Method for improving molecular weight uniformity of polycarbosilane

Abstract

The invention discloses a method for improving molecular weight uniformity of polycarbosilane, and relates to the field of materials and chemical engineering. A packed distillation tower is additionally arranged in a polycarbosilane (PCS) reaction kettle, low-boiling-point small molecules in the reaction kettle in the synthesis process are distilled and separated out through the packed distillation tower, precise separation of the small molecules is achieved in combination with three-stage heating and a corresponding distillation mode, and the low-medium-boiling-point small molecules are removed in real time through rectification separation. The content of low-boiling-point components easily causing molecular weight fluctuation in the PCS is reduced, the randomness of the softening point of the PCS product is remarkably reduced, and the quality stability of the PCS is improved. And the temperature uniformity is improved, a reaction kettle with a larger volume is adapted, the problems of random molecular weight distribution, unstable softening point and the like of the existing PCS are effectively solved, and a reliable process is provided for preparing the high-quality PCS.

Description

TECHNICAL FIELD

[0001] The application relates to the material and chemical industry field, in particular to a method for improving the uniformity of the molecular weight of polycarbosilane. BACKGROUND

[0002] Polycarbosilane (PCS) is an important precursor for synthesizing silicon carbide (SiC) ceramics by the precursor method, and is widely used in the preparation of continuous SiC fibers and SiC ceramic matrix composites. Professor Yajima of Japan adopted polydimethylsilane (PDMS) as raw material to directly synthesize PCS in an autoclave [Yajima S, Hasegawa Y, Hayashi J, et al. Synthesis of continuous SiC fibers with high tensile strength and modulus [J]. J Mater Sci. 1978. 13: 2569-2576]. Subsequently, domestic scientific research units improved the synthesis method of Professor Yajima, and adopted a two-step method to synthesize PCS, that is, PDMS is first pyrolyzed into liquid silane (LPS), and then LPS is heated again to synthesize PCS, and the reaction process is as shown in the following formula:

[0003]

[0004] The PCS synthesized by the method still contains a certain amount of low molecular weight part, which cannot meet the precursor demand, and the PCS can be used as a precursor only after the low molecular weight part is removed [Xie ZF, et al. High temperature and high pressure synthesis and reduced pressure distillation of polycarbosilane [J]. Journal of National University of Defense Technology, 2006, 28 (2): 39-43].

[0005] When the PDMS is heated and decomposed into LPS, the Si-Si bond in the PDMS molecule is broken to form LPS, and the heating and breaking process of the Si-Si bond is accompanied by a certain randomness, resulting in that the LPS cracked out of the PDMS is a liquid mixture of a plurality of different molecular weight silane molecules containing different Si-Si bonds [Song Yongcai et al. Thermal decomposition of polydimethylsilane [J]. Acta Polymerica Sinica, 1995, 6: 753-757]. In the reaction of synthesizing PCS from LPS, the Si-Si in the LPS is heated and rearranged into the Si-C-Si structure and the Si-H structure in the PCS, and this rearrangement reaction is a bimolecular free radical reaction, and the matching between two molecules is random. Since the molecular weight of the LPS is distributed in a certain way, this rearrangement reaction causes the molecular weight distribution of the synthesized PCS to be wide, and a considerable part of the product is liquid polycarbosilane (LPCS) and gas, and the molecular structure of the PCS is complex; even if the same process conditions are used to synthesize the PCS, the softening point and the molecular weight distribution of the PCS also have a certain randomness.

