Coating Composition

One hundred wt. parts of polyol silicate B, 4 wt. parts of strontium chromate (jet milled), 50 wt. parts of zinc flake (minus 325 mesh and made by ball milling zinc dust in aromatic solvent dispersion), 100 wt. parts of toluene and 5 wt. parts of mixed nitropropane solvent were mixed to form a coating composition.

This coating composition was stable for several months without hard settling yet gave a good hard adhesive coating which had exceptional life in the ASTM B117 salt fog test even at a 0.2-mil, dry-film thickness and provided excellent weld-thru properties. In addition, exposure (south with tipping at a 45.degree. angle) to atmospheric conditions (the Adrian Michigan, atmosphere) for 10 months produced no rusting of any kind to a 0.1-mil dry film on scored, cold-rolled clean steel.

The polyol silicate A, which leaves no residue other than silica, since it had no residual polyol such as polyvinyl butyral in it, was used to make a composition otherwise the same A as Example 1. The salt fog test of this composition showed the desirability of using the residual polyol in the preparation of the polyol silicate.

It was found that the more polyvinyl butyral in the polyol silicate the better the salt fog antirusting characteristics (with relatively high strontium chromate content) of the zinc flake coating composition. This shows the value of the residual polyol; however, it was noted that if the residual polyol content (polyvinyl butyral content) of the dry-cured film was much greater than about 7% (e.g. 20%) the chemical, and solvent insolubility resistance was greatly impaired. This was because such coating was soluble in certain solvents. At lower ratios the coating was insoluble. While the insolubility of the coating is important in some applications, it is unimportant in others, thus the range of residual polyol can be adjusted to fit the needs of the final use of the coatings. It was also noted that the degree of reaction of the polyvinyl butyral and silicate was dependent on the amount of strong acid catalyst e.g. HCl present. At high levels of catalyst the polyvinyl butyral and silicate may over condense to gel at high temperatures, possibly thru excessive crosslinking, however prior to gelling the acid can be neutralized by addition of a basic filler so that the reaction is stopped. It was also noted that sufficient alcohol or monofunctional OH compound must be present to also inhibit gelling.

It was found that zinc chromate was not nearly as effective as strontium chromate in the coating composition of the invention. A coating composition using basic zinc chromate was substantially poorer in preventing rust than a comparable coating composition without any chromate composition.

In the absence of strontium chromate or other chromate composition in the coating composition, a thin film in the ASTM B117 salt fog test starts rusting in 1 or 2 days but with only a 0.2-mil thickness of the coating composition containing strontium chromate there was no rusting after several months.

Using polyol silicate B, a series of coating compositions were prepared identical to Example 1, except that 6, 8, 10, 12, 15, 20, and 40 wt. parts of strontium chromate were used instead of the 4 wt. parts used in Example 1. The compositions provided coatings with excellent rust prevention, particularly as thin film for which the higher chromate content is preferred. In another series the zinc flake content was doubled with excellent results. Much less zinc flake is necessary than similar coatings using zinc dust. If the strontium chromate was left out of the coating composition, rusting occurred rapidly in ASTM B117 salt fog test; the coating failed in as little as 2 days when it was 0.05 to 0.1 mil dry film thickness on clean cold rolled steel. The composition, as a coating of such thickness, protected for more than 2 months if the strontium chromate was present.

Coatings of compositions of this invention prepared with zinc flake were directly compared with those made with zinc dust. Such coating on steel panels were exposed to the atmospheric and to the ASTM B117 salt fog exposure test. It was found that on a thickness basis zinc flake is about 3 times as protective as zinc dust. Coating compositions with zinc flake or zinc dust do not perform well as thin coatings unless the chromate composition is also present. Zinc flake films are highly protective down to 0.05 to 0.2 mil thickness whereas the zinc dust films need to be about greater than 0.4 mil thickness to be as protective. Zinc flake films are less flexible and hence will not undergo forming as well as zinc dust coatings at the same thicknesses. Zinc flake gives superior coverage and works best at low levels (i.e., 1-5 lbs. of zinc/gal. of paint, i.e., of coating composition), whereas zinc dust is desirably present in a content of at least 3 lbs./gal. and appears to work much better when an inert extending filler, such as iron phosphide, is also present in the film.

