US2142024A - Coating surfaces of zinc-base - Google Patents

Description

Patented Dec. 27, 1938 UNITED STATES COATING SURFACES F ZINC-BASE William E. Hall, Hillside, N. J., assignor to The Egyptian Lacquer Manufacturing Company No. Drawing. Application September 30, 1936, Serial No. 103,362

7 Claims.

corrode upon exposure to the weather and leave much to be desired from the standpoints of appearance and durability unless subjected to further treatment. Consequently it has been heretofore customary practice to paint the galvanized metal either before or after it is in place.

Paint; tends to peel after a short time of service when applied to untreated galvanized surfaces so that it has been proposed heretofore to etch or sandblast the galvanized surfaces preparatory to applying the paint. Etching, however, has-not proved entirely satisfactory because zinc, and

25 particularly zinc in crystalline form on galvanized surfaces, tends to etch in an irregular and spotty manner which deleteriously affects both the appearance and the resistance to weather of the finished surface.

3o Spray application of primer and paint to galvanized metal, especially galvanized metal roofing which is coated at the factory in large lots, is highly desirable because of the resultant saving in labor. But efforts to employ the heretofore cus- 3 tomary etching agents or primers", such as aqueous solutions of hydrochloric acid, to the galvanized metal by means of sprays has only aggravated the aforementioneddifliculties. When applied with sprays such primers do not flow uni- 40 formly over the galvanized roofing but tend to Etching occurs imcollect as isolated droplets. mediately adjacent the droplets but, unless an, excessive amount of primer is sprayed on, much of the surface area remains unetched while other 45 portions are excessively corroded. When the surface is etched in this uneven manner, the lack of uniformity in appearance persists even after the surface is painted, probably because the more deeply etched surfaces absorb more of .the paint have less lustre than adjacent areas. Such spots of lower lustre are similar in appearance to those known as hot spots on a plaster wall which has been painted without proper priming.

Brush application of the heretofore customary vehicles than other portions and form spots which primers has yielded fair results, but unless much time and effort is spent in working the primer over the surface, the primed surface is spotty and the spotty character persists after the surface is painted, injuring its appearance and decreas- 5 ing its resistance to weathering.

As a result of my investigations I have discovered a coating composition or primer for surfaces of zinc and other water-repellant metals, for

example, galvanized metal which not only gives 10 excellent results when applied by spraying and entirely avoids the difficulties due to the spotty finish of prior primers, but also imparts to the surface a uniform and durable finish of any desired color and shade. This finish is not affected substantially it! under the conditions to which ordinary roofing is exposed. Tests indicate that the finish given by the priming treatment of my invention will last for many years even when no coat of paint is placed over the primed surface. However, since 20 the priming treatment of my invention places the galvanized metal in excellent condition to receive and retain paint,'I prefer, especially in the case of galvanized roofing, to use both paint and primer, employing a primer which imparts to the roofing the same color as the subsequently applied paint, thus increasing the protection afforded to the roofing. Eventually the paint may come off the roofing in more exposed portions thereof, but inasmuch as the priming treatment has imparted the same or nearly the same appearanceto the underlying galvanized surface, repainting is necessary at infrequent intervals, if at all, and the roofing never acquires the leprous appearance often acquired by ordinary painted galvanized iron. I

The primer of my invention comprises a mixture containing water, a water-soluble etching agent or water-repellant metal surfaces, a readily volatile, non-viscous, water-miscible organic liq- 4Q uid which wets the surface more easily than does water and which does not react readily with the aqueous solution of the etching agent in the primer, and a pigment in the form of'a water-insoluble compound of a heavy metal which does not react readily with the aqueous solution of the etching agent in the primer.

I prefer to employ hydrochloric acid as the etching agent, but the other mineral acids, by which "I mean sulphuric acid, nitric acid and phosphoric acid, may also be used advantageously.

The readily volatile, water-miscible and nonviscous organic liquid in the primer acts as a wetting agent. I have found that acetone, methyl ethyl ketone, mono-ethyl ether of ethylene glycol and the common lower alcohols such as methyl alcohol, ethyl alcohol and propyl alcohol are among themost satisfactory wetting agents to employ in the practice of my invention.

