Catalytic Sensitization Of Substrates For Metallization

Abstract

Metal films of improved uniformity are formed on substrates having their surfaces sensitized by the deposition of palladium on tin by buffering the palladium salt solution in contact with the glass at a pH from 6 to 9. Buffering is preferably accomplished by contacting the substrate with an aqueous buffering solution and an acidic palladium salt solution.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

This invention is related to electroless metal deposition and to the processes disclosed in co-pending applications here listed: "Transparent Metal-Boron Coated Glass Articles", Ser. No. 57,575; "Wet Chemical Method of Producing Transparent Metal Films", Ser. No. 57,451; "Solution for Depositing Transparent Metal Films", Ser. No. 57,754; and "Wet Chemical Method of Producing Transparent Metal Films", Ser. No. 57,527, all filed July 23, 1970. Application Ser. No. 57,575 is now abandoned; application Ser. No. 57,754 is now U.S. Pat. No. 3,674,517 issued July 4, 1972; application Ser. No. 57,451 is now U.S. Pat. No. 3,723,158 issued Mar. 27, 1972, and application Ser. No. 57,527 is now U.S. Pat. No. 3,723,155 issued Mar. 27, 1972.

BACKGROUND OF THE INVENTION

This invention relates to chemical plating referred to in the art as electroless plating, and more particularly, it relates to a new method of sensitizing a substrate surface to produce a catalytic surface receptive to the deposition of metal-boron containing film.

In the art of metallizing substrates, particularly non-conductive materials such as non-metals, for example glass and plastics, it has been found desirable to prepare the substrate surface to make it more receptive to metal deposition.

Bergstrom in U.S. Pat. No. 2,702,253 teaches an effective method of sensitizing a substrate surface and thereafter metallizing it utilizing electroless plating solution. In the method taught by Bergstrom the substrate is first treated with a tin salt (a step now referred to in the art as "sensitization"), then treated with a palladium salt (a step now referred to as "supersensitization") and finally treated with the salt of the metal desired to be deposited in a suitable electroless plating bath.

While the method of Bergstrom has been widely accepted by workers in the art, the method has not been free of problems. When depositing relatively thick films on substrates by immersing the substrates in a series of baths according to the teachings of Bergstrom, it has been possible to produce satisfactory films. However, attempts using the method of Bergstrom to deposit relatively thin transparent films, particularly by economical and rapid spray techniques suited for the production of transparent metallized articles in commercial quantities, have resulted in imperfect films. There has been a tendency of the films to be streaked, to have many fine pinholes through the film, to have bands of thicker and thinner film as visually observed and to have a thinner film along the leading edge of a moving substrate passing under sprays for contacting the substrate with chemical filming solutions.

SUMMARY OF THE INVENTION

It has been discovered that by buffering a palladium salt super-sensitizing solution in contact with a substrate to be sensitized for chemical metal film deposition to a pH between about 4 and about 9 that uniform metal films may be obtained when thin, transparent metal films are subsequently deposited on the sensitized surface from electroless plating baths. The presence of a colloidal suspension of palladium hydroxide in contact with the substrate being treated enhances the uniformity of subsequently deposited films. A pH within the range of from 6 to 9 for the palladium solution in contact with the substrate provides assurance of this desired condition for palladium treatment.

A suitable process of metallizing a plurality of substrates, such as flat glass articles, is to convey the articles in series past several spray, drip or flood contacting stations. The process includes: rinsing the glass, contacting the glass with a tin salt solution, rinsing away the residue of tin salt solution, contacting the glass with a palladium salt solution, rinsing away the residue of the palladium salt solution and contacting the glass with one or more sets of metal salt solutions and boron-containing reducing solutions followed by rinsing, drying and inspection of the metallized glass. By including a suitable buffer, such as sodium bicarbonate, in a rinse between the tin salt solution and the palladium salt solution, residue buffer is retained on the glass to buffer the pH of the contacting palladium salt solution to a pH of from 4 to 9 and preferably from about 7 to about 9.

Other buffering agents may be successfully used rather than sodium bicarbonate. Typical, commercially available buffers which may be used include water soluble alkali metal carbonates, alkali metal bicarbonates, alkali metal phosphates, ammonium hydroxide, ammonium carbonate, ammonium bicarbonate, ammonium phosphate and ammonium borate. The dibasic phosphates are preferred as contrasted with other phosphates. Alkali metal salts of organic acids may be effectively employed as well; sodium acetate, for example, is effective to control uniform supersensitization. Also, mixtures of buffers, such as phosphates, and alkali metal hydroxides are effective buffers in this invention.

