Microspheric opacifying agents are provided by admixing an aqueous, partially condensed, aldehyde condensation product with an oily material containing an emulsifying agent thereby forming a water-in-oil emulsion, admixing an amphiphilic acid catalyst with the emulsion and polymerizing the condensation product to form discrete, substantially spherical, opaque, solid particles. Additionally, the solid particles may be separated from the oily continuous phase and admixed with water for the further removal of oil material, and thereby causing the formation of spherical agglomerates or "super particles" having a substantially greater opacity than the aforesaid discrete particles. The super particles are hollow, substantially spherical particles having substantially spherical, discontinuous walls composed of agglomerated substantially spherical particles.

This invention relates to a process for the preparation of opaque, 
substantially spherical, microscopic opacifying particles, and to the 
particles produced by such process. More specifically, this invention 
relates to the preparation of opaque, substantially spherical particles 
formed of a formaldehyde condensation product and to spherical 
agglomerates having an unusually high opacifying ability. 
The preparation of microscopic particles formed from natural and synthetic 
polymeric materials has been described, for example, in U.S. Pat. No. 
3,585,149 and U.S. Pat. No. 3,669,899 to A. E. Vassiliades et al. The 
opacifying agents described in the Vassiliades et al patents comprise 
discrete, air-containing microcapsules having substantially continuous, 
solid walls, and an average particle diameter in the below 2 micron range. 
Generally, the microcapsules described in the aforesaid patents are formed 
by preparing an oil-in-water emulsion and coating the emulsion droplets 
with a film-forming material, which is subsequently hardened. The oily 
core material is expelled from the microcapsules to form air-containing 
microcapsular opacifying agents. The expulsion of the oily material from 
the microcapsules is accomplished be heating the microcapsules, for 
example, after the capsules have been coated by means of an aqueous 
dispersion onto a paper substrate. Alternatively, the oily core material 
may be expelled by subjecting the capsules to a spray-drying operation 
under relatively high temperature conditions. In any case, the expulsion 
of the oily material presented a number of difficulties. For example, one 
problem faced by the operators of such a system is that of the care 
necessary to insure that all of the oil is completely expelled from the 
capsules. This was generally time consuming and required relatively severe 
temperature conditions. Additionally, there is the problem of oily solvent 
recovery, which is both an environmental problem as well as an economic 
one. Thus, some of the oily solvent materials employed are fairly toxic 
and could endanger the health of the operators of the paper machine 
dryers. Additionally, a substantial portion of the oily material is lost 
to further use in the system using the evaporative recovery systems 
employed. Such losses, of course, play a role in the economics and 
commercialization of such systems. 
It has now been discovered that opaque, microscopic, pigment particles may 
be produced in a system which avoids many of the drawbacks of the 
aforesaid systems utilized in the production of microcapsular opacifying 
agents. Thus, it has been discovered that opaque, substantially spherical 
particles may be produced by a process which comprises admixing an 
aqueous, partially condensed, aldehyde condensation product with an oily 
material containing an emulsifying agent thereby forming a water-in-oil 
emulsion. Thereafter, an amphiphillic acid catalyst is admixed with the 
emulsion and the condensation product is polymerized, thereby forming 
substantially spherical, solid, opaque polymeric particles 
According to one aspect of the present invention, the polymerized particles 
are separated from the bulk of the oily material and admixed with an 
aqueous liquid, preferably water, under increased temperature conditions 
in order to remove the remainder of the oily material from the particles 
and form microscopic agglomerates of said substantially spherical 
particles, which agglomerates have an unexpectedly high opacifying power. 
A further aspect of the present invention involves the formation of 
agglomerates having substantially spherical walls, which walls are formed 
of substantially spherical, microparticles. 
The process of the present invention avoids many of the drawbacks of the 
previous systems wherein an oil-in-water emulsion is formed, since the 
system of the present invention permits the easy separation and recovery 
of the oily material by simple centrifugation or distillation, rather than 
solvent evaporation. Thus, the oily material of the present invention may 
be easily and readily recycled back to the emulsification operation thus 
providing both safe handling and an economic system. 
According to a preferred aspect of the present invention, the 
microparticles are provided by a process which comprises forming a 
prepolymer of urea and formaldehyde and admixing an aqueous solution of 
the prepolymer with an oily material containing an emulsifying agent. A 
water-in-oil emulsion is thereby formed and an amphiphilic acid catalyst 
is admixed with the emulsion causing the prepolymer to polymerize. The 
resultant particles are admixed with an aqueous liquid, such as water, 
under conditions of brisk agitation while heating the particles to remove 
residual oily material. Optionally, the particles may be separated from 
the aqueous liquid.