I prefer to use mineral pigments in the primer of my invention. By mineral pigments I mean finely divided, water-insoluble compounds of heavy metals (both natural and synthetic) such as .the oxides of iron, chromium and titanium, barium sulphate, and complex pigments such as v lithopone and titanated lithopone.

As indicated hereinbefore, the pigment employed should not be one which will be dissolved in large part by the aqueous solution of the etching agent in the primer under normal conditions of storage and use. 'I'hus inert pigments such as barium sulphate are satisfactory, although I prefer to use pigments which are not substantially insoluble in the aqueous solution ofthe etching agent in the primer. The oxides -of iron, titanium and chromium (although not completely inert to the action of the mineral acids) are morev satisfactory than inert pigments, because they become more firmly affixed to the treated surface. This I believe to be-due to the fact that slight reaction between the etchingagent and the pigment aids in fastening the latter to the treated surface. This may be due either to a cementing action'of the reaction products formed or it may be due to an alteration. ofthe surface of the pigment particles which makes them 'ad- 2 here more firmly to zinc-base surfaces.

In addition to water, the etching agent, the

wetting agent and the pigment, the primer of my invention preferably contains two other in gradients-a deposition agent and a grease- ,cutting agent. By deposition agent I mean a water-soluble salt of a metal less electro-positive than zinc (or other water-repellant metal to be coated), such as copper sulphate, in which the metal, for example copper, tends to replace zinc and deposit on the zinc-base metal surface. The use of this deposition agent is not essential but aids in obtaining a uniform surface for the application of paint.

Practically all galvanized iron and other fabricated forms of zinc coated metal have a thin non-uniform and usually invisible film of oil or grease upon them (at least in patches) which has attached to the metal during fabrication or during subsequent handling. In order for the Pr mer to act upon the metal surface it is necessary that it penetrate or remove this film of rease. or oil. Otherwise the film will interfere with uniform etching. The grease-cutting agent enables. the primer to penetrate the film. The agent should be an organic solvent for hydrocarbons of the paraffin series. In addition it must be miscible with the wetting agen'v in the primer. Toluol, carbon tetrachloride and their homologues are preferred examples of grease-cutting agents to be employed.

In practically all industrial applications it is safer to include the grease-cutting agent in the primer of my invention. If, however, it is certain that the metal surface to be treated has no grease or oil on it, the grease-cutting agent may be eliminated from the primer and replaced by an of mineral particles some of which have non-metallic surfaces (quartz, feldspar) and others of which have metallic surfaces (pyrite, galena) are agitated vigorously in the presence of water and a flotation agent (pine oil), the flotation agent attaches to the particles having metallic surfaces and to air bubbles resulting from the agitation andare thus buoyed up in a froth which iseasily skimmed. g

Metal surfaces of zinc, cadmium, ,magnesium and alloys of these metals of which zinc, cadmium or magnesium predominate have a .greasy" quality even when they are absolutely clean. In other words, such surfaces are water-repellant. In consequence, when a strictly aqueous liquid is sprayed on such a surface-it tends to collect thereon as minute drops and does not tend to spread into a uniform film. This is especially true of galvanized sheets and the like where the coarsely crystalline character of the metal makes the surface irregular and bumpy: and thus introduces a further obstacle to the flow or spread of the drops over the surface.

However, when a suitable wetting agent for the particular metal, say zinc, is included in the sprayed liquid the drops spread upon contact with the surface of the metal, eventually flow into each other and produce a continuous liquid film through which the etching agent and the pigment particles may flow freely and thus become distributed on the surface, with substantial uniformity. This results in an even distribution and an intimate relationship between the etching agent in the aqueous solution and the pigment particles, with the result that the metal surface it etched uniformly.

I believe that the wetting agent in the primer, in addition to overcoming the difficulties. described hereinbefore in a manner analogous to that of a flotation agent in ore dressing, also aids in dispersing the pigments in the primer. This is manifested by the tendency of the pigmented primer of my invention to become smooth with much less stirring than is required with a suspension of pigments in a fluid which does not contain a wetting agent. The improved dispersion of pigments in the primer of my invention may be due to the more thorough wetting of the pigments by the fluids, but whatever be the explanation, the improved dispersion which is attained in the practice of my invention facilitates the uniform application of the primer by spraying and precludes the presence of large agglomerates of pigment particles.

