1 Introduction
In the conventional polymer synthesis process, liquid solvent is often an indispensable part of the reaction medium. Except for bulk polymerization (which does not require solvent), other polymerization reactions generally take place in solution (solution polymerization), droplets (suspension polymerization), micelles and latex particles (emulsion polymerization). Compared with the solvent-free polymerization system, the solvent is a very good heat transfer medium, which can control and reduce the polymerization temperature and avoid the decomposition of monomers and products. In addition, the small molecules produced in the polymer reaction process can be removed by azeotropy or reaction with the solvent, so that the reaction becomes stable and easy to control. Therefore, liquid solvents have been an important research object with the development of polymer materials science, and the chemical problems related to polymer condensed state in liquid solvents have been deeply and extensively studied, such as reaction types, reaction mechanisms, reaction kinetics, and the formation and evolution of the morphological structure of polymerization products.
However, with the rapid development of science and technology, the application field of polymer resin materials has been expanding, the consumption has been increasing, but the production capacity has been lagging behind (especially in China), which has not been reversed so far. On the other hand, with the continuous expansion of the industrial production scale of polymer materials, the problems of "solvent recovery" and "solvent residue removal in polymer materials" after polymerization have become increasingly prominent and serious.Especially with the growing voice of green environmental protection for industrial production, the production cost of polymer resin has risen sharply. Under this research background, the research work of solvent-free polymerization has been paid attention to and developed, and some solvent-free polymerization production technologies have been widely used through technological innovation. Their efficient production capacity and outstanding environmental protection effect make people full of expectations for their future development prospects
[1⇓⇓⇓~5].
Melt polycondensation (1930s), reactive extrusion (1960s) and solid phase polycondensation (1960s) are typical representatives of these solvent-free resin synthesis technologies
[6⇓~8][9⇓⇓~12][13⇓~15]. Compared with the conventional resin production technology in which polymerization occurs in solution, no solvent is used in the reaction process, which is the biggest common feature and advantage of these three technologies.But also the content of by-products and impurities in the crude product after the polymerization reaction is completed is less, so that the complicated purification and separation steps can be omitted, the production cost is greatly reduced, and the purposes of energy saving and emission reduction can be achieved. For example, the production of 150,000 tons of SBS (styrene-butadiene-styrene block copolymer) using the conventional production process requires the use of 600,000 tons of organic solvent. If the entire solvent used for production is intended to be recovered, ~ 24 billion kilocalories of energy need to be consumed, which is equivalent to the energy produced by burning ~ 2.8 million cubic meters of natural gas or ~ 2.8 million kW · H of electricity. However, if reactive extrusion processing technology is used to produce SBS, it can not only avoid a series of problems such as high energy consumption and environmental pollution caused by the use of solvents, but also meet the requirements of the global sustainable development strategy of "carbon neutrality"
[5].
Most of the polymerization reactions without solvent are exothermic reactions. Because there is no solvent as a heat transfer medium, when the polymerization reaction reaches a certain level, the temperature of the material will rise sharply.It will reach more than 300 ℃ in a few minutes, and the corresponding polymer is prone to degradation and carbonization, so the polymerization heat generated in the reaction process must be removed in time. In order to solve this key problem and obtain higher quality polymer resins, in addition to the basic problems and new technologies of chemical engineering and engineering thermophysics,More attention should be paid to the types of chemical reactions, reaction mechanisms and related reaction kinetics of polymer preparation without solvent, which are all chemical problems under the change of condensed state. For example, the change of condensed state of materials in the production of polymer resins by melt polycondensation, reactive extrusion and solid state polycondensation has been closely related to the chemical reactions occurring during polymerization.The research on the interaction and influence between condensed state change and chemical reaction can provide better theoretical guidance for the development of corresponding production technology and promote the breakthrough and innovation of solvent-free production technology. In this paper, the three polymerization methods and mechanisms are briefly analyzed and introduced by summarizing the previous work and related experience, and the relationship among the three methods is further straightened out through some typical examples of polymer resin production.The basic problems between the change of polymer condensed state and chemical reaction in the three solvent-free polymer preparation technologies are revealed, which provides some valuable references for the vast scientific research work.
2 Brief Introduction of Three Typical Polymer Resin Production Technologies without Solvent Participation
In this paper, the three polymerization methods will be briefly introduced one by one in the order of melt polycondensation (melt state), reactive extrusion (melt state) and solid polycondensation (solid state), and then the differences and connections between them will be comprehensively compared and analyzed to show the correlation of the development of these three polymerization methods.
2.1 Melt polycondensation technique
Melt polymerization (MP) refers to the production and preparation process of synthesizing polymer resin by placing the monomer or oligomer participating in the polymerization reaction in a reaction system with a certain temperature and pressure, and making it undergo polycondensation, ring-opening or addition reaction in the molten state, which belongs to the category of bulk polymerization. In the melt polymerization process, the polymerization temperature in the reaction system is generally controlled to 10 to 25.d egree. C. above the melting point (T
m) of the target polymer product
[1,6⇓~8]. Melt condensation polymerization (MCP) is the most important and representative polymerization process in the Melt polymerization process. Because of its important position in the history of melt polymerization process and its wide application in industrial production, people usually use "melt condensation polymerization" instead of "melt polymerization" in many cases.
In 1908, polyimide (PI) first became a synthetic resin material that could be prepared by melt self-polycondensation, but at this time, the melt polycondensation method had not received much attention
[6]. It was not until the outbreak of the Second World War that melt polycondensation technology officially stepped onto the historical stage of synthetic resins to meet the huge demand for military applications of polyamide materials, demonstrating its simple and efficient production capacity. Since then, this technology has been widely used in the actual industrial production of polyester, unsaturated polyester, polyurethane and other polymer materials, and has become a hot spot in the field of resin production and technology research, such as the polycondensation of ethylene terephthalate (PET polyester).Polyester), polycondensation of butylene terephthalate (PBT polyester), polycondensation of caprolactam (nylon 6, nylon) and other polymer resins can be industrially produced through melt polycondensation technology. At present, the production capacity of a single melt polycondensation plant has reached tens of thousands of tons per year
[16][17,18][19].
The advantages of melt polycondensation are: (1) high product purity: because melt polycondensation does not use solvent, the reactant concentration is high, the reaction rate is fast, and there is little chance to introduce impurities, so the product quality is more uniform, the product is pure, the molecular weight distribution is narrow, and no additional product purification process is needed; (2) the production process is simple: because the melt polymerization does not use a solvent, a solvent separation process with complex operation and high energy consumption is avoided, and the production process is greatly simplified; (3) easy large-scale production: because there is no solvent, the volume ratio of reaction equipment is high, so the production capacity is large; And (4) continuous production is facilitated: the product after the polycondensation reaction is completed is still in a molten state, so that the polymer melt can be directly spun, formed into a film or pelletized by a casting belt, the energy consumption is reduced, the processing process is shortened, and the production efficiency is improved.
