Fast crystallizing polyester compositions

Claim 1

A fast crystallising polyester composition obtained by reacting a polyester which is a block copolyester containing the repeating polymeric segments A and B, wherein A is a polymeric segment of ethylene terephthalate units and B is a polymeric segment having a glass-transition temperature of less than 0° C. the concentration of the segments of A being at least 90 mole % and the concentration of the segments of B being between 0.05 and 10 mole % of the block copolyester, with an ionisable metal salt of an organic compound having at least one acidic proton and at least one aromatic ring in which the ionisable metal salt is one which, when present in polyethylene terephthalate at a concentration of 1% by weight, will: (a) give a difference in the crystallisation peak temperatures (ΔT=Tc-Tn) of at least 100° C. measured by differential scanning calorimetry on heating (Tn) and on cooling (Tc) using heating and cooling rates of 10° C./minute, and (b) has a melt degradation value (K value) measured in polyethylene terephthalate according to the method given in Makromol Chem 179, 2201-2209 (1978) of less than 60.