Process for preparing polymer emulsions and polymers formed therefrom

Abstract

A process for preparing multi-stage emulsion polymers is provided. The process is capable of producing multi-stage emulsion polymers having low dry-bulk density. These polymers are useful in coating compositions such as paints and paper coatings.

We claim:  
     
         1 . A process for preparing emulsion polymer particles comprising: 
 a) providing an aqueous emulsion of 
 i) multi-stage emulsion polymer, comprising a core stage polymer and a shell stage polymer, 
 wherein the core stage polymer comprises, as polymerized units, from 5 to 100 percent by weight, based on the weight of the core stage polymer, of hydrophilic monoethylenically unsaturated monomer, and from 0 to 95 percent by weight, based on the weight of the core stage polymer, of at least one nonionic monoethylenically unsaturated monomer; and  
 wherein the shell stage polymer comprises, as polymerized units, at least 50 percent by weight of nonionic monoethylenically unsaturated monomer;  
 
 ii) monomer at a level of at least 0.5 percent by weight based on the weight of the multi-stage emulsion polymer; and  
 iii) swelling agent under conditions wherein there is no substantial polymerization of the monomer; and  
   b) reducing the level of monomer by at least fifty percent.    
     
     
         2 . A process for preparing emulsion polymer particles comprising: 
 a) providing an aqueous emulsion of 
 i) multi-stage emulsion polymer, comprising a core stage polymer and a shell stage polymer, 
 wherein the core stage polymer comprises, as polymerized units, from 5 to 100 percent by weight, based on the weight of the core stage polymer, of hydrophilic monoethylenically unsaturated monomer, and from 0 to 95 percent by weight, based on the weight of the core stage polymer, of at least one nonionic monoethylenically unsaturated monomer; and  
 wherein the shell stage polymer comprises, as polymerized units, at least 50 percent by weight of nonionic monoethylenically unsaturated monomer;  
 
 b) adding an effective amount of one or more polymerization inhibitors or reducing agents to substantially stop any polymerization;  
 c) providing monomer at a level of at least 0.5 percent by weight based on the weight of the multi-stage emulsion polymer;  
 d) adding swelling agent; and  
 e) reducing the level of monomer by at least fifty percent.  
   
     
     
         3 . The process of    claim 2    wherein the one or more polymerization inhibitors or reducing agents are added in an amount of from 25 to 5,000 ppm based on polymer solids.  
     
     
         4 . The process of    claim 2    or    claim 3    wherein the one or more polymerization inhibitors are selected from the group consisting of N, N-diethylhydroxylamine, N-nitrosodiphenylamine, 2,4-dinitrophenylhydrazine, p-phenylenediamine, Phenothiazine, alloocimene, triethyl phosphite, 4-nitrosophenol, 2-nitrophenol, p-aminophenol, 4-hydroxy TEMPO, hydroquinone, p-methoxyhydroquinone, tert-butyl-p-hydroquinone, 2,5-di-tert-butyl-p-hydroquinone, 1,4-naphthalenediol, 4-tert butyl catechol, copper sulfate, copper nitrate, cresol and phenol.  
     
     
         5 . The process of    claim 1    or    2    wherein the monomer at a level of at least 0.5 percent by weight based on the weight of the multi-stage emulsion polymer is one or more of the monomers used to prepare the multi-stage emulsion polymer.  
     
     
         6 . The process of    claim 1    or    2    wherein the monomer at a level of at least 0.5 percent by weight based on the weight of the multi-stage emulsion polymer is nonionic monomer.  
     
     
         7 . The process of    claim 1    or    2    wherein the swelling agent is selected from volatile base, fixed base, and combinations thereof.  
     
     
         8 . The process of    claim 1    or    2    wherein the level of monomer is reduced to less than 10,000 ppm based on polymer solids by polymerizing said monomer.  
     
     
         9 . An aqueous polymer emulsion comprising water and swollen multi-stage emulsion polymer wherein the dry bulk density of the swollen multi-stage emulsion is less than 0.77 g/cc when the swollen multi-stage emulsion polymer has a particle size below 275 nm; less than 0.74 g/cc when the swollen multi-stage emulsion polymer has a particle size of from 275 to 500 nm; less than 0.59 g/cc when the swollen multi-stage emulsion polymer has a particle size of from 501 to 750 nm; less than 0.46 g/cc when the swollen multi-stage emulsion polymer has a particle size of from 751 to 1300 nm.