Related Background Art Liquid coatings applied to vertical surfaces are susceptible to defects commonly known as sagging, running and curtaining, as described in Percy E. Pierce and Clifford K. Schoff, Coating Film Defects (Federation of Societies for Coatings Technology, 1988), the disclosure of which is incorporated by reference herein. These defects arise when the action of gravitational forces on the coating film results in downward flow of the film. The conventional remedies involve modification of the viscosity or thickness of the film. Reducing the film thickness decreases the flow velocity in the liquid coating, allowing the

coating to dry or cure before noticeable defects arise. However, the decreased film thickness requires multiple applications of coating to achieve the same overall coating thickness. Increasing the coating viscosity also decreases the flow velocity, and is typically accomplished by adding thickeners or thixotropes, or by using a solvent that evaporates relatively quickly.

Modifying the coating formulation in this way adds to the cost of the coating and may be detrimental to other properties of the coating.

A method for producing a substantially even coating on a vertical surface without the use of, or with a reduction in the amount of additives would be extremely useful.

SUMMARY OF THE INVENTION This invention is directe to a method for producing a coating of substantially even thickness on a coating area of a vertical substrate surface. The method comprises the steps of: (a) applying a coating formulation comprising a carrier and a resin to the coating area from a first edge of the coating area of a substrate surface to a second edge of the coating area, said coating formulation being applied to produce a coating formulation thickness which increases continuously from the first edge to the second edge; and (b) situating at a higher vertical position whichever of the first edge or the second edge at which a surface tension of the coating formulation will be greater as the carrier evaporates than at the other edge.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side cross section of a substrate coated with a solvent-based coating formulation.

Figure 2 is a side cross section of a substrate coated with a water-borne coating formulation.

DETAILED DESCRIPTION OF THE INVENTION The method of the present invention produces a coating on a vertical surface with reduced tendency to sag, run or curtain. Without being bound to theory, this is believed to be accomplished by creating an approximate balance between the force due to surface tension and the force of gravit.

A coating formulation is applied unevenly across a predetermined area to be coated so that the thickness of the applied coating formulation (henceforth,"film") increases continuously. After or during application of the coating formulation, the edge of the predetermined area at which the film thickness is a maximum is situated either above or below the edge at which the film thickness is a minimum. The relative position of the edges is determined by the relative surface tensions of the components of the coating formulation.

Typically, as the carrier evaporates, the surface tension of the film either increases or decreases.

This change in surface tension will vary with the film thickness. It is desired in the method of this invention to place in a higher vertical position the edge at which the surface tension of the film will be greater as the carrier evaporates.

For example, in a coating formulation in which the carrier is a typical non-aqueous solvent having a

relatively low surface tension, e. g., a hydrocarbon or ketone, the surface tension of the carrier is typically lower than that of the other components of the formulation. Consequently, during drying of the film, evaporation of the solvent from the film increases the concentration of the higher surface tension components, especially in regions of lower film thickness, thus increasing the surface tension of the film.

Accordingly, a solvent-based coating is purposely applied to a substrate unevenly and the substrate oriente so as to produce a film which increases in thickness towards the lower edge. This comprises an embodiment of this invention illustrated in Figure 1, wherein the film 2 is applied unevenly on substrate 1, which is oriente so that the edge at which the coating is thicker is at the bottom. Under conditions of uniform temperature and pressure, evaporation of solvent will be dependent only on surface area, and not on thickness. However, in regions of lower thickness 3, the proportional effect of surface evaporation on the lower-thickness area of the film will be greater than in regions of greater thickness 4, thus increasing the surface tension more in region 3 than in region 4.

As the solvent evaporates, the surface tension will thus be a maximum at the edge at which the thickness is a minimum. Hence, in the method of this invention, a surface unevenly coated with a solvent-based film is situated with the edge at which thickness is a minimum in a higher vertical position than the edge at which thickness is a maximum. This produces a slight upward force on the coating formulation which results in upward flow to give a substantially uniform film thickness.

For the case in which the carrier has a relatively high surface tension, e. g., when the carrier is water, the surface tension of the carrier is typically higher than

that of the other components of the formulation.

Consequently, evaporation of the carrier in this case increases the concentration of the lower surface tension components, and thereby decreases the surface tension of the film. If, as in an embodiment of this invention illustrated in Figure 2, the applied coating formulation 2 is not of uniform thickness on the substrate 1, the decrease in surface tension will vary with thickness. As in the case of a carrier having a low surface tension, the change in surface tension with evaporation of the carrier is greater in regions of lower thickness 3 than in regions of greater thickness 4. The surface tension in the regions of lower thickness will decrease more than the surface tension in the regions of greater thickness, leading to a higher surface tension in the regions of greater thickness, i. e., the surface tension will be higher in region 4 than in region 3. Hence, a surface coated with a water-borne film whose thickness increases from one edge to another edge is situated with the edge at which thickness is a maximum in a higher vertical position than the edge at which thickness is a minimum, as shown in Figure 2. This produces an upward force on the coating formulation which retards downward flow due to gravitational forces to give a substantially uniform film thickness.