Although other tin and palladium salts may be used, the halides are preferred. Stannous chloride (SnCl.sub.2) is the preferred tin salt for initial substrate surface sensitization. Palladious chloride (PdCl.sub.2) is the preferred palladium salt for supersensitization of the substrate surface, although palladic chloride (PdCl.sub.4) may be suitably employed.

It is preferred that water used for preparing the aqueous sensitizing and chemical filming solutions and the water for rinsing the substrate be softened, deionized or otherwise purified since in the commercial production of metallized articles film quality may be detrimentally affected by the uncontrolled chemical composition of process water entering a plant from public water supplies. Throughout the present discussion, use of the words "water", "demineralized water" or the like shall mean water substantially free of organic and inorganic contaminants, for example an indicated resistance of more than one megaohm/cm at 25.degree.C. would indicate pure water. Throughout words, such as "tap water", shall mean water of unknown quality as typically obtainable from public supplies.

It is preferred that the palladium chloride contacting solution has a pH from about 1 to 4, preferably from about 2 to 3.9, and most preferably from about 3.0 to 3.6.

The buffer may be included in the intermediate rinse between tin sensitization and palladium supersensitization, or the substrate may be rinsed with water and then contacted, as by a spray or drip, with an aqueous buffer solution immediately prior to the palladium salt solution contacting the substrate. In the latter embodiment buffer solutions of 0.1 to 70.0, preferably 0.5 to 5 and most preferably 0.5 to 1.5 grams of buffer per liter of water, when dripped on a horizontally disposed glass sheet being conveyed under spray contacting stations, suitably buffer the immediately following palladium salt solution in contact with the glass.

The present discovery provides a method by which more uniform, mottle-free, metal-containing films may be deposited on a catalyzed non-metallic substrate than has been possible before. Bands of non-uniform film thickness, streaks and pinholes through the deposited metal-containing films are substantially reduced and commonly are absent from films produced by the method of this invention. Thinning out of the film toward the leading edge of substrates metallized while moving past contacting stations is substantially reduced and generally eliminated by this method. It has also been discovered that the adhesion of the deposited film to the glass or other substrate is improved by applying film by this method. This results in films of improved durability when the films are applied according to this invention.

Any metal suited for chemical filming on a catalytic sensitized substrate may be deposited according to the method of this invention. Chromium, manganese and the metals of Groups VIII and IB of the Periodic Table may be applied to substrates according to this invention. Of particular interest are the lightest elements of Group VIII, iron, cobalt and nickel. The method of this invention is particularly suited for the application of iron, cobalt or nickel to substrates which require sensitization, such as glass.

The method has particular utility for producing thin, transparent metal-containing films on transparent substrates, for example glass or plastics, such as polycarbonate, acrylics and the like. The desirability of uniform films is particularly critical for transparent viewing closures. Visible transmittance of flat, clear, lime-soda-silica glass metallized using the present method may be as low as 8 percent and yet be uniform. Transparency is determined as the percent of light having a wave length from 380 to 760 nanometers which is transmitted through the coated article relative to the transmittance through air as measured by a Beckman Model DK 2A Spectrometer.

The mechanism by which the buffering of palladium salt sensitization solutions promotes film uniformity is not fully understood. In fact, the mechanism by which palladium catalytically sensitizes substrates is not understood with universal agreement among those skilled in the art. This lack of agreement and understanding persists despite the many years of using palladium sensitization.

Despite a lack of full understanding as to why the present invention is effective, the following observations may be made. Palladium chloride solubility in water is influenced by pH. High concentrations of palladium chloride in a sensitizing solution improve sensitization of the surface, presumably by increasing the density of catalytic sites at the sensitized substrate surface. The preferred pH range of a suitably concentrated palladium chloride solution is from 1 to 4 with higher concentrations attainable at lower pH. At a pH of 3.9 some incipient Pd(OH).sub.2 precipitation is noted. A colloidal suspension apparently is present upon adjusting pH from 6 to 9 as indicated by a yellow coloration typical of colloidal suspensions; at pH 6 to 7 the characteristics of colloidal suspension are apparent; at pH 7 to 9 a colloidal suspension remains with the apparent particle size of the precipitate smaller at pH 8 than at pH 7.8. It appears that precipitation and formation of a relatively stable colloidal suspension of palladium hydroxide in the supersensitization solution when contacting the substrate is at least partially responsible for the beneficial results obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A plurality of commercial soda-lime-silica glass plates (for example, 28 inch .times. 66 inch .times. 0.25 inch) are metallized or coated with a nickel-boron film using the apparatus illustrated schematically in FIGS. 1 and 2. The apparatus comprises five basic sections, designated as the glass loading and cleaning section, 100, the glass sensitizing or activating section 200, the metallizing or film deposition section, 300, the glass drying section, 400, and the inspection and unloading section, 500.

