A polyethylene powder, its preparation and use

Abstract

The present application relates to the technical field of high polymer materials, in particular to a kind of polyethylene powder and its preparation method and application.The molecular weight distribution of the polyethylene powder is 1.0~4.0, the number of short chain branches per thousand carbon atoms is 25~85, the mass fraction of the powder with particle size less than 200μm is 15.0%~20.0%, the critical fluidization velocity of this part of powder is 0.2m / s~0.4m / s;The mass fraction of the powder with particle size of 200~600μm is 65%~75%, the critical fluidization velocity of this part of powder is 0.6m / s~0.75m / s.The powder has a unique molecular structure and powder morphology, and the photovoltaic adhesive film made therefrom has excellent interfacial adhesion, effectively improving the long-term sealing reliability of photovoltaic modules.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a polyethylene powder, its preparation method, and its application. Background Technology

[0002] Photovoltaic encapsulant film is a key lamination material for solar cell module encapsulation, undertaking the core functions of bonding the cells, glass, and backsheet, and isolating them from moisture and corrosion. Currently, the mainstream encapsulant film materials on the market are EVA (ethylene-vinyl acetate copolymer) and POE (polyolefin elastomer). EVA material has a significant cost advantage, but the vinyl acetate units in its molecular chain are prone to hydrolysis under humid heat and ultraviolet radiation conditions. The generated acetic acid can corrode the cell grid lines and solder ribbons, and can also induce sodium ion migration in the glass, leading to accelerated module power degradation; the material itself also faces the risk of yellowing after long-term use. In contrast, POE material, based on polyolefins, performs better in terms of hydrolysis resistance, moisture barrier properties, and resistance to potential-induced degradation, and has become an important development direction for high-performance photovoltaic module encapsulation.

[0003] However, POE materials are composed entirely of non-polar hydrocarbon segments, resulting in poor compatibility with polar additives such as peroxide crosslinking agents and silane coupling agents, which are essential in encapsulant film formulations. During encapsulant film processing and long-term module use, additives are prone to migration and surface precipitation. This not only weakens the formation and stability of the crosslinking network but also directly leads to a decrease in the interfacial adhesion strength between the encapsulant film and the glass / backsheet, reducing the sealing reliability of the encapsulation structure. Although existing technologies have attempted to improve compatibility through physical blending modification, chemical grafting to introduce polar groups, or designing multilayer structures, they often face challenges such as complex processes, increased costs, or limited long-term effects. Therefore, providing a polyolefin material that can fundamentally improve compatibility with polar additives while maintaining the excellent bulk properties of POE, thereby obtaining a photovoltaic encapsulant film with superior interfacial sealing durability, is of great significance for promoting the improvement of photovoltaic module reliability. Summary of the Invention

[0004] This invention provides a polyethylene powder, its preparation method, and its application, to solve the problem that the poor compatibility and severe migration of additives in existing photovoltaic encapsulant polyolefin materials lead to a decrease in the interface sealing performance after encapsulation.

[0005] In a first aspect, the present invention provides a polyethylene powder, wherein the polyethylene powder satisfies the following conditions:

[0006] a. Its molecular weight distribution (PDI) is 1.0~4.0; for example, it can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, this invention will not exhaustively list the specific values ​​included in the range, but is further preferably 2.0~3.0;

[0007] b. The number of short branches (SCB) per thousand carbon atoms is 25 to 85; for example, it can be 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, this invention will not exhaustively list the specific values ​​included in the range, but it is more preferably 40 to 70.

[0008] c. The particle size distribution satisfies the following: the mass fraction of powder with a particle size less than 200 μm is 15.0%~20.0%, and the critical fluidization velocity of this portion of powder is 0.2 m / s~0.4 m / s; the mass fraction of powder with a particle size of 200~600 μm is 65%~75%, and the critical fluidization velocity of this portion of powder is 0.6 m / s~0.75 m / s. For example, the mass fraction of powder with a particle size less than 200 μm can be 15%, 16%, 17%, 18%, 19%, 20.0%, and specific values ​​between these points; the critical fluidization velocity of this portion of powder can be 0.20 m / s, 0.25 m / s, 0.30 m / s, 0.35 m / s, 0.40 m / s, and specific values ​​between these points. The mass fraction of powder with a particle size of 200~600μm can be 65%, 68%, 70.0%, 72%, 74%, 75%, and specific values ​​between these values; the critical fluidization velocity of this portion of powder can be 0.60m / s, 0.65m / s, 0.70m / s, 0.75m / s, and specific values ​​between these values. For space limitations and for the sake of brevity, this invention will not exhaustively list all the specific values ​​included in the aforementioned ranges.

