CA1103824A

CA1103824A - Process for the production of hot melt coating compositions containing microcapsules

Info

Publication number: CA1103824A
Application number: CA298,332A
Authority: CA
Inventors: Robert A. Austin; Dale R. Shackle
Original assignee: Mead Corp
Current assignee: Mead Corp
Priority date: 1977-04-29
Filing date: 1978-03-06
Publication date: 1981-06-23
Also published as: DE2818976A1; JPS6055320B2; JPS53135720A; US4171981A; DE2818976C2; GB1603531A

Abstract

ABSTRACT OF THE DISCLOSURE
This invention relates to a process for the pro-duction of a hot melt coating composition containing micro-capsules. The process comprises the steps of preparing a dispersion of substantially discrete microcapsules in a volatile solvent continuous phase. A hot melt suspending medium is prepared, the hot melt suspending medium having a melting point of from about 50°C. to about 140°C. and a melting point range of less than about 15°C, the hot melt suspending medium being in a fluid state. The dispersion of substantially discrete microcapsules in a volatile solvent is then mixed with the fluid hot melt suspending medium with turbulent agitation to form an intimate mixture of the dispersion of microcapsules and the hot melt suspending medium. Heat and a vacuum are applied to the mixture, while maintaining the agitation, until the volatile solvent is substantially removed from the mixture to form a dispersion of substantially discrete microcapsules in the liquid hot melt suspending medium. The heat is applied at a rate sufficient to maintain the mixture at a temperature above the melting point of the melt suspending medium, the temperature also being above the boiling point of water at the vacuum level. This invention further relates to the coating composition produced by the process of this invention.

Description

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B~CKGROUND OF TIIE INVENTION
_ Field of the Invention This invention relates to the production o~
coating compositions comprising à hot melt suspending medium and having capsular material dispersed therein.
In particular, it is concerned with a process o~ making hot melt coating compositions containing microcapsules starting with volatile solvent m;crocapsular dispersions.
The hot melt coating compositions so produced are parti- -cularly useful, for example, for the production of pressure-sensitive carbonless copy papers.

Prior Art .
Carbonless copy paper, briefly stated, is a standard type of paper whereln during manuaoture the backside of a paper substrate is coated with what is referred to as a CB coating, the CB coating containing one or more color precursors generally in capsular, - ~ -and more preferably microcapsular, foxm. At the same time, the front side of the paper substrate is coatea during manufacture with what is referred to as a CF :
coating which contains one or more color developers.
Both the color precursor and the color developer remain in the coating compositions on the respective back and front surfaces of the paper in colorless form.
This is true until the C~ and CF coatings are brought ~3B24 into abuttin~ relationship and su~Eicient pressure, as by a typewr:iter, is applied to rupture the CB
coating to release the color precursor. At this time the color precursor trans~ers to the CF coating and reacts ~ith the color developer therein to form an image.
Carbonless paper has proved to be an exceptionally valuable image transfer medium for a variety of reasons only one of which is the fact that until a CB coating is placed next to a CF coating both the CB and the CF
`10 are in an inactive state as the co reactive elements are not in contact with one another. Patents relatîng to carbonless paper proaucts are:
U. S. Patent 2,712,507 (1955) to Green U. S. Patent 2,730,456 (1956) to Green et a~
U. S. Patent 3,455,721 (1969) to Phillips et al U. S~ Patent 3,466,184 (1969j to Bowler et al ~ -U. S. Paten~ 3,672~935 (1972) to Miller et al .
A third genPration product which is in an advanced ~
.~ . : . .
stage of development and commercialization at -this time - and which is available in some business sectors is referred to as self-contained paper. Very generally stated self-contained paper refers to an imaging s~stem wherein only one side of the paper needs to be coated and the one coating contains both the color precursor, generally in encapsulated form, and the color developer.
Thus when pressure is applied, again as by a typewriter or other writing instrument~ the color precursor capsule 3~2~L ~

is ruptured and reacts with the surrounding color developer to form an imageO Bo~h the carbonless paper image transfer system and the self-contained system have been the subject of a great deal of patent activity.
~ t~pical autogeneous record material system, earlier sometimes referred to as "self-contained" because all elements Eor making a mark are in a single sheet, is disclosed in U. S~ Patent 2,730,457 (1956) to Green.
A disadvantage of coated paper products such as carbonless and self-contained SteTnS -from the necessity of applying a liquid coating composition containing the color forming ingredients during the manufacturing process. In the application of CB coatings, the coating composition generally comprises a dispersion of micro-capsules in an aqueous medium containing also a binder for the microcapsules. This aqueous coating composition requires removal of th excess water by dryin~ which drying entails the use of complex and expensive equlp-ment and a high input vf heat energy to continuously dry a substrate coated with an agueous coating com-position.
The application of heat not only is expensive, making the total product manufacturing operation less cost effective, but also is poten-tially damaging to the color forming ingredients which are generally coated onto the paper substxate during manufacture. High degrees of temperature in the drying step require i3~2~ ' specific for~ulcltion o~ wall-forming compounds which permit the use of excess hea~ The problems encountered in the actual coating step are ~enerally attributable to the necessity fox a heated drying step following the coating operation.
~any of the par-ticular advantages of the process of this invention are derived from the fact that a hot melt coating composition is produced which may be used to coat the paper substrate. This is in contrast to the coatings used by the prior ar-t which have generally required an aqueous or organic solvent coating. For purposes oE this application the term "100% solids coating"
will sometimes be used to describe the coating operation and should be understood to refer to the fact that a hot melt coating composition is used and -therefore the drying step usually present in the m~nufacture of paper and in coating has been eliminated.
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- The use of hot melt CB coatings i5 known having been disclosed, for examplet in:
~ U. S. Patent 3,016,308 (1962) to~Macauley U. S. Patent 3,079,351 (1963~ to Staneslow et al U. SO Patent 3,684,549 (1972) to Shank ~ ~ ~
The use of hot melt CB coatings containing microcapsules is mentioned in Staneslow et al and the use of free flowing powders of discrete microcapsules in preparing hot melt CB coating compositions is disclosed in Macauley.
The abo~e prior art processes of preparing the hot melt CB co~ltln~ compositions have the disadvclntage o~ requirillcJ a sepclra-te step to produce a powder of microcapsules with accompanying dif:~iculties of handling and s-toring the microcapsular powder without damaying S the dry microcapsules. Addi-tionally, the dry micro-capsules have to be dispersed in the hot melt suspending medium.
The preparation of hot melt CB coating compositions containing microcapsules by the process of the instant invention involves the process of mixing together a hot melt suspending medium and a volatile solvent dispersion of microcapsules and applying heat and vacuum to the mixture to produce the finished coating composition.
Furthermore, damage to the microcapsules in handling of the powd~r is substantially eliminated and in its preferred form, the process may be operated continuously.
The use of heat in combination with vacuum permits the effective removal of water from the microcapsular material without requiring an excess of either. For purposes of this application the term vola-tile solvent shall be used to refer to aqueous and non-aqueous solvents and would include among many others water and organic solvents.

