Process for preparing a heat resistant polylactic acid foamed article
Some embodiments of the present invention provides a method of preparing a thermoformed article composed of polylactic acid that includes the steps of providing a foam sheet comprising polylactic acid having a D-lactic acid stereo-isomer content that is about 3 mole % or less, exposing the sheet to a temperature of at least about 190° C., and forming the heated sheet in a mold at a mold temperature that is at least about 50° C. and for a length of time to form a thermoformed article having a PLA crystalline content of at least about 10%. Surprisingly, it has been discovered that heat resistant thermoformed articles can be prepared in accordance with embodiments of the invention wherein the time required for forming the foam sheet into a molded article is less than about 10 seconds.
1. A method of preparing a thermoformed article comprising polylactic acid, the method comprising the steps of:
providing a foam sheet comprising polylactic acid having a D-lactic acid stereo-isomer content that is about 3 mole % or less, from about 0.1 to 5 percent by weight of nucleating agent, and from about 0.5 to about 4 percent by weight of a chain extender agent;
exposing the sheet to a temperature of at least about 190° C.; and
forming the heated sheet in a mold at a mold temperature that is 50° C. to 79.4° C. and for a length of time of less than 10 seconds to form a thermoformed article having a PLA crystalline content of at least about 10%, and wherein the steps of exposing the sheet and forming the heated sheet in the mold is completed in a total length of time of less than 10 seconds.
2. The method according to claim 1 , wherein the form time for the forming steps is less than about 5 seconds.
3. The method according to claim 1 , wherein the step of exposing the sheet to a temperature of at least about 190° C. comprised heating the sheet in an oven having an oven temperature is from about 190° to 250° C.
4. The method according to claim 3 , wherein the oven temperature is from about 215° to 246° C.
5. The method according to claim 3 , wherein the mold temperature is from about 52° to 65.6° C.
6. The method according to claim 1 , wherein the process of preparing the thermoformed article is conducted in the absence of the step of subjecting the thermoformed article to a cold mold treatment.
7. The method according to claim 1 , wherein the foam sheet is substantially free of crosslinking.
8. The method according to claim 1 , wherein the thermoformed article is in the form of a tray.
9. The method according to claim 1 , wherein the foam sheet comprises at least about 85 percent by weight of polylactic acid having a D-lactic acid stereo-isomer content that is about 3 mole % or less.
10. The method according to claim 1 , wherein the foam sheet comprises from about 85 to 95 percent by weight of polylactic acid having a D-lactic acid stereo-isomer content that is about 3 mole % or less.
11. The method according to claim 1 , wherein the foam sheet has a density of less than about 0.1 g/cm 3 .
12. The method according to claim 1 , wherein the thermoformed article resulting from the crystalline step has a crystalline content greater than about 25%.
13. A method of preparing a thermoformed article comprising polylactic acid, the method comprising the steps of:
providing a foam sheet comprising polylactic acid polymer having a crystalline half life of from about 6 to 12 seconds;
heating the sheet in an oven at an oven temperature that at least 190° C.; and
forming the heated sheet in a mold at a mold temperature that is 50° C. to 79.4° C. and for a length of time of less than 10 seconds to form a thermoformed article having a PLA crystalline content of at least about 20%, wherein the steps of heating the sheet and forming the heated sheet in the mold is completed in a total length of time of less than 10 seconds, and wherein the foam sheet comprises
from about 85 to 95 percent by weight of polylactic acid having a D-lactic acid stereo-isomer content that is about 3 mole % or less; and
from about 0.1 to 5 percent by weight of nucleating agent;
from about 0.5 to about 4 percent by weight of a chain extender agent; and
wherein the foam has a heat resistance after 5 minutes of exposure of about 74° C. or more, and is substantially free of crosslinks formed by a crosslinking agent.
14. The method according to claim 13 , wherein the time for the forming steps is less than about 5 seconds, and wherein the oven temperature is from about 190° to 250° C.
15. The method according to claim 13 , wherein the mold temperature is from about 50° to 65.6° C.
16. The method according to claim 13 , wherein the process of preparing the thermoformed article is conducted in the absence of the step of subjecting the thermoformed article to a cold mold treatment, a crosslinking agent and a synthetic polymer.
17. The method according to claim 13 , wherein the foam sheet is substantially free of crosslinking.
18. The method according to claim 13 , wherein the thermoformed article is in the form of a tray.
19. The method according to claim 13 , wherein the nucleating agent is Talc.
20. The method according to claim 13 , wherein the oven temperature is from about 215° to 246° C., and the mold temperature is from about 52° to 65.6° C.
21. The method according to claim 1 , wherein a total cycle time for the steps of heating the sheet and forming the heated sheet in the mold is completed in a length of time of less than 8 seconds.
22. The method according to claim 1 , wherein a total cycle time for the steps of heating the sheet and forming the heated sheet in the mold is completed in a length of time of less than 6 seconds.