Expandable microspheres and methods of making and using the same

Abstract

Expandable microspheres formed by suspension polymerization using a shot growth method are provided. The microspheres are formed of a continuous, gas impermeable shell surrounding a blowing agent. The shell includes a first polymer layer formed from primary monomers and a second layer that includes a chemically reactive monomer or a high Tg monomer. To form the microspheres, the primary monomers are polymerized in a reaction vessel to an approximate 90% polymerization, at which time a secondary monomer that is either a monomer having a Tg of at least 85° C. or a chemically reactive monomer, is added to the reaction vessel to drive the polymerization reaction to completion. The outer layer thus contains either a larger amount of the high Tg monomer or a chemically reactive monomer that possesses the ability to covalently bond a cationic species. The microspheres may be used in papermaking processes to increase the paper bulk.

Having thus described the invention, what is claimed is: 
     
         1 . An expandable microsphere comprising:
 a gas impermeable shell encapsulating at least one blowing agent, said gas impermeable shell including:
 a first polymeric layer surrounding said at least one blowing agent; and 
 a second polymeric layer at least substantially encapsulating said first polymeric layer, and 
   wherein said second polymeric layer includes at least one monomer selected from a monomer having a Tg of at least 85° C. and a chemically reactive monomer.   
     
     
         2 . The expandable microsphere of  claim 1 , wherein said first polymeric layer comprises polymers formed from monomers selected from nitrile containing monomers, acrylic ester monomers, methacrylic ester monomers, vinyl esters, vinyl halide monomers and combinations thereof. 
     
     
         3 . The expandable microsphere of  claim 2 , wherein said first polymeric layer comprises at least one nitrile containing monomer, at least one methacrylic ester monomer, and at least one vinylidene halide monomer. 
     
     
         4 . The expandable microsphere of  claim 2 , wherein said at least one monomer has a Tg less than 85° C. 
     
     
         5 . The expandable microsphere of  claim 2 , wherein said first polymeric layer further comprises a crosslinking monomer. 
     
     
         6 . The expandable microsphere of  claim 2 , further comprising a cationic species covalently bonded to said second polymeric layer. 
     
     
         7 . The expandable microsphere of  claim 2 , wherein said second polymeric layer has a larger amount of said monomer having a Tg of at least 85° C. compared to an amount of said monomer having a Tg of at least 85° C. present in said first polymeric layer. 
     
     
         8 . The expandable microsphere of  claim 1 , wherein said first and second polymeric layers are covalently attached. 
     
     
         9 . The expandable microsphere of  claim 1 , wherein said expandable microsphere has an expanded volume average diameter of less than 50 microns. 
     
     
         10 . The expandable microsphere of  claim 1 , wherein said expandable microsphere has a temperature of onset expansion from about 60° C. about 105° C. 
     
     
         11 . The expandable microsphere of  claim 1 , wherein said expandable microsphere has a temperature of shrinkage greater than about 105° C. 
     
     
         12 . A method of forming an expandable microsphere comprising:
 mixing primary monomers selected from nitrile containing monomers, acrylic ester monomers, methacrylic ester monomers, vinyl esters, vinyl halide monomers and combinations thereof, at least one blowing agent, a crosslinking monomer, a polymerization initiator, and a stabilizer system in a reaction vessel for a period of time sufficient to achieve an approximate 90% polymerization of said primary monomers and form a first polymeric layer surrounding said blowing agent; and   adding a secondary monomer selected from monomers having a Tg of at least 85° C. and chemically reactive monomers to said reaction vessel to form a second polymeric layer at least substantially surrounding said first polymeric layer and form an expandable microsphere.   
     
     
         13 . The method of  claim 12 , wherein said reaction vessel further includes at least one member selected from a salt, a phase partitioner, an inhibitor an acid and water. 
     
     
         14 . The method of  claim 12 , further comprising:
 purging said reaction vessel to remove oxygen.   
     
     
         15 . The method of  claim 12 , further comprising:
 adding a cationic species to said microsphere such that said cationic species covalently bonds to said second polymeric layer.   
     
     
         16 . A composition comprising the expandable microsphere of  claim 1  and a plurality of cellulose fibers. 
     
     
         17 . A paper comprising:
 a web of cellulose fibers; and   a plurality of expandable microspheres, said expandable microspheres including:
 a gas impermeable shell encapsulating at least one blowing agent, said gas impermeable shell including:
 a first polymeric layer surrounding said at least one blowing agent; and 
 a second polymeric layer at least substantially encapsulating said first polymeric layer, 
 
 wherein said second polymeric layer includes at least one monomer selected from a monomer having a Tg of at least 85° C. and a chemically reactive monomer. 
   
     
     
         18 . A method of making a paper substrate, comprising
 contacting a plurality of cellulose fibers with at least one expandable microsphere according to  claim 1  prior to or at a head box, at the size press, or at a coater.   
     
     
         19 . A method of enhancing the bulk of a paper substrate, comprising
 contacting a plurality of cellulose fibers with at least one expandable microsphere according to  claim 1  prior to or at a head box, at the size press, or at a coater.   
     
     
         20 . A method of enhancing the retention of expandable microspheres in a web of cellulose fibers, comprising
 contacting a cationic species to the second layer of at least one expandable microsphere according to  claim 1  to form a modified expandable microsphere having a zeta potential that is greater than the zeta potential of the microsphere prior to said contacting step, the zeta potential being measured at a pH of about 9.0 or less at an ionic strength from 10-6 M to 0.1 M; and   contacting a plurality of cellulose fibers with at least one modified expandable microsphere prior to or at a head box, at the size press, or at a coater.   
     
     
         21 . An expandable microsphere comprising:
 a gas impermeable shell encapsulating at least one blowing agent, said gas impermeable shell including:
 a first polymeric layer surrounding said at least one blowing agent; and 
 a second polymeric layer at least substantially encapsulating said first polymeric layer, and 
   wherein said second polymeric layer has a weight average Tg that is greater than a weight average Tg of the first polymeric layer.