Document Byb2B5eQOjDXpz2YnoQ79p12m

N39909 K N -6 4 -5 A GOLD-APPEARING FLAKE PIGMENT BASED ON SILVER-CO ATED M IC A AND GLASS By: A. R. Hanke E. I. DU PONT DE NEMOURS & COMPANY 256 VANDERPOOL STREET NEWARK, NEW JERSEY Serial No. KN 64-5 Copy No. l \ NEWARK PLANT f PIGMENT COLOR RESEARCH REPORT K \ A g o l d -a p p e a r in g f l a k e p ig me n t b a s e d o n s il v e r -c o a t ed mic a a n d g l a s s I Period Covered September, 1962 to June, 1963 \ NJ 28626 FILE: DATE: 2/19/64 DUP050053948 KN No. 64-5 Copy No. I 1. Numerical Pile 2. Research Office Pile No. 5. library Pile No, sathQ 4. I. J. Krchma, Pigments - Wilmington 5. A. A. Brlzzolara 6. W, S. Struve/A.Siegel 7. L. 0. Wise, Central Research, Wilm. 8. P, J, Monahan, (X Pile) 9. A. R. Hanke 10. Extra 11. Extra 12. Extra 13. Extra 14. Extra 15. Extra NEWARK PLANT PIGMENT COLOR RESEARCH REPORT SUBJECT: A Gold-Appearing Plaice Pigment Based on Silver Coated Mica and Glass PERIOD COVERED; September, 1962 to June, 1963. ABSTRACT Flake pigments comprising the layered system glass/silver/ hydrous SlOg have been prepared with an Si03 layer thick enough (0.17u) to produce weak first order interference colors. Flake pigments comprising a mica or glass substrate have been prepared, possessing an intense gold color. This is done by coating a silvered flake substrate with hydrous SiO, hydrous TiOg, or hydrous Pea03. DATE SUBMITTED: 2/14/64 DATE RELEASED: 2/19/64 DUP050053949 TABLE OP CONTENTS PAGE I. INTRODUCTION AND OBJECT 1 II. SUMMARY AND CONCLUSIONS 1 to cd to - ! o cntncncniei^ie w w III. EXPERIMENTAL AND DISCUSSION 2 A. Method of Silver Coating Mica and Glass Flakes 2 1. General Discussion 2 2. Details of Silver Coating; a. Preparation of Silvering Solns. b. Pre-treatment of Flakes o. Silver Coating Treated Lg.Flakes d. Silver Coating Treated Sm.Flakes B. Description of Silver Coating C. Coating Silvered Flakes with Hy.SiOa 1. General Discussion 2. Details of Co-Addition Method 3. Details of Hydrolysis Method 4. Obtaining A Silica Coating Thick Enough to Show Interference Colors 5. Formation of a Silica Coating To Produce an Intense Gold Color D. Goating Silvered Flakes with Hy. HO2 E. Coating Silvered Flakes with Hy,FeaQ3 F. Miscellaneous Observations 1. Formation of an Odd Silvered Batch of Mica 9 2. Effect of Silver Sulfide on the Color of Silica-Coated Flakes 9 IV. INVENTION RECORD 9 DUP050053950 I. INTRODUCTION AND OBJECT This work followed the work on preparing flake pigments possessing high intensity interference colors by vacuum evaporation and reported in KN-64-1. Metal deposition by chemical means Is more economical than by vacuum evaporation and since silver is easy to deposit by chemical means, this work was undertaken to explore the possibilities of obtaining high intensity interference colors by chemical deposition. The same optical principles guided the chemical deposition work that guided the vacuum deposition work The major portion of the work centered around the deposition of silver on mica and then top coating with hydrous silica. Diffi culty was experienced in obtaining silica coatings thick enough to show light interference. During the attempt to form hydrous silica layers thick enough to show interference colors, it was observed that a gold color could be produced by silica coating a properly silvered mica flake. The emphasis of the work then shifted to an exploitation of this unexpected gold color. II. SUMMARY AND CONCLUSIONS 1. It has been shown that it is possible, though difficult, to produce a coating of hydrous silica on silvered glass to yield first order interference colors. These colors are dark and much less intense than the interference colors obtained by vacuum evaporation and described in KN-64-1. 2. If a silver-coated glass flake or mica flake is coated with hydrous silica by hydrolysis, an intense gold color is produced under certain circumstances. The percent silver must be within certain limits, dependent on the specific surface of the substrate, in order to obtain this gold color with maximum intensity. There is also an optimum amount of hydrous silica to achieve maximum color intensity. 