Referring now to FIG. 1, a water-immiscible oily liquid is introduced by 
means of line 10 for admixture with recycled oily material and a 
surface-active emulsifying agent 12. Suitable water-immiscible oily 
materials for use in the present invention include, for example, any 
organic solvent capable of acting as the continuous phase of a 
water-in-oil emulsion. Suitable solvents include aliphatic and aromatic 
solvents, such as petroleum ethers, naphthas, mineral spirits, toluene, 
xylene, turpentine or the like. Similarly, ketones, esters, halogenated 
hydrocarbons etc., may be suitably utilized in the process of the present 
invention. The preferred solvents are those having a relatively low cost 
and a low toxicity, such as mineral spirits or xylene. 
The emulsifying agent is admixed with oily material in amounts sufficient 
to provide, for example, between about 0.005 and about 0.2 part by weight 
of emulsifying agent per part of oily material, preferably between about 
0.02 and about 0.08 part per part of oily material. 
Suitable surface-active emulsifying agents are those capable of promoting 
the formation of a water-in-oil emulsion. Such materials include, for 
example, lanolin, lanolin derivatives, sorbitan monooleate, polyol 
oleates, ethylene oxide adducts of fatty acids, fatty alcohols, fatty 
amines and fatty amides, cholesterol derivatives, fatty acid diethanol 
amides, ethylene oxide-propylene oxide block copolymeric condensation 
products and the like, such surface-active agents being well known in the 
art. The preferred emulsifying agents are the ethylene oxide-propylene 
oxide block copolymeric condensation products commercially available from 
BASF-Wyandotte Corporation under the names "Pluronic" and "Tetronic." 
Meanwhile, a urea-formaldehyde prepolymer is provided by introducing urean 
and formaldehyde by means of line 14 into reactor 16 at a mole ratio of 
formaldehyde to urea in the range of between about 1:1 to 2.5:1, 
preferably between about 1.2:1 and 1.5:1. The reaction takes place in an 
aqueous solution at about 50% solids, at a pH of 9-10, and a temperature 
between about 50 and 120.degree. C. The reaction time is controlled so as 
to produce a substantially clear prepolymer solution when the reaction 
mixture is cooled to room temperature. Such reactions are well known in 
the art. Although the foregoing discussion has been directed towards a 
urea-formaldehyde prepolymer, any suitable partially condensed aldehyde 
condensation product may be employed for the formation of an internal 
phase of the emulsion. Accordingly, for example, any carbamide-aldehyde 
condensation product that is compatible with aqueous solution 
polymerization is suitable for use in the present invention. Accordingly, 
other acid or base-catalyzed co-reactants may be employed including 
condensation reaction products of formaldehyde with phenols, such as, 
hydroxybenzene (phenol), m-cresol and 3, 5-xylenol carbamides, such as, 
urea; triazines, such as, melamine; amino and amido compounds, such as 
aniline, p-toluenesulfonamide, ethyleneurea and guanidine; ketones, such 
as, acetone and cyclohexanone, or combinations of these materials, with 
the provision that the prepolymer be insoluble in the water-immiscible 
phase. Additionally, the prepolymer may be provided in any suitable 
aqueous medium including water, glycerol, poly (ethylene oxide), glycols, 
or the like, may be suitably employed. Ureaformaldehyde is the preferred 
prepolymer. However, the substitution of melamine for 1 to 2% by weight of 
urea provides a prepolymer will better storage stability. The expression 
"prepolymer" as utilized herein is intended to mean the initial, water 
soluble, reaction product of the carbamide and the aldehyde. In the case 
of urea and formaldehyde, the prepolymer includes methylol ureas and the 
oligomers of methylol ureas. 
The prepolymer in the aqueous medium is withdrawn form the reactor 16 by 
means of line 18 and admixed with the water-immiscible solvent-emulsifying 
agent mixture in line 20, and the resulting admixture is passed by means 
of line 22 into mixer 24 wherein a water-in-oil emulsion is formed under 
conditions of brisk agitation. The water-in-oil emulsion may be prepared 
batch-wise, utilizing a tank with a high shear agitation, or continuously 
by combining the oil and water phases into an in-line mixer, e.g., a 
Homomixer or Sonulator. Preferably, the agitation is conducted in a manner 
such that the emulsion droplets have an average particle diameter below 
about 5 microns, preferably in the range of between about 0.5 and about 2 
microns. Alternatively, a suitable inorganic opacifying pigment, such as 
TiO.sub.2, Al.sub.2 O.sub.3, barytes (BaSO.sub.4), clay, ZnO, 
Ca(SO.sub.4).sup.2, talc, and the like may be provided in the emulsion. 
