Ethylene / Α-olefin random copolymer for photovoltaic adhesive film, and use thereof

Abstract

Provided are an ethylene / α-olefin random copolymer and the use thereof. The ethylene / α-olefin random copolymer has the following features: (i) the molar content of a soluble fraction (SF) at 35°C or less, measured by means of elution under gradient heating, is 0.5-3%, wherein the soluble fraction has a weight-average molecular weight of less than or equal to 20,000 g / mol; and (ii) the total branch chain number per thousand carbon atoms in the chain structure of the copolymer is 25-85, and a relationship of highly linear fitting is formed between the branch chain number per thousand carbon atoms of chain segments with different molecular weights and the logarithms of the molecular weights of the chain segments of the copolymer, the determination coefficient R2 of fitting being greater than or equal to 0.75. The copolymer has a fast cross-linking speed and a high cross-linking degree when used in photovoltaic adhesive films, has improved processability, and enables obtained photovoltaic adhesive films to have high light transmittance and good sealing performance.

Description

Ethylene / α-olefin random copolymer for photovoltaic film and its application Technical Field

[0001] The present application relates to the technical field of photovoltaic films, for example, an ethylene / α-olefin random copolymer and its application. Background Art

[0002] Polyolefin elastomers (POE) are random copolymers of ethylene and α-olefins. Common α-olefin comonomers include 1-butene and 1-octene. Typically, the α-olefin comonomer accounts for no less than 20 wt% of the random copolymer.

[0003] In vinyl copolymers, the insertion of α-olefin comonomers disrupts the regularity of the methylene sequence, introduces short-chain branches, and reduces the crystallinity of the polymer, resulting in good transparency while maintaining the elasticity of rubber and the processability of plastic. Polyolefin elastomers have a non-polar carbon-hydrogen skeleton, a low number of tertiary carbon atoms, and excellent weathering and UV aging resistance. Due to their excellent insulation and water vapor barrier properties, they are widely used in photovoltaic films, accounting for nearly 30% of the overall photovoltaic film market. In recent years, the photovoltaic industry has developed rapidly, with photovoltaic installed capacity increasing year by year. As a sealing material for photovoltaic modules, POE has broad market prospects in the photovoltaic industry.

[0004] Currently, EVA holds a significant market share in the photovoltaic film market due to its superior cost advantages. However, over time, in humid environments, the polar components in EVA can hydrolyze and produce acidic substances, which can corrode the transparent glass in photovoltaic modules. Furthermore, EVA produces easily ionized sodium ions, leading to leakage current and a decrease in PID (Protection Deficit / Pollution) resistance.

[0005] Although POE materials, which have a non-polar hydrocarbon skeleton composition, have excellent aging resistance and electrical insulation properties and can largely avoid the occurrence of the above-mentioned problems, in order to further improve the mechanical strength and aging resistance of the film, polar additives such as cross-linking agents are usually required during the processing process. Due to its non-polar nature, POE has poor compatibility with polar additives, which affects the cross-linking rate during processing, further affecting the sealing performance of the film on photovoltaic cell modules, and also causing water vapor intrusion and attenuation of the power generation efficiency of the cell modules. Therefore, for POE films, poor compatibility with cross-linking additives, additive migration problems, and the adverse effects of low cross-linking rates on processing efficiency have always been pain points in the photovoltaic film industry.

[0006] With the development of photovoltaic module structure changes, thin-walled cells, and larger sizes, higher requirements are placed on the electrical insulation and processing performance of photovoltaic films. Therefore, there is an urgent need to obtain an olefin copolymer with improved processing performance for use in the photovoltaic film field.

[0007] Summary of the Invention

[0008] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0009] In response to the above technical problems, the present application provides an ethylene / α-olefin random copolymer for photovoltaic film and its application; the ethylene / α-olefin random copolymer described in the present application has a low soluble fraction (SF) content below 35°C and a uniform branch distribution, so that its cross-linking speed in photovoltaic film applications is fast, the cross-linking degree is high, and it has improved processing performance, and at the same time the resulting photovoltaic film has high light transmittance and excellent sealing performance.

