US1534738A - Method of drying varnish - Google Patents

Description

April 21, 19.25.` 1,534,738`

C. H. ROE

METHOD OF DRYING VARNISH Filed March 31, 1920 Ca z/as Qzy/ a@ @51g i@ @wcm/nuff t Patented Apr. l21, 1925.

UNITED STATES PATENT OFFICE.

CHARLES H. ROE, OF TARRYTOWN, NEW YORK, ASSIGNOR TO GENERAL MOTORS COB/- PORATION, F DETROIT, MICHIGAN, A CORPORATION OF DELAWARE 'Y METHOD OF DRYING VARNISH.

Application led March 31, 1920. Serial No. 370,278.

To all 'whom t may concern:

Be it known that I, CHARLES H. Ron, a citizen of the United States, and a resident ofTar town, county o f Westchester, and

State o New York, have invented certain new and useful Improvements in Methods of Drying Varnish, of which the following is a full, clear, concise, and exact descri tion, such as will enable others skilled in tlife art to which the invention relatesl to make and use the same.

My invention relates to the art of coating articles with varnish, japan, enamel, paint or like protective coating material, and particularly to the. drying and hardening of a film of coating material applied to the articles in the liquid form; and the character of which material is such that the articles being coated have to be heated to a comparathe drying and oxidation of the film, and a satisfactory nished coating for the article.

The object of my invention is to provide an improved method or process for accomplishing the dryingand hardening of a film of liquid coating composition applied to the articles and by the use of which process a better adhering, a more durable, a better appearing and an otherwise more satisfactory coating and protection for the articles will be secured; my invention being particularly adapted for use in the enameling or varnislling of metallic sheet metal parts used in the construction of bodies for automobiles.

Varnishes, japans, paints and like liquid coating compositions used for the coating of metal articles such, for example, as the various metal parts of the body of an automobile, may be defined enerally as com osed of a suit-able liquid ve icle in which suitable `gums and resins, together with pigments which determine the color of the coating, are dissolved and held in suspension; the liquid vehicle being composed 1n part of a drying oil which is oxidized during the settingand hardening of the coating film and in part of a volatile solvent or material employed for the purpose of thinning the liquid coating composition so that the same may be the more readily applied to the article or surface to be coated; and which solvent or thinning liquid must be driven off from the film of coating material before the same can become dry and hard as the same has no drying qualities as that term tively high temperature in order to secure gasolene, benzine, alcohol, or in fact an easily volatile liquid which will mix wit the more viscous drying oil and gums which, particularly in the case of japans, is so thick as to render the application thereof to articles to be coated difiicult unless a thinning liquid isl first mixed therewithft The film of coating material applied'in the liquid form to the article to be coated must obviously. become dry and hard before the film becomes fixed upon the surface to which itis applied and assumes the` character of -a permanent and strongly adherent coating necessary in actual practice.

The ultimate drying and hardening of an initial film of liqu1d coating composition involves two steps or operations which are for the most part essentiallyseparate and distinct from one another, although the end of the first or 'drying step proper in a sense mer es with the beginning .of the second or har ening step; the first of said steps involving the evaporation or the driving off therefrom of the volatile solvent or thinning ingredient of the coating film, While the second of said steps is characterized by the absorption of oxygen from a surrounding body of air which accomplishes the oxidation and hardening of the linseed or equivalent drying oil ingredient which enters into the composition of thecoating film.

Heretofore to the' best of my knowledge and belief no attempt has been made to se'parate the two phases above referred to through which the liquid coating film passes in attaining its final form of a dry, hard, resistant, and strongly adhering coating or finish upon the article or surface to be protected; althou h said hases are essentially separate and istinct rom one another and involve entirely different processes, the first being a urely, physical and the second an essential y chemical process. After the Vns japan, varnish or paint has been applied to the articles they' have heretofore commonly been passed into an oven or enclosure heated to `a comparatively high temperature, and left therein for a time which experience dictated as that roper to secure a dry and sufficiently har coating upon the surface or article being treated; all as a single and uninterru ted process or operation.

It wi l be appreciated that while the volatile solvent constituent of the liquid coating film must be'completely eliminated before the resultant coating will be of sufficient hardness to afford the requisite degree of protection to the article, even though the drying oils and gums thereof may ultimately become completely oxidized, there is no necessary connection between the primary volatile liquid driving oi' step and the subserlent oxidizing step of the entire drying an hardening process re arded as starting with the freshly applied m of liquid coat# ing material, and ending with the dry and hard. layer of protecting material secured yin varnishing processes.

