Process for continuously manufacturing microcapsules

Method for the continuous manufacture of microcapsules Förfarande för kontinuerlig framställning av mikrokapslar

This invention relates to a process for the continuous manufacture of microcapsules for use in pressure sensing systems in particular, but not exclusively.

In the most widely used pressure-sensitive reproduction system, the lower surface of the upper sheet is coated with microcapsules containing a colorless color former solution (this upper sheet is known as CF-coated sheet or CB sheet), and the upper surface of the lower sheet is coated with ). Usually, a number of intermediate sheets are also used, each of which has a lower surface coated with microcapsules and an upper surface with a color developer (these intermediate sheets are known as front and back coated or CFB sheets). When pressure is applied to a series of sheets, for example by hand or typing, the microcapsules break, and from the color form the solution is released to the color developer on the adjacent lower sheet, followed by a chemical reaction in which the color of the color former develops.

Numerous microencapsulation methods are known in the art. Most of them are batch mode. In principle, continuous methods offer advantages over batch applications, and several continuous methods have been proposed, see, for example, U.S. Patents 3,821,056 and 3,816,331. U.S. Patent 3,821,056 discloses a method in which liquid-liquid phase separation is performed in a circular encapsulating pipeline having one inlet and one outlet. The method disclosed in U.S. Patent 3,816,331 is similar to that of U.S. Pat. These methods are limited by the fact that the encapsulating medium must be introduced under turbulent flow conditions to prevent agglomeration of the capsule elements formed through the tube. A temperature gradient must also be maintained along the tube to surround the core material of the intended capsule with the wall material. This method discloses methylated methylol melamine for use as a capsule wall material precursor.

One of the main problems with these previously known continuous encapsulation methods is the need to keep the material components of the encapsulation system uniformly turbulent to prevent agglomeration of the capsules formed along the entire length of the encapsulation tube. These continuous encapsulation methods also require the generation and maintenance of temperature gradients along the tube to form a separate capsule wall material phase, to make the separated phase the most viscous, to encapsulate the materials of the dispersed capsules with the separated phase and to solidify and / or gel the wall material.

Another earlier proposal can be found in Japanese Patent Publication No. 55-124534. It relates to a process for the continuous preparation of microcapsules in a multi-stage series of reaction vessels, and uses an interface polymerization process to form the capsule wall.

Yet another proposal can be found in U.S. Patent 4,105,823. Example 8 of this patent relates to the use of this method for the manufacture of capsules by a continuous process in which the capsule-making medium flows through a series of vessels under the influence of gravity through overflow tubes. The performance of this method is such that the material takes two hours to pass through the system.

In fact, the methods of Japanese Patent Publication No. 55-124534 and U.S. Patent No. 4,105,823 are only a series of batch processes in which the product in each step of the corresponding process flows by gravity alone to the next step of the process. In these methods, first-introduced (old) droplets of the desired nuclear material have a greater tendency to obtain thicker walls at the expense of later (new) droplets. Such inhomogeneity of the reaction conditions results in capsules of uneven quality; i.e., some capsules have thick walls and others have thin walls.

One of the better batch encapsulation processes that has been used successfully commercially is disclosed in U.S. Patent 4,100,103. In this method, encapsulation is performed by in situ reaction in an aqueous medium of melamine and formaldehyde and / or by polycondensation of a monomeric methylol melamine or etherified methylol melamine (EMM), or a low molecular weight polymeric aliphatic aliquot with a carboxylate, a desired capsule core substance and an aqueous supernatant. in the presence of.

However, when the EMM is used in this type of batch encapsulation process, the polymerization begins the moment the EMM and the negatively charged polyelectrolyte come into contact with each other. In scale production equipment of this method, where emulsification can take 35-40 minutes and batch temperature can be about 45 ° C, the addition of EMM and other system components prior to emulsifying the liquid capsule core in question results in capsule wall formation and deposition before reaching the correct capsule core. This premature wall formation results in the utilization of room wall materials, with some of the resulting microcapsules also being outside the acceptable size range.

