Polyethylene resin for rotational molding, process for its preparation and use

Abstract

The application relates to a polyethylene resin for rotational molding and a preparation method and application thereof, and belongs to the technical field of polyethylene resins. The polyethylene resin is a copolymer of ethylene and alpha-olefin; the content of a fraction greater than 120 DEG C obtained by continuously performing self-nucleation annealing thermal fractionation test on the polyethylene resin is 45-55 wt%, the content of a fraction of 116-120 DEG C is 15-20 wt%, the content of a fraction of 111-115 DEG C is 10-15 wt%, the content of a fraction of 100-110 DEG C is 11-15 wt%, and the content of a fraction less than 98.5 DEG C is 8-10 wt%. The polyethylene resin has excellent impact resistance, environmental stress cracking resistance and slow crack growth resistance, and is suitable for military packaging boxes.

Description

Technical Field

[0001] This invention belongs to the field of polyethylene resin technology, specifically relating to a polyethylene resin for rotational molding, its preparation method, and its application. Background Technology

[0002] Military packaging boxes are primarily used for the packaging and storage of military supplies. Traditional military packaging boxes are mainly wooden and iron boxes. However, as the requirements for packaging materials during the transportation and storage of military products become increasingly stringent, traditional wooden and iron boxes are no longer adequate due to their susceptibility to corrosion and decay, poor impact resistance, bulky appearance, and inconvenient loading and unloading. Therefore, a new generation of rotomolded military packaging boxes has been developed and is gradually being deployed to the military.

[0003] Rotational molding is a commonly used method for processing plastic products, particularly suitable for molding large hollow containers. Rotationally molded products have advantages such as low internal stress and seamless construction. The main raw material for rotational molding is polyethylene resin. While current resins can meet molding and appearance requirements, they still have serious deficiencies in certain key properties such as impact resistance, resistance to environmental stress cracking, and resistance to slow crack propagation. It has been reported that during critical earthquake relief efforts, there were instances where airdropped rotationally molded military packaging boxes suffered severe damage due to poor impact resistance or insufficient crack resistance, resulting in the loss of vital disaster relief supplies.

[0004] Patent CN106699943B discloses a polymerization process for high-rigidity polyethylene rotational molding resin. The process involves adding a mixture of dry powder catalyst and slurry catalyst to a nitrogen-protected container, then introducing hydrogen, ethylene, and butene into the container. The molar percentages of hydrogen, butene, and ethylene are controlled at 8-12%, 4-7%, and 30-40%, respectively. Polymerization is carried out at 80-100°C for 3-9 hours under a polymerization pressure of 0.05-0.1 MPa. The resin is then terminated, flashed, centrifuged, dried, and sieved. Both the dry powder catalyst and the slurry catalyst are zinc (Zn) catalysts. The polyethylene resin obtained by this method exhibits high tensile strength and high rigidity, but the resistance to environmental stress cracking and slow crack propagation of the polyethylene resin has not been investigated.

[0005] Patent CN107108993A discloses a rotational molding article comprising at least one metallocene-catalyzed polyethylene resin, said polyethylene resin comprising at least two metallocene-catalyzed polyethylene fractions A and B, wherein said polyethylene resin comprises: polyethylene fraction A at 25-55% by weight based on the total weight of said polyethylene resin, wherein fraction A has a melt index MI2 of at least 25.0 g / 10 min as determined according to ISO 1133, condition D, at 190°C and under a load of 2.16 kg, and a density at least 0.005 g / cm³ higher than that of said polyethylene resin.3 The density; and the polyethylene resin thereon has a density of 0.938-0.950 g / cm³ as measured at 23°C according to ASTM D-1505. 3 The density; such as the melt index MI2 of 1.0-25.0 g / 10 min as determined according to ISO 1133, condition D, at 190°C and under a load of 2.16 kg. The rotomolded polyethylene resin products prepared by this patent have improved mechanical properties and processability, but the environmental stress cracking resistance and slow crack propagation resistance of the polyethylene resin still need further improvement. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this invention is to provide a polyethylene resin for rotational molding, its preparation method and application, wherein the polyethylene resin has excellent impact resistance, environmental stress cracking resistance and slow crack propagation resistance, and is suitable for use in military packaging boxes.

[0007] To achieve the above objectives, according to one aspect of the present invention, a polyethylene resin for rotational molding is provided, wherein the polyethylene resin is a copolymer of ethylene and α-olefin; the content of fractions above 120°C obtained by continuous self-nucleation annealing thermal grading test of the polyethylene resin is 45-55 wt%, the content of fractions above 116-120°C is 15-20 wt%, the content of fractions above 111-115°C is 10-15 wt%, the content of fractions above 100-110°C is 11-15 wt%, and the content of fractions below 98.5°C is 8-10 wt%.

