October 25, 2020

What is biodegradable polylactic acid? What role does it play in plastics manufacturing?

1. Polylactic acid

Polylactic acid (PLa) is a synthetic polymer material with excellent biocompatibility and biodegradability. PLa, a linear thermoplastic biodegradable aliphatic polyester, is derived from starch extracted from some plants such as corn, wheat, and cassava. It is enzymatically decomposed to obtain glucose, which is then fermented by lactic acid bacteria to become lactic acid, and then chemically synthesized. A high purity polylactic acid is obtained. After being discarded, the polylactic acid product will be completely decomposed into CO2 and H2O under the action of microorganisms, water, acid and alkali within 30 days, and then become the starting material of starch under sun photosynthesis. The environment is polluted and is therefore a completely natural circulating biodegradable material.

1.1 Preparation of polylactic acid

At present, there are two main routes for the production and preparation of polylactic acid: (1) indirect method, ie, lactide ring-opening polymerization (ROP method); (2) direct polymerization (PC method). Both types of methods use lactic acid as a raw material. The lactide ring-opening polymerization method firstly polycondenses lactic acid into an oligomer, and the oligomer undergoes intramolecular transesterification under conditions of high temperature and high vacuum, and depolymerizes into a cyclic dimer 2 lactide of lactic acid. The lactide is subjected to ring-opening polymerization to obtain polylactic acid, and high purity lactide is required in this method. The direct method uses a high-efficiency dehydrating agent to dehydrate the lactic acid or its oligomer, and prepares the polylactic acid by bulk or solution polymerization.

1.2 Basic properties of polylactic acid

Since lactic acid has optical activity, there are three corresponding polylactic acids: PDLa, PLLa, PDLLa (racemic). Commonly available are PDLLa and PLLa, which are obtained by the racemic or left-handed system of lactic acid or lactide, respectively.

Polylactic acid (PLa) is a kind of real bioplastic. It is non-toxic, non-irritating, has good biocompatibility, can be biodegraded and absorbed, has high strength, does not pollute the environment, has good plasticity and is easy to be processed. Due to its excellent biocompatibility, polylactic acid can participate in human metabolism. It has been approved by the US Food and Drug Administration (FDa) and can be used as medical surgical sutures, injection capsules, microspheres and implants.

At the same time, the disadvantages of polylactic acid are: (1) polylactic acid has a large number of ester bonds, poor hydrophilicity, and reduced its biocompatibility with other substances; (2) the relative molecular weight distribution of the product obtained by polymerization is too wide, Lactic acid itself is a linear polymer, which makes the strength of polylactic acid materials often fail to meet the requirements, high brittleness, low heat distortion temperature (54 ° C under 0146 MPa load), poor impact resistance; (3) difficult to control the degradation cycle; 4) The price is too expensive, the price of lactic acid and the polymerization process determine the cost of PLA is higher. This has prompted people to conduct in-depth research on the modification of polylactic acid.

2. Modification of polylactic acid

Due to the above-mentioned shortcomings of polylactic acid, the mechanical properties of polylactic acid are improved by modifying, copolymerizing, blending and compounding polylactic acid, and the hydrophilicity is improved, and the degradation property is not affected. This will better meet biomedical and environmentally friendly applications.

2.1 Plasticization modification

At present, biocompatible plasticizers such as citrate ether, glucose monoether, partial fatty acid ether, oligomeric polyethylene glycol (PEG), oligomeric polylactic acid (OLa), glycerol are widely studied. Improve the flexibility and impact resistance of polylactic acid. Thermal analysis and mechanical property characterization of the plasticized polylactic acid were carried out to study the changes in glass transition temperature (Tg), elastic modulus, elongation at break, etc., to determine the effectiveness of the plasticizer. A large number of studies have shown that the more effective plasticizer is OLa and low molecular weight PEG (PEG400). Adding 20% ​​(wt) PEG400 and OLa can reduce the glass transition temperature of polylactic acid from 58 °C to 12 ° C and 18 ° C.

