CA1316035C - Process for preparing a photographic emulsion containing tabular grains exhibiting high speed - Google Patents

Abstract

PROCESS FOR PREPARING A PHOTOGRAPHIC EMULSION
CONTAINING TABULAR GRAINS EXHIBITING HIGH SPEED

ABSTRACT OF THE DISCLOSURE
Process for preparing a photographic emulsion containing tabular silver halide grains, which exhibit high speed upon sensitization having a thickness of about 0.05 to 0.5 µm, average grain volume of about 0.05 to 1.0 µm3 and mean aspect ratio of greater than 2:1 comprising A. adding silver nitrate to a vessel containing dispersing medium/bromide mixture, initial bromide ion concentration 0.08 to 0.25 N, to form tabular seed grains;
B. adding an ammoniacal base solution, to achieve 0.002 to 0.2 normal of the base (e.g., after at least 2% of total silver nitrate has been added); and C. adding additional silver nitrate and halide, i.e., Br- or BrI- by balanced double jet procedure. The emulsions are used in photographic elements for x-ray, graphic arts, etc.

4246k

Description

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PROCESS FOR PR~PARING A PHOTOGRAPHIC EMULSION

DESCRIPTI~N

Thi6 inven~ian relates to a proce6~ for preparing a photographic emulsion containing tabular silver halide grainfi. More particula~ly thi6 invention relate~ ~o a pro~e~s ~or preparing a photographic emul~ion wherein the tabular silver ; halide grain~ exhibi~ high Epeed upo~ 6ensitiza~ion.
BACKGROUND OF THR INVENTION
Tabular ~ilver halide grains, eheir preparatio~ and use in photographic emul6ions, are widely known. They have been exten~ively ~tudied in the literature 6ince photographic emulsion6 containing these grains appeared to offer 60me significant advantage~ over photographic emulsions containing round or globular grain ~e.g., 6pla6h prepared types). Generally, tabular g~ain6 are large, flat ~ilver halide grains that are prepared by employing lo~g ripening time~ or by balanced double jet (BDJ) precipi~ation ~e~hod~. Commercial ~: emulsions u~ing tabular grains are conventionally made ~y u6ing a BDJ proces~. The tabular grain~
usu211y have triangular parallel crystal face~ each of which is u~ually Iar~ger than a~y other cry~tal face o~ the grain and are conventionally defined by their a~pect ratio (AR) which i~ the ratio of the dia~ete~ of the grain to the thickne6s. Larger A~
grain~, e.g., at lea6t 8:1, have diame~ers of at least 0.6 ~m a~d thickne~es oP less than 0.3 ~m.
~ These laLger ~abular grains have certain commercial ;~ adva~tage6 apparen~ to those of normal Ekill in the ~ 35 art. For example, they have a larger ~urfac~ area 13~l60~

and thus can accept more ~ensitizing dye. Since the~e tabular g~ains u~ually are dye sen6itized, when emul6ions u6ing ~uch tabular grains are present in medical x-ray element~ an increase in sharpne66 can result. In addi~ion, ~ince the tabular~grains no~mally lie flat when coated from an emul~ion on a support, the covering power i~ usually ~rèater and thus the emulsion can be coated at ,a lower coaeing weight and i6 therefore le~s costly. It i6 desired to prepare photographic e~ul~ions containing ~abular grains exhibiting high speed.
SUMMARY OP THE_INVENTION
In accordance with this invention there i~
provided a proce~6 for the prepa~ation of a photo-graphic emulfiion containing tabular ~ilver halide grains, which exhibit high speed upon sen~iti2ation, having a thickne~s of about 0.05 to 0.5 ~m, average grain volume oP about 0.05 to 1.0 ~m3, and a mean a~pect ratio of greater than 2:1 comprising A. adding silver nitrate to a ves~el containing a dispersing medium~bromide mixture wherein the ini~ial bromide ion concentration is 0.08 to 0.25 normal whereby tabular ~eed grains are focmed:
B. adding an ammoniacal base solution to achieve Q.002 to Q.2 normal of the base after at ~o least 2% of the total silver nitrate ha6 been added to the ves6el: and C. adding 6ilver ni~rate and halide ta~en from the group 3s consi6ting of Br and BrI
by balanced double jet -3- ~3~6~3~

procedure whereby tabular grains are formed.

