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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a microencapsulation system, to a method for
preparing the microencapsulation system and to pressure-sensitive transfer
sheets employed in this system.
At the present time, microencapsulation systems including
pressure-sensitive transfer sheets utilizing microcapsules containing an
oily liquid and a colorless dye intermediate are well known in the art. In
these systems, the sheet is formed of a suitable substrate such as paper
having coated thereon microcapsules comprising a polymeric wall which
surrounds an oil droplet containing the colorless dye intermediate.
Generally, the microcapsules are coated on the substrate by utilizing a
polymeric binder composition. In use, the coated surface of the transfer
sheet is positioned against an underlying copy sheet and pressure is
supplied to the uncoated surface of the transfer sheet to rupture the
capsules and effect transfer of the dye intermediate to the underlying
copy sheet. The copy sheet contains a composition which is reactive with
the dye intermediate to form visible colored marks at that portion of the
surface of the copy sheet adjacent to the capsules which have been
ruptured and from which the dye intermediate has been transferred.
While this transfer system has proven to be satisfactory in applications
wherein it is desired merely to form an image on the copy sheet
corresponding to the image formed under pressure on the uncoated surface
of the transfer sheet, less than satisfactory results have been obtained
in other applications. For example, in a transfer system wherein it is
desired not only to form the copy on the copy sheet but it is also
desirable to permit the user to write on the coated surface of the
transfer sheet with commonly employed oil-based inks such as are employed
in ball-point pens, the written image obtained is often incomplete. This
is because the pressure needed to effect the writing is sufficient to
rupture the microcapsules thereby releasing oil which admixes with the
applied ink and contaminates the printing press or the ball-point pen tip
thereby interrupting the ink flow. Thus, attempts to write on the coated
surface with a ball-point pen typically result in clogging and skipping,
similar to the effect observed when trying to write on a greasy or oily
surface.
Prior attempts to solve these problems including changing the nature of the
polymeric composition used to form the microcapsules or applying a
protective coating over the microcapsules have proven to be unsatisfactory
in that they cause a material reduction in the transfer of oil and dye
intermediate from ruptured capsules to the copy sheet under normally
employed writing pressure. Accordingly, it would be desirable to provide a
means which would permit printing or writing with oil-based inks on the
coated surface of the transfer sheet without adverse effects on the
transfer of the oil-based dye intermediate from ruptured microcapsules.
SUMMARY OF THE INVENTION
The present invention is based upon the discovery that an improved transfer
sheet is obtained by coating a substrate with pressure-rupturable
microcapsules containing an oil and an oil-soluble dye intermediate, and
with particulate, oil-absorptive material which is non-reactive with the
dye intermediate and is situated with respect to the microcapsules such
that oil released from said microcapsule can be absorbed by said
particulate material. The resulting composite surface permits writing
thereon without interference from the oil and permits transfer of the dye
intermediate to an underlying copy sheet under normally employed writing
pressure on the uncoated sheet surface. The transfer sheet comprises (i) a
substrate such as paper, (ii) a coating thereon comprising microcapsules
each having a continuous polymeric wall encapsulating an oil and an
oil-soluble dye intermediate and (iii) a particulate oil-absorptive
material situated with respect to the microcapsules such that oil released
from said microcapsules, either upon rupture or by diffusion, can be
absorbed by said particulate material. The particulate oil-absorptive
material is unreactive with the dye intermediate in that it does not form
a colored image when contacted with the dye intermediate.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The transfer sheet of this invention is particularly useful in applications
wherein it is desired to maintain a master copy of paper originals and
wherein it is desired to permit writing on both surfaces of the originals
such as in bank check manifolds in which a copy of each bank check is
retained on the master.
