This invention discloses, as an article of manufacture, a capsule having a hollow interior and an enclosing wall, referred to as a membrane. The membrane is not soluble in water, but is permeable to water or an aqueous solution. The membrane is comprised of a first material and can also include a second material. When the membrane is comprised of the first material and the second material, then it is referred to as a composite material wherein the first material forms sporting matrix having the second material fixed in the matrix. The first material is a urethane/vinyl hybrid polymer and is not soluble in, reactive with or a solvent for the second material. The second material is a particulate solid, different from the first material, and is not soluble in, reactive with or a solvent for the first material. The capsule of this invention is useful to maintain in the hollow interior thereof a solid, water-soluble first chemical composition which is not soluble in, reactive with or a solvent for the enclosing wall of the capsule. In operation, the exterior of the capsule, having the first composition maintained in the interior thereof, is placed in contact with a water-soluble second chemical composition which is not soluble in, reactive with or a solvent for the enclosing wall of the capsule, but which is reactive with the first chemical composition.

DISCLOSURE OF INVENTION

1. Summary of the Invention

This invention is an article of manufacture and a method of using the article to treat a chemical composition in contact with the exterior of the article. The article of this invention is, thus, a capsule comprised of a hollow interior and an enclosing wall which is permeable to water or an aqueous solution. A first, water-soluble, chemical composition is enclosed, that is, encapsulated, in the hollow interior of the capsule by the wall of the capsule.

The permeable wall of the capsule, which is referred to herein as a membrane, is at least comprised of a first material and can be comprised of the first material together with a second material, in which case, the membrane is a composite material. When the membrane is a composite material the first material forms a supporting matrix, and the second material is fixed in the supporting matrix.

The first material is a urethane/vinyl hybrid polymer. It is not reactive with, soluble in or a solvent for the encapsulated first chemical composition, the second material, the liquid in contact with the exterior surface of the membrane or the chemical composition (referred to herein and in the appended claims as the second chemical composition) in contact with the exterior of the membrane.

The second material is a particulate solid and is different from the first material. The second material is not reactive with, soluble in or a solvent for the encapsulated first chemical composition, the first material, the liquid in contact with the exterior surface of the membrane or the second chemical composition.

The encapsulated first chemical composition can be substantially any water-soluble material which is not reactive with, soluble in or a solvent for the first material or the second material, but is reactive with and is, thus, useful to alter the chemical and/or physical properties of the second chemical composition in contact with the exterior surface of the membrane. The chemical nature of the encapsulated first chemical composition can include enzymes, and organic and inorganic acids, bases, salts and oxidizing agents. The encapsulated first chemical compositions are useful as medicines, pesticides, algaecides, herbicides, cosmetics, laundry products, pigments, polymerization initiators, cross linking agents and viscosity reducing agents. The encapsulated first chemical composition can be an additive for adjusting the setting properties of hydraulic cement and can be a breaker for a fracturing fluid, wherein the breaker can be a chemical selected from water-soluble enzymes, and organic or inorganic acids, bases, salts and oxidizing agents.

The second chemical composition, which as disclosed, is in contact with the exterior surface of the membrane can be substantially any water-soluble material which is not reactive with, soluble in or a solvent for the first material or the second material, but is reactive with the encapsulated first chemical composition.

2. Description of the Preferred Embodiments

The article of this invention is a capsule comprised of a solid, water-soluble first chemical composition preferably enclosed, that is, encapsulated, in a composite material. The composite material is comprised of a first material, which forms a matrix, in combination with a second material, different from the first material, which is fixed in the matrix. The phrase "fixed in the matrix," when employed in connection with the second material, means that the second material, while being firmly attached to the matrix, can be completely embedded within and, therefor, completely surrounded by, the matrix, or it can be partially embedded within and partially exposed at the interior surface or at the exterior surface of the matrix. It is understood that the interior surface of the matrix is the interior surface of the membrane and the exterior surface of the matrix is the exterior surface of the membrane. As mentioned previously, the first material, which forms the supporting matrix of the membrane, is not reactive with, soluble in or a solvent for the encapsulated first chemical composition, the second material, the second chemical composition or a liquid in contact with the exterior surface of the membrane.