The apparatus comprises in section 100 a plurality of belts 1 for carrying and advancing glass plates 3 through the section. In sections 200-500, the apparatus comprises a plurality of rollers 2 for carrying and advancing the plates through the sections. The belts and rollers are rotated by conventional means (not shown) to advance the plates at about 3 to 6 feet per minute.

In operation, plates 3 are loaded onto the belts 1 and advanced through section 100. In this section, four rotating blocks 101 comprised of brushes gently abrade the upper surface of the plates 3 with a mixture of amorphous silica and water or cerium oxide and water to loosen and remove any dirt. The blocks are rotated on shafts 102 at a rate of about 350 rpm and are oscillated in a direction transverse to the motion of the advancing plates at a frequency of about 15 cycles per minute and an amplitude of about 4 inches. Four rotary cup brushes 104 are arranged on 12-inch centers in a line traversing the belt and roller conveyor orientation such that the longitudinal distance between the blocks 101 and the rotary cup brushes is about 9 inches. The rotary cup brushes are rotated at about 350 rpm and oscillated in a transverse direction with a frequency of about 15 cycles per minute and an amplitude of about 4 inches. A spray of water, the purity of which is not critical, is used to remove residual silica or cerium oxide. A rotary cylinder brush 105 is disposed transversely across the conveyor and is rotated to wipe away excess water and residue silica or cerium oxide.

After cleaning, the plates are advanced into section 200 for surface sensitization. As the plates pass through section 200, each is first rinsed with water which is substantially pure or demineralized. The rinse is accomplished by employing a cross-fire spray. A mutually opposed pair of spray guns, 201 and 202, are supported from a carriage 203 and reciprocated transversely of the plates on a track 204 at a rate of about 40 to 60 cycles per minute. During the reciprocation of carriage 203, rinse water is fed to guns 201 and 202 in alternating intermittent fashion such that water is sprayed only from gun 201 when the carriage moves in one direction (from bottom to top of FIG. 1), and water is sprayed only from gun 202 when the carriage is moving in the opposite direction. The guns 201 and 202 are tilted slightly toward each other to provide a cross-fire effect or sweeping action tending to wash excess water from the surface of the plates.

After undergoing the initial rinse with demineralized water, each plate is advanced beneath a spray gun 211 which is mounted on the reciprocating carriage 203. A stannous chloride solution (a preferred composition is given below) is fed to spray gun 211 which directs the solution against the top surface of each advancing plate.

Each plate then advances beneath an intermediate set of cross-fire rinse guns 212 and 213. These guns are similar to guns 201 and 202 in structure, mounting and operation. Water containing a sufficient buffering agent, preferably sodium bicarbonate, to buffer the later applied palladium supersensitizing solution is fed to guns 212 and 213 and onto the top surface of each plate. Alternatively, a drip applicator 218 comprising a pipe having a plurality of orifices disposed along its length is positioned between guns 212 and 213 and spray gun 214 and a buffering solution is applied to each plate through such an applicator 218 following rinsing with demineralized water from guns 212 and 213.

In any event, buffer-containing rinse water is retained on each plate as it advances beneath spray gun 214. Spray gun 214 is also mounted on carriage 203 to reciprocate across each plate. A palladium chloride solution (a preferred composition is given below) is fed to spray gun 214 which directs the solution against the top surface of each advancing plate. The palladium chloride solution, as it is sprayed from the gun 214, is acidic, generally pH 3.9 or less, in order to maintain a high concentration of palladium chloride in solution. The solution is atomized exiting the spray gun 214 and contacts the plate below mixing into the residue rinse water containing an effective amount of buffer to adjust the pH of the mixed solutions on the plate to at least 4 and preferably to pH 6 to 9, preferably about pH 8.

Each plate then advances past cross-fire rinse spray guns 215 and 216, which are similar in construction, mounting and operation to cross-fire spray guns 201 and 202 and 212 and 213. Demineralized water is fed to guns 215 and 216 though the water quality is less critical than the rinse prior to palladium supersensitization.