DETAILED DE~SCRlPTION OF THE INVENTION
The process of this invention results in photog~raphic emulsions containing tabular silver halide grains which are higher in speecl when compared to tabulargrains prepared according to known processes. The resultant high speed could nothave been predicted from the state of the art. It is also possible that the tabular silver halide grains can be prepared having a narrow size distribution as taught in Nottorf, Canadian Application Serial No. 548,682, filed October 6, 1987. The narrow size distribution is achieved by using a silver halide solvent such as ammonia, ammonia derivatives, etc., and by stopping the initial silver nitrate addition for a time period of 1 to 60 minutes at a bromide ion concentration in the range of 0.005 to 0.05 normal (N).
The excess ammoniacal base present upon completion of the final addition of silver nitrate and halide can be neutraliæd with acid. Optionally the neutral;zed emulsion containing the final tabular grains may be ripened further by the addition of a thiocyanate salt ripening agent, e.g., alkali metal thiocyanate, for a period of about 1 to 20 minutes. The speed advantage of the emulsion is achieved after it has been chemically and spectrally sensitized by procedures known to those skillcd in the art. The thinner grains exhibit higher speed.

The tabular silver halide grains which upon sensitization exhibit high speed are of the silver bromide or silver bromoiodide types. The grains have ,~

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an average thicknes~ of about 0.05 to 0.5 ~m, preferably O.OS ~o 0.2 ~m and more preferably 0.1 ~m or below; an average grain volumle of 0.05 to 1.0 ~m3, ereferably 0.1 to 0.3 ~m3 and a mean a6pec~ ratio of greater than 2:1, preferably greater than 5:1, and more preferably greater than 8:1.
The grain charactsristic~ de6~ribed above of the ~ilver halide emul~ions o~ this invention can be readily a~cectained by procedurefi well known to tho~e skilled in the art. As employed herein the te~m "aspect ratio" refers to the ratio of ~he diameter of the ~rain to it~ thicknes6. The "diameter" of the grai~ is in turn defined as the diameter of a circle having an area equal to the projected area of the grain a~ viewed in a photomicrograph or an electron micrograeh o~ an emulsion ~ample. From ~hadowed electron micrograph6 of emul~ion 6ampl2s it i8 pos6ible to determine the thickness and diameter of ~ each grain. From this ~he aspect ratio of each ; 20 tabular grain can be calculated, and the aspect ra~ios of all the tabular grain6 in the ~ample can be averaged to obtai~ their mean aspect ratio. By this ~ definition the ~ea~ a~pect ratio i6 the average of : individual tabular grai~ aspect ratios. In practice it is u~ually ~impler to obtain an average thicknees and an avecage diameter of the tabular grain6 having a thickne~ of les6 than 0.5 ~m and to calculate the mean a~eect ratio as the ratio of the e two average~. Whether the averaged individual aspect ratios or the averages of thickness and diameter are u~ed to determine the mean aspec~ ratio, within the tolerance6 of grain mea~urements contemplated, the mean afipect ratios obtained do not significantly dif~er. The average qrain volume of a tabular grain 35 i8 de~ermined a~ taught by Holland et al. PS and E, 5 131~
volume 17, NoO 3 ~1973~, page 295 et ~eq. Normally the6e determinations are ~ade u~ing ~abular grain6 which are in the gLain diameter range of 0.5 to Z.5 ~m and appear tabular at 2,500 timeE; magnifica~ion.
In the preparation of the 1:abular grains described above the ~ollowing proceclure i6 used.
Into a conven~ional reaction ve6~el for 6ilver halide precipiSation equipped wi~h a stirri.ng mechanism i8 introduced a disper~ing medium/bromide mixture wherein the initial bro~ide ion concentration i~ 0.08 to 0.25 N, which i~ the known range to produce tabula~ grain~. Preferably the bromide ivn concentration i~ 0.1 to 0.2 N. The bromide salt ~re6ent i~ typ;cally in the for~ of an aqueou6 ~alt 60lution, e.g., one or more ~oluble ammonium, alkali metal, e.g., ~odium, potas6ium: alkaline earth metal.
e.g., magne6ium or calcium. Suitable disper~ing media initially pre~ent in the reaction vessel include water and a peptizer, e.g., gelatin, including alkali-treated gelatin (cattle bon~ or hide :~ gelatinS, acid-treated gelatin (pig6kin gelatin), gelatin derivatives, e.gO, ace~ylated gelatin, phShalated gelatin, etc.; protein6, protein i deri~a~ives, c011U108e derivatives, e.g., cellulo~e e6ters; p~ly6accharides, e.g., dextran, gum arabi~, : zei~, ca~ein, pectin, collagen derivative~, agar-agar, ar~owroot, albu~in, etc. Mixtures of peptizer6 may be u~ed. A preferred peptizer i&
gela~in or a~gelatin deriva~ive.
OtheE material~ commonly employed in combination with hydrophilic colloid peptizer~ a ~: vehicle~ (including vehicle e~tender~, e.g., materials in the ~orm of latices) include 6ynthetic :~ polymeri~ pep~izers, carrier~ and/o~ binders 6uch as 35 poly(vinyl lactams~, acrylamide polyme~, polyvinyl alcohol and its derivative~, poly~inyl acetals, ' S