2.2 Copolymerization modification

Copolymerization is currently the most studied method to improve the flexibility and elasticity of polylactic acid. Its main purpose is to introduce another molecular chain into the main chain of polylactic acid, which reduces the regularity and crystallinity of PLLa macromolecular chain. At present, the copolymerization modification of polylactic acid can be mainly divided into the following aspects:

2.2.1 Copolymer of lactide and glycolide Polyglycolide (PGa) is the simplest linear aliphatic polyester. As early as 1970, PGD sutures were commercialized as "Dexon", but PGA was hydrophilic. It has good properties and degrades too fast. At present, monomeric lactic acid or lactide is copolymerized with glycolic acid or glycolide to obtain an amorphous rubber-like ductile material. The rate of degradation of the material can be controlled by adjusting the ratio of LLaPGa, which has been obtained as a surgical suture. For clinical applications, copolymers of L2 lactide and glycolide Ga have been commercialized.

2.2.2 Block copolymer of polylactic acid and polyethylene glycol (PEG) Polyethylene glycol (PEG) is the simplest low polyether macromolecule with excellent biocompatibility and blood compatibility, hydrophilic Sex and softness. Zhu Kangjie et al. synthesized a triblock copolymer of PLa2PEG2PLa by ring-opening polymerization under the conditions of stannous octoate as a catalyst. Such block copolymers have hydrophilic PEG segments and hydrophobic PLa segments. By changing the copolymer composition, the hydrophilic and hydrophobic properties and degradation rate of the materials can be greatly adjusted [7]. Ge Jianhua et al. [8] of South China University of Technology and Co., Ltd. [8] copolymerized a biodegradable polymer polylactic acid with a polyethylene glycol having a hydrophilic segment to obtain a block copolymer. Under certain reaction conditions, the contact angle of the material was The 46° drop to 10° to 23° significantly improves the hydrophilicity of the polylactic acid material.

2.2.3 Co-polymerization of lactide and caprolactone (CL) Poly(ε2 caprolactone) (PCL) is a biomedical polymer with good biocompatibility and degradability, which degrades faster than polylactic acid. Slowly, LaPCL block copolymers have been prepared to achieve controlled degradation rates. LaPCL block copolymers have received extensive attention in recent years due to their excellent biodegradability and biocompatibility, and are mainly used in the biomedical field. Jeon et al. prepared poly L2 lactide and poly(ε2 caprolactone) multi-block copolymers to further improve their processing and degradation properties.

2.2.4 Lactide Copolymerized with ether segment and cyclic ester ether Polyether polymer has excellent blood compatibility, but its water solubility is too large to limit its application. Polypropylene glycol and ethylene oxide addition polymer (PEO2PPO2PEO) (polyether) Pluronic has been approved by the US Food and Drug Administration for food additives and pharmaceutical ingredients, Xiong et al. successfully grafted PLA to Pluronic copolymer The amphiphilic P(La2b2EO2b2PO2b2EO2b2La) block copolymer containing short PLa segments was obtained, and the results show that the block copolymer retains the thermal responsiveness of the original Pluronic system and is effectively introduced by the introduction of the PLa segment. The critical micelle concentration was reduced, and a hydrophilic drug was used as a model to observe sustainable release, which is highly promising for drug controlled release. In addition, the hydrophilicity of lactide and a cyclic ester ether such as p-dioxanone, which is a surgical suture material having excellent flexibility and elasticity, can be improved.

2.2.5L2 lactide copolymerized with starch Chen et al. synthesized a starch grafted poly L2 lactic acid copolymer. This graft copolymer can be directly used in starch 2 poly(ε2 caprolactone) and starch 2 polylactic acid blends. Thermoplastic and two-phase compatibilizer. Tu Kehua and other studies found that starch 2 polylactic acid graft copolymer can effectively increase the compatibility of starch and polylactic acid, thereby improving the water resistance and mechanical properties of the blend system.

2.2.6 Other He, etc. The natural metabolite containing double bond, malic acid (hydroxybutyric acid, maleic acid), is introduced into the main chain or side chain of the polylactic acid macromolecule to obtain both degradability, mechanical properties and reaction. Sexual functional materials that can be used as targeting and controlled release carriers as well as scaffold materials for tissue and cell engineering.

Luo Yanfeng of Chongqing University synthesized a new modified polylactic acid (BMPLa) based on maleic anhydride modified polylactic acid (MPLa) to improve the hydrophilicity of polylactic acid and completely overcome the modification of polylactic acid and maleic anhydride. The polylactic acid is acidic during degradation and provides active groups for further introduction of biologically active molecules such as polypeptides and collagen. Butane diamine modified polylactic acid is expected to have excellent cell affinity and has important application potential in tissue engineering.

