A process for manufacturing a solid composite layer, said process comprising the following steps: (i) providing a mixture comprising: - at least one functional fluoropolymer [polymer] comprising recurring units derived from at least one fluorinated monomer and at least one (meth)acrylic monomer of formula: wherein each of R 1, R 2 and R 3, equal to or different from each other, is independently a hydrogen atom or a C 1 -C 3 hydrocarbon group, and R OH is a C 1 -C 5 hydrocarbon moiety comprising at least one hydroxyl group, - at least one metal compound of formula [compound]: X 4-m AY m wherein X is a hydrocarbon group, optionally comprising one or more functional groups, m is an integer from 1 to 4, A is a metal selected from the group consisting of Si, Ti and Zr, and Y is a hydrolysable group selected from the group consisting of an alkoxy group, an acyloxy group and a hydroxyl group, and - at least one inorganic filler material [filler] having on its surface reactive groups towards compound, in an amount of from 50% to 95% by weight, with respect to the total weight of said polymer and said filler; (ii) reacting at least a fraction of said hydroxyl group(s) of at least one polymer with at least a fraction of said hydrolysable group(s) Y of at least one compound, so as to provide a fluoropolymer composition comprising at least one grafted fluoropolymer [polymer (Fg)] comprising pendant groups of formula -Y m-1 -AX 4-m, wherein m, Y, A and X have the same meaning as defined above; and (iii) processing the fluoropolymer composition obtained in step (ii), so as to obtain a solid composite layer. (i) providing a mixture comprising: - at least one functional fluoropolymer [polymer] comprising recurring units derived from at least one fluorinated monomer and at least one (meth)acrylic monomer of formula: wherein each of R 1, R 2 and R 3, equal to or different from each other, is independently a hydrogen atom or a C 1 -C 3 hydrocarbon group, and R OH is a C 1 -C 5 hydrocarbon moiety comprising at least one hydroxyl group, - at least one metal compound of formula [compound]: X 4-m AY m wherein X is a hydrocarbon group, optionally comprising one or more functional groups, m is an integer from 1 to 4, A is a metal selected from the group consisting of Si, Ti and Zr, and Y is a hydrolysable group selected from the group consisting of an alkoxy group, an acyloxy group and a hydroxyl group, and - at least one inorganic filler material [filler] having on its surface reactive groups towards compound, in an amount of from 50% to 95% by weight, with respect to the total weight of said polymer and said filler; - at least one functional fluoropolymer [polymer] comprising recurring units derived from at least one fluorinated monomer and at least one (meth)acrylic monomer of formula: wherein each of R 1, R 2 and R 3, equal to or different from each other, is independently a hydrogen atom or a C 1 -C 3 hydrocarbon group, and R OH is a C 1 -C 5 hydrocarbon moiety comprising at least one hydroxyl group, - at least one metal compound of formula [compound]: X 4-m AY m wherein X is a hydrocarbon group, optionally comprising one or more functional groups, m is an integer from 1 to 4, A is a metal selected from the group consisting of Si, Ti and Zr, and Y is a hydrolysable group selected from the group consisting of an alkoxy group, an acyloxy group and a hydroxyl group, and - at least one inorganic filler material [filler] having on its surface reactive groups towards compound, in an amount of from 50% to 95% by weight, with respect to the total weight of said polymer and said filler; (ii) reacting at least a fraction of said hydroxyl group(s) of at least one polymer with at least a fraction of said hydrolysable group(s) Y of at least one compound, so as to provide a fluoropolymer composition comprising at least one grafted fluoropolymer [polymer (Fg)] comprising pendant groups of formula -Y m-1 -AX 4-m, wherein m, Y, A and X have the same meaning as defined above; and (iii) processing the fluoropolymer composition obtained in step (ii), so as to obtain a solid composite layer.