3. The gold color is not due to light interference brought about by light reflection from the front and back surface of the silica coating, 4. The gold color is not due to the formation of silver silicate, silver oxide or other silver salts. This is established by the fact that the spec trophotometrie curve of these compounds is different from that of the gold appearing coated silver. The gold color is also produced by coating with liquids such as acetone, immersion oil, oleic acid, or paint vehicles. The gold color disappears upon the removal of the liquid. DUP050053951 -2 - 5. The cause of the gold color is not clearly understood but it is believed to be due to the particulate nature of the silver making up the film and light interference set up by light transmission through the particulate silver film. 6. Rydrous titanium dioxide and ferric oxide also produce an intense gold color when coated on silvered mica, it is be lieved that any transparent coating will produce this gold color. 7. No color intensity is lost when the colored flake is immersed in a paint vehicle or in vinyl. 8. The silica coating furnishes some measure of protection to the silvered flake but severe darkening occurs on outdoor exposure in High Gloss Acrylic and less severe darkening in 30J alkyd. III. EXPERIMENTAL AND DISCUSSION A. Method of Silver Coating Mica and Glass Flakes 1. General Discussion For the early work on silver coating, the Rochelle salt method of silver reduction was used. The two solutions were prepared according to the method described in the Rubber Hand Book, This is given on p. 3429 of the 44th Edition, and is the method recommended for the half-silvering of glass. In some cases the glass and mica flakes were pre-cleaned by slurrying in warm dilute cleaning solution, but later trials showed that this step was not necessary. The flakes were always given a pre-treatment of stannous chlorldd solution prior to silvering in order to allow the silver ions to preferentially reduce on the stannous chloride flake surface. For later work, the silvering process was modified by using formaldehyde as the reducing agent rather than Rochelle salts. The solutions were sinkier to prepare and the results every bit as satisfactory as those obtained with the Rochelle salt method. It was observed that to obtain optimum sparkle in the coated flakes, the silvering conditions had to be modified depending on the specific surface of the flakes. The controlling factor appear te be the relationship between rate of silver deposition and total flake surface available to accept the silver. For a reaction rate too great for the available flake surface, dull reflecting surfaces are obtained. If the rate is too great for a given flake surface, a flake with a higher specific surface can be sub stituted but tliis causes an increase in rate of deposition tending to negate the beneficial effect of a greater surface. To compen sate for this effect, the concentration of the solution must be decreased by dilution. It is possible to obtain silvered flakes with high reflectance over a wide range of specific surfaces by controlling the concentration of the silvering solution and the ratio of quantity of flakes to quantity of solution. These re lationships have to be determined by trial and error. DUP050053952 --3-- The amount of silver deposited on the flakes is critical. If too little is deposited, the subsequent gold color is dark and dull. If too much silver is deposited, the gold color is light and weak. The proper amount depends on the specific sur face and is also found by trial and error. Two examples are given for the silvering of flakes. One example utilizes mica from the "Integrated Mica Conany" and is typical of the conditions required for flakes with a specific surface of about one square meter per gram. Hie other example utilizes a PR-2 type mica from the Concord Mica Co. The flake size is such that almost all of it passes through a 400 mesh screen and its specific surface is around 4.5 square meters per gram by nitrogen adsorption. Mica in this size range is suitable for use in automotive finishes. 2. Details of Silver Coating a. Preparation of Silvering Solution Solution I 30 grams silver nitrate 500 ml. water 30 ml. ammonium hydroxide (28$ NIfe) Solution II 32.5 ml. formaldehyde (40$ by volume) 500.0 ml. water b. Pre-treatment of Flakes Mica from the Integrated Mica Company is in the form of large thick sheets. These sheets are broken down by placing 10 g. portions in a Waring blender along with 100 ml. water. Run at high speed for 30 sec. If the mica is already in small flake form, this step is unnecessary. Clean the flakes in dilute cleaning solution by slurrying 100 g. flakes in 1 liter of water. Add 250 ml. of cleaning solution made by adding 4 kg. cone, sulfuric acid to 100 ml. of sodium bichromate solution (69$ NaaCrgO?, 2HgO). Slurry for 2 min. and wash chromate-free. MOTE: wood results have been obtained without this cleaning step. Whether