PLA, a non-natural polyester, is considered one of the most promising "green plastics" due to its excellent properties, such as biocompatibility, biodegradability, and high mechanical strength. PLA can be prepared by extracting starch from renewable plants, producing lactic acid through biological fermentation, and finally through chemical synthesis. PLA has good degradability and can be completely degraded by microorganisms. Products made of PLA can be wholly degraded into CO2 and water after use and are non-toxic and non-irritating. PLA has similar mechanical properties to polypropylene. Its gloss, clarity, and processability are identical to polystyrene, and its processing temperature is lower than that of polyolefin. Plastic processing methods are processed into various packaging materials, fibers, non-wovens, etc. PLA has been widely used in disposable plastic products. In addition, PLA can be commonly used in chemical, medical, pharmaceutical, and 3D printing industries. There is growing recognition that PLA polyester will play a key role in tackling plastic pollution.
The production method of PLA
The production method of PLA can be generally divided into the direct polycondensation method and the ring-opening polymerization method (lactide method). The direct polycondensation method, also known as the PC method or one-step method, is to use the activity of lactic acid to remove carboxyl and hydroxyl groups in the presence of dehydration groups so that the lactic acid molecules can be polycondensed to form low-molecular polymers. Then the molecules can be directly dehydrated by high temperature. Condensation into PLA, one of the processes, often adopts melt polymerization, solution polymerization, and melt-solid phase polymerization, among which melt polymerization is the most widely used. The ring-opening polymerization method is also called the ROP method. The lactic acid monomer is dehydrated and cyclized first to synthesize lactide, and then the recrystallized lactide is polymerized to obtain PLA. This method can get PLA with a very high molecular weight. It is about 700,000-1 million (low-molecular-weight PLA can be rapidly degraded, which is conducive to drug release and is suitable for the medical field; high-molecular-weight PLA has crucial commercial value in the fiber, textile, plastic, and packaging industries), so it is the current industry The polylactic acid synthesis process mainly used in the world. The production process of lactide is highly demanding, especially because the purification is complex and the production scale in our country is small. Currently, my country has a small production capacity of polylactic acid, mainly because the production process requirements from lactic acid to lactide are relatively high. Lactic acid has two optical isomers, L-lactic acid, and D-lactic acid, due to its chiral structure. Therefore, four polylactic acid polymers will be produced after the polycondensation reaction, namely L-polylactic acid, D-polylactic acid, DL-polylactic acid, and meso-polylactic acid. The content of different types of polylactic acid will lead to differences in the performance and characteristics of PLA products. At present, L-lactic acid is generally used as raw material to synthesize lactide in foreign countries to further produce L-type polylactic acid, and the relative molecular weight of the product is relatively high. In China, mainly the mixture of D-lactic acid and L-lactic acid is used as the raw material, and the synthesized lactide will contain meso-lactide. This kind of meso-lactide is tough to purify during the production process, resulting in production. The rate is meager, which greatly increases the production cost of lactide. Polylactic acid has high strength, modulus, transparency, and air permeability. Still, its crystallization rate is too slow during processing, resulting in a prolonged processing cycle and poor heat resistance, significantly limiting the application fields of polylactic acid products. The most common way to improve the performance of polylactic acid is to add a nucleating agent. In actual enterprise processing applications, talc powder is the most commonly used inorganic nucleating agent for polylactic acid, which can improve the stretching, bending, etc., of polylactic acid. Mechanical properties enhance its heat resistance.
Current status of the PLA industry
At present, the global PLA production capacity is about 653,500 tons, and major PLA manufacturers are mainly concentrated in the United States, China, Thailand, Japan, and other countries. Nature Works of the United States is the largest PLA manufacturer in the world, with an annual production capacity of 180,000 tons, accounting for about 30% of the global PLA production capacity. my country's PLA production started relatively late, and the main lactide raw material mainly relies on imports. Due to technical reasons or lack of raw material lactide, some PLA units cannot operate stably or are shut down. The actual adequate production capacity is about 48,000 tons/year, and the output is about 18,000 tons/year.
PLA Market Outlook:
PLA has a wide range of applications and has been successfully applied in plastic packaging, biomedicine, and textile fibers. The harmless properties of PLA make it widely used in the field of packaging, mainly for food packaging, product packaging, and agricultural mulch. PLA has a smooth surface, good transparency, and excellent barrier properties. It can completely replace PS (polystyrene) and PET (polyethylene terephthalate) in many places, reducing plastic pollution. PLA degradable fiber combines degradability, moisture permeability, flame retardancy, molding, application, and degradability and is widely used in the field of textile fibers. At the same time, PLA has excellent biocompatibility and good physical properties. After its degradation, it generates carbon dioxide and water, which is harmless to the human body and can be degraded naturally. Therefore, it is increasingly used in the field of biomedicine, such as tissue consolidation (such as bone screws, fixation plates, and plugs), wound dressing (e.g., artificial skin), drug delivery (e.g., diffusion control), and wound closure (e.g., application of sutures). Polylactic acid does not produce pollution in the production process and can be recycled in nature. It is an ideal green polymer material. At the same time, it can also be used in 3D printing extrusion, injection molding, film drawing, and other processing methods. It is also widely used in food packaging and industrial and agricultural fabrics.
