Polylactic resin compositions for paperboard coating and paperboard coating processes using the compositions

1. A method for making a modified polylactide composition, comprising the steps of

I. forming a reaction mixture by combining:

i) 85 to 99.9 weight percent, based on the combined weight of components i) and ii), of a starting linear polylactide resin or mixture of linear polylactide resins wherein the starting linear polylactide resin or mixture of linear polylactide resins has a relative viscosity of 2 to 4.5, the ratio of L-lactic repeating units to D-lactic acid repeating units in the starting linear polylactide resin or mixture of linear polylactide resins is >8:92 and <92:8 and the starting linear polylactide or mixture of linear polylactide resins contains carboxyl end groups and

ii) 15 to 0.1 weight percent, based on the combined weight of components i) and ii), of a thermoplastic epoxy group-containing polymer having an epoxy equivalent weight of 1000 to 6000, an average of at least three epoxy groups per molecule, a melting temperature of 100° C. or lower and a glass transition temperature of 0° C. or lower;

II. reacting a portion of the carboxyl groups of the starting linear polylactide resin or mixture of linear polylactide resins with a portion of the epoxy groups of the thermoplastic epoxy group-containing polymer to form a composition that includes a reaction product of components i) and ii) and a quantity of unreacted component i) and

III. after step II, melt-blending the reaction mixture or composition formed in step II with iii) at least one branched polycaprolactone, wherein the at least one branched polycaprolactone has a glass transition temperature of 0° C. or lower and constitutes 30 to 60 weight percent of the combined weights of components i), ii) and iii).

2. The method of claim 1 wherein step II is performed by heating the reaction mixture to a temperature of 185° C. to 240° C. for a period of 30 seconds to 5 minutes under shear at a shear rate of at least 100 s −1 and then cooling the composition formed in step II to a temperature below 185° C.

3. The method of claim 2 wherein in step I, i) 85 to 98 weight percent, based on the combined weight of components i) and ii), of the starting linear polylactide resin or mixture of linear polylactide resins are combined with 15 to 2 weight percent, based on the combined weight of components i) and ii), of the thermoplastic epoxy group-containing polymer.

4. The method of claim 3 wherein components i) and ii) are provided in amounts to provide 1 to 3 epoxy groups per carboxyl group provided by component i).

5. The method of claim 3 wherein at least 50% of the end groups of the starting linear polylactide resin or mixture of polylactide resins are carboxyl groups.

6. The method of claim 5 wherein at least 55% of the end groups of the starting linear polylactide resin or mixture of polylactide resins i) are carboxyl groups and at least a portion of the linear polylactide resin molecules have two terminal carboxyl groups and no terminal hydroxyl groups.

7. The method of claim 1 wherein the thermoplastic epoxy group-containing polymer is a copolymer of monomers that include ethylene and at least one of glycidyl acrylate and glycidyl methacrylate.

8. The method of claim 1 , wherein the modified polylactide composition contains a) 4 to 56 weight percent of the reaction product of components i) and ii), b) 30 to 60 weight percent of component iii) and c) 12 to 72 weight percent of the starting linear polylactide resin or mixture of linear polylactide resins, based on the combined weights of a), b) and c).

9. The method of claim 1 , further comprising the step of blending the modified polylactide composition with at least one additional linear polylactide resin to form a diluted modified polylactide composition.

10. A modified polylactide composition made in accordance with the method of claim 1 .