US3713873A

US3713873A - Electrostatic spray coating methods

Info

Publication number: US3713873A
Application number: US00090746A
Authority: US
Inventors: F Fish
Original assignee: Gillette Co LLC
Current assignee: Gillette Co LLC
Priority date: 1970-11-18
Filing date: 1970-11-18
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30

Abstract

A method of electrostatic spray coating an article includes the steps of establishing an electrostatic field having an average gradient less than 10 kilovolts per centimeter between a mist generator and the article to be coated; and maintaining a quiescent zone in the atmosphere adjacent the article to be coated, the distance of the article from the output of the mist generator being sufficient to permit substantial dispersion of the spray particles. Particles of less than 10 microns in size are continuously generated and they are accelerated under the influence of the electrostatic field for deposit on the surface to be coated. The electrostatic field carries the particles through the quiescent atmosphere so that a substantial portion of the mist particles which would not otherwise have been deposited on the article and liquid are deposited on the article. Relative movement between the mist generator and the article surface transverse to the general direction of mist particle movement during particle deposition is continuous.

Description

United States Patent n 1 Fish [ Jan. 30, 1973 [541 ELECTROSTATIC SPRAY COATING 3,169,883 2/1965 .luvinall ..i 17 9342 METHODS Primary ExaminerAlfred L. Leavitt [75] Inventor. FMrzisnsk Hamilton Fish, Westwood, Assistant Examiner john H. Newsome Attorney-Willis M. Ertman [73] Assignee: The Gillette Company, Boston,

Mass. [57] ABSTRACT [22] Filed: Nov. 18, 1970 A method of electrostatic spray coating an article ineludes the steps of establishing an electrostatic field [2]] Appl' 90746 having an average gradient less than 10 kilovolts per Related Appncation Data centimeter between a mist generator and the article to I i be coated; and maintaining a quiescent zone in the at- [62] Division of Ser. No. 786,318, Dec. 23, I968, Pat. No. mosphere adjacent the article to be coated, the distance of the article from the output of the mist generator being sufficient to'permit substantial disper- [52] "117/914 117/132 g $373 sion of the spray particles. Particles of less than 10 microns in size are continuously generated and they hilt. Cl. are accelerated under the influence of the electro [58] held of "117/934, z. static field for deposit on the surface to be coated. 17/9443 132 118/621 627; The electrostatic field carries the particles through the 239/3 15 quiescent atmosphere so that a substantial portion of the mist particles which would not otherwise have [56] References C'ted been deposited on the article and liquid are deposited UNITED STATES PATENTS on the article. Relative movement between the mist generator and the article surface transverse to the 2,796,832 6/1957 Pritchard ..ll8/62l X general direction of mist particle movement during 3,5l8,l Fischbein R particle deposition is continuous 3,083,l2l 3/1963 Gauthier ..l l7/93 4 R 12 Claims, 4 Drawing Figures K 0 24 g 22 i I 52 1 I 58 l J 2O 3 IO :l i i,

l VOLTAGE H l l SOURCE i L & 9 I 60 I8 PATENIEDJAN 30 m3 3.713. 73

HJJIIlI/ll/l/j////////////Ill/l1 8O E I 1 I I I III/[llI/l/IJIIIIII/Ill/Ill ELECTROSTATIC SPRAY COATING METHODS This application is a division of my co-pending patent application Ser. No. 786,318, filed Dec. 23, 1968, now US. Pat. No. 3,5 89,606.

SUMMARY OF INVENTION This invention relates to electrostatic spray coating methods for the application of coating materials to provide a coating on a surface and more particularly to methods particularly useful for placing a shaving facilitating coating on the cutting edges of razor blades during the manufacturing operation.

ln present razor blade technology, it has been found that fluorocarbon materials, when properly applied to the shaving edge of the razor blade, significantly improved the shavability characteristics of the razor blade. A thin uniform coating over the entire surface of the final facet is desired, which coating can be obtained by spraying the coating material ina uniform distribution of extremely fine particles that are deposited with liquid. A particularly advantageous coating material for such an application is an aqueous dispersion of a fluorocarbon telomer and it is a particular object of this invention to provide novel and improved methods for applying a thin uniform coating of such material to the sharpened edges of razor blades in a production process.

Another object of the invention is to provide novel and improved methods for applying coating material to the sharpened edges of razor blade elements economically and ata rate consistent with present-day mass production requirements, which methods are capable. of operating automatically with minimum amount of supervision and maintenance while assuring a product of high quality and uniformity.

A further object of the invention is to provide novel and improved methods for applying coating materials through the use of electrostatic techniques.

Another object of the invention is to provide novel and improved methods for applying a telomer material in thin coatings of uniform coverage efficiently and economically.

