A process for the production of ultra-high molecular weight polyethylene

Abstract

This invention belongs to the field of polymer materials technology, specifically relating to a production process for ultra-high molecular weight polyethylene (UHMWPE). By optimizing the catalyst system and polymerization process, this invention improves the performance and production stability of UHMWPE. A co-catalyst composed of alkylaluminum and organosiloxane modifiers, acting synergistically with external electron donors, significantly enhances catalyst activity and stereoselectivity, resulting in a more stable polymerization process, higher molecular weight and narrower molecular weight distribution, and significantly enhanced mechanical properties. The multi-stage polymerization process achieves precise control of the polymerization process through segmented temperature control, coupled with a dual-circulation heat removal system to effectively remove reaction heat, avoid localized overheating, and ensure a uniform and stable product structure. An integrated solvent recovery and molecular sieve drying system enables efficient recycling of hexane, controlling moisture content to ≤5ppm, preventing catalyst poisoning, ensuring stable continuous production, and overall improving the process's economy and reliability.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a production process for ultra-high molecular weight polyethylene. Background Technology

[0002] Ultra-high molecular weight polyethylene (UHMWPE) possesses irreplaceable application value in high-end medical implants, high-performance fibers, bulletproof materials, and industrial wear-resistant components due to its excellent wear resistance, high impact strength, self-lubricating properties, and chemical stability. Currently, industrially, it is mainly produced using the Ziegler-Natta catalyst system for slurry polymerization. However, traditional processes generally suffer from problems such as low catalyst activity, uneven exothermic polymerization leading to excessively wide molecular weight distribution, and unstable product mechanical properties. Furthermore, in continuous production processes, trace amounts of residual moisture during solvent recovery and recycling can easily cause catalyst poisoning and deactivation, affecting the continuity and economic efficiency of production.

[0003] In existing technologies, to improve catalyst efficiency, external electron donors are often added or the type of alkylaluminum is adjusted. However, problems such as insufficient stereoselectivity control and unsatisfactory polymer chain structure regularity still exist. Furthermore, polymerization processes often operate in a single-temperature zone, which is not conducive to the gradual formation and control of molecular chain structure, and the heat removal efficiency is limited, easily leading to localized overheating and polymer degradation. Regarding solvent recovery, conventional stripping and condensation processes are insufficient to reduce the moisture content in the circulating solvent below the catalyst's tolerance threshold, resulting in a significant decrease in activity over long-term operation.

[0004] Therefore, there is an urgent need to research and develop a production process for ultra-high molecular weight polyethylene in order to solve the problems existing in the current technology. Summary of the Invention

[0005] The purpose of this invention is to address the existing problems by providing a production process for ultra-high molecular weight polyethylene.

[0006] This invention is achieved through the following technical solution:

[0007] A production process for ultra-high molecular weight polyethylene includes the following steps:

[0008] S1. Catalyst preparation:

[0009] A composite cocatalyst containing alkylaluminum cocatalyst and organosiloxane modifier, an external electron donor, and purified hexane are mixed online, and then mixed online with a titanium-containing main catalyst to form a catalytic slurry.

[0010] S2, multi-stage polymerization reaction:

[0011] The catalytic slurry, refined hexane, and ethylene are continuously fed into the polymerization reactor to carry out a slurry polymerization reaction.

[0012] S3. Post-treatment and solvent recovery:

[0013] The polymerized slurry is degassed by flash evaporation and then subjected to steam stripping. The vapor phase generated at the top of the stripping tower is condensed and separated into layers. The resulting aqueous phase is discharged, and the resulting organic phase is dehydrated and returned as a hexane for recycling. The stripped polyethylene wet powder is dried, graded, sieved, and blended to obtain ultra-high molecular weight polyethylene products.

[0014] Further, in step S1, the water content of the purified hexane is ≤5ppm, the mass ratio of the composite co-catalyst to the titanium-containing main catalyst is (2~8):1, the molar ratio of the organosiloxane modifier to the alkylaluminum co-catalyst is (0.05~0.5):1, the molar ratio of the external electron donor to the titanium-containing main catalyst is (0.1~1.0):1, and the concentration of the titanium-containing main catalyst in the catalytic slurry is 0.01~0.1g / L.

[0015] Further, the alkylaluminum cocatalyst mentioned in step S1 is selected from one or more of triethylaluminum, triisobutylaluminum, or diethylaluminum chloride;

[0016] The organosiloxane modifier is selected from one or more of methoxysilane, ethoxysilane, or propoxysilane;

[0017] The external electron donor is selected from one or more of dimethyldimethoxysilane, diethyldimethoxysilane, methyl tert-butyldimethoxysilane, or cyclohexylmethyldimethoxysilane.