Preferred inorganic pigments for the purpose of the present invention are 
TiO.sub.2, Al.sub.2 O.sub.3, BaSO.sub.4, clay and ZnO, with TiO.sub.2 
being especially preferred. 
The addition of the inorganic pigment particles by means of line 26 results 
in the incorporation of the pigment into the ultimate structure of the 
opacifying particles of the present invention. Alternatively, the 
inorganic pigment may be added to the emulsion by means of line 27 where 
it migrates to the oil/water interface. When the particles of the present 
invention are ultimately formed, as thereinafter described, the inorganic 
opacifying pigment becomes incorporated in the polymeric structure at a 
point depending upon its position during the polymerization step, e.g, at 
the particle-solvent interface or homogeneously distributed throughout the 
polymer phase. 
Although it is possible to incorporate an inorganic pigment particles into 
the structure of the polymeric particles of the present invention, highly 
opaque particles may be provided in the absence of such inorganic pigment 
particles. Accordingly, since the resulting polymeric pigment is opaque 
and not transparent, the practice of the present invention may be 
conducted without the use of the aforesaid inorganic opacifying pigments. 
The ratio of the internal, aqueous phase, to the external, water-immiscible 
solvent phase, is preferably in the range of between about 0.4 and about 3 
parts by weight, preferably between about 1 to 2 parts by weight of the 
internal phase per part by weight of the external phase. Although it is 
possible to utilize a higher ratio of internal to external phase, it is 
preferred to use an approximately 2 to 1 ratio. The resulting emulsion 
that is withdrawn from the mixer 24 by means of line 28 have a viscosity 
in the range of between about 2 and 2000 centipoises. Preferably, the 
viscosity of the resulting emulsion is low and water-like. 
Next, the emulsion is introduced to the polymerization reactor 30 along 
with an amphiphilic, acidic polyermization catalyst, having an ionization 
constant greater than about 10.sup.-4, which is introduced by means of 
line 32. Suitable polymerization catalysts for the purpose of the present 
invention include, for example, polymerization catalysts that are soluble 
in the continuous oily phase, but which have a significant affinity for 
the internal, or water phase, such as anhydrous hydrochloric acid, 
SO.sub.2, SO.sub.3, BF.sub.3, BF.sub.3 etherate, titanium tetrachloride, 
phosphoric acid, phosphorous pentachloride, silicontetrachloride, 
phosphorous trichloride, sulfuryl chloride, and the like; organic 
carboxylic acids such as formic acid, acetic acid, trichloroacetic acid, 
and the like; alkyl acid phosphates, such as monoethyl acid phosphate, 
monoamyl acid phosphate, monobutyl acid phosphate, diethyl acid phosphate, 
and the like; substituted sulfamic acids, etc. Preferably, the acid 
catalyst is employed in amounts necessary to bring the final pH of the 
prepolymer phase to a pH of between about 0.5 and about 4, preferably 
between about 1 and about 2. 
The catalyst is added at about ambient temperatures under mild agitation, 
preferably in the temperature range of about 10.degree. to about 
25.degree. C. The polymerization reaction is exothermic, resulting in a 
temperature rise, so that the polymerization reaction occurs at 
temperatures in the range of between about 30.degree. and about 70.degree. 
C, preferably in the range of between about 40.degree. and about 
50.degree. C. Since the acid-catalyzed condensation polymerization is 
exothermic, cooling means (not shown) must be utilized in connection with 
reactor 30 in order to keep the polymerization temperatures in the 
preferred range. Accordingly, costly heating means for reactor 30 are not 
needed. The polymerization reaction is conducted for between about 0.25 
and about 4 hours, preferably between about 0.5 and about 2 hours. 
As employed herein, the term "amphiphilic catalyst" means that the catalyst 
possesses at least some significant affinity for both the aqueous and the 
oily phase of the emulsion, and thus is neither completely hydrophilic nor 
completely lipophilic. The employment of an amphiphilic polymerization 
catalyst permits the addition of said catalyst after the formation of the 
emulsion, since it is capable of passing through the continuous solvent 
phase to the water-oil interface, effectively catalyzing the 
polymerization of the urea-formaldehyde. Thus, the use of such catalysts 
permits the control of the emulsification operation including the emulsion 
droplet side without concern of any premature gelling, which might occur 
if a catalyst were introduced into the prepolymer solution before 
emulsification. Another advantage of using such a catalyst is that it will 
not prematurely precipate any added inorganic opacifying pigments, such as 
titanium dioxide, out of the suspension in the prepolymer solution, which 
could occur if a water-soluble acid polymerization catalyst were added 
along with the aqueous, internal phase prior to the formation of the 
emulsion. 
