Suitable processing methods include: vacuum molding, injection molding, bottle blowing machine, transparent film, plastic film, plastic film, paper coating, melt spinning and so on.
The raw materials of polylactic acid (PLA) are mainly natural raw materials such as corn, which reduces the dependence on petroleum resources and indirectly reduces the emission of pollutants such as nitrogen oxides and sulfur oxides during crude oil refining. In order to get rid of the dependence on the increasingly exhausted petroleum resources, it has become a research hotspot to develop environmentally friendly biodegradable polymers to replace petroleum-based plastic products. According to China's sustainable development strategy, using renewable resources as raw materials and biotechnology to produce biodegradable polylactic acid (PLA) has great market potential. Deep processing of grain products and production of high value-added products are important measures to realize the leap-forward development of economy.
Analysis of domestic polylactic acid market;
China is a big producer and consumer of plastic resin materials, with an annual output of nearly190,000 tons. It is imperative to vigorously develop and produce environmental protection EDP plastic products, which is conducive to reducing the environmental pollution of petroleum-based plastic products and reducing the dependence and consumption of non-renewable petroleum resources. At present, many enterprises and institutions in China are engaged in the research and application of "PLA" polyester materials, and PLA development projects are also listed as key scientific research projects in the Ninth Five-Year Plan, the Tenth Five-Year Plan, the 863, the 973, the Torch Plan, the Spark Plan, the Eleventh Five-Year Plan and the national medium-and long-term scientific and technological development plan. However, at present, the pace of PLA industrialization in China is slow. After years of research and development, only more powerful enterprises and institutions such as Zhejiang Haizheng Group and Shanghai Tongjieliang Biotechnology Co., Ltd. have achieved more results. Jiangyin Gaoxin has also developed products such as granules, fibers and non-woven fabrics. PLA polyester materials mainly rely on foreign imports. Due to the high import price of PLA raw materials, it also limits the application and development of PLA polymer materials in China.
With China's accession to the WTO, advanced production technology, equipment and new products have entered the domestic market in large quantities, which has also prompted some domestic enterprises, institutions, group companies and lactic acid production enterprises to set about establishing PLA industry, and to compete with foreign PLA products by taking advantage of the rich domestic resources, technological advantages and human resources advantages of scientific research institutions, thus successfully forming a consumer market represented by PLA products in China and earning foreign exchange through export.
Economists and environmentalists point out that the development of high-performance EDP materials, as one of the measures to control environmental pollution in China, is gradually supported by the government. The country has listed EDP plastics in the key areas of national priority development of high-tech industries (packaging materials, agricultural application materials, medical materials, etc.). ), and the development of EDP plastic packaging materials is also included in the agenda of China 2 1 century. Biomass plastics are being pushed to the market, which has great market potential in agriculture, packaging, daily use, medical care and other fields.
In 2005, the demand for plastic packaging materials in China will reach 5.5 million tons. If 1/3 is disposable plastic packaging materials and products that are difficult to collect, the waste will reach 1.8 million tons. According to the forecast of the Ministry of Agriculture, the coverage area of plastic film will reach 65.438+0.7 billion mu in 2005, and the demand for plastic film, compost bags, seedling pots, plastic film, wood chips, boxes and other agricultural and sideline products will reach 65.438+0.2 million tons. Garbage bags, construction nets, non-woven fabrics, medical and health materials and other disposable daily necessities are also difficult to collect. It is estimated that the waste will reach 4.4 million tons. If 50% of them are replaced by EDP plastics, the market demand of EDP plastics will reach 2.2 million tons. In addition, the total domestic demand for EDP plastics will reach 2.6 million tons in 2005. On the other hand, the quality of EDP plastic products in China is guaranteed and the cost is relatively low. In recent years, some countries, such as Australia, Japan and South Korea, are optimistic about the biodegradable polyolefin plastics market with high starch content in China and have come to China to discuss trade and cooperation. At present, the export volume entering the international market has reached 20,000 tons, and it is estimated that the export volume will reach 200,000 tons in 2005. Accordingly, the total demand for EDP plastics at home and abroad will reach 28 million tons in 2005, accounting for 1 1.2% of the planned total output of plastic products (250 million tons). This is basically consistent with the development trend abroad. Therefore, EDP plastics is a developing new industry with great market potential. The average annual growth rate of demand from 2005 to 20 10 is 20%, and the market demand will reach 6.9 million tons in 20 10.