[0009] This invention obtains polyethylene powder with specific physicochemical properties through polymerization. Compared with traditional polyolefin materials, this powder enables photovoltaic encapsulant films to exhibit significantly superior sealing performance. Specifically, through precise control of the polymerization process, this invention obtains polyethylene powder with a molecular weight distribution (PDI of 1.0~4.0) and the number of short branches (SCB 25~85 per 1000 carbon atoms) within specific ranges. Simultaneously, this powder possesses an optimized particle size distribution and matching critical fluidization characteristics (powder content less than 200μm is 15.0%~20.0%, critical fluidization velocity 0.2m / s~0.4m / s; powder content of 200~600μm is 65%~75%, critical fluidization velocity 0.6m / s~0.75m / s). The aforementioned molecular structure characteristics are beneficial for improving the compatibility of the material with polar additives and inhibiting additive migration; while the unique powder form ensures its uniform flow and melting during processing, ultimately forming a dense and uniform encapsulation layer inside the photovoltaic film, and establishing a strong interfacial bond between the film and glass, solar cells, and backsheet, thereby significantly improving the bonding strength and long-term sealing reliability, effectively solving the problem of decreased interfacial sealing performance caused by poor compatibility and easy migration of additives in existing polyolefin films.

[0010] The following is a list of common terms:

[0011] Unless otherwise defined, the terms used in this disclosure should be considered to have the ordinary meaning that would be understood by one of ordinary skill in the art. Some common terms are introduced and explained below:

[0012] Unless otherwise specified, the terms "include" or "include" as used herein are open-ended rather than restrictive, meaning that unlisted categories, elements, or methods or steps are not excluded.

[0013] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0014] In one alternative embodiment, the polyethylene powder further satisfies one or more of the following conditions:

[0015] d. Its weight-average molecular weight M wThe value ranges from 20,000 g / mol to 200,000 g / mol; for example, it can be 20,000 g / mol, 30,000 g / mol, 40,000 g / mol, 50,000 g / mol, 60,000 g / mol, 70,000 g / mol, 80,000 g / mol, 90,000 g / mol, 100,000 g / mol, 110,000 g / mol, 120,000 g / mol, 130,000 g / mol, 140,000 g / mol, 1500 g / mol. 00 g / mol, 160000 g / mol, 170000 g / mol, 180000 g / mol, 190000 g / mol, 200000 g / mol, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific values ​​included in the range. Preferably, it is 30000 g / mol to 150000 g / mol, and more preferably 80000 g / mol to 130000 g / mol; the weight-average molecular weight (M) of this invention... w Number-average molecular weight (M) n The molecular weight distribution (PDI) was obtained by gel permeation chromatography (GPC), using polystyrene as a standard; the PDI was M. w / M n ;

[0016] e. It is obtained by catalytic polymerization of a catalyst with a single active center;

[0017] f. Its density is 0.850 g / cm³. 3 ~0.910g / cm 3 For example, it could be 0.850 g / cm³. 3 0.855g / cm 3 0.860 g / cm 3 0.862 g / cm 3 0.865g / cm 3 0.868 g / cm 3 0.870 g / cm 3 0.872 g / cm 3 0.875g / cm 3 0.878 g / cm 3 0.880 g / cm 3 0.882 g / cm 3 0.885g / cm 3 0.888g / cm 3 0.910 g / cm 3And the specific point values ​​between the above-mentioned point values, due to space limitations and for the sake of brevity, this invention will not exhaustively list the specific point values ​​included in the range, preferably 0.865~0.900 g / cm³. 3 Further optimization was performed using a concentration of 0.870~0.900 g / cm³. 3 For example, the density of the polyethylene powder is obtained by testing according to the methods in ASTM D792-2021 or ASTM D792-13;

[0018] g. Its melt index (or melt flow rate) MI at 190°C and 21.6 kg load is 0.1 g / 10 min to 30.0 g / 10 min, preferably 1.0 g / 10 min to 3.0 g / 10 min. For example, it can be 0.1 g / 10 min, 1 g / 10 min, 2 g / 10 min, 3 g / 10 min, 5 g / 10 min, 7 g / 10 min, 9 g / 10 min, 10 g / 10 min, 15 g / 10 min, 20 g / 10 min, 25 g / 10 min, or 30 g / 10 min, as well as specific values ​​between the above values. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific values ​​included in the range. Exemplarily, the melt index (MI) of the polyethylene powder is obtained by the method in ASTM D1238-13 (190°C, 2.16 kg).

