Stock material or miscellaneous articles – Composite
Reexamination Certificate
1998-09-11
2002-11-05
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Composite
C428S416000, C428S423100, C428S425800, C428S457000, C428S458000, C428S463000, C428S522000, C523S404000, C523S414000, C528S093000, C528S103000, C528S124000
Reexamination Certificate
active
06475621
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention pertains to an aqueous primer containing a silane adhesion promoter for use on metal surfaces to which composite or metal adherends will be bonded utilizing structural adhesives. More particularly, the subject invention pertains to a primer comprising an aqueous dispersion of a solid thermosetting resin, a silane adhesion promoter and a solid curing agent which contain little or no volatile organic solvent.
2. Description of the Related Art
In the current manufacture of composite structures, particularly in the aerospace and transportation sectors, it is frequently necessary to bond a fabricated metallic structure to metal or composite adherends utilizing structural adhesives, generally structural film adhesives, or to laminate one or more plies of thermosetting resin impregnated fibrous reinforcement to the fabricated metal structure. In general, to ensure the greatest level of adhesive strength, the metal surface(s) are scrupulously cleaned of dirt, soil, grease, and metal oxidation products immediately prior to bonding. Unfortunately, this procedure cannot be generally used most times, as the cleaning and bonding operations are often separated by long periods of time. During such periods, the metal surface may become hydrolyzed, lessening the adhesive strength of the bond. One alternative to overcome this difficulty is to use a primer on the cleaned metal surface. However, many of the commercially available primers do not provide the desired durability or performance required when exposed to harsh environmental conditions.
Primers have often been prepared from one or two component thermosetting resins diluted to sprayable levels with volatile organic solvents. However, the use of primers containing large amounts of volatile organic compounds (“VOCs”) is under increasing scrutiny for both toxicological and environmental reasons.
It has been proposed to utilize electro-depositable primers. Such primers contain resin particles bearing a charged group which migrate to an oppositely charged substrate from aqueous solution or dispersion. Such compositions often contain a lower but still undesirable level of VOCs and do not have ideal properties due to the restraints placed on the resin structure. Other ways of applying primers to metal surfaces include spraying, brushing or dipping the metal in a bath of primer.
In addition, liquid epoxy esters were proposed for use in coatings by C. K. Thorstad, “Emulsions—Why and How They are Used”,
Modern Plastics
, pp. 83-84, July 1995, in compositions containing either water or the liquid epoxy ester itself as a vehicle, together with polyvinylacetate, polyacrylic, or poly(butadiene/styrene) lattices. Acid curing agents, for example dimethyl acid pyrophosphate or boron trifluoride, are cited for use in these applications.
In U.S. Pat. No. 2,872,427 it has been proposed to utilize aqueous emulsions of liquid epoxy resins in textile treating. Additional suggested uses are in paints and enamels. The emulsions contain one or more emulsified liquid epoxy resins dispersed in an aqueous dispersing phase containing an alkali and acid stable non-ionic emulsifying agent and a water-dispersible binding colloid. For those applications where curing agents are also required, the curing agent is dissolved into the epoxy resin prior to dispersion. Such emulsions are storage stable with respect to sedimentation or coagulation, but suffer from the defect that since the epoxy and curing agent are intimately associated, premature curing may take place, particularly when exposed to elevated temperatures during shipment or storage. Shelf life of such emulsions is extremely limited. Furthermore, the protective colloids utilized are not themselves thermally stable, and may cause high temperature performance of the composition to suffer. Finally, such coatings suffer from lack of solvent resistance.
Raghavon and Lewis in the article “Epoxy Water-Borne Primer: Low-Temp Cure and Zero VOCs”,
Modern Paint and Coatings
, July 1991, evidences the need for water-borne primers without VOCs and suggests use of an aqueous dispersion. However, no details of a suitable composition for such use are given.
U.S. Pat. No. 5,001,173 describes basecoats for metal surfaces which are to be latex primed and painted. The basecoat comprises an aqueous epoxy resin dispersion also containing chromium trioxide and preferably phosphoric acid. The use of aromatic amine or other solid epoxy curing agents is not disclosed. The basecoat is followed by a solvent borne primer containing substantial amounts of polyesters. In the article “Guidelines to Formulation of Waterborne Epoxy Primers”, M. A. Jackson,
Polymer Paint Color Journal
180, (4270), 1990 at pages 608-621 discloses two component primer systems containing as one component an epoxy resin dispersion in water and solvent together with various corrosion inhibitors, and, as the second component, a water-reducible amine catalyst in water. Such formulations, however, require mixing or metering of two separate components and are not storage stable.
In the article “Waterborne Epoxy Dispersions Provide Compliant Alternatives”, R. Buehner et. al.,
Adhesives Age
, December 1991, described waterborne liquid and solid epoxy resin dispersions cured with dicyandiamide and water-soluble 2-methylimidazole catalyst for use as adhesives. However, the shelf life of such dispersions is indicated as only several days or less.
Another significant consideration in the use of primers is the durability and corrosion resistance of the joint formed between the metal surface and the material bonded to the metal surface. This is particularly important in structural applications, such as aircraft structures, because these joints are exposed to a wide range of environmental conditions with extreme temperatures, high humidity, and highly corrosive marine environments. To avoid the failure of the joint as well as to meet stringent commercial passenger and cargo aircraft standards, the adhesive-bonded joint of the structural component must withstand harsh environmental conditions, and, in particular, resistance to corrosion and disbanding in humid, salt-laden environments, especially those resulting from sea spray or deicing materials. Failure of these joints often starts with diffusion of water through the adhesive followed by corrosion of the underlying metal structure. Thus, it is desirable to have a method and composition useful for bonding metal surfaces that delays onset of corrosion and exhibits stability in aqueous and salt-laden environments.
One method to improve the environmental durability of the metal interface in aerospace applications is etching the metal surface in acid and then anodizing it in phosphoric acid or chromic acid. This is particularly necessary for aerospace applications which involves protection of the aluminum oxide surface of aluminum adherends from moisture-induced hydrolysis. It has been found that aluminum substrates etched in acid and then anodized in phosphoric or chromic acid have shown superb environmental durability with either organic or aqueous based primers. However, if the metal surface is merely abraded, or only given the acid etch treatment, only the organic solvent-based primers show superior results. In the case of steel adherends the aqueous-based primers have been found to give unacceptable durability no matter how the surface is treated.
However, chromate and phosphate-based anodizing surface treatments have been found to have a number of disadvantages in that these processes typically use large amounts of water to neutralize the treated metal surfaces, and to rinse off the corrosive acids used for anodization of the metal surface. Disposal of the phosphate or chromate containing waste water is expensive and can be environmentally hazardous. Commercial anodizing processes also require large amounts of electricity to sustain an anodizing current in the anodizing bath, particularly for large metal components, and require expensive equipment su
Dickerson Elaine
Kohli Dalip
Cytec Technology Corp.
Thibodeau Paul
Zacharia Ramsey
LandOfFree
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