Patent Description:
Various processes for microencapsulation, and exemplary methods and materials are set forth in various patents such as Schwantes (<CIT> (<CIT> (<CIT> (<CIT> (<CIT>(<CIT>(<CIT>. (<CIT> (<CIT>(<CIT> (<CIT> (<CIT> (<CIT> and <CIT>(<CIT>(<CIT> (<CIT>; <CIT>; <CIT> and <CIT> (<CIT>; <CIT> and <CIT> (<CIT> (<CIT> and <CIT> (<CIT>(<CIT> (<CIT>(<CIT>(<CIT> (<CIT>. (<CIT>), among others and as taught by Herbig in the chapter entitled "<NPL>.

Other useful methods for microcapsule manufacture are: <CIT> and <CIT> describing a reaction between urea and formaldehyde; <CIT> describing reaction between melamine and formaldehyde; and <CIT> describing a process for producing microcapsules having walls produced by polymerization of melamine and formaldehyde in the presence of a styrene sulfonic acid. Forming microcapsules from urea-formaldehyde resin and/or melamine formaldehyde resin is disclosed in <CIT>; <CIT>; <CIT>; <CIT>; and<CIT>. Alkyl acrylate-acrylic acid copolymer capsules are taught in <CIT>.

Interfacial polymerization is a process wherein a microcapsule wall such as polyamide, an epoxy resin, a polyurethane, a polyurea or the like is formed at an interface between two phases. Riecke, <CIT> discloses an interfacial polymerization technique for preparation of microcapsules. The core material is initially dissolved in a solvent and an aliphatic diisocyanate soluble in the solvent mixture is added. Subsequently, a nonsolvent for the aliphatic diisocyanate is added until the turbidity point is just barely reached. This organic phase is then emulsified in an aqueous solution, and a reactive amine is added to the aqueous phase. The amine diffuses to the interface, where it reacts with the diisocyanate to form polymeric polyurethane shells. A similar technique, used to encapsulate salts which are sparingly soluble in water in polyurethane shells, is disclosed in<CIT>. <CIT> teaches polymerization reactions in which the material to be encapsulated, or core material, is dissolved in an organic, hydrophobic oil phase which is dispersed in an aqueous phase. The aqueous phase has dissolved materials forming aminoplast (amine and aldehyde) resin which upon polymerization form the wall of the microcapsule. A dispersion of fine oil droplets is prepared using high shear agitation. Addition of an acid catalyst initiates the polycondensation forming the aminoplast resin within the aqueous phase, resulting in the formation of an aminoplast polymer which is insoluble in both phases. As the polymerization advances, the aminoplast polymer separates from the aqueous phase and deposits on the surface of the dispersed droplets of the oil phase to form a capsule wall at the interface of the two phases, thus encapsulating the core material. Urea-formaldehyde (UF), urea-resorcinol-formaldehyde (URF), urea-melamine-formaldehyde (UMF), and melamine-formaldehyde (MF), capsule formations proceed in a like manner. In interfacial polymerization, the materials to form the capsule wall are in separate phases, one in an aqueous phase and the other in an oil phase. Polymerization occurs at the phase boundary. Thus, a polymeric capsule shell wall forms at the interface of the two phases thereby encapsulating the core material. Wall formation of polyester, polyamide, and polyurea capsules also typically proceeds via interfacial polymerization.

Common microencapsulation processes can be viewed as a series of steps. First, the core material which is to be encapsulated is typically emulsified or dispersed in a suitable dispersion medium. This medium is typically aqueous but involves the formation of a polymer rich phase. Most frequently, this medium is a solution of the intended capsule wall material. The solvent characteristics of the medium are changed such as to cause phase separation of the wall material. The wall material is thereby contained in a liquid phase which is also dispersed in the same medium as the intended capsule core material. The liquid wall material phase deposits itself as a continuous coating about the dispersed droplets of the internal phase or capsule core material. The wall material is then solidified. This process is commonly known as coacervation.

In melamine formaldehyde processes using alkylated melamine precondensates such as methylated methylol melamine, during core material emulsification, premature polymerization of the methylated methylol melamine precondensate is often a problem. To combat this tendency, in prior art processes the pH is raised to the highest level at which emulsification can still be effected. The problem with the prior attempts is that elevating the pH can destroy intended acidic cores making encapsulation of acidic materials difficult to realize.

