Abstract:
Methods of making microcapsules and microcapsules comprising a core material and a shell material with substantially different dielectric constants and dissipation factors. Exposure to appropriate electromagnetic energy selectively (a) heats the core material with the higher dielectric constant and dissipation factor, directly or indirectly fusing the shell material and forming microcapsules, or (b) hardens polymerized shell material, which has a high dielectric constant and dissipation factor.

Description:
The present application claims the benefit of U.S. Provisional Application Ser. No. 60/312,704, filed Aug. 16, 2001, pending. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to methods of making microcapsules and microcapsules comprising a core material and a shell material with substantially different dielectric constants and dissipation factors. Exposure to appropriate electromagnetic energy selectively (a) heats the core material with the higher dielectric constant and dissipation factor, directly or indirectly fusing the shell material and forming microcapsules, or (b) hardens polymerized shell material, which has a high dielectric constant and dissipation factor. 
     BACKGROUND 
     Most current microencapsulation methods are used to produce a uniform coating on an irregular shaped particle. Such methods typically require relatively complex equipment and/or a substantial input of labor. Such methods typically are very expensive. Simpler, less expensive microencapsulation methods are needed for encapsulating core materials having relatively non-uniform shapes. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention provides a method of microencapsulation comprising:
         providing core material effective to absorb electromagnetic energy at a given wavelength and frequency;   providing shell material ineffective to absorb electromagnetic energy at said given wavelength and frequency;   forming a precursor mixture comprising said core material and said shell material; and   exposing said precursor mixture to said electromagnetic energy at a power and for a time effective to microencapsulate said core material with said shell material.       

     In another aspect, the invention provides a method of microencapsulation comprising:
         providing a core material having a first dielectric constant and a first dissipation factor;   providing a shell material having a second dielectric constant that is substantially different than said first dielectric constant and having a second dissipation factor that is substantially different than said first dissipation factor;   forming a precursor mixture comprising said core material and said shell material;   exposing said precursor mixture to electromagnetic energy under conditions effective to microencapsulate said core material with said shell material.       

     In a preferred embodiment, the second dielectric constant is a magnitude or more different than the first dielectric constant, and the second dissipation factor is a magnitude or more different than the first dissipation factor. 
     In one embodiment, the second dielectric constant is substantially less than the first dielectric constant and the second dissipation factor is substantially less than the first dissipation factor. Preferably, “substantially less than” is a magnitude or more less than. In this embodiment, said “exposing” heats the core material and causes fusing of the shell material immediately adjacent to the heated core material. The core material preferably comprises particles having an average diameter x. The shell materials preferably have an average diameter of about 0.1x, more preferably 0.01x. 
     In another embodiment, the second dielectric constant is substantially greater than the first dielectric constant and the second dissipation factor is substantially greater than the first dissipation factor. Preferably, “substantially greater than” is a magnitude or more greater than. In this embodiment, exposing the precursor mixture to electromagnetic energy preferably induces polymerization of the shell material. 
     In another aspect, the invention provides microcapsules comprising a core material effective to absorb electromagnetic energy at a given wavelength and frequency and a shell material that is ineffective to absorb said electromagnetic energy. 
     In another aspect, the invention provides microcapsules comprising a core material having a first dielectric constant and a first dissipation factor encapsulated by a shell material having a second dielectric constant and a second dissipation factor. The first dielectric constant and the first dissipation factor are substantially different than, preferably at least a magnitude different than, the second dielectric constant and the second dissipation factor, respectively. 
     In another aspect, the invention provides microcapsules comprising a core material having a first dielectric constant and a first dissipation factor encapsulated by a shell material. The shell material has a second dielectric constant and a second dissipation factor less than said first dielectric constant and said first dissipation factor, respectively. 
     In yet another aspect, the invention provides microcapsules comprising a core material having a first dielectric constant and a first dissipation factor encapsulated by a shell material having a second dielectric constant and a second dissipation factor. The second dielectric constant and the second dissipation factor are greater than the first dielectric constant and the first dissipation factor, respectively. 
    
