Process for manufacturing particles comprising polylactic acid

Abstract

A process for manufacturing particles including a stereocomplex of poly-D-lactide (PDLA) and poly-L-lactide (PLLA), including the steps of: extruding a melt including 30-70 wt. % of PDLA and 70-30 wt. % of PLLA through an sc-PLA formation zone in a twin-screw extruder, wherein the formation zone is operated at a barrel temperature above the melting temperature of the PDLA and PLLA and below 220? C.; wherein the sc-PLA formation zone is followed by a finishing zone which is operated at a barrel temperature below 160? C.; wherein the finishing zone is followed by the end of the extruder which has a die-head resistance of 0; and recovering solid stereocomplex particles from the end of the extruder. The stereocomplex particles find use in various applications, e.g., in fracking fluids, as filler, as nucleating agent, in particular in the molding of semi-crystalline PLA, or as a starting material for the manufacture of sc-PLA products.

Claims

1. Process for manufacturing particles comprising a stereocomplex of poly-D-lactide (PDLA) and poly-L-lactide (PLLA), comprising the steps of extruding a melt comprising 30-70 wt. % of PDLA and 70-30 wt. % of PLLA through an sc-PLA formation zone in a twin-screw extruder, wherein the sc-PLA formation zone is operated at a barrel temperature of at least 170? C. and below 220? C., for a residence time of at least 1 minute and not more than 10 minutes, and at an L/D ratio of at least 10 and not more than 30, wherein the sc-PLA formation zone is followed by a finishing zone, wherein the finishing zone is operated at a barrel temperature below 160? C., wherein the finishing zone is followed by the end of the extruder, wherein the end of the extruder has a die-head resistance of 0, and recovering solid stereocomplex particles from the end of the extruder.

2. Process according to claim 1, wherein the melt comprising 30-70 wt. % PDLA and 70-30 wt. % PLLA is obtained by the steps of providing solid particles of PDLA and solid particles of PLLA to a feeder zone of the extruder, and melting the PDLA and PLLA in a melting zone in the extruder located prior to the sc-PLA formation zone.

3. Process according to claim 1, wherein the melt comprises 40-60 wt. % PDLA and 60-40 wt. % PLLA.

4. Process according to claim 1, wherein the barrel temperature in the sc-PLA formation zone is at least 170? C. and at most 210? C.

5. Process according to claim 1, wherein the finishing zone is operated at a barrel temperature below 140? C.

6. Process according to claim 1, wherein the solid stereocomplex particles have a particle size distribution which is such that its mean volume diameter is below 2 mm.

7. Process according to claim 1, wherein the solid stereocomplex particles show a single melting peak between 195 and 250 degrees Celsius.

8. Process according to claim 1 which further comprises one or more steps selected from milling, grinding, and sieving.

Description

Example 1: Manufacture of Sc-PLA Powder (I)

(1) A starting PDLA (Luminy? D070, Corbion) was selected with an absolute weight-average molecular weight of 45 kg/mol, and a melt flow index of 12 g/10 min (ISO 1133-A, 190? C./0.325 kg). The stereochemical purity was >99% (D-isomer) and the melting point was 175? C. (DSC).

(2) A starting PLLA (Luminy? L105, Corbion) was selected with an absolute weight-average molecular weight of 65 kg/mol, and a melt flow index of 22 g/10 min (ISO 1133-A, 190? C./2.16 kg). The stereochemical purity was >99% (L-isomer) and the melting point was 175? C. (DSC).

(3) The two materials were provided in a 1:1 ratio to the gravimetric feeder of a Berstorff 400 rpm ZE40A-38D co-rotating twin-screw extruder. Temperature settings of the extruder barrel were as shown in the scheme below. Z1 is the feeding zone. Melting takes place in Z2-Z3. The majority of the sc-formation takes place in Z4-Z5, and Z6 through Z8 make up the finishing zone. Screw rotation speed was set at 70 rpm and the throughput rate was about 40 kg/h. The torque level during stable operation was 60-70%. The L/D values for the respective zones were as follows: Z1: L/D is 4; Z2-Z3: L/D is 10; Z4-Z5: L/D is 10; Z6-Z8: L/D is 14. Total L/D was 38.

(4) TABLE-US-00001 Zone Feed Die Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Barrel Temperature set value (? C.) 40 120 195 195 195 160 100 100 NA Actual barrel temperature values 39 117 198 186 194 154 114 100 NA (? C.)

(5) The temperature of the collected sc-PLA powder leaving the extruder barrel was approximately 170? C. At the end of the extruder, no die was present. The end of the extruder had a die-head resistance of 0. Material temperature in zones Z3-Z8 was between 170? C. and 220? C.