But the disadvantage is also obvious: the main reason is that the molecular weight is not high enough. The reasons for this result are as follows: (1) In the process of melt polycondensation, with the increase of the degree of polymerization, the viscosity of the reaction system (molten state) in the kettle increases sharply.As a result, the low molecular by-products produced in the same reaction period are difficult to be removed from the high viscosity reaction system, and with the extension of the polycondensation reaction time, these by-products that cannot be removed lead to the aggravation of the polymer degradation reaction. (2) In the later stage of the reaction, the viscosity of the reaction system increases with the increase of the degree of polymerization, and the heat transfer in the reaction system becomes more difficult, which is prone to local overheating, resulting in the increase of degradation reaction and limiting the further increase of molecular weight; (3) In the polymerization reactor, the high-viscosity fluid is easy to be in the "dead zone", resulting in serious sticking to the reactor. A long stay at high temperature will cause degradation and other side reactions, which will deepen the color of the product, deteriorate the quality, make it more difficult to discharge and easily carbonize and blacken. It is generally believed that the key to solve the above problems lies in the optimization and improvement of the reactor design. As long as the flow and mixing state of the melt system in the polycondensation reactor is good, the heat and mass transfer efficiency is high, and the process meets the plug flow, it is expected to obtain resin products with high molecular weight and uniform quality.
2.2 Solid state polycondensation technique
Solid state polycondensation (SSP), also known as Solid state polycondensation, is one of the most important solid state polymerization techniques: the monomer or prepolymer is placed in a vacuum or inert gas flow environment, and heated at a temperature below the melting point (Solid state polycondensation) of the monomer, prepolymer and target polymer to undergo polycondensation reaction (generally 15 ~ 30 ℃ lower than the melting point of the monomer or prepolymer)
[14,20⇓~22].
In the late 1930s, when studying the melt polycondensation of nylon, Paul, an American scientist, found that after the nylon obtained by melt polycondensation was cooled to a solid state, if it was maintained at a certain temperature, the polycondensation reaction between these nylon molecules could continue to occur, and the long-chain nylon polymer with higher molecular weight could be grown
[15]. Because the temperature of this reaction is below the melting point of nylon and above the glass transition temperature, the reactants are still in a solid state macroscopically (in the elastic state of crystalline polymers) when the polycondensation reaction is carried out, so this reaction is vividly named solid phase polycondensation (SSP). However, in that era, based on the knowledge and understanding of melt polycondensation, people often thought that monomer molecules in the solid state were relatively frozen, lacking collision opportunities with each other, and it was difficult to carry out polymerization, let alone polymers with a certain molecular weight, so this work did not attract much attention. In 1945, it was reported that high molecular weight polyacrylamide was successfully obtained by radiation-initiated solid-state acrylamide polymerization, and the research on "solid-state polymerization process" began to receive a certain degree of attention in the polymer science community
[23]. From the late 1950s to the 1960s, this method was used to prepare high molecular weight nylon, polyester and other polycondensation polymer materials in industry, which promoted the development of systematic research covering the control method, reaction mechanism, physical chemistry and other contents of solid phase polycondensation reaction process, and a large number of reports were made
[20,24⇓~26]. In the 1970s, DuPont Company of the United States successfully used solid state polycondensation (also known as solid state polycondensation or solid state tackifying) technology to develop bottle-grade polyester chips, which were put into industrial production. The polyester produced by this technology has light weight, high strength, good gas permeability and gas barrier properties, which has made the application of solid phase polycondensation technology in the industry develop rapidly
[8]. At the same time, Wegner took the lead in preparing macroscopic single crystals of polydiacetylenes by Solid-state polymerization, which provided an ideal one-dimensional crystal model for the study of the relationship between polymer structure and properties.Solid state polymerization has the characteristics of low activation energy, high purity, low cost, no induction period, obvious after-effect of polymerization and local regularity control of molecular structure before and after polymerization.Since then, the research on solid-state polymerization has become a hot topic in the scientific and industrial circles, and "solid-state polymerization" covering solid-state polycondensation has also become an important branch of solid-state chemistry
[27]. With continuous development, solid-state polymerization can be initiated by more ways, such as high-energy radiation or ultraviolet radiation, heat and electrodes, and the polymerization mechanism is no longer limited to the initial stepwise polymerization (polycondensation), and then extended to addition polymerization with the characteristics of free radical and ionic chain reactions, and the types of polymers that can be prepared are expanding.
At present, the generally accepted principle of solid phase polycondensation is that when the temperature is above the glass transition temperature and below the melting point of the polymer, although the macromolecular chain of the polymer is fixed, the terminal groups of the molecular chain have certain activity and can react with each other through diffusion, and the generated small molecules are brought out by vacuum or airflow, so that the reaction can continue; Therefore, the solid state polycondensation reaction generally goes through the following four stages: (1) the active groups approach and collide with each other; (2) reversible chemical reaction between functional groups; (3) the generated small molecule by-product diffuses from the inside to the surface of the particle; (4) Small molecules diffuse out of the system from the surface into the inert gas or vacuum. There are more and more reports on the related mechanism, polymerization kinetics and its influencing factors.
Because the polycondensation reaction is carried out in the solid state (belonging to homogeneous polycondensation), it is a polymer resin preparation technology to achieve molecular weight increase in the solid state. Because this technology can make polymer chips (such as PET chips) undergo polycondensation reaction in the solid state, so as to achieve the purpose of increasing polymer viscosity (macroscopically) and molecular weight, it is also vividly called "solid phase tackifying" technology.
Compared with the melt polycondensation technology, the solid phase polycondensation technology has the following advantages: (1) the reaction temperature is lower, so the degradation reaction and the side reaction in the polymerization process can be effectively reduced, and the product quality problems such as product discoloration or molecular weight reduction can be avoided; (2) the relative molecular mass of the polymer can be obviously improved, so that the physical and mechanical properties and the molding processing performance of the polymer are improved, and the application field of the product is further widened; (3) because a stirring device is not needed in the solid phase polycondensation reaction process, the reaction kettle has a simple structure, thereby avoiding the phenomena that a high-viscosity polymer is difficult to stir and discharge in the later stage of melt polycondensation, and because the required reaction temperature is low, the energy consumption required by production can be effectively reduced; (4) The solid phase polycondensation method requires mild reaction conditions and does not use liquid raw materials in the whole process. This not only reduces the process of solvent recovery, saves the production cost, but also avoids the environmental pollution caused by the leakage of liquid raw materials. (5) The solid phase polycondensation reaction process is relatively simple and flexible, and can be operated continuously or intermittently.