[0018] Furthermore, the titanium-containing main catalyst mentioned in step S1 is a supported titanium catalyst with magnesium chloride as the support, and the mass fraction of titanium element is 3.0~4.5% based on the total mass of the catalyst.

[0019] Furthermore, the preparation of the titanium-containing main catalyst includes the following steps:

[0020] (1) Under N2 protection, anhydrous magnesium chloride powder was added to purified hexane at a mass-volume ratio of 1g:10~12mL, and stirred to form a suspension. The system temperature was raised to 50~60℃ and stirred at a constant temperature for 1~2h.

[0021] (2) Add ethyl benzoate to the above suspension. The amount of ethyl benzoate added is 1 / 40 of the volume of purified hexane. Stir at a constant temperature for 30-40 min. Then add titanium tetrachloride dropwise at a uniform rate over 2 h. The amount of titanium tetrachloride added is 1 / 4 of the volume of purified hexane. Control the reaction temperature to be maintained at 50-55℃. After the addition is complete, raise the reaction temperature to 70℃ and continue stirring for 2-3 h.

[0022] (3) After the reaction is complete, stop heating, let it stand and settle, remove the supernatant (mother liquor), wash the solid product with 100 mL of purified hexane at 60 °C, repeat the washing 5 times until no white precipitate is detected by AgNO3 solution in the last washing liquid, and dry the washed solid product at 60 °C and vacuum degree -0.095 MPa for 6~8 h.

[0023] Further, in step S2, the ratio of the ethylene feed flow rate to the hexane feed flow rate is (0.1~0.3):1;

[0024] The polymerization reaction is carried out in a first temperature zone and a second temperature zone in sequence. The reaction temperature in the first temperature zone is 50~65℃, the pressure is 0.3~0.5MPaG, and the reaction time is 0.2~1h. The reaction temperature in the second temperature zone is 70~85℃, the pressure is 0.5~0.8MPaG, and the reaction time is 1.5~4h.

[0025] During the polymerization reaction, a dual circulation system including jacketed circulating cooling and in-reactor condensation reflux circulation is used for heat removal. The inlet temperature of the cooling water in the jacketed circulating cooling is 30~35℃ and the outlet temperature is 40~45℃. The amount of gas phase extracted from the in-reactor condensation reflux circulation is 10~30% of the total gas phase in the polymerization reactor, and the reflux temperature of the condensate is 25~35℃.

[0026] Furthermore, step S2 also includes the step of introducing hydrogen gas as a molecular weight regulator in the later stage of polymerization in the second temperature section, and the introduction time is the last 1 / 3 stage of the reaction time in the second temperature section.

[0027] Furthermore, the concentration of hydrogen in the reaction system is 80~150ppm. When preparing general-purpose ultra-high molecular weight polyethylene with a viscosity-average molecular weight greater than 1 million and less than 5 million, the hydrogen concentration is 120~150ppm; when preparing medical-grade ultra-high molecular weight polyethylene special material with a viscosity-average molecular weight greater than or equal to 5 million, the hydrogen concentration is 80~100ppm.

[0028] Further, the flash degassing conditions in step S3 are a temperature of 60~70℃ and a pressure of 0.01~0.03MPaG; the flash vapor generated by flash evaporation is first pre-cooled to 40~50℃ by heat exchange with the cold material to be entered into the polymerization reactor, and then condensed and recovered.

[0029] The conditions for steam stripping are a temperature of 85~95℃ and a pressure of 2~3kPa, with the steam flow rate to slurry mass ratio being (0.5~1.0):1; the vapor phase generated at the top of the stripping tower is condensed to 30~40℃ and then separated into layers.

[0030] Furthermore, the dehydration of the organic phase in step S3 is performed using a molecular sieve dryer. The molecular sieve dryer uses 3A or 4A molecular sieves, the regeneration temperature of the molecular sieve is 200~250℃, and the regeneration cycle is 8~12h.

[0031] Furthermore, the drying conditions for the polyethylene wet powder in step S3 are: drying at 100~110℃ under a nitrogen atmosphere for 2~4 hours, and the moisture content of the polyethylene powder after drying is ≤0.15%;

[0032] The mesh size range of the grading and screening is 20~200 mesh, and the mass fraction of particles below 40 mesh after screening is ≤2.0%.

