A method for molecular weight distribution control for low pressure high density polyethylene production

Abstract

This invention discloses a method for controlling the molecular weight distribution in the production of low-pressure high-density polyethylene, which relates to the field of polymer material polymerization production technology, including the following steps: (1) Catalyst pre-activation: Select Ziegler-Natta catalyst or chromium-based catalyst and activate it at 90-110℃ for 2-4 hours under an inert atmosphere; (2) Preparation of composite molecular weight regulator: It is prepared by compounding α-methylstyrene dimer and thiol compounds at a weight ratio of 80-90:10-20; (3) Two-stage polymerization: The temperature of the first stage is 70-80℃, and the hydrogen concentration is 0.5-1.0 mol%, generating high molecular weight segments; the temperature of the second stage is 85-95℃, and the hydrogen concentration is 1.5-2.5 mol%, and the composite regulator is added online to generate low molecular weight segments; (4) Post-treatment: After degassing, drying and granulation, HDPE products with controllable molecular weight distribution are obtained, which solves the problems of low control precision, large fluctuation and poor compatibility of traditional processes.

Description

Technical Field

[0001] This invention relates to the field of polymer material polymerization production technology, specifically to a method for controlling the molecular weight distribution in the production of low-pressure high-density polyethylene, which is particularly suitable for the precise adjustment of the molecular weight and molecular weight distribution of HDPE products under low-pressure polymerization processes such as slurry polymerization and gas-phase polymerization. Background Technology

[0002] High-density polyethylene (HDPE) is one of the most widely produced and applied general-purpose plastics in the modern petrochemical and new materials industries. With its high mechanical strength, excellent resistance to environmental stress cracking, good chemical stability, and moderate processing fluidity, HDPE is widely used in high-end industrial fields such as municipal water supply and drainage pipes, pressure pipelines, packaging films, hollow blow-molded containers, automotive injection molded parts, and cable sheaths. As downstream applications increasingly demand higher material performance, the molecular weight and distribution of polyethylene resin have become core indicators determining the final performance and processing suitability of products.

[0003] In the industrial production of low-pressure high-density polyethylene (HDPE), molecular weight directly affects the material's strength, toughness, and heat resistance, while the molecular weight distribution width determines melt flowability, molding and processing stability, product appearance, and long-term service life. The distribution width is typically characterized by the molecular weight distribution index (PDI, the ratio of weight-average molecular weight to number-average molecular weight). For pipe and hollow products, a wider molecular weight distribution is needed to ensure melt strength and crack resistance; for injection molding and film products, a moderate or narrower distribution is required to ensure dimensional accuracy and optical properties. If the molecular weight distribution is too wide, problems such as uneven melt strength, a narrow processing temperature window, rough product surface, and uneven crystalline region distribution may occur. If the molecular weight distribution is too narrow, it will lead to poor processing flowability, high extrusion torque, insufficient product toughness, and decreased resistance to environmental stress cracking. Therefore, achieving predictable, adjustable, and stable control of the molecular weight distribution in the continuous production of low-pressure HDPE is a key technology for producing high-end specialty materials.

[0004] Currently, industrial plants both domestically and internationally widely use hydrogen as a chain transfer agent to regulate the molecular weight of polyethylene, controlling the molecular chain length by altering the partial pressure of hydrogen in the polymerization system. However, single-agent hydrogen regulation has significant limitations: First, hydrogen has a uniform effect on molecular chain transfer, making it impossible to achieve differentiated control between high-molecular-weight and low-molecular-weight components; second, changes in hydrogen concentration have a drastic impact on reactivity, yield, and melt index, easily causing fluctuations in product quality; third, relying solely on hydrogen makes it difficult to achieve wide-range, high-precision distribution regulation, especially in adjusting the distribution width while maintaining a relatively constant molecular weight.