However, the solid state polycondensation technology itself also has some inherent shortcomings: (1) the reaction rate is slow and the reaction time is very long; The reaction activation energy of solid state polycondensation is much higher than that of melt polycondensation, but the reaction temperature is lower than the melting point of the polymer, so the reactants are in a solid state, the space for movement is limited, and the energy obtained by the end group of the molecular chain is less, the reaction activity is reduced, resulting in slower diffusion rate and reaction rate. (2) The solid phase polycondensation has good synergistic effect, but the reproducibility is poor and the product quality is unstable. (3) Solid phase polycondensation in an open system, which uses a continuous stream of inert gas to take small molecular by-products away from the system, can effectively promote the reaction to the positive direction, but it will increase production costs and consume a lot of resources; Although the solid phase polycondensation in closed system saves energy and is easy to operate, it will limit the balance of polycondensation reaction and can not effectively improve the relative molecular mass of the polymer.
2.3 Reactive extrusion technique
Reactive extrusion (REX), also known as extrusion reaction, is a new polymer resin production technology that combines polymer chemical reaction with extrusion process, and integrates polymer synthesis, modification, processing and molding. A (screw) extruder is used as a reaction container, various raw materials (reaction monomer, initiator, polymer, auxiliary agent, etc.) To be reacted are respectively put into the screw extruder, and under certain temperature and pressure,The polymerization system is fully mixed, dispersed and pushed forward due to the pushing and shearing action of the screw, and the predetermined chemical reaction is completed simultaneously in the continuous melt extrusion process to produce the required polymer material
[10⇓~12,28⇓~30]. This technology effectively combines the two production processes of reaction and molding processing into a complete continuous process, and is also an effective method for high viscosity polymer reaction.
This technology was born in the 1950s and was initially regarded as a new hybrid technology
[10,12,28]. It was not until the late 1960s that it was first applied to the research of polymer synthesis and polymer modification in developed countries and regions such as Europe, the United States and Japan, especially in the preparation and development of new polymer materials.Therefore, the major research institutions and extrusion equipment manufacturers are highly concerned about the technology, so that it has developed rapidly. At present, reactive extrusion technology has become a research direction of high concern in the world
[31⇓~33]. Menges of Achen University in Germany, White of Akron University in the United States and Homsky of Brumel University in the United Kingdom published articles in 1986 and 1994 respectively, which comprehensively analyzed the effects of various processing parameters and condition factors on the synthesis of nylon-6 by twin-screw reactive extrusion, and pushed the research and development of reactive extrusion nylon-6 to a new height. Waymouth, an American scientist, prepared thermoplastic elastomer fibers directly from propylene monomers by controlling the microstructure of polyolefins and using continuous reaction molding technology. Bodolus of the United States reported that the impact strength of the copolymer prepared by the copolymerization of acrylate and acrylonitrile in the extruder was 10 times higher than that of the general acrylate-acrylonitrile copolymer in the production of nitrile rubber. It was not until the early 1980s that this technology really came into the sight of academic and industrial workers in China, and people began to engage in research and discussion in this field. Through the persistence and unremitting efforts of scientific researchers, reactive extrusion processing has also been widely used in the preparation, grafting, coupling/crosslinking, degradation and other aspects of polymer alloys, and some aspects of reactive extrusion technology in China have reached the world's advanced level
[34].
Compared with the traditional polymerizer-type production technology which needs polymerization, separation, purification, re-extrusion granulation molding and the like, the reactive extrusion bulk polymerization technology integrates the continuous bulk polymerization of raw materials and the melting molding of polymer products into a whole,Make full use of the limited residence time in the screw extruder to make the polymerization reaction "one-step" to get the required product, so it has obvious technical advantages: (1) the reaction process can be carried out continuously, by adjusting the temperature setting, screw speed and feeding position, the precise control of the reaction process can be realized, and the best time to start and stop the polymerization reaction can be adjusted
[2,5,29,35,36]; (2) no or a small amount of solvent can be used, and no separation operation is needed after polymerization, thereby not only saving energy, but also reducing environmental pollution; (3) that residence time, the reaction temperature, the material distribution and the like can be control by adjusting the combination of the extrude cylinder, the threaded block structure, the barrier ring and the like, which is beneficial to obtain a product with narrow molecular weight distribution and uniform performance; (4) The screw structure and extrusion process can be flexibly adjusted to adapt to different material systems, and it has the flexibility to produce a variety of products; (5) a novel polymer with specific properties can be obtained by controlling both the physical structure and the chemical structure through polymerization; (6) the reaction process which is difficult to operate or cannot be realized in the conventional reactor can be realized, and the method is particularly suitable for a system with high reaction rate, high viscosity and large thermal effect and a reaction process involving solid-liquid phase change in the reaction process; (7) Polymerization (or modification) and molding (granulation or final product) are simultaneously realized on the same production line, which not only simplifies the process flow, but also reduces the production cost. In addition, by changing the structure of the screw block and the rheological behavior of the material, the physical texture of the product can be evolved according to the predetermined requirements, so as to obtain high-performance polymers and polymers with special molecular chain structures which can not be obtained by traditional methods. Because of its high cost performance and significant social and economic benefits, reactive extrusion technology is bound to become one of the most economical and promising polymerization methods.
Of course, there are still some problems to be solved in reactive extrusion technology. (1) Technical difficulty: For different materials, it is not only necessary to study the formula and process conditions, but also to design new reaction extruders for different reactions. Therefore, for new systems, there are often problems of large investment in research and development and long research and development time. (2) It is difficult to observe and detect in the reaction process: when the materials involved in the reaction are extruded and reacted in the extruder, they are always in a dynamic and closed high temperature and high pressure environment, and the operator cannot judge the specific reaction degree by observing and detecting the state changes of the materials; In the extrusion process, the residence time of the material in the barrel is relatively short (generally only a few minutes), but the chemical reaction in the barrel must be completed quickly. If the reaction lasts more than 20 minutes, more side reactions will occur, and the application significance of the reactive extrusion technology will be lost. (3) High technical content: Reactive extrusion technology involves many disciplines such as polymer materials, polymer physics, polymer chemistry, chemical engineering, polymerization reaction engineering, rubber and plastic machinery, polymer molding and processing, mechanical processing, electronics, materials, etc. To achieve certain research results, it is necessary to go through a long period of research and get support and cooperation from many fields. To sum up, reactive extrusion technology has the characteristics of high R & D investment, high technology content and high product profit. Although there are many difficulties in the R & D stage, it has obvious advantages in industrial application.
