase contrast microscope at 1000 magnifications, said gel layer consisting essentially of quaternary ammonium chloride having a ratio of carbon to nitrogen of from 10:1 to 30:1 and having a structural formula selected from the group consisting of:

in which R is a hydrocarbon radical and X is a divalent aliphatic hydrocarbon radical containing from 1 to 10 carbon atoms, and said gel layer having a thickness of from about 1000 A. to about 25,000 A. and a resistivity not exceeding 10 ohms per square, said quaternary ammonium chloride being responsible for the formation of the required gel structure, and projecting electrostatically charged coating particles toward said surface While dissipating the electrostatic charge on said surface to maintain said surface at a relatively opposite polarity with respect to the electrostatic charge on said coating particles.

18. A method as recited in claim 17 in which said quaternary ammonium chloride contains a single essentially straight chain alkyl substituent of from 10-22 carbon atoms.

19. A method as recited in claim 17 in Which said quaternary ammonium chloride is:

References Cited by the Examiner UNITED STATES PATENTS 2,463,282 3/1949 Kang.

2,593,787 4/1952 Parker 260666.5 2,626,877 1/1953 Carnes.

2,723,921 11/1955 Starkey 11757 X 2,955,958 10/1960 Brown 117-113 2,992,139 7/1961 Arhart 11793 X FOREIGN PATENTS 774,807 5/1957 Great Britain.

RICHARD D. NEVIUS, Primary Examiner,

Claims

1. A METHOD OF ELECTROSTATICALLY COATING AN ARTICLE HAVING AN ELECTRICALLY INSULATING SURFACE INCLUDING FUNCTIONAL GROUPS ADAPTED FOR CHEMISORPTION WITH ELECTRICALLY CHARGED IONS COMPRISING DEPOSITING UPON SAID ARTICLE A THIN ELECTRICALLY CONTINUOUS CONDUCTIVE GEL SURFACE LAYER CONTAINING POLAR LIQUID OF HIGH DIELECTRIC CONSTANT AND STABLE UNDER NORMAL ATMOSPHERIC CONDITINS, SAID GEL LAYER CONSISTING ESSENTIALLY OF IONIZABLE ORGANIC COMPOUND CAPABLE OF IONIZING TO FORM A POSITIVELY CHARGED ORGANIC ION CAPABLE OF CHEMISORPTION WITH SAID FUNCTIONAL GROUPS IN SAID SURFACE, AND STRONGLY ELECTRONEGATIVE MONOVALENT IONS, AND HAVING A THICKNESS OF AT LEAST ABOUT 1000 A. AND A RESISTIVITY OF NOT MORE THAN 10**10 OHMS PER SQUARE, SAID IONZABLE ORGANIC COMPOUND BEING RESPONSIBLE FOR THE FORMATION OF THE REQUIRED GEL STRUCTURE, AND PROJECTING ELECTROSTATICALLY CHARGED COATING PARTICLES TOWARD SAID SURFACE WHILE DISSIPATING THE ELECTROSTATIC CHARGE ON SAID SURFACE TO MAINTAIN SAID SURFACE AT A RELATIVELY OPPOSITE POLARITY WITH RESPECT TO THE ELECTROSTATIC CHARGE ON SAID COATING PARTICLES.