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~olymer~ o~ alkyl and ~ulfoalkyl acrylates and methacrylates, hyd~olyzed polyvinyl acetates, polyamide6, polyvinyl pyridine, a~rylic acid eolymers, maleic anhydride copolymer6, polyalkylene oxide~, methacrylamide copolymer6, maleic acid copolymers~ vinylamine copolymer6, ~lethacrylic acid copolymer~, ~ulfoalkylacrylamide copolymer6, polyalkyleneimine copolymer~, polyamines, N,N-dialkylaminoalkyl acrylates, vinyl imidazoIe copolymer~, vinyl ~ulfide ~opolymer~, halogenated ~tyrene poly~ers, a~inea~rylamide polymer6, polypeptide~ etc. The~e additional material6 need ;~ not be present in the reaction ve66el during sil~er halide precipitation, but can be added to the 15 emul6ion prior to coating on a 6upport. The tempeeature of the contents in the reaction ve6sel is preferably in the range of 40 to 80C. Silver nitrate i~ then added at a 6teady rate into the ~eac~ion ve~sel containing the disper~ing 20 medium/bromide mixture whereby ~abular 6eed grains begi~ to form.
After approximately at lea6e 2 percen~ o~
the total ~ilver nitrate has been added to ensure ; proper size tabular 6eed grains have been permanently 25 ~ormed, an ammoniacal ba~e 601ut~ion i8 added to ~he reaction ves6el to achîe~e about 0.002 to 0.2 N, preferably 0.002 to 0.1 N, of the ba6e in the ve6~el.
Upon achieving a de6ired bromide io~
concentration in the ~eaction ve~6el, silver nitrate 30 i8 continually added into the ve~6el together with a halide compound which introduce6 additional bromide ions or bromoiodide ions by a balanced double jet (BDJ) ~roceaure known to tho6e ~killed in the art thereby maintaining the desired bromide ion ; 35 con~entration. It i~ i~n thi~ 6tep ~ha~ the ~abular 7 ~ 3 ~
grains achieve the desired propertie~ including mean a~pect ratio. In ~he event that bromoiodide ion~ are added during the B W procedure, the! a~ount o~ iodide prefient in the emulsion i~ in the range of about 0.01 to 10.0 mol percent, preferably O.Cl to 2.0 mol percent basQd o~ total silver.
A~ stated above and in order to ~pe~ifically control the grai.n ~ize di~tribution, when the de ired bromide ion concentration in the range of 0.005 to 0.05 N i~ achieved, and in the pre~ence of a silver halide solvent, e.g., ammonia, ammonia derivativ~, : etc., the initial silver nitrate addition optionally may be stopped for a period of time in the range of 1 to 60 ~inute~.
After completion of the total ~ilver nitrat2 and halide addition by the BDJ procedure, any excess ba6ic 601ution present in the emulsion optionally can ~ be neutralized ~ith a~id, e.g., acetic acid, ~ulfuric :~ acid, ni~ric acid, hydrochloric acid, etç. The p~
achieved i~ in the range of abou~ 5.0 ~o 7.0, preferably ~bout 6Ø Ae thi6 st~ge, the tabular grains ~ay be further Lipened for a time period of l : to 20 minute~ by the addition o~ a ~hiocyana~e salt to the emulsion. UseFul ~hiocyanate ~alt6 include alkali metal thiocyanates and ammonium thiocyanate, e.g.. in an amount of 0.1 to 20 g 6alt/mole 6ilver ~ halide. Other ri~ening agents include thioether, ; etc., a~ well a~ o~hers known to those ~killed in the art.
The tabular grain emul~ion6 are pre~erably washed to remove ~oluble ~alt~. Wa6hing technique6 are known to tho~e skilled in the art. The wa~hing i6 advantageou6 in terminating ripening o~ ~he ~abular grai~6 after completion of precipitation ~o avoid increa~ing their thickneEs and reducing their -13~3~

