A process for preparing a composition comprising: a) a (meth) acrylic copolymer or a mixture of two (meth)acrylic copolymers (A1a) and (A1b) comprising at least 50wt% of polymeric units coming from methyl methacrylate, b) a filler or a mixture of two fillers and c) a halogen containing thermoplastic polymer characterized that the glass transition temperature Tg of the (meth) acrylic copolymer or the mixture of two (meth)acrylic copolymers (A1a) and (A1b) is less than 105°C and that the quantity of the filler F or the mixture of two fillers and is between 1phr and 250phr relative to the halogen containing thermoplastic polymer, wherein the filler or the mixture of two fillers and is a mineral filler chosen from calcium carbonate, titanium dioxide or calcinated clay, silica (fumed or precipitated), clay, Montmorillonite (nano-clay), zeolite, perlite, and wherein the glass transition temperature Tg of the (meth)acrylic copolymer or the mixture of two (meth)acrylic copolymers (A1a) and (A1b) is between 60°C and 105°C, said glass transition temperature Tg is measured as described in the specification, and wherein the mass average molecular weight Mw of the (meth)acrylic copolymer or the two (meth)acrylic copolymers (A1a) and (A1b) comprising at least 50wt% of polymeric units coming from methyl methacrylate is at least 300 000g/mol as measured with by size exclusion chromatography, and comprising between 0.01phr and 20phr of the (meth)acrylic copolymer or the mixture of two (meth)acrylic copolymers (A1a) and (A1b) relative to the halogen containing thermoplastic polymer, said process comprises either the step of blending a compositions P1 with a halogen containing polymer and a filler or, characterized in that the composition P1 comprises an (meth)acrylic copolymer or a mixture of two (meth)acrylic copolymers (A1a) and (A1b) and a filler or; or said process comprises the step of blending two compositions P1 and P2 characterized that the composition P1 comprises an (meth)acrylic copolymer or mixture of two (meth)acrylic copolymers (A1a) and (A1b) and a filler F or F1 and the composition P2 comprises a halogen containing polymer and a filler F or F2; wherein the glass transition temperature Tg of the (meth)acrylic copolymer is between 60°C and 105°C, said glass transition temperature Tg is measured as described in the specification and that the quantity of the filler or and together is between 1phr and 250phr in view of the halogen containing thermoplastic polymer a) a (meth) acrylic copolymer or a mixture of two (meth)acrylic copolymers (A1a) and (A1b) comprising at least 50wt% of polymeric units coming from methyl methacrylate, b) a filler or a mixture of two fillers and c) a halogen containing thermoplastic polymer blending a compositions P1 with a halogen containing polymer and a filler or, characterized in that the composition P1 comprises an (meth)acrylic copolymer or a mixture of two (meth)acrylic copolymers (A1a) and (A1b) and a filler or; blending two compositions P1 and P2 characterized that the composition P1 comprises an (meth)acrylic copolymer or mixture of two (meth)acrylic copolymers (A1a) and (A1b) and a filler F or F1 and the composition P2 comprises a halogen containing polymer and a filler F or F2;