In the practice of the method of the invention with this system, after establishment of an electrostatic field having an average' gradient in the range of 4-9 kilovolts per centimeter between a workpiece support and a mist generator, and a quiescent atmosphere adjacent the workpiece support, particles of the coating material less than microns in size are continuously generated for acceleration under the influence of the electrostatic field for impact on the surfaceto be coated. Relative movement between the surface to be coated and the mist generator is produced at a continuous and uniform rate and a coating material is deposited and produces a coating which covers the entire surface to be coated. The surface to be coated in particular embodiments is preheated as a function of the coating material. Such preheat is desirable for slow drying dispersants to speed, the drying of drops on the edge of a blade so that the solids are deposited uniformly and dry sufficiently rapidly to avoid coalescing of the drops and formation of a bead of solid material along the facet of the blade away from the ultimate edge and yet not dry so fast that spots of solids are scattered with bare spaces between them. Freon 113 dispersions have been found to require no preheat, satisfactory results have been obtained with 4 percent aqueous dispersions with a l50F preheat, and 1% percent aqueous dispersions with drops of less than 10 micron size preferably are used with a 205F preheat at a blade movement rate of 50 cm/min. and dispersion consumption rate of 1 l0 cc/hr.

The outlet port of the mist generator is preferably so spaced from the blade edges (the surface to be coated) that the angular difference between the length of the outlet port and the length of the blade edge is less aretan 0.1 so that the particles of coating material are accelerated by the electrostatic field along generally parallel paths from the outlet port to the, blade edges; In such a system the particles are introduced into the electrostatic field at low velocity, preferably less than 2 meters per second. This geometry of material movement provides a uniform and efficient deposition of material on the blade edges.

A razor blade coating system constructed in accordance with the invention includes a workpiece transport for a stack of blades, the blades in the stack being disposed in faceto face relation with their sharpened edges aligned and exposed to the coating source. The system also includes an aspirating system for generating a mist of coating material in which the particles have a size of less than ten microns, this mist generator producing an output of low velocity particles and including an outlet passageway of relatively large cross-sectional area. The mist generator is electrically insulated from ground and connected in electric circuit with the workpiece support to establish anelectrostatic field of magnitude in the range of 4-9 kilovolts per centimeter and preferably about -5 kilovolts'per centimeter. The mist generator and the workpiece support structure are movable relative to one another so that the workpiece may be moved past the outlet passage of the mist generator while the output mist is accelerated under the influence of the electrostatic field for impingement on the edge surface to be coated.

In a particular embodiment for the coating of razor blades, the system further includes apparatus for heating the blade prior to the application of the coating material. The mist generator in that embodiment includes an outlet passageway having an elongated inclined surface on which larger particles impinge and are removed from the output particle stream and an outlet port structure of electrically insulating material, the port being of horse-collar configuration and having a plane surface disposed parallel to the blade edges t be coated.

While this method provides a particularly convenient and efficient'method of applying a coating material to the sharpened razor blade edges as a shave-facilitating coating it may be practiced with a variety of workpiece surfaces and with a variety of coating materials including electrolytes. It is particularly useful in applying an aqueous dispersion of a fluorocarbon telomer to provide a shave-facilitating film of thickness in the order of 2,000A or less on the sharpened edges of razor blades, which blades exhibit excellent shaving characteristics.

Other objects, features and advantages of the invention will be seen as the following description of particular embodiments of the invention progresses, in conjunction with the drawings, in which:

FIG. 1 is a sectional view of apparatus employed in the practice of the invention;

FIG. 2 is an enlarged view of the aspirator structure employed in the mist generator shown in FIG. 1;

FIG. 3 is an end view taken along the line 3-3 of FIG. 1 of the configuration of the output channel of the mist generator; and 7 FIG. 4 is a diagrammatic top view of an arrangement of a furnace and mist generators in a system constructed in accordance with the invention.

DESCRIPTION OF PARTICULAR EMBODIMENT centimeters cross-sectional area, the base of which is disposed twelve centimeters above the base of chamber 16. An outlet tubular passage 22 connected to port 20 has an extension 24 of polypropylene (of oval or horse collar cross-sectional configuration as indicated in FIG. 3) mounted thereon. The total length of passage 22 and extension 24 is 6 centimeters, its heighth is 2.7 centimeters and its width (at its widest point) is 1.7 centimeters. The workpiece support 12 is disposed so that the center of the blades 14 is in alignment with the axis of passage 22.