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~;TATEME2~T OF THE INVENTION
This invention relates to a process for the pro-duction of a hot melt coating composition containing micro-capsules. The process comprises the steps of preparing a dispersion of substantially discrete microcapsules in a volatile solvent continuous ph~se. The microcapsules con-tain an oily solution oE a chromogenic material, and are substantially oil and water impermeable. A hot melt sus-pending medium is prepared, the hot melt suspending medium having a melting point of from about 50C. to about 140C.
and a melting point range of less than about 15C, the hot melt suspending medium being in a fluid state. The disper-sion of substantially discrete microcapsules in a volatile solvent is then mixed with the fluid hot melt suspending medium with turbulent agitation to form an intimate mixture of the dispersion of microcapsules and the hot melt suspen-ding medium. Heat and a vacuum are applied to the mixture, while maintaining the agitation, until the volatile solvent is substantially removed from the mixture to form a disper-sion of substantially discrete microcapsules in the li~uid hot melt suspending medium. The heat is applied at a rate sufficient to maintain the mixture at a temperature above the melting point of the hot melt suspending medium, the temperature also being above tha boiling point of water at the vacuum level. This invention further relates to a liquid hot-melt coating composition comprising (1) a hot melt suspending medium characterized by: (i) being sub-stantially water soluble; (ii) being characterized by the presence of one or more functional yroups selected from the group consisting of: carboxyl, carbonyl, hydroxyl, ether, methoxy, ethoxy, ester, amide, amine, heterocyclic groups and combinations thereof to impart polarity thereto; (iii) having a melting point of from about 60C to about 140C

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and a melting point range of less -than about 15C; and (2) an encapsulated chromogenic material substantially dispersed thereln, said hot melt suspending medium being compa-tible with colour forming characteristics oE said capsular, chromo-genic material.

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Brief Description of the Drawings Figures 1 through ~ are photomicroyraphs of four hot melt dispersions evaluated according to the test descri-bed herein, Fig~re ~ appearlng on the same sheet as Figure 2.
Detailed D0sc_ ~ n of the Invention The process of this invention involves the produc-tion of a hot melt coating composition contalning micro-capsules. In its broadest form the process is accomplished by mixi.ng a microcapsular dispersion in a volatile solvent with a fluid hot melt suspending medium and applying a vacuum while maintaining mixing and a temperature high enough to keep the hot melt suspending medium in a liquid state and to evaporate the volatile solvent by heating un-til substantially all of the solvent is removed from the mixture. In the most preferred process and product of this invention the volatile solvent is water. For purposes of -this application the term "fluid" as applied to a hot melt suspending medium will be used to describe a liquid or a pourable particulate powder.
An essential feature of the process of this inven- -tion is the condition under which the continuous volatile solvent phase of the dispersion of microcapsules is ex changed for the hot melt suspending medium as the continuous phase. This is accomplished by the application of heat and vacuum. The particular difficulty lies~ in the sensitivity of microcapsules to heat, particularly moist heat, and high shear conditions.

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.r ~3B2~ -In order Eor th:is exch~ncJe to he success:Eul, it must kak~ place w.ithout .rup~ure or substanti.al deteriorat.ion of the m.icrocapsules to the point where the micro-capsules are unctionally ineffective. We have found that by controlling the conditions of the exchange as follows, a dispersion of microcapsules in a hot melt suspending medium can be successrully produced.
l. The microcapsular dispersion in a volatile solvent is a dispersion of substantially discrete microcapsules.

2. The temperature throughout the mixing step is low enough to prevent substantial heat deterioration of the microcapsules during the mixing step.

3. The vacuum is high enough to substantia11y lower the boiling temperature yet not hi~h enough to rupture the microcapsules. ~ .

4. The combined effect of the ~emperature and vacuum is evaporation of the.volatile~s~olvent and the temperature is maintained above ~he melting point of the hot melt suspending medium and also above the boiling point of the volatile solvent. ~ .
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5. Mixing of the microcapsular dispersion in the volatile solvent and the liquid hot melt suspending med1um i5 by low shear, turbulent agitation. .~.

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These conditions and their effect on the resul-tant microcapsular dispersions are explained in detail later in this description.
The coating compositions produced by the process of this invention are dispers~ions of substantially dis~
crete microcapsules in a hot melt suspending medium. On applying the coating compositions of this invention to a substrate and allowing the composition to set by cooling, the hot melt suspending medium acts as a binder for the microcapsules to facilitate adherence of the microcapsules to the substrate. Microcapsules containing pharmaceutical agents, aromas, perfumes, flavoring agents, insecticides, dyes, pigments and color precursors may be dispersed in hot melt suspending medium b~ the process of this invention and coated on a variety of substrates including papers, fabrics and plastic films.
A preferred embodiment of this invention is the production of hot melt microcapsular coating compositions which may be used in the preparation of pressure-sensitive carbonless copy papers. The preparation of these carbon-less copy papers is described in detail in a commonly assigned, co-pending application entitled, "Pressure-Sensitive Carbonless Transfer Sheets Using Novel Hot Melt Systems and Process for the Production Thereof", Canadian Application Serial No. 277,564 filed April 28, 1977.

