CA1097031A - Pigmentary bright primrose yellow monoclinic bismuth vanadate and processes for the preparation thereof - Google Patents

Abstract

ABSTRACT
A pigmentary, bright primrose yellow monoclinic bismuth vanadate and processes for the preparation thereof by reacting bismuth nitrate with and alkali vanadate under controlled condi-tions to obtain a bismuth vanadate gel followed by aqueous diges-tion or calcination to produce pigmentary, bright primrose yellow monoclinic bismuth vanadate.

Description

~097031 BACKGROUND OF THE INVENTION
This invention relates to pigmentary bright primrose yello~ monoclinic bismuth vanadate and processes for the preparation thereof, A pigment is an insoluble small particle s~lid which is incorporated in palnts~ plastics~ and inks to impart color and~or opaclty. Ideall~, a pigment should have strength, that is, a lo~ color pigment requirement when mixed with a white pigment to make tints; intensity, that is, purity of shade or absence of dullness or grey-ness; llghtfastness, that is, resistance to color change when exposed to sunlight in a pigmented article; and resistance to bleed, that is, low migration o~ color in a pigmented article In addition~ for many applications~
it is desirable that a pigment haYe go~d hlding power, that is, ability to efficiently opacif~ a pigmented article.
The ma~or lntense yellow pigments currently used are lead chromate~ cadmlum sulfide, nickel titanate~ and a number o~ organic materials such as certain azo compounds.
Lead chromate and cadmium sulfide have good hiding; however~
at present there is a trend away from using these compounds as plgments because of their possible toxicity~ Nlckel titanate has good hiding power but it does not have the strength and high intensity desired in a high-performance pigment. The organic pigments generally exhibit high strength but poor hiding and frequently poor bleed resistance.
Thus~ there is a need for a nontoxic~ hlgh hidlng, yellow pigment whlch also has good strength, high intensity, good lightfastness, and no bleed in organic solvents or vehicles.

~ 2 _ ~097031 Bismuth vanadate occurs in nature in an ortho-rhombic form commonly called pucherite. Pucherite is a dull yellow-brown mineral, which is not useful as a pigment.
Pucherite is similar in color to limonite, a dull yellow-brown iron oxide, which has ne~er beèn produced as a bright, high intensity plgment despite repeated attempts.
Various réferences have described the synthetic preparation of bismuth vanadate, for example~ I,M, Gottlieb and C.R, Rowe, "Preparation and Thermal Propertles of Bismuth ~rthovanadate", Thermal Analysis, Vol, 2, Proceed_ ings Third ICTA DAVOS (1971)~ pp, 303-311; R. S, Roth and J.W. ~aring, "S~nthesis and Stability of Bismutotantalite, Stibiotantalite and Chemically Similar AB04 Compounds", The American Mineralogist, Vol, 48 (No~_Dec,, 1963), pp.
1348-56; H. E. Swanson et al., "Standard X-Ray Diffraction Powder Patterns", National Bureau o~ Standards Report, No.
7592 (Aug., 1962~; Eduard Zintl and Ludwig Vanino, "Process For The Manufacture Of Pure Bismuth Vanadate~'; German Patent No. 422,947 (1925), However, as in the case of yellow iron oxide, none of the above were successful in pre-paring bismuth vanadate as a bright yellow pigment.
SUMMARY OF THE INVENTION
In accordance with the present lnvention~, there is provided a pigmentary monoclinic blsmuth vanadate which is primrose yellow in shade, is nontoxic~ and has high hiding, ~ood strength, high intensity, good lightfastness, and no bleed in organic solvents or vehicles. This pigmen-tary bright primrose yellow bismuth vanadate has a pure monoclinic crystal structure as determined by X-ray diffrac-3 tion, exhibits an increase in re~lectance from 450 to 525 ~097~3:1 nm of at least about 65 units using Type I illumination and a green filter reflectance in paint of about 60%
(preferably 64%), has a sur~ace area of about 2-22 m2/g, and a masstone light~astness in paint such that it loses about 11% or less reflectance during 44 hours Fade-Ometer~ exposure, There is also provided a process for preparing the bismuth vanadate pigment of thls inventlon, which pro-cess comprises mixing a solution o~ Bi(NO3)3-5H2O in nitric acid with a solution of alkall vanadate, preferably Na3VO4, in an aqueous base selected ~rom sodium hydroxide and potassium hydroxide while controlling the mixing conditions to assure equimolar incremental combination of the solu-tions under turbulent conditions to obtain bismuth vanadate gel suspended ln a solution containing dissolved alkali nitrate~ the molar ratio of Bi3~ to V043- in this sus-pension is ~rom about 0,95:1~00 to 1.10;1,00 and the normalities of the acid and base solutions are adjusted prior to mixing so that the pH of the suspension will be about 1.0 to 8.o and preferably 1,5 to 2,0; ad~usting, i~ necessary, the pH of the suspension to about 1,9_3,6 with an alkall hydroxide selected from sodlum hydroxide or potassium hydroxlde or as the case may be wlth an acid selected ~rom nitric acid or sulfuric acid; separating the gel ~'rom the suspension; washing the gel with water until it contain8 about 2Q% or les~ alkali nltrate based on the theoretical yield of bismuth vanadate; and cal-cinlng the gel at about 200_500C. ~or from about 0,4 hour to about 3 hours~