[0008] In some embodiments, the α-olefin is selected from at least one of butene, hexene, and octene.

[0009] In some embodiments, the α-olefin content in the polyethylene resin is 1.18-1.92 mol%.

[0010] In some embodiments, the polyethylene resin has one or more of the following characteristics:

[0011] The density of the polyethylene resin is 0.931-0.935 g / cm³. 3 ;

[0012] The weight-average sequence length distribution index of the polyethylene resin is 1.15-1.18;

[0013] The melt flow rate (MFR) of the polyethylene resin under test conditions of 190°C and 2.16 kg load is 4-9 g / 10 min.

[0014] According to another aspect of the present invention, a method for preparing the above-mentioned polyethylene resin for rotational molding is also provided, comprising the following steps:

[0015] (1) Methylaluminoxane and supported silica are mixed at a mass ratio of 1-2:1, and a toluene solution of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidyl)zirconia is added. The mixture is stirred evenly, washed, filtered, and dried to obtain a metallocene catalyst.

[0016] (2) Add the metallocene catalyst to the Unipol gas-phase process polyethylene device, introduce α-olefin and ethylene with a molar ratio of 0.01-0.05:1, introduce hydrogen as a chain transfer agent, and carry out gas-phase polymerization reaction under the conditions of polymerization temperature of 80-90℃ and polymerization pressure of 2.0-2.5MPa to obtain the polyethylene resin for rotational molding.

[0017] In some embodiments, in step (1), the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindioyl)zirconia is 1:0.1-0.5.

[0018] In some embodiments, in step (1), the molar ratio of the sum of methylaluminoxane and bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindioyl)zirconia is 20-500:1.

[0019] In some embodiments, in step (2), the concentration of hydrogen is 500-1000 ppm.

[0020] In some embodiments, in step (2), the partial pressure of ethylene in the gas-phase polymerization reaction is 70-80%.

[0021] According to another aspect of the present invention, the application of the above-described rotomolding polyethylene resin or the rotomolding polyethylene resin prepared according to the above preparation method in military packaging boxes is also provided.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] (1) The present invention creatively designs a special structure of polyethylene resin for rotational molding. By controlling the microstructure of the polyethylene resin, the fraction content in different temperature ranges obtained by continuous self-nucleation annealing thermal grading test of the resin can be controlled, thereby making the polyethylene resin have excellent impact resistance, environmental stress cracking resistance and slow crack propagation resistance, and suitable for use in military packaging boxes.

[0024] (2) The present invention also provides a method for preparing polyethylene resin, in particular, by controlling the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindioyl)zirconia, a specific metallocene catalyst is designed, thereby enabling the polyethylene resin obtained by using the catalyst to have excellent impact resistance, environmental stress cracking resistance and slow crack propagation resistance. Attached Figure Description

[0025] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein...

[0026] Figure 1 This is the melt curve of polyethylene resin after continuous self-nucleation annealing and thermal grading in Example 1 of the present invention.

[0027] Figure 2 This is the melt curve of polyethylene resin after continuous self-nucleation annealing and thermal classification in Comparative Example 1 of the present invention. Detailed Implementation

[0028] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0029] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0030] In this document, when describing embodiments or examples, it should be understood that it is not intended to limit the invention to those embodiments or examples. Rather, all alternatives, modifications, and equivalents of the methods and materials described herein are covered within the scope defined by the claims.

[0031] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0032] This invention provides a polyethylene resin for rotational molding, wherein the polyethylene resin is a copolymer of ethylene and α-olefin; the content of the fraction with a temperature greater than 120°C obtained by continuous self-nucleation annealing thermal grading test of the polyethylene resin is 45-55 wt%. It is understood that this content can be any specific value among 45 wt%, 46 wt%, 47 wt%, 48 wt%, 49 wt%, 50 wt%, 51 wt%, 52 wt%, 53 wt%, 54 wt%, and 55 wt%, or 45 wt%. The thickness of the lamellar crystals in the fractions above 120°C obtained by continuous self-nucleation annealing thermal grading test of the polyethylene resin is 12-13 nm. It can be understood that the thickness can be any specific value among 12 nm, 12.1 nm, 12.2 nm, 12.3 nm, 12.4 nm, 12.5 nm, 12.6 nm, 12.7 nm, 12.8 nm, 12.9 nm, and 13 nm, or any value in the range of 12-13 nm.

[0033] The content of the fraction at 116-120℃ obtained by continuous self-nucleation annealing thermal grading test of the polyethylene resin is 15-20 wt%. It can be understood that its content can be any specific value among 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, and 20 wt%, or any value within the range of 15-20 wt%. The lamellar thickness of the fraction at 116-120℃ obtained by continuous self-nucleation annealing thermal grading test of the polyethylene resin is 9.5-10 nm. It can be understood that its thickness can be any specific value among 9.5 nm, 9.6 nm, 9.7 nm, 9.8 nm, 9.9 nm, and 10 nm, or any value within the range of 9.5-10 nm.

