GB1603448A - Microcapsules and their use - Google Patents

Description

(54) MICROCAPSULES AND THEIR USE (71) We, THE MEAD CORPORATION, a Corporation organized and existing under the laws of the State of Ohio, United States of America, of Mead World Headquarters, Courthouse Plaza Northeast, Dayton, Ohio 45463, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement : This invention relates to the production of microcapsules containing an oil and having a primary capsule wall of a reaction product of hydroxypropylcellulose and a cross-linking agent and a secondary capsule wall formed by the reaction of a phenol and an aldehyde. The microcapsules produced by the process of this invention are particularly adaptable for use in preparing pressure-sensitive carbonless transfer papers.

A process for the production of microcapsules containing oils using coacervation is disclosed in U. S. Patent 2,800,457 (1957) to Green et al. The process described therein involves the coating of oil droplets with a liquid wall of gelatin-gum arabic colloidal material produced by coacervation. The liquid wall is hardened by treatment with formaldehyde. Microcapsuies made by the process of Green et al have had wide commercial acceptance, particularly in the field of pressure-sensitive carbonless copy papers.

Since that time, microcapsules have been made using many types of wall- forming materials, wall-forming processes and wall structures. The microcapsules produced have been suggested for many uses, including the encapsulation of aromas, perfumes, flavouring agents, adhesives, reactants, colour reactants, pharmaceuticals, pigments and as opacifying agents. The nature of the material to be encapsulated and ultimate use of the microcapsules many times dictated the materials, structure and process of making them.

Dual-walled microcapsules having inner (primary) walls and outer (secondary) ualls of different wall-forming materials have been proposed in the following: U. S. Patent 2,969,331 (1961) Brynko et al U. S. Patent 3,551,346 (1970) Breen et al U. S. Patent 3, 578, 605 (1971) Baxter It has also been proposed in the following that single walled microcapsules could be made in which the wall-forming material is a condensation polymer of resorcinol and an aldehyde : U. S. Patent 3, 755, 190 (1973) to Hart et al U. S. Patent 3, 816, 331 (1974) to Brown, Jr. et al Canadian Patent 879,043 (1971) to Bayless Additionally, the following patents disclose processes for producing an aqueous dispersion of microcapsules containing an oil. The microcapsules have hydrophilic (water-swellable) polymeric capsule walls which are impregnated with or otherwise contain resorcinol which is then reacted with formaldehyde.

U. S. Patent 3,576,660 (1971) to Bayless et al U. S. Patent 3,726,803 (1973) to Bayless et al U. S. Patent 3,803,046 (1974) to Matsukawa et al Carbonless copy paper, briefly stated, is a standard type of paper wherein during manufacture the backside of the paper substrate is coated with what is referred to as a CB or transfer coating, the CB coating containing one or more chromogenic materials, generally in capsular form. At the same time the front side of the paper substrate is coated during manufacture with what is referred to as a CF coating, which contains one or more chromogenic material. Both the chromogenic materials remain in the coatings on the respective surfaces of the paper in substantially colourless form. This is true until the CB and CF coatings are brought into overlying relationship and sufficient pressure, as by a typewriter, is applied to rupture the CB coating to release the encapsulated chromogenic material. At this time the chromogenic materiat contacts the CF coating and reacts with the chromogenic material therein to form a coloured image. Carbonless copy paper has proved to be an exceptionally valuable image transfer media 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 coatings are in an inactive state as the coreactive elements are not in contact with one another until pressure is applied. Patent proposas relating to carbonless copy paper products include : U. S. Patent 2,550,466 (1951) to Green et al U. S. Patent 2, 712, 507 (1955) to Green U. S. Patent 2, 730,456 (1956) to Green et al U. S. Patent 3,016,308 (1962) to Macaulay U. S. Patent 3,170,809 (1965) to Barbour U. S. Patent 3,455,721 (1969) to Phillips et al U. S. Patent 3, 466, 184 (1969) to Bowler et al U. S. Patent 3,672,935 (1972) to Miller et al U. S. Patent 3, 955,025 (1976) to Matsukawa et al U. S. Patent 3, 981, 523 (1976) to Mallouf A disadvantage of coated paper products such as carbonless transfer papers stems from the necessity of applying a liquid coating composition containing the colour forming ingredients during the manufacturing process. In the application of such coatings, volatile organic solvents are sometimes used which then in turn requires evaporation of excess solvent to dry the coating thus producing volatile solvent vapours. An alternate method of coating involves the application of the colour forming ingredients in an aqueous slurry, again requiring removal of excess water by drying. Both such methods suffer from serious disadvantages. In particular the organic solvent coating method necessarily involves the production of generally volatile solvent vapors, creating both a health and a fire hazard in the surrounding environment. When using an aqueous solvent system the water must be evaporated which involves the expenditure of significant amounts of energy.