aspect ratio. Hhile 6ub~tantially all ~he grains aretabular in form the emul~ion i6 not affected by the pre~ence of a minor amount of nontabular grainæ.
Tabular grains of any aspect ratio can be made according to the described proces~; for exa~ple.
large, thin tabular grain6 or, al~ernatively, thicker, smaller tabular grain& can be prepared.
The emulsion containing tabular grain6 prepared according to this invention is generally fully dispersed and bulked up with gelatin or other disper~ion of peptizer de~cribed above and ~ubjected to any of the known ~ethodc for achieving optimum sen~iCivity whereby the high speed of the tabular ~ilver halide grain~ i6 achieved. Preferably optimum chemical ~en~itization i8 achieved by the additior~ o~
sultur and gold. Other sensitizer6 include:
~elenium, tellurium, platinum, palladium, iridium, osmium, rhodium, rhenium or phosehorous ~ensitizers or combination6 thereof at 10 8 to 10 10 N silver 20 (pAg 8 to 10), pH of 6.0 to 7.0 and temperatu~es of from 50 to 60C. Chemical ~en~itization can oecur in ::
the pre~ence of ~odifier6, e.g., compound~ known to supyre~ fog and increace speed when pre6ent during chemicai sensitization, such a~ azaindene6, azapyridazines, azapyrimidine~, benzothiazolium ~alt6, and ~en6i~izerR having one or more heterocyclic nuclei. The tabular grain silver halide emulsion~ are also spectrally sen~itized. U~eful sensitizing dyes are those dyes that exhibit ~; 30 ab60rption ~axima in the blue and minu~ blue ~i.e., green and red~ portion~ of the visible ~pectrum. In addition for ~pecialized appli~ation6, spectral en~itizinq dyes can~be employed which have improved spectral re~pon~e beyond the vi~ible 6pectrum, e.g.
infrared absorbing spectral sensitizer6. Example~ of :

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dye~ include ~ho~e di6closed in U.S. Paten~ No.
.42S.~26, col. 16, line 52 to col. 19, line 42, The tabular grai~ e~ulsion6 are u~eful in photoqraphic film element~. An emul6ion c-an ~e coa~ed in the nocmal manner on any o~ the conventional ~uppo~t~, e.g~. preferably polyethylene teeephthalate ~ubbed in a eonventional ~anner. Any of the other ~uppor~s known to the a~t can al~o be used. Coating, wetti~g aides, aneifoggant6, antistatic agent~0 etc., common t9 mo~t 6ilver halide elements, can also be u6ed in the preparation of the film element~.
Since element6 prepared from the emul~ion3 made u6ing the process of thi~ invention are eminently ~uitable for u6e in x-ray elements, usually the elements are coated on both sides of the support which usually i~ tinted with a blue dye as i6 known to tho~e 6killed in the x-ray art. The support may, and preferably doe~ have the conventional reain-type ~ub a~plied to the ~uppo~t and the sublayer i8 the~
u~ually overcoated with a thi~ ~ub6tra~um of gelatin ~ over which the e~ul~ion i~ ~hen applied. ~he :~ emulsion may be applied at coating weights of le6s tha~ 5 g Agi~2, preferably les6 than 4 g Aq/m2, for example, and then an abra6ion layer of ha~dened gelatin applied thereto to provide protection for the silver ~ontaining layer This element i8 conventionally expo6ed in a typical ~a~ette with a : 30 paic of x-ray inten~ifying ~creens as i5 well known.
Of cour6e, this i8 only a preferre~ element e~ploying emul~ions o~ thi~ inYention. The emulsion can be u6ed conveniently in any of the well-known photosen~itive ~ystem~ a~ noted below. A pre~erred mod~ of the invention:i~ de~cribed in Exa-ple 5.