[0005] To improve distribution control capabilities, existing technologies propose two main approaches: First, a dual-reactor series process is used, generating high-molecular-weight fractions and low-molecular-weight fractions separately in different reactors before physical mixing. While this method broadens the distribution, it involves high equipment investment, complex processes, and difficulty in matching residence time distributions, easily leading to uneven mixing and fractional precipitation. Second, a composite catalyst system is used, utilizing the kinetic differences of different active sites to control the distribution. However, this method is sensitive to catalyst purity, formulation, and pretreatment conditions, easily resulting in activity imbalance, large bimodal fluctuations in molecular weight, and poor batch stability. Furthermore, traditional processes lack online fine-tuning and flexible switching molecular weight regulators, making it impossible to quickly adjust the distribution index according to downstream demand, and thus difficult to adapt to the trend of flexible production with multiple grades, small batches, and high added value.

[0006] In summary, existing low-pressure high-density polyethylene production technologies still have the following technical shortcomings:

[0007] Single hydrogen regulation is insufficient for precise control of molecular weight distribution and cannot achieve flexible regulation over a wide range;

[0008] Solutions such as dual reactors and composite catalysts are complex, costly, and have poor compatibility with operating conditions.

[0009] The regulator system is singular and lacks a composite regulation method that is low in toxicity, highly efficient, and does not affect polymerization activity;

[0010] The lack of a synergistic matching mechanism between process parameters and regulators can easily lead to fluctuations in product performance.

[0011] It is difficult to adjust the molecular weight distribution while maintaining the stability of key indicators such as melt index and density. Summary of the Invention

[0012] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for controlling the molecular weight distribution in the production of low-pressure high-density polyethylene, using the following technical solution:

[0013] Includes the following steps:

[0014] (1) Catalyst pre-activation: Select ZieglerNatta catalyst (ZieglerNatta is a coordination polymerization catalyst) or chromium-based catalyst, and activate it at 90-110℃ for 2-4h under an inert atmosphere;

[0015] (2) Preparation of composite molecular weight regulator: It is prepared by compounding α-methylstyrene dimer and thiol compounds in a weight ratio of 80-90:10-20;

[0016] (3) Two-stage polymerization: The first stage temperature is 70-80℃, and the hydrogen concentration is 0.5-1.0 mol%, to generate high molecular weight segments; the second stage temperature is 85-95℃, and the hydrogen concentration is 1.5-2.5 mol%, with a composite regulator added online to generate low molecular weight segments.

[0017] (4) Post-processing: HDPE products with controllable molecular weight distribution are obtained by degassing, drying and granulation.

[0018] Furthermore, the thiol compounds are selected from one or more combinations of n-octylthiol, dodecylthiol, and hexadecylthiol, providing stronger chain transfer capabilities and precise fine-tuning of the proportion of low molecular weight segments. In synergy with AMSD, the molecular weight dispersion index can be continuously adjusted in a wide range of 3.0-15.0, while not affecting catalyst activity or producing strong odors, ensuring food contact / medical grade applicability.

[0019] Furthermore, the mass concentration of the composite regulator in the polymerization system is 50-300 ppm. The regulator, which is a compound of α-methylstyrene dimer (AMSD) and thiols in a ratio of 80-90:10-20, is added online to the second stage polymerization reaction at a dosage of 50-300 parts per million relative to the total mass of the system.

[0020] Furthermore, the partial pressure of ethylene polymerization in the first stage is 0.8-1.2 MPa, and the residence time is 60-90 min. This is intended to provide a stable supply of monomers and sufficient chain growth time for the reaction, ensuring the full generation of high molecular weight chain segments, guaranteeing the mechanical properties of the product, and laying a stable foundation for the second stage to regulate low molecular weight components and achieve a wide molecular weight distribution.

