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. is continuously advanced in a hori trated solution. Ideal compositions include about 2.5 lontal plane, on a conveyor, it is highly desirable to pounds per gallon of 70% sorbitol and I9 ml of formal )rovide one or more sets of spray guns which are dehyde (40% solution), per gallon. nounted for swinging movement back and forth above In commerical use, solutions A, B and C may be he surface of the glass. each set of spray guns being made up separately and held in separate solution tanks irranged to apply the respective solutions substantially A, B and C. These solutions may be separately con~ :oncurrently to substantially the same location, on the ducted to separate guns and separately sprayed upon ‘.urface of the glass. The sprays of different solutions, the surface of the substrate in which the mixing occurs ndependently delivered to the situs and independently just prior to reaching the substrate. On the other hand, )rojected are then mixed with one another and concur» it is sometimes considered desirable to mix solutions A 'ently distribute themselves upon the surface of the and B with one another. and to conduct them jointly glass, whereupon the silver reduction reaction takes through one gun while conducting solution C through a Jlace at a delicately controlled rate, in a manner to separate gun for separate spraying upon the substrate. deposit the silver uniformly and tenaciously on the Other modi?cations and forms of procedures and appa turface of the glass, depositing a major percentage of ratus may be substituted, provided however that the ihe original silver contained in the silver solution upon reducer solution C should highly preferably be kept ‘.he glass. before the solution runs off into the collecting separate from the other solutions until subtantially the trough. time of actual contact with the substrate itself. Desirably, two or more solutions are separately pro It has been found in accordance with this invention vided and these are delivered to spray guns which are that the presence of ammonium hydroxide in the so mounted for traversing across the upwardly exposed dium hydroxide solution B increases the brilliance of surface of the glass which is traveling continuously on ~ silver deposition when solutions A and B are combined the conveyor. Each gun separately provides a single with a polyhydric alcohol in accordance with this in solution to the surface of the glass. Separate solution vention. tanks that contain either concentrated or dilute solu It is highly preferable to maintain the pH of the react tions are preferably provided. each connected to its ing solutions at a value of about l2'/2 or higher, prefer associated gun. In this manner, the individual solutions ably 12.6 or higher, in order to obtain optimum effi are concurrently applied to the glass as heretofore ciency in accordance with the method of this invention. described. Desirably, the spray guns are provided in a Solutions in accordance with this invention are highly succession or series of banks, in order to apply succes effective at a wide variety of temperatures. This is an sive layers of silver, each upon its predecessor. important factor, because it is desirable to use puri?ed This invention is applicable universally to systems 45 or deionized water that is available at the particular using hydraulic pumps for the delivery and projection silvering plant in question, regardless of the season of of the solutions. and to systems utilizing air pressure for the year without the extreme heating of the water that actuating the sprays. is the practice at many plants. Thus, there is consider It is highly desirable in accordance with this inven able ?uctuation in many geographic locations with tion to provide at least three solutions which are con respect to water temperature, and it is highly desirable currently applied to the substrate. These are conve in accordance with this invention to provide a process niently referred to as solutions A, B and C. However. which operates with optimum efficiency throughout there are some circumstances when it is preferable to this wide range of temperature. The polyhydric alco mix two of these solutions together. hols utilized in accordance with this invention are sta With respect to solution A, this solution contains a ble enough to maintain high rates of silver deposition silver salt, desirably silver nitrate, mixed with amm throughout the range from as low as 20°C up to as high monia to provide a soluble silver ammine compound, as 50°C and above. having a formula of the general type Ag,O.NH,-, or It is important in accordance with this invention to [Ag(NH‘-,)=]NO,, etc. a colorless solution. The quanti store the polyhydric alcohol, particularly sorbitol, in ties of silver nitrate per gallon may vary within the 60 the absence of highly alkaline media such as sodium general range of about 30 to 36 ounces avoirdupois per hydroxide solutions, except for relatively short periods gallon, and the amount of ammonia may also vary, for of time. For example, in approximately a half-hour, the example between 45 to 75 fluid ounces of ammonium e?'iciency of sorbitol to reduce silver diammine ions to hydroxide (28° Baume) per gallon. pure metallic silver is greatly decreased, and the film With respect to solution B, which might be referred 65 produced tends to become spotty upon the substrate. to as an activator solution, this solution desirably con Accordingly, although it is not necessary to feed all of tains an alkali and ammonia. For example, suitable the solutions A, B and C individually for application to alkalis include sodium hydroxide and potassium hy the substrate, it is desirable to avoid mixing solution C 3,983,266 5 6 with either solution A or solution B. It is entirely per missible, however, to premix the solutions A and B, as EXAMPLE m has heretofore been discussed. A plant run was conducted, utilizing an air-atomized system, and using guns mounted on transverse arms EXAMPLE I moving back and forth over a moving glass plate which The rate of silver deposition on a con?ned glass sur was running (downstream) on a continuous conveyor. face was measured using various reducers from a react Several sets of spray guns were utilized, in series, ing solution that also contained silver diammine ions spaced about one foot apart, each set spraying just and sodium hydroxide. downstream of its predecessor. In this run, solutions The following reducers were tested: sodium potas were used as follows: sium tartrate (Rochelle Salt), fructose, dextrose, so Solution A: AgNO_-,:33 oz avoir/gal + 70 ? oz NHJJH dium glucoheptonate and glucitol (sorbitol). 28° Baume All variables were maintained at the following values: Solution B: (activator) 1.70 lbs/gal NaOH; l3 ? oz NH_1OH 28° Baume Solution C: (reducer) 2.5 lbs/gal sorbitol. 19 ml (40% solution) preservative Silver diammine ions .05 moles per liter The rate of application of these solutions was varied Sodium hydroxide concentration .l5 moles per liter Reducer concentrations .05 moles per liter by varying the pump setting to control and regulate the Reaction time 2 minutes speed of the pump providing the concentrated solu Reaction temperature 30° C tions, utilizing a constant rate of coating in terms of square feet of glass coated per minute. After establish Deposition of silver was l55 mg/sq ft for d-glueitol ing steady state conditions. silver deposition of 70 while sodium gluchoheptonate gave only 148. Compa mg/sq ft was obtained using solutions A. B and C sepa rately, having the compositions de?ned above, and this rable ?gures for other reducers were: dextrose I25, 25 fructose I07 and NaK tartrate I03. D-glucitol exempli produced a deposition rate of 44 sq ft per min utilizing ?es a markedly increased rate of silver deposition in a pump setting providing 33 ml/min of concentrated comparison to conventional reducers. solution. In contrast, a solution was prepared containing the EXAMPLE ll same amount of sodium glucoheptonate instead of 30 The rate of silver deposition on a con?ned glass sur d-glucitol. A pump setting of 44 ml/min of concen face was measured using various reducers at different trated solution was required to produce a coating rate concentrations. The following constant parameters of 44 sq ft per min, with silver deposition of 70 ml/sq ft. The resulting silver ?lm was very bright and evenly were used: applied. The pump settings used for the sorbitol solu 35 tion were lower than the pump settings used with so Silver diammine ions .05 moles per liter dium glucoheptonate. The silver ?lm was heavier than Sodium hydroxide .lS moles per liter that produced by the glucoheptonate reducers. and Reaction time 2 minutes Reaction temperature much brighter. Salt spray tests were conducted based on a modi?ed 40 ASTM test method No. B ll7-72 on mirror samples The reducers tested were fructose. sodium glucohep~ taken from these runs. The ?lm was protected in the tonate and d-glucitol (sorbitol). Only the identity of the usual manner with standard mirror backing paint and reducer itself and the concentration of that reducer the results showed excellent adherence of silver to were varied. glass. There was no lea?ng of the silver from the glass. The results appear in the following table as mg/sq ft 45 The samples of these mirrors were placed in salt of silver deposited versus reducer concentration. spray. After 637 hours of continuous salt spray expo sure (in accordance with a modi?ed version of ASTM Table l method B ll7-72 for mirrors), the silver still adhered