The particulate absorptive material can be applied to the microcapsules
either by direct admixture therewith or by coating the absorptive material
as a substantially separate layer over the layer containing the
microcapsules. In any event, the particles are applied to the substrate in
a manner such that they are retained thereon with a polymeric binder. If
desired, the particles can be applied to the substrate either with the
microcapsules or subsequent to applying the microcapsules to the
substrate. In any event, the composition containing the particles must be
compatible with the microcapsules so that the resultant composition either
contains a binder for the particles or comprises a reaction system which
will form a binder for the particles while not rupturing the microcapsules
or weakening the walls of the microcapsules thereby preventing substantial
migration of oil from unruptured microcapsules.
Useful coatings on the substrate which contain both the microcapsules and
the particles can be obtained by employing any one of a number of
conventional coating techniques. In a broad aspect of the process of this
invention, the binder for the particles can be formed either by
incorporating a water soluble polymeric material in admixture with the
particles, incorporating a material with the particles which acts as a
cross-linking agent for a polymeric component used to form the
microcapsules or by a combination of these techniques. Depending upon the
particular coating technique employed, the absorptive particles can be
directly admixed with the microcapsules or they can be separated from the
microcapsules by a polymeric barrier.
When forming an admixture of microcapsules and particles, the composition
comprising the microcapsules and particles can contain a binder and/or a
cross-linking agent for the polymer forming the microcapsule wall. This
composition can be applied to a substrate in a one-step process. When it
is desired to include a polymeric barrier, it can be formed either in a
two-step coating process or in a three-step coating process. When
employing the two-step coating process, the microcapsules are coated on
the substrate in a first step. Thereafter, a composition comprising the
absorptive particles and a polymeric binder with or without a
cross-linking agent for the polymer forming the microcapsule wall is
applied over the microcapsular coating. When employing a three-step
coating process, the microcapsules are coated on the substrate in a first
step. An aqueous solution of a polymeric composition is applied over the
microcapsule coating to form the polymeric barrier. Thereafter, a
composition containing the oil absorptive particles is coated over the
polymeric barrier in a manner such that the barrier retains the particles.
Transfer sheets made by incorporating absorptive particles with the
microcapsules but without a cross-linking agent, are effective in that
they do not adversely affect writing with oil-based inks. Their use is
somewhat limited, however, since in some cases, over a period of time, it
has been found that oil migrates by diffusion from the interior of the
microcapsules and is absorbed by the absorptive particles so that the
exposed surface of the particles contains some oil. When this occurs,
writing thereon with oil-based inks may be adversely affected. Such oil
migration can be effectively reduced by employing the polymeric barrier
described hereinabove. However, care must be taken to ensure that the
barrier coating does not become too thick so that migration of the oil
from ruptured microcapsules may be seriously reduced. A preferred
embodiment of this invention comprises a coating process wherein a
cross-linking agent for an uncross-linked polymer in the microcapsule
walls is incorporated with the particles so that when the cross-linking
agent comes in contact with the microcapsules, a thin barrier is formed
between the microcapsules and the absorptive particles which materially
reduces migration of the oil by diffusion through the microcapsule walls
but does not adversely affect migration of oil when the microcapsules are
ruptured under normal writing pressure.
Regardless of the coating process employed to form the transfer sheet, the
microcapsules are formed prior to contact with the absorptive particles.
The particular method by which the microcapsules are formed is not
critical to the present invention. Accordingly, the microcapsules can be
formed by any known technique including coacervation or by forming the
microcapsules in an emulsion whereby an oily material is dispersed as
microdroplets in an aqueous continuous phase. The aqueous phase and the
oily material each contain a reactant which reacts at the oil-water
interface to form a polymeric, mechanically-stable capsule wall.
Representative suitable methods for forming microcapsules and for coating
them on a substrate such as paper are disclosed in U.S. Pat. No. 3,779,941
issued on Dec. 18, 1973, and U.S. Pat. No. 3,875,074 issued on Apr. 1,
1975, both of which are incorporated herein by reference.