    
     
       DETAILED DESCRIPTION OF THE FIGURES 
         FIG. 1  is an optical micrograph of the microcapsules formed in Example 1 after two minutes of microwave exposure in a 600 W microwave oven. 
         FIG. 2  is an optical micrograph of the sharp edges of the microcapsules of  FIG. 1  showing efficient coating of the sharp edges of the TPP/carbon black particle. 
         FIG. 3  is a graph of the temperature versus time during electromagnetic heating at 2.45 GHz of pure H 2 O, pure hydroxyethylmethacrylate, pure acrylic acid, and pure methacrylic acid, respectively. 
         FIG. 4  is a graph of temperature versus time during electromagnetic heating at 2.45 GHz of aqueous solutions of acrylic acid, hydroxyethylmethacrylate, and methacrylic acid, respectively. 
         FIG. 5  is an optical micrograph of the microcapsules formed in Example 4. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides simple, efficient, and relatively inexpensive methods for forming microcapsules. The methods of the present invention use electromagnetic energy to form microcapsules from a core material having a “first” dielectric constant and a “first” dissipation factor, and a shell material having a “second” dielectric constant and a “second” dissipation factor. The first dielectric constant is “substantially different” than the second dielectric constant and the first dissipation factor is “substantially different” than the second dissipation factor. As used herein, the term “substantially different” is defined as a difference sufficiently great to permit microcapsules to form under the conditions of operation. Preferably, the difference between the first and second dielectric constant and the first and second dissipation factor is about one magnitude or more. 
     Microwaves interact with materials through electric and magnetic fields generated in the wave guide. Materials, which absorb energy from an electromagnetic (EM) field, can be heated by exposure to such a field. The absorption of electromagnetic energy also may drive a reaction, such as a polymerization reaction. The selective microwave absorption depends on the power density of the EM field and on the absorptive characteristics of the materials, as defined by the complex dielectric constant or dissipation factor (loss tangent). Absorption of microwave energy is high when the dissipation factor or loss tangent is high. The loss tangent (tan δ) is defined as: 
         tan   ⁢           ⁢   δ     =       ε   ″       ε   ′           
 
wherein ε*=jε″+e′ is the complex dielectric constant.
 