Further, the necessity of a drying step requires the use of complex and expensive apparatus to continuously dry a substrate which has been coated with an aqueous coating compound. The application of heat not only is expensive, making the total paper manufacturing operation less effective, but also is potentially damaging to the encapsulated chromogenic materials which are generally coated on to the paper substrate during manufacture. High degrees of temperature in the drying step require specific formulation of coating compositions which permit the use of excess heat. The problems encountered in the actual coating step are generally attributable to the necessity for a heated drying step following the coating operation.

In an attempt to overcome difficulties associated with the coating and drying of microcapsular compositions, attempts have been made to use liquid coatings containing substantially no volatile solvent. Notable among these attempts are the use of hot melt coating compositions as suggested in the following patents: U. S. Patent 3, 016, 308 (1962) to Macaulay U. S. Patent 3, 079, 351 (1963) to Staneslow et al U. S. Patent 3,864,549 (1972) to Shank The use of hot melt coatings containing microcapsules for CB transfer coatings is disclosed in both Staneslow et al and Macaulay. Macaulay also discloses preparation of hot melt CB coating compositions using spray dried microcapsuies, Improved coating compositions containing microcapsules having improved dispersion of the microcapsules within the hot melt suspending media are disclosed in our co-pending British Patent Application No. 19423/77 (Serial No. 1, 581,757).

The present invention provides a novel method by which novel dual-walled microcapsules can be produced. Use of these microcapsules is particularly advantageous in the preparation of hot melt coating compositions for pressuresensitive carbonless copy papers. The microcapsules are produced in an aqueous continuous phase and the microcapsules can be easily removed from the aqueous dispersion by vacuum filtering. The still wet microcapsules can be easily incorporated into a hot helt in a manner disclosed in the above co-pending application. Additional advantages of the microcapsules are high strength and their capability of being more efficiently used than most prior art microcapsules in the production of CB transfer coatings. Reductions in microcapsular coat weights are thus possible.

According to a first aspect of the present invention, there is provided a process for preparing filterable microcapsules comprising the steps of: (a) preparaing an aqueous dispersion of microcapsules containing an oil, said microcapsules having a primary capsule wall comprising a reaction product of hydroxypropylcellulose and at least one cross-linking agent for hydroxypropylcellulose, said primary capsule wall being substantially oil and water impermeable ; (b) adding and mixing into said aqueous dispersion of microcapsules a phenol and an aldehyde ; and (c) maintaining said mixing while said phenol and said aldehyde react together to form a condensation polymer, said condensation polymer being insoluble in said aqueous dispersion of microcapsules thereby precipitating said condensation polymer on said primary capsule wall as a secondary capsule wall.

In a second and alternative aspect of this invention, we provide microcapsules containing an oil, said microcapsules having a primary capsule wall and a secondary capsule wall, said primary capsule wall comprising a reaction product of hydroxypropylcellulose and at least one cross-linking agent for hydroxypropylcellulose, said primary capsule wall being substantially impermeable to oil in water, said secondary capsule wall comprising a condensation polymer of a phenol and an aldehyde.

The invention also extends to the production of pressure sensitive carbonless transfer sheets utilising said said microcapsules or microcapsules produced by the said process.

The embodiments of our process described hereinafter involve a further treatment of known microcapsules having substantially impermeable walls comprising the reaction product of hydroxypropylcellulose and at least one oil soluble cross-linking agent for hydroxypropylcellulose. The treatment involves adding to an aqueous dispersion of microcapsules, as defined above, a phenol and an aldehyde. The phenol and aldehyde react with each other to form a condensation polymer. The condensation polymer, being insoluble in the aqueous continuous phase of the dispersion, precipitates on the primary walls of the microcapsules as secondary walls. Microcapsules produced by such a process are effectively dual walled in that a secondary capsule wall of precipitated condensation polymer deposits on the primary cross-linked hydroxypropylcellulose wall.

