Method for regulating and controlling entanglement degree of nascent UHMWPE

Abstract

The invention discloses a nascent state UHMWPE entanglement degree regulation and control method, and relates to the field of polyethylene resin preparation. According to the method, nascent-state ultra-high molecular weight polyethylene powder is placed in an annealing furnace, the time of raw materials in a temperature field is accurately regulated and controlled, then a system is slowly quenched to the room temperature, and ultra-high molecular weight polyethylene is obtained. By accurately controlling the annealing temperature and time, ultra-high molecular weight polyethylene is promoted to overcome crystal nucleation energy barriers, enthalpy drives molecular chains in an amorphous region to participate in crystallization, meanwhile, woven entanglement overcoming energy barriers which are difficult to open in the amorphous region are converted into non-woven entanglement which is easy to open, the content of entanglement molecular chains in the amorphous region is reduced, and the content of the entanglement molecular chains in the amorphous region is reduced. Molecular chains penetrate through each other to form topological constraint structures, the processing difficulty of ultra-high molecular weight polyethylene in compression molding, solution spinning and extrusion molding is reduced, and the yield and superior product rate of ultra-high molecular weight polyethylene products are improved.

Description

Technical Field

[0001] This invention belongs to the field of polymer material performance regulation, specifically relating to a method for regulating the degree of entanglement in nascent UHMWPE. Background Technology

[0002] Ultra-high molecular weight polyethylene (UHMWPE) typically refers to polyethylene with a viscosity-average molecular weight greater than 100 × 10⁻⁶. 4 Linear polyethylene with a molecular weight of g / mol. Its extremely high molecular weight gives it excellent mechanical properties, strong chemical inertness, biocompatibility, high wear resistance and self-lubricating properties, making it widely used in special fields such as bulletproof armor, cut-resistant gloves, wire ropes, and lithium battery separators.

[0003] Currently, nascent UHMWPE exhibits extremely long molecular chains, resulting in numerous chain entanglements that restrict molecular chain orientation, diffusion, and relaxation. This not only leads to extremely high melt viscosity and exceptionally difficult melt processing, but also results in mechanical properties that are only one-third of the theoretical values, far below the expected performance. Furthermore, due to the high melt viscosity of UHMWPE, industrial solution processing of UHMWPE requires the use of large amounts of organic solvents such as white oil or decahydronaphthalene. These small-molecule solvents swell and dissolve the UHMWPE, improving its processing performance. In solution processing of UHMWPE, the mass fraction of UHMWPE is typically only around 10%, resulting in low production efficiency.

[0004] To reduce the initial entanglement level of UHMWPE and thus improve its processing performance, traditional methods include controlling the polymerization process, such as modifying catalysts to prepare low-entanglement polyethylene and lowering the polymerization temperature. To this end, invention patent CN103193908B discloses a method for preparing low-entanglement UHMWPE and its related catalyst. This catalyst is prepared by loading a homogeneous catalyst onto polysilsesquioxane (POSS) particles with a size of 0.01-10 nm, and then loading them onto a porous support. The steric hindrance of the POSS increases the distance between active centers, suppressing the overlapping effect of molecular chains during polymerization, thus preparing low-entanglement ultra-high molecular weight polyethylene. However, this technology involves catalyst design and preparation, making the process more complex. Patent CN104053684B authorizes a method for polymerizing low-entanglement UHMWPE at 25-50℃. This method prepares low-entanglement UHMWPE at a relatively mild temperature by using an optimized ratio of FI catalyst / co-catalyst. However, the method of lowering the temperature used in this approach is not suitable for traditional Ziegler-Natta catalysts because it would reduce the activity of ethylene polymerization, resulting in a decrease in molecular weight and yield. Summary of the Invention

[0005] The technical problem this invention aims to solve is to provide a method for controlling the degree of entanglement in nascent UHMWPE, addressing the limitations of existing technologies. This method utilizes a high-temperature vacuum annealing process to reduce the degree of entanglement in nascent UHMWPE, resulting in superior processing performance and allowing for higher processing concentrations in solution processing, thereby improving production efficiency. This is significant for reducing industrial production costs. The method of this invention can control the degree of entanglement in UHMWPE within a wide range and is applicable to UHMWPE of different molecular weights.

