Background of the Invention Coating materials to protect and to improve the surface qualities of objects such as metallic fasteners have enjoyed widespread usage and acceptance, especially in the aerospace field where stringent requirements often exist. Among them are to provide a surface with a low coefficient of friction, to be strongly adherent to the substrate, and to be amenable to the addition of additives for various purposes such as corrosion resistance and coloration.

Numerous products have been developed and sold for these purposes, and have been and remain useful to this day for many important applications. However, especially in the aerospace frame industry, performance requirements for these materials have been made increasingly stringent to the point where existing compositions are unable to meet all of them.

One such requirement relates to coatings for fasteners which are to make an interference fit in a hole in a metal sheet or plate. It is not unusual for a fastener pin to have a diameter as much as 0.00762 or greater centimeters (0.003 or greater inches) larger than the diameter of the hole into which it is to be pressed. Because the solid pin resists compression more than the hole resists expansion, the wall of the hole must be expanded by the pin as it enters the hole creating an elastic deformation of the metal surrounding the hole. This results in a strong preloaded gripping force on the fastener pin which exerts a frictional force on the pin while it is being pressed or driven into the hole. This resistance can, of course, be overcome by the exertion of sufficient push-in force, but these larger forces create problems of their own. As an illustrative example, push-in forces of 907.1848 kg (2,000 pounds) or less to force a 0.635 cm (0.2500 inch) diameter titanium pin into a 0.6223 cm (0.245") hole in an aluminum workpiece have previously been regarded as acceptable. These lower allowable forces required the use of an additional lubricant. Because of problems which these lubricants may create, the requirement has now been changed to an insertion force of 680.3886 kg (1,500 pounds) or less on a 0.3175 cm (1/8") diameter pin, but without an additional lubricant. No available coating known to the inventor herein can meet this new requirement without resorting to the use of supplementary lubricants.

One of the most successful known coatings, in widespread use for decades is shown in United States patent No. 3,979,351, which issued on September 7,1976 to Sekhon. This is the well-known Hi- Kote coating sold by Hi-Shear Corporation of Torrance, California.

This patent is incorporated herein by reference for its teaching of the art, and as one of the most effective existing coatings for the purposes intended by this invention. The instant invention is an improvement over the Sekhon patent. While the Hi-Kote material can meet many requirements without a supplementary lubricant, it cannot meet the 680.3886 kg (1,500 pound) push-in force requirement without the use of a supplementary lubricant. The coatings according to this invention can provide this capacity, without any additional external lubricant.

The most critical requirement is lubricity. The lower the coefficient of friction, the lower will be the required insertion force to overcome the frictional restraining force caused by the interference fit. In the Sekhon patent, lubricity was principally supplied by Teflon which is incorporated into the composition. As stated, the lowest coefficient of friction created by that composition without a supplementary lubricant is not low enough for the newer requirements. In fact, it has been common shop practice to dip a fastener already coated with Hi-Kote material in a liquid lubricant such as a solution of cetyl alcohol. Then the Hi-Kote covered fastener can meet the 680.3886 kg (1,500 lb.) push-in force limitation. A problem with the use of cetyl alcohol is that it has been thought to contribute to corrosion or perhaps wet the sheet surrounding the hole, and sometimes poor paint adhesion. For these reasons objections have arisen to the use of any supplementary lubricant, or of any supplementary lubricant.

It is an object of this invention to provide a solid coating which fulfills the new requirement without the use of a supplementary lubricant.

Of course, other additives to reduce the coefficient of friction of a surface are known. Examples pertinent to this invention are polyolefin such as polyethylene. Additives to reduce coefficient of friction are well-known and include polyethylene. Examples are Slip-Ayd products Nos. SL 31 and SL 50 sold by Daniel Products Company, Inc., of Jersey City, New Jersey.

These products are widely used to increase lubricity of coating compositions, and to create surfaces which because of their lubricity actually resist being polished.

However, this inventor's efforts to substitute polyethylene for polytetrafluoroethylene have proved disappointing, and to the best of his present knowledge a solid film lubricant type coating utilizing only polyethylene cannot be formulated to provide all of the benefits of this invention.

It has come as a surprise to the inventor herein that the inclusion of both polytetrafluoroethylene and polyethylene as additives to the coating improves the lubricity and adherence of the composition so much that the 680.3886 kg (1,500 pound) push-in force objective can be reached without supplementary lubrication, this in a coating which also provides other advantages expected from such a coating. Acting together, synergisticably the combination does achieve the unexpected and unpredictable result of a lesser coefficient of friction without a supplementary lubricant.

