Aluminum Plating Process

Abstract

Described is a closed loop method of producing a volatile aluminum hydride trimethylamine complex, depositing aluminum from the complex onto a substrate, and recycling trimethylamine vapor released during deposition to produce additional aluminum hydride trimethylamine complex.

Description

BACKGROUND OF THE INVENTION

This invention relates to an electroless process for the plating of aluminum on various substrates. More particularly it pertains to the conversion of a substantially nonvolatile aluminum hydride to a volatile aluminum hydride and the plating of metallic aluminum from vapors of the volatile aluminum hydride.

It is known that metallic aluminum can be plated from aluminum hydrides by contacting a substrate with such hydrides at or about the decomposition temperature of the aluminum hydride. Such a process usually requires a relatively high temperature to cause decomposition of the aluminum hydride; however, employment of a catalyst can effectively reduce the decomposition temperature of aluminum hydride. A process for catalytic decomposition of aluminum hydride is described in U.S. Pat. No. 3,462,288. A process for vapor plating aluminum onto a substrate by cyclically contacting a heated substrate with a heat decomposable aluminum-containing compound, such as the aluminum hydride trimethylamine complex, AlH.sub.3.sup.. (CH.sub.3).sub.3 N, is described in U.S. Pat. No. 3,206,326. However, this patent teaches the employment of vapors of a heat decomposable aluminum compound as the starting aluminum source material. The suggested aluminum hydride trimethylamine complex is generally extremely sensitive to moisture and also has an offensive odor. Additionally the aluminum hydride aminate generally contains less than about 30 percent by weight aluminum. The combination of these properties not only make it difficult, but generally uneconomical to transport the aluminum hydride aminate from the supplier to the coating operation. Moreover, the trimethylamine formed, as aluminum is deposited on the substrate, is frequently lost to the atmosphere or otherwise not recovered.

It is an object of this invention to provide an electroless aluminum deposition process which employs a relatively nonvolatile aluminum hydride as a starting material.

It is another object of this invention to provide an aluminum vapor deposition process which employs an aluminum hydride compound having a low vapor pressure as the starting material and a recirculatory system for usable by-products.

Other objects and advantages of this invention will become apparent during the course of the following discussion.

SUMMARY OF THE INVENTION

It has been found that a substrate can be satisfactorily plated with aluminum and the aforementioned objects achieved in the hereinafter described process. A substantially nonvolatile solid aluminum hydride-ether complex is contacted with trimethylamine vapor. The aluminum hydride-ether complex is characterized as being reactive with trimethylamine vapor to form a volatile aluminum hydride trimethylamine adduct and the ether. Illustrative of such adduct is aluminum hydride trimethylamine, AlH.sub.3.sup.. N(CH.sub.3).sub.3, aluminum hydride bistrimethylamine, AlH.sub.3.sup.. [N(CH.sub.3).sub.3 ].sub.2, and the like.

Due to the sensitivity of most volatile aluminum hydride compounds to the presence of moist air, it is usually desirable that the application of the aluminum hydride plate be conducted in a substantially anhydrous, inert atmosphere. The substrate is, therefore, preferably inserted into the closed loop plating system through a substantially gas impervious interchange means such as generally described in U.S. Pat. Nos. 3,474,823 and 3,519,398. However, the plating system can be dried after the substrate is positioned in said system. The substrate can be heated to at least the decomposition temperature of the volatile aluminum hydride trimethylamine adduct either before or subsequent to insertion into the closed loop system by any means known to those skilled in the art. The heated substrate is then contacted with the adduct for a sufficient time to decompose the adduct and deposit aluminum onto the substrate and simultaneously release hydrogen and trimethylamine vapor. The aluminum coated substrate is then removed through the gas impervious interchange means in substantially the same manner in which it was inserted.