[0021] Furthermore, the partial pressure of ethylene polymerization in the second stage is the same as that in the first stage, and the residence time is 40-60 min. This ensures stable reaction conditions and balanced monomer concentration in both stages, maintaining the melt index and density essentially unchanged. Controlling the residence time to 40-60 min allows for the full generation of low molecular weight segments under high temperature, high hydrogen, and composite regulators. These segments are then rationally proportioned with the high molecular weight segments in the first stage, ultimately achieving a wide range, precise, and stable control of the HDPE molecular weight distribution.

[0022] Furthermore, the molecular weight distribution index (PDI) of the final product is 3.0-15.0. By controlling the PDI of the final product within the range of 3.0-15.0, continuous and stable control from narrow distribution to ultra-wide distribution can be achieved. Under the premise of keeping the melt index and density basically unchanged, it can independently meet the performance and processing requirements of different high-end special materials such as injection molding, film, pipe, blow molding, and pressure pipeline, significantly improving the resistance to environmental stress cracking and reducing processing torque.

[0023] Furthermore, the method is applicable to slurry or vapor phase low-pressure HDPE production processes, and is well compatible with the operating conditions of mainstream industrial plants at home and abroad. It can be implemented without modifying equipment, and achieves wide-range control of molecular weight distribution while ensuring stable production, thus possessing significant industrial application value.

[0024] Furthermore, the method can independently adjust the molecular weight distribution width while maintaining stable melt index and density, avoiding the defects of large fluctuations in melt index and density when adjusting the distribution in traditional hydrogen conditioning processes. It can accurately match the requirements of molecular weight distribution for different application scenarios while stabilizing the basic physical properties of the product, significantly improving the product's processing performance and mechanical properties.

[0025] Furthermore, the prepared HDPE can be used for high-end specialty materials such as pipes, films, injection molding, blow molding, and pressure pipelines, indicating that the method of the present invention can meet the differentiated requirements of different application scenarios for material mechanical properties, processing fluidity, and service life by controlling the molecular weight distribution over a wide range. It can achieve stable, efficient, and flexible production of high-end polyethylene specialty materials, and has significant industrial application value and market prospects.

[0026] Furthermore, the composite regulator has no irritating odor and is suitable for the production of food contact grade and medical grade polyethylene. It can overcome the shortcomings of traditional thiol regulators, such as strong odor, high residue, and inability to meet hygiene and safety requirements. While achieving wide-range molecular weight distribution control, it ensures that the product is non-toxic, odorless, environmentally friendly and safe, and can meet the stringent requirements of high-end hygiene-grade application scenarios such as food packaging and medical materials.

[0027] The present invention has the following advantages:

[0028] This invention employs four key methods in synergistic coupling: catalyst pre-activation, composite molecular weight regulator, segmented temperature control, and graded hydrogen concentration. These methods control the uniformity of active centers, chain transfer strength, proportion of high molecular weight segments, and proportion of low molecular weight segments, respectively. This achieves precise, independent, wide-range, and continuously adjustable control over the molecular weight distribution of HDPE, ultimately ensuring that PDI stabilizes within the 3.0-15.0 range. This solves the problems of low control precision, large fluctuations, and poor compatibility in traditional processes. Attached Figure Description

[0029] Figure 1 This is a comparison chart of the molecular weight distribution curves of GPC in the embodiments and comparative examples of the present invention. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] This invention provides a method for controlling the molecular weight distribution in the production of low-pressure high-density polyethylene, comprising the following steps:

[0032] Catalyst pre-activation: Select ZieglerNatta catalyst or chromium-based catalyst and activate it at 90-110℃ for 2-4 h under an inert atmosphere to remove adsorbed water and residual alcohol, unify active centers, make the polymerization rate more stable, avoid local over-polymerization / under-polymerization, and ensure that the ratio of high molecular weight segment to low molecular weight segment is controllable. Function: to lay a solid controllable foundation and ensure that subsequent regulation does not go astray.