Although polycondensation of alkylated melamine formaldehyde resins such as alkylated methylol melamine resins in the presence of protective colloids is known, a need continues to exist for durable encapsulates of acidic material, such as organic acids.

The condensation of melamine formaldehyde resin can proceed under acid conditions. However, encapsulation of acidic materials with melamine formaldehyde has been difficult due to the reactivity of melamine formaldehyde resin.

For better reaction control, alkylated melamine formaldehyde resins such as alkylated methylol melamine formaldehyde resins have been prepared. These resins react in the presence of acidic materials such as citric acid, formic acid, acetic acid, oxalic acid, toluene sulfonic acid, hydrochloric acid, phthalic acid, maleic acid, sulfuric acid, trichloroacetic acid, p-toluene sulfonic acid or phosphoric acid, and the like, making their use in encapsulation of acidic materials problematic.

Until the invention, it has been challenging to attempt to form melamine formaldehyde core-shell microcapsules encapsulating an acidic material.

<CIT> discloses "an encapsulated curable adhesive composition especially adapted for use as a structural adhesive.

The present invention teaches a method of forming core-shell microcapsules encapsulating an acidic material, the microcapsules obtained by condensation of a fully alkylated melamine resin in the presence of a protective colloid. The method comprises preparing an aqueous dispersion in water of an acrylic acid-alkyl acrylate copolymer, a fully alkylated melamine resin, and adjusting the pH of the aqueous dispersion to be pH <NUM> or less. An intended acidic core material is added to the aqueous dispersion while applying high shear agitation to form an emulsion with droplet or particle size of less than <NUM> microns, or even less than <NUM> microns, or even less than <NUM> microns. Adding a sulfate salt to the emulsion aids in increasing the hydrophilicity of the emulsion and helps driving the forming wall material out of solution. Polycondensation of the alkylated melamine resin is effected with heating, thereby enwrapping particles or droplets of the acidic core material with polymeric shells of the polycondensed alkylated melamine resin. With further heating the microcapsule wall is further cured and hardened.

The acidic core material can be added to the aqueous dispersion as a solid particulate, or alternatively, the acidic core material can even be added to the aqueous dispersion as an acid dissolved or dispersed in an oil phase. If desired, a formaldehyde scavenger such as ammonia can be added after microcapsule curing and the pH can be adjusted to be alkaline. Preferably the formaldehyde scavenger is added after microcapsule curing. The alkylated melamine resin in certain embodiments can be a fully methylated methylol melamine resin. In the process of the invention the pH of the aqueous dispersion in the first step is adjusted to be pH <NUM> or less, or even pH <NUM> or less, or even pH <NUM> or less. The pH can be adjusted with addition of acid such as citric acid. During emulsification, the droplet or particle size of the emulsion is mixed under high shear agitation preferably achieving a size of <NUM> microns or less. Heating in the polycondensation step is from about <NUM> to about <NUM> over an extended time period. Further heating to cure is from <NUM> to <NUM> over several hours.

In Example <NUM>, herein, initial heating was at <NUM> with ramp to <NUM> over <NUM> minutes. This was followed by curing at <NUM> over eight hours. In Example <NUM>, initial heating was at <NUM> and then from <NUM> to <NUM> over <NUM> minutes. This was followed by curing at <NUM> over eight hours. Desirably a protective colloid such as acrylic acid-butyl acrylate copolymer is also employed. Alternative protective colloid copolymers can be selected from the group of co-polymers consisting of acrylic acid-butyl acrylate, acrylic acid-ethyl acrylate, acrylic acid-propyl acrylate, acrylic acid-amyl acrylate, acrylic acid-hexyl acrylate, acrylic acid-cyclohexyl acrylate, and acrylic acid-ethylhexyl acrylate. The intended acidic core material can be any of various organic acids such as carboxylic acid. For example, the acidic core material can be selected from terephthalic acid or oleic acid.

