Abstract:
Kaolin (clay) platelets in water are flocculated with an acid (pH 3.0) to form &#34;structured agglomerates&#34;, which are stabilized by reacting with an urea-formaldehyde prepolymer. The urea-formaldehyde prepolymer forms polymeric agglomerated particles when it comes in contact with the catalytic, acidic kaolin &#34;structured agglomerates.&#34; The urea-formaldehyde polymeric agglomerated particles cause adhesion or stabilizing of the structured kaolin agglomerates to form a three-dimensional network of &#34;open structure&#34; with a large number of microvoids (pores). The urea-formaldehyde polymer in the stabilized product varies from 5 to 95% by weight based on the total weight of the final product. 
     The size of the primary particles of the urea-formaldehyde polymer is from 0.1 to 0.7μ (micron). The size of the kaolin platelets is from 0.1 to 10μ (microns). The structured agglomerates of kaolin-urea-formaldehyde possess a size ranging from 0.25 to 40μ (microns).

Description:
This invention relates to structured kaolin agglomerates and methods of making the same and particularly to structured kaolin agglomerates which are stabilized with ureaformaldehyde polymer. 
     BACKGROUND OF THE INVENTION 
     Natural and calcined clays are used as opacifying pigments and extenders of TiO 2  (titanium dioxide) in paper filler applications, paper coating and paint formulations. The opacifying strength of a pigment is expressed by a measurable property, the so-called &#34;Pigment Scattering Coefficient,&#34; Sp, which is based on the Kubelka-Munk theory. The pigment scattering coefficients are determined by measuring the absolute reflectances, R 0  (black body backing) and R w  (white body backing of known reflectance), of thin, uniformly deposited layer of pigment on a substrate of known reflectances. Clay pigments are much less expensive than prime pigments, such as TiO 2  ; and their pigment scattering coefficients, that is, opacifying strength, are considerably lower than those of TiO 2  (see Table I, Pigment Scattering Coefficients, Sp): 
     
                       TABLE I______________________________________PIGMENT SCATTERING COEFFICIENTS (Sp)         cm.sup.2 /g.______________________________________Clays             670-1,250Calcined Clays  1,800-2,150TiO.sub.2 (Anatase)           4,000-4,800TiO.sub.2 (Rutile)           4,600-5,100U/F Pigments    3,500-4,200______________________________________ 
    