Apparently, during the etching process the pigment particles become affixed to the surface. I believe that the adhesion of the pigmentparticles to the metal surface without any conventional film forming ingredients to hold them there is due to adsorption. In any case, the fact remains that the pigment particles become affixed to the surface even in the absence of any I first make up a clear primer which contains:

. Parts by Ingredient volume 30% copper sulphate solution (2% lbs. (iiSO4.5ll 1w!" gallon watorl 354 Commercial concentrated hydrochloric acid (contn about 35% by weight HCl) 5 (Tarbon tetrachloride"... 5 'loluol 25 Cellosolve mono-ethyl other of ethylene glycol. ill I Denatured ethyl alcohol 51% Total 100 primer may be varied within moderately wide limits without impairing the final results. In general, however, the tota volume of water, etching agent and deposition agent should range from 5 to of the total. The wetting agents should comprise from 60 to 75% of the total volume, and the grease-cutting agents should be about 25 to 35% of the total volume. In this connection it should be noted that the mono-ethyl ether of ethylene glycol is one of the most desirable wetting agents to incorporate, to some extent at least, in the primer because it acts as a blending agent for the other liquid ingredients and permits a wider variation in the proportions of the other liquid ingredients in the primer.

To the clear primer is added the mineral pigments to give the desired color to the treated surface. The pigment should be acid-resistant and for optimum results it should be fine enough to pass completely through a screen having 325 meshes to the inch. In general, the mineral oxide pigments such as chrome green, red iron oxide, black iron oxide and titanium dioxide are to be preferred, but acid resistant sulphates such asbarium sulphateand compound pigments such as lithopone and titanated lithopone may be employed advantageously. The following tabulation gives the preferred proportions of some pigments to clear primer:

Ounces of dry Color Pigments g f g a gig primer Green Chrome oxide 10 Red Red iron oxide... 8

Black Black iron oxide. 3

Black iron oxide 6 l slam {Titanium dioxide. 2 l

Red iron oxide I 6 Brown {Chromeoxide 3 9 In mixing the clear primer and the pigment the latter is first worked. into a paste with a small proportion of the primer. Working is continued until the paste is uniform and free of lumps, after which the balance of the primer is added and the whole is stirred together until the mixture is uniform and of a thin consistency.

Coating the metal surfaces The pigmented primer may be applied by brushing or by dipping, but spray application is preferable. My invention, therefore, contemplates a process for producing a protective coating on the surface of zinc or similar water-repellant metal or alloy which comprises spraying thesurface with a fluid mixture containing water, mineral acid, a mineral pigment and a volatile non-viscous and water-miscible organic liquid which does not react readily with the aqueous solution of the mineral acid in the mixture and which wets the surface more easily than does water, and permitting the surface to dry, whereby the surface becomes etched with substantial uniformity and the pigment adheres thereto.

During the spray application of the fluid mixture. the pigment should be kept in suspension. This is preferably accomplished by agitating the fluid mixture in the reservoir which fills the spray.

Preferably also, the grease-cutting agent or the deposition agent hereinbefore described are included in the fluid mixture which is sprayed.

In greater detail, the application of the pigmented primer to the water-repellant metal surface. for example, galvanized sheet iron shingles. should be conducted as follows:

Any visible grease or oil spots on the surface of the shingles is first removed by washing with an appropriate solvent such as naphtha.

The pigmented primer is charged into an acid proof container, such as a wooden keg. equipped with an agitator and acting as a reservoir for a pneumatic spray gun having an acid proof nozzle and needle. vThe pigmented primer in the reservoir should be agitated throughout the period of spray application.

The clean shingle surface is then sprayed as uniformly as possible. The spray should be fine and evenly distributed in a fan shape. With the customary paint spray gun the air pressure when the spray is open should be approximately 40 pounds, and the valve which supplies the pigmented primer to the spray should be opened only slightly; otherwise an excessive spray results. The sprayed coat of primer while wet should show definitely the pattern of the spangles on the surface of the galvanized shingles. If the pattern of the spangles is obscured, too much primer has been used.