When this production-scale batch process is modified so that the EMM is added after the emulsification is complete, partial emulation occurs in the emulsion, resulting in increase in droplet size of the liquid nucleus. Techniques used to minimize the droplet size increase of this emulsion include adding EMM to the mixture along with a portion of the negatively charged polyelectrolyte material and maintaining the entire batch at lower temperatures.

Another problem with batch processes, such as that described in U.S. Patent 4,100,103, is that in order to perform the method within a reasonable time, the vessel in which the encapsulation is performed must be heated. This heating is carried out either by the heating mantle of the vessel or by an immersion heater in the vessel. Due to the preparation of several batches in these vessels, solids accumulate on the surfaces where the heated part comes into contact with the encapsulating medium. These accumulations must be removed from time to time by maintenance cleaning maintenance. This necessary cleaning both increases the production cost of the capsules and, due to the time spent, leads to a reduced production volume of the capsules.

It has now been found that the encapsulation technique generally described in U.S. Patent 4,100,103 in connection with a batch process is suitable for use in a continuous process, and that this continuous method avoids many, if not all, of the disadvantages associated with the batch process just described and previously known continuous processes.

Thus, the present invention is based on the surprising finding that a continuous encapsulation process can be performed in a tube that does not require a temperature gradient or turbulent flow without size distribution to produce narrow, high quality single core microcapsules. if the capsule wall material is prepared by in situ polymerization of methylol melamine or etherified methyl ethyl imelamine in the presence of certain negatively charged carboxyl-substituted straight aliphatic hydrocarbon polyelectrolyte materials.

According to the present invention, there is provided a method of continuously preparing microcapsules comprising continuously emulsifying substantially water-insoluble capsule core material particles or droplets in a manufacturing aqueous medium comprising:

(a) a mixture of a water-soluble capsule wall precursor selected from monomeric methylol melamine or etherified methylol melamine or low molecular weight polymers thereof, or any combination thereof; and

b) continuously passing a negatively charged polymeric polyelectrolyte having a straight aliphatic hydrocarbon backbone containing an average of two carboxyl groups per four to six backbones and passing the emulsion thus formed through a tubular reactor at an elevated temperature of flow rate, temperature and reactor length, that the residence time in the reactor is sufficient for the capsule wall precursor to polymerize to form walls around the capsule core particles or droplets so that the dispersion of the finished microcapsules in the residual preparation medium leaves the tubular reactor.

In a preferred embodiment of this invention, the water-soluble capsule wall precursor in aqueous medium, the substantially water-insoluble capsule core material, and the polyelectrolyte in aqueous solution are mixed together to form a dispersion before being fed to an emulsifier such as an in-line emulsifier. This can be done by pumping each of these encapsulation components from the tanks into the pipe leading to the emulsifier so that they mix before emulsification. The droplet size of the substantially water-insoluble capsule core material is determined, in part, by the flow rate of the encapsulation components through the emulsifier, which depends on the rate at which they are pumped into the tube leading to the emulsifier. Flow meters can be connected to the device of the preferred embodiment to measure the flow rate of the encapsulation components.

The resulting emulsion flows from the emulsifier to the tubular reactor. In a preferred embodiment, the tubular reactor is maintained at one and the same elevated temperature, preferably 40-95 ° C, more preferably 60-95 ° C. To monitor the physical conditions of the tubular reactor, it is recommended to connect temperature and pressure sensors to the tubular reactor.

Of the possible capsule wall precursors, etherified methylol melamine is recommended.

The negatively charged polymeric polyelectrolyte is preferably poly (ethylene-co-maleic anhydride), poly (acrylic acid), poly (methyl vinyl ether-co-maleic anhydride), poly (propylene-co-maleic anhydride), poly (butadiene-co-maleic anhydride) or -maleic anhydride).

Using the method of the present invention, it will be seen that many of the disadvantages identified by previously known encapsulation methods have been eliminated.

The method of the present invention does not require the turbulent flow required by previous continuous encapsulation processes, which is neither advantageous nor, in many cases, even practical. Likewise, there is no longer a need to maintain a cumbersome temperature gradient, because using this method, one elevated temperature can be maintained in the reactor, which is simpler and more suitable than providing a temperature gradient.