A dispersion of the polymerized urea-formaldehyde particles and water 
droplets in the water-immiscible oily solvent, i.e., a "solvent 
dispersion," is withdrawn from reactor 30 by means of line 34 and passed 
to a vessel 36 by means of valve 35 and line 37. In vessel 36 the solvent 
dispersion is separated into a substantially clear supernatent phase 
comprising the oily solvent, e.g., xylene, and most of the emulsifying 
agent. This phase is removed from vessel 36 by means of the line 40. The 
remaining phase is a heavy phase which comprises residual solvent and the 
solid polymer particles. The heavy phase is termed an "inverted sludge" 
phase, since the system fed to the vessel 36 had been in the form of a 
water-in-oil emulsion, and the system has now inverted to an oil-in-water 
emulsion. 
The phase separation in vessel 36 may be accomplished by various means 
including heating the solvent dispersion to a temperature, for example, in 
the range of between about 35.degree. and about 70.degree. C, preferably 
between about 40.degree. and about 50.degree. C. Alternatively, the 
solvent dispersion may be subjected to direct centrifugation. Still 
another means for effecting the phase separation is by diluting the 
solvent dispersion by additing of solvent, e.g., xylene, by means of 
process line 38. The phase separation can also be accomplished by 
subjecting the solvent dispersion to high shear mixing. In any event, 
after treatment, the solvent dispersion is subjected to settling and 
decantation or centrifugation (by conventional means not shown). 
Regardless of the phase separation means employed, it is important to 
retain at least a small amount of the solvent, e.g., xylene, in the 
inverted sludge. Suitable amounts include between about 0.2 and about 2, 
perferably between about 0.5 and about 1 parts by weight solvent per part 
of polymer solids. 
As previously indicated, the supernatent liquid comprising the solvent and 
most of the emulsifying agent is withdrawn from the separator 36 by means 
of line 40 and the solvent is passed to a recovery system 42 which 
includes a liquid-liquid separator wherein the organic solvent is 
separated from residual water and catalyst and recycled by means of line 
44 for admixture with a solvent make-up present in line 10. Thus, in this 
manner, the solvent or external phase may be easily recovered without the 
use of exotic recovery equipment normally associated with the collection 
of volatilized solvents, and may be easily recycled for reuse in the 
process. Water is withdrawn from recovery system 42 by means of line 46 
and may be subjected to waste treatment for recycle of the water for use 
in the process, or alternatively, the water may be passed to disposal. 
The resultant inverted sludge containing solid polymerized particles is 
withdrawn from the separator 36 by means of line 48, and the particles may 
be passed by means of valve 50 and line 52 to drier 54 wherein residual 
solvent is removed and the particles are obtained in line 56 in a dry 
randomly agglomerated form. If the acidic catalyst present in the inverted 
sludge in line 52 is neutralized by addition of a base, such as an alkali 
metal hydroxide, such as sodium hydroxide, introduced by means of line 53, 
the resultant dried particles are substantially discrete when removed from 
drier 54. The resultant particles have an average particle size of below 
about 2 microns, preferably between about 0.5 and about 1.0 microns and 
may be employed as opacifying pigments, to provide a relatively high 
opacity in the form of an opaque coating which is white, in the case of 
urea-formaldehyde. 
According the another and more preferred aspect of the present invention, 
the solid polymeric particles present in the inverted sludge are withdrawn 
from separator 36 by means of line 48, three-way valve 50 and, line 58 and 
are passed to solvent removal tank 60, wherein an aqueous liquid such as 
water is introduced by means of line 62 for diluting the inverted sludge 
and further removal of the water-immiscible organic solvent. The polymeric 
particles in vessel 60 are subjected to agitation and heating either 
directly employing live steam or indirectly using conventional heating 
means to a temperature in the range of between about 120.degree. and about 
250.degree. F, preferably between about 190.degree. and about 212.degree. 
F for the removal of residual solvent by steam distillation, and to 
provide additional acid-catalyzed curing of the polymeric particles. 
Surprisingly, under the influence of shear, acid and heat redispersion of 
the polymeric particles in the aqueous medium results in the formation of 
substantially spherically agglomerated opacifying particles wherein the 
walls of the particles are themselves formed of micropheres. Temperatures, 
for example, in the range of between about 120.degree. and about 
250.degree. F are utilized to "set" the spherical agglomerate structure. 