[0034] The content of the fraction at 111-115℃ obtained by continuous self-nucleation annealing thermal grading test of the polyethylene resin is 10-15 wt%. It can be understood that its content can be any specific value among 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, and 15 wt%, or any value within the range of 10-15 wt%. The lamellar thickness of the fraction at 111-115℃ obtained by continuous self-nucleation annealing thermal grading test of the polyethylene resin is 7.5-8.0 nm. It can be understood that its thickness can be any specific value among 7.5 nm, 7.6 nm, 7.7 nm, 7.8 nm, 7.9 nm, and 8.0 nm, or any value within the range of 7.5-8.0 nm.

[0035] The content of the fraction at 100-110℃ obtained by continuous self-nucleation annealing thermal grading test of the polyethylene resin is 11-15 wt%. It can be understood that its content can be any specific value among 11 wt%, 12 wt%, 13 wt%, 14 wt%, and 15 wt%, or any value within the range of 11-15 wt%. The lamellar thickness of the fraction at 100-110℃ obtained by continuous self-nucleation annealing thermal grading test of the polyethylene resin is 6.0-7.0 nm. It can be understood that its thickness can be any specific value among 6.0 nm, 6.1 nm, 6.2 nm, 6.3 nm, 6.4 nm, 6.5 nm, 6.6 nm, 6.7 nm, 6.8 nm, 6.9 nm, and 7.0 nm, or any value within the range of 6.0-7.0 nm.

[0036] The content of the fraction with a temperature below 98.5°C obtained from the continuous self-nucleation annealing thermal grading test of the polyethylene resin is 8-10 wt%. It can be understood that its content can be any specific value among 8 wt%, 9 wt%, and 10 wt%, or any value within the range of 8-10 wt%. The lamellar thickness of the fraction with a temperature below 98.5°C obtained from the continuous self-nucleation annealing thermal grading test of the polyethylene resin is 4.5-5.0 nm. It can be understood that its thickness can be any specific value among 4.5 nm, 4.6 nm, 4.7 nm, 4.8 nm, 4.9 nm, and 5.0 nm, or any value within the range of 4.5-5.0 nm.

[0037] In this invention, the continuous self-nucleation annealing thermal classification (SSA) test is a method for classification based on the different crystallization temperatures of different branched segments in polyethylene. The continuous self-nucleation annealing thermal classification generally includes the following steps: (1) heating the sample to above the melting temperature (at least 25°C above the melting temperature) and holding it at that temperature for a certain time to eliminate thermal history; (2) cooling the sample at a certain cooling rate to a pre-set minimum temperature (this minimum temperature ensures that the sample can crystallize at this temperature) and holding it at that minimum temperature for a period of time; (3) heating the sample at a certain heating rate to the first pre-set self-nucleation temperature T. S (Generally 25°C above the melting temperature), and hold at that temperature for a period of time; (4) Repeat step (2); (5) Heat the sample at a certain heating rate to the next set self-nucleation temperature T. S (relative to the previous self-nucleation temperature T) S The next self-nucleation temperature T S (2.5 or 5°C) and keep the temperature constant for a period of time, repeating this cycle (the temperature range of the entire thermal grading process is similar to the melting range of the sample); (6) heat the sample to the melting temperature set in step (1) at a certain heating rate and record the heating and melting curve.

[0038] In the continuous self-nucleation annealing thermal grading test of polyethylene, fractions with higher melting temperatures correspond to lamellar crystals with less branching content, thicker lamellar crystals, and longer methylene sequences. The higher the content of fractions with higher melting temperatures, the greater the rigidity of the material, manifested in higher yield tensile stress and higher flexural modulus in mechanical properties. This invention designs a specific metallocene catalyst by controlling the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia, thereby controlling the microstructure of the synthesized polyethylene resin. This allows for the control of fraction content within different temperature ranges obtained from the continuous self-nucleation annealing thermal grading test, resulting in polyethylene resin exhibiting excellent impact resistance, environmental stress cracking resistance, and slow crack propagation resistance.

[0039] In some embodiments, the α-olefin is selected from at least one of butene, hexene, and octene, preferably 1-hexene. In some embodiments, the content of α-olefin in the polyethylene resin is 1.18-1.92 mol%. It is understood that the content can be any specific value among 1.18 mol%, 1.2 mol%, 1.3 mol%, 1.4 mol%, 1.5 mol%, 1.6 mol%, 1.7 mol%, 1.8 mol%, 1.9 mol%, and 1.92 mol%, or any value within the range of 1.18-1.92 mol%.