In a preferred embodiment of this invention, the microcapsules are formed
so that the walls thereof include a polymeric material which can be
cross-linked by means of a cross-linking agent provided by the absorptive
particle composition. These polymeric materials also can be partially
cross-linked during the formation of the microcapsules. Representative
suitable polymeric materials which can be subsequently completely
cross-linked include the hydroxyl-containing polymers such as polyvinyl
alcohol, methylcellulose, starch, carboxymethyl cellulose and the like;
amino-containing materials such as proteins and mixtures of hydroxy-
and/or amino-containing materials or the like. Polyvinyl alcohol is the
preferred polymeric material for forming the microcapsules particularly
those grades known as 88% (nominal) hydrolyzed high molecular weight
products (e.g. commercially available as Covol 97-40 from Corn Products
International (CPI) or Elvanol 50-42 from E. I. Dupont de Nemours & Co.).
However, any of the available water-soluble grades either fully or
partially hydrolyzed, whether of high or low molecular weight, can be
utilized.
Substituted starches are the preferred form of starch for use in the
present invention and can be provided by any suitable process. For
example, they may be provided by an etherification of the starch in
granular form under non-gelatinizing conditions with monofunctional
etherifying agent which provides the starch with etherlinked hydrophobic
groups. Thus, the starch granule will become more oleophilic due to the
presence of a high percentage of hydrophobic groups. Thus, the term
"substituted starch" as employed herein refers to a starch that has been
rendered more oleophilic due to an increase in hydrophobic groups.
The etherification reaction is conducted until the starch becomes more
hydrophobic and essentially non-gelatinizable. Finally, the starch is
fragmented and reduced to submicron sized particles by treatment with
steam under pressure. The starch is not swollen or cooked but is reduced
to very fine particles which are mainly in the microscopic or colloidal
size range. Such starches are described, for example, in U.S. Pat. No.
3,462,283 to Hjermstad et al., the disclosure of which is incorporated
herein by reference.
When forming the microcapsules, the polymeric component can be partially
cross-linked with an oil-soluble cross-linking agent dissolved in the oil
microdroplets. Subsequently, cross-linking can be completed with a
cross-linking agent which is added with the absorptive particles in an
aqueous medium. Representative suitable cross-linking agents include
sodium borate (borax), formaldehyde, glyoxal, formaldehyde condensation
products, e.g. urea formaldehyde, melamine formaldehyde, or the like.
As described above, one alternative process for forming the transfer sheet
of this invention involves overcoating the microcapsular layer with a
cross-linkable polymer composition such as the hydroxy-containing polymers
set forth above to form the polymeric barrier. This overcoating is quite
thin so as to minimize its effect on oil transfer from ruptured
microcapsules to the copy sheet. Generally, it is applied to the substrate
in an amount of between about 0.05, and about 1.0 pounds per ream, (3300
sq. ft.) preferably between about 0.1 and about 0.5 pounds per ream. The
polymer generally is applied as a dilute aqueous solution of about 0.5 to
10 wt. percent, preferably from about 1 to 3 wt. percent. Advantageously,
a wetting agent can be included in the aqueous solution to insure complete
wetting of the capsule coating thereby insuring continuity of the
polymeric barrier formed therefrom. The wetting agent comprises between
about 0.005 and 0.1 wt. percent preferably between about 0.01 and 0.03 wt.
percent based upon the total weight of the aqueous polymer solution.