It is not clearly understood how these three ingredients of my coating composition cooperate to make such a highly protective coating, even as a very thin coat. It is thought that the strontium chromate or other chromate composition in the presence of metallic zinc and my polyol silicate, functions in a dual role--that of creating a cell that has excellent electrical contact with the steel surface to sacrifice the zinc as necessary, to protect the steel from rusting, and even more important to passivate the steel (or other ferrous) surface so that the electrical flow is just sufficient to protect the steel surface so that the coating functions much longer than conventional coatings.

The coatings of this invention can be applied over clean steel, cold rolled steel, etched steel, pickled steel, chronic-zinc passivated steel, sand, shot or water blasted steel, over or under inorganic chromate inhibitive compositions including those containing polyol silicate with and without metallic zinc, or over coatings from solutions of chromic acid in collodial silica solutions containing strontium or calcium carbonate, oxalate or oxide. The coating compositions of the invention are particularly useful to retard rusting by a touch-up painting of old zinc-coated surfaces, and by painting over inhibited surfaces after blasting. The casting adheres well to epoxy zinc coatings, inorganic zinc silicate coating of all kinds and even over porous ceramic coated steel surfaces.

To show the effect of various contents of strontium chromate in the coating compositions of this invention as cured films, coating compositions were made in which the metallic zinc content and the content of the other ingredients (except strontium chromate) of the composition, to form a cured dry film, were kept constant. Each composition, being a single-package paint, was made by high shearing into 200 wt. parts of the polyol silicate B in the following order: 400 wt. parts of iron phosphide (average particle size of 2-3 microns; 50 wt. parts of zinc oxide (fine pigment grade) 5 wt. parts of finely divided (1 micron) plately talc; 200 wt. parts of dry toluene. Eight compositions were prepared. Strontium chromate was not added to the first composition. It was added in the amount of 1, 5, 10, 20, 40, 80, and 160 wt. parts to the second through eighth compositions. It was noted that all were paintable excpet the eighth composition to which was added 160 wt. parts of strontium chromate. That composition was too thick to paint.

The first through eight compositions as cured dry films contained, calculated from the coating compositions, on a weight percent basis: zinc dust in the amount of 29, 29, 28.8, 28.6, 28.2, 27.6, 27.4, 26.0 and 23.6, respectively; zinc oxide in the amount of 7.3, 7.2, 7.2, 7.2, 7.0, 6.8, 6.5 and 5.9, respectively; strontium chromate in the amount of 0, 0.14, 0.7, 1.4, 2.8, 5.5, 10.4 and 18.8, respectively, calculated SiO.sub.2 (from polyol silicate) in the amount of 3.5, 3.5, 3.4, 3.4, 3.3, 3.1 and 2.8, respectively; polyvinyl butyral in the amount of 1.4, 1.4, 1.4, 1.4, 1.4, 1.4, 1.3, and 1.2, respectively; talc in the amount of 0.7 down to 0.6, and Ferrophos in the amount of 58.1, 58.0, 57.6, 57.6, 57.2, 56.4, 52 and 47.1, respectively.

The first 7 of these 8 compositions were used to provide a scored film of 0.8-mil thickness on clean 22-gage steel panel. These panels were subjected to the ASTM B117 salt fog test until rust appeared. These coated panels of the first 7 compositions started to rust at 24, 36, 144, 990, 2,100, 3,100, and 4,200 hours, respectively.

Some of these compositions appear to age affer a substantial storage time. As a result, for certain compositions of the present invention it is desirable to furnish the composition to the user in the form of a two-package system mentioned earlier. In that case, for the foregoing compositions of this example the zinc dust is in one package and it is added to the rest of the composition, furnished as a second package, just prior to use. This prevents excess passivation of the metal zinc dust.

