Preparation of capsules

Abstract

Prior art processes for producing protein-based capsules (for example, capsules for use in electrophoretic media) tend to be wasteful because they produce many capsules outside the desired size range, which is typically about 20 to 50 μm. Capsule size distribution and yields can be improved by either (a) emulsifying a water-immiscible phase in a preformed coacervate of the protein; or (b) using a limited coalescence process with colloidal alumina as the surface-active particulate material.

1 . A process for encapsulating a water-immiscible phase in a protein coacervate, the process comprising: 
 forming a coacervate of the protein and a coacervating agent in an aqueous medium; and    emulsifying the water-immiscible phase in the aqueous medium comprising the coacervate under conditions effective to cause deposition of the coacervate around the water-immiscible phase, thereby forming capsules of the water-immiscible phase surrounded by capsule walls of the coacervate.    
     
     
         2 . A process according to  claim 1  wherein the protein is gelatin.  
     
     
         3 . A process according to  claim 1  wherein the coacervating agent is acacia.  
     
     
         4 . A process according to  claim 1  wherein the water-immiscible phase comprises an aliphatic hydrocarbon.  
     
     
         5 . A process according to  claim 4  wherein the water-immiscible phase further comprises solid particles suspended in the hydrocarbon.  
     
     
         6 . A process according to  claim 5  wherein the solid particles are electrically charged.  
     
     
         7 . A process according to  claim 6  wherein the water-immiscible phase comprises two different types of solid particles bearing charges of opposite polarity.  
     
     
         8 . A process according to  claim 1  wherein at least part of the emulsification/capsule formation step is conducted at a temperature below about 35° C.  
     
     
         9 . A process according to  claim 1  wherein the capsules formed are thereafter treated with a cross-linking agent effective to cause cross-linking of the protein.  
     
     
         10 . A process according to  claim 9  wherein the cross-linking agent is an aldehydes.  
     
     
         11 . A process according to  claim 1  wherein the capsules formed are thereafter mixed with a polymeric binder and the capsules/binder mixture coated on to a substrate and dried to form a coherent layer of capsules on the substrate.  
     
     
         12 . A process for encapsulating a water-immiscible phase in a protein coacervate, the process comprising: 
 forming an aqueous phase comprising a colloidal alumina suspension and a promoter;    emulsifying the water-immiscible phase in the aqueous phase under conditions effective to cause the formation of an unstable emulsion comprising small droplets of the water-immiscible phase in the aqueous phase; and    admixing the emulsion with the protein and a coacervating agent under conditions permitting coalescence of the emulsion and the formation of capsules of the aqueous phase surrounded by capsule walls of the coacervate.    
     
     
         13 . A process according to  claim 12  wherein the alumina comprises from about 0.1 to about 1.0 percent by weight of the aqueous phase.  
     
     
         14 . A process according to  claim 12  wherein the promoter comprises a polyacid.  
     
     
         15 . A process according to  claim 14  wherein the promoter comprises a copolymer of a carboxylic acid and an olefin.  
     
     
         16 . A process according to  claim 12  wherein the protein comprises gelatin.  
     
     
         17 . A process according to  claim 12  wherein the coacervating agent comprises an anionic polymer.  
     
     
         18 . A process according to  claim 17  wherein the coacervating agent comprises a polyanionic polymer having a vinyl main chain and a plurality of anionic groups bonded to the main chain.  
     
     
         19 . A process according to  claim 18  wherein the polyanionic polymer comprises any one or more of poly(acrylic acid); poly(methacrylic acid); copolymers of poly(acrylic acid) and/or poly(methacrylic acid) with esters of the same acids; styrene sulfonate copolymers with styrene; methyl vinyl ether or vinyl acetate copolymers with (meth)acrylic acid; copolymers of alkyl-substituted olefins, methyl vinyl ether and vinyl carboxylate with maleic acid, maleic esters, and maleic half ester, half acids.  
     
     
         20 . A process according to  claim 12  wherein the wherein the water-immiscible phase comprises an aliphatic hydrocarbon.  
     
     
         21 . A process according to  claim 20  wherein the water-immiscible phase further comprises solid particles suspended in the hydrocarbon.  
     
     
         22 . A process according to  claim 21  wherein the solid particles are electrically charged.  
     
     
         23 . A process according to  claim 22  wherein the water-immiscible phase comprises two different types of solid particles bearing charges of opposite polarity.  
     
     
         24 . A process according to  claim 21  wherein the hydrocarbon has dispersed therein a di-block copolymer or an aromatic-substituted alkene and an alkene.  
     
     
         25 . A process according to  claim 12  wherein the capsules formed are thereafter treated with a cross-linking agent effective to cause cross-linking of the protein.  
     
     
         26 . A process according to  claim 25  wherein the cross-linking agent is an aldehyde.  
     
     
         27 . A process according to  claim 12  wherein the capsules formed are thereafter mixed with a polymeric binder and the capsules/binder mixture coated on to a substrate and dried to form a coherent layer of capsules on the substrate.