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The Procc-~ss of this invention wiLl hereinafter be des-cribed in detail with respect to this preEerred e~bodiment~
In the preferred embodiment, the hot melt coating composition i5 essentially a dispersion oE an encapsulated chromogenic material in a hot melt system. ~or purposes of this application, "chromogenic material" will refer to color precursors, color formers, color developers and the like. The encapsulated chromoyenic material is usually an oil solution o one or more color pre-cursors. The coating composition can contain, in addition to the encapsulated chromogenic material, S I :z I hq , ~illers, ~}}~naterial such as Arrowroot starch granules and dispersing agents. The type and amount - 15 of such additional ingredients in the coating composition are strictly a matter of choice and are generally dependent on the desired inal product.
Although any of the color precursors or color~
formers known in the prior art can be used, the color precursors most useful in the practice of the pre- ~ -erred embodiment of this invention are the color~
- precursors of the electron-donatlng ~ype~ The preferred group of electron donating color precursors include the lactone phthalides, such as crystal violet lactone, and 3,3-bis-~1'-ethyl-2-meth~lindol-3"-yl) phthalide, the lactone fluorans, such as 2-dibenzylamino-6-diethylaminofluoran and 6-diethylamino-.
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l, 3-d:imethyl:El.uorclns, the lactone xanthenes, the leucoauramines, the 2--(ome~a substituted vinylene) -3, 3-d.isubstituted-3-H-indo.les and l,3,3-trialkyl-indolinosp:irans. ~lixtures of these colo.r precursors S can be used if desired. In the preferred process of this invention microencapsulated oi.l solutions of . -~
color precursors are used. The color precursors are preferably present in such oi.l solutions in an amount of from about 0.5% to about 20.0% based on the weight of the oil solution, and the most preferred range . bèing from about 2% to about 7%.
In the practice of this invention the micro-capsules are prepared in the form of a dispersion in a volatile solvent. In general, the more uniform the microcapsular dispersion the better the final : product. As has been defined hereinabo~e, the volatile solvents useful in the practice of this invention include both aqueous and non-aqueous solvents. O~
the non-aqueous solvents the most preferred are ~he~ -organic solvents such as: benzene, xylene,~toluene, mineral spirits, carbon tetrachloride, chloroform, methylene dichloride, cyclohexane, n-hexane, n-butylacetate and diethylether.
The ho~ melt suspending media. generaIly useful in the pract1ce of this invention include waxes and resins.
A preferred group o~ compounds useful as hot melt sus-pending media include: deresinated, oxidized mineral waxes ~3~3Z4 .such as the montan ~axes, amide waxes such as bisstearamide wax, stearamide wax, behenamic~e wax, fatty acid ~axes, hydroxylated :Eatky acid waxes, hydroxyst~arate waxes, oxazoline waxes and mixtures thereof.
~nother type of hot melt suspending media is a non-polar hydrocarbon wax, such as Be Square 170/175 from Bareco Division of Petrolite Corporation which includes a small amount of a dispersing agent. The di~persing -agent may, for instance, be sulfated castor oil, more commonly known as Turkey ~ed Oil.
A characteristic of the preferred hot melt suspendiny media useul in the practice of this invention is a melting point of from about 50C~ to ahout 140C. A more preferred melting point or the waxes or resins useful in the practice of this invention is from ahout 70~C. to about 100C.~Also relative to the meltiny point, it lS necessary for the ~
coating composition of ~his invention to~set rapidly after application to the particular substrate. More particularly, a practical melting range limitation, or in other words :
range of temperature in which the liquid hot melt compositon sets into a solid composition, is from about 1.0C. to about 15C.
The hot melt waxes and resins of this invention preferably also ha~e a low viscosity when in a molten state in order to facilitate ease of spreading on a substrate~
In general, it is desirable that the hot mel~ suspending media have a viscosity of less than about 120 centipoises ~33~

at a tenlperature of approximately 5C. abov~ the meltin~
point oE a particulclr hot melt suspenclin~ medium. In ad- ~
dition, it is preferred th~t the hot melt wax or hot melt suspending media of this invention have a light color in order to be compatible with the final paper or plastic product being produced. This means that it is preferred for the hot melt to be whi-te or transparent after appll-cation to the particular substrate bein~ coated.
The pxeferred waxes, resins and other hot melt sus-pendlng media of this invention preferrable are polar.
By polar it is meant that the preEerred waxes are char-acteriæed by a certain amount of polarity, the polar compositions being characterized by the presence of func-tional groups selected from the group consisting OL:
carboxyl, carbonyl, hydroxyl, ether, methoxy, ekhoxy, ester, amide, amine, hetercyclic gxoups and combinations thereof.
Another group of compounds whi h are useful in the practice of this invention are the water soluble waxes and . .
resins such as, for example, acetamide, acetanilide, tri methylol propane, hydantoin, urea, ammonium nitra~e, benzene sulfonabide, poly(vinylpyrolidone) ~GAF Corp~ration New . ~ ~ ~o~e ~ ~r ~ :
L~ York, NY), polyethylene glycol~waxes, Carbowax~4000 and ~000 (Unlon Carbide Corporation, New ~ ~ G~ ~ d ~ t~oxy ;
polyethylene glycol waxes, Carbowax ~ 000 and 5000. Of these compounds, the polyethylene glycols are preferred.
The dispersibility of any particular microcapsule .

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system in any particular hot melt syst~m is a ~unc~ion of the chemical compakability oE the two systems. It has been shown that a subjective, yet r~produceable, numerical rating in "dispersion units" can be ass.igned to any micro-capsular/hot melt system to evaluate its commercial potential. To illustrate this applicant has pro~ided an example of various dispersion ratings the photomicrographs labelled Figures 1 through 4 and made a part hereof.
Applicant has devised several dispersion charactexistics such as agglomeration, microcapsules per unit area and flowability of various microcapsular-hot melt activation systems. In evaluating these systems a numerical figure of from 0 to 10 is assigned to each system which represents dispersion units. The number 0 would represent a non- ~
dispersed system wherein essentially a larqe agglomerated mass of microcapsules exist as is shown specif1cally ~v Figure 4. At the other end of the subjective spectrum of dispersibility is a uniform dispersion~of individual microcapsules 1n a hot con~inuous medium. Thi~ is il- :
lustrated by Figure5 1 and 2. Whi1s~1Ower dispersion characteristics are acceptable for msny produat5 a~ high degree of dispersibility is~essential for the effective~
production of carbonless paper.
- It has been experimentally determined that a dispersion characteristic rating of from about~ 6 to about 10 1S~ com-mercially acceptable while a rating of from about 8 to about 10 is preferred. A most prsferxed dispersion ~