[0019] In one optional embodiment, the polyethylene powder is a copolymer of ethylene and C3-C20 α-olefins; optionally, the C3-C20 α-olefins include at least one selected from propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-octene, 1-decene, and 1-dodecene; optionally, the molar percentage of ethylene structural units is 5% to 50% based on the total molar number of structural units in the copolymer. For example, it can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%, and specific values ​​between the above values. For the sake of space and brevity, the present invention will not exhaustively list the specific values ​​included in the range, but preferably 5% to 45%, more preferably 5% to 40%, and more preferably 5% to 35%.

[0020] In one optional embodiment, the C3-C20 α-olefin can be an α-olefin of C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, C15, C16, C17, C18, C19, or C20. Introducing an appropriate amount of C3-C20 α-olefin comonomer can effectively regulate the crystallinity, density, and chain flexibility of the polymer by introducing short branches while maintaining the rigidity of the polyethylene backbone. This molecular design helps to optimize its compatibility with polar auxiliaries (such as crosslinking agents) while maintaining the basic mechanical properties of the material, reducing auxiliary migration, and improving the toughness and interfacial adhesion of the final film. More preferably, it is at least one of propylene, 1-hexene, and 1-octene, and more preferably 1-butene.

[0021] In an optional embodiment, the single active site catalyst comprises at least one of a metallocene catalyst and a non-metallocene transition metal catalyst. Preferably, it is at least one of dimethylsilylbridged diindylzirconium dichloride, dimethylsilylbis(2-methylindyl)zirconium dichloride, bis(2-methylindyl)zirconium dichloride, dimethylsilylbis(2-methyl-4-phenylindyl)zirconium dichloride, dimethylsilylbis(2-methyl-4-phenylindyl)dimethylzirconium, dimethylsilyl(cyclopentadiene)(9-fluorenyl)zirconium dichloride, dimethylsilylbis(2-methyl-4,5-benzo[A]indyl)zirconium dichloride, dimethylsilane(tert-butylamino)tetramethylcyclopentadienyldimethyltitanium, and a supported catalyst obtained after loading the catalyst described above.

[0022] Secondly, the present invention also provides a method for preparing the above-mentioned polyethylene powder, comprising the following steps: mixing monomer, hydrogen, a single active center catalyst and a co-catalyst, and carrying out a polymerization reaction under polymerization reaction conditions to obtain polymer powder.

[0023] In one alternative embodiment, the monomer comprises ethylene and a comonomer; optionally, the comonomer is at least one of C3-C20 α-olefins.

[0024] And / or, the cocatalyst is an organoaluminum compound; optionally, the organoaluminum compound includes at least one of triethylaluminum, triisobutylaluminum, and ethyl sesquialuminum chloride;

[0025] And / or, the molar ratio of the co-catalyst to the single active site catalyst is (10~1000):1;

[0026] And / or, the concentration of hydrogen in the reaction system is 1 ppm to 200 ppm;

[0027] And / or, the pressure of the polymerization reaction is 2200 kPag to 2413 kPag;

[0028] And / or, the polymerization reaction temperature is 40℃~100℃, preferably 55~75℃;

[0029] And / or, the polymerization reaction time is 0.5h to 4h. For example, it can be 0.5h, 1.0h, 1.5h, 2.0h, 2.5h, 3.0h, 3.5h, and 4.0h, etc. It should be noted that the specific reaction residence time needs to take into account the main catalyst and match the reaction kinetic curve of the main catalyst.

[0030] In the preparation method of this invention, the absolute amount of a single active catalyst is not specifically limited, which is consistent with the conventional practice and understanding of those skilled in the art. This is because, in continuous or semi-continuous olefin polymerization processes, the catalyst feed rate is dynamically adjusted mainly based on the production rate (space-time yield) of the target polymer, the inherent activity of the catalyst, and the desired polymer properties. The "effective amount" of catalyst is essentially a process variable related to multiple factors such as reactor type, scale, monomer concentration, and reaction conditions (temperature, pressure), rather than an independent, fixed formulation parameter. Those skilled in the art understand that, in order to achieve a specific polymerization rate and ensure that product properties (such as molecular weight and molecular weight distribution) fall within the range defined by this invention, it is entirely possible to determine a suitable catalyst feed rate through conventional experimental optimization without creative effort. Therefore, limiting the scope of protection to the reaction conditions jointly defined by key process parameters such as monomer, hydrogen, pressure, temperature, residence time, and the molar ratio of co-catalyst to main catalyst is sufficient to clearly and completely define the method of this invention and ensure that those skilled in the art can repeat its implementation. Conversely, adding an absolute amount of catalyst that is detached from the specific process system may unduly limit the scope of protection of this invention or introduce unnecessary uncertainties in implementation.