3 Typical Application of Solvent-free Resin Production Technology
Polythylene terephthalate (PET) and polylactic acid (PLA) resins are selected as examples to introduce and analyze the application of the three technologies in practical production and the existing problems.
3.1 Solvent-free production of PET resin
3.1.1 Introduction to PET
PET, commonly known as polyester, is a highly crystalline polymer, mostly milky white or light yellow, with a smooth and shiny surface. Its structural formula is shown in Figure 1. Its glass transition temperature (Tg) is 83 ° C, melting temperature (Tm) is about 255 – 265 ° C, and it can be used for a long time in the temperature range of 120 ° C. Because of its strong barrier to gas, water, oil and odor, high transparency, good gloss, good safety and relatively low price, it is widely used in fiber manufacturing, film production, bottle packaging and engineering plastics, and has a huge demand in the global market. Although the annual output of PET in the world is increasing rapidly year by year, it has been unable to meet the growing demand for applications. Improving PET production capacity, improving PET resin viscosity, and developing PET recycling and recycling technology have become the research focus in the field of PET resin production and development.
3.1.2 Industrial Synthesis of Polyethylene Terephthalate Resin
At present, the main technical routes for the production of PET resin in the world are transesterification (also known as DMT method) and direct esterification (PTA method). Dimethyl terephthalate (DMT) is transesterified with ethylene glycol (EG) and then polycondensed into PET. The PTA method is based on the direct esterification of high-purity terephthalic acid or medium-purity terephthalic acid (MTA) with EG, followed by polycondensation. The above two methods have their own advantages and disadvantages in the actual production process, but since the recrystallization method of high-purity PTA has been industrialized, the production cost of PTA technology has been greatly reduced, and its own advantages have become more prominent compared with the transesterification method. At present, PTA method has become the most widely used PET production method in the world by virtue of many advantages, such as low raw material consumption, small EG recovery system, no by-product methanol, gentle reaction rate, short production process, stable production control, high production safety, low engineering investment and low production cost.
The production mode of polyester can be divided into "continuous production" and "batch production". Since the 1980s, the continuous production of polyester by PTA method has become the main production process of PET polyester materials. By 2000, the PTA route has been basically adopted
[20,37]. Compared with the batch production, the continuous production equipment occupies a smaller area and is simpler to operate, and the reaction conditions of uniform temperature and concentration can be achieved inside the equipment, so the quality of the product can also be kept uniform. With the application of PET materials in polyester bottle chips, industrial yarn and other fields, the device for increasing the viscosity of PET is also being developed and applied
[38,39].
3.1.3 Typical PET melt polycondensation process
Taking the melt polycondensation process of PET polyester as an example, the process mainly includes the following two reactions: the first reaction is the esterification reaction between PTA and EG to produce ethylene terephthalate (BHET) and a small amount of prepolycondensation raw material; The second step is the polycondensation of BHET in the presence of a catalyst to produce PET resin. The specific process is as follows: firstly, solid TPA, polymerization catalyst (antimony compound) and liquid EG are mixed to prepare slurry, wherein the molar ratio of EG to TPA is 2:1. In order to shorten the reaction time, the reaction pressure in the reactor was higher than the atmospheric pressure (0. 05 ~ 0.2 MPa), and the reaction temperature was also higher than the boiling point of EG (258 ~ 263 ℃). By observing the change of the condensed state in the system at this stage, it can be found that the solid TPA can only be partially dissolved in the liquid EG under the reaction conditions, so in the early stage of the esterification reaction,The reaction system in the reactor is a "solid-liquid heterogeneous" system, at this time, the esterification reaction first occurs between TPA and EG dissolved by diffusion, and the homogeneous esterification reaction begins to remove water and produce BHET. As the reaction goes on, the content of BHET becomes higher and higher, and TPA begins to dissolve in the EG/BHET (and a small amount of short-chain oligomer prepolymer) mixed system due to its good solubility in BHET.Moreover, the dissolution rate increases with the increase of oligomers, and the system gradually changes from heterogeneous phase to homogeneous phase, from turbidity to transparency. When TPA is completely dissolved, the mixed system in the reactor appears a "clear point", which indicates that the solid TPA disappears and the homogeneous reaction begins. The experimental results show that the "clear point" generally appears when the esterification rate reaches about 85%. Subsequently, the production process begins to transit from esterification to the second step of polycondensation. The second step is the polycondensation reaction of BHET, which belongs to the chain propagation reaction in polyester synthesis. The polycondensation between two β-hydroxyethyl groups is reversed, and one molecule of EG is removed. The reaction formula is as follows. The reaction temperature of polycondensation in this stage must be higher than the melting temperature (Tm=245~260℃) of PET resin, but at the same time, the temperature must be lower than 300 ℃ (PET resin begins to degrade above this temperature), so the most suitable temperature range of melt polycondensation of PET is 275 ~ 290 ℃ (20 ~ 30 ℃ above the melting point is generally selected empirically). The reaction time of the polycondensation process is at least 2 H, depending on the reactor. The reaction constant of this reaction is small, so the ethylene glycol produced in the reaction must be removed as soon as possible in the reaction process to break the reaction balance and promote the reaction to continue to the right, otherwise not only the reaction rate will be affected, but also the degree of polymerization will not be improved. Therefore, the polycondensation should be carried out under vacuum, especially in the later stage of polycondensation under high vacuum, and the evaporation surface should be increased as much as possible.
Because the reaction temperature of the melt polycondensation is above the melting point of the polyester and the melt polycondensation is bulk polymerization without any medium, the obtained product is pure, does not need a separation medium, has low production cost, and is suitable for simple, large-scale and continuous production processes. In addition, the discharge state of the polyester after melt polycondensation is generally polyester melt, so as long as the performance and quality of the polyester melt meet the requirements, the polyester can be directly processed and formed, which is very economical and convenient. However, it is difficult to obtain PET polyester with high intrinsic viscosity by melt polycondensation technology, because in the process of melt polycondensation, with the increase of the viscosity of the reaction system, it is more difficult to remove the low molecular by-products generated by the reaction.At the same time, it is easy to cause local overheating of the reaction system, which leads to the increase of degradation reaction and limits the further increase of molecular weight, so only some PET resins with low intrinsic viscosity can be produced. Therefore, in order to obtain resin materials with higher molecular weight, further polymer tackifying treatment is needed.