In a preferred embodiment of this invention, microcapsules having a primary capsule wall are produced according to the process disclosed in U. S. Patent 4,025,455 issued May 24, 1977 to Dale Richard Shackle and assigned to The Mead Corporation of Dayton, Ohio. Microcapsules as disclosed in U. S. Patent 4,025,455 have a cross-linked hydroxypropylcellulose primary wall formed by the reaction of hydroxypropylcellulose in an aqueous continuous phase with at least one crosslinking agent for hydroxypropylcellulose in a dispersed oil phase. the microcapsules may contain a chromogenic material dissolved in the oil phase. The resultant primary wall is substantially impermeable to the microencapsulated oil and the water in the aqueous continuous phase. The hydroxypropylcellulose, hereinafter sometimes referred to as HPC, is a film-forming cellulosic ether polymer soluble in cold water, but insoluble in hot water. The commercially available polymers have a molar substitution (M. S.) of about three to five hydroxypropyl units to each cellulose unit. A particular group of hydroxypropylcelluloses are manufactured by Hercules, Inc. and sol by them under the trade name of Klucel. These polymers precipitate out of a water solution at a temperature of about 45 C and preferably at from 45 C to 52 C as a finely divided solid precipitate. The polymers are available in a variety of viscosity types.

The lower viscosity types, G, J, L and E are preferred for use in the practice of this invention. Of these, Type L having a molecule weight of approximately 75,000 and an M. S. of approximately three has been found to be particularly useful.

The aqueous phase, containing HPC, can be prepared by dissolving the HPC in water at a temperature below 40 C, preferably from IO C-30 C. To facilitate the preparation, the HPC can be dispersed in the water at 45 C or above prior to lowering the temperature of the dispersion to below 40 C to dissolve the HPC. The concentration of the HPC is not critical, but would depend on the ratio of aqueous phase to the material to be encapsulated, the size of the desired oil solution droplets and the desired thickness of the microcapsular walls. Aqueous solutions containing from 0.2% to 6%, preferably 1. 5'. to 4% of HPC are suitably used in the preparation of the aqueous phase.

The oil phase is prepared by combining an oil soluble cross-linking agent for the HPC wall-forming compound with an oil. The oil solution can be prepared by adding and stirring the oil soluble cross-linking agent to the oil while the mixture is cool, preferably below 15 C. The choice of oil depends largely on the final utilization of the microcapsules. If, for example, the microcapsules are to be used in preparing pressure-sensitive paper, the oil should be a carrier oil for a chromogenic material. Such carrier oils are preferably monoisopropylbiphenyl (MIPB), chlorinated biphenyls, alkylnaphthalenes, kerosene, petroleum naphtha or mixtures thereof.

The oil soluble cross-linking agents used in this embodiment for forming the oil phase are those containing more than one group capable of reacting with hydroxyl groups thus providing the desired cross-linkage with the HPC wall- forming compound. The cross-linking agents must be soluble in the oil phase and not reactable with the oil or interfere with the desired function of any component of the oil phase. For example, if an oil solution of a colour precursor is desired to be encapsulated and coated on paper, the cross-linking agent should not interfere with the colour producing function of the resultant paper. In general polyfunctional isocyanates, acyl chlorides, sulphonyl chlorides, alkylene bischloroformates or mixtures thereof can be used. If microcapsules for use in the preparation of pressure-sensitive papers are desired, the polyfunctional isocyanates or prepolymers containing more than one reactive isocyanate group are preferred.

The concentration of the oil soluble cross-linking agent in the oil phase is not critical. The degree of cross-linking desired is dependent on the end utilization of the microcapsules. For example, if the microcapsules are to be incorporated into an aqueous coating composition, sufficient reactive groups must be present to react with available hydroxyl groups of the wall-forming compound to render the wall- forming compound water insoluble. We have, however, shown that the superiority of microcapsules formed by the presently described process result from the use of HPC and not from the particular oil soluble cross-linking agent. In particular, any of the cross-linking agents exhibiting the above recited characteristics can be used in combiation with HPC to produce the desired microcapsules.

Emulsification of the oil phase in the aqueous phase can be accomplished bv adding the oil phase to the aqueous phase with vigorous mixing. Mixing may be by stirring, shaking or milling. During the mixing operation, the temperature of the mixture must be kept below the temperature at which the HPC wall forming compound precipitates. A small amount of an emulsifier, such as about 0.2t% b) weight Turkey Red Oil (sulphonated castor oil) can be added to the aqueous phase prior to mixing. However, the hydroxypropylcellulose acts as an emulsifier and an acceptable dispersion of droplets can be obtained in the absence of an additional emulsifier. Droplet sizes of from I micron to 100 microns can be produced.