[0031] In one optional embodiment, the polymerization reaction is terminated by adding an inactivating agent to the reaction system. Optionally, the inactivating agent includes at least one of carbon monoxide (CO), carbon dioxide (CO2), water, and C1-C6 fatty alcohols. Optionally, the inactivating agent is added after being mixed with an inert gas (e.g., argon, nitrogen, etc.). The purpose of mixing the inactivating agent with the inert atmosphere is to prevent the inactivating agent from condensing upon cooling during the quenching process, which could lead to problems such as polymer powder holding up to liquid, incomplete quenching of active centers, and powder agglomeration. Furthermore, incomplete quenching can cause further polymerization reactions at the active centers in the quenching chamber, resulting in a series of product quality problems such as unqualified polymer product indicators. On the other hand, quenching enables reasonable control of the polymerization reaction time, ensuring that the polymer product indicators are qualified, and simultaneously controlling the particle size distribution of the polymer. Of course, other methods can also be used to regulate the relevant characteristics of the obtained polymer. For example, by controlling the aspect ratio and circulation ratio of the reactor, the temperature distribution and concentration distribution within the reaction system can be adjusted. Therefore, the above method is only a specific example of obtaining polyolefin materials with the specific properties mentioned above, and is not the only technical means for those skilled in the art to obtain the above products;

[0032] And / or, the polymerization reaction is carried out under anhydrous and oxygen-free conditions;

[0033] And / or, the polymerization reaction is carried out in one or more reactors including a slurry loop reactor, a fluidized bed gas-phase reactor, and a stirred tank reactor.

[0034] Thirdly, the present invention also provides a photovoltaic film, which is obtained by molding and processing the above-mentioned polyethylene powder.

[0035] In one optional embodiment, the molding process includes at least one of hot pressing, extrusion molding, blow molding, and rotational molding;

[0036] And / or, prior to the molding process, the step further includes blending the polyethylene powder with additives.

[0037] In one optional embodiment, the additive includes at least one of an antioxidant, a crosslinking agent, and a silane coupling agent;

[0038] And / or, the total amount of the additives added is 500ppm to 5000ppm of the polyethylene powder mass.

[0039] In one optional embodiment, during the preparation of the photovoltaic film, during hot pressing, the polyethylene powder is first blended with an antioxidant and then processed at a processing temperature of 30°C to 40°C; during extrusion molding, the polyethylene powder is first blended with additives including antioxidants, release agents, or lubricants and then melt-processed at a processing temperature of 70°C to 100°C; during blow molding, the polyethylene powder is first blended with additives including antioxidants, release agents, and lubricants and then processed at a processing temperature of 75°C to 90°C. For the specific polyethylene powder of this invention, a relatively mild processing temperature is used, which is beneficial for maintaining the morphological stability of the powder during processing, reducing the adverse effects of thermal history on the material structure and additive system, thereby better maintaining and utilizing its excellent sealing performance in the final film.

[0040] The technical solution of this invention has the following advantages:

[0041] The polyethylene powder provided by this invention has its molecular weight distribution (PDI 1.0~4.0) and the number of short branches (SCB 25~85 per 1000 carbon atoms) controlled within specific ranges through a polymerization process. Simultaneously, the powder possesses a key particle size distribution and matching critical fluidization characteristics (powder content of particles smaller than 200 μm is 15.0%~20.0%, critical fluidization velocity 0.2 m / s~0.4 m / s; powder content of particles with a size of 200~600 μm is 65%~75%, critical fluidization velocity 0.6 m / s~0.75 m / s). These molecular structure characteristics are beneficial for improving the compatibility of the material with polar additives in photovoltaic films, effectively inhibiting the migration and interfacial precipitation of additives during processing and use; while the optimized powder morphology ensures uniform flow, spreading, and melting during subsequent molding processes, thereby forming a dense and uniform film layer. Therefore, the photovoltaic encapsulant film made from this powder can significantly improve the interfacial bonding strength and long-term sealing reliability between the encapsulant film and glass, solar cells, and backsheet. It fundamentally solves the problem of reduced encapsulation and sealing performance caused by poor compatibility and easy migration of additives in existing polyolefin encapsulant films, thereby greatly improving the long-term operational stability and service life of photovoltaic modules. Detailed Implementation

[0042] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0043] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0044] Example 1

[0045] This embodiment provides a method for preparing polyethylene powder, the specific steps of which are as follows:

[0046] (1) Preparation of polymerization raw materials: Ethylene with a purity of 99.95% and 1-butene with a purity of 97% were prepared as comonomers, hydrogen with a purity of 99.95% was prepared as a molecular weight regulator, and purified nitrogen (purity of 99.95%) was prepared as a carrier gas and dilution gas. Dimethylsilylbridged diindenyl dichloride was used as a single active center catalyst, and triethylaluminum was used as a cocatalyst.

