Method for purifying and modifying polypropylene carbonate based on supercritical CO2

Abstract

The invention discloses a method for purifying and modifying polypropylene carbonate based on supercritical CO2. The method is characterized by comprising the following steps: carrying out dry mixing on a polypropylene carbonate raw material and an amino-modified nano filler which accounts for 5-10wt% of the mass of the polypropylene carbonate raw material; adding the mixed material into a screw extruder through a feeder, controlling the operation temperature of the screw extruder to be 60-120 DEG C, injecting supercritical CO2 with the pressure of 8-30 MPa and the temperature of 40-120 DEG C into the middle rear end of the screw extruder, feeding the material treated by the screw extruder into a drop-type devolatilization device, controlling the cavity temperature of the drop-type devolatilization device to be 60-120 DEG C and the vacuum degree to be 20-80 kPa, and carrying out vacuum drying, so as to obtain the high-temperature-resistant low-temperature-resistant low-temperature-resistant low-temperature-resistant low-temperature-resistant low-temperature-resistant low-temperature-resistant low-temperature-resistant low-temperature- The method has the advantages that the content of the propylene carbonate is remarkably reduced, and meanwhile, the mechanical property and the thermal stability of the material are improved.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a method for purifying and modifying polypropylene carbonate based on supercritical CO2. Background Technology

[0002] Polypropylene carbonate, also known as polymethyl ethylene carbonate, or PPC for short, is a fully biodegradable and environmentally friendly plastic synthesized from carbon dioxide and propylene oxide. It can be used in meat preservation films, biodegradable foam materials, sheets, disposable tableware, disposable medical and food packaging materials, and can also be blended with PPAT (biodegradable plastic) to improve the toughness of materials.

[0003] PPC can be polymerized using bulk polymerization of carbon dioxide and propylene oxide. However, as the molecular weight of the polymer increases, coupled with the strong intermolecular interactions of carbonate molecules, the viscosity of the reaction system rises rapidly, hindering the complete reaction and easily leading to catalyst and byproduct (cyclic small molecules) residues. These residues not only impair the mechanical properties and service life of the material but may also pose a threat to human health. According to food safety standards, the total migration of volatile organic compounds (VOCs) in food-grade plastics must not exceed 10 mg / dm².

[0004] However, while polymeric monomers and low-boiling-point solvent molecules are relatively easy to remove, residual cyclic small molecules with boiling points as high as 250℃ are extremely difficult to remove. PPC is unstable and easily decomposes when heated; heating will decompose it into cyclic small molecules, which will actually increase the residue content. Therefore, propylene carbonate (PC) is both a degradation product and a synthesis byproduct, and its content control is particularly critical. When using bulk synthesis processes, screw extruders are often used in industry to maintain the flowability of high-viscosity PPC for devolatilization. However, even with vacuum operation, high temperatures are still required to remove high-boiling-point PC, which will trigger new degradation reactions and form a vicious cycle. In the PPC production process, due to the high viscosity and easy degradation of the material, traditional PPC post-processing sections often use screw extruders. If the extruder temperature is too low, the devolatilization efficiency is poor; if the extruder temperature is too high, it will cause PPC to degrade and produce PC, which will actually increase its residue.