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1V97~)3~L

This invention relates to a pigmentary monoclinic bismuth vanadate and processes for the preparation thereof.
The pigmentary bismuth vanadate is bright primro~e yellow and has high intensity, good strengthJ and good light-fastness.
The pigmentary bismuth vanadate of the pre~ent lnven~ion ~s entirely in the monoclinlc crystal phase a~
determined using X-ray dlffraction. X-ray diffraction measurements are made with a Debye-Scherrer* powder camera using CuK~ radiatlon from a Norelco* Type 12045B unit at 40 kV/20mA and a 6-hour exposure.
The pigmentary bismuth vanadate of the present inven~ion exhibits a large increase in reflectance in the range of the visible sp~ctrum between 450-525 nm which shows that lt is primrose yellow in color and has high intensity and good strength. Re~lectance as used herein is a comparison of the specular and di~use re~lectance o~ a known standard with the sample to be tested using Type I illumination.
Re~lect~nce is measured on a Cary* Model 14 spectrophoto-meter equipped with an integrating sphere, painted with East~an* white re~lectance paint No. 6080. The sample to be te~ted is prepared b~ mlxlng 10 w~lght percent o~ the blsmuth vandate o~ the present inventlon wlth 90 weight percent of barium sulfate, Eastman Re~lectance Standard No. 6091, until a u~i~orm powder ls obtalned. Then the reflectance of the ~nown stand~rd) Eastman Reflectance Standard No. 6091, is compared with that of the sample.
In the range of the vislble spectrum Yrom 450 to 525 nm, the sample containing the plgmentary blsmuth vanadate of the present invention exhiblts an increase of at least about 65 *- denotes trade mark.

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~097031 re~lectance units on a scale in which the Eastman Re~lectance St~ndard No. 6091 exhibits a re~lectance of 100 unitæ o~er the entire visible spectrum. In fact, many of the samples exhibit increases in reflectance of at least about 70 units or greater. The greater the magnitude of the change in reflectanc~ within the speclfied spectral range, the greater the intensity and strength of the sample.
The intensity and light~astness of the bismuth vanadate pigment in a paint are measured from p~int draw-downs made to complete hiding. The paint is made by d~s-persing the pig~ent into a binder in Q 2:1 ratio by weight of pi~ment to binder uslng a Hoover* muller. The binder ls composed of 98.9 parts by weight #2 transparent varnish made by the Superlor Varnish and Drier Company, Merchants-vllle, New Jer~ey, 1 part by welght as lead, o~ lead ~uodexD drler (contains 24~ lead), and 0.1 part by welght as manganese, o~ manganese Nuodex~ drier (contains 6~
manganese). The drawdowns are dried for about 72-120 hours ln a well-ventilated room at 25C. and less than about 50%
humidity.
Intensit~ 1~ determlned by the green filter reflectance of a masstone drawdown. Intensity is e~pres~ed ~s a percent of green fllter re nect~nce measured with a G~rdiner* Multipurpose Reflectometer~ Serial ~o. 40, (Gardiner Laboratory, Inc.3 Bethesda, Md.) using a white re~lect~nce standard at a setting of 86.1 and light from a General Electric* CVS pro~ector lamp passed through the grèen tristi~ulus ~ilter supplied wlth the Gardlner Rerlectometer. The Gardin~r Multipurpose R~flectometer is descrlbed in Nation~l Bureau of Standards Research Paper *- denotes trade mark.

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RP 1345, dated Nov., 1940, by Rlchard S. Hunter, and National Bureau of Standards Circular C429, da~ed July 30, 1942, by Richard S. Hunter. The high~r the percent reflectance of the draw~own, the more intense the color.
The intensity of the bismuth vanadate of the present invention is such that it has a green filter re~lectance of at least about 60%, preferably 64%.
Light~astness is determined a~ter the dried paint drawdowns have been continuously exposed for 44 hour~ in an Atlas Color Fade-Ometer~ ~ype FDA-P. A Gardiner Multi-purpose Reflectometer i8 used as described above to measure green filter re~lectance within one hour prior to and within on~ hour after exposure. The percent lowering of reflectance after exposure based on initial reflectance i8 percent Fade-Ometer~ darkening. The lower the percent Fade-Omete~D darkenlng, the better the lightfastness of the pigment. The bismuth vanadate o~ the present inventlon will generally exhibit a percent Fade-Ometer~ darkening of ~bout 11~ or less, preferably about 7~ or less, after 44 hours of exposure.
The ~urface area of the bismuth vanadate is about

2-22 m2/g. For use in film forming compositlons, such a8 palnts and ink~, a fiurface area of about 8-22 m /g. i9 preferred. For use in plastic compo~ltionsJ a æur~ace area of a~out 2-4 m /g. i~ pre~erred because of increased heat ~tabllity. The sur~ace area is measured on a Perkin-Elmer Shell* Model 212C Sorp~ometer ~sing the technique recommended by the ~anu~acturer.
Pigmentary bismuth vanadate is prepared by mixing a solution o~ up to about o.8M Bi(~3)3.5H20 in p *- denQtes trade m~rk.