     Peter Economou, John F. Hardy and Alfred Renner, in a recent U.S. Pat. No. 3,909,348, described the high opacifying properties of urea-formaldehyde pigments in paper filler applications. The pigment scattering coefficients, Sp, of urea-formaldehyde pigments approach those of TiO 2 , anatase grade. However, the urea-formaldehyde pigments are equally as expensive as TiO 2 . 
     Dominic Simone, of NL Industries, Inc., describes in his U.S. Pat. No. 3,912,532 the opacifying properties of clay coated with urea-formaldehyde particles in paper filler and paper coating applications. Simone in his invention describes clay particles which are encapsulated with a coating of finely divided particles of urea-formaldehyde polymer, which have been formed and polymerized onto the surface of the clay particles. Simone pretreats well dispersed clay particles in water with urea-formaldehyde prepolymer and then adds the acid catalyst to polymerize the ure-formaldehyde prepolymer. 
     The present invention relates to the high opacifying strength of flocculated clay particles (as opposed to dispersed clay mentioned in the Simone patent) which are treated with an acid to form &#34;structured agglomerates,&#34; which are then stabilized with urea-formaldehyde polymeric particles. The acid used to flocculate the clay particles is also the catalyst for the urea-formaldehyde polymerization. Due to the very thixotropic character of the flocculated clay particles in water and, in an attempt to produce products with &#34;open structure&#34; or maximum possible number of microvoids of desirable size, th solids of clay slurries and of the urea-formaldehyde prepolymer of this invention are considerably lower than the concentrations stated in Simone&#39;s examples. As a result of these major differences between Simone&#39;s invention and this invention, the pigment scattering coefficients of products made by this invention are considerably higher than the pigment scattering coefficients of the products made by Simone&#39;s inventioon, when both products have the same clay/urea-formaldehyde polymer weight ratio. 
     SUMMARY OF THE INVENTION 
     Natuurally occurring or mechanically delaminated clay platelets in water suspensions are treated with an acid to a pH less than 3.0 to form a very viscous flocculated clay slurry. The acid treatment causes the clay platelets to flocculate preferably by &#34;face&#34; to &#34;edge&#34; contact and form &#34;structured agglomerates,&#34; which possess high void volume and high power to scatter light. These &#34;structured agglomerates&#34; of clay are then stabilized (or frozen in position) by the addition of a urea-formaldehyde prepolymer under strong agitation. Accordingly by the term structured agglomerates, I mean a structure of separate clay particles agglomerate in a manner such that a substantial number of particles are in edge to face contact forming a porous structure. The urea-formaldehyde prepolymer, upon contact with the acidic clay agglomerates (which act as a catalyst), polymerizes almost instantaneously to form essentially spherical sub-micron size polymeric particles which glue and stabilize the clay &#34;structured agglomerates.&#34; The resulting product is a three-dimensional network of agglomerates of urea-formaldehyde polymeric particles and clay platelets of &#34;open structure&#34; which possesses a large number of microvoids (pores). The high number and controlled size of microvoids contribute to high scattering coefficient (Sp) or opacity. 
     The amount of urea-formaldehyde polymer in the final product can be varied from 5 to 95% by weight based on the total weight of the product. The urea-formaldehyde polymer is composed of primary particles essentially spherical and non-porous of 0.1 to 0.7 micron (μ) size. The clay platelets are of 0.1 to 10 microns (μ) size. 
     The final product consists of &#34;open structure&#34; agglomerates of urea-formaldehyde essentially spherical particles and clay platelets. The said agglomerates possess a size of 0.25 to 40 microns. The detailed method of preparing these products is described hereafter. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The products are made by flocculating clay slurries in water with an acid to form &#34;structured agglomerates.&#34; The clay agglomerates then are stabilized or frozen by treatment with an urea-formaldehyde prepolymer. 
     The urea-formaldehyde prepolymer water solution consists of 25 to 50% solids by weight and a molar ratio of urea to formaldehyde from about 1:1.3 to about 1:1.8. 
     The clay platelets of size 0.1 to 10μ are slurried in water to about 10 to 40% solids by weight and treated with an acid to form &#34;structured agglomerates&#34; of viscous clay slurry. The amount of acid used for the clay treatment is sufficient to bring the pH below 3.0 and, preferably, below 2.5. The flocculated form of the clay slurry is heated preferably to 50° C., however, any temperature from 30° to 80° C. is suitable. To this mixture an urea-formaldehyde prepolymer solution at preferably 50° C. (or 30° to 80° C.) is added under strong agitation to stabilize or freeze the &#34;structured agglomerates&#34; of clay platelets. The urea-formaldehyde prepolymer, upon contact with the acidic clay slurry, polymerizes very rapidly to form a gel-like mixture. The gel-like mixture is then left to cure for 1 to 3 hours in order to increase the polymer yield. During the gelation step and curing period, the temperature of the mixture is increasing by 5° to 20° C. because the condensation polymerization of the urea-formaldehyde system is an exothermic reaction. The amounts of urea-formaldehyde polymer employed in the gel mixture are from 5 to 95% by weight based on the total weight of clay and urea-formaldehyde particles. The final total solids of the gel mixture is between 20 and 45% by weight. 
     After the curing period, the gel-like mixture is crushed and slurried with approximately an equal volume of water. The slurry is then neutralized by a base to a pH of 6.0 to 8.0. The slurry then is filtered and the filter cake is washed with water. The filter cake is then dried in an oven or flash dryer and micropulverized. The agglomerate size of the micropulverized final product ranges from 0.25 to 40μ. 
    
    
     In the foregoing I have outlined certain objects, purposes and advantages of this invention. Other objects, purposes and advantages of this invention will be apparent from the following examples and the drawings in which: 
     FIG. 1 is a flow sheet illustrating the process of this invention compared with the prior art process of Simone U.s. Pat. No. 3,912,532. 
     FIG. 2 is a scanning electron micrograph at 4,800 magnifications of a structured kaolin agglomerate stabilized with urea-formaldehyde as prepared by this invention. 
     FIG. 3 is a scanning electron micrograph at 10,000 magnifications of a portion of the same product as in FIG. 2. 
    
