US3073721A - Method of hot coating - Google Patents

Description

Jan. 15, 1963 J. POKORNY METHOD OF HOT COATING Jan. 15, 1963 J. PoKoRNY METHOD OF' HOT COATING 3 Sheets-Sheet 3 Filed May 1B, 1959 INVENTOR.

Lgs??? 0760@ United States Patent 3,073,721 METHGD F HOT CGATRNG Joseph Pokorny, Lincolnwood, Ill., assigner to G. S. Biakeslee Co., Cicero, Ill., a corporation of lilinois Filed May 18, 1959, Ser. No. 313,867 1 Claim. (Ci. 117-1051) This invention relates to painting or coating articles by projecting onto them a coating composition or paint comprising a volatile solvent having a comparatively low boiling point and a film forming or coating substance, and has to do with means for and a method of performing such an operation.

t is known to paint articles by spraying them with paint containing chlorinated hydrocarbon solvents such as trichlorethylene, perchlorethylene and the like, but is dithcult to obtain satisfactory results. Such paints do not, under ordinary conditions, readily coalesce and level of to form a smooth and continuous coating. Spraying with resultant atomizing of the paint, produces multitudinous speaks upon the article, which become pimples or craters causing a rough and unsatisfactory coating surface. lt has been proposed to avoid that diculty by preheating the article to about the temperature of the vapor of the paint solvent, spray the preheated article in the solvent vapor with paint heated to the boiling point of the solvent at atmospheric pressure, and dry the applied coating by rapid evaporation of the solventcontent thereof by the heat of the article. In such a method, the preheated article is at a temperature but several degrees higher than that of the solvent vapor, at best, and the paint is cooled incident to spraying thereof to below its boiling point. That incurs risk of cooling of the article, while in the vapor zone, to a temperature lower than the solvent vapor with resultant condensation of solvent vapor upon the sprayed article. When that happens solvent wash-off, i.e., removal of the paint from the article by the solvent condensate thereon, occurs rendering it impossible to obtain a uniform and satisfactory coating. Accordingly, in that method precise control of temperatures, often diiiicult to attain, is essential and the time during which the article or work piece can remain in the vapor-spray zone without solvent wash-off occurring is limited. That limits the possible maximum thickness of the paint coating which can be applied to the article, and accurate timing also becomes a critical factor. In view of the necessity for precise control of temperature and of timing, the method referred to is not best suited to quantity production in industrial operations.

`It has also been proposed to spray articles, at room temperature of approximately 70 F., lwith full bodied or unthinned paint heated, while maintained under pressure, to a temperature of at least 135 F., but lower than the boiling point, at atmospheric pressure, of at least 50% of the solvent content of the paint. The paint is sprayed under pressures of at least 100 p.s.i. preferably 200 to 600 p.s.i., in a chamber at 20 F. above room temperature, i.e. approximately 90 F. In actual practice the paint is heated to a temperature of about 150 F. to 250 F. and is sprayed under a pressure of from 250 to 700 psi., so that it is tho-roughly atomized and mechanically subdivided into tine particles. The solvent is released from the paint and the temperature of the latter is substantially reduced, incident to spraying and atomizing thereof under high pressure. The small particles of paint deposited on the relatively cold article or work piece condense thereon and are, for a short time, at a slightly higher temperature so that they coalesce to form a smooth coating. The solvent released from the atomized paint mixes with the air present in the chamber and the spraying of the article occurs in a zone of solvent vapor laden air. The article "ice is at room temperature of 70 F. as it enters the spray chamber, and the latter is at a temperature of approximately F. Assuming the paint solvent to be trichlorethylene, the temperature of the solvent vapor, lowered somewhat by spraying of the paint possibly would be around F. and it would condense on the relatively cold article causing solvent wash-off. In this method, as in the rst method referred to above, precise control of temperatures and accu-rate timing are critical factors and the maximum possible thickness of the applied coating of paint is limited by the shortness of the time the article being spray painted can safely remain in the solvent vapor spray zone Without risk of solvent wash oif.