Film adhesion to a substrate is an important requirement, not only in the fastener field but in other fields in which a reliable coating is needed. Coatings on fasteners which must be inserted into an undersized hole are generally on the order of about 0.000508 cm (0.0002 inches) to 0.00127 cm (0.0005 inches) thick.

They are tested for adhesion by wrinkling the substrate or by isolating small patches of the coating and attempting to remove them by a designated powers. These tests, while crude, are accepted and are functionally sufficient.

Neither polytetrafluoroethylene nor polyethylene is especially suited for adhesion to a substrate, and the entire composition is formulated to create a film which adhere at least partially to the substrate substances in place. It is surprising that the combination of polyethylene polytetrafluoroethylene, neither of which is well-suited for adhesion, provides a suitably adherent coating.

Brief Description of the Invention A film forming composition according to this invention includes a resin, fillers to accomplish various objectives such as lubricity, corrosion resistance and color, and a solvent to enable the composition to be applied.

The stringent requirements for this coating are best met by a thermosetting resin, which include polyesters, since phenolics, epoxies and polyurethanes. The phenol-formaldehyde resins are preferred, when heat-cured in situ they provide an adherent, chemically-resistant coating. The inherent brittleness of the phenolics may be decreased by flexibilizing it with a variety of resins or plasticizers, such as polybutylene and DOP (dioctylpthalate).

Fillers can be added to provide features such as corrosion resistance, lubricity and color, as will be described below.

According to this invention, lubricity is provided by the inclusion of both polytetrafluoroethylene and a polyolefin, for example, polyethylene, polypropylene, a copolymer of polyethylene and polypropylene, and their mixtures.

A volatile solvent or solvents will be added to the pre-cured resin and filler mixture to enable it to be applied to the substrate by dipping or spraying, after which the coating will be dried and cured.

The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: Brief Description of the Drawings Fig. 1 is an oblique view of a coated fastener according to this invention; Fig. 2 is an axial cross-section of a coated fastener according to this invention; Fig. 3 is a cross-section of a plate having a hole into which the fastener of Fig. 1 is to be pressed.

Detailed Description of the Invention The problem solved by this invention is shown in Figs. 1-3.

A metal workpiece 15 (Fig. 3) has a drilled hole 16 through it that has a diameter 17. A fastener 10 (Fig. 1) with a head 11 and a cylindrical shank 12 is to be pressed into hole 16. The diameter 13 of the shank is, however, larger than diameter 17 of the hole. The force required to insert the larger pin into the smaller hole must overcome the frictional force developed between the pin and the wall. This force is a function of the spring-back force exerted by the expanded wall and the coefficient of friction between the shank and the wall of the hole. Evidently, the push- in force will be reduced as a direct function of reduction of the coefficient of friction.

The coefficient of friction between the bare metal shank and the bare wall of the hole in an aluminum plate or sheet is so large as to obviate the use of a metal-to-metal interference fit.

A surface treatment is required to reduce the coefficient of friction, such as the cited Hi-Kote coating to enable the pin to be inserted into the interference fit hole.

Adhesion of the coating to the surface of the pin is essential. A coating according to this invention provides a film of which at least the base part is so adherent that it cannot be mechanically removed except along with some of the underlying metal of the shank. It thereby provides a reliably slick surface with a coefficient of friction low enough to enable a fastener to be pressed in with remarkably lower forces.

For purposes of illustration, Fig. 2 shows fastener 10 with its metal body surrounded by a coating 20 according to this invention. The coating will have a dimension of thickness 21 established in part by the viscosity of the liquid mixture when it is being applied. Its preferred thickness will usually be on the order of about 0.000762 cm (0.0003 inches) to 0.001016 cm (0.0004 inches).

The film-forming composition, which provides a sufficiently low coefficient of friction, comprises a cured mixture of the following components: a thermosetting resin, fillers, and a solvent.

The resin is a thermosetting type which when cured adheres to the metal substrate and forms a tough outer cladding for the substrate. It resists wrinkling and lift-off removal, and is tolerant of the additives that are generally used. It is temperature resistant, resisting stresses caused by thermal cycling, and is resistant to solvents that are likely to come into contact with it.