[0006] To achieve the above objectives, the first aspect of the present invention provides a method for controlling the degree of entanglement of nascent ultra-high molecular weight polyethylene (UHMWPE), comprising the following steps: placing nascent UHMWPE powder in an inert gas protection or vacuum environment, setting the temperature within the range of 90-140 ℃, controlling the time the nascent UHMWPE powder acts in the temperature field, and then slowly quenching the system to room temperature using a programmed cooling path to obtain annealed UHMWPE, thereby achieving control of the degree of entanglement; wherein the time the powder acts in the temperature field is 2-60 hours.

[0007] Furthermore, the viscosity-average molecular weight of the nascent ultra-high molecular weight polyethylene powder is 1.0 × 10⁻⁶. 6 -2.0×10 7 g / mol.

[0008] Optionally, when a vacuum environment is used, the vacuum level of the vacuum environment is set to 10. -3 Pa to 10 -1 Within the Pa range, the preferred temperature setting for the vacuum environment is 120-140 ℃.

[0009] Optionally, when an inert gas is used to protect the environment, the inert gas used in the inert gas protection environment is one or more of nitrogen, helium, neon, argon, krypton or xenon; the pressure of the inert gas protection environment is between 0.5 and 2 atmospheres.

[0010] Furthermore, the annealing process takes place over a period of 12-60 hours. The cooling rate of the programmed cooling path is 5-30 °C / min.

[0011] Secondly, the present invention provides an ultra-high molecular weight polyethylene obtained according to the aforementioned method.

[0012] Furthermore, the degree of entanglement of ultra-high molecular weight polyethylene was characterized by DSC heating and cooling program: the UHMWPE was first heated from 30°C to 5-10°C below the melting point, then held at that temperature for 30-60 min, then cooled to 30°C, and then heated again to 170-180°C and cooled to 30°C, wherein the heating and cooling rate was 10°C / min. The secondary heating curve of the ultra-high molecular weight polyethylene obtained by the DSC heating and cooling program shows a bimodal distribution, and the peak area ratio A1 of the low temperature melting peak at 110-138℃ is 40-90%.

[0013] Furthermore, the degree of entanglement of annealed ultra-high molecular weight polyethylene (1-A1 / (A1+A2)) is reduced by 10-60% compared with nascent ultra-high molecular weight polyethylene powder, where A2 is the peak area of ​​the melting peak at 135-155℃ in the secondary heating curve.

[0014] Compared with the prior art, the advantages of the present invention are as follows: This invention performs thermal annealing in an inert gas environment or a high-vacuum annealing system, which eliminates the influence of impurities in the air and effectively avoids oxidation. By controlling the cooling rate, it avoids stress concentration and defects inside the material that may be caused by rapid cooling, which is beneficial to improving the quality of UHMWPE.

[0015] This invention successfully controlled the degree of entanglement in nascent UHMWPE over a wide range by performing an annealing process at an appropriate temperature. In an inert gas environment or a high-vacuum annealing system, nascent ultra-high molecular weight polyethylene powder was placed in a precisely controlled temperature field. By precisely controlling the annealing temperature and time, the ultra-high molecular weight polyethylene overcame the crystal nucleation energy barrier. Enthalpy drove the molecular chains in the amorphous region to participate in crystallization. Simultaneously, the difficult-to-open woven entanglements in the amorphous region overcame the energy barrier and transformed into easily openable non-woven entanglements. The content of entangled molecular chains in the amorphous region decreased, and the number of interpenetrating molecular chains forming topological constraints decreased, thereby achieving the goal of reducing the degree of entanglement in nascent ultra-high molecular weight polyethylene. The degree of entanglement (1-A1 / (A1+A2)) of ultra-high molecular weight polyethylene after annealing was reduced by 10-60%.

[0016] The resulting nascent UHMWPE powder obtained through the above technical solution has a lower degree of entanglement and better processing performance in subsequent processing. Attached Figure Description

[0017] Figure 1 These are the DSC secondary heating curves of ultra-high molecular weight polyethylene obtained in Examples 1-4 and Comparative Example 1.

[0018] Figure 2 These are the DSC secondary heating curves of ultra-high molecular weight polyethylene obtained in Examples 5-8 and Comparative Example 2.