[0033] The present invention has the following advantages over the prior art:

[0034] The ultra-high molecular weight polyethylene (UHMWPE) production process provided by this invention achieves significant improvements in product performance and stable, efficient production through optimized catalyst system and polymerization process control. First, a composite cocatalyst comprising an alkylaluminum cocatalyst and an organosiloxane modifier, combined with the synergistic effect of an external electron donor, significantly enhances catalyst activity and stereoselectivity. The organosiloxane modifier effectively disperses active centers and prevents aggregation, while the external electron donor regulates the stereoregularity of polymer chains. The synergistic effect of both makes the polymerization reaction more stable and controllable, resulting in polyethylene products with higher molecular weight and narrower molecular weight distribution, and significantly enhanced mechanical properties such as tensile strength and impact strength. Second, the multi-stage polymerization process achieves precise control of the polymerization process by setting reaction stages with different temperatures and pressures. The low-temperature stage is conducive to the formation of active centers and chain initiation, while the high-temperature stage promotes chain growth and molecular weight increase. Combined with a dual-circulation cooling system, the heat of reaction is effectively controlled, avoiding molecular chain breakage or cross-linking caused by localized overheating, further ensuring the uniformity of product structure and performance stability. Finally, the integrated solvent recovery and drying system achieves efficient recycling of hexane solvent through multi-stage treatment including flash evaporation, stripping, and molecular sieve dehydration, while strictly controlling moisture content to extremely low levels. This not only reduces raw material consumption and production costs, but more importantly, it avoids the poisoning of catalyst activity by moisture, ensuring the stability of continuous production and product consistency, thus improving the overall economic efficiency and reliability of the technology. Attached Figure Description

[0035] Figure 1 The tensile strength comparison results of ultra-high molecular weight polyethylene in Example 2 and each comparative example are shown.

[0036] Figure 2 The impact strength comparison results are shown for ultra-high molecular weight polyethylene in Example 2 and each comparative example. Detailed Implementation

[0037] To further explain the present invention, the following specific embodiments are described.

[0038] Example 1

[0039] A production process for ultra-high molecular weight polyethylene includes the following steps:

[0040] S1. Catalyst preparation:

[0041] A composite cocatalyst containing an alkylaluminum cocatalyst and an organosiloxane modifier, an external electron donor, and purified hexane are mixed online, and then mixed online with a titanium-containing main catalyst to form a catalytic slurry; the purified hexane has a water content ≤5ppm, the mass ratio of the composite cocatalyst to the titanium-containing main catalyst is 2:1, the molar ratio of the organosiloxane modifier to the alkylaluminum cocatalyst is 0.05:1, the molar ratio of the external electron donor to the titanium-containing main catalyst is 0.1:1, and the concentration of the titanium-containing main catalyst in the catalytic slurry is 0.01g / L;

[0042] The alkylaluminum co-catalyst is triethylaluminum;

[0043] The organosiloxane modifier is a methoxysilane;

[0044] The external electron donor is dimethyldimethoxysilane;

[0045] The preparation of the titanium-containing main catalyst includes the following steps:

[0046] (1) Under N2 protection, anhydrous magnesium chloride powder was added to purified hexane at a mass-volume ratio of 1g:10mL, and stirred to form a suspension. The system temperature was raised to 50℃ and stirred at a constant temperature for 1h.

[0047] (2) Add ethyl benzoate to the above suspension. The amount of ethyl benzoate added is 1 / 40 of the volume of purified hexane. Stir at a constant temperature for 30 min. Then add titanium tetrachloride dropwise at a uniform rate over 2 h. The amount of titanium tetrachloride added is 1 / 4 of the volume of purified hexane. Control the reaction temperature to be maintained at 50 °C. After the addition is complete, raise the reaction temperature to 70 °C and continue stirring for 2 h.

[0048] (3) After the reaction is complete, stop heating, let it stand and settle, remove the supernatant (mother liquor), wash the solid product with 100 mL of purified hexane at 60 °C, repeat the washing 5 times until no white precipitate is detected by AgNO3 solution in the last washing liquid, and dry the washed solid product at 60 °C and vacuum degree -0.095 MPa for 6 h.

[0049] S2, multi-stage polymerization reaction:

[0050] The catalyst slurry, purified hexane, and ethylene are continuously fed into the polymerization reactor for slurry polymerization. The feed flow rate of ethylene to that of purified hexane is 0.1:1. The polymerization reaction proceeds sequentially through a first temperature stage and a second temperature stage. The reaction temperature in the first temperature stage is 50°C, the pressure is 0.3 MPaG, and the reaction time is 0.2 h. The reaction temperature in the second temperature stage is 70°C, the pressure is 0.5 MPaG, and the reaction time is 1.5 h. During the polymerization reaction, a dual circulation system, including jacketed circulating cooling and in-reactor condensation reflux circulation, is used for heat removal. The cooling water inlet temperature of the jacketed circulating cooling system is 30°C, and the outlet temperature is 40°C. The amount of gas phase extracted from the in-reactor condensation reflux circulation system is 10% of the total gas phase in the polymerization reactor, and the reflux temperature of the condensate is 25°C.