Applications for the types of coating formulations deposited according to the method of this invention inclue, but are not limited to, automobile coatings and refinishes, general industrial coatings, architectural coatings, inks, paper coatings, general metal coatings, wood coatings, coil coatings, appliance coatings and container coatings.

Coating formulations for use in this invention comprise at least a carrier and a resin. The formulations may

also include pigments, catalysts, flow modifiers, preservatives, stabilizers, buffers and any other components typically used in coating formulations.

The types of resins suitable for use in this invention inclue, but are not limited to, polyesters, acrylics, alkyds, isocyanates, epoxies, butyrates, cellulose acetate, ethyl cellulose, alkylated melamine- formaldehyde, nitrocellulose, polyvinyl butyral, phenolics, urea-formaldehyde and urethanes.

Suitable carriers for use in this invention inclue, but are not limited to one or more of water, ethylene glycol, xylene, methyl isoamyl ketone, mineral spirits, n-butanol, methyl isobutyl ketone, methyl ethyl ketone and n-butyl acetate.

The coating formulations may be applied by any technique suitable for unevenly applying a coating formulation to a surface. A suitable technique comprises placing the surface in a horizontal position, applying a coating formulation to the surface, drawing the coating formulation on the surface down with a wire wound rod supporte on one side with a shim to produce a thickness gradient, and then repositioning the surface vertically in the proper orientation, depending on the identity of the carrier. The thickness gradient is defined in terms of the increase in thickness with distance along the surface, and is measured in mils/inch of surface. Suitable gradients of coating thickness across the surface will vary with the nature of the coating formulation.

The Examples which follow are intended as illustrations of certain preferred embodiments of the invention, and no limitation of the invention is implied.

EXAMPLE 1 Solvent-Based Coating Formulation The resin of the coating was a 70: 30 weight ratio mixture of high-solids polyester (available from McWhorter Technologies, Carpentersville, IL, as 057- 5776) and melamine resin Resimene 747 (available from Solutia, Inc., Springfield, MA). Titanium dioxide pigment was added at 250 parts on a total formulation of 701 parts. Nacure 3525 catalyst (available from King Industries, Norwalk, CT) was added in an amount sufficient to provide 1.6% based on the total resin solids (TRS). Modaflow 2100 flow agent (available from Solutia, Inc., Springfield, MA) was added in an amount sufficient to provide 0.22% based on TRS. A mixture of methyl isoamyl ketone, xylene and n-butanol in a volume ratio of 68: 23: 9 was added at 33 parts on a total formulation of 701 parts. After the addition, the spray viscosity of the formulation was 23 seconds in a #4 Ford Cup with 1: 1, xylene: isopropanol.

COMPARATIVE EXAMPLE 1 Solvent-Based Coating with Uniform Thickness The coating formulation of Example 1 was applied to a 4x12 inch Bonderite panel using a 3.6 mil wire wound rod. This resulted in a coating with a uniform thickness of 3.6 mils. The panel was placed in a vertical position for 10 minutes and was then placed in an oven in a horizontal position and cured at 260°F for 15 minutes.

COMPARATIVE EXAMPLE 2 Solvent-Based Coating with Greater Thickness at the Top The coating formulation of Example 1 was applied to a 4x12 inch Bonderite panel using a #22 (2.2 mil) wire wound rod supported on one twelve-irsch side of the panel by a strip of tape with a thickness of 3.5 mils.

This resulted in a coating thickness of 5.7 mils on the twelve-inch side near the tape and 2.2 mils on the other twelve-inch side, with the gradient running across a distance of four inches. The panel was placed in a vertical position with the twelve-inch side having a 5.7 mil coating thickness at the top. The panel was kept in a vertical position for 10 minutes and was then placed in an oven in a horizontal position and cured at 260°F for 15 minutes.

EXAMPLE 2 2.2-5.7 mil Solvent-Based Coating with Greater Thickness at the Bottom A panel was coated in the same manner as described in Comparative Example 2, but was placed in a vertical position with the 5.7 mil thickness along the bottom edge. The panel was placed in a vertical position for 10 minutes and was then placed in an oven in a horizontal position and cured at 260°F for 15 minutes.

EXAMPLE 3 3.5-5.5 mil Solvent-Based Coating with Greater Thickness at the Bottom The coating formulation of Example 1 was applied to a 4x12 Bonderite panel using a 3.5 mil wire wound rod supporte on one end by a strip of tape with a

thickness of 2 mils. This resulted in a coating thickness of 5.5 mils on the side near the tape and 3.5 mils on the other side. The panel was placed in a vertical position with the 5.5 mil thickness along the bottom edge. The panel was placed in a vertical position for 10 minutes and was then placed in an oven in a horizontal position and cured at 260°F for 15 minutes.

EXAMPLE 4 Thickness Measurements on Cured Solvent-Based Films Film thickness measurements of the cured films obtained in Examples 2-3 and Comparative Examples 1-2 were performed using an Elcometer 256 film thickness gage (available from Elcometer Inc., Rochester Hills, MI).