In a similar series of formulations the zinc dust content was decreased to 14%, keeping the other ingredients in the same relative proportions, with salt fog test results similar--when the coating thickness was 1 mil. At lower amounts of zinc dust than 10% in the cured film the results were less desirable. At higher levels of zinc dust in the cured film (46%), a 1-mil film on a cold rolled clean (not blasted) steel panel was still rust-free after 6,000 hours. A 0.5 mil film (with 10.4% strontium chromate) lasted over 3,000 hours with no rusting even in the scored areas. At a film thickness of less than 0.5 mil, the salt fog protection was less effective. Outdoor exposure of a 0.7 mil film for a period of one year showed no rusting, even in the scored areas. It was also noted that the position of the panel in the salt fog tester had no effect on the performance that was equivalent whether it was vertical, or horizontal, or at an angle throughout its surface. Ordinary inorganic zinc films on the other hand change drastically with position in the salt fog apparatus (upper and lower surfaces).

It was noted that at the higher level of strontium chromate (10.4%) very thick (6-12-mil) coatings did not mud-crack. They were not nearly as hard as the thinner coatings or those coatings having lower amounts of the strontium chromate and therefore, had poorer abrasion resistance. It was concluded that, if thick coatings are applied, a lower content of strontium chromate would preferably be used (about 1.5-3% by weight based on the cured dry film), whereas, for very thin coatings (e.g., 0.5 mil thickness) a higher strontium chromate content is preferred as better performance is obtained and the hardness is not reduced.

It was further found in similar experiments that the talc was not necessary or advisable in some formulations. Zinc oxide had an improving effect on performance particularly when zinc chromate was used as the chromate composition, and in that case a substantial amount of zinc oxide was required to obtain a long period of protection in the salt fog exposure.

The polyvinyl butyral, as an illustrative residual polyol, had a definite promoting effect on the corrosion inhibiting effect, actually extending the period of time before rusting started.

The following example illustrates the rust protection provided by the compositions of the invention containing zinc chromate and zinc oxide as the chromate composition. The illustrative composition is compared with a composition that is identical in weight percent of ingredients except that it does not contain any zinc chromate. The composition is also compared with one in which the ingredients are the same except to contain basic zinc chromate instead of zinc chromate.

Three coating compositions were prepared for comparison. Each contained 200 wt. parts of zinc oxide, 400 wt. parts of polyol silicate B, 100 wt. parts of zinc dust (3 micron average particle size and containing 5% zinc oxide), 600 wt. parts of Ferrophos (2-3 micron particle size), and 200 wt. parts of dry toluene. In addition, the second composition contained 40 wt. parts of basic zinc chromate while the third composition contained instead of basic zinc chromate, 40 wt. parts of zinc chromate (yellow).

These three compositions were used to provide 1.8 mil scored films on steel that were then subjected to the ASTM salt fog test mentioned above. The first rust was noticed after 24 hours when using the films obtained from the first and second compositions, whereas, the first rust was noticed only after 2000 hours when using the film from the third composition, namely, that composition containing zinc chromate. The cured films from these three compositions contained, on a calculated weight basis, about 20% zinc oxide, about 5% SiO.sub.2 (from polyol silicate B), about 2% polyvinyl butyral (also from polyol silicate B) about 50% Ferrophos, and about 10% zinc dust. The second and third composition contained about 4% basic zinc chromate and zinc chromate, respectively.

100 grams of polyol silicate C, 40 g. 2-nitro-propane 10 g. strontium chromate, jet milled pigment grade, and 200 g. finely divided Ferrophos (less than 5 micron) were high sheared together and 200 g. more of polyol silicate C. added. 200 g. zinc dust, 5 microns particle diameter, and 2 g. of pyrogenic silica (Cabosil) were blended in to produce a single package, non-gassing, non-hard settling paint having excellent self-curing properties when applied as a coating on steel surfaces as a 1 mil dry film protecting from rust and rust undercutting for a period of 3 months in the ASTM B 117 salt fog test.

The polyvinyl butyral residue content of the film is preferably over 0.1% and performs well up to 8%.

The metallic zinc content is not limited to low value when used with polyol silicate and chromate composition. The higher the metallic zinc content, up to about 90%, the better the performance, particularly in very thin films or for those coatings where exceptional long life is desired. For instance a composition containing 91% by weight in a 4-mil dry-cured coating) of zinc and 4% by weight of strontium chromate gave a coating which lasted without rusting for over 2 years in the ASTM B117 salt fog test. As a 0.5-mil coating it was at least 5 times as effective when strontium chromate is present than the composition without it. These compositions with strontium chromate are exceptionally protective against rusting even at low film thickness levels.