[0051] S3. Post-treatment and solvent recovery:

[0052] The polymerized slurry is flash-degassed at 60°C and 0.01 MPaG. The flash vapor generated is pre-cooled to 40°C by heat exchange with the cold material to be introduced into the polymerization reactor, and then condensed and recovered. The flash-degassed slurry is then stripped by steam at 85°C and 2 kPa, with the steam flow rate to slurry mass ratio being 0.5:1. The vapor phase generated at the top of the stripping tower is condensed to 30°C and then separated into layers. The aqueous phase is discharged, and the organic phase is dehydrated by a molecular sieve dryer to a water content ≤5 ppm and then returned for recycling as hexane. The stripped polyethylene wet powder is dried at 100°C under a nitrogen atmosphere for 2 hours to a water content ≤0.15%, and then sieved and blended through a 20-mesh sieve to obtain ultra-high molecular weight polyethylene product. The molecular sieve dryer uses 3A or 4A molecular sieves, the regeneration temperature of the molecular sieve is 200°C, and the regeneration cycle is 8 hours.

[0053] Example 2

[0054] A production process for ultra-high molecular weight polyethylene includes the following steps:

[0055] S1. Catalyst preparation:

[0056] A composite cocatalyst containing an alkylaluminum cocatalyst and an organosiloxane modifier, an external electron donor, and purified hexane are mixed online, and then mixed online with a titanium-containing main catalyst to form a catalytic slurry; the purified hexane has a water content ≤5ppm, the mass ratio of the composite cocatalyst to the titanium-containing main catalyst is 5:1, the molar ratio of the organosiloxane modifier to the alkylaluminum cocatalyst is 0.3:1, the molar ratio of the external electron donor to the titanium-containing main catalyst is 0.5:1, and the concentration of the titanium-containing main catalyst in the catalytic slurry is 0.05g / L;

[0057] The alkylaluminum co-catalyst is triethylaluminum;

[0058] The organosiloxane modifier is a methoxysilane;

[0059] The external electron donor is dimethyldimethoxysilane;

[0060] The preparation of the titanium-containing main catalyst includes the following steps:

[0061] (1) Under N2 protection, anhydrous magnesium chloride powder was added to purified hexane at a mass-volume ratio of 1g:11mL, and stirred to form a suspension. The system temperature was raised to 55℃ and stirred at a constant temperature for 1.5h.

[0062] (2) Add ethyl benzoate to the above suspension. The amount of ethyl benzoate added is 1 / 40 of the volume of purified hexane. Stir at a constant temperature for 35 min. Then add titanium tetrachloride dropwise at a uniform rate over 2 h. The amount of titanium tetrachloride added is 1 / 4 of the volume of purified hexane. Control the reaction temperature to be maintained at 52℃. After the addition is complete, raise the reaction temperature to 70℃ and continue stirring for 2.5 h.

[0063] (3) After the reaction is complete, stop heating, let it stand and settle, remove the supernatant (mother liquor), wash the solid product with 100 mL of purified hexane at 60 °C, repeat the washing 5 times until no white precipitate is detected by AgNO3 solution in the last washing liquid, and dry the washed solid product at 60 °C and vacuum degree -0.095 MPa for 7 h.

[0064] S2, multi-stage polymerization reaction:

[0065] The catalyst slurry, purified hexane, and ethylene are continuously fed into the polymerization reactor for slurry polymerization. The feed flow rate of ethylene to that of purified hexane is 0.2:1. The polymerization reaction proceeds sequentially through a first temperature stage and a second temperature stage. The reaction temperature in the first temperature stage is 58°C, the pressure is 0.4 MPaG, and the reaction time is 0.5 h. The reaction temperature in the second temperature stage is 78°C, the pressure is 0.6 MPaG, and the reaction time is 2.5 h. During the polymerization reaction, a dual circulation system, including jacketed circulating cooling and in-reactor condensation reflux circulation, is used for heat removal. The cooling water inlet temperature of the jacketed circulating cooling system is 32°C, and the outlet temperature is 42°C. The amount of gas phase extracted from the in-reactor condensation reflux circulation system is 20% of the total gas phase in the polymerization reactor, and the condensate reflux temperature is 30°C.

