US2728686A - Method for coating objects - Google Patents

Description

Dec. 27, 1955 M. J. BORUSHKO METHOD FOR COATING OBJECTS Filed Dec. 25, 1952 United States Patent METHOD FOR COATING OBJECTS Michael J. Borushko, Detroit, Mich, assignor to Harding Manufacturing Company, Detroit, Mich., a corporation of Michigan This invention relates to a new and'useful' improvement in a coating process adapted for use in applying various types of coatings to various types of articles, particularly to metallic articles, and to an apparatus for practicing said process. 1

This invention is an improvement over that disclosed in my patent on a Coating Process, Patent No. 2,515,- 489, issued July 18, 1950, and is related in some respects to the invention disclosed in my copendingapplication Serial No. 327,639, filed of even date herewith, and by reference thereto the disclosure in said copending application is hereby made a part of the disclosure in this application. The process and apparatus described is adapted for applying coatings of paints, varnishes, lacquers, enamels, wax coatings, and so forth, which are all characterized as consisting of one or more suitable film-forming components dissolved or dispersed in one or more volatile solvents which may also contain pigments or dyes and other useful additional agents, such as driers or plasticizers.

The invention described in my Patent No. 2,515,489 related to an improvement in applying such organic coatings by the hot dip method. In practicing the invention of Patent No. 2,515,489 with a conveyorized line, such as is commonly used in large scale production, wherein articles to be painted are hung or held on a conveyor and carried thereby through the painting or other processes, it is sometimes necessary to have a very long coating tank. For example, when the conveyor is moving at relatively high speeds, and where the article being processed is of rather heavy cross section or great mass, or where the coefiicient of heat absorption of the article is relatively low, or wherever heat transfer of the liquid to the article is reduced, it is necessary to have a corresponding relatively long coating tank so that the temperature of the article immersed in the coating liquid of highly chlorinated hydrocarbons.

can be raised to the desired point before removing it from the liquid within the tank.

On the other hand, it is most desirable to keep the paint tank as short as possible to keep to a minimum the amount of coating liquid or paint in process. Furthermore, since the coating material is dispersed in a volatile solvent, which in some instances constitutes 80% of the weight of the liquid in the tank, the solvent losses can be materially reduced by employing as short a tank as possible. Still further, costs and difliculties of construction encountered in tanks of increased length are also materially reduced by employing tanks of minimum length.

in my copending application, I have disclosed that by preheating an article prior to its immersion in the coating liquid some of the disadvantages referred to above may be obviated. Reference is made to my copending application Serial No. 327,639 filed of even date herewith and the disclosure therein is hereby made a part of the disclosure in this application. It is one of the objects of this invention to provide a novel method and means for preheating the article to be coated prior to its immersion in the coating liquid.

, 2,728,686 vPatented Dec. 27, 1955 'ice For many years the greatest hazard encountered in coating operations has been the danger of fire or explosion. Such fire or explosion may occur where flammable solvents are used even when the best of safety practices are followed, for unforseen conditions or accidents may create an explosive concentration of vapor near a suitable source of ignition.

As is now well known, this fire hazard can be entirely eliminated by using as a solvent component of the coating solution a non-flammable coating solvent of the class For example: carbon tetrachloride, trichlorethylene, perchlorethylene. However, these solvents are very expensive in comparison with other flammable solvents. And because of the fact that there is always a certain amount of losses of solvents to the atmosphere, there has been a reluctance to use such solvents primarily because of economic reasons. Furthermore, these highly chlorinated hydrocarbon solvents are much more toxic than the flammable solvents and are more volatile at room temperatures. Thus, any use of these highly chlorinated hydrocarbons must be accompanied by careful ventilation to protect the people working in the areas where the coating solutions are being applied, and, also, careful temperature control must be provided so as to reduce losses to the atmosphere which may occur due to the rapid volatilization of the solvents. These problems of ventilation and temperature control are also important in the areas where the solvents are being evaporated from the wet film that has been deposited on the articles.

The highly chlorinated hydrocarbons above referred to have also found wide use in the cleaning of metal articles, principally for the removal of solvent-soluble surface contaminants such as oils, greases, or waxes, and also for the removal of insoluble particles which may be adhering to the metal surfaces because of the presence of oil, grease, or .wax film which serves as an adhesive.

