The invention relates to a process for increasing the melt viscosities of polyester resins. This process comprises a step of heating the polyester resin in the temperature range comprised between the melting temperature of the polyester resin and the degradation temperature of the polyester resin with the addition of at least one modifier compound which has at least three glycidyl side chains. The improved polyester resins, having such improved properties, are used for the production of polyester foamed materials.