1~)97031 4.ON nitrlc acid, preferably about 0.2M Bi(N03)3 5H20 ln about l.ON nitric acid, with a solut~on of alkali vanadate, preferably sodium vanadate in a concentration up to about 0.32M in up to about 1.6N aqueous base selected from sodium hydroxide and potassium hydroxide. UpPer concentrat~on limits for the reactant solution are determined by solubility;
however, control of equimolar incremental mixing is easier with more dilute solutions. In a preferred embodiment a solution Or about 0.2M Na3V04 in about l.ON aqueous sodium hydroxide is employed. The solutions are preferably mixed at a temperature o~ from about 20C. to 30C., but could be mixed at 10-100C.
The molar ratio of Bi3+ to V043 can be about 0.95:1.00 to about 1.10:1.00, preferably from about 0.98:1.00 to 1.02:1.00. As indlcated, it has been found that slight molar excess o~ Bi3~ or V043- may be used.
The normalities of the acid and base solutions are adJusted prior to mixing so that the pH of the reaction mixture will be about 1. o~8 Ø A pH of about 1.5-2.0 is preferred because it yields a gel from which pigmentary monoclinic bismuth ~anadate product having optimum intenslty and/or lightfastness can be obtained.
The preferred method of mixing the solutions is in a ~low reactor which gives a very rapid incremental combination under high turbulence conditions. Incremental comblnation means that small, essentially stoichiometric quantities are reacted at one time. This prevents relatively large gradients and stoichiometric imbalance that result when large quantities of reactants are mixed at once. Such lmbalance results in undesirable side reactions. ~pparatus which conyeniently achieve this result are tee mixers or ~low reactors.
When the solutlons of Bi(N03)3-5H20 and alkali vanadate are mixed under the aboYe conditions, a suspen-sion of bismuth vanadate gel in alkali nitrate (sodium nitrate, potassium nitrate, or both~ solution is formed.
Bismuth ~anadate ~el as used herein means a hydrous~ X-ray amorphous bismuth vanadium oxide precipitate containing occluded water, After formation of the gel the pH of the suspension must be adjusted to a pH of ~rom 1.9 to 3.6 if the suspension pH is found to be outside o~ this range, Thus, if the pH of the sus~ension is less than about 1.9 lt must be adJusted up-ward wlth aqueous sodlum hydroxide or potassium hydroxide, and if it is greater than about 3.6 lt must be ad~usted downward with an acid selected ~rom nitric acld or sulfuric acid.
Preferably the pH of the suspension will be ad~usted to a pH of from 3.3 to-3.5 as this produces a gel which~ when calcined, gives pigmentary monoclinic bismuth Yanadate o~ desirable lightfastness and thermal stability. Prompt ad~ustment o~ the pH o~ the suspension to the final ranges listed above is advisable, although under some circumstanceS the bismuth vanadate gel is stable ~or up to several hours at room temperature prior to pH adJustment.
~-1 The gel is then remo~ed from the suspension, pre~erably by filtrat~on The gel is collected and, if ; it contalns more than about 20 percent alkali nitrate based on the theoretical yield of bismuth vanadate, the gel must be washed with water untll lt contains about 20 _ 9 -10gi703~

percent or less. Pref~rably~ the alkali nitrate remain-lng in the gel will be about 5-7%.
The gel is then calcined at a temperature of about 200C. to 500C. for about 0~4-3 hours and preferably at about 380C. to 460C~ for about one hour. The bismuth vanadate pigment is entirely in the monoclinic phase as identified by X-ray diffraction.
Another process for preparing pigmentary mono-clinic bismuth vanadate comprises mixing a solution of up to about o.8M Bi(N03)3 5H20, preferably about 0.2M, in aqueous nitric acid of up to 4.oNI preferably about l.ON, with a solution of alkali vanadate (sodium vanadate or potassium vanadate), preferably up to about 0.32M Na3V04, in up to about 1.6N aqueous base selected from sodium hydroxide or potassium hydroxide, more preferably about 0.2M Na3V04 in about l.ON sodium hydroxide. The solutions are preferably mixed at 20-30C., but could be mixed at 10-100C.
:
The molar ratio of Bi3+ to V043 can be about 0.90:1.00 to 1,10:1.00, preferably about 0.98:1.00 to 1.00:1.05, which indicates that either an excess of Bi3+
or V043- may be used. The symbol V043- is used herein to denote a pentavalent vanadium species in an aqueous solu-tion~ but it does not mean a specific pentavalent vanadium species present at a given pH and vanadium concentration.
The normallties of the acid and base solutions are ad~usted prior to mixing so that the pH of the reaction mlxture will be about 1.0-11,0. A pH of about 1.5-4.0 is ; preferred because it yields a gel from which a pigmentary monoclinic blsmuth vanadate product having optlmum 1()97031 intensity and~or lightfastness can be obtained.
The solutions must be mixed in accordance with the mixing procedure described ~or the first process to achieve an equimolar incremental combination of the solu-tions under high turbulence conditions and result in a suspension o~ b~smuth vanadate gel in alkali nitrate solu-tion, Preferably the gel should not be removed from the suspension until the pH of the suspension has been stabilized at about 2~2~6.0~ Thus~ if the pH of the SUS-pension is less than about 2.~, it may be adjusted upward with aqueous sodium hydroxlde or potasslum hydroxide, and 1 it is greater than about 6,o, it ma~ be ad~usted downward with an acid selected from nitric acid or sulfuric acid.
Prompt adjustment of the pH of the suspension to the final range listed above is advisable, although under certain circum-stances the bis~uth vanadate gel is stable for up to several hours at room temperature prior to pH ad~ustment~
The gel is then removed from the suspension, pre-ferably by filtration. The gel is collected and, if itcontains more than about 10 percent alkali nitrate based on the theoretlcal yleld o~ bismuth vanadate, the gel must be washed with water until it contains about 10 percent or less.
The gel is then heated ln water at a temperature of from about 60C~ to about 200C. for at least 0.2 hour, and preferably at a temperature of from about 90C. to about 100C. for about one to two hours to convert the ~el to pigmentary monoclinic bismuth vanadate. During the aqueous digestlon the p~ of the suspension should be . .