[0010] In a first aspect, an ethylene / α-olefin random copolymer for a photovoltaic film is provided, wherein the ethylene / α-olefin random copolymer comprises ethylene monomer units and C3 to C8 α-olefin monomer units; the ethylene / α-olefin random copolymer has the following characteristics:

[0011] (i) a molar content (molar percentage) of a soluble fraction (SF) below 35° C. as measured by gradient temperature elution is 0.5 to 3% (e.g., 0.55%, 0.6%, 0.8%, 1.0%, 1.2%, 1.5%, 2.0%, 2.5%, 2.8%), optionally 0.5 to 1.5%, wherein the weight average molecular weight of the soluble fraction is less than or equal to 20,000 g / mol (e.g., 18,000 g / mol, 15,000 g / mol, 12,000 g / mol, 10,000 g / mol, 8,000 g / mol, 6,000 g / mol, 5,000 g / mol, 4,000 g / mol, 2,000 g / mol, 1,000 g / mol, 500 g / mol);

[0012] (ii) the total number of branches per thousand carbon atoms in the chain structure of the copolymer is 25 to 85 (e.g., 26, 28, 30, 32, 35, 38, 40, 42, 45, 48, 50, 55, 60, 65, 70, 75, 80, 82, 84); and a good linear fit is formed between the number of branches per thousand carbon atoms of each molecular weight segment as the dependent variable y (i.e., the number of branches per thousand carbon atoms) and the logarithm of the molecular weight of each copolymer segment as the independent variable x, with a fitting coefficient of determination R 2Greater than or equal to 0.75 (for example, 0.8, 0.85, 0.9, 0.94, 0.96, 0.965, 0.97, 0.975, 0.98, 0.985, 0.99), optionally greater than or equal to 0.90.

[0013] In some embodiments, the linear relationship between x and y can be expressed as: y = ax + b; wherein a ranges from -3 to 3 (for example, -2.8, -2.5, -2.2, -2.0, -1.8, -1.5, -1.4, -1.2, -1.0, -0.5, 0, 0.5, 1, 2, 2.5); b ranges from 45 to 80 (for example, 46, 48, 50, 52, 55, 58, 60, 62, 65, 70, 75, 78).

[0014] Furthermore, in the above feature (ii) possessed by the copolymer, the molecular weight absorption intensity of the copolymer as the dependent variable y' and the logarithm of the molecular weight of each copolymer segment as the independent variable x are normally distributed. In some embodiments, the molecular weight absorption intensity of the copolymer as the dependent variable y' and the logarithm of the molecular weight of each copolymer segment as the independent variable x are (negatively) skewed, wherein the mean μ may be 4.4 to 4.9 (e.g., 4.45, 4.5, 4.55, 4.6, 4.65, 4.7, 4.75, 4.8) and the standard deviation σ may be 1.00 to 1.20 (e.g., 1.02, 1.05, 1.08, 1.1, 1.12, 1.15, 1.18).

[0015] In the linear relationship fitting between x and y, the fitting determination coefficient R 2 It is an effective indicator used to measure the correlation between the independent variable and the dependent variable; its calculation formula is:

[0016] In the formula, yi is the actual value of each dependent variable, is the average value of the actual value yi of each dependent variable, is the regression value of the linear equation of the dependent variable.

[0017] In this application, the molecular weight of the entire copolymer is the average molecular weight of all copolymer segments, and the molecular weight of each copolymer segment refers to the molecular weight of each specific copolymer segment.