These highly chlorinated hydrocarbons, when used as cleaning solvents, are used in what has come to be known in the trade as vapor degreasers. In a vapor degreaser a zone of hot vapor of the solvent is created, and this vapor is kept from escaping into the work room by suitably disposed condensing surfaces, generally above the vapor zone. As the relatively cold metal article is low ered into the hot solvent vapor, the vapor condenses upon the cold metal article and the condensate thus formed runs down the article, dissolves the soluble surface contaminants, and carries them away. After a sufficient time of contact with the hot solvent vapor the article itself becomes heated to the temperature of the hot solvent vapor, through heat transfer from the vapor, and condensation ceases. At such time, the article is withdrawn from the vapor, and as it rises above the heated solvent vapor the heat held by the article serves to drive 01f any remaining solvent on it; thislatter volatilized solvent is then condensed upon the previously mentioned condenser surfaces and is returned to the degreaser. When these chlorinated solvents are used in such a manner, the loss of solvent per unit area cleaned is generally considered negligible, and because the solvent loss is relatively low, the problem of providing satisfactory ventilation is not great. Various modifications of the above basic procedure have been developed for increased efiiciency of cleaning or for special purposes,

such as solvent slushing in cold condensed solvent after preliminary exposure to the hot vapor, followed by a second exposure to the hot vapor. Another common variation is to spray the hot vapor or liquid upon the metal article.

Now, the use of highly chlorinated hydrocarbon solvents in the coating liquid was retarded for the economic reasons above noted; namely, the expensiveness of the solvent, the necessity of additional equipment to eliminate or reduce the toxic efiects which the solvents might have upon people in the immediate vicinity, and the necessity of careful temperature control in the immediate vicinity of their use. In an effort to devise an economically feasible method of using these solvents in coating liquids, I devised the process disclosed in my Patent No. 2,515,489. In the practice of the process disclosed in said patent, there is practically no loss of solvent from the coating solution being applied for the reasons explained therein.

Thus, by substantially reducing the loss of solvent in the application of organic coatings, l have made it economically feasible to use expensive solvents where it may be desirable for some particular property it may possess. Thus, the cost need no longer be a major determinant in the selection of a solvent for a coating application. At the same time, it is simpler to use with safety those solvents whose vapors are highly toxic, because very, very little vapor escapes from the dip coating tanks described in my above-mentioned patent.

Articles which are to be coated with organic coatings must have their surfaces thoroughly cleansed of all oils, greases, waxes or like substances, if the best possible service performance of the applied organic coating is to be obtained. The teachings of my above-mentioned Patent No. 2,515,489 are also applicable with respect to the use of cleaning solvents and, therefore, the use of highly chlorinated hydrocarbons for cleaning has also been made economically feasible.

Thus, it is another object of this invention to provide a machine for both cleaning and painting an article, which machine is so constructed as to reduce as much as possible the losses of expensive solvents used therein.

A further object of this invention is to provide a machine in which both cleaning and painting are accornplished therein and wherein highly chlorinated hydrocarbon solvents may be used for both the cleaning of the work prior to coating and as the solvent or suspending medium for the film-forming materials, and wherein the same solvent may be used for both purposes.

Still another object of this invention is to provide a novel method of cleaning and painting articles whereby the article is preheated prior to its immersion in the coating liquid in a manner such as to effect substantial savings in the amount of solvents used in the cleaning and painting operations.

Further objects and advantages of this invention will become apparent as the following description proceeds and the features of novelty which characterize this invention will be pointed out with particularity in the claims annexed to and forming part of this specification.

A preferred embodiment of the invention is shown in the accompanying drawings, in which:

Figure l is a diagrammatic elevation view of the new apparatus by means of which the novel method is practiced;

Figure 2 is a diagrammatic crosssection view taken on line 22 of Figure l; and

Figure 3 is a diagrammatic cross-section view taken substantially on line 3-3 of Figure l.

The essence of my present invention is the use of heated cleaning solvent vapor as the preheater' of the articles prior to their immersion into the coating fluid. Furthermore, by using a single vessel for housing the tanks of the cleaning solvent and the coating liquid, the number of openings through which solvent losses and heat losses may be suffered are materially reduced from the number of solvent-loss openings which would be present if separate tanks were provided.

