PLA: the most promising bio-based degradable plastic

PLA (polylactic acid) is a new type of degradable material that 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 completely degraded into CO2 and water after use and are non-toxic and non-irritating. They are widely recognized as environmentally friendly materials.

PLA has similar mechanical properties to polypropylene, and its gloss, clarity and processability are similar to polystyrene, and its processing temperature is lower than that of polyolefin. Plastic processing methods are processed into various packaging materials, fibers and non-wovens, etc., widely used in industry, agriculture, medical and civilian fields.

1. PLA production method
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, and 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; that is, 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 obtain 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 important 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.

Lactic acid has two optical isomers, L-lactic acid and D-lactic acid, due to its chiral structure. Therefore, after the polycondensation reaction, four polylactic acid polymers will be produced, 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.

Polylactic acid has high strength, high modulus, and good transparency and air permeability, but its crystallization rate is too slow during processing, resulting in a prolonged processing cycle and poor heat resistance, which greatly limits the application fields of polylactic acid products. At present, 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 improve its heat resistance.

2. Current status of 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. The main production enterprises in my country include Zhejiang Hisun Biotechnology, Anhui Fengyuan Group, Henan Jindan Technology, etc. The PLA project under construction has a production capacity of about 300,000 tons/per year, and the long-term planned production capacity is about 2 million tons/per year. With my country's Jindan Technology, Fengyuan Group, etc. relying on the accumulation of technology in the field of biological fermentation to successfully produce qualified lactide raw materials, it will provide the possibility for the rapid expansion of PLA production capacity in my country in the future.

3. PLA market prospect
PLA has a wide range of applications and has been successfully applied in the fields of 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, thereby reducing plastic pollution. PLA degradable fiber combines degradability, moisture permeability, and flame retardancy, as well as 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).

Nutrients ingested by the human body must be "processed" by enzymes before they can be converted into directly absorbed nutrients. Among the three meals a day, starchy foods are decomposed into glucose by amylase; proteins are decomposed into amino acids by protease; various lipids are decomposed into fatty acids by lipase. Otherwise, it cannot be used by the body, and it will also burden the body and cause diseases.