Full Analysis of Polylactic Acid (PLA) Properties
Polylactic acid (PLA) is one of the more mature biodegradable plastics. Its raw materials come from renewable plant fibers, corn, agricultural by-products, etc., and have good biodegradability. PLA has excellent mechanical properties, similar to polypropylene plastics, and can replace PP and PET plastics in some fields.
1 Production status of PLA
At present, there are two synthetic routes for PLA. One is the direct polycondensation method, that is, the direct dehydration polycondensation of lactic acid at high temperature and low pressure. The production process is simple and the cost is low, but the molecular weight of the product is uneven, and the practical application effect is poor. The other is the lactide ring-opening polymerization method, which is the current mainstream production method.
2 Degradability of PLA
PLA is relatively stable at room temperature, but is easily degraded into CO2 and water rapidly in slightly higher temperature environment, acid-base environment, and microbial environment. Therefore, PLA products can be safely used within the validity period by controlling the environment and fillers, and can be degraded in time after disposal.
The factors affecting the degradation of PLA mainly include molecular weight, crystalline state, microstructure, environmental temperature and humidity, pH value, light time and environmental microorganisms. Blending PLA with other materials can affect the rate of degradation. For example, adding a certain amount of wood flour or corn stover fiber to PLA can greatly accelerate the degradation rate.
3 Barrier properties of PLA
Barrier refers to the ability of a material to prevent the transmission of gas and water vapor. Barrier properties are very important for packaging materials. At present, the most common degradable plastic bags on the market are PLA/PBAT composite materials. Improving the barrier properties of PLA films can broaden the field of application.
The factors affecting the barrier properties of PLA mainly include its own factors (molecular structure and crystalline state) and external factors (temperature, humidity, external force).
1) Heating the PLA film will reduce its barrier properties, so PLA is not suitable for food packaging that requires heating.
2) Stretching PLA within a certain range can increase the barrier properties. When the draw ratio is increased from 1 to 6.5, the crystallinity of PLA is greatly improved, so the barrier properties are improved.
3) Adding some barriers (such as clay and fibers) to the PLA matrix can improve the barrier properties of PLA. This is because the barrier prolongs the tortuous path of the water or gas permeation process of small molecules.
4) Coating on the surface of PLA film can improve the barrier properties.
4 Mechanical properties of PLA
PLA has good strength, but lacks toughness and is easily deformed by bending, so it usually needs to be toughened and modified. In order to ensure the biodegradability of PLA, biodegradable resin is usually used for blending and toughening modification. Substances such as PBAT, PBS, PCL, and natural rubber can improve the toughness of PLA. 5 Optical properties of PLA PLA has transparency and gloss that are rare in other degradable plastics, and is comparable to cellophane and PET. It is especially suitable for visual packaging and has a good decorative effect. Generally, the transparency and gloss of PLA do not need to be improved. It should be noted that when modifying other aspects, its good transparency should not be reduced as much as possible to ensure its packaging visibility and decorative effect.
6 Thermal properties of PLA
The thermal stability of PLA material is comparable to that of PVC, but lower than that of PP, PE and PS. The processing temperature is generally controlled between 170 and 230 ° C. It is suitable for injection, stretching, extrusion, blow molding, 3D printing and other processing processes.
In the actual processing process, the crystallization rate of PLA is relatively slow, and it generally needs to be modified. Due to the slow crystallization rate and low crystallinity, the heat distortion temperature of PLA is low, which limits its application in hot filling or heat sterilization product packaging.
In order to improve the crystallization rate and crystallinity of PLA, the optical purity of PLA can be improved as much as possible during production. Annealing treatment is also a method to improve the crystallinity of PLA. In addition, a nucleating agent can be added to improve the crystallization behavior, increase its crystallinity, and then increase the heat distortion temperature and improve its heat resistance.
7 Antibacterial properties of PLA
PLA can form a weak acid environment on the surface of the product, which has antibacterial and antifungal effects. If other antibacterial agents are used as an auxiliary, the antibacterial rate of more than 90% can be achieved, which can be used for antibacterial packaging of products.
Commonly used inorganic antibacterial agents mainly include ions or oxides of metals such as silver, copper, and zinc. Commonly used organic antibacterial agents in packaging are vanillin or ethyl vanillin compounds. The food safety of other antibacterial agents needs to be studied. Generally, organic antibacterial agents are The agent has poor heat resistance and short validity period.
8 Electrical properties of PLA
PLA can prepare conductive polymer composites by filling conductive particles such as carbon black (CB), carbon nanotubes (CNTs), carbon fiber (CF), or graphene. Conductive polymer composite materials are widely used in antistatic plastics, electromagnetic shielding materials, self-controlled temperature heating materials, positive temperature coefficient materials and environmentally sensitive devices.
The composite conductive polymer materials based on PLA also have degradability and biocompatibility, and can be used in special antistatic packaging, electromagnetic shielding packaging and smart packaging, etc. PLA-based conductive polymers can be used for gas or liquid sensors to detect food quality information.