My invention is directed to an apparatus for and a method of painting articles whereby the diiiiculties of the known methods, referred to above, are eliminated. The apparatus comprises a paint chamber and associated means for producing therein a saturated vapor zone and for projecting onto the articles, while in that zone, unatomized substantially continuous and solid streams or jets of coating material or paint including a chlorinated hydrocarbon solvent such as perchlorethylene, trichlorethylene, or the like. Means is provided for heating the paint under pressure to a temperature substantially above its boiling point at atmospheric pressure. The pressure to which the paint is subjected and the temperature to which it is heated are so related that the temperature of the paint impinging upon the article is effective for maintaining the surface thereof at a temperature higher than that of the surrounding vapor of the paint solvent, so as to eliminate possibility of solvent wash-oit. The method practiced by such apparatus comprises projecting onto the articles unatomized substantially continuous and solid streams of paint, while the article is surrounded by a substantially saturated vapor of paint solvent, maintaining the surface of the article at a temperature above that of the surrounding solvent vapor by heat derived from the paint impinging upon the article, and rapidly drying the painted article by evaporation of the solvent of the paint by the heat thereof supplemented by the surface heat of the article. Further objects and advantages of my invention will appear from the detail description.

in the drawings: v

FIGURE l is a semidiagrammatic lengthwise sectional view of painting means embodying my invention;

FIGURE 2 is a transverse sectional view, on an enlarged scale and in part diagrammatic, of the paint charnber and associated means for projecting jets of hot paint onto the articles; and l FIGURE 3 is a lengthwise sectional view of a modified form of machine for practicing the method of my invention.

The apparatus and the method of my invention may be used for coating articles with paint, varnish, lacquer; enamel and other materials properly formulated with chlorinated hydrocarbon solvents such as trichlorethylene, perchorethylene and solvents of like character. It will be understood, therefore, that the term paint is used herein in its generic sense and includes coating corn-v positions of the general character referred to comprising a film or coating forming material and an appropriate solvent `which is evaporated from or given olf by the coating composition in the drying or setting thereof.

The apparatus shown in FIGURES 1 and 2 is generally similar to degreasing machines of conventional type, though differing therefrom in respects particularly adapting it for painting in accordance with the method of my invention, and may appropriately be referred to as a machine. It has an enclosing housing 10 providing an entrance passage or tunnel 11 leading at an inclination downward to a solvent tank 12 provided with a bottom sump 13 having therein heating means, conveniently steam coils 14. The inner end wall 15 of tank 12 is joined to the inner end wall 16 of a paint chamber 17, providing a bafe 18 therebetween. The housing 10 further includes an intermediate passage 19 leading from solvent tank 12 and opening downward into the paint chamber 17, which is preferably, though not necessarily, of substantially less height than the solvent tank portion of housing 10. Cooling coils 26 and a coolingT jacket 21, through which a suitable coolant is circulated, extend about the top of solvent tank 12. A shallow trough 23 underlies the cooling coils and is provided with suitable means (not shown) for draining,7 solvent condensate back into sump 13.

The top portion of paint chamber 17 is substantially reduced in width, between t'ne ends portions thereof, to provide an upwardly projecting extension 24 of inverted channel cross section, as is shown more clearly in FIG- URE 2, opening at its inner end into passage 19. A cooling jacket 25 extends about paint chamber 17, at the end portions and along the sides of extension 24 thereof, substantially in the plane of cooling jacket 21. The outer end of extension 24 opens into an exit passage 26 leading at an inclination upward from the top edge of the outer end wall 27 of paint chamber 17. The inclined bottom wall 23 of passage 26 is provided with a cooling jacket 29 through which a suitable coolant is circulated in a known manner. The exit passage 26 is also provided, at each side thereof, with a cold plate 30.

It may be assumed for purposes of description, that the paint solvent is trichlorethylene, which has a boiling point of 188 F. at atmospheric pressure, and that sump 13 contains a body 32 of the same solvent. Suitable means may be provided for withdrawing contaminated solvent from sump 13 and replacing it with pure solvent, and means may also be provided for spraying articles in tank 12 with pure solvent; as is frequently done in degreasing machines. The solvent in sump 13 is heated to its boiling point and the evolved solvent vapor provides a concentrated solvent vapor zone 33, indicated by stippling, overlying the liquid solvent 32 in sump 13. The top of the vapor zone 33, i.e., the vapor-air interface, indicated by the dash and dot line 34, is maintained substantially constant by the cooling coils 2t), at or adjacent the midheight thereof, the condensate owing back into sump 13.