[0066] S3. Post-treatment and solvent recovery:

[0067] The polymerized slurry is flash-degassed at 65°C and 0.02 MPaG. The flash vapor generated is pre-cooled to 45°C by heat exchange with the cold material to be introduced into the polymerization reactor, and then condensed and recovered. The flash-degassed slurry is then steam-stripped at 90°C and 2.5 kPa, with the steam-to-slurry mass ratio being 0.7:1. The vapor phase generated at the top of the stripping tower is condensed to 35°C and then separated into layers. The aqueous phase is discharged, and the organic phase is dehydrated in a molecular sieve dryer to a water content ≤5 ppm and then returned for recycling as hexane. The stripped polyethylene wet powder is dried at 105°C in a nitrogen atmosphere for 3 hours to a water content ≤0.15%, and then sieved and blended through a 100-mesh sieve to obtain ultra-high molecular weight polyethylene product. The molecular sieve dryer uses 3A or 4A molecular sieves, the regeneration temperature of the molecular sieve is 220°C, and the regeneration cycle is 10 hours.

[0068] Example 3

[0069] A production process for ultra-high molecular weight polyethylene includes the following steps:

[0070] S1. Catalyst preparation:

[0071] A composite cocatalyst containing an alkylaluminum cocatalyst and an organosiloxane modifier, an external electron donor, and purified hexane are mixed online, and then mixed online with a titanium-containing main catalyst to form a catalytic slurry; the purified hexane has a water content ≤5ppm, the mass ratio of the composite cocatalyst to the titanium-containing main catalyst is 8:1, the molar ratio of the organosiloxane modifier to the alkylaluminum cocatalyst is 0.5:1, the molar ratio of the external electron donor to the titanium-containing main catalyst is 1.0:1, and the concentration of the titanium-containing main catalyst in the catalytic slurry is 0.1g / L;

[0072] The alkylaluminum co-catalyst is triethylaluminum;

[0073] The organosiloxane modifier is a methoxysilane;

[0074] The external electron donor is dimethyldimethoxysilane;

[0075] The preparation of the titanium-containing main catalyst includes the following steps:

[0076] (1) Under N2 protection, anhydrous magnesium chloride powder was added to purified hexane at a mass-volume ratio of 1g:12mL, and stirred to form a suspension. The system temperature was raised to 60℃ and stirred at a constant temperature for 2h.

[0077] (2) Add ethyl benzoate to the above suspension. The amount of ethyl benzoate added is 1 / 40 of the volume of purified hexane. Stir at a constant temperature for 40 min. Then add titanium tetrachloride dropwise at a uniform rate over 2 h. The amount of titanium tetrachloride added is 1 / 4 of the volume of purified hexane. Control the reaction temperature to be maintained at 55℃. After the addition is complete, raise the reaction temperature to 70℃ and continue stirring for 3 h.

[0078] (3) After the reaction is complete, stop heating, let it stand and settle, remove the supernatant (mother liquor), wash the solid product with 100 mL of purified hexane at 60 °C, repeat the washing 5 times until no white precipitate is detected by AgNO3 solution in the last washing liquid, and dry the washed solid product at 60 °C and vacuum degree -0.095 MPa for 8 h.

[0079] S2, multi-stage polymerization reaction:

[0080] The catalyst slurry, purified hexane, and ethylene are continuously fed into the polymerization reactor for slurry polymerization. The feed flow rate of ethylene to that of purified hexane is 0.3:1. The polymerization reaction proceeds sequentially through a first temperature stage and a second temperature stage. The reaction temperature in the first temperature stage is 65°C, the pressure is 0.5 MPaG, and the reaction time is 1 h. The reaction temperature in the second temperature stage is 85°C, the pressure is 0.8 MPaG, and the reaction time is 4 h. During the polymerization reaction, a dual circulation system including jacketed circulating cooling and in-reactor condensation reflux circulation is used for heat removal. The cooling water inlet temperature of the jacketed circulating cooling system is 35°C, and the outlet temperature is 45°C. The amount of gas phase extracted from the in-reactor condensation reflux circulation system is 30% of the total gas phase in the polymerization reactor, and the condensate reflux temperature is 35°C.

[0081] S3. Post-treatment and solvent recovery:

[0082] The polymerized slurry is flash-degassed at 70°C and 0.03 MPaG. The flash vapor generated is pre-cooled to 50°C by heat exchange with the cold material to be introduced into the polymerization reactor, and then condensed and recovered. The flash-degassed slurry is then steam-stripped at 95°C and 3 kPa, with the steam flow rate to slurry mass ratio being 1.0:1. The vapor phase generated at the top of the stripping tower is condensed to 40°C and then separated into layers. The aqueous phase is discharged, and the organic phase is dehydrated by a molecular sieve dryer to a water content ≤5 ppm and then returned for recycling as hexane. The stripped polyethylene wet powder is dried at 110°C under a nitrogen atmosphere for 4 hours to a water content ≤0.15%, and then sieved and blended through a 200-mesh sieve to obtain ultra-high molecular weight polyethylene product. The molecular sieve dryer uses 3A or 4A molecular sieves, the regeneration temperature of the molecular sieve is 250°C, and the regeneration cycle is 12 hours.

