Research status of polybutylene succinate
Polybutylene succinate (PBS) is an important biodegradable polymer, which is prepared by polycondensation of aliphatic diol 1,4-butanediol (BDO) and aliphatic dibasic acid 1,4-succinic acid (SA). It is an emerging polymer material that is completely biodegradable. Its chemical structure is shown in Figure 1:
The synthesis of PBS and other aliphatic copolyesters was pioneered by Carothers et al. in 1931. However, due to the technical conditions at that time, the molecular weight of the synthesized PBS was less than 5000, and the mechanical properties were average, so it did not receive corresponding attention. It was not until the 1990s that high molecular weight PBS with practical application value was produced. As early as 1993, Showa Corporation of Japan created a PBS production line with high relative molecular mass, with an annual output of 3000 tons. In the production process designed by it, isocyanate is used as a chain extender to increase the molecular weight of PBS. The molecular weight of the product produced can reach 2×10^5, which has great practical application value.
Synthesis of PBS
The traditional synthesis method of PBS roughly includes two methods according to the different sources of its monomer raw materials: biological fermentation method and chemical synthesis method.
Biological fermentation method
It mainly uses biomass raw materials existing in nature to convert them into biomass monomers through biological fermentation, and then synthesizes polybutylene succinate (PBS). Its raw materials can be obtained entirely by biological fermentation. For example, succinic acid is an intermediate metabolite of the tricarboxylic acid cycle, and its main production strain is Escherichia coli; and another monomer raw material 1,4-butanediol can also be produced by fermentation of biomass. In general, the biological fermentation method has the advantages of mild reaction conditions and low pollution, and has a high recycling rate and reduces carbon emissions. However, the process of this method is relatively complicated and the production cost is high, which is not conducive to large-scale promotion.
Chemical synthesis method
The commonly used PBS synthesis methods currently include direct esterification, ester exchange and chain extension. In the process of chemical synthesis of PBS, catalysts are generally required to promote the reaction. The catalysts commonly used are strong acid catalysts, such as p-toluenesulfonic acid; and titanate catalysts, such as tetrabutyl titanate.
Direct esterification method
The direct esterification method is a process in which dicarboxylic acids and dialkyl alcohols are directly polymerized without chain expansion. The method can be described as: esterifying diols with dicarboxylic acids under low temperature reaction conditions, wherein diols are added in excess to ensure the synthesis of terminal carbonyl prepolymers, and then diols are removed under high temperature and high vacuum conditions by the action of a catalyst to obtain PBS polyester products. The reaction equation is shown in Figure 2:
The solution polymerization method is to add a solvent during the esterification process of succinic acid butanediol monomer in the first step of PBS synthesis. The main role of the solvent is to take away the low molecular weight substances (mainly water) generated during the reaction. After the esterification reaction is completed, the reaction temperature is increased to carry out the second step of polycondensation to obtain PBS. Solvents commonly used in the solution polymerization method include toluene, xylene or decahydronaphthalene.
Using xylene as a solvent, SnCl2 as a catalyst for the reaction, and adding 4A molecular sieves for further dehydration, the relative molecular mass of the final PBS reached 24800, but the polycondensation process took a long time, lasting 70h.
Using decahydronaphthalene as a solvent, succinic acid and butanediol were mixed with the catalyst SnCl2, first reacted at a temperature of 150-160°C for 1-2h until the esterification was complete, and then the temperature was raised to 190-200°C for polycondensation. After 10-12h of polycondensation reaction, a PBS polyester product with a relative molecular mass of 78912 was obtained. Compared with the melt polycondensation method, the solution polymerization method reduces the low molecular weight products generated during the esterification reaction and polycondensation reaction in the process of synthesizing PBS to a certain extent due to the presence of solvents. Therefore, this method can reduce the reaction temperature relatively and prevent the oxidation of PBS products. However, the drawback is that the corresponding reaction time will be prolonged, and the molecular weight of the product is not very ideal. In addition, solvents are used in the synthesis process, so this method has great limitations in environmental protection and large-scale practical application.
The ester exchange method is usually used in the synthesis of polyethylene (PE). The ester exchange method can also be said to be one of the earliest methods used in the synthesis of PBS. In the process of synthesizing PBS by the ester exchange method, PBS is synthesized by melt polymerization of 1,4-butanediol and dimethyl succinate under the condition of a catalyst, and the molar ratio is usually 1,4-butanediol: dimethyl succinate = 1.0-1.1:1.0. The reaction process is divided into two steps: transesterification and polymerization. The first step of transesterification is to carry out transesterification in an inert gas environment (usually nitrogen) at 150-190°C. After the reaction is complete, the water and methanol in the system are removed, and then the second step of polycondensation is carried out at 200°C high temperature and high vacuum. The relative molecular mass of PBS synthesized by transesterification can reach 5.95×10^5. The reaction equation for synthesizing PBS by transesterification is shown in Figure 3:
Compared with the direct esterification method, the synthesis of PBS by the ester exchange method also includes two steps of esterification and polycondensation. The difference between the two is that in the first step of the esterification reaction, 1,4-butanediol is used to react with dimethyl succinate through ester exchange to remove methanol, while the direct esterification method is to remove water through the alcohol acid reaction to complete the esterification.