Representative suitable wetting agents include anionic compounds, such as
fatty acid salts, salts of higher alcohol sulfates, alkylbenzene
sulfonates, alkylnaphthalene sulfonates, or salts of poly(oxyethylene)
sulfates; nonionic compounds, such as polypropylene oxide-polyethylene
oxide block copolymers, poly(oxyethylene) alkyl ethers, poly oxyethylene
alkylphenol ethers, sorbitol fatty acid esters, poly(oxyethylene) sorbitol
fatty acid esters, poly(oxyethylene) alkyl esters or fatty acid
monoglycerides; and cationic compounds, such as, quaternary ammonium salts
with long chain alkyl group(s) or pyridinium salts. Preferred surfactants
are sodium lauryl sulfate or polypropylene oxide-polyethylene oxide block
copolymers or the like. The preferred polymers useful for forming the
polymeric barrier are the fully hydrolyzed medium or high molecular weight
polyvinyl alcohols such as Elvanol 72-60 (available from E. I. du Pont de
Nemours), Covol 9870 (CPI) or Vinol 125 (available from Air Products
Corp.). However, any fully or partially hydrolyzed polyvinyl alcohol is
satisfactory for this purpose. Other barrier materials can be used to form
the polymeric barrier layer in this invention including other
water-soluble polymers such as starch, modified starch, proteins, natural
or artificial gums or any polymer capable of being made subsequently water
insoluble by chemical reaction. The water solubility of the polymeric
barrier coating should be eliminated before or during application of the
particle coat. This can be achieved by adding a cross-linking agent such
as glyoxal or a low molecular weight, water soluble urea formaldehyde or
melamine formaldehyde resin to the aqueous polymer solution.
Alternatively, a material such as borax which almost instantly
insolubilizes or cross-links the barrier polymer can be added with the
particles. The borax instantaneously forms a gel with polyvinyl alcohol,
and this preserves the integrity of the barrier until the coating
composition containing the particles has dried. The polymeric barrier
coating also can comprise a solution of a material or materials which form
a continuous, water insoluble film upon dehydration. Representative
suitable materials include low molecular weight, water-soluble urea
formaldehyde, or melamine formaldehyde resins which form highly
cross-linked polymeric matrices upon dehydration.
The polymeric barrier coating also can comprise a solution of
poly-functional material capable of cross-linking at least one of the
materials making up the capsule walls and/or the capsule coating binder.
Examples of such cross-linking agents are formaldehyde, glyoxal, borax,
glutaraldehyde and the urea formaldehyde or melamine formaldehyde resins
described above. After applying this overcoating, a separate coating of
the absorptive particles and a cross-linking agent can be applied. When
this latter coating contacts the surface of the microcapsules,
cross-linking thereof is initiated so that a cross-linked polymeric
barrier is formed between the absorptive particles and the microcapsules
thereby minimizing or preventing migration of oil by diffusion from the
unruptured microcapsules to the absorptive particles.
While the present invention has been described above with reference to the
use of a cross-linking agent which cross-links a polymeric material either
incorporated with the microcapsules or provided as a separate layer, it is
to be understood that suitable binders for the absorptive particles can be
employed which can form an effective barrier to oil migrating by diffusion
from the microcapsules without a cross-linking agent. Thus, any
water-soluble or water dispersible polymer which functions as a stable
binder upon drying can be employed in the present invention. The binder
can be added to the coating either with the microcapsule-forming layer or
with the absorptive particles. In any event, the concentration of binder
is such as to substantially reduce contact of the absorptive particles
with oil that may migrate by diffusion from unruptured microcapsules but
in a concentration less than that which substantially reduces migration of
oil and dye intermediate from ruptured microcapsules from the transfer
sheet to the copy sheet during use under normal writing pressures.
Representative suitable binders which need not be cross-linked include
polyvinyl chloride, vinyl chloride-vinylidene chloride copolymer,
polyvinylidene chloride, nitrile rubber polymer latices or the like.
The absorptive particles useful in the present invention are those which
are unreactive with the dye intermediates employed in the oil phase of the
microcapsules. Representative suitable absorptive particles include paper
coating clays such as kaolin, bleached kaolins, or pigments such as
calcium carbonate, barium sulfate, talc, silica, calcium sulfate, titanium
dioxide, mixtures thereof and the like. These absorptive particles
normally have a size between about 0.1 and about 5 microns, preferably
between about 0.25 and about 2 microns.