[0040] In some embodiments, the density of the polyethylene resin is 0.931-0.935 g / cm³. 3 Understandably, its density could be 0.931 g / cm³. 3 0.932g / cm 3 0.933g / cm 3 0.934 g / cm 3 0.935g / cm 3 Any specific value in the range is either 0.931-0.935 g / cm³. 3 Any value within the range.

[0041] In some embodiments, the weight-average sequence length distribution index of the polyethylene resin is 1.15-1.18. It is understood that it can be any specific value among 1.15, 1.16, 1.17, and 1.18, or any value within the range of 1.15-1.18. The weight-average sequence length distribution index can characterize the uniformity of lamellar thickness distribution. The larger the index, the more uneven the distribution of comonomers, and the worse the regularity of the material.

[0042] In some embodiments, the Z-mean sequence length distribution index of the polyethylene resin is 1.25-1.28. It can be understood that it can be any specific value among 1.25, 1.26, 1.27, and 1.28, or any value within the range of 1.25-1.28.

[0043] In some embodiments, the melt flow rate (MFR) of the polyethylene resin under test conditions of 190°C and 2.16 kg load is 4-9 g / 10 min. It is understood that this can be any specific value among 4 g / 10 min, 5 g / 10 min, 6 g / 10 min, 7 g / 10 min, 8 g / 10 min, and 9 g / 10 min, or any value within the range of 4-9 g / 10 min. In some preferred embodiments, the melt flow rate (MFR) of the polyethylene resin under test conditions of 190°C and 2.16 kg load is 4.5-5.6 g / 10 min.

[0044] According to another aspect of the present invention, a method for preparing the above-mentioned polyethylene resin for rotational molding is also provided, comprising the following steps:

[0045] (1) Methylaluminoxane and supported silica are mixed at a mass ratio of 1-2:1, and a toluene solution of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidyl)zirconia is added. The mixture is stirred evenly, washed, filtered, and dried to obtain a metallocene catalyst.

[0046] (2) Add the metallocene catalyst to the Unipol gas-phase process polyethylene device, introduce α-olefin and ethylene with a molar ratio of 0.01-0.05:1, introduce hydrogen as a chain transfer agent, and carry out gas-phase polymerization reaction under the conditions of polymerization temperature of 80-90℃ and polymerization pressure of 2.0-2.5MPa to obtain the polyethylene resin for rotational molding.

[0047] In some embodiments, in step (1), methylaluminoxane and carrier silica are mixed at a mass ratio of 1-2:1. It is understood that the mass ratio can be any specific value among 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, and 2:1, or any value within the range of 1-2:1. The present invention can use conventional carrier silica in the art, including but not limited to spherical mesoporous silica carriers and rod-shaped mesoporous silica carriers, with an average silica particle size between 20-100 micrometers. In some embodiments, in step (1), the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is 1:0.1-0.5. It is understood that the molar ratio can be any specific value among 1:0.1, 1:0.2, 1:0.3, 1:0.4, and 1:0.5, or any value within the range of 1:0.1-0.5. In some embodiments, in step (1), the molar ratio of methylaluminoxane to the sum of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is 20-500:1, preferably 20-100:1. In some embodiments, the washing can be performed using hexane.

[0048] In some embodiments, in step (2), the molar ratio of the metallocene catalyst to ethylene is 0.2-0.4. In some embodiments, in step (2), the concentration of hydrogen is 500-1000 ppm, preferably 600-800 ppm.

[0049] In some embodiments, in step (2), the partial pressure of ethylene in the gas-phase polymerization reaction is 70-80%. It is understood that the partial pressure can be any specific value among 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, and 80%, or any value within the range of 70-80%.

[0050] According to another aspect of the present invention, the application of the above-described rotomolding polyethylene resin or the rotomolding polyethylene resin prepared according to the above preparation method in military packaging boxes is also provided.

[0051] The present invention will be described in detail below through embodiments. It should be understood that the following embodiments are only used to further explain and illustrate the content of the present invention by way of example, and are not intended to limit the present invention. Among them, Embodiment 1 of the present invention is the preferred embodiment.

[0052] The chemical additives used in the embodiments and comparative examples of this invention are all commercially available, and the specific information is as follows:

[0053] Methylaluminoxane: purchased from AkzoNobel; Silica: purchased from Grace Davison; Bis(1-butyl-3-methylcyclopentadienyl)zirconium dichloride and dimethylsilylbis(2-methyl-4-phenylindinyl)zirconium dichloride: purchased from Wuxi Yaodexin Chemical Products Co., Ltd.; Butene, 1-hexene, octene, toluene, and hexane: analytical grade, purchased from Aladdin Reagent Co., Ltd.