.let means, comprising heads 36 and jet nozzles 37, of suitable known type, is mounted in paint chamber 17 at each side thereof. The bottom wall 38 of chamber 17 is inclined from each end downward at 39 to a fiat central area 49, which is inclined downward toward one side of chamber 17 to an outlet conduit provided with a hand valve 42. Valve 42 normally is open, and paint is discharged from conduit $1, as will be explained more fully later, into a paint mixing and storage or supply tank 43 adjacent one side of chamber 17. The paint enters tank 43, which normally is closed, adjacent the top thereof and flows downward therein elective for agtating and mixing the paint so as to mantain a uniform mix. An outlet conduit 44, provided with two hand valves t5 and 46, connects tank 43, adjacent the bottom thereof, to the intake of a suitable pump 47, driven by an electric motor 48. The discharge of pump 47 is connected by a conduit 49 to the inlet of a steam jacketed heat exchanger Si) of conventional type. Steam under appropriate pressure is delivered from a suitable source to heat exchanger Si) through a conduit 51, condensate and steam being discharged through a conduit 52 including a trap 53. The outlet of heat exchanger Si) is connected by a short circuit 54 to one arm of a cruciform fitting 5S, a second arm of which is connected by a short conduit 56 to a pressure responsive valve 57 of conventional type in the steam supply conduit S1. The valve 57 regulates the flow of steam to the heat exchanger 56 responsive to the pressure of the paint in a manner assuring that the paint will be maintained at the desired temperature, as will be understood. A pressure gauge 58, of suitable known type, is connected to a third arm of fitting the fourth arm of which is connected by a conduit 59 and branch conduits 66, each provided with a hand valve 61, to the jet heads 36.

The articles to be coated or painted are passed through the machine, from the entrance to the exit end thereof, by an endless conveyor 65 of suitable known type, the lower run of which underlies the roof of housing generally' parallel therewith, the portion of the lower run overlying solvent tank 12 being offset downward as shown. The articles 65 may be of any suitable size and shape and are shown, by way of example, as of flat rectangular shape. They are suspended from the lower run of conveyor 65 in any suitable manner, conveniently by hangers 67 attached thereto. The hangers 67 are of such length that the articles 65 are immersed in and pass through the concentrated solvent vapor of the vapor zone 33 in tank 12. They are at room temperature as they enter the machine and the solvent vapor quickly condenses upon the relatively cold articles. The solvent condensate ilows downward over the articles and drips therefrom into sump 13, effectively removing from the articles any grease and particles of foreign materials thereon. rIhe articles then pass upward out of the vapor zone 33, lbeing then in a thoroughly clean and degreased condition well suited for painting. They then pass over the baille 13 and downward into paint chamber 17 and through the latter between the jet means at opposite sides thereof by which they are painted. The painted articles then pass upward through the exit passage or tunnel 26 and leave the machine in coated and dried condition, as will appear more fully presently.

The paint, containing trichlorethylene as the solvent thereof, is heated to approximately 224 F. in the heat exchanger 50 and is subjected to comparatively low pressure, on the order of from 9 to l2 p.s.i. lts temperature as it is discharged from the nozzles 37 is approximately 210 F. Due to the low pressure employed, the paint solvent is not atomized in its entirety as the paint leaves the nozzles 37 and the paint is projected therefrom in continuous solid streams or jets, which impinge upon the article and rapidly spread over the surfaces thereof to provide a continuous coating enveloping the article. The relatively hot paint impinging upon the article rapidly raises the surface or skin thereof to a temperature sub- Stantially higher than the boiling point of the solvent at atmospheric pressure and maintains it at that temperature, or approximately so, during the painting operation, particularly if the article is not of substantial mass. In painting metal articles of substantial mass there may be a tendency for ow of heat from the surface of the article to the interior thereof such as would tend to lower the surface temperature of the article to appreciable extent. In such cases, the paint may be heated to a temperature somewhat higher than it is heated for articles of less mass, effective for compensating for the tendency toward lowering of the surface temperature of the article produced by the hot paint. Briey, the hot paint projected onto the article in solid streams quickly heats the surface thereof to a temperature above the boiling point of the solvent at atmospheric pressure and maintains it at such temperature during the painting operation.

