Polystyrene and polylactic acid blends

Abstract

Polymeric blends of polystyrene (PS) and polylactic acid (PLA) are described, in the preparation of which a compatibilizing agent was added, preferably a PS-PLA block copolymer. Such compatibilizing agents act controlling phase separation of the blending compounds, and preventing excessive growth of polystyrene domains scattered in the PLA matrix; and this results in blends with good mechanical and thermal resistance.

Claims

1. A polystyrene and polylactic acid blend, characterized by having: a) a mixture of polystyrene (PS)/polylactic acid (PLA), wherein the polystyrene is a high impact polystyrene (HIPS), having as its base a matrix of styrene-based polymer and, dispersed in this matrix, a rubber phase made of discrete particles of rubber based on butadiene and/or styrene-butadiene, and where the polystyrene has a concentration from 70% m/m to 30% m/m, and the PLA has a concentration from 30% m/m to 70% m/m; and b) PS-PLA block copolymer in concentrations of 0.1% to 15% in relation to the mixture of polystyrene/PLA.

2. The polystyrene and polylactic acid blend according to claim 1, wherein the polystyrene is in concentrations from 60% m/m to 40% m/m in the mixture of polystyrene/polylactic acid.

3. The polystyrene and polylactic acid blend according to claim 1, wherein the polylactic acid concentrations are in the range from 40% m/m to 60% m/m in the mixture of polystyrene/poly-lactic acid.

4. The polystyrene and polylactic acid blend according to claim 1, wherein the PS-PLA copolymer is in concentrations from 3% m/m to 12% m/m in relation to the mixture of polystyrene/polylactic acid.

5. The polystyrene and polylactic acid blend according to claim 1, wherein the PS-PLA copolymer concentrations are from 5% m/m to 10% m/m in relation to the mixture of polystyrene/polylactic acid.

6. The polystyrene and polylactic acid blend according to claim 1, wherein the concentration by mass of the rubber phase in the polymeric matrix varies between 3% and 15%, in relation to the total mass of the polystyrene.

7. The polystyrene and polylactic acid blend according to claim 1, wherein the rubber phase is constituted by rubber particles representing a morphology of the types core shell or salami, or even a mixture of those in different proportions.

8. The polystyrene and polylactic acid blend according to claim 7, wherein the average diameter of the rubber particles range is from 0.1 microns to 8 microns over a distribution curve of particle sizes.

9. The polystyrene and polylactic acid blend according to claim 1, wherein the styrene-based polymeric matrix comprising styrene presents average weighted molar mass (Mw) between 120,000 g/mol and 300,000 g/mol.

10. The polystyrene and polylactic acid blend according to claim 1, wherein the appropriate PLA for this blend is obtained through lactic ring opening polymerization.

11. The polystyrene and polylactic acid blend according to claim 1, wherein the PLA present different D or L isomers, that is, it can generate PLLA, PDLA or even a mixture of isomers, generating PDLLA is present as PLLA, PDLA, and/or PDLLA.

12. The polystyrene and polylactic acid blend according to claim 1, wherein the average PLA number molar mass (Mn) is from 30,000 g/mol to 200,000 g/mol.

13. The polystyrene and polylactic acid blend according to claim 1, wherein the average PLA number molar mass (Mn) is from 50,000 g/mol to 160,000 g/mol.

14. The polystyrene and polylactic acid blend according to claim 1, wherein the PS-PLA block copolymer is obtained in two steps, the first step the PS block with OH thermal functionality (hydroxyl group) is synthesized via atom transfer radical polymerization (ATRP), and using tribromoethanol as the trigger; and in the second step the PS-OH block is used to initiate the polymerization via opening of the lactic ring, forming the PS-PLA block copolymer.

15. The polystyrene and polylactic acid blend according to claim 14, wherein the average number molar mass (Mn) of the PS block is from 2,000 g/mol to 20,000 g/mol, and the average number molar mass (Mn) of the PLA block is from 2,000 g/mol to 20,000 g/mol.

16. The polystyrene and polylactic acid blend according to claim 14, wherein the average number molar mass (Mn) of the PLA block is from 5,000 g/mol to 15,000 g/mol.

17. The polystyrene and polylactic acid blend according to claim 14, wherein the average number molar mass (Mn) of HIPS is from 5,000 g/mol to 15,000 g/mol.

18. A pelleted composition prepared by extruding the blend according to claim 1, in a single or double-screw extruder, at temperatures between 160 C. and 230 C., followed by cooling and then pelletizing.

19. The pelleted composition according to claim 18, wherein the temperature of the blend in the extruder is between 165 C. and 210 C.

20. The pelleted composition according to claim 18, wherein the temperature of the blend in the extruder is between 170 C. and 190 C.

21. The pelleted composition according to claim 18, wherein the polylactic acid in the polystyrene/polylactic acid mixture is PLLA.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

(2) FIG. 1: Optical microscope image of a non-compatibilized HIPS and PLA blend (800 magnification)

(3) FIG. 2: Optical Microscope image of a compatibilized HIPS and PLA blend (800 magnification)

DETAILED DESCRIPTION OF THE INVENTION

(4) Generally speaking, the invention covers the polystyrene blends (PS), especially high-impact polystyrene (HIPS) with polylactic acid (PLA) produced via melting, having the PS-PLA block copolymer as a compatibilizing agent.

