A process for producing an expandable pelletized material comprising A) a polymer matrix composed of A1) at least 55% by weight of polyester (based on the entirety of components A1) and A2)) with a total enthalpy of fusion in the range from 1 to 37.5 J/g, optionally with one or more melting points in the range from 130 to 290°C and with one or more glass transition temperatures in the range from 25 to 170°C, where component A1) comprises a mixture made of one or more polycarbonates selected from bisphenol A polycarbonate and polypropylene carbonate, and one or more polycarboxylic esters selected from polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, and copolyesters thereof, and/or comprises transesterification products of said mixture; and A2) from 0 to 45% by weight (based on the entirety of components A1) and A2)) of one or more thermoplastic polymers different from component A1); B) a physical blowing agent component, and C) optionally further additives, comprising the following steps: a) provision of the polyester A1) and optionally of polymer component A2) in a molten state, b) mixing to incorporate physical blowing agent component B) and optionally one or more additives C) into the melt, c) extrusion, and d) underwater pelletization of the melt containing blowing agent at elevated pressure of from 1 to 20 bar. A) a polymer matrix composed of A1) at least 55% by weight of polyester (based on the entirety of components A1) and A2)) with a total enthalpy of fusion in the range from 1 to 37.5 J/g, optionally with one or more melting points in the range from 130 to 290°C and with one or more glass transition temperatures in the range from 25 to 170°C, where component A1) comprises a mixture made of one or more polycarbonates selected from bisphenol A polycarbonate and polypropylene carbonate, and one or more polycarboxylic esters selected from polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, and copolyesters thereof, and/or comprises transesterification products of said mixture; and A2) from 0 to 45% by weight (based on the entirety of components A1) and A2)) of one or more thermoplastic polymers different from component A1); A1) at least 55% by weight of polyester (based on the entirety of components A1) and A2)) with a total enthalpy of fusion in the range from 1 to 37.5 J/g, optionally with one or more melting points in the range from 130 to 290°C and with one or more glass transition temperatures in the range from 25 to 170°C, where component A1) comprises a mixture made of one or more polycarbonates selected from bisphenol A polycarbonate and polypropylene carbonate, and one or more polycarboxylic esters selected from polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, and copolyesters thereof, and/or comprises transesterification products of said mixture; and A2) from 0 to 45% by weight (based on the entirety of components A1) and A2)) of one or more thermoplastic polymers different from component A1); B) a physical blowing agent component, and C) optionally further additives, a) provision of the polyester A1) and optionally of polymer component A2) in a molten state, b) mixing to incorporate physical blowing agent component B) and optionally one or more additives C) into the melt, c) extrusion, and d) underwater pelletization of the melt containing blowing agent at elevated pressure of from 1 to 20 bar.