[0054] Example 1

[0055] The method for preparing a polyethylene resin for rotational molding described in this embodiment includes the following steps:

[0056] (1) Methylaluminoxane and supported silica are mixed at a mass ratio of 1:1, and a toluene solution of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is added. The mixture is stirred until homogeneous, washed with hexane, filtered, and dried at 50°C for 1 h to obtain a metallocene catalyst. The molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is 1:0.2, and the molar ratio of methylaluminoxane to the sum of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is 100:1.

[0057] (2) The metallocene catalyst is added to the Unipol gas-phase polyethylene device, and 1-hexene and ethylene are introduced in a molar ratio of 0.01:1. The molar concentration ratio of the metallocene catalyst to ethylene is 0.20. Hydrogen is introduced as a chain transfer agent. The partial pressure of ethylene in the polymerization reaction is adjusted to 75%, the hydrogen concentration is 600 ppm, and the gas-phase polymerization reaction is carried out at a polymerization temperature of 85°C and a polymerization pressure of 2.0 MPa to obtain the polyethylene resin for rotational molding.

[0058] Example 2

[0059] The method for preparing a polyethylene resin for rotational molding described in this embodiment includes the following steps:

[0060] (1) Methylaluminoxane and supported silica are mixed at a mass ratio of 2:1, and a toluene solution of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is added. The mixture is stirred until homogeneous, washed with hexane, filtered, and dried at 60°C for 1 h to obtain a metallocene catalyst. The molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is 1:0.1, and the molar ratio of methylaluminoxane to the sum of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is 20:1.

[0061] (2) The metallocene catalyst is added to the Unipol gas-phase polyethylene device, and butene and ethylene are introduced in a molar ratio of 0.05:1. The molar concentration ratio of the metallocene catalyst to ethylene is 0.20. Hydrogen is introduced as a chain transfer agent. The partial pressure of ethylene in the polymerization reaction is adjusted to 70%, and the hydrogen concentration is 500 ppm. The gas-phase polymerization reaction is carried out at a polymerization temperature of 80°C and a polymerization pressure of 2.5 MPa to obtain the polyethylene resin for rotational molding.

[0062] Example 3

[0063] The method for preparing a polyethylene resin for rotational molding described in this embodiment includes the following steps:

[0064] (1) Methylaluminoxane and supported silica are mixed at a mass ratio of 1.5:1. A toluene solution of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is added, stirred evenly, washed with hexane, filtered, and dried at 50°C for 1 h to obtain a metallocene catalyst. The molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is 1:0.5, and the molar ratio of methylaluminoxane to the sum of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia is 500:1.

[0065] (2) The metallocene catalyst is added to the Unipol gas-phase polyethylene device, and octene and ethylene are introduced in a molar ratio of 0.03:1. The molar concentration ratio of the metallocene catalyst to ethylene is 0.20. Hydrogen is introduced as a chain transfer agent. The partial pressure of ethylene in the polymerization reaction is adjusted to 80%, and the hydrogen concentration is 1000 ppm. The gas-phase polymerization reaction is carried out under the conditions of polymerization temperature of 90°C and polymerization pressure of 2.0 MPa to obtain the polyethylene resin for rotational molding.

[0066] Comparative Example 1

[0067] The preparation method of the polyethylene resin for rotational molding in this comparative example is exactly the same as that in Example 1, except that the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindinitro)zirconia in step (1) is 1:0.05; that is, the detailed steps of step (1) are as follows: methylaluminoxane and carrier silica are mixed at a mass ratio of 1:1, and bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindinitro)zirconia are added. A toluene solution of bis(1-butyl-3-methylcyclopentadienyl)zirconia was stirred until homogeneous, washed with hexane, filtered, and dried at 50°C for 1 h to obtain a metallocene catalyst; wherein the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia was 1:0.05, and the molar ratio of methylaluminoxane to the sum of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia was 100:1.

[0068] Comparative Example 2

[0069] The preparation method of the polyethylene resin for rotational molding in this comparative example is exactly the same as that in Example 1, except that the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia dichloride and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia dichloride in step (1) is 0.2:1; that is, the detailed steps of step (1) are as follows: methylaluminoxane and carrier silica are mixed at a mass ratio of 1:1, and bis(1-butyl-3-methylcyclopentadienyl)zirconia dichloride and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia dichloride are added. A toluene solution of bis(1-butyl-3-methylcyclopentadienyl)zirconia was stirred until homogeneous, washed with hexane, filtered, and dried at 50°C for 1 h to obtain a metallocene catalyst; wherein the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia was 0.2:1, and the molar ratio of methylaluminoxane to the sum of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindidine)zirconia was 100:1.