[0005] Chinese patent application publication number CN114773587A discloses a propylene oxide removal process for PPC production. This process includes a customized SCP unit composed of a horizontal self-cleaning SCP reactor. A discharge hopper is installed at the lower discharge end of the SCP unit, which is connected to the feed inlet of a twin-screw extruder, making the SCP unit and the twin-screw extruder a single integrated twin-screw PO removal unit. The main purpose of this process is to remove residual PO and other low-boiling-point organic solvents from the PPC process, while simplifying the PPC post-processing. However, it does not provide specific solutions for the high-temperature thermal degradation of PPC and the removal of high-boiling-point byproducts. Chinese patent application publication number CN101928387A discloses a method and apparatus for washing, coagulating, and devolatilizing aliphatic polycarbonate. The main feature of this method is the disclosure of the washing, coagulating, and devolatilizing apparatus, which includes a polymerization reactor, a flash evaporation tank, a coagulation system consisting of one or more washing and coagulating reactors connected in series, a solid-liquid separation system, a polymer drying system, a gear pump, and connecting pipes. The patent also discloses a method for washing, coagulating, and devolatilizing aliphatic polycarbonate solutions suitable for this apparatus. This patent requires specialized equipment, resulting in high costs. Furthermore, the coagulation and washing process consumes a large amount of solvent, which requires subsequent removal and recovery, further increasing costs. Finally, the final product is obtained through refining, sedimentation, and drying processes, resulting in a long process route and high energy consumption. Existing technologies such as CN114773587A and CN101928387A have not resolved the fundamental contradiction between high-temperature degradation and the removal of high-boiling-point byproducts.

[0006] While supercritical fluid extraction technology has been widely reported in the field of polymer devolatilization (e.g., US2004 / 0116690A1), it is mostly applied to heat-stable materials such as polyolefins, and its process purpose is singular. Applying it to heat-sensitive PPC systems faces challenges such as difficulty in dispersing and mixing CO2 in viscous melts, narrow process parameter windows, and a tendency to lead to uncontrolled foaming. Furthermore, relying solely on physical removal has limited efficiency for PC molecules deeply encapsulated within polymer chains. In summary, PPC post-processing suffers from drawbacks including degradation due to high temperatures, difficulty in removing high-boiling-point PC, long process routes, and limited functionality. Therefore, there is an urgent industrial need to develop a proprietary technology that targets the characteristics of PPC, enabling low-temperature, efficient, and deep PC removal, potentially leading to additional performance improvements. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for purifying and modifying polypropylene carbonate based on supercritical CO2 that significantly reduces the content of propylene carbonate while improving the mechanical properties and thermal stability of the material.

[0008] The technical solution adopted by this invention to solve the above-mentioned technical problems is as follows: a method for purifying and modifying polypropylene carbonate based on supercritical CO2, comprising the following steps: dry mixing polypropylene carbonate raw material with 5-10 wt% of amino-modified nanofiller; feeding the mixture into a screw extruder through a feeder, controlling the operating temperature of the screw extruder at 60-120°C, and injecting supercritical CO2 at a pressure of 8-30 MPa and a temperature of 40-120°C into the rear end of the screw extruder; the material after being processed by the screw extruder enters a strip devolatilizer, controlling the chamber temperature of the strip devolatilizer at 60-120°C and the vacuum degree at 20-80 kPa, and the residence time of the sample in the strip devolatilizer at 10-100 s, thereby obtaining purified and modified polypropylene carbonate.

[0009] Furthermore, the nanofiller is organomontmorillonite, graphene oxide, or polysilsesquioxane. The modification method of the nanofiller is as follows: a substitution reaction is carried out using an amino-containing modifier. The modified nanofiller can improve its reaction with the carbon-based or terminal hydroxyl groups of PPC, thereby improving its compatibility with PPC and achieving uniform dispersion. Because these fillers possess good heat resistance and mechanical properties, their uniform and stable dispersion in the PPC matrix, through the construction of strong interfacial interactions, enhances the thermal stability and mechanical properties of PPC.

[0010] Furthermore, the operating temperature of the screw extruder is 100–110°C.

[0011] Furthermore, the supercritical CO2 is injected at a pressure of 15–25 MPa and a temperature of 90–110 °C. Supercritical CO2 is injected into the devolatilization unit to fully mix with the filler-containing PPC melt, forming a homogeneous and micro-dispersed system. Under the set temperature and pressure, supercritical CO2 exerts the following synergistic effects: 1) Plasticizing and permeation enhancement: reducing the viscosity of the PPC melt and greatly promoting the diffusion rate of PC molecules to the melt-gas interface; 2) Synergistic removal: supercritical CO2 has good solubility for PC, carrying PC to form an enriched phase, which separates from the bulk polymer phase. The further processed material undergoes gas-solid or gas-melt separation in a strip devolatilization unit to obtain purified PPC; the separated CO2 is compressed, condensed, and recycled.

