Amorphous Hydrated Metal Oxide Primer for Organic Adhesively Bonded Joints

The FIGURE illustrates a perspective view of a three-point flexure strength test apparatus for determining the adhesive bond strength of a fiber reinforced resin composite to a ceramic adherend.

Any metal alkoxide that hydrolyzes to give an amorphous hydrated metal oxide (i.e. a monohydroxy metal oxide) may be used in the practice of this invention. Metal alkoxides having the formula M.sub.x (OR).sub.y where x is 1 and y is 3 or 4 are preferred in forming the metal oxide primer of this disclosure, y being determined by the particular valence of the metal. y should not be 2 as typically a valence of at least 3 is necessary to form a monohydroxy metal oxide. M is any metal capable of forming a stable alkoxide, which can be purified by, for example, a distillation or crystallization without decomposition; as y is defined above essentially all metals meet this requirement. It is preferred that the metal is selected from the group consisting of titanium, zirconium, silicon, iron, nickel and aluminum. It is especially preferred that the metal is aluminum, magnesium or silicon or mixtures thereof. Typically, R can be any organic radical that can be distilled at temperatures below about 300 degrees centrigrade (0.degree. C.). Since the alkoxide ((OR) moiety) is not incorporated in the primer, the important criteria associated with it is that the resultant alcohol can be volatilized at temperatures that are not high enough to damage the primer or substrate. It is preferred that R is an alkane radical from C.sub.1 to C.sub.10. It is especially preferred that R is methyl, ethyl, propyl or sec-butyl as these radicals are volatilized as alcohols at relatively low temperatures. In addition, the alkoxides can be modified by incorporation of varying amounts of additives such as phosphate, chromate or magnesium oxide without affecting the primer properties. Mixtures of the above metal alkoxides may be used in the practice of this invention. The composition comprising a metal alkoxide cordierite precursor (cordierite: 2 MgO, 2 Al.sub.2 O.sub.3, 5 SiO.sub.2) is a preferred example of such a composition.

The above metal alkoxides hydrolyze to amorphous hydrated metal oxides (primer) when exposed to moisture such as atmospheric moisture or moisture on the ceramic substrate surface and optionally heat as described below. An exemplary reaction believed to occur is that of aluminum alkoxide to alumina. The initial hydrolysation reaction of aluminum alkoxides is empirically illustrated as

This reaction proceeds rapidly with further hydrolysation-polymerisation to ##STR1## to incorporate n aluminum ions, i.e. Al.sub.n.sub.- 1 (OH).sub.(n+2)-x (OR).sub.x assuming linear polymerisation for simplicity. As the reaction proceeds the number of OR groups, i.e. x, relative to n decreases to a value depending on the hydrolysis temperature and available moisture concentration. Under normal application conditions, the ratio of residual OR groups as designated by x is less than 4 and n is 28 or greater. Such low levels of OR do not impede the performance of the primer. In contrast, zirconium alkoxide is believed to hydrolize to a hydrated oxide, i.e. ZrO.sub.2. 1.7 H.sub.2 O having no residual--OR or OH groups.

The thickness of this primer layer can vary effectively from about 0.15 microns (.mu.) to about 120 .mu.. Preferably the thickness is about 0.3.mu. to about 1.0.mu.. Above about 10.mu., the layer can be so thick as to create stress risers and to form a weak boundary layer. Below about 0.15.mu., the layer does not provide the properties such as crack propagation resistance at the levels typically required. In addition, it is preferable to apply the primer to the ceramic surface with a plurality of layers of metal alkoxide as this facilitates removal of volatiles and solvent which can be more difficult to achieve from a single thick application.

The articles of this invention comprise nonmetallics hereinafter defined as ceramics which inherently have an oxide surface. Ceramics is herein defined as glasses, glass ceramics, "traditional ceramics" and fiber reinforced bodies of the above. Typical exemplary glasses are borosilicates and silicas. Typical exemplary glass ceramics are lithium aluminum silicates and magnesium aluminum silicates. Typical exemplary "traditional ceramics" are oxide ceramics such as alumina or zirconia and silicon based ceramics such as silicon carbide or silicon nitride which naturally exist with a silica film. However, this list is not meant to be exhaustive and those skilled in the art will know others. In addition, a primed ceramic article can be bonded to a solid polymeric article or conventional fiber reinforced polymeric matrix composite such as an epoxy, polyimide, polyester, acrylic, urethane, cellulosic, rubber or phenolic based composite. Examples of fibers include glass, alumina, silicon carbide, graphite, amides and Kevlar.RTM. fiber (DuPont DeNemours, E.I., Co., Wilmington, Del.). Also, any primed ceramic article can be bonded to any metal article which may optionally be treated with the inorganic primer of this invention.

Typically, the surface is cleaned prior to primer application to provide the ceramic surface with an adhesive receptive quality. For example, conventional surface cleaning agents are various freons, methylene, chloride, trichloroethylene, etc. In addition, the surface is preferably abraded to enhance surface roughness.

Typical surface treatments for preparing a resin matrix composite surface include solvent cleaning, abrasion (e.g., emery cloth, sand blasting) followed by solvent washing.

Any conventional adhesive can be used for the practice of this invention that is useful for bonding articles, particularly ceramic, organic composite or metallic articles. For example, epoxide, polyurethane, acrylic or polyimide adhesives are used as these provide the properties most desired such as good strength. It is especially preferred to use epoxy or urethane adhesives as they resist environmental stresses, are strong and are frequently chosen for aerospace applications.