     Microwave heating creates non-uniform thermal or energy absorption gradients which, in general, are a disadvantage. In the present process, the formation of non-uniform thermal or energy absorption gradients helps in inducing a selective absorption of energy either by the core material immediately adjacent to the shell material, or by the shell material itself. The result is microcapsules of the core material bearing a coating of the fused or polymerized shell material. 
     With appropriate process control on the amount of microwave energy supplied, appropriate wave guide design, and suitable selection of core and shell ingredients, the invention offers unique methods to coat materials of non-uniform shapes. The methods require only localized heating and therefore require a lower net temperature increase in the substrate. The methods have improved reaction specificity. The electromagnetic energy can be delivered rapidly at a specific time and place. The method provides increased processing speed, high coating efficiency, and the ability to prepare microcapsules with different payloads. Although batch, the methods are inexpensive because of short processing time and the ability to handle high production volumes. 
     Higher Dielectric Characteristics Core Material 
     In a preferred embodiment, the second dielectric constant and the second dissipation factor (of the shell material) are substantially less than, preferably a magnitude or more less than the first dielectric constant and the first dissipation factor (of the core material). In this embodiment, the core material either inherently has a relatively high “first” dielectric constant and a relatively high “first” dissipation factor, or contains one or more absorption enhancer(s) to increase the absorption of electromagnetic energy by the core material. The core material, alone, or in combination with one or more absorption enhancer(s) and/or one or more surface wetting agent(s), is mixed with a shell material that has a relatively low dielectric constant. The mixture is exposed to electromagnetic energy at a wavelength and frequency that are ineffective to heat the shell material, but that are effective to heat the core material and to induce melting of the shell material immediately adjacent to and surrounding the core material. Upon cooling, the previously melted shell material forms a fused, intact shell around the core material. The remaining shell material may be simply sieved or washed away, for example by density separation, leaving the resulting microcapsules. 
     Because water absorbs microwave energy effectively at 2.456 Hz, suitable core materials with a relatively high dielectric constant and a relatively high dissipation factor include, but are not necessarily limited to materials that comprise water, including, but not necessarily limited to salts, hydrates, oxides, water soluble materials, water absorbing materials, and hydrophilic materials. The purity of the compound and the amount of water present may influence the loss factor and microwave frequency of absorption, thus playing an important role during the microencapsulation process. 
     Specific examples of suitable core materials with a relatively high dielectric constant include, but are not necessarily limited to alumina, acidulants, citric acid, ascorbic acid, antiperspirants, solid bleaches, coffee solids, fruits, nuts, leavening agents, oxidizers, pigments, sodium bicarbonate, sweeteners, salts, activated carbon, pesticides, fungicides, fumigants, seeds, antiperspirants, bleaches, deodorants, inks, active metals, catalysts, curing agents, chemoluminence agents, corrosion inhibitors, deodorants, flame retardants, organometallics, phase change materials, curing agents, sealants, and resins. 
     It is not always necessary to mix a core material with a relatively high dielectric constant and a relatively high dissipation factor with an absorption enhancer. However, depending upon the core material and the shell material, it may be desirable to use one or more absorption enhancer(s) to increase absorption of electromagnetic energy by the core material. A variety of absorption enhancers may be used, including but not necessarily limited to chlorides, carbonates, nitrates, phosphates, carbon black, silicates, sodium silicates, calcium silicates, CABOSIL, silica aerogels, silica, silicon dioxides, talc, starches, maltodextrins, mica, bentonite, and other clay particles. Preferred absorption enhancers include, but are not necessarily limited to carbon black, talc, and silicates. The excess of core material to absorption enhancer is at least about 2:1, preferably 10:1, more preferably about 20:1. 
     One or more absorption enhancer(s) may be used to increase the absorption of electromagnetic energy throughout a given core material. Or, the absorption enhancer(s) may be used to increase absorption of electromagnetic energy immediately adjacent to the shell material. In order to accomplish this, spheronization preferably is conducted. “Spheronization” is a method by which micronized particles of absorption enhancer physically adhere onto the core particle by powder blending, semi-wet, or wet blending methods, including but not necessarily limited to ball-mill mixing, impaction, or compressive compaction. If an absorption enhancer is used, the absorption enhancer may be mixed with a surface wetting agent to provide improved surface adhesion to the core particle and/or to increase the efficiency of the spheronization process. Suitable wetting agents include, but are not necessarily limited to anionic, cationic, or non-ionic surfactants. Examples of suitable wetting agents include, but are not necessarily limited to sodium dodecyl sulphate, PLURONICS™, TWEENS™, SPANS™, phospholipids, and the like. Where an absorption enhancer is used, the core material is mixed with the absorption enhancer. After adequate mixing, by kneading, hand mixing, dough mixing, powder blending, or ball-mixing techniques, the mixture of absorption enhancer and core material is sieved to remove excess absorption enhancer, preferably carbon black. 
     The core material is mixed with an excess of suitable shell material, preferably a micronized shell material. Where the core material has been spheronized, the sieved mixture is added to an excess of the shell material. The ratio of sieved mixture to shell material is about 1:5, preferably about 1:10, more preferably about 1:100 or greater. The particles of core material should be well dispersed in the shell material. 
     Suitable shell materials preferably exhibit a “sharp” phase transition, i.e., melt point, gel point, softening point crystallization point, glass transition temperature, and the like. In this context, the term “sharp” means that the shell system exhibits a change in specific heat and exhibits a latent heat (Δλ) or change in enthalpy of polymerization (ΔH p ). The shell particles preferably are much smaller than the core particles, preferably only 0.1x, more preferably only 0.01x, where x is the average diameter of the core particles. Preferred shell particles are micronized, and have an average diameter of from about 1 μm to about 500 μm. Micronized materials are available from source companies or can be produced by conventional spray congealing methods. 
