US2986480A - Method for depositing metals on nonconducting substrates - Google Patents

Description

y 1961 F. A. REISS 2,986,480

METHOD FOR DEPOSITING METALS ON NONCONDUCTING SUBSTRATES Filed May 18, 1960 INVENTOR. FELIX A. REIS BY WW ATTOR.NEY$

United States Patent IVIETHOD FOR DEPOSITING METALS ON NON- i CONDUCTING SUBSTRATES Felix A. Reiss, Ann Arbor, Mich., assignor to the United States of America as represented by the Secretary of the Air Forcev Filed May 18, 196 0, Ser. N0.'30,038

3 Claims. (Cl. 117-227) This invention relates generally to coating nonconducting substrates and, more particularly, to methods and apparatus for applying a uniform metal coating to a nonconducting substrate by chemical reduction.

Various methods of the prior art for coating nonconducting substrates have proved to be inadequate due to disadvantages inherent in such methods. For example, if the coating of fibrous glass yarn with a silver solution were carried out in a tank or vessel of any shape or size, the bulk of the silver deposition would take place on the walls of the containing vessel and only a minimal fraction of the available silver, if any would deposit on the glass fiber. Shibsequent methods eliminated the use of a containing vessel by applying, drop by drop, a silver solution and a reducing solution, respectively, to afibrous glass strand advancing horizontally underneath the two solutions. Such methods were severely limited by the relatively slow rate of speed at which it was necessary to advance the fibrous glass and. were further characterized by the relatively nonuniform electro-conductivity of the coated product produced thereby.

It is, therefore, an object of the present invention to provide a method for depositing metals on nonconductingsubstrates.

It is a further object of the present invention to provide a method for applying a uniform coating of silver to fibrous glass yarn.

It is another object of the present invention to provide a method and apparatus for rapidly and continuously applying a uniform coating of silver to a nonconducting substrate.

A still further object of the present invention is to provide a method and apparatus for depositing coherent and adherent silver films on nonconducting filaments or yarns at fairly high speeds with a minimum loss of silver.

The novel features characteristic of the present invention, as well as additional objects and advantages thereof, will be better understood in the following detailed description of the certain form of apparatus for carrying out the process, when read in connection with the accompanying drawings in which:

Fig. l is a view of apparatus for coating a fibrous glass yarn with silver.

Referring now to Fig. 1, a fibrous glass yarn 1 is unwound from a package 2 and passes through conventional pretreatment equipment, indicated generally at 9, where the yarn undergoes conventional pretreating processes, such as wetting, sensitizing, etc. As the fibrous glass yarn 1 emerges from the pretreatment equipment 9, it enters an excess liquid collection tank 3 by means of idler rolls 7. From collection tank 3, the yarn rises perpendicularly to another idler roll 7 and continues on to pass through conventional and drying equipment, indicated generally at 10. The yarn is then wound onto a takeup drum 15, which is revolving in the direction of the arrow shown thereon to pull the yarn through the entire aforementioned process. A pair of reactant containers, 11 and 12, respectively, are positioned adjacent the upper idler roll 7 and each is pro- "ice vided with a metering valve 13 for metering the reactant contained therein, as will be hereinafter described.

In operation, reactant container 11 is supplied with an aqueous silver complex solution comprising 120 grams of silver nitrate and approximately 30% ammonium hydroxide to dissolve the brown precipitate formed thereby, plus a sufiicient amount of water to make up one liter? of: solu-v tion. Reactant container 12 is supplied with an aqueous reducing solution consisting of 20 gramsof hydrazine, 45 grams of sugar and 5 grams of gelatin plus a suflicient amount of water to make one liter. of solution. The sugar and gelatin are added to the reducing solution to promote the mechanism of droplet. formation, aswill be hereinafter described. As the fibrous glass strand is, advanced perpendicularly upward to a height of. approximately 50 feet, as shown by the arrow Y, at a speed of approximately 100 ft./min., the silver complex solution and the reducing solution are metered drop by drop from reactant containers 11 and 12, respectively, by. means of metering valves 13. Metering valves: 13: are positioned with respect to the advancing fibrous glass. yarn 1 sov that the droplets of the respective soltuion beingmetered therefrom contact and slide downwardly along the upwardly advancing yarn in the diretcion shown by the arrow L. No attempt is made to bring aboutmixing of the respective solutions at the point of application, but rather it has been proved in practice advantageous to space the points of application severalinchesv apart.

The mixing of the two solutions and the consequent silvering of the upwardly. advancing fibrous glass yarn takes place while the solutions and yarn'move: in opposite directions. It is this perpendicularly: opposed motion of the substrate and solutions that induces substantially increased capillary action between the globules of the respective solutions in the, interstices of the fibrous glass yarn. Y.

It has been discovered that the initial drops or globules of each of the solutions, upon coming into contact with the fast moving yarn, break up into a series of uniformly spaced small droplets that seem to travel with the yarn. The next drop of one of the two solutions coming down along the yarn is very large relative to the clinging drop lets and swallows on its way down the yarn one droplet after another, thereby increasing its size as it travels downwardly. When this oversize globule reaches a certain volume, it begins to spout a tail, which seems to cling to the yarn more tenaciously than the spherical drop. The formation of a tail on a downwardly falling drop is further induced by the force of gravity operating on the drop. The tail section of the drop is further elongated by the oppositely moving yarn. At this point in the sequence of events, the action of the downwardly traveling drop with its elongated tail section is difficult to predict. Sometimes the tail section consumes the drop entirely; sometimes the tail section breaks off to release a medium sized drop that continues to travel downwardly picking up droplets, and further growing and dividing. The tail section clings to the yarn like a thin sheath of liquid and, as it reaches a certain length, it breaks up into many small droplets that are picked up again by the next large drop of either solution coming down from above. This sequence of events, though well defined with respect to the mechanics involved, is entirely random with respect to the chemical composition of the droplets and any individual large drop. Since this phenomenon repeats itself over the entire fifty feet of yarn, the resulting mixing of the silver complex solution and the reducing solution is complete and uniform. The excess liquid from this process is caught in the collection tank 3 and is subsequently drained therefrom by means of a faucet 5.