3.1.4 Viscosity increasing method of PET
Studies have shown that the properties of PET polyester are greatly affected by its molecular weight. The molecular weight of PET resin increases, which is reflected in the increase of intrinsic viscosity, and the mechanical properties of PET resin also increase. As shown in Table 1, the molecular weight of polyester directly determines its different application fields. Ordinary molecular weight PET resin (intrinsic viscosity [η] of 0.55 ~ 0.64 dL/G) can be used to produce conventional polyester fibers, while high molecular weight PET polyester can be used to produce polyester bottles and engineering plastics.Ultra-high molecular weight PET polyester can be used to make crystalline polyester, cable (such as tire cord), woven fabrics (coated fabrics, canvas, filter cloth, etc.), reinforcement materials, asbestos substitutes, protective products (such as bulletproof vests), sporting goods (tennis rackets, fishing rods, bats, racing boats, etc.).
Therefore, it is not difficult to find that most non-fiber applications and some fiber applications require polyester with higher intrinsic viscosity, except for low-viscosity PET resin for textile products. PET resin with high intrinsic viscosity can be obtained by tackifying conventional low-viscosity PET resin to achieve the purpose of increasing molecular weight, so as to meet various needs. At present, there are three main methods of polyester tackifying: melt polycondensation, solid state polycondensation and chemical reaction extrusion tackifying
[40,41].
(1) PET melt tackifying technology
Viscosity increasing by melt polycondensation refers to a method of increasing the degree of polymerization by prolonging the time of melt polycondensation
[8,42]. Compared with the common melt polymerization device, the biggest difference of the polymerization production device is that a polycondensation kettle is added after the final polycondensation kettle of the conventional melt polycondensation, so as to increase the molecular weight by prolonging the melt polycondensation reaction time. The reaction temperature in the prepolycondensation reactor is usually 270 ℃, and the reaction temperature in the postpolycondensation reactor is 270 ~ 280 ℃. During the final polymerization, a small amount of stabilizer is added to improve the thermal stability of the solution. In addition, the polycondensation reaction must be carried out under high vacuum (residual pressure not more than 266 Pa) and strong stirring to obtain polyester with higher molecular weight. However, the intrinsic viscosity of the general product is not more than 0.9 dL/G, and the product quality is not high. In addition, the high viscosity polyester produced by this long time melt polycondensation has a high acetaldehyde content, so it is not suitable for the production of bottle polyester.
Generally speaking, the cost of producing high intrinsic viscosity PET resin by melt polycondensation is low, which is suitable for single variety and large-scale continuous production process. However, the improvement of the intrinsic viscosity of the product is limited, the product quality is poor, and the acetaldehyde content is high, so it is not suitable for the production of PET for bottles. Therefore, since the 1980s, this process has been less used, and the only large companies using this process are DuPont and Allied in the United States.
(2) PET solid phase tackifying technology
Solid state polycondensation is a very good and popular method for producing high intrinsic viscosity, high quality PET
[38,43]. The SSP of PET is a polycondensation reaction in which the monomer or low molecular weight prepolymer is heated to a temperature above the glass transition temperature and below the melting point. At this time, it is difficult for the macromolecular chain segment to move. By vacuumizing or introducing inert gas, the terminal functional group can obtain sufficient activity to cause solid phase polycondensation.A transesterification reaction between two hydroxyl end group and an esterification reaction between a hydroxyl end group and a carboxyl group (fig. 2),The generated small molecular by-products such as water and ethylene glycol are removed from the reaction system by vacuumizing or introducing nitrogen, so that the solid phase polycondensation reaction is carried out in the positive reaction direction, the intrinsic viscosity of the product is continuously improved, and finally the PET polyester with the required molecular weight is obtained
[20,22,44].
For example, PET prepolymer (chip) with lower molecular weight is heated, and when the temperature is between the melting point and the glass transition temperature, the degree of polymerization of PET is increased by vacuumizing or introducing protective gas. In this method, vial grade PET with intrinsic viscosity of 0. 70 ~ 0.81 dL/G is usually used, and condensed at 210 ℃ for 15 ~ 20 H to obtain high viscosity and high quality PET resin. Because the solid state polycondensation reaction has low temperature, slow speed and long process flow, it will have high cost if it is put into large-scale production.
In the current industrial production at home and abroad, solid state polymerization is divided into batch and continuous (Table 2). The two processes are roughly the same, generally through the steps of pre-drying, pre-crystallization, polycondensation and cooling. Pre-crystallization, drying and solid state polycondensation can be carried out in the same equipment, and the main reactors are vacuum drum and moving bed respectively. The technological process of the intermittent solid phase polycondensation is as follows: after screening and metal separation, the prepolymer slices pass through a drying crystallization device with air or nitrogen as a heating medium, and then enter a vacuum drum reactor after crystallization, and then undergo solid phase polycondensation after vacuumizing. Cooling to 160 deg C after the reaction is finished, discharging after the vacuum state is relieved by using nitrogen, or performing drying crystallization, solid phase polycondensation and cooling in sequence in a rotary drum reactor by adopting program process control to finish the whole reaction
[45]. The small molecular by-products such as water and ethylene glycol produced by the reaction are removed by vacuumizing, the process flow is short, the operation is convenient and flexible, and the intrinsic viscosity can be adjusted according to the required industrial yarn performance and use. Intermittent process is a traditional process route with low heat transfer efficiency and production efficiency of chips, strict requirements on vacuum degree, and greater flexibility in changing chips, so it is suitable for small batch and multi-variety production.
The continuous solid phase polycondensation process is roughly as follows: the conventional spinning grade PET chip as the prepolymer is firstly screened and separated from the metal, and then enters a drying crystallization device with hot air or nitrogen as the medium, and the crystallized chip enters a continuous solid phase polymerization reactor. The materials uniformly and slowly move downwards in a piston shape in the vertical reactor by means of gravity, and exchange heat and mass with the hot nitrogen flow introduced from the lower part of the reactor,Polycondensation by-products such as ethylene glycol and water diffused from the inside of the chip are enriched by hot nitrogen flow and flow out from the upper part of the reactor, and the chip after reaction is sent to the product condenser from the lower part and then to the product storage tank, or directly sent to spinning from the lower part
[46]. The small molecular by-product is removed by means of inert gas, the process flow is long, the heat transfer efficiency and the production efficiency of the chip are high, the intrinsic viscosity of the chip after solid phase polycondensation is uniform, the flexibility of viscosity change is good, and products with different intrinsic viscosities can be produced by depending on different spinning process conditions. Compared with the traditional batch process, the continuous solid state polycondensation is more suitable for large-scale modern production because of its high production efficiency and automation control. Most of the domestic continuous production processes are imported from foreign companies such as Bepex Company of the United States, Gemma Company of Germany or Toray Company of Japan.