Once the formation of spherically agglomerated opacifying particles has 
been achieved, residual oily solvent can be removed at lower temperatures 
under reduced pressure is desired, with no impairment of the opacifying 
power of the final product. The resulting agglomerated particles are 
illustrated in FIG. 2 of the drawings. 
Alternatively, the solvent dispersion from the Reactor 30 can be discharged 
through line 34 through valve 35 and line 39 to line 58 and into vessel 60 
where water is added (line 62) and the mixture agitated under high shear 
to form an oil-in-water emulsion. The oily solvent is then removed by 
steam distillation, as described in the preceeding paragraph, to produce 
the spherically agglomerated particles illustrated in FIG. 2. 
FIG. 2 is a photomicrograph illustrating the nature of a preferred 
opacifying particle of the present invention which may be termed a 
"super-agglomerate," since it is composed of a shell formed of 
agglomerated secondary particles, which are substantially spherical and 
substantially solid throughout, and which are themselves formed of 
clusters of substantially spherical primary particles. As seen in FIG. 2, 
the super-agglomerate is hollow on the inside of the outer shell. Although 
it is not intended to limit the invention to any particular theory, it is 
believed that the secondary particles are derived from the polymerization 
of an aqueous prepolymer droplet. Thus, the size of the secondary 
particles is controlled by the droplet size of the initial water-in-oil 
emulsion droplets formed during the initial emulsification step. The 
aqueous redispersion system in vessel 60 involves an oil-in-water emulsion 
with the secondary particles concentrated at the solvent-water interface. 
The subsequent heating of this material during the initial phase of the 
distillation step results in the post-curing and fusion of the particles 
into a rigid substantially spherical structure. 
The secondary particles are irregular and bumpy and are composed of smaller 
primary particles. Although the primary particles have been described as 
substantially spherical, with increased formaldehyde-to-urea ratio this 
differentiation becomes less distinct, so that the secondary particles 
have a smoother, only slightly pebbled surface. FIG. 3 is a sectional 
photomicrograph illustrating the interior of a super-agglomerate and 
demonstrates that the particles are hollow. 
The super-agglomerate particles have an opacity which provides a two to 
three-fold increase of that of the discrete, substantially spherical 
secondary particles that are withdrawn by means of line 52, which 
particles have not been subjected to heating under shear. The 
superagglomerates have an average particle diameter of between about 1 and 
about 20 microns, preferably between about 2 and about 7 microns. 
Solvent vapor is steam distilled from the solvent removal vessel 60 in the 
form of a mixture of residual xylene with water by means of line 63 and 
passed through a condenser (not shown) and then on to join process line 
40. Meanwhile, the aqueous dispersion of the substantially spherical 
agglomerates is withdrawn from vessel 60 by means of line 64 and valve 66 
and is withdrawn by means of line 68. The aqueous slurry that is withdrawn 
by means of line 68 may be employed as a coating for the direct 
application of the opacifying agents onto the desired substrate, such as 
paper, with the incorporation of a suitable binder material. The resulting 
substrate is then dried under conventional paper drying conditions for 
removal of the moisture and the resulting coated substrate has a high 
degree of opacity. The Kubelka-Munk light scattering coefficient of such a 
coating formulated from 100 parts by weight of these pigment particles and 
10 parts by weight of a conventional paper coating binder is between about 
2,000 and about 6,000 cm.sup.2 per gram. Likewise, the resulting slurry 
may be incorporated in a surface finish, such as paint, to provide a high 
degree of opacity thereto. 
Alternatively, the aqueous slurry withdrawn from vessel 60 by means of line 
64 may be passed by means of line 70 to a solid-liquid separator 72, such 
as, for example, a centrifuge, for water removal by means of line 74, and 
the resultant particles may be passed by means of line 76 to dryer 54 in 
order to produce agglomerates in powder form. It has been found that the 
adjustment of the pH to a value greater than 8 prior to drying greatly 
facilitates the production of a free-flowing, non-caking powder during the 
drying process. The resultant polymeric opacifying agents may be 
incorporated in paint or may be redispersed in a suitable aqueous or 
nonaqueous liquid with the addition of a binder and employed in the 
coating of paper or some other substrate, such as plastic, fabric or 
textile webs wherein it is desired to increase the opacity of such 
substrate. If desired, the product from line 76 can be passed to line 77 
to wash tank 78 for washing to remove residual emulsifying agents by 
resuspending it in additional water in vessel 78. The washed 
super-particles are then passed by means of line 79 to separator 72. 
These organic opacifying pigment particles in the form of the aqueous 
slurry from line 68, the wet cake from line 76, or the dry powder from 
line 56 can also be added to paper furnish, that is, the slurry of 
cellulose pulp fibers, sizing agents and other additives, and used to 
produce paper by conventional papermaking techniques, providing a paper 
with greatly increased opacity. 
