Polylactic acid (PLA) is one of the more maturely researched and applied degradable plastics. 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, and also has good gloss, transparency, feel and certain anti-bacterial properties.
1. Current status of PLA production
At present, there are two synthetic routes for PLA, one is direct condensation method, that is, direct dehydration condensation of lactic acid at high temperature and low pressure, the production process is simple and low cost, but the product molecular quality is not uniform, and the practical application effect is poor. The other is the propylene-crosslinked ester ring-opening polymerization method, which is the current mainstream production method.
2. Degradability of PLA
PLA is more stable at room temperature, but it is easily and rapidly degraded to CO2 and water in slightly higher temperature environments, acidic and alkaline environments, and microbial environments. Therefore, it is possible to control the environment and fillers so that PLA products can be used safely during their validity period and degraded in time after disposal.
The factors affecting PLA degradation include molecular mass, crystalline state, microstructure, environmental temperature and humidity, pH, light time and environmental microorganisms. The blending of PLA with other materials can affect the degradation rate. For example, the addition of 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 the material to prevent gas and water vapor from passing through. Barrier properties are very important for packaging materials, and the most common biodegradable plastic bags on the market today are PLA/PBAT composites. Improving the barrier properties of PLA films can broaden the application areas.
The factors affecting the barrier property of PLA mainly include its own factors (molecular structure and crystalline state) and external factors (temperature, humidity, external forces).
（1）Heating PLA films reduces their barrier properties, making PLA unsuitable for food packaging that requires heat.
（2）Stretching PLA within a certain range can increase the barrier property. When the stretching ratio was increased from 1 to 6.5, the crystallinity of PLA was greatly increased, so the barrier property was improved.
（3）The addition of some blockers (e.g. clay and fiber) to the PLA matrix can improve the barrier properties of PLA. This is because the barrier extends the bending path of the water or gas permeation process of small molecules.
（4）Coating the surface of PLA films can improve the barrier properties.
4. Mechanical properties of PLA
PLA has good strength, but lack of toughness, very easy to bend deformation, usually need to be toughened modified. To guarantee the biodegradability of PLA, it is usually modified with biodegradable resins for blending toughening. substances such as PBAT, PBS, PCL, and natural rubber can improve the toughness of PLA.
5. Optical properties of PLA
PLA has the transparency and gloss of other biodegradable plastics, which is comparable to cellophane and PET, and is especially suitable for visual packaging with good decorative effect. Generally the transparency and gloss of PLA do not need to be improved, what needs to be taken care of is not to reduce its good transparency as much as possible when modified in other aspects to ensure its packaging visibility and decorative effect.
6. Thermal properties of PLA
The thermal stability of PLA material is comparable to PVC, but lower than PP, PE and PS, etc. The processing temperature is generally controlled between 170~230℃, which is suitable for injection, stretching, extrusion, blow molding, 3D printing and other processing processes.
In actual processing, PLA has a slow crystallization rate and generally requires modification. The slow crystallization rate and low crystallinity lead to low heat deflection temperature of PLA, which limits its application in hot filling or hot sterilization product packaging.
In order to improve the crystallization rate and crystallinity of PLA, the optical purity of PLA can be increased as much as possible during the production. Annealing treatment is also a way to improve the crystallinity of PLA. In addition, nucleating agents can be added to improve the crystallization behavior and increase its crystallinity, which in turn increases the heat deflection temperature and improves its heat resistance.
7. Antibacterial properties of PLA
PLA can make the product surface form a weak acidic environment, with antibacterial and anti-mold effect, if auxiliary use of other antibacterial agents can reach more than 90% antibacterial rate, can be used for antibacterial packaging of products.
Commonly used inorganic antimicrobial agents are mainly silver, copper, zinc and other metal ions or oxides, packaging commonly used organic antimicrobial agents are vanillin or ethyl vanillin compounds, other antimicrobial agents food safety to be studied, the general organic antimicrobial agents heat resistance is poor, the expiration date is short.
8. Electrical properties of PLA
PLA can be prepared as conductive polymer composites by filling with 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-temperature control heating materials, positive temperature coefficient materials and environmentally sensitive devices.
PLA-based composite conductive polymer materials are also degradable and biocompatible, and can be used for special antistatic packaging, electromagnetic shielding packaging and smart packaging, etc. PLA-based conductive polymers can be used for gas or liquid sensors to detect quality information of food products.