    
     Referring to the drawings, I have illustrated in flow sheet form the process of this invention compared with the Simone process, the differences being noted in italics. In FIGS. 2 and 3, the scanning micrographs show the highly structured, porous agglomerate form of the product of this invention which produces the unusual properties herein described. 
     In the following examples I have compared the product of this invention with the product of Simone U.S. Pat. No. 3,912,532, which I believe is probably the most pertinent available art. Those examples designated with the suffix &#34;A&#34; were carried out according to the present invention while those designated with the suffix &#34;B&#34; were carried out according to the teachings of Simone U.S. Pat. No. 3,912,532. 
     EXAMPLE 1 A 
     729 g. of an aqueous formaldehyde solution of 37% concentration were neutralized with a sodium hydroxide solution to a pH of 8.5. 270 g. of urea were added and the mixture condensed for 1/2 hour at 90° C. At the end of this period, 90 g. of urea was added and the temperature adjusted to 70° C. This urea-formaldehyde prepolymer is an exact replica of the U/F prepolymer described in Example 1 of Simone&#39;s U.S. Pat. No. 3,912,532. The total reactants solids are 57.85% by weight with a final molar ratio of urea of formaldehyde, 1:1.5. This U/F prepolymer was used in Example 1 A to stabilize clay &#34;structured agglomerates&#34; of this invention, as well as in Example 1 B to treat the same clay but in dispersed condition, as it is described in Example 1 of Simone&#39;s U.S. Pat. No. 3,912,532, for the purpose of comparing the performance differences between the two products prepared by this invention and Simone&#39;s invention. 
     The condensation reaction was then continued for an additional half hour to form the urea-formaldehyde prepolymer. 
     420 g. of kaolin clay were slurried in 400 g. of deionized water. No dispersant, such as sodium hexametaphosphate, was used. The clay slurry was then treated with a solution of 6.4 g. of sulfamic acid in 364 g. of water. The clay slurry, at this point, becomes very viscous due to the formation of &#34;structured agglomerates&#34; of clay platelets. 1.5 g. of oxalic acid crystals were added to the acid treated clay and the temperature was adjusted to 50° C. It has been found that oxalic acid, 0.1 to 0.5% by weight based on clay solids, reduces the viscosity of the flocculated clay slurry to some extent, and it assists the mixing of the viscous clay slurry when the urea-formaldehyde prepolymer is added to stabilize the clay agglomerates. It is the opinion of this inventor that oxalic acid acts as a digestive agent, it breaks down large clay agglomerates into smaller ones. Also, it has been found that the oxalic acid is not needed if the clay slurry is further diluted with water. 
     The flocculated clay slurry at 50° C. was then mixed with 360 g. of the urea-formaldehyde prepolymer, also at 50° C. The condensation polymerization of the urea-formaldehyde prepolymer takes place as soon as it is mixed with the acidic flocculated clay slurry and the exothermic reaction increases the temperature of 68° C. The gel-like mixture was then allowed to cure at this temperature adiabatically for 2 hours. 
     The gel-like mixture was crushed to small pieces and mixed with 1 liter of water, neutralized with a NaOH solution to pH 8.0, filtered, and the filter cake was washed with 100 ml. of water. The filter cake was dried at 70° C. and the dried product was then micropulverized with an impact stud pulverizer. 
     The initial clay to urea-formaldehyde prepolymer weight ratio of this example was 66.9 to 33.1, and the final weight ratio of clay to urea-formaldehyde polymer, as it was determined from % ash analysis, was 73.9 to 26.1. This difference, between the initial weight ratio of clay to urea-formaldehyde prepolymer before gelation and that of the final product, is due to the formation of water from the condensation polymerization of the urea-formaldehyde prepolymer. 
     The total solids at the final gelation step of the mixture was 41% by weight based on the total weight of the components. 
     The clay particles used in this example had an average particle size of 0.77μ. The urea-formaldehyde primary particles had an average size of 0.25μ. The clay-urea-formaldehyde polymer agglomerates had an average size of 5μ. 
     The properties of Example 1 A product are listed in Table II with the properties of Example 1 B product for comparison. 
     EXAMPLE 1 B 
     In this example the same raw materials were used as in Example 1 A except for the procedure, which is an exact repeat of the Simone&#39;s U.S. Pat. No. 3,912,532, Example 1. 
     420 g. of kaolin clay were dispersed in 200 g. of deionized water containing in solution 0.42 g. of sodium hexametaphosphate as a dispersant. 
     The clay dispersion was mixed with 360 g. of the urea-formaldehyde prepolymer solution and the temperature of the slurry was adjusted to 50° C. 
     A solution of 6.4 g. of sulfanic acid in 364 g. of deionized water was prepared and heated to 50° C. 
     The clay dispersion in the urea-formaldehyde prepolymer solution was mixed with the acid solution and the precipitated resin formed a gel which on standing was allowed to cure for 2 hours at 65° C. 
     The gelled mass was broken up and was dispersed in 1 liter of water, neutralized with NaOH solution to pH 7.5,  filtered, washed on the filter with some additional water, then dried at 75° C. The dried product was then passed through an impact stud pulverizer. 
     The initial clay to urea-formaldehyde prepolymer weight ratio of this example was the same as in Example 1 A (66.9 to 33.1) and the final clay to urea-formaldehyde polymer weight ratio, as it was determined from % ash analysis, was 73.7/26.3. 
     The clay particles used in this example had an average particle size of 0.77μ. The urea-formaldehyde particles coating the clay particles had an average size of 0.2μ. The coated clay particles had an average size of 6μ. 
     The products of Examples 1 A or 1 B were used in a paper coating formulation as follows: 
     100 parts of either product from Example 1 A or 1 B was mixed with 16 parts of a styrene-butadiene latex (50% solids). A third coating formulation containing 100 parts of the untreated clay was prepared as a control. 
     A 28 lbs./book ream base stock paper was coated on one side with the three coating compositions at a coat weight of 2.3 lbs./book ream. 
     The coated papers were calendered one at 400 pli and at a speed of 37 feet/minute. 
     The same products of Examples 1 A and 1 B were used as filler pigments in paper. The pulp used for this evaluation was a 50/50 Hardwood/Softwood Kraft bleached pulp of Canadian Standard Freeness of 40 c.c. The paper formed contained 2, 4,  and 6% by weight of the filler pigment based on the total weight solids of paper. The basis weight of the paper was 50 lbs./book ream. (A book ream is 500 sheets of 25&#34;×28&#34; or 475,000 square inches or about 3300 square feet. ) 
     The results of these comparative evaluations of the products of Examples 1 A and 1 B, as well as other pigment characteristic properties, are tabulated in Table II. 
     