In a word, due to the low reaction temperature (at the melting point of PET), although PET resin will also undergo thermal degradation in the tackifying process, its side effects are significantly reduced compared with the melt polycondensation process, so that the intrinsic viscosity of the tackified PET resin product is not only significantly increased, but also the acetaldehyde content is significantly reduced. In addition, solid phase polycondensation can also improve the crystallinity and melting point of PET resin, so that the applicable temperature range of PET resin for bottles is also increased. However, the solid state polycondensation method also has obvious disadvantages: because the reaction temperature is between the glass transition temperature and the melting point, the PET macromolecule cannot move and can only rely on the movement and reaction of the molecular chain in the amorphous region, so the reaction speed is very slow and the reaction time is long, generally 15 to 20 hours; Such a low reaction rate will lead to the combination of low polyester particles, which is not conducive to the elimination of small molecules, making it difficult for the condensation to continue the reaction, and affecting the molecular weight and quality of the product. Moreover, the steps of solid phase polycondensation are relatively complicated, the required process is long, and the input equipment is large, so the production cost is high, and the solid phase polycondensation is more suitable for industrial mass production, mainly for industrial research, and rarely for laboratory research.
(3) PET chain extension reactive extrusion tackifying technology
Chain extension reaction usually refers to the processing of polymers containing active reactive groups at the chain end by means of chain extender or hot melting, so that the molecular weight can be significantly increased through the chemical reaction between the active chain ends in a short time, which is manifested by the increase of the length of the polymer molecular chain (degree of polymerization) and the increase of the intrinsic viscosity of the polymer. This process of polymer backbone growth is called chain extension, and most chain extension reactions use chain extenders, but the amount is very small (generally about 1% by mass). The chain extender is usually a bifunctional compound, and can easily react with the active groups at the end of the two polymer chains, which can accelerate and effectively shorten the reaction time, so it is also very suitable for reactive extrusion processing on extruders. According to the mechanism, the chain extension reaction can be divided into the following four types: (1) condensation between living end-polymers; (2) condensing with a low molecular coupling agent; (3) utilizing a chain exchange reaction in a polycondensation reaction; (4) anionic polymerization initiated by living anionic chains.
Chain extension reaction has very important practical application value in the synthesis and modification of polymer materials, for example, its great application value has emerged in the recycling of PET resin. Nowadays, PET bottles are widely used in beverage, food, chemical, pharmaceutical packaging and other fields, which leads to the recycling problem of waste beverage bottles. Chain extension reactive extrusion technology is an effective way to solve the recycling problem of PET bottles, which can make waste PET bottles as a new raw material resource and alleviate the contradiction of insufficient PET raw materials in China
[5,48]. The molecular weight or intrinsic viscosity of the recovered PET can be significantly increased by the chain extension reaction, and the increase in molecular weight also compensates for the loss during the reaction. Specifically, in the process of melt extrusion of the recycled PET resin, a certain amount of chain extender is added to react with at least two end groups of the PET macromolecular chain to increase the intrinsic viscosity of the PET and achieve the goal of increasing the molecular weight. Currently, PET chain extenders generally employ addition compounds that can react with either — OH or — COOH or with both functional groups simultaneously, such as oxadiazoline, bicyclic derivatives, dianhydrides, etc
[49⇓~51]. However, some compounds are accompanied by side reactions during the reaction, producing by-products, such as diphenyl carbonate (DPC) condensation chain extender and polyester reaction (Figure 3), which have high boiling points and produce less volatile molecules of phenol, which are difficult to remove and limit the reaction
[52]. Therefore, there are still many problems to be further studied and solved.
3.2 Solvent-free production of PLA resin
3.2.1 Introduction to PLA
PLA is a typical biodegradable and renewable polymer material with biological function. Because of its non-toxicity, biocompatibility and tissue absorbability, it can be completely degraded into carbon dioxide and water, and has been certified by the Food and Drug Administration (FDA) for a long time, and was first applied to the medical industry
[53]. At present, as a green degradable material, low molecular weight polylactic acid is generally used as a drug delivery material, while high molecular weight polylactic acid can be processed into plastic, fiber, film and other polymer materials, which are widely used. Therefore, the synthesis, preparation, processing and application of polylactic acid are the most active. People not only hope that PLA itself has the characteristics of green environmental protection, but also hope to realize the green synthesis of PLA from the production process, and the new preparation technology can further reduce the production cost of PLA, so that it can be widely used like general plastics.
3.2.2 Production route of PLA resin
The preparation methods of PLA are mainly divided into ring-opening polymerization and direct polycondensation
[54]. The ring-opening polymerization of PLA, also known as "lactide ring-opening polymerization": the first step is to prepare lactic acid oligomer with low molecular weight by polycondensation of lactic acid at 140-180 ℃, and then crack and dehydrate the oligomer at a high temperature above 200 ℃ to obtain a crude product of lactide, and then recrystallize the crude product to prepare high-purity lactide; In the second step, the refined high-purity lactide is used to prepare high-molecular weight polylactic acid resin by ring-opening polymerization, and the reaction equation is shown in fig. 4, so it is also called "two-step method".
Because of the high molecular weight and good product performance of the prepared PLA, the two-step method is the most commonly used industrial preparation method of polylactic acid in industry at present, such as Cargill Dow, Shimadzu, Dupont and other international chemical enterprises use this method to produce PLA. Therefore, lactide (catalytic ring-opening) polymerization is also the most mature and oldest method for the preparation of polylactic acid. The relative molecular weight of the polylactic acid prepared by the method is generally more than 100,000. High molecular weight poly (lactic acid) (HMW PLA) with a molecular weight of up to 1 million can be obtained in only 2 ~ 5 H by using Sn(Oct)
2 as catalyst and adjusting the reaction conditions
[55]. In addition to the use of appropriate catalyst, the factors affecting the effect of lactide ring-opening polymerization include polymerization vacuum, temperature, time, etc., but only the purity of lactide monomer is the most critical factor in the whole polymerization process.PLA resin products with high molecular weight, controllable chemical structure and good mechanical properties can be synthesized only by using high-purity lactide. However, high-purity lactide can only be obtained by repeated recrystallization operations, which not only consumes a large amount of solvent, but also has a low product yield. Therefore, although the two-step preparation of PLA is relatively easy to achieve in industrial production, and the produced PLA resin has high molecular weight and good quality, the high production cost seriously hinders the wide application of PLA.
