United States Patent 1191 lll] 3,983,266 Bahls [451 Sept. 28, 1976 [54] METHOD FOR APPLYING METALLIC 3.776.740 [2/1973 Sivcrz ct al. .......................... .. l06/l SILVER TO A SUBSTRATE OTHER PUBLICATIONS [75] Inventor: Harry Bahls, Wayne, Pa. lvanov et al., Chem. Abs. 43:2548c, I949, [73] Assignee: Peacock Laboratories, Inc., Philadelphia, Pa. Primary Examiner-Ralph S. Kendall [22] Filed: Oct. 9, I974 I 57] ABSTRACT [2!] ,Appl. No.: 513,417 High efficiency deposition of silver on the surface of a substrate is obtained by providing a solution contain [52] [1.8. CI ............................... .. 427/164; 427/165; ing reducible dissolved silver in the presence of an al 427/168; 427/424; 427/426; l06/l kali metal hydroxide and ammonia, all of which are [51] Int. CLZ. ......................................... .. C23C 3/02 applied to the substrate in the presence of an aqueous [58] Field of Search .............. .. l06/l; 427/l68. I69, solution of a moderating reducer containing :1 poly 427/165, I64, 426, 304, 125, 425 hydric alcohol of the formula CH2OH(CHOH),,C H,OH, where n is an integer from 1 to 6. Preferably [56] References Cited the polyhydric alcohol is sorbitol, and in a preferred UNITED STATES PATENTS embodiment a moderator is the form of a thio glycerol is present. 2,996,406 8/l96l Weinrich ........... ............ .. 427/168 3,772,078 ll/l973 Polichcttc ct al ................. .. l06/l X l5 Claims, No Drawings 3,983,266 1 Other objects and advantages of this invention, in METHOD FOR APPLYING METALLIC SILVER TO cluding the economy of the same, and the case with A SUBSTRATE which it may be applied to existing silver coating equip ment and apparatus, will further become apparent BRIEF SUMMARY OF THE INVENTION 5 hereinafter. This invention relates to the application of metallic silver to the surface of a substrate, by reducing the DETAILED DESCRIPTION OF THIS INVENTION silver from a solution in which the silver is present as a It has been discovered that striking improvement in soluble silver salt. This invention relates particularly to reduction efficiency can be obtained by the provision the electroless deposition of silver and particularly to of a reducer solution in the form of an aqueous solution the deposition of silver with high efficiency upon a of a polyhydric alcohol of the formula CH._.OH( stationary object or a continuously moving surface of a CHOH),,CH2OH, where n is an integer from 1 to 6. The substrate such as glass, plastic or ceramic, for example, sharp improvement of efficiency is reflected in a con in the coating of mirrors, decoration of objects and the siderable saving of valuable silver in the usual commer like. cial processing plant, wherein the unreacted (unre duced) silver which remains in solution over?ows into BACKGROUND OF THE INVENTION a trough or other collecting chamber, which may then It is already known in the art to provide methods and be reclaimed. The process of reclamation is compli compositions for the electroless deposition of metallic cated by the presence of sludges and other undesirable silver on various substrates, such as glass plate in the 20 side reaction products, and is complicated to such an manufacture of mirrors, and on other glass objects such extent that in many silvering estabishments the waste as Christmas tree ornaments, etc. over?ow solution and sludge is kept in a holding tank The patent to Sivertz et al US. Pat. No. 3,776,740, for a very substantial number of months before repro granted Dec. 4, 1973, discloses one method for the cessing is attempted. This results in the storage of waste electroless deposition of silver, utilizing a reducing 25 materials containing silver of very substantial value, agent in the form of an aldonic acid such as gluconic which is of no use whatsoever during the entire period acid, for example. It is pointed out in the Sivertz et al of storage. patent that various mild reducing agents have been The polyhydric alcohols in accordance with this in used, such as formaldehyde, glucose or invert sugar, vention are also known as alditols, and are acyclic, but that the use of these materials have caused prob 30 polyhydric alcohols that are normally derived from lems in the art, particularly since the silvering solutions aldoses or ketoses by the known reduction of the corre are short-lived and the reducing agents tend to be un sponding carbonyl group. For example, glucose may be stable, often evolving hydrogen or decomposing to reduced in the presence of a nickel catalyst to form form sludge or interfering products. Using the method dextro-glucitol, which is a hexitol and is a member of of the Sivertz et al patent, the components of the silver 35 the alditol series. ing solution are brought together just before contact Alditols may be divided on the