                                           TABLE II__________________________________________________________________________          PAPER COATINGSPigment Properties          Uncalendered   Calendered     PAPER FILLER    Scattering     Scattering     Scattering                                        Opacity, %                                                 BrightnessBright-  Coefficient,          Opacity               Bright-                   Coefficient,                         Opacity                              Bright-                                  Coefficient,                                        % Pigment                                                 % PigmentExampleness    Sp cm.sup.2 /g.          %    ness                   Sp cm.sup.2 /g.                         %    ness                                  Sp cm.sup.2 /g.                                        2  4  6  2  4  6__________________________________________________________________________ 1 A 89.3    2550  89.8 76.2                   3220  87.8 74.3                                  2490  81.2                                           85.5                                              88.4                                                 83.6                                                    85.3                                                       86.71 B  88.2    2300  88.4 74.9                   2310  87.0 73.2                                  2080  80.2                                           82.9                                              85.8                                                 82.9                                                    84.2                                                       85.2Clay 84.8    1020  86.3 73.0                   1160  85.3 70.5                                  1150                                        Pulp % Opacity                                                 Pulp Brightness                                        77.2     82.0__________________________________________________________________________ 
    
     The results in Table II indicate clearly the differences in product performance between the product of Example 1 A of this invention and the product of Example 1B of Simone&#39;s invention (U.S. Pat. No. 3,912,532). Example 1A product has 1.1 points higher brightness and 10.9% higher pigment scattering coefficient than Example 1B product. In paper coating applications of uncalendered and calendered sheets, the Example 1A product gives higher % opacity by 1.4 and 0.8 points correspondingly than the Example 1B product. The brightness of the coated paper is also higher by 1.3 and 1.1 points, and the pigment scattering coefficients of coatings differ by 39.4% and 19.7% corresponding to uncalendered and calendered sheets. 
     In paper filler applications, the Example 1A product increased the % opacity of paper by 1.0 point higher than the Example 1B product at the 2% pigment level and by 2.6 points at the 6% pigment level. Differences in the brightness of the filled papers of 0.7 point at 2% pigment level and 1.5 points at 6% pigment level are observed too. 
     The superior performance of the Example 1A product of this invention over that of the Example 1B product of Simone&#39;s invention in both paper filler and coating applications is due to the unique &#34;open structure&#34; of clay &#34;structured agglomerates,&#34; which were stabilized with agglomerates of sub-micron size urea-formaldehyde polymeric particles. The clay &#34;structured agglomerates&#34; are called stabilized because slurries of the products of this invention hold their original structure under any normal dispersing conditions with any common dispersing agents. 
     The bulk density of Example 1A product (0.206 g/c.c.) is 28% lower than the bulk density of Example 1B (0.264 g/c.c.). 
     EXAMPLE 2A 
     1332 g. of an aqueous formaldehyde solution of 37% concentration were neutralized with a sodium hydroxide solution to a pH of 8.0. 657 g. of urea were added and the mixture solution of urea and formaldehyde was reacted for 45 minutes at 70° C. During this period the pH was maintained between 7.0 and 7.5. At the end of the 45 minute period, 568 g. of deionized cold water were added and the temperature was decreased and maintained at 50° C. until the urea-formaldehyde prepolymer was used. This urea-formaldehyde prepolymer, preferably used in the present invention, is a simplified single step method and less expensive than Simone&#39;s method. The final solids of this urea-formaldehyde prepolymer are 45% by weight based on the total weight of the reactants, and the molar ratio of urea to formaldehyde is 1:1.5. 
     350 g. of kaolin clay were slurried in 701 g. of deionized water. No dispersant, such as sodium hexametaphosphate, was used. The clay slurry was then treated with 60 g. of 9% by weight solution of sulfamic acid, 0.7 g. of oxalic acid, and the temperature was adjusted to 50° C. 
     The acid flocculated clay slurry at 55° C. was then mixed with 333 g. of the urea-formaldehyde prepolymer, also at 55° C. The condensation polymerization of the urea-formaldehyde prepolymer takes place as soon as it is mixed with the acidic flocculated clay slurry and the exothermic reaction increases the temperature to 63° C. The gel-like mixture was then allowed to cure at this temperature adiabatically for 2 hours. 
     The gel-like solid mixture was crushed to small pieces and mixed with 750 ml. of water, neutralized with a NaOH solution to pH 8.0, filtered and the filter cake was washed with 80 ml. of water. The filter cake was dried at 75° C. and the dried product was then micropulverized with an impact stud pulverizer. 