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~i3~3~4 i rat:ing or us~ in carbonless ~?aper systems would he from 9 to about 10 as lllustrated by Figures 1 and 2 at-tachea h~reto. A ratin~ of 9 to 10 i9 ~escribed here.in as "a dispersion of substantially discrete microcapsules".
Figure 3 illustrates a dispersion which would be given a rating o 4 on the dispersion characteristic test of applicants. As such this type of a dispersion may be satisfactory for products other than carbonless paper.
However, poor dispersion characteristics in carbonless paper result in an unsatisfactory product which do not image properly and which suffèr ~rom feathering and from incomplete and irre~ular line and image formation. Thus, disper-sibility is considered a key characteristic of any hot lS melt activation system including microcapsules. Dis-persibility can be attained by several methods although use of extreme process conditions such as high-shear agitation or heat are generally not ~onsldered feasible ~ in carbonless paper manufacture. The dispersion char-acteristics mos-t preferred for carbonless paper are attained .
by using a hot melt activation system and microcapsular system whi~h are chemically compatible to promote dispersibility.
An alternate but preferred embodiment of this invention includes the use of a dispersing àgent with the~waxes of this invention. In this embodiment, the dispersing agent is - added to the aqueous dispersion of mlcrocapsules prior to combining the microcapsules with the hot~melt suspending medium. A preferred group of dispersing agents are the :
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anionic dispersing agents, many of which are commercially available. A preferred group of anionic dispersing agents includes the sodium salts of condensed naphthalene sulfo-nic acid, the sodium salt of polymeric carboxylic acid, the free acids of complex organi.c phosphate esters, sulfa-ted castor oil, poly-(methylvin~l ether/maleic anhydride) and combinations thereoE. rrhe dispersing agent is added to the microcapsules in an amount of from about 0.1~ to about 10% based on the dry weight of the microcapsules.
A preferred range of addition is from about 0.5~ to about 5.0~ based on the dry weight of the microcapsules while a most preferred range is from about 1.0~ to about 3.0%
based on the dry weight of the microcapsules.
In some instances the dispersing agent and the wall-forming mater.ial are one in the same and the wall-forming material not actually used in the microcapsule wall formation i5 present in hot melt coating dispersions as a dispersing agent. Although, as described above, many of the well-known, commercially available dispersing agents can ~e used in the process and product of this in-vention, a group of secondar~ dispe~sing agents that may be present as excess wall forming material includes:
hydroxypropylcellulose, gum arabic, gelatin, polyvinyl alcohol, carboxymethylcellulose, and mixtures of the above.

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~3~Z4 The par-ticular wall-~orming materials or the particular encapsulated chromogenic material are not asser-ted to be an inventive feature herein. Rather, there are described in the patent literature various capsular chromo-genic materials which may be used. Such chromogenic Materials have been encapsulated in gelatin wall-forming materials (see U. S. Patents Nos. 2,730,~56 and 2,800,457) including gum arabic, polyviny:L alcohol, carboxymethyl-cellulose, resorcinolformaldehyde wall-formers (see U. S.
Patent No. 3,755,190), isocyanate wall-formers (see U. S.
Patent No. 3,914,511) isocyanate-polyol wall-formers (see TJ. S. Patent No. 3,796,669) and hydroxypropylcellulose (see U. S. Patent No. 4,025,455) in addition to mi~tures of the above. Microencapsulation has been accomplished by a variety of known techniques including coacervation, interfacial polymerization, polymerization of one or more monomers in an oil, various melting dispersing and cooling methods. Compounds which have been found preferable for use as wall-forming materials in the various microencapsu-lation techniques included: hydroxypropylcellulose, methyl-cellulose, carboxymethylcellulose, gelatin, melamine-formaldehyde, polyfunctional isocyanates and prepolymers thereof, polyfunctional acid chlorides, polyamines, polyols, epoxides and mixtures thereof.
Particularly well-suited to use in the present invention are microcapsules of hydroxypropylcellulose (HPC) material and isocyanate-pol~ol materials. This is because .

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SUClI microcapsul~s carl be dispersed in most hot melt media.
In addi.tion, th~ HPC ancl isocyanate-polyol capsules have good pe:r~eab.ility, strength, and temperature characteristics.
In ~eneral, the micxocapsules prepared as indicated above are in the form of an.aqueous dispersion of the microcapsules although most can also be used in the form of a dispersion of microcapsules in a volatile organic solvent. In the prior art methods of making hot melt dispersions of microcapsules starting with a~ueous dis-persions of microcapsules, the microcapsules were removed from the aqueous medium by spray drying, filtering or drying at elevated temperatures. The dried microcapsules . were then redispersed by mechanical means in a hot melt suspending medium~
Using the process of the instan-t invention, the con-version of the microcapsular dispersion in a volatile solvent to a dispersion of microcapsules in a hot melt suspending medium i.s accomplished in one exchange process ~-step~ The process may be either batch or continuous.
In the batch process~ the volatile solvent dispersion of microcapsules and the hot melt suspendi.ng medium,.
either in the forms of.a finely ground powder or preferably as a melted liquid, can be mechanically mixed together in a closed environment, heated to a temperature above melting point of the suspending medium and a vacuum is applied to the closed environment. The temperature must also be above the boiling point of the volatile solvent at the particular - , - lg `

vacuum ws~d. In practice, sucll an en~ironment can be conven1e~ntly prc~duced in a closed vessel such as a resin kettle and in a variety oE additional colNmerc~ally available closed containers where the application of heat and vacuum can be controlled. In this apparatus, a mixture of the dispersion of microcapsules in a volatile solvent and the melted hot melt suspending medium can be in-troduced into the kettle b~tchwise and the heat and vacuum can be ap-plied and maintained until substantially all of the solvent is removed from the system. Dependin~ on the size of the batch and the rate of transfer of heat into the batch, this may take a matter of minutes to several hours~
Turbulent mixing of the low shear type, such as by a rotating paddle, of the mixture in the kettle materially reduces the time of batch treatment and improves the .. . .
dispersion of the microcapsules. For purposes of this application -the term "low-shear" shall be understood to refer to the shear sufficient to perform satisfactory turbulent mixing without at the same~time rupturiny or otherwise causing substantial deterioration of the micro-capsule. It should further be understood that~the shear which can be used satisfactorily will vary depending among other things on the type of microcapsule used.
A preferred form o~ the process can be obtained using a thin film evaporator. Such evaporators are generally tubular in construction with the evaporating section of the tube being equipped with rotating wiper blades.