[0033] The compound molecular weight regulator is composed of α-methylstyrene dimer (AMSD) and thiol compounds in a weight ratio of 80-90:10-20. This is the most crucial step in achieving a continuously adjustable PDI from 3.0 to 15.0. AMSD (main): mild chain transfer, does not affect catalyst activity, odorless, suitable for food grade. Thiol (auxiliary): strong chain transfer, fine-tunes the proportion of low molecular weight. After compounding: the proportion of low molecular weight can be continuously and smoothly changed by adding concentration → PDI is continuously adjustable from narrow (3.0) to ultra-wide (15.0). The thiols are selected from one or more of n-octyl mercaptan, dodecyl mercaptan, and hexadecyl mercaptan. This regulator can selectively control the generation of low molecular weight components, does not affect the growth of high molecular weight segments, does not produce irritating odor, and is suitable for the production of food contact grade and medical grade polyethylene.

[0034] Two-stage polymerization: The first stage, at 70-80℃, results in slow chain growth and minimal chain transfer, strengthening the high molecular weight segment. A hydrogen concentration of 0.5-1.0 mol% generates long-chain, high molecular weight components, providing strength and toughness. The second stage, at 85-95℃, enhances chain transfer and accelerates chain growth, strengthening the low molecular weight segment. A hydrogen concentration of 1.5-2.5 mol% generates short-chain, low molecular weight components, providing flowability and processability. An online composite regulator is added to generate low molecular weight segments. Hydrogen achieves a bimodal framework of high and low molecular weights, forming a broad distribution structure. The dual enhancement of the bimodal structure by temperature and hydrogen results in a wider and more stable distribution.

[0035] Post-processing: HDPE products with controllable molecular weight distribution are obtained through degassing, drying, and granulation.

[0036] Raw materials and equipment

[0037] Ethylene: Polymer grade, purity ≥99.95%; ZieglerNatta catalyst: Titanium-magnesium supported catalyst; Chromium-based catalyst: Chromium oxide / silica gel supported type; Composite regulator: AMSD + dodecyl mercaptan (mass ratio 85:15); Hydrogen: High purity ≥99.99%; Hexane solvent: Industrial grade, dehydrated and deoxygenated; Additives: Phenolic antioxidant 1010, Phosphite-assisted antioxidant 168, Calcium stearate.

[0038] Polymerization unit: Two-stage continuous slurry polymerization reactor, effective volume 5L / stage, equipped with temperature control, hydrogen regulation, and online feeding system. Testing equipment: High-temperature gel permeation chromatography (GPC), melt indexer, electronic tensile testing machine, simply supported beam impact testing machine, and environmental stress cracking resistance tester.

[0039] Example 1 (Narrow distribution injection molding grade, PDI≈3.5)

[0040] Catalyst: ZieglerNatta, activated at 100℃ for 3 hours;

[0041] Regulator dosage: 80 ppm;

[0042] First stage: 75℃, H2 0.6 mol%, stay for 75 min;

[0043] Second stage: 90℃, H2 1.8 mol%, stay for 50 min;

[0044] Post-processing: Degassing → Drying → Granulation.

[0045] Example 2 (medium-distribution thin film level, PDI≈6.0)

[0046] Catalyst: ZieglerNatta;

[0047] Regulator dosage: 150 ppm;

[0048] First section: 75℃, H2 0.6 mol%;

[0049] Second paragraph: 90℃, H2 2.0 mol%

[0050] Target PDI≈6.0.

[0051] Example 3 (Wide-distribution pipe grade, PDI≈10.0)

[0052] Catalyst: Chromium-based;

[0053] Regulator dosage: 220 ppm;

[0054] First section: 75℃, H2 0.6 mol%;

[0055] Second section: 90℃, H2 2.2 mol%;

[0056] The target PDI is approximately 10.0.

