[1]Util'ted States Patent Office 32046@128 3,046,128 THERMALLY DEVELOPABLE DIAZOTYPE PHOTOPP,INTING MATERIAL AND PRODUCTION TP.EREOF Robert J. Kiimkowski, Chicigo, Ill., and John W. Krue. ger, Cott,-ge C-iove, Wis., assignors to Eugene Dietzgen Co., Chicago, fil., a corporation of Delawire Filed July 3, 1958, Ser. No. 746,544 6 Ci-ims. (Cl. 96-49) This invention relates to the production of diazotype photoprinting materials, aiid to new materials and niethods. More part-'@cularly, the invention relates to improved thermally developable photoprinting materials charae'Lerized by good development and good storage or shelf life. The diazotype reproduction processes are b@,sed on light-se-,isitive diazo compounds coated on a suitable base, and a colored, positive azo dye image is formed on the base j'rorii a positive master. Exposure to ultraviolet light u-.ider a translucent master or original, such as an engineering dra,,ving, photo,-raphic positive, typed material or other des;gn, will decompose the diazo com@pound where it has been struck by light, and retain the diazo coinpound where it has been protected from light by the opaque areas of the original. The print is then developed by reactir,,- the retained diazo compound with a coupler, to 'Lorrii a highly colored azo dyestuit, in precise duplicatioia of the original. The diazotype reproduction processes are divided into two categories, the "moist process" and the "dry process." The moist process is a one-compo-tient diazotype process ir, which the diazo is on the base while the coupler is applied to the coqted surface in a developer solution. The dry process is a two-cortiponent process in which both th-- diazo and the cotipler are on the base and in which develop-rnent is achieved without wettin.-, stich as through the use of moist ammo@iia vapors to i@iduce the colipling reaction. The present inventioll is concerned with improvemeiits in the dry process printing materials and in their manufacture and use. Each of the above system@ s, though in commercial usage today, has serious disadvantages. The ammonia developed system is handicapi)ed by the pungent, lachrymatory nature of the animonia employed. Consequently the majoritv of ammonia-developed mar-hines reqi-iire special v-,ntin.- in order to eliminate this problem. In spite of these efforts, the solution to this problem is unsatisfactory. Ammonia n-iust be handled in order to fill the machine, a7id venting does not allow a portable machine. The second system (liquid developer), thou-,h eliminating the odor of amr@ionia and consequently the need for venting, is hampered by the fact that the developing solution nitist be mixed frequently and once mixed it is si-isceptible to oxidation. Fi-irthermore, eva-poratio-@l of water fror@i the developer induces crystalliza:tioi, and t@is combined with the af--rer@ientioned oxidation often leav,-s th(-- developin-, section of a macl-iine a brown sticky niass. Attempts have b@-en made iTi the past to obtain a diazotype printin- paper develoi)able bv heat, but they liave met vvith lilltl,- or no success. These metliods have included incorroratioil oi. tlae heat developin.- chemicals into or onto th@ sensitized side of the base employed. Inasmuch as the materials employed were capable of causin.- the sonsit-lzed coating to develop rapidly, the problem of stora.@e life of the sensitized product was ol' sr,,-Ii a raagnittide that commercial prodtiction of the above was not feasible. Whenever satisfactory sto@-age life has b@-en obtained, Patented July 24, 1962 2 it has been accomi-)Iished at the expe@ise of clevelo-vrnent. Acbievement of storage life has been possible only by such heavy stabilization that development wotild yie@ld only a faint, weak reprodtiction in no way comparable to that obtained from commercial diazotype papers. It is t ' herefore an object of the present invention to provide improved diazotype photoprir.-tin- materia's and methods which overcome the disadvantages of the prior systerns, and particularly, thermally developable photo10 printing materials which have both -,ood development and gogo stora.-c characteristics. A more particular object is to provide diazotype materials which are developed solely by the application of heat. 15 A further object, is to provide a single photoprinti-tig material for both exposure. and developrient, with no need for other materials or solutions. An additional object is to provide a completely dry photoprinting material and process which produce excel20 lent permanent diazotype prints, not subject to darkeriing, discoloration or other uidesirable changes th--reafter, whether or not exposed to ii.