The following examples illustrate the production of the opacifying pigments 
of the present invention and constitute the best modes contemplated for 
carrying out the present invention. 
EXAMPLE 1 
Ninety grams of urea are added to a solution of 165 grams of 37% aqueous 
formaldehyde and 45 grams of water, adjusted to pH 9.3 with NaOH and 
heated for 1 hour at 65.degree. to yield a prepolymer solution containing 
about 50% solids. Using a Waring Blendor, 140 grams of this prepolymer 
solution are emulsified in a solution of 6 grams of a polyethylene 
oxide-polypropylene oxide block copolymer ("Pluronic L 122 from 
BASF-Wyandotte Company) emulsifier dissolved in one-hundred grams of 
toluene to produce a low-viscosity water-in-oil emulsion. 
The emulsion then is treated with 4 milliliters of a 33% by weight solution 
of titanium tetrachloride in toluene, resulting in an exothermic reaction 
which raises the temperature from about 28.degree. to about 50.degree. C. 
After stirring for 2 hours, the resulting "solvent dispersion" consists of 
water droplets and solid polymer particles dispersed in the oil phase, 
with little or no tendency for coagulation of the polymer particles. This 
is separated into a clear supernatant phase, containing the oily solvent 
and some of the emulsifying agent, and a heavier phase, the "inverted 
Sludge", containing about 40% solids and about 20% oily solvent. The phase 
separation is accomplished by centrifugation. The inverted sludge is 
redispersed in about 200 grams of water and is subjected to high-shear 
agitation while heating to steam distill off the oily solvent as a mixture 
with a portion of the excess water. 
The resulting product is free of toluene and consists of an aqueous 
dispersion of 0.25 to 2.mu. polymer particles, which have fused together 
into substantially spherical agglomerates (super-particles) 1 to 5.mu. in 
diameter. The super-particles are made basic with ammonia and blended with 
a carboxylated styrene-butadiene rubber (SBR) latex paper coating adhesive 
(Dow 620 SBR latex, ten parts by weight latex solids to one-hundred parts 
polymer solids) and coated on a paper substrate. 
The Kubelka-Munk scatter coefficient of the paper coating is measured using 
a Huygen model 2100 digital opacimeter and computational methods described 
in the literature. A scatter coefficient of approximately 4000 cm.sup.2 
/gram is obtained. Formulated and coated under the same conditions, a 
water-dispersable paper-coating grade of anatase TiO.sub.2 gives coatings 
with a scatter coefficient of 3,800 to 4,000 cm.sup.2 /gram. 
EXAMPLE 2 
Two hundred grams of a prepolymer solution prepared by heating 165 grams of 
37% aqueous formaldehyde, 45 grams of water, 89 grams of urea and 1 gram 
of melamine, adjusted to pH 9.3 with sodium hydroxide, at 70.degree. C for 
one hour are emulsified in a solution of six grams of a polyethylene 
oxide-polypropylene oxide block copolymer attached to a central 
amine-functional group (Tetronic 1502 from BASF-Wyandotte Company), which 
acts as an emulsifying agent, dissolved in one-hundred grams of xylene. 
0.06 milliequivalents of sulfur dioxide (as a 4 normal solution in xylene) 
are added to the water-in-oil emulsion, resulting in a temperature rise 
from about 28.degree. to about 48.degree. C. After one hour the resulting 
solvent dispersion is centrifuged, the inverted sludge phase is mixed with 
water and steam distilled under high-shear agitation. The product is free 
of residual xylene, and a coating on paper, prepared as described in 
Example 1, has a scatter coefficient of about 4,500 cm.sup.2 /gram. 
EXAMPLE 3 
A urea-formaldehyde-melamine prepolymer solution is prepared from 165 grams 
of 37% aqueous formaldehyde, 45 grams of water, 89 grams of urea and 1 
gram of melamine, adjusted to pH 9.3 and heated for 1 hour at 65.degree. 
C. One-hundred seventy grams of this prepolymer solution are emulsified in 
a solution of six grams of Tetronic 1502 dissolved in one-hundred grams of 
xylene, and 0.06 milliequivalents of sulfur dioxide (about 4 normal in 
xylene solution) is added, resulting in an exothermic reaction. After 
stirring for 1 hour, the solvent dispersion has little or no precipitate. 