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INDUSTRIAL APPLICABILITY
Pho~og~aphic ~ilver halide film elemen~s having at iea~ one layer of an emulsion ~ontaining the high ~peed tabular 6ilver halide grain~ prepa~ed according to ~he pcoees6 of thi~ invention are u6eful in conventional areas of photograph~y. The photographic element~ are particularly useful a~
x-ray film6, e.g., ~upport ~oated on each ~ide, in cooperation with x-ray inten~ifying 6creen6.
Sensitization can be in the green or blue portion of the spectrum. Other u~e6 include: graphic arts film~ color photographic fil~, etc~
E~AMPLE S
The following example~ illu6trate but do not limit the invention. In the Control and Example6 the percentage~ are by weight. N mean6 normal.

To 450 liters of a well-Rtirred aqueous olution, which Gontained 1.6 percent photog~aphic gelati~ and 9.1 kilogram6 of potafi6iu~ bromide and which wa6 ~aintained at 60C, wa6 added a 1 N AgN03 solution at con~tant flow until the bromide io~
concentration was lswerea to 0.075 N. Double-jet addition of a 3 N Ag~03 601u~ion and a 3 N K~r ~lution was ~hen initiated, with the AgN03 flow increasing 200 ~lf~in/min for 20 minute6 and the KBr flow increa~ing to maintain a growth bromide ion concen~ration of 0.075 N. The KBr 601ution wa~
halted and the AgN03 ~olution continued at a con6tant flow until ~he bromide ion concentration was lowered to 0.025 N, which wa~ then maintained by double-jet addition of K~r and AgN03 at con~tant flow until 540 molefi of AgN03 wa~ di~pen~ed.
~ollowing precipitation, 675 gram~ of sodium : 35 thiocyanate were added and the emul~ion ripened at ~ 31S~

60C for 15 minutes. The smlllsion was then cooled to 40C and washed by a coagulatio~ process three ~imes.
~ The re~ultan~ tabular grain AgBr e~ul6ion : had an average grain volume o~ 0.1~5 ~m , an S average grain thicknes6 o 0.10 ~m and an average AR of 1~
E~AMPLE 1 To 550 liter~ o~ a well-s~i-red aqueou~
~olution. which contained 1.6 percent ehotographic gelatin and 11.1 kilogra~s of pota~sium bromide and which was maintained at 60C, wa~ added a 3 N AgN03 solution at constaRt flow. When the bromide ion concen~ra~ion wa~ lowered to 0.058 N, 1210 gra~s of a 23 parcen~ ammonia 601utio~ were added. The AgN03 flow continued until the bro~ide ion concentration was lowered to 0.020 N, at which time the AgN03 ~olution wa6 halted. After 3 minutes double-jet addition of a 3 N AgN03 solution and a 3 ~ XBr solution wa~ initiated, with the AgNO3 flow increasing 3 7 ml~in/min for 15 ~inutes and the ~Br flow increasing eo maintain a growth bromide ion concentration of 0.020 N. Double-jet addition continued at constant flow until 660 moles of AgN03 were dispen6ed. Following precipitation, 925 grams of glacial acetic acid were fiest added to neutralize all remaining ammonia and then 625 grams of sodiu~
thiocyanate were added and the emulsion ripened at 60C for 15 minute6. The e~ulsion wa6 then cooled to 40C and wa~hed by a coagulation prnces6 three time6.
: 30 The resultant tabular grain AgBr emulsion had an average grain volume of 0.09 ~m3, an average grain thickne66 of 0.15 ~m and an a~erage M of 6:1.
E~A~PLE 2~
To 550 liters of a well-6tirred aqueous ~olutio~, which ~ontained 1.6 per~en~ photographic : ;

