Reinforced fluoropolymer

A curable fluoropolymer composition comprising

• a crosslinkable fluorine-containing polymer, and
• a filler selected from surface-reacted calcium carbonate, ultrafine calcium carbonate, or a mixture thereof,
• wherein the surface-reacted calcium carbonate is a reaction product of natural ground calcium carbonate or precipitated calcium carbonate with carbon dioxide and one or more H3O+ ion donors, wherein the carbon dioxide is formed in situ by the H3O+ ion donors treatment and/or is supplied from an external source,
• wherein the filler comprises a surface-treatment layer on at least a part of the filler surface,
• wherein the surface-treatment layer is formed by contacting the filler with at least one surface-treatment agent in an amount from 0.07 to 9 mg/m2 of the filler surface, and
• wherein the at least one surface treatment agent is selected from the group consisting of mono- or di-substituted succinic anhydride containing compounds, mono- or di-substituted succinic acid containing compounds, mono- or di-substituted succinic acid salts containing compounds, saturated or unsaturated fatty acids, salts of saturated or unsaturated fatty acids;
• unsaturated esters of phosphoric acid, salts of unsaturated phosphoric acid esters, functionalized poly- and/or perfluorinated alkyl compounds, abietic acid, salts of abietic acid, and mixtures thereof and reaction products thereof.

A method of producing a cured fluoropolymer product, comprising the steps of

• i) providing a crosslinkable fluorine-containing polymer,
• ii) providing a filler selected from surface-reacted calcium carbonate and a mixture of surface-reacted calcium carbonate and ultrafine calcium carbonate,
• wherein the surface-reacted calcium carbonate is a reaction product of natural ground calcium carbonate or precipitated calcium carbonate with carbon dioxide and one or more H3O+ ion donors, wherein the carbon dioxide is formed in situ by the H3O+ ion donors treatment and/or is supplied from an external source,
• wherein the natural ground calcium carbonate is selected from the group consisting of marble, chalk, limestone, and mixtures thereof, or
• the precipitated calcium carbonate is selected from the group consisting of precipitated calcium carbonates having an aragonitic, vateritic or calcitic crystal form, and mixtures thereof,
• iii) combining the crosslinkable fluorine-containing polymer of step i) and the filler of step ii) to form a curable fluoropolymer composition, and
• iv) curing the curable fluoropolymer composition of step iii).