- - __ 2 well with no signs of peeling from the glass. Concentration Fructose Sodium Glucoheptonate d-Glucitol 50 EXAMPLE [V 0.25 l40 I45 l50 0.50 l20 l75 l85 The following tests were conducted to determine the 0.75 90 I80 I90 1.00 67 I80 I90 amount of silver deposition on a glass beaker, utilizing 1.25 - I80 I90 different concentrations of sorbitol but using a concen 150 -— 180 l90 55 tration of silver nitrate solution of 33 oz avoir/gal (0.244 g/ml) with a 30-l dilution ratio of water to concentrate silver, and using sodium hydroxide con D-glucitol thus provides greatly improved rates of centrations of l lb/gal and 1.5 lb/gal. The following silver deposition versus conventional reducers. The results were obtained using 10 milliliters each of the rate of deposition rose quickly and was maintained at a 60 diluted solutions A, B and C: high rate for various ranges of reducer concentration. Table ll lSorbitol] Amount of Ag Deposition on Concentration Test No. (molar concentration) glass beaker (mg/sq ft) Comments on Film Quality

NaOH = 3 X it)’2 molar I .25 M 0.25 Good. light deposit 2 .50 M 0.25 Good, light deposit 3 .75 M 0.30 Good. light deposit 3,983,266 7 8 Table ll-continued [Sorbitol] Amount of Ag Deposition on Concentration Test No (molar concentration) glass beaker (mg/sq ft) Comments on Film Quality 4 1.00 M 0.25 Good‘ light deposit 5 1.25 M 0.35 Ag" brighter than above is 1.50 M 030 Ag" ?lm better 7 2.00 M 050 Good ?lm NaOH = 5 X 10 2 8 .25 030 Ag" ?lm: goodv light deposit molar ‘J .50 030 Ag" ?lm: good. light deposit 10 .75 0.35 Ag” ?lm: good, light deposit 1 l 1.00 0.30 Ag" ?lm: good. light deposit 12 1.25 040 Ag" ?lm: good. light deposit 13 1.50 035 Ag" ?lm: good. light deposit 14 2.00 050 Ag" ?lm: good. light deposit NHJJH = 10 ’ 15 .25 0.25 Ag“ ?lm clean and bright and molar I6 .50 0.35 noticeably different from above NaOH = 5 x 10 I 17 .75 0.35 samples molar 114 1.00 0.40 19 1.25 0.40 Ag“ ?lm appeared good 20 1.50 0.35 21 2.00 0.35 22 3.00 0.35 Excellent ?lm appearance

