Multiphase emulsion polymers for aqueous coating compositions containing little or no organic solvents

1. A polymer latex binder having a VOC level of less than about 50 g/L comprising a multiphase emulsion polymer in the form of latex particles, water and at least one emulsifier, said multiphase emulsion polymer comprising:

at least one soft polymer phase having a glass transition temperature of from 0 to −70° C. comprised of a soft phase polymer and at least one hard polymer phase comprised of a hard phase polymer, said multiphase emulsion polymer prepared by a multi-stage emulsion polymerization,

wherein the hard phase polymer has a glass transition temperature of from 10° C. to 100° C. which is more than 50° C. higher than that of the soft phase polymer,

wherein said multi-stage emulsion polymerization comprises the steps of forming the at least one hard polymer phase as a first stage polymer, and subsequently polymerizing the at least one soft polymer phase in the presence of the first stage polymer, and

wherein the hard phase polymer comprises: (i) copolymer of at least one C1-C4 alkyl (meth)acrylate, methacrylic acid and at least one wet adhesion monomer, or (ii) copolymer of butyl acrylate, methyl methacrylate, methacrylic acid and at least one of hydroxyethyl ethylene urea methacrylate and diacetone acrylamide,

wherein the soft polymer phase represents from 65% to 75% by weight of the multiphase emulsion polymer and the hard polymer phase represents from 25% to 35% by weight of the multiphase emulsion polymer, the total weight amount of soft polymer phase and hard polymer phase equaling 100%,

wherein the soft phase polymer is a homopolymer or copolymer formed from one or more monomers selected from the group consisting of methyl (meth)acrylate, ethyl(meth)acrylate, butyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, and diacetone acrylamide,

wherein the multiphase emulsion polymer is in the form of latex particles, and

wherein the latex particles comprise an outer phase comprising the hard polymer phase and an inner phase comprising the soft polymer phase and have an average particle size less than 150 nm, and

the polymer latex binder has a minimum film forming temperature of less than 15° C.

2. The latex polymer binder of claim 1 , wherein the copolymer of the hard phase is comprised of 1 to 12% by weight of the methacrylic acid.

3. The latex polymer binder of claim 1 , wherein the hard phase polymer is a copolymer of at least one C1-C4 alkyl (meth)acrylate, methacrylic acid and at least one wet adhesion monomer.

4. The latex polymer binder of claim 1 , wherein either the hard phase polymer or the soft phase polymer or both further comprises a multifunctional crosslinking unsaturated monomer.

5. The latex polymer binder of claim 1 , wherein the hard polymer phase comprises a copolymer of butyl acrylate, methyl methacrylate, methacrylic acid and at least one of hydroxyethyl ethylene urea methacrylate and diacetone acrylamide.

6. The latex polymer binder of claim 1 , wherein the soft polymer phase comprises a copolymer of butyl acrylate and methyl methacrylate.

7. A latex coating composition, comprising the polymer latex binder of claim 1 and at least one pigment.

8. A method of making a polymer latex binder having a VOC level of less than about 50 g/L, comprising:

(a) copolymerizing an aqueous emulsion of (i) at least one C1-C4 alkyl (meth)acrylate, methacrylic acid and at least one wet adhesion monomer, or (ii) butyl acrylate, methyl methacrylate, methacrylic acid and at least one of hydroxyethyl ethylene urea methacrylate and diacetone acrylamide,

to form a hard phase polymer having a glass transition temperature of from 10° C. to 100° C.,

(b) then polymerizing one or more additional monomers in the presence of the hard phase polymer to form a soft phase polymer having a glass transition temperature of from 0 to −70° C.;

wherein the glass transition temperature of the hard phase polymer is more than 50° C. higher than that of the at least one soft phase polymer,

wherein the soft phase polymer is a homopolymer or copolymer formed from one or more monomers selected from the group consisting of methyl (meth)acrylate, ethyl acrylate, butyl(meth)acrylate, 2-ethylhexylacrylate, and diacetone acrylamide,

wherein the soft phase polymer represents from 65% to 75% by weight of the multiphase emulsion polymer and the hard phase polymer represents from 25% to 35% by weight of the multiphase emulsion polymer, the total weight amount of soft phase and hard phase polymer equaling 100%,

wherein the polymer latex binder is in the form of latex particles, and

wherein the latex particles comprise an outer phase comprising the hard polymer phase and an inner phase comprising the soft polymer phase and have an average particle size less than 150 nm.

9. The method of claim 8 , wherein the copolymer of the hard phase polymer is comprised of 1 to 12% by weight of the methacrylic acid.

10. The method of claim 8 , wherein the hard phase polymer is a copolymer of butyl acrylate, methyl methacrylate, methacrylic acid and at least one of hydroxyethyl ethylene urea methacrylate and diacetone acrylamide.

11. The method of claim 8 , wherein either the hard phase polymer or the soft phase polymer or both further comprise a multifunctional crosslinking unsaturated monomer.

12. The method of claim 8 , wherein the soft phase polymer is a copolymer of butyl acrylate and methyl methacrylate.