-ht, huniidity or other conditions which are commonly encou-iitered. Thus, the materials and photoprinting process oi' the inveiitio@i are 25 eminently suitable for office and shop use. Another object is to provide photoprinting materials which are readily and econom:ically prepared usliig coinmonly available and substantially stable materials. A further object is to provide methods for producing 30 photoprinting materials and for producing photoprints therefrom which do not require complex equipment or involved operations. These and other objects, advantages, and functions of the invention will b-- apparent on reference to the 35 specification and to the attached drawing illustrative of several preferred embodirnents of the invention, described hereinafter, The invention provides a thermally developable diazo40 type photoprinting material con-iprised of a vapor perwieable base, a diazotype sensitizer on one side Ciaereof, and on the reverse side, a corndotind which pr<)duces alkaline vapprs when heated. The photoprinting process en-iploying the material of the inveption involves mereiy exposin.- the sensitized side of the material followed by 45 application of heat to the reverse or alkali ge-@lerating side for the production of alkaline vapors and development of an image. The invention provides an effective barrier in the form of the base, such as paper, between the lirht-sensitive 50 compound and the alkaline materials, to eliminate the possibility of their uniting prematurely with consequent undesirable precoupling. The photoprinting materials thus have the desired storage life i'Or unexposed lightsensitized material. At the same time, the provision of 55 a vapor permeable base coated on the reverse side with a materia pro cing development. The compounds which produce alkaline vapors when heated are preferably those which yield ammonia by 60 decomposition or dissociation. The compounds include, for example, the preferred urea, guanidine, alkyl substituted ureas and the like. Other compounds which may be employed are the ammonium salts of weak acids such as ammonium acetate, ammonium carbonate, ammonium diglycolate, and ammonium oleate. Volatile organic 65 bases may also be, employed, for example, ethylene diamine, N'-(2-hydroxyethyl)-2-methyl-1,2 -propanediamile, and -oth.ers. They do not liberate ammonia but themselve s possess the requ ired basicity. It will be apparent that the alkaline acting COM7 70 pounds must not have an apprecia@ble alkaline vapor pressure at ambient temperatures, or precoupling v@iR vapors resu ts in exce ent
[2]3,046,12S 3 take place. It is therefore preferred that such vapor pressure be insubstantial at atmospheric temperatures and Lip to about 200' F. and proferably up to a@bout 225' F. However, it is possible to employ compounds havina moresubstantial vapor pressure, by enclosing them in a resinous qlm or the like, as subsequently described. Satisfactory storage life can then be achieved with the interposition of the paper or other base. At the same time, it is pre-ferred that substantial alkaline vapor pressure sufficient for coupung be achieved below about 450' F. Otherwise, there is danger of decomposing the diazo compound. The upper temperature limit will vary with the thermal stability of the diazo compound. In a preferred embodiinent, the stability and shelf life may be enhanced by incorporating in the alkali generating layer, a compatible acidic stabilizing material, of a type and quantity which will not interfere with development. Weak preferably organic arids are most advantageously employed, suich as ',.tartaric, citric, diglycolic, oxalic, acetic, and malic ar-ids. Those acids which decompose on heating are further preferred. In a preferred practice of the invention, a light-sensitive diazo compound, an azo dye coupling component and adjuncts are applied to the side of the base opposite to the side containing the alkali generating materialThe materials are applied to the respective sides in any order. The diazo compound, coupling component, and adjuncts are not critical factors in the practice of the invention. Any diazo compound and coupler suitable for producing dry process diazotype materials may be used. Exemplary diazo compounds are the diazonium salts obtained by diazotization of the following amines: p-Amino-N,N-dimethylaniline p-Amino-N,N-diethylanfline p-Amino-N-ethylaniline p-Amino-N-ethyl-N-p-hydroxyethylaniline p-Amino-N-methyl-N-,e-hydroxyethylaniline p-Amino-N,N-di-p-hydroxyethylaniline p-Amino-m-ethoxy-N,N-diethylaniline p-Amino-N-ethyl-o-toluidine p-Amino-N-ethyl-m-toluidine p-Amino-N,N-diethyl-m-toluidine p-Amino-N-ethyl-N-;benzylaniline p-Amino-N-ethyl-N-p-hydroxyethyl-m-toluldine N-p-amino-phenylmorpholine p-Amino-diphenylamine 3-aminocar@bazole and the -like. These diazonium salts are commonly employed as their stabilized double salts, e.g., their zinc chloride, cadmiurn chloride or stannic chloride double salts. Exemplary suitable azo coupling components are the follo,,ving: 2,3-dihydroxynapthalene 2,3-dihydroxynaphthalene-6-sulfonic acid Resorcinol Phloroglucinol Acetoacetanilide 7-hydroxy-1,2-naphthimidazole I-phenyl-3-methyl-5-pyraz6lone 2-naphthol-3,6-disulfonic acid, and the like It is also,contemplated that in providing the azo dye coupling component on the sensitized surface of the base, a coupler capable of thermal transfer or vapor distillation may be provided on the unsensitized side with the alkaline material for transfer therewith to the sensitized side upon application of heat. The diazo compoiinds and coupling components are applied to the base in conventional coating solutions and at standard rates of application. For example, diazo compounds are conventionally applied to diazotype paper at the rate of 10 to 50 grams per 1000 square feet. The various adjuncts usual in the manufacture of lightsensitive diazotype materials may be employed in the Eensitizin.