After the primer is sprayed on the shingles, they should be stacked carefully in a dry atmosphere and permitted to dry for at least 5 hours and preferably overnight. On rainy days or when the humidity is exceptionally high this drying period should be prolonged up to a maximum of about 24 hours.

Onev spray coat of primer is suflicient, but in no case should paint be applied over the primer surface until it is completely dry. One gallon of the primer will cover 450 to 500 square feet of surface if the primer is properly applied.

The containers, spray pots, spray guns and hose which are used for applying the primer should not be used for any other purpose, such as for spraying paint. Admixture of the primer with paint spoils the paint and vice versa.

When changing from one color of primer to another, the interior of the entire spray apparatus should be cleaned with water.

As hereinbefore indicated, it is not necessary to apply paint over the primed surface because the primer itself imparts to the roofing a durable coating of any desiredcolor or shade. However, the primed surface furnishes an excellent base for paint, particularly for the paint described hereinafter and presents an outstanding advantage over primers proposed heretofore in that paint can be applied directly over the dry primer coat, without first washing the primed surface or otherwise treating it to remove reaction products. Consequently, to give additional protection for the surface I prefer to apply at least one spray coat of paint over the primed surface, using paint of the same color as the pigmented primer. The paint gradually chalks from the surface durin exposure to the weather, buteven after the paint is entirely gone the roof will still give many years of service and will appear unaltered in color both during the time when the paint is chalking oil and thereafter.

Almost any paint may be used to cover the primed shingle surface. However, I have discovered that it is best to employ paint which chalks when exposed to the weather, i. e. paint which fails gradually and washes off the painted surface as an impalpable powder or mud during long periods of exposure. A paint of this character is desirable because it does not fail in patches and therefore gives additional security that the roof will not acquire a spotty appearancedue to the fact that on some portions of it paint still remains while on other portions the paint has been removed,- exposing a primed surface which is of slightly different appearance.

I have developed an especially suitable paint which comprises a varnish base made by cooking China-wood uli with a phenol formaldehyde resin together with a high proportion of filler such as talc or barytes. This paint is especially applicable in the practice of my invention not only because it does not fail in patches but also because it adheres well to the primed surface and give an unusually uniform finish.

The varnish is prepared by cooking about 4 parts by weight of phenol formaldehyde resin with about 16 parts by weight of China-wood oil for about half an hour at a temperature not to exceed 480 F. The mixture thus formed is al-. lowed to cool and 26 parts by weight of mineral spirits is mixed with it thereafter. About 1 part by weight ofdryer (for example a mixture of powdered cobalt, manganese and lead in the proportion 416-80 by weight) is then added to the varnish.

The pigment employed in the paint should be the'same in fineness and color as the pigment in the primer. However, the chemical composition of the pigment in the paint need not be the same as that in the primer because the pigment in the paint need not be acid resistant to the same degree. Thus, when green shingles are desired, chrome green should be used as the pigment in the primer, but a mixture of Prussian blue and lead chromate which will give the same shade and color of green may be used in the paint.

A preferred formula for the paint to apply over the primed surface is as follows:

A portion of the varnish is used as a grinding medium for the pigment and filler. Grinding may be carried out conveniently in a pebble mill or the like. A'stifi paste is thus formed. Thereafter the balance of the liquid ingredients of the paint is added. v

The above produces approximately 1'78 gallons of heavy paint. Before use this should be thinned with one part of naphtha or mineral thinner for each three parts of the paint.

The thinned paint should be applied over the dry primed surface with an ordinary pneumatic spray gun, using suflicient paint to cover the surface adequately. It has been found that one gallon of the paint describedhereinbefore will adequately cover about 400 square feet. Only one coat of this paint is necessary.

Good results have been obtained with an air pressure of from 30 to pounds per squareinch when the gun is open and with the paint valve on the gun open but slightly so as to produce a fine spray.

After painting, the finished articles should be dried for at least 24 hours before they are packed for shipment. The paint described above acquires a fiat and dry appearance within a few minutes after it is applied, but it does not harden to the extent required for handling until it has dried at room temperature for at least 12 hours.