300 ml. Polyol silicate C, 450 g. Ferrophos 100 g. zinc dust and 30 ml of 2-nitropropane were briefly high sheared together to make a single component paint and a panel coated, cured and scored to test in ASTM B 117 salt fog. The 1 mil thick coating failed in 48 hours. It was concluded that such a low level of zinc could never be used in actual paint for preventing rusting of steel without the aid of an additional synergistic inhibitor. This is confirmed in the remainder of the example in which increasing amounts of barium metaborate were added with highly increased salt fog resistance. Example 7a was identical to 7 except that an additonal 10 g. barium metaborate was added. A coating and panel made identically to 7 passed the ASTM B 117 salt fog test for only 8 days. Example 7b was identical to 7 except 20 g. barium metaborate was added and failed the salt fog in 12 days. With identical tests using respectively 30, 50 and 80 g. barium metaborate in Ex. 7 the ASTM salt fog time to first rust was 15 days, 21 days, and 30 days exposure to first rust. In example using the 80 g. barium metaborate, was further added 10 g. strontium chromate and similar films showed a reduction of white zinc rust over those with no strontium chromate added with also greater protection against red rust. There was a synergistic effect.

This series demonstrates the effect of zinc phosphate as an inhibitor in these zinc-polyol silicate compositions. 300 g. of polyol silicate C, 40 g. 2-nitropropane, 100 g. zinc dust and 450 g. Ferrophos (Fe.sub.2 P) were high sheared together to form a single component paint which when applied as a 1 mil dry film coating to a clean steel panel produced a hard coating, which when scored and exposed to ASTM B 117 salt fog test, lasted only 2 days until first red rust. In similar experiments using identical ingredients but with increasing additions of finely divided zinc phosphate the 1 mil coating gave the following salt fog test results: 10 g. additional zinc phosphate-2 days to first red rust; 20 g.-3 days; 30g-10 days;50 g. 16 days, 80 g. 39 days. It was apparent that substantial amounts of zinc phosphate must be present to improve protection, but improvement is very apparant even with the reduction in volume of zinc in the dry film. At the high levels of zinc phosphate in the cured film, strontium chromate improved the inhibition synergistically.

Zinc phosphate was evaluated in larger amounts in the cured film. 200 g. polyol silicate C and 120 g zinc phosphate were high sheared together and applied as a coating at a thickness of 1 mil (dry film) to a clean cold rolled steel panel. After curing and scoring, the panel was exposed to ASTM salt fog B 117 test-failing in less than one day. In identical experiments but using increasing amounts of zinc dust in the formulation and applying the 1 mil film and testing in ASTM B 117 gave the following results to first red rust: 10 g additional zinc dust-3 days; 20 g. zinc dust 11 days; 30 g. zinc dust 23 days; 50 g. zinc dust 41 days; 100 zinc dust 98 days, and 150 zinc dust-test is still going after 128 days. It was also noted that no white zinc rust appeared in the zinc phosphate-containing coatings as it did in the barium metaborate containing inhibitor, exposure tests.

In other experiments small amounts of kaolin, and talc were added to give better suspension and smoother coatings with improved results. The presence of even very small amounts of pyrogenic silica (Cabosil) prevented hard settling of either the pigments or the zinc. A compilation of a large number of these tests revealed that a minimum of about 40% to 50% zinc in the cured film gives about as good a life test when the proper inhibitor is present as films having 80% or more zinc without the inhibitors. Strontium chromate improves the performance of other red rust inhibitors and visa versa many of the other inhibitors improve the performance of strontium chromate inhibitor in the polyol silicate zinc paint composition. The barium metaborate, even in amounts much lower than optimum for inhibitive performance in these formulations with zinc and polyol silicates, improved suspension, and inhibited gassing also. Zinc phosphate, in large amounts, is a better antirust. The zinc phosphate was not so useful in small amounts unless other inhibitors were also present.