Alkylated methylol melamine resins generally are the products of the condensation of <NUM> molar proportion of melamine with up to <NUM> molar proportions of formaldehyde, then further alkylated with up to <NUM> molar proportions of a lower aliphatic alcohol, and <NUM> molar proportions of aliphatic alcohol to obtain fully alkylated methylol melamine.

The fully alkylated melamine resin precondensates useful in the invention are alkylated methylol melamine precondensate, and particularly fully alkylated methylol melamine precondensate such as fully methylated methylol melamine precondensate. Alkyl groups are preferably of <NUM> to <NUM> carbons.

Aqueous melamine formaldehyde precondensates have found use in a variety of applications. One such use is in microencapsulation of the core-shell type, where a wall material is formed of the condensed melamine formaldehyde precondensate by condensing over a period of time at elevated temperatures and/or addition of acid to promote the condensation reaction.

The invention describes an improved process for preparing an encapsulated acidic material within a core-shell microcapsule.

Encapsulation of acidic materials within shells formed of melamine formaldehyde resins has been difficult as acidic materials tend to promote polymerization of melamine formaldehyde precondensates such as alkylated melamine resin precondensates.

In the process of the invention a core-shell microcapsule of the melamine formaldehyde type is used to encapsulate an acidic material, such as an organic acid. The melamine formaldehyde resin is an alkylated melamine precondensate, and particularly a fully alkylated melamine resin precondensate.

The melamine formaldehyde precondensate, such as an alkylated melamine formaldehyde precondensate is used in an amount of from <NUM> to <NUM> parts per <NUM> parts by weight of the intended core material.

pH of the emulsion can be adjusted with materials such as citric acid, formic acid, acetic acid, oxalic acid, toluene sulfonic acid, hydrochloric acid, phthalic acid, maleic acid, sulfuric acid, trichloroacetic acid, p-toluene sulfonic acid or phosphoric acid.

For hardening of the capsules, curing was effected at elevated temperature, such as <NUM> over <NUM> hours.

The concentration of precondensate in the aqueous medium is from about <NUM>% to <NUM>% by weight. For capsule formation, the precondensate is dispersed in water. Desirably a protective copolymer such as an acrylic acid-alkyl acrylate copolymer in an amount of from about <NUM> to <NUM> parts by weights of the precondensate wall material is employed. A <NUM> : <NUM> to about a <NUM> : <NUM> proportion of copolymer to precondensate ratio by weight can be useful.

The intended core material is emulsified into the water dispersion forming desirably a low viscosity emulsion. The target droplet size of the core material is less than <NUM> microns, or even less than <NUM> microns, or even the majority of the droplets are in a range from <NUM> to <NUM> microns, or an even narrower size distribution.

Generally, the pH of the emulsion of precondensate, protective colloid and core material is maintained at a range of pH from about <NUM> to <NUM>.

Shell formation by curing proceeds with heating and the ramp in heating is in the range of from about <NUM> to <NUM>, or even to <NUM>, or even to <NUM>, or even to <NUM>, or higher, over a period of hours. The examples herein further illustrate the heating steps and heating ramp, meaning rate of heat increase over time.

Sodium sulfate or other sulfate salt is added to promote phase separation of the condensing alkylated melamine formaldehyde precondensate. The amount of sodium sulfate is from about <NUM> to about <NUM> parts by weight precondensate and colloid.

Condensation and deposit of the alkylated melamine formaldehyde around and onto a particle or droplet of the intended core material is accomplished by adjusting the pH and/or heating to promote the condensation reaction.

Melamine formaldehyde precondensates mean alkylol melamine prepolymers such as mono to hexamethylol melamines or a mixture of methylol melamines, all of which are further alkylated forming fully alkylated methylol melamines.

Optionally anionic surfactants, such as sodium dodecylbenzene sulfonate or other alkylaryl sulfonates or salts of aliphatic acids, can be included in addition.

The acidic core materials, which can be encapsulated by the invention, include terephthalic acid, oleic acid and other acids which are either solids or liquids dispersible in the oil phase. In particular, various oil soluble or oil dispersible organic acids can be encapsulated. Solid organic acids can also be encapsulated by the process of the invention.