The advantage of the ester exchange method is that the energy required for the reaction is low, which makes the temperature required during the reaction low, and in the process of synthesizing PBS, the end-capping caused by unreasonable raw material ratio can be avoided, so that the structure of the final synthetic product PBS can be better controlled. However, when synthesizing PBS by the ester exchange method, dimethyl succinate must be prepared first, which increases the production cost compared with the direct esterification method, and the methanol removed during the synthesis process will cause certain pollution to the environment if it is not handled properly, while the water produced by the direct esterification method is pollution-free to the environment and the cost is relatively reduced.
Chain extension method
The chain extension method for synthesizing PBS is to use the active groups in the chain extender to react with the terminal hydroxyl groups in the prepolymer to increase the relative molecular weight of the polyester. Chain extenders suitable for terminal carboxyl structures generally have two functional groups and can react with the terminal carboxyl groups of prepolymers to achieve the effect of chain extension. Commonly used chain extenders include isocyanates, oxazolines, anhydrides and toluene diisocyanates. High molecular weight polybutylene succinate was prepared using chain extenders. Studies have shown that the crystallinity and melting point of PBS obtained by chain extension reaction decreased, but the molecular weight was greatly improved, and the tensile strength was improved. PBS chain extension products were obtained using dibenzoyl peroxide (BPO) as a chain extender. The relative molecular weight of the PBS product after chain extension increased by about 70%, from 13800 to 23500. Although the crystallinity of the PBS product obtained by the chain extension method decreased, its mechanical properties were relatively improved to a certain extent. However, the PBS prepared by using chain extenders has the disadvantage of reduced biosafety and biodegradability compared to the product prepared by direct esterification. The synthesized PBS is not suitable for the fields of medicine and food packaging with high requirements for biosafety, but is more suitable for thin films and recyclable packaging bottles in industry and agriculture.
Modification of PBS
Although PBS is a good biodegradable polymer material, it still has the disadvantages of poor processing performance and insufficient mechanical properties in practical applications in various fields, which limits its further application. Therefore, the modification of PBS has also been a research hotspot in recent years. In general, the methods used can be divided into chemical copolymerization modification and physical blending modification.
Copolymerization modification of PBS
The copolymerization modification of PBS is mainly achieved by introducing aliphatic or aromatic diols or dibasic acids to copolymerize with succinic acid and butanediol during the synthesis of PBS, thereby changing the structure of PBS and changing the performance of PBS. In the copolymerization modification of PBS, commonly used aliphatic modification monomers include adipic acid, hexanediol, ethylene glycol, etc. The advantage of introducing aliphatic modification monomers is that the prepared modified PBS copolyester can improve the mechanical properties and chemical properties without sacrificing biodegradability.
It is worth mentioning that in terms of the biodegradability of aromatic monomer modified PBS copolyester, when an appropriate aromatic structure is introduced into the molecular chain of PBS, the product can still be guaranteed to have good biodegradability. Hexanediol was used as the modification monomer and polycondensed with butanediol succinate in decahydronaphthalene to synthesize butanediol succinate-hexanediol succinate copolymer. After adding the modification monomer, the synthesized product has a higher molecular weight, and the elongation at break of the product is improved, the crystallinity is reduced, and the product has good thermal stability. A series of random copolymers PBST were prepared using dimethyl terephthalate. By adjusting the amount of dimethyl terephthalate, the prepared products had good mechanical properties and different biodegradation rates, and the crystalline melting point of the prepared copolymers conformed to the Flory equation of random copolymers. Isosorbide and glucose were used in the modification of PBS. The isosorbide-modified copolyester containing bicyclic sugar groups had better oxygen barrier properties and mechanical properties than the glucose-modified copolyester containing monocyclic sugar groups.
Physical blending modification
Using PBS as the matrix, adding different polymers, inorganic materials, fibers, etc. to prepare PBS composite materials by blending is also a common method for PBS modification. The PBS composite materials prepared by physical blending have many advantages. Their mechanical properties can be greatly improved, and many blending components also have good biodegradability, such as natural polymer materials such as starch, wood powder, and hemp; biodegradable materials such as PBAT and PLA, etc. The above materials will not destroy the biodegradability of PBS when modifying it. At the same time, by changing the blending components, the use cost of PBS can also be reduced to a certain extent. A series of composite materials were prepared by blending PBS with waxy starch and corn starch. The tensile properties and processing properties of the products modified by blending the two starches with PBS were greatly improved, and the effect of waxy starch was better.
A series of PBS/PLA blends were prepared by adding tetrabutyl titanate (TBT) for reactive extrusion. With the addition of TBT, the compatibility of the blend was greatly improved, the impact performance was increased by 25.6 times, the tensile strength did not change much, and the elongation at break was increased by 2.0 times. Cellulose acetate butyrate (CAB) was blended with PBS as a modified component. When 10% CAB was added, the tensile strength at break of the blended product was 35MPa and the elongation at break was 547%. In addition, the biodegradation rate of the blend was affected, and no degradation occurred within 60 days.
Application fields of PBS
PBS started relatively late and has only become a hot topic in biomaterial research in recent years. However, as a biodegradable material with good comprehensive performance, PBS and various PBS modified materials have been widely used in various fields, such as packaging bags and packaging bottles in the field of food packaging, mulch films, compost bags and medical materials in the field of agriculture, etc.
