Study on Modification of Poly (lactic acid) PLA: Toughening, Barrier, Transparent
1. Study on modification of polylactic acid
Currently, there are two main methods for toughening and modifying polylactic acid (PLA): chemical copolymerization and physical blending. However, because chemical copolymerization requires harsh experimental conditions, complex experimental operations, and difficult industrial production, while physical blending only requires low experimental equipment and simple operations, with high economic efficiency, physical blending is easier to achieve large-scale industrial production.
Blending modification is the blending of lactic acid with other compounds or polymer materials. The principle is to introduce polymers containing polar and rigid groups into PLA to improve the interaction between polymer chains, thereby improving the performance of materials. Blending agents are mainly biodegradable high molecular materials because other compounds with high glass transition temperatures and high heat resistant polymer materials cannot be degraded. Due to the relatively simple operation method of blending modification, it has become the main method of PLA modification at present.
Study on Toughening Modification
Polylactic acid (PLA) is a hard and brittle polymer material with high tensile strength, low elongation at break, and low impact resistance. Molecular weight is an important parameter that determines a material. The greater the molecular weight, the greater the mechanical strength of PLA. Shady doubled the molecular weight of PLA and found that its tensile modulus also doubled, while its tensile strength increased several times. The higher L-type content in the film results in higher tensile strength. Farrer's research found that the yield elongation of 98% of L-shaped PLLA is higher than that of 94% of L-shaped PLLA, and the breaking elongation of 94% of L-shaped PLLA is 7 times higher than that of 98% of L-shaped PLLA, indicating that 94% of L-PLLA is more plastic.
However, when PLLA is used as a plastic film, its brittleness makes it impractical. The optical purity of PLLA is closely related to the performance of the product. Pure PLLA has a crystallinity of about 60%, a melting point of 180 ° C, a glass transition temperature of 60 ° C, a tensile strength of more than 60MPa, a tensile modulus of more than 3GPa, and an elongation at break of about 3-6%, presenting characteristics such as rigidity and vulnerability to brittle fracture.
Blending modification is one of the most effective ways to improve the brittleness of PLLA. Qin Yuyue et al. added 2% - 10% natural plant antibacterial essential oil such as clove essential oil, cinnamon essential oil, fennel essential oil and lemon grass essential oil to PLA, and then blended it with poly (trimethylene carbonate) phenol. The tensile strength of the prepared PLA blend film can reach 11-27MPa, and the elongation at break can reach 120% - 200%. The antibacterial rates against Escherichia coli and Staphylococcus aureus exceeded 90% and 95%, respectively. It is a high-performance antibacterial packaging material.
Study on barrier modification
Poly (lactic acid) (PLA) has low barrier properties to water vapor, oxygen, carbon dioxide, and nitrogen, and is not used in food packaging with high barrier requirements. However, its good air permeability and stable water vapor permeability just meet the standards for modified atmosphere packaging of fruits and vegetables. In order to study the gas barrier property of PLLA, Auras et al. measured that the water vapor permeability coefficient of PLLA film was reduced to 1.34 × 10-11g.m/m2.s.Pa。
When a chitosan/nano clay blend was coated on the surface of PLLA, the oxygen transmittance per unit time decreased from 736 cc/m2. d to 4 cc/m2.24 h, and the water vapor transmittance also decreased from 556 g/m ²• It decreased to 431 g/m2.24 h in 24 h. At the same time, the content of D-lactic acid also has a certain impact on the permeability of PLLA films. The presence of a small amount of D-lactic acid can promote the formation of regular molecular structures in PLLA. The molecular chain arrangement is more orderly, and the permeability of PLLA will decrease.
Study on transparency modification
PLA has a rare transparency and glossiness of other degradable plastics, comparable to cellophane and polyester (PET), and is particularly suitable for use as visual packaging, with good decoration effects. The transparency and glossiness of a single polylactic acid material do not need to be improved, but polylactic acid is prone to poor interfacial compatibility during toughening modification, resulting in a decrease in the transparency of polylactic acid blends. Therefore, maintaining the original transparency based on the toughening and modification of polylactic acid is a problem worthy of attention.
One of the main factors affecting the transparency of blends is interfacial compatibility. Good interfacial compatibility is beneficial to improving the transparency of the blends. In fully compatible polymer blends, they can be considered isotropic materials due to the formation of homogeneous phases. If the two component polymer is transparent, the blend is transparent. In incompatible (partially compatible) polymer blends, phase separation generally occurs, in which one polymer is distributed as a dispersed phase in the continuous phase of another polymer. If the refractive indices of the two polymers are different, the resulting interface generates light scattering, and even if the two homopolymers are transparent, their blends are also opaque.
The solution is to add compatibilizers and compatibilizers to the blend or adjust the refractive index of the polymer through copolymerization or grafting. Li Zhaoxin prepared a graft copolymer (iPP-g-PLLA copolymer) of polypropylene (PP) and poly (PLLA) with high graft density for toughening PLA. The semi-crystalline or rubbery main chain of iPP is designed to improve the toughness of the copolymer and maintain high strength, while the grafted PLLA branch chain is designed to ensure high compatibility with the PLA matrix and improve phase boundary properties through the ion aggregation of imidazole, The resulting modified PLA blend exhibits significantly improved elongation at break while maintaining very high strength and excellent transparency.