In my improved varnish drying process (by which term I mean all the steps involved in the conversion of a freshly applied film of liquid coating composition into a finished oxidized, dry, hard, and resistant pro-l tecting la er or coating, as distinguished from the drying which involves the final oxidation of the dr ing oil ingredient of the coni sition) the fivrstxstep or phase which invo ves the evaporation of the volatile solvent or thinning liquid ingredient of the film is isolated from the second step or phase which involves the absorption of oxygen from atmospheric air by the drying oil constituent ofthe varnish and the hardening or oxidation of the said film. f

Inasmuch as the evaporation of the volatile solvent constituent of the liquid coating film is commonly accomplished under conditions such that the articles being coated are surrounded by a gaseous medium containin oxygen, and under conditions such that t ere will be a simultaneous oxidation of the drying oil constituent ofthe film; and in order that the resulting finished layer of varnish material may be roughened 4as little as possible by minute bubbles of thev volatile constituent breaking through a surface of the film which has -taken up some oxygen from the gaseous medium and has be n to harden; I referably drive of the vo atile liquid ingre ient as rapidly as possible durin the first phase or step of my improved rying process in order that the complete or substantial elimination of such liquld may be accomplished beforeY the drys ing oil constituent has had time to become oxidized to any materialdegree, and before a. surface layer has been formed which would be broken or roughened bythe escape of volatile material from the inner portions of the ilm from which the volatile ingredient is being eliminated. The subsequent or second phase of the process during which the drying oiland gums become oxidized by the absorption of oxygen `from a' surrounding gaseous medium and a finished coating is secured is then permitted to proceed more slowly, thus increasing the hardness, the adherent property, and the general excellence of the resulting finished layer of coating material upon the article being treated. v

i The irst or primary stage of my im proved varnish drying process during which the volatile solvent material of the film is eliminated is obviously in no way dependent upon the presence of oxygen in the gaseous medium surrounding the articles and through which heat is commonly communicated thereto, to accomplish the vaporization of the volatile products, whereas the subsequent or secondary stage of the process during which the oxidation of the coating remaining upon the articles is accomplished necessitates the presence of oxy gen 1n such gaseous medium; and such final stage proceeds the more rapidly, and is the dance, and at a much lower temperature, to vthe 'space' surrounding the articles during the' secondary or oxidizing state of the process. Such a method of procedure se` cures an effective andreticient application of heat to the article during the volatile liquid eliminating stage and reduces the heat required to drive off the volatile products during the first or primary stage of the process; and Kat the same time. provides oxygen in abundance, and even 1n excess, during the secondary or oxidizing stage, so that the oxidation of the drying oil and gums may'proceed rapidly andl eectively.

Although the process wherein my invention consists may be performed byvarious forms of apparatus or device, I have in the accompanying drawing illustrated one form oi? apparatus capable of performing the same, and which Iyhave found to be satisfactory for that purpose. In said drawing;

Figure l is a perspective view broken of the an apparatus or device capable ',of use inperforming the second step of my improved process.

In erforming my improved process the essentlal steps of which have been hereinbefore enumerated the articles being treated are first coated with japan, varnish, paint or equivalent liquid coating material in any way, as by dipping them in a bath of such material, by spraying the same thereupon, by means of a brush, or otherwise; after which the said articles are placed within a closed chamber such, for example, as the chamber or oven 1, Figure L'having .ha heating coil 2, and an outlet fiue 3 through whichV such gases as may be produced may pass off and heated to a temperature, and for a period sufficient to vaporize the volatile solvent or thinning constituent of the film of coating material. It is preferable that there be no supply of fresh air to the heating chamber during this rst or evaporating stage rocess, as the oxidation of the film should be as far as possible prevented during this phase of the process; and the temperature should be high in order to accomplish the vaporization of the volatile products,y quickly, and reduce the time during which some slight and unavoidable oxidation of the film may occur.

The evaporating step of the process may be hastened byA heating the artic-les in a chamber in which a' reduced pressure is maintained by means of a vacuum producing pump or eguivalent device; and the oxidation which to va degreev is unavoidable during the' first stage of the process may be reduced by heating in an atmosphere having a reduced oxygen content, should such refinement in the process be deemed desirable and of sufficient practical importance to justify the somewhat greater diiliculty and expense incident to the performance thereof tunder reduced pressure, and in a gaseous medium in ywhich the oxygen is reduced in quantity. The heating of an uny changing mass of air on equivalent aseous medium` may obviously be the more e cientl accomplished than would be the case if resh'air was su plied to the heating chamber during( the first or driving oi stage of the process durin which the volatile constituents of the fi m of liquid coating material are eliminated. I

After the volatile solvent ingredient has been completely or sufficiently eliminatedv from the film of liquid coating material an abundant 'supply of oxygen is su plied to the article being coated, so that t e oxidau tion and hardening of the residual portion of the film will take place. This second step of my process is preferably performed jin a separate chamber such, for example, as

the chamber 4, Figure 2, to which the articles are transferred and through which chamber an abundant supply qf air is caused to flow as by means of 'a fan blower'5 which discharges thereinto through the conduit 6,

or other type of vair forcing means, the temperature of such stream of air being usually materially- 'lower than the temperature to which the articles were subjected during the first or evaporating stagev ofthe process, although such air will ordinarily be heated to a comparatively moderate degree as by the heating coil 7; as the important consideration during this second stage or phase of the process is to provide an abundant supply of oxygen so that the oxidation of the drying oil will'occur with rapidity and efficiency, rather than a high temperature as no volatile constituents of the film remain to be eliminated; such constituents having been eliminated duringthe first stage of the process.