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-3~4 The wiper blades may contact the cyllndrical walls of the evaporator or thele~ may be a slight gap in the order o~
several thousandths of an inch between the wiper blades and the wall. In either case, a thin ~ilm of the liquid to be S trea-ted is formed on the cylinder wall by the centrifugal action and wiping o~ the rotating blades. The rotating blades continuously agitate the thin ~ilm material being treated and keep it in a turbulenk condition as it passes through the evaporating section. Treatment times are in the ordex of a ~ew seconds. Heat necessary for the evaporation o~ the volatile solvent is applied through the walls of the evaporator. Thus, the temperature of the material being -treated can be maintained at the desired temperature by ~ontrolling the temperature of the applied heat.
Both horizontally and vertically mounted khin film evaporators have been used successfully in the process o this invention. B~ horizontall~ mounted is meant that the axis of the tube and ro-tating wiper blades is horlzontal.
Likewise, in vertically mo~nted thin film evaporators the axis of the tubes and rotating wiper blades is vertlcal.-This thin film evaporator apparatus has the advantage of being capable of operating in a manner in which the aqueous dispersion of microcapsules and liquid hot melt suspending medium can be continuously introduced either separately or as a premix ahead of the rotating wiper blades and withdrawing the dehydrated dispersion of micro-capsules in the hot melt suspendin~ medium at a point after ' :.

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passin~ throucJh the rotating wiper blades of -the evaporator.
~ signi~`:icant aclvaIItage :is thclt the dwell time of the mixture in the evaporator can be a matter oE seconds which ma-terially reduces the poss:ibility o~ degradation and/or deterioration of the microcapsules. In practice the inlet and outlet ports may be located j~st within the rotating blade section of the tube. The particular construction of the evaporator is not asserted to be an inventive feature of this invention The dispersion of microcapsules in the hot melt suspending medium can be withdrawn from the evaporator either continuously or intermittently, as desired, using any con~enient means of removal such as by pumping.
In the preferred form o~ thls process, streams of the lS aqueous dispersion of microcapsules~and the hot melt suspending medium are continuously introduced into a thin film evaporator at the beginning of the rotating blade section The two streams are preferably introduced ~ ~
separately whereby the mixing is done by the rotating wiper blades. These blades may rotate at speeds of,~for example, 600 to 1000 rpm. Turbulent, low shear agitation is main-tained during the evaporation by the rotating wiper blades.
Throughout the preferred process of this invention the temperature is maintalned about the meLting point of the particular hot ~elt suspendin~ medium otherwise the hot melt will solidif~ and not flow through the apparatus used in the process. The process temperature 338~

must also be maintained at a temperature above the boilin~
point o~ water at the vacuum conclitions in the evaporator to prov:icle quick evaporation of the water. Heating the hot me:Lt suspending medium to a temperature above its melting point keEore mixing with the aclueous dispersion of microcapsu1es is essential. The aqueous dispersion of microcapsules is preferably added at room temperature or only slightly above. Maintaining too high a temperature can deteriorate and effectively prohLbit thè ability of the microcapsules to function properly~ High temperatures cause the m.icrocapsules to agglomerate and in some cases cause the microcapsule wall to swell to the point where they lose their contents by permeation or rupture. The tempera-- ture at which this deterioration occurs varies widely depending on the interac-tion of the particular wall-forming material used in making the microcapsules and the parti-cular hot melt suspending medium.
~ Deterioration of the microcapsules~is observed by taking a sample of the hot melt coating composition as ~
it comes from the evaporator and deterMining the permeability of the microcapsules. A permeabillty of 5% is commerclally acceptable. ~ ~
Permeability as herein used is expressed as percent and lS actually 100 times the ratio of the quantity Gf dye obtained by extraction of the capsules by the oil of the internal phase to the total quantity of dye obtained by this extraction plus that obtained by extraction of the ::
, ~ 23 _ ~38Z4 m:icrocapsul~s with c~ material which destroys the capsule wall. In each case, the color o:E the dye was developed by stann.ic chloride and the quantity oE dye was determined spectrophotométrically.
In practice, the preferred wall temperature of the evaporator i5 as high as possible without degrading the microcapsul.es. Under this temperature condition both deterioration of the microcapsules and temporary solidi-fication of the hot melt suspending medium which may appear as small particles due to cooling of the mixture by evaporation of the water .is minim.ized.
The vacuum used in this operation is to reduce the boiling point thus permitting rapid removal of the volatile solvent by evaporation without prolonged exposure of the.
capsules to high temperatures particularly when in contact with water. Microcapsules tend to deteriorate.rapidly with prolonged exposure to water at, say, 100C. Using the wiped film evaporator, the dwe11~time of the mi~ro-capsules in contact with the hot water can be materially reduced being on the average only a few seconds before the . .
water lS evaporated. By metering the flow of the liguid hot melt suspending medium and the aqueous dispersion, the relative ratio of microcapsules to hot melt suspending medium in the resulting hot melt coating composition can be controlled as desired. ~ ~
Mixing as app:lied to the process o~ this invention is of the low shear, turbulent type. The purpose of this .

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turbulent mixing is to brin~ together in intimate contact the di.sper~ion oE t.he microcapsules in a volatile solvent and the hot melt suspending medium. Further, it prevents agglomeration of the microcapsules during the critical exchange oE the continuous phase o~ the microcapsule dis-persion for the continuous hot melt phase.
The followin~ examples illustrate but do not limit lQ the process of the invention. Example 1 illustrates a batch process. Examples 2 an~ 3 illustrate the preerred orm o this process using thin ~ilm evaporators.
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Example 1 A 100 ml. resirl ket~:LQ tYclS Eitted with a paddle stlrrer and vacuum talce-o~E and immersed in a hot water B hath ~ 5 -98C ~ Then 60 gm. each oE an oxazoline wax (Oxawax TS-254AA ohtalned Erom IMC Chemical Group, Inc.
Terre Haute, Indiana~ and a deresinated oxidized montan wax (Hoechst "S" wax obtained from American Hoechst Corp., Chemicals and Plastics Div., Somerville/ New Jersey) were melted together and puk in the kettle. The combined wax had a Fisher-Johns meltin~ p~int of 61-64C and a Brook-field viscosity at 95C of 62 centipoise using a No. 1 spindle at 60 rpm. To the melted wax, 210 gm. of a capsule emulsion containing 38~ by weight isocyanate-polyol micro-capsules wer~ added slowly over a 15 minute period to pre-vent cooling and solidification of the wax. The micro-capsules contained an oil solution of crystal violet~lactone.
A smooth, viscous, cream colored mixture resulted. Restricted water aspirator vacuum was immediately applied with stirring.
~acuum was regulated to cause rapid boiling without causing ~ -excessive foaming of the mass and to maintain -the wax in a fluid state. ~fter boilin~ subsided, full aspirator vacuum was applied, and some solldiiicat~on of the compo- ~ -sition on the stirrer occurred. When boiling was complete, a light tan colored, fluid hot melt coating composition~
had formed. The hot melt coating composition was coated on a 13.5 lb per 1300 square foot paper with a heated metal , , 3~

blade. Tlle coat wei~ht appliecl was abou-t 1.3 lb per 1300 scluare foot oE paper. Upon cooling, the coating hardened and had a ~axy ~eel. When ;maged agains-t a paper coated with a phenol~ormaldehyde novolak resin clear, blue images were formed.

