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1. A method of reinforcing rovings or ribbons of natural fibers comprising impregnating the rovings or ribbons of natural fibers with an aqueous polymer dispersion of polymer particles comprising at least one polymer selected from the group consisting of an amorphous polymer having a Tg ranging from 50° C. to 175° C. and a semi-crystalline polymer having a melting point ranging from 70° C. to 220° C., wherein said polymer: is incorporated into a core of a bundle of fibers making up said rovings or ribbons of natural fibers and thus binding said fibers to...
1. A method of reinforcing rovings or ribbons of natural fibers comprising impregnating the rovings or ribbons of natural fibers with an aqueous polymer dispersion of polymer particles comprising at least one polymer selected from the group consisting of an amorphous polymer having a Tg ranging from 50° C. to 175° C. and a semi-crystalline polymer having a melting point ranging from 70° C. to 220° C., wherein said polymer: is incorporated into a core of a bundle of fibers making up said rovings or ribbons of natural fibers and thus binding said fibers to one another,is present in said aqueous polymer dispersion in a content by weight ranging from 5% to 50% with the polymer particles in dispersion having a number-average size of less than 10000 nm, andis selected from the group consisting of copolyamides, polyesters, polyurethanes, poly(meth)acrylates including copolymers, polyolefins, and fluorinated polymers including fluorinated copolymers. 2. The method of claim 1, wherein said polymer is selected from the group consisting of poly(meth)acrylates including copolymers functionalized with acid functions and fluorinated polymers including fluorinated copolymers grafted with reactive functions, and wherein said aqueous polymer dispersion is an aqueous polymer dispersion obtained by emulsion polymerization in the presence of a surfactant and wherein said reactive functions can react with said natural fibers. 3. The method of claim 1, wherein said polymer is a polyurethane formed from an ionic polyisocyanate prepolymer dispersed in water with a chain extension in an aqueous medium. 4. The method of claim 1, wherein said polymer is dispersible or dispersed in powder form in an aqueous medium without surfactant and said polymer in the powder state bears ionic groups or groups that are precursors of ionic groups. 5. The method of claim 4, wherein said polymer is a copolyamide. 6. The method of claim 5, wherein said copolyamide bears amine groups. 7. The method of claim 5, wherein said copolyamide bears carboxy groups which are neutralized in salt form by a base. 8. The method of claim 5, wherein said copolyamide is semi-crystalline and has a melting point ranging from 70° C. to 150° C. 9. The method of claim 5, wherein said copolyamide comprises at least one repeating unit selected from the group consisting of: 5.9, 5.10, 5.12, 5.13, 5.14, 5.16, 5.18, 5.36, 6, 6.9, 6.10, 6.12, 6.13, 6.14, 6.16, 6.18, 6.36, 9, 10.6, 10.9, 10.10, 10.12, 10.13, 10.14, 10.16, 10.18, 10.36, 11, 12, 12.6, 12.9, 12.10, 12.12, 12.13, 12.14, 12.16, 12.18, 12.36, 6.6/6, 11/10.10 and mixtures thereof. 10. The method of claim 5, wherein said copolyamide is selected from the group consisting of: PA 6/6.6/12, PA 6/6.6/11/12, PA 6/12, PA 6.9/12, PA Pip.9/Pip.12/11, PA 6/IPD.6/12, PA IPD.9/12, PA6/MPMD.12/12, PA 6/6.12/12, PA 6/6.10/12, PA 6/Pip.12/12, PA 6/6.6/6.10/6.1, PA 6.10/Pip.10/Pip.12, PA 6/11/12, PA Pip.12/12, PA IPD.10/12, PA Pip.10/12, PA 6/11, PA Pip.10/11/Pip.9, PA 6/6.6/6.10, PA 6/6.10/6.12 and mixtures thereof. 11. The method of claim 5, wherein said copolyamide is semi-aromatic amorphous and is selected from the group consisting of: (i) 6.1, 8.1, 9.1, 10.1, 11.1, 12.1, 6.1/9.1, 9.1/10.1, 9.1/11.1, 9.1/12.1, 9/6.1, 10/6.1, 11/6.1, 12:6.1, 10/9.1, 10/10.1, 10/11.1, 10/12.1, 11/9.1, 11/10.1, 11/11.1, 11/12.1, 12/9.1, 12/10.1, 12/11.1, 12/12.1, 6.10/6.1, 6.12/6.1, 9.10/6.1, 9.12/6.1, 10.10/6.1, 10.12/6.1, 6.10/9.1, 6.12/9.1, 101/61, 10.10/9.1, 10.12/9.1, 6.10/10.1, 6.12/10.1, 9.10/10.1, 9.12/10.1, 10.10/10.1, 10.12/10.1, 6.10/12.1, 6.12/12.1, 9.10/12.1, 9.12/12.1, 10.10/12.1, 11/6.1/9.1, 11/6.1/10.1, 11/6.1/11.1, 11/6.1/12.1, 11/9.1/10.1, 11/9.1/11.1, 11/9.1/12.1, 11/10.1/11.1, 11/10.1/12.1, 11/11.1/12.1, 6.1/10.1, 6.1/11.1, 6.1/12.1, 10.1/11.1, 10.1/12.1, 11.1/12.1, 12/6.1/10.1, 12/6.1/11.1, 12/6.1/12.1, 12/9.1/10.1, 12/9.1/11.1, 12.9.1/12.1, 12/10.1/11.1, 12/10.112.1, 12/11.1/12.1, 12/11.1/12.1, (ii) terpolymer polyamides from (i) with 12/ replaced with 9/, 10/, 6.10/, 6.12/, 10.6/, 10.10/, 10.12/, 9.10/ and 9.12/, (iii) polyamides (i) or (ii), with isophthalic (1) partially replaced up to 40 mol % with terephthalic (T), naphthalene-2,6-dicarboxylic and/or with 1,3- or 1,4-CHDA (cyclohexanedicarboxylic acid), optionally with the linear aliphatic diamines being replaced with branched aliphatic diamines, and (iv) polyamides from (i) or (ii) where isophthalic (1) is partially or totally replaced with a linear or branched C6 to C18 aliphatic diacid and at the same time with total or partial replacement of the aliphatic diamine with bis(3-methylaminocyclohexyl)methane, bis(aminocyclohexyl)-methane and/or isophorone diamine. 12. The method of claim 1, wherein said polymer is semi-crystalline with a melting point ranging from greater than 90° C. up to 220° C., and wherein the polymer particles of said aqueous polymer dispersion have a number-average size ranging from 50 to 5000 nm. 13. The method of claim 1, wherein the content by dry weight of said polymer relative to the dry weight of said fibers and polymer ranges from 0.5% to less than 50%. 14. The method of claim 13, wherein said content by weight ranges from 0.5% to 10%. 15. The method of claim 13, wherein said content is greater than 25% and less than 50%. 16. The method of claim 1, wherein the viscosity of said aqueous polymer dispersion at 23° C. ranges from 10 to 1000 mPa·s. 17. The method of claim 1, wherein said polymer is also biobased. 18. The method of claim 1, wherein said fibers are long flax fibers with L/D>2000. 19. The method of claim 1, wherein said rovings or ribbons are based on flax fibers having a tex ranging from 10 to 10,000. 20. The method of claim 1, wherein said rovings or ribbons of fibers are reinforcing fibers.
