# Poly(lactic acid)-Based Blends: A Comprehensive Review
## 4 Composites of Poly(lactic acid) with Poly(butylene adipate-co-terephthalalate)  
Poly(butylene adipate-co-terephthalalate) (PBAT) is a biodegradable statistical copolymer from the class of polyesters, in particular the copolymer of adipic acid, 1,4-butanediol, and terephthalalic acid (from dimethyl terephthalalate). The structural formula of the PBAT monomer units is shown in Figure 7.  
In terms of technological and operational properties, PBAT is similar to polyethylene; however, its properties can vary in a wide range depending on the composition and molecular weight of the copolymer (a number of adipic acid and terephthalalic acid units) [57]. A modulus of elasticity increases, and an elongation at break decreases with an increase in the content of terephthalalic units [58]. It has been defined that a tensile strength of PBAT increases while the elongation at break decreases with an increase in molecular weight [59; 60]. On average, PBAT is characterized by the following parameters: a tensile strength of 20-22 MPa, an elongation at break of 600-700%, a bending strength of 7-8 MPa, and a modulus of elasticity at a bending of 120-130 MPa. The monomeric composition of PBAT has a significant effect on the rate of biological degradation (mineralization) of the material. It has been shown that the decomposition rate of PBAT under industrial compost at 60 \({}^{\circ}\)C depends on the content of terephthalalic acid units and aromatic sequences [61]. With an increase in the proportion of terephthalalic acid units, a rate of biological decomposition decreases. Longer aromatic oligomers can be biodegradable in the compost conditions only at elevated temperatures. The statistical structure of PBAT causes an absence of structural ordering, which affects a low degree of crystallinity up to its absence. A flexible-chain nature of the polymer and a low degree of crystallinity, in turn, causes a low modulus of elasticity and stiffness, but high flexibility. Precisely, the flexibility of this polymer makes it promising for mixing with other rigid polymers, for example, with PLA, to give flexibility while maintaining a complete biodegradability of the blends [62]. In addition, PBAT is used for the lamination of cardboard and paper products used as a part of the flexible films for developing highly filled composites with nano- and micro-sized particles, including jobbased fillers. The most well-known commercial trademarks of the materials based on PLA/PBAT blends are Ecolze and Ecovio, both produced by BASF Corporation. A number of works consider the problems of combining PLA and PBAT [63], as well as a mutual influence of the polymers on the structure and properties of the blends on their basis. [PERSON] et al. [64] have reported that the solubility parameters (\(\delta\)) of poly(L-lactide) (PLLA) and PBAT are 19.70 and 19.83 \(J^{0.5}/\)cm\({}^{1.5}\), respectively. The proximity of these values suggests that these two polymers are potentially miscible. According to the results of the study, the highest miscibility has been demonstrated for the blend with 25 wt.% of PBAT. However, in subsequent studies, the researchers have pointed out a limited solubility of PBAT in the PLA matrix when added up to 2.5 wt.% [63; 65]. Above this amount, it is possible to distinguish droplets (domains) separated by phases, the so-called \"sea island\" morphology [66]. [PERSON] et al. [57] have concluded that PLA/PBAT blends exhibit multi-phase behavior due to the incompatibility of the characteristics of each component. It has been shown that a high difference in solubility parameters between PLA and PBAT indicates the immiscibility of this pair of polymers [67; 68]. [PERSON] et al. [69] have reported that an average diameter of dispersed PBAT domains increased with increasing PBAT content in the blend. In addition, a fracture pattern under tension for specimens changed from brittle to ductile  
Figure 7: The chemical structure of poly(butylene adipate-co-terephthalalate). fracture with increasing PBAT content.