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Equal parts of a deresinated oxidized montan wax (Hoechst "S" wax) and an oxazoline wax (Oxawax (Trade Mark) TS-254AA) were melted together and stored in a reservoir heated to 95C. The combined wax had a Fisher-Johns mel-ting pOillt oE 61-64C and a ~rookfield viscosity at 95C
of 62 centi.poise using a No. 1 spindle at 60 rpm. An aqueous capsule dispersion containing 40%, by weight, hydroxypropylceLlulose microcapsules was prepared and stored in a second unheated reservoir. The hydroxypropyl-cellulose capsules contained an oil solution of crystal violet lactone.
Both the melted wax and aqueous microcapsule dispersion were fed to a thin film evaporator using two Zenith (Trade Mark) metering pumps turned by the same gear drive. The pumps delivered the wax at 4.7 grams per minute and the capsule emulsion at 8.2 grams per minute. The two liquid streams were introduced into a steam jacketed, glass, wiped fi:Lm evaporator (Scientific Glass and Instruments, Inc., Houston, Texas, Catalog No. 12000) at two separate locations just above the vertical wiper blades which rotate at 600 rpm. A temperature of about 100C.
was maintained on the evaporator walls by continuously passing steam at atmospheric pressure through the steam jacket. The pressure within the evaporator was maintained at an absolute pressure of 120 mm. Hg. Vigorous boiling was observed at the point where the streams entered the evaporator and the boiling became less vigorous -2~-' ~ 3~

as the mixture proc,~ressecl clown the evaporator walls.
~o:ilincJ was almost complete]y absent where the ho-t li~ht tan colored dr~ liquid le-t the evaporator. The liquid collected at the bottom of the evaporator had a Brookfield viscosity of 900 centipoise as measured at 90~C with the No. 3 spindle at 60 rpm. The microcapsules appeared to be well dispersed and the dispersion ~as smooth and free '' of lumps. The permeability of the microcapsules in the dispersion was less than 3%.
A 1.3 lb. per 1300 square foo-t coating of the,hot melt , product was applied to a 13.5 lb. per 1300 square foot bond paper using a hot metal blade. The cooled, set coating had a slightly wàx~ feel. When pressure imaged against a novolak resin coated record sheet, a clear, blue, well defined image resulted.

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Example 3 Carnauba wax (No. 3 N.C. Light refined, Dura Commodities Corp., Harrison, N.Y.) was melted and stored in a reservoir heated to 95C. The wax had a Fisher-Johns meLting point oE 81-84C and a Brookfield viscosity at 95C of 25 centipoise using a No. 1 spindle at 60 rpm.
An aqueous capsule dispersion containing 40~, by weight, HPC microcapsules and 3%, by dry weight based on the dry weight of the microcapsules, of a dispersing agent, Tamol (Trade Mark) 731, (Rohm and Haas Company, Philadelphia, Pennsylvania) was prepared and stored in a second unheated reservoir. The HPC capsules contained an oil solution of crvstal violet lactone.
Both the melted wax and aqueous microcapsule dispersion were fed to a thin film evaporator using two Zenith (Trade Mark) metering pumps turned by the same gear drive. The pumps delivered the wax at about 37 grams per minute and the capsule emulsion at about 65 grams per minute. The two liquid streams were introduced into a steam jacketed, horizontally mounted thin film evaporator (one square foot Rototherm(Trade Mark) "V", model (DB) Artisan Industries, Waltham, Mass.) at two separate loca-tions just wlthin the wiped area. The horizontal wiper blades rotated at 1000 rpm. and were mounted with 30 thousandth inch clearance~from the vaporator walls. A
temperature of about 105C. was maintained on the evapo-rator walls by maintaining steam at 3 psi gauge pressure - -in the steam ~

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-jacket. ~he pres5u~e w:i.thin tllc evaporcltor was maintained at an absol.ut:e pressul-e of 25mm. Hg. Boiling was almost completel.~ abse.nt where the ho~ tan colored dry liquid left the evaporator. The li~uid collected at the bottom oE the evapor~tor had a Brookfield viscosity of about 400 centipoise as measured at 90C with the No. 3 spindle at 60 rpm The hot melt coating composition contained about 31~ microcapsules.
As viewed on a hot glass slide under a microscope the micro-capsules appeared to be well dispersed having a dispersion characteristic rating o:E 9 or better. No evidence of rupture of the microcapsules was observed.
A 1~3 lb. per 1300 square foot coating of the hot :
melt product was applied to a 13.5 lb. per 1300 square foot bond paper using a hot metal blade. The cooled, set coating had a slightly waxy feel. When pressure imaged agains~
a novolak resin coated record sheet, a clear, well de~ined image resulted.
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Example 4 To 8 parts o~ a 50% water slurry of titanium di-oxide, 30 parts of 90~ arrowroot starch particles, and 63 parts of a ~0% HPC capsules (containing an oil solution oE
color precursor) aqueous slurry, 56 par-ts of Carbowax (Tracle Markj~000 were added with stirrin~ by paddle stir-rer in a lO00 ml. resin kettle fitted with water aspirator take-off. At room temperature, most oE the Carbowax (Trade Mark) particles dissolved in the water of the capsule slurry. ~ 75C hot water bath was applied to the resin kettle, and the remaining Carbowax (Trade ~ark) dissolved as the temperature of the mass in the resin kettle reached 50C. Water aspirator vacuum of about 90mm. Hg absolute pressure was applied with continued stirring, causing rapid boiling. After about two hours r boiling nearly ceased. The microcapsules appeared to be well dispersed and the dispersion was smooth and free of lumps. The viscosity as measured by a Brookfield viscometer at 100 rpm with a No. 7 spindle was 7800 centip~se at 68C.
The dispersion was coated by a hot blade draw-down on a sheet of 13.5 pounds per 1300 square foot bond ~ ~
paper. The cooled~ set coating had a waxy feel. When ~ -pressure imaged against a phenolformaldehyde novolak resin coated record sheet, a clear, well defined image resulted.