or not it is necessary will depend on how clean the flakes are. The wet presscake is added to 1 liter of stannous chloride solution (10 g* SnClg.2Hg0 + 40 ml. cone. HC1 + HgO to make 1 liter). Stir for about 5 minutes. Filter and was free of SnClg. DUP050053953 -4 - c. Sliver Coating Treated Large Flakes Dilute 200 ml. of Solution I to 400 ml. with water. Add 30 grams of treated "Integrated Mica" flakes. Dilute 200 ml. of Solution II to 400 ml. with water. Add rapidly to the mica slurry. After two minutes or longer, filter, wash, and dry. The flakes should show a brilliant sparkle and contain close to 22# silver. NOTE: This amount of silver for integrated mica flakes will yield an optimum gold color when subsequently silica-coated. d. Silver Coating Treated Small Mica Flakes (PR-2 type mica) Dilute 170 ml. of Solution I to 1,360 ml, with water. Add 4.5 g.treated PR-2 type mica. Dilute 170 ml. of Solution II to 1,360 ml. with water. Add rapidly to the mica slurry. After four minutes or longer, filter, wash, and dry, NOTE: The flakes should contain about 57# silver. This will yield an optimum gold color for flakes in this size range when subsequently coated with hydrous silica, B, Description of Silver Coating Electron micrographs show that the silver is deposited on nuclei. These nuclei grow with a roughly spherical shape and can vary in size from 0.05u to 0.5p. Some coalescence occurs but at the 20# silver level for "Integrated Mica", most of the nuclei appear to be particulate. See Fig. 1. It is believed that the particulate nature of the coating is in some way re sponsible for the gold color that develops when the silver-coated flake is coated with hydrous silica or other oxides. To develop the gold color, it is not necessary to apply a solid coating, A liquid coating will also develop the color. This holds for acetone, immersion oil, oleic acid, or paint vehicles. When the coating is removed, the gold color disappears . The intensity of the gold color is related to the refractive index of the coating. The higher the refractive index of the immersion liquid, the more intense the color. A hydrous TiOg coating (ref, index 2.2) gives a more intense color than a hydrous Si02 coating (ref. index 1.4). Note also that the refractive index of most vehicles is around 1.55 hence when a silicacoated silver flake is immersed in a vehicle, it is surrounded by a medium of Mgfrar refractive index whereas if it is immersed in air. it is surrounded by a medium of lower refractive index (n l). If interference ef light reflected from the front and back surface of a silica layer is responsible for the gold color when Immersed in air, the color should become a pale blue when immersed in a vehicle because of the phase reversal when changing the surrounding medium from a lower refractive index to a higher refractive index. That such a color ohange does not occur, is proof that the observed gold color is not due to this type of light interference. DUP050053954 -5To rule out the effect of colored silver salts as "being responsible for the gold color, spectrophotometric curves of silver silicate and silver hydroxide were obtained. These, if present in a silica-coated product, could impart a yellow color to the flake. The curves were sufficiently different from that of the coated gold flake to rule out these compounds as the cause of the gold color. Furthermore, if the flakes were coated with silica in the presence ef a stannous salt, a gold color was still obtained. It is unlikely that this gold color is due to a silver salt because such a salt would be reduced to metallic silver by the stannous ions. G. Coating Silvered Flakes with Hydrous SiOa 1. general Discussion The methods used are based on those developed by R. K, Her of the Industrial and Biochemical Department. One methodoinvolves the co-addition of sulfuric acid and a silicate solution with a mole ratio Na20:S102 of 1:3.3. The other method simply relies on the hydrolysis of a dilute solution of Na20: Si02 1:3.3 mole ratio. Following is a detailed description of the two methods. 