1~9703I

maintained at a pH of from 2.2 to 6.o, and preferably from a pH of 3.0 to 3.5, by adding a suitable acid or base as needed. Acids suitable for use include nitric, sulfuric, hydrochloric, hydrobromic or phosphoric. Bases suitable for use include aqueous sodium hydroxide or potassium hydroxide. I~t will be understood that if temperatures above about 100C~ are used, the heating must be carried out under pressure. When an excess of vanadium is used~ especially Bi3~ to ~043 mole ratio range of O.90_0.95:1.00, the pig~ent must be washed after the heating step by suspending it in water for 15 to 30 minutes at 50C. to lOaC. while holding the pH at 8.o 9.5 with an aqueous base selected from sodium hydroxide and potasslum hydroxlde.
After digestion is complete the pigment is filtered and dried, e.g., at about 130C. The product is bright primrose yellow bismuth vanadate which is entirely in the monoclinic phase as identified by X-ray diffraction.
In each of the above processes, the pH adjustment can be made after the gel has been removed from the sus-pension. However, in order to easily ad~ust the pH of the gel, the gel should be resuspended in water. Once the pH of the gel has been properly adJusted, the gel under certain circumstances is stable for up to about a week at room temperature.
In each of the above processes it is possible to prepare the alkali vanadate solutions by dissolving a penta-valent vandium compound such as V205, Na3V04, Na4V207, NaV03, or K3V04 in an aqueous base~selected from sodium

3 hydroxlde and potassium hydroxide.

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After the bismuth vanadate pigment has been prepared by either of the aforementioned processes, its lightfastness may be improved by encapsulation in a dense amorphous coating of silica or a treatment with aluminum pyrophosphate. Coatings with silica can be applied in accordance with known processes, such as those tau~ht in RE 27,818 (reissue of U.S. Patent 3,437,502~ to Werner;
U.S. 3,639,133 to Linton; U.S. 3,370,971 to Linton; and of U.S. 3,885,366 to Iler.
The following examp]es illustrate the present invention. All parts, p~rcentages, and proportions ~re by wei~ht unless ~therwise spccified.
13Xl~MrLF 1 ~ine samplcs arc prcpared accordinq to the following px~c~dur~:
A solutlon o~ Na3V~4 ~7.58 g.~ dissolved in 200 ml. l.ON NaOH is poured, over about a two-minute period, into a stirring sblution of Bi(N03)3 5H20 (20 g.) dissolved , ~ ~
in 200 ml. of l.ON HN03. The mixture is stirred vigorously and a fine ~ellow-orange precipitate is immediately formed.
; The pH of the mixture is then adJusted to 3 . 5 with 2.ON
; NaOH. The mixture ls ~tlrred for 0. 5 hour, filtered, washed with 400 ml. of distilled water, and refiltered. A
bismuth vanadate ~el whiGh re~ains on the filter paper is collected and calcined at 40a~oc~ ~or one hour to produce pigmentary monoclinic BiV04.
The a~erage green filter reflectance of these ; nlne samples of~ bismuth vanadate is 68,o and their average percent ~ade~Ometer~ darkening a~ter 44 hours of exposure ls 11,0, ~ 13 10~703~

Eight samples are prepared according to the following procedure:
The procedure above is repeated except that the Na3VO4 dissolved in NaOH and the Bi(NO3)3~5H2O dissolved in HNO3 are comblned by using a lmm ID mixing tee at 40 psi over a period o~ about two m~nutes to obtain turbulent e~uimolar mixing.
The average green ~ilter reflectance of these eight samples of bismuth vanadate plgment is 69,0 and their average percent ~ade~Ometer~ darkening after 44 hours of exposure ~s 6~7~
This example shows that turbulent equimolar mixing improves the llghtfastness o~ bismuth vanadate pig-ment.
; EXAMpLE 2 A mixture is prepared by combining the followingingredients in a 1 mm ID mixing tee at 40 psi over a period , ~ of about two mlnutes: .
:
(i) Bi(NO3)3~5H2O (78.4 g~) dissolved in 200 ml.
20 ~ o~ 4.0N HN03, then diluted with 6a ml. o~
2,ON HNO3 and 545 ml. of water, and (ii) Na3~04 (30.3 g.) dissol~ed in 500 ml~ of 1.6N NaOH, then dlluted to 800 ml. with water.
Sufficlent HNO3 is added to ingredlent (~) or a~ueou~ NaOH to ingredient (il) so that the pH of t.he . ~ :
;: mixture of the tw~ wlll be ~ithin the range of about 1.5_2Ø

The mixture is squirted into a three-liter beaker contain_ lng 1200 ml. of:water ad~us.ted to pH 2.3 with 0..5N HN03.

~he con.tents of the beaker are stirred for one minute and 14.~

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the pH is ad~usted to 3~2 with 0.5N NaOH, Then the pH is adJusted to 3.4 and kept there ~or 30 minutes~ The contents are filtered, washed ~ith 250 ml o~ water, and again washed with 250 ml of w~ter, ~fter filterlng, a bismuth vanadate gel is collected from the filter paper and is divided into three equal samples. Each sample is converted to pigmentar~ monoclinlc BiVO4 by the methods of Table I
and tested for the listed properties.

TABLE I
Lightfastness ) Surface % Fade-Ometer~ Area Sample Method of Convcrsion _Darkening(m2/g~

~ A Washed with 250 ml 2.1 13.6 - of water, then boiled for 4 hours B Calcined at 390C. 4.7 6.8 for 1.5 hours C Calcined at 450C. 6.9 2.3 for 1.5 hours 1) Porccnt r~flectance cllangc aftcr 44 hours of Fado-OmctcrC3 ~xposure Thls shows that monoclinic bismuth vanadate pig-~ ment prepared by heating in water has better light~astne~s ; than samples prepared by calcinlng. Further, bismuth .
vanadate pigment prepared by heating in water has a higher sur~ace area which means that it has greater strength per unit weight than calclned samples.
;~ EXAMPLE 3 A mixture is prepared by combining the following ingredients in a 1 mm ID mixing tee at 40 psi over a ~`` period of about two mlnutes:
3 (i) Bi(N03)3~5H2O (80.0 g.) dissolved in 204 ml.