- and coupling compositions, sucli as metal salts for intensification of the ima.-e, stabilizing agents, acids acting to retard precoupling, and others. In a novel embodiment of the inve@ition, a useful adjunct is a heat fugitive acidic stabilizer pr<)vided in the sensitizing solution. Thus, stannic chloride may be employed, and it will volatilize upon the application of heat. Citric iacid also decomposes upon the application of heat. The efficiency of the photoprinting material may be im10 proved thereby, it bein@a possible to rnaintain high stability until such time as low stability and coliplin.- of the diazo comuound is desired. After exposure of the sensitized side of the material under a design to be reproduced, heat is applied to the 15 reverse side for the production of alkaline vapors and development of an image of a design. The time and temperature of heating are selected for the system employed, to produce the requisite vaporization of all@aline material without - undesira-ble decomposition of the diazo 20 compound. The results are generally advanta.-eous when the alkali generating materials are subjected to a relatively high @temperature of about 300' F. to 400' F. for a short time, a period of about several seconds to about a minute. While the materials can be heated to a lower 25 temperature for a -longer period of time, the former procedure is preferred. Upon heating, the ammonia-producin,a materials evolve ammonia which permeates the base throu,-hout the same and completely and intirnately contacts the sensitized 30 layer, where the pH is raised sufficiently to produce the coupling reaction between the unexposed diazo compound and the coupling component, which produces the colored image. Certain of the compounds or their decomposition prod35 ucts, such as urea, guanidine, and their d@-rii7atives, apparently undergo other reactions including possible recombination to urea and combination to other compounds. It appears that urea may be reconstituted on the sensitized side of the base, and it there further assists 40 in the development. When employing a volatile amine, it appears that a triazene may be formed intermediately by reaction with the diazo compound. The triazenes under the influence of heat decompose to the diazo compound which reacts 45 with the coupler to produce ttie desired azo dye. In the event that an amine should form a stable triazene under the conditions of operation it would not be used, since the desired color reaction would not be obtained, althou.-h the triazene might be colored. The new photoprinting material is preferably employed 50 in combiination with means for maintaining the alkaline vapors in contact with the sensitizer. Such means include provision during the heating and developing of apparatus for confining or restraining the vapors adjacent the seii55 sitizer, e.g., a plate or the like, and/or a vapor-restraining film on the sensitized side of the base. Such a film may be produced by incorporating a suitable resinous or equivalent material in the sensitizing solution, or by coating such a material over the sensitized layer, the resinous n-iaterial 60 being one of those more particularly described below in connection with the application of the alkaline material. In this manner, contact of the alkaline vapors with the sensi d ayer is more complete and longer lasting, and the amount of alkaline material may be limited. However, iB5 it is also possible to effect development without restraining the vapors in such a manner, but a relatively high concentration of alkaline material may be required. The sensitized layer may also include finely divided silica, of the type subsequently described, to enhance the dye ima.-e 70 and also assist in the penetration of the layer by the alkaline vapors. It is preferred to incorporate the alkaline reacting material on the reverse side of the base with a film-forming resinous material or the like, which assists in preventing 75 precoupling, and also prevents crystallization of certain of