After the desired droplet size is obtained, the mixed emulsion is heated with mild agitation to a temperature above the precipitation temperature of the HPC wall-forming compound. The wall forming compound precipitates on the droplets to form microcapsule watts. The temperature increase promotes cross-linking of the HPC wall forming compound with the oil soluble cross-linking agent. The temperature can be maintained with continued agitation of the mixture for a sustained period of time, preferably more than I hour, and most preferable 3 to 16 hours, if microcapsules of low oil and water permeability are desired. However, longer periods do not appear to be detrimental. The exact temperature is not critical so long as the precipitation temperature of the HPC wall forming compound is reached. Satisfactory microcapsules may be made using a hydroxypropylcellulose where a temperature of at least 45 C, preferably 45 C to 52 C, can be maintained for a period of 1 to 16 hours.

The preferred phenols for use in this invention are, for example, resorcinol, hydroquinone, catechol, o-cresol, m-cresol, p-cresol, Bisphenol A, naphthalenediols, gallic acid, phloroglucinol, pyrogallol, guaiacol, digallic acid, tannic acid, the chlorophenols, the xylenols, eugenol, the hydroxybiphenols and the naphtols. Mixtures of these phenolic materials may also be used. The phenol most preferred is resorcinol.

The preferred aldehydes are, for example, acetaldehyde, formaldehyde, glyoxal, glutaraldehyde, and furfural. Mixtures of these aldehydes may also be used. The aldehyde most preferred is formaldehyde which is preferably used in its commercial form as an aqueous solution of 37% formaldehyde.

If resorcinol is used, the weight ratio of resorcinol to dry microcapsules is suitably from 0.015 to 0. 15. The preferred range is from 0.02 to 0. 10 and the most preferred range is 0.03 to 0.07. The ratio of formaldehyde to resorcinol is suitably from 0.2 to 2.0. The preferred range is from 0.4 to 1. 5, and the most preferred range is from 0.6 to 1. 3.

The phenols and aldehydes may be added to the aqueous dispersion of microcapsules in any manner which provides thorough mixing of the added ingredients with the aqueous dispersion. A preferred manner of adding them is in the form of aqueous solutions. An aqueous phenol solution is preferably added and mixed into the aqueous dispersion prior to adding and mixing in an aqueous aldehyde solution. An aqueous solution of urea may be added, if desired. At this point, the phenol and aldehyde react together to form a condensation polymer. The condensation polymer is insoluble in the aqueous dispersion of microcapsules and precipitates out in the primary capsule walls to form a secondary capsule wall.

Conditions which promote the formation of acceptable secondary walls are as follows : The pH of the aqueous dispersion containing the phenol and aldehyde is preferably 6 or below. A most preferred pH range is from 2 to 4. The reaction temperature is suitably between 25 C and 70 C. The preferred range is 35 C to 55 C, and the most preferred rnage is 45 C to 50 C. Under these conditions, the condensation polymer can be substantially completely formed in from I hour to 16 hours and under preferred conditions from 2 hours to 5 hours.

Although the addition of the phenol may bring the pH of the aqueous dispersion below a pH of 6, it is desirable in most cases to add a small amount of dilute acetic acid to the aqueous dispersion to bring the pH into the preferred or most preferred range. Sulphuric acid or glacial acetic acid may also be used instead of dilute acetic acid.

The reaction of the phenol and aldehyde to form a condensation polymer is also accompanied by an interaction of the phenol and/or aldehyde with the ingredients in the primary wall to provide a tight bond between the primary and secondary walls of the microcapsule. The secondary wall surrounds and adheres tightly to the primary capsule wall.

After the phenovaidehyde reaction is substantially complete, the pH of the aqueous dispersion of microcapsules is suitably raised to 7 or above by the addition of a base such as sodium hydroxide. At this point, the microcapsules can be filtered, such as by vacuum filtration, and air dried, if desired, to a microcapsular powder. The microcapsular powder can, for example, be dispersed in a liquid (melted) hot melt suspending medium to form a hot melt coating composition in the manner described in our British Patent Application 19423/77 (Serial No.