[0006] In the existing PCS synthesis process, the low-boiling small molecules condensed from the reaction kettle are not separated but directly returned to the reaction kettle, so that the PCS material in the reaction kettle always contains a certain amount of small molecules, the molecular weight dispersion coefficient of the PCS is large, and this brings certain difficulties to the subsequent PCS post-processing for adjusting the softening point; the low-boiling small molecules are returned to the reaction kettle for evaporation again, and this process continues, which takes out a large amount of heat energy from the reaction kettle, resulting in the need for a large power for heating the reaction kettle and a large energy consumption. SUMMARY

[0007] The present application aims at the problems that the PCS synthesized by the existing method contains a certain amount of small molecules, the molecular weight dispersion coefficient is large, and the softening point and the molecular weight distribution have randomness, and provides a method for improving the uniformity of the molecular weight of polycarbosilane. The present application introduces a packed distillation column between the synthesis reaction kettle and the small molecule reflux condenser, a part of the material in the PCS synthesis kettle is converted into steam, the low-boiling small molecular weight part and the gas product in the steam are removed through the packed distillation column, and the part with a larger molecular weight is returned to the reaction kettle, so as to separate the low-boiling small molecules directly in the PCS synthesis reaction, improve the uniformity of the molecular weight of the PCS, and improve the quality of the PCS product.

[0008] To achieve the above-mentioned application purposes, the present application provides the following technical solutions.

[0009] A method for improving the uniformity of the molecular weight of polycarbosilane, comprising the following steps:

[0010] 1) PDMS is pyrolyzed into LPS at 390℃-450℃ under normal pressure in an inert atmosphere, then the LPS is transferred to a reaction kettle with a distillation column, the reaction kettle is vacuumed and replaced with an inert atmosphere, then the reaction kettle is pressurized to the required pressure for the reaction, and heated to the boiling point of the material in the reaction kettle, then heated to 340℃-360℃ while maintaining full reflux in the distillation column;

[0011] 2) The small molecule portion distilled and condensed from the distillation column is received into a receiving tank, and the material temperature is raised to 380-410℃ by controlling the reflux ratio;

[0012] 3) When the material temperature rises to 380-410℃, open all the outlets of the distillation column, change the reflux distillation mode of the distillation column to simple distillation mode, receive all the distilled small molecules into the receiving tank, and continue to heat at a certain heating rate to the final holding temperature;

[0013] 4) The material is heated to 420-470℃, the simple distillation mode of the distillation column is maintained, and after holding for 3-20h, the reaction material is discharged from the reaction kettle and cooled, and the reaction is completed.

[0014] By adding a distillation column to the polycarbosilane (PCS) reaction kettle, the low-boiling point small molecules generated during the synthesis of PCS are distilled and separated, reducing the content of low-boiling point components that can cause fluctuations in molecular weight, significantly reducing the randomness of the softening point of PCS products, and improving the stability of the quality of PCS.

[0015] The distillation column uses packed distillation column, the packing used in the distillation column is Pall ring, Raschig ring, stainless steel mesh theta ring or regular stainless steel mesh packing, as preferred, stainless steel mesh theta ring or regular stainless steel mesh packing is selected, the theoretical tray number of the distillation column packing is 1-5;

[0016] The atmosphere for PDMS pyrolysis and PCS synthesis reaction is anhydrous and oxygen-free inert atmosphere such as nitrogen, helium, argon, etc.; the synthesis pressure range is 0-1.0 MPa (gauge pressure), as preferred, normal pressure reaction is adopted;

[0017] The reaction material has three heating stages during synthesis, and the distillation column also has three corresponding working modes:

[0018] When the material temperature rises from room temperature to 200–310°C, and then to 340–360°C, the distillation column adopts a total reflux mode, where a portion of the low-boiling-point small molecules condensed from the condenser are refluxed back to the distillation column, and the small molecule receiving tank does not receive low-boiling-point small molecules. When the temperature rises from 340–360°C to 380–410°C, the distillation column adopts a partial reflux mode, where a portion of the low-boiling-point small molecules condensed back from distillation are discharged to the small molecule receiving tank, with a reflux ratio of 1:2 to 1:30. The reflux ratio can be adjusted in real time according to changes in the reaction. When the temperature rises from 380–410°C to the final holding temperature of 420–470°C, and in the final holding section of 420–470°C, the distillation column adopts a simple distillation mode, where all the small molecules condensed back from distillation are discharged to the small molecule receiving tank.