Thus, in my new machine, the cleaning tank and the coating tank are placed end to end and suitably enclosed, so that as the article being processed passes from the cleaning unit to the painting unit it is not exposed tothe room atmosphere. I thus achieve two major gains. The number of openings by which solvent loss can he suffered is cut by half as compared with performing the two operations in two separate units and thus the total solvent loss is reduced. Second, the heat taken up by the article in the vapor degreasing stage is mostly retained, as the work passes immediately from the dcgreaser into the coating solution, and, therefore, less immersion time in the hot paint is required to bring the article to the temperature that will accomplish satisfactory evaporation of the solvent from the applied coating film, as compared with coating a cold article. Therefore, hird advantage is obtained, the coating tank can be comparatively short.

Referring now to the drawings, Figures 1, 2 and 3 will show the general mode of construction and operation of my painting and coating machine.

Referring now to Figure 1, it will be seen that the machine consists of an elongated substantially vapontight vessel It having only two openings, the entrance 12 through which the Work being processed enters the vessel 1i and the exit 14 through which the finished work leaves the vessel 10.

Shown also is a conveyor 3.6 from which the articles 18 are suspended to be carried through the vessel it The conveyor 16 is an endless conveyor and may carry the parts on to other stations in the plant, if desired.

In Figures 2 and 3, this conveyor is shown as a roller and link-chain conveyor, but cable-type or other types of conveyors can be used. The articles 1% may be suspended from the conveyor 16 by a hook 29 or any other suitable means.

The vessel 10 has formed in the lower portion thereof a tank generally indicated at 22 for cleaning solvent and a tank generally indicated at 24 for coating liquid. The vessel' 10 is also provided with a cover 23 which extends above tanks 22 and 24 and has formed therein the entrance 12 and the exit 14 through which pass the articles to be coated. The tanks 22 and 24 are spaced from each other longitudinally along the vessel 10, with the clean ing tank 22 adjacent the entrance 12 and the coating tank between tank 22 and the exit 14. The vessel 19 is so shaped'between tanks 22 and 24, as indicated generally at 25, that the bodies of liquid and vapor zones therein are maintained separate from each other.

As the conveyor. 16 enters the vessel in it begins to descend so that the articles 13 suspended therefrom enter into the fog of solvent vapor 26 in the cleaning tank 22. This solvent vapor fog 7.6 is created by boiling the liquid solvent layer 28 in the bottom of the tank 22, and as shown in. Figure. 2, steam coils 3% may be used to attain the boiling. Other means of heating can be used, for example, electric immersion or plate heaters. The hot vapor zone 26 exists between the top of the boiling liquid 28 and the bottom of condenser 32 adjacent the upper portion of the tank 22. The condenser 32 bounds a substantially vapor-free zone 36 wherein solvent-vapor is condensed. This vapor-free zone also serves to prevent the hot vapors from rising to the level of entrance 12 from whence they could escape into the room. For the condenser 32 there is shown a doubled wall construction adjacent the upper portions of the lateral walls of tank 22 with cold water circulating between the double walls. Other well known means of condensation may be used. For example, pipe coils may be suspended around the top of the tank through which cold water or other cooling media may be circulated. There is also a gutter 34 to catch the condensed solvent and return it back to the body of liquid solvent. If desired, the condensed solvent may be passed through a water separator (not shown). This is desirable to remove atmospheric moisture that will at times, condense on the condensing surfaces.

As the articles 18 descend into the hot solvent 'vapor 26, the vapor condenses into liquid solvent upon the cold surfaces of the articles, and the liquid runs off, falling back into the hot liquid layer 28 carrying the soluble surface contaminants with it. Thus, in effect, the clean solvent 26 is always condensing upon the surface, for none of the contaminants with which we are concerned vaporize with the solvent. Many, and possibly all of the devices and techniques used in vapor degreasing equipment are adaptable for use in this sectionof the apparatus.

The time of exposure of the articles 18 to the hot solvent vapor 26 should be great enough to allow the articles 18 to reach the temperature of the vapor 26 or come close to it. Then as the articles 18 proceed forward it is raised through the vapor zone 26 and carried into a substantially vapor-free zone 36 above the vapor zone 26, wherein any liquid film remaining on the articles surface are evaporated. The solvent volatilized in the vapor-free zone 36 is condensed on the condensing surfaces and returned to the liquid layer 28.