As illustrated, the first, second and third cross-fire rinse guns, as well as the tin gun and the palladium gun, i.e., all of the guns in section 200, are mounted from a single boom that reciprocates in the transverse direction at a rate of about 54 single passes per minute. Each of the rinse guns comprises a single UniJet-T8001 or T8002 spray nozzle (Spraying Systems Co., Bellwood, Ill.) operated at a pressure of about 40 psig. and an average flow rate of about 0.12 to 0.20 gallon of rinse water per minute. Each of the tin and palladium guns comprises a single type C spray gun equipped with a Paasche U2, F2-8 nozzle (Paasche Air Brush Co., Chicago, Ill.) operated at an air pressure of about 30 to 70 psig. and at a flow rate of about 800 milliliters of the solution described below per minute. The distance between the rotary cylinder brush and the first cross-fire rinse guns 201 and 202 is 36 inches, while the distance from each gun or gun set in section 200 to the next respective gun or gun set is 18 inches.

Each plate is then advanced through section 300, wherein a metal-boron containing coating is deposited on the now catalytically activated or sensitized surface of each plate by simultaneously spraying and intermixing a metal-containing solution and a boron-containing reducing solution onto the activated surface such that the metal ions present in the contemplated metal solution became reduced to a transparent boron-containing metal film which tenaciously adheres to the activated surface. For the sake of illustration, section 300 is shown to have four gun sets 301-304 each comprising a metal-containing solution gun and a mutually opposed reducing solution gun. Section 300 also includes a mutually opposed pair of water spray guns 305 and 306 arranged for cross-fire rinsing. As shown, the guns 301-304 are supported for transverse reciprocating movement in the manner described above. However, it should be noted that the gun sets in section 300 are generally reciprocated in the transverse direction at a rate greater than the reciprocation rate in section 200, for example, a rate of 74 single passes per minute. During operation, each of the metal deposition gun sets in section 300 is maintained at a pressure of about 40 psig. and a flow rate of about 800 milliliters of solution per minute, while the final cross-fire rinse guns 305-306 are operated at a pressure of about 40 psig. and an average flow rate of about 0.12 to 0.20 gallon of rinse water per minute.

The gun sets in section 300 of the apparatus are spaced apart from those in section 200 such that the distance between the last rinse guns in section 200 and gun set 301 is about 54 inches. In addition, gun set 301 is spaced apart from gun set 302 such that the sprays generated from these sets (301-302) are overlapped, and such that the residence time of each plate in the spray area defined by gun sets 301 and 302 (spray area I) provides for deposition of a controlled film thickness. The residence time in the dwell area between gun set 302 and gun set 304 (dwell area I), the residence time in the spray area of gun set 304 (spray area II) and the residence time in the dwell area between gun set 304 and rinse guns 305-306 (dwell area II) also are established to control film thickness. Gun set 303 may optionally be used to apply additional metal salt and reducer solution. The metal-reducer gun sets (301-304) typically employed have Paasche U2, F2-8 nozzles, while the rinse guns 305-306 each comprise a single UniJet-T8001 or T8002 spray nozzle.

After undergoing a final water rinse under guns 305 and 306, each plate advances into section 400 of the coating apparatus where it is dried with an air knife 401 comprising an elongated metal housing having an 0.002 inch delivery channel extending along the length thereof. The knife 401 is disposed at a 45.degree. angle relative to the advancing plate and has its centermost portion spaced about 48 inches from the final rinse guns. The air knife is operated at about 5 psig. and an air flow rate of about 4,000 to 6,000 cfm. After passing beneath the air knife, each plate passes beneath a Gardner-Large Area Hazemeter 501 which measures and records the luminous transmittance of the coated plates.

The ambient air temperature during film deposition is about 80.degree. Fahrenheit, while the temperature of the demineralized and tap water used throughout these examples is generally about 65.degree. Fahrenheit. The temperature of the metal and reducer solutions is about 80.degree. Fahrenheit. On the basis of a liter of solution, each of the prepared aqueous solutions employed had the following composition:

NICKEL SOLUTION PREFERRED RANGE Nickelous acetate 5 grams 4-10 grams Boric acid 2.5 grams 2-5 grams Sodium gluconate 9.0 grams 7-18 grams Hydrazine sulfate 0.5 gram 0.4-1.0 grams Water added to 1 liter Ammonium hydroxide added to pH 7.2 pH 7.0-7.6 Ethomeen C-20* 1 drop per liter of solution 0-2 drops Acetone 0.01 gram 0-100 grams *Ethomeen C-20 (trademark of Armour and Company) is a cocoamine having an average molecular weight of 645 and the following generalized formula: wherein R is derived from a cocoamine and x+y equals 10.

REDUCING SOLUTION PREFERRED RANGE Sodium borohydride 0.5 gram 0.4-1.0 grams Water added to 1 liter added to 1 liter Sodium hydroxide added to pH 11.5 pH 11-11.6 Ethomeen C-20 one-half drop per liter 0-2 drops

The pH of the intermixed nickel and borohydride solutions is about 7.7.