The stringent requirements for the coating suggest the use of a thermosetting resin. Families of commercially available resins which are at least potentially useful are polyesters, phenolics, epoxies, and polyurethanes. Phenol-formaldehyde resins condensed to the A state where it is soluble in certain solvents is the preferred resin.

To reduce the brittleness of the cured resin, it may be flexibilized by the addition of known and commercially available resins, the best example of which is polybutylene. Liquid plasticizers can also be used, for example DOP.

The above will provide a suitable coating useful for many purposes, but to accomplish the objectives of this invention, fillers must be provided as follows.

For corrosion resistance one may choose from the class of chromates, borates, and molybdates. Chromates are generally preferred, and the chromates of barium, strontium and zinc are used. Strontium chromate is often preferred because its lesser solubility tends toward a longer life of its function.

To reduce the coefficient of friction of the cured coating, quantities of polytetrafluoroethylene and of an olefin based on either polyethylene or polypropylene selected from the group consisting of polyethylene, polypropylene, a copolymer of polyethylene and polypropylene, and their mixtures are incorporated into the formulation prior to curing.

Should color be desired, for example to make the location of the fastener more visible, or to check the completeness of the coating, colorizing materials may be added as appropriate to the desired color. Solvent-soluble dyes and pigments are suitable.

Aluminum may be used should a metallic color be desired.

The resin and fillers will be incorporated into a solvent carrier. The solvent must be volatile, and be able to dissolve the uncured resin. The thermosetting resin is partially reacted to form a solvent soluble oligomer into which the fillers are stirred and milled to form a paste. Then this paste is dissolved in the solvent. Suitable solvents are aromatic hydrocarbons such as xylene or toluene; alcohol such as methyl, ethyl or a propyl alcohol ketone or methyl isobutyl ketone; or other suitable solvents or their any of their combinations.

After the resin and fillers are mixed in, or dissolved in, the solvents, the coating mixture may be sprayed onto the substrate, or the substrates may be dipped into it. They are then dried at a moderate temperature to evaporate the solvent and create a non-tacky surface. Thereafter the coated article is heated, for example for about one hour at 204.44 OC (400 degrees F), to cure the coating to its final thermoset condition.

The ranges of useful concentrations of the components of the presently-preferred uncured coating formulation are as follows.

The percentages are by weight of the formulation: Phenolic resin 10-20% Polybutylene 3-5% Strontium chromate 1.5-3.0% Aluminum leaf 0-3. 0% Plasticizer 0-1.0% Fluorocarbon polymer 10-40% Polyolefin 1-10% Volatile solvent To make 100% Should corrosion resistance not be required, the strontium chromate may be eliminated, all other components remaining in the same ratio to one another. Similarly, the aluminum may be eliminated or replaced by another pigment to obtain a suitable color.

The presently-preferred compositions, percentages by weight, useful for corrosion resistance and coloration as well as for a low coefficient of friction are as follows: Example Example Example Example I II III IV Phenol-formaldehyde resin 14.80 14.01 12.98 10.64 Polybutylene 4.34 4.06 3.63 3.09 Strontium chromate 2.22 2.10 1.87 1.54 Aluminum leaf 2.00 1.89 1.69 1.44 DOP 0.57 0.69 0.61 0.52 Teflon 15.06 20.30 28.79 39.86 Toluene 20.25 18.91 16.98 14.44 Absolute ethyl alcohol 40.73 38.03 34.03 29.02 Polyethylene-2.00 1.00 2.00 Polypropylene 2.00-1.00- A useful source for the resin is Georgia Pacific BKS 2600.

It is as a resol solution supplied into which the other constituents are added. The other components are readily commercially available from a number of sources.

The examples given above show that various percentages of Teflon in the product can be utilized to provide different degrees of coefficient of friction. The amount of any of the ingredients can be adjusted to suit the objectives of the coating. The numbers, especially of the ranges, are"about". Some relatively small extensions beyond the range, either above or below it, are within the scope of this invention, so long as it is not deleterious to the desired function.

Although the coatings described herein have been described with particular reference to use as coatings for fasteners, the utility is not limited to fasteners, but they may be also be applied to other surfaces requiring a lower coefficient of friction. They are applicable to metals other than titanium, for example to steel and to aluminum.

This invention is not to be limited by the embodiments shown in the drawings and described in the description, which are given by way of example and not of limitation, but only in accordance with the scope of the appended claims.