Melt blending of PBAT and PLA is an effective way to improve the brittleness of PLA and enhance the processing performance of PLA. However, the melt blending of PLA and PBAT is a physical modification, and the performance of the blend depends on the compatibility of the two blends. Therefore, the compatibility of PLA and PBAT is a key factor affecting the performance of composite materials.
The compatibilization and enhancement modification methods of PLA/PBAT composite materials will be introduced below.
Compatibility of PLA/PBAT composite materials
Many current studies have shown that PLA and PBAT are thermodynamically incompatible systems. However, when only a small amount of PBAT is added, PLA and PBAT will be partially compatible; when PBAT is added in excess, the phase morphology of the blend is an obvious two-phase structure.
Due to the poor compatibility of PLA and PBAT, the mechanical properties of the blend made by melt blending of PLA and PBAT are poor. In order to improve the mechanical properties of PLA/PBAT blends, it is necessary to introduce one or more compatibilizers during the melt blending process. In this way, the tension of the two-phase interface between PLA and PBAT can be reduced, the adhesion of the interface can be improved, and the compatibility of the blending system can be improved. At present, there are mainly three types of compatibilizers commonly used: plasticizers, reactive compatibilizers, and tough high-molecular polymers.
Compatibility modification method of PLA/PBAT composite material
1. Add reactive compatibilizer
PBAT and PLA belong to the polyester family, and the end groups are both hydroxyl and carboxyl groups, so they can react with the end groups of the polyester by introducing a ring-opening reaction. In addition, the resulting copolymer can improve the compatibility of the two-phase interface, that is, the reactive compatibilizer, thereby enhancing the compatibility of the blending system.
Currently, multifunctional epoxy compatibilizers (ADR), cage polysilsesquioxanes (POSS), hydrogen peroxide and isocyanate compounds are mainly used as compatibilizers.
2. Add plasticizer
Studies have found that plasticizers have a good effect on improving the flexibility of PLA, and plasticizers can also plasticize PBAT, so it can be inferred that plasticizers can improve the flexibility of PLA/PBAT blends.
Adding 0.5% tetrabutyl titanate (TBT) to PLA/PBAT, the elongation at break of the blend reaches 298%, and the impact strength can be increased by 76%. With the addition of 0.5% acetylated tributyl citrate (ATBC), the elongation at break of the blend reached 263%, and the toughness was significantly improved.
3. Add tough polymer
At present, in addition to using organic or inorganic small molecules as compatibilizers, some tough high-molecular polymers can also be added to the PLA/PBAT blend system to achieve the purpose of compatibilization. Among them, the high-molecular polymer that is commonly used as a compatibilizer is PCL.
The mechanical properties of PCL are equivalent to those of polyethylene, and it has good thermal stability, low melting point and ultra-low glass transition temperature. Therefore, in addition to the toughening effect of PLA/PBAT blends, PCL can also make the blends obtain better thermal stability and mechanical properties.
The study found that with the increase of PCL addition, the tensile strength and impact strength of composite materials showed a trend of first increasing and then decreasing. When the PCL content is 2%, the tensile strength and impact strength of the composite material reach their maximum values, which are 66.44 MPa and 6.87 kJ/m2, respectively.
Reinforced modification of PLA/PBAT composite materials
PLA/PBAT blending can absorb the advantages of PBAT’s excellent thermal stability, so that the blending system has better toughness and excellent heat resistance at the same time. However, with the addition of PBAT, the mechanical strength of PLA will be lacking, which limits the application of PLA/PBAT blends. Therefore, the blends must be enhanced and modified to improve their practicability.
Adding minerals (such as glass fiber, montmorillonite, hydroxyapatite, calcium carbonate, silica, etc.) or fiber-like natural polymer can improve the mechanical properties of PLA/PBAT composites. However, in order not to damage the biodegradability of composite materials, natural fiber reinforced modification is the most ideal modification method.
In addition, the use of inorganic nanoparticles/reactive compatibilization synergistic technology to toughen and compatibilize composite materials will be a trend in the future. It realizes the enhancement of nanoparticles and heterogeneous nucleation, and combines organic reactive compatibilization and toughening with the enhancement and promotion of crystallization of inorganic particles. The synergistic effect produced by the two makes the comprehensive performance of the material further improved.