EXAMPLE V the diluted sample. This corresponds to 7.2 ?uid ounces NH4OH per gallon of NaOH solution. Further tests were conducted utilizing different con Very favorable concentrations were found in Run centrations of sodium hydroxide. and also utilizing No. 33, from the viewpoint of overall appearance. l-thio glycerol in a concentration of 10'“ molar, with 25 speed and deposition efficiency, while also considering the results obtained as stated in Table 111 which follows. the amount of sorbitol used. The sorbitol which was used in the tests reported in EXAMPLE V1 Table III had the following characteristics: This test was directed to stability of sorbitol at 30-1 ' _ 30 dilution of NaOH and sorbitol with H20, and feeding Y‘“'‘“§“Y. ‘00 ‘Pt (2,5 cl conjunctively. Sorbitol (0.75 M) was mixed in 30-1 Hashpin point point above 300300» pF dilution. . and reacted with. silver.. Then the sorbitol. was g?irli?s P01!“ d :"ga‘i gf'iéy'ml allowed to stand for a minimum of 6 hours to determine Hygroscopicitye ractlon In ex Medium.. lower than glycerin the CffeCIIVFlICSS- of reducing- capabilitiesv - - f0!” Ago . or glycols 35 The stab|1|ty test of diluted reducer included the following steps: 1. Sorbitol concentrate ( 1.25 M) was diluted one Table Ill Test Sorbitol Ag" Deposition Concentration Run No. concentration (M) [mg/sq ft) Comments

INaOH) = B X10" 23 .25 0.30 O. K. molar concen~ 24 .50 040 Silver ?lm brighter trate (6 ml di- 25 .75 0.30 Bright silver ?lm luted 5-1! to 26 1.00 0.35 Bright silver film 90 ml 11,0 27 1.25 0.35 Brightest. clearest ?lm of all 28 1.50 0.35 Good ?lm l0" molar NH1OH 29 2.00 0.35 O. K. added to NaOH 30 3.00 0.45 O. K. lNaOHl = 6 X 10‘2 ll .25 0.40 Very bright. Ag” ?lm thick molar 32 .50 050 Film much heavier 33 .75 0.45 Very bright; best overall 10‘1 molar NH.0H 34 1.00 045 Good ?lm added to NuOH solu- 35 1.25 0.50 Same results tion 36 1.50 0.45 Results similar to L25 M of sorbitol 37 2.00 0.55 for remaining reactions 38 3.00 0.65 [NaOH] = 6 X 10" 39 0.50 0.20 Doubling the NH.0H concentration molar 40 0.75 0.30 decreased the amount of Ag“ deposited 41 1.25 Not titrated but very thin nearly to half 2 X l0-I NI'LOH added 42 2.00 Not titrated but very thin to NaOH

The addition of ammonium hydroxide to sodium hydroxide de?nitely improved the rate of reaction. it 60 also produced a silver film that was very much brighter part sorbitol with 30 parts water. The reducer was and cleaner in appearance. The sodium hydroxide concentration is also of im mixed with silver and NaOH and the reaction took portance. An increase of NaOH concentration in place in a glass beaker (1.5 mm). The ?lm was dis creased the deposition of silver — to a point. 65 solved in nitric acid and titrated with KCNS. The optimum silver deposition occurred at an NaOH 2. Six hours later the same reaction was conducted concentration of l.75 lbs/gal (in the concentrated form using the diluted samples, and the Ag ?lm was titrated of solution) with 0.3 ml of NH,OH added to 90 ml of against KCNS to obtain a direct comparison. 3,983,266