Care should be taken to assure that the primed surface is absolutely dry before paint is applied over it and also to assure that the paint does not come in contact with the wet primer at any time. Thismeans that the containers, hose, spray gun and any other apparatus with which the paint comes in contact should not be used for handling the primer.

I .claim:

1. A process for producing a protective coating on a water-repellant metal surface selected from the group consisting of zinc, cadmium, magnesium and alloys of these metals which comprises spraying said surface with a fluid mixture containing water, mineral acid, a volatile, nonviscous and water-miscible organic liquid which does not react readily with the aqueous solution of the mineral acid in the mixture and which readily wets the surfaceand more easily than does water, and a substantially water-insoluble mineral pigment which is resistant to the acid in the mixture, and permitting said surface to dry, whereby a reaction occurs which etches the surface with substantial uniformity and causes the pigment to adhere thereto.

2. A process for producing a protective coating on a water-repellant surface of metal selected from the group consisting of zinc, cadmium and magnesium and alloys in whichone or more of these metals constitute a major ingredient which comprises spraying said surface with a fluid mixture containing water, a mineral acid, a volatile non-viscous and water-miscible organic liquid which does not react readily with the mineral acid in the mixture and readily wets said surface and more readily than does water and a pigment in the form of a water-insoluble oxide of a heavy metal, said pigment being resistant to the acid in said mixture, permitting the resulting primed surface to dry, whereby a reaction occurs which causes the surface to become etched and the pigment to adhere. to the etched surface and while retaining on the dry surface a reaction product of the metal and the mineral acid spraying the dry surface with a paint of substantially the same color as the pigment and which fails by chalking upon continued exposure to the weather.

3. A process for producing a protective coating on a greasy galvanized metal surface which comprises spraying the surface with a fluid primer containing water, a mineral acid, a wetting agent in the form of a volatile non-viscous and watermiscible organic liquid which readily wets zinc and more easily than does water and'does not react readily with the mineral acid in the mixture at room temperature, an organic solvent for the grease on the galvanized metal surface which is miscible with the wetting agent and a pigment in the form of a water-insoluble oxide of a heavy soluble acid-resistant mineral pigment into a fluid mixture containing water, mineral acid, a volatile non-viscous and water-miscible organic liquid which does not react readily with the aqueous solution of the mineral acid in the mixture' and which readily wets the surface and more easily than does water, spraying the resulting pigmented mixture on said surface while maintaining the pigment in suspension in the mixture, and permitting said surface to dry,

'whereby the surface becomes etched with substantial uniformity and the pigment adheres firmly to the etched surface.

fiaProcess according to claim 4 in which the pigment mixture is agitated in a reservoir before being sprayed and the agitation is continued sub- 5 stantially throughout the time of spraying.

6. A process for producing a protective coating on water-repellant surfaces of metals and alloys containing a metal selected from the group consisting of zinc, cadmium and magnesium whichcomprises incorporating a mineral pigment selected from the group consisting of iron, chromium and titanium oxides, barium sulphate and lithopone into a fluid mixture containing water, mineral acid, a volatile non-viscous and watermiscible organic liquid which does not react readily with the aqueous solution of the mineral acid in the mixture and which readily wets the surface and more easily than does watercth proportions of pigment to liquid in the mixture eing 20 about 10 ounces per gallon, spraying the resulting pigmented mixture on said surface while main-"1 taining the pigment in suspension in the mixture, and permitting said surface to dry whereby the surface becomes etched with substantial uni- 25 formity and the pigment in the mixture adheres to the etched surface. r

7. A process for producing a protective coating on a spangled galvanized surface which comprises incorporating a mineral pigment selected 30 from the group consisting of iron, chromium and titanium oxides, lithopone and barium sulphate into a fluid mixture containing water, mineral acid, a volatile non-viscous and water-miscible organic liquid which does not react readily with the aqueous solution of the vmineral acid in the mixture and which readily wets the surface and more easily than does water, spraying the resulting pigmented mixture on the surface while maintaining the pigment in suspension in the mixture, the coating of primer being applied to the surface 4 in such quantity that while the surface is still wet the spangles are apparent, and permitting said surface to dry, whereby the surface becomes etched and the pigment adheres to the etched surface. 45

WILLIAM E. HALL.