According to experts' prediction, in order to realize the strategy of sustainable resource development, China has planned to establish a national biomass plastic production base. In the next 5 ~ 10 years, China will form a large-scale sales market dominated by PLA degradable plastics, with an annual output value of tens of billions of yuan. In terms of drug controlled release materials, bone fixation materials and human tissue repair materials, if we can successfully make several drug controlled release systems, bone fixation materials and minimally invasive catheter materials and enter the market, the annual output value will be at least several billion yuan. In terms of eco-fiber products, the development and production of high-quality fiber products will have a market sales space with an annual output value of 654.38+000 billion yuan. In terms of degradable plastic products, China has a larger consumer market space, and its annual sales will reach tens of billions of yuan. In terms of disposable medical supplies, if we can develop environmentally-friendly disposable medical devices that can not only self-destruct functionally but also decompose and destroy the environment, then the market space and profit will be huge and its significance will be far-reaching.
Polylactic acid (PLA) is a kind of polyester material which has no toxic effect on human body, and has good biocompatibility, biodegradability and bioabsorbability. In various pharmaceutical and biomedical applications, polylactic acid, polyglycolic acid (PGA) and lactic-glycolic acid * * * polymer (PLGA) can be degraded by enzymes or chemicals, and do not need to be removed by surgery after completing their target tasks, so they are widely used as biomedical polymer materials such as drug sustained release, surgical suture and fracture internal fixation materials. Polylactic acid is stable at room temperature, and its degradation product is lactic acid, which is an environmentally renewable resource and will not pollute the environment. It can also be used as an environmentally friendly polymer material, and can be made into films, sheets, foams, injection-molded products, hollow blow-molded bottles, etc. By using common plastic processing methods, such as extrusion, injection molding and hollow molding.
At present, there are two synthetic methods of PLA. One is the direct polycondensation of lactic acid (PC method), and the commonly used polymerization methods are melt polycondensation, melt polycondensation-solid phase polymerization and solution polycondensation. The other is ring-opening polymerization (ROP method), that is, lactide (3,6-dimethyl-1, 4- dioxane-2,5-dione) is synthesized by dehydration and cyclization of lactic acid monomer, and then high molecular weight polylactic acid is obtained by ring-opening polymerization of lactide.
Polylactic acid has great application prospects, but its physical defects, such as brittleness and slow crystallization speed, will hinder the processing and molding of PLA. There are many researches on polylactic acid and its modified products abroad. In recent years, China is also vigorously studying polylactic acid. In this paper, the synthesis methods and modification of polylactic acid in recent years are reviewed in detail.
Method for synthesize 1 polylactic acid
Direct synthesis of 1. 1 polylactic acid
1. 1. 1
Direct synthesis method is to dehydrate and condense lactic acid or lactic acid oligomer into high molecular weight polylactic acid by using efficient dehydrating agent and catalyst. Figure 1 (omitted) shows the direct synthesis process of polylactic acid. The raw material lactic acid synthesized by direct method is abundant, which greatly reduces the cost and is beneficial to the popularization of polylactic acid materials. However, the PLA obtained by this method has low molecular weight and poor mechanical properties, which inhibits the practical application of PLA obtained by this method.
The key of direct polymerization is to remove raw materials and small molecules (water) produced in the reaction process and control the reaction temperature. Because the increase of reaction temperature is conducive to the forward progress of the reaction, when the temperature is too high, the oligomer will undergo cracking cyclization and depolymerization into cyclic dimer of lactic acid-lactide Under high vacuum, water molecules are taken away, and lactide produced by depolymerization is also taken away, which promotes the reaction depolymerization and is not conducive to the production of high molecular weight polylactic acid. Therefore, on the one hand, the reaction should remove water molecules, on the other hand, it should inhibit the loss of lactide, which is the key.
1. 1.2 melt polycondensation method
The temperature of the reaction system is higher than the melting point of the polymer, and the reaction is carried out in the molten state. It is a by-product (water, lactide, etc.) formed by bulk polymerization without any medium. ) is continuously removed by inert gas or by the vacuum degree of the system. The advantages are that the product is pure, and there is no need to separate media; The disadvantage is that the relative molecular weight of the product obtained by melt polycondensation is not high. Because with the progress of the reaction, the viscosity of the system is getting bigger and bigger, and it is difficult to discharge small molecules and reach equilibrium in the polymerization direction. In the process of melt polymerization, catalyst, reaction time, reaction temperature and vacuum degree have great influence on the relative molecular weight of the product.