[0083] Example 4

[0084] Compared with Example 2, Example 4 replaces the alkylaluminum cocatalyst with triisobutylaluminum, while the other steps and parameters are the same as in Example 2.

[0085] Example 5

[0086] Compared with Example 2, Example 5 replaces the triethylaluminum cocatalyst with diethylaluminum chloride, while the other steps and parameters are the same as in Example 2.

[0087] Example 6

[0088] Compared with Example 2, Example 6 replaces the organosiloxane modifier with ethoxysilane instead of methoxysilane, while the other steps and parameters are the same as in Example 2.

[0089] Example 7

[0090] Compared with Example 2, Example 7 replaces the organosiloxane modifier with propoxysilane instead of methoxysilane, while the other steps and parameters are the same as in Example 2.

[0091] Example 8

[0092] Compared with Example 2, Example 8 replaces the external electron donor with diethyldimethoxysilane instead of dimethyldimethoxysilane, while the other steps and parameters are the same as in Example 2.

[0093] Example 9

[0094] Compared with Example 2, Example 9 replaces the external electron donor with methyl tert-butyl dimethoxysilane instead of dimethyl dimethoxysilane, while the other steps and parameters are the same as in Example 2.

[0095] Example 10

[0096] Compared with Example 2, Example 10 replaces the external electron donor with cyclohexylmethyldimethoxysilane instead of dimethyldimethoxysilane, while the other steps and parameters are the same as in Example 2.

[0097] Comparative Example 1

[0098] Compared with Example 2, this comparative example removes the organosiloxane modifier from the composite cocatalyst, while the other steps and parameters are the same as in Example 2.

[0099] Comparative Example 2

[0100] Compared with Example 2, Comparative Example 2 does not add an external electron donor, but the other steps and parameters are the same as in Example 2.

[0101] Comparative Example 3

[0102] Compared with Example 2, Comparative Example 3 replaced the polymerization reaction with a single-stage polymerization, with a reaction temperature of 68°C, a pressure of 0.5 MPaG, and a reaction time of 3 hours. The total duration was the same as in Example 2, and the other steps and parameters were the same as in Example 2.

[0103] Comparative Example 4

[0104] Compared with Example 2, in Comparative Example 4, the organic phase after stripping is not dehydrated by a molecular sieve dryer and is directly returned for recycling as purified hexane. Other steps and parameters are the same as in Example 2.

[0105] Comparative Example 5

[0106] Compared with Example 2, in the preparation of the titanium-containing catalyst, the ethyl benzoate in step (2) is omitted in Comparative Example 5, while the other steps and parameters are the same as in Example 2.

[0107] Performance testing

[0108] (1) The weight-average molecular weight, molecular weight distribution, melt index, and density of the ultra-high molecular weight polyethylene of Examples 1-10 and Comparative Examples 1-5 were tested, and the test methods are as follows:

[0109] Weight-average molecular weight (Mw): Tested according to ASTM D6474 standard / method;

[0110] Molecular weight distribution (PDI): Tested according to the standard / method of ASTM D6474;

[0111] Melt flow index (MI): Tested according to ASTM D1238 (190°C, 21.6 kg) standard / method;

[0112] Density: Tested according to ASTM D1505 standard / method.

[0113] The test results are shown in Table 1.

[0114] Table 1

[0115] Grouping <![CDATA[Mw(×10 6 g / mol)]]> PDI (Mw / Mn) MI (g / 10min) <![CDATA[Density (g / cm 3 ).]]> Example 1 3.2 5.8 0.05 0.935 Example 2 4.5 5.2 0.03 0.938 Example 3 5.8 6.0 0.02 0.940 Example 4 4.3 5.3 0.04 0.937 Example 5 4.0 5.5 0.05 0.936 Example 6 4.6 5.1 0.03 0.939 Example 7 4.4 5.4 0.04 0.938 Example 8 4.5 5.3 0.03 0.938 Example 9 4.7 5.0 0.02 0.940 Example 10 4.8 4.9 0.02 0.941 Comparative Example 1 3.0 6.5 0.10 0.930 Comparative Example 2 2.8 7.0 0.12 0.928 Comparative Example 3 3.5 8.2 0.08 0.932 Comparative Example 4 3.8 6.0 0.06 0.934 Comparative Example 5 2.5 9.0 0.15 9.925

[0116] As shown in Table 1, with the increase of the ratio of composite catalyst to main catalyst, the proportion of external electron donor, and the catalyst concentration in the catalyst formulation, Mw increases significantly, and the molecular weight distribution slightly widens. The decrease in melt index indicates a decline in flow properties, making it more suitable for high-strength and high-toughness applications. The significant improvement in mechanical properties indicates that high molecular weight is beneficial for enhancing toughness.