[0018] According to the ethylene / α-olefin random copolymer provided herein, in some embodiments, the ethylene / α-olefin random copolymer further has the following characteristics:

[0019] (iii) the copolymer has a melt flow rate of 4 to 35 g / 10 min (190° C., 2.16 kg), for example, 5 g / 10 min, 6 g / 10 min, 8 g / 10 min, 10 g / 10 min, 12 g / 10 min, 15 g / 10 min, 16 g / 10 min, 18 g / 10 min, 20 g / 10 min, 25 g / 10 min, 30 g / 10 min, and 32 g / 10 min; and a density of 0.865 to 0.890 g / cm 3 , for example, 0.868 g / cm 3 、0.870g / cm 3 , 0.872g / cm 3 、0.875g / cm 3 , 0.880g / cm 3 , 0.882g / cm 3 、0.885g / cm 3 , 0.889g / cm 3 ;

[0020] (iv) The copolymer has a weight average molecular weight of 45,000 to 80,000 g / mol (e.g., 48,000 g / mol, 50,000 g / mol, 52,000 g / mol, 54,000 g / mol, 55,000 g / mol, 60,000 g / mol, 65,000 g / mol, 70,000 g / mol, 72,000 g / mol, 75,000 g / mol, 78,000 g / mol), and a molecular weight distribution (Mw / Mn) of less than or equal to 3 (e.g., 1.5, 2, 2.2, 2.4, 2.5, 2.6, 2.8), and optionally less than or equal to 2.6.

[0021] In some embodiments, the α-olefin monomer is selected from one or more of propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene.

[0022] In some embodiments, the ethylene / α-olefin random copolymer is a product obtained by continuously introducing an alkane solvent, an ethylene monomer, an α-olefin monomer, and a polymerization catalyst into a polymerization reactor for mixing and contacting to carry out a solution polymerization reaction.

[0023] In the solution polymerization reaction, for example, a polymerization catalyst, an ethylene monomer, and an α-olefin monomer are dispersed, dissolved, and contact-reacted in the presence of an organic solvent in a stirred tank reactor to obtain a polymer reaction solution.

[0024] In some embodiments, the alkane solvent is one or more of C6-C10 straight-chain alkanes, C6-C10 isoalkanes, C6-C10 cycloalkanes and C6-C10 aromatic alkanes, optionally selected from one or more of n-hexane, cyclohexane, methylcyclohexane, n-heptane, isoheptane, n-octane, isooctane, n-decane, toluene and xylene.

[0025] In some embodiments, the polymerization catalyst includes a main catalyst and a co-catalyst.

[0026] In some embodiments, the main catalyst is a Group IVB-IIB transition metal compound, optionally selected from dimethylsilyl-bridged bisindenyl zirconium dichloride, dimethylsilyl-bridged bis(2-methyl-indenyl) zirconium dichloride, bis(2-methyl-4-phenyl-indenyl) zirconium dichloride, dimethylsilyl-bridged bis(2-methyl-4-phenyl-indenyl) zirconium dichloride, dimethylsilyl-bridged bis(2-methyl-4-phenyl-indenyl) zirconium dimethyl, dimethylsilyl-bridged bis[2-methyl-4-(3,5-dimethylphenyl)-indenyl] zirconium dimethyl, isopropyl-bridged bis(2-methyl-4-phenyl-indenyl) dimethyl hafnium, isopropyl-bridged bis[2-methyl-4-(3,5-dimethylphenyl)-indenyl] dimethyl hafnium, biscyclopentadienyl dimethyl At least one of methyl hafnium, dimethylsilyl-bridged bis-tetramethylcyclopentadienyl titanium dimethyl, dimethylsilyl-bridged (cyclopentadienyl-fluorenyl) zirconium dichloride, isopropyl-bridged (cyclopentadienyl-fluorenyl) zirconium dichloride, isopropyl-bridged [cyclopentadienyl-(3,6-di-tert-butyl-fluorenyl)] zirconium dichloride, isopropyl-bridged (fluorenyl-tert-butylamino) titanium dichloride, dimethylsilyl-bridged (3,6-di-tert-butylfluorenyl-tert-butylamino) titanium dimethyl, isopropyl-bridged (tetramethylcyclopentadienyl-tert-butylamino) titanium dimethyl, dimethylsilyl-bridged (3-pyrrolylindenyl-tert-butylamino) titanium dimethyl, bis(3-methylsalicylidene-pentafluorophenylimino) titanium dichloride, and bis(salicylidene-phenylimino) titanium dichloride.