Any method of bonding may be used for the practice of this invention that provides an amorphous, hydrated metal oxide coated ceramic article bonded to another article with an adhesive. If a plurality of ceramic articles are to be bonded, it is preferred to prime each ceramic article. It is also preferred to apply a layer of metal alkoxide to the ceramic article(s) by a solvent casting, dipping or spraying procedure. The metal alkoxide coated ceramic articles are then maintained at a temperature of about 25.degree. C. to about 300.degree. C. in the presence of moisture. Below 25.degree. C., the reaction kinetics are typically too slow and above 300.degree. C. crystalization of the oxide primer surface may occur with an accompanying loss of mechanical strength. The application of the inorganic primer for field repair situations is also possible with the use of the lower conversion temperatures.

The adhesive is applied by conventional methods to the articles and they are joined together with the application of conventional pressures, temperatures and times appropriate for the adhesive used.

Slip cast silica ceramic adherends were solvent cleaned by flushing with freon solvent and then E-8385 aluminum alkoxide available from Stauffer Chemical Company, (Fairfield, Conn.), was diluted to 1% with toluene and applied to the silica adherends by solvent casting (brushed on). The aluminum alkoxide was converted to amorphous alumina by solvent evaporation at room temperature after which a coating of EA-934.TM. epoxy adhesive, available from Hysol (Pittsburgh, California) was applied to the amorphous alumina coated adherends. A strip of Kevlar.TM. (DuPont DeNemours, Inc., Wilmington, De.) reinforced epoxy composite, 60 mils thick, was solvent cleaned, sand blasted with 60 grit alumina followed by solvent cleaning and mated to the ceramic adhesive coated article in a suitable jig. A stop was placed between the composite and jig supports to insure a bondline thickness of 10 mils. The articles were clamped and allowed to stand seven days to cure the adhesive. The bonded part was removed from the jig, excess fillet removed and immersed in boiling water for 40 hours. Testing of the wet specimens was carried out using the test fixture depicted in the Figure. The results are listed in Table I.

In a similar manner, the ceramic article was treated with a 1% toluene solution having the metal alkoxide precursor composition of cordierite (2 MgO, 2 Al.sub.2 O.sub.3, 5 SiO.sub.2) E 8385.6 composition available from Stauffer Chemical Company (Fairfield, Ct). After application of the primer, the articles were heated at 120.degree. C. for 10 minutes to facilitate solvent removal and hydrolysis of the alkoxide. The Kevlar-epoxy composite was similarly treated as in Example 1 and the two adherends bonded with the EA-934. The bonded articles were allowed to stand for seven days to effect adhesive cure. The bonded articles were immersed in boiling water for 24 hours and tested using the test fixture depicted in the Figure. Test results are listed in Table II.

As a control sample specimens of the silica ceramic were obtained which were primed with an organic primer, HT-424.TM. available from American Cyanamid (Wayne, N.J.). The treatment of the Kevlar-epoxy composite surface was the same as described in Example 1. The two articles were bonded together using the Hysol EA-934 adhesive. After seven days, the bonded specimens were immersed in boiling water for 24 hours and tested using the test fixture depicted in the FIGURE. The results are listed in Table III.

These adhesively bonded aluminum joints are particularly resistant to crack propagation. A clear understanding of this may be had by reference to the Figure and Tables.

The tables compile data on a three-point flexure strength test. The Figure illustrates the test apparatus used to supply the data. According to the Figure, a composite 1 is supported on two cylinders 3 which are separated by a 4.76 cm span. The cylinders in turn rest on a suitable support 6. A ceramic adherend 9 which is bonded to the composite 6 lies underneath the composite 1 and between the cylinders 3. Application of pressure to a third cylinder 12 (which rests across the top center of the composite 1) results in debonding of the ceramic adherend from the composite by crack formation at the ceramic-adhesive interface. A measure of the adhesive bond strength is obtained by the pressure required to initiate a crack in this three-point flexure strength test.

It is believed the results listed in Tables I, II and III that the inorganic primers provide a 100% increase in load to initiate a shear crack (wet condition) compared to the organic control; bond strengths below 50 lbs. (measured as load to initiate a shear crack) produces all adhesive/ceramic interfacial failure; and the inorganic primers (2 coats) provide mixed failure modes of adhesive/composite and adhesive/ceramic interfaces.

This primer may be used to advantage in bonding ceramic to other ceramics or to resin composites, metal, fiber reinforced ceramics or solid plastics. In addition, it may be applied to articles to aid in adhesion of coatings such as with plastic packaged microelectronic devices. While this invention has been described in terms of a metal alkoxide a mixture of various metal alkoxides can be used.

This primer coating provides improved crack propagation resistance in moist environments. The resulting properties such as tensile and T-peel strengths of bonded joints made with the inorganic primer are at least equivalent to bonded joints made using conventional organic primers. Yet the inorganic primers can be utilized at thinner layers than the 5.mu. to 10.mu. layers typical of organic primers. Thicker layers tend to set up stress rises (i.e. weak boundary layer) as the components segregate. Also, because of its thermal stability the inorganic primer can be used equally as well with high temperature adhesives such as polyimides or with low temperature adhesives such as epoxy systems unlike organic primers which are typically temperature specific. Another major advantage of the inorganic primer is that it can be used on ceramic surfaces which have been treated by a variety of surface treatments and provide the same high level of crack propagation resistance. In contrast, organic primers produce different results dependent upon the surface pretreatments employed. In addition, conventional organic primers use strontium chromates as corrosion inhibitors and these cause toxicity problems in their manufacture and use. This invention provides an amorphous hydrated metal oxide primer for adhesively bonded joints that results in greatly increased crack propagation resistance. Thus, it makes a significant advance in the aerospace industry by providing new technology relating to adhesively bonded joints.

It should be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the spirit and scope of this novel concept as defined by the following claims.