The first material is comprised of a urethane latex system and, more specifically, an aqueous polyurethane-vinyl polymer dispersion. The vinyl element preferred for use in this invention is acrylic in nature. Accordingly, the dispersion is more particularly referred to herein as a urethane/acrylic hybrid polymer. The aqueous polyurethane-vinyl polymer dispersion employed herein is disclosed in U.S. Pat. No. 5,173,526 to Vijayendran et al. According to Vijayendran et al, the polyurethane-vinyl polymer dispersion is prepared by the simultaneous polymerization of a vinyl monomer and chain extension of an isocyanate-terminated polyurethane prepolymer in the presence of water. More particularly, Vijayendran et al disclose that, "the aqueous polymer is prepared by (a) forming a carboxy-containing, water dispersible, isocyanate-terminated polyurethane prepolymer, (b) adding a vinyl monomer mixture which contains a polyethylenically unsaturated monomer to the prepolymer to make a prepolymer/monomer mixture, (c) adding a tertiary amine to the prepolymer/monomer mixture, (d) dispersing the prepolymer/monomer mixture in water, (e) adding an oil soluble free radical initiator and a chain extender to the aqueous dispersion, and (f) polymerizing the vinyl monomers and completing the chain extension of the prepolymer by heating the aqueous dispersion." The process is said to provide an intimacy of interaction that cannot be achieved by blending. Accordingly, the first material, as defined above, is not a mere blend of a polyurethane and an acrylic polymer.

A urethane/acrylic hybrid polymer dispersion useful herein is commercially available in the form of an aqueous polymer dispersion from Air Products and Chemicals, Inc., under the Hybridur family of trademarks, two of which are Hybridur 540 and Hybridur 580.

The aqueous polyurethane-vinyl polymer dispersion employed herein is disclosed in Vijayendran et al to be useful as a protective coating for a solid substrate. It is disclosed that the dispersion is applied to the substrate by conventional flexographic or gravure methods and that the finished product protects the substrate from solvents, corrodants, and abrasives and that it exhibits good gloss and flexibility. The mentioned solid substrates include paper, metals, plastics and wood.

The first material can also be a reaction product. Accordingly, the urethane/acrylic hybrid polymer can be cross linked with a polyaziridine, carbodiimides, epoxies or metal ion cross linkers. An example of a polyaziridine polymer useful herein is pentaerythritol-tris-[.beta.-(aziridinyl)proprianate]. An example of a carbodiimide useful herein 1,3-dicyclohexyl-carbodiimide. The urethane/acrylic hybrid polymer is preferably reacted with an aziridine cross linking agent to form the first material in the composite material employed in this invention.

The second material in the composite material is a particulate solid present in the composite material in an amount in the range of from greater than about 0 to about 50, preferably 10 to about 40 and still more preferably from about 20 to about 30 percent particulate solid by total weight of composite material. The particulate solid can have a size in the range of from about 1 to about 15 and preferably from about 2 to about 5 microns. As explained below in connection with the description of the capsule release mechanism, the particle size of the particulate solid in the composite material is preferably not less than 1 micron. In a manufacturing operation it is, of course, difficult to completely exclude all particulate solids having a size of less than 1 micron, however, it is preferred that concentration of particulate solids having a size less than 1 micron should not exceed about 25 percent by weight of particulate solids present in the composite material.

The second material can include silica, calcium carbonate, titanium dioxide, barium sulfate, calcium sulfate, similar such materials and mixtures thereof.

The first chemical composition encapsulated within the composite material of this invention can be substantially any water-soluble material, limited as previously stated, selected from the group consisting of enzymes, and organic and inorganic acids, bases, salts and oxidizing agents. More specifically, the chemical compositions preferably include alkali, alkaline earth metal and ammonium halides, oxides, hydroxides, carbonates, bicarbonates, perborates, peroxides, percarbonates, bisulfates, bromates and sulfates. The chemical agents can also include liquids which have been adsorbed on solid subtrates, such as, diatomaceous earth and then encapsulated.