[0027] In some embodiments, the cocatalyst is an alkylaluminoxane, optionally selected from at least one of methylaluminoxane and modified methylaluminoxane. The specific type of the cocatalyst can be selected conventionally in the art and will not be described in detail here.

[0028] In the present application, the amount of the polymerization catalyst, the ratio of the main catalyst to the co-catalyst, etc. can be conventionally selected in the art and are not particularly limited here.

[0029] In the present application, the solution polymerization reaction can be a conventional operation in the art. In some embodiments, the process conditions of the solution polymerization reaction include: a reaction temperature of 110 to 180° C. (e.g., 115° C., 120° C., 125° C., 130° C., 140° C., 150° C., 160° C., 170° C.), and a reaction pressure of 3 to 10 MPaG (e.g., 3.2 MPaG, 3.5 MPaG, 4 MPaG, 5 MPaG, 6 MPaG, 8 MPaG, 9 MPaG).

[0030] In the present application, a deactivator can be added to the polymer reaction liquid obtained by the polymerization reaction to control the polymerization reaction time or the residence time in the polymerization system. For example, in the present application, the polymerization time (or the residence time of the material in the polymerization reaction system) is controlled to be 3 to 15 minutes (for example, 4 minutes, 5 minutes, 6 minutes, 8 minutes, 10 minutes, 12 minutes). In some embodiments, the polymer reaction liquid obtained by the solution polymerization reaction (for example, at the outlet of the polymerization reactor) is deactivated by a deactivator; the deactivator can be a C6 to C8 long-chain alcohol.

[0031] The C6-C8 long-chain alcohol may be a C6-C8 aliphatic alcohol; for example, it may be selected from but not limited to at least one of n-hexanol, n-heptanol, and isooctyl alcohol.

[0032] In the present application, the amount of the inactivator may not be specifically limited, as long as the amount added can stop the polymerization reaction within a corresponding time; for example, the amount of the inactivator may range from 2 to 5 times the mass of the polymerization catalyst added.

[0033] Generally speaking, various technical means can be used to achieve the copolymers defined in this application with a low soluble fraction content range and a uniform branch distribution. In this application, for example, in some embodiments, the polymer reaction solution obtained by the polymerization reaction is deactivated to control the polymerization reaction time within a suitable range, thereby ultimately achieving control of the low soluble fraction content, resulting in a copolymer product with a low soluble fraction content and a uniform branch distribution.

[0034] In some embodiments, the product stream after the inactivation treatment is heated (e.g., by heating in a heater), and the resulting high-temperature reaction liquid is flash-evaporated in a flash evaporation system (e.g., a three-stage flash evaporation) to remove volatile components, solvent, and unreacted monomers. The flash evaporation system used herein can be conventional equipment in the art, and the various stages of the flash evaporation process can also be conventionally selected in the art and will not be described in detail here.

[0035] In some embodiments, the unreacted monomers and solvents are recycled as raw materials, and the resulting polymer product stream is melt-extruded and pelletized to obtain copolymer particles. The melt-extrusion can be a conventional operation in the art.

[0036] In a second aspect, a use of the above-mentioned ethylene / α-olefin random copolymer in the field of photovoltaic encapsulation films is provided.

[0037] The specific process for applying the ethylene / α-olefin random copolymer to the photovoltaic encapsulation film field herein can be conventional in the art and will not be further described. For example, the ethylene / α-olefin random copolymer can be mixed with an additive to form a photovoltaic encapsulation film composition, which can then be processed into a photovoltaic encapsulation film using conventional processing techniques in the art.

[0038] The auxiliary agents mentioned here can be any of the common auxiliary agents in the art, such as cross-linking agents, cross-linking auxiliary agents, antioxidants, and the like.

[0039] Compared with the related art, the positive effects of this application are:

[0040] The olefin copolymer provided in this application has the characteristics of low soluble matter content (for example, 0.5 to 3 mol%) and uniform branching distribution for polymer segments of different molecular weights, which gives it improved processing performance. When the copolymer is used as a photovoltaic film, it has a fast cross-linking speed, a high degree of cross-linking, and a high sealing strength. At the same time, the resulting film has good light transmittance.