Ren Jie of Tongji University and others invented the method of preparing polymer polylactic acid by direct melting. Under the protection of inert gas, a chain extender containing two active functional groups is added to the polylactic acid prepolymer, one functional group is easy to react with hydroxyl group and the other functional group is easy to react with carboxyl group, such as 1, 2- epoxy octanoyl chloride, epichlorohydrin, 2,4-toluene diisocyanate and tetramethyl diisocyanate. Then polylactic acid is prepared by reactive extrusion, so that the intrinsic viscosity of polylactic acid obtained by reaction is different from that of prepolymer.
Yu Muhuo of Donghua University invented a method for preparing high molecular weight polylactic acid by melt polycondensation. Lactic acid prepolymer with carboxyl groups at both ends was prepared from lactic acid and aliphatic dibasic acid, and then a certain proportion of epoxy resin was added to prepare high molecular weight polylactic acid under certain temperature and pressure conditions. By optimizing the conditions, the polymer with viscosity-average molecular weight of 65438+300,000-220,000 can be obtained.
In the aspect of catalyst selection, the commonly used esterification catalysts are medium-strong acid H2SO4, H3PO4, etc. Transition metals and their oxides and salts, such as tin, zinc, tin dioxide, zinc oxide, stannous chloride, stannous chloride, etc. Metal organics, such as stannous octoate, triethylaluminum, etc. In our research group, lactic acid was catalyzed by rare earth oxides Y2O3, Nd2O3 and Eu2O3 which are easily separated from the product, and polylactic acid with viscosity-average molecular weight of 8. 157× 103g/mol was synthesized by direct polycondensation. In the follow-up study, poly (lactic acid) with high viscosity-average molecular weight (1.39× 104g/mol) was synthesized directly by using rare earth solid superacid so42-/TiO 2-Ce4+.
1. 1.3 melt polycondensation-solid state polymerization method
Firstly, the reactant monomer lactic acid was decompressed, dehydrated and polycondensed to synthesize low molecular weight polylactic acid, and then the prepolymer was polycondensed at a temperature higher than the glass transition temperature but lower than the melting point. In the low molecular weight lactic acid prepolymer, the macromolecular chain is partially "frozen" to form a crystalline region, while the functional group end group, the small molecular monomer and the catalyst are excluded from the amorphous region, so that sufficient energy can be obtained, and the polymerization reaction can continue through diffusion and effective collision. Small molecular by-product ice in the reaction system is carried away by vacuum or inert gas, which makes the reaction balance move in the positive direction and promotes the further increase of the molecular weight of prepolymer. Because the reaction is carried out under mild conditions, side reactions at high temperature can be avoided, thus improving the purity and quality of polylactic acid. Xing et al. firstly melted the condensed L- lactic acid to obtain low molecular weight L- lactic acid prepolymer. After isothermal crystallization, the prepolymer can not be melted under the condition of solid-state polymerization at higher temperature, and the depolymerization of PLA is greatly inhibited during solid-state polymerization. In the presence of molecular sieve, vacuum solid-state polymerization was carried out to obtain polylactic acid with weight average molecular weight of 65438+ 10,000-150,000.
1. 1.4 solution polycondensation method
Solution polycondensation is a polycondensation reaction of reactants in inert solvents. Its advantages are low reaction temperature, few side reactions and easy to obtain high molecular weight products. However, a large amount of solvent is needed in the reaction, so it is necessary to increase solvent purification and recovery equipment. Ren Jie of Tongji University and others invented a reaction device for solution polycondensation, which can realize the repeated reflux of solvent, and is not only suitable for the reaction with solvent density less than water, but also suitable for the reaction with solvent density greater than water, thus greatly reducing the reaction cost. In the reaction process, the solvent can effectively reduce the viscosity of the reaction system, absorb the heat released by the reaction, and make the reaction process stable; The solvent can dissolve the raw material monomer lactic acid, so that the growing polylactic acid can be dissolved or swollen, which is beneficial to the continuous growth reaction; The solvent can also form a * * boiling substance with the small molecular by-product water produced in the polycondensation process, and take away the small molecules in time. Zhong Wei of Fudan University used anisole as solvent to synthesize polylactic acid. Li Li et al. synthesized high molecular weight polylactic acid by boiling solution with xylene as solvent. Wang Chaoyang of South China University of Technology and others synthesized polylactic acid with diisocyanate as chain extender and tetrahydrofuran as solvent, and all obtained satisfactory results.