In order to reduce the manufacturing cost of PLA, the direct polycondensation method (also known as one-step method), which uses lactic acid monomers with relatively low price as the basic raw material, through mutual dehydration and esterification, to gradually polycondense into PLA, has attracted people's attention.However, due to the inherent problems such as the kinetic control of dehydration equilibrium, PLA with high molecular weight could not be obtained by "direct polycondensation" before 1995
[56]. As shown in Figure 5, the "direct polymerization method" of PLA belongs to the stepwise polymerization mechanism. Through analysis, it can be seen that the polycondensation reaction is a reversible reaction, and water molecules will be continuously generated during the polymerization process. If the generated water can not be diffused and removed in time, the forward polymerization will be seriously affected, and high molecular weight polylactic acid can not be obtained. Studies have shown that if there is no water removal in the reaction process, the molecular weight of PLA can only be about 10,000
[56,57]. In 1995, Ajioka et al. Of Mitsui Company in Japan used a large amount of organic solvent with high boiling point (200 ~ 250 ℃) in the direct polycondensation of PLA. The purpose was that when lactic acid and polylactic acid were dissolved in the reaction medium with high boiling point, the water was easy to form phase separation without coagulation, and then was removed. Finally, PLA resin with molecular weight up to 300,000 was prepared
[58]. This work made researchers realize that changing the reaction conditions, reducing the viscosity of the system, and enhancing the diffusion effect of water molecules can effectively promote the direct polycondensation of polylactic acid, thus obtaining polylactic acid with higher molecular weight.
3.2.3 Solvent-free production technology of PLA resin
(1) preparation of PLA resin by melt polycondensation
The key to prepare high molecular weight polylactic acid resin by direct polycondensation of lactic acid monomer is water removal. In order to promote the forward direction of the polymerization reaction, a solvent with a high boiling point must be used in the production of polylactic acid by solution polycondensation, and the water molecules and by-products continuously produced in the polycondensation process are removed through the azeotropic principle, so that the reaction balance is continuously broken, and the forward direction of the polymerization reaction is promoted. However, the existence of high boiling point solvents makes the post-treatment process of polymerization products relatively complex. In addition, these high boiling point solvents often contain aromatic benzene ring structure, such as toluene, anisole, etc. In industrial production, a large number of these non-green (or toxic) solvents are used, which will inevitably bring serious harm to human health and living environment in the long run.
The melt polycondensation process for preparing the polylactic acid can avoid using any solvent medium, only needs to add the monomer and the catalyst, and is protected by inert gas (nitrogen), so that the polymerization reaction is carried out at a temperature 15 to 20 deg C higher than the melting point of the monomer and the target polymer, and the whole reaction system is in a molten state.By-products such as water and the like generated by the reaction are continuously discharged through flowing inert gas or vacuum operation to promote the reaction to move in the positive direction, so that not only pure products can be obtained,The post-treatment process is also relatively simplified, so it can further reduce the production cost of PLA, improve productivity and reduce environmental pollution, which is one of the best green synthesis methods of PLA, a green degradable material
[53,59].
However, in the production process of melt polycondensation, with the extension of reaction time, the viscosity of the melt system in the kettle will become larger and larger, and the small molecules produced in the polymerization process will become more and more difficult to be discharged.As a result, the reaction equilibrium is difficult to move to the polymerization direction, especially the melt polycondensation of lactic acid without catalyst, although high-purity PLA resin can be obtained, the relative molecular mass is low, and it is difficult to meet the practical requirements. In addition, there is a serious problem that the preparation of polylactic acid from lactic acid by direct polycondensation is a dynamic equilibrium reaction, and side reactions such as cleavage reaction and cyclization reaction occur in the chain propagation reaction, mainly including acidolysis reaction, alcoholysis reaction, hydrolysis reaction, (lactide) cyclization reaction, and (functional group) elimination reaction (fig. 7). The occurrence of these side reactions has a serious impact on the preparation of high molecular weight PLA resin.
Therefore, there are the following problems in the actual production of PLA by melt polycondensation of lactic acid: (1) In order to achieve high molecular weight, the reaction time of melt polycondensation must be long enough, generally up to 20 ~ 25 H.However, the depolymerization reaction is serious due to the long time of high temperature melt polycondensation, and the lactide by-product produced by the depolymerization reaction is continuously brought out, which accelerates the depolymerization, resulting in a low yield of polylactic acid, the highest reported in the literature is only about 60%
[60]. (2) Long-term high temperature melt polycondensation is easy to darken the color of the polymer, affecting its appearance quality and transparency, and limiting its application; (3) The long time of high temperature melt polycondensation makes the racemization of polylactic acid more serious, which leads to the acceleration of the degradation of polylactic acid and the reduction of mechanical strength; (4) The most important thing is that the molecular weight of PLA obtained by melt polycondensation is still not high enough to meet the application requirements, especially the PLA obtained by melt polycondensation of lactic acid without catalysis.
In order to achieve the purpose of synthesizing high molecular weight PLA resin by melt polycondensation, researchers have carried out a lot of fruitful research work from three key points: kinetic control, effective removal of water and inhibition of degradation. It was not until 2001 that the Japanese research team led by Moon et al. Was able to prepare PLA resin with a molecular weight of more than 100,000 in the laboratory
[61]. In the past 20 years, although there are still further breakthroughs and developments in this research direction, the strategy of preparing high quality and high molecular weight PLA resin by melt polycondensation process alone can not fully meet the requirements of actual production and application. In order to improve the molecular weight of polylactic acid, reactive extrusion, chain extension and solid state polymerization were studied on the basis of melt polycondensation.
(2) reactive extrusion to prepare PLA resin
Faced with the problem of low molecular weight of PLA produced by melt polymerization technology, the United States, as one of the major producers of PLA resin, first proposed the upgrading scheme of melt polymerization technology-reactive extrusion technology, which uses lactic acid oligomer or dimer lactide as raw material and twin-screw extruder as reactor.Through the combination of extrusion modules of the extruder and the regulation and control of process parameters, a proper reaction environment is provided for polymerization, and under the action of an initiator and a catalyst, a polymerization reaction of a polymeric monomer (lactide or oligomer) is completed in a short time during the extrusion process through the extruder, so that the PLA resin with high molecular weight is obtained.