12 ~31~3~
gelatin and 11.1 kilogram6 of potas6ium bromide and which wa~ ~aintained at 60C, wa~ added a 3 N AgN03 solution at constant flow. When thle bromide ion concentration was lowered to O.OS~ N, 3226 grams of a S 23 percen~ ammonia solution were added. The AgN03 flow continued until the bromide ion concentration was lowered to 0.010 N, a which time ~he AgN03 ~olution was halted. After 3 minu~e~ double-jet addition of a 3 N AgN03 solution and a 3 N KBr solution waQ initiated, with ehe AgN03 flow increasing 413 ml~min/min for lS minute~ and the ~Br ; flow increasing to maintain a gLow~h bLo~ide ion concentration of O o O10 N. Double~jet addition continued at con~tant flow until 660 moles of AgN03 lS were di6pensed. Following preci~itation, 2440 grams of glacial acetic acid were fi~st added to neutralize all eemaining ammonia and then 825 qram6 of ~odium thiocyanate were added and the emulsion ripened at 600C for 15 minutes. The emulsion ~a& then cooled to 40C and washed by a coagulati~n process three ~imes.
The resultant tabular qrain AgBr emulsion had an average grain volume of 0.13 ~m3, an averase grain ~hickne6s of 0.35 ~m and an average AR of 2~
l'he emul~ions of Control 1 and E~amples 1 and 2 were che~ically 6en6itized with sulfur and gold and spectrally senRitized to ~he green portion of the spect~um with 1.3 g/Ag mole of anhydro-9-ethyl-5,5'-dichloro-3,3'-bi6(4-sulfobutyl)oxacarbocyanine hydroxide, triethylamine 8alt ~ensitizing dye and 0.2 g~Ag mols of ~otas6ium iodide. The ~en~itized : emulsions were coated on both 6ide~ of a.polyethylene terephthalate film support and given a SOkVp, lOOm~, 40 m6 exposur~ 40 inehe6 tlOl.6 cm) from a standard tung~ten x-ray source through a continuou6 aluminum 13 ~3~3~
~epwedge and through a ~tandard green light emitting 6c~een such as Du Pont Quanta~V and proce~ed in a conventional radiogra~hic element procee~or, Du Pon~
QC-I~T, u~ing a 6tandard developer solution, Du Pont HSD.
The propertie~ of the resultant image~ are 6ummarized in Table 1 belo~.
Table 1 Cont.
1~ or Grain Grain ~x. Ba~ic Thick. Vol.Rel. PER
No. Geowth ~L_ (um3~AR SPeed SPeed Cl No 0.10 0.16 14:1 225 100 El Yes 0.15 0.09 6:1 245 175 E2 Yes 0.35 0.13 2:1 260 140 1 Pro~ec~ed Equivolume Relative (PER) Speed i6 the average of the speed predicted by a~u~ing speed i~ proportional to ~olume and ~hat predicted by a~uming speed is proportional to ~olume to the 2/3.