1, 581, 757) mentioned supra. The coating composition can be applied to a substrate, and allowed to cool to a solid coating. The hot melt medium binds the microcapsules to the substrate. Alternatively, the aqueous dispersion of microcapsules, preferably containing additionally a water dispersible binder, such as latex or starch, can be formed into an aqueous coating composition. This composition can be coated onto a substrate and air dried. The substrate can be paper, plastics film or fabric. A preferred substrate for both the hot melt composition and aqueous composition is paper. Stilt material may be added, if desired, to prevent premature breakage of the microcapsules. The coating compositions can be set to a solid coating by cooling or by drying.

In a preferred arrangement, the microcapsules can contain an oil solution of a chromogenic material, such as a colour precursor, colour former or colour developer. Chromogenic materials most useful for this purpose are the electron donor type colour precursors. Microcapsules coated papers containing oil solutions of these chromogenic materials can be used as pressure-sensitive carbonless copy papers. Microscopic examination of the microcapsules has shown the presence of significant amounts of polynuclear microcapsule particles in the resultant dispersion. This phenomenon is apparently due to the aggregation (clustering) of the smaller microcapsules having a primary wall prior to formation of the secondary wall. The secondary wall envelopes the whole cluster causing the formation of the polynuclear microcapsule particles. The larger primary walled microcapsules were not observed to cluster. The ratio of polynuclear microcapsule particles to mononuclear microcapsules particles can be controlled by the reactants, the reaction temperature and pH-value. Higher temperatures, lower pH, and larger amounts of reactants, for example, resorcinol, during the condensation step favour clustering and therefore, increase the formation of the polynuclear microcapsule particles. The process described herein can substantially eliminate the smaller microcapsules particles by the formation of such polynuclear microcapsule particles and thus can control the particle size of the final microcapsules to a narrow range. For coating on a paper substrate, the particle size distribution is preferably between 3 microns and 25 microns, with the average particle size being preferably between 7 to 12 microns.

A comparison of examples of microcapsules containing chromogenic material before and after provision of the secondary wall demonstrates the advantages obtained by this practice of this invention. Microcapsules provided with the secondary wall showed an improvement in typewriter intensity of a microcapsular coated fibrous substrate for a given microcapsular coat weight over that of the secondary wall untreated (i. e. prior art) microcapsules. This improvement apparently is due to the clustering effect of the treatment process which promotes the aggregation of the smaller microcapsules during the formation of the secondary wall. In the case of the untreated (prior art) microcapsules, these smaller microcapsules appear to be small enough to penetrate the surface of a paper substrate, lodging in the interstices between the paper fibres, and do not rupture under ordinary writing or typing pressure. In contrast to this, the clustered polynuclear microcapsule particles do not penetrate the paper surface and are thus available for rupturing during pressure imaging. Additionally, the treated microcapsules are easily filtered using a vacuum filter whereas the untreated microcapsules cannot be filtered. The treated microcapsules are also stronger and more heat resistant. Thus, they can be used for a variety of uses. A preferred use is in the production of hot melt coatings for pressure-sensitive paper.

The following examples illustrate the production of these filtrable microcapsules and pressure-sensitive carbonless copy papers made using the filterable microcapsules.

Example I A carrier oil solution of colour precursors was prepared by dissolving 7 grams of crystal violet lactone, 0.9 grams of 3,3-bis (l'-ethyl-2'-methylindol-3 yl) phthalide, 1. 8 grams of 3-N, N-diethyl-amino-7- (N, N-dibenzylamino) fluoran, and 2. 9 grams of 3-N, N-diethylamino-6, 8-dimethyl-fluoran in 150 ml of monoisopropylbiphenyl (MIPB) at 90 C. This oil solution was then cooled to 15 C. To this carrier oil solution was added 2.5 grams of Desmodur N 100 (a liquid biuret reaction product of hexamethylene diisocyanate and water in 3 to I molar ratio made and sold by Mobay Chemical Co.-'Desmodur'is a Registered Trade Mark), 2.5 grams of NIAX SF-50 (a trifunctional aromatic polyurethane prepolymer having a free isocyanate content of 32.5 . ; made and sold by Union Carbide-'NIAX'is a Registered Trade Mark) and I drop of dibutyl tin laurate as a catalyst for the hydroxypropylcellulose-polyisocyanate reaction. An aqueous solution of hydroxypropylcellulose was prepared by dissolving 5 grams of hydroxypropylcellulose (Klucel L-Hercules, Inc) in 215 ml of distilled water at room temperature.