During the painting operation the solvent is released from the paint, some of the solvent being released as the paint is discharged from the nozzles 37 but the major portion of the solvent being released from the paint as it impinges upon and spreads over the article being painted. The released solvent rapidly expands, with a resultant drop in temperature, producing solvent vapor which provides a first or main paint solvent vapor zone 70 filling the paint chamber 17, such vapor being at a temperature about the same as the boiling point of the solvent at atmospheric pressure and substantially lower than the surface or skin temperature of the article being painted. Thereafter, the solvent released from the paint creates a second and localized paint solvent vapor zone 71, indicated by the area of denser stippling, surrounding the article being painted and at a somewhat higher temperature than the main body of paint solvent vapor in the paint chamber 17. The cooling jacket 25 maintains the solvent vapor in chamber 17 at a substantially constant level with the top thereof at about the midheight of the jacket 25, as indicated by the dash-dot line 72. The article is thus painted in the concentrated vapor of the solvent, which is conducive to levelling oi of the paint and the production of a smooth and uniform coating of paint on the article. Due to the fact that the hot paint maintains the surface of the article at a ternperature higher than that of the solvent vapor, there is no risk of solvent vapor condensing upon the article while it is being painted. The localized zone 71 of superheated paint solvent vapor extends to the exit passage or tunnel 26 and to the top surface of the main body 70 of vapor in chamber 17. The painted articles are thus surrounded by the localized zone 71 of superheated vapor, in their passage from the painting zone out of the chamber 17 and into exit tunnel 26. The vapor of zone 71 is derived from solvent released from the paint and is at a lower temperature than the surface temperature of the article, as above noted and as will be understood. That effectively guards against possibility of the painted surface of the article dropping to a temperature such that condensation of solvent vapor thereon can occur in chamber 17. Solvent vapor is never present in condensible amount in exit tunnel 26 and there is no risk there or" condensation of solvent vapor upon the painted article. The article may safely remain in the paint chamber 17 until the applied coating of paint attains any desired thickness, within practicable limits. It is thus possible to apply to different articles paint coatings of a wide range of thickness, by retaining them in the paint chamber for different periods of time, without necessity for precise temperature and time control. That renders the method of my invention well suited to large volume production in industrial operations.

rl`he solvent is rapidly evaporated from the paint projected onto the article by the surface heat of the latter and by the contained heat of the paint, at a temperature substantially higher than the boiling point of the solvent at atmospheric pressure. That assures rapid drying of the paint coating on the article, which coating may be completely dry, or substantially so, when the painted article enters the exit tunnel or passage 26. If an exceptionally thick coat of paint is applied to the article, it may be substantially dry but somewhat tacky as it enters tunnel 26, but will be completely dried when the article passes out of the exit end of the machine. The excess paint with the included solvent projected onto the article drips therefrom to the bottom wall of the paint chamber 17, and paint projected from the jet nozzles 37 which does not strike the article also falls to the bottom wall of the paint chamber. Such paint is returned to the tank 43, in the manner above described.

Preferably, though not necessarily, the pump supply conduit 44 is connected inwardly of valve 45 thereof, by a cross conduit 75, provided with a hand valve 76, to conduit 41 inwardly beyond valve 42 thereof. In the normal painting operation above described, valve 76 is closed and valves 42, 45 and 46 are open, as will be understood. By closing valves 42 and 46 and opening valve 76, paint or solvent from chamber 17 may be delivered directly to the intake of pump 47, for a purpose to be explained presently. The conduit 41 is provided with a union coupling 77 disposed between tank 43 and valve 42, and conduit 44 is provided with a similar coupling 7S disposed between valves 45 and 46. The couplings 77 and 7S provide means whereby the tank 43 may quickly and easily be disconnected from chamber 17 and pump 47 and replaced by a similar tank. That renders it possible to change from one color of paint to another color, or from one coating composition to another, as frequently is desirable, with a minimum of delay and work. In making a change of tanks, the valves 42, and 46 are closed preliminary to uncoupling of the tank, to prevent leakage of the paint, as will be understood. A suitable solvent may then be circulated through the paint heating and jetting systems, by opening valve 76 and setting pump 47 into operation, for removing any remaining paint therefrom, if desired or necessary. The replacement tank, containing the desired paint or material, being coupled to the conduits 41 and 44, valves 42, 45 and 46 are opened and valve 76 is closed, and the painting operation may then be resumed.