200 g. polyol silicate D concentrate, 400 g. toluene and 18 g. polyvinyl butyral were heated to 90.degree. C. and 1.7 ml. of 40% zinc chloride in 2-ethoxy ethanol added. After cooling, 300 ml of this binder was blended with 1 g. finely divided chromium oxide green pigment, 20 g. strontium chromate, 1 g. pyrogenic silica, 150 g. zinc flake and 50 g. 2-nitropropane. Upon application, the coating dried to touch in about 1 minute, and cured very hard in a few hours. Another coating was made by brushing the steel to apply a very thin coating filled with voids and allowed to cure. The coating was less than 0.1 mil was extremely flexible and could be formed; it lasted for 3 months in the ASTM B 117 salt fog test without red rust even in the voids. It was apparent that a synergistic highly protective action was taking place by the proper use of the inhibitors with the zinc.

1 lb. Ferrophos, 3/4 lb. barium metaborate, 1.2 lbs. polyol silicate D, 1 lb. zinc dust, 50 ml. 2-ethanol acetate 40 ml. of 2-nitropropane, and 2 and a half g. pyrogenic silica (Cabosil) were blended to yield a stable non settling single component paint which performed exceptionally well in high humidity (100%) for a period of 4 months. In a similar experiment 30 g. of strontium chromate pigment was added with improved inhibitive (to red rust) synergistic action, particularly when the film was exposed to ASTM Salt Fog tests.

200 g. polyol silicate E, 200 g. xylene, 100 zinc flake, 80 barium metaborate, 2 g. pyrogenic silica and 30 mil. 2-nitropropane were blended together to form a stable paint, having excellent protective properties on outside exposure, at film thickness of only 0.3 mil.

When Calcium borosilicate compositions known as Halox CW 221 and CW 22 were added in place of the barium metaborate similar inhibitive action was noted.

500 g. Polyol silicate D concentrate which was especially prepared of ethylene glycol/Ethyl Silicate 40 of 1.2/3 and using 3 mil. concentrated HCL 37%) as catalyst was mixed with 1000 toluene and 35 g. polyvinyl butyral added. This blend was heated to 80.degree. C. and 5 ml. of a 40 wt.% solution of zinc chloride in 2-ethoxy ethanol added. After cooling this blend, 100 g. strontium chromate, 2.5 g. chrom oxide green, and 200 mll 2-nitropropane were high sheared together. 770 g. of zinc flake and 2 g. pyrogenic silica was added. This single component paint was stable over several months giving good fast hardness when applied and giving superior protection life to ferrous surfaces. In a similar experiment 8% by weight of binder of clay (air floated kaolin) was added to the above mix with good performance results.

To demonstrate the ultility of a two package system comprising stable blends of the polyol silicate vehicle and suspending agent and inhibiting fillers were prepared as follows: 44 g. air floated kaolin clay, 43 ml. alcohol and 30 ml. of polyol silicate were high sheared and 35 g. strontium chromate (jet milled pigment grade) and 30 ml 2-nitro propane sheared in; 700 ml. more of polyol silicate C was added to form a non hard-settling slurry for a 2 package paint -- to which various amounts of zinc flake or zinc dust could be added as desired before use. This allows shipment of a ready to use extremely stable vehicle to which heavier zinc dust or flake can be compounded or stirred in at will for whatever desired usage demanded. Similarly in an identical mix an additional 1000 g. of Ferrophos was sheared into the mix to produce one package of a two package system to which zinc dust or zinc flake or other materials can be added prior to use. In another system dispersions of either the barium metaborate inhibitor or the zinc phosphate inhibitor were found to be stable, in the polyol silicate vehicles, with or without the Ferrophos pigment. The advantage here is the stability and versatility in the amount and type of zinc pigment added for a particular formulation for coating application. Obviously these zinc free coating packages can be used with other fillers, or as is as a ceramic top coat over the base zinc coat, a system particularly useful for exceptionally long life protection in immersion service such as in fuel tanks or ballast -salt brine tanks where porous but protective coatings are important.