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Example 5 A mixture of 62.5 parts of a ~0~ HPC capsule slurry, 6 parts of a 50% aqueous dispersion of titanium dioxide, and 11.1 parts of 90% Arrowroot starch was stir-red together in a 5000 ml. resin kettle by a paddle stir-rer. In a beaker, 44 parts oE Carbowax (Trade Mark) 5000,

6 parts of Bakelite (Trade Mark) AyAc (a polyvinyl acetate, Union Carbide Corporation, New York, N.Y.), and 12 parts of Arochem (Trade Mark) 650 (a polyester resin, Ashland Chemicals, Columbus, Ohio) were heated to 120C on a hot plate and stirred with a propeller stirrer. When all materials had melted and the mixture was homogeneous, it was allowed to cool to 95C and added to the mixture in the resin kettle with stirring. The Arochem (Trade Mark) resin was caused to precipitate. A white viscous slurry resulted. Heat was applied by an electric heating mantle and water aspirator vacuum of about 25mm. Hg absolute pressure caused rapid boiling until almost all the water was removed. A temperature of about 65C was maintained in the kettle during rapid boiling. When the temperature rose to 95C, water removal was substantially complete.
After 0.5 hours more, the mixture was smooth and~uniform and appeared to be free of lumps. The viscosity as mea-sured by a Brookfield viscometer at 60rpm. with a No. 4 spindle was 10,000 centipolse at 92C. The coating was applied to a paper substrate and imaged as in example 4.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the preparation of a hot melt coating composition containing microcapsules comprising the steps of:
(a) preparing a dispersion of substantially dis-crete microcapsules in a volatile solvent continuous phase said microcapsules containing an oily solution of a chromo-genic material, said microcapsules being substantially oil and water impermeable;
(b) preparing a hot melt suspending medium, said hot melt suspending medium having a melting point of from about 50°C to about 140°C and a melting range of less than about 15°C, said hot melt suspending medium being in a fluid state;
(c) mixing together with turbulent agitation said dispersion of substantially discrete microcapsules in a vola-tile solvent continuous phase and said hot melt suspending medium;
(d) applying heat and a vacuum to said mixture, while maintaining said agitation, until said volatile solvent is substantially removed from said mixture to form a dis-persion of substantially discrete microcapsules in said hot melt suspending medium, said heat being applied at a rate sufficient to maintain said mixture at a temperature above the melting point of said hot melt suspending medium, said temperature also being above the boiling point of water at said vacuum.

2. A process for the preparation of a hot melt coating composition containing microcapsules comprising the steps of:

(a) preparing a dispersion of substantially dis-crete microcapsules in a volatile solvent continuous phase said microcapsules containing an oily solution of a chromo-genic material, said microcapsules being substantially oil and water impermeable;
(b) preparing a hot melt suspending medium, said hot melt suspending medium having a melting point of from about 50°C to about 140°C and a melting range of less than about 15°C;
(c) heating said hot melt suspending medium to a temperature above the melting point of said hot melt sus-pending medium to form a liquid hot melt suspending medium;
(d) mixing together with turbulent agitation said dispersion of substantially discrete microcapsules in a vola-tile solvent continuous phase and said liquid hot melt sus-pending medium to form an intimate mixture of said dispersion of microcapsules and said hot melt suspending medium;
(e) applying a vacuum to said mixture, while maintaining said agitation and said temperature, until said volatile solvent is substantially removed from said mixture to form a dispersion of substantially discrete microcapsules in said liquid hot melt suspending medium, said temperature being above the boiling point of water at said vacuum level.

3. The process of Claim 2, wherein said hot melt suspending medium is characterized by the presence of one or more functional groups selected from the groups consisting of: carboxyl, carbonyl, hydroxyl, ether, methoxy, ethoxy, ester, amide, amine, heterocyclic groups and combinations thereof to impart polarity thereto.

4. The process of Claim 2, wherein said volatile solvent is water.

5. The process of Claim 4, wherein said hot melt suspending medium is water insoluble.

6. The process of Claim 4, wherein said hot melt suspending medium is water soluble.

7. The process of Claim 2, wherein said mixture contains a dispersing agent.

8. The process of Claim 2, wherein metered quan-tities of said dispersion of microcapsules and said liquid hot melt suspending medium are continuously added with mix-ing to an evaporator provided with a source of heat, vacuum and continuous turbulent agitation and said mixture with substantially all of the volatile solvent removed is conti-nuously withdrawn from said evaporator.

9. The process of Claim 8, wherein said mixing of said dispersion of microcapsules and said liquid hot melt suspending medium occurs within said evaporator.

10. The process of Claim 2, wherein said micro-capsules are prepared by encapsulating an oil solution of a chromogenic material in a wall forming compound selected from the group consisting of hydroxypropylcellulose, carboxy-methylcellulose, gelatin, melamine-formaldehyde, polyfunc-tional isocyanates and prepolymers thereof, polyfunctional acid chlorides, polyamines, polyols, epoxides and mixtures thereof.

11. The process of Claim 6, wherein said step of mixing together with agitation said aqueous dispersion of substantially discrete microcapsules and said liquid hot melt suspending medium comprises dissolving said liquid water soluble hot melt suspending medium in said aqueous continu-ous phase of said aqueous dispersion of microcapsules, said dissolving resulting in the formation of a homogeneous mixture.