The organic acids useful as a core material can include carboxylic acid, salts of carboxylic acid, di-, tri- and polycarboxylic acid and can include formic acid, citric acid, oxalic acid, lactic acid, maleic acid, benzoic acid, stearic acid, salicylic acid, ascorbic acid, gallic acid, lactic acid, phthalic acid, sorbic acid, sulfonilic acid, tannic acid, tartaric acid, succinic acid and the like.

The term oil phase as used herein refers to generally hydrophobic oils and can include by way of illustration and not limitation, various hydrocarbons and hydrocarbon solvents such as ethyldiphenylmethane, butyl biphenyl ethane, benzylxylene, alkyl biphenyls such as propylbiphenyl and butylbiphenyl, dialkyl phthalates e.g. dibutyl phthalate, dioctylphthalate, dinonyl phthalate and ditridecylphthalate; <NUM>,<NUM>,<NUM>-trimethyl-<NUM>,<NUM>-pentanediol diisobutyrate, alkyl benzenes such as dodecyl benzene; but also carboxylates, ethers, or ketones such as diaryl ethers, di(aralkyl)ethers and aryl aralkyl ethers, ethers such as diphenyl ether, dibenzyl ether and phenyl benzyl ether, liquid higher alkyl ketones (having at least <NUM> carbon atoms), alkyl or aralky benzoates, e.g., benzyl benzoate, alkylated naphthalenes such as dipropylnaphthalene, partially hydrogenated terphenyls; high-boiling straight or branched chain hydrocarbons, arenes and alkaryl hydrocarbons such as toluene, glycerides, tri-glycerides, vegetable oils such as canola oil, soybean oil, corn oil, sunflower oil, or cottonseed oil, methyl esters of fatty acids derived from transesterification of canola oil, soybean oil, cottonseed oil, corn oil, sunflower oil, pine oil, lemon oil, olive oil, or methyl ester of oleic acid, vegetable oils, esters of vegetable oils, e.g. soybean methyl ester, straight chain saturated paraffinic aliphatic hydrocarbons of from <NUM> to <NUM> carbons; C8 - C42 esters, ethyl hexanoate, methyl heptanoate, butyl butyrate, methyl benzoate, methyl nonoate, methyl decanoate, methyl dodecanoate, methyl octanoate, methyl laurate, methyl myristate, methyl palmitate, methyl stearate, ethyl heptanoate, ethyl octanoate, ethyl nonoate, ethyl decanoate, ethyl dodecanoate, ethyl laurate, ethyl myristate, ethyl palmitate, ethyl stearate, isopropyl myristate, isopropyl palmitate, ethylhexyl palmitate, isoamyl laurate, butyl laurate, octyl octanoate, decyl decanoate, butyl stearate, lauryl laurate, stearyl palmitate, stearyl stearate, stearyl behenate, behenyl behenate and the like. Mixtures of the above can also be employed.

After capsule formation, further heating such as at <NUM> for several hours can be used to effect curing of the capsules.

By adjusting the rate of high shear agitation, particularly during or following precondensate agitation, to mill rates of less than <NUM> rpm, agglomeration can be enhanced. In some applications agglomerates are desired to create more concentrated domains or clusters, such as for certain coated substrates where it is desirable to raise the cluster above the substrate. Single capsules, although useful, in certain applications such as porous paper, are less preferable than agglomerates, which can bridge gaps. Agglomeration can be adjusted by reduced rate of high shear agitation and/or combined with slower rates of addition of the acid and/or amount or rate of addition of the sulfate salt.

