Polylactic acid (PLA) is one of the more mature degradable plastics in research and application. Its raw materials come from renewable plant fibers, corn, agricultural by-products, etc., and have good biodegradability. PLA has excellent mechanical properties and is similar to polypropylene plastics. It can replace PP and PET plastics in some fields and has good gloss, transparency, feel and certain antibacterial properties.
1- Production status of PLA
At present, there are two synthetic routes of PLA. One is the direct polycondensation method, the direct dehydration and polycondensation of lactic acid under high temperature and low pressure. The other is the lactide ring-opening polymerization method, which is the current mainstream production method. The PLA ring-opening polymerization method has high production efficiency, good product quality and high cost.
2- Degradability of PLA
LA is relatively stable at room temperature, but it is easy to rapidly degrade into CO2 and water in a 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 being discarded.
Factors affecting the degradation of PLA mainly include molecular weight, crystallization state, microstructure, ambient temperature and humidity, pH value, light time, and environmental microorganisms.
Blending PLA with other materials can affect the degradation rate. For example, adding some wood powder or corn stalk fiber to PLA can greatly accelerate the degradation rate.
3- Barrier properties of PLA
Barrier property refers to the ability of a material to prevent the transmission of gas and water vapor. Barrier properties are very important for packaging materials. The most common degradable plastic bags on the market are PLA/PBAT composite materials. Improving the barrier properties of PLA films can broaden the application fields.
Factors affecting the barrier properties of PLA mainly include internal factors (molecular structure and crystallization 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 property. When the stretch ratio is increased from 1 to 6.5, the PLA's crystallinity greatly increases, so the barrier properties are improved.
3) Adding some barriers (such as clay and fibers) in 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 for small molecules.
4) Coating on the surface of PLA film can improve the barrier property.
4- Mechanical properties of PLA
PLA has good strength and similar mechanical properties to PP, but it lacks toughness and is easy to bend and deform, so toughening modification is usually required.
In order to ensure the biodegradability of PLA, biodegradable resins are 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, comparable to cellophane and PET, and is especially suitable for visual packaging with good decorative effects. Generally, the transparency and gloss of PLA do not need to be improved. It should be noted that its good transparency should not be reduced as much as possible when modifying other aspects, so as to ensure its packaging visibility and decoration effect.
6- TThermal properties of PLA
The thermal stability of PLA material is equivalent to that of PVC but lower than that of PP, PE and PS. The processing temperature is generally controlled between 170~230 °C, and it is suitable for injection, stretching, extrusion, blow molding, 3D printing and other price processes.
In the actual processing process, the crystallization rate of PLA is slow and 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-fill or heat-sterilized product packaging.
In order to improve PLA crystallization rate and crystallinity, the optical purity of PLA can be increased 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 to improve its heat resistance.
7- Antibacterial properties of PLA
PLA can form a weakly acidic environment on the surface of the product and has antibacterial and antifungal effects. If it is supplemented with other antibacterial agents, it can achieve an antibacterial rate of more than 90% and can be used for the antibacterial packaging of products.
Commonly used inorganic antibacterial agents mainly include metal ions or oxides such as silver, copper, and zinc. Organic antibacterial agents commonly used in packaging include vanillin or ethyl vanillin compounds. The food safety of other antibacterial agents needs to be studied. The heat resistance of the agent is poor and the validity period is short.
8- Electrical properties of PLA
PLA can be used to prepare conductive polymer composites by filling conductive particles such as carbon black (CB), carbon nanotubes (CNTs), carbon fibers (CFs) or graphene. Conductive polymer composite materials are widely used in antistatic plastics, electromagnetic shielding materials, self-controlling heating materials, positive temperature coefficient materials and environmentally sensitive devices.
CPCS based on PLA is also degradable and biocompatible and can be used in special antistatic packaging, electromagnetic shielding packaging and smart packaging, etc. For example, PLA-based conductive polymers can be used for gas or liquid sensors to detect food quality information.
