CA1067761A - Capsules with a shell of water-soluble polymer cross linked by precondensates - Google Patents

Abstract

ABSTRACT
A method of encapsulating finely divided solid or liquid particulate material to produce microcapsules in which small particles or drops of the material are surrounded by polymer shells, wherein, dispersion of the material is prepared in an aqueous medium containing a water-soluble urea-formaldehyde precondensate, a water soluble melamine-formaldehyde precondensate and a water soluble polymer which is capable of being cross-linked by said precondensates, and condensing said precondensates by acid catalysis with resultant cross linking of said polymer about said particulate material, thereby to form said polymeric shells.

Description

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mis invention relates to a microcapsules and their ~Production.
Microcapsules afford a convenient means of holding reactive or v~latile materials in readiness for subsequent llCP. The encapsulated materials retain their prDperties until-the article to which they are 5 applied is used. Amongst the materials which it has been proposed to encapsulate are flavourings, adhesives, sealants, drugs, fertiIizers and dyes.
: Hbwever, the st widespread use of micm capsules to date has probably been in certain kinds of pressure sensitive copying systems.
In one such system, usually kncwn as a transfer system, an upper sheet ;
is coated on its lower surface with microcapsules aontaining a solution of oolourless colour former,. and a lower sheet is ooated on its upper surface with a colour developing co-reactant material, e.g. an acidic clay, a phenolic resin or certain organic salts. ~or most applications, a number of inter~ediate sheets are also provided, each of which is .
ooated on its lower surface.with microcapsules and on its upper surface with acidic material. P.ressure exerbed on the sheets by writing or typing ruptures the microcapsules, thereby releasing the aolour fonmer ~ .
solution on to the acidic material on the next lower sheet and giving ~ 20 rise to a chemical reaction which develops the aolour of the colour ~ ~.
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~ In another such system, usually know.n as a self-oontained system," .
microcapsules and:colour developing oo-reactant mat~rial are coated on to the same surfaoe of a sheet of paper, and writing or tyPing on a :~
~ 25 sheet plaoed above the coated sheet causes the capsules to rupture and :~ release the oolour former, which then reacts with the oo-reactant ...:: .. .
material on the sheet to produce a oolour.
~: Hitherto, the st oommonly u æd method of naking microcapsules has been by coaoe rvation from an aqeous solution of hydrophilic oolloids, such as gelatin, gum arabic Polyvinyl~methyl ether~raleic anhydride . ~
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oopolymer and carkoxymethyl cellulose. However, coacervation techniques have a number of disadvantages. Firstly, encapsulation can only be ` carried out using an aqueous solution of low solids content, bPcause ooacervation only occurs at low colloid con oe ntrations and because at S high solids contents the viscosity of the system tends to beoome un-workably high. In consequence,-encapsulation by ooa~ervation techniques ` has been carried out typically in the past at from 18 to 23% solids oontent. SecDndly, hydrophilic oolloids tend to be expensive, parti-cularly gelatin which is the material mDst oommanly used. Thirdly, for optimum operation, the characteristics and purity of the oolloids used are critical and have to be precisely specified. For example a high salt content ~LSt not be present as this would inh~bit ooa oe rvation.
Fourthly, since many of the hydrophilic colloids used are natural products or derivatives of natural products, they tend to bo attacked by ~~ 15 micro-organisms. This li~its the time for which the colloids can be r' stored, both as raw materials prior to ooaoervation and as finished capsules. Fifthly, the colloidal walls of capsules made by ooa oe rvation may not ke oompletely impermeable, which nay result in scme cases in gradual leaching out of capsule aontents. m is may se~etimes ke so . 20 serious as to prevent the enaapsulati~n of oertaLn materials. Sixthly, ; ~ the future availability at reasonabIe oost of na~lral materials such as ~?; gelatin and gu~ arabic has on occasions appeared uncertain.
Despite the disadvantages listed abcve, ooacervation techniques have been widely and succesffully used for encapsulating oily solutions of ¢olourless oolour formers for use in pressure sensitive oopying ~i papers. Hbwever, the use of in situ polymerization techniques for ~` making microcapsule walls oould theoretioally overoame many of the i ~ .
disadvantages~of ooacervation techniques. ffl r example, higher wall material ooncentrations are theoretically usable because the prablem ; "
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of coaoervation inhibition dces not arise. MbreoNer, amunDplast wall material, which are inherently suited to in situ polymerization techni-ques, are generally cheap corp~rcd with the hydrophilic oolloids used hitherto. A further advantage is that they-are likely to continue to be widely available and that their characteristics need not be specified 90 ~-~
p w isely as that of hydrophilic colloids for use in coacervation techniques. Synthetic wall materials are also generally less susceptible to attack by micrororganisms,~and capsules move from them are potentially less likely to allcw leaching out of microcapsule contents than are microcapsules made using naturally occurring wall materials such as ~, :''', gelatin.
It is not surprising therefore that a number of proposals for in ~ situ polymerization techniques have been made. Hbwever, altou~h capsules !. have bean made on a oommercial scale by an in situ polymerization tebhnique for a nunber of years now, ooaoervation techniques have remained domin~nt.
An early propo&al for an in situ polymerization technique is to be found in Example r~ of U.S. Patent 3fO16,308. Liquid to be encapsulated is dispersed in an aqueous solution of a water soluble urea for~aldehyde pre<ondensate and further oondensatian and s~bsequent precipitation of ; urea formaldehyde resm is brought about by the addition of hydrochloric , acid. A small amount of carboxymethyl cell~ se is present, presumably as an emulsifying agent.
Another technique using urea and fonmaldehyde (or a urea fonmalde-hyde pre-polymer) is-described in V.S. Patents 3,516,846 and 3,516,941.
mese stress the inportanoe of-careful con~luI of the acidification step if capsules having aoceptable prqperties are to be obtained. These ~-; patents also asseLt that a wetting agent such as c~rboxymethyl oellulose is inoompatible with the fonmation of useful capsules.