Stevels et al. Studied the polycondensation of L-lactide in a screw extruder
[62]. When the polycondensation temperature was 180 ℃, the rotation speed of the screw extruder was 40 R/min, the residence time in the extruder was 5 min, and the amount of catalyst was 0. 5 wt%, the number-average molecular weight of polylactic acid was 68 000. In 1996, Miyoshi et al. Of JSW Company in Japan reported the melt polycondensation of lactic acid intensified by reactive extrusion, which realized the synthesis and molding of polylactic acid in one step.Specifically, the continuous melt polycondensation experiment of lactic acid was carried out by combining a batch stirring reactor and a twin-screw extruder, and the polylactic acid with a relative molecular mass of 150,000 was successfully obtained from lactic acid through continuous melt polycondensation
[63,64]. The extruder used is specially designed, the reaction cylinder is divided into five sections, the temperature of each section can also be controlled independently, each section is equipped with a vacuum reflux device, on the one hand, it can be used to remove water, on the other hand, it can simultaneously return the lactide or L-lactic acid oligomer produced in the reaction into the cylinder by reflux. Because the surface renewal speed of the twin-screw extruder is greatly improved, the molecular weight of the polylactic acid is also greatly improved, and finally the polylactic acid is directly made into a film product through a T-shaped head, so that the polymerization and molding of the polylactic acids are realized in one step. China started this research relatively late, but it has developed rapidly and has reached the world level. For example, in 2006, Yu Muhuo of Donghua University invented another method to prepare high molecular weight polylactic acid by melt polycondensation, which uses lactic acid and aliphatic dibasic acid as the initial raw materials.Lactic acid prepolymer with carboxyl groups at both ends was obtained, and then a certain proportion of epoxy resin was added to obtain polylactic acid with viscosity-average molecular weight of 130 ~ 220 000 under certain temperature and pressure conditions
[65].
In general, reactive extrusion can realize the polymerization and molding of poly (L-lactic acid) in one step, which has a good prospect for industrialization. The rational design of the extruder is the key to its success, but at the same time, it should be pointed out that there may be great difficulties in realizing large-scale production of this method, mainly because it is difficult to increase the output and the equipment investment is large.
(3) preparing PLA resin by solid phase polymerization
Due to the low temperature of solid phase polymerization, the side reaction of PLA degradation caused by high temperature in the process of melt polycondensation can be significantly reduced, and researchers have also carried out research on the preparation of high molecular weight PLA resin by solid phase polycondensation, and the reaction mechanism is shown in Figure 9: because the polymerization occurs at a temperature lower than the Tm and higher than the Tg,Therefore, in the low molecular weight lactic acid prepolymer (chips, powder, etc., mainly referring to PLA prepared by melt polycondensation), the macromolecular chain is partially "frozen" to form a crystalline region.The active groups at the end of the molecular chain, small molecular monomers and catalysts in the amorphous region can obtain enough energy to approach each other through diffusion, and then collide effectively to promote the polymerization reaction to proceed again. In the process of reaction, small molecular by-products (such as water) produced in the reaction system can be taken away by vacuum operation or flowing inert gas, so that the reaction equilibrium can be carried out in the forward direction to obtain PLA with higher molecular weight.
When the prepolymer is subjected to solid phase polycondensation, PLA with high molecular weight is continuously formed, the crystallinity of the PLA is gradually increased, and small molecular substances (catalyst and lactide) and macromolecular active end groups in the system are gathered in an amorphous region,Further esterification reaction can occur, which is beneficial to the forward reaction and promotes the continuous growth of the molecular chain to obtain products with higher relative molecular mass. These growing molecular chains aggregate at the edge of the crystalline and amorphous regions, which in turn increases the crystallinity of the polymer. Therefore, the crystallinity and crystalline morphology of the prepolymer and the evolution of the crystallinity and crystalline morphology of polylactic acid in the solid state polycondensation process will have a great impact on the solid state polycondensation.
In 2001, Moon et al. Prepared oligomers of lactic acid without catalyst, then added catalyst SnCl
2·2H
2O(0.4 wt%) and p-toluenesulfonic acid (TSA, equimolar to SnCl
2·2H
2O), heated to 180 ° C under mechanical stirring, and gradually reduced the pressure to 10 Torr within 1 H, and cooled after 5 H to obtain a white solid
[66]; The solid was crushed and preheated at 105 ℃ for 1 ~ 2 H, and then subjected to solid phase polycondensation at 150 ℃ and 0. 5 Torr for 20 H to obtain polylactic acid with a molecular weight of up to 670 000. At the same time, the yield of PLLA is more than 90%, the crystallinity is obviously improved, and the racemization of polylactic acid is effectively inhibited.
In 2002, Wang Chaoyang et al. Studied the process conditions of solid phase polymerization of polylactic acid with L-lactic acid monomer as raw material, and obtained a prepolymer with a relative molecular mass of 5000 by melt polymerization, and then carried out solid phase polymerization
[67,68]. The results showed that the viscosity-average molecular weight of polylactic acid was increased to 5. 3 times (265 000) of that before solid state polymerization by using SnCl
2 as catalyst and controlling the reaction temperature at 60 Pa. At the same time, the effects of solid state polymerization time, temperature and temperature control on the relative molecular mass were studied. In the same year, Yu Xingxing et al. Verified the effectiveness of the solid state polymerization method through experiments. First, a prepolymer with a relative molecular mass of 8000 was prepared, and then a polymer with a viscosity-average relative molecular mass of 30,000 was obtained with stannous octoate as a catalyst
[69]. The mechanism of solid state polymerization was discussed by analyzing the crystallinity and melting point of the product of solid state polymerization. Because the low molecular weight of catalyst and the terminal hydroxyl and carboxyl groups of macromolecules in the system gathered in the amorphous region,Further esterification reaction is carried out in the direction of generating polymer, so that the molecular chain is further increased to obtain a product with higher relative molecular mass. In 2004, Qian Gang et al. Studied the feasibility of applying water absorbent to the solid phase polycondensation of polylactic acid, and obtained polylactic acid with a relative molecular weight of 250,000. By changing the catalyst concentration, prepolymer particle size and reaction time, they studied the effects of reaction conditions on the polycondensation process, indicating that the degradation side reaction is still an important factor in controlling the final relative molecular weight of polylactic acid
[70].
Melt-solid phase polycondensation can greatly increase the relative molecular weight of polylactic acid, and the whole process is green production, so it is a promising preparation method, but this method needs to reduce the reaction temperature, the process is long, and the equipment is complex, so there is still a broad space for exploration.
4 Summary and Prospect
From the above discussion, we can see that the solvent-free polymer resin production technology represented by melt polymerization, reactive extrusion and solid state polymerization is a new science and technology integrating polymer resin synthesis, material processing and engineering.It is the frontier field of material science and represents the inevitable trend of the development of polymer resin production technology. In addition to using conventional research methods to carry out these research work, we also hope that you can pay attention to the interaction between the formation and change of condensed state (molten liquid state, general elastic state, solid state) and chemical reaction in the "reaction process", which is precisely the related chemical problems of polymer condensed state worthy of our attention. Without the participation of solvent medium, the interaction between polymer condensed state and chemical reaction becomes more direct and prominent, which is quite different from our previous research work.Therefore, more and more detailed basic research work is needed in this area, especially to understand the relationship between polymer condensed structure and chemical reactions, and to find new problems and solutions from the perspective of condensed chemistry.
Jilin Province Science and Technology Department Science Fund(20200404177YY)
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