The aspirator structure 30 includes a tubular body section 32, at the top of which is supported a vertically adjustable output regulator plate 34. The bottom of tube 32 is open to the supply chamber 16. Disposed in tube 32 is an aspirator assembly 36, details of which may be seen with reference to FIG. 2. That assembly includes an outlet port 40, an air supply passage 42, and a fluid supply passage 44. Jet passage 46 (0.5 mm. in diameter) connects the air supply passage 42 to the outlet port 40 and jet passage 48 0.5 mm. in diameter) connects the fluid supply passage 44 to outlet port 40. Passage 44 is connected to fluid tube 50 which extends through the upper wall of chamber 16 to the'bottom thereof, Air inlet passage 42 is connected to tube 52 which is connected through a filter 54 and a pressure regulator 56 to a source of compressed air 58. Airunder pressure (suitable pressure being in the range of 18-30 psi) is applied to passage 42 and aspirates the coating material from container 16 for dispersion in a mist downwardly through tube 32, together with air drawn in through a port below regulator plate 34. The

heavier aspirated particles are all returned to the supply of the coating material 18 while the smaller particles flow upwardly and through outlet port 20 and the inclined passage 22 for discharge at low velocity (in the order of 1 meter per second) into the atmosphere. The inclined surface of passage 22 collects particles of intermediate size and returns those particles to chamber 16 so that only particles of less than ten microns in size are dispersed into the air.

Mists of a variety of coating materials may be generated with this apparatus, including solids such as telomers as dispersions in water, alcohols, freons, or various fluorocarbon liquids. A preferred coating material is an aqueous dispersion of tetrofluoroethylene telomer of the type disclosed in the US. Pat. No. 3,658,742. Operation of this mist generator consumes that coating material at the rate of approximately 135 cubic centimeters per hour, of which about 25 cubic centimeters is due to loss of water by evaporation.

A high voltage power supply 60 (in a particular embodiment a Spellman Model 2,040 power supply which has a variable output with a range of 0-40 kv. at 1 milliampere current) is connected with its positive terminal grounded and connected to the blade support structure 12 and its negative terminal connected to the nickel plated brass aspirator structure 30. The container 16 is supported on an electrical insulator. The blade support ispositioned relative to container 16 so that the sharpened edges of the blades 14 are 7 /2 centimeters from the container body and a voltage gradient of 5.33 kv. per centimeter is established. The angular difference (FIG. 1) between the length of the outlet port 24 and the length of the razor blades 14 (3.7 cm.) with this spacing is are tan 0.067.

With reference to FIG. 4, an enclosure is provided in which six mist generators 10-1 10-6 are disposed. A blade transport structure, which includes a rail 72 and a chain 74 driven by a variable speed reversible motor mounted outside of enclosure 70, supports a series of blade holders 12 in the form of knives which run through the central slots in the blades. Typically, there are 600l,200 blades in each stack and a lava spacer 76 is used to separate the blades from the clamps 78 mounted on each knife 12. Adjacent enclosure 70 is an oven 80 through which the blade transport passes. In this embodiment, the blades are heated to a temperature of 205F prior to entry into enclosure 70.

In operation, the aqueous dispersion 18 is placed in chamber 16 preferably only half full to minimize the influence of foaming and liquid entrainment on the formation of mist. The electrostatic field is established and mist is generated while the blade transport is advanced through enclosure 70 at a rate of 50 centimeters per minute. past the six mist generators 10 so that a fluorocarbon coating in the thickness range of 2,000A is deposited on both edges of the blades. This dispersion isconsumed at a rate of about 660 cc/hour; The blades are then heated to cure the coating material. Razor blades with coatings applied by this method and apparatus compare favorably from the standpoint of product uniformity and economy, as well as shaving characteristics, with commercially available razor blades that have been coated with other spray techniques.

While a particular embodiment of the invention has been shown and described, various modifications thereof will be apparent to those skilled in the art and therefore it is not intended that the invention be limited to the disclosed embodimentorto details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

What is claimed is: A

1. A method of electrostatic spray coating an article comprising the steps of continuously generating a mist of particles of less than 10 microns in size by aspirating coating material from a supply with a downwardly directed gas stream so that the aspirated particles are directed in a downward direction, the heavier particles returning to the supply and the lighter particles flowing upwardly through an outlet port, establishing an electrostatic field having an average gradient less than kilovolts per centimeter between said mist generator outlet port as one electrode and the article to be coated as a second electrode; 5

maintaining a quiescent zone in the atmosphere adjacent the article to be coated, the distance of the article from the output of the mist generator being sufficient to permit substantial dispersion of the spray particles that pass through said outlet port;

accelerating said particles under influence of said electrostatic field for deposit on the surface to be coated, said electrostatic field carrying said particles through said quiescent atmosphere so that a substantial portion of the mist particles which would not otherwise have been deposited on said article are deposited on said article,

and effecting continuous relative movement between said mist generator and the article surface transverse to the general direction of mist particle movement during particle deposition.