It is also found that in the continuous process of this invention, each addition batch introduces fresh, particles of the desired core material and fresh reactants, giving all oil droplets the same opportunity to obtain a wall material, resulting in a high homogeneity of the capsule product.

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The continuous process of this invention eliminates the disadvantages of previous batch processes. In the present invention, all components are mixed before the emulsification of the core material is started without adverse effects on the microcapsule size, since the total time between the addition of the EMM and the completion of the emulsification can only be calculated in seconds (13.5 seconds in the preferred embodiment). Complete mixing of the components in a continuous emulsifier mixer and a higher temperature of the effluent emulsion results in rapid polymerization of the EMM and subsequent deposition of wall material to form microcapsules without destabilizing the emulsion. The droplet size distribution of the microcapsules prepared by the method of this invention is narrow and small, and compared to previous experience, the result is indeed surprising.

An additional advantage of this method is that no solids accumulate in the reaction vessel, thus avoiding costly and time-consuming regular maintenance to remove this deposit.

It should be noted that the wall-forming chemistry of this process is substantially similar to U.S. Patent 4,100,103 and its GB equivalent 154,2058. These patents disclose additional information about the raw materials that can be used in this process, including, for example, capsule core materials.

To facilitate understanding of the method of the present invention, reference is now made to the drawing, which schematically and by way of example shows one embodiment thereof, and which schematically shows a flow chart of the method.

Referring then to the drawing, in which the feed tanks 1 and 3 holding the various components used in the process are provided with respective tubes 4 leading to a common pipeline 6. Corresponding pumps 5 are arranged in each tube 4. The tube 6 leads to a continuous emulsion mixer 7. 8 in a tubular reactor. The tubes 8 pass through the reactor 9 to the outlet 10. Part of the tube inside the reactor 9 is in the form of a coil 9 to prolong the time that the substance in the tube 8 is in the reactor.

9. Reactor 9 is essentially the water jacket of the coil 11. In use, the components from the tanks 1, 2 and 3 are continuously led from the pipes 4 to the pipe 6 using pumps 5. The resulting mixture passes to a continuous emulsifier mixer 7, where complete mixing and emulsification takes place to prepare an emulsion of the desired capsule core material in a medium containing the other components of the encapsulation system. This emulsion then passes from the emulsifier mixer 7 to the tube 8 of the tubular reactor 9, where it is kept at the same elevated temperature by the hot water in the reactor 9. Single-core capsules with a droplet size distribution dispersed in the manufacturing medium are removed from the outlet 10 of the encapsulation tube 8.

The invention will now be described by the following example, in which the method just described is used with reference to the drawing, and in which all parts and percentages are by weight unless otherwise indicated.

Tank 1 contained an aqueous solution of a mixture of poly (ethylene comaleic anhydride) materials according to Table 1. After this solution was prepared, its pH was adjusted to 4 with 20% sodium hydroxide and maintained at about 25 ° C.

table 1

<td>Concentration</td><td>Substance</td><td>Molecular weight</td><td>Supplier</td>

<td> 1,4%</td><td>EMA-31</td><td> 75 000 - 90 000</td><td>Monsanto Chemical Co. St. Louis, Missouri</td>

<td> 2,6%</td><td>EMA-1103</td><td> 5 000 - 7 000</td><td>Monsanto Chemical Co. St.Louis, Missouri</td>

Container 2 contained the solution of chromogenic compounds according to Table 2 as the intended capsule core material.

Table 2

<td>Concentration</td><td>Chromogenic substance</td>

<td> 1,7 %</td><td>3,3-Bis (p-dimethylaminophenyl) -6-dimethylaminophthalide</td>

<td> 0,55 %</td><td> 2¹-anilino-3¹-methyl-6¹-diethylaminofluorane</td>

<td> 0,55 % 1</td><td>3,3-Bis (1-ethyl-2-methyl-indol-3-yl) -phthalide</td>

The solvent for the solution of chromogenic compounds was a mixture of 65 parts of C 1-4 alkylbenzene and 35 parts of benzyl xylene. Tank 2 was maintained at a temperature of about 100 ° C.