The oily materials used to form the oily nucleus of the microcapsules are
those conventionally employed in the prior art and are water-immiscible
and unreactive with respect to the dye-forming system employed. In the art
of making a transfer sheet record material, a low viscosity-low vapor
pressure oil is preferred. The viscosity of the oily medium is a
determining factor in the speed with which the markings can be transferred
to the copy sheet since low viscosity oils will transfer more quickly than
oils of higher viscosity. The vapor pressure should be sufficiently low to
avoid substantial losses of oil through evaporation during the
encapsulation process. Suitable oily materials which may be employed
include the aliphatic and aromatic hydrocarbon oils, such as kerosene,
mineral spirits, naphtha, xylene, toluene, substituted biphenyls,
terphenyls, napthalenes, diphenylmethanes and the like; terpenes, such as
turpentine; esters, such as dimethyl phthalate, dioctyl phthalate,
dimethyl azelate, methyls, 2-ethyl hexanoate, 2-ethylhexyl acetate or the
like.
The amount of polymeric material used to form the microcapsule walls
relative to the oily nucleus material employed will vary over a wide range
depending upon the particular system under consideration. However,
suitable amounts include between about 5 and 100 parts of polymeric
material per 100 parts by weight oil, preferably between about 10 and
about 50 parts of polymeric material per 100 parts by weight oil.
In forming the transfer sheet record material, known processes can be used
to encapsulate an oily printing ink, such as may be used in smudge-proof
typewriter ribbons or carbon papers. In such a use, it has been found
expedient to encapsulate a colorless, water-insoluble dye intermediate
dissolved in the oil. Colorless dye intermediates are wholly conventional
in such utilities and are well known in the art. Exemplary of the
colorless dye intermediates which have been contemplated for use in this
invention are leuco dyes, such as crystal violet lactone and derivatives
of bis(p-dialkylaminoaryl) methane such as disclosed in U.S. Pat. Nos.
2,981,733 and 2,981,738 which are incorporated herein by reference. These
dye intermediates are colorless in an alkaline or neutral medium and react
to form a visible color in an acidic medium. Thus, when a capsule
containing such a compound is ruptured and the compound is discharged onto
an absorbent, acidic electron-acceptor material, such as a paper web
coated with an organic or an inorganic acid material, a visible color
appears on the absorbent material at the point of contact.
Optionally, inhibitors can be dispersed in the oily material with the dye
intermediates. Such materials are helpful in preventing the light and heat
degradation of the intermediates during the encapsulation procedure,
especially when elevated temperatures are required, such as when a fat is
encapsulated. Inhibitors are also considered to aid in the stabilization
of the colored marking on the copy sheet against the effects of the
atmosphere. A small amount (generally about 1 to 10 percent by weight of
the dye) of an inhibitor, such as N-phenyl-2-naphthylamine, can be used.
The leuco dye intermediates which are mentioned above are, in general, oil
soluble. Oils which are inert with respect to the dye and in which the dye
has appreciable solubility, e.g. above 0.5 grams of dye per 100 grams of
oil, are preferable.
Microcapsules having diameters ranging from 0.1 to several hundred microns
can be employed with capsules having diameters in the range of 3.0 to 5.0
microns being preferred.
The emulsion containing the microcapsules can be either coated directly
onto a web material and dried or the microcapsules can be separated from
the emulsion by some physical means such as filtration or centrifugation;
washed, if desired; redispersed in a solution of a binder; coated onto a
web material and dried. Suitable binders include methyl cellulose, starch,
casein, polyvinyl alcohol, polyvinyl acetate latex, and styrene butadiene
latex. Alternatively, materials such as urea-formaldehyde or
melamine-formaldehyde condensates can be employed. The coating operation
is performed by conventional means, such as by use of an air knife. The
capsule coatings are dried at temperatures ranging from about 40.degree.