     The initial clay to urea-formaldehyde prepolymer weight ratio of this example was 70 to 30, and the final weight ratio of clay to urea-formaldehyde polymer, as it was determined from % ash analysis, was 77 to 23. The total solids at the gelation step of this mixture was 35% by weight based on the total weight of the components. 
     The clay particles used in this example had an average particle size of 0.2μ. The urea-formaldehyde primary particles had an average size of 0.3μ. The clay-urea-formaldehyde polymer agglomerates had an average size of 6μ. 
     The properties of Example 2A product of this invention are listed in Table III with the properties of the Example 2B product, made by Simone&#39;s method, for comparison. 
     EXAMPLE 2B 
     In this example the same raw materials were used as in Example 2A except for the procedure, which is the same as that described by Simone in his U.S. Pat. No. 3,912,532. 
     350 g. of kaolin clay were dispersed in 233 g. of deionized water containing in solution 0.35 g. of sodium hexametaphosphate as a dispersant. The clay dispersion was mixed with 333 g. of the urea-formaldehyde prepolymer solution (the same U/F prepolymer as in Example 2A) and the temperature of the mixture was adjusted to 55° C. 
     A solution of 60 g. of 9% sulfamic acid (by weight) was heated to 55° C. 
     The clay dispersion in the urea-formaldehyde prepolymer was mixed with the sulfamic acid solution and the precipitated resin formed a gel which on standing was allowed to cure for 2 hours at 71° C. adiabatically. 
     The gelled mass was broken up and was dispersed in 750 ml. of water, neutralized with NaOH solution to pH 7.5, filtered, washed with 80 ml. of water, then dried at 75° C. The dried product was then passed through an impact stud pulverizer. 
     The initial clay to urea-formaldehyde prepolymer weight ratio of this mixture was the same as in Example 2A (70 to 30) and the final clay to urea-formaldehyde polymer weight ratio, as it was determined from % ash analysis, was 77 to 23. 
     Again, the clay particles used in this example had an average particle size of 0.2μ. The urea-formaldehyde particles coating the clay particles had an average size of 0.2μ. The coating clay particles had an average size of 6μ. 
     The products of Examples 2A and 2B were used in a paper coating formulation as follows: 
     100 parts of either product from Example 2A or 2B was mixed with 16 parts of a styrene-butadiene latex (50% solids). 
     A 28 lbs./book ream base stock paper was coated on one side with the two coating compositions at a coat weight of 2.7 lbs./book ream. 
     The coated papers were calendered one at 400 pli and at a speed of 37 feet/minute. 
     The same products of Examples 2A and 2B were used as filler pigments in paper. The pulp used for this evaluation was a 50/50 blend of Hardwood/Softwood Kraft bleached pulp of 400 cc. Canadian Standard Freeness. The paper formed contained 5%, 10% and 15%, by weight of the filler pigment based on the total solids of paper. The basis weight of the paper was 50 lbs./book ream. 
     The results of these comparative evaluations of the products of Examples 2A and 2B, as well as other pigment optical properties, are tabulated in Table III. 
     
                                           TABLE III__________________________________________________________________________                 PAPER COATINGSPigment Properties    Uncalendered     CalenderedBulk       Scattering       Scattering       ScatteringDensity    Coefficient                 Opacity    Coefficient                                  Opacity    CoefficientExampleg./c.c.     Brightness           Sp, cm.sup.2 /g                 %    Brightness                            Sp, cm.sup.2 /g                                  %    Brightness                                             Sp, cm.sup.2 /g__________________________________________________________________________2 A  0.130     91.0  2,190 89.9 77.5  2,430 87.8 74.3  1,7802 B  0.199     90.0  1,810 88.5 76.3  2,180 86.8 73.3  1,580Clay 0.240     86.9  1,280__________________________________________________________________________                    PAPER FILLER                                         Pigment Scattering                    % OPACITY  BRIGHTNESS                                         Coefficient cm.sup.2 /g                    % Pigment  % Pigment % Pigment               Example                    5  10   15 5  10  15 5  10  15__________________________________________________________________________               2 A  86.7                       89.4 91.1                               83.3                                  84.7                                      85.5                                         2,520                                            2,240                                                2,170               2 B  85.8                       88.2 89.8                               83.1                                  84.1                                      84.5                                         1,910                                            1,880                                                1,780                    Pulp % Opacity 81.7                               Pulp Brightness 80.5                                         Pulp Scattering                                         Coefficient__________________________________________________________________________                                         338 
    