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All three U.S. Patents ~entioned above stress the Lmportanoe of rapidly agitating the dispersion during the condensation reaction. If this is not done, the dispersion de-stabli æs, i.e. the droplets ooalesce.
mis tendency for ooaleæcence to occur mRkes it very difficult to oDntr~l drcplet size (ianl hence macrocapKule size) and also prevents ; very s~all micr~capsules being~produced other than in very tiny a~ounts.
Reproducible control of droplet size is very important in achieving a pressure sensitive oopying paper of reproducible copying capability.
Mbreover, the need to maintain the dispersion in existence by rapid ~ 10 agitation conEumes large a~ounts of energy and hence adds to production oosts.
~; Proposals for overocrdng the problems of poor dispersion stability have been made in British Patents 1,I56,725, 1,301,052, iand 1,355,124, : .
all of which disclose the use of reactive surfaae ac*ive agents or "tensides~. These are polyner precursors, for example amincplast pre-cursors, which are chemirally mcdified so as to be surface active.
They thus offer the potential of forming a mare stable dispersion of drcplets to be encapsulated. The ahemical nodification wDuld however . appear to increase the oost of the precurscrs oompared with the un-mcdified m2terials. So far as we are aware, capsules nade using re-aotave surfaoe active agents as described in British Patents 1,156,725, , ~ 1,301,052 and 1,355,124 have not been used on~a oommer~iAl scale.
A ~ifferent a~proach to mz~ing capsules having walls of fonmaldehyde ... . .
oondensation products invDlves the use of aqueous thermDsetting resins which on dilution with water have the prcperty of separating out of solution to fonn a polymeric phase. Such a dilution step wust limit the ~ solids content at which microoapsuies can be prepared. MDreover, since '~ dilution is emphasized~as being the finalrprooess operation, it is not clear to what extent the resins will "set" bD fonm imlen~eable capsule walls, particularly sinoe the process dbes not apparently in~Dlve '' ' . ~ . .

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the use of a hardening agent or of pH adjustment. It also seems likely that adequate stirring of the dispersion before addition of the dilution water will be difficult, in view of the likely high visaosity of a ~. .
"resin syrup".
et another approach to the production of synthetic walled micro-capsules has been by so-called "interfacial polymerization" techniques.
ese invDlve tw3 reactive polymer precursors, one of which is present ' in droplets of the material to be encapsulated and the other of which is present in the medium in which the droplets are disper~ed. me pre- ~

cursors then react at the interfaces between the droplets and the medium ;
to form the microcapsule walls. A disadvantage of such a technique is ; that once a very thin "skin" has formed at the boundary of the droplet, .. " . .
the precursors are separated from one another and further reaction is inhibited. mis means that it is difficult to form walls of adequate strength. ~roposals have been made for overaoming this problem but, so .~, . . .
far as we are aware, they have not been sucoessful to an extent suffi-cient to displaoe the microcapsules manufacturing techniques hitherto used commercially.
It is an object of the present invention to provide a method of microcapsule production which obviates at least in part some of the disadvantages of the aoacervation and other techniques described above.
Acaording to the invention, a method of encapsulating finely ;
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divided particulate material to produce microcapsules in which the particulate material is surrounded by polymeric shells, wherein a dispersion of said particulate material is prepared in an aqueous medium aontaining a water-soluble urea formaldehyde precondensate, is charac-terized by the steps of inaorporating in the aqueous medium a water-soluble melamine-formaldehyde precondensate and a water-soluble polymer which is capable of being cross-linked by said preaondensates, and ~-condensing said preaondensates by acid ,~ .
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catalysis with resultant cross-linking of said polymer-about said particulate material, thereby to form said polymeric shells. It is thought that the cn~ss-linking action of the preoondensates is primarily the result of the presenoe of methylol groups, but other groupings may possibly be involved.
me invention also ewbraoe s mircocapsules Produced by the method defined in the preoeding paragraPh and sheet material carrying such microcapsules. Such sheet material may be paper, and the microcapsules may be present as a coating or in the interstioe s between the fibres of the paper.
me finely divided particulate ~aterial may b~ a solid or it may be ninute drDplets of a water-immiscible liquid. In the latter case, the ., .
polymer must be dispersion stabilizing as defined hereafter.
e expression "water-inmiscible liquid" includes not only liquids which are absolutely inmiscible with water but also those which are largely inniscible with water bur nevertheless have a slight solubility.
m e water-soluble polymer preferably oontains alcohol, amine, amide, acid or acid derivative groups. Preferred examples of such polymers are cellulose derivatives e.g. carboxy methyl cellulose and -.
~ethylhydroxy proply cellulse, starch, a starch ~rivative, a polyacid, a polyester, a polyanhydride oopolymer-e.g. polyvinvl methyl ether/
maleic anhyrdide cop~}ymer or polyethylene naleic/anhydride oopol~mer a polyacrylamide or an acryLamide copolymer. A p3rticularly advantageous poly~er is an acrylamide/acrylic acid copolymer. Capsules made using such a ccpolymer have been found to-be particularly resistant to-ageing.
By a dispersion-st~bilizing poly~er is meant a polymer which in solutian is dispersian stabilizing on its own, or if that is not the case, is dispersion stabilizing-in the presenoe of at least ane of the preccndcnststcs. An example of the latter type-of polymer is the :- ,. -' ; '' ~ .
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preferred acrylamide/acrylic acid copoly~er, which is not dispersion ,~
stablizing on its own, but when muxed with the urea formaldehyde pre-; condensate will stabilize a dispersion. In order that the polymer is dispersion-stabilizing, the polymer is desirably charged, and is pre-ferably anionic.
whilst in principle the chemical characteristics of the preconden-sates are not critical, there are certain practical oonstraints on the particular materials to be used. Firstly, the ~reoondensates must be soluble in water. Secondly, some precondensates prevent the formation of a stable dispersion of the dkoplets, even in the presence of the ~-dispersion-stabilizing polym2r. me choioe of suitable preaondensates which avoid the problems just discussed does not present any difficulty to the experienced encapsulation-technologist.
; The urea formald~yde preoondensate is preferably cationic, and the melamine form~ldchyde-precondbnsate is preferably a methylated melamine formaldehyde preoondensate. Instead of a single melamine formaldehyde precondbnsate, a mixture of tWD or ~re such materials may be used.
SimiLarly, a mix*ure of two or more urea formaldehyde precondensates may be used.
If only polymer and urea formaldehyde preoondensate are used i.e.
` no melamine fornaldehyde^is present, capsules can still be formed, but~ they tend~to be too weak~to~withstand the drying operations inv~lved in ''A~ paper coating. Even if they can withstand such operations, it has been , . . ..
found that-they weaken unac oeptably on ageing.

