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 40 to 70 wt % of acrylonitrile, from 5 to 40 wt % of methacrylonitrile, from above 20 to 50 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, and from 0.1 to 10 wt % of one or more crosslinking multifunctional monomers, and said propellant comprising from 50 to 100 wt % of isobutane.

2. Microspheres as claimed in claim 1 , wherein said ethylenically unsaturated monomers comprises from 45 to 65 wt % acrylonitrile.

3. Microspheres as claimed in claim 1 , wherein said ethylenically unsaturated monomers comprises from 10 to 35 wt % of methacrylonitrile.

4. Microspheres as claimed in claim 1 , wherein said propellant comprises from 75 to 100 wt % isobutane.

5. Microspheres as claimed in claim 4 , wherein said propellant consists essentially of isobutane.

6. Microspheres as claimed in claim 5 , wherein said propellant consists of isobutane.

7. Microspheres as claimed in claim 1 , wherein said ethylenically unsaturated monomers comprises from above 20 to 50 wt % of methyl acrylate.

8. Microspheres as claimed in claim 1 , wherein said ethylenically unsaturated monomers are substantially free from or comprise less than 10 wt % of halogen containing monomers.

9. Microspheres as claimed in claim 1 , wherein T start is from 50 to 110° C.

10. Thermally expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers consisting essentially of from 45 to 65 wt % of acrylonitrile, from 10 to 35 wt % of methacrylonitrile, from above 20 to 50 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, and from 0.1 to 10 wt % of one or more crosslinking multifunctional monomers, and said propellant comprising from 50 to 100 wt % isobutane.

11. 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 40 to 70 wt % acrylonitrile, from 5 to 40 wt % methacrylonitrile, from above 20 to 50 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, and from 0.1 to 10 wt % of one or more crosslinking multifunctional monomers, and said propellant comprising from 50 to 100 wt % of isobutane.

12. 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 40 to 70 wt % of acrylonitrile, from 5 to 40 wt % of methacrylonitrile, from above 20 to 50 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, and from 0.1 to 10 wt % of one or more crosslinking multifunctional monomers, and said propellant comprising from 50 to 100 wt % of isobutane.

13. Aqueous slurry as claimed in claim 12 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, polyetyleneimine, polyethylene/polypropylene oxides, polyurethane, aminoplast, phenoplast precondensates and polyamidoamine epichlorohydrin resins.

14. 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 40 to 70 wt % of acrylonitrile, from 5 to 40 wt % of methacrylonitrile, from above 20 to 50 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, and from 0.1 to 10 wt % of one or more crosslinking multifunctional monomers, and said propellant comprising from 50 to 100 wt % of isobutane.

15. Printing ink comprising thermally expandable thermoplastic microspheres comprising a polymer she made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers consisting essentially of from 40 to 70 wt % of acrylonitrile,

from 5 to 40 wt % of methacrylonitrile, from above 20 to 50 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, and from 0.1 to 10 wt % of one or more crosslinking multifunctional monomers, and said propellant comprising from 50 to 100 wt % of isobutane.

16. 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 40 to 70 wt % of acrylonitrile, from 5 to 40 wt % of methacrylonitrile, from above 20 to 50 wt % of acrylate monomers selected from the group consisting of methyl acrylate, ethyl acrylate and mixtures thereof, and from 0.1 to 10 wt % of one or more crosslinking multifunctional monomers, and said propellant comprising from 50 to 100 wt % of isobutane.

17. Process as claimed in claim 16 , wherein the thermally expandable microspheres are added in the form of an aqueous slurry comprising said thermally expandable microspheres.