[0012] Furthermore, the cavity temperature of the strip-type devolatilizer is 90–110°C, and the vacuum degree is 60–80 kPa.

[0013] Furthermore, the screw extruder outlet is connected to the inlet of the stripper. The material enters the stripper through the screw extruder. The temperature inside the stripper chamber is controlled by a temperature control system and the pressure is controlled by a vacuum pump. The processed material enters the receiving tank at the bottom of the stripper and is finally sent to the next stage for direct discharge or recycled through the screw extruder for multiple stripper processes to achieve the predetermined PPC product quality indicators.

[0014] Compared with existing technologies, the advantages of this invention are as follows: This invention provides a method for purifying and modifying polypropylene carbonate (PPC) based on supercritical CO2. Utilizing the strong penetration and dissolving capabilities of supercritical CO2, the core devolatilization temperature is reduced from 150-200℃ in traditional processes to below 120℃, fundamentally eliminating the thermal degradation of PPC and breaking the vicious cycle of "high-temperature removal-high-temperature degradation." The PC removal rate is more than 10 times higher than that of simple thermal vacuum removal. Secondly, the strip devolatilization method is a static devolatilization process, without mechanical cutting of the material. At the same temperature, PPC is not easily degraded. Furthermore, different vacuum levels can be adjusted within the strip devolatilization chamber, which facilitates the vaporization and rapid departure of supercritical CO2 after depressurization. Finally, considering the low glass transition temperature and poor mechanical properties of PPC, modified nanofillers are used. These are first premixed with PPC and then fed into a twin-screw extruder. Under the strong shearing and meshing action of the screws, the uniform dispersion and good interfacial bonding of the nanofillers within the PPC matrix are ensured.

[0015] In summary, this invention provides a method for purifying and modifying polypropylene carbonate (PPC) based on supercritical CO2. Modified nanofillers enhance the PPC matrix through interfacial interactions, improving its mechanical properties and thermal stability. CO2 penetrates into the polymer under high pressure and temperature, carrying away residual monomers and promoting filler dispersion. Vacuum devolatilization further removes residues, stabilizing the material structure and preventing volatilization or aging during subsequent use. The filler provides a reinforcing framework, CO2 acts as a "cleaner" and "dispersant" to promote devolatilization and dispersion, and the devolatilizer acts as a "stabilizer." The combination of these three elements forms a physicochemical synergistic devolatilization enhancement system, significantly improving the purity and performance of PPC. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the process flow of the present invention; wherein 1-screw extruder, 2-strip devourer, 3-observation window, 4-camera, 5-vacuum pump, 6-receiving tank, 7-temperature control system. Detailed Implementation

[0017] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified, specific conditions in the embodiments are performed under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0018] The following methods were used to test the structure or properties of the polypropylene carbonate produced in the examples described: PC content: Headspace gas chromatography was used to infer the volatile content in the polymer by detecting the composition of the gas above the headspace vial.

[0019] Glass transition temperature ( T g ): Samples tested using DSC T g .

[0020] Mechanical property testing: Tensile strength and Young's modulus were tested using a universal testing machine in accordance with GB / T 1040.3 standard.

[0021] Nanofillers: Organomontmorillonite, graphene oxide, or polysilsesquioxane can be modified by substitution reactions using conventional amino-containing modifiers. Examples of amino-containing modifiers include amino-modified silicone oils, water-soluble amino-modified alkyd resins, silane coupling agents (such as 3-aminopropyltrimethoxysilane), amino-modified polydimethylsiloxane, or amino-modified silicones. Amino-containing polysilsesquioxanes and amino-modified montmorillonite fillers can also be purchased directly from Maclean's Reagents Company.