The trimethylamine vapor produced during decomposition of the aluminum hydride trimethylamine adduct is generally contaminated with the by-product hydrogen and the ether (unless the ether is previously removed). The ether and optionally the hydrogen can be separately or simultaneously removed from the trimethylamine vapor effecting purification of said vapor. If the explosion hazard of hydrogen is unimportant, the hydrogen can remain mixed with the vapor and used as a carrier gas. The purification can be carried out by selective refrigeration and condensation of the gases or by other methods known to those skilled in the art. The trimethylamine vapor is recirculated to the original contacting step where the aluminum hydride trimethylamine adduct was formed. Recirculation of the vapor is preferably carried out by use of a standard commercially available pump. However, recirculation can be accomplished by condensing the vapor in a cylinder and vaporizing the trimethylamine prior to introducing it into the original contacting step.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The described aluminum hydride-ether complex has the general formula (AlH.sub.3).sub.n.sup.. R, wherein n has a numerical value of from 1 to 5 inclusive, and R is an ether group. The ethers usually employed are the lower aliphatic ethers such as ethyl, propyl or butyl ethers, but those containing an aromatic group such as methylphenyl ether, ethylphenyl ethers, propylphenyl ether or the alicyclic ethers are also useful. Diethylether, tetrahydrofuran and dioxane, which respectively form aluminum hydride etherate, (AlH.sub.3).sub.3.sup.. C.sub.4 H.sub.10 O, aluminum hydride tetrahydrofuranate, AlH.sub.3.sup.. C.sub.4 H.sub.8 O and aluminum hydride dioxinate, AlH.sub.3.sup.. C.sub.4 H.sub.8 O.sub.2, are preferred herein. The above aluminum hydride-ether complexes are in a suitable physical form to react with trimethylamine to provide a volatile aluminum hydride trimethylamine complex, such as a porous body, particulate or a block having a large surface area. Naturally, the rate of chemical reaction to produce the aluminum hydride aminate is directly proportional to the aluminum hydride-ether complex surface area exposed to the trimethylamine vapor.

Substantially any normal solid material is suitable as a substrate in the aforedescribed process. For example, metals such as iron, magnesium, brass and copper; polymers such as polyolefins, polyamides and polymeric fluorocarbons, glass, paper, cloth, carbon and graphite, wood, ceramics and the like are all platable with aluminum by the process of this invention. The nature of the surface being plated determines to a large extent the brightness of the aluminum plate. In general, the use of a smooth, nonporous surface, as found on most metals and some polymer films, produces a brighter plate than a relatively porous surface, as those encountered with paper or cloth. On the surfaces of some polymers, such as polyethylene, polytetrafluoroethylene, acrylonitrile-butadienestyrene polymers and polypropylene, it has been found that even better adhesion of the aluminum plate is achieved if the surface has been made more polar, that is by sulfonation, corona discharge and the like, prior to plating with the aluminum. To increase the vapor flow rate and improve the contact between the vapor and aluminum hydride-ether complex, it is oftentimes desirable to mix a substantially chemically and physically inert carrier gas with the vapor prior to contact of the vapor with said complex. Examples of suitable carrier gases are hydrogen, nitrogen, helium, neon, argon, krypton, xenon and the like. The rate of reaction between the trimethylamine vapor and the aluminum hydride-ether complex to form the volatile aluminum hydride trimethylamine adducts can be increased by heating the trimethylamine vapor prior and/or simultaneously with contact of said complex with said vapor. Desirably the vapor is heated to a temperature of from about 25.degree.C. to about 65.degree.C.

The rate of aluminum deposition on the heated substrate can be controlled by adjusting the rate at which the aluminum hydride trimethylamine is formed. The aluminum deposition rate can additionally be controlled by variance of the pressure within the aluminum deposition chamber. Since the volatilization rate of aluminum hydride trimethylamine is inversely proportional to the pressure, a low pressure, especially lower than atmospheric, will increase the vaporization rate of the adduct and consequently increase the aluminum deposition rate onto the substrate. Preferably the pressure is from about 0.2 to about 15 pounds per square inch (guage).

As before described, the substrate should be heated to at least the decomposition temperature of the aluminum hydride trimethylamine vapor. This temperature is generally from about 120.degree.C. to about 400.degree.C. It has, however, been discovered that when in contact with certain transition metal catalysts, volatilizable aluminum hydrides can be used to produce plating of metallic aluminum at temperatures substantially below the usual decomposition temperature of such hydrides, i.e., about 25.degree.C. to about 100.degree.C. and preferably about 50.degree.C. to about 100.degree.C. The use of such catalysts permits the deposition of a uniform adherent plate or coating of metallic aluminum, usually in the form of a bright plate, on substantially any substrate at relatively low temperatures. Transition metal decomposition catalysts useful herein are compounds of titanium, zirconium, hafnium, vanadium, niobium and tantalum. In instances where the catalyst is applied to the substrate in a solvent, it is preferable that the metal be in the form of a compound which is soluble to the extent of at least 1 .times. 10.sup.-.sup.6 weight percent of the solvent employed. For example, such compounds as ZrCl.sub.4, NbCl.sub.5, VOCl.sub.2, VOCl.sub.3, TiCl.sub.3, TiCl.sub.4.sup.. 2[(C.sub.2 H.sub.5).sub.2 O], TiCl.sub.4, TiBr.sub.4, TiI.sub.4, VCl.sub.4, Ti(OC.sub.2 H.sub.5).sub.2 Cl.sub.2, TiCl.sub.2.sup.. (i-OC.sub.3 H.sub.7).sub.2, TiCl.sub.2.sup.. 2[(C.sub.2 H.sub.5).sub.2 O], and Ti(BH.sub.4).sub.3.sup.. 2[(C.sub.2 H.sub.5).sub.2 O] have proven effective. Some of the transition metal catalysts defined herein have a more pronounced effect than others in lowering the decomposition temperature of the aluminum hydride. The chlorides, bromides, iodides and oxychlorides of titanium, niobium, vanadium and zirconium generally seem to be more effective than compounds of the other transition metals. TiCl.sub.4 has been found to be particularly effective in achieving lower decomposition temperatures of aluminum hydride compounds.