[0019] The advantages and beneficial effects of this invention are as follows:

[0020] 1. By combining the distillation column with the reaction vessel, the small molecules generated in the synthesis reaction are continuously separated in the PCS synthesis using the distillation column, thereby reducing the content of small molecules in the PCS and further improving the molecular weight uniformity of the PCS product.

[0021] 2. The use of a distillation column reduces the amount of small molecule liquid boiling and evaporating in the synthesis vessel, lowers the heating power of the reactor, improves the temperature uniformity of the reactor, and allows for the use of a larger volume reactor as the synthesis vessel.

[0022] 3. This method reduces the small molecule content of the synthesized PCS, which is beneficial for PCS post-processing. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the PCS synthesis apparatus without a distillation column, serving as a comparative example. The markings in the diagram are: 1. Reactor; 2. Condenser; 3. Small molecule receiving tank; 4. Small molecule reflux valve; 5. Tail gas discharge valve (the reactor pressure control system is not shown); 6. Vacuum valve; 7. Nitrogen valve.

[0024] Figure 2 This is a schematic diagram of the PCS synthesis apparatus with a packed distillation column according to an embodiment of the present invention. The markings in the diagram are as follows: 1. Reactor; 2. Condenser; 8. Packed distillation column; 9. Small molecule discharge valve; 10. Small molecule receiving tank; 5. Tail gas discharge valve (the reactor pressure control system is not shown); 6. Vacuum valve; 7. Nitrogen valve. Detailed Implementation

[0025] The following embodiments, in conjunction with the accompanying drawings, further illustrate the present invention, but are not intended to limit the scope of protection of the present invention. Equipment, reagents, operating procedures, etc., not specifically described in these embodiments can all be achieved using conventional techniques existing in the art.

[0026] The implementation process of the method for improving the molecular weight uniformity of polycarbosilane according to the present invention is as follows:

[0027] 1) PDMS is pyrolyzed to LPS at 390–450 °C under an inert atmosphere at atmospheric pressure. The LPS is then transferred to a reactor equipped with a packed distillation column. After evacuating and replacing the inert atmosphere, the reactor is pressurized to the required reaction pressure and then heated to begin the PCS synthesis reaction. The inert atmosphere includes nitrogen, argon, or helium. The pressure range of the synthesis reaction is 0–1.0 MPa (gauge pressure), and preferably, the reaction is carried out at atmospheric pressure.

[0028] Since the evaporation rate of small molecules changes during the PCS synthesis reaction, a packed distillation column with high operational flexibility is used. In the reactor with the packed distillation column, the inner diameter of the distillation column can be 1 / 5 to 1 / 3 of the inner diameter of the reactor. The packing used in the distillation column is a bulk packing such as Raschig rings, Pall rings, or θ stainless steel mesh rings, or a structured packing made of stainless steel mesh. Preferably, a structured packing made of θ stainless steel mesh rings or stainless steel mesh is selected. The theoretical plate number of the packing can be 1 to 5, and the packing layer height is 2 to 6 times the inner diameter of the column.

[0029] As the material in the reactor heats up to 200–310°C and boils and evaporates, the steam passes through the distillation column, is condensed by the condenser, and then refluxes back into the distillation column. The heating power of the reactor is adjusted to control the evaporation rate of the material and to maintain total reflux distillation in the distillation column. As the LPS rearrangement reaction proceeds and small molecules are distributed from bottom to top in the distillation column according to their molecular weight decreasing, i.e., low-boiling-point components accumulate at the top of the distillation column, the reactor temperature is slowly increased to 340–360°C.

[0030] 2) Collect the condensed small molecule fraction distilled from the distillation column into the receiving tank, and control a certain reflux ratio to raise the material temperature to 380℃~410℃; the reflux ratio is 1:2~1:30, and the reflux ratio can be adjusted in real time according to the reaction changes.

[0031] 3) When the material temperature in the reactor reaches 380-410℃, turn on the discharge of the distillation column to the full opening, collect all the distilled small molecules into the receiving tank, that is, change part of the reflux distillation mode of the distillation column to simple distillation mode, and continue to heat up to the final holding temperature.

