The invention relates to a polyether-polyester copolymer comprising: (i) polyether segments wherein at least one polyether segment contains at least one polytetramethylene oxide segment, (ii) polyester segments, (iii) bridging elements of the structure —CO—R2-CO—, wherein R2 represents an optionally substituted bivalent hydrocarbon residue consisting of 1 to 100 carbon atoms; (iv) one or two end-caps R1-O—(C2-C4—O—)e—*, wherein R1 is an optionally substituted hydrocarbon residue and e is an integer of from 0 to 1000.