C. to 75.degree. C. At these temperatures, no appreciable degradation of
the capsules, and in particular, the leuco dye intermediate, takes place.
The web material commonly used in transfer sheet record material is paper
and is, therefore, preferable in the practice of this invention. However,
the microcapsules also are capable of being coated onto other materials
such as plastic and fabric or textile webs. When using a web material
having a high degree of porosity, it is advisable to pre-coat the web with
a material which will reduce seepage of the microcapusle-containing
coating through the web. Impregnating the web material with polyvinyl
alcohol or a butadiene-styrene latex is the conventional practice for
producing an essentially impervious substrate.
The following examples illustrate the present invention and are not
intended to limit the same. Unless otherwise stated, all percentages and
parts are by weight.
EXAMPLE I
This example illustrates the one-step coating process for making the
transfer sheet of this invention wherein a binder and a cross-linking
agent are incorporated into the coating composition.
250 Grams of a 6 percent aqueous solution of a medium molecular weight, 88
percent hydrolyzed poly (vinyl alcohol) (Dupont Elvanol 50-42) were used
to emulsify an oily solution comprising 2.1 grams crystal violet lactone,
1.8 grams of benzoyl leuco methylene blue, eight grams of a polyfunctional
isocyanate comprising an adduct of tolylene diisocyanate with
trimethylolpropane, (Mobay CB-75), four grams of diethyl phthalate and 90
grams of an alkylated aromatic oil. Emulsification was effected by
subjecting the mixture to high shear mixing at 25.degree. C. for 1 minute.
The resulting emulsion was diluted with 116 grams of water and cured at
60.degree. C. for two hours, after which time suspended microcapsules were
formed. The microcapsules ranged in size between 1 and 7 microns, with an
average of about 4 to 5 microns.
80 Grams of these microcapsules, still suspended in water, were mixed with
30 grams of kaolin clay, 20 grams of water, 1.6 grams of a wet strength
resin (Virginia Chemicals Virset 656-4) and 2.1 grams of 60 percent
aqueous glyoxal for 5 minutes at about 25.degree. C. The resultant
composition was coated on a paper substrate at a coat weight of six to
eight pounds per ream (3300 square feet). The resulting carbonless
transfer paper sheet gave an image intensity of 69.6 on an underlying copy
sheet, based on an arbitrary intensity scale. After heating the transfer
sheet for three hours at 100.degree. C., the image intensity produced
under the same conditions still had a value of 54.1, indicating retention
of most of its activity. The intensity scale is a logarithmic absorptivity
scale read from the transfer image using a microdensitometer. The paper
continued to produce strong images even after several months.
Attempts to write with a ball-point pen on the microcapsule-coated side of
a transfer sheet made with capsules similar to those described above but
without the clay were generally unsatisfactory. Considerable skipping and
interruption of the ink line were observed in almost every case. In some
cases, the pen stopped writing completely and had to be cleaned by writing
on uncoated paper before the ink flow would resume. Fine point ball-point
pens were particularly susceptible to this type of contamination. In
contrast, the ball-point pen writability of the clay-capsule coated paper
made by the above-described procedure was excellent. The ball-point pen
produced an unbroken ink line and continued to write, even when
considerable pressure was used.
EXAMPLE II
This example further illustrates the one-step coating process for making
the transfer sheet of this invention wherein a binder and a cross-linking
agent are incorporated in the coating composition.
250 Grams of a 6 percent aqueous solution of a medium molecular weight, 88
percent hydrolyzed poly (vinyl alcohol) (Dupont Elvanol 50-42) were used
to emulsify an oily solution comprising 2.1 grams crystal violet lactone
1.8 grams of benzoyl leuco methylene blue, eight grams of a polyfunctional
isocyanate (Mobay CB-75), four grams of diethyl phthalate and 90 grams of
an alkylated aromatic oil. Emulsification was effected by subjecting the
mixture to high-shear mixing at 25.degree. C. for 1 minute. The resulting
emulsion was diluted with 116 grams of water and cured at 60.degree. C.
for two hours, after which time suspended microcapsules were formed. The
microcapsules ranged in size between 1 and 7 microns, with an average size
of 4-5 microns.
