A process is disclosed for performing encapsulation, en masse, by an in situ polymerization reaction to yield capsule wall material. The reaction comprises the polymerization of urea and formaldehyde, monomeric or low molecular weight polymers of dimethylol urea or methylated dimethylol urea, melamine and formaldehyde, monomeric or low molecular weight polymers of methylol melamine or methylated methylol melamine, in an aqueous vehicle and the reaction is conducted in the presence of mixtures of poly-electrolyte material, esp. poly(acrylic acid), and polystyrene sulfonic acid and/or salts thereof in certain critical proportions. The disclosed encapsulation process provides improved resistance of the emulsion of intended capsule core material to destabilization and permits the manufacture of micro-capsules with improved drop size distribution.

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Inventors:	Brown; Robert W. (Appleton, WI); Bowman; Richard P. (Appleton, WI)
Assignee:	Appleton Papers Inc. (Appleton, WI)
Appl. No.:	460704
Filed:	January 24, 1983

 

Current U.S. Class:	264/4.7; 428/402.21
Intern'l Class:	B01J 013/02
Field of Search:	264/4.7 428/402.21

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References Cited [Referenced By]

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U.S. Patent Documents

4001140	Jan., 1977	Foris et al.	264/4.
4087376	May., 1978	Foris et al.	264/4.
4089802	May., 1978	Foris et al.	264/4.
4100103	Jul., 1978	Foris et al.	428/320.
4353809	Oct., 1982	Hoshi et al.	264/4.
4409156	Oct., 1983	Hoshi et al.	428/402.
Foreign Patent Documents
0070528	Jan., 1983	EP.
2277621	Feb., 1976	FR.
2062570	May., 1981	GB.

Primary Examiner: Lovering; Richard D.
Attorney, Agent or Firm: McKinney; E. Frank, Phillips, Jr.; Paul S.
 

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Claims

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We claim:

1. A process for preparing microcapsules in an aqueous manufacturing vehicle which comprises enwrapping particles of intended capsule core material, substantially insoluble in said vehicle, with polymeric shells produced by in situ polymerization of melamine and formaldehyde, methylol melamine, methylated methylol melamine, urea and formaldehyde, dimethylol urea or methylated dimethylol urea in the presence of a mixture of poly(acrylic acid) and polystyrene sulfonic acid or a salt thereof, wherein said mixture is present as about 0.75 to about 10 percent, by weight, of the aqueous maufacturing vehicle and said polystyrene sulfonic acid or salt is present in an amount of about 6 to about 50 percent by weight based on the weight of said mixture.

2. The process of claim 1 wherein the polymeric shell is produced by in situ polymerization of methylated methylol melamine or urea and formaldehyde.

3. The process of claim 1 wherein the polymeric shell is produced by in situ polymerization of methylated methylol melamine.

4. The process of claims 1, 2 or 3 wherein the amount of polystyrene sulfonic acid or salt is about 20 to about 40 percent.

5. The process of claim 4 wherein the amount of polystyrene sulfonic acid or salt is about 30 percent.

6. The process of claim 4 wherein the polymerization is conducted at a temperature of about 40.degree. C. to about 95.degree. C.

7. The process of claim 6 wherein the polymerization is conducted at a temperature of about 50.degree. C. to about 70.degree. C.

8. A process for preparing microcapsules in an aqueous manufacturing vehicle which comprises enwrapping particles of intended capsule core material, substantially insoluble in said vehicle, with in situ polymerized methylated methylol melamine in the presence of a mixture of poly(acrylic acid) and polystyrene sulfonic acid, wherein said mixture is present as about 0.75 to about 10 percent, by weight, of the aqueous manufacturing vehicle and said polystyrene sulfonic acid is present in an amount of about 6 to about 50 percent by weight based on the weight of said mixture.

9. The process of claim 8 wherein the amount of polystyrene sulfonic acid is about 20 to about 40 percent.

10. The process of claim 9 wherein the amount of polystyrene sulfonic acid is about 30 percent.

11. The process of claims 8, 9 or 10 wherein the polymerization is conducted at a temperature of about 40.degree. C. to about 95.degree. C.

12. The process of claim 11 wherein the polymerization is conducted at a temperature of about 50.degree. C. to about 70.degree. C.

