Zinc Dust-Coated Steel Ligatures

This invention relates to steel ligatures, and particularly to steel strapping, coated with zinc particles, or dust, the particles being bonded to each other and to the surface of the strapping with a resinous composition comprising a mixture of an epoxy resin and a phenolic resin. The invention also relates to a method for applying such coatings to steel ligatures.

The coating of steel to provide protection against corrosion caused by exposure to the elements has been known for many years. Organic resinous films provide protection, but weather resistance is limited, particularly when the film is thin. Pigmentation of the organic resinous film is somewhat beneficial, but weather resistance is still inadequate.

The incorporation of zinc dust into organic resinous films to enhance weather resistance is known, but these coatings, when heavily loaded with zinc, generally lack the flexibility, toughness and abrasion resistance essential to use on steel strapping.

In Lucas U.S. Pat. No. 3,242,001 there is disclosed a steel strapping coated with a heavily loaded zinc dust-containing organic resinous material of particular composition which is durable and flexible and has weather resistance substantially superior to that of prior steel strapping and adequate for most uses. Specifically, the Lucas patent teaches steel strapping coated with a thin and flexible film comprising 80 to 96 parts by weight of finely divided zinc particles, and 4 to 20 parts by weight of a resinous composition comprising a heat-cured oily butadiene polymer having a molecular weight from about 1,000 to about 30,000 and a high molecular weight linear condensation product of epichlorohydrin and bisphenol A having an average molecular weight of approximately 100,000.

While the coated steel strapping of the Lucas invention provides excellent corrosion protection for most uses, it is not satisfactory for uses in which the strapping is to be exposed to severe weathering conditions over an extended period. Some makers of bricks, for example, use steel strapping to make bundles of bricks and then store the bundles out of doors, exposed to the weather, for weeks or months until they are sold and used. Failure of the protective coating under these severe conditions causes discoloration of the bricks by the rusting of the strapping, making the bricks unsatisfactory for many purposes. Failure of the protective coating can also lead ultimately to the weakening and failure of the strapping itself to perform its function.

The use of steel strapping under conditions which expose it to acid is also a severe use. Some fabricators of aluminum ingots, for example, wrap steel strapping around bundles of ingots and then immerse the bundles into an acid pickling bath while suspending them by the strapping. Packets of acid may remain in the narrow spaces between the strapping and the ingot surfaces after the ingots are withdrawn from the acid bath and the prolonged exposure of the coated strapping to this acid may weaken the strapping and ultimately cause failure.

In accordance with one aspect of the instant invention steel ligatures, such steel strapping or wire, are provided which have coated thereon a thin film of a resinous composition containing a high loading of zinc particles (at least about 80% by weight) and comprising a mixture of from about 8 to 65 weight percent of a phenolformaldehyde resin and from about 35 to 92 weight percent of an epoxy resin and particularly a low molecular weight reaction product of epichlorohydrin and bisphenol A.

The resin mixture is unique in that neither the phenolic resin, by itself, nor the epoxy resin, by itself, will product a satisfactory coating when heavily loaded with zinc dust, the phenolic being too brittle for satisfactory adhesion on a flexible ligature and the epoxy providing poor corrosion resistance. Furthermore, the substitution of a urea-formaldehyde resin or a melamine-formaldehyde resin for the phenol-formaldehyde resin does not provide a coating with equivalent resistance to corrosion.

The method of this invention involves the application of a zinc dust-containing solution of the resin composition in an organic solvent to the surface of the ligature as a thin film followed by heating to drive off the solvent. Further heating to cure the resin composition may be employed, if desired, but is not essential.

Suitable coatings in accordance with this invention require an extremely finely divided zinc dust having an average particle size of 10 microns, or less, and preferably less than 5 microns. In the coatings, zinc dust comprises from about 80 to about 96 weight percent of the coating, the remainder being binder. Zinc dust in concentrations below about 80 weight percent are so encased in the resinous binder that they are unable to provide any appreciable galvanic effect. Coatings having zinc dust in concentrations above about 96 weight percent have so little binder that they lack the necessary toughness to survive as a protective coating in normal ligature usage. The preferred concentration of zinc dust in the coatings is from about 83 to about 95 weight percent.

The phenolic resin used in the binder composition is an alkali-catalyzed phenol-formaldehyde condensation product of low molecular weight and preferably having an average molecular weight in the range of about 800 to about 2000. It is an A-stage product (as described at pages 117-8 in "The Chemistry of Commercial plastics" by Reginald L. Wakeman - Reinhold Publishing Corp., 1947) which is soluble in alcohols and ketones.

As stated above, the binder composition may contain as little as 8 weight percent or as much as 65 weight percent of the phenolic resin. However, for optimum results the concentration of the phenolic resin should be in the range of from about 30 to about 40 weight percent of the binder composition.