mg. Ag deposited Mannitol Run No. concentration Gms. Ag‘I Comments Results: Sorhitol (fresh) 0.40 Sorhitol (6 hours) 0.40 5 4] 0.25 M .0024 Immediate deposit. no sludge. clear film 44 0.50 M .0034 Clean ?lm. good deposit 45 0.75 M .0040 Heavier Ag" deposit This test demonstrates that sorbitol does not lose 46 1-00 M W37 ?'lcimp?lm- am“! As" ' . ( epnslt efficiency after 6 hours. it has been found that such 0 47 025 M mm Mlwcd “mixing Rwcmm solutions can be used after storage ovemlght, wlthout was allowed to proceed for r due ‘ of e ' t 5% nun to completion 6 “on f?clency 48 0.50 M .0095 Allowed sludgl'ng. Reaction was allowed to proceed for EXAMPLE V" 6 min to completion. Solutions of sorbitol were made 70% at 1.25 M and t5 potassium sorbate pellets and powder samples were added. Potassium sorbate was added at concentrations EXAMPLE IX of 0. 1% and 0.0l%. The potassium sorbate was used as a mold inhibitor, used in place of formaldehyde. Reaction of sorbitol at reduced temperatures: The following tests were run: 20 1. Solutions were cooled to 20° C (68° F) and reacted in ISO ml beakers. Solutions were also run using potas sium sorbate pellets. Sample No. Sorhitol concentration K Sorbate concentration 2. Reaction time: l .5 min. KS powder 125 M m“ (by WH powder 3. Standard [Ag]; NaOH = 1.70 lbs/gal. 13 fl. ozs. KS powder 1.25 M 0.0m (by wt.) powder 25 NH4OH were used as stock solutlons and dlluted 30:l KS pellets 1.25 M 0J0‘; (by wt.) pellets w‘ ‘ ' KS pellets 1.25 M 0.0% (by wt.) pellets “h dlsmled "20' Temperature variance v. rate of reaction Results: Reaction occurred quickly at the reduced tempera There was no indication of interference with the 30 tures and the samples were set aside for titration analy reaction and the presence of sorbate did not increase or sis. The ?lms of silver which had been produced at promote sludging or sludge time. reduced temperatures were similar in appearance to EXAMPLE Vlll those run at room temperature. Mannitol solution preparation (MTL = mannitol) 3-5 Sample Description No. grams/beaker Molar COII- Pellets (K Sorbate 1) sngbiwh I325 M -0024 Sample No. centrat'lon Grams/100 ml concentration) 20 C *“lumms 2) Sorbitol: l.25 M .0025 MTL 25 0.25 M 4.55 -0.l0q “0 21° c KSP 0.1% MTL 50 0.50 M 91! —0.l0el 3) Sorbitul: l.z5 M .0025 MTL vs 0.15 M lam —0.l0% 27° c KSP 0.0m MTL 1.00 1.00 M ls.22 —0.l0‘k MTL l.2s 1.25 M 22.70 —0.l0% MTL l.s0 1.50 M 21.33 -0.l0'/, 45 A nearly 10 a C drop ln_ temperature had a negllglble_ _ effect on the rate of deposition of silver versus the rate Procedure. of reaction at 27° C for samples 2 and 3. Among the polyhydric alcohols and particularly the l. Tests were run at [.5 min in 100 ml beakers. At aldltols,_ the following. classes crust:_ that time excess solutions were discarded and beakers rinsed. 2. Concentrations of reactants: ll Emmi“ cmoll cllptl

Ag(Nl-i,),+ 33 av. oz. AgNOJgal and 70 0. cu. moon/gal 55 ll-CP" "0C" NaOH l.70 lb. of NaOH/gal. __ __ _ Nl-LOH 13 H. oz. of 28' Be per gal. H Co“ H C 0“ Mannitol variable (as above) _ Temperature 30' C H CO“ CH'OH Erythritol D-Threitol

. . . . . 60 CH 0H cl-l OH All solutions were dlluted 30-l wlth delorllzed water. | ' | ' l-thio glycerol was added in the concentration of "0?" "$0" 0.004% by weight. (‘SH-0H HO—CI—H Procedure: . . CH CH CH on The number of mg. of precipitated Ag was calculated 65 ' ' using the Volhard titration method for quantitative D. L Threitol analysis of Ag“ using standardized KSCN. 2) Pentium The results were as follows: 3,983,266

-continued ~continued (;H._.OH tin-ion CH.,0H CH,OH C'H,0H CH,0H Hot‘ H-(‘OH H-coH H—("—0H H-(|‘—0H HO-C-H H-CQH H—C—oH HO-C-H 5 H-(i-OH H0-L|"-H H—-C-—0H H-CoH H-C-UH H—(I‘—()H H—C|‘—0H H0—C|‘—H H—L|‘—0H (‘HIGH inc-0H CinoH H—("-()H Ho-C—H H—c"-0H D>Arabintol Rihitul Xylitul H-’("-OH H‘_C'-OH H-C_OH iD-Lyxitul] H) | CH,0H CH,0H CH,0H Volemitol Meso—Glycero-altro D-GlycermDaltro D-glucose has been converted into penitols in yield of LH'OH 35 — 409%. it is obtained through synthesis by reduction 15 H0-C'-H of D-arabinose or D-lyxose by sodium amalgam or I sodium. borohydride.. L-urablnltol. . has been prepared by Ho-C-Hl reduction of L-arabinose and also the Cannizzaro reac- H0"(|‘_H tion with l.-arabinose in the presence of nickel. HO-C-H In view of the compatibility of the polyhydric alco- 2" hols with each other, they may be used individually or H_C_OH in any desired combination with each other. CH,OH D-Glycero-L Allo 25 51 ()ctitols I) Hcxituls ie D-erythro-D~Galacto-octitol: the only il carbon (at Allitol (b) Galactitol (c) D~Glucllul chained alditol that occurs naturally. (Mcso) (Meso) ("H,0H CH.,oH CH,0H l l l 30 EXAMPLE X H-(‘OH Con H-C-OH l_ l Ho 0 H Tests were conducted on the effect of temperature H (| OH HO (; H on rate of silver deposition reaction, and to compare H'—tI‘-OH HO—(l‘-'H H-f-OH directly the reducing capacities of other reducers, H_(___0H (._0H H_C_0H 35 namely, fructose. dextose, glucomc acid and sodium | heptagluconate versus D-glucitol and D-manmtol. K‘HtOH (‘Ht-ml (‘"20" Concentrations of chemicals:

fAgHNOa)+ = 3 X H)“ moles/liter NaOH = l5 X 10“ moles/liter NHiOH = t X 10'2 moles/liter Reducer = 3 X It) " moles/liter Reaction time = I45 min. (90 sec.)