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of 4.0~ HNO3, then diluted with 20 ml of 2.ON HNO3 and 581 ml. of water, and (ii) Na3VO4 (30.3 g.) dissolved in 500 ml. of 1.6N NaOH, then diluted to 800 ml. with water Sufficient HNO3 is added to ingredient (i~ or aqueous NaOH
to ingredient (ii~ so that the pH of the mixture of the two will be about 1.5-2.0, The mixture is squirted into a 3~
liter beaker containing 1200 ml. Or water adJusted to a pH
of 2.3 with 0.5N HNO3. The contents of the beaker are stirred for one mlnute and the pH is ad~usted to 3.2 with 0.5N NaOH. Then the pH ls ad~usted to 3.4 and kept there for 30 minutes. After filtering, a bismuth vanadate gel is collected from the filter paper and is divided into four equal samples~ Samples A and B are not washed. Sample C
is washed twice on the filter, each time with 250 ml. of water. Sample D i5 resuspended twice, each time with 500 ml. of water, refiltering each time. Each sample is con-verted to monoclinic bismuth vanadate by calcining at 20~ ~450~C. ~or on~e hour. (The exact amounts of water and method u~ed to wash the bismuth vanadate gel should be adJusted so that the amounts of NàNO3 specified in Table II remain in the samples~. ~

ABLE II

Amount ~ NaNO3 I - I ln sample based on dry weight Or plgment Surface Area t%) (m2 A 44 1.4 `~ ~ B ~44 1.4 C ~ ~10 20 3.5 30 ~ D tV 3 4,9 16 ~

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The results reported in Table II demonstrate the effect of residual sodium nitrate in the gel prior to cal-cining. As indicated above it has been observed that the surface area of the pigmentary bismuth vanadate prepared from the gel decreases as the amount of sodium nitrate in the gel increases with a corresponding decrease in strength per unit. Preferred particle size for thermal stability is 2-4 m2/g.

A mixture is prepared by combining the following ingredients in a 1 mm ID mixing tee at 40 psi over a period of about two minutes:
(i) Bi(NO3)3-5H2o (80.0 g.) dissolved in 204 ml.
of 4.ON HNO3, then diluted with 20 ml. of 2.ON HNO3 and 581 ml. of water, and (ii) Na3VO4 (30.3 g.) dissolved in 500 ml. of 1.6N
NaOH, then diluted to 800 ml. with water.
Sufficient HNO3 is added to ingredient (i) or aqueous NaOH
to ingredient (ii) so that the pH of the mixture of the ; 20 two will be about 1.5-2Ø The mixture is squirted into a 3-liter beaker containing 1200 ml. of water adjusted to pH 2.3 with 0.5N HNO3. The contents of the beaker are stirred for one minute and the pH is adjusted to 3.2 with 0.5N NaOH. Then the pH is adjusted to 3.4 and kept there for 30 minutes. The contents, a suspension of bismuth vanadate gel, are filtered and washed on the filter with - two 500 ml. portions of water. After filtering, the bis-muth vanadate gel which is collected from the filter paper is converted to monoclinic bismuth vanadate by calcining at 450C. for one hour. This material is labelled Sample A.

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Sample B i5 prepared according to the pr~cedure usecl for Sample A, except tha~ after the su~penslon is filtered, the gel i8 resuspended in 500 ml. of water, filtered, again resuspended in 500 ml. of water, and refiltered.
Sample C is prepared by res~pending 10 g. of Sample A ln 500 ml. of water, filtering, again ~uspending in 500 ml. o~ water, f~ltering, and drying at 140C.
Table III Qhows the relative heat stability of the plgment ln pla~tic with varying sodium nitrate content.
TABLE-III
Amount of ~aN0 in ~ample based ~n Relative Heat dry weight of plgment Stabllity of the Sample _ (~) Plgment ln Plastic A 6.4 ~ost stable B 0.9 Le~s stable than A but more stable than C
;~ 20 C 0 Le~s stable than A or B
D 44* Least ~table Heat ~tabllity tests performed separately in poly~tyrene and ln ABS u6ing a Brabender~*Plastograph**Type PL ~000 wlth a Banbury head. For polystyrene, the BiV0l 18 dlspersed into tho poly~tyrene at 205C. at 40 rpml~ ~o~ 5 minutes, heated t~ 316C. and held there for 15 minutes at 100 rpm's. For ABSJ
the Bi~0l is dispersed into the ABS at 205C. at 40 rpm's ~or ~ ~ 5 minute~, heated t~ 260 C . and held there for 10 minutes at ;~ 30 100 rpm's. Lack of ~ta~ility 18 indicatéd by darkening.
*The washlng ~tep to remove NaN0 ~as not performed in the prep~ration of this sample. 3 ~- .
The re~ults reported in Table III demonstrate that th~ absence of 8GdiU~ nitrate in the gel prior calcina-tion has an adYerse afrect on the heat stablllt~ of the mùth`va~a~ate produced by calcining the gel. When the ~* deno~es trade mark. - 18 -., A~' ' - -, : ~
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11)~7031 results reported in Tables II and III are considered it will be appreciated that control of the sodium nitrate level in the gel prior to calcining is essential to the production of high quality pigmentary bismuth vanadate, particularly when heat stability for use in pigmentation of plastics is desired.

