Progress in Biodegradable Plastics (2)
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At present, aliphatic polyesters have not been able to be used alone as plastic products because their melting points are mostly lower than 100°C. Under normal circumstances, the melting point of polyester is only higher than 100°C. After processing and molding, the obtained plastic products have the use value; only the melting points of several aliphatic polyesters are higher than 100°C. They are poly(oxalate). Alcohol ester) melting point 159°C, poly(butylene oxalate) melting point 103°C, poly(neopentyl glycol oxalate) melting point 111°C, poly(ethylene glycol succinate) melting point 102°C, poly(butylene) Butylene glycol ester) melting point 113°C. The thermal stability of polyoxalates is lower, the molecular weight of the polyester obtained by direct polycondensation does not exceed 5000, while succinic acid-based polyesters are much more stable and can obtain higher molecular weights. Such polyesters are already in Showa, Japan. Highpolymer company put into production, the annual output of 3000t, trade name BIONOLLE, about its synthesis method, performance measurement and application of foreign development has been a lot of reports abroad, but domestic research on this area is still very little, this is a great potential for development Research direction.
There are two main methods for synthesizing biodegradable polyesters, namely microbial fermentation and chemical synthesis. The use of microbial fermentation method is currently mainly used to synthesize poly(amino acid esters), such as poly(3-hydroxybutyrate)[P(3HB)], P(3HB-co-3HV) and 3-hydroxybutyric acid. Copolyester of 4-hydroxybutyric acid [P(3HB-co-4HB)]. Chemical methods mainly include condensation polymerization and ring-opening polymerization. Molecular methods can be chemically designed to synthesize biodegradable polyesters of various structures. For example, P(3HB) and P(3HB-co-4HB) can also be synthesized by ring-opening polymerization. The polymer composition and stereoregularity can be achieved. Sex and microbiological fermentation methods are exactly the same. Biosynthetic costs are higher, and condensation polymerization has long been industrialized. Various conditions have been well established. The raw materials used are also petroleum products, and the cost is low. Chemical synthesis of biodegradable polyester has the following progress:
1. Condensation polymerization is mainly used for the synthesis of aliphatic polyesters. Polycondensation reactions are often performed at higher temperatures. If the polyester obtained by the polycondensation method has a low molecular weight, the molecular weight thereof must be further increased to obtain a polymer material having good properties. Increasing the molecular weight is often achieved by the chain extension reaction. The reactive group of the chain extender reacts with the hydroxyl end or carboxyl group of the polyester to increase the molecular weight of the polyester. The chain extenders of hydroxy-terminated polyesters include dianhydride and diisocyanate; carboxy-terminated polyesters can be used as chain extenders such as oxazolidine, aziridine derivatives, diepoxy compounds, and divalent metal ions. It has been reported in the literature that after the condensation of polybutylene succinate-1,4-butanediol ester (PBS) to a certain molecular weight, a certain amount of diisocyanate is added, the viscosity, ie, the molecular weight, will be rapidly increased without gelation. .
PBS is a thermoplastic aliphatic polyester and is a semi-crystalline linear polymer. Due to the flexibility of the molecular structure and excellent processing performance, it is difficult to achieve the use requirements of homopolymer properties, and copolymerization or blending methods are generally adopted to overcome this defect.
1 Synthetic initial addition of comonomer to synthesize random copolymer Masatsugu et al. found that the synthesis of butanediol succinate and ethylene succinate copolymer P (BS-co-ES) has the same dimorphism The two monomers are similar in structure, similar in volume and compatible. With the increase of ethylene glycol content, the glass transition temperature of the copolymer increases, the melting point has a lowest point, and the crystal index has a lowest point. In the crystallization process, PBS forms a monoclinic system, PES is a positive commutator cell, and a low content component grows in a high content component lattice. Biodegradability is best when the ES content is about 50%. M-Nagata et al. synthesized butanediol succinate and butylene terephthalate copolymer P (BS-co-BTA). As the content of terephthalic acid increased, the glass transition temperature and melting point were similar to the above changes. , Tensile strength and degradation rate have increased. There are also many studies on the copolymerization of PBS with various monomers.
