The present invention consists of the use, in a chlorinated and filled thermoplastic composition containing at least one chlorinated thermoplastic resin and at least one mineral filler, as an agent improving the thermal stability and UV resistance of the said composition, of a linear amphiphilic copolymer characterized in that: it consists of at least one hydrophilic monomer and at least one hydrophobic monomer, it has an average molar mass by weight of between 5,000 g/mol and 20,000 g/mol and a polymolecularity index less than or equal to 3, and preferentially an average molar mass by weight of between 10,000 g/mol and 15,000 g/mol, and a polymolecularity index less than or equal to 2.5. It also concerns a manufacturing process for a chlorinated and filled thermoplastic composition implementing the said copolymer, as well as the composition obtained directly by the said process.