Aqueous polymer dispersions modified with solvent-softened nanoparticles

1. An aqueous polymer dispersion comprising a mixture of

(a) a primary aqueous dispersion containing primary polymer particles formed from a polymer having a lower glass transition temperature and

(b) solvent softened polymer nanoparticles formed from a polymer having a higher glass transition temperature, the higher glass transition temperature being at least 10° C. greater than the lower glass transition temperature, the polymer nanoparticles being softened with a softening solvent,

wherein the aqueous dispersion is made by a process in which the solvent softened polymer nanoparticles are softened with the softening solvent before being combined with the primary aqueous dispersion so that the amount of softening solvent present in the aqueous polymer dispersion is substantially less than would be present in an otherwise identical aqueous polymer dispersion in which the solvent softened polymer nanoparticles were solvent softened after combining with the primary aqueous dispersion.

2. The aqueous polymer dispersion of claim 1 , wherein the solvent softened polymer nanoparticles are made by combining the polymer nanoparticles with a softening solvent, and further wherein the softening solvent is removed from the softened polymer nanoparticles before they are combined with the aqueous primary dispersion.

3. The aqueous polymer dispersion of claim 2 , wherein the solvent softened polymer nanoparticles are made by adding the softening solvent to an aqueous nanoparticle dispersion containing the polymer nanoparticles, and further wherein the liquid phase of the nanoparticle dispersion is separated from the solvent softened nanoparticles before the solvent softened nanoparticles are combined with the aqueous primary dispersion.

4. The aqueous polymer dispersion of claim 1 , wherein the solvent softened polymer nanoparticles are made by adding the softening solvent to an aqueous nanoparticle dispersion containing the polymer nanoparticles, and further wherein the mixture of softening solvent and aqueous nanoparticle dispersion so made is combined with the primary aqueous dispersion thereby forming the aqueous polymer dispersion.

5. The aqueous polymer dispersion of claim 4 , wherein the total amount of softening solvent in aqueous polymer dispersion is 5 wt. % or less based on the weight of aqueous polymer dispersion.

6. The aqueous polymer dispersion of claim 5 , wherein the polymer nanoparticles are solvent softened by adding no more than 10 wt. % softening solvent to an aqueous nanoparticle dispersion containing the polymer nanoparticles, the percent being based on the weight of the aqueous nanoparticle dispersion.

7. The aqueous polymer dispersion of claim 6 , wherein the polymer nanoparticles are solvent softened by adding no more than 5 wt. % softening solvent to an aqueous nanoparticle dispersion containing the polymer nanoparticles, the percent being based on the weight of the aqueous nanoparticle dispersion, and further wherein the amount of aqueous nanoparticle dispersion combined with the primary particle dispersion is about 0.5 to 40 wt. %, based on the weight of the aqueous polymer dispersion.

8. The aqueous polymer dispersion of claim 7 , wherein the amount of aqueous nanoparticle dispersion combined with the primary particle dispersion is about 1.0 to 30 wt. %, based on the weight of the aqueous polymer dispersion.

9. The aqueous polymer dispersion of claim 1 , wherein the polymer nanoparticles are solvent softened by allowing a mixture of a softening solvent and an aqueous dispersion of the nanoparticles to soak for 6-20 hours.

10. The aqueous polymer dispersion of claim 1 , wherein the higher glass transition temperature is at least 40° C. greater than the lower glass transition temperature.

11. The aqueous polymer dispersion of claim 10 , wherein the primary polymer particles are made from a polymer having a glass transition temperature of 10° C. or less.

12. The aqueous polymer dispersion of claim 11 , wherein the primary polymer particles have an average particle size of about 0.1 to 2μ, and further wherein polymer nanoparticles, before softening, have an average particle size of 50 nm or less.

13. The aqueous polymer dispersion of claim 1 , wherein the aqueous polymer dispersion contains about 0.5 and 40 wt. % softened polymer nanoparticles.

14. The aqueous polymer dispersion of claim 1 , wherein polymer nanoparticles are made from an acrylic polymer.

15. The aqueous polymer dispersion of claim 1 , wherein the softened polymer nanoparticles are softened by contacting an aqueous dispersion of the polymer nanoparticles with an organic solvent prior to combining the nanoparticles with the primary polymer particles.

16. The aqueous polymer dispersion of claim 1 , wherein

the difference between the glass transition temperatures of the polymers forming the primary polymer particles and the polymer nanoparticles, as well as

the amount of polymer nanoparticles included in the aqueous polymer dispersion, as well as

the extent to which the polymer nanoparticles are solvent softened before being combined with the primary polymer particles

are selected so that a protective coating formed from the aqueous polymer dispersion exhibits at least one mechanical property selected from hardness, strength and flexibility which is significantly enhanced relative to an otherwise identical protective coating made from an otherwise identical aqueous polymer dispersion except that the polymer nanoparticles forming this otherwise identical aqueous dispersion are not solvent softened before being combined with the primary polymer particles.

17. The aqueous polymer dispersion of claim 15 , wherein the mechanical property is tensile strength.

18. A polymer protective coating formed by depositing the aqueous polymer dispersion of claim 1 on a substrate and allowing the aqueous polymer dispersion to dry.