[0047] (2) Gas-phase polymerization: Polymerization was carried out using a continuous gas-phase fluidized bed polyethylene unit with a designed capacity of 4 kg / h. Under anhydrous and oxygen-free conditions, ethylene was fed at a feed rate of 2000 g / h, and 1-butene was fed at a feed rate of 1000 g / h. The concentration of hydrogen in the reaction recirculation gas was controlled to be approximately 50 ppm by adjusting the hydrogen flow rate. A single-active-center catalyst and a co-catalyst (the molar ratio of co-catalyst to catalyst was approximately 200:1) were continuously injected into the reactor.

[0048] (3) Reaction condition control: The temperature of the polymerization reactor bed was controlled at 65℃ and the pressure at 2413 kPag (gauge pressure). The average residence time of the material in the reactor was controlled at 2.5 hours.

[0049] (4) Reaction Termination and Post-treatment: After the polymerization reaction is completed, purified nitrogen gas with a water content of 5000 ppm is introduced into the reactor discharge system as an inactivating agent to terminate the reaction and remove dissolved hydrocarbons from the polymer powder. The powder is then discharged and packaged to obtain white polyethylene powder.

[0050] Example 2

[0051] This embodiment provides a method for preparing polyethylene powder, the specific steps of which are as follows:

[0052] (1) Preparation of polymerization raw materials: Prepare ethylene with a purity of 99.95% and 1-butene with a purity of 97% as comonomers, hydrogen with a purity of 99.95% as a molecular weight regulator, and purified nitrogen as a carrier gas and dilution gas. Dimethylsilylbridged diindenyl dichloride is used as a single active center catalyst, and triethylaluminum is used as a cocatalyst.

[0053] (2) Gas-phase polymerization: Polymerization was carried out using a continuous gas-phase fluidized bed polyethylene unit with a designed capacity of 4 kg / h. Under anhydrous and oxygen-free conditions, ethylene was fed at a feed rate of 2000 g / h, and 1-butene was fed at a feed rate of 1000 g / h. The concentration of hydrogen in the reaction recirculation gas was controlled to be approximately 50 ppm by adjusting the hydrogen flow rate. A single-active-center catalyst and a co-catalyst (the molar ratio of co-catalyst to catalyst was approximately 200:1) were continuously injected into the reactor.

[0054] (3) Reaction condition control: The temperature of the polymerization reactor bed was controlled at 75℃ and the pressure at 2413 kPag (gauge pressure). The average residence time of the material in the reactor was controlled at 2.5 hours.

[0055] (4) Reaction Termination and Post-treatment: After the polymerization reaction is completed, purified nitrogen gas with a water content of 10,000 ppm is introduced into the reactor discharge system as an inactivating agent to terminate the reaction and remove dissolved hydrocarbons from the polymer powder. The powder is then discharged and packaged to obtain white polyethylene powder.

[0056] Example 3

[0057] This embodiment provides a method for preparing polyethylene powder, the specific steps of which are as follows:

[0058] (1) Preparation of polymerization raw materials: Prepare ethylene with a purity of 99.95% and 1-butene with a purity of 97% as comonomers, hydrogen with a purity of 99.95% as a molecular weight regulator, and purified nitrogen as a carrier gas and dilution gas. Dimethylsilylbridged diindenyl dichloride is used as a single active center catalyst, and triethylaluminum is used as a cocatalyst.

[0059] (2) Gas-phase polymerization: Polymerization was carried out using a continuous gas-phase fluidized bed polyethylene unit with a designed capacity of 4 kg / h. Under anhydrous and oxygen-free conditions, ethylene was fed at a feed rate of 2000 g / h, and 1-butene was fed at a feed rate of 700 g / h. The concentration of hydrogen in the reaction recirculation gas was controlled to be approximately 50 ppm by adjusting the hydrogen flow rate. A single-active-center catalyst and a co-catalyst (the molar ratio of co-catalyst to catalyst was approximately 200:1) were continuously injected into the reactor.

[0060] (3) Reaction condition control: The temperature of the polymerization reactor bed was controlled at 60℃ and the pressure at 2413 kPag (gauge pressure). The average residence time of the material in the reactor was controlled at 2.5 hours.

[0061] (4) Reaction Termination and Post-treatment: After the polymerization reaction is completed, purified nitrogen gas with a water content of 5000 ppm is introduced into the reactor discharge system as an inactivating agent to terminate the reaction and remove dissolved hydrocarbons from the polymer powder. The powder is then discharged and packaged to obtain white polyethylene powder.

