A polymeric flake obtained by polymerisation, in a suspension or dispersion in an aqueous solvent, of a mixture comprising A) at least one lipophilic monomer and B) at least one monomer selected from: a halogenated lipophilic monomer, an ionisable acid monomer and an ionisable basic monomer: (i) with at least one neutral hydrophilic monomer or at least one hydrophilic monomer having a fixed charge, or (ii) in the presence of at least one polymeric stabiliser; wherein: said at least one lipophilic monomer is selected in the group consisting of methyl methacrylate, 2-hydroxyethyl methacrylate, butyl acrylate, butyl methacrylate, hexyl acrylate, hexyl methacrylate, lauryl acrylate, lauryl methacrylate, vinyl acetate, acrylonitrile and styrene; said at least one lipophilic halogenated monomer is selected in the group consisting of 2,2,2-trifluoroethyl methacrylate (TFEMA), 1,1,1,3,3,3-hexafluoroisopropyl methacrylate (HFIPMA), heptadecafluorodecyl methacrylate (HDFDMA) and pentadecafluorononyl methacrylate (PDFNMA), vinyl chloride monomer and 1,1-dichloroethene; said at least one neutral hydrophilic monomer or said at least one hydrophilic monomer having a fixed charge is selected in the group consisting of 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPS), potassium 3-sulphopropyl methacrylate, vinylsulphonic acid, vinylphosphonic acid, (3-acrylamidopropyl)trimethylammonium chloride, (3-methacryloylamino)propyl-trimethylammonium chloride, (vinylbenzyl)trimethylammonium chloride, 2-(methacryloyloxy)ethyl-trimethylammonium chloride (MADQUAT), 2-(acryloyloxy)ethyl-trimethylammonium chloride, N,N-dimethylacrylamide, polyethylene glycol derivatives, 2-hydroxyethyl acrylate, styrenesulphonic acid, N-2-hydroxypropyl methacrylamide, acrylamide and 2-aminoethyl methacrylate; said at least one ionisable acid monomer is selected in the group consisting of methacrylic acid, acrylic acid, mono-2-(methacryloyloxy)ethyl succinate, mono-2-(acryloyloxy)ethyl succinate and itaconic acid; said at least one ionisable basic monomer is selected in the group consisting of 2-(dimethylamino)ethyl methacrylate (DMAEMA), N,N-dimethylacrylamide, 2-aminoethyl methacrylate and 2-aminoethyl acrylate; said polymeric stabiliser is selected from polyvinyl alcohol, polyvinyl pyrrolidone, poly(methacrylic acid) and polyethylene glycol; and wherein said polymerisation, according to point (i) or (ii), takes place in the presence of at least one cross-linking agent and at least one polymerisation initiator and is performed by using said at least one monomer selected from: a halogenated lipophilic monomer, an ionisable acid monomer and an ionisable basic monomer in an amount comprised between 30 and 90% by weight, preferably between 40 and 80% by weight relative to the total weight of the polymeric flake. (i) with at least one neutral hydrophilic monomer or at least one hydrophilic monomer having a fixed charge, or (ii) in the presence of at least one polymeric stabiliser; said at least one lipophilic monomer is selected in the group consisting of methyl methacrylate, 2-hydroxyethyl methacrylate, butyl acrylate, butyl methacrylate, hexyl acrylate, hexyl methacrylate, lauryl acrylate, lauryl methacrylate, vinyl acetate, acrylonitrile and styrene; said at least one lipophilic halogenated monomer is selected in the group consisting of 2,2,2-trifluoroethyl methacrylate (TFEMA), 1,1,1,3,3,3-hexafluoroisopropyl methacrylate (HFIPMA), heptadecafluorodecyl methacrylate (HDFDMA) and pentadecafluorononyl methacrylate (PDFNMA), vinyl chloride monomer and 1,1-dichloroethene; said at least one neutral hydrophilic monomer or said at least one hydrophilic monomer having a fixed charge is selected in the group consisting of 2-acrylamido-2-methyl-1-propanesulphonic acid (AMPS), potassium 3-sulphopropyl methacrylate, vinylsulphonic acid, vinylphosphonic acid, (3-acrylamidopropyl)trimethylammonium chloride, (3-methacryloylamino)propyl-trimethylammonium chloride, (vinylbenzyl)trimethylammonium chloride, 2-(methacryloyloxy)ethyl-trimethylammonium chloride (MADQUAT), 2-(acryloyloxy)ethyl-trimethylammonium chloride, N,N-dimethylacrylamide, polyethylene glycol derivatives, 2-hydroxyethyl acrylate, styrenesulphonic acid, N-2-hydroxypropyl methacrylamide, acrylamide and 2-aminoethyl methacrylate; said at least one ionisable acid monomer is selected in the group consisting of methacrylic acid, acrylic acid, mono-2-(methacryloyloxy)ethyl succinate, mono-2-(acryloyloxy)ethyl succinate and itaconic acid; said at least one ionisable basic monomer is selected in the group consisting of 2-(dimethylamino)ethyl methacrylate (DMAEMA), N,N-dimethylacrylamide, 2-aminoethyl methacrylate and 2-aminoethyl acrylate; said polymeric stabiliser is selected from polyvinyl alcohol, polyvinyl pyrrolidone, poly(methacrylic acid) and polyethylene glycol;