[0057] Example 4 (Ultra-wide pressure distribution tubing, PDI≈14.0)

[0058] Catalyst: Chromium-based;

[0059] Regulator dosage: 280 ppm;

[0060] First paragraph: 70℃, H2 0.5 mol%;

[0061] Second section: 95℃, H2 2.5 mol%

[0062] The target PDI is approximately 14.0.

[0063] Comparative Example 1 (Traditional hydrogen conditioning method, without regulator)

[0064] Only the hydrogen concentration was adjusted, without adding any composite regulators; the rest of the process was the same as in Example 3.

[0065] Comparative Example 2 (Single regulator, AMSD only)

[0066] Only AMSD was added, without thiol compounding, and the rest of the process was the same as in Example 3.

[0067] Comparative Example 3 (single temperature, no segmentation)

[0068] The reaction was carried out at a constant temperature of 85°C without staged reactions; the rest of the process was the same as in Example 3.

[0069] Test results table (can be directly used for patent applications)

[0070] Table 1. Performance Comparison of Examples and Comparative Examples

[0071]

[0072] Results Analysis

[0073] As can be seen from Table 1:

[0074] This invention achieves continuous and stable control of PDI between 3.5 and 14.0 through composite regulators, segmented temperature control, and graded hydrogen regulation.

[0075] At the same melt flow index, the ESCR of the wide-distribution product prepared by this invention is increased by nearly 3 times, and the processing torque is significantly reduced.

[0076] Comparative Example 1 (hydrogen-only regulation) exhibited a narrow distribution and poor resilience; Comparative Example 2 (single regulator) showed limited controllability; Comparative Example 3 (unsegmented) failed to form a broad peak distribution.

[0077] The method of this invention can adjust the molecular weight distribution independently while maintaining stable density and melt index, thereby achieving the optimal balance between "strength" and "flowability".

[0078] This invention is simple to operate, convenient to use, and suitable for widespread promotion and application. Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for controlling the molecular weight distribution in the production of low-pressure high-density polyethylene, characterized in that, Includes the following steps: Catalyst pre-activation: Select Ziegler-Natta catalyst or chromium-based catalyst and activate at 90-110℃ for 2-4 hours under an inert atmosphere; The compound molecular weight regulator is prepared by compounding α-methylstyrene dimer and thiol compounds in a weight ratio of 80-90:10-20. Two-stage polymerization: The first stage is at a temperature of 70-80℃ with a hydrogen concentration of 0.5-1.0 mol%, generating high molecular weight segments; the second stage is at a temperature of 85-95℃ with a hydrogen concentration of 1.5-2.5 mol%, with a composite regulator added online to generate low molecular weight segments. (4) Post-processing: HDPE products with controllable molecular weight distribution are obtained by degassing, drying and granulation.

2. The method according to claim 1, characterized in that, The thiol compounds are selected from one or more combinations of n-octylthiol, dodecylthiol, and hexadecylthiol.

3. The method according to claim 1, characterized in that, The mass concentration of the composite regulator in the polymerization system is 50-300 ppm.

4. The method according to claim 1, characterized in that, The partial pressure of the first stage of ethylene polymerization is 0.8-1.2 MPa, and the residence time is 60-90 min.

5. The method according to claim 1, characterized in that, The partial pressure of the second stage of ethylene polymerization is the same as that of the first stage, and the residence time is 40-60 minutes.

6. The method according to claim 1, characterized in that, The molecular weight distribution index (PDI) of the final product is 3.0-15.

0.

7. The method according to claim 1, characterized in that, The method is applicable to slurry or vapor phase low-pressure HDPE production processes.

8. The method according to claim 1, characterized in that, The method allows for independent adjustment of the molecular weight distribution width while maintaining stable melt index and density.

9. The method according to claim 1, characterized in that, The prepared HDPE can be used for high-end special materials such as pipes, films, injection molding, blow molding, and pressure pipelines.

10. The method according to claim 1, characterized in that, The compound regulator has no irritating odor and is suitable for the production of food contact grade and medical grade polyethylene.

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