The present metbod may be carried out in a number of ways. For ; exa0ple, the urea formaldehyde precondensate, the melamIne fo .. ~ldehyde precondensate and the water-soluble polymer may all be present in the aqueous medium before addition of the liquid to be encapsulated, and the acid for bringing about condensation of the precondensates may be added suboequfntly. Alternatively, however, only the urea formaldehyde .. . .

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precondensate and the water-soluble polymer may be present in the aqueous medium when the liquid to be encapsulated is added. m e melamine fonmaldehyde preoondensate is then added subsequently. -The stage at which acid is added is not cru:cial. It is preferred that the acid is added on oe the material to be encapsulated has been added, either before or after addition o the meLamine fonmaldehyde precondensate. It is of course important that excessive condensation of the precondensates dces not occur before the droplets are addbd, and that excessive candensation-of the urea formaldehyde precondensate dbes not occur before-addition of the mela~ine fonmaldehyde precondensate, if ;; the latter is added after the acid. Conditians under which such ex-; oe ssive aondensation is avDided, are readily detcsnined by experiment.
The optimum pH for the condensation and cross-linking reactions depends to scme extent on the precondensates and the water-soluble poly~er used. For-example, for the preferred acryIamide/acrylic acid ; oopDly~er a pH in the range 3.5 to 5.0 is preferred, more preferably 4.0 ;' to 4.5, for example, 4.15. However, for a different polymer, vinyle methyl ether/m21eic anhydride copolymer, a pH of 5.0 to 5.5 is pre-.,;
ferred. The acid used for pH adjustment is not critical, and may for example be acetic acid or hydrochloric acid. If the water soluble :! ' pDlymer has a high natural acidity, the amount of acid used for pH
;~ adjustment will be less, and oould possibly be zero. An example of a p~lymer having a high natural acidity is ethylene~raleic anhydride oopDlymer.
-~ 25 m e order in which the capsule wall materials are added influen oe s capsule p~oduction. At least certain melamine~formaldehyde precon-densates tend to derstablize a dispersion of matorial to be encapsu-~ s lated, which leads to coalescence of small droplets to fonm larger droplets. m us, if the melamine formaldehyde addition is delayed until ~` , ' ~: . ., .;

: 1067761 sore ti~e after acidification, less coalescence cccurs. The ooalescenoe-inducing effect of melamine fonmaldehyde precondensate affords a degree of control over the droplet size of the capsules. For example, it is possible to-add the melamine formaldehyde precondensate in small katches over a period of time, for example one hour. me m~re the number of separate additions, the greater the degree of coalescence cbserved.
For minimizing coalescence, the dispersion nay be chllled before addition of the melamine formaldehyde preaDndensate and before addition of acid for bringing ab~ut condensation. Chilling is preferably to . ..
; 10 below 20C, for example to 15C. Chilling to any temperature below 30C
has however been found to have scme effect.
Acidification may be followed by-maintaining the nuxture in a wanm state, for example at 55-C for twD hw rs. If the melamine formaldehyde preoondbnsate is added after addition of the material to be encapsula-ted, without a chilling step, it is preferred to adjust the pH of the ; dispersion before addition of-the melamine fonmaldehyde precondenFate , .: . .
- and then to maintain the dispersion at 55C for two hours after the 1 addition of the melamine forr~ldchyde preccndcns~te. Adjustment may be ....
in tw~ stages, one before and the other after addition of melamine ~ -formaldbhyde precondensate.
`~ If a temperature lower than 55C is used, capsules can still be obtainedj but their production takes~longer. For example, if the mix is maintained at 35C rather than 55C, the t~me~taken to produce equi-valent capsules may~be as much as fifteen hours. Temperatures greater than 55C may be used, for example up to 85C, ~ut such capsules have SD
far been found to be somewhat more su~oeptible to premature rupkure onoe they are ooated on tD a paper sheet.
After condensation and c~vss-linking of the wall ntaterials has oocurred, the pH of the dispersion maSy be raised to aLkalInity to stop further raction of the wall materials.

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m e capsule size is deter~ined, within limits, by-the size of the -d m plets being encapsulated, and thus the capsule size can be varied by suitably milling a mixture of the capsule contents and a solution of one or more of the capsule wall materials.
- 5 m e present method may p m duce single oil drop-capsules or capsule clusters, depending to a considerable extent on the materials and oonditions used. For exaTple, whereas the preferred acryla~ide/ acrylic acid copolymer gave predbminantly sinqle oil drop capsules, use of :: .
poly~inyl methyl ether/Taleic anhydride copolymer instead gave pre~
do~inantly capsule clusters. Both single oil drop capsules and capsule clus~rs an~ well known in the mic m encapsulation art, and so will not be described further herein.
` m e extent to which the dispersion of materials to ~e encapsulated is stirred also influences the type of capsule p m duced. In general, increasing s*irring results in production of a higher proportion of ~ single oil droplet capsules.
;~ The relative prcpartions of the-preoondensates and the polymer can ~ be varied quite considbrably whilst still cb~YI~Iing capsules, although ; functional p m perties of-the capsules are affected. An~Example illus-. .
trating the use of different relative proportions of the preornden~tes and the polymer is described subsequently.
é phase ratio employed is ~enerally around 6:1 if capsules for a transfer pressure sensitive system are reqyired, but this is not critical - ~the phase ratio is the weight ratio of the oil droplets bo cap d e wall ....
:j~ 25 ma~rial in the aqueous solution). ~ hiqher phase ratio nay be used if it is desired to encapsulate at a higher solids content. If it is desined to produce stron~er capsules, for example for use in a self-~Y~ contained pressune-sensitive copyin~ system, for which the capsules ....
; ~ should be more robust, a lower phase ratio sh3uld be used.