[0041] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0043] FIG1 shows the relationship curve between the torque and (treatment) time after vulcanization of the polymers obtained in various examples and comparative examples, ie, the vulcanization curve.

[0044] Figure 2 shows the linear fitting relationship between the number of thousand-carbon branches of each different molecular weight segment in the polymer obtained in Example 2 (as the dependent variable y) and the logarithm of the molecular weight of each copolymer segment (as the independent variable x), as well as the normal distribution relationship between the copolymer molecular weight absorption intensity (as the dependent variable y') and the logarithm of the molecular weight of each copolymer segment (as the independent variable x); in this figure, the dotted straight line represents the theoretical linear fitting line. DETAILED DESCRIPTION

[0045] In order to understand the technical features and content of the present application in detail, the preferred embodiments of the present application will be described in more detail below. Although the preferred embodiments of the present application are described in the embodiments, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments described herein.

[0046] Unless otherwise specified, the experimental procedures used in the following examples are conventional methods.

[0047] The materials and reagents used in the following examples can all be obtained from commercial sources. The specific information of some raw materials is shown in Table 1 below:

[0048] Table 1

[0049] Polymerization method of ethylene / α-olefin random copolymer

[0050] The continuous solution polymerization reaction is carried out in a high-pressure stirred reactor with PLC control. After thorough mixing of dehydrated, deoxygenated, purified C8-C10 mixed isoparaffins (Isopar E), ethylene monomer, α-olefin monomer, and cocatalyst (n-octyl-modified aluminoxane), the mixture is continuously fed into a 2.0-liter high-pressure stirred reactor equipped with a temperature-controlled jacket and internal thermocouple. The ethylene monomer feed rate is metered by a gas mass flow controller, while the flow rates of the C8-C10 mixed isoparaffins, α-olefin monomer, and cocatalyst (n-octyl-modified aluminoxane) are controlled by a diaphragm metering pump. Prior to entering the reactor, the temperature of the solution of the C8-C10 mixed isoparaffins, monomers, and cocatalyst (n-octyl-modified aluminoxane) is controlled by a heat exchanger, ensuring that the solution enters the bottom of the reactor at -20-25°C. The prepared main catalyst solution is separately fed into the bottom of the reactor at room temperature via a diaphragm metering pump.

[0051] The reaction temperature in the reactor was stably controlled by the feed temperature and the reactor jacket temperature, as shown in Table 1. The reactor pressure was controlled by a heat exchanger outlet pressure regulating valve. The reactor was operated with full liquid at a stirring speed of 500-1000 rpm, and the reactor pressure was 3.5 MPa. The reaction liquid obtained after polymerization flowed out through the outlet pipeline at the top of the reactor. The outflowing reaction liquid was treated with a deactivator and then entered a heat exchanger and heated to 260° C. The high-temperature reaction liquid obtained by heating was depressurized by a pressure control valve and then sequentially entered a three-stage low-pressure flash evaporation system for flash evaporation treatment (wherein the three-stage flash evaporation temperatures were all 240-260° C., and the three-stage flash evaporation pressures were 1-3 barG, 0.2-1 barG, and 3-5 kPaA, respectively). The small amount of ethylene removed was discharged, and the removed α-olefin monomer and solvent were reused.

[0052] The obtained polymer product is extruded and pelletized to obtain polymer particles.

[0053] The detailed polymerization conditions in each embodiment and comparative example are shown in Tables 2 to 4.

[0054] Table 2 Continuous solution polymerization conditions

[0055] Al / M represents the molar ratio of the cocatalyst to the main catalyst. The flow rate of the cocatalyst (n-octyl modified aluminoxane, calculated as a solution with an Al content of 7%) is 3 to 12 g / h.

[0056] Table 3 Main catalyst types and polymerization activities in continuous solution polymerization

[0057] Table 4 Reaction conditions for continuous solution polymerization

[0058] In each embodiment and comparative example, the amount of the deactivator used is based on the amount that can deactivate the catalyst in the polymerization reaction system.

