Background of the Invention

This invention relates to corrosion resistant fasteners and more particularly relates to compositely coated fasteners.

Typical drill screws are widely used, for example, in automotive applications for attachment of a variety of fixtures to sheet metal and similar panels. Particularly in such automotive applications, corrosion resistance is an important requirement of such screws. These screws have been successfully coated with decorative and protective pigmented coating coating such as polymer paint, and particularly corrosion resistant screws have been developed in which epoxy coatings are applied by electrodeposition on typical metal screws such as galvanized steel or aluminum core screws.

In accordance with this invention, these polymeric coated screws are provided with further improved corrosion resistance.

Summary of the Invention

In accordance with the present invention, corrosion resistant fasteners and similar articles are compositely coated in which an epoxy base coating is applied on an inner metallic core and a top coating is applied on the epoxy coating, with the composition of the top coating comprising ester of hydrocarbon fatty acid. The epoxy coat is preferably applied on the metallic core by cathodic electrodeposition.

In preferred embodiments, the hydrocarbon fatty acid ester of the top coat is a formulation comprising a polymer of ethylene and propylene in hydrocarbon solvent. Particularly preferred top coat formulations additionally include a salt of sulfamic acid or a derivative such as cyclohexane sulfamic acid and/or salts of barium such as barium carbonate and barium

sulfate. The composite coatings provide improvement in corrosion resistance of the fasteners.

Detailed Description

In accordance with the present invention, a variety of metallic articles, for example, fasteners such as screws and nails, can be compositely coated to achieve particularly effective corrosion resistance. Broadly, metallic articles, such as a fastener core typically fabricated from steel, zinc or aluminum is first coated by applying an epoxy primer or base coat to the metallic core. In a preferred base coating procedure, an epoxy paint formulation is applied to the metallic core by electrodeposition. Suitable epoxy paint formulations for such electrodeposition are commercially available, for example, from PPG Industries. Best corrosion protection has been obtained by cathodic electrodeposition of epoxy formulations on the metallic fastener core. The cathodic electrodeposition process is conducted by immersing the metallic part in the epoxy paint formulation and then placed in contact with the negatively charged electrode (cathode) . When high voltage DC power is applied, positively charged epoxy paint ions migrate and attach to the negatively charged electrode and metallic part.

Particularly suitable epoxy paint formulations for cathode electrodeposition are commercially available, such as those formulations supplied by PPG Industries in grades, for example, representatively designated epoxy CR-500. These epoxy paints are formulated with typical isocyanate crosslinking agents and the epoxy base is solubilized by attachment to amine groups and reacted with acetic acid. The cathodic electrodeposition results in positively charged epoxy paint ions reacting with hydroxyl ions at the negatively charged electrode and metallic parts to form insoluble amide while acetic acid is generated at the positive anode. Following the electrodeposition, the epoxy coated parts are rinsed to remove non-deposited and water-soluble paint residues. Thereafter,

the coated parts are placed in a cure oven for sufficient time at an elevated temperature, for example in the range 350-400°F, to crosslink and cure the epoxy coating on the parts. The thickness of the epoxy base coating can be, for example, in a range of apporoximately 0.0005-0.001 in. when applied on typical screw cores.

Following the cure, a top coating or finish coating having a composition including ester of hydrocarbon fatty acid is applied over the epoxy coating, which results in particularly effective resistance to corrosion of the parts even in severely corrosive conditions such as exposure to salt spray. Preferably, the finish coating is applied at a temperature in the range of approximately 100-150 ^β F during cooling of the parts from the epoxy curing operation, after which the finish coated parts stand in ambient air for 3 to 5 minutes. Thereafter, any excess finish coating formulation is removed from the parts, for example by centrifuging. Suitable hydrocarbon fatty acid ester for the top coating includes formulations of polymeric ester, such as polymers of ethylene, which is applied in a conventional dip and spin coating application. Particularly uniform coating and good adhesion to the epoxy undercoat has been achieved in dip and spin coating of polymeric esters derived from ethylene/propylene based terpolymer resins for example, the hydrocarbon fatty acid ester formulations commercially supplied by Mitchell Bradford International Corp. under the tradename RUST PEL such as grades designated Rust Pel 47, 51 and 52. Similar hydrocarbon fatty acid ester formulations are commercially supplied by Cortec Corporation of St. Paul, Minnesota, under the tradename CORTEC in a grade designated VCI 368. These hydrocarbon fatty acid ester formulations include esters of ethylene/propylene based terpolymer resins in hydrocarbon solvent which evaporates slowly at room temperature which promotes very complete coverage of the epoxy undercoat and a self-healing coverage action tending to cover any minute gaps in the top coating prior to complete drying and solvent removal.

Best results in adherence to the epoxy base coat and

corrosion resistance have been obtained with a top coat resulting from application of Rust Pel 51 and Rust Pel 52 generally as ester of ethylene/propylene based terpolymer (the third constituent believed to be diene such as 1,4-hexadiene) formulated in Stoddard solvent mineral spirits. The slow evaporation rate of Stoddard solvent is characterized by a flash point of 100 ^β F with autoignition at 450*F and a composition which boils (90%) below 375 ^β F. The Rust Pel 51 and 52 formulations also contain salts of sulfamic acid and small amounts of barium carbonate and barium sulfate which are additives believed to retard the drying rate of the solvent. The formulations can be blended or thinned with additional solvent to achieve modification of the specific gravity and viscosity for thorough coating as governed by the configuration of the part coated.

The excellent wettability of the Stoddard solvent- polymeric ester formulation evidenced in the Rust Pel 51 formulation applied over the epoxy coating is further characterized by contact angle of the wet formulation with the epoxy coating substrate, achieving a contact angle of zero indicating complete wettability and coverage of the undercoat. In contrast, a water based acrylic latex formulation commercially supplied by Cortec in a grade designated VCI 376 tends to bead up on equivalent cured epoxy coated parts exemplifying a high contact angle and poor wettability in coverage; furthermore, the dried acrylic latex readily flaked off the epoxy undercoat and was completely unsatisfactory as a finish or top coating.

EXAMPLE

The following table reflects the particularly enhanced corrosion resistance provided by the polymeric hydrocarbon ester top coat in accordance with the invention in comparison with the typical acrylic latex similarly applied in a conventional dip and spin application on identically cured and cathodic electrodeposition of epoxy coated screws having a

steel 1/4-20 x 1 in. hex head core configuration, under standard salt spray corrosion testing according to ASTM B117. The commercially obtained Rust Pel 51 polymeric ester formulation had a specific gravity in the range 0.0-0.82 as determined by the weight ratio of equal volumes of Rust Pel 51 and distilled water. The commercial Rust Pel 51 formulation was thinned with additional Stoddard solvent mineral spirits to a specific gravity in the range 0.78-0.80.

The epoxy coated screws were placed on a plastic panel 15 degrees from vertical, placed in the standard exposure cabinet and subjected to the salt spray for the indicated length of time before removal, rinsing, drying and evaluation. The following comparative results were obtained.

TABLE

While particular embodiments of the present invention have been described herein, it will be obvious to those skilled in the art that changes and modifications in various aspects may be made without departing from the broad scope of the invention. Consequently, the scope of the invention is not limited by any particular embodiment but is defined by the appended claims and the equivalents thereof.