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The present method has so far been found to produce ca~sules at solids contents of at least 50% if the mnst preferred water-soluble polymers and the most ~referred precondensates are used.
It will be realized from the foregoing that the present invention inv~lves a number of variables, both in the wall ~aterials used and the choioe of process parameters, i.e. the experimental oonditions and the ~-sequenoe in which~the various prDcess steps are carred out. Por any particular com~ination of wall materials therefore, it is necessary to experiment to find the optin~n way of practising the invention so as to obtain capsules having good prop~rties. This experi~entation is in the nature of aoquiring expertise and kncw-how and will present little g difficulty to the skilled encapsulation technologist.
; In order to illusbr~te the production of microcapsul~s by the present method, reeerence will now be made to Figures l, 2 and 3 of the acooqpanying drawings which illustrate diagrammatically and by way of example three en}sdl~ents thereof, and in which:-Figure 1 depicts a pro~ess in which liquid to be encapsulated is added to an unacidified solution of urea formaldehyde preoondensate, mela~ine fonmaldehyde preoondensate and acrylamide/acrylic acid copolymer;
Figure 2 depicts a process in which liquid to be encapsulated is added to an acidified solution of urea formaldehyde preoondensate and acrylamide/acrylic acid copolymer; and Figure 3 depicts a prDcess in which liquid to be encapsulated is added to an unacidified solution of urea fonmaldehyde preoondensate and ;~ 25 acrylamide/acrylic acid copolym~r, and the resulting nixture is ~ ~.

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chilled before addition of melamine formaldehyde preoondensate and subsequent acidification; and Figure 4, which is a diagrammatic side view of a oDntinuDus en-; capsulation apparatus.
Referring first to Figure 1, an aqueous solution of urea formal- ;~
deh~de preoDndensate, melamine fonmaldehyde preoDndensate and acryla-mide/acrylic acid oDpDlymer is first prepared and then milled with material to be encapsulated until a desired droplet size is achieved, for example 2 to 3~m. The dispersion is then diluted with mDre water and stirred for a period of, for example, half an hour. The dispersion is then acidified with a oetic acid to a pH of abDut 4.7, raised to a temperature of abDut 55C and maintained at that temperature for abDut 2 hours. me dispersion is then allowed to coDl and is left stirring overniqht, after which usable capsules are found to be present. me pH
was then raised to about 8.5 with sodium hydnaxide solution.
Referring now to Figure 2, an aqueous solution of urea formaldehyde preoondensate and acrylamide/arcylic acid oopDlymer is prepared and its pH adjusted to abDut 4.4. After stirring for a short time, for example ; 5 minutes, material to be encapsulated is added, and the dispersion S~D
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obtained milled to a desired dnDplet siæ, for example 2-3~ m. The dispersion is then raised to a temperature of abDut 55C and maintained at that temperature for a period of, for example, from half an hour to three hours. Melamine formaldehyde preoDndensate is then added, and the dispersion is maintained at 55C for a further t~D hours. me disper-sion is then allowed to a~Dl and is left stirring overnight, after which usable capsules are found to be present. Finally, the pH of the dis-persion was raised to abDut 8.5 with sodium hydroxide solution.
Referring now to Figure 3, an aqueous solution of urea formaldehyde preoDndensate and acrylamide/acrylic acid oDpDlymer is prepared. After stirring for a short time, for example five minutes, liquid to be encapsulated is added and the dispersion so obtained milled to a rB