     The results in Table III show, again, substantial differences in product optical properties and performance characteristics between the product of Example 2A of this invention and the product of Example 2B of Simone&#39;s invention (U.S. Pat. No. 3,912,532). Example 2A product has 1.0 point higher brightness and 21% higher pigment scattering coefficient (Sp) than Example 2B product. Also, the bulk density of product 2A is 35% lower than product 2B. In paper coating applications of uncalendered and calendered sheets, again Example 2A product gives higher % opacity by 1.4 and 1.0 points correspondingly than the Example 2B product. The brightness of the coated paper with Example 2A product is also higher by 1.2 and 1.0 points than the coated paper with Example 2B product, and the pigment scattering coefficients of coatings differ by 11.5% and 12.6% corresponding to uncalendered and calendered sheets. 
     In paper filler applications product 2A of this invention imparted higher opacity, brightness and pigment scattering coefficients than the 2B product of Simone&#39;s invention. 
     EXAMPLE 3A 
     450 g. of kaolin clay were slurried in 1,000 g. of deionized water. No dispersant, such as sodium hexametaphosphate, was used. The clay slurry was then treated with 70 g. of 9% by weight solution of sulfamic acid, 1.5 g. of oxalic acid, and the temperature was adjusted to 50° C. 
     The acidic flocculated clay slurry at 50° C. was then mixed with 430 g. of 45% urea-formaldehyde prepolymer, also at 50° C. (The urea-formaldehyde prepolymer is the same as the one used in Example 2A). The condensation polymerization of the urea-formaldehyde prepolymer takes place as soon as it is mixed with the acidic flocculated clay slurry and the exothermic reaction increases the temperature to 63° C. The gel-like mixture was then allowed to cure at this temperature adiabatically for 2 hours. 
     The gel-like solid mixture was crushed to small pieces and mixed with 1 liter of water, neutralized with a NaOh solution to pH 8.0, filtered, and the filter cake was washed with 100 ml. of water. The filter cake was dried at 75° C. and the dried product was then micropulverized with an impact stud pulverizer. 
     The initial clay to urea-formaldehyde prepolymer weight ratio of this example was 70 to 30, and the final weight ratio of clay to urea-formaldehyde polymer, as it was determined from % ash analysis, was 79 to 21. The total solids at the gelation step was 33.3% by weight based on the total weight of the reactants. 
     The clay particles used in this example had an average particle size of 0.4μ. The urea-formaldehyde primary particles had an average size of 0.35μ. The clay-urea-formaldehyde polymer agglomerates had an average size of 6.5μ. 
     The properties of Example 3A product of this invention are listed in Table IV with the properties of the next Example, 3B product, which was made by Simone&#39;s method for the purpose of comparing the two methods. 
     EXAMPLE 3B 
     In this example the same raw materials were used as in Example 3A except for the process, which is the same as that described by Simone in his U.S. Pat. No. 3,912,532. 
     450 g. of kaolin clay were dispersed in 300 g. of deionized water containing in solution 0.45 g. of sodium hexametaphosphate as a dispersant. 
     The clay dispersion was mixed with 430 g. of 45% urea-formaldehyde prepolymer solution (the same U/F prepolymer as in Example 3A), and the temperature of the mixture was adjusted to 50° C. A solution of 70 g. of 9% sulfamic acid (by weight) was heated to 50° C. 
     The clay dispersion in the urea-formaldehyde prepolymer was mixed with the sulfamic acid solution and the precipitated resin formed a gel which on standing was allowed to cure for 2 hours at 68° C. adiabatically. 
     The gelled mass was broken up and was dispersed in 1 liter of water, neutralized with NaOH solution to pH 8.0, filtered, washed with 100 ml. of water, then dried at 75° C. The dried product was then passed through an impact stud pulverizer. 
     The initial clay to urea-formaldehyde prepolymer weight ratio of this mixture was the same as in Example 3A (70 to 30) and the final clay to urea-formaldehyde polymer weight ratio, as it was determined from % ash analysis, was 78 to 22. 
     Again, the clay particles used in this example had an average particle size of 0.4μ. The urea-formaldehyde particles coating the clay particles had an average size of 0.2μ. The coated clay particles had an average size of 6.5μ. 
     The products of Examples 3A and 3B were used in a paper coating formulation as follows: 
     100 parts of either product from Example 3A or 3B was mixed with 16 parts of a styrene butadiene latex (50% solids). 
     A 28 lbs./book ream base stock paper was coated on one side with the two coating compositions at a coat weight of 2.3 lbs./book ream. 
     The coated papers were calendered once at 400 pli and at a speed of 37 feet/minute. 
     The same products of Examples 3A and 3B were used as filler pigments in paper. The pulp used for this evaluation was 50/50 blend of Hardwood/Softwood Kraft bleached pulp of 400 c.c. Canadian Standard Freeness. The paper formed contained 5%, 10%, and 15%, by weight of the filler pigment based on the total solids of paper. The basis weight of the paper was 50 lbs./book ream. 
     The results of these comparative evaluations of the products of Examples 3A and 3B, as well as other pigment optical properties, are tabulated in Table IV. 
     