Run Concentration of No. reducer Gms. Ag" Comments 49 0.75 M (Mannitol) .0057 Deposition at 50° C t I22’ F). A good, heavy ?lm was produced 50 0.75 M (Mannitol) .0057 T‘ = 45° C ( I04‘ F] 5| 0.25 M (Mannitol) .0028 30°C 52 0.50 M (Mannitol) .0034 30°C 53 0.75 M (Mannitol) .0038 30°C 54 L00 M (Mannitol) .0034 30° C 55 0.50 M (Mannitol) .0033 25° C 56 0075 M (Mannitoll .0036 25“ C

(d) D-lditol (e) D-Munnitol (f) D-Altritol C|H,0H CIHDH C'FhOH EXAMPLE Xl HO-(II-H "0-? Ho-c'" An object of this example is to demonstrate rates of H_C_OH HO_C H_C_OH reactions of other reducer solutions for a direct com | I 0H 69 parison with sorbitol. The compounds tested in this HO"C_H H_(‘:_°H H C case were the following: (1) potassium sodium tartrate, H-C-OH H-C-OH H-C-OH (2) fructose, (3) gluconic acid, (50% P?zer) (4) so CH‘OH CHIOH CHIC“ drum heptagluconate, (5) sorbitol, (6) mannltol, (7) dextrose. 65 All variables in this test were maintained constant except for the nature of the reducing material. The . silver solution employed was a silver diammine solution 4) Heptitols and the constants were as follows: 3,983,266 13 14 3. 3. section: 3 min. reaction time before final rinse of silvering solution with water. AgNQ, 4 X It)“2 moles/l 4. Silvering was accomplished by an airless spray NaOl-l l5.0 X [0"! moles/l +0.3 m1 NHtOH (28%) Reducer 4 X 10‘2 moles/l + l~thio glycerol (004% system. The console was provided with metered flow by wt.) which was set at settings 9 and l l. (Flow rates for these Reaction time l20 sec. Reaction 30° C settings are listed below.) temperature 5. The conveyor was 68 inches wide X 72 inches/min. linear speed. A day‘s run was conducted, utilizing ap Run No. Reducer Grams Ag" deposited proximately l500 sq. ft./hr of glass surface, for l0 57 Gluconic acid-509i .003l hours. This silvers 34 sq. ft./min = 2040 sq. ft./hr. = 58 Sodium glucoheptonate .0037 59 Fructose .0032 20,400 sq. ft. l day. 60 Potassium sodium tar- .0032 6. Spray tips: trate 6i Dextrose .0030 62 Sorbitol-70% .0043 No. 9 No. l l setting setting The gluconic acid 50% sample of Run No. 57 had lst spray set: .050" ori?ce about 300 about .125 ml/min ml/ntin changed color because of limited shelf life from a straw 2nd, 3rd, 4th .0l0" ori?ce about I75 about 200 colored substance to a brown color. The effectiveness spray sets: ml/min ml/‘niin of this material as a reducer was most probably im 20 (for each gun) (for each gunl peded due to this degradation. However, the sodium glucoheptonate sample which was derived from a salt powder showed no signs of degradation. The results of Various settings of ?owmeter were used. including these tests show conclusively the greater reducer capa setting “9" which is usually used for window glass, and 25 setting “II“ which is usually used for providing a bilities of sorbitol. thicker silver coating on plate glass. Each solution of reducer was modi?ed or “buffered" to retard sludge formation with l-thio glycerol. The Run a dextrose solution was the only reducer to sludge in the 120 sec. reaction time. A dilution of 40:] (5 gals. H2O to 1 pt. concentrate Fructose produced a highly re?ective coat but its ?lm 30 ABC) was used. The other reaction conditions were: weight was about one-third less than that obtained with sorbitol. Temperature Potassium sodium tartrate produced a yellowish cast Reaction time Meter setting on silver ?lm and the ?lm also had pinholes. This re~ Silver deposit 70 mg/sqft. No. 9 setting ducer is totally unacceptable. 35 7! tng/sql't. No. ll setting Dextrose produced a good ?lm but the sludging in this solution was quite difficult to control to obtain optimum reducing capabilities. Further, the weight of It was observed that the silver deposited very evenly, silver deposited was signi?cantly lower than the corre~ cleanly (without sludge) and quickly. sponding average weight deposited by an equimolar 40 solution of sorbitol. Run b Sorbitol, a hexahydric alcohol (alditol), accordingly, Another run using the same equipment was con has a much greater ability to reduce silver to a metallic ducted under the following conditions: state than gluconic acid and a salt of a monocarboxylic acid such as sodium glucoheptonate. 45 Dilution 40: l EXAMPLE X" Temperature I20“ F Meter setting No. l = 9; No. 2 = H A run was conducted in a continuously running con Silver deposit 71 mgs/sqft. No. 9 setting: veyor line for the production of mirrors, utilizing a 73 mgslsqtt. No. ll setting three part sludgeless process using solutions A, B and C 50 as follows: Concentrations: Results and observation: the silver deposit increased A. Silver diammine 33 av. ozs./gal + 60 ?. ozs. with a 52° C increase of temperature. This shows that the reducer of this invention can be used either hot or B. Sodium hydroxide l.5 lbs/gal + l ?. ozs. Nl-LOl-ll cold. This increase of temperature, though drastic, did gal. not produce a drastic increase of silver ?lm weight at C. Sorbitol 2.5 lbs/gal (of 70% sorbitol solution) 19 this dilution. ml formaldehyde/gal (40% solution CHZO) 0.] ml. Run c thiovanol The procedure was as follows: 60 This is a comparative example, using dextrose solu 1. Mix solutions in the ratio of 29 gals. of water to tion as solution C. All operating conditions, including one gal. each of silver, caustic and reducer. speed and reaction time, were the same as in Runs (a) 2. Solutions were sprayed at 125° — 130° F. All solu and (b). tions passed through a heated coil which was im 65 mersed in a heated bath at a temperature of 125° — Dilution Ratio (gal 130° F. Rinse water was also heated at same tem Flow Temp- Ag deposit Hgolgal concentration perature and sprayed on the glass prior to silvering setting erature tmglsq.t‘t.) rate ABC) in order to increase the glass temperature. Dextrose l 1 I30“ F 90 29:l 3,983,266 15 16 -continued Dilution Ratio (gal Temperature: 125‘I F Flow 'fcmp— Ag deposit HEO/gal concentration Reaction time: 180 sec. setting craturc tmglsqlfl.) rate ABC) Meter setting: Nov 1 at 9, No. 2 at ll ‘ 5 Silver deposit: 73 mgs/sq ft No. l Sorhitnl l 1 80° F 95 33:] 80 mgs/sq ft No. 2