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desired droplet size, for exanple 2 to 3 ~ . The dispersion is then chilled to a temperature of akout 15C and ~elamine fonmaldehyde pre-condensate is then added. After adjusting the pH to about 4.2, the dispPrsion is heated bo about 55C and maintained at that temperature for about 2 hours. The pH of the dispersion is then raised to about 8.5 with sodium hydroxide solution, and the dispersion is then allowed -to oool.
Although pH values of 4.7, 4.4 and 4.2 are referred to above, the sa~e techniques can be used for pH values in the range 3.5 to 5.5.
In the event that formaldbhyde fumss are ev~lved from the capsule dispPrsion as a result of formalin having been present in the preoonden-sates, it may be quenched by the addition of an amwanium ccmpound or other material which wlll cansu~e ths formaldehyde.
me present invention enables a stable dispersion of liquid to be e encapsulated to b~ formed~ without the need for continual agitation to prevent drqplet ooalescence. This facilitates accurate oon~.~l of the droplet size, and also permits the production of small capsules. Rela-tively cheap raw materials can bs used in the process, and the ~roaess . .
itself can be made relatively short oompared with nany~previously used i~ 20 prooesses. The fact that the wall materials are synthetic means that ;~ they are not grsatly susceptible bo attack by micrororganisms, and the purity and chemiaal characteristics of the materials are not as critical as those used in aoaoervation systews for example. A furthsr advantage of the present invention is-that the ~apsules aan be nade stronger and less permeable than capsules resulting-from coacervation systems. This means that it is possible to enc~psulite highly polar materials which in general are difficult or impossible to encapsulate satisfactorily by means of aoacervation systems. Examples of such highly polar materials . are esters, e.g. phthalates. A further advantage of the present method ; 30 is that the ~', . -14-~ ' , . , . . :
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of the dispersion need not be less than about 4.0 which is not low ;;
enough to produce a significant premature oolour former developrent in the case of microcapsules for use in pressure-sensitive copying systems.
-~ me mat~rial to be encapsulated may be any liquid or solid which can be dispersed in an aqueous medium and which is substantially inert to that medium. As previously stated, the present method is particu-larly suitable for the encapsulation of materials for use in pressure sensitive copying systems. m ese mateirials generally oomprise a sol-ution of one or more leuoo dye derivative oolour fonmers in an oily solvent. Examples of such solvents are partially hydrogenated terphenyls, chlorinated paraffins, biphenyl derivatives, aLkyl naphthalenes, diaryl 4' methane derivatives, and dibenzylbenzene derivatives. Examples of suitable leuoo dye derivative colour fonmers are phthalide deirivatives e.g crystal violet lactone, fluDran derivatives, diphenyLamine deriva-tives, spirDpyrane derivatives and phthalimidine derivatives. Such solvents and colour formers are well kncwn in the pressure-sensitive copying paper art, and so will not be described further herein.
me invention will now be illustrated by the follcwing examples.
Trade marks are in quotation marks at their first use:-EX~MPLE I
,~ m e following were first mixed:-I (a) 95 g of "BC 77" cationic urea-formaldehyde precondensate having a reactive resin content of approximately 45% and a solids content of approximately 35% (~C 77 is supplied by British Industrial Plastics Limited);
~i (b) 60 g of "BC 336" methylated melamine formaldehyde precon-densate having a reactive resin content of about 76% and a solids con-tent of approxImately 71% (~C 336 is supplied by British Industrial Plastics Limited);

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- ~067761 (c) 240 g of "R1144" oopolymer (a 20% solution of an acryLamide acrylic acid copolymer supplied by Allied Colloids Limited and having a viscosity average molecular weight of 400,000 and an acrylic acid ;
oontent of 42%); and r' (d) ~50 g de-ionized water.
~ 200 g of de-ionized water were then added to 800 g of the -~ mixture described above, and the mixture was milled with 800 g of material to be encapsulated until a mean droplet size of 2 to 3~ m was reached.
me material to be encapsulated, which will hereafter be referred to as 'linternal phase", was a oolour former solution. The solvent for :. . - .
the colour former solution was a 4:1 W/w mixture of kerosene and "HB40", , the latter being a mixture of a partially hydrcgenated terphenyls sold ` by Mbnsanto Limited (HE40 is also known as "Santasol 340"). The colour ; 15 formers were crystal violet lactone and benzoyl leucomethylene blue, present in amDunts of 1.7~ W/w and 1.4% W/w respectively.
The remainder of the mixture was then added, follcwed by 1405 g of de-ionized water as a diluent. me resulting composition was stirred ; 20 for 30 minutes, after which its pH was lowered to 4.7 by adding aoetic acid. Stirring was then carried out for a further 30 minutes. The ..... ...
temperature was then raised to 55C using a water bath and the mixture stirred for 2 hours at that temperature, after which the composition was allowed to cool and left stirring overnight. Next morning capsules were '~!
seen to have formed, and the pH was raised to 10Ø me capsules -obtained were subsequently coated onto paper using a laboratory Mbyer .:, ....
bar ooater. When the sheet was placed on a oolour-developing sheet and written upon, a clear blue copy was develoQed on the colour-developing sheet.

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EX~LE II
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mis Example illustrates the use of polymers other than the acryla-mide/acrylic acid copoly,mer used in Example I, and also the use of a cationically modified acrylamide copolymer.
The followqng were first mixed:-~- a) lOS g of BC 77 cationic urea formaldehyde precondensate;
b) 50 g of BC 336 methylated melamine formaldehyde precondensate; and c) 650 g of de-ionized water.
me above mixture, which had a pH of 7.8, was then emulsified with 800 ml of the internal phase used in Example I and the emulsion obtained was diluted with 1605 g de-ionized water. It was neoessary to agitate the mixture vigorously to prevent the emulsion from de-stabilizing.
me pH was then raised to 8.7 by adding sodium hydroxide solution, with the result that internal phase clusters of mean diameter 10 to 15 ~m were formed.
Ihe emulsion was then divided into five parts, and an addition was made to each part as follows:-Part (i) nothing;
Part (ii) 50 g of 20% solution of a cationically modified acrylamide aopolyrer ("R1148"
supplied by Allied Cblloids Limited ' as a 20% solution);
' 25 Part (iii) 50 g of a 20% solution of R1144 aopolymer (as a aontrol):
Part (iv) 200 g %5 polyvinylmethyl ether/maleic anhydride oopolymer (PUMyM~
Part (v) 200 g of 5~ sodium carboxymethyl cellulose solution ("B10", supplied by I.C.I. Limited) ' '. "

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~` 1067761 Each mixture was then stirred for one hour at ambient temperature, ; after which the pH was lowered to 4.5 by adding acetic acid, follawed by ; further stirring for 1 hour. The temperature of each ~ix was then ~; raised to 55C and maintained at that te~perature for 1 1/2 hours with S stirring. The nux was then ooated onto paper as described in Example I.Mix (i) gave an oily sheet, indicating that capsules had not been formed. The remaining sheets appeared satisfactory, and when superim~
posed on a aolour-developing sheet and written upon, a clear blue o~y was developed on the oolour-devel-oping sheet.
EXPJ!~LE III :
This illustrates the use of different relative proportions of the preoondensates and the polymer while maintainLng the total weight of precondensates and polyrer const nt.
; me encapsulation prooedure in each case was as described in Example I, exoept that the pH of the mixture was first adjusted to 8.7, ; and subYcqyently lowered to 4.5 instead of 4.7. me quantities of ~aterial used are shown below:-Wt. of BC 77 Wt. of BC 336Wt. of R1144 ~) ~q) , ,(d~y~ ' (q) t~L~q) .. ' .