[0059] Test method:

[0060] (1) The polymerization activity is calculated as the ratio of the weight of the dry polymer product obtained from the polymerization reaction to the number of moles of the transition metal compound (primary catalyst) added during the reaction.

[0061] (2) The content of polymer soluble fraction (SF) was tested by temperature gradient interaction high temperature chromatography (TGIC) with 1,2,4-trichlorobenzene as the mobile phase, the elution temperature was 10-150℃, and the heating rate was 5℃ / min. The test process was as follows: 2-3 mg of the polymer sample to be tested was dissolved in 1,2,4-trichlorobenzene to form a uniform solution phase, which was pumped into the chromatographic column. The chromatographic column was eluted with trichlorobenzene mobile phase under different temperature conditions of 10-150℃. The polymer components were fractionated according to the different adsorption capacities of the chromatographic column for the polymer components under different temperature conditions. The different polymer components obtained after fractionation were detected by infrared detector to obtain the ratio (content), molecular weight and degree of branching of different fractions.

[0062] (3) The molecular weight (Mw), molecular weight distribution (PDI), and 1,000 carbon branching number of the polymer were determined using a high-temperature gel permeation chromatography-infrared spectrometer (GPC-IR) with 1,2,4-trichlorobenzene as the mobile phase and polystyrene as the standard at 150°C. The standard concentration was 0.1 mg / mL, the solvent flow rate was 1.0 mL / min, the standard parameters were K = 59.1, α = 0.69, and the parameters of the sample were K = 14.1, α = 0.70.

[0063] (4) The melt flow rate (MFR) of the polymer is measured using a melt indexer (MI-4). Under the test conditions of 190°C and a load of 2.16 kg, the weight of the molten polymer to be tested is extruded through a die with a length of 8 mm and an inner diameter of 2.095 mm for a specified time. The unit is MFR, which is g / 10 min.

[0064] (5) The density of the polymer is tested using a density meter (immersion method METTLER). The mass of the test sample suspended by a metal wire with a diameter not greater than 0.5 mm is weighed in air; the mass of the sample is not greater than 10 g, accurate to 0.1 mg, and the mass of the sample is recorded. The sample suspended by the thin metal wire is immersed in a beaker filled with immersion liquid placed on a fixed bracket. The temperature of the immersion liquid should be 23℃±2℃; use the thin metal wire to remove bubbles adhering to the sample; and weigh the mass of the sample in the immersion liquid to an accuracy of 0.1 mg.

[0065] (6) The crosslinking degree and peel strength of the sample were tested by xylene soluble content test and peel tensile tester test respectively. The specific steps are as follows:

[0066] First, the copolymer particles prepared in each embodiment and comparative example are mixed with diisopropylbenzene peroxide (cross-linking agent). The resulting mixture is processed into a photovoltaic film with a thickness of 500 to 700 μm by a casting machine, and then pressed by a laminator under negative pressure conditions at 70°C for 5 minutes (cross-linking occurs during this process). Finally, the cross-linked photovoltaic film is taken out and the degree of cross-linking is obtained using the xylene soluble content, and the peel strength (peel strength with glass) is measured.

[0067] (7) The crosslinking rate of the sample is characterized by a vulcanization test using a rotorless vulcanizer. The polymer sample to be tested is mixed with a crosslinking agent (i.e., dicumyl peroxide) and the resulting mixture is placed in a completely sealed mold cavity, maintained at a test temperature of 145°C, and vulcanized for 15 minutes. The mold cavity is divided into two parts, one of which oscillates with a small linear reciprocating motion or angular swing. The oscillation causes shear strain in the sample, and the reaction torque of the sample on the mold cavity is measured. This torque depends on the shear modulus of the sample.

[0068] The mixed sample with cross-linking agent undergoes cross-linking reaction at 145℃. As the degree of cross-linking increases, the shear modulus of the sample increases until it reaches a stable value or maximum value. The relationship curve between torque and (treatment) time, i.e., the vulcanization curve, is obtained, as shown in Figure 1. Generally, the cross-linking speed is expressed by T 90 It represents the time when the degree of vulcanization reaches 90%. The smaller the value, the faster the cross-linking speed.