When the silvering process is completed, the reaction products, which are a mixture of silver, silver hydroxide conditioned yarn, but show very low or no electrical conductivity. However, when they are heated in open air to about 600 F. for about one ortwo seconds,the'y change intoa strongly adherent coating of very good conductivity; the change in conductivitybeing accompanied by a change in color from black to a yellowish-brown. The silver coated yarn is brought to a bright metallic silver luster by washing the yarn in water. The conditioning or sensitizing of the fibrous glass yarn prior to the silvering process is essential for a successful operation, and is accomplished by pulling the yarn through a strongly acidic solution, for example, hydrochloric acid, of stannous chloride and subsequent washing in tap water. The fibrous glass yarn is thus exposed to .theaction f the stannous chloride solution for about one or two seconds to remove all inhibiting impurities, such as sizing preparations used in the yarn making process, from the surface of the glass fibers.

By means of the process of the present invention, it is possible to apply a continuous and uniform coating of silver to a nonconducting substrate, such as fibrous glass yarn, at a speed more than three times that disclosed by the prior art and with aminimum. loss of silver. Since the silvering process repeats itself over a fifty-foot span of yarn, the globules of the silvercomplex solution and the reducingsolution are thoroughly and uniformly, distributed throughout the interstices of the fibrous glass yarn. The reduction reactionis completed by the appli cation ofheat, as previously described. The resulting silver coated yarn is uniformly electroconductive, which, in addition to its decorative efiect, can be used in any situation demanding such a product. 1

It will be readily apparent to persons skilled in the art that metals other than silver, such as gold, copper, mercury, etc., and .any nonconducting substrate in addition to fibrous glass yarn, such as nylon, can be used without departing from, the spirit of the present invention.

Although the process of the present invention hasbeen .described with respect to a specific embodiment, it should be understood that obvious modifications may be made within the spirit and scope of the appended claims.

I claim:

1. A method for depositing coherent and adherent silver films on nonconducting yarns at fairly high speeds with a minimum loss of silver, comprising continuously advancing the yarn upwardly in a substantially vertical path at a speed of approximately lGOfeet per minute to a height of approximately fifty feet, dropping a globule of silver complex solution from a first applicator, directing the downwardly falling globule of silver complex solution to contact the upwardly advancing yarn, dropping a globule of a reducing solution from a second applicator, directing the downwardly falling globule of reducing solution to contact the upwardly advancing yarn, allowing the globules of the silver complex solution and the globule of the reducing solution to slide downwardly along the upwardly advancing yarn for mixing the two solutions and distributing the mixture throughout the interstices of the yarn by capillary action, and allowing the reduction reaction to: proceed to completion to effect a substantially continuous deposit of silver upon the surface of the yarn.

' 2. The method of claim 1 wherein the silver complex solution comprises 120 grams silver nitrate per liter and an amount of ammonia suflicient to dissolve the precipitate formed by the reduction reaction and the reducing solution comprises 20 grams of hydrazine, 45 grams of sugar and 5 grams of gelatin per liter.

3. The method of claim 1 wherein the silver coated yarn is subsequently heated in open air to approximately 600 F. for approximately l-2 seconds to substantially improve the electrical conductivity thereof.

References Cited in the file of this patent

Claims (1)

1. A METHOD FOR DEPOSITING COHERENT AND ADHERENT SILVER FILMS ON NONCONDUCTING YARNS AT FAIRLY HIGH SPEEDS WITH A MINIMUM LOSS OF SILVER, COMPRISING CONTINUOUSLY ADVANCING THE YARN UPWARDLY IN A SUBSTANTIALLY VERTICAL PATH AT A SPEED OF APPROXIMATELY 100 FEET PER MINUTE TO A HEIGHT OF APPROXIMATELY FIFTY FEET, DROPPING A GLOBULE OF SILVER COMPLEX SOLUTION FROM A FIRST APPLICATOR, DIRECTING THE DOWNWARDLY FALLING GLOBULE OF SILVER COMPLEX SOLUTION TO CONTACT THE UPWARDLY ADVANCISING YARN, DROPPING A GLOBULE OF A REDUCING SOLUTION FROM A SECOND APPLICATOR, DIRECTING THE DOWNWARDLY FALLING GLOBULE OF REDUCING SOLUTION TO CONTACT THE UPWARDLY ADVANCING YARN, ALLOWING THE GLOBULES OF THE SILVER COMPLEX SOLUTION AND THE GLOBULE OF THE REDUCING SOLUTION TO SLIDE DOWNWARDLY ALONG THE UPWARDLY ADVANCING YARN FOR MIXING THE TWO SOLUTIONS AND DISTRIBUTING THE MIXTURE THROUGHOUT THE INTERSTICES OF THE YARN BY CAPILLARY ACTION, AND ALLOWING THE REDUCTION REACTION TO PROCEED TO COMPLETION TO EFFECT A SUBSTANTIALLY CONTINUOUS DEPOSIT OF SILVER UPON THE SURFACE OF THE YARN.

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