FIELD OF THE INVENTION

Provided are aqueous primer compositions useful for protecting a substrate from corrosion. The provided primer compositions can be used for protecting primary structures on an aircraft.

BACKGROUND

Manufacturers of airplanes and rotorcrafts go to great lengths to ensure the long-term durability of their aircraft. With many aircraft structures made from metal, protecting these parts from corrosion is a foremost concern. Corrosion is the deterioration of metal by chemical or electrochemical attack, whereby the metal is converted into a metallic compound such as an oxide, hydroxide or sulfate. Chemical corrosion can be caused by exposure to caustic cleaning agents, while electrochemical corrosion can be induced by exposure to conductive liquids such as salt water.

One way to protect against corrosion is to apply a primer coating (or film) that acts as a barrier layer on the surfaces of vulnerable metal structures. For added protection, it is common to add corrosion inhibitors to such primer films. Conventionally, corrosion inhibitors for aluminum alloys are derived from hexavalent-chromium compounds, which are used in both surface preparation and organic primer compositions. These chromium compounds generally include chromates, which are salts with an anion containing chromium and oxygen, such as CrO42−.

While chromates can inhibit corrosion, these compositions are becoming increasingly disfavored because of environmental, health, and safety concerns. Evolving regulatory regimes have caused manufacturers to explore use of fundamentally new primer systems capable of meeting new regulatory standards.

SUMMARY

Amid concerns related to use of chromium and its compounds, pre-treatments and primers free of these chemistries have been explored by industrial manufacturers. Efforts in developing non-chromated systems have focused on both replacement of the chromate conversion coating with a non-chromated surface preparation as well as replacement of the chromated epoxy primer system with a non-chromated system.

With respect to the latter, certain non-chromated primer systems are based on water-based compositions. While these compositions are low in volatile organic compounds (VOCs) and thus desirable for environmental reasons, many do not afford the same levels of corrosion resistance as their solvent-based counterparts. When epoxy resins are dispersed in water, solid epoxy resin particles can undergo sedimentation, where these particles stack upon each other as the water evaporates. If these resins are used in combination with solid curatives in the primer formulation, the inhomogeneous nature of the epoxy and curative can significantly decrease the corrosion resistance of the cured primer.

It was discovered that blending both soluble and insoluble epoxy curatives can provide a solution to this technical problem. This not only improves the homogenous of the cured thermoset system, but also does not significantly impair the low water absorption properties brought by the insoluble solid curatives. By balancing these curatives, it was possible to achieve a superior primer system.

Further, the average particle sizes of the thermosetting resin and associated curative was observed to influence the performance of the cured final product as a protective layer on a substrate. Commercially available curatives, such as epoxy curatives, tend to have an average particle size well above the optimal range. While such particles can be reduced in size using traditional grinding methods such as jet milling, ball milling, or sand milling, it is difficult to get a small particle size comparable to that of the thermosetting resin dispersion. Disclosed herein are methods of milling the curatives for the thermoset resins to fine particles without need for added surfactants.

In sum, the provided primer compositions can have particle sizes that increase solvent resistance, mechanical performance and salt spray corrosion resistance of the cured primer, and further overcome technical difficulties associated with prior art methods and achieve performance that compare favorably with those of solvent based primer systems.

In one aspect, a primer composition is provided. The primer composition comprises: a thermosetting resin dispersion in water; a coalescent solvent in which the thermosetting resin is at least partially soluble; a first curative that is substantially water-insoluble; and a second curative that is substantially water-soluble.

In a second aspect, a primer composition is provided, comprising: an epoxy resin dispersion in water; a coalescent solvent in which the epoxy resin is at least partially soluble; an epoxy silane; a first curative that is substantially water-insoluble; and optionally, a second curative that is substantially water-soluble, wherein the primer composition is substantially free of any liquid or water-soluble curatives containing a primary or secondary amine group.

In a third aspect, a method of making a primer composition is provided, comprising: milling an aqueous mixture to obtain a precursor dispersion, the aqueous mixture comprising: a thermosetting resin dispersion in water; a coalescent solvent in which the thermosetting resin is at least partially soluble; and a first curative that is substantially water-insoluble; and mixing the precursor dispersion with a second curative that is substantially water-soluble to obtain the primer composition.

In further aspects, cured primer films and primed articles based on the aforementioned primer compositions are herein provided.

Definitions

As used herein:

“ambient conditions” means at a temperature of 25° C. and a pressure of 1 atmosphere (i.e., 101.3 kPa);

“ambient temperature” refers to a temperature of 25° C.;

“average” refers to a number average;

“cure” refers to chemically crosslinking, such as by exposing to radiation in any form, heating, or allowing to undergo a chemical reaction that results in hardening or an increase in viscosity (e.g., under room temperature or heated conditions);

“polymer” refers to a molecule having at least one repeating unit and can include copolymers;

“solvent” refers to a liquid capable of dissolving a solid, liquid, or gas, such as silicones, organic compounds, water, alcohols, ionic liquids, and supercritical fluid;

“substantially” refers to a majority of, or mostly, as in an amount of at least 50%, 60, 70, 80, 90, 95, 96, 97, 98, 99, 99.5, 99.9, 99.99, or 99.999%, or 100% of a composition based on weight or volume;

“substantially free of” means having a trivial amount of, such that a composition is 0 wt % to 5 wt % of a given component, or 0 wt % to 1 wt %, or 5 wt % or less than, equal to, or greater than 4.5 wt %, 4, 3.5, 3, 2.5, 2, 1.5, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.01, or 0.001 wt %, or 0 wt % of the component; and

“substituted” refers to the state in which one or more hydrogen atoms contained therein are replaced by one or more non-hydrogen atoms.

