Concentrated microcapsule suspensions for reaction copying papers

A process for the production of aqueous suspensions containing from 35 to 60% by weight of microcapsules by interfacial polyaddition from polyisocyanates and H-active compounds, wherein PA0 (a) an isocyanaurate-modified aliphatic polyisocyanate is used as the polyisocyanate, and PA0 (b) the suspension is adjusted to a pH-value of .ltoreq.7 after the polyaddition reaction, which microcapsules optimally contain dye precursors and can be used for the production of completely or partly coated copying systems by the rotogravure or flexograph processes.

This invention relates to a process for the production of concentrated 
microcapsule suspensions by interfacial polyaddition for polyisocyanates 
and H-active compounds which is characterised in that 
isocyanurate-modified aliphatic polyisocyanates are used as the 
polyisocyanate and in that the aqueous suspension is adjusted to a 
pH-value of .ltoreq.7 immediately after the polyaddition reaction. 
More particularly, the present invention relates to concentrated 
microcapsule suspensions containing dye precursors in encapsulated form 
and to their use for the production of partly coated systems by the 
rotogravure and flexograph processes. 
The production of microcapsules by interfacial polyaddition using 
polyisocyanates is known, for example from German Offenlegungsschrifts 
Nos. 21 90 921; 22 42 920; 23 42 066 and 23 11 712. 
Particular commercial interest is attributed to the encapsulation of leuco 
dyes for the production of reaction copying papers, as described in German 
Offenlegungsschrifts Nos. 25 37 982; 24 34 406 and 21 09 335 and in U.S. 
Pat. No. 3,900,669, for example. 
Reaction copying papers are also known (cf. M. Gutcho, Capsule Technology 
and Microencapsulation, Noyas data Corporation, 1972, pages 242-277; G. 
Baxter in Microencapsulation, Processes and Applications, published by J. 
E. Vandegaer, Plenum Press, New York, London, pages 127-143). 
Reaction copying papers preferably consist of two or more sheets of paper 
placed loosely on top of one another, the upper sheet being coated 
underneath with a donor layer and the lower sheet being coated on top with 
a receiving layer. In other words, a donor layer and a receiving layer are 
in contact with one another. The donor layer contains microcapsules, of 
which the core material is a solution of a dye-forming compound in an 
organic solvent, whilst the receiving layer contains a material which 
develops the dye-forming compound to form the dye. 
When copying papers of this type are written on, the capsules are destroyed 
under the high pressure of the writing instrument and the core material 
which flows out impinges on the receiving layer so that a copy is formed. 
The receiving layer generally contains binders and pigments, for example 
absorbents, such as kaolin, attapulgite, montmorillonite, bentonite, acid 
fuller's earth or phenolic resins. For example, acid-activatable dyes may 
be used in the donor layer whilst acid-reacting components may be used in 
the receiving layer. 
The donor layer is generally applied to the paper substrate over its entire 
surface by conventional methods, for example as described in German 
Offenlegungsschrift No. 19 34 457 or 19 55 542, in other words aqueous 
coating compositions are required. The water has to be removed after 
coating and thus, the amount of energy consumed is greater, the lower the 
concentration of microcapsules in the coating composition. 
Donor layers covering the entire surface of the paper substrate are 
generally unnecessary because in many cases only parts of the copying 
system are written on. 
Accordingly, attempts have been made to apply donor layers to a part of a 
paper substrate. Thus, aqueous coatings have been applied to part of a 
paper substrate by the rotogravure or flexograph process (German 
Offenlegungsschrift No. 25 41 001; U.S. Pat. Nos. 3,016,308 and 
3,914,511). Aqueous flexograph pastes are very highly concentrated and 
therefore microcapsule dispersions for this process have first to be 
concentrated. 
The amount of energy consumed in the production of the microcapsule 
dispersions themselves, based on the quantity of microcapsules, is again 
greater, the lower the content of capsules, because the entire dispersion 
always has to be intensively heated or cooled. 