Wu et al. synthesized a novel amphiphilic chitosan polylactide graft copolymer, which can form a core-shell micelle with a hydrophobic polylactide segment as the core and a hydrophilic chitosan segment as the outer shell in an aqueous medium. structure. It is expected to be used for trapping and controlled release of hydrophobic drugs. Luo et al. synthesized a novel amphoteric diblock copolymer of low molecular weight poly N2 vinylpyrrolidone (PVP) and poly D, L2 lactide. This diblock copolymer can self-assemble into micelles in aqueous solution and is expected to be used. A pharmaceutical carrier for parenteral injection of a drug.

Breitenbach et al. grafted a copolymer of polylactic acid and ethylene glycol (PLG) onto hydrophilic polyvinyl alcohol (PVa) to obtain biodegradable comb-shaped polyester PVa2g2PLG, which regulates the length, composition and PVa molecular weight of PLG. It can effectively control the degradation rate, avoid the hydrophobic polymer to denature hydrophilic macromolecular drugs, and can be used for the parenteral drug delivery system of hydrophilic macromolecular drug proteins, peptides and low (poly)nucleotides.

Lo et al. synthesized a graft copolymer of poly-DL lactide with a core-shell structure and a copolymer of poly N2 isopropyl acrylamide and methacrylic acid [PLa2g2P(NIPam2co2Maa)], which is temperature sensitive and pH sensitive. A pharmaceutical carrier that can be used for intracellular delivery of anticancer drugs.

2.3 Blending modification

The most common and important biodegradable polymers are aliphatic polyesters such as polylactic acid (PLa), poly(ε2 caprolactone) (PCL), polyethylene oxide (PEO), polyhydroxyalkanoate (PHB). , polyglycolic acid (PGa). However, any one of them has some shortcomings that limit its application. Blending modification is another effective way to improve the mechanical and processing properties of materials and reduce the cost of PLa. The preparation method of the blend sample is widely used in the following ways: melt blending method, solution casting film forming method, dissolved P sedimentation method, water as a foaming agent, and a single screw or twin screw extruder to prepare a foaming material.

Polylactic acid and another type of biodegradable polymer such as polyhydroxyalkanoate (PHa) synthesized by microorganisms, chemically synthesized poly(ε2 caprolactone) (PCL), polyethylene oxide (PEO), poly N2 vinyl Pyrrolidone (PVP), soluble calcium phosphate glass particles, dextran and natural polymer starch constitute a fully biodegradable blending system, which is committed to fundamentally solve the environmental pollution problems caused by plastic consumption. The other type, polylactic acid and non-biodegradable polymers such as polyurethane, polystyrene [, polyisoprene glycol grafted polyvinyl acetate copolymer rubber, p-vinyl phenol (PVPh), polymethyl methacrylate ( PMMa) [30], polymethyl acrylate (PMa), linear low density polyethylene (LLDPE) components of biodegradable blends, such systems can not fundamentally solve environmental pollution problems.

2.4 Compound modification

The combination of polylactic acid and other materials is aimed at solving the problem of brittleness of polylactic acid, and achieving the purpose of enhancement, so that it can be used as a material for internal fixation of fractures. Currently can be divided into the following composite systems:

2.4.1 Polylactic acid and fiber composite The polylactic acid matrix and polylactic acid fiber are molded by fiber bundling to obtain polylactic acid self-reinforcing material; the carbon fiber reinforced PLLa composite material has an initial bending strength of 412 MPa and a modulus of 124 GPa, which is equivalent. Carrying capacity; Oksman and other natural linen fiber reinforced PLA, compared with the traditional polypropylene P linen composite, the preparation method is similar, but the composite strength is much better than PPP linen composite; Shi Zongli and other preparation can be arbitrarily controlled degradation The calcium polyphosphate (Calcium Polyphosphate CPP) fiber with good mechanical properties, compatibility and toxicological properties was then developed. The CPPPPLLa cartilage tissue engineering three-dimensional connected microporous scaffold composite was developed with the fiber as a reinforcing material. Sun Kang, Shanghai Jiaotong University, developed a modified chitin fiber reinforced polylactic acid composite material, in which acylation modification can effectively improve the solubility and meltability of chitin derivatives, and the composite interface is well combined, which reduces the degradation rate of Pla. And it has better strength retention, which can better meet the application of fracture internal fixation materials.

2.4.2 Polylactic acid and hydroxyapatite composite hydroxyapatite (HaP) is the basic component of human bones, tightly binds to collagen and cells, connects soft and hard tissues, and guides the growth of bone, but made porous

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