[0022] The equipment involved includes a screw extruder 1, a stripper 2, a temperature control system 7, and a vacuum pump 5. The outlet of the screw extruder 1 is connected to the inlet of the stripper 2. The material enters the stripper 2 through the screw extruder 1. Inside the screw extruder 1, it first undergoes vacuum heating and devolatilization to remove most of the low-boiling-point volatile components. Then, in the stripper 2, it is further heated and vacuum extracted to remove the high-boiling-point volatile component PC. The temperature inside the stripper 2 is strictly controlled by the temperature control system 7, and the pressure is controlled by the vacuum pump 5. The processed material enters the receiving tank 6 at the bottom of the stripper 2. Finally, the material is sent to the next stage for direct discharge or recycled through the screw extruder 1 for multiple devolatilization cycles to achieve the predetermined PPC product quality indicators. The temperature control system 7 uses conventional temperature sensors and controllers for precise temperature control. The drop-strip devolatilizer 2 is a static devolatilization device with a simple structure. It forms the devolatilized material into thin strips, which then fall to the bottom of the devolatilization tank by gravity. The devolatilization effect is not affected by excessively high material viscosity, making it particularly suitable for the devolatilization of high-viscosity polymer melts. The drop-strip devolatilizer 2 is designed for visualization. An observation window 3 and a camera 4 are installed on the side wall of the drop-strip devolatilizer 2. A distributed control system (DCS) screen connected to the camera 4 allows for on-site or remote observation of the sample strip's condition, including continuity, melting, and bubbling.

[0023] Example 1 Synergistic devolatilization of amino-containing polysilsesquioxane filler and supercritical CO2: Take PPC raw material (M w =65k) and 10wt% of amino-containing polysilsesquioxane filler (by weight of PPC raw material) were dry-mixed; the mixture was fed into screw extruder 1 through a feeder, and the operating temperature of screw extruder 1 was controlled at 100℃. At the end of screw extruder 1, supercritical CO2 at a pressure of 15MPa and a temperature of 90℃ was injected through a high-pressure metering pump. The treated material entered the strip devolatilizer 2, and the chamber temperature of strip devolatilizer 2 was controlled at 100℃, and the vacuum degree of strip devolatilizer 2 was 80kPa. The obtained strip devolatilized samples were subjected to DSC, headspace and tensile tests, and the test results are shown in Table 1.

[0024] Example 2 PPC raw material (Mw=65k) was dry-mixed with 5wt% of amino-containing polysilsesquioxane filler. The mixture was fed into screw extruder 1 via a feeder, and the operating temperature of screw extruder 1 was controlled at 100℃. At the end of screw extruder 1, supercritical CO2 at a pressure of 20MPa and a temperature of 100℃ was injected through a high-pressure metering pump. The treated material entered a strip devolatilizer 2, and the chamber temperature of strip devolatilizer 2 was controlled at 100℃, and the vacuum degree of strip devolatilizer 2 was 60kPa. The obtained strip devolatilized samples were subjected to DSC, headspace, and tensile tests. The test results are shown in Table 1.

[0025] Example 3 PPC raw material (Mw=65k) was dry-mixed with 8wt% of amino-containing polysilsesquioxane filler. The mixture was fed into screw extruder 1 via a feeder, and the operating temperature of screw extruder 1 was controlled at 110℃. At the end of screw extruder 1, supercritical CO2 at a pressure of 25MPa and a temperature of 110℃ was injected through a high-pressure metering pump. The treated material entered a strip devolatilizer 2, and the chamber temperature of strip devolatilizer 2 was controlled at 110℃, and the vacuum degree of strip devolatilizer 2 was 60kPa. The obtained strip devolatilized samples were subjected to DSC, headspace, and tensile tests. The test results are shown in Table 1.