Specific examples of chemical agents useful herein as the first chemical composition include hemicellulase, sodium bisulfate, calcium chloride, lithium hydroxides, potassium carbonate, salts of phosphonic acid, sodium persulfate, ammonium persulfate, magnesium oxide, citric acid, fumaric acid, sodium citrate, sodium fumarate, polyglycolic acid, sulfamic acid, potassium bromate, sodium bromate and tetraethylpentamine.

In one embodiment, the first chemical composition can be a breaker for an aqueous fracturing fluid, examples of which include the oxidizing agents, sodium persulfate and potassium persulfate. In another embodiment, the first chemical composition can be an aqueous viscosifying agent such as guar gum, hydroxy propyl guar, hydroxy ethyl cellulose and xanthan gum. In still another preferred embodiment, the first chemical composition can be an additive for adjusting the setting properties of hydraulic cement such as accelerators, retarders and viscosifiers, examples of which include calcium chloride, calcium lignosulfonate and hydroxy ethyl cellulose. In yet another embodiment, the first chemical composition can be a pH adjusting material such as lithium hydroxide and potassium carbonate.

The capsule of this invention can be made by utilizing a fluidized bed process, one version of which is referred to as the Wurster process and a modification thereof which employs a top spray method. Equipment employed to produce the capsule is available from Glatt Air Techniques, Inc. Ramsey, N.J. A specific apparatus available from Glatt Air Techniques is the WSG 5 fluidized bed coater, the use of which is illustrated in the examples set out below.

In a preferred method of manufacture, the capsule of this invention is made by first admixing the second material, the particulate solid, with the first material, the urethane/acrylic hybrid polymer, in the proportions above disclosed. The cross linker can then be admixed with the hybrid polymer and solid in an amount in the range of from about 0 to about 5, and preferably from about 1 to about 3, percent cross linker by weight of hybrid polymer. Thereafter, utilizing the fluidized bed process, the first chemical composition to be encapsulated is introduced into a spray chamber and suspended therein by a flow of gas, such as air, while the mixture of hybrid polymer, solid and cross linker are sprayed against the suspended chemical composition to thereby form the capsule. The combination of chemical composition to be encapsulated and composite material is adjusted to produce a capsule having in the range of from about 10 to about 50, and preferably from about 20 to about 40 percent composite material by weight of capsule.

As previously indicated, the capsules of this invention can be made by well known fluidized bed encapsulation techniques wherein solid particles of the first chemical composition are sprayed with the composite material while the particles are suspended in a flow of air or other gas within a spray chamber. There is no known limitation on the particle size of the chemical composition being encapsulated, but a particle size in the range of from about 10 to about 60 mesh US Sieve series is conveniently employed. Thus, with respect to the above range, particles of a size which do not pass a 10 mesh screen and particles which do pass a 60 mesh screen are preferably not employed. To maintain product uniformity with respect to performance, the first chemical composition, prior to encapsulation, is preferably sized to remove a substantial portion of any fines or agglomerations of chemical particles. In this manner, the thickness of the membrane wall of the capsules will have, within a relatively narrow range, a substantially uniform thickness and the capsules will exhibit generally uniform release control properties.

Generally, the capsule of this invention is prepared having a membrane coating consisting of the cross linked urethane/acrylic hybrid polymer and particulate mixture, i.e. the composite material, of a certain thickness and permeability to obtain the desired controlled release of the first chemical composition for a particular dissolved second chemical composition to be treated. The quantity and size of the particulate present in the composite material will significantly effect the permeability of the created membrane coating.

The particle size of the capsule varies depending upon the desired amount of chemical composition to be released and the desired rate at which the composition is to be released. For example, the thicker the membrane, generally the slower the release since it takes longer for an aqueous liquid in contact with the exterior surface of the capsule to diffuse, i.e., permeate, through the wall of the membrane to dissolve the agent and then diffuse, i.e., permeate, back through the membrane. This relationship, however, can be modified by changing the particle size of the second material present in the matrix. If the second chemical composition to be treated by the first chemical composition is a fracturing fluid containing a proppant, then it is preferred that the particle size of the capsule be substantially equal to or smaller than the particle size the proppant.