[0019] Figure 3 The relative entanglement degree of ultra-high molecular weight polyethylene obtained in Examples 1-8, Comparative Example 1, and Comparative Example 2. Detailed Implementation

[0020] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0021] The specific information regarding the raw materials used in the embodiments and comparative examples of this invention is described below: UHMWPE-1, viscosity-average molecular weight 4×10 6 g / mol, Manufacturer: Daehan Oil & Chemical UHMWPE-2, viscosity-average molecular weight 9×10 6 g / mol, Manufacturer: Shanghai Lianle Chemical The following analytical testing methods were used in the following embodiments and comparative examples: Entanglement degree characterization: The entanglement degree of ultra-high molecular weight polyethylene is usually characterized by DSC heating and cooling program. First, the temperature is raised from 30℃ to 138℃, then held for 30 min, then lowered to 30℃, then raised to 170℃ and lowered to 30℃, with the heating and cooling rate being 10℃ / min.

[0022] Using the DSC program described above, two peaks can be observed in the secondary heating curve of UHMWPE. Under the same annealing conditions, the proportion of the low-temperature peak area in the secondary heating curve of DSC can be used to characterize the initial entanglement state of UHMWPE. The smaller the ratio of the peak area A1 of the melting peak at 110-138℃ to the peak area A2 of the melting peak at 135-155℃, the higher the initial entanglement density of the ultra-high molecular weight polyethylene. Therefore, 1-A1 / (A1+A2) can be used to characterize the relative magnitude of the entanglement degree. The peak areas A1 and A2 of the melting peak in the secondary heating curve are obtained by integration calculation using Origin software.

[0023] The UHMWPE described in this invention is a typical semi-crystalline polymer with a linear, straight-chain molecular chain structure, exhibiting excellent flexibility and regularity, and therefore typically possesses a high degree of crystallinity. However, due to the near lack of structural coherence in the amorphous regions, quantifying the degree of entanglement is challenging.

[0024] In semi-crystalline polymers, a single molecular chain is partially located in the crystalline phase and partially in the amorphous phase. Therefore, during the enthalpy-driven melting and separation of the molecular chain from the crystalline surface, it is strongly influenced by the topological constraints formed by the entanglement of the molecular chain in the amorphous region with surrounding molecular chains. In other words, the kinetics of the enthalpy relaxation process of UHMWPE molecular chains during annealing largely depend on the degree of entanglement in the amorphous regions of the newly formed semi-crystalline polymer. Therefore, with the aid of a specific DSC procedure, the degree of molecular chain entanglement in the amorphous region can be indirectly characterized by the amount of crystalline component that melts after holding the polymer at 5-10°C below its melting point for a certain time. The area of ​​the low-temperature melting peak, normalized by the total area of ​​the two peaks, indicates the degree of entanglement in the amorphous regions of the semi-crystalline polymer obtained from the reactor.

[0025] A specific DSC procedure results in heterogeneity in the crystalline regions of UHMWPE. One type consists of low-melting-point crystals formed by the recrystallization of molecular chains detached from the surface of the nascent lamellar crystals at the annealing temperature during subsequent cooling. The other type consists of incompletely melted nascent lamellar crystals with higher melting points. The low-temperature peak represents crystals formed by the subsequent cooling of molecular chains detached from the surface of the nascent crystals at 138°C, while the high-temperature peak represents the nascent lamellar crystals. Since UHMWPE is a typical semi-crystalline polymer, crystalline and amorphous regions coexist. The degree of molecular chain entanglement in the amorphous region affects the molecular chain movement in the crystalline region. The lower the degree of molecular chain entanglement in the amorphous region, the stronger the molecular chain movement in the crystalline region, resulting in more molecular chains detaching from the surface of the nascent crystals at 138°C and a higher proportion of the low-temperature peak.

[0026] Example 1 Set the temperature of the vacuum annealing furnace to 120℃ and the vacuum level to 10. -1 Pa. Add UHMWPE-1 virgin powder, keep warm for 24 hours, then set the oven cooling rate to 30℃ / min to cool UHMWPE to room temperature. After cooling, UHMWPE particles with different degrees of entanglement are finally obtained.

[0027] The prepared UHMWPE powder showed a low-temperature peak ratio of 58.3% and an entanglement degree of 41.7% under the above-mentioned DSC specific procedure test results.