Coating compositions or paints employing as solvents chlorinated hydrocarbons, such as perchlorethylene, trichlorethylene, and solvents of like character, arev known and available on the market under different trade names. One of such paints used by me is an asphalt base paint containing approximately 50% (by weight) of trlchlorethylene. Another paint which I have used is a modied alkyd paint containing 50% (by weight) of trichloretl'iylene.v The above and other paints or coating compositions containing approximately 56% solvent, which may be termed full-bodied paints, have been used in the apparatus and the method of my invention with `satisfactory results. ln painting articles having depressions or recesses which the paint may tend to either fill or bridge, `it may be desirable to thin the paint. l have found that the paint may be thinned up to approximately by weight of solvent. Thinning of the paint assures that it will cover all of the surfaces of the articles and is conducive to ready spreading of the paint to produce a coating of smooth and uniform thickness. Thinning of the paint also has the advantage of permitting increase in the temperature to which it may be heated without risk of deterioration while also promoting ready low of the paint over the articles. That is of advantage in jet painting articles of substantial mass, in which cases the paint is heated to a sutlciently high temperature to assure that the surface or skin temperature of the articles will be maintained, by the hot paint projected thereon, such as effectively to eliminate possibility of condensation of the solvent vapor on the objects being painted. That renders it possible to hold the articles in the spray zone, in which they are immersed in a concentrated vapor of the solvent, until a coating of any desired thickness, within practicable limits, has been attained, as above explained.

In the above description of the method of my invention, it has been assumed that the solvent used in the paint is trichlorethylene, which is heated to a temperature of approximately 224 F., well above its boiling temperature of approximately 188 F. at atmospheric pressure. lf the paint solvent is perchlorethylene, which has a boiling point of approximately 248 F., the paint should be heated under pressure to approximately 285 F., or higher. Likewise, in using paints using other chlorinated hydrocarbon solvents, or solvents of like character, the paint should be heated under pressure to a temperature well above the boiling point of its solvent, adequate to assure that the articles being painted will be maintained at a surface temperature well above that of the surrounding solvent vapor, as and for the reasons above stated. In all cases the temperature to which the paint is heated and the pressure of pump 47 are maintained at such Values as to assure that the paint will be projected onto the articles in solid streams or jets and will not be atomized or subdivided into small particles.

In the above description of the method of my invention, it is assumed that the articles are degreased in the vapor zone in the solvent tank 12 of FiGUlE l, that the solvent in such tank is the same as the paint solvent. If desired, the solvent in tank 1.2 may be diterent from the paint solvent and have a higher boiling point, at atmospheric pressure, than the latter. For example, the solvent in tank 12 may be perchlorethylene and the paint solvent may be trichlorethylene. The articies are preheated by the vapor in tank 3 2, incident to degr 'ng and cleaning of the articles preparatory to jet pain g there of. If the solvent in tank 12 is the same as the paint Solvent, the articles are preheated to a temperature approXimately the same as tne boiling point, at atmospheric pressure, of the paint solvent, and if the solvent in tank 'i2 has a higher boiling point than the paint solvent, the articles may be preheated to a temperature substantially higher than the boiling point of the paint solvent at atmospheric pressure. The preheating of the articles incident to degreasing and cleaning thereof, is desirable, though not essentialL to the method of my invention, as expediting the jet painting operation. the articles are in condition for painting without being passed through tank 12, the degreasing step therein and the preheating incident thereto may be omitted. Accordingly, the jet painting method of my invention may be performed by passing the articles through passage 19 into paint chamber 17, through the latter and thence through exit passage 26. As will be understood, the degreasing step, accompanied by preheating of the articles, is not essential to the jet painting method of my invention and may be omitted. Likwise, the degreasing tank 12 is not essential to the apparatus or machine of my invention and may also be omitted where degreasing of the articles is not required7 the apparatus or machine, in such cases, comprising the paint chamber i7 and associated parts for performing the jet painting operation.