2. Details of the Co-Addition Method Add 3.90 g. of cone. HaSO* to 334 ml. water. Add 23.3 g. of silicate solution Na20:S102 1:3,3 mole ratio (28.4# Si02) to 334 ml. water. Add 10 g, of silvered flakes to 334 ml. water contained in a four-necked flask equipped with a stirrer, con denser, and two dropping funnels. Pre-adjust the pH of the aqueous flake slurry to 9.5-10 by adding a small amount of dilute silicate solution. Heat to 95C, Add the dilute sulfuric add and the dilute silicate solution simultaneously to the slurry by means of the dropping funnels. Adjust the rate to around 4 ml. per minute. Check the pH at around 30 min. intervals. Avoid letting the pH get too low. If this occurs, a small amount of dilute ammonium hydroxide is added to bring the pH up to the 9.5-10 range. Make pH measure ments on a cooled portion of the slurry and not while the slurry is hot. Continue heating for about 30 mins, after complete addition of solutions. Filter, wash, and dry. This method can be proportionately scaled up or down for different flake quantities. 3. Details of the Simple Hydrolysis Methods Add 22 g. of silicate solution Na20:$i02 1:3.3 mole ratio (28.4# SiOa) to about 300 ml. water. Add 100 ml. of a one normal NaaS04 solution. Make volume to 625 ml. with water. The pH should be around 10.7. Add 25 g. silver-coated DUP050053955 - 6 * mica flakes. . .. Heat to reflux (<^103C.) and allow to reflux for around 7 hrs. (balance of an 8 hr, day). Hie pH will rise from around 10.7 to 11.0. Hie pH should be measured on a portion of the cooled solution and not on the slurry at reflux temperature. Filter and wash thoroughly. Continue the coating for an overnight period by slurrying the washed flakes in a double-sized batch made from 44 grams con centrated silicate solution, 200 ml. of one normal NaaSO* solu tion made to 1350 ml. with water. Allow this to reflux overnight (---' 16 hrs.). Do not attempt to reflux over a week-end. All of the hy drous silica does not coat on the flakes and eventually the free silica forms a gel making filtration impossible. Sometimes such a batch can be saved by cautiously adding 100-200 ml. of 10# sodium hydroxide solution and warming. No more should be added than will dissolve the colloidal silica and render the slurry filterable. This procedure always dissolves some of the silica coating and hence lengthens the time schedule for adequate coat ing. This procedure can be scaled up or down for different flake quantities. 4. Obtaining A Silica Coating Thick Enough to Show Interference Colors. ______ Class flakes from Coming Class Co. with a specific surface estimated at around 1 sq. meter per gram were carried through the silica coating procedure for an extended period of time. The co-addition method was used. Hie coating was started using the co-addition method on 10 grams of silvered glass flakes. The coating procedure was re peated on the same batch of flakes for a total of five times. As coating progressed, the flakes first took on a pale gold color and then turned dark gold. A spectrophotometrie reflectance curve of the flakes showed an interference band at such position that would give a calculated thickness of around 0.10a assuming a refractive index of 1,4 for the hydrous SiOa. In immersion oil, the gold color turned to dull dark blue as a result of light phase reversal as explained on p. 4 . It was further observed that after about the third coating cycle, there seemed to be no further increase in the thickness of the coating. The coating procedure was modified to the extent that the 30 min. heating period was changed to an overnight reflux. On weekends, it was also allowed to reflux. After approximately 120 hrs. reflux time, a sample was withdrawn and a reflectance curve obtained. The sample had a blue color and from the position of the interference bands, the hydreus silica film thickness was estimated at 0.13a. The coating schedule was continued and the color slowly advanced to a dull green. After a total reflux time of around 780 hrs., the color was a dull yellow green. From the interference band position, the coating thickness was estimated at around 0.17a. Judging from the dull DUP050053956 -7color and the broad Interference bands, the coating was not very uniform and there was evidence that the flakes had broken down to a smaller size during the long reflux period. Fig. 2 shows a light reflectance curve of the coated flake at the termination of the coating experiment, 5, Formation of a Silica Coating to Produce an intense Cold Color_________ ' To produce the maximum gold color requires the proper amount of silver on the flake and also requires that the silica coating not be prolonged to the point where it is thick enough to produce interference colors. For mica with a specific surface estimated around one square meter per gram, the proper amount of silver is around 2.2$, For mica estimated around 4.5 square meters per gram, the proper amount of silver is around 60$. The "simple hydrolysis method" described on p,5 is used. The quantity of