Controlled release granule with water resistant coating

1. A controlled release composition comprising an active water-soluble material in the form of granules having a protective barrier coating capable of releasing the active water-soluble material in an aqueous environment, wherein release of the active water-soluble material after 1 hour at 70° C. is less than 2.2% by weight, said protective barrier coating comprising a polymer blend of two latex polymers, said blend comprising 60 to 80 wt % high glass-transition temperature (Tg) latex polymer and 20 to 40 wt % low Tg latex polymer, wherein the high Tg latex polymer has a Tg at least 20° C. higher than the Tg of the low Tg latex polymer, whereby a Tg is determined by heating from a temperature of −40° C. at a rate of 3° C./min with a superimposed sine oscillating temperature of +/−1° C. with a period of 1 min.

2. The composition according to claim 1 , wherein the high Tg latex polymer has a Tg at least 40° C. higher than the Tg of the low Tg latex polymer.

3. The composition according to claim 1 , wherein the high Tg latex polymer is a styrene-acrylate copolymer.

4. The composition according to claim 3 , wherein the high Tg styrene-acrylate polymer is a polymerized combination of monomers comprising styrene and ethyl acrylate monomers having a styrene content in the range of 52 to 63 wt % and ethyl acrylate content in the range of 9 to 13 wt %, and from 10 to 15 wt % of at least one cross-linker monomer, up to a total of 100 wt %.

5. The composition according to claim 4 , wherein the at least one cross-linker monomer is a combination of glycidyl methacrylate and methacylic acid.

6. The composition according to claim 3 , wherein the high Tg styrene-acrylate polymer has a Tg of 80° C. or higher.

7. The composition according to claim 1 , wherein the low Tg styrene-acrylate polymer is a polymerized combination of monomers comprising styrene and ethyl acrylate monomers having a styrene content in the range of 9 to 13 wt % and ethyl acrylate content in the range of 70 to 76 wt %, and from 10 to 15 wt % of at least one cross-linker monomer, up to a total of 100 wt %.

8. The composition according to claim 7 , wherein the low Tg styrene-acrylate polymer has a Tg of 40° C. or lower.

9. A method for breaking an aqueous fracturing fluid, said method comprising introducing into a subterranean formation a controlled release breaker composition comprising a breaker for the aqueous fracturing fluid in the form of granules with a barrier coating according to claim 1 , and wherein the release conditions are the exposure to the fracturing fluid that results in osmotic rupturing of the barrier coating, or subjecting said controlled release breaker composition to high shear or crush forces sufficient to rupture the barrier coating, wherein the high Tg latex polymer preferably has a Tg at least 40° C. higher than the Tg of the low Tg latex polymer.

10. A method according to claim 9 , wherein the granules of the breaker are oxidizer breakers which are released in an amount of less than 10 wt % of said breaker after exposure to water for 1 hour at 70° C., when the granules have a 22 wt % coating, based on the total weight of the granule.

11. A sprayable coating formulation comprising water and a polymer blend of two latex polymers, said blend comprising 60 to 80 wt % high glass-transition temperature (Tgl latex polymer and 20 to 40 wt % low Tg latex polymer, wherein the high Tg latex polymer has a Tg at least 40° C. higher than the Tg of the low Tg latex polymer, wherein both latex polymers have a gel content of 50% or greater, wherein the sprayable formulation has a polymer solids content in the range of 10 to 40 wt %, and wherein the sprayable formulation further contains an organic solvent having a boiling point in the range from 110° C. to 225° C. and the weight ratio of solvent to polymer solids is in the range from 0.5:1 to 0.1:1.

12. A method of coating a water-soluble material in the form of granules with a sprayable formulation according to claim 11 in a particle coating apparatus, said method comprising: (1) introducing the water-soluble material in the form of granules into the particle coating apparatus; (2) introducing the sprayable formulation into the particle coating apparatus and maintaining conditions sufficient for a barrier coating to build up on the water-soluble material and form coated particles, and to remove water from the apparatus until the coated particles contain a predetermined amount of barrier coating and the barrier coating is substantially free of water; and (3) subjecting the coated particles to conditions sufficient to anneal the barrier coating and substantially remove the organic solvent from the barrier coating.

13. A method according to claim 12 , wherein step (2) is carried out at an initial temperature below 50° C. and step (3) is carried out at a temperature at or above 100° C.