[0032] 4) When the material is heated to the final temperature of 420-470℃, continue to maintain the simple distillation mode of the distillation column, keep it at the temperature for 3-20 hours, and then discharge the reaction material from the reactor to cool it down. The reaction ends.

[0033] Comparative Example 1

[0034] like Figure 1A 300L reactor 1 is used, along with a condenser 2 and a small molecule receiving tank 3. The small molecule receiving tank 3 directly returns the small molecules to the reactor 1 through the small molecule reflux valve 4 at the bottom.

[0035] 1) Pyrolysis of PDMS to LPS obtained by distillation at 420℃ in a nitrogen atmosphere is performed. 200kg of LPS is added to reactor 1. The reactor is evacuated using vacuum valve 6 and then purged with nitrogen three times using nitrogen valve 7. Nitrogen valve 7 and exhaust valve 5 are kept open to maintain nitrogen flow at atmospheric pressure. Reactor 1 is heated slowly. As the temperature of the material in reactor 1 rises to 280℃, the material boils and evaporates. All vapor is condensed by condenser 2 and returned to the small molecule receiving tank 3. The heating power of reactor 1 is controlled to ensure that all small molecules received in small molecule receiving tank 3 are returned to reactor 1. As the reaction proceeds, the temperature of the material in reactor 1 is increased to 350℃ at a rate of 25℃ / h.

[0036] 2) When the temperature of the material in reactor 1 rises to 350℃, the small molecule reflux valve 4 is adjusted to close slightly. A portion of the small molecules distilled from reactor 1 and condensed by condenser 2 are retained in the small molecule receiving tank 3. As the reaction continues, the temperature of the material in reactor 1 rises to 400℃ at a rate of 10℃ / h.

[0037] 3) When the material in reactor 1 slowly heats up to 400℃, close the small molecule reflux valve 4. All the small molecules condensed in condenser 2 are retained in the small molecule receiving tank 3. Continue heating reactor 1, allowing the material in reactor 1 to heat up to the final temperature of 460℃ at a rate of 5℃ / h.

[0038] 4) After holding at 460℃ for 5 hours, stop heating, discharge the material from reactor 1 and cool it in a flowing nitrogen atmosphere.

[0039] The prepared PCS weighed 112 kg, with a number-average molecular weight Mn = 912, a weight-average molecular weight Mw = 2354, a dispersion index D = 2.58, and a softening point of 138℃. In the PCS, components with a molecular weight below 500 accounted for 21%, and components with a molecular weight below 1000 (including those below 500) accounted for 35%.

[0040] Example 1

[0041] like Figure 2 The entire reaction system mainly consists of a 300L reactor 1, a packed distillation tower 8 with a diameter of 100-250mm and filled with θ stainless steel rings of 5-20mm in diameter, wherein the packing height is 500-1500mm, a condenser 2, a small molecule discharge valve 9, a small molecule receiving tank 10, and a tail gas discharge valve 5, a vacuum valve 6, and a nitrogen valve 7.

[0042] 1) Add 200 kg LPS to reactor 1 and evacuate the reactor by vacuuming with vacuum valve 6 to replace the nitrogen gas, then keep nitrogen valve 7 open. As in step 1) of Example 1, heat reactor 1 to raise the temperature. The material boils and evaporates. All vapor rises through distillation column 8 to condense in condenser 2 and is then refluxed back to distillation column 8. Control the heating power of reactor 1 to maintain full reflux distillation. As the reaction proceeds, the material in reactor 1 heats up to 350°C at a rate of 25°C / h.

[0043] 2) When the material temperature in reactor 1 reaches 350℃, open the small molecule discharge valve 9 appropriately to allow a portion of the distilled reflux small molecules to be discharged into the small molecule receiving tank 10. Adjust the small molecule discharge valve 9 to control the reflux ratio at 1:20, and heat reactor 1 slowly to 400℃ at a heating rate of 10℃ / h.

[0044] 3) When the material in reactor 1 slowly heats up to 400℃, fully open the small molecule discharge valve 9 to discharge all the small molecules that have been condensed from distillation column 8 to condenser 2 into small molecule receiving tank 10. Heat reactor 1 to the final holding temperature of 460℃ at a heating rate of 5℃ / h.