40 Grams of these microcapsules, still suspended in water, were mixed with
15 grams of a paper coating grade calcium carbonate, 7.5 grams of water,
0.6 grams of a wet strength resin (Cyanamide Parez 613) and 2.1 grams of
50 percent aqueous glutaraldehyde for 5 minutes at about 25.degree. C. The
resultant composition was coated on a paper substrate at a coat weight of
six to eight pounds per ream (3300 square feet). The resulting carbonless
transfer paper sheet gave an image intensity of 65-70 on an underlying
copy sheet, based on an arbitrary intensity scale. After heating the
transfer sheet for three hours at 100.degree. C., the image intensity
produced under the same conditions still had a value of 55-60, indicating
retention of most of its activity. The intensity scale is as defined in
Example I. The paper continued to produce strong images even after several
months.
Attempts to write with a ball-point pen on the microcapsule-coated side of
a transfer sheet made with capsules similar to those described above but
without the calcium carbonate were generally unsatisfactory. Considerable
skipping and interruption of the ink line were observed in almost every
case. In some cases, the pen stopped writing completely and had to be
cleaned by writing on uncoated paper before the ink flow would resume.
Fine point ball-point pens were particularly susceptible to this type of
contamination. In contrast, the ball-point pen writability of the calcium
carbonate-capsule coated paper made by the above-described procedure was
excellent. The ball-point pen produced an unbroken ink line and continued
to write, even when considerable pressure was used.
EXAMPLE III
This example still further illustrates the one step coating process for
making the transfer sheet of this invention wherein a binder and a
cross-linking agent are incorporated in the coating composition and
wherein the microcapsules are formed by coacervation.
The microcapsules are prepared by the coacervation process used to form
microcapsules containing an oily solution of crystal violet lactone from a
gelatin-gum arabic emulsion as follows:
At 50.degree. C. a solution of twenty grams of gum arabic dissolved in 160
grams of water is used to emulsify a solution of 2.1 grams of crystal
violet lactone and 1.8 grams of benzoyl leuco methylene blue in 80 grams
of alkylated naphthalene oil by shearing in a Waring blender. The emulsion
is mixed with a solution of 20 grams of gelatin (isoelectric point pH8) in
160 grams of water at 50.degree. C. The pH is adjusted to 8 with 10
percent sodium hydroxide and the emulsion is diluted with 500 grams of
water at 50.degree. C. after which the pH is slowly adjusted back to pH
4.5 with 10 percent acetic acid. The mixture is kept continuously agitated
throughout all of these operations. 5 grams of 37 percent aqueous
formaldehyde then are added, and the mixture then is cooled to 10.degree.
C., with agitation over a one-half hour period. The pH finally is adjusted
to 9 with 10 percent sodium hydroxide.
80 Grams of these microcapsules are mixed with 25 grams of kaolin clay, 200
grams of 5 percent poly(vinyl alcohol) (Dupont Elvanol 50-42) and 3 grams
of 50 percent aqueous glyoxal for 5 minutes at about 25.degree. C. The
resultant composition is coated on a paper substrate at a coat weight of 4
to 5 pounds per ream (3300 square feet). The resulting carbonless transfer
paper sheet gives an image intensity of 65-70 on an underlying copy sheet,
based on the arbitrary intensity scale of Example I.
Attempts to write with a ball-point pen on the microcapsule coated side of
a transfer sheet made with complex coacervate capsules similar to those
described above but without the clay were generally unsatisfactory.