[0069] The performance test results of the copolymers obtained in various examples and comparative examples are shown in Tables 5 to 7. The performance test results of the films made of the copolymers obtained in various examples and comparative examples are shown in Table 8.

[0070] Table 5 Performance index test results of copolymers

[0071] Table 6 Performance index test results of copolymers

[0072] For the polymer prepared in Example 2, a good linear fit relationship was formed between the number of thousand-carbon branches (i.e., the number of branches per thousand carbon atoms) of each molecular weight segment, as the dependent variable y, and the logarithm of the molecular weight of each copolymer segment, as the independent variable x. Furthermore, a normal distribution relationship was formed between the molecular weight absorption intensity of the copolymer, as the dependent variable y', and the logarithm of the molecular weight of each copolymer segment, as the independent variable x. The changing relationships between x and y, and x and y', are shown in FIG2 , in which:

[0073] The fitting coefficient of determination R of the linear fit of x and y 2 =0.952; the MMD on the left ordinate (y') represents the molecular weight absorption intensity of the copolymer, the SCB / 1000C on the right ordinate (y) represents the number of thousand-carbon branches of each different molecular weight segment, and the LogM on the abscissa (x) represents the logarithm of the molecular weight of each polymer segment.

[0074] The total number of branches per thousand carbon atoms in the chain structure of the polymer prepared in Example 2 can be calculated based on the relationship between x and y (y') shown in FIG2 .

[0075] Table 7 Performance index test results of copolymer

[0076] Table 8 Test results of performance indicators of films made from copolymers

[0077] The polymerization results of the Examples and Comparative Examples demonstrate that the present application utilizes a controlled inactivation process to effectively regulate the polymer composition. The resulting polymers exhibit low soluble fraction content, uniform distribution of short branches within segments of varying molecular weight, and a good linear fit between the number of branches and the logarithm of the molecular weight. Polymers with these characteristics, when used in photovoltaic films, maintain high light transmittance while exhibiting rapid crosslinking, high crosslinking degrees, and high peel strength from glass.

[0078] Through the preparation method of olefin polymers shown in the above embodiments, the copolymer prepared by the method provided in this application has low soluble matter content and uniform branch distribution, high cross-linking degree under the same cross-linking conditions, fast cross-linking speed, and high peel strength, showing improved processing performance in photovoltaic film applications.

Claims

1. An ethylene / α-olefin random copolymer for a photovoltaic encapsulant film, the ethylene / α-olefin random copolymer having ethylene monomer units and α-olefin monomer units of C3 to C8; wherein, The ethylene / α-olefin random copolymer has the following characteristics: (i) The molar content of the soluble fraction below 35 °C measured by temperature rising elution fractionation is 0.5-3%, wherein the weight average molecular weight of the soluble fraction is less than or equal to 20,000 g / mol; (ii) The total number of side chains per thousand carbons in the chain structure of the copolymer is 25 to 85; and a good linear fitting relationship is formed between the number of side chains per thousand carbons of each different molecular weight chain segment as the dependent variable y and the logarithm of the molecular weight of each copolymer chain segment as the independent variable x, and the fitting determination coefficient R 2 is greater than or equal to 0.

75.

2. The ethylene / α-olefin random copolymer according to claim 1, wherein, The ethylene / α-olefin random copolymer further has the following characteristics: (iii) The melt flow rate of the copolymer is 4 to 35 g / 10 min (190 °C, 2.16 kg), and the density is 0.865 to 0.890 g / cm 3 ; (iv) The weight average molecular weight of the copolymer is 45,000-80,000 g / mol, and the molecular weight distribution (Mw / Mn) is less than or equal to 3.

3. The ethylene / α-olefin random copolymer according to claim 1 or 2, wherein, The α-olefin monomer is selected from one or more of propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene and 1-octene.

4. The ethylene / α-olefin random copolymer according to any one of claims 1 to 3, wherein The ethylene / α-olefin random copolymer is a product obtained by continuously introducing an alkane solvent, an ethylene monomer, an α-olefin monomer and a polymerization catalyst into a polymerization reactor for mixing and contacting to carry out a solution polymerization reaction.

