Microspheres

1. Thermally expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers consisting essentially of from 20 to 70 wt % of acrylonitrile, from 20 to 80 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, from 0 to 10 wt % of methacrylonitrile, from 0 to 25 wt % of monomers selected from the group consisting of esters of methacrylic acid, and optionally a small amount of one or more crosslinking multifunctional monomers, the total amount of acrylonitrile and acrylate monomers constituting from 50 to 100 wt % of said ethylenically unsaturated monomers, and said propellant comprising isobutane.

2. Microspheres as claimed in claim 1 , wherein said ethylenically unsaturated monomers comprise from 30 to 80 wt % of said acrylate monomers.

3. Microspheres as claimed in claim 1 , wherein said ethylenically unsaturated monomers comprising from 30 to 65 wt % acrylonitrile and from 35 to 70 wt % of said acrylate monomers.

4. Microspheres as claimed in claim 2 , wherein the amount of acrylonitrile and acrylate monomers constitutes from 65 to 100 wt % of said ethylenically unsaturated monomers.

5. Microspheres as claimed in claim 1 , wherein said ethylenically unsaturated monomers comprises from 20 to 80 wt % of methyl acrylate.

6. Microspheres as claimed in claim 4 , wherein said ethylenically unsaturated monomers comprises one or more crosslinking multifunctional monomers in an amount from 0.1 to 3 wt % of the ethylenically unsaturated monomers.

7. Microspheres as claimed in claim 6 , wherein said ethylenically unsaturated monomers comprises one or more crosslinking monomers that are at least tri-functional.

8. Microspheres as claimed in claim 7 , wherein said ethylenically unsaturated monomers comprises one or more crosslinking multifunctional monomers in an amount from 0.1 to 1 wt % of the ethylenically unsaturated monomers.

9. Microspheres as claimed in claim 1 , wherein said ethylenically unsaturated monomers are substantially free from halogen containing monomers.

10. Microspheres as claimed in claim 6 , wherein said propellant comprises from 50 to 100 wt % of isobutane.

11. Microspheres as claimed in claim 10 , wherein said ethylenically unsaturated monomers comprise from 0 to 10 wt % of monomers selected from the group consisting of esters of methacrylic acid.

12. Thermally expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers consisting essentially of from 20 to 70 wt % of acrylonitrile, from 20 to 80 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, from 0 to 2 wt % of methacrylonitrile, from 0 to 25 wt % of monomers selected from the group consisting of esters of methacrylic acid, and from 0 to 1 wt % of one or more crosslinking multifunctional monomers, the total amount of acrylonitrile and acrylate monomers constituting from 65 to 100 wt % of said ethylenically unsaturated monomers, and said propellant comprising from 75 to 100 wt % of isobutane.

13. Microspheres as claimed in claim 12 , wherein said acrylate monomer is methyl acrylate and said esters of methacrylic acid is methyl methacrylate, wherein the amount of methyl methacrylate is in the range from 0 to 10 wt % of the ethylenically unsaturated monomers.

14. Process for the production of thermally expandable microspheres comprising polymerising ethylenically unsaturated monomers in the presence of a propellant to yield microspheres comprising a polymer shell encapsulating said propellant, said ethylenically unsaturated monomers consisting essentially of from 20 to 70 wt % of acrylonitrile, from 20 to 80 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, from 0 to 10 wt % of methacrylonitrile, from 0 to 25 wt % of monomers selected from the group consisting of esters of methacrylic acid, and optionally a small amount of one or more crosslinking multifunctional monomers, the total amount of acrylonitrile and acrylate monomers constituting from 50 to 100 wt % of said ethylenically unsaturated monomers, and said propellant comprising isobutane.

15. Aqueous slurry comprising thermally expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers consisting essentially of from 20 to 70 wt % of acrylonitrile, from 20 to 80 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, from 0 to 10 wt % of methacrylonitrile, from 0 to 25 wt % of monomers selected from the group consisting of esters of methacrylic acid, and optionally a small amount of one or more crosslinking multifunctional monomers, the total amount of acrylonitrile and acrylate monomers constituting from 50 to 100 wt % of said ethylenically unsaturated monomers, and said propellant comprising isobutane.

16. Aqueous slurry as claimed in claim 15 further comprising at least one thickener being an at least partially water soluble polymer selected from the group consisting of starch, gums, celluloses, chitins, chitosans, glycans, galactans, pectins, mannans, dextrins, co-polymers made from monomers comprising acrylic acid or salts thereof, homo- and co-polymers made from monomers comprising esters or amides of acrylic acid, homo and co-polymers made from monomers comprising methacrylic acid, esters or amides thereof, rubber latexes, poly(vinyl chloride) and copolymers, poly(vinyl esters) and co-polymers, poly(vinyl alcohol), polyamines, polyethyleneimine, polyethylene/polypropylene oxides, polyurethane, aminoplast precondensates, phenoplast precondensates and polyamidoamine epichlorohydrin resins.

17. Expanded microspheres obtained by expanding thermally expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers consisting essentially of from 20 to 70 wt % of acrylonitrile, from 20 to 80 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, from 0 to 10 wt % of methacrylonitrile, from 0 to 25 wt % of monomers selected from the group consisting of esters of methacrylic acid, and optionally a small amount of one or more crosslinking multifunctional monomers, the total amount of acrylonitrile and acrylate monomers constituting from 50 to 100 wt % of said ethylenically unsaturated monomers, and said propellant comprising isobutane.

18. Printing ink comprising thermally expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers consisting essentially of from 20 to 70 wt % of acrylonitrile, from 20 to 80 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, from 0 to 10 wt % of methacrylonitrile, from 0 to 25 wt % of monomers selected from the group consisting of esters of methacrylic acid, and optionally a small amount of one or more crosslinking multifunctional monomers, the total amount of acrylonitrile and acrylate monomers constituting from 50 to 100 wt % of said ethylenically unsaturated monomers, and said propellant comprising isobutane.

19. Process for the production of paper comprising the steps of adding thermally expandable thermoplastic microspheres to a stock containing cellulosic fibres, dewatering the stock on a wire to obtain paper, and drying the paper by applying heat and thereby also raising the temperature of the microspheres sufficiently for them to expand and increase the bulk of the paper, said expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers consisting essentially of from 20 to 70 wt % of acrylonitrile, from 20 to 80 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, from 0 to 10 wt % of methacrylonitrile, from 0 to 25 wt % of monomers selected from the group consisting of esters of methacrylic acid, and optionally a small amount of one or more crosslinking multifunctional monomers, the total amount of acrylonitrile and acrylate monomers constituting from 50 to 100 wt % of said ethylenically unsaturated monomers, and said propellant comprising isobutane.

20. Process as claimed in claim 19 , wherein the thermally expandable microspheres are added in the form of an aqueous slurry comprising said thermally expandable thermoplastic microspheres.