Polyhydroxycarboxylic acid and its production process

A polyglycolic acid obtained by ring-opening polymerization of glycolide or glycolide with other cyclic comonomer(s) in an amount of up to 10% by weight of all the charged monomers and having melt stability, which has:

• (I) a retention of melt viscosity of at least 40% as defined by a proportion of the viscosity (η60) of said polyglycolic acid measured after a 60-minute retention at 250° C. to the initial viscosity (η0) of said polyglycolic acid measured after a 5-minute preheating at 250° C. ((η60/η0)×100), and
• (II) a yellowness index (YI) of 40 or less as measured using a sheet obtained by press molding and crystallization of said polyglycolic acid.

A polyhydroxycarboxylic acid production process, comprising ring-opening polymerization of a cyclic ester at 120 to 250° C. for 3 minutes to 50 hours, and an additional polymerization that is carried out at a temperature 10 to 50° C. lower than the temperature for said ring-opening polymerization for 1 to 50 hours.

A polyhydroxycarboxylic acid production process, comprising ring-opening polymerization of a cyclic ester at 120 to 250° C. for 3 minutes to 50 hours, and an additional polymerization that is carried out at a temperature 10 to 50° C. lower than the temperature for said ring-opening polymerization for 1 to 50 hours, wherein a polyhydroxycarboxylic acid having:

• (a) a weight-average molecular weight (Mw) in the range of 10,000 to 1,000,000,
• (b) a molecular weight distribution in the range of 1.0 to 2.5 as represented by a weight-average molecular weight-to-number-average molecular weight ratio (Mw/Mn), and
• (c) a yellowness index (YI) of up to 40 as measured using a sheet obtained by press molding and crystallization of said polyhydroxycarboxylic acid is produced.

A process for producing a polyglycolic acid having melt stability, comprising steps of:

• (1) subjecting glycolide or glycolide with other cyclic comonomer(s) in an amount of up to 10% by weight of all the charged monomers to ring-opening polymerization in a molten state at a temperature of 140 to 230° C.,
• (2) converting a polymer product from the molten state to a solid state by (i) cooling the polymer down from the polymerization temperature at which the polymer is in the molten state, (ii) precipitating or crystallizing and solidifying the polymer by carrying out the molten-state polymerization at a temperature lower than the melting point of the final polymer, or (iii) adding a nucleating agent to the polymer, and
• (3) hot kneading the solid-state polymer product with application of heat thereto at a temperature of 220 to 250° C. in a time range of 5 to 20 minutes.