Polylactic acid stereocomplex compositions and methods for making and using same
View Patent ↗Thermoformed PLA stereocomplex parts are made using a PLA stereocomplex composition having a highest crystallization melting temperature from 200 to 215° C. The stereocomplex composition preferably has less than 5 J/g of lower melting (160 to 190° C.) crystallites. The stereocomplex can be pre-annealed in various ways to reduce thermoforming cycle times. The stereocomplex forms parts with low haze and good thermal resistance, at reasonable cycle times.
1. A process comprising
a) forming a mixture of starting PLA resins in which a high D PLA resin constitutes from 25 to 75% by weight of the weight of the PLA resins and a high L PLA resin constitutes from 75 to 25% by weight of the PLA resins, and heating the mixture to a temperature above its highest crystalline melting temperature for a period of from about 5 minutes to 3 hours in the presence of a transesterification catalyst, and then blending the resulting transesterified PLA resins with an additional quantity of a high L PLA resin or high D PLA resin to form a blend provided that the proportions of PLA resins, including the starting PLA resins and the additional PLA resin, are such that a high D PLA resin and a high L PLA resin each constitute at least 25% of the PLA resins; then
b) extruding the mixture to form a thermoformable sheet, and
c) thermoforming the sheet.
2. The process of claim 1 wherein, after step b), the thermoformable sheet is cooled to below the glass transition temperature of the PLA resins prior to conducting step c).
3. The process of claim 2 wherein step c) is conducted by c1) heating the thermoformable sheet to a temperature between the glass transition temperatures of the PLA resins and their highest crystalline melting temperature and c2) thermoforming the heated sheet on a mold.
4. The process of claim 3 wherein, prior to step c1) the sheet is pre-annealed at a pre-annealing temperature between the glass transition temperature of the PLA resin but not higher than 140° C. prior to thermoforming the sheet.
5. The process of claim 4 wherein the pre-annealing temperature is from 90 to 120° C.
6. The process of claim 4 , wherein the pre-annealed sheet is quenched by cooling it below the glass transition temperatures of the PLA resins, prior to performing step c1).
7. The process of claim 4 wherein the pre-annealed sheet contains less than 5 J per gram of PLA resin of crystallites having a crystalline melting temperature of from 160 to 190° C., and a total crystallinity of not greater than 15 J per gram of PLA resin.
8. The process of claim 3 wherein step c1) is conducted at a temperature of from 121 to 150° C.
9. A process comprising
a) forming a mixture of PLA resins in which a high D PLA resin constitutes from 25 to 75% by weight of the weight of the PLA resins and a high L PLA resin constitutes from 75 to 25% by weight of the PLA resins, and heating the mixture to a temperature above its highest crystalline melting temperature for a period of from about 5 minutes to 3 hours in the presence of a transesterification catalyst and then blending the resulting transesterified PLA resins with (1) an additional quantity of a high L PLA resin or high D PLA resin to form a blend provided that the proportions of PLA resins, including the starting PLA resins and the additional PLA resin, are such that a high D PLA resin and a high L PLA resin each constitute at least 25% of the PLA resins; then
b) extruding the mixture to form a thermoformable sheet,
c) heating the sheet to a temperature above its glass transition temperature but below 180° C. for a period of time sufficient to produce a semi-crystalline sheet having from 15 to 40 J per gram of PLA resin of crystallites that have a melting temperature of from 200 to 215° C. and then
d) thermoforming the sheet.
10. The process of claim 9 , wherein the sheet prior to thermoforming has a thickness of from 5 to 50 mils.
11. The process of claim 10 , wherein the sheet is thermoformed on a mold that is at a temperature below the glass transition temperature of the PLA resin.
12. The process of claim 10 , wherein the sheet is thermoformed on a mold that is at a temperature above the glass transition temperature of the PLA resin but not higher than 130° C.