EXAMPL~_3 To 4.375 liter6 of a well-stirred aqueous 601ution, which contained I.6 percent photographic gel~ and 88.51 gra~ of poeassiu~ bromide and ; 25 which wae maintained at 60C, wa~ added a 3 N AgN03 601ution a~ constant 10w. When the broMide ion concentration Wa8 10WeEed tO 0 . 058 N, 3 . B m1S Of a 23 ;percent ammonia 601utio~n were added. The AgN03 flow cuntinued until the bromide ion concentration ~ 30 was lowered to 0.010 N, at which ~lme double-jet :: addi~ion of a 3 N Ag~03 601ution and a 3 N KBr eolution was initiated, with the AgN03 ~low increa~ing 3.0 ml/min~min for 15 minute~ and ~he KBr flow increa~ing to maintain a growth bromide io~
concentration of 0.010 N. Double-jet addition . .
: ~ 13 14 ~3~3~
continued at constant flow until 5~25 moles of AgN03 were dispensed. Following precipitation, glacial acetic acid was fir6t added to neutralize all remaining ammonia and then 6.5S grams of ~odium thiocyanate were added and ~he emulsion ripened at 60C for 15 minutes. The emulsion was ~hen ~ooled to 40C and wa~hed by a coagulation proce6~ three time~.
The resultant tabular grairl AgBr emulsion had an ave~age grain volume of 0.11 ~m3~ an av~rage grain thickne~6 of 0.15 ~m and an average AR of 6:1.
E~AMPLE_4 To 4.375 liters of a well~stirred aqueou~
solution, which contained 1.6 percent photograehic gelatin and 88.51 grams of pota~sium bromide and which wa6 maintained at 60C, was added a 3 N AgN03 solution at constant flow. When the bromide ion cQncentration was lowered to O.Q58 N, 9.4 mls of a 23 percent ammonia fiolution were added. The AgN03 flow continued until the bromide ion concen~ration was lowered ~o 0.020 N. at which time the AgN03 solution was halted. After 3 ~inutes double-jet :~ addition of a 3 N AgN0 ~olution and a 3 N KBr solu~:ion was initiated, with the AgN03 flow in~rea6ing 3.0 ml/min/min fo.r 15 minutes and the KBr ; flow increasing to maintain a growth bromide ion concentratio~ of O.Q20 N. Double-jet addition continued at constant flow until 5.25 moles of ~; : AgN03 were dispensed. Following precipitation, : 30 glacial a~etic acid ~as first added to neutralize all ~emaining ammonia and th~en 6.55 grams of sodium - thiocyanate were added and the e~ul~ion ripened at 69C for 15 minute~. The emulsion wa~ then cooled to 40CC and washed by a coa~ulation process three times.

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The resultant tabular grain AgBr emulsion had an average grain volume o~ O.:L5 ~m , an average grain thickness of 0.15 ~Dn~ and an average AR of 7:1.

To 4.375 liters of a well-stirred aqueou ~olution, which ~o~ained 1.6 percent photogsaphic gelatin and 08.51 gram~ of pota~ium bromide and which was maintained at 60C, wa~ added a 3 N AgN03 ~olution at constant flow. When the bromide io~
concentration wa~ lowered ~o 0.058 N, 7.0 mls of a 23 percent ammonia 601ution were added. The AgN03 flow continued until the bromide ion concentra~ion wa~ lowered to 0.090 N, at which time the AgN03 solution wa6 halted. After 3 minutes double-jet addition of a 3 N AgN03 ~olution and a 3 N KBr solution was initiated, with the AgN03 flo~
increasing 3.2 ml/min/min for 15 minutes and the KBr flow increasing to maintain a growth bro~ide ion concentration of 0.040 N. Double-jet addition continued at constant flow for 14 minutes, at which ; time the KBr solutiQn was hal~ed and the AgN03 ~olution continued a~ con6tant flow until ~he bromide ion concent~ation was lowered ~o O.OlS N, which was then maintained by double-jet addition of ~Br and : AgN03 at con~tant flo~ until 5.25 ~ole6 of A~N0~
wa6 dispen~ed. ~ol}owing precipi~ation, glacial acetic acid was fir~t added to neutralize all remaining a~onîa and then 6.55 grams of &odium thiocyanate were added and the emul~ion ripened at 60C for lS minu~es. The emul~ion wa~ then cooled to 40C and wa~hed by a coagulation process three time~.
The resultant tabular grain AgBr emulsion had an average grai~ volum~ of 0.23 ~m3, an average grain thickne~ of 0.10 ~m, and an average AR of 17:1.