Ring-opening polymerization of 1.2 polylactic acid
Figure 2 (omitted) shows the process of synthesizing polylactic acid by ring-opening polymerization. Firstly, intermolecular dehydration of lactic acid generates low molecular weight polylactic acid; Then, the oligomer depolymerized at 180-230℃ to generate cyclic lactide (LA); Finally, lactide was ring-opening polymerized to produce polymer. Polylactic acid with a relative molecular weight of 700,000 ~ 6,543.8+0,000 can be obtained by this method.
There are three commonly used polymerization methods: cationic polymerization, anionic polymerization and coordination polymerization. Among them, the initiator used for cationic polymerization is protonic acid, such as RSO3H. Lewis acids, such as SnCl2, MnCl2, Sn(Oct)2, etc. Alkylation reagents, such as trifluoromethylsulfonic acid (CF3SO3CH3) and other acidic compounds. In the anionic polymerization of LA, the anionic catalysts used in the reaction generally have strong nucleophilicity and alkalinity, such as alkali metal alkoxides. Kasperczyk et al. used lithium tert-butoxide to catalyze the polymerization of rac-LA, and studied the stereocontrollability of rac-LA polymerization. The initiators commonly used in LA coordination ring-opening polymerization are tin carboxylate, aluminum isopropoxide, aluminum alkoxide or bimetallic alkoxide. Among them, stannous carboxylate, especially stannous octoate [Sn(Oct)2] has been put into industrial production, which is easy to handle and can be mixed with organic solvent and molten LA monomer in LA polymerization, so its catalytic activity is high. And stannous octoate has been recognized as a food additive by FDA.
In order to make PLA more widely used in biomedical field, scientists have developed a series of related catalysts containing bioabsorbable metals, such as Mg, Ca, Fe, Zn and other metal catalysts, which are used in the living polymerization research and industrial production of LA, especially Zn salt compounds. So far, zinc lactate is the best catalyst for LA polymerization among zinc compounds, which can control the molecular weight of PLA well, with high LA conversion and narrow PDI. Oota et al. used cyclic imines, such as succinimide, glutamine, phthalimide, etc. As a polymerization initiator in the ring-opening polymerization of lactide. Under the protection of nitrogen flow, the reaction temperature is lower (100- 190℃) and the catalyst content is lower (the mole percentage of stannous octoate is 0.000 1%-0%), thus avoiding the problem of high reaction temperature (180-230℃).
Study on Modification of Polylactic Acid
* * Modification of 2. 1 polylactic acid
e？ Answer? Frexman invented a thermoplastic polylactic acid composition toughened by random ethylene polymer containing glycidyl groups, which made the polylactic acid composition easy to melt and process into various products with acceptable toughness. An ethylene polymer refers to a polymer derived from ethylene and at least two other monomers. The * * * monomer in the modified polylactic acid can also be glycolide, bicyclic glycolate, ε-lactone, etc. The modification method of * * * polymerization is to use the similar activity and polarity of two monomers to mix them and get random * * * polymer through free radical polymerization. If the two monomers are similar in activity, but opposite in polarity, and the reactivity ratio is r 1→0 or r2→0, an alternating polymer can be obtained by free radical polymerization. Zhang Qian et al. synthesized a biomedical polymer alternating polyglycolide, which has excellent characteristics of polyglycolide (PGA) and polylactic acid.
In recent years, the preparation of block * * * polymers or grafted * * * polymers by chemical reaction of polymers has attracted people's attention. Kazuki Fukushima et al. synthesized high molecular weight stereoregular block D, L- polylactic acid: firstly, low molecular weight D- polylactic acid and L- polylactic acid were synthesized by melt polycondensation; Then PLA 1: 1 were mixed in the same melting state to form a three-dimensional composite. Finally, the amorphous polylactic acid chain was extended to regular block racemic polylactic acid with high molecular weight by cooling the molten solid complex for solid-state polymerization. The results show that the graft copolymer of starch D, L- lactide synthesized from starch and D, L- lactide can be completely degraded by acid, alkali and microorganism, and its mechanical properties are good. Because starch is rich and cheap, the cost of synthesizing graft polymer is greatly reduced, which is beneficial to the popularization of this material.
Mixed Modification of 2.2 * * Polylactic Acid
The poor mechanical properties and flexibility of polylactic acid limit its application, while other important polyesters, such as poly ε-2 caprolactone (PCL), polyethylene oxide (PEO), polyhydroxy fatty acid butyl ester (PHB) and polyglycolic acid (PGA), etc. , all have defects that limit their wide application, but * * * mixed modified materials can make up for their respective applications. * * * Mixed modified materials have the advantages of several materials, expanding the application range of polyester materials.