Thus, when the articles 18 come' out of this section, they are hot, clean and dry, and they immediately begin to descend into the hot coating solution 38 in the coating tank 24, the principles of whose operation are thoroughly described in my aforementioned Patent No. 2,515,489. In this invention, there is an additional advantage over the patent, as the articles 18 are already hot, they need not be held inthe hot coating solution 38 for any length of time, but need merely be thoroughly immersed so that the surfaces to be coated are wetted with the coating liquid. In this way a very substantial shortening of the coating tank 24 is effected.

For an example: with certain cold articles being coated,

it was found necessary to allow one minute of complete I immersion in the hot coating solution to bring the cold articles up to the temperature of the solution, and it was desired to have a conveyor speed of 10 feet per minutethe coating tank would then have to be long enough to allow 10 feet of horizontal travel of the work in the hot coating solution. In the present machine, the horizontal travel of the work can be eliminated in most cases, and very substantially reduced in others.

The general features of construction and operation of the coating tank 24 are as shown in Figures 1 and 3, but are further described as follows:

The condenser 33 around the substantially vapor-free zone 37 adjacent the top of the tank 24 condenses the solvent vapor which rises above the solvent vapor zone 42 which extends between the surface of the hot coating solution 38 and the bottom of condenser 33. The condenser 33 condenses the solvent vapor which is volatilized from the film of coating solution deposited on the article as it emerges from the hot coating solution 38 and is carried into vapor-free zone 37. The condensed solvent may be allowed to run off the condenser directly back to the lower portion of the tank; or may be directed by means of a gutter 35, such as that shown, and piping, and through a water trap (not shown), back into the body of liquid The condenser shown is of doubled walled construc- 1 tion with cold water or other cooling liquid circulation between the walls but many other constructions are suitable to obtain the required condensation. In either the coating tank 24 or in the cleaning tank 22, it is not necessary that the condensers be integral parts of the tanks as shown for convenience. They may be separately constructed and suspended at the correct levels to obtain the eifects described.

Since the articles dipped into the coating liquid composition 38 carry the film-forming material deposited thereon out of the vessel 10, while most of the solvent is recovered within the vessel and returned to the body of coating liquid 38, the concentration of film-forming material in the coating liquid is continually being depleted. Accordingly, means are provided (not shown) for replenishing the film-forming material in said liquid composition.

The coating solution is shown being heated by steam coils 44. Here again other means of applying heat are satisfactory, such as steam jacketing, electric immersion heaters, and so forth. Many methods of applying heat in such situations are very well known. The insulation 46 shown adjacent the hot portions of each tank are very desirable for reducing heat losses, but not necessary to the operation of the machine.

As I have related in my Patent No. 2,515,489, as the coated article emerges from the hot coating solution the heat it has absorbed from the hot liquid serves to drive the solvent out of the wet film upon the work; within a matter of seconds, the film is totally free of solvent and the work comes out of the tankwith a solvent-free film of paint, varnish, lacquer, resin, enamel, oil, wax, grease or other coating. In contrast, other coating application methods require supplementary oven facilities for the removal of solvent, and the removed solvents are lost or collected at great expense.

I have, therefore, described a new and useful coating machine and method of operation whereby the heat imparted to the work article in the cleaning stage (which heat must generally be dissipated when coating by other methods) is taken advantage of to efiect economies in the coating stage. Furthermore, to retain as much as possible of this heat during the transfer from one tank to another, I have brought the two together, end to end, and housed them, and by reducing the openings available for escape of solvent, I have appreciably reduced the solvent loss which accompanies all uses of organic solvents.

While there has been shown and described a particular embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and, therefore, it is intended in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new, and desire to secure by Letters Patent of the United States, is:

1. A method of applying a filmlike coating to the surface of an article, which film-like coating is derived from a liquid body of coating composition including a volatile liquid solvent and a film-forming material dispersed therein; said method comprising the steps of maintaining said body of coating liquid at an elevated temperature approximating the boiling point of the solvent therein; providing a liquid body of cleaning solvent with a fog of heated cleaning solvent vapor thereabove and maintaining said cleaning liquid and vapor at an elevated temperature approximating the boiling point of the cleaning solvent, the cleaning solvent being the same solvent as the solvent in said coating composition; preheating said article by passing it through a fog of heated cleaning solvent vapor until the temperature of the article is substantially the same as the boiling point of said solvent; thereafter removing the article from the cleaning solvent vapor and immediately immersing said preheated article into said liquid coating composition for a time sufficient to wet the surfaces to be coated; removing said article from said body of coating liquid; and drying said article by rapidly volatilizing with the heat content of the article the solvent content of the layer of liquid Wetting the article.