Accordingly. even at a lower temperature and It was observed that the silver deposit increased greater dilution, the sorbitol solution reduced a greater to somewhat with the increase in concentration of the quantity of silver on the glass substrate. reactants. Sorbitol (d-glucitol) does react quickly enough to provide a sufficient silver ?lm that is acceptable to EXAMPLE XlV today's mirror manufacturers at room temperature Investigation was made of other polyhydroxy alditols (68° F or 20° C). and has an advantage in that the 15 of varying carbon chain lengths (erythritol and xylitol ). dilution ratio is 40 to 1 versus 28 to 1 of other solutions, a. Concentration of solutions: This provides 12 additional gallons of ready-to-spray AgNO3: 4 X 10‘2 moles/l material and thus reduces the cost of solutions very NaOH: lS X 10‘2 moles/l +0.3 ml NH,OH (28%) signi?cantly. Reducer: 4 X 10‘2 moles/l Accordingly, in this example, dextro-glucitol was 20 l-thio glycerol: 0.004% by weight shown to improve sharply the rate of silver deposition. Reaction time: 120 sec. In a conveyor run of 10 hours, the substitution of dex Reaction temperature: 20° C tro-glucitol for dextrose results in a saving of approxi mately 1 gallon of silver concentrate per day, for a run of about 20,000 sq ft of glass. This is due to the much 25 Sample Reducer Gms Ag“ higher efficiency of the reducer solution, where effi 63 Erythritol .0024. The reaction was somewhat ciency is de?ned as the number of milligrams of silver slower than with sorbitol but a good Ag“ ?lm was produced. deposited divided by the number of milligrams of silver 64 Xylitol .0033 The reaction was much faster sprayed, times 100. gms. than with erythritol and pro 30 duced a good. clean ?lm, hut EXAMPLE Xlll xylitol was found to be not as ef?cient as d-glucitol per mol Further tests were conducted using the equipment in these runs. described in Example Xll but with respect to dilution ratio, as follows: 1. Dilution ratio: 40 parts water to 1 part concen 35 Conclusions: trate. However. NaOH concentration was increased Both erythritol and xylitol do react with silver diam slightly to a NaOH concentrate of 1.75 lbs/gal instead mine in an alkaline environment to produce a silver of 1.50 lbs/gal. Also, 10 ml of NH4OH was added. No mirror. Their rates of deposition are clearly less than other constitutents were changed in concentration - d-glucitol (rate of reaction probably can be increased only NaOH. with an increase in concentration) but they produce excellent ?lm quality. EXAMPLE XV Temperature: 125° F Reaction time: I80 sec. Tests were run for comparison of conventional re Meter setting: No. l at 9. No. 2 at 11 Silver deposit: 72 mgs/sq ft No, l ducers with d-glucitol at varying reducer concentra 73 mgs/sq ft No. 2 tions. These tests included a study of rates of silver deposition using the following materials as reducers: fructose, sodium glucoheptonate and mannitol. It was observed that silver ?lm increased slightly with The following concentrations were used: an increase of NaOH concentration. Good silver depo- 5 sition was observed and it occurred very quickly. The silver film is excellent in quality. Silver diammine ions 5 X 10" moles/liter Sodium hydroxide 15 X l0‘I moles/liter 2. Dilution ratio: Same NaOH concentration as Reaction time 120 sec. above ( 1 ), same silver concentration as throughout and Temperature 25° C 100 ml additional of 1.25 M sorbitol solution. 55 Reducer Film Test No. Reducer Concentration mgms Ag“