[0070] Comparative Example 3

[0071] The preparation method of the polyethylene resin for rotational molding in this comparative example is exactly the same as that in Example 1, except that in step (2), the molar ratio of 1-hexene to ethylene is 0.005:1; that is, the detailed steps of step (1) are as follows: the metallocene catalyst is added to the Unipol gas phase process polyethylene device, 1-hexene and ethylene with a molar ratio of 0.005:1 are introduced, the molar concentration ratio of the metallocene catalyst to ethylene is 0.20, hydrogen is introduced as a chain transfer agent, the partial pressure of ethylene in the polymerization reaction is adjusted to 75%, the hydrogen concentration is 600ppm, and the gas phase polymerization reaction is carried out under the conditions of polymerization temperature of 85°C and polymerization pressure of 2.0MPa to obtain the polyethylene resin for rotational molding.

[0072] Comparative Example 4

[0073] The preparation method of the polyethylene resin for rotational molding in this comparative example is exactly the same as that in Example 1, except that in step (2), the molar ratio of 1-hexene to ethylene is 0.1:1; that is, the detailed steps of step (1) are as follows: the metallocene catalyst is added to the Unipol gas phase process polyethylene device, 1-hexene and ethylene with a molar ratio of 0.1:1 are introduced, the molar concentration ratio of the metallocene catalyst to ethylene is 0.20, hydrogen is introduced as a chain transfer agent, the partial pressure of ethylene in the polymerization reaction is adjusted to 75%, the hydrogen concentration is 600ppm, and the gas phase polymerization reaction is carried out under the conditions of polymerization temperature of 85°C and polymerization pressure of 2.0MPa to obtain the polyethylene resin for rotational molding.

[0074] Performance testing

[0075] (I) Routine performance test: The polyethylene resins for rotational molding obtained in Examples 1-3 and Comparative Examples 1-4 were subjected to routine performance tests according to the following methods. The specific results are shown in Table 1.

[0076] (1) Density: Tested according to GB / T1033.2-2010;

[0077] (2) Melt mass flow rate (MFR): The melt flow rate (MFR) was determined according to GB / T3682.1-2018 at 190℃ and a load of 2.16 kg.

[0078] (3) α-olefin content in polyethylene resin: calculated by nuclear magnetic resonance method;

[0079] (4) Tensile yield stress: Tested according to GB / T1040.2-2022, using type I spline, with a tensile speed of 50 mm / min;

[0080] (5) Nominal strain at break: Tested according to GB / T1040.2-2022, using type I specimens, with a tensile speed of 50 mm / min;

[0081] (6) Flexural modulus: Tested according to GB / T9341-2008;

[0082] (7) Notched impact strength of simply supported beam at room temperature: tested according to GB / T1043.1-2008;

[0083] (8) Vicat softening temperature: Tested according to GB / T1633-2008A120 method;

[0084] (9) Environmental stress cracking time: Tested according to GB / T1842-2008, using condition B;

[0085] Table 1. Conventional performance data of Examples 1-3 and Comparative Examples 1-4

[0086]

[0087]

[0088] As shown in Table 1, the polyethylene resins in Examples 1-3 of this invention exhibit excellent nominal strain at fracture, notched impact strength at room temperature, and environmental stress cracking time, indicating superior impact resistance, environmental stress cracking resistance, and slow crack propagation resistance. Compared to Example 1, Comparative Example 1 shows that the catalyst preparation in Comparative Example 1 had a higher content of bis(1-butyl-3-methylcyclopentadienyl)zirconium dichloride, resulting in significantly worse performance indicators for the obtained polyethylene resin, including nominal strain at fracture, notched impact strength at room temperature, and environmental stress cracking time, indicating deteriorated impact resistance, environmental stress cracking resistance, and slow crack propagation resistance. Compared with Example 1, Comparative Example 2 showed a higher content of dimethylsilylbis(2-methyl-4-phenylindidine)zirconium dichloride in the catalyst preparation process. This resulted in significantly worse performance indicators of the obtained polyethylene resin, including nominal strain at break, notched impact strength at room temperature, and environmental stress cracking time. This indicates that its impact resistance, environmental stress cracking resistance, and slow crack propagation resistance were also deteriorated. Compared with Example 1, Comparative Example 3 showed an insufficient amount of 1-hexene, resulting in significantly worse nominal strain at break, notched impact strength at room temperature, and environmental stress cracking time in the obtained polyethylene resin. Compared with Example 1, Comparative Example 4 showed an excessive amount of 1-hexene, resulting in significantly worse tensile yield stress and flexural modulus in the obtained polyethylene resin.