The epoxy resin component is preferably a condensation product of epichlorohydrin and bisphenol A having an average molecular weight in the range of about 1200 to about 5000 and most preferably in the range of about 1500 to about 2700.

The resinous components of the binder composition are intimately blended by solution in a suitable solvent. HIgh boiling solvents and solvent systems are preferred, the preferred solvents being cellosolve acetate, carbitol and mixtures thereof. Ketones, such as methyl ethyl ketone and methyl isobutyl ketone are also suitable solvents, but their use involves a flammability hazard.

The coating composition used in this invention is generally applied to the surface of the ligature in a concentration of about 60 to 80 weight percent of solids and about 20 to 40 weight percent of solvent. Preferably the percentage of solvent is from about 22 to about 30 weight percent. The coating composition may be applied by brushing or spraying but is preferably applied by passing the ligature through a reservoir containing the coating composition and then wiping off the excess liquid. The coating composition is generally applied to the ligature in thicknesses from about 0.6 to about 1.2 mils to produce a final coating after solidification from about 0.3 to about 0.6 mils.

After application of the coating composition the ligature is passed through one or more ovens for heating to drive off the solvent and, if desired, to cure the resins. Passage of the ligature through the oven or ovens is preferably in a vertical direction to avoid the possibility of producing an uneven coating due to sag of the coating composition by gravitational effect before solidification.

The heating of the ligature coated with the liquid coating composition drives off the solvent and effects at least a partial cure of the resinous composition. It is a distinctive feature of this invention, however, that satisfactory coatings are produced even under mild heating conditions under which curing is so minimal that the coatings are soluble in solvent systems similar to those from which the coatings are formed. Satisfactory coatings have been produced, for example, at temperatures as low as 300.degree.F. and at heating times at such low temperatures as low as 20 seconds.

When a fully cured coating is desired which is insoluble in solvents, heating at higher temperatures, of the order of 600.degree.F. to 800.degree.F. are employed. Heating times at such higher temperatures may be as low as 15 seconds but may, if desired, be extended to 90 seconds, or more.

To determine the effect of different levels of phenolic resin and the effect of different curing temperatures and times on the resistance of coated steel strapping to salt spray exposure, a series of tests were run. In each of the tests the predominant resinous component in the coating composition was a condensation product of epichlorohydrin and bisphenol A having an average molecular weight in the range of 2000 to 2700. The coating compositions comprised 86 weight percent of zinc dust and 14 weight percent of the resinous binder composition. It was applied to short lengths of steel strapping 1/2 inch wide .times. 0.020 inches thick in amounts sufficient to produce a coating of about 0.4 mil thickness. On each sample an "X" mark was scratched through the coating to the bare metal with a carbide scribe. Resistance to rusting at the "X" mark was an indication of galvanic protection by the zinc since the purely mechanical protection of an organic coating seal does not protect underlying steel against corrosion once the seal is broken by a scratch.

In Examples 1 to 3, the binder consisted of 100% of epoxy resin and no phenolic resin; in Examples 4 to 12, the binder consisted of 92% of epoxy resin and 8% of phenolic resin having a molecular weight in the range of 800 to 1200; in Examples 13 to 21, the binder consisted of 83% of epoxy resin and 17% of the same phenolic resin; and in Examples 22 to 30, the binder consisted of 65% of epoxy resin and 35% of the same phenolic resin.

A series of steel strapping samples were coated with a coating composition comprising 86 weight percent of zinc dust and 14 weight percent of a binder comprising 35 weight percent of phenolic resin and 65 weight percent of a condensation product of epichlorohydrin and bisphenol A having an average molecular weight of 1500 to 2000. The coatings were generally similar to those described in connection with Example 1 to 30. The heating temperatures (.degree.F.), times (seconds), and the salt spray test results were as follows:

Another series of steel strapping samples were coated with the coating composition of Examples 31 to 39. The heating temperatures (.degree.F.), times (seconds), and the salt spray test results were as follows:

For comparison purposes another series of steel strapping samples were coated with a coating composition comprising as the resinous ingredient a mixture of 15 weight percent of a melamine-formaldehyde condensation product and 85 weight percent of the low molecular weight epoxy resin of Examples 1 to 30. The heating temperatures (.degree.F.), times (seconds), and the salt spray test results were as follows:

For comparison purposes another series of steel strapping samples were coated with a coating composition comprising as the resinous ingredient a mixture of 79 weight percent of the low molecular weight epoxy resin of Examples 31 to 39 and 21 weight percent of a flexible ureaformaldehyde condensation product. The heating temperatures (F.degree..), times (seconds), and the salt spray test results were as follows:

This invention has been described with respect to a specific embodiment thereof, however, it will be apparent to those skilled in the art that numerous modifications and variations may be effected without departing from the spirit and scope of this invention.