Temperature: 125° F 65 Fructose 0.25 M 3.85 Reaction time: 180 sec. 66 Fructose 0.50 M 340 Meter setting: No. l at‘),No. 2a! 11 67 Fructose 0.75 M 2.50 Silver deposit: 73 mgs/sq ft No. 1 68 Fructose 1.00 M 190 74 mgs/sq ft No. 2 69 Sodium glucoheptonate 025 M 3.90 70 Sodium glucoheptonate 0.50 M 4.90 71 Sodium glucoheptonate 0.75 M 4.90 72 Sodium glucoheptonate 1.00 M 4.90 73 Sodium glucoheptonate 1.25 M 4.90 It was observed that there was an increase of reducer 74 Sodium glucoheptonate 1.50 M 4.95 concentration along with increased NaOH concentra 65 75 Mannitol 0.25 M 4.10 tion increased the rate of silver deposition slightly. 76 Mannilol 0.50 M 5.06 77 Mannitol 0.75 M 5.06 3. Dilution: 37 parts water to l of concentrate A, B, 78 Mannitol 1.00 M 5.10 C. 79 Mannitol 1.25 M 5.10 3,983,266 17 -continued Further, sorbitol has been shown to be stable in stor age without loss of reducing ability in this clectroless 80 Mannitol LSO M S. l 5 method of depositing silver. Further, it can be stabi lized for a prolonged time with the aid of formalde hyde, potassium sorbate or other materials. EXAMPLE XVl Sorbitol has the ability to withstand variations of temperatures of l0° C and more, without any substan Another run was conducted in a continuously run tial loss of efficiency. At water temperatures lower than ning conveyor line for the production of mirrors utiliz 20° C, the efficiency of this reducer can be increased by ing a three part sludgeless silvering solution whose heating the water. Sorbitol is stable upon heating. Heat concentrations were as follows: ing to 50° C is possible without adverse effects such as 1. Silver Diammine: 33 av. ozs/gal. + 60 fl. oz. disintegration or decomposition. Nl-LOH/gal. indeed. sorbitol can be used at quite high tempera~ 2. Sodium Hydroxide: l.5 lbs/gal. — l0 ?. 02s. tures (up to 130° F, for example) without decomposi NH‘Ol-l/gal. ‘ tion or loss of its reducing capabilities. This increases 3. Sorbitol: 2.5 lbs/gal. (70% solution), 19 ml. for the rate of silver deposition. Even at increased temper maldehyde (40% sln.), 0.1 ml. thiovanol. atures, sludging is readily controlled. Procedure: it is important to observe that although the highly 1. The solutions were pumped from containers that preferred moderator compound is l-thio glycerol, as held the concentrated solutions and the ?ow rate of the 20 described heretofore in the speci?cation and examples, solutions was metered. various other compounds may be used within the for 2. Solutions were heated to 120°F. and ?owed at a mula CH2OH(CHOH),,,CH2SH, where m is an integer rate of 24 ml. of concentrate A, B, C each per min. from 0 to l. 3. A conveyor was used which was 48 inches wide In view of the fact that these thio compounds or and traveled at a linear speed of 85 inches/min. which 25 mercaptans are quite compatible with each other, they corresponds to 28 sp. ft/min. of glass. may be used individually or as admixtures with each 4. Silvering Section: reaction time of 40 sec. before other. Regardless of the thio compound that is utilized the silvering solutions were rinsed with water. There in accordance with this invention, it should be present were four sets of spray guns (?rst set 0.039 inch) ori in the reducer solution in a proportion of 0.004 to lice and the 2nd, 3rd, and 4th set of guns — 0.050 inch 0.020% by weight of thio compound, based upon the ori?ce). weight of the water, and the percentage of polyhydric 5. Silvering was accomplished by an airless spray alcohol of the formula CH,OH(CHOH),,CH,OH, system. The console contained a flow meter to regulate where n is an integer from I to 6, should be present in the quantity of silvering solutions applied to the glass. 35 the same solution in the quantity of 5 to 35% by weight EXAMPLE 1 of said polyhydric alcohol, based upon the weight of the water. The thio compound and the polyhydric alco Flow rate: 24 ml. of concentrate A, B, and C per min. hol are quite compatible with each other and may be Temperature: 120° F. stored for an inde?nite period in a common solution. Reaction Time: 40 secs. With respect to the silver solution that has been de Silver deposit: 73 mgs/sp. ft. scribed herein as being desirably silver nitrate mixed It was observed that silver deposited evenly and with ammonia to form a soluble silver ammine com cleanly. During this run, samples of mirrors were taken pound, it will be appreciated that a wide variety of for a scotch tape adhesion test. Here, a piece of scotch other silver solutions may be used as well, including tape was applied on top of each silver ?lm and pressed 45 well known silver compounds or salts, including com ?rmly with the fingers. The tape was then quickly re plexes, coordination compounds and the like. How moved and observed for the removal of the silver film. ever, it is highly preferred as has heretofore been stated In this test, no silver was removed. This is considered to provide a silver complex with ammonia. However, severe testing. The result attests to the tenacity of the this invention is not intended to be limited to any par adherence of this silver ?lm to the glass. 50 ticular silver solution inasmuch as the novel moderat ing reducer compositions in accordance with this in SUMMARY vention are highly e?‘ective with various types of elec It has been discovered, accordingly, that great advan troless silver deposition solutions. In this connection it tages can be enjoyed commercially with the use of should be pointed out that although this invention has alditols in the manner described in this speci?cation. 55 been preferably carried out with three solutions A, B Sorbitol is demonstrably a superior reducer as com and C as heretofore described, the novel reducer pared to gluconic acid and other such related materials, modi?er solution in accordance with this invention i.e., fructose, dextrose, etc. Sorbitol has shown conclu may be applied with great success to a single silver sively the ability to reduce silver to its pure metallic solution from which the silver is to be deposited. state in a more efficient manner than gluconic acid and 60 The relative proportions of reactants in the silvering other reducers of conventional use. This means signi? solutions may vary over a wide range. As has been cant savings in the quantity of silver required to silver disclosed in the examples, the ratio of the reducer an object. modifier solution to the silver compound may also vary Further, it has been found that the silver deposited over a wide range, preferably within the molar ratio from solutions according to this invention adheres te 65 range of about I to l. Below this range the rate of naciously to the substrate. Even using such a severe test deposition of silver upon the substrate suffers, and as the "scotch tape” test, the silver adhered to the above this range the rate of reduction reaction does not substrate and was not lifted off by the scotch tape. materially increase. 3,983,266