28.5 46 60.5 32.5 42.5 57.5 46.5 31 2539.5 32 _ __ .,~
The resulting muxes were each aoated on to paper as descriked in ; Example I. W.hen each of the ooated sheets was superimposed on a oolour developing sheet and~written upon, a clear blue copy was developed on the colour developing sheet.

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`` 1~67761 .
EX~MPLE IV
4;
- Example 1 was repeated except that different reaction pH values - were employed, namely 5.5, 5.0, 4.5, 4.0, 3.5, 5.2, 5.0, 4.8, 4.6, and 4.4. m e resulting mixes were each ooated on to paper as described in Example 1. When each of the coated sheets was superimposed on a oolour developing sheet and written upon, a clear blue oopy was developed on the colour developing sheet.

~ ~,, V
Example 1 was repeated except that in plaoe of R1144 copolymer the -; 10 same quantity of each of the following polymers were used:-- a) Cationic Starch ("Krystal Kote" sold by Laing National Ltd.) b) "Dispex N40" (a polyacrylate containing polymer sold be Allied Colloids Ltd.) c) Coating starch d) Methyl hydroxypropyl cellulose ""Methofas PM" sold by ICI Ltd.) e) "Versicol X13" (a non-ionic polyelectrDlyte sold by Allied Colloids Ltd.) me resulting mixes were each coated on to paper as described in ' EXample I. When each of the ooated sheets was superimposed on a oolour developing sheet and written upon, a clear blue copy was developed on the colour developing sheet.
EXAMPLE VI
. 25 mis illustrates the fact that addition of methylated melamine formaldehyde precondensate can take plaoe after formation of a dispersion of internal phase.
me following were first mixed~-a) 19 g of BC 77 cationic urea formaldehyde precondensate. ~;~
b) 42 g of R1144 copolymer.
c) 180 g of de-ionized water.
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~067761 ..
- oolour developing sheet.

EX~MPLE VII
::.
This illustrates a technique for swiftly achieving a stable dis-persion of a droplet size of approxImately 4~1n or lower. ~- -me follawing were first muxed~
a~ 19 g of BC 77 cationic urea formQldehyde preoondensate.
, b) 42 g of R1144 copolymer.
c) 180 g of de-ionized water.
mis mixture was heated for 1/2 hour with constant stirring at a temperature of 55C, after which was added:
c) 12 g of BC 336 methylated mela~ine formaldehyde preoondensate.
The pH of this nixture was reduced to 4.4 by addition of 14.7%
aoetic acid solution and stirred for a further S m~nutes after which ; were added:-lS e) 154 g of internal phase.

me resulting mLxture was milled until a dnDplet si~e of 4 ~m was achieved ~the milling ti~e by this method was reduoed from 2 minutes as in EXample Vl to lO seoonds). After millLng, the temperature of the ,. . .. .
dispersion was reduced to 35C and stirred at constant temperature for 1 hour, after which the temperature was raised to 55C and held for 2 ... .
. ~ . .
~ hours. Mbintaining-the constant stirring, the capsule dispersion which `
;~ had formed was allowed to oool overnight and the pH adjusted to be in the range from 8.0 to 10Ø

If desined, extra methylated melamine formaldehyde preoondensate ' may be added after the internal phase has been dispersed and the temr perature drqpped to 35C. This gives the advantage of producing tougher capsules but at a oost of necessitating the addition of extra dilution water and henoe a lawering in solids content and-phase ratio.
, The above prodecure was then repeated using diffenent phase ratios, namely 5.9:1, 4.8:1 and 2.6:1. The 5.9:1 phase ratio nux : . .: ' ' : .
.~ .

, : , ~ , .... . .. .. : .. . . . .. .

154 g of internal phase were addbd and the pH was lowered to 4.4.
After 5 m mutes stirring the nuxture was milled to give a mean droplet size of 4~Dn. The dispersion was then divided into five parts, and 12 g of BC 336 ~ethylated melamine formaldehyde resLn were added to each part as follows:-Part 1 - i~mediately Part 2 - after heating at 55C (water bath) for 1/2 hour Part 3 - after heating at 55C (water bath) for 1 hour ~ .
Part 4 - after h atLng at 55C (waber bath) for 2 hours Part 5 - after heating at 55C (water bath) for 3 hours Each part was then maintaLned at 55C, for a further twD hours, and then left to cool overnight, with stirring.~ Next morning, the pH was raised to 8.5 with sodiu~ hydroxide solution.
, The mixes were then diluted to 40% solids content and ooated onto paper using a laboratory Mbyer bar ooater. The visoosity and solids contents of the nixes before dilution are shown below:-Part Nb. Solid aontent Visoosity cp ' 20 ~ obefore ilu~ion)

2 41 842

3 43 572

4 49 540 ; 45 ~16 ,~, .....
*As measured using a Brookfield Viscometer, ~odel RVT, ~' Spindle 2 at 10 rpm.
The visoosity was low compared with that which is observed for oonventional gelatin-based capsular mixes of comparable solids oontent.
When each of the coated sheets was superimposed on a colour deve-30 loping sheet and written upon, a clear blue Lmage was developed on the ~
. : .