pH of the microcapsule dispersion can be adjusted after capsule formation such as with ammonium hydroxide to elevate the pH to the alkaline side and to help scavenge for formaldehyde. Optionally, other conventional formaldehyde scavengers can be adapted, such as acetoacetamide, urea, ammonium bisulfite, melamine, lysine, sodium bisulfite, ethylene urea, cysteine, cysteamine, glycine, serine, carnosine, histidine, glutathione, <NUM>,<NUM>- diaminobenzoic acid, allantoin, glycouril, anthranilic acid, methyl anthranilate, methyl <NUM>- aminobenzoate, ethyl acetoacetate, acetoacetamide, malonamide, ascorbic acid, <NUM>,<NUM>- dihydroxyacetone dimer, biuret, oxamide, benzoguanamine, pyroglutamic acid, pyrogallol, methyl gallate, ethyl gallate, propyl gallate, triethanol amine, succinamide, thiabendazole, benzotriazol, triazole, indoline, sulfanilic acid, oxamide, sorbitol, glucose, cellulose, poly( vinyl alcohol), partially hydrolyzed poly(vinylformamide), poly( vinyl amine), poly(ethylene imine), poly(oxyalkyleneamine), poly( vinyl alcohol)-co-poly(vinyl amine), poly(<NUM>-ammostyrene), poly(l-lysine), chitosan, hexane diol, ethylenediamine- N,N'-bisacetoacetamide, N-(<NUM>-ethylhexyl)acetoacetamide, <NUM>-benzoylacetoacetamide, N- (<NUM>-phenylpropyl)acetoacetamide, lilial, helional, melonal, triplal, <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-cyclohexanedione, <NUM>,<NUM>-dimethyl-<NUM>-cyclohexenecarboxaldehyde, <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-dioxan-<NUM>,<NUM>-dione, <NUM>-pentanone, dibutyl amine, triethylenetetramine, ammonium hydroxide, benzylamine, hydroxycitronellol, cyclohexanone, <NUM>-butanone, pentane dione, dehydroacetic acid, or a mixture thereof.

These scavengers can be included as part of the formed microcapsule slurry or optionally included in the core material in capsule formation.

In the following examples, the chemicals correspond to the following materials.

The water and Kemecal <NUM> was mixed in a <NUM> steel jacketed reactor for <NUM> minutes at 30C using a <NUM>-tip flat mill blade at about 750rpm. The water phase pH was adjusted to <NUM> with <NUM>% citric acid solution. The Cymel <NUM> resin was added over the course of about <NUM> minutes. After <NUM> additional minute of mixing, the core was added to the reactor over a period of <NUM> minutes. Mill to target size. The target mill size was <NUM> microns, and milling was started at <NUM> rpm. Milling was continued for <NUM> minutes, and was gradually increased to <NUM> rpm to achieve target size. After milling was completed, mixing was done with a <NUM>" propeller, run at <NUM> rpm. The sodium sulfate was added. The batch was heated from 30C to 95C in <NUM> minutes and held at 95C for <NUM> hours before cooling back to room temperature. Ammonia was added to a pH target of <NUM>.

Water phase materials were combined in a plastic beaker and mixed at room temperature with a magnetic stir bar. The water phase pH was adjusted to pH <NUM> with <NUM>% citric acid. The water phase was added to a <NUM> jacketed steel reactor (at 20C), and mixed at <NUM> rpm with a <NUM>-tip flat mill for <NUM> minutes. The core was added over <NUM> minutes and then mixed for <NUM> minutes at <NUM> rpm. Cymel <NUM> was added over <NUM> minutes, then mixed for an additional <NUM> minute at <NUM> rpm. Mixing was continued for <NUM> minutes at <NUM> rpm. Sodium sulfate was added, mixing was begun at <NUM> rpm with a <NUM>" propeller blade. The batch was heated from <NUM>-22C rapidly, then heated from <NUM> to 95C in <NUM> minutes and held at 95C for <NUM> hours. The batch was cooled back to room temperature, where the aqua ammonia was added.

Claim 1:
A method of forming core-shell microcapsules encapsulating an acidic material, the microcapsules being obtained by condensation of a fully alkylated melamine resin precondensate in the presence of a protective colloid, the method comprising:
(a) preparing an aqueous dispersion in water of an acrylic acid-alkyl acrylate copolymer, a fully alkylated melamine resin precondensate, and adjusting the pH of the aqueous dispersion to be pH <NUM> or less;
(b) adding an acidic core material to the aqueous dispersion while applying high shear agitation to form an emulsion with droplet or particle size of less than <NUM> microns;
(c) adding a sulfate salt to the emulsion;
(d) heating to effect polycondensation of the alkylated melamine resin precondensate, thereby enwrapping particles or droplets of the acidic core material with polymeric shells of the polycondensed alkylated melamine resin, and,
(e) further heating to cure the microcapsules.