Considerable skipping and interruption of the ink line were observed in
almost every case. In some cases, the pen stopped writing completely and
had to be cleaned by writing on uncoated paper before the ink flow would
resume. In contrast, the ball-point pen writability of the clay-capsule
coated paper made by the above-described procedure was excellent. The
ball-point pen produced an unbroken ink line and continued to write, even
when considerable pressure was used.
EXAMPLE IV
This example illustrates the two-step coating process for making the
transfer sheet of this invention wherein a binder without a cross-linking
agent is incorporated in the coating composition.
250 Pounds of a 6 percent aqueous solution of a medium molecular weight, 88
percent hydrolyzed poly(vinyl alcohol) (Dupont Elvanol 50-42) were used to
emulsify an oily solution comprising 2.1 pounds crystal violet lactone,
1.8 pounds of benzoyl leuco methylene blue, 8 pounds of a polyfunctional
isocyanate (Mobay CB-75), 4 pounds of diethyl phthalate and 90 pounds of
an alkylated aromatic oil. Emulsification was effected by subjecting the
mixture to high-shear mixing at 25.degree. C. for 1 minute. The resulting
emulsion was diluted with 116 pounds of water and cured at 60.degree. C.,
for 2 hours after which time, suspended microcapsules were formed. The
microcapsules ranged in size between 1 and 7 microns, with an average of
about 5 microns, 4.9 pounds of the microcapsules per ream (3300 square
feet) were coated on a 33 pound per ream paper substrate.
A clay composition was prepared by mixing 100 pounds kaolin paper coating
clay and 200 pounds of a 7 percent solution of poly(vinyl alcohol) Airco
Vinol 125. The resultant composition was overcoated upon the microcapsule
coated paper coated at a coat weight of 2.1 pounds per ream (3300 square
feet). The resulting clay-coated transfer paper sheet gave an image
intensity of 43.7 on an underlying copy sheet, based on the arbitrary
intensity scale set forth in Example I.
Attempts to write with a ball-point pen on the microcapsule-coated side of
a transfer sheet made as described above but without the clay, were
generally unsatisfactory. Considerable skipping and interruption of the
ink line were observed in almost every case. In some cases, the pen
stopped writing completely and had to be cleaned by writing on uncoated
paper before the ink flow would resume. Fine ball-point pens were
particularly susceptible to this type of contamination. In contrast, the
ball-point pen writability of the clay-capsule coated paper was excellent.
The ball-point pen produced an unbroken ink line and continued to write,
even when considerable pressure was used.
After heating the clay-coated sheet for 3 hours at 100.degree. C., the
image intensity was below 10 due to oil slowly leaching through the
capsule walls. The same result was obtained after the clay-coated paper
was allowed to stand for several weeks at normal room temperature.
EXAMPLE V
This example further illustrates the two-step coating process for making
the transfer sheet of this invention wherein a binder and a cross-linking
agent are incorporated in the coating composition.
250 Pounds of a 6 percent aqueous solution of a medium molecular weight, 88
percent hydrolyzed poly(vinyl alcohol) (Dupont Elvanol 50-42) were used to
emulsify an oily solution comprising 2.1 pounds crystal violet lactone,
1.8 pounds of benzoyl leuco methylene blue, 8 pounds of a polyfunctional
isocyanate (Mobay CB-75), 4 pounds of diethyl phthalate and 90 pounds of
an alkylated aromatic oil. Emulsification was effected by subjecting the
mixture to high shear mixing at 25.degree. C. for 1 minute. The resulting
emulsion was diluted with 116 pounds of water and cured at 60.degree. C.
for 2 hours, after which time suspended microcapsules were formed. The
microcapsules ranged in size between 1 and 7 microns, averaging about 5
microns, 4.5 pounds of the microcapsules per ream (3300 square feet) were
coated on a 33 pound per ream paper substrate.
A clay composition was prepared by mixing 100 pounds kaolin paper coating
clay and 32 pounds of a 50 percent solids styrene-butadiene latex binder
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