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~067761 -:

(43% solids content) was produoe d as described above, without the final :
addition of extra preoondensate and dilution water. The 4.8:1 and 2.6:1 phase ratio mixes (40~ and 25% solids content repsectively), were produced by a final addition of 8.0 g and 44.0 respectively of pre-condensate and of 30 g of 420 g de-ionized water respectively.
All four mixes were then coated on to paper as described in Example I. When each of the resulting sheets was superimposed on a colour developing sheet and written upon, a clear blue copy was developed on ;
the colour developing sheet.
EXAMPLE VIII
This illustrates the production of capsules by a oontinuous process rather than by the batch processes so fæ described. r~he process will be described with referenoe to Figure 4. A dispersion of internal phase was prepared in an aqueous solution of the wall mHterials as described in Example I, and after pH adustment, was fed into a hopper A. This was then allowed to drip at a constant rate into a first tank B which was ; heated by a water jacket C to a oonstant temperature of 55C. An overflow pipe P was provided in the t~nk B, so that when the level of dispersion reached the pipe P, it flowed out into a second tank D also heated by a water jacket E which maintained the dispersion at 55C. An overflow pipe Q was prDvided in the tank D, for similarly conveying the dis-persion to a third tank F, heated to 55C by a water jacket G. An -overflow pipe R was provided in the tank F, for oonveying dispersions to a reoeiving vessel H. Capsules were found to be present in vessel H, and the pH was adjusted to 10.0 with sodium hydroxide and the capsulæ
mux stored.
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Onoe all three tanks were full, the throughput was adjusted so that the dispersion took on average 2 hours to go through the system. By aon-tinually topping up tank A capsules were produced for a period of 3 hours after stability had been attained.
Samples were withdrawn one, two and three hours after the system had reached an equilibrium state, and were ooa W on to paper as described - in Exam@le I. When each of t~e resulting sheets was superimposed on a aolour developing sheet and written upon a clear blue copy was developed on the colour developing sheet.
' 10 EX~MPLE LX : ' This illustrates an encapsulation procedure in which a chilling step is carried out before addition of ~elamine fonmaldehyde precon-densate.
84 g of R1144 copolymer was dissolved in 400 g of de-ionized water, and the solution was heated to 55C. 38 g of BC 77 cationic urea-formaldehyde precondensate were added, and the nuxture was stirred for 40 ninutes at 55C. me mixture was then milled with 189 g of internal phase, as described in Example I.
The resulting emulsion was chilled to 15C, and 45 g of BC 336 mela~ine formaldehyde precondensate were added. me ~H was then lowered to 4.15 by addition of 14.7~ aoetic acid, and the emulsion was allowed to stand for one hour with stirring. me temperature was then raised to 55C, and stirring was continued at this temperature for tWD hours. The .
emulsion was then allowed to cool to ambient temperature, after which the pH was raised to 10 with odium~hydrcxide solution.
~ The capsules obtained were coated on to paper as descrbed in ; Example I. When the resulting ooabed sheet was superinçosed on a ; colour developing sheet and written upDn, a clear blue image was . , ~ '.

: . . .
:: .

10677~;1 developed on the colour developing sheet.
me chilling step mentioned above helps to prevent droplet aoale-scence and formation of undesirably large capsules, which may lead to formation of blue spots when in use in pressure-sensitive oopying system. It is preferred to chill to a temperature in the range 15 to 30C, about 15C being preferred.
EXAr~ X
84 g of R1144 acrylic acid/acrylamide oopolymer were dissolved in 550 g of de-ionized water. 38 g of BC 77 cationic urea-formaldehyde preoondensate were added and the mixture stirred for 5 minutes. The mixture was then milled with 189 g of internal phase as described in Example I to a droplet size of 4 m.
m e resulting emulsion was chilled to 15C and split into two equal parts. To one half were added 40 g of "BC 355", a methylated melamine ; 15 formaldehyde precondensate having a reactive resin o~ntent of 76%, and ,' to the other half were added 34 g of "BC 309", a methylated melamine formaldehyde preoondensate having a reactive resin oontent of 90% (both BC 355 and BC 309 are supplied by British Industrial Plastics Limited).
Both emulsions were lowered in pH to 4.15 by addition of 14.7%
aoe tic acid and held for 1 hour before heating to 55C. mis tempera-ture was maintained for 2 hours after which the pH was raised to 8.5 with 25% caustic soda.
Each batch of capsules obtained was ooated on to paper as described in Example I. When each of the resulting ooated sheets were super-imposed on a colour-developing sheet and written upon, a clear blue oopy was developed on the colour dev,eloping sheet.
E~MPLE X:[
.
42 g of R1144 acrylic acid/acrylamide aopolymer were dissolved in 275 g of de-ionized water. 19 g of "BC 55", a cationic urea-formal-..

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. . .

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dehyde precondensate having a reactive resin oontent of 45~ were added and the mixture stirred for 5 minutes (~C 55 is supplied by British '~
Industrial Plastics Limited). The mixture was then ~illed with 189 g of internal phase as described in Example I to a dr~plet size of 4 ~.
e resulting emulsion was chilled to 15C and 40 g of BC 336 i: .
methylated melamine fonmaldehyde precondensate were added. me pH was dropped to 4.15 by the addition of 14.7% acetic acid. After holding for 1 hour it was heated to 55C and maintained at that temperature for 2 hours. The pH was raised to 8.5,with 25% caustic soda. -ffle capsule mix so obtained was coated on to pap~r as described in Example I. When the resulting ooated paper was superimposed on a oDlour developing sheet and written upon, a clear blue oopy was developed on ' the colour developing sheet.
EXAMPLE XII
. . .
This example illustrates the use of further water soluble polymers.
~a) 84 g of a 10% gelatin solution were mixed with 233 g of '' deionized water and the solution maintained at 40C. 19 g of BC 77 cationic urea form21dehyde precondensate were added and the nuxture stirred for 5 minutes. It was then-milled with I89 g of internal phase as described in Example I to a droplet size of 8 ~m. 40 g of BC 336 methylated melamine fonmaldehyd,e precondensate were added and the pH was , dropped to 4.15 with 14.7% aoetic acid. After heating for 2 hours at ' 55C, the pH was raised to 8.5 with 25% caustic soda after which usable , capsules had been pnoduced.
ob) 8-4 g of a polyethylene/maleic anhydride oD,polymer was n~xed with 160 g of de-ionized water and heated to 90C (the copolymer used as EM~ 31 supplied by Mon&anto Ltd.). After chilling to 20C the solution was nixed with 250 g of de-ionized water and 19 g of BC 77 cationic ,' urea formaldehyde ,~recondensate. After S minutes stirring this wuxture ;~

:. , , . .: . ..... .. . ..