silica and the dilution used are independent of the specific surface of the silvered flake. This holds true because a considerable excess of silica is used and although Increasing the flake surface will increase the rate with which silica is deposited, because there are more nuclei, the rate of growth of the coating thickness will stay approximately the same. The optimum gold color is reached after a reflux period of around three "day runs" and two "night runs". This can vary at times and experience is necessary to stop the reflux at the point of optimum gold color. Some silver is lost during the reflux period, but this loss becomes slight after a silica coat ing is established. To minimize the silver loss, refluxing should begin with a day run rather than with a night run. A modification of this method, in which the silica coating was carried out in the presence of stannous ions, resulted in a product slightly less gold but more lightfast. it was thought that the presence of stannous ions would Insure that no silver salts formed. For 25 grams of silvered flakes, 50 ml. of stannous chloride were added (10 g. SnCla made to 1 liter with water). The solution was quite cloudy because of hydrolysis. In addition to this, 2 ml. of 10# sodium hydroxide were added to counteract the acidity of the stannous chloride. This observa tion furnishes further proof that the gold color is not due to the formation of a yellow silver compound because such a com pound would be reduced to metallic silver by the stannous ions. The hydrous silica coated mica could be heated to around 600C. for 3 min. and still retain a gold sparkle. At 800C., most of the sparkle is lost. Heating silvered mica without a coating of silica to 600C. destroys the sparkle. DUP050053957 ~ 8 * D. Coating Silvered Flakes with Hydrous TlOg A gold color, even more Intense than thateproduced by silica coating, can be produced by coating a silvered flake with hydrous TiOa. To accomplish this, it is necessary to use dilute solutions of titanium salts. If the solution is too concentrated, the acidity is too high and silver will dissolve from the flake surface. During hydrolysis, some free hydrous H02 always forms but since the flakes are so much larger than the colloidal TiOa, separation can be easily affected by decantation and filtration. The following method uses so-called "Anatase Concentrate" but other quadrivalent titanium salts can be used. Dilute 12.5 ml. of "Anatase Concentrate" (Titanyl sulfate containing the equivalent of around 15# TlOg) to 3.13 liters with water below 18C. Add 25 grams of silvered mica of specific surface around 1 sq. Vi/g. and containing around 22# silver. Stir while allowing the slurry to warm up to room temperature. Very little hydrolysis begins until c-the temperature reaches 20 C. Allow to stir for several hours. The length of time depends on the shade of gold desired. The longer the hydrolysis the redder the gold. The time is usually between 3 to 6 hours. In air, the gold color often appears tinted with other color: suggesting that the Ti02 coating is thick enough to produce inter ference colors. In immersion oil, it is always the gold color that predominates and this color is believed to be related to the particulate nature of the silver coating and not to light interference brought about by reflection from the front and back surface of the TiOa coating. Heating the TiOa-coated silvered flake to 6000, for 30 minutes does not destroy the sparkle but the color becomes a pale violet. Heating to 800*0. destroys the sparkle and the color. Without the SPiOa coating, the silvered flake loses sparkle at 600 C, . Coating Silvered Flakes with Hydrous Pea03 To achieve a coating, the hydrolysis of basic ferric acetate is used. There is considerable free hydrous ferric oxide but this could be removed by decantation and filtration. Make up a ferric chloride solution by dissolving 10 g. FeaCk (anhyd.) + 10 ml. cone, hydrochloric acid in water to make 100 m3 Heat to obtain more rapid solution. Make up a solution of sodium acetate by dissolving 4 g. Na acetate in 100 ml. water. Add 2.5 ml. of the ferric chloride solution to 10 ml. of the sodium acetate solution. Make to 100 ml. with water. The pH will be around 6. Add 2 g. silvered mica flakes with specific surface around 1 sq. H^g. and containing around 22# silver. Heat the slurry to around 65*0. and stir for about 45 minutes. Decant several times to get rid of the colloidal FegOa, Filter, wash, dry. The flakes are a brilliant red gold color. DUP050053958  9 F Miscellaneous