A mixture is prepared by combining the following ingredients in a mixing tee:
(i) Bi(NO3)3-5H2o (10.731 lb.) dissolved in 12.78 l. of 4.ON HNO3, then diluted to 51.1 1. with water, and (ii) V2O5 (2.049 lb.) dissolved in 31.94 1. of water containing 7.210 1~. NaOH, then diluted to 51.1 1. with water to give a solution of sodium vanadate plus sodium hydroxide.
The mixing tee has inside diameters of 0.118 in. for the bismuth nitrate opening, 0.192 in. for the sodium vanadate opening, and 0.192 in. for the exit. The bismuth nitrate enters on the leg. The solutions are mixed in about 15 minutes when a pressure of about 15 psi is applied across the tee. Sufficient HNO3 is added to ingredient (i) or aqueous NaOH to ingredient (ii) so that the pH of the mix-ture of the two will be about 1.5 to 2Ø The mixture is squirted into a 45 gal. tank containing 7 gal. of water which has been adjusted to a pH of 2.2 with HNO3. The contents of the tank are stirred for several minutes, and the pH is adjusted to 3.2 with 2.ON NaOH. The contents are then stirred for 15 minutes, filtered, and washed with 20 gal.
o~ water and refiltered. The bismuth vanadate gel is :

~97~31 collected from the filter cloth, a.nd i5 stlrred into a tank contain~ng 70 1~. of water, which has been ad~usted to a pH of 3.1 with HN03. The contents of the tank are heated to 95C, in about 40 minutes and held at that temperature and a pH of 3.1 for 120 minute~. me product, pigmentary bismuth vanadate, i6 isolated by flltration, waæhed twice ~ith 20 gal. of water, and dried at 140C.
The above procedure is repeated four times to yield about 28 lb. of pigment. Twenty-five pounds of the pigment are dispersed in 104 lb. of water containing 567 g. of Fischer* 28% sodium sllicate ~olution (40-42 ~aume) ~y passing it through a ~aulin* Submicron ~omogeni~er, rirst at 2000 p8i, and again at 5000 psi. The mixture is then he~te~ to ~nd held at 90C. and ad~usted to a pH of 9.6 with 2.0N NaOH.
A solution o~ 14.497 lb. of Fisch~r 28% sodium silicate in 62.5 lb. of water is added to the mixture over a four-hour perlod. At the ~ame time, a dilute sulfuric acid solution ; (2.2991b. of concentrated sulfuric acid in 71.25 lb. of water) is added t~ the mixkure at a rate such that the pH of the mixture drops to 9.4 in about 45 minutes. The pH of the mixture i8 immediately raised to 9.6 with 2.0N NaO~. Thls pH control procedure i8 continued over the four-hour :~ silicate addition perlod.
A~ter the four-hour addltion period, the pN
~s reduced to 9.0 with H2S04. held ther~ for 60 minutes, `~ and then reduced to 7Ø Then, while the mixture 18 ~till ~t 90C., a ~olution o~ 3.75 lb. o~ A12(S04~3.18H20 in 15 lb. of water is added over a ten-minute period and ths pH ralls to 2.0~3.0 The pH is then raised to 6.0 with 2,0N NaOH in about 20 mlnutes. The product is *- denotes trade mark.

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~97031 filtered hot, washed twice with 30 gal. of water and dried at 140C. Chemical analysis and electron micrographs show that the pigment particles have been coated with silica. Intensity and lightfastness measurements on the pigment before and after coating are shown in Table IV.

TABI.E IV

Before Coating After Coating IntensityLightfastnessl)IntensityLightfastness % Green Filter % Fade-Ometer~% Green Filter % Pade-Ometer~

ReflectanceDarkening Reflectance Darkening 1067.9 6.0 67.4 2.4 -)Percent reflectance change after 44 hours of Fade-Ometer~ exposure This example shows that encapsulating pigmentary bismuth vanadate in silica improves its lightfastness as determined by Fade-Ometer~ exposures.

A mixture is prepared by combining the following i~ ingredients in a 1 mm ID mixing tee at 40 psi over a period -~ ~ of about two minutes:
(i) Bi(NO3)3 5H2O (40.0 g.) dissolved in 200 ml.
of 2.ON HNO3, then diluted to 405 ml. with water, and (ii) Na3VO~ (15.16 g.) dissolved in 200 ml. of 2.ON NaOH, then diluted to 400 ml. with water.
Sufficient HNO3 is added to ingredient (i) or aqueous NaOH to ingredient (ii) so that the pH of the mixture of ~;~ the two will be about 1.5 to 2Ø The mixture is squirted into a 4-liter beaker containing 1200 ml. of water which has been adjusted to a pH of 3.0 with HNO3. The contents of the beaker are stirred for one minute and the pH is adjusted to . .