In the method of operation, an aqueous liquid in contact with the exterior surface of the capsule gradually passes, by diffusion, through the membrane into the hollow interior of the capsule and therein contacts and dissolves the first chemical composition encapsulated therein to form a solution of the composition in water. It is to be understood that the aqueous solution in contact with the exterior surface of the capsule can be present in the contact location as a natural constituent of the environment or introduced into the environment prior to, together with or subsequent to the introduction of the capsule.

The solution formed within the capsule then gradually passes, by diffusion, from the hollow interior of the capsule through the membrane to the exterior of the capsule to there contact and react with the second chemical composition.

The described operating mechanism, which consists in diffusion of an aqueous liquid in contact with the exterior of the capsule into the capsule and diffusion of a formed solution out of the capsule, requires an extended period of time to be completed to thereby avoid release of all of the first chemical composition over a very short span of time.

It is believed that the operating mechanism of this invention is driven by very small pressure differences between the interior and exterior of a capsule. Thus, when the pressure in the interior of the capsule is less than the pressure on the exterior of the capsule, the aqueous liquid passes through the membrane to the interior of the capsule to contact and dissolve the first chemical composition. Upon dissolution, pressure within the interior of the capsule increases to a value greater than the pressure on the exterior of the capsule. Accordingly, upon this pressure increase, the formed solution passes through the membrane to the exterior of the capsule. In some instances, depending upon the nature of the aqueous liquid and/or the nature of the agent, a gas may be produced within the interior of the capsule. In spite of the described pressure actuated mechanism, it is believed that the pressure stresses do not cause the capsule to rupture, break, dissolve or disintegrate. The capsule remains intact during the entire diffusion process.

The particle size of the second material, which is fixed in the matrix, plays an important role in the diffusion process. In this regard, it is believed that capsules which contain particles having a size of less than about 1 micron, i.e., submicron particles, do not operate to dissipate internal pressure, generated as described above, at a rate sufficiently great to prevent rupture of the capsule. Capsules which contain particles having a size of 2 microns and greater, preferably about 5 microns, fixed in the matrix, do operate to dissipate internal pressure at a rate sufficient to prevent rupture of the capsule.

The capsule of this invention functions to slowly release all of the encapsulated first chemical composition over an extended period of time as contrasted with a rapid release of all of the encapsulated agent over a very short period of time. The capsule operates as stated, without change in the desired rate of release of the encapsulated composition, in the presence of liquids at temperatures in the range of from about 50 to about 325.degree. F. and at pH values in the range of from about 2 to about 11.

The above disclosure has concentrated on the permeable wall of the capsule of this invention as consisting solely of a defined first material or as a composite material consisting of a combination of the first material and a defined particulate second material. However, in another aspect, the membrane of this invention, as defined above, can serve as an undercoat or a primer coat for a third material, such as an acrylic, which would not be compatible with the encapsulated first chemical composition if placed in direct contact therewith. It has been discovered that the membrane of this invention can successfully function as a barrier between an encapsulated chemical and a coating which, for some reason, would not ordinarily be compatible with the encapsulated chemical.
 

Claim 1 of 19 Claims

1. An article of manufacture comprising a capsule and a first chemical composition, said capsule having a hollow interior and an enclosing membrane wall having an interior surface and an exterior surface, wherein said first chemical composition is enclosed within said hollow interior of said capsule; said membrane is permeable to water and aqueous solutions, but is not soluble in aqueous liquids, and includes at least a first material comprised of a polyurethane-vinyl polymer dispersion prepared by the simultaneous polymerization of a vinyl monomer and chain extension of an isocyanate-terminated polyurethane pre-polymer in the presence of water to thereby form a urethane/vinyl hybrid polymer; and said first chemical composition is comprised of a solid, water-soluble chemical composition which is not reactive with, soluble in or a solvent for said membrane.
 

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