5. The ethylene / α-olefin random copolymer according to claim 4, wherein, The alkane solvent is one or more of straight-chain alkanes with C6-C10, isoparaffins with C6-C10, cycloalkanes with C6-C10 and aralkyl hydrocarbons with C6-C10.

6. The ethylene / α-olefin random copolymer according to claim 5, wherein, The alkane solvent is selected from one or more of n-hexane, cyclohexane, methylcyclohexane, n-heptane, isoheptane, n-octane, isooctane, n-decane, toluene and xylene.

7. The ethylene / α-olefin random copolymer according to claim 4, wherein, The polymerization catalyst includes a main catalyst and a cocatalyst.

8. The ethylene / α-olefin random copolymer according to claim 7, wherein, The main catalyst is a transition metal compound of Group IVB-IIB; The cocatalyst is an alkylaluminoxane.

9. The ethylene / α-olefin random copolymer according to claim 8, wherein, The main catalyst is selected from at least one of dimethylsilyl-bridged bis(indenyl)zirconium dichloride, dimethylsilyl-bridged bis(2-methyl-indenyl)zirconium dichloride, bis(2-methyl-4-phenyl-indenyl)zirconium dichloride, dimethylsilyl-bridged bis(2-methyl-4-phenyl-indenyl)zirconium dimethyl, dimethylsilyl-bridged bis[2-methyl-4-(3,5-dimethylphenyl)-indenyl]zirconium dimethyl, isopropylidene-bridged bis(2-methyl-4-phenyl-indenyl)hafnium dimethyl, isopropylidene-bridged bis[2-methyl-4-(3,5-dimethylphenyl)-indenyl]hafnium dimethyl, dicyclopentadienylhafnium dimethyl, dimethylsilyl-bridged bis(tetramethylcyclopentadienyl)dimethyltitanium, dimethylsilyl(cyclopentadienyl-fluorenyl)zirconium dichloride, isopropylidene-bridged (cyclopentadienyl-fluorenyl)zirconium dichloride, isopropylidene-bridged [cyclopentadienyl-(3,6-di-tert-butyl-fluorenyl)]zirconium dichloride, isopropylidene-bridged (fluorenyl-tert-butylamino)titanium dichloride, dimethylsilyl(3,6-di-tert-butylfluorenyl-tert-butylamino)dimethyltitanium, isopropylidene-bridged (tetramethylcyclopentadienyl-tert-butylamino)dimethyltitanium, dimethylsilyl(3-pyrrolylindenyl-tert-butylamino)dimethyltitanium, bis(3-methylsalicylidene-pentafluorophenylimino)titanium dichloride, bis(salicylidene-phenylimino)titanium dichloride.

10. The ethylene / α-olefin random copolymer according to claim 8, wherein, The cocatalyst is selected from at least one of methylaluminoxane and modified methylaluminoxane.

11. The ethylene / α-olefin random copolymer according to any one of claims 4-10, wherein, The process conditions of the solution polymerization reaction include: the reaction temperature is 110-180 °C, and the reaction pressure is 3-10 MPaG.

12. The ethylene / α-olefin random copolymer according to any one of claims 4 to 11, wherein, The polymer reaction solution obtained from the solution polymerization reaction is inactivated by an inactivator; the inactivator is a long-chain alcohol with 6 to 8 carbon atoms.

13. The ethylene / α-olefin random copolymer according to claim 12, wherein, The product stream after the inactivation treatment is heated, and the obtained high-temperature reaction solution is subjected to flash evaporation treatment through a flash evaporation system to remove the volatile components, solvents, and unreacted monomers therein.

14. The ethylene / α-olefin random copolymer according to claim 13, wherein, The obtained unreacted monomers and the solvents are recycled as raw materials, and the obtained polymer product stream is melt-extruded and pelletized to obtain copolymer particles.

15. Use of the ethylene / α-olefin random copolymer according to any one of claims 1-14 in the field of photovoltaic encapsulation films.

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