By controlling the substrate temperature when it is contacted with the aluminum hydride trimethylamine vapor and by addition of the transition metal decomposition catalyst to only selected areas of substrate, it is possible to form an aluminum plate only on such selected areas. In this manner, ornamental designs, outlines, printed circuits and the like may be produced. Likewise, all or a portion of a selected substrate may be coated or plated with aluminum to enhance the ability of such surface to adhere to other metals. Of particular utility is the aluminum coating of glass, ceramic, metal or polymer surfaces to enhance their bonding to adhesive polymers and copolymers, such as the copolymers of ethylene and acrylic acid. Once the desired form and quantity of decomposition catalyst is applied to the substrate, the catalyzed surface is heated and contacted with the aluminum hydride trimethylamine vapor.

Particular embodiments of the present method can be carried out as hereinafter described.

A closed plating system is maintained in a dry box atmosphere having a maximum moisture content of less than about 1 part per million. A glass beaker, heated to 150.degree.C., is inserted within a plating chamber in the system through a gas impervious interchange. Trimethylamine vapor is then passed through an enclosed conduit to a container. Said container is enclosed at two opposite ends by screens having a mesh of adequate size to prohibit passage of 200 mesh particulate. Aluminum hydride etherate, (AlH.sub.3).sub.3.sup.. C.sub.4 H.sub.10 O, particulate (200 mesh) is periodically supplied to the container, through a gas impervious interchange, while the trimethylamine flows through the particulate to form aluminum hydride trimethylamine. An aluminum coating is deposited on the glass beaker as aluminum hydride trimethylamine flows, at atmospheric pressure, through the plating chamber and therein contacts the heated glass. A uniform aluminum plate is formed after about one hour of aluminum hydride trimethylamine contact. Diethylether is removed from the trimethylamine released during the plating step by liquefying the ether at about -10.degree.C. Hydrogen is then removed by liquefying the trimethylamine at about -78.degree.C. The so purified trimethylamine is volatilized and recirculated by pumping, through the aluminum hydride etherate. The trimethylamine is replenished as necessary.

Employing substantially the same procedure as described above a glass article is catalyzed by exposure to TiCl.sub.4 vapors before being placed in the plating chamber. Deposition of an adherent aluminum plate on the glass occurs at about 25.degree.C.

Claims

1. A closed loop process for plating aluminum onto a substrate comprising:

a. contacting a substantially nonvolatile solid aluminum hydride-ether complex with trimethylamine vapor to form a volatile aluminum hydride trimethylamine adduct and ether;

b. contacting the substrate, heated to at least the decomposition temperature of the adduct, with the adduct in a substantially oxygen and moisture free environment for a sufficient time to deposit aluminum onto the substrate and produce hydrogen and trimethylamine vapor; and

c. recirculating the trimethylamine vapor of step (b) to step (a) to

2. The process of claim 1 wherein the trimethylamine vapor is mixed with a carrier gas prior to step (a), the gas being substantially chemically and physically inert to the vapor, substrate and products formed in the

3. The process of claim 1 wherein the trimethylamine vapor is heated to

4. The process of claim 1 wherein the contacting of step (a) is carried out at a pressure of from about 0.2 to about 15 gauge pounds per square inch.

5. The process of claim 1 wherein the ether is selected from the group

7. The process of claim 1 including the additional step of removing hydrogen from the trimethylamine vapor decomposition product of step (b).

8. The process of claim 1 including the additional step of removing ether

9. The process of claim 1 wherein the contacting of step (b) is carried out with a substrate catalyzed with a transition metal decomposition catalyst effective in achieving a lower decomposition temperature of the adduct selected from the group consisting of compounds of titanium, zirconium,

10. The method of claim 3 wherein the contacting of step (a) is carried out at a pressure of from about 0.2 to about 15 pounds per square inch, the ether is diethylether and the ether is removed from the decomposition products of step (b).