Deposition of silver layer on nonconducting substrate

Details Deposition of silver layer on nonconducting substrate Deposition of silver layer on nonconducting substrate

1999-08-13

2001-05-01

Talbot, Brian K. (Department: 1762)

Stock material or miscellaneous articles

Composite

Of metal

C428S465000, C428S434000, C604S265000, C604S328000, C604S905000

Reexamination Certificate

active

06224983

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to the deposition of silver films on nonconducting substrates, and, more particularly, to deposition of such films that are very thin.
Thin films of certain metals on nonconducting substrates can have important commercial applications. Thin films of conducting metals on transparent substrates are used in electronic display devices. Thin films can be used to reflect heat in solar shading or other solar devices, and to filter radiation from sunlight. A thin film can reduce the incidence of infection caused by a device that is introduced into the human body, when the film, is coated onto the device before introduction into the body. Thin films are used in packaging as a vapor barrier coating. These applications are only illustrative of the thousands of uses of thin films, and are not limiting of their uses.
In one particular application, films of silver or silver-containing compounds are particularly effective in reducing microbial infection in the human body. U.S. Pat. No. 4,404,197 describes the use of silver-containing salts in reducing the likelihood of infection of burn victims. U.S. Pat. No. 4,581,028 describes the use of antimicrobial silver salts in, implants, and U.S. Pat. No. 4,603,152 describes other such devices utilizing silver compounds to resist infection. U.S. Pat. Nos. 4,054,139 and 4,483,688 disclose the use of a pure silver metallic coating on medical devices to reduce the incidence of infection. Thus, it is well established that coatings of silver or silver compounds are effective in reducing the chances of infection caused by medical devices that are implanted or inserted into the body.
Although the value of using silver to avoid infection is well established, there is less knowledge as to effective approaches to the best approach to providing the silver on the surface. Electrodeposition might be used, but in most cases the medical instruments are made of nonconducting materials which cannot be readily coated electrolytically on a commercial scale, with a thin, adherent coating. The '139 patent suggests that coating “of the type deposited by electroless deposition” would be operable, but gives no details of operable processes. The '688 patent describes the use of large 300 mesh particulate silver to coat catheters.
Silver can be coated -onto nonconducting substrates by electroless processes. . One example is the process used to coat silver onto mirrors, but such coatings are comparatively thick. Another example is dry deposition techniques such as vapor deposition or sputtering, but these cannot be used to coat irregularly shaped substrates, or the insides of long bores.
Because silver can be toxic in some circumstances, and is expensive, it is preferable to coat the silver as a very fine layer onto the electrically nonconducting substrate. The coating should be strongly adherent to the substrate, because loss of the coating might result in infection or passage of the silver into the body. There are not presently known techniques for depositing silver onto various types of nonconducting substrates that permit the deposition of a very thin, but uniform, transparent layer of silver, on the order of 2 to 2000 Angstroms thick, produce a highly adherent layer with good mechanical properties, and are readily adapted to large scale commercial manufacturing of coated products.
There therefore exists a need for such a coating technology. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a process for depositing thin, uniform layers of silver onto a wide variety of nonconducting substrates. The silver layer is adherent and effective in various uses, including, for example, antimicrobial medical applications, barrier packaging, and optical filters. The process can be performed at ambient temperature or, at most, slightly elevated temperature, using conventional industrial chemical procedures. It is highly controllable and reproducible, producing virtually identical layers on large numbers of substrates. Tests have shown that the yields of good quality coated parts using the approach of the invention is very high.
In accordance with the invention, a process for depositing a uniform thin layer of silver onto the surface of an electrically non-conducting substrate comprises the steps of activating the surface in an aqueous activating solution containing at least about 0.001 grams per liter of a salt containing stannous tin ions; and depositing silver onto the surface from a deposition solution of a silver-containing salt, a reduction agent in a concentration sufficient to reduce the silver salt to form metallic silver at the surface of the substrate, and a deposition control agent in a concentration sufficient to prevent the silver in the solution from precipitating from the solution, and to permit it to deposit upon the surface of the substrate, the step of depositing being accomplished in darkness. If necessary, the surface of the substrate may be cleaned prior to processing. Preferably, the silver layer is stabilized after deposition, but before use.
The preferred approach deposits a thin, uniform layer of silver, preferably 2 to 2000 Angstroms thick, at the rate of about 5-7 Angstroms per second in the deposition solution. The thickness of the surface layer is readily controlled. The resulting silver layer is adherent to the surface of the nonconducting substrate. Other features and advantages will be apparent from the following more detailed description of the preferred embodiments, which illustrate by way of example, the principles of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with a preferred aspect of the invention, a process for depositing a uniform thin layer of silver onto the surface of an electrically non-conducting substrate comprises the steps of cleaning the surface in an aqueous cleaning solution; activating the surface in an aqueous activating solution containing at least about 0.001 grams per liter of a salt containing stannous tin ions; depositing silver onto the surface from a deposition solution having a pH of not less than about 8, and containing silver nitrate, a reduction agent selected from the group consisting of formaldehyde, hydrazine sulfate, hydrazine hydroxide, and hypophosphoric acid, in a concentration sufficient to reduce the silver salt to form metallic silver at the surface of the substrate, and a deposition control agent selected from the group consisting of invertose, succinate acid, sodium citrate, sodium acetate, sodium hydroxide, potassium hydroxide, and ammonia in a concentration sufficient to prevent the silver in the solution from precipitating from the solution, and to permit it to deposit upon the surface of the substrate, the step of depositing being accomplished in darkness; and stabilizing the deposited silver by contacting the surface upon which colloidal silver has deposited for at least about 5 seconds with a stabilizing solution of at least about 0.001 grams per liter of a salt of a metal selected from a member of the platinum group and gold dissolved in dilute nitric acid, the resulting solution having a pH value of from about 3.0 to about 4.8.
The present invention is operable in depositing a colloidal metallic silver layer upon the surfaces of many different nonconducting substrate materials. The substrate may itself be a flexible film, or may be a rigid solid. Such materials include, by way of example and not limitation, latex, polystyrene, polyester, polyvinylchloride, polyurethane, ABS polymers, ceramics such as aluminum oxide, glass, polyamide, polyimide, polycarbonate, and synthetic rubber. The nature of the surfaces of these materials varies widely, but the present approach is applicable for all.
It is important that the surface of the substrate be sufficiently clean that it can be wetted by subsequent activation, deposition, and stabilization solutions. Contaminant layers of grease, oil, dirt, chemicals, and other ma

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