The aqueous solution of hydroxypropylcellulose was charged into a Sunbeam blender and the speed was set at 8. The carrier oil solution was slowly added to the blender and stirred thereafter for about) minute. The emulsion formed in the blender was transfered to a 600 ml metal beaker and the beaker was put in a 50 C water bath. The emulsion was stirred with paddle stirrer for about I hour during which time the microcapsules were formed. The aqueous dispersion of microcapsules was treated as follows : A solution of 0.7 grams of urea and 2.7 grams of resorcinol in 50 ml of water was warmed to 50 C and charged to an addition funnel. A solution of 5. 4 ml of 37% aqueous formaldehyde was dissolved in 10.8 ml of water at room temperature and charged to a second addition funnel. The urea-resorcinol solution was slowly dripped into the aqueous dispersion of microcapsules while stirring over a period of about 25 minutes. The reaction mixture was kept at 45-50 C. The formaldehyde solution was then slowly dripped into the reaction mixture and the reaction mixture was maintained at 45-50 C with stirring for 3.5 hours.

The resulting dispersion of microcapsules was coated on a paper substrate and dried, and the dried coated paper performed well as the CB part of a carbonless form using a phenolic (novolac) resin coated paper as the CF part.

Example 2 The procedure of Example I was repreated except that the pH of the reaction mixture was adjusted to 2.5 with 0.5 ml of 10% sulphuric acid prior to the addition of the formaldehyde solution. After a 3.5 hour reaction time, the pH of the reaction mixture was readjusted to 9.6 with sodium hydroxide thus stopping the reaction.

About half of the reaction mixture was vacuum filtered at about 30 mm Hg absoute pressure and the resultant filter cake was washed twice with 100 ml of distilled water. A light gray filter cake of microcapsules was obtained. 40 grams of methoxy polyethylene glycol (Carbowax 5000-Union Carbide-'Carbowax'is a Registered Trade Mark) was melted on a hot plate and 12 grams of the still wet capsules were added to the melted Carbowax while stirring. The melted Carbowax containing the microcapsules was coated on to a 34.5 pound per 3300 square foot paper substrate using a hot glass rod. The resulting coated paper substrate was cooled and the coated paper substrate performed well as the CB part of a carbonless form using a phenolic (novolac) resin coated paper as the CF part.

Example 3 All amounts are in parts by weight. A carrier oil solution of colour precursors was prepared by dissolving 53.2 parts of crystal violet lactone, 6.9 parts of 3,3bis (l'-ethyl-2'-methylindol-3-yl) phthalide, 13. 5 parts of 3-N, N diethylamino-7- (N, N-dibenzylamino) fluoran, and 16. 1 parts of 3-N, N diethylamino-6, 8-dimethylfluoran in 1142 parts of monoisopropylbiphenyl (MIPB) at 90 C. This oil solution was then cooled to H C. To this carrier oil solution was added 50 parts of Desmodur N-100, 18 parts of NIAX SF-50 and 0.05 parts of dibutyl tin dilaurate catalyst. An aqueous solution of hydroxypropylcellulose was prepared by dissolving 50 parts of hydroxypropylcellulose in 2082 parts of water at room temperature.

The carrier oil solution was emulsified into the aqueous solution of hydroxypropylcellulose for about 45 minutes. The resultant emulsion was heated to 47 C while stirring and the temperature was maintained between 47 C and 50 C for a period of 2 hours to form the microcapsules. At this point 160 parts of the aqueous dispersion of microcapsules was removed from the process and designated "Comparative Sample A". The remaining aqueous dispersion of microcapsules was treated as follows : A solution of 7.6 parts of resorcinol in 37 parts of water was added to the aqueous emulsion in a period of 3 minutes. A solution of 7 parts of glacial acetic acid in 37 parts of water was added in a period of 4 minutes. A solution of 18. 3 parts of 37% aqueous formaldehyde in 24.6 parts of water were added in a period of 3 minutes. The reaction mixture was maintained at 46.8 to 48.2 C with stirring for 2 hours. A solution of 5 parts of sodium hydroxide in 19. 8 parts of water was added to the reaction mixture to bring the reaction mixture to a pH of 7.8 and stop the resorcinol-formaldehyde reaction. The aqueous dispersion of treated microcapcules was designated"Treated Sample B".