[0062] Example 4

[0063] This embodiment provides a method for preparing polyethylene powder, the specific steps of which are as follows:

[0064] (1) Preparation of polymerization raw materials: Prepare ethylene with a purity of 99.95% and 1-butene with a purity of 97% as comonomers, hydrogen with a purity of 99.95% as a molecular weight regulator, and purified nitrogen as a carrier gas and dilution gas. Dimethylsilylbridged diindenyl dichloride is used as a single active center catalyst, and triethylaluminum is used as a cocatalyst.

[0065] (2) Gas-phase polymerization: Polymerization was carried out using a continuous gas-phase fluidized bed polyethylene unit with a designed capacity of 4 kg / h. Under anhydrous and oxygen-free conditions, ethylene was fed at a feed rate of 2000 g / h, and 1-butene was fed at a feed rate of 1000 g / h. The concentration of hydrogen in the reaction recirculation gas was controlled to be approximately 50 ppm by adjusting the hydrogen flow rate. A single-active-center catalyst and a co-catalyst (the molar ratio of co-catalyst to catalyst was approximately 200:1) were continuously injected into the reactor.

[0066] (3) Reaction condition control: The temperature of the polymerization reactor bed was controlled at 55℃ and the pressure at 2413 kPag (gauge pressure). The average residence time of the material in the reactor was controlled at 2.5 hours.

[0067] (4) Reaction Termination and Post-treatment: After the polymerization reaction is completed, purified nitrogen gas with a water content of 20,000 ppm is introduced into the reactor discharge system as an inactivating agent to terminate the reaction and remove dissolved hydrocarbons from the polymer powder. The powder is then discharged and packaged to obtain white polyethylene powder.

[0068] Example 5

[0069] This embodiment provides a method for preparing polyethylene powder, the specific steps of which are as follows:

[0070] (1) Preparation of polymerization raw materials: Prepare ethylene and 1-hexene as comonomers, hydrogen, and purified nitrogen. Dimethylsilyl bis(2-methyl-4,5-benzo[A]indenyl)zirconium chloride is used as a single active center catalyst and triisobutylaluminum is used as a cocatalyst.

[0071] (2) Gas-phase polymerization reaction: A continuous gas-phase fluidized bed reactor was used. Under anhydrous and oxygen-free conditions, ethylene was fed at a rate of 2000 g / h, and 1-hexene was fed at a rate of 1000 g / h. The concentration of hydrogen in the reaction recirculation gas was controlled to be approximately 10 ppm by adjusting the hydrogen flow rate. The catalyst and co-catalyst were continuously injected (the molar ratio of co-catalyst to catalyst was 10:1, lower limit).

[0072] (3) Reaction condition control: The polymerization reaction temperature was controlled at 65℃ and the pressure at 2200 kPag (gauge pressure). The average residence time of the material in the reactor was controlled at 0.5 hours.

[0073] (4) Reaction termination and post-processing: After the polymerization reaction is completed, refined nitrogen gas with a water content of 100 ppm is introduced into the discharge system for quenching to obtain polyethylene powder.

[0074] Example 6

[0075] This embodiment provides a method for preparing polyethylene powder, the specific steps of which are as follows:

[0076] (1) Preparation of polymerization raw materials: Prepare ethylene and 1-butene as comonomers, hydrogen, and purified nitrogen. Use dimethylsilylbis(2-methyl-4-phenylindinyl)dimethylzirconium as a single active center catalyst and ethyl sesquialuminate chloride as a cocatalyst.

[0077] (2) Slurry polymerization reaction: A stirred tank slurry polymerization reactor was used. Isobutane was added to the reactor as a solvent under anhydrous and oxygen-free conditions. Ethylene was fed at a rate of 2000 g / h, and 1-butene was fed at a rate of 1000 g / h. The hydrogen flow rate was adjusted to control its partial pressure in the reaction system to correspond to a concentration of approximately 200 ppm. The catalyst and co-catalyst were added (the molar ratio of co-catalyst to catalyst was 1000:1).

[0078] (3) Reaction conditions control: The polymerization reaction temperature was controlled at 55℃ and the pressure at 2413 kPag (gauge pressure). The polymerization reaction time was controlled at 4 hours.

[0079] (4) Reaction termination and post-processing: After the reaction is completed, nitrogen gas mixed with carbon dioxide is introduced into the reactor for quenching. After separation and drying, polyethylene powder is obtained.

[0080] Comparative Example 1

[0081] This comparative example provides a method for preparing polyethylene powder, which differs from Example 1 only in that the average residence time of the material in the reactor is controlled to be 0.5 hours in step (3), while all other conditions are exactly the same as in Example 1.

[0082] Comparative Example 2

[0083] This comparative example provides a method for preparing polyethylene powder, which differs from Example 1 only in that the average residence time of the material in the reactor is controlled to be 7 hours in step (3), while all other conditions are exactly the same as in Example 1.

