With the continuous development of social economy and the continuous growth of global population, environmental and energy problems such as serious exploitation of earth resources, global warming and depletion of oil resources are becoming more and more serious. Among them, high molecular compound products synthesized from oil and other resources are also increasingly polluting the environment in the process of production, consumption and waste, People have realized the importance of protecting the environment. Therefore, in recent years, non petroleum based degradable materials have attracted more and more attention. Among many biodegradable materials, polylactic acid, as a new environmentally friendly polymer material, has gradually attracted people's attention.

Polylactic acid (PLA), also known as polylactide, is a polyester polymerized from lactic acid. Polylactic acid has excellent biodegradability, compatibility and absorption. Polylactic acid is a non-toxic and non irritating synthetic polymer material. Its raw material is lactic acid, which mainly comes from the fermentation of starch (such as corn and rice). It can also be obtained from cellulose, kitchen waste or fish waste. PLA has a wide range of raw materials, and the products made from it can be directly composted or incinerated after use, which can finally completely reduce CO2 and H2O and meet the requirements of sustainable development. PLA is widely used because of its good transparency, certain toughness, biocompatibility and heat resistance. In addition, PLA is thermoplastic and can be used in many fields. The products prepared with it, such as packaging materials and fibers, are mainly used in disposable products, such as disposable tableware and packaging materials, car doors, footmats and seats, clothing, electrical appliances and medical hygiene (orthopedic internal fixation materials and removable surgical suture, etc.). Compared with traditional petrochemical products, the energy consumption in the production of polylactic acid is only 20% ~ 50% of petrochemical products, and the carbon dioxide produced is only 50% of petrochemical products. Therefore, the development of polylactic acid degradable materials is very necessary to alleviate the global environmental and energy problems.

1. Biodegradability

Compared with traditional plastics, polylactic acid can be degraded into CO2 and H2O by microorganisms and light. Its degradation products are non-toxic and harmless, and will not pollute the environment. The monomer for producing polylactic acid is lactic acid, which can be produced by fermentation of crops or agricultural and sideline products such as wheat, rice and sugar beet. Therefore, the raw materials for the production of polylactic acid are renewable. As a new biodegradable material, polylactic acid is widely used.

2. Biocompatibility and absorbability

Polylactic acid can be hydrolyzed by acid or enzyme to produce lactic acid in human body. As a metabolite of cells, lactic acid can be further metabolized by enzymes in the body to produce CO2 and H2O. Therefore, polylactic acid is non-toxic and harmless to human body, and has good biocompatibility and bioabsorbability. Polylactic acid has passed the certification of the U.S. Food and Drug Administration and can be used as a biomaterial implanted into human body.

3. Physical processability

As a thermoplastic polymer material, polylactic acid has good plasticity and physical processing properties, high melting point and crystallinity, good elasticity and flexibility, and excellent thermoformability. Like polypropylene (PP), polystyrene (PS) and polyphenylene oxide resin (PPO), polylactic acid materials can be extruded, stretched and injection blown.

At present, there are two main synthetic methods of polylactic acid: direct polymerization and lactide ring opening polymerization. Direct polymerization means that in the presence of dehydrating agent, the carboxyl and hydroxyl groups are removed by the activity of lactic acid molecules, so as to polycondensate between lactic acid molecules to form low molecular polymer, and then the molecules are directly condensed by the action of catalyst or high-temperature dehydration to finally obtain polylactic acid. Ring opening polymerization is also called ring opening polymerization (ROP), that is, lactide is synthesized by dehydration and cyclization of lactic acid monomer, lactide is repeatedly purified, and then the recrystallized lactide is obtained by ring opening polymerization.

1. Direct polymerization: direct polymerization mainly includes melt polymerization, solution polymerization and melt solid phase polymerization. Polylactic acid synthesized by direct polymerization is shown in the figure below:

1) Melt polymerization: under the action of catalyst, lactic acid molecules directly synthesize polylactic acid through polycondensation reaction, which is called melt polymerization. The method has the advantages of low cost, high yield and no separation of substances, but the relative molecular weight of the product is not high.

2) Solution polymerization: an organic solvent is added to the reaction system, which can dissolve the polymer but does not participate in the reaction, and can form azeotrope with the water in the reaction system. Through azeotropic reflux, the water can be continuously taken out of the reaction system to ensure the positive direction of the reaction, so as to synthesize polylactic acid, which is called solution polymerization. Solution polymerization needs a lot of solvents and is easy to pollute the environment.