The results are presented in Table 1. Each film thickness in mils is presented as the average of several measurements (av. film thick.), along with the standard deviation (s. dev.) and degrees of freedom (deg. free., the number of measurements minus one).

Table 1

av. film s. dev. deg. thick. free. Comp. Ex. 1 top 1.28 0.10 5 middle 80.05 bottom 1.52 0.20 9 Comp. Ex. 2 top 1.39 0.09 7 7- bottom 0.78 0.02 6 Ex. 2 top 0.77 0.04 7 middle 0.83 0.06 9 bottom 1.25 0.06 7 Ex. 3 top 1.24 0.07 11 middle 1.25 0.06 9 bottom 1.37 0.13 13 The average film thicknesses for the coating of Comparative Example 1, a control in which a film of uniform thickness was applied, show that considerable sag occurred, leading to a much greater thickness at the bottom of the panel. In Comparative Example 2, the coating was applied in the opposite of the desired direction, i. e., with the thicker end up, resulting in sagging of the top half of the coating so that the thickness was much greater in the top half of the panel. In Examples 2 and 3, the coating was applied with the thicker end down. A pronounced thickness gradient, with much greater thickness at the bottom, is present in the coating of Example 2 because the coating formulation flowed downward in this case. It is apparent that, in this application of the coating formulation, forces due to surface tension did not completely overcome those due to gravit. However, in the coating of Example 3, the cured film thickness is substantially even, showing only a slight increase in thickness at the bottom of the panel.

EXAMPLE 5 Water-Borne Coating Formulation The resin used in the coating was a mixture of a water- reducible polyester 72-7203 (available from McWhorter Technologies, Carpentersville, IL) and melamine resin Resimene 747 (available from Solutia, Inc., Springfield, MA) in a 7: 3 ratio based on solids content. The coating formulation, after addition of distille water, pigment, amine buffer and flow modifier, contained 22.20% polyester and 6.75% melamine resin. Titanium dioxide pigment was present in the coating formulation at 20.68% by weight of the formulation, and Modaflow AQ-3000 flow agent (available from Solutia, Inc., Springfield, MA) was present at 0.52% by weight of the formulation. Nacure 1040W catalyst (available from King Industries, Norwalk, CT) was added to the coating formulation in an amount sufficient to provide 0.8% catalyst based on the total resin solids (TRS).

COMPARATIVE EXAMPLE 3 Water-Borne Coating with Uniform Thickness The coating formulation of Example 5 was applied to a 4x12 inch Bonderite panel using a #34 (3.4 mil) wire wound rod. This resulted in a coating with a uniform thickness of 3.4 mils. The panel was placed in a vertical position for 10 minutes and was then placed in an oven in a vertical position and cured at 270°F for 20 minutes.

EXAMPLE 6 Water-Borne Coating with Greater Thickness at the Top The coating formulation of Example 5 was applied to a 4x12 inch Bonderite panel using a #34 (3.4 mil) wire wound rod supporte on one side by a strip of tape with a thickness of 2.4 mils. This resulted in a coating thickness of 5.8 mils on the side near the tape and 3.4 mils on the other side. The panel was placed in a vertical position with the 5.8 mil thickness along the top edge. The panel was placed in a vertical position for 10 minutes and was then placed in an oven in a vertical position and cured at 270°F for 20 minutes.

COMPARATIVE EXAMPLE 4 Water-Borne Coating with Greater Thickness at the Bottom A panel was coated in the same manner as described in Example 6, but was placed in a vertical position with the 5.8 mil thickness along the bottom edge. The panel was placed in a vertical position for 10 minutes and was then placed in an oven in a vertical position and cured at 270°F for 20 minutes.

EXAMPLE 7 Thickness Measurements on Cured Water-Borne Films Film thickness measurements of the cured films obtained in Comparative Examples 3-4 and Example 6 were performed using an Elcometer 256 film thickness gage.

The results are presented in Table 2. Each film thickness in mils is presented as the average of several measurements (av. film. thick.) along with the

standard deviation (s. dev.) and degrees of freedom (deg. free.).

Table 2 av. film s. dev. deg. thick. free. Comp. Ex. 3 top 0.64 0.10 8 middle 0.82 0.25 8 bottom 0.78 0.19 9 Ex. 6 top 1.50 0.05 7 middle 1.76 0.05 8 bottom 1.50 0.08 7 Comp. Ex. 4 top 1.21 0.08 7 middle 1.68 0.09 6 bottom 1.75 0.04 8 The average f lm thicknesses for the coating of Comparative Example 3, a control in which a film of uniform thickness was applied, show that sag occurred, leading to a greater thickness at the middle and bottom of the panel. In Example 6, the coating was applied in the desired direction, i. e., with the thicker end up, resulting in a fairly uniform thickness. Although the coating is slightly thicker in the middle, thickness is more uniform than in the control, despite the much greater thickness of the coating in Example 6. In Comparative Example 4, the coating was applied in the opposite of the desired direction, i. e., with the thicker end down. The thickness gradient in this coating is clearly greater than optimum because sagging occurred in this case.

Other variations and modifications of this invention will be obvious to those skilled in this art. This invention is not to be limited except as set forth in the following claims.