[0089] (II) Continuous self-nucleating annealing heat classification (SSA) test: The polyethylene resins obtained in Examples 1-3 and Comparative Examples 1-4 for rotational molding were subjected to a continuous self-nucleating annealing heat classification (SSA) test. The specific steps are as follows:

[0090] Using a differential scanning calorimeter (TAQ200, TA Instruments, USA), under a nitrogen atmosphere, the polyethylene sample was heated from room temperature to 160°C at a rate of 10°C / min, held at that temperature for 3 min to eliminate thermal history; then cooled to 0°C at a rate of 10°C / min, held for 5 min; and finally heated to the nucleation temperature T at a rate of 10°C / min. S The temperature was then lowered to 0℃ at a rate of 10℃ / min and held for 5 minutes to complete one nucleation process. The fractionation results of self-nucleation annealing of polyethylene at 129℃, 125℃, 120℃, 114℃, 108℃, 103℃, 98℃, 94℃, and 89℃ were recorded. After fractionation, the temperature was finally raised to 160℃ at a rate of 10℃ / min, and the final melt curve was recorded. The relative peak area represents the mass fraction of the crystalline fraction. Specific results are shown in Table 2 and... Figure 1-2 As shown.

[0091] Table 2. SSA data of polyethylene resins in Examples 1-3 and Comparative Examples 1-4

[0092]

[0093]

[0094]

[0095] Figure 1 This is the melt curve of polyethylene resin after continuous self-nucleation annealing and thermal classification in Example 1. Figure 2 This is the melt curve of polyethylene resin after continuous self-nucleation annealing and thermal classification in Comparative Example 1 of the present invention. Figure 1 and Figure 2 In the graph, the horizontal axis represents temperature, and the vertical axis represents heat flow rate, which indicates the amount of heat that each gram of sample needs to absorb per second at a given temperature when the temperature is increased at a set rate. Figure 1 and Figure 2 The ordinate of the melting curve is negative, indicating that the sample is endothermic at this point. Each peak represents a melting peak, and the peak area represents the mass fraction of the crystalline fraction. A larger peak area indicates a larger mass fraction of the crystalline fraction within the corresponding temperature range. As the melting temperature decreases, it indicates a gradual increase in the content of short-chain branches, thinner lamellar crystals, and shorter methylene sequences.

[0096] Through Table 2 and the Figure 1-2Peak area integration reveals that the content of fractions above 120°C obtained from continuous self-nucleation annealing thermal fractionation of polyethylene resin in Examples 1-3 of this invention is 45-55 wt%, the content of fractions above 116-120°C is 15-20 wt%, the content of fractions above 111-115°C is 10-15 wt%, the content of fractions above 100-110°C is 11-15 wt%, and the content of fractions below 98.5°C is 8-10 wt%. Examples 1 and Comparative Examples 1 and 2 show that the content of fractions above 120°C in Comparative Example 1 is less than 42 wt%, and the content of fractions above 120°C in Comparative Example 2 is greater than 71 wt%, indicating that the content of fractions with fewer branches in the polyethylene resin is either too low or too high. Furthermore, the content of fractions at other temperatures in Comparative Examples 1 and 2 differs significantly from that in Example 1. As can be seen from Example 1 and Comparative Examples 3 and 4, the content of polyethylene resin fractions above 120°C in Comparative Example 3 is greater than 80 wt%, and the content of polyethylene resin fractions above 120°C in Comparative Example 4 is less than 45 wt%, indicating that the content of less branched fractions in polyethylene resin is either too high or too low.

[0097] (III) Lamellar Thickness Distribution Index: The lamellar thickness distribution index of the polyethylene resins obtained in Examples 1-3 and Comparative Examples 1-4 for rotational molding is shown in Table 3. The lamellar thickness distribution index characterizes the uniformity of lamellar thickness distribution. The larger the index, the more uneven the distribution of comonomers, and the worse the regularity of the material.

[0098] Table 3. Lamellar thickness distribution index of polyethylene resin in Examples 1-3 and Comparative Examples 1-4

[0099]

[0100] As shown in Table 3, the weight-average sequence length distribution index of the polyethylene resins in Examples 1-3 of this invention is between 1.15 and 1.18, and the Z-average sequence length distribution index is between 1.25 and 1.28, indicating that the polyethylene resins of this invention have a more uniform lamellar thickness distribution. As shown in Examples 1 and Comparative Examples 1-4, the weight-average sequence length distribution index and Z-average sequence length distribution index of the polyethylene resins in Comparative Examples 1-4 are higher than those in Example 1, indicating that the regularity of the polyethylene resins in Comparative Examples 1-4 is poor.

[0101] (IV) Full-Notch Creep Tensile Test (FNCT): The full-notch creep tensile test (FNCT) is an effective means of evaluating the resistance of materials to slow crack growth. Full-notch creep tensile tests (FNCT) were conducted on samples prepared from the rotomolded polyethylene resin sheets used in Examples 1-3 and Comparative Examples 1-4. The tests were performed according to ISO 16770:2004(E). The sample dimensions were (90×6×6) mm, and a circumferential notch was made on the square cross-section of the sample, with all four notches on the same plane and a notch depth of 1 mm. The samples were placed in a 50°C, 2% TX-10 solution to apply constant stress, and the failure time of the samples under different stresses was measured. Specific results are shown in Table 4.