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Description

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This invention relates to a process for manufacturing minute capsules, en masse, in a liquid manufacturing vehicle. The process of the invention involves liquid-liquid phase separation of a relatively concentrated solution of polymeric material to be used in the formation of walls for the minute capsules. More particularly, the process of this invention involves the polymerization of urea and formaldehyde, monomeric or low molecular weight polymers of dimethylol urea or methylated dimethylol urea, melamine and formaldehyde, monomeric or low molecular weight polymers of methylol melamine or methylated methylol melamine, in an aqueous vehicle and the reaction is conducted in the presence of mixtures of polyelectrolyte material and polystyrene sulfonic acid and/or salts thereof.

A method of encapsulating by in situ polymerization, including a reaction between urea and formaldehyde or polycondensation of monomeric or low molecular weight polymers of dimethylol urea or methylated dimethylol urea in an aqueous vehicle conducted in the presence of negatively-charged, carboxyl-substituted, linear aliphatic hydrocarbon polyelectrolyte material dissolved in the vehicle, is disclosed in U.S. Pat. Nos. 4,001,140, 4,087,376 and 4,089,802.

A method of encapsulating by in situ polymerization, including a reaction between melamine and formaldehyde or polycondensation of monomeric or low molecular weight polymers of methylol melamine or etherified methylol melamine in an aqueous vehicle conducted in the presence of negatively-charged, carboxyl-substituted linear aliphatic hydrocarbon polyelectrolyte material dissolved in the vehicle, is disclosed in U.S. Pat. No. 4,100,103.

British Pat. No. 2,062,570, published May 28, 1981, discloses a process for producing microcapsules having walls produced by polymerization of melamine and formaldehyde in the presence of a styrenesulfonic acid polymer which becomes incorporated in the system. Other anionic high molecular electrolytes, such as polyacrylic acid, are disclosed for use in possible combination with the styrenesulfonic acid polymer. This same disclosure teaches that the styrenesulfonic acid polymer is present as 67-100% by weight of the mixture of the styrene sulfonic acid polymer and the anionic high molecular electrolyte.

The most widespread use of microcapsules to date has been in certain kinds of pressure-sensitive copying systems. In one such system, disclosed in U.S. Pat. No. 2,730,456 and commonly known as manifold record material, an upper sheet is coated on its lower surface with microcapsules containing a solution of a colorless chromogenic material, and a lower sheet is coated on its upper surface with a color developing coreactant material, e.g. an acidic clay, a phenolic resin or certain organic salts. For applications which require more than two plies in the record material, a number of intermediate sheets are also provided, each of which is coated on its lower surface with microcapsules and on its upper surface with acidic material. Pressure exerted on the sheets by writing or typing ruptures the microcapsules, thereby releasing the chromogenic material solution on to the co-reactant material on the next lower sheet and giving rise to a chemical reaction which develops the color of the chromogenic material.

In another such system, known as a self-contained system and disclosed in U.S. Pat. Nos. 2,730,457 and 4,197,346, microcapsules, containing a chromogenic material solution, and a co-reactant material are coated on the same surface of a sheet of paper. Pressure exerted on the sheet by writing or typing causes the capsules to rupture and release the chromogenic material, which then reacts with the co-reactant material on the sheet to produce a color.

Microcapsules for use in the above-described pressure-sensitive copying systems have a series of stringent property requirements so as to produce an optimum copying system. Some of these properties are capsule strength, size distribution range and wall integrity (impermeability).

The processes according to U.S. Pat. Nos. 4,001,140, 4,087,376, 4,089,802 and 4,100,103 have been successfully used to encapsulate solutions of chromogenic materials for use in pressure sensitive copying papers. Of the eligible carboxyl group system modifiers disclosed in said patents, the hydrolyzed maleic anhydride copolymers are preferred. Among the hydrolyzed maleic anhydride copolymers disclosed, the most preferred is poly(ethylene-co-maleic anhydride) (hereinafter referred to as EMA) because of the balance of properties provided to the encapsulation processes.

The cost of EMA has recently been rising rapidly, producing a consequent rise in the cost of the microcapsules manufactured by processes in which EMA constitutes the system modifier. Because of cost and availability considerations, poly(acrylic acid) (hereinafter referred to as PAA), is a logical substitute for EMA as the system modifier. While microcapsules made from processes according to U.S. Pat. Nos. 4,001,140 and 4,100,103, in which PAA constitutes the system modifier, are of commercial quality for use in pressure-sensitive copying paper, they do not possess the optimum balance of properties obtained when EMA is utilized.

One function of the system modifier in said patents is to take an active part in the control or moderation of the polymerization reaction of the starting materials used to form the condensation polymer which makes up the resulting capsule walls.