Particle size determinations were made on Samples A and B by means of a Coulter Counter (a commercially available electromc particle size counter made and sold by Coulter Electronics, Inc., Hialeah, Fla.). The results were as follows : Particle Size (Diameter of Microcapsules Percent by in Microns) Weight Comparative Sample A Treated Sample B 25 9.1 and above 12. 3 and above 50 6.6 and above 9.7 and above 75 4.7 and above 7.7 and above 100 1.3 and above 3.1 and above As shown in the above table, the size of the microcapsules increased during treatment of the aqueous dispersion of microcapsules with resorcinol and formaldehyde. Microscopic examination of Samples A and B showed an aggregation (clustering) of the smaller microcapsules in Treated Sample B whereas in Comparative Sample A substantially no clustering was observed.

Example 4 The procedure of Example 3 was repeated except that the resorcinol was reduced to 6 parts instead of 7.6 parts as in Example 3. Samples of the aqueous dispersion of microcapsules before and after treatment with resorcinol and formaldehyde were obtained. These samples were designated respectively "Comparative Sample C"and"Treated Sample D". A coating composition was prepared using each of Samples C and D using the following general formulation at 39.7% solids in water. The amounts are given in parts on a dry weight basis Amount Microcapsuies 51 Cooked starch binder 18 Uncooked starch binder 31 Each coating composition was applied to the back side of a 34 pound per 3300 square foot commercial CF paper. The CF paper contained a coating of a zinc modified phenol formaldehyde novolac resin on the front side. Typewriter intensity determinations of papers coated with several different weights of microcapsules were as follows : Coating Composition Microcapsule Coat Weight Typewriter Contains (Pounds per 3300 square ft) Intensity Comparative Sample C 1. 84 66 Comparative Sample C 1. 98 63 Comparative Sample C 2.26 60 Treated Sample D 1.48 65 Treated Sample D 1.56 64 The above test results showed that for a given microcapsule coat weight the coating composition containing the Treated Sample D performed better than the coating composition containing the Comparative (untreated) Sample C.

Examinations of the coated papers revealed that significant amounts of clustered (polynuclear) microcapsules were present in the papers coated with the compositions containing Treated Sample D and that such clustered microcapcules were substantially absent in the papers coated with the compositions containing Comparative Sample C.

As used herein, the typewriter intensity is a contrast ratio and is equal to 100 times the ratio for the reflectance of a printed character divided by the background refTectance. A typewriter intensity value of 100 indicates a not discernible print and a lower value indicates a more intense print. Specifically, the backside of the papers made in Example 4 and a CF sheet coated with a phenolformaldehyde novolac resin were placed with the coated sides together and a series of closespaced characters were typed on the test paper. Reflectance readings were than taken of the background and also the printed characters transferrred to the reactant sheet and the contrast ratio was calculated.

Claims (24)