Hitherto, it has not been possible by interfacial polymerisation directly 
to produce microcapsule suspensions which have a capsule content of more 
than 35% by weight and, at the same time, a low viscosity. The reason for 
this is said to be that the polyfunctional wall-forming monomers or 
oligomers in the individual droplets of the dispersed organic phase and 
the similarly polyfunctional reactants in the continuous phase enter into 
agglomerate-forming polymerisation reactions between the individual 
droplets where the content of organic phase is high. 
In general, dispersions produced by interfacial polymerisation have to be 
thermally aftertreated for prolonged periods because, in view of the first 
capsule wall formation, the reactants are separated from one another and 
can only be made to react further at elevated temperature. 
This thermal aftertreatment also increases agglomerate formation. On the 
other hand, only a fully reacted capsule wall would appear to have all the 
properties required for practical application. 
The present invention is based on the discovery that concentrated 
suspensions containing from 35 to 60% by weight of microcapsules can be 
obtained by the polyaddition process provided that isocyanurate-modified 
aliphatic polyisocyanates are used as the polyisocyanates and provided 
that the suspensions are adjusted to a pH-value of 7 or lower immediately 
after the polyaddition reaction. 
Accordingly, the present invention provides a process for the production of 
aqueous suspensions containing from 35 to 60% by weight of microcapsules 
by interfacial polyaddition from polyisocyanates and compounds containing 
active hydrogen which is characterised in that an isocyanurate-modified 
aliphatic polyisocyanate is used as the polyisocyanate and in that the 
suspension is adjusted to a pH-value of 7 or below immediately after the 
polyaddition reaction. 
More particularly, the present invention relates to the production of 
highly concentrated microcapsule dispersions containing dye precursors in 
encapsulated form and to the use of these suspensions for producing partly 
coated copying systems by the rotogravure and flexograph processes 
printing 
The aliphatic isocyanurate-modified isocyanates used in the process 
according to the invention based on 1,4-xylylene diisocyanate, 
trimethylene diisocyanate, tetramethylene diisocyanate, 
propylene-1,2-diisocyanate, butylene-1,2-diisocyanate, ethylidene 
diisocyanate, cyclohexyl-1,4-diisocyanate, m-xylylene diisocyanate, 
hexamethylene-1,6-diisocyanate, 4,4'-diisocyanato-dicyclohexyl methane and 
isophorone diisocyanate. 
Processes for the production of isocyanurate-modified polyisocyanates 
containing more than 2 terminal isocyanate groups are already known. The 
production of isocyanurate-modified polyisocyanates based on hexamethylene 
diisocyanate is specifically described in German Offenlegungsschrift No. 
28 39 133. The others can be similarly obtained. To produce the 
microcapsules by polyaddition, the isocyanate may first be dissolved in 
the core material of what will subsequently become the capsule and the 
resulting organic phase emulsified in the continuous aqueous phase 
containing protective colloid and, optionally, emulsifiers. An aqueous 
polyamine solution is added to the resulting emulsion in a stoichiometric 
quantity, based on the polyisocyanate in the organic phase. 
The hydrophobic core materials to be encapsulated may be, for example, 
synthetic oils, such as alkyl naphthalenes, alkylated diphenyls, alkylated 
diphenyl alkanes, hexahydro-terphenyl, triaryl dimethanes, chlorinated 
paraffins, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, 
dibutyl maleate, toluene, dichlorobenzene or benzyl alcohol, or natural 
oils, such as cottonseed oil, soya bean oil, corn oil, for example maize 
oil, castor oil fish oil or lard. 
In addition, the core material may be a synthetic and/or natural perfume 
oil or a solution thereof, an adhesive or an adhesive solution (for 
example acrylates, methacrylates, polyvinyl acetates) or a solution of 
natural rubber. 
It is also possible to encapsulate plant protection agents and their 
solutions, for example various fungicides, insecticides and herbicides. 
Encapsulated plant protection agents reduce the acute toxicity of the 
active components during handling and also prolong the duration of their 
activity. 
Solutions of dye precursors are preferably used as the core material in the 
process according to the invention. 