~0~7031 3.4 with 0.5N NaOH. The mixture is then stirred for 15 minutes, filtered, washed with 500 ml. of water, and re-filtered. A bismuth vanadate gel is collected from the filter paper, and is divided into two equal samples, Sample A and Sample B.
Sample A is stirred into a beaker containing 800 ml. of water and the pH of the mixture is adjusted to 3.3 to 3.4 with HBr or NaOH. The mixture is boiled for 90 minutes while the pH is held at 3.3 to 3.4. A bismuth vanadate pigment is isolated by filtration, washed with 500 ml. of water, and heated to 130C. until dry.
Sample B is calcined in an open crucible at 400C.
for 90 minutes.
Sample C is prepared according to the procedure used for Sample A except that 42.0 g., instead o,f 40.0 g., of Bi(NO ? 5H20 is used.
Sample D is prepared according to the procedure used for Sample B, except that 42.0 g. instead of 40.0 g. of 3)3 5H2O is used.
~0Sample E is prepared by combining the following ingredients in a 1 mm ID mixing tee at 40 psi over a period ;~ of about 2 minutes:
(iii) ~i(NO3)3-5H2o (88.0 g.) dissolved in 52 ml.
of concentrated HNO3 and 400 ml. of water, ~; then diluted to 800 ml. with water, and ~- (iv) V2O5 (15.0 g.) dissolved in 600 ml. of water -; containing 53.0 g. of NaOH, then diluted to 800 ml~ with water to give a solution of ~; sodium vanadate plus sodium hydroxide.
; 30 Sufficient HNO3 is added to ingredient (iii) of aqueous .

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NaOH to ingredient (iv) so that the pH of the mixture of the two will be about 1.5 to 2Ø The mixture is squirted into a 4-liter beaker containing 1200 ml. of water which has been adjusted to a pH of 2.2 with HNO3. The contents of the beaker are stirred for one minute and the pH is adjusted to 3.3 with 0.5N NaOH. The mixture is then stirred for 15 minutes, filtered, washed with 500 ml. of water, and refiltered. A bismuth vanadate gel is collected from the filter paper and is stirred into a beaker con-taining 500 ml. of water. The pH of the mixture isadjusted to 3.3 to 3.4 with HNO3 or NaOH and is boiled for 90 minutes while the pH is kept constant at 3.3 to 3.4. A
bismuth vanadate pigment is isolated by filtration, washed with 500 ml. of water, and heated at 130C. until dry.
Sample F is prepared according to the procedure used for Sample B, except that 44.0 g., instead of 40.0 g., of Bi(NO3)3-5H2o is used.
~; Sample G is prepared according to the procedure used for Sample A, except that 46.0 g., instead of 40.0 g., of Bi(NO3)3 5H2O is used.
Sample H is prepared according to the procedure used for Sample 9, except that 46.0 g., instead of 40.0 g., of Bi(No3)3-5H2o is used.
Sample 1 is prepared by combining the following ingredients in a 1 mm ID mixing tee at 40 psi over a period of two minutes.
(v) Bi(NO3)3-5H2O (72.0 g.) dissolved in 56 ml.
concentrated HNO3 and 400 ml. water, then diluted to 800 ml. with water, and (vi) V2O5 (15.0 g.) dissolved in 600 ml. water , : - .

~0~703~

containing 53.30 g. NaOH, then diluted to 800 ml. with water to give a solution of sodium vanadate plus sodium hydroxide.
Sufficient HNO3 is added to ingredient (v) or aqueous NaOH
to ingredient (vi) so that the pH of the mixture of the two will be about 1.5 to 2Ø The mixture is squirted into a

4-liter beaker containing 1200 ml. of water which has been adjusted to a pH of 2.2 with HNO3. The contents of the beaker are stirred for 1 minute and the pH is adjusted to 3.3 with 0.5N NaOH. The mixture is then stirred ~or 15 minutes and filtered. A bismuth vanadate gel is collected from the filter paper. The gel is stirred into a beaker con-taining 500 ml. of water and the pH of the mixture is adjusted to 3.1 with HNO3 or NaOH. ~he mixture is boiled for 120 minutes while its pH is held at 3.1. The bismuth vanadate precipitate is isolated by filtration and resuspended in 400 ml. of water. The temperature is then raised to 50C.
and the pH raised to 8.0 and held there for 10 minutes. A
~
~- bismuth vanadate pigment is isolated by filtration, washed ~; 20 with 500 ml. of water, and heated at 130C. until dried.
Sample J is prepared by combining the ollowing ingredients in a 1 mm mixing tee at 40 psi over a period ; ~ of about 2 minutes:
(vii) Bi(No3)3-5H2o (72.0 g.~ dissolved in 56 ml.
of conc, HNO3 and 400 ml. water, then dis-tilled to 800 ml. with water, and ~ ,:
(viii) V2O5 (15.0 g.) dissolved in 600 ml. water containing 53.30 g. NaOH, then diluted to 800 ml. with water to give a solution of sodium vanadate plus sodium hydroxide.
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1097~1 Sufficient HNO3 is added to ingredient (vii) or aqueous NaOH to ingredient (viii) so that the pH o the mixture of the two will be about 1.5 to 2Ø The mixture is squirted into a 4-liter beaker containing 1200 ml. of water which has been adjusted to a pH of 2.2 with HNO3. The contents of the beaker are stirred for 1 minute and the pH is adjusted to 3.3 with 0.5N NaOH and the mixture is stirred for 15 minutes, filtered, washed with 500 ml. of water, and refiltered. A bismuth vanadate gel is collected from the filter paper. The gel is calcined in an open crucible at 400C. for 60 minutes.
The intensity and lightfastness of the above samples are compared in Table V.
TABLE V
3~ 3_ Intensity Lightfastnessl) Bi to V04% Green Filter% Fa~e-Ometer~
Samples Molar RatioReflectanceDarkening Aqueous A 1.00/1.0071.2 4.5 C 1.05/1.0069.0 6.4 :~ E 1.10/1.0073.9 8.0 G 1.15/1.0074.2 14.5 I: 0.90/1.0061.7 4.7 Calcined B 1.00/1.007S.0 6.4 : D 1.05/1.0078.4 7.9 : F 1.10/1.0078.2 10.0 :~ H 1.15/1.0076.4 12.5 J 0.90/1.005~.9 15.1 ; 30 l)Percent reflectance change after 44 hours of Fade-Ometer~ exposure The data reported in Table V demonstrates that preparation of bismuth vanadate by use of molar ratios out-side the limits disclosed herein adversely affect the .