'~ was milled with 189 g of internal phase as described in Example I to a ::
droplet size of 4~Im. 40 g of.BC 336 methylated melamine formaldehyde preccndbns~te was added and the pH dropped to 4.15 with 14.7% aoetic :
acid. Af er heating at 55~C for 2 hours, the pH was raised to 8.5 with - -: .25% caustic soda after which usable capsules had been ,produced. The ~`' capsule mixes from ~a~ and ~b) were-each ooated on to a paper as des- ..
~ .
., cribed in Example I. When the.resulting coated papers were superimposed .. ~ .
'. on a colour-developing sheet.and written upon, a clear blue copy was $~p developed on the colour developing sheet in each case.
i~
.' 10 . EXPMPLE XIII
'~ This~Example demanstrates the ew apsulation of a phosphate ester, i~ .
'' which is a highly polar liquid and which is difficult or inpossible to ." encapsulate by ccnventiona~.ooacervation techniques using hydro,Dhilic , colloids such as gelatin. . ' ' ' ,' 15 42 g of R1144 acrylic acid~acrylamide oopolymer were dissolved in '. . ..
170 g of de~ionized water and heated at 50C. 19 g of BC 77 cationic ~ ..
"I urea formaldbhyde precondensates were added and the temperature main-,'1 tained at 50C for 40 ~inutes. lOS g aold de-ionized water were.stirred '~
1 in and the nixture milled.with 189 g of internal phæse to a droplet size ',~i 20 of 4 JJn. me internal phase-consisted of aolour for~ers as detailed in Example I dissolved in a 1:1 mi~ture of.kerosene~and phosphate ester.
e resulting emLlsion was chilled to 15C and 40 g of BC 336 ' ~ethylated melamine form21dehyde,preoandensat~e were adde~. The pH was : dropped to 4.15 by the addition o 14.7% acetic acid. After h~lding for - :
l-hour the nixture was heated to 55C and n~intained at that temperature ~`.',~ for 2 hours. The pH was raised to 8.5 with 25% caustic soda and usable . . ~ .
, capsules-were obtained. me capsules were ooated on to paper as des-,, cri~ed in Example I. When the aoated sheet was superimposed on a aolour ~, develcping sheet and written uFon, a clear blue aopy was developed on .` 30 the aolour developing sheet.
. .

-

Claims (17)

The embodiments of the invention in which an exclusive pro-perty or privilege is claimed are defined as follows:

1. A method of encapsulating finely divided particulate material to produce microcapsules in which the particulate material is surrounded by polymeric shells, wherein a dispersion of said particulate material is prepared in an aqueous medium containing a water-soluble urea-formaldehyde precondensate, characterized by the steps of incor-porating in the aqueous medium a water-soluble melamine-formaldehyde precondensate and a water-soluble polymer which is capable of being cross-linked by said precondensate, and condensing said precondensates by acid catalysis with resultant cross-linking of said polymer about said particulate material, thereby to form said polymeric shells.

2. A method as claimed in claim 1 wherein said particulate material comprises minute droplets of water-immiscible liquid and said water-soluble polymer is dispersion stabilizing on its own or in the presence of at least one of said precondensates.

3. A method as claimed in claim 1, wherein said polymer contains alcohol, amine, amide, acid or acid derivative groups.

4. A method as claimed in claim 3, wherein said polymer is a cellulose derivative, starch, a starch derivative, a polyacid, a poly-ester, a polyanhydride copolymer, a polyacrylamide or a polyacrylamide copolymer.

5. A method as claimed in claim 4 wherein said polymer is an acrylamide/acrylic acid copolymer.

6. A method as claimed in claim 1 wherein said polymer is ionic.

7. A method as claimed in claim 6 wherein said polymer is anionic.

8. A method as claimed in any one of claims 1 to 3 wherein said urea formaldehyde precondensate is cationic.

9. A method as claimed in any one of claims 1 to 3 wherein said melamine formaldehyde precondensate is a methylated melamine formaldehyde precondensate.

10. A method as claimed in claim 1 wherein said urea formaldehyde precondensate, said melamine formaldehyde precondensate and said water-soluble polymer are all present in said aqueous medium before addition of the liquid to be encapsulated to form said dispersion, and wherein acid for bringing about condensation of the precondensates is added after formation of said dispersion.

11. A method as claimed in claim 1 wherein only said urea formaldehyde precondensate and said water-soluble polymer are present in said aqueous medium before addition of the liquid to be encapsulated, and wherein said melamine formaldehyde precondensate is added subsequently to form said dispersion.

12. A method as claimed in claim 10, wherein said melamine formaldehyde precondensate is added in batches.

13. A method as claimed in claim 10 or claim 11, wherein acid for bringing about condensation of said pre-condensates is added to said aqueous medium before addition of melamine formaldehyde precondensate.

14. A method as claimed in claim 10 or claim 11 wherein said aqueous medium is chilled after addition of the liquid to be encapsulated and before addition of melamine formaldehyde precondensate.

15. A method as claimed in any one of claims 1 to 3 wherein heat is supplied to accelerate condensation of said precondensates.

16. A method as claimed in claim 1 wherein after microcapsule formation, the aqueous medium is made alkaline.
17. A microcapsule comprising a finely divided particulate material surrounded by a polymeric shell, the polymeric shell comprising urea-formaldehyde, melamine formaldehyde and a polymer cross linked by both the urea formaldehyde and the melamine formaldehyde, said polymer being water-soluble prior to the cross linking.
18. A sheet material carrying microcapsules as claimed in

claim 17.