~3~L6~3~
1~
The emulsions of Examples 3 to S were chemically sensitized, spectrally 6ensitized, coated ~ingle 6ide on a sueport, and expo6ed a6 de~cribed prior to Table 1.
The properties of the resultant images are summarized in Table 2 below.
Table ?
Halt Grain Grain : E~. Ripen- Thick. Vol. Rel. PEa No~ inq (um) (~m3) AR Speed Speed E3 no 0.15 .116:1 250 100 E~ y~s 0.15 .157:1 335 105 E5 yes 0.10 .2317:1 600 130 1 Projected Equivolume Relative ~PER~ Speed is the average of the speed predicted by as6uming speed is proportional to volume and that -predicted by assuming speed i6 proportional to volume~to the 2/3.

~ 0 ,:~

~ 25 .

;
. 30 ~'~

: : 35 ';
.
~ 16

Claims (19)

1. A process for the preparation of a photographic emulsion containing tabular Silver halide grains, which exhibit high speed upon sensitization, having a thickness of about 0.05 to 0.5 µm, average grain volume of about 0.05 to 1.0 µm3, and a mean aspect ratio of greater than 2:1 comprising A. adding silver nitrate to a vessel containing a dispersing medium/bromide mixture wherein the initial bromide ion concentration is 0.08 to 0.25 normal whereby tabular seed grains are formed;
B. adding an ammoniacal base solution to achieve 0.002 to 0.2 normal of the base after at least 2% of the total silver nitrate has been added to the vessel; and C. adding silver nitrate and halide taken from the group consisting of Br- and BrI-by balanced double jet procedure whereby tabular grain are formed.

2. A process according to claim 1 wherein when the bromide ion concentration is in the range of 0.005 to 0.05 N, the initial silver nitrate addition is stopped for a time period of 1 to 60 minutes.

3. A process according to claim 1 wherein after Step C the excess base is neutralized with acid.

4. A process according to claim 3 wherein a thiocyanate salt ripening agent is added and the emulsion is then ripened for 1 to 20 minutes.

5. A process according to claim 1 wherein the emulsion is chemically and spectrally sensitized.

6. A process for the preparation of a photographic emulsion containing tabular silver halide grains, which exhibit high speed upon sensitization, having a thickness of about 0.05 to 0.2 µm, average grain volume of about 0.1 to 0.3 µm3, and a mean aspect ratio greater than 8:1 comprising A. adding silver nitrate to a vessel containing a gelatino/bromide mixture wherein the initial bromide ion concentration is 0.1 to 0.2 N
whereby tabular seed grains are formed;
B. adding an ammoniacal base solution to achieve 0.002 to 0.1 normal of the base after at least 2% of the total silver nitrate has been added to the vessel; and C. adding silver nitrate and halide taken from the group consisting of Br- and BrI- by balanced double jet procedure whereby tabular grains are formed.

7. A process according to claim 6 wherein when the bromide ion concentration is in the range of 0.005 to 0.05 N, the initial silver nitrate addition is stopped for a time period of 1 to 60 minutes.

8. A process according to claim 6 wherein after Step C the excess base is neutralized with acid.

9. A process according to claim 8 wherein a thiocyanate salt ripening agent is added and the emulsion is then ripened for 1 to 20 minutes.

10. A process according to claim 6 wherein the emulsion is chemically and spectrally sensitized.

11. A photographic film element comprising a support having coated thereon at least one silver halide emulsion prepared according to claim 1.

12. A photographic film element comprising a support having coated thereon at least one silver halide emulsion prepared according to claim 5.

13. A photographic film element comprising a support having coated thereon at least one silver halide emulsion prepared according to claim 6.

14. A photographic film element comprising a support having coated thereon at least one silver halide emulsion prepared according to claim 10.

15. A photographic film element according to claim 11 wherein the support is coated on each side with a layer of said silver halide emulsion.

16. A photographic film element according to claim 12 wherein the support is coated on each side with a layer of said silver halide emulsion.

17. A photographic film element according to claim 13 wherein the support is coated on each side with a layer of said silver halide emulsion.

18. A photographic film element according to claim 14 wherein the support is coated on each side with a layer of said silver halide emulsion.

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