                                           TABLE IV__________________________________________________________________________                 PAPER COATINGSPigment Properties    Uncalendered     CalenderedBulk       Scattering       Scattering       ScatteringDensity    Coefficient                 Opacity    Coefficient                                  Opacity    CoefficientExampleg./c.c.     Brightness           Sp, cm.sup.2 /g                 %    Brightness                            Sp, cm.sup.2 /g                                  %    Brightness                                             Sp, cm.sup.2 /g__________________________________________________________________________3 A  0.240     88.3  2,300 89.3 76.5  3,290 87.3 73.4  2,3503 B  0.256     85.1  1,470 87.7 74.2  1,980 86.1 72.3  1,790Clay 0.280     83.8  1,120__________________________________________________________________________                    PAPER FILLER                                         Pigment Scattering                    % OPACITY  BRIGHTNESS                                         Coefficient cm.sup.2 /g                    % Pigment  % Pigment % Pigment               Example                    5  10   15 5  10  15 5  10  15__________________________________________________________________________               3 A  86.3                       89.1 91.0                               82.6                                  83.7                                      84.3                                         2,310                                            2,240                                                2,140               3 B  85.1                       87.8 89.2                               81.4                                  81.9                                      82.2                                         1,660                                            1,630                                                1,570                    Pulp % Opacity 81.7                               Pulp Brightness 80.5                                         Pulp Scattering                                         Coefficient__________________________________________________________________________                                         338 
    
     Table IV shows again that the 3A product of this invention has lower bulk density, higher brightness (by 3.2 points) and higher pigment scattering coefficient (by 36%) than the 3B product of Simone&#39;s invention. Also, in paper coatings and paper filler applications, the opacity, brightness and pigment scattering coefficients of product 3A are substantially higher than those of product 3B. 
     EXAMPLE 4A 
     The urea-formaldehyde prepolymer used in this example is of this invention and it is the same as it was described in Example 2A. 
     The kaolin clay used in this example is the same as the one used in Examples 1A and 1B. The only difference between Examples 1A and 4A is that in Example 4A the urea-formaldehyde propolymer of this invention (45% solids) was used, while in Example 1A the urea-formaldehyde prepolymer of Simone&#39;s invention (57.85% solids) was used. 
     The product performance of this Example 4A, made by the process of this invention and using the urea-formaldehyde prepolymer (45% solids) of this invention, is compared in Table V to the product performance of Example 1B, which was made by Simone&#39;s process using the urea-formaldehyde prepolymer (57.85% solids) of Simone&#39;s invention. Both products had the same initial gelation weight ratio of clay to urea-formaldehyde prepolymer (66.9 to 33.1) and the same final weight ratio of clay urea-formaldehyde polymer (74 to 26). 
     420 g. of kaolin clay were slurried in 594 g. of deionized water. No dispersant, such as sodium hexametaphosphate, was used. The clay slurry was then treated with 71 g. of 9% by weight solution of sulfamic acid, 1.5 g. of oxalic acid, and the temperature was adjusted to 50° C. 
     The acidic flocculated clay slurry at 50° C. was then mixed with 462 g. of 45% urea-formaldehyde prepolymer of this invention, also at 50° C. The condensation polymerization of the urea-formaldehyde prepolymer takes place almost immediately and the exothermic reaction increases the temperature to 66° C. The gel-like mixture was then allowed to cure at this temperature adiabatically for 2 hours. 
     The gel-like solid mixture was crushed to small pieces and mixed with 750 ml. of water, neutralized with a NaOH solution to pH 8.0, filtered, and the filter cake was washed with 80 ml. of water. The filter cake was dried at 75° C. and the dried product was then micropulverized with an impact stud pulverizer. 
     The pigment properties and paper coatings performance of Example 4 A product of this invention are listed in Table V with those of the Example 1 B product, which was prepared using Simone&#39;s urea-formaldehyde prepolymer and process. 
     The coating formulation is the same as in Example 1 A. 
     