Examples of the dye precursors are triphenyl methane compounds, diphenyl 
methane compounds, xanthene compounds, thiazine compounds and spiropyrane 
compounds. Suitable triphenyl methane compounds are 
3,3-bis-(p-dimethylaminophenyl)-6 dimethylaminophthalide ("crystal violet 
lactone" or "C.V.L.") and 3,3-bis-(p-dimethylaminophenyl)-phthalide 
("malachite green lactone"). Suitable dimethyl methane compounds are 
4,4'-bis-dimethylamino-benzhydryl benzyl ether, N-halogen phenyl 
leucolamine, N-.beta.-naphthyl leucolamine, N-2,4,5-trichlorophenyl 
leucolamine, N-2,4-dichlorophenly leucolamine. Suitable xanthene compounds 
are rhodamine-.beta.-anilinolactam, 
rhodamine-.beta.-(p-nitroaniline)-lactam, 
rhodamine-.beta.-(p-chloroaniline)-lactam, 7-dimethylamine-2-methoxy 
fluorane, 7-diethylamine-3-methoxy fluorane, 7-diethylamine-3-methyl 
fluorane, 7-diethylamine-3-chloroflorane, 7-diethylamine-3-chloro-2-methyl 
fluorane, 7-diethylamine-2,4-dimethyl fluorane, 
7-diethylamine-2,3-dimethyl fluorane, 
7-diethylamine-(3-acetylmethylamine)-fluorane, 7-diethylamine-3-methyl 
fluorane, 3,7-diethylamine fluorane, 
7-diethylamino-3-(dibenzylamine)-fluorane, 
7-diethylamine-3-(methylbenzylamine)-fluorane, 
7-diethylamine-3-(chloroethylmethylamino)-fluorane, 7-diethylamine-3 
(dichloroethylamine)-florane and 7-diethylamine-3-(diethylamine)-fluorane. 
Suitable thiazine compounds are N-benzoyl leucomethylene blue 
o-chlorobenzoyl leucomethylene blue, p-nitrobenzoyl leucomethylene blue. 
Lastly, a suitable spiro compound is 
3-methyl-2,2-spiro-bis-(benzo(f)-chromene). 
Solvents which dissolve these dye-forming compounds are, for example, 
chlorinated diphenyl, chlorinated paraffin, cottonseed oil, peanut oil, 
silicone oil, phthalate esters, phosphate esters, sulphonate esters, 
monochlorobenzene, also partially hydrogenated terphenyls, alkylated 
diphenyls, alkylated naphthalenes, aryl ethers, aryl alkyl esters, 
higher-alkylated benzene and others which may be used either individually 
or in combination. 
Diluents such as, for example, kerosene, n-paraffins and isoparaffins are 
frequently added to the solvents. 
To produce the microcapsules by polyaddition, the isocyanate may be 
dissolved in the hydrophobic core materials mentioned above and the 
resulting organic phase emulsified in the continuous aqueous phase which 
contains protective colloid and, optionally, emulsifiers. An aqueous 
polyamine solution may be added to the resulting emulsion in a 
stoichiometric quantity to the polyisocyanate in the organic phase. 
Diamines suitable for reaction with the isocyanates mentioned above are 
aliphatic, primary or secondary polyamines. such as for example 
1,2-ethylene diamine, bis-(3-aminopropyl)-amine, hydrazine, 
hydrazine-2-ethanol, bis-(2-methylaminoethyl)-methylamine, 
1,4-diaminocyclohexane, 3-amino-1-methyl-aminopropane, N-hydroxyethyl 
ethylene diamine, N-methyl-bis-(3-aminopropyl)-amine, 
1,4-diamino-n-butane, 1,6-diamino-n-hexane, 1,2-ethylene diamine-N-ethyl 
sulphonic acid (in the form of an alkali salt); 1-aminoethyl-1,2-ethylene 
diamine and bis-(N,N'-aminoethyl)-1,2-ethylene diamine. Hydrazine and its 
salts are also regarded as diamines in the present context. 