3) Melt solid phase polymerization: first, lactic acid is melt polycondensated to obtain a low molecular weight prepolymer, and then the prepolymer is further polymerized under the condition that the prepolymer is higher than its glass transition temperature and lower than its melting point temperature, so as to obtain polylactic acid with higher relative molecular weight, which is called melt solid phase polymerization. The mechanism of further polymerization is that the low molecules in the amorphous region react with the end groups of macromolecules to form high molecular polymers, which significantly improves the molecular weight and crystallinity of the synthesized polylactic acid. The polylactic acid prepared by this method has high relative molecular weight, but the reaction time is long.

2. Lactide ring opening polymerization

According to different initiators and reaction mechanisms, lactide ring opening polymerization can be divided into anionic ring opening polymerization, cationic ring opening polymerization and coordination ring opening polymerization. Polylactic acid is synthesized by lactide ring opening polymerization, as shown in the figure below:

1) Anionic ring opening polymerization: the mechanism of anionic ring opening polymerization is the carbonyl carbon in lactide

Under the attack of catalyst anion nucleophile, acyloxy bond breaks to form lactone anion in the active center, which is inserted into the main chain to trigger chain growth, and finally polylactic acid is prepared. The initiators of anionic polymerization are mostly strong bases, such as sodium alcohol, potassium alcohol, butyl lithium and potassium aluminum hydride. This reaction has the advantages of high activity and high speed, but it is prone to racemic reaction and difficult to prepare high molecular weight polylactic acid.

2) Cationic ring opening polymerization: the mechanism of cationic ring opening polymerization is that the catalyst cation first reacts with the oxygen atom in the monomer to form oxonium ions. Then, the alkoxy bond breaks, and acyl positive ions are produced by single molecule ring opening reaction, which leads to chain growth, and finally polylactic acid is prepared. There are many initiators for cationic polymerization, mainly Lewis metal salts or their hydrates, such as AlCl3, ZnBr2, SnCl2, etc; Proton acids and alkylating reagents, such as p-benzenesulfonic acid, HCI, HBr, etc

3) Coordination ring opening polymerization: at present, coordination ring opening polymerization is the most widely used and studied synthetic method of polylactic acid at home and abroad. Polylactic acid synthesized by coordination ring opening polymerization has high molecular weight and strength. The mechanism is that the carbonyl oxygen on lactide is coordinated with the metal in the initiator, and the acyl oxygen bond of the monomer enters the coordination bond for chain growth. The catalysts for coordination ring opening polymerization mainly include metal alkyl compounds, such as snph4, cdet2, etc; Tin salt compounds, such as stannous octanoate, stannous isooctanoate, etc; Rare earth compounds, such as rare earth alkoxy complexes, rare earth aminates, etc.

1. Biomedical field

In the biomedical field, polylactic acid materials can be used as drug transport materials and tissue engineering scaffolds, bone repair materials, etc. The medical field is an early application field of PLA and its complexes. At present, it has carried out extensive basic research and clinical application in the fields of bone surgery, thoracic surgery, maxillofacial surgery, tumor targeted therapy and so on.

2. Industry and agriculture

Polylactic acid has good plasticity, heat resistance and physical processing performance. It can be processed into agricultural plastic film to make up for the fragile and non degradable defects of traditional plastic film. It can also be processed into accessories engineering materials, construction ropes, pesticide and fertilizer slow-release materials in the automotive industry. Comparative study of ordinary polyethylene (PE) film, 18m and 15 μ The degradability of m-thick polylactic acid film and the growth of cotton under different film mulching showed that polylactic acid film began to degrade in about 20 days, the degradation area could reach about 80% during cotton harvest, and the degraded film showed good thermal insulation performance. The effects of common PE film and polylactic acid (PLA) film on watermelon planting were compared. The experiment showed that PLA film was degradable and would not cause environmental pollution. Covering PLA film could promote the growth and development of watermelon.

3. Food packaging materials

Compared with polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP) and other materials, polylactic acid has good biodegradability, excellent antibacterial and mildew resistance. The cold fresh meat was vacuum packed with plla-pva-pcl composite film and packaging material added with nisin. The shelf life of the packaged meat was much longer than that of the cold fresh meat packed with PE fresh-keeping film, and the meat maintained relatively good color and quality. Under different gas conditions, the fresh-keeping effects of unpacked, PLA modified atmosphere packaging, PE modified atmosphere packaging, PLA vacuum packaging and PE vacuum packaging on carambola showed that PLA film packaging bag could well preserve the appearance quality and nutritional components of carambola and prolong the shelf life of carambola.