[3]the materials. The film-formin- ingredients may be either hydrophilic or hydrophobic, but hydrophobic materials are preferred. The hydro,)hobic natural and synthetic resins are preferred, such as rosin derivatives, and syntheticsubstantially water-insoluble filmforrning synthetic resins. The preferred resins are hydrophobic synthetic filmformiing thermoplastic addition polymers of at least one mono-olefinic monomer having apolymerizable CH2=C< group, such as the polystyrene resins, the polyacrylic resins and the polyvinyl resins. The polystyrene resins include, for example, homo- and copolymers of styrene, and ring and side chain-substituted styrene, substituted with halo, alkyl or aryl radicals, Thus, the polymers may be prepared from vinyl toluene, alpha and beta-chloro and bromo styrene or alpha and beta-methyl and ethyl styrene. The polyacrylic resins include the pre.erred polymers of acrylic esters, such as methyl and ehyl methacrylate, and methyl and ethyl acrylate. The polyvinyl resins include the vinyl esters such as vinyl acetate, the vinyl halides such as vinyl chloride, and the vinylidene halides. Especially preferred are the butadiene-styrene copolymers and polyvinyl acetate. The polyacrylates and polystyrene are also very useful. The resins are preferably employed finely dispersed or emulsified in an aqueous coating compositio@i for application to a base. In this form, they are generally believed to be in or near the colloidal particle size range, e.g., about 10 microns or below. About 1 or 2 percent of an emulsifier may be employed to aid in forming a stable emulsion of the resin particles, in known manner. One or more resins may be provided in the coating composition and resulting layer. The compound producin.- alkaline vapors is then dissolved or dispersed in the aqueous resin composition to provide a solution thereof for appl;cation to the unsonsitized side of the base. It is further prefei-red that the foregoing developing composition also include finely divided silica, which apparently assists physically in the develop.-nent. Colloidal silica is preferred, having a particle size of about 0.1 micron or below. The particle size may, however, range up to 10 microns. Aqueous finely divided silica dispersions may be prepared in known manner froni a silica powder, and thedispersion may contain an agent to stabilize the colloidal sus ension. Also, one of the various p commercially available preparations of aqueous finely di,iided silica may be employed, including available niaterials ranging in particle size from 0.007 to @O@.1 micron and greater. The concentration of the alkaline reacting material in the composition applied to the base and the rate of aDplication to the base should be sufficieit to elevate the pH. of the sensitized layer to that required for coudling, and an excess should be provided to compensate for losses and variations in distribution. For exapiple, the adplication of about 500 grams to 2000 grams, and at times to 3000 grams, of urea per 1000@ square feet of diazotype paper is recomrnended. Other compounds producing alkaline, vapors may be applied at rates corresponding thereto on the basis of the amrr@onia or other bas,@'c vapor evolved. The proportion of the alkaline material in its composition should of course not be so small that the vapors would be hindered from evolvin.- throu,-h the permeable base. Thus, it is -enerallv j)referred that the compound producing alkaline vapors be present in a proportion greater thail about Z parts per part by weight of film-forming mat,-rial. Silica rnay be employed in a proportion up to about 5 parts per part by weight of resin, there preferably bein,@ employed about 0.1 part to 2.0 parts per part of resin. An acidic material for enhancing the stability of the alkali generating material, as previously described, may be incorporated in a suitableproportion depending upon the nature of the acidic material -and 6f the alkali generator@ For example, about 8 to 20 grams of citric acid 3,046,128 6 or about 10 to 40 grams of tartaric acid may be added to the alkaline composition of Example I which follows, the compos-ition containing 125 grams of urea and 50 grams of guanidine carbonate. Citric acid aloric has a yellowing tendency, and the preferred addition to thecomposition is a combination of 0.8 to 5 grams of citric acid with 10 to 15 grams of tartaric acid. The proportiod of acidic material is thus g@nerally minor relativ-. to the alkali generating material so as not to intcrj'ere with de10 velOPment. The several materials are deposited on the unsensitized side of the base from an aqueous solution or dispersion having a concentration of about 20% to 80% by weight of dispersed and dissolved materials. 