[0117] Compared with Example 2, Examples 4 and 5 show that by changing the type of alkylaluminum cocatalyst, triethylaluminum exhibits the best performance, followed by triisobutylaluminum, and diethylaluminum chloride is the worst. The alkylaluminum structure affects the formation and stability of the catalyst's active center, which in turn affects the molecular weight and distribution.

[0118] Compared with Example 2, Examples 6 and 7 show that by changing the type of organosiloxane modifier, methoxysilane performs better than ethoxysilane and propoxysilane. This indicates that short-chain alkyl groups are beneficial to improving catalyst dispersibility and activity, thereby obtaining a more uniform polymer.

[0119] Compared with Example 2, by changing the type of external electron donor, cyclohexylmethyldimethoxysilane showed the best performance and the narrowest molecular weight distribution in Examples 8-10, indicating that the external electron donor can adjust the stereoselectivity of the catalyst and improve the regularity of the molecular chain.

[0120] (2) To further compare the technical effects of the present invention, the tensile strength and impact strength of the ultra-high molecular weight polyethylene of Example 2 and Comparative Examples 1 to 5 were tested.

[0121] The testing method is as follows:

[0122] Tensile strength: Tested according to ASTM D638 standard / method;

[0123] Impact strength: Tested according to ASTM D256 standard / method;

[0124] Test results are as follows Figure 1 As shown.

[0125] From Table 1 and Figure 1 As can be seen, compared with Example 2, Comparative Example 1 removed the organosiloxane modifier in the composite cocatalyst, resulting in a significantly wider polymer molecular weight distribution and a comprehensive decrease in mechanical properties. This indicates that the organosiloxane modifier is crucial for maintaining the dispersion of active centers and preventing agglomeration. Its absence leads to uneven catalyst efficiency, poor controllability of the polymerization reaction, and difficulty in forming high molecular weight products with uniform structure.

[0126] Comparative Example 2, without the addition of an external electron donor, produced the product with the lowest molecular weight, the widest molecular weight distribution, and the worst performance. This confirms the crucial role of external electron donors in regulating the stereoselectivity of the catalyst and guiding the directional insertion of ethylene monomers. Without external electron donors, the polymer chain stereoregularity is poor, making it impossible to form the regular structure required for high performance.

[0127] Comparative Example 3 changed the multi-stage polymerization to single-stage polymerization. Although the total reaction time was the same, the molecular weight distribution of the product deteriorated sharply, and the performance declined. This shows that a simple single-stage process cannot achieve precise control of the polymerization process. The multi-stage polymerization used in this invention, through the segmented setting of temperature and pressure, can orderly control the chain initiation and growth process, which is the key to obtaining narrow-distribution, high-performance products.

[0128] Comparative Example 4 omits the molecular sieve dehydration step in solvent recovery. While the short-term impact on molecular weight is manageable, this process leads to the continuous accumulation of moisture in the circulating solvent. Trace amounts of moisture can undergo irreversible reactions with the alkylaluminum co-catalyst, deactivating it. Long-term operation will result in a continuous decline in catalyst efficiency, making stable production impossible, highlighting the necessity of deep dehydration for continuous and stable production.

[0129] Comparative Example 5 omitted the internal electron donor, ethyl benzoate, during the preparation of the main catalyst. This resulted in an incomplete active site structure, extremely low catalytic efficiency, and the smallest molecular weight and worst performance of the resulting polymer. This demonstrates, conversely, that a complete main catalyst preparation process, especially the introduction of an internal electron donor, is fundamental to forming a highly active and selective catalytic system.

[0130] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A production process for ultra-high molecular weight polyethylene, characterized in that, Includes the following steps: S1. Catalyst preparation: A composite cocatalyst containing alkylaluminum cocatalyst and organosiloxane modifier, an external electron donor, and purified hexane are mixed online, and then mixed online with a titanium-containing main catalyst to form a catalytic slurry. S2, multi-stage polymerization reaction: The catalytic slurry, refined hexane, and ethylene are continuously fed into the polymerization reactor to carry out a slurry polymerization reaction. S3. Post-treatment and solvent recovery: The polymerized slurry is degassed by flash evaporation and then subjected to steam stripping. The vapor phase generated at the top of the stripping tower is condensed and separated into layers. The resulting aqueous phase is discharged, and the resulting organic phase is dehydrated and returned as a hexane for recycling. The stripped polyethylene wet powder is dried, graded, sieved, and blended to obtain ultra-high molecular weight polyethylene products.

