US3393172A - Polyvinyl butyral pretreatment coating for metals - Google Patents

Description

United States Patent "ice 3,393,172 POLYVINYL BUTYRAL PRETREATMENT COATING FOR METALS Joseph Spitz Breskman, Narberth, Pa., assignor to Sentry Paint & Chemical Company, Darby, Pin, a corporation of Pennsylvania No Drawing. Filed July 22, 1966, Ser. No. 567,383

3 Claims. (Cl. 260-33.4)

This invention relates to a pretreatment coating for metals, and more particularly, to a pretreatment coating for preparing metals for subsequent coating.

A long-felt problem has existed concerning practical finishes for present day metallic building materials, such as tin, aluminum, galvanized metal, stainless steel, tin plate and cadmium metal coatings. Such materials cannot be properly coated with conventional paints, primers and enamels since such coatings readily fail due to lack of adhesion. The problem is particularly acute in the case of non-ferrous metals, wherein conventional paint systems have been known to fail in as little as three months after application.

Another disadvantage of conventional paints, primers and enamels is the requirement for a relatively thick film. Thus, conventional finishes generally require a film on the order of the 1.0 mil thickness in order to give effective coverage.

Still another shortcoming of known paints, primers and enamels is relatively short shelf life due to instability. Thus, after a relatively short period of storage, many known coating compositions lose their properties, and become useless.

This invention has as an object the provision of a pretreatment coating for metals which provides great adhesion to metallic surfaces, including those set forth above.

This invention has as another object the provision of a pretreatment coating which gives effective coverage and adhesion in films of only 0.2 mil thickness.

This invention has as a still further object the provision of a pretreatment coating for metals which is characterized by ample stability, as evidence by a long shelf life.

This invention has as yet another object the provision of a pretreatment coating which is contained in a single package, and which is inexpensive to produce.

This invention has as another object the provision of a pretreatment coating which may be dispensed from an aerosol container.

Other objects will appear hereinafter.

The pretreatment coating of the present invention consists essentially of from about 3.0 to 3.6 weight percent of polyvinyl butyral, a chromate salt of a divalent metal equivalent to between about 1.5 and 2.25 weight percent of lead chromate, about 0.265 to 0.75 weight percent of talc, about 0.9 to 1.3 weight percent of 85% H PO about 0.9 to 1.3 weight percent of water, with the remainder being an alcohol having from 1 to 6 carbon atoms. Optimally, the polyvinyl butyral should be present in the amount of about 3.3 weight percent, the, chromate salts of the divalent metal should be present in an amount equivalent to about 2.0 weight percent of lead chromate, the talc should be present to the extent of about 0.5 Weight percent, the 85 weight percent phosphoric acid should be present to the extent of about 1.1 weight percent, the water should be present to the extent of about 1.1 Weight percent, with the remainder being the alcohol.

The polyvinyl buyral resin is commercially available.

3,393,172 Patented July 16, 1968 We have obtained excellent results using a polyvinyl butyral having a molecular weight of approximately 34,- 000, and an approximate weight percent composition of:

Percent Vinyl 54.4 Butyraldehyde 3 8.3 Acetate 0.3 Hydroxyl 7.0

The aforesaid polyvinyl butyral resin is commercially available from Shawinigan Resins Corporation under the trademark Butvar B-98, and is provided in the form of a free-flowing powder. The properties of this material are described in Bulletin on Butvar Polyvinyl Butyral Properties and Uses, fourth printing, copyright 1963, Shawinigan Resin Corporation.

The function of the polyvinyl butyral is as a binder or vehicle for the primer. Although I do not wish to be bound by any theory, it is my belief that the free hydroxyls in the polyvinyl butyral effect alignment of the pretreatment coating with the metal substrate to which it is applied and produce added adhesion of the pretreatment coating.

A wide variety of chromate primers may be used in a composition of the present invention. Thus, I have found the chromates of the divalent metals lead, zinc, strontium, barium, and calcium to be most satisfactory. However, other divalent metals may be used. Load chromate is to be preferred because of its color and stability characteristics.

The function of the talc is to toughen the film, to control the gloss, and to provide some physical roughness or tooth for adhesion of finish coats above the pretreatment composition of the present invention. A wide variety of suitable talcs are commercially available. I have obtained excellent results using the tale formed of magnesium silicate and sold under the trademark Nytal 300 by R. T. Vanderbilt Co., Inc.

The phosphoric acid is believed to form a complex ion With a metal substrate in conjunction with the chromate of the divalent metal. The water that is present permits the systems to operate as an ionic system.

A wide variety of alcohols having from 1 to 6 carbon atoms may be used, either singly or in admixture. Examples of suitable alcohols include methanol, ethanol, propanol, isopropanol, butanol, isobutanol, pentanol, and hexanol. The function of the alcohol is to serve as a solvent for the polyvinyl butyral, and as a vehicle for the pretreatment coating composition.

It is necessary that the solids used in the preparation of the pretreatment compositions of the present invention be finely divided such as to the order of an average particle size of 25 microns.

The precise mechanism by which the composition of the present invention obtains adhesion on metal surfaces is not known. It is suspected, however, that the metallic pigment and phosphoric acid form a complex ion with the metal to be coated. Thus, a possible complex ion is:

The Pb CrO and PO ions, it is suspected, react in the vessel in which the composition is retained, thereby leaving available negative charges to react with a metallic substrate upon application. It should be apparent that complexes similar to that set forth above could be formed with non-ferrous metals.