Without any preliminary phase separation, the product is mixed with water 
to give about 16% total solids, and this "aqueous redispersion" is steam 
distilled under high-shear agitation to yield a product free of of 
residual xylene, closely resembling that described in Example 1. A paper 
coating prepared as described in Example 1 has a scatter coefficient of 
about 4,000 cm.sup.2 /gram. 
The aqueous dispersion of spherical agglomerates is centrifuged, the 
aqueous supernatant phase is decanted, the precipitate is mixed with fresh 
water at about 10% solids and stirred for one hour. The washed product is 
concentrated by centrifugation and formulated into a paper coating as 
described in Example 1. A scatter coefficient of about 4,800 cm.sup.2 
/gram is observed. 
EXAMPLE 4 
Eighty grams of a urea-formaldehyde prepolymer solution prepared as 
described in Example 1 are emulsified in a mixed emulsifier solution of 
four grams of Pluronic L 122, 2 grams of sorbitan monooleate (Span 80 from 
ICI America), 1 gram of Pluronic L 63 (a lower molecular polyethylene 
oxidepolypropylene oxide than Pluronic L 122, containing a higher percent 
polyethylene oxide) and one-hundred grams of xylene. The resulting 
water-in-oil emulsion is treated with two milliliters of a 50% (by weight) 
solution of monobutyl acid phosphate in xylene and stirred for 2 hours. 
The resulting solvent dispersion is centrifuged, and the heavier inverted 
sludge phase is dried in an oven at 80.degree. C to obtain a material free 
of solvent. This is redispersed in water made basic with ammonia and mixed 
with an SBR latex adhesive (ten parts latex solids to one-hundred parts 
polymeric opacifier solids) to give a coating which exhibits a scatter 
coefficient of about 2,000 cm.sup.2 /gram. 
EXAMPLE 5 
One-hundred grams of a urea-formaldehyde prepolymer solution, prepared as 
described in Example 2, are emulsified in a solution of two grams of 
stearic acid diethanolamide (Schercomid ST from Scher Bros., Inc.) 
dissolved in one-hundred grams of xylene. Four milliliters of a 33% by 
weight solution of titanium tetrachloride in xylene are added, the 
exothermic reaction occurs and the dispersion is stirred for two hours. 
The products is centrifuged, and the heavier inverted sludge phase is 
mixed with about two-hundred grams of water and steam distilled under 
high-shear agitation. A coating prepared from the solvent-free product as 
described in Example 1 has a scatter coefficient of about 3,000 cm.sup.2 
/gram. 
EXAMPLE 6 
One-hundred-forty grams of a prepolymer solution prepared as described in 
Example 2 are emulsified in a solution of six grams of tall oil fatty acid 
diethanolamide (Schercomid TO-1 from Scher Bros., Inc.), dissolved in 
one-hundred grams of a chiefly aliphatic solvent (Shell-Sol 70 from Shell 
Oil Co., 98% saturates, 2% olefins and aromatics; initial boiling point 
160.degree. C, end point 180.degree. C). Seven milliliters of about 4 
normal sulfur dioxide in xylene solution are added to the water-in-oil 
emulsion, resulting in a temperature rise from about 30.degree. to about 
50.degree. C. After stirring for about two hours, there is no significant 
precipitate. The emulsion is centrifuged for forty-five minutes, and the 
inverted sludge is redispersed in about two-hundred grams of water 
containing two grams of polyoxyethylene sorbitan monolaurate (Tween 20 
from ICI America). The organic solvent is distilled off under high-shear 
agitation to give polymeric opacifiers which are formulated into a paper 
coating as described in Example 1. This has a scatter coefficient of about 
1,400 cm.sup.2 /gram. 
EXAMPLE 7 
A urea-formaldehyde prepolymer solution is prepared by heating an aqueous 
solution of two-hundred-twenty grams of 37% formaldehyde and 82 grams of 
urea, adjusted to pH 8 with triethanolamine, at 70.degree. C for one hour. 
Fifty grams of this prepolymer solution are mixed with thirty grams of a 
50% by weight dispersion of titanium dioxide in distilled water, and 
adjusted to pH 6 with dilute sulfuric acid. The aqueous dispersion is 
emulsified in a solution of six grams of Pluronic L122, three grams of 
sorbitan monooleate (commerically available as Span 80 from ICI America) 
and 1.5 grams of a poly(oxyethylene)-poly(oxypropylene) block copolymer 
with 30% poly(oxyethylene) and a molecular weight of about 2500 
(commercially available as Pluronic L 63) in one-hundred grams of xylene 
to yield a water-in-oil emulsion. This is treated with four milliliters of 
a 50% by weight solution of monobutyl acid phosphate in toluene and 
stirred for two hours. 