TIN SOLUTION PREFERRED RANGE Stannous chloride 0.2 gram 0.02-0.4 gram Hydrochloric acid (12N) 0.04 milliliter 0.04-0.06 milliliter Water added to 1 liter added to 1 liter PALLADIUM SOLUTION Palladious chloride 0.02 gram 0.02-0.04 gram Hydrochloric acid (12N) 0.04 milliliter 0.02-0.04 milliliter Water added to 1 liter added to 1 liter

The pH of the palladium solution is generally about 3.

BUFFERED RINSE WATER PREFERRED RANGE Water 1 liter 1 liter Sodium bicarbonate 0.5 gram 0.1-70 grams

The amount of buffered rinse water sprayed onto each plate is adjusted so that the pH of the mixture of the palladium solution and buffered rinse on the plate is about pH 8.

Table I summarizes a series of metallizing runs following the procedure described above, except as indicated.

TABLE I ______________________________________ Run No. Rinse After Tin Solution pH After PdCl.sub.2 Spray* Luminous Transmittance** Film Quality ______________________________________ 1 Demineralized Water 3.7 20 percent Mottled, leading portion of plate with less film 2 Buffered Water (1 gm/liter, NaHCO.sub.3) 7.8 20 percent Smooth, uniform fine grain 3 Demineralized Water followed by drip with 1 gram/liter (NH.sub.4).sub.2 CO.sub.3.sup.. H.sub.2 O in water 8.0 20 percent Smooth, uniform fine grain 4 Buffered Water (0.25 gm/liter NaHCO.sub.3) 20 percent Smooth, uniform fine grain, but leading portion somewhat lighter in film ______________________________________ *Estimated by one twenty-fifth dilution of PdCl.sub.2 spray solution. Other rinses in the system were varied as demineralized water and buffere water without effect. **Determined using Beckman DK 2A Spectrometer for visible light, 380-760nm.

Sodium carbonate is substituted for the sodium bicarbonate of run 2 in an amount of 1 gram per liter of rinse solution. The resulting pH is estimated as 11.9. The leading edge film deficiency is eliminated as with sodium bicarbonate. However, the film texture is more coarsely textured than when sodium bicarbonate is used.

Substitution of sodium acetate in an amount of 2 grams per liter improves film uniformity and texture but not as markedly as does sodium bicarbonate.

Ammonium bicarbonate, dibasic sodium phosphate, ammonium borate and a commercially available mixed buffer of monobasic sodium phosphate and sodium hydroxide (Fisher Scientific "pH Seven") are each used in an amount of one gram per liter of rinse as in run 3. Each buffer eliminates a leading edge film deficiency, each improves texture of the film and all but the borate result in an extremely fine grain film.

While the present invention is described with particular reference to specific embodiments, it is not intended to be limited by specific chemicals or means of application of buffer in the preparation of the substrate to be coated.

Claims

We claim:

1. In the electroless deposition of metal on a non-conductive substrate wherein a substrate is contacted successively with a tin-salt solution, then with a palladium salt solution to activate it, making its surface catalytic, and then is contacted with a mixture comprising a solution of metal salt of the metal to be deposited and a solution of a reducing agent for reducing such metal, the improvement comprising buffering the palladium salt solution in contact with the substrate at a pH from about 7 to about 9.

2. In the method of coating a non-conductive substrate surface with a metal-containing deposit comprising the steps of:

first, contacting the substrate with a solution of tin salt and then rinsing the same;

second, contacting the substrate with a solution of palladium salt and thereafter contacting the substrate with a solution containing the desired metal to be deposited and reducing agent for such metal, whereby a non-uniform metal-containing deposit is formed, the improvement comprising the steps of:

rinsing the tin salt contacted substrate with a rinse of water containing an alkaline buffering compound in an effective amount to provide a pH of about 7 to about 9 in the mixture of palladium salt solution and remaining rinse in contact with the substrate whereby a uniform metal-containing deposit results.

3. The method of claim 2 wherein the contacting solutions are sprayed against the substrate.

4. The method of claim 2 wherein the alkaline buffer is selected from the group consisting of ammonium hydroxide, ammonium borate and water-soluble ammonium, alkali metal and alkaline earth metal carbonates, bicarbonates and phosphates and mixtures thereof.

5. The method of claim 4 wherein the rinse contains as an esential ingredient from about 0.01 to about 7 percent by weight of the aqueous rinse, sodium bicarbonate.