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.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention refers to polystyrene (PS) and poly-lactic acid (PLA) blends having as compatibilizing agent a PS-PLA block copolymer. Due to the thermodynamic incompatibility between PS and PLA, their blends result in heterogeneous materials with inadequate mechanical, thermal and processability properties for the intended applications; this can be reversed by adding a compatibilizing agent, as described in this invention patent application 
       THE BASIS OF THE INVENTION 
       [0002]    The use of raw materials from renewable sources replacing the ones from fossil sources in polymer production has been growing rapidly in the last years, driven by society&#39;s demand for environmentally responsible products. However, polymers from renewable sources are still expensive, not easily available, and their properties and processability characteristics fail to meet market needs. 
         [0003]    Polymers that are made through petrochemical processes, from fossil sources, on the other hand, are inexpensive, easily available and have properties and processibility characteristics that can accommodate diverse applications. 
         [0004]    Therefore, an alternative to overcome the deficiency of the polymers from renewable sources is to produce blends with polymers from fossil sources. However, in some cases, there is incompatibility between the polymers in those blends, which results in products of poor quality and with inadequate chemical and mechanical properties for the intended applications. 
         [0005]    In order to make it easier to obtain polymeric blends containing polymers from renewable sources, most of the available literature mentions the use of compatibilizing agents. 
         [0006]    Patent application number US 2009/0123728 describes blends of aromatic alkenyl polymers such as styrenic polymers (for example, polystyrene and high-impact polystyrene) and biopolymers or biodegradable polymers (polylactic acid, polyglycolic acid (PGA), polyhydroxyalkanoate (PHA), polybutylene succinate (PBS) and polycaprolactone (PCL) compatibilized with a styrene-based copolymer (styrene-ethylene-butylstyrene block copolymer (SEBS), SEBS functionalized with a maleate group, styrene-anhydride maleic copolymers, styrene-methyl methacrylate copolymers (SMMA)) or a mixture of two or more copolymers based on styrene, such as SEBS and SMMA. 
         [0007]    Document number WO 2010/120673 presents blends of polyolefins with a biodegradable polymer, such as polylactic acid (PLA) and polyhydroxybutyrate, which are compatibilized by a copolymer constituted by an olefin functionalized with a methacrylic group. 
         [0008]    Similarly, we can also mention document WO 2012/109144, which covers polylactic acid blends. Such blends are produced through a process involving the contact of a polyolefin with a polylactic acid and a reactive modifier acting as a compatibilizing agent. The modifier is a glycidyl methacrylate-grafted polypropylene (PP-g-GMA). 
         [0009]    Therefore, the technique still needs compatibilizing agents for the PLA and polystyrene blends, since they are thermodynamically incompatible, as per the detailed description below. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides blends of polystyrene and polylactic acid with polystyrene making use of a compatibilizing agent, in this case the PS-PLA block copolymer. 
         [0011]    Such blends are constituted by a PLA/HIPS mixture, where PLA mass percent is between 30% and 70% and HIPS mass percent is between 70% and 30%, with a compatibilizing agent being added to the mixture, in this case the PS-PLA block copolymer in a concentration of 0.1% to 15% in relation to the mass of PS-PLA mixture. 
         [0012]    The compatibilizing agent, the PS-PLA block copolymer, allows the control and the morphological stability of the HIPS/PLA mixture. If no compatibilizing agent is added to HIPS/PLA polymeric blends, separation of the HIPS and PLA mixture phases occurs, resulting in a heterogeneous material with unstable mechanical and thermal properties. 
         [0013]    Property instability occurs because HIPS and PLA have different mechanical and thermal resistances; therefore each phase will behave as a separate compound. In other words, the PLA phase will have mechanical and thermal properties inferior to those of the HIPS phase. 
         [0014]    In the situation of blends using the compatibilizing agent, which is the object of this invention, it is possible to control phase separation after the mixture of melted compounds cools down through the compatibilizing agent, which does not allow excessive growth of the discrete domains. This morphological control generates a homogeneous material with stable mechanical and thermal properties; in other words, their properties do not change due to cooling and processing. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]      FIG. 1 : Optical microscope image of a non-compatibilized HIPS and PLA blend (800× magnification) 
           [0016]      FIG. 2 : Optical Microscope image of a compatibilized HIPS and PLA blend (800× magnification) 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    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. 
         [0018]    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%.       
 
         [0021]    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). 
         [0022]    Such rubber particles are dispersed in the polymeric matrix in concentrations by mass between 3% and 15% of the total composition. 
         [0023]    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. 
         [0024]    The adequate PLA for this blend can be obtained by lactic ring opening polymerization, such as mentioned in patents 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&#39;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. 
         [0025]    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. 
         [0026]    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. 
         [0027]    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. 
         [0028]    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. 
         [0029]    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 . 
         [0030]    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. 
         [0031]    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. 
         [0032]    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 
       [0033]    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. 
         [0034]    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 
       [0035]    This example illustrates the preparation of a blend which is the object of this invention. 
         [0036]    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 
       [0037]    This example shows the preparation of the blend which is the object of this invention. 
         [0038]    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.