     Suitable shell materials with a relatively low dielectric constant and a relatively low dissipation factor include, but are not necessarily limited to waxes, fats, resins, and low melting polymers. Examples of suitable waxes include, but are not necessarily limited to paraffins, polyalkylenes, polyakylene glycols, polyalkylene oxides, edible waxes, mineral wax, shellac, and other natural waxes. Examples of suitable fats include, but are not necessarily limited to monoglycerides, diglycerides, triglycerides, lipids, fatty acids, and fatty alcohols. Examples of low melting polymers include, but are not necessarily limited to polyethylene oxide, polyethylene glycol, polyethylene, polypropylene, and polytetrafluoroethylene. Preferably, the shell materials are micronized, exhibit a sharp melt point, and do not absorb the electromagnetic energy used to heat the core material. 
     After mixing the core material with an excess of shell material, the mixture is then exposed to electromagnetic energy to “cure” the mixture and to microencapsulate the core material. The electromagnetic field produces heat by means of the dielectric loss properties of the core material. The use of an electromagnetic field having substantially any frequency or substantially any wavelength should operate according to the present invention; however, certain frequencies or wavelengths will be preferred in certain circumstances, as outlined herein. 
     Where the core material has the relatively higher dielectric constant and the relatively higher dissipation factor, a preferred frequency range heats a selected core material but the intensity of heating is localized. In other words, the absorption intensity is sufficiently high to cause the micronized shell material immediately adjacent to the core material to fuse, leaving the remainder of the micronized shell material in micronized form. 
     The preferred frequency of operation depends on the absorption efficiency of the core so that the first dielectric constant and the first dissipation factor are maximized at that specific frequency range. The frequency dependence on heating is also influenced by the size of the core particle. When the size of the core particle is small, shorter wavelengths of microwave energy may be required for effective localized heating by Marangoni&#39;s dissipation of the core particle that has a high dielectric constant and dissipation factor. Preferably, the wavelength of the electromagnetic wave should be less than the largest dimension of the core particle, most preferably less than 0.1x of the size of the core. In such a case, energy absorption occurs by Marangoni&#39;s heat dissipation. 
     The electromagnetic field may be used to quickly raise the temperature of the core material to a desired point, and then to maintain the core material at that temperature for the desired “cure” period. 
     Preferred electromagnetic energy sources for use in this embodiment have a frequency region selected from the group consisting of a millimeter wave region and a microwave region. A millimeter wave energy source has a frequency in the range of from about 30 GHz to about 300 GHz, more preferably in the range of from about 30 GHz to about 50 GHz. A microwave energy source has a frequency in the range of from about 0.5 GHz to about 30 GHz, more preferably in the range of from about 1 GHz to about 10 GHz. The energy source preferably has a power in the range of from about 250 W to about 5 kW, most preferably in the range of from about 500 kW to about 1500 kW. 
     The “fusion time” for a given mixture is defined as the period of time required to induce fusion of the micronized shell material immediately adjacent to the core material in the mixture and to produce a continuous shell of fused material around the core material. The “fusion time” will depend on the type of core material, the electromagnetic energy source and its power, the shell material, absorption efficiency of the particle and wave guide design, and other reaction conditions. Suitable “fusion time” periods are in the range of from about 2 seconds to about 500 seconds, preferably in the range of from about 10 seconds to about 100 seconds, and most preferably in the range of from about 20 seconds to about 60 seconds. 
     Lower Dielectric Characteristic Core Material 
     Alternately, a core material that has a relatively low dielectric constant and a relatively low dissipation factor is mixed with a shell or monomer material that polymerizes to form a shell which has a relatively high dielectric constant and a relatively high dissipation factor. In this case, the mixture is exposed to electromagnetic energy at a wavelength and frequency that harden polymerized shell material adsorbed to the surface of emulsified droplets of core material. The polymerized shell material forms a fused, intact shell around the core material. The remaining shell material may be simply washed away, leaving the resulting microcapsules. 
     According to a preferred embodiment, a “precursor mixture” to be exposed to electromagnetic energy is prepared by producing an emulsion or suspension of the core material in an immiscible solvent. The emulsion or suspension may be in a hydrophilic solvent or a hydrophobic solvent. The type of solvent used will depend upon the miscibility of the core material. Preferred solvents are hydrophilic and include, but are not necessarily limited to water, aqueous solutions, N-methyl pyrrolidone, acetone, methyl ethyl ketone, methyl isobutyl ketone, heptanol, and octanol. The use of an aqueous solution of increases the efficiency of absorption of microwave energy. In a preferred embodiment, the solvent is aqueous, and one or more shell monomers/precursor(s) which are miscible with the aqueous solvent are added to the emulsion/suspension to form a “precursor mixture.” Examples of suitable water-miscible monomeric shell materials include, but are not necessarily limited to styrene oxide, acrylic acid, methacrylic acid, glycols, and acrylate monomers. Most preferred shell materials are acrylic acid, methyacrylic acid, and acrylate monomers. The “precursor mixture” may contain a polymerization catalyst. In a preferred embodiment, the polymerization catalyst is a weak acid or base which alters the pH of the solution, thereby accelerating the polymerization reaction. Without limiting the invention to a particular theory of operation, it is believed that, as it polymerizes, the polymerizing shell material becomes increasingly less susceptible to microwave absorption. As polymerization occurs, phase separation is induced by the like interactions, which allows deposition of the formed polymer onto the surface of the core droplets. Exposure of the mixture to electromagnetic energy accelerates polymerization and hardens the adsorbed polymeric shell material, completing the formation of the microcapsules. A cross-linking agent may be added before application of microwave energy. 
     Preferred electromagnetic energy sources for use in this embodiment are the same as previously described. The following microwave heating rates are expected for the following monomers at the following concentrations: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                   
                 Microwave 
               