                                           TABLE V__________________________________________________________________________                    PAPER COATINGSPigment Properties  Coat Uncalendered   CalenderedBulk     Scattering               Weight        Scattering     ScatteringDensity     Bright-         Coefficient               lb./Book                    Opacity                         Bright-                             Coefficient                                   Opacity                                        Bright-                                            CoefficientExampleg./c.c.     ness         Sp, cm.sup.2 /g               Ream %    ness                             Sp, cm.sup.2 /g                                   %    ness                                            Sp, cm.sup.2 /g__________________________________________________________________________4 A  0.190     90.3         2,600 1.4  89.1 75.6                             5,180 87.4 73.7                                            4,1701 B  0.264     88.2         2,300 2.3  88.4 74.9                             2,310 87.0 73.2                                            2,080Clay 0.288     84.8         1,020__________________________________________________________________________ 
    
     The superior performance of products prepared by the present invention: forming &#34;structured agglomerates&#34; by an acid treatment of clay platelets and stabilizing the said &#34;structured agglomerates&#34; with a solution of urea-formaldehyde prepolymer of the present invention, over the products prepared according to Simone&#39;s invention and his urea-formaldehyde prepolymer, is very clear. 
     Table V shows that 4 A product of this invention has 28% lower bulk density than Simone&#39;s product 1 B. The brightness of product 4 A is 2.1 points higher than the brightness of product 1 B, and the pigment scattering coefficients differ by 13% in favor of product 4 A. 
     Paper coating performance of Example 4 A product is superior to Example 1 B product, even though the coat weight of Example 1 B product is higher by 64% than the coat weight of Example 4 B product. The superior performance in paper coatings of the product 4 A of this invention over the that of product 1 B of Simone&#39;s invention is associated with the superior optical characteristics of the &#34;structured agglomerates&#34; of clay platelets stabilized with the urea-formaldehyde polymeric submicron particles. The said urea-formaldehyde polymer clay &#34;structured agglomerates&#34; contribute to coating film structure more than the products prepared by Simone&#39;s invention. 
     EXAMPLES 5 A, 5 B thru 7 A, 7 B 
     All these examples were prepared with the same urea-formaldehyde prepolymer as in Example 2 A of this invention. The clay used in these examples had a mean particle size of 0.42 and the brightness through a leaching process was improved from 82.8  in Examples 5 A and 5 B to 88.0 in Examples 7 A and 7 B. All 5 A through 7 A products of this invention have lower bulk densities, higher brightness and higher pigment scattering coefficients than the corresponding 5 B through 7 B products of Simone&#39;s invention. In paper coating applications, again, the A products of this invention outperformed the corresponding B products of Simone&#39;s invention. The results of these comparative evaluations of the products of Examples 5 A and 5 B thru 7 A and 7 B, as well as other pigment optical properties, are tabulated in Table VI. 
     
                                           TABLE VI__________________________________________________________________________Initial  Initial    PIGMENT PROPERTIES                                PAPER COATINGSClay/Urea    Clay/Urea          Scat-    Uncalendered Calendered   Formal-    Formal-          Total        tering           Scatter-     Scatter-   dehyde    dehyde          Solids               Bulk    Coef-                           Coat         ing Coef-    ing Coef-Ex-   Prepolymer    Polymer          at   Den-    ficient                           Weight   Opac-                                        ficient  Opac-                                                     ficientam-   Weight    Weight          Gelation               sity                   Bright-                       Sp, lbs/book                                Bright-                                    ity Sp   Bright-                                                 ity Spple   Ratio Ratio Step, %               g./c.c.                   ness                       cm.sup.2 /g.                           Ream ness                                    %   cm.sup.2 /g.                                             ness                                                 %   cm.sup.2__________________________________________________________________________                                                     /g5 A   70/30 78/22 33.3 0.145                   87.7                       2810                           1.4  76.2                                    88.9                                        4520 72.3                                                 85.8                                                     28405 B   70/30 78/22 41.9 0.211                   86.2                       2195                           1.4  75.1                                    88.2                                        3870 73.0                                                 86.4                                                     2405Clay   --    --    --   0.280                   82.8                       1120                           --   --  --  --   --  --  --6 A   70/30 79/21 35.1 0.178                   88.6                       2385                           1.3  75.8                                    87.6                                        4010 72.5                                                 86.2                                                     25806 B   70/30 78/22 52.0 0.269                   86.7                       1230                           1.3  74.0                                    86.6                                        3435 71.4                                                 85.1                                                     1980Clay   --    --    --   0.288                   83.2                       1110                           --   --  --  --   --  --  --7 A   70/30 78/22 35.1 0.176                   90.0                       2620                           1.5  76.8                                    89.1                                        3655 72.9                                                 86.4                                                     22357 B   70/30 79/21 52.0 0.272                   88.3                       1710                           1.5  75.1                                    87.9                                        3060 71.9                                                 85.6                                                     1970Clay   --    --    --   0.283                   88.0                       1115                           --   --  --  --   --  --  --__________________________________________________________________________ 
    
     In the foregoing specification I have set out certain preferred practices and embodiments of this invention, however, it will be understood that this invention may be otherwise embodied within the scope of the following claims.