15 As is well known, it is necessary for water to be present in contact with the diazo compound and coupler for development to take place. The water requirement is not great, and frequently is supplied by the natural moisti-ire content of the photoprinting material. Certain of the 20 alkaline materials may evolve water vapor upon decomposition, such as ammonium carbonate. To assist in the incorporation of water in the photoprinting material, a humectant may be included on either side of the base. A hiimectant or hygroscopic materie,,l in 25 the sensitized layer will also assist in absorbing the alkaline vapors. Suitable htimectants include such materials as zinc chloride and polyhydric alcohols, for exarnple, glycerine, ethylene glycol and diethylene glycol. The presence of silica may,also assist in retai-iiing moisture in the print30 ing materials. The vapor permeable base is most advantageously paper, porous cloth or the like as employed in drafting and printmaking, or similar materials. It is necessary that the paper or other base provide a physical barrier between 35 the respective light-sensitive and alkalin.- sides or layers of the photoprinting material pr'@Or to development, while being freely permeable to vapors. Standard commercial diazo process paper is suitable, preferably all sulfite paper stock. Paper having a rag content may also be used, al40 thou-h with increasing proportions of rag, it may be advisable to precoat the paper to inereas@- the thickness of the barrier and/or reducepenetrability. With the diazo process paper, the diazo sensitizer may per@etrate to about one-thirtieth of the depth of the paper, and the reverse coating of alkahne miterial will not meet the diazo layer. 45 The coating times and other condit@'ons effecting penctration may be regulated to insure tha "; the re.spective layers do not @come in contact througli the base. Penetration may also be controlled by coating the base on either side with a vapor permeable precoat or layer, 50 preferably of a resinous material and further preferabl-y of a resin-silica layer of the type described above in connection with the application of the alkaline material. A precoat on the sensitized side also improves image density. A precoat solution may be applied which contains, for 55 example, 5-35% of one of the resinous materials described above, and preferably also about 5-35% of the describedfinelydividedsilica. ThereSintOSiliCaTati0ispre@ferably within the range of about 4:1 to 1:4. Several preferred embodiments of the @invention are 60 schematically illustrated in the affached drawings, each of which is an enlarged fragrnentary cross section of coated paper. In FIGURE 1, a sheet of paper I is illustrated as provided with a dried coat or layer 2 of a dry process lightsensitive diazotype composition, which may or may 65 not @also contain a resin whi(-h provides a vapor-restraining film and silica, as described above. On the reverse side of the paper, an alkali generating layer 3 is provided which contains a compound prodticing alkaline vapors when heated. 'Fhis layer also may or may not 70 contain a resinous material and silica. FIGURE 2 illustrates the sheet of pap,-r 1 containing a dried precoat layer 4 of resin preferably also containing silica, on one side of the paper. The alkali generating layer 3 is provided over the precoat layer. The diazo 75 layer 2 is provided on the reverse side of the paper in con-
[4]7 tact therewith. Alternatively, the diazo layer and ti-ie al,kaline layer may be transposed, so that the former is over the precoat and the latter is in contact with the reverse side of the paper. FIGURE 3 illustrates the sheet of paper I having the diazo layer 2 and the alkali generating layer 3 on opposite sides of the paper. A vapor barrier film 5 is also provided over the diazo layer, which may constitute a resinous niaterial as previotisly described. The following examples illustrate the materials and methods of the iiivention. It will be understood that the invenlion is not limited to the examples nor to the components, proportions, conditions and procedures set forth therein. Example 1 A sensitizing solution for producing blueline diazotype prints has the follolving composition: Water ---------------------------------- ml-- loo Ethylene glycol ------------------------- ml-- 15 Citric acid ------------------------------- g-- 0.75 Zinc chloride ----------------------------- g-- 5.0 Thiourea -------------------------------- 5.0 2,3-dihydroxynaphthalene-6-sodium sulfonate --g-- 4.0 p-Diazo-N-ethyl-N hydroxy-ethylaniline zinc chloride salt -- ----------------------------- g-- 1.4 Sapon-in -- ------------------------------- g-- 0.2 The sensitized solution is coated on a sheet of 100% sulfite diazo process paper, the excess being meel-ianically removed. The paper is then hot @air-dried at 140' F. for about 2 minutes. A coating composition containing a compound whiel-i produces alkaline vapors when heated is composed of the followi-ng ingedients: Water ---------------------------------- ml-- 100 30% aqueous icolloidal silica, 0.015 micron particle size (Ludox LS) ------------------------ ml-- 25 Urea ------------------------------------ g-- 125 Guanidine carbonate ----------------------- g-- 50 Butadiene-styrene (weight ratio, 40:60) copolymer aqueous emulsion, 48% solids (Borden Polyco 35OW) ------------------------------- mi__ 50 Both the urea and the guanidine carbonate produce ammonia when heated. In place o'L the btitadiene-styrene copolymer, the same quantity of a cationic polyvinyl acetate emulsion containing 50% solids, particle size 10 micr6ns (Xyno Y-99) may be used. The combination of urea and guanidine carbonate is preferred, for excellence of development and considerations of solubilization and crystallization of -the alkali ,- enerators. To enhance the stability and shelf life, there rnay be ,added to the alkali -enerating layer 15 grams of tartaric acid, 10 gtams of citric acid, or preferably, 10 grams of tartaric acid and 1.2 grams of citric acid. The alkaline composition is co,,ited on the unsensitized side of the papet, applying one coat by dipping and mechanically removing the excess, followed I>y drying at 140' F. for 2 minutes. The application deposits about 1250 grams o'L urea and 500 grams of guanidine carbonate per 1000 square feet of paper surface.. The diazo compound is present on the reverse side in a quantity of about 15 grams per 1000 square feet. Alternatively, the respective coatings may be @applied in the reverse order. The drying ter.,iperatures may be increased at shorter drying times. Th-Lis, it is found that the se-iisitizing layer may be dried for several seconds with hot air, at 290' F., the alkali generatin.- lay-,r being present on -Lhe reverse side, without precotipliiig. The photoprinting material is exposed to ultraviolet light under a design to be reproduced. The image is developed by applying heat to the alkaline side, such as by a hot metal plate, while confining the vapors evolved on the sensitized side by means of an additional metal plate 3,046,128 8 applied thereto. The temp'erature m@ aintained on the alkaline side is about 350' F. for about 25 seconds. Intense blue reproductions are obtained in this manner. Exan2ple 2 5 instead of confining the vapors evolved by a plate or the like as described in Example 1, the sensitized surface of the paper may be sprayed with a resinous filmformin.- composition. A suitable composition is a poly10 acrylic lacquer containing 10% solids, 55% toluene and 35% ethyl alcohol. Example 3 An alternative method of confining the alkaline vapors in contact with the diazo compound and coupler is to in15 clude a film-forming resinous material such as polyvinyl acetate in the sensitizer composition of Example 1. For example, the ser,.sitizer composition may ad,-4itio-,ially coiitain about 20 mi. of the Xyno X-99 emulsion. 20 Example 4 A sensitizing compositioia which may be substituted for that of Example 1 to produce blackline diazotype prints may have the following composition: 25 Water ---------------------------------- ml-- l@00 Ethylene .-lycol --------------------------- ml-- 15 Citric@ acid -------------------------------- g-- 0.7 Zinc chloride ------------------------------ g-- 5.0 Thiourea -------------------------------- g-- 6.0 30 Resorcinol -------------------------------- g-- 0.2 77% thiourea-23% acetoacetanilide ---------- g-- 0.45 2,3-dihydroxyn aphthalene-6-sodium sulfonate--@g-- 2.05 p-Diazo-N,N-diethylaniline zinc chloride salt --- g-- 1.4 II% silica emulsion, .0 15-.020, micron (Cab-o35 sil) ---------------------------------- ml-- 25 Exainple 5 The following composition may be substituted for the sensitizer composition of Example 1, to produce rich 40 sepia prints: Water ---------------------------------- ml-- 100 Ethylene glycol --------------------------- ml-- 10 Citric acid -------------------------------- g-- 5.5 Boric acid -------------------------------- g-- 2.85 45 Zinc chloride ------------------------------ g-- 10 'Ibiourea --------------------------------- g-- 6.5 Resorcinol ------------------------------- g--. 4.75 p-Diazo-N-methyl-N-hy&oxyethylaniline zinc chloride salt -------------------------------- g-- 6.0 50 Saponin --------------------------------- g-- 0.3 11% silica emulsion (Cab-o-sil) ------------ nii-- 10 The several photoprinting materials may also be made from up to 1100% rag paper, depending upon the sizing, 55 finish, and density of the sheet, with possible need for a precoat as desrribed above. Example 6 The alkaline composition of Example I may be sub60 stituted by the following composition: Water ---------------------------------- ml-- loo Urea ------------------------------------- g-- 50 Polyvin@yl acetate emulsion, 55% solids, about I micron (Vinac HF) ----------------------- ml-- 25 65 Example 7 The alkaline composition of Example I may be substituted by the following composition: 70 mi. Water -------------------------------------- 100 30% colloidal silica emiilsion (Ludox LS) -------- 25 Ethylenediamine -------------- --------------- So Butadiene-styrene copolymer emulsion (Polyco 75 35OW) ----------------------------------- 50