To emulsify and stabilise the emulsion formed, protective colloids and 
emulsification aids are added to the aqueous phase. Example of such 
products acting as protective colloids are carboxy methyl cellulose, 
gelatin and polyvinyl alcohol. Examples of emulsifiers are ethoxylated 
3-benzyl hydroxy biphenyl, reaction products of nonyl phenol with 
different quantities of ethylene oxide and sorbitan fatty acid esters. 
The microcapsules may be produced continuously or in batches. Dispersion 
machines capable of generating a shear gradient are generally used. 
Example of machines such as these are paddle, rotor-cage and high-speed 
stirrers, colloid mills, homogenisers, ultrasonic dispersers, jets, steel 
jets and supraton machines. The intensity of the turbulence generated 
during mixing is the major determining factor for the diameter of the 
microcapsules obtained. It is possible to produce capsules ranging from 1 
to 2000 .mu.m in diameter are preferred. 
The capsules do not agglomerate and have a narrow particle size 
distribution. The ratio by weight of core material to shell material is 
from 50-90 to 50-10. 
In the process according to the invention, the formulations for producing 
the microcapsules are adjusted in such a way that suspensions containing 
from 35 to 60% by weight of capsules are obtained 
After the amines have been added, the suspension is adjusted with an acid 
to a pH-value of 7 or lower. For example, it is possible to use aqueous 
mineral acids, for example hydrochloric acid, and also organic acids, such 
as acetic acid, oxalic acid and others. It is also possible, for example, 
to use acid anhydrides or acid-reacting, gaseous compounds, for example, 
carbon dioxide in particular. 
After another brief aftertreatment, the highly concentrated suspension may 
be directly used for the particular purpose envisaged. 
The suspensions may be converted into agglomerate-free capsule powders, for 
example by spray drying. Spray drying is particularly rational with 
suspensions as concentrated as these because the amount of water which has 
to be evaporated is comparatively small. Microcapsule dispersions 
containing dye precursors may be used for example for the production of 
carbonless copying papers. 
To this end, the suspensions according to the invention are provided with 
binders and spacers and coated onto a web-form paper substrate. The 
production of spreading dyes of this type has long been known. 
However, highly concentrated spreading compositions may advantageously be 
used in the process according to the invention and the drying of the 
coating compositions made more rational. 
In particular, the process according to the invention makes it possible to 
use other coating techniques, for example, coating with blade coaters or 
by means of gravure rolls. Although coating techniques such as these are 
well known, they have never been rationally used for capsule-containing 
coating compositions. These coating techniques enable the coating 
compositions to be applied considerably more quickly than for example, 
air-knife coating, as has hitherto normally been used in the production of 
carbonless copying paper. 
Coating with gravure rolls is already a modified, wet "rotogravure" 
process. 
By virtue of their high capsule content, the suspensions according to the 
invention may also be directly used for the production of aqueous 
flexograph pastes without any need for expensive concentration. The 
production of capsule-containing flexograph pastes and the production of 
completely or partly coated copying papers by the flexograph process 
printing are already known. The advantage of using the suspensions 
according to the invention lies in the fact that there is no longer any 
need for the hitherto essential concentration steps. 
Suitable reactants for the dye precursors in the microcapsules are dye 
developers known per se, for example, acid clays, montmorillonite, 
bentonites and smectites or synthetic phenolic resins. 
In copying sets, the donor component is generally the back of the top 
sheet. The front of the next sheet is coated with the reaction component. 
This layer is known as the receiving component. In copying sets, the 
receiving component is the top of the second sheet of paper. In the case 
of multiple copying sets, the following donor sheets have to carry a 
receiving layer on the other side. The production of receiving layers such 
as these is known and is also described in German Offenlegungsschrifts 
Nos. 19 34 457 and 19 55 542.

The process according to the invention is further illustrated by the 
following Examples. 
EXAMPLE 1 
300 g of isocyanurate-modified hexamethylene diisocyanate 
(NCO-content=20.5%, monomeric hexamethylene diisocyanate content &lt;0.7%) 
are dissolved in 1700 g of toluene. This organic phase was emulsified by 
means of an ultrasonic pipe in 1605 g of an aqueous phase containing 2% of 
88% hydrolysed polyvinyl acetate, the droplet size of the emulsion being 
adjusted to around 7 .mu.m. 50.4 g of diethylene triamine dissolved in 
344.6 g of water were then added. After addition of the amine, the 
suspension was adjusted to a pH-value of 6.5 by the dropwise addition of 
10% hydrochloric acid, followed by tempering for 1 hour at 60.degree. C. 
