Method used to protect a thermoplastic polymer consisting of superposing a protective layer (I) comprising an acrylic copolymer comprising, by weight (the total making 100%): from 80 to 99.8% of methyl methacrylate (MMA), from 0 to 20% of at least one comonomer that can be copolymerized with MMA by a radicalar method, and from 0.2 to 10% of maleic anhydride or 15% of acrylic and/or methacrylic acid and possibly anhydride groups with a formula: in which R1 and R2 denote H or a methyl radical, a layer of at least one thermoplastic polymer (II), all these steps being performed in order by coextrusion, by hot compression or by multi-injection.