2. The method as claimed in claim 1 wherein the coating material is in the form of a dispersion of solids in a liquid.

3. The method as claimed in claim 1 wherein said step of generating said particles imparts a velocity to said particles of less than two meters per second.

4. The method as claimed in claim 1 wherein said outlet port is so spaced from the blade edges that the angular difference between the length of the outlet port and the length of the article surface to be coated is less than are tan 0.1 so that the coating material moves along substantially parallel paths from the outlet port to said article surface.

5. The method of applying a shave facilitating coatin g to razor blades which comprises the steps of:

permit substantial dispersion of the spray particles;

accelerating said particles under the influence of said electrostatic field for deposit on the edges of said blades, said said electrostatic field carrying said particles through said quiescent atmosphere so that a substantial portion of the mist particles which would not otherwise have been deposited on said blade edges are deposited thereon; and effecting continuous relative movement between said mist generator and said blade stack transverse to the general direction of mist particle movement during particle deposition.

6. The method as claimed in claim 5 wherein said outlet port is so spaced from the blade edges that the angular difference between the length of the outlet port and the length of the blade edge is less than are tan 0.1 so that the coating material moves along substantially parallel paths from the outlet port to said blade edges.

7. The method as claimed in claim 5 wherein said step of generating said particles imparts a velocity to said particles of less than 2 meters per second.

8. The method as claimed in claim 5 wherein the coating material is in the form of a dispersion of solids in a liquid.

9. The method as claimed in claim 8 wherein said particles flow through said port at a velocity of less than two meters per second.

10. The method as claimed in claim 8 wherein said coating material is a fluorocarbon telomer.

11. The method as claimed in claim 10 including the step of preheating said stack of razor blades.

12. The method as claimed in claim 5 wherein the particle generating rate and said relative movement are coordinated so that said coating material is deposited on the edges of said blades to a thickness in the order of 2,000A or less.

Claims

1. A method of electrostatic spray coating an article comprising the steps of continuously generating a mist of particles of less than 10 microns in size by aspirating coating material from a supply with a downwardly directed gas stream so that the aspirated particles are directed in a downward direction, the heavier particles returning to the supply and the lighter particles flowing upwardly through an outlet port, establishing an electrostatic field having an average gradient less than 10 kilovolts per centimeter between said mist generator outlet port as one electrode and the article to be coated as a second electrode; maintaining a quiescent zone in the atmosphere adjacent the article to be coated, the distance of the article from the output of the mist generator being sufficient to permit substantial dispersion of the spray particles that pass through said outlet port; accelerating said particles under influence of said electrostatic field For deposit on the surface to be coated, said electrostatic field carrying said particles through said quiescent atmosphere so that a substantial portion of the mist particles which would not otherwise have been deposited on said article are deposited on said article, and effecting continuous relative movement between said mist generator and the article surface transverse to the general direction of mist particle movement during particle deposition.

4. The method as claimed in claim 1 wherein said outlet port is so spaced from the blade edges that the angular difference between the length of the outlet port and the length of the article surface to be coated is less than arc tan 0.1 so that the coating material moves along substantially parallel paths from the outlet port to said article surface.

5. The method of applying a shave facilitating coating to razor blades which comprises the steps of: disposing a multiplicity of razor blades in a stack with their sharpened edges aligned parallel with one another, said edges together defining a plane; providing a dispersion of coating material; continuously generating a mist of particles of less than 10 microns in size from said dispersion of coating material by aspirating material from said dispersion with a downwardly directed gas stream so that the aspirated particles are directed in a downward direction, the heavier particles returning to said dispersion and the lighter particles flowing upwardly through an outlet port, said outlet port facing said plane; establishing an electrostatic field having an average gradient in the range of 4-9 kilovolts per centimeter between said mist generator outlet port as one electrode and said stack of razor blades as a second electrode; maintaining a quiescent zone in the atmosphere adjacent said stack, the distance of said stack from the output of the mist generator being sufficient to permit substantial dispersion of the spray particles; accelerating said particles under the influence of said electrostatic field for deposit on the edges of said blades, said said electrostatic field carrying said particles through said quiescent atmosphere so that a substantial portion of the mist particles which would not otherwise have been deposited on said blade edges are deposited thereon; and effecting continuous relative movement between said mist generator and said blade stack transverse to the general direction of mist particle movement during particle deposition.

6. The method as claimed in claim 5 wherein said outlet port is so spaced from the blade edges that the angular difference between the length of the outlet port and the length of the blade edge is less than arc tan 0.1 so that the coating material moves along substantially parallel paths from the outlet port to said blade edges.