(6) A particulate product was recovered from the end of the extruder. The white, free-flowing powder had the following particle size distribution: D [4,3]=430 micron, D [0.1]=102 micron, D [0.5]=330 micron and D [0,9]=896 micron. Thermal characterization using DSC (scanning rate 10K/min) showed a single melting point at 231? C. with a melting enthalpy of 73 J/g. (See DSC graph overlay in FIG. 1)

Example 2: Manufacture of Sc-PLA Powder (II)

(7) A similar powder production was realized as in Example 1, in the same extruder with identical throughput (40 kg/h) and screw speed (70 rpm) and the same PLLA/PDLA granulate mixture. The temperature settings were adjusted and shown in the scheme below. Material temperature in zones Z3-Z8 was between 170? C. and 220? C.

(8) TABLE-US-00002 Zone Feed Die Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Barrel Temperature set value (? C.) 40 120 195 195 195 140 50 50 NA Actual barrel temperature values 38 119 210 188 192 147 108 70 NA (? C.)

(9) Under these settings, a powder produced with the following characteristics: D [4,3]=469 micron, D [0.1]=101 micron, D [0.5]=354 micron and D [0,9]=996 micron. The DSC thermograph (scanning rate 10K/min) confirmed the existence of just a single melting peak at 231? C.

Comparative Example 1: Conventional Blend Strands

(10) As in Example 1, a blend of 50% Luminy? D070 and 50% Luminy? L105 was fed to the twin-screw extruder of Example 1 at a feed rate of 20 kg/h and a screw speed of 227 rpm. Now the extrusion zones were set to the temperature scheme below and a conventional double-strand die head was mounted.

(11) TABLE-US-00003 Zone Feed Die Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Barrel; Temperature set value 60 120 195 215 215 215 205 205 230 (? C.) Actual barrel temperature 57 120 200 211 214 215 203 204 228 values (? C.)

(12) This classical compounding operation proceeded in a steady state at a torque of 15% and produced a homogeneous, clear polymer melt with temperature of 237? C. The viscous melt did not crystallize and as a result, a conventional transparent double strand was extruded. This example shows that too high (material) temperatures and the presence of a die do not constitute the proper conditions for continuous manufacture of sc-PLA powder.

Comparative Example 2: Extruder Blocks

(13) Using the same materials and extruder set-up as in Comparative Example 1, the temperature settings below were now chosen.

(14) TABLE-US-00004 Zone Feed Die Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9 Temperature set value (? C.) 60 120 195 195 195 175 170 165 160

(15) A lower screw speed of 100 rpm had to be used, to prevent the mixture in the extruder from overheating and thus still leaving the extruder as a viscous polymer melt. An opaque melt was extruded from one of the two orifices of the die head, while the other orifice was jammed. At these settings, it could thus be shown that crystallization was incomplete and moreover resulted in a (partial) blocking of the extrusion die, and thus no feasible, continuous operation.

Example 3: Use as Hydrolytically Degrading Particles

(16) To mimic the behavior of sc-PLA powder in fracking applications, the powder produced in Example 1 was subjected to hydrolytical degradation.

(17) To this end, 12 grams of the powder were mixed in a 600 mL reactor (Parr Instrument Company Series 4760 General Purpose Vessel) with 200 mL of demineralized water. A nitrogen pressure of 7 bar was applied and the temperature was raised and maintained to 155? C. This temperature was chosen as it mimics a typical high temperature degradation profile as used in the fracking industry.

(18) The experiment was allowed to run for 16 h, after which the remaining solid was filtered over Whatman #3 paper filter and was subsequently dried at 40? C. until constant weight. The mass loss of the powder was determined to be 82%. The sc-PLA powder as such shows higher hydrolytical degradation resistance than typical poly(L-lactide) powders, which consistently fully degrade under identical circumstances.

Example 4.: Use as Nucleating Agent

(19) To test the use of the powder of Example 2 as nucleating agent, a compound was made of 5 wt. % of the powder and 95 wt. % Luminy? L130 (Corbion). Using DSC analysis, the differences in crystallization behavior were analysed. Samples of both PLLA L130 and PLLA L130 with 5% of the powder of example 2 were subjected to the following DSC protocol: Equilibrate at 20? C., heat at 10K/min to 200? C., hold at 200? C. for 3 minutes and cool to 0? C. at 5K/min. In the case of the pure PLLA L130 sample, a very broad crystallization peak was observed, with about 12 J/g crystallinity being created. This indicates slow and incomplete crystallization from the melt. For the compound with 5% of the sc-PLA powder, a sharp crystallization peak was observed with a maximum at 110? C., forming about 37 J/g of crystals. This shows that the sc-PLA powder acts as a nucleating agent in crystallizable PLA compounds.