# Recent Approaches to the Plasticization of Poly(lactic Acid) (PLA) (A Review)
## 3 Compounding with Flexible Polymers (Blends)
Relatively low levels of PBS addition improved the melt flow characteristics of the blend. An important parameter when compounding polymers is the close values of melt viscosity. In most cases, at processing temperatures of 170-200 \({}^{\circ}\)C, the melt viscosity of flexible polymers (PCL, PBAT and PBSA) is lower than the melt viscosity of PLA. Therefore, to create systems with the maximum possible phase interface areas, it is preferable to use PLA as the dominant (larger) phase. For example, it was found in [45] that for PLA/PBSA mixtures, the size of PLA domains in a sample with 30 wt% PLA was larger than that of PBSA domains in a sample with 70 wt% PLA since a PBSA melt of lower viscosity is not capable of deforming (dispersing) the PLA phase. It was found that with a content of 30 wt% PBSA in the PLA matrix, the blend is characterized by a structure with a maximum specific interfacial surface area of PBSA domains, providing the maximum plasticizing effect on PLA. The improvement of mechanical properties is considered a determining parameter for the effectiveness of plasticization. Flexible polyesters such as PCL, PBSA and PBAT have low glass transition temperatures, so at room temperature, their macromolecules are able to move freely, exhibiting flexibility [46]. The elongation at the breakup parameters for samples based on these polymers comprises 400-600% [47]. Therefore, the addition of PCL, PBSA or PBAT to PLA has a positive effect on its rigidity and brittleness. In addition, when flexible PBAT and PBSA are introduced into the structure of rigid PLA, the impact strength increases, which reduces the brittleness of such materials [48]. [PERSON] et al. [49] showed an increase in the proportion and perfection of PLA crystallites in the presence of small PCL additives. An increase in impact strength parameters was shown with the introduction of small PBSA additives (10-20 wt%) [50] in PLA compared to parameter additive values, which reduces the fragility of materials. With a higher content of the plasticizing polymer in the blend, a negative deviation from the additive parameters was observed. At the same time, [PERSON] et al. [46] found that for PLA/PBSA blends, the optimal ratio was 70/30, which corresponds to increased relative elongation and impact strength. In [51], it was found that the elongation at break for PLA/PBAT mixtures increases to 600% with PBAT content of 15 wt%. A negative aspect of adding flexible PCL, PBAT and PBSA to PLA is the deterioration of optical properties, namely a decrease in the transparency of the latter [49]. To increase the miscibility of PLA and plasticizing polyesters, compatibilizers are used. Thanks to compatibilizers, chemical interactions and cross-links are formed at the polymer-polymer interface, which makes the material less defective and more homogeneous. It should be noted that compatibilizers do not affect the mutual solubility of polymers. The role of these additives is reduced to increasing the \"connectivity\" of the polymers, increasing the bond strength in the interfacial layer and, as a result, increasing the mechanical homogeneity of the mixture as a whole. Triblock and diblock copolymers from blocks of compoundable polymers, epoxystyrene-acrylic oligomers [46], as well as low molecular weight tributyl citrate [49; 52], glycidyl methacrylate (GMA), dicumyl peroxide, triphenyl phosphite, and lysine triiso-cyanateare used as compatibilizers. Such compatibilizers can enhance the effect of the plasticizer polymer. There is also the practice of using PLA copolymers with grafted compatibilizers. For example, the study described in [53] describes the compositions of PLA/PBAT/PLA-g-GMA = 70/30/(1-10) wt%. When the compatibilizer PLA-g-GMA was more than 8 wt%, the elongation at breakup increased more than 200%. Adding poly(L-lactide-block-carbonate copolymers (PLA-b-PC) to 80 PLA/20 PCL blends reduced the PCL droplet size.