[0084] Comparative Example 3

[0085] This comparative example provides a method for preparing polyethylene powder, which differs from Example 1 only in that the purified nitrogen gas introduced into the reactor discharge system in step (4) has a water content of 50,000 ppm, while the other conditions are exactly the same as in Example 1.

[0086] Comparative Example 4

[0087] This comparative example provides a method for preparing polyethylene powder, which differs from Example 1 in that the catalyst used in step (1) is a multi-active-center catalyst system, specifically a mixture of dimethylsilylbridged diindene dizirconia and dimethylsilylbridged dicenyl dizirconia (mass ratio 1:1). All other conditions are exactly the same as in Example 1.

[0088] Comparative Example 5

[0089] This comparative example provides a method for preparing polyethylene powder, which differs from Example 1 only in that the feed rate of the comonomer 1-butene in step (2) is adjusted to 300 g / h, while all other conditions are exactly the same as in Example 1.

[0090] Test case

[0091] The polyethylene powders prepared in the above embodiments and comparative examples were subjected to performance characterization and film performance testing according to a unified standard testing method. The specific testing methods are as follows:

[0092] (1) Performance testing of polyethylene powder:

[0093] Molecular weight, molecular weight distribution, and insertion rate of α-olefins: Gel permeation chromatography (GPC) was used for determination. The chromatographic column was a PLOlexis, the solvent was trichlorobenzene (TCB), the flow rate was 1.0 mL / min, the sample concentration was 1.0 mg / mL, the injection volume was 200 μL, the column temperature was 160 °C, the detector was an Agilent high-temperature RI detector, and the standard was polystyrene (calibrated using a cubic function). The weight-average molecular weight (M...) was determined. w Number-average molecular weight (M) n And calculate the molecular weight distribution (PDI=M) w / M n The results are summarized in Table 1.

[0094] Density: Tested according to the ASTM D792-2021 standard method, the results are shown in Table 1.

[0095] Melt flow index (MI): Tested according to ASTM D1238-13 standard at 190℃ and 2.16kg load. The results are shown in Table 1.

[0096] Branching degree (SCB): The number of methyl groups per thousand carbon atoms (CH3 / 1000C) was determined by nuclear magnetic resonance as a characterization of the number of short branches. The results are shown in Table 1.

[0097] Particle size distribution and critical fluidization velocity (Umf): Powder was sieved through a standard sieve to obtain samples with different particle size ranges. Critical fluidization velocity was determined using a transparent cold-modulated fluidized bed apparatus: Powder with a specific particle size range was added to the apparatus, the inlet gas rate was gradually adjusted, and the apparent gas velocity at the start of fluidization was visually observed and recorded. The results are summarized in Table 2.

[0098] (2) Photovoltaic encapsulant film preparation and performance testing:

[0099] Film Preparation: The polyethylene powder obtained in each example and comparative example was mixed uniformly with dicumyl peroxide (DCP) crosslinking agent (added at 2000 ppm by weight of polyethylene powder) in a high-speed mixer. The mixture was then fed into a casting machine and processed into a film sheet with a thickness of approximately 600 μm at 75°C, according to the conventional thickness range of photovoltaic films (e.g., 300 μm to 800 μm). It should be understood that this thickness is only an exemplary test thickness; in actual production, processing parameters can be adjusted according to the specific requirements of module encapsulation (such as cell type, glass thickness, process route, etc.) to prepare films of different thicknesses. The film sheet was placed in a laminator and hot-pressed for 5 minutes at 70°C and a vacuum of -0.09 MPa to induce a crosslinking reaction. After cooling, the photovoltaic film sample to be tested was obtained.

[0100] Crosslinking performance test: Crosslinking degree: Refer to the Soxhlet extraction method according to ASTM D2765 standard. A known mass of the film sample was refluxed with xylene solvent for a certain time to dissolve and remove uncrosslinked molecular chains. The remaining insoluble gel was dried and weighed. Crosslinking degree (%) = (mass of gel after extraction / mass of sample before extraction) × 100%, results are shown in Table 3. Crosslinking rate (T90): The crosslinking degree of the film was monitored at different hot-pressing times, and a crosslinking degree-time curve was plotted. The crosslinking rate T90 is defined as the shortest time required to reach 90% crosslinking degree, results are shown in Table 3.

[0101] Interface sealing performance test (peel strength): The 180° peel strength between the above-mentioned film sample and standard photovoltaic glass was tested using a peel strength tester according to standard methods. The results are expressed in N / cm and are shown in Table 3. Peel strength is a core indicator for evaluating the reliability of the interfacial seal of the film.