Polyol silicate vehicles using less polar solvents meeting Rule 66 requirements were prepared by heating 700 ml. of polyol silicate concentrate C having about 25% silica with 400 m. 2-nitropropane, 400 ml. toluene and 600 ml of high flash naphtha, to about 80.degree. C. and adding 60 g. polyvinyl butyral. The resultant binder was compounded with fillers similar to those in preceding examples with similar results.

Another polyol silicate vehicle was prepared by mixing 700 ml polyol silicate concentrate C, 300 ml 2-ethoxy ethanol acetate 400 toluene, 200 ml. of 2-nitropropane and 500 ml high flash naphtha, heating to 80.degree. C. and incorporating therein 16 g polyvinyl butyral. The resulting binder meeting Rule 66 requirements performed nearly identically as the other binders in the examples of this invention. Other solvents include ethyl benzene, cyclohexane, butyl cellosolve acetate, alcohol methyl ketones such as methyl butyl or amyl ketone, and other hydrocarbons such as naphthol spirits.

This example demonstrates improvements in red rust inhibiting activity of the polyol silicate-zinc-strontium chromate-Ferrophos-vehicle coating by high shearing the strontium chromate, Ferrophos and vehicle to obtain a more intimate dispersion and then adding the balance of the polyol silicate and zinc flake without shear to prevent its breakup. These data show that the strontium chromate is most efficient in its rust preventative action if intimately broken up by the sharp Ferrophos particles during mixing whereas the zinc flake is most efficient in rust protection if not high sheared to cause breaking up of the flat platlets of zinc.

Thus 100 g. polyol silicate C, 25 g. strontium chromate and 600 g. Ferrophos were high sheared together to obtain an intimite dispersion. Varying amounts of zinc flake and 200 g. more polyol silicate C were then stirred in carefully as follows: 17-a. 40 g. zinc flake to 550 g of the polyol silicate mix; giving a salt fog (ASTM B 117) test to first rust of about 14 days for a 1 mil dry film; 17b. 80 g. zinc flake and 17c -- 120 g. zinc flake was added in each case to a 550 g. portion of the polyol silicate sheared mix to produce 1 mil coatings which respectively lasted over 31/2 months and are still aging in the salt fog without rust. The zinc content was calculated at only 17 and 27% zinc based on solids in the coating.

When mixing was not carried out as above the 1 mil coatings lasted about 1 week in the ASTM B 117 salt fog test.

It was also observed that minor amounts of organic acids such as benzoic greatly deteriorates the inhibitive effect of the chromates. If the zinc flake has an acid lubricant on its surface it should either be neutralized or a buffering pigment such as barium metaborate added to keep the coating from rusting prematurely. EXAMPLE 16

200 g. polyol silicate E, 200 g. xylene, 100 g. zinc flake, 80 g. barium metaborate, and 8 g. nitrobenzene were blended together. 2 g. pyrogenic silica (Cabosil) was blended in to make a stable, non-hard-settling single component paint having excellent protection for cold rolled steel surfaces.

In a similar test the addition of 10 g. strontium to the above formula had a synergistic protective effect on preventing red rust. In this and other tests it was noted that generally the barium metaborate and strontium chromate were highly synergistic in antirust protection with the zinc-polyol silicate system, whereas small amounts of zinc phosphate with the strontium chromate may even have a deleterious effect on rust inhibition, a although working very well alone. The calcium borosilicate inhibitor was not as effective as the strontium chromate, or barium metaborate. The zinc chloride-strontium chromatechrom oxide co-inhibitor was touchy and deteriorated on aging and may have to be applied as a 2 pkg.

The red lead-calcium carbonate co-inhibitor paint was formulated as follows: 300 g. polyol silicate E, 300 g. finely divided dry calcium carbonate, 25 g. neutral talc, 500 g. zinc dust and 50 g. red lead oxide were blended together and applied as a one mil film to cold rolled steel panels; the film protected for a period of 31/2 months without red rust whereas if the red lead were left out the coating lasted only 11/2 months, and if the calcium carbonate was omitted lasted only 1 month in the ASTM B 117 salt fog test. The red lead did not appear compatable with the strontium and calcium chromate inhibitors and caused them to fail.

The foregoing description has been presented solely for the purpose of illustration and not by way of limitation of the invention because the latter is limited only by the claims that follow.