Three Modification Methods of Polylactic Acid PLA
At present, there are two main methods of 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 requires only low experimental equipment, simple operations, and high economic efficiency, physical blending is easier to achieve on a large scale industrial production.
Blending modification is to blend 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, so as to improve the durability of the material. performance. The blending agent is mainly a degradable polymer material, because other high glass transition temperature compounds and high heat-resistant polymer materials cannot be degraded. Because the operation method of blending modification is relatively simple, it has become the main method of PLA modification at present.
Research 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 to determine the material, the larger 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-fold. Higher L-form content in the film results in higher tensile strength. Farrer's research found that the yield elongation of 98% L-shaped PLLA is higher than that of 94% L-shaped PLLA, and the elongation at break of 94% L-shaped PLLA is 7 times that of 98% L-shaped PLLA, which shows that 94% L-PLLA is more plastic.
But 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. The crystallinity of pure PLLA is about 60%, the melting point is 180°C, the glass transition temperature is 60°C, the tensile strength is higher than 60MPa, and the tensile modulus is higher than 3GPa. The elongation is about 3 to 6%, showing the characteristics of rigidity and brittle fracture.
In order to improve the brittleness of PLLA, blending modification is one of the most effective ways. Qin Yuyue and others added natural plant antibacterial essential oils such as clove essential oil, cinnamon essential oil, fennel essential oil and lemongrass essential oil with a mass fraction of 2%-10% to PLA, and then blended them with polytrimethylene carbonate , The tensile strength of the prepared polylactic acid blend film can reach 11-27MPa, and the elongation at break can reach 120%-200%. The antibacterial rates against Escherichia coli and Staphylococcus aureus are over 90% and 95% respectively. It is a high-performance antibacterial packaging material.
Barrier Modification Research
Polylactic acid (PLA) has low barrier properties to water vapor, oxygen, carbon dioxide and nitrogen, so it is not used in food packaging that requires high barrier properties. However, its good air permeability and stable water vapor permeability just meet the standards of modified atmosphere packaging for fruits and vegetables. In order to study the barrier property of PLLA to gas, Auras et al. measured that the water vapor permeability coefficient of PLLA film was reduced to 1.34×10-11g.m/m2.s.Pa.
Coating the chitosan/nanoclay blend on the surface of PLLA, the oxygen transmission rate per unit time is reduced from 736cc/m2.d to 4cc/m2.24h, and the water vapor transmission rate is also reduced from 556g/m²·24h 431g/m2.24h. At the same time, the content of D-lactic acid also has a certain influence on the air permeability of the PLLA film. The presence of a small amount of D-lactic acid can promote the formation of regular molecular structure of PLLA. The molecular chain arrangement is more orderly, and the gas permeability of PLLA will be reduced.
Transparent Modification Research
PLA has the rare transparency and glossiness of other degradable plastics, comparable to cellophane and polyester (PET), and is especially suitable for visual packaging, with better decorative effects. The transparency and gloss of a single polylactic acid material do not need to be improved, but polylactic acid is prone to poor interfacial compatibility during toughening modification, which leads to a decrease in the transparency of polylactic acid blends. Therefore, on the basis of the toughening modification of PLA, maintaining the original transparency is a problem worthy of attention.
One of the main factors affecting the clarity of the blend is interfacial compatibility. Good interfacial compatibility is beneficial to improve the transparency of the blend. In a fully compatible polymer blend, it can be considered an isotropic material because a homogeneous phase is formed. If the two-part polymers are clear, the blend is clear. In immiscible (partially compatible) polymer blends, there is generally a phase separation situation where one polymer is distributed as a dispersed phase in a continuous phase of the other polymer. If the refractive indices of the two polymers are different, the resulting interface will scatter light, and even if the two homopolymers are transparent individually, their blends will be opaque.
The solution is to add compatibilizer, compatibilizer to the blend or adjust the refractive index of the polymer by copolymerization or grafting. Li Zhaoxin prepared a graft copolymer of polypropylene (PP) and polyPLLA (iPP-g-PLLA copolymer) with high graft density for PLA toughening, where the semi-crystalline or rubbery backbone of iPP was designed to enhance The copolymer is tough and maintains high strength, while the grafted PLLA branches are designed to ensure high compatibility with the PLA matrix and improve the phase boundary properties through the ionic aggregation of imidazole, the resulting modified PLA blend Exhibits significantly improved elongation at break while maintaining very high strength and excellent clarity.
Liu et al. successfully introduced PDLA into PLLA/PMMA blends, and formed stereopolylactic acid (SC-PLA) in situ in PLLA. The study found that SC-PLA was uniformly dispersed in PMMA, with an average diameter of 720nm to 760nm. When the SC-PLA content is higher than 20wt%, agglomeration occurs, and the transparency decreases to about 80%. It can be found that polymers with a similar refractive index can be added in large quantities and have little effect on the transparency of the material. The material can be modified to adjust the refractive index, so that the refractive index between the mixed materials is similar, thereby improving the transparency of the blend.