Tank 3 contained a 57% aqueous solution of methylated methylmellamine (Resimene 714, Monsanto Chemical Company, St. Louis, Missouri) maintained at about 25 ° C.

Tubes 4 were made of stainless steel and had an outer diameter of 9.53 mm (3/8 inch) and an inner diameter of 7.75 mm (0.305 inch). Pumps 5 were valveless, variable flow submersible pumps, supplied by Fluid Metering Inc., Oyster Bay,

New York, which are¹ described in one or more of U.S. Patents 15 3,168,872, 3,257,953 and 4,008,003.

Each pump was immediately followed by a flow meter that monitored the flow rate of the fluid being pumped. Tube 6 was made of stainless steel and had an outside diameter of 9.53 mm (3/8 inch).

The contents of the tanks 1, 2 and 3 were fed through the pipes 4 into the pipe 6 by means of pumps 5. The flow rates were as shown in Table 3.

Table 3

<td>Component</td><td>Temperature</td><td>Relative flow rate</td><td>Typical flow rate</td>

<td>Contents of tank 1</td><td>25 ° C</td><td> 1,00</td><td>6.66 gs ¹ , (400 gmin)</td>

<td>Contents of tank 2</td><td>100 ° C</td><td> 0,72</td><td>4.8 gs ¹(288 gmin)</td>

<td>Contents of tank 3</td><td>25 ° C</td><td> 0,28</td><td>1.86 gs, (112 gmin)</td>

As a result of the flow effect of the encapsulation components into the tube 6, the components were partially mixed prior to arrival in the continuous emulsifier mixer 7. The emulsion mixer was a continuous automatic mixer 2 572 049 from patents,

600 569, 2,679,866 and 3,081,069. The submersible pumps 5 provide a pressurized and measured flow of the encapsulation components through the mixing chamber of the emulsion mixer 7. The combination of fluid flow rate, fluid temperature, and rotor speed determines the intended distribution (i.e., droplet size) of the capsule core fluid in the manufacturing medium. One skilled in the art can easily determine the required parameters with little experimentation.

The emulsion of the desired capsule core material in the preparation medium flowed from the emulsion mixer at a temperature of 55-60 ° C through temperature and pressure sensors to the portion of the tube 8 inside the reactor 9 in a water bath maintained at 70-75 ° C. The section of the tube 8 inside the reactor 9 consisted of a 45.72 m (150 ft) rubber hose. The first 15.24 m (50 ft) of hose had an inside diameter of 19.05 mm (3/4 inch) and the remaining 30.48 m (100 ft) had an inside diameter of 15.88 mm (5/8 inch). ).

Table 4 shows the physical dimensions of the tube 8 from which the residence time of each of the encapsulation components in the reactor 9 can be calculated.

Table 4

<td>Hose inner diameter.</td><td>Volume/ meter</td><td>Volume/ foot</td><td>Length in meters (feet)</td><td>Total- volume</td>

<td>19.05 irm (3/4 inch)</td><td>287 en?</td><td>87.5 cm³</td><td> 15,24 (50)</td><td>4350 cm³</td>

<td>15.88 mm (5/8 inch)</td><td>198 cm³</td><td>60.5 cm³</td><td> 30,48 (100)</td><td>6050 cm³10400 cm³</td>

Calculated in Table 3, the flow rate of all components was 13.33 grams per second (800 grams per minute). The specific gravity of the component mixture was about one, and thus the residence time of each specific part of the encapsulation component mixture in the tube 8 of the reactor 9 was 780 seconds (13 minutes).

The finished capsule dispersion exited the outlet end 10 of the tube 8 at a temperature of about 70 ° C.