Xiong Huiming et al. synthesized a ternary * * * mixed polymer of L- polylactic acid (L-PLA), polystyrene (PS) and polymethylmethacrylate (PMMA) with high areal density. They first synthesized hydroxyl-functionalized PS-PMMA complex in emulsion, and then used this complex as molecular initiator and triethylaluminum as catalyst to insert L- lactide for polymerization, thus greatly improving the toughness of the polymer. Ran Xianghai invented a ternary composite polylactic acid composite material. The material is made of polylactic acid, polypropylene carbonate (PPC), poly 3- hydroxybutyrate (PHB) and various additives. The thermoplastic composite material prepared by using the ternary composite polylactic acid-based composite material as the master batch improves the formability, heat resistance, tear strength and dimensional stability of polylactic acid products.
2.3 Composite Modification of Polylactic Acid
Brittleness of polylactic acid is one of the important reasons that prevent it from being used as orthopedic fixation material. Composite modification of PLA with other materials can solve the brittleness problem of PLA.
Hydroxyapatite is a kind of colloidal calcium phosphate, which is mainly distributed in bones and teeth in human body, so it can be used as a carrier material for bone defect repair and bone tissue engineering, but the mechanical properties of hydroxyapatite alone are not suitable for bone transplantation. Hydroxyapatite (HA) is compounded with polylactic acid, and HA/PLLA composites with excellent mechanical properties can be obtained by thermal calcination, hot pressing and tape casting.
Sun Kang of Shanghai Jiaotong University invented a modified chitin fiber reinforced polylactic acid composite. The acylated modified chitin fiber prepared by wet spinning process passes through an impregnation tank containing polylactic acid glue and is wound into a weft-free prepreg by a winding machine, and then the dried and properly cut prepreg is molded. The composite material has good interface bonding and biocompatibility. Compared with polylactic acid, it reduces the degradation rate and has a better strength retention rate, which can better meet the use requirements of fracture internal fixation materials.
2.4 Plasticizing Modification of Polylactic Acid
Plasticizing polylactic acid is to improve the flexibility and impact resistance of polylactic acid by adding biocompatible plasticizer. The changes of glass transition temperature (Tg), elastic modulus and elongation at break of plasticized polylactic acid were studied by thermal analysis and mechanical properties characterization to determine the efficacy of plasticizer.
Li Boxin was mixed with diphenylmethane-4,4'-diisocyanate (MDI) in L- polylactic acid, thus improving the thermal and mechanical properties of PLA. By differential scanning calorimetry and thermogravimetric analysis, when the molar ratio of -NCO of MDI to -OH of L- PLA is 2: 1, the glass transition temperature of PLA increases from 55℃ to 64℃, and the tensile strength increases from 4.9MPa before modification to 5.8MPa.
3 Conclusion
To sum up, the research and material application of polylactic acid and its modified polymers abroad are relatively mature, and China is still in its infancy. Although polylactic acid material has the advantages of non-toxicity and environmental protection, it has not been widely used in China, mainly because the production cost of polylactic acid remains high and it has no advantage compared with similar materials in price. Therefore, the main research direction is to reduce the production cost of polylactic acid, so that this environmentally friendly material can be really applied to our lives and medical undertakings. Although high molecular weight polylactic acid can be obtained by ring-opening polymerization of lactide, the process is complicated and the cost is high. Therefore, developing a low-cost direct synthesis method of lactic acid is beneficial to the real application of polylactic acid in people's production and life. At the same time, the synthesis process of PLA will directly affect the properties of PLA. Therefore, the future research direction is mainly to optimize the synthesis conditions of PLA and find new catalysts with low toxicity, high catalytic activity and recycling. In addition, pure polylactic acid has poor mechanical properties and is easy to break, which limits its application range. Therefore, it is also a main direction of PLA research to improve its mechanical properties and thermal properties by means of * * * poly, * * blending and compounding.
Domestic research on polylactic acid mostly focuses on the synthesis of high molecular weight polylactic acid, and the molecular weight distribution of the synthesis is wide. High molecular weight polylactic acid can be used to make products with high mechanical strength, such as bone internal fixation materials; However, the carrier drug delivery system needs low molecular weight polylactic acid, so it is necessary to strengthen the research on controlled polymerization of polylactic acid, and prepare polylactic acid with narrow molecular weight range and controllable molecular weight by selecting catalyst, initiator, polymerization time, temperature and solvent, so as to expand and optimize the application of polylactic acid materials.
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