12. A process for the continuous preparation of a hot melt coating composition containing microcapsules com-prising the steps of:
(a) preparing a dispersion of substantially dis-crete microcapsules in an aqueous continuous phase said micro-capsules containing an oily solution of a chromogenic material, said microcapsules being substantially oil and water imper-meable;
(b) preparing a water insoluble hot melt suspen-ding medium, said hot melt suspending medium having a melting point of from about 50°C to about 140°C and a melting range of less than about 15°C; said hot melt suspending medium being characterized by the presence of one or more functional groups selected from the group consisting of: carboxyl, carbonyl, hydroxyl, ether, methoxy, ethoxy, ester, amide, amine, heterocyclic group and combinations thereof to impart polarity thereto;
(c) heating said water insoluble hot melt suspen-ding medium to a temperature above the melting point of said hot melt suspending medium to form a liquid hot melt suspen-ding medium;
(d) mixing together with turbulent agitation said aqueous dispersion of substantially discrete microcapsules and said liquid hot melt suspending medium to form an inti-mate mixture of said aqueous dispersion and said liquid hot melt suspending medium;
(e) applying a vacuum to said mixture, while main-taining said turbulent agitation and said temperature, until water is substantially removed from said mixture to form a dispersion of said substantially discrete microcapsules in said liquid water insoluble hot melt suspending medium, whereby said agitation, said temperature and said vacuum are sufficiently low to prevent substantial deterioration of said microcapsules, said temperature being above the boiling of water at said vacuum level.

13. The process of Claim 12, wherein said mixture additionally contains a dispersing agent.

14. A process for the continuous preparation of a hot melt coating composition containing microcapsules comprising the steps of:
(a) preparing a dispersion of substantially dis-crete microcapsules in an aqueous continuous phase said microcapsules containing an oily solution of a chromogenic material, said microcapsules being substantially oil and water impermeable;
(b) preparing a water-soluble hot melt-suspending medium, said hot melt suspending medium having a melting point of from about 50°C to about 140°C and a melting range of less than about 15°C; said hot melt suspending medium being characterized by the presence of one or more functional groups selected from the group consisting of: carboxyl, carbonyl, hydroxyl, ether, methoxy, ethoxy, ester, amide, amine, heterocyclic groups and combinations thereof to impart polarity thereto;
(c) heating said water soluble hot melt suspending medium to a temperature above the melting point of said hot melt suspending medium to form a liquid water soluble hot melt suspending medium;
(d) mixing together with turbulent agitation said aqueous dispersion of substantially discrete microcapsules and said liquid water soluble hot melt suspending medium until said liquid water soluble hot melt suspending medium has dissolved into said aqueous continuous phase of said dispersion of microcapsules, said dissolving resulting in the formation of a mixture; and (e) applying a vacuum to said mixture, while maintaining said turbulent agitation and said temperature, until water is substantially removed from said mixture to form a dispersion of said substantially discrete microcap-sules in said liquid water soluble hot melt suspending medium, whereby said agitation, said temperature and said vacuum are sufficiently low to prevent substantial deteriora-tion of said microcapsules, said temperature being above the boiling point of water at said vacuum level.

15. The process of Claim 14, wherein said mixture additionally contains a dispersing agent.

16. A liquid hot-melt coating composition compri-sing:
(1) a hot melt suspending medium characterized by:
(i) being substantially water soluble;
(ii) being characterized by the presence of one or more functional groups selected from the group consisting of: carboxyl, carbonyl, hydroxyl, ether, methoxy, ethoxy, ester, amide, amine, heterocyclic groups and combinations thereof to impart polarity thereto;
(iii) having a melting point of from about 60°C
to about 140°C and a melting point range of less than about 15°C; and (2) an encapsulated chromogenic material substan-tially dispersed therein, said hot melt suspending medium being compatible with colour forming characteristics of said capsular, chromogenic material.

17. A liquid choromogenic coating composition comprising:
(a) a hot melt suspending medium, said hot melt sus-pending medium being water soluble and having a melting point of from about 50°C to about 140°C and a melting point range of less than about 15°C, said hot melt suspending medium being further characterized by the presence of one or more functional groups selected from the group consisting of: carboxyl, carbonyl, hydroxyl, ehter, methoxy, ethoxy, ester, amide, amine, heterocyclic groups and combinations thereof to impart polarity thereto; and (b) a microencapsulated chromogenic material, said chromogenic material being a color precursor of the electron donating type said chromogenic material being mixed with a carrier oil to form an oil solution of said chromogenic color pre-cursor material, said oil solution being micro-encapsulated by combination with one or more wall forming compounds.

18. The coating composition of Claim 17 wherein said color precursor is selected form the group consisting of: lactone-phthalides, actone fluorans, lactone xanthenes, leucoauramines, l-(omega sustituted vinylene) 3,3-disubstituted-3-H-indoles,1,3,3-trialkylindolinospirans and mixtures thereof.

19. The coating composition of Claim 17, wherein said one or more wall forming compounds are selected from the group consisting of: hydroxypropylcellulose, carboxymethylcellulose, gelatin, methylcellulose, melamine-formaldehyde, polyfunctional isocyanates and prepolymers thereof, polyfunctional acid-chlorides, polyamines, polyols, epoxides and mixtuers thereof.

20. A liquid chromogenic coating composition comprising:
(a) a hot melt suspending medium, said hot melt sus-pending medium being characterized by (i) being substantially water soluble, (ii) being characterized by the presence of one or more functional groups selected from the group consisting of: carboxyl, carbonyl, hydroxyl, ether, methoxy, ethoxy, ester, amide, amine, heterocyclic groups and com-binations thereof to impart polarity thereto, (iii) having a melting point of from about 50°C to about 140°C and a melting range of from about 0°C to about 15°C, (b) a microencapsulated chromogenic material, said chromogenic material being a color precursor of the electron donating type sald chromoqenic material being mixed with a carrier oil to form solution of said chromogenic color precursor material, said oil solution belng micro-encapsulated by combination with one or more wall forming compounds selected from the group consisting of: hydroxypropylcellulose, methyl-cellulose, carboxymethylcellulose, gelatin, melamine-formaldehyde, polyfunctional isocyanates and prepolymers, polyfunctional acid chlorides, polyamines, polyols, epoxides and mixtures thereo.

21. A pressure-sensitive carbonless transfer sheet com-prising:
(a) a paper substrate having a front and a back surface; and (b) a coating composition adhered to at least one of said front and back surfaces, said coating composition being set to a flexible, tack-free coat, said coating composition including:
(1) a hot melt suspending medium characterized by:
(i) being substantially water soluble;
(ii) being characterized by the presence of one or more functional groups selected from the group consisting of: carboxyl, carbonyl, hydroxyl, ether, methoxy, ethoxy, ester, amide, amine, heterocyclic groups and combinations thereof to impart polarity thereto;
(iii) having a melting point of from about 60°C to about 140°C and a melting point range of less than about 15°C; and (2) an encapsulated chromogenic material sub-stantially dispersed therein, said hot melt suspending medium being compatible with color forming characteristics of said capsular, chromogenic material.