The finished capsule dispersion was then tested for capsule wall formation. The test used to determine the wall formation of the capsules is the so-called CF response test (drawdown test). The finished capsule dispersion was coated on reactive CF paper with a reactive coating of a metal-modified phenolic resin, as disclosed in U.S. Patent 3,732,120. It is observed that if encapsulation has not taken place or is incomplete or the walls of the capsules are very weak, a color is formed as a result of the reaction between the dye and the CF coating. However, if wall formation has occurred, no discoloration occurs, or it is only mild. The amount of staining is determined by reflectance measurements using an opacity meter. The coating using capsules prepared as just described had a reflectance of 72 as measured by an opacimeter. This shows excellent satisfactory capsule wall formation after only a residence time of 13 minutes in the reactor.

For comparison, capsules were prepared by the batch process of Example 18 of U.S. Patent 4,100,103, in which the reactants were treated at 70 ° C for one hour to obtain a reflectance of 70 with an opacimeter.

Since the higher reflectance numbers represent less discoloration and correspondingly better wall formation, the method of the present invention obtained better capsules at a comparable temperature in a much shorter time.

The finished capsule dispersion was also tested by forming and coating a CB sheet and testing it first with a metal sheet of a metal-modified phenolic resin in a so-called Typewriter Intensity Test and then in an oven storage test.

The Typing Intensity Test (TI) tests the ability of capsules to break and form color when used in a pressure-sensitive de-icing system and calculates a value that indicates a reflectance ratio - , and the paper background, respectively:

_ font reflectance x 100 background reflectance

A high value indicates little color development, and a low value indicates good color development.

The oven storage test measures the amount of capsule-coated paper that loses its ability to produce transfer marks in a typing test when stored in an oven at a specified temperature for a specified period of time. (The oven storage test is intended to simulate what happens over a long period of time at room temperature). The CB / CF pair is typed, and then the CB sheet is placed in a 95 ° C oven for 18 hours, after which the CB / CF pair is rewritten after storage. During such oven storage, poor capsules lose most or all of their ability to form a transfer mark, while good capsules withstand such storage with little or no loss of ability to form color. In these tests, the initial typing intensity was 56, and the typing intensity after overnight storage of the CB sheet at 95 ° C in the oven was 59. These data show that the capsules have a long shelf life as well as good stability and good shelf life.

U.S. Patents 3,812,056 and 3,816,331 define the turbulent flow of fluid in a pipe when a dimension called the dimensionless Reynolds number is greater than about 2000. The Reynolds number is denoted by R in the following equation:

R = h

where D is the diameter of the pipe,

V is the cross-sectional velocity of the front surface of the fluid passing through the tube, p is the density of the flowing fluid, and μ is the viscosity of the flowing fluid.

When calculating the Reynolds number for the example of the present invention just described, the first section of the tube 8 has the following values:

D = 1.9 cm = 3/4 inch

V = flow / cross section

3-1 3-1 flow = 800 cm min = 13.3 cm s cross section = irr² = 3,14(0,95)² = 2.83 cm² „ 13,3 _Λ -1 ^V = 2,83 = ⁴'^{7 CmS} , -3 p = 1 gcm μ = 100 centipoise = 1 boy or 1 g (cm-s) _{R =} (1&gt;9) (4,7) (1) _{= 8},₉

If the turbulent flow requires a Reynolds number of 2000 or greater, the flow in the 19.05 mm portion of the tube 8 of the device of the present invention is undoubtedly non-turbulent.

An analogous calculation for the second section of pipe 8 shows:

D = 1.6 cm = 5/8 inch, ο o 3-1 flow = 13.3 cm s cross section =? Rr² = 3,14(0,8)² = 2.01 cm²

V = '/ o, = 6.6 cms'¹_{5 R =} 11-6)(6,6)(1) _{= 10},₆

The flow in the second section of the pipe 8 of the preferred embodiment of the present invention is undoubtedly also non-turbulent.

Using similar equations, it was calculated that in order for turbu10 to flow into tube 8 of the apparatus of the present invention using the same flow rate and residence time, the tube should be 0.085 mm in diameter and 1825 kilometers (1130 miles) in length.

Thus, it is clear that the parameters of the method of the present invention are both very far from U.S. Pat. Nos. 3,812,056 and

3816331 corresponding, that it would also be highly impractical, if not impossible, to make the apparatus of the present invention operate within the framework of such prior art continuous encapsulation methods.