A reinforcing element, having a cross section with a flattened overall shape and extending in a main direction and comprising at least one lateral edge made of a polymeric composition comprising a thermoplastic polymer, the lateral edge extending in a general direction substantially parallel to the main direction is treated by heating at least a part of the lateral edge, during which at least a part of the lateral edge is subjected to a plasma flow so as to raise the temperature of the part of the lateral edge above the melting point of the thermoplastic polymer.