[0028] Example 2 Set the temperature of the vacuum annealing furnace to 120℃ and the vacuum level to 10. -3 Pa. Add UHMWPE-1 virgin powder, hold at the temperature for 12 hours, then set the oven cooling rate to 30℃ / min to cool UHMWPE to room temperature. After cooling, UHMWPE powder with different degrees of entanglement is finally obtained.

[0029] The test results of the prepared UHMWPE powder under the above-mentioned DSC specific procedure are as follows: Figure 2 As shown, the low-temperature peak accounts for 63.4% and the entanglement degree is 36.6%.

[0030] Example 3 Set the temperature of the vacuum annealing furnace to 126℃ and the vacuum level to 10. -1 Pa. Add UHMWPE-1 virgin powder, keep warm for 48 hours, then set the oven cooling rate to 30℃ / min to cool UHMWPE to room temperature. After cooling, UHMWPE particles with different degrees of entanglement are finally obtained.

[0031] The test results of the prepared UHMWPE powder under the above-mentioned DSC specific procedure are as follows: Figure 1 As shown, the proportion of low temperature peaks is 76.2%, and the degree of entanglement is 23.8%.

[0032] Example 4 Set the temperature of the vacuum annealing furnace to 128℃ and the vacuum level to 10. -1 Pa. Add UHMWPE-1 virgin powder, hold at the temperature for 36 hours, then set the oven cooling rate to 30℃ / min to cool the UHMWPE to room temperature. After cooling, UHMWPE particles with different degrees of entanglement are finally obtained.

[0033] The prepared UHMWPE powder showed a low-temperature peak ratio of 80.5% and an entanglement degree of 19.5% under the aforementioned DSC specific procedure test.

[0034] Example 5 Set the temperature of the vacuum annealing furnace to 120℃ and the vacuum level to 10. -3 Pa. Add UHMWPE-2 virgin powder, hold at the temperature for 12 hours, then set the oven cooling rate to 30℃ / min to cool the UHMWPE to room temperature. After cooling, UHMWPE particles with different degrees of entanglement are finally obtained.

[0035] The test results of the prepared UHMWPE powder under the above-mentioned DSC specific procedure are as follows: Figure 2 As shown, the proportion of the low-temperature peak is 40.6%, and the degree of entanglement is 59.4%.

[0036] Example 6 Set the temperature of the vacuum annealing furnace to 129℃ and the vacuum level to 10. -1 Pa. Add UHMWPE-2 virgin powder, keep warm for 24 hours, then set the oven cooling rate to 30℃ / min to cool UHMWPE to room temperature. After cooling, UHMWPE particles with different degrees of entanglement are finally obtained.

[0037] The test results of the prepared UHMWPE powder under the above-mentioned DSC specific procedure are as follows: Figure 2 As shown, the proportion of low temperature peaks is 44.7%, and the degree of entanglement is 55.3%.

[0038] Example 7 Set the temperature of the vacuum annealing furnace to 128℃ and the vacuum level to 10. -2 Pa. Add UHMWPE-1 virgin powder, hold at the temperature for 36 hours, then set the oven cooling rate to 30℃ / min to cool the UHMWPE to room temperature. After cooling, UHMWPE particles with different degrees of entanglement are finally obtained.

[0039] The test results of the prepared UHMWPE powder under the above-mentioned DSC specific procedure are as follows: Figure 2 As shown, the low-temperature peak accounts for 65.8% and the entanglement degree is 34.2%.

[0040] Example 8 Set the temperature of the vacuum annealing furnace to 131℃ and the vacuum level to 10. -1 Pa. Add UHMWPE-1 virgin powder, keep warm for 48 hours, then set the oven cooling rate to 30℃ / min to cool UHMWPE to room temperature. After cooling, UHMWPE particles with different degrees of entanglement are finally obtained.

[0041] The test results of the prepared UHMWPE powder under the above-mentioned DSC specific procedure are as follows: Figure 2 As shown, the low-temperature peak accounts for 92.2% and the entanglement degree is 7.8%.

[0042] Comparative Example 1 Set the temperature of the vacuum annealing furnace to 160℃ and the vacuum level to 10. -2 Pa. Add UHMWPE-1 virgin powder, keep warm for 48 hours, then set the oven cooling rate to 30℃ / min to cool UHMWPE to room temperature. After cooling, UHMWPE particles with different degrees of entanglement are finally obtained.