In manufacturing the instant composition, it has been found that a practical procedure is as follows:

The polymer is solutioned in the alcohol solvent under positive agitation. Approximately of the solution and all of dry components (primer and talc) are charged into a mill for grinding. A suitable type of mill is a pebble mill, half full of flint pebbles, although other well-known mills may be used if desired. Milling continues until the average size of the insoluble particles is 25 microns or less. Thus, in the case of the above-mentioned pebble mill, the milling operation takes from sixteen to eighteen hours. After milling, the milled components are discharged into the remaining 90% of the solution, and the phosphoric acid and water are added.

In usage, it has been found that the composition of the present invention is readily dispensed from a conventional aerosol or other spray apparatus. In the case of aerosol application, the coating composition would be packaged with any well-known compatible propellant. Such propellant per se, forms no part of the present invention.

The following examples illustrate the compositions of the present invention:

EXAMPLE 1 Component: Parts by weight Isopropyl alcohol, 99% 85.9 Normal butyl alcohol 4.8 Butvar B-98 3.3 1010 Yellow* 3.2 Nytal 300 .5 Water 1.1 85% H PO 1.1

1010 Yellow is a commerlcally available PbCrOr containing 62.5 weight percent PbCrO4.

EXAMPLE 2 The solvents may be changed in either direction until solubility is lost.

Pretreatment coatings were prepared in which the lead chromate portion of the composition was replaced with equivalent amounts of zinc chromate or strontium chromate or barium chromate or calcium chromate. In each case, a satisfactory composition was obtained.

The following example illustrates the manner of making a quantity of pretreatment coating in accordance with the present invention, as illustrated in Example 1.

EXAMPLE 4 A batch of 154.50 lbs. of polyvinyl butyral is mixed with 3,972 lbs. of isopropyl alcohol 99% and 225 lbs. normal butyl alcohol in a vat at room temperature and under agitation. Four hundred thirty-five pounds of the resulting solution was drawn ofl? and charged into a closed pebble mill half full of flint pebbles with 147 lbs. of yellow 1010 lead chromate and 24 lbs. of magnesium silicate. The mill was rotated until the average particle size in the resulting mix was 25 microns, and then discharged into the remaining alcohol solution and the resultant mix mildly agitated to obtain mechanical mixing. Immediately thereafter, 49.75 lbs. of Water and 49.75 lbs. of phosphoric acid were added at room temperature. The yield was 4,622.00 lbs. (about 666 gallons) of finished product.

The aforementioned batch may be conveniently dispensed from an aerosol. By way of example, 273 grams of the above composition may be very satisfactorily dispersed from an aerosol can containing a No. 640 nozzle and having 181 grams of Freon-12 propellant, such propellant being essentially dichlorodifiuoromethane.

The ability of the instant composition to be applied by means of spray devices, particularly aerosol containers, makes its usage in connection with home maintenance handy and inexpensive. It has been found that on a per pound basis, the present composition costs somewhat less than products presently available in aerosol containers. Its economy, however, lies not only in such difference in price, but also in the fact that only 0.2 mil thickness of the composition gives the desired adhesion. Moreover, usage of the instant composition may be expected to bring about still further economies, due to the fact that conventional paint systems applied over such composition may be expected to provide several years of service, free from peeling, flaking or blistering. Thus, whereas repainting with conventional systems is normally done every year, usage of the present composition would increase the interval between paintings to three to four years. The present composition has also been found to possess a long shelf life, such as on the order of eighteen months.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. A pretreatment coating composition consisting essentially of from about 3.0 to 3.6 weight percent of polyvinyl butyral, a chromate salt of a divalent metal equivalent to between about 1.5 and 2.25 weight percent of lead chromate, about 0.265 to 0.75 weight of tale, about 0.9 to 1.3 weight percent of 85% phosphoric acid, about 0.9 to 1.3 weight percent of water, and with the remainder being at least one alcohol having from 1 to 6 carbon atoms.

2. A pretreatment coating in accordance with claim 1 in which the chromate is PbCrO 3. A pretreatment coating in accordance with claim 1 consisting of about 3.3 weight percent of polyvinyl butyral, 2.0 weight percent of lead chromate, 0.5 weight percent of talc, 1.1 weight percent of water, 1.1 weight percent of phosphoric acid, with the remainder being at least one alcohol having from 1 to 6 carbon atoms.

References Cited UNITED STATES PATENTS 2,525,107 10/1950 Whiting et al. 1486.16 2,798,015 7/1957 Lesser 148--6.16 2,798,016 7/1957 Lesser 1486.16 3,325,432 6/ 1967 Kellert et al 260-334 MORRIS LIEBMAN, Primary Examiner.

L. T. JACOBS, Assistant Examiner.

Claims (1)

1. A PRETREATMENT COATING COMPOSITION CONSISTING ESSENTIALLY OF FROM ABOUT 3.0 TO 3.6 WEIGHT PERCENT OF POLYVINYL BUTYRAL, A CHROMATE SALT OF A DIVALENT METAL EQUIVALENT TO BETWEEN ABOUT 1.5 AND 2.25 WEIGHT PERCENT OF LEAD CHROMATE, ABOUT 0.265 TO 0.75 WEIGHT OF TALC, ABOUT 0.9 TO 1.3 WEIGHT PERCNET OF 85% PHOSPHORIC ACID, ABOUT 0.9 TO 1.3 WEIGHT PERCENT OF WATER, AND WITH THE REMAINDER BEING AT LEAST ONE ALCOHOL HAVING FROM 1 TO 6 CARBON ATOMS.