2 Various homopolymers were dissolved in solvents and blended or mixed to prepare blends. T. Uesaka et al. Blended PBS with cellulose acetate to obtain homogeneous membranes; Yong used PHB and PBS together. The mixture was found to be incompatible, and the glass transition temperature did not change with the change of composition. Tianzhong used PBS to co-extrude with plant fibers and rare earth metal salts to obtain a blend with good molding and good degradability.
3 homopolymer melt blending, transesterification block copolymers This is the most commonly used method, the resulting change in performance is generally similar to the first method, in particular, reduced crystallinity, improved ductility, The melting point decreases slightly, and the degradability increases. The disadvantage is that the blending temperature is high, the polymer is easily degraded, discolored, and the molecular weight is reduced. Therefore, the processing time generally does not exceed a few minutes.
2. The polyester synthesized by the ring-opening polymerization polycondensation reaction has a molecular weight of only tens of thousands, and the ring-opening polymerization method uses a polyester having a molecular weight of several hundred thousand. The ring-opening polymerization method can synthesize various types of polyhydroxyalkanoate, and the obtained PHA structure is also different depending on the cyclic monomer. The lactide monomer (such as glycolide, lactide) can get poly(2-hydroxyalkanoate); the monomer of cyclic lactone (such as β-propiolactone, γ-butyrolactone, δ-) For example, valerolactone, ε-caprolactone, and the like, poly(3-, 4-, 5-, 6-hydroxyalkanoate) can be obtained.
The ring-opening polymerization of lactones and lactide compounds generally has three modes of anion, cation and coordination polymerization. Because the activity center of coordination ring-opening polymerization is lower than that of common anionic polymerization, such as alkoxy negative ions can inhibit the intramolecular and intermolecular acid exchange, and prevent the polymer molecular weight distribution from broadening, so coordinating type initiators are often used.
VI. Degradation mechanism Factors Influencing Biodegradability These polymers are natural and synthetic polymers containing easily hydrolyzed ester bonds, amide bonds, urea bonds, ether bonds, etc. In addition to the following factors:
1 The main chain hooks and the flexibility main chain are highly flexible, and the enzymes and microorganisms are easy to find offensive points, and the degradation rate is also large. For example, the aliphatic polyester which is the main chain is soft and easy to be degraded by various lipases and microorganisms. Duanshui/fumaric acid and aromatic diacids are introduced into the main chain, and the biodegradation rate becomes slow. In fact, the aromatic polyester is difficult to be biodegraded. It has been concluded from experiments that the length of the chain between ester bonds is a factor that affects biodegradability. Polyesters with about 6 carbon atoms spaced are more susceptible to enzymatic degradation than polyesters with less or more carbon atoms. effect.
2 Molecular weight and its distribution Many of the polymers involved in microbial degradation begin with end groups. High molecular weight polymers have a low rate of degradation due to the small number of end groups. For polymers with broad molecular weight distribution, the low molecular weight moiety is always degraded first.
3 Polymer Morphology Amorphous polymers are more biodegradable than crystalline ones.
2. The degradation process can be divided into hydrolysis and enzymatic degradation. The microorganism first secretes hydrolytic enzymes in vitro and binds to the surface of the material, and hydrolyzes the polymer chains on the surface of the material to generate small-molecular-weight compounds (organic acids, sugars, etc.), which are then degraded. The product is taken up by microorganisms and synthesized into microorganisms or transformed into active energy of microorganisms and converted to CO2 under oxygen-consuming conditions. According to this mechanism, biodegradation evaluation methods for plastics include: reduction of plastic weight during biodegradation, consumption of oxygen, generation of carbon dioxide, and accumulation of degradation products (intermediate).