As a matter of course the temperature employed during the primary and secondar stages of my process, the duration of suc i.

essential steps of which are the primary step y or stage during which the volatile products are eliminated, followed by the secondary stage or step during which oxidation of the drying oil p-resent in the film is accomplished; the two steps or phases being separated from one another and each limited, so far as is practicable, to the particular end to be attained during such stage. In the practical operation of my process upon metallic body members, fenders therefor, hood structures, and like sheet metal automobile body parts I haver secured satisfactory results by first dipping said articles in a bath of japan' or equivalentivarnish; then permitting excess varnish to drain from said members and fiow back into the tank containing the bath; then transferring said articles to`an oven or chamber which so far .as feasible is closed, and to which there is no designed supply of fresh air; then heating'said articles within the chamber aforesaid to a^temperature of approximately 500 chamber until the oxidation of the coating an unchanging mass of air is maintained thu-s avoiding the waste incident to the heating of a large volume of air to the high temperature necessary to accomplish the vaporization of the volatile constituents of the film; as no fiow of air heated to the temperature necessary to Vaporize such products is maintained past the 1 articles throughout the drying of the film `or for a considerable part of the drying process. The heating of the articles to a high temperature and in a quiescent body of air during the evaporating stage of the process reduces surface oxidation to a minimum during such first stage and secures the elimination of the volatile constituents of the filmbefore any material oxidation thereof can take place; so that the partially dried and hardened layer is not broken or disturbed by the escape of volatile constituents after such first step and during the oxidizing phase; because the volatile products have already been eliminated and because the articles are heated to a much lower temperature durin the second stage of the process; from w ich it follows that asmooth surface is secured in the resulting layer or varnish at the end of the process. Furthermore, the carrying out of the oxidizing step of the process by means of a large volume of air heated to a moderate temperature, or in fact at atmospheric temperature in many cases, results in a more completely oxidized and in a harder and more satisfactory, and a better appearing coating upon the articles being coated.

Having thus described and explained the process wherein my invention consists, 1 claim and desire to secure by Letters Patent:

1. The method of drying a liquid coating composition containing a volatile ingredient and a drying oil applied as a film to an article to be coated which consists in heat-ing the article to thereby evaporate the volatile constituent of said film as a first step or phase; and oxidizing the remaining portion of said film as a second ste or phase se arate and distinct from said rst mentione evaporating stage.

mentioned phase.

2. The method of drying a liquid coating composition containing a vola-tile ingredient the article to a comparatively high temperature, and for; a comparatively short time,l

to thereby rapidly dr1ve ofi' the volatile 'constituent of said film therefrom as a first Step or phase; and heating the article being coated to a materially lower temperature, and for a comparatively long time, to thereby accomplish the oxidation of the drying` oil constituent of said film as a step or phase separate and distinct from said first mentioned stage.

3. The method of drying a liquid coating composition containing a volatile ingredient and a dryingoil applied as a film to an article to be coated which consists in heatingthe article Within a chamber in which a limited amount of oxygen is present, to therebyevaporate the volatile constituent from said film, as a first step or phase; and then oxidizingfthe dryin oil constituent of said film in a chamber Wiere'in an abundant supply of oxygen is present as a second step or phase separate and distinct from said first 4. The method of drying a liquid coating composition containing a volatile ingredient and a drying oil applied as a film to the article to be coated which consists in heating the article being coated to a comparatively high temperature, a-nd for a comparatively short time, within a chamber containing a quiescent body of gaseous medium, to thereb drive off the volatile constituent of said fi m as a first step or phase; and then subjecting the article being coated to the action of a comparatively large volume of air to thereby oxidize the remaining portion of said film as a second stepseparate and dis-` tinct from said first mentioned step.

5. The method of drying a liquid coating composition containing a volatile ingredient and a drying oil applied as a film to an article to be coated which consists in heating the article in a chamber containing a limited uantity of air to a high temperature, and or a comparativel short time, to thereby rapidly evaporate t e volatile constituent of said film as a first stepl or phase; and then causing a stream of air eated to a moderate temperature to flow past thearticle being coated to thereby'oxidize the remaining portion of the film as a second Astep or phase separate and distinct from said first mentioned'phase. y

In testimony whereof I affix my signature.

CHARLES H. ROE.

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