Further. the ratio of silver salt to the other reactants S. The method de?ned in claim 4, wherein said thio may vary widely. Preferably the ratio of alkali metal compound is l-thio glycerol. hydroxide to silver salt is about 3:] on a molar basis‘ 6. The method de?ned in claim 5, wherein said poly Various other parameters and factors in connection hydric alcohol is sorbitol. with the reaction. including concentrations, tempera 7. The method de?ned in claim I, wherein said sub tures, ratios, methods of application. spraying. pouring, strate is glass. etc. are considered to be well within the capabilities of 8. The method de?ned in claim 1. wherein said dis those skilled in the art. However. it is highly desirable solved silver. said hydroxide and said ammonia are in the actual performance of the process that the vari mixed together before being applied as a mixture to ous reactant solutions are brought together just before said substrate, and wherein said polyhydric alcohol is separately applied at substantially the same location contact with the substrate to which the silver is to be upon said substrate as the location to which said mix applied‘ This. of course, is attained in a manner that is ture is applied. familiar to persons skilled in the art. 9. The method de?ned in claim 1, wherein said dis Although this invention has been described with ref solved silver and said ammonia are mixed to form a erence to certain particular compounds. processing solution (A) separately applied to said substrate, conditions and other parameters, it will be appreciated wherein a solution (B) of ammonia and said hydroxide that many variations may be made without departing is separately applied to said substrate, and wherein said from the spirit and scope of the invention as de?ned in reducer solution (C) comprising polyhydric alcohol is the appended claims. 20 also separately applied to said substrate, all solutions The following is claimed: (A), (B), and (C) being substantially concurrently 1. In a method of applying a coating of metallic silver applied at substantially the same location on the sur to the surface of a substrate, wherein a silver solution face of said substrate. containing reducible dissolved silver in the presence of 10. The method de?ned in claim 1, wherein said an alkali metal hydroxide and ammonia is applied to 25 reducer solution also contains about 5 to 15 ml of for the surface of said substrate‘ the improvement which maldehyde per pound of said polyhydric alcohol. comprises concurrently applying to the surface of said ll. The method de?ned in claim I, wherein said substrate and mixing in situ with said silver solution a substrate is plastic. separate aqueous solution of a moderating reducer 12. The method de?ned in claim 1, wherein said comprising about 5 to 35%, by weight of the water, of 30 substrate is ceramic. a polyhydric alcohol of the formula CH2OH( 13. In a method of modifying the silver reduction CHOH),,CHZOH where n is an integer from 1 to 6. reaction in the electroless deposition of metallic silver 2. The method de?ned in claim 1. wherein said alco as a ?lm upon a substrate. wherein said silver is present hol is selected from the group Consisting of erythritol, in aqueous solution as a silver diammine and is applied threitol. arabinitol, ribitol, xylitol. allitol. galactitol, 35 continuously to a moving substrate. the step which glucitol. sorbitol, iditol. mannitol, altritol. volemitol, comprises concurrently applying to the surface of said glycero-altro hepitol, erythro-galacto octitol and dex moving substrate and mixing with said aqueous silver tro and laevo isomers thereof. diamine solution an aqueous solution of a polyhydric 3. The method de?ned in claim 1, wherein said poly alcohol of the formula CH,OH(CHOH ),,CH,Ol-l hydric alcohol is sorbitol. wherein n is an integer from 1 to 6. 4. The method de?ned in claim I, wherein a modera 14. The method de?ned in claim 13, wherein said tor is concurrently applied to said substrate surface. alcohol is de?ned in claim 2. said moderator comprising a thio compound of the 15. The method de?ned in claim 13, wherein said alcohol is sorbitol. formula CH2OH—(CHOH),,,—CH2SH where m is an it‘ * * * ‘ll integer from 0 to lv 45

50

55

60

65