[0043] The test results of the prepared UHMWPE powder under the above-mentioned DSC specific procedure are as follows: Figure 1 As shown, the proportion of low temperature peaks is 18.7%, and the degree of entanglement is 81.3%.

[0044] Comparative Example 2 Set the temperature of the vacuum annealing furnace to 120℃ and the vacuum level to 10. -3 Pa. Add UHMWPE-2 virgin powder, hold at the temperature for 100 hours, then set the oven cooling rate to 30℃ / min to cool the UHMWPE to room temperature. After cooling, UHMWPE particles with different degrees of entanglement are finally obtained.

[0045] The test results of the prepared UHMWPE powder under the above-mentioned DSC specific procedure are as follows: Figure 2 As shown, the proportion of the low-temperature peak is 37.4%, and the degree of entanglement is 62.6%.

[0046] The above provides a detailed description of the method for controlling ultra-high molecular weight polyethylene provided by this invention. Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of these embodiments are merely for the purpose of helping to understand this invention and its core ideas. It should be noted that those skilled in the art can make various improvements and modifications to this invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims of this invention.

Claims

1. A method for controlling the degree of entanglement in nascent ultra-high molecular weight polyethylene, characterized in that, Includes the following steps: Nascent ultra-high molecular weight polyethylene (UHMWPE) powder is placed in an inert gas protection or vacuum environment, and the temperature is set within the range of 90-140 ℃. The time during which the nascent UHMWPE powder is exposed to the temperature field is controlled. Subsequently, the system is slowly quenched to room temperature using a programmed cooling path to obtain annealed UHMWPE, thereby achieving control over the degree of entanglement. The time during which the powder is exposed to the temperature field is 2-60 hours.

2. The method for controlling the degree of entanglement of nascent ultra-high molecular weight polyethylene according to claim 1, characterized in that, The viscosity-average molecular weight of the nascent ultra-high molecular weight polyethylene powder is 1.0 × 10⁻⁶. 6 -2.0×10 7 g / mol.

3. The method for controlling the degree of entanglement of nascent ultra-high molecular weight polyethylene according to claim 1, characterized in that, The vacuum level of the vacuum environment is set at 10. -3 Pa to 10 -1 Within the Pa range, the temperature of the vacuum environment is set in the range of 120-140 ℃.

4. The method for controlling the degree of entanglement of nascent ultra-high molecular weight polyethylene according to claim 1, characterized in that, The inert gas used in the inert gas protective environment is one or more of nitrogen, helium, neon, argon, krypton, or xenon; the pressure of the inert gas protective environment is between 0.5 and 2 atmospheres.

5. The method for controlling the degree of entanglement of nascent ultra-high molecular weight polyethylene according to claim 1, characterized in that, The annealing process takes between 12 and 60 hours.

6. The method for controlling the degree of entanglement of nascent ultra-high molecular weight polyethylene according to claim 1, characterized in that, The cooling rate of the programmed cooling path is 5-30 °C / min.

7. An ultra-high molecular weight polyethylene obtained by the method according to any one of claims 1 to 6.

8. The ultra-high molecular weight polyethylene according to claim 7, characterized in that, The degree of entanglement of ultra-high molecular weight polyethylene was characterized by DSC heating and cooling program: the ultra-high molecular weight polyethylene was first heated from 30℃ to 5-10℃ below the melting point, then held at that temperature for 30-60 min, then cooled to 30℃, and then heated again to 170-180℃ and cooled to 30℃, with heating and cooling rates of 10℃ / min. The secondary heating curve of the ultra-high molecular weight polyethylene obtained by the DSC heating and cooling program shows a bimodal distribution, and the peak area ratio A1 of the low temperature melting peak at 110-138℃ is 40-90%.

9. The ultra-high molecular weight polyethylene according to claim 8, characterized in that, The degree of entanglement of annealed ultra-high molecular weight polyethylene (1-A1 / (A1+A2)) is reduced by 10-60% compared with that of nascent ultra-high molecular weight polyethylene powder, where A2 is the peak area of ​​the melting peak at 135-155℃ in the secondary heating curve.

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