basis of the number of with the substrate to be silvered. This may be achieved, carbon atoms found in the normal chain. Hence, al for example, by pouring or pumping the component ditols in accordance with this invention include tetri solutions such that they meet just before contact with tols, pentitols, hexitols, heptitols, octitols, etc. the substrate. Alternatively, the component solutions 40 Although the polyhydric alcohols having from 3 to 10 may be air-atomized or hydraulically sprayed prior to carbon atoms exist in a wide variety of forms, some of or simultaneously with intermixing at the surface of the the most available forms include erythritol, threitol, substrate. Othr reducing agents such as dextrose, fruc arabinitol, ribitol, xylitol, allitol, galactitol, glucitol, tose, arabinose rid invert sugar are referred to in the iditol, manitol, altritol, volemitol, glycero-altro hep‘ disclosure of the Sivertz et al patent. 45 titol, erythrogalacto octitol and dextro and laevo iso mers thereof. In accordance with this invention, it is OBJECTS OF THE INVENTION highly preferred to use dextro-glucitol, or sorbitol, It is an object of this invention to provide a method which is normally available as a crystalline material or and solution for the electroless deposition of silver on a as a 70% solution in water. Further, dextro-mannitol is substrate, with higher efficiency than has heretofore 50 a preferred reducer as well as all hexitols including all been achieved by any of the procedures of the prior art. possible stereoisomers. It is also possible to use the A further object of this invention is to provide a greater laevo isomers, and the dextro-laevo mixtures. For ex deposition e?iciency in the reduction of silver from an ample, laevo-glucitol and laevo-mannitol are highly ionic solution in which it is contained. Still another desirable reducers in accordance with this invention. object is to provide a method of reducing silver from 55 In view of the fact that certain of the alditols have a solution in a stationary manner or in a continuously tendency to undergo chemical change upon storage, it running process, with a considerable saving of the silver is highly desirable to incorporate into the aqueous solu applied to the substrate as a coating. Still another ob tion a minor amount of a preservative such as fonnal ject of this invention is to provide a method for apply dehyde or the like. The function of the formaldehyde in ing a superior coating of silver to a stationary or contin 60 this connection should not be confused with the disclo uously running substrate, without utilizing any addi sures of the prior art which indicate that formaldehyde tional silver solution. Still another object of this inven may be used as a mild reducing agent in the electroless tion is to provide a continuous method for the electro deposition of silver, as referred to in the aforemen less deposition of silver upon a continuously running tioned Sivertz et al US. Pat. No. 3,776,740, inasmuch substance, wherein the silver is continuously reduced 65 as the quantity of formaldehyde is limited to about 5 to to the metallic state in a reaction, the rate of which is 15 ml per pound of polyhydric acohol, which latter controlled by the presence of particularly effective constitutes the reducing agent in accordance with this reducing and modifying agents. invention, quantities being expressed as a 40% solution. 3,983,266 3 4 Further, in accordance with this invention. it is highly droxide, and it is preferred to use a concentrtion of .esirable to combine a moderator compound with the about I to Wt pounds of sodium hydroxide per gallon iolyhydric alcohol and to apply the moderator to the of activator solution, together with about 0 to 20 fluid urfacc of the substrate concurrently with the applica ounces of 28° ammonium hydroxide per gallon of solu» ion of the silver solution. Preferably, the moderator tion. Desirable speci?c compositions include L70 ompound is a thio compound or mercaptan of the pounds per gallon of sodium hydroxide and I3 ?uid ormula CH2OH(CHOH),,,CH2SH. where m is an inte» ounces of ammonium hydroxide (28° Baume). ,er from t) to l. The highly preferred thio compound is Regarding solution C, which might be referred to as -thio glycerol. or mercapto glycerol. a reducer solution, this solution contains about l/é to 2 A wide variety of manipulative procedures are avail 0 pounds per gallon of sorbitol (100% sorbitol) when ible in the actual practice of this invention. When the sorbitol is the polyhydric alcohol selected for use in ilvering process is to be applied to a conveyorized accordance with the invention, and about 15 to 25 ml ystem in which a substrate. such as a sheet or plate of of formaldehyde (40% solution) per gallon of concen {lass for example.