2. The production process for ultra-high molecular weight polyethylene according to claim 1, characterized in that, In step S1, the water content of the purified hexane is ≤5ppm, the mass ratio of the composite co-catalyst to the titanium-containing main catalyst is (2~8):1, the molar ratio of the organosiloxane modifier to the alkylaluminum co-catalyst is (0.05~0.5):1, the molar ratio of the external electron donor to the titanium-containing main catalyst is (0.1~1.0):1, and the concentration of the titanium-containing main catalyst in the catalytic slurry is 0.01~0.1g / L.

3. The production process for ultra-high molecular weight polyethylene according to claim 1, characterized in that, The alkylaluminum cocatalyst mentioned in step S1 is selected from one or more of triethylaluminum, triisobutylaluminum, or diethylaluminum chloride; The organosiloxane modifier is selected from one or more of methoxysilane, ethoxysilane, or propoxysilane; The external electron donor is selected from one or more of dimethyldimethoxysilane, diethyldimethoxysilane, methyl tert-butyldimethoxysilane, or cyclohexylmethyldimethoxysilane.

4. The production process for ultra-high molecular weight polyethylene according to claim 1, characterized in that, The titanium-containing main catalyst mentioned in step S1 is a supported titanium catalyst with magnesium chloride as the support, and the mass fraction of titanium element is 3.0~4.5% based on the total mass of the catalyst.

5. The production process for ultra-high molecular weight polyethylene according to claim 1, characterized in that, In step S2, the ratio of the ethylene feed flow rate to the hexane feed flow rate is (0.1~0.3):1; The polymerization reaction is carried out in a first temperature zone and a second temperature zone in sequence. The reaction temperature in the first temperature zone is 50~65℃, the pressure is 0.3~0.5MPaG, and the reaction time is 0.2~1h. The reaction temperature in the second temperature zone is 70~85℃, the pressure is 0.5~0.8MPaG, and the reaction time is 1.5~4h. During the polymerization reaction, a dual circulation system including jacketed circulating cooling and in-reactor condensation reflux circulation is used for heat removal. The inlet temperature of the cooling water in the jacketed circulating cooling is 30~35℃ and the outlet temperature is 40~45℃. The amount of gas phase extracted from the in-reactor condensation reflux circulation is 10~30% of the total gas phase in the polymerization reactor, and the reflux temperature of the condensate is 25~35℃.

6. The production process for ultra-high molecular weight polyethylene according to claim 5, characterized in that, Step S2 also includes the step of introducing hydrogen gas as a molecular weight regulator in the later stage of polymerization in the second temperature range, with the introduction time being the last 1 / 3 of the reaction time in the second temperature range.

7. The production process for ultra-high molecular weight polyethylene according to claim 6, characterized in that, The concentration of hydrogen in the reaction system is 80-150 ppm. When preparing general-purpose ultra-high molecular weight polyethylene with a viscosity-average molecular weight greater than 1 million and less than 5 million, the hydrogen concentration is 120-150 ppm. When preparing medical-grade ultra-high molecular weight polyethylene special material with a viscosity-average molecular weight greater than or equal to 5 million, the hydrogen concentration is 80-100 ppm.

8. The production process for ultra-high molecular weight polyethylene according to claim 1, characterized in that, The conditions for flash degassing in step S3 are a temperature of 60~70℃ and a pressure of 0.01~0.03MPaG. The flash vapor generated by flash evaporation is first pre-cooled to 40~50℃ by heat exchange with the cold material to be entered into the polymerization reactor, and then condensed and recovered. The conditions for steam stripping are a temperature of 85~95℃ and a pressure of 2~3kPa, with the steam flow rate to slurry mass ratio being (0.5~1.0):1; the vapor phase generated at the top of the stripping tower is condensed to 30~40℃ and then separated into layers.

9. The production process for ultra-high molecular weight polyethylene according to claim 1, characterized in that, The organic phase dehydration in step S3 is performed using a molecular sieve dryer. The molecular sieve dryer uses 3A or 4A molecular sieves, and the regeneration temperature of the molecular sieve is 200~250℃, with a regeneration cycle of 8~12h.

10. The production process of ultra-high molecular weight polyethylene according to claim 1, characterized in that, The drying conditions for the polyethylene wet powder in step S3 are: drying at 100~110℃ under a nitrogen atmosphere for 2~4 hours, and the moisture content of the polyethylene powder after drying is ≤0.15%; The mesh size range of the grading and screening is 20~200 mesh, and the mass fraction of particles below 40 mesh after screening is ≤2.0%.

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