Full Analysis of the Performance of Polylactic Acid

Polylactic acid (PLA) is one of the biodegradable plastics with mature research and application. Its raw materials come from renewable plant fibers, corn, agricultural by-products, etc., which have good biodegradability. PLA has excellent mechanical properties, similar to polypropylene plastics, and can replace PP and PET plastics in some fields. At the same time, it has good gloss, transparency, feel and certain antibacterial property.

1. Current status of PLA production

At present, there are two synthesis routes of PLA, one is direct condensation, that is, lactic acid is directly dehydrated and condensed under high temperature and low pressure. The production process is simple and low cost, but the product molecular mass is uneven, and the practical application effect is poor. The other is the ring opening polymerization of lactide, which is the mainstream production method at present.

2. Degradability of PLA

PLA is stable at room temperature, but it is easily and rapidly degraded into CO2 and water in slightly higher temperature environments, acid-base environments, and microbial environments. Therefore, by controlling the environment and fillers, PLA products can be safely used within the validity period and degraded in time after disposal.

The factors affecting the degradation of PLA mainly include molecular mass, crystalline state, microstructure, environmental temperature and humidity, pH value, light time, and environmental microorganisms. Blending of PLA with other materials can affect the degradation rate. For example, adding a certain amount of wood flour or corn straw fiber to PLA can greatly accelerate the degradation rate.

3. The barrier property of PLA

Barrier refers to the ability of a material to prevent gas and water vapor from passing through. Barrier is very important for packaging materials. At present, the most common biodegradable plastic bag on the market is PLA/PBAT composite. The improved barrier properties of PLA films can widen the application field. The factors affecting the barrier properties of PLA mainly include internal factors (molecular structure and crystalline state) and external factors (temperature, humidity, and external force).

1) Heating the PLA film will reduce its barrier property, so PLA is not suitable for food packaging that requires heating.

2) PLA can be stretched in a certain range to increase the barrier property. When the stretch ratio is increased from 1 to 6.5, the crystallinity of PLA is greatly increased and therefore the barrier property is improved.

3) Adding some barriers (such as clay and fibers) to the PLA matrix can improve the barrier property of PLA. This is because the barrier prolongs the curved path of the water or gas permeation process of small molecules.

4) Coating on the surface of PLA film can improve the barrier property.

4. Mechanical properties of PLA

PLA has good strength, but it lacks toughness and is extremely susceptible to bending and deformation, which usually requires toughening modification. In order to ensure the biodegradability of PLA, it is usually toughened by blending with biodegradable resin. PBAT, PBS, PCL, natural rubber and other substances can improve the toughness of PLA.

5. Optical properties of PLA

PLA has a transparency and gloss that are rare in other degradable plastics, which is comparable to cellophene and PET. It is especially suitable for visual packaging, and the decoration effect is better. In general, the transparency and gloss of PLA do not need to be improved, and attention should be paid to not reduce its good transparency as much as possible when modifying other aspects to ensure its packaging visibility and decoration effect.

6. Thermal 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 ℃ and 230℃, which is suitable for injection, stretching, extrusion, blow molding, 3D printing and other processing processes.

In the actual processing process, the PLA crystallization rate is slow and generally requires modification. Due to the slow crystallization rate and low crystallinity, the thermal deformation temperature of PLA is low, which limits its application in hot filling or heat sterilization product packaging.

In order to increase the PLA crystallization rate and crystallinity, the optical purity of PLA can be increased as much as possible at the time of production. Annealing treatment is also a method to improve the crystallinity of PLA. In addition, nucleating agents can be added to improve the crystallization behavior and crystallinity, thereby increasing the thermal deformation temperature and improving its heat resistance.

7. Antibacterial properties of PLA

PLA can make the surface of the product form a weakly acidic environment, and has antibacterial and mild-proof effects. If the auxiliary use of other antibacterial agents can achieve more than 90% antibacterial rate, it can be used for antibacterial packaging of products.

The commonly used inorganic antibacterial agents mainly include metal ions or oxides such as silver, copper, and zinc. The commonly used organic antibacterial agents for packaging include vanillin or ethyl vanillin compounds. The food safety of other antibacterial agents needs to be studied.

8. Electrical properties of PLA

PLA can be prepared as conductive polymer composites by filling conductive particles such as carbon black (CB), carbon nanotubes (CNTs), carbon fibers (CF), or graphene. Conductive polymer composites are widely used in antistatic plastics, electromagnetic shielding materials, self-controlled temperature heating materials, positive temperature coefficient materials and environmentally sensitive devices.

Pla-based conductive polymer composites also have degradability and biocompatibility, which can be used in special antistatic packaging, electromagnetic shielding packaging and intelligent packaging. Pla-based conductive polymer can be used for gas or liquid sensors to detect the quality information of food.