1. WHAT WE CLAIM IS :- 1. A process for preparing filtrable microcapsules comprising the steps of: (a) preparing an aqueous dispersion of microcapsules containing an oil, said microcapsules having a primary capsule wall comprising a reaction product of hydroxypropylcellulose and at least one cross-linking agent for hydroxypropylcellulose, said primary capsule wall being substantially oil and water impermeable ; (b) adding and mixing into said aqueous dispersion of microcapsules a phenol and an aldehyde ; and (c) maintaining said mixing while said phenol and said aldehyde react together to form a condensation polymer, said condensation polymer being insoluble in said aqueous dispersion of microcapsules thereby precipitating said condensation polymer on said primary capsule wall as a secondary capsule wall.
2. 2. The process of Claim 1, wherein said phenol is selected from resorcinol, hydroquinone, catechol, o-cresol, m-cresol, p-cresol, Bisphenol A, naphthalenediols, gallic acid, phloroglucinol, pyrogallol, guaiacol, digallic acid, tannic acid, chlorophenols, xylenols, eugenol, hydroxybiphenols, naphthols, and mixtures thereof.
3. 3. The process of Claims I or 2, wherein said aldehyde is selected from acetaldehyde, formaldehyde, glyoxal, glutaraldehyde, furfural, and mixtures thereof.
4. 4. The process of any of Claims I to 3, wherein said cross-linking agent for said hydroxy-propylcellulose is selected from the polyfunctional isocyanates, acyl chlorides, phosphoryl chlorides, sulphonyl chlorides, alkylene bischloroformates, and mixtures thereof.
5. 5. A process according to any preceding claim, wherein step (b) inclues the addition of sufficient acid to bring said aqueous dispersion of microcapsules to a pH of 6 or below, said phenol, said aldehyde and said acid each being in the form of aqueous solutions.
6. 6. The process of Claim 2 or any claim appendant thereto, wherein said phenol is resorcinol, said resorcinol being present in said aqueous dispersion of said microcapsules in a ratio of 0.015 parts to 0.15 parts of said resorcinol to I part, dry weight, of said microcapsules having a primary capsule wall.
7. 7. The process of Claim 3 or any claim appendant thereto, wherein said aldehyde is formaldehyde, said formaldehyde being present in said aqueous dispersion of said microcapsules in a ratio of 0.2 parts to 2.0 parts of said formaldehyde to I part, by weight of said phenol.
8. 8. A process according to both Claim 6 and Claim 7, wherein step (c) comprises heating, with mixing, said aqueous dispersion of microcapsules to a temperature of from 25 C to 70 C for a period of I hour to 16 hours while said resorcinol and said formaldehyde react together to form said condensation polymer ; and wherein the pH of said aqueous dispersion of microcapsules is thereafter adjusted to a pH of 7 or above.
9. 9. A process according to any preceding claim, wherein said oil contains a chromogenic material.
10. 10. A process according to Claim 1 for preparing filterable microcapsules substantially as hereinbefore described with reference to the Examples.
11. ll. Microcapsules whenever produced by a process according to any preceding claim.
12. IZ. Microcapsules containing an oil, said microcapsules having a primary capsule wall and a secondary capsule wall, said primary capsule wall comprising a reaction product of hydroxypropylcellulose and at least one cross-linking agent for hydroxypropylcellulose, said primary capsule wall being substantially impermeable to oil and water, said secondary capsule wall comprising a condensation polymer of a phenol and an aldehyde.
13. 13. The microcapsules of Claim 12, wherein said phenol is selected from resorcinol, hydroquinone, catechol, o-cresol, m-cresol, p-cresol, Bisphenol A, naphthalenediols, gallic acid, phloroglucinol, guaiacol, digallic acid, tannic acid, chlorophenols, xylenols, eugenol, hydroxybiphenols, naphthols, and mixtures thereof.
14. 14. The microcapsules of Claims 12 or 13, wherein said aldehyde is selected from acetaldehyde, formaldehyde, glyoxal, glutaraldehyde, furfural, and mixtures thereof.
15. 1$. Microcapsules according to Claim 12, wherein said secondary capsule wall comprises a condensation polymer of resorcinol and formatdehyde ; and wherein said microcapsules have significant amounts of polynuclear microcapsule particles present, said polynuclear microcapsule particles being a cluster of smaller microcapsules enveloped by said secondary capsule wall.
16. 16. The microcapsules of any of Claims 12 to 15, wherein said oil contains a chromogenic material, said chromogenic material being soluble in said oil.
17. ! 7. Microcapsutes according to Claim 12 and substantially as hereinbefore described.
18. 18. A process for the production of a pressure-sensitive carbonless transfer sheet comprising the steps of adding a binder to microcapsules according to Claim 16 or to microcapsules produced according to Claim 9 to form a coating composition; applying said coating composition to a substrate; and setting said coating composition to a solid coating.
19. 19. A process for the production of a pressure-sensitive carbonless transfer paper comprising the steps of adding a binder to the aqueous dispersion of microcapsules resulting from the process of Claim 9 to form an aqueous coating composition ; applying said aqueous coating composition to a paper substrate; and drying said aqueous composition to a solid coating.
20. 20. A process for the production of a pressure-sensitive carbonless transfer paper comprising the steps of filtering the aqueous dispersion of microcapsules resulting from the process of Claim 9 to obtain said microcapsules ; dispersing said microcapsules in a liquid hot melt suspending medium to form a hot melt coating composition ; apply said coating composition to a paper substrate; and cooling said hot melt suspending medium to a solid coating.
21. 21. Substantially as hereinbefore described with reference to the Examples, a process according to any one of Claims 18, 19 or 20 for the production of a pressure-sensitive carbonless transfer paper.
22. 22. A pressure-sensitive carbonless transfer paper comprising the microcapsules of Claim 16 coated on a paper substrate.
23. 23. A pressure-sensitive carbonless transfer paper comprising microcapsules produced according to Claim 9 coated on a paper substrate.
24. 24. A pressure-sensitive carbonless transfer paper whenever produced according to any one of Claims 18 to 21.