Observations 1, Formation of an Odd Silvered Batch of Mica In developing the Ideal conditions for producing a silver coating giving maximum sparkle, a silver-coated batch of mica was prepared that was quite different in behavior from the normally so-called "good" batches. This batch had a smoky dull gold appearance. In immersion oil, the color became a dull violet. The specific surface of the starting mica was estimated at around 1 sq.Iyg. and the sample analyzed 27# silver. This behavior suggests a coating on the silver of something thick enough to produce Interference colors and with a refractive index below that of immersion oil (1.51). This batch was not studied sufficiently to achieve a good explanation of the nature of this coating, but the batch did not develop a gold color when coated with hydrous silica, The failure of this batch to yield a gold color Is not due to the 27# silver. Other batches of silvered flakes containing 27# silver did not have the appearance of this odd batch and did yield gold products. This points up the need for proper control in the silver coating process in order to achieve a product that will yield a good gold color upon subsequent coating. 2. Effect of Silver Sulfide on the Oder of Silica- Coated Flakes _ _______. Some silvered mica flakes were treated with ammonium sulfide plus a little ammonium polysulfide. This pro duced a surface of silver sulfide and appeared as a sparkly dark grey. This was then coated with hydrous silica in a manner that would produce a gold color in a properly silvered flake. No gold color was produced. The sample retained its appearance as a sparkly dark grey. IV. INVENTION RECORD In September, 1962, work was started to produce layered systems of metal/dielectric/metal using chemical deposition methods. Glass flakes were used as the substrate. The metal was silver and the dielectric was hydrous SIOa. The silver was deposited by chemical reduction using Rochelle Salts and later on in the work, formaldehyde,as the reducing agent. The hydrous silica was deposited by hydrolysis. The layered system con sisted of glass/Ag/SiOa/Ag. Interference colors were obtained but they were dull. It was observed, however, that by silica coating a properly silvered glass or mica flake, a brilliant gold could be obtained and the remainder of the work was devoted to the coating of silvered flakes with hydrous oxide of silicon, titanium, and iron to produce this brilliant gold color. DUP050053959 10 1737-8,9.11,15.18,19 ( 9/15/62 t.o 2/17/65) Silvered glass and mica flakes were coated with hy drous silica and topped with stannous chloride, paladium, and silver. Low intensity interference colors were obtained. After a prolonged silica coating experiment, the thickness of the silica coating reached that of a first order green. 1737-20,21,24,28,29,30; 1751-2,3,4,6,7,8,9,11,12,13,14,15 16,18 ,21,22,25,24,25, (12/6/62 to 5/27/63 _______ __ The details for preparing a mica/Ag/SiOa flake system possessing an intense gold color were worked out. The inclusion of stannous tin in the hydrolyzing silicate solution was found to increase lightfastness. Methods for silver coating and silica coating were worked out and applied to large flakes in the order of one square meter per gram, and also small flakes in the order of 5 square meters per gram, 1737-27 (2/8/65) A silver sulfide-coated mica flake was prepared and coated with hydrous silica. It did not develop a gold color. The appearance was a shiny dark grey, 1751-20 (4/17/65) Electron micrographs of silver-coated mica with and without a silica coating were obtained. The particulate struc ture of the silver coating was clearly evident. 1751-29 (6/17/63) A coated system comprising mica/Ni/Ag was prepared by chemical reduction of nickel sulfate followed by chemical re duction of silver nitrate. The product showed good sparkle but was very little better than the mica/Ni without the silver. Considerable work was subsequently done with nickel coating of mica and glass. This constitutes the subject of another report, 1737-15,22,25,26 (11/15/62 to 2/6/65) Silver-coated mica was coated with hydrous Ti02 to yie: an intense gold product. It was necessary to work with a very dilute solution of a titanium salt to minimize the solution of silver in acid. 1751-27 (5/13/63 to 5/17/65) Silver-coated mica was coated with hydrous Ee203 to produce an intense gold colored flake. A dilute solution of ferric chloride solution in sodium acetate was used. DUP050053960 l SILVEH OOATm WGA MAS, 77,000X O.lix DUP050053961 WAVELENGTH Mju DUP050053962