Determination of the dry matter content of the suspension thus obtained 
revealed a capsule content of 48.5% by weight. 
The suspension had a Brookfield viscosity of 240 mPas, as measured at 100 
r.p.m. 
EXAMPLE 2 
22.5 g of isocyanurate-modified isophorone diisocyanate (NCO-content 18%) 
are dissolved in 127.5 g of xylene. Following the addition of 133 g of 
water in which 2% by weight of 88% hydrolysed polyvinyl acetate were 
dissolved, the product was emulsified using a Kotthoff mixing siren (8900 
r.p.m.). 
3 g of ethylene diamine in 47 g of water were then added. 
After addition of the amine, a pH-value of 7 was adjusted by the addition 
of 5% aqueous sulphuric acid. The suspension was then heated to 50.degree. 
C. and left at 50.degree. C. for 2 hours. 
A suspension was obtained having a measured capsule content of 45% by 
weight the average capsule diameter was 10 .mu.m. 
EXAMPLE 3 
46 g of crystal violet lactone and 15 g of N-benzoyl leucomethylene blue 
were dissolved in 1372 g of diisopropyl naphthalene and 328 g of 
isohexadecane. 300 g of the isocyanurate-modified hexamethylene 
diisocyanate of Example 1 were added to this core material. This organic 
phase was emulsified by means of an ultrasonic pipe in 1605 g of water 
containing 2% by weight of 88% hydrolysed polyvinyl aceate in solution, 
and the size of the droplets was adjusted to 7 .mu.m. 
65 g of isophorone diamine and 19 g of hydrazine hydrate in 311 g of water 
were then added. After addition of the amine, carbon dioxide was passed 
with stirring through the suspension until a pH-value of 6.9 was reached. 
The slurry was then heated to 60.degree. C. and stirred for 2 hours at 
that temperature. Determination of the dry matter content of the slurry 
revealed a capsule content of 49.5%. The slurry has a Brookfield viscosity 
(cf. Example 1) of 370 mPas. 
EXAMPLE 4 
The procedure was as described in Example 3, except that after addition of 
the amine, a 10% aqueous hydrochloric acid solution was added to the 
suspension instead of carbon dioxide. The slurry was aftertreated as 
further described in Example 3. The resulting suspension had a measured 
capsule of 49% and a viscosity of 170 mPas. 
EXAMPLE 5 
The microcapsule dispersions were coated with a 30 .mu.m wire doctor onto a 
paper substrate and a cover sheet of a carbonless copying paper was 
produced in this way. 
A sample of the paper thus produced was placed with its coated side on a 
receiving paper, after which another 7 sheets of paper were placed on top. 
Using a typewriter adjusted to a constant striking pressure, the letter 
"w" was typed on as closely as possible over an area measuring 4.times.4 
cm. The copy of the paper sample visible on the lower receiving paper is 
examined for its clarity of impression by measuring the loss of reflection 
against a clean sheet of paper using a remission gauge (Zeiss Elrephomat). 
The following clarity of impression was measured: 
______________________________________ 
Remission value: 
______________________________________ 
Example 3 40.6% 
Example 4 37.0% 
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EXAMPLE 6 
A flexograph paste was prepared as follows using the suspension described 
in Example 3: 
20 parts of shellac are stirred with 5 parts by weight of ammonia and 15 
parts of water and then heated, hydrolysed and made soluble in water. 60 
parts of the suspension described in Example 3 are stirred into this 
solution. 
The paste was partly applied to a paper substrate by means of a soft rubber 
sheet in a flexograph machine. The rate of the coating amounted to 
approximately 6 g/m.sup.2. 
Measurement of the clarity of impression in the same way as described in 
Example 5 produced a remission value of 25%.