[0102] Table 4. FNCT results (h) for polyethylene resin tablets prepared in Examples 1-3 and Comparative Examples 1-4.

[0103]

[0104]

[0105] As shown in Table 4, the polyethylene resins synthesized in Examples 1-3 of this invention have relatively long failure times under different loads, indicating that the polyethylene resins of this invention have excellent resistance to slow crack growth. As shown in Examples 1 and Comparative Examples 1-4, the polyethylene resins obtained in Comparative Examples 1-4 have significantly shorter failure times under different loads than those in Example 1, indicating that the polyethylene resins in Comparative Examples 1-4 have poor resistance to slow crack growth.

[0106] Therefore, this invention provides a special method for preparing polyethylene resin, particularly by designing a specific metallocene catalyst through controlling the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindioyl)zirconia, thereby controlling the microstructure of the synthesized polyethylene resin. This allows for the control of fraction content within different temperature ranges obtained from continuous self-nucleation annealing thermal grading tests, resulting in polyethylene resin exhibiting excellent impact resistance, environmental stress cracking resistance, and slow crack propagation resistance, making it suitable for use in military packaging boxes.

[0107] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A polyethylene resin for rotational molding, characterized in that, The polyethylene resin is a copolymer of ethylene and α-olefin; the content of fractions above 120℃ obtained by continuous self-nucleation annealing thermal classification test of the polyethylene resin is 45-55 wt%, the content of fractions above 116-120℃ is 15-20 wt%, the content of fractions above 111-115℃ is 10-15 wt%, the content of fractions above 100-110℃ is 11-15 wt%, and the content of fractions below 98.5℃ is 8-10 wt%. The method for preparing the polyethylene resin for rotational molding includes the following steps: (1) Methylaluminoxane and supported silica are mixed at a mass ratio of 1-2:1, and a toluene solution of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindinyl)zirconia is added. The mixture is stirred evenly, washed, filtered, and dried to obtain a metallocene catalyst. (2) Add the metallocene catalyst to the Unipol gas phase process polyethylene device, introduce α-olefin and ethylene with a molar ratio of 0.01-0.05:1, introduce hydrogen as a chain transfer agent, and carry out gas phase polymerization reaction under the conditions of polymerization temperature of 80-90℃ and polymerization pressure of 2.0-2.5MPa to obtain the polyethylene resin for rotational molding.

2. The polyethylene resin for rotational molding according to claim 1, characterized in that, The α-olefin is selected from at least one of butene, hexene, and octene.

3. The polyethylene resin for rotational molding according to claim 1, characterized in that, The α-olefin content in the polyethylene resin is 1.18-1.92 mol.

4. The polyethylene resin for rotational molding according to claim 1, characterized in that, The polyethylene resin has one or more of the following characteristics: The density of the polyethylene resin is 0.931-0.935 g / cm³. 3 ; The weight-average sequence length distribution index of the polyethylene resin is 1.15-1.

18.

5. A method for preparing the polyethylene resin for rotational molding according to any one of claims 1-4, characterized in that, Includes the following steps: (1) Methylaluminoxane and supported silica are mixed at a mass ratio of 1-2:1, and a toluene solution of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindinyl)zirconia is added. The mixture is stirred evenly, washed, filtered, and dried to obtain a metallocene catalyst. (2) Add the metallocene catalyst to the Unipol gas phase process polyethylene device, introduce α-olefin and ethylene with a molar ratio of 0.01-0.05:1, introduce hydrogen as a chain transfer agent, and carry out gas phase polymerization reaction under the conditions of polymerization temperature of 80-90℃ and polymerization pressure of 2.0-2.5MPa to obtain the polyethylene resin for rotational molding.

6. The method for preparing the polyethylene resin for rotational molding according to claim 5, characterized in that, In step (1), the molar ratio of bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindinyl)zirconia is 1:0.1-0.

5.

7. The method for preparing the polyethylene resin for rotational molding according to claim 5, characterized in that, In step (1), the molar ratio of the sum of methylaluminoxane and bis(1-butyl-3-methylcyclopentadienyl)zirconia and dimethylsilylbis(2-methyl-4-phenylindioyl)zirconia is 20-500:

1.

8. The method for preparing the polyethylene resin for rotational molding according to claim 5, characterized in that, In step (2), the concentration of hydrogen is 500-1000 ppm.

9. The method for preparing the polyethylene resin for rotational molding according to claim 5, characterized in that, In step (2), the partial pressure of ethylene in the gas-phase polymerization reaction is 70-80%.

10. The use of the polyethylene resin for rotational molding according to any one of claims 1-4 or the polyethylene resin for rotational molding prepared by the preparation method according to any one of claims 5-9 in military packaging boxes.

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