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properties o~ the resultant bismuth vanadate and render the product nonpigmentary as defined herein.

Samples prepared in the previous examples were also characterized by reflectance spectra and differential thermal analysis (DTA) measurements. The results of these measurements, together with lntensity values ror drawdowns are glven in Table VI.

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A mixture is prepared by combining the following ingredients in a mixing tee: -(i) Bi(N03)3 5H2O (10.731 lbs.) dissolved in 12.78 llters of 4.ON HN03, then diluted to 51.1 llters with Nater, and (ii) V205 (2.049 lb.) dissolved in 31.94 liters of water containing 7.210 lb. NaOH, then diluted to 51.1 liters with water, to glve a solution of sodium vanadate plus sodium hydroxlde.
The m~xing tee has inside diameters of 0.118 in. for the bismuth nitrate opening, 0.192 in. for the sodium vanadate opening, and 0.192 in. for the exit. The bismuth nitrate enters on the leg. The solutions are mixed in about 15 -mlnutes when a pressure of about 15 psi is applied across the tee. Sufficient HNO3 is added to ingredient (i) or . :
aqueous NaOH to ingredient (il) so that the pH of the mix-~ ture Or the two will be about 1.5 to 2Ø The mixture ls i~20~ 5quirted into a 45 ~al. tank contalnln~ 7 gal. Or water : ~ :
which has been ad~usted to a pH of 2.2 with HN03. The contents of the tank are stirred for se~eral mlnutes, and the pH is ad~usted to 3.2 with 2.ON NaOH. The contents are~then stirred for 15 minutes, filtered and washed with 20 gal. ~f water and reflltered.
Slx 100 g. samples are suspended in slx 300 ml.
portlons o~ water. ~he pHs of the suspensions are adJusted to the values indicated in Table VII by additions of HN03 or NaOH as necessary and held at the indlcated pH for i 3 about 15 mlnutes. The suspenslons are then flltered, the .

gel is removed and calcined for one hour at 400C~ The crystal phase, intensity and lightfastness are measured and reported in Table VII.

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Claims (14)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A pigmentary bright primrose yellow bismuth vanadate having a pure monoclinic crystal structure which when uniformly mixed with 90 weight % barium sulfate exhibits an increase in reflectance from 450 to 525nm of at least 65 units using Type I illumination, which has a surface area of from 2-22 m2/g and which has a masstone lightfastness in paint such that it loses 11% or less reflectance during 44 hours exposure in an Atlas Color Fade-Ometer? Type FDA-P.

2. The bismuth vanadate of Claim 1 wherein the increase in powder reflectance is at least about 70 units.

3. The bismuth vanadate of Claim 1 wherein the lightfastness in paint is such as to give a reflectance drop of less than about 7 percent in 44 hours of Fade-Ometer? exposure.

4. The bismuth vanadate of Claim 1 wherein the intensity in paint gives a green filter reflectance of at least about 60 percent.

5. The bismuth vanadate of Claim 1 wherein the intensity in paint gives a green filter reflectance of at least about 64 percent.

6. The bismuth vanadate of Claim 1 with sufficient lightfastness in paint to give a reflectance drop of less than about 7% in 44 hours of Fade-Ometer? exposure, and sufficient intensity in paint to give a green filter reflec-tance of at least about 60%.

7. The bismuth vanadate of Claim 2 with sufficient lightfastness in paint to give a reflectance drop of less than about 7% in 44 hours of Fade-Ometer? exposure, and sufficient intensity in paint to give a green filter reflect-ance of at least about 64%.

8. A process for preparing a pigmentary bright primrose yellow monoclinic bismuth vanadate comprising:
A. mixing a solution of Bi(NO3)3 .5H2O in nitric acid with a solution of alkali vanadate in an aqueous base selected from sodium hydroxide and potassium hydroxide, to precipitate a bismuth vanadate gel suspended in a solution containing dissolved alkali nitrate, wherein the molar ratio of Bi3+ to VO43- is from about 0.95:1.10:1.00, and wherein the normalities of the acid and base solutions are adjusted prior to mixing so that the pH of the mixture will be from about 1.0-8.0;
B. adjusting the pH of the suspension to about 1.9 to 3.6;
C. removing the gel from the suspension;
D. washing the gel with water until it contains about 20 weight percent or less alkali nitrate, based on the theoretical yield of bismuth vanadate; and E. calcining the gel at a temperature of from about 200-500°C. for about 0.4-3 hours.

9. The process of Claim 8 wherein the molar ratio of Bi3+ to VO43- is from about 0.98:1.00 to 1.02:1.00.

10. The process of Claim 8 wherein the pH in Step B is adjusted to about 3.3 to 3.5.

11. The process of Claim 8 wherein the gel is washed with water until it contains about 5-7 weight percent alkali nitrate.

12. The process of Claim 8 wherein the gel is calcined at about 380-460°C. for about one hour.

13. The process of Claim 8 wherein the gel is of Bi3+ to VO43- is about 0.98:1.00 to 1.02:1.00, the pH of the mixture will be about 1.5 to 2.0,the pH in Step B is adjusted to a pH of from about 3.3-3.5, the gel is washed with water until it contains about 5.7% alkali nitrate, and the gel is calcined at a temperature of from about 380-460°C. for about one hour.

14. The process of Claim 13 in which the Na3VO4 is dissolved in aqueous sodium hydroxide.