[0045] 4) Same as step 4 in Example 1.

[0046] The prepared PCS weighed 115 kg, with a number-average molecular weight (Mn) of 1107, a weight-average molecular weight (Mw) of 2405, a dispersion coefficient (D) of 2.17, and a softening point of 195°C. In the PCS, components with molecular weights below 500 accounted for 4%, and those below 1000 (including those below 500) accounted for 20%. By comparison, the PCS synthesized in Example 2 showed a significant reduction in the content of small molecules with molecular weights below 1000, particularly below 500. This reduction, along with the decrease in the dispersion coefficient (D), improved the uniformity of the PCS's molecular weight and enhanced its stability. At the same heating rate, the heating power of Example 2 was significantly lower than that of Example 1.

[0047] Example 2

[0048] like Figure 2 The system uses a 300L reactor 1, a packed distillation column 8 with an inner diameter of 150mm and filled with 10mm stainless steel mesh θ-ring packing (packing height 800mm, theoretical plate number 3), and a condenser 2. The system also includes a small molecule discharge valve 9, a small molecule receiving tank 10, a tail gas discharge valve 5, a vacuum valve 6, and a nitrogen valve 7.

[0049] 1) Add 200kg LPS to reactor 1, evacuate through vacuum valve 6, and then purge with nitrogen three times through nitrogen valve 7 to maintain normal pressure nitrogen flow; heat reactor 1, and when the material reaches 290℃, it boils and evaporates. The steam rises through packed distillation tower 8 to condenser 2 and is then refluxed back into the tower. Control the heating power to maintain full reflux and raise the temperature to 350℃ at 25℃ / h.

[0050] 2) When the material temperature reaches 350℃, open the small molecule discharge valve 9 to control the reflux ratio of 1:20 (partial reflux mode) and raise the temperature to 400℃ at 10℃ / h.

[0051] 3) When the temperature reaches 400℃, fully open the small molecule discharge valve 9 to switch to simple distillation mode and raise the temperature to 450℃ at 5℃ / h.

[0052] 4) After holding at 450℃ for 8 hours, stop heating and discharge the material to cool in a nitrogen atmosphere.

[0053] The prepared PCS weighed 116 kg, with a number-average molecular weight Mn = 1185, a weight-average molecular weight Mw = 2520, a dispersion index D = 2.13, a softening point of 202℃, and components with molecular weights below 500 accounted for 3.2%, while those below 1000 (including those below 500) accounted for 18%. The product's molecular weight uniformity and stability were further improved, and the heating power was reduced by approximately 22% compared to Example 1.

[0054] Example 3

[0055] like Figure 2 The system uses a 300L reactor 1, a packed distillation column 8 with an inner diameter of 100mm and filled with 5mm regular stainless steel mesh packing (packing height 500mm, theoretical plate number 1), and a condenser 2.

[0056] 1) Add 200 kg LPS to reactor 1, evacuate and replace with argon gas, then pressurize to 0.5 MPa (gauge pressure); heat reactor 1 to 310°C until the material boils, and keep the packed distillation column 8 under full reflux, raising the temperature to 340°C at 20°C / h.

[0057] 2) When the temperature reaches 340℃, open the small molecule discharge valve 9 to control the reflux ratio of 1:2 (partial reflux mode) and raise the temperature to 380℃ at 8℃ / h.

[0058] 3) When the temperature reaches 380℃, switch to simple distillation mode and increase the temperature to 420℃ at 4℃ / h.

[0059] 4) After holding at 420℃ for 20 hours, cool and discharge the material.

[0060] The prepared PCS weighed 110 kg, with a number-average molecular weight Mn = 1050, a weight-average molecular weight Mw = 2380, a dispersion coefficient D = 2.27, a softening point of 188℃, and components with molecular weights below 500 accounted for 5.5%, while those with molecular weights below 1000 (including those below 500) accounted for 22%. This verifies that the method can still effectively improve molecular weight uniformity under boundary conditions.

