Process for forming polylactide expanded bead foam

Abstract

Expanded poly(lactide) (PLA) beads are made by pressurizing PLA beads with carbon dioxide at approximately room temperature, heating the beads under pressure to 90 to 160 C to saturate and partially crystallize the beads, and then depressurizing and cooling the beads. The PLA beads contain a blend of PLLA and PDLA in certain ratios. The beads are useful for making expanded bead foam.

Claims

1. A process for making expanded poly(lactide) bead foam, comprising: (a) pressurizing unfoamed beads of a PLA resin blend having a crystallinity of no greater than 10 J/g as measured by differential scanning calorimetry with carbon dioxide to a superatmospheric pressure while maintaining the temperature at or below the 40° C.; (b) heating the pressurized unfoamed beads to a temperature between 90° C. and 160° C. to at least partially crystallize the PLA resin blend to form carbon dioxide-saturated, partially crystallized, unfoamed beads; and then (c) depressurizing the carbon-dioxide-saturated, partially crystallized, unfoamed beads such that the carbon dioxide at least partially vaporizes to expand the beads; and then (d) simultaneously and/or after step (c), cooling the expanded beads to below 40° C., wherein the PLA resin blend includes: i) a first PLA polymer containing at least 88% L-lactic units, based on the total weight of the first PLA polymer; ii) a second PLA polymer containing at least 88% D-lactic units based on the total weight of the second PLA polymer; the first and second PLA copolymers being present in the PLA resin blend at a weight ratio of 75:25 to 98:2 or 25:75 to 2:98.

2. The process of claim 1 wherein the first PLA polymer contains at least 92% L-lactic units and the second PLA polymer contains at least 92% D-lactic units.

3. The process of claim 1 wherein the first PLA polymer contains at least 95% L-lactic units and the second PLA polymer contains at least 95% D-lactic units.

4. The process of claim 1 wherein the superatmospheric pressure in step (a) and the superatmospheric pressure in step (b) each is 3 MPa gauge to 10 MPa gauge.

5. The process of claim 4 wherein the temperature in step (b) is 110° C. to 140° C.

6. The process of claim 5 wherein in step (b) the pressurized unfoamed beads are maintained at a temperature of 110° C. to 140° C. for a period of 5 to 30 minutes.

7. The process of claim 1 wherein the carbon-dioxide-saturated, partially crystallized, unfoamed beads formed in step (b) have an extent of crystallization of 10% to 35% based on the total weight of the PLA resin blend in the beads, as measured by DSC.

8. The process of claim 1 wherein PLA stereocomplex crystals and PLA homocrystals form during step (b).

9. The process of claim 8 wherein the PLA stereocomplex crystals constitute 10% to 50% of the combined weight of the PLA stereocomplex crystals and PLA homocrystals.

10. The process of claim 1 wherein the first and second PLA copolymers are present in the PLA resin blend at a weight ratio of 85:15 to 98:2 or 15:85 to 2:98.

11. Expanded poly(lactide) beads made in the process of claim 1.

12. An expanded poly(lactide) bead comprising a cellular blend PLA resin blend, wherein the expanded poly(lactide) bead has a density of 0.025 to 0.35 g/cm.sup.3 and a volume of 0.5 to 1500 mm.sup.3, and the PLA resin blend contains PLA stereocomplex crystals having a crystalline peak melting temperature of 200-240° C. and PLA homocrystals having a peak melting temperature of 140-180° C.

13. A method for producing a molded article, comprising introducing expanded poly(lactide) beads of claim 12 into a mold and sintering and optionally further expanding the expanded poly(lactide) beads in the mold to form the molded article.

Description

EXAMPLES 1 AND 2 AND COMPARATIVE SAMPLE A

(1) PLA Blend 1 is made by melt blending 85% of a PLLA resin containing 95.5% L-lactic units and 4.5% D-lactic units and having a relative viscosity of about 4 g/dL with 15% of a PDLA having a weight average molecular weight of 70,000 g/mol (GPC, relative to polystyrene). Pellets of the starting resins are melted in a twin-screw extruder to a melt temperature, extruded through a strand die into a water bath where the strands are immediately quenched to a temperature below 40° C. The strands are chopped into spherical beads having a diameter of about 1 to 2 mm.

(2) PLA Blend 2 is made in the same manner, but at a weight ratio of 95% PLLA and 5% PDLA.

(3) Example 1 is made by expanding PLA Blend 1. Example 2 is made by expanding PLA Blend 2. Comparative Sample A is made by expanding pellets of the PLLA.

(4) In each case the beads are expanded as follows: A 1-L autoclave is filled with 800 mL room temperature water. 20 g of the beads and 1 g of a suspension aid are added. The autoclave is sealed and pressured to 6 MPa gauge with carbon dioxide at room temperature. The contents of the sealed autoclave are then heated to 123-135° C. and held at that temperature for 15 minutes to saturate the beads with carbon dioxide and partially crystallize them. The autoclave is then depressurized to atmospheric pressure over several seconds. Cooling to room temperature takes place simultaneously with the depressurization.

(5) In Example 1, the beads expand 4 to 5 times their original volume. In Example 2, the beads expand to about 40 times their original volume. SEM micrographs of the expanded Examples 1 and 2 reveal a uniform, regular cell structure. Comparative Sample A expands to 4 to 5 times its original volume. It has a highly irregular cell structure characterized by many large cells.

(6) DSC thermographs are taken of the expanded beads. Example 1 is found to contain both PLA stereocomplex crystals (about 34 J/g) and about 7.7 J/g of PLLA homocrystals. Its total crystallinity is about 36%.

(7) Example 2 is found to contain about 4 J/g of PLA stereocomplex crystals and about 21 J/g of PLLA homocrystals. Total crystallinity is about 26%.

(8) Comparative Sample A contains only PLA homocrystals. Total crystallinity is about 17%.

(9) Examples 1 and 2 sinter easily in a steam chest molding machine, at a range of temperatures between about 150° C. and 200° C., to produce moldings. Comparative Sample A, on the other hand, is molded with difficulty, with small variations in pressure and temperatures leading to the production of defects in the molded part.