The solids are separated from the continuous phase by centrifugation and 
dried at 80.degree. C in a forced draft oven. The residue is redispersed 
in water, yielding discrete, spherical particles averaging 2-3 microns in 
diameter, with titanium dioxide particles clearly visible inside each 
sphere. No free titanium dioxide is discernable in the aqueous phase at 
1000X magnification. 
The aqueous dispersion is formulated with a latex adhesive as described in 
Example 1 and coated on paper to yield a scatter coefficient of about 1700 
cm.sup.2 /gram. Electron micrographs of the dry product show approximately 
spherical particles; X-ray analysis of single particles on a scanning 
electron microscope show high titanium loading. 
EXAMPLE 8 
A urea-formaldehyde prepolymer is prepared in a continuous manner by 
blending together a stream of 37% aqueous formaldehyde, adjusted to pH 9.5 
with sodium hydroxide and a stream of 67% aqueous urea, heated to 
65.degree. C to prevent crystallization, in a ratio corresponding to a 
formaldehyde to urea ratio of 1.3 to 1, and then passing this mixture 
through a heated coil at a temperature of about 95.degree. C, with a 
residence time in the reactor of about 3 minutes. Upon leaving the heated 
reactor coil the prepolymer solution is cooled in a heat exchanger and 
then emulsified in a solution of six parts of Tetronic 1502 in one-hundred 
parts xylene, in a ratio of one-hundred forty parts of prepolymer solution 
to one-hundred parts of xylene. The resulting emulsion is cooled to about 
20.degree. C in a heat exchanger and then passed into a mixing zone where 
0.06 milliequivalent of sulfur dioxide is added to about 250 parts of the 
emulsion. The catalyzed emulsion passed through a tube reactor and, after 
a reactor residence time of about one-half hour, is steam distilled with 
excess water under high-shear. Coatings prepared from the product and from 
washed product as described in Examples 1 and 3 have scatter coefficients 
of about 4000 and about 5000 cm.sup.2 /gram, respectively. 
EXAMPLE 9 
Paper handsheets were prepared using microcapsular opacifiers prepared as 
described in Example 1 and anatase titanium dioxide as fillers, added to 
the furnish to enhance opacity and brightness. 
Three samples of a 300 gram "air dry" mixture of 50% pine and 50% hardwood 
pulps, each, were disintegrated in a dynapulper and were refined in a 
valley beater to a Canadian Standard Freeness of 250-350. After each batch 
was pressed and shredded individually, the three batches were combined and 
the composite was shredded until a 10.0 gram sample gave a Canadian 
Standard Freeness of 275-325. Moisture of the composite was obtained by 
dispersing several 10.0 gram samples in 100 milliliters of distilled water 
under a Hamilton Beech dispersator for 2-3 minutes, forming pads in a 
Buchner Funnel, drying the pads on a hot plate, and calculating the 
percentage moisture. This result was used to calculate "bone dry" weights 
of fiber for paper furnish formulations. 
Nine handsheets were prepared for each of the filler pigment samples (three 
at 5, 10, and 15% as bone dry weight, respectively) by weighing out 3.0 
grams "bone dry" samples of fiber into plastic bottles, adding the 
appropriate weight of filler (0.15, 0.30, and 0.45 grams "bone dry") as 
necessary, diluting with distilled water to 110 milliliters total volume, 
dispersing under the Hamilton Beech dispersator for 3 minutes, and forming 
in a Noble and Wood sheet mold, with the following results. 
______________________________________ 
G. E. Brightness 
Description F/W Tappi Opacity 
______________________________________ 
78.9/79.7 76.6 
5% TiO.sub.2 81.4/82.2 81.1 
10% TiO.sub.2 82.9/83.6 84.5 
15% TiO.sub.2 84.1/85.4 87.1 
5% microcap. opac. 
81.5/82.2 80.5 
10% microcap. opac. 
83.7/85.2 84.6 
15% microcap. opac. 
84.5/86.1 86.6 
______________________________________ 
The microcapsular opacifiers compared favorably with anatase titanium 
dioxide in this application. 
EXAMPLE 10 
A sample of an aqueous dispersion of microcapsular opacifiers prepared as 
described in Example 2 (coating scatter coefficient about 4500 cm.sup.2 
/gram) is adjusted to pH 8.3 with sodium hydroxide and heated to dryness 
in an 80.degree. C forced draft oven. Ten grams of the dry powder are 
dispersed in fifty grams of water, adjusted to pH 9 with ammonia and 
treated with two grams of Dow 620 latex (about 50% solids) to give a paper 
coating which had a scatter coefficient of about 4700 cm.sup.2 /gram.