(5) Such blends are constituted by PLA and polystyrene, with the addition of a compatibilizing agent, with the following mass proportions: a) Mixture of polystyrene/PLA, where the polystyrene concentration range is from 70% m/m to 30% m/m, preferably from 60% m/m to 40% m/m, and the PLA concentration range is from 30% m/m to 70% m/m, preferably from 40% m/m to 60% m/m; b) PS-PLA block copolymer in concentrations of 0.1% to 15% in relation to the polystyrene/PLA mixture, preferably from 3% to 12%, but ideally from 5% to 10%.

(6) A polystyrene obtained through bulk or solution polymerization is used for the polystyrene/PLA mixture. Preferably, the polystyrene is a high-impact polystyrene (HIPS), having as its base a matrix of styrene-based polymer and, dispersed in this matrix, a rubber phase made of discrete particles of rubber based on butadiene and/or styrene-butadiene copolymer with a different microstructure (cis, trans and vinyl).

(7) Such rubber particles are dispersed in the polymeric matrix in concentrations by mass between 3% and 15% of the total composition.

(8) The morphology of the rubber particles useful for the invention is known as core Shell or salami, or even as a mixture of these in different proportions, and its average diameter may vary from 0.1 microns to 8 microns over a distribution curve of particle sizes. The styrenic matrix presents, in general, a weighted average molar mass (Mw) between 120,000 g/mol and 300,000 g/mol.

(9) The adequate PLA for this blend can be obtained by lactic ring opening polymerization, such as mentioned in U.S. Pat. No. 7,507,561 and U.S. Pat. No. 6,326,458, without limitation to those. PLA may present different D or L isomers and can generate PLLA, PDLA or even a mixture of isomers, generating PDLLA, with PLA's average number molar mass (Mn) ranging from 30,000 g/mol to 200,000 g/mol, preferably from 50,000 g/mol to 160,000 g/mol.

(10) Due to the thermodynamic incompatibility of the polystyrene/PLA mixture, a compatibilizing agent is added to it, in this case, a PS-PLA block copolymer, which is obtained in two phases.

(11) In the first phase, the PS block with OH terminal functionality (hydroxyl group) is synthesized via an atom transfer radical polymerization (ATRP), with tribromoethanol as initiator.

(12) In the second phase, the PS-OH block is used to initiate the polymerization by opening the lactic ring, forming the PS-PLA block copolymer, where the average number molar mass (Mn) of the PS block is from 2,000 g/mol to 20,000 g/mol, preferably from 5,000 g/mol to 15,000 g/mol, and the number average molar mass (Mn) of the PLA block is 2,000 g/mol to 20,000 g/mol, preferably from 5,000 g/mol to 15,000 g/mol.

(13) The PLA block used to obtain the PS/PLA copolymer used as compatibilizing agent can present different D or L isomers, in other words, the block can be PLLA, PLDA or even a mixture of PDLLA isomers. Preferably, PS and PLA blocks have the same molar mass, but they may also have different molar masses.

(14) Polystyrene/PLA polymeric blends produced through a compatibilizing agent such as in this invention, present smaller HIPS domains, indicating that there was morphological control due to the use of the compatibilizing agent, controlling phase separation between the components of the blend and not allowing excessive growth of the dispersed HIPS domains in the PLA matrix, as illustrated in FIG. 2.

(15) This morphological control does not take place in blends produced without the compatibilizing agent, as observed in FIG. 1, where it is possible to verify the heterogeneity of the HIPS domains scattered in the PLA matrix, evidencing the phase separation that occurs after the molten mixture cools down.

(16) The polymeric blends which are the object of this invention can be produced through a mixture of molten PS and PLA, with the addition of a PS-PLA block copolymer. The mixture can be made in a single or double-screw extruder. After the mixture, the compatibilized blend is cooled in a water bath and pelletized.

(17) The mixture temperature at the extruder must be between 160 C. and 230 C., preferably between 165 C. and 210 C., and ideally between 170 C. to 190 C. After being pelleted, the compatibilized blend can be processed later via extrusion to generate a thermoforming plate or be injected into a mold which will define its final shape.

Example 1

(18) This example illustrates the preparation of a blend of 40% m/m of HIPS containing 6% m/m of polybutadiene (Innova R 870E) and 60% of PLLA without the compatibilizing agent.

(19) The materials were mixed, according to the indicated proportions, by extrusion, at a temperature of 180 C., with the resulting blend being cooled in a water bath. In this case, there was a phase separation and the heterogeneity of the mechanical and thermal properties of the blend was preserved as can be seen in FIG. 1.

Example 2

(20) This example illustrates the preparation of a blend which is the object of this invention.

(21) A mixture was prepared by melting 60% m/m of HIPS containing 6% m/m of polybutadiene (Innova R 870E) and 40% m/m of PLLA, with 10% m/m of a PS-PLA compatibilizing agent in relation to the total HIPS and PLA. The materials were blended by extrusion at 210 C. and the resulting blend was cooled in a water bath. There was no phase separation, as shown in FIG. 2, and the mechanical and thermal properties were homogeneous. Izod impact resistance (as per ASTM D 256) was 18 J/m, and heat deflection temperature (HDT, as per ASTM D 684) was 73.6 C.

Example 3

(22) This example shows the preparation of the blend which is the object of this invention.

(23) A mixture was prepared by melting 40% of HIPS containing 6% of polybutadiene (Innova R 870E) and 60% of PLLA, with 10% of a PS-PLA compatibilizing agent in relation to the total HIPS and PLA. The materials were mixed via extrusion at 180 C., and the resulting blend was cooled in a water bath. There was no phase separation and the mechanical and thermal properties of the blend were homogeneous. Izod impact resistance (as per ASTM D 256) was 15 J/m, and heat deflection temperature (HDT, as per ASTM D 684) was 56 C.