[0102] Table 1

[0103]

[0104] Table 2

[0105]

[0106] Table 3

[0107]

[0108] According to the test results above, the photovoltaic films made from the polyethylene powder prepared in Examples 1 to 6 of this invention exhibit excellent crosslinking performance (crosslinking degree ≥85%, T90 ≤15.7 minutes) and interfacial sealing performance (peel strength ≥82 N / cm). In contrast, Comparative Example 1, due to its short residence time, resulted in an excessively high content of fine powder (45%), poor flowability, and a significant decrease in the crosslinking degree and peel strength of the film. Comparative Example 2, due to its excessively long residence time, resulted in a wider molecular weight distribution, and the excessive particle growth led to an abnormally high fluidizing gas velocity, resulting in lower crosslinking degree and peel strength compared to Examples 1 to 6. Comparative Example 3, due to its excessively high water content in the inactivating agent, resulted in poor control of the polymerization reaction, a significantly wider molecular weight distribution of the product, and deterioration of the film performance. Comparative Example 4, due to the use of a multi-active-center catalyst, resulted in an excessively high PDI in the powder; although the crosslinking degree of the film was acceptable, the peel strength was severely insufficient. Comparative Example 5 had too low content of comonomer (1-butene), resulting in insufficient branching of the product, which led to too few crosslinking points in the film, extremely slow crosslinking speed, and unsatisfactory crosslinking degree and peel strength.

[0109] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A polyethylene powder, characterized in that, The polyethylene powder meets the following conditions: a. Its molecular weight distribution (PDI) is 1.0~4.0; b. The number of short branches (SCB) per thousand carbon atoms is 25-85; c. The particle size distribution satisfies the following: the mass fraction of powder with a particle size less than 200 μm is 15.0%~20.0%, and the critical fluidization velocity of this portion of powder is 0.2 m / s~0.4 m / s; the mass fraction of powder with a particle size of 200~600 μm is 65%~75%, and the critical fluidization velocity of this portion of powder is 0.6 m / s~0.75 m / s; d. It is obtained by catalytic polymerization of a catalyst with a single active center; e. The polyethylene powder is a copolymer of ethylene and C3~C20 α-olefins.

2. The polyethylene powder according to claim 1, characterized in that, The polyethylene powder also meets one or more of the following conditions: f. Its weight-average molecular weight M w The value ranges from 20,000 g / mol to 200,000 g / mol. g. Its density is 0.850 g / cm³. 3 ~0.910g / cm 3 ; h. Its melt index (MI) at 190℃ and 21.6kg load is 0.1g / 10min~30.0g / 10min.

3. The polyethylene powder according to claim 1 or 2, characterized in that, The molecular weight distribution (PDI) of the polyethylene powder is 2.0~3.0; And / or, the number of short branches (SCB) per thousand carbon atoms in the polyethylene powder is 40 to 70.

4. The polyethylene powder according to claim 2, characterized in that, The single active site catalyst includes at least one of metallocene catalysts and non-metallocene transition metal catalysts.

5. A method for preparing polyethylene powder according to any one of claims 1 to 4, characterized in that, The process includes the following steps: mixing monomers, hydrogen, a single-active-center catalyst, and a co-catalyst, and carrying out a polymerization reaction under polymerization conditions to obtain polymer powder; The molar ratio of the co-catalyst to the single active site catalyst is (10~1000):1; The concentration of hydrogen in the reaction system is 1 ppm to 200 ppm; The polymerization reaction temperature is 40℃~100℃; The polymerization reaction pressure is 2200 kPag ~ 2413 kPag; The polymerization reaction takes 0.5 h to 4 h. The monomers include ethylene and comonomers; the comonomers are at least one of the C3 to C20 α-olefins.

6. The preparation method according to claim 5, characterized in that, The cocatalyst is an organoaluminum compound.

7. The preparation method according to claim 5 or 6, characterized in that, The polymerization reaction is terminated by adding an inactivating agent to the reaction system; And / or, the polymerization reaction is carried out under anhydrous and oxygen-free conditions; And / or, the polymerization reaction is carried out in one or more reactors including a slurry loop reactor, a fluidized bed gas-phase reactor, and a stirred tank reactor.

8. A photovoltaic encapsulant film, characterized in that, It is obtained by molding and processing the polyethylene powder according to any one of claims 1 to 4.

9. The photovoltaic encapsulant film according to claim 8, characterized in that, The molding process includes at least one of hot pressing, extrusion molding, blow molding, and rotational molding; And / or, prior to the molding process, the step further includes blending the polyethylene powder with additives.

10. The photovoltaic encapsulant film according to claim 9, characterized in that, The additives include at least one of antioxidants, crosslinking agents, and silane coupling agents; And / or, the total amount of the additives added is 500ppm to 5000ppm of the polyethylene powder mass.

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