In the modified form of machine shown in FEGURE 3 an open top tank Sii, rectangular in plan, contains a body of paint S1 in its lower portion and is there provided with suitable heating means, such as steam coiis 32. Tank titl constitutes the paint chamber and is provided at its upper portion with an exterior cooling jacket 83, interior cooling coils 84 and a trough 85 underlying coils 84 for receiving condensate therefrom and returning it to tank 8d. The paint 81 is heated to its boiling point and the evolved solvent vapor fills the area of tank Sti between the body of paint Si. and the midheight of cooling coiis 84 providing therein a main body 86 of solvent vapor, indicated by stippling, the upper surface of which, constituting the vaponair interface is indicated by the dotwash line 36. The intake of pump d'7 is connected to tank 80, at one end thereof and bel w the level of tie paint 81 therein, by a conduit 87 preferably, though not necessarily, provided with a hand valve S8. 'The die charge of pump 47 is connected, by a conduit 89, to the inlet of heat exchanger t) is connected by conduit 9G, fitting 55 and conduits 91 and 92 to pressure gauge 53, pressure responsive valve 57 in the steam supply conduit 51 for the heat exchanger 5G, and headers 36 of jet nozzles 37 of ,iet means at opposite sides of tank Sti; as before, heat exchanger 5@ having a discharge conduit 52 including a trap 53.

An article 65 to be painted is suspended, by a cord or cable 67a within tank 8) between the jet means. The

hot paint passing through heat exchanger 5! is heated lu cases Where The outlet of heat exchanger Cilhas attained the desired thickness, the painted article is withdrawn from tank 8u upward through the localized zone 93 of superheated solvent vapor, the temperature of which is higher than that of the vapor of the main body 86 and effectively eliminates possibility of condensation of solvent vapor on the painted article. The larticle may thus remain in the painting zone, in the solvent vapor, until a coating of paint of any desired thickness, within practicable limits, has been applied, and then withdrawn from tank 80, without risk of solvent wash The paint of the jets spreads smoothly over the article and quickly dries to substantial extent thereon by evaporation of `the solvent by the skin or surface heat of the article supplemented by the heat of the paint. 'The painted article dries rapidly as it is withdrawn upward through the superheated solvent vapor of zone 93 and ordinarily is completely dried, or nearly so, when it has reached a position clear of the tank 80. If the paint coating `applied to the article is quite thick, it may be somewhat tacky when the article has been completely withdrawn from the tank. in such cases the coating quickly dries completely in the atmosphere by rapid evaporation of the then slight amount of solvent remaining in the paint coating at the surface thereof.

It is assumed that in using the machine of FIGURE 3, the articles are in a clean condition suitable for painting. In such cases, it is not necessary to degrease the articles and the paint itself may be utilized, preliminary to operation of the jetmeans, as the source of the main vapor zone in which lthe articles are painted. In the machine of FIGURES 1 and 2, the main vapor body or zone in which the articles are jet painted in chamber 17 may be created by use 4of the jet means for a short time, after which the articles are jet painted as above described. In either case, the painting method is essentially the same, with the advantages above pointed out, and is well suited for use in large volume industrial operations, the machine of FIGURES 1 and 2 being particularly suitable in that respect, when degreasing and cleaning of the articles is required preliminary -to painting thereof.

As above indicated, and as will be understood from what has been said, changes in detail may be resorted to without departing from the held and scope of my invention, both as to the machine and the method, and i intend to include all such variations as fall within the scope of the appended claim, in this application in which the preferred `forms only of my invention have been disclosed.

I claim:

The method of coating articles with paint containing organic solute dissolved iu an organic solvent, which cornprises disposing the articles within a space at approximately atmospheric pressure, heating the paint to a temperature higher than the boiling point of said solvent at atmospheric pressure, projecting upon the articles the hot paintrin the form of unatomized substantially continuous and solid jets at a temperature above the boiling point of the solvent at `atmospheric pressure until the surface tempertature of the articles is increased above said normal boiling point of the solvent, and drying the paint on the articles by the surface heat of the latter.

References Cited in the tile of this patent UNITED STATES PATENTS 1,763,866 Rodd et al June 17, 1930 2,515,489 Borushko July 18, 2,728,686 Borushko Dec. 27, 1955 2,754,228 Bede July l0, 1956 v2,763,575 Bede Sept. 18, 1956 2,791,516 Chambers et al. May 7, 1957 2,864,404 Thiessen Aug. 27, 1957 2,861,897 Hendrixson Nov. 25, 1958

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