[0061] This invention adds a packed distillation column to the polycarbosilane (PCS) reactor to separate low-boiling-point small molecules during the synthesis process, thereby reducing the content of low-boiling-point components that easily cause molecular weight fluctuations in PCS, significantly reducing the randomness of the softening point of PCS products, and improving the stability of PCS quality.

[0062] The above embodiments are merely preferred embodiments of the present invention and should not be considered as limiting the scope of the present invention. All equivalent variations and improvements made within the scope of the present invention should still fall within the patent coverage of the present invention.

Claims

1. A method for improving the molecular weight uniformity of polycarbosilane, characterized in that... Includes the following steps: 1) Polydimethylsilane (PDMS) is heated to 390℃~450℃ under normal pressure and in an inert atmosphere to pyrolyze it into LPS. The LPS is then transferred to a reactor equipped with a distillation column. After evacuating and replacing the inert atmosphere, the reactor is pressurized to the pressure required for the reaction and heated until the material in the reactor boils. The distillation column is kept under full reflux and the temperature is increased to 340℃~360℃. 2) Collect the condensed small molecule fraction distilled from the distillation column into the receiving tank, and control a certain reflux ratio to raise the material temperature to 380-410℃; 3) When the material temperature rises to 380-410℃, turn on the discharge of the distillation tower and change the reflux distillation mode of the distillation tower to the simple distillation mode. Collect all the distilled small molecules into the receiving tank and continue to heat them at a certain heating rate until the final holding temperature is reached. 4) Heat the material to 420-470℃, maintain the simple distillation mode of the distillation column, keep it at this temperature for 3-20 hours, then discharge the reaction material from the reactor to cool it down, and the reaction ends.

2. The method for improving the molecular weight uniformity of polycarbosilane as described in claim 1, characterized in that... By adding a distillation column to the polycarbosilane (PCS) reactor, the low- and medium-boiling-point small molecules generated during the PCS synthesis process are separated by distillation. This reduces the content of low-boiling-point components in PCS that are prone to molecular weight fluctuations, lowers the randomness of the softening point of PCS products, and improves the stability of PCS quality.

3. The method for improving the molecular weight uniformity of polycarbosilane as described in claim 1, characterized in that... In step 1), the distillation column is a packed distillation column, and the packing used in the packed distillation column is Pall rings, Raschig rings, stainless steel mesh θ rings or structured stainless steel mesh packing; the theoretical number of plates of the distillation column packing is 1 to 5.

4. The method for improving the molecular weight uniformity of polycarbosilane as described in claim 1, characterized in that... In step 1), the atmosphere for the PDMS pyrolysis and PCS synthesis reaction is an anhydrous and oxygen-free inert atmosphere such as nitrogen, helium, or argon; the synthesis pressure range is 0–1.0 MPa.

5. The method for improving the molecular weight uniformity of polycarbosilane as described in claim 4, characterized in that... The synthesis pressure is atmospheric pressure.

6. The method for improving the molecular weight uniformity of polycarbosilane as described in claim 1, characterized in that... The synthesis process involves three heating stages for the reactants, and the distillation column also has three corresponding operating modes: When the material temperature is raised from room temperature to 200-310℃, and then to 340-360℃, the distillation column adopts a total reflux mode. The low-boiling-point small molecules condensed from the condenser are all refluxed back to the distillation column, and the small molecule receiving tank does not receive low-boiling-point small molecules. During the process of heating from 340-360℃ to 380-410℃, the distillation column adopts a partial reflux mode, that is, a portion of the low-boiling-point small molecules that are distilled and condensed back are discharged to the small molecule receiving tank. The reflux ratio is 1:2 to 1:30, and the reflux ratio is adjusted in real time according to the reaction changes. When the temperature is raised from 380-410℃ to the final holding temperature of 420-470℃, and the final holding section of 420-470℃, the distillation column adopts a simple distillation mode, and all small molecules condensed back from the distillation are discharged to the small molecule receiving tank.

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