               
                   
                   
                 Concentration 
                 heating 
               
               
                   
                 Monomer 
                 (vol. %) 
                 Rate (J/s) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Acrylic Acid 
                  25 
                 14 
               
               
                   
                   
                  40 
                 30.6 
               
               
                   
                   
                  50 
                 32.6 
               
               
                   
                   
                  60 
                 27.3 
               
               
                   
                   
                  75 
                 14.7 
               
               
                   
                   
                 100 
                 2.0 
               
               
                   
                   
                  50 in conc. HCl 
                 17.8 
               
               
                   
                   
                  50 in 50% HCl 
                 13.6 
               
               
                   
                   
                  75 in conc. HCl 
                 32.6 
               
               
                   
                 2-Hydroxyethyl 
                  25 
                 19.7 
               
               
                   
                 Methacrylate 
                  50 
                 32.1 
               
               
                   
                   
                  75 
                 23.7 
               
               
                   
                   
                 100 
                 17.6 
               
               
                   
                 Methacrylic Acid 
                  25 
                 12.6 
               
               
                   
                   
                  50 
                 24.0 
               
               
                   
                   
                  75 
                 10.1 
               
               
                   
                   
                 100 
                 0.9 
               
               
                   
                 Deionized water 
                 100 
                 5.8 
               
               
                   
                   
               
             
          
         
       
     
     The “hardening time” for a given shell material is defined as the period of time that a polymerization mixture must be exposed to electromagnetic energy in order to produce a hardened, continuous shell of hard polymer around the core material. The “hardening time” will depend on the type of core material, the electromagnetic energy source and its power, the shell material, absorption efficiency of the particle and wave guide design, and other reaction conditions. Suitable “hardening times” are from about 10 seconds to about 500 seconds, preferably in the range of from about 20 seconds to about 100 seconds, and most preferably in the range of from about 30 seconds to about 60 seconds. 
     The invention will be better understood with reference to the following examples, which are illustrative only, and should not be construed as limiting the present invention to any particular embodiment. 
     EXAMPLE 1 
     An organic catalyst, quaternary phosphonium salts (TPP, Sigma), was selected as a core. The TPP initially was mixed with carbon black. Spheronization with carbon black was conducted by ball-milling to provide uniform surface adhesion to the catalyst surface. In a simplified experiment, TPP was well mixed with carbon black in a ratio of 3:1 using a spatula. After adequate mixing, the mixture was sieved to remove the excess carbon black. The sieved TPP/carbon black mixture was later added to microcrystalline mineral wax (Petrolite™ C-1035) in the ratio of 1:5 such that the particles were well dispersed in the micronized wax. The resultant mixture was exposed to microwave energy in a conventional microwave oven (600 W) at full power for 2 minutes. The resulting microcapsules were observed under microscope to exhibit coated sharp edges of uniform thickness of from about 10 μm to about 20 μm. 
     EXAMPLE 2 
     10 g of citric acid crystals obtained from Sigma, having an approximate particle size ranging from about 250 μm to about 1000 μm, are added to polyethylene wax (POLYWAX™ 500) at a ratio of 1:4. The particles of acidulant are well dispersed in the micronized wax. The resultant mixture is exposed to microwave energy in a conventional microwave oven (750 W) at full power for about 3.5 minutes. The resulting microcapsules are observed under the microscope to exhibit coated sharp edges of fused wax, and complete encapsulation. 
     EXAMPLE 3 
     Type A-5 alumina (Sigma) is dispersed in 10 mL of SPAN 80 to improve surface wetting characteristics. Excess SPAN 80 is filtered in a Buckner&#39;s funnel to form a cake. The resultant cake is blended with ULTRAFLEX WHITE (Petrolite) microcrystalline wax at a ratio of 1:20 to obtain a dry, free-flowing powder. The resultant mixture is subjected to a conventional 750 W microwave oven for period of about 1 minute. Microspheres are separated upon subsequent washing in water. The encapsulated alumina settles at the bottom, while excess wax floats to the top. The resultant capsules are observed under a microscope to exhibit several alumina particles in one microsphere. All microspheres are well coated; however, the particles are not spherical. 
     EXAMPLE 4 
     A 100 g. mixture of 5% wt hydroxymethacrylate (Fischer Scientific) in water was prepared. Methyl ethyl ketone (MEK) Peroxide at 1% concentration and 0.5% ethyl glycol dimethacrylate (EGDMA) was added as an initiator and cross-linking agent, respectively. The resultant solution was then redispersed in silicon fluid (200 cP) and subjected to 750 W microwave heating for 5 minutes. Microbeads were prepared in size ranging from about 10 μm to 50 μm. The results are shown in  FIGS. 3 and 4 . 
     Persons of ordinary skill in the art will recognize that many modifications may be made to the present invention without departing from the spirit and scope of the present invention. The embodiment described herein is meant to be illustrative only and should not be taken as limiting the invention, which is defined in the claims.