[0026] Example 4 PPC raw material (Mw=65k) was dry-mixed with 8wt% of amino-modified montmorillonite filler. The mixture was fed into screw extruder 1 via a feeder, and the operating temperature of screw extruder 1 was controlled at 110℃. At the end of screw extruder 1, supercritical CO2 at a pressure of 20MPa and a temperature of 110℃ was injected through a high-pressure metering pump. The treated material entered a strip devolatilizer 2, and the chamber temperature of strip devolatilizer 2 was controlled at 110℃, and the vacuum degree of strip devolatilizer 2 was 60kPa. The obtained strip devolatilized samples were subjected to DSC, headspace, and tensile tests. The test results are shown in Table 1.

[0027] Example 5 PPC raw material (Mw=65k) was dry-mixed with 5wt% (by weight) of amino-modified montmorillonite filler. The mixture was fed into screw extruder 1 via a feeder, and the operating temperature of screw extruder 1 was controlled at 110℃. At the end of screw extruder 1, supercritical CO2 at a pressure of 15MPa and a temperature of 90℃ was injected through a high-pressure metering pump. The treated material entered a strip devolatilizer 2, and the chamber temperature of strip devolatilizer 2 was controlled at 90℃, and the vacuum degree of strip devolatilizer 2 was controlled at 80kPa. The obtained strip devolatilized samples were subjected to DSC, headspace, and tensile tests. The test results are shown in Table 1.

[0028] Comparative Example 1 PPC raw material (Mw=65k) was fed into screw extruder 1 through a feeder. The operating temperature of screw extruder 1 was controlled at 150℃. The obtained sample was subjected to DSC, headspace and tensile tests. The test results are shown in Table 1.

[0029] Comparative Example 2 PPC raw material (Mw=65k) was dry-mixed with 10wt% of amino-containing polysilsesquioxane filler. The mixture was fed into screw extruder 1 through a feeder, and the operating temperature of screw extruder 1 was controlled at 100℃. The resulting samples were subjected to DSC, headspace, and tensile tests, and the test results are shown in Table 1.

[0030] Comparative Example 3 Similar to Example 1 above, the difference lies in omitting the step of injecting supercritical CO2 at a pressure of 15 MPa and a temperature of 90°C through a high-pressure metering pump at the end of the screw extruder 1. The obtained samples were subjected to DSC, headspace, and tensile tests, and the test results are shown in Table 1.

[0031] Table 1. Test results of propylene carbonate samples

[0032] As shown in Table 1 above, functional fillers can improve the glass transition temperature ( T g And its role in improving the mechanical properties of PPC, such as in Comparative Example 1 (no filler, no CO2). T g The temperature was 24.4℃, while in Comparative Example 2 (with filler, no CO2) it was 24.4℃. T g The temperature reached 31.2℃, indicating that the amino-containing polysilsesquioxane filler can significantly improve the thermal stability of PPC. The tensile strength and modulus of Comparative Example 2 (7.47 MPa, 11.34 MPa) were much higher than those of Comparative Example 1 (3.34 MPa, 3.35 MPa), indicating that the filler has a significant reinforcing effect on PPC. The role of supercritical CO2: significantly reducing residual propylene carbonate content: the peak area of ​​propylene carbonate in Comparative Example 2 (without CO2) was 1008.193 pA·s, while in Example 1 (with CO2) it was only 84.704 pA·s, indicating that supercritical CO2 can effectively extract residual monomers or small molecules, playing a role in physical devolatilization and diffusion promotion. Assisting in filler dispersion and devolatilization: the high diffusivity and permeability of supercritical CO2 help promote filler dispersion during melt blending and remove volatile substances. Furthermore, the strip-type devolatilizer 2 has the functions of devolatilization and material structure stabilization: by controlling the vacuum degree and temperature, the strip-type devolatilizer 2 can further remove residual monomers, CO2 and other volatile substances, stabilize the microstructure of the material, and thus improve the mechanical properties.