Another function of the system modifier in said patents is to act as an emulsifying agent to promote and maintain the separation of the individual droplets of the intended capsule core material in the aqueous manufacturing vehicle. When PAA is utilized as the system modifier, emulsification of the intended capsule core material requires more energy input and time and produces a poorer drop size distribution than when EMA is employed. The poorer emulsifying power of PAA can be offset in the case of the process of U.S. Pat. No. 4,100,103 by mixing in, prior to emulsification, the starting materials (e.g. methylated methylol melamine) employed in the in situ polymerization reaction to form the condensation polymer which makes up the resulting capsule walls. The presence of methylated methylol melamine or a low molecular weight polymer thereof, (hereinafter referred to as MMM) during the intended core material emulsification step can result in the premature polymerization of the MMM. This tendency of the MMM to prematurely react under these circumstances is reduced by raising the pH of the PAA-MMM solution to the highest level at which emulsification of the intended core material can be obtained. Once a satisfactory intended core material emulsion is obtained, the pH of the emulsion must be reduced in order to obtain the deposition of satisfactory capsule walls in a reasonable amount of time. This process has been further improved by the addition of certain salts as disclosed in copending application Ser. No. 370,323, now U.S. Pat. No. 4,444,699, of Donald E. Hayford.

It has now been learned that when the processes of U.S. Pat. Nos. 4,001,140, 4,087,376, 4,089,802 and 4,100,103 are practiced using PAA as the system modifier in combination with polystyrene sulfonic acid or a salt thereof (hereinafter referred to as PSA) in which the amount of PSA is about 6% to about 50% by weight of the PAA/PSA mixture, unexpected benefits are produced. Improved emulsification of intended capsule core material and an unexpected resistance of said emulsion to destabilization due to the presence of aminoplast precondensate intended capsule wall materials are two of the principal benefits. Additionally, the completed microcapsule slurries possess lower viscosities which has benefit in transferring and coating said slurries. The minimum required amount of PSA is based upon the presence of sufficient PSA to provide the improved emulsification. Above the maximum preferred amount of PSA, the resistance of the emulsion to destabilization is unacceptably lowered.

It is, therefore, an object of the present invention to provide a capsule manufacturing process wherein emulsion of intended capsule core material of improved drop size distribution is produced.

It is another object of the present invention to provide a capsule manufacturing process wherein the emulsion of intended capsule core material possesses improved resistance to destabilization resulting from the addition of aminoplast precondensate intended capsule wall materials to the manufacturing system.

It is a specific object of this invention to provide an encapsulating process wherein the capsule wall material comprises a urea-formaldehyde polymeric material or a melamine-formaldehyde polymeric material generated by an in situ polymerization reaction in the presence of a negatively-charged, carboxyl-substituted polyelectrolyte material and polystyrene sulfonic acid and/or sodium salts thereof dissolved in the manufacturing vehicle.

These and other objects and advantages of the present invention will become more apparent to those skilled in the art from a consideration of the following specification and claims.

The starting materials used to form the condensation polymer which makes up the resulting capsule walls and the procedures described in U.S. Pat. Nos. 4,001,140, 4,087,376, 4,089,802 and 4,100,103, which are hereby incorporated by reference, are eligible for use in the present invention. As indicated in U.S. Pat. No. 4,001,140, the encapsulating system should include from about 0.75 percent to about 10 percent of the system modifier. In addition to the materials and procedures described in the abovereferenced patents, the process of the present invention involves the use of poly(acrylic acid) (PAA) as the system modifier in combination with polystyrene sulfonic acid and/or sodium salts thereof (PSA) in a certain relative amount range. This combination is made prior to completion of the polycondensation of the starting material used to form the condensation polymer which makes up the resulting capsule wall. It has been found that the specific useful range of amounts of the mixture PAA and PSA is that in which the amount of PSA is about 6% to about 50% by weight of the PAA/PSA mixture. More preferred is about 20% to about 40% by weight PSA of the PAA/PSA mixture. Most preferred is about 30% by weight PSA of the PAA/PSA mixture.

The process is operable over a wide range of temperatures but a temperature range of about 40.degree. C. to about 95.degree. C. is preferred. More preferred is the temperature range of about 50.degree. C. to about 70.degree. C.

Under certain circumstances the inclusion of one of the salts disclosed in copending application Ser. No. 370,323 now U.S. Pat. No. 4,444,699, of Donald E. Hayford (supra) provides a further improvement in the wall integrity of the resulting microcapsule. However, the use of such salts is not required to practice and demonstrate the beneficial properties of the claimed invention.

The following examples are given merely as illustrative of the present invention and are not to be considered as limiting. All parts and percentages throughout the application are by weight, unless specified otherwise. All solutions, unless otherwise designated, are aqueous solutions.