2. A method of applying a film-like coating to the surface of an article, which film-like coating is derived from a liquid body of coating composition including a volatile liquid solvent and a film-forming material dispersed therein; said method comprising the steps of maintaining said body of coating liquid at an elevated temperature approximating the boiling point of the solvent therein; providing a liquid body of cleaning solvent with a fog of heated cleaning solvent vapor thereabove and maintaining said cleaning liquid and vapor at an elevated temperature approximating the boiling point of the cleaning solvent, the cleaning solvent being the same solvent as the solvent in said coating composition; preheating said article by passing it through a fog of heated cleaning solvent vapor until the temperature of the article is substantially the same as the boiling point of said solvent; thereafter removing the article from the cleaning solvent vapor and immersing said article into said liquid coating composition for a time sufficient to wet the surfaces to be coated; removing said article from said body of coating liquid; and disposing the article in an enclosed substantiall'y vapor-free chamber, said enclosed chamber surface of an article, which film-like coating is derived from a liquid body of coating composition including a volatile liquid solvent and a film-forming material dispersed therein; said method comprising the steps of maintaining said body of coating liquid at an elevated temperature approximating the boiling point of the solvent therein; providing a liquid body of cleaning solvent with a fog of heated cleaning solvent vapor thereabove and maintaining said cleaning liquid and vapor at an elevated temperature approximating the boiling point of the cleaning solvent, the cleaning solvent being the same solvent as the solvent in said coating composition; preheating said article by'passingit through a fogof heated cleaning solvent vapor until the temperature of the article is substantially the same as the boiling point of said solvent; thereafter removing the article from the cleaning solvent vapor and disposingsaid' article in an enclosed substantially vapor-free chamber, wherein the heat of the article volatilizes the liquid solvent thereon; then immediately immersing said preheatedarticle into said liquid coating composition for a time suffi'cient to wet the surfaces to be coated; removing said article from said body of coating liquid; drying said article in an enclosed substantially vapor-free chamber, wherein the heat of the article volatilizes the solvent content of the layer of liquid Wetting the article; condensing the solvent volatilized in said vapor-free chambers; and returning the solvent condensate to the bodies of liquid".

References Cited in the file of this patent UNITED STATES PATENTS 1,780,566 Pedersen Nov. 4, 1930 2,298,513 Rogers Oct. 13, 1942 2,380,968 Kimmig et al. Aug. 7, 1945 2,411,042 King et al. Nov. 12, 1946 2,413,144 King Dec. 24, 1946 2,515,489 Borushko July 18, 1950 2,547,047 Saums. et al. Apr. 3, 1951 2,552,612 Adams et al. May 15, 1951

Claims (1)

1. A METHOD OF APPLYING A FILM-LIKE COATING TO THE SURFACE OF AN ARTICLE, WHICH FILM-LIKE COATING IS DERIVED FROM A LIQUID BODY OF COATING COMPOSITION INCLUDING A VOLATILE LIQUID SOLVENT AND A FILM-FORMING MATERIAL DISPERSED THEREIN; SAID METHOD COMPRISING THE STEPS OF MAINTAINING SAID BODY OF COATING LIQUID AT AN ELEVATED TEMPERATURE APPROXIMATING THE BOILING POINT OF THE SOLVENT THEREIN; PROVIDING A LIQUID BODY OF CLEANING SOLVENT WITH A FOG OF HEATED CLEANING SOLVENT VAPOR THEREABOVE AND MAINTAINING SAID CLEANING LIQUID AND VAPOR AT AN ELEVATED TEMPERATURE APPROXIMATING THE BOILING POINT OF THE CLEANING SOLVENT, THE CLEANING SOLVENT BEING THE SAME SOLVENT AS THE SOLVENT IN SAID COATING COMPOSITION; PREHEATING SAID

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