[0033] The combined process of functional packing + critical CO2 + dropper 2” has a synergistic effect: 1. Significantly reduced residue content: The peak area of ​​propylene carbonate in Example 1 (packing material + CO2 + devolatilizer) was 84.704 pA·s, which was much lower than that of Comparative Example 1 (1156.019) and Comparative Example 2 (1008.193), and even significantly lower than that of Comparative Example 3 (967.306), indicating that the combination of the three can significantly improve the devolatilization efficiency.

[0034] 2. Significant improvement in mechanical properties: The tensile strength of Example 1 is 8.12 MPa and the modulus is 12.19 MPa, which are higher than those of Comparative Example 2 (with filler but no CO2) and all other examples, indicating that the synergistic effect of filler + CO2 + devolatilizer can maximize the improvement of the mechanical properties of PPC.

[0035] 3. Enhanced thermal stability: The Tg of Example 1 was 33.2℃, which was higher than all comparative examples, indicating that the synergistic effect of the filler and CO2 can further increase the glass transition temperature of the material and enhance its high-temperature stability.

[0036] In summary, the method disclosed in this invention significantly reduces the propylene carbonate residue content in polypropylene carbonate (PPC) while simultaneously improving its glass transition temperature and tensile properties. Traditional PPC post-processing often employs a screw extruder (Comparative Example 1). Excessive extruder temperature results in poor devolatilization efficiency, while excessively high extruder temperatures cause PPC degradation, producing PC and increasing its residue. Experiments have shown that the combined process of "functional filler + critical CO2 + strip-type devolatilizer 2" can utilize low-temperature operating conditions to better remove high-boiling-point volatiles and improve the overall performance of PPC.

[0037] The foregoing description is not intended to limit the invention, nor is the invention limited to the examples given. Any changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the invention should also be considered within the protection scope of the invention.

Claims

1. A method for purifying and modifying polypropylene carbonate based on supercritical CO2, characterized in that... The process includes the following steps: dry mixing polypropylene carbonate raw material with 5-10 wt% of amino-modified nanofiller; feeding the mixture into a screw extruder via a feeder, controlling the operating temperature of the screw extruder at 60-120°C, and injecting supercritical CO2 at a pressure of 8-30 MPa and a temperature of 40-120°C into the rear end of the screw extruder; the material processed by the screw extruder enters a strip devolatilizer, controlling the chamber temperature of the strip devolatilizer at 60-120°C and the vacuum degree at 20-80 kPa, with the sample residence time in the strip devolatilizer at 10-100 s, to obtain purified and modified polypropylene carbonate.

2. The method for purifying and modifying polypropylene carbonate based on supercritical CO2 according to claim 1, characterized in that: The nanofiller is organomontmorillonite, graphene oxide, or polysilsesquioxane. The modification method of the nanofiller is as follows: a substitution reaction is carried out using an amino-containing modifier.

3. The method for purification and modification of polypropylene carbonate based on supercritical CO2 according to claim 1, characterized in that: The operating temperature of the screw extruder is 100-110℃.

4. The method for purifying and modifying polypropylene carbonate based on supercritical CO2 according to claim 1, characterized in that: The supercritical CO2 is injected at a pressure of 15–25 MPa and a temperature of 90–110 °C.

5. The method for purifying and modifying polypropylene carbonate based on supercritical CO2 according to claim 1, characterized in that: The cavity temperature of the strip-type devolatilizer is 90-110℃, and the vacuum degree is 60-80kPa.

6. A method for purifying and modifying polypropylene carbonate based on supercritical CO2 according to any one of claims 1-5, characterized in that: The screw extruder outlet is connected to the inlet of the stripper. The material enters the stripper through the screw extruder. The temperature inside the stripper chamber is controlled by a temperature control system and the pressure is controlled by a vacuum pump. The processed material enters the receiving tank at the bottom of the stripper. Finally, the material is sent to the next stage for direct discharge or recycled through the screw extruder for multiple stripper processes to achieve the predetermined PPC product quality indicators.

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