Polyethylene rotational molding product with low warping deformation and preparation method thereof

Abstract

The invention relates to the field of rotational molding products, and discloses a polyethylene rotational molding product with low warping deformation and a preparation method thereof. The polyethylene rotational molding product comprises the following raw materials in parts by weight: 5-30 parts of metallocene linear polyethylene with a melt flow rate ratio of less than 18, 70-95 parts of high melt index polyethylene with a melt index higher than that of the metallocene linear polyethylene, and 0.02-0.2 part of toner, the melt flow rate ratio is the ratio of the melt rolling rates at 190 DEG C when the nominal loads are 21.6 kg and 2.16 kg respectively. The polyethylene rotational molding product provided by the invention can realize a relatively low warping deformation degree under the condition of adding the toner.

Description

Technical Field

[0001] The invention relates to the field of roto-molded products, and in particular to a polyethylene roto-molded product with low warpage deformation and a preparation method thereof. Background Art

[0002] Roto-molding, also known as rotational molding, rotational molding, etc., is a thermoplastic hollow molding method. The method is to first add plastic powder into the mold, then the mold is continuously rotated along two vertical axes and heated, gradually and evenly coated, melted and adhered to the entire surface of the mold cavity, formed into the required shape, and then cooled and shaped into a product. Roto-molding is suitable for the processing of large-volume, complex-structured hollow plastic products, and is widely used in engineering machinery, automobiles, medical, military, home appliances, amusement and other fields.

[0003] The rotational molding process generally uses natural cooling and air cooling to cool down. The cooling time is long, which easily leads to a large temperature difference between the mold surface and the air surface of the product, causing the plane of the product to warp and deform after the product leaves the mold. In addition, various colors are often added to polyethylene rotational molded products through dry mixing and extrusion. Some color powders have obvious nucleation effects on polyethylene. Combined with the process characteristics of slow heating and slow cooling of rotational molding, the nucleation effect of color powders and the similar "annealing" effect during the slow cooling process of rotational molding lead to the worsening of the warping and deformation of some colored polyethylene rolling parts.

[0004] In addition, the warping deformation of polyethylene roto-molded products is different from the warping deformation caused by other thermal processing techniques (such as injection, blow molding, etc.): polyethylene is a semi-crystalline thermoplastic, and oriented crystallization will occur under shear flow and tensile flow, resulting in crystallization anisotropy. For example, tensile oriented crystallization makes its strength parallel to the stretching direction greater than the vertical direction. In terms of molding, different degrees of vertical and horizontal crystallization will cause warping deformation. Therefore, except for roto-molding (zero shear process), the cause of warping deformation of other polyethylene products is mainly due to the different degrees and types of anisotropic crystallization caused by shear, stretching and other flow fields, which is different from the cause of warping deformation in polyethylene roto-molded products. Therefore, the methods and mechanisms for reducing warping deformation in other thermal processing techniques are not applicable to polyethylene roto-molded products.

[0005] At present, there are generally several ways to reduce the warpage caused by the rotational molding process: (1) When designing and manufacturing the rotational molding mold, set reinforcing ribs on the large surface; (2) After the rotational molded product is separated from the mold, it is immediately cooled and shaped by using tooling and ventilation cooling inside the mold; (3) Use natural cooling to minimize the warpage caused by temperature difference. Although the above methods can reduce the warpage of polyethylene rotational molded products to a certain extent, the effect is very limited. Summary of the invention

[0006] In order to solve the technical problem that polyethylene rotomolded products have a large degree of warping deformation (especially when using color powder), the present invention provides a polyethylene rotomolded product with low warping deformation and a preparation method thereof. The polyethylene rotomolded product of the present invention can achieve a lower degree of warping deformation when adding color powder.

[0007] The specific technical scheme of the present invention is: In the first aspect, the present invention provides a polyethylene rotational molded product with low warpage deformation, comprising the following raw materials in parts by weight: 5 to 30 parts of a metallocene linear polyethylene having a melt flow rate ratio of less than 18, 70 to 95 parts of a high melt index polyethylene having a melt index higher than that of the metallocene linear polyethylene, and 0.02 to 0.2 parts of a color powder; the melt flow rate ratio is the ratio of the melt rolling rates at 190°C when the nominal loads are 21.6 kg and 2.16 kg respectively.

[0008] According to the crystallization kinetics of polyethylene, during the cooling process of polyethylene melt after rotational molding, the polyethylene molecular chains fold to form 10-20nm thick lamellae. Since the folded polyethylene chains still have available relaxation length (10%-20% of the amorphous regions in the lamellae accommodate folding and loops), the relaxed length can undergo secondary crystallization during the slow cooling process of rotational molding. When color powder is added to polyethylene, the nucleation effect of the color powder increases the crystallization starting temperature. Combined with the slow cooling and pressure-free characteristics of rotational molding, the above-mentioned secondary crystallization will induce severe warping deformation of polyethylene rotational molded products.

[0009] Aiming at the cause of warping deformation of the above-mentioned polyethylene rotomolded products, the present invention introduces a metallocene linear polyethylene with a specific melt flow rate ratio, and uses it in combination with high melt index polyethylene to effectively inhibit the warping deformation induced by color powder in polyethylene rotomolded products. Specifically: (1) The melt index (melt index) of metallocene linear polyethylene is lower than that of high melt index polyethylene, that is, the molecular weight is larger, so the number of folded chains in the lamellae increases, which can inhibit its secondary crystallization during the slow cooling process of rotational molding, thereby reducing the degree of warping deformation of polyethylene rotational molded products.

[0010] (2) The present invention team found that the melt flow rate ratio of low melt index polyethylene materials will affect its effect of reducing the warpage deformation of polyethylene roto-molded products. Compared with other types of polyethylene, metallocene linear polyethylene has a lower melt flow rate ratio, and for metallocene linear polyethylene, the smaller the melt flow rate ratio, the narrower the molecular weight distribution, and the more uniform the distribution of comonomers (vinyl and other alkenyl groups, such as butenyl, hexenyl, octenyl) in the molecule. Therefore, in metallocene linear polyethylene with a low melt flow rate ratio (less than 18), the comonomers (vinyl and other alkenyl groups, such as butenyl, hexenyl, octenyl) have a more significant inhibitory effect on secondary crystallization during the slow cooling process of roto-molding.

[0011] In addition, since low melt index metallocene linear polyethylene has a high melt viscosity and is difficult to process, compounding it with high melt index polyethylene can solve processing problems while reducing the warping deformation of polyethylene rotational molded products induced by color powder.

[0012] Preferably, the high melt index polyethylene has a melt index of 5 to 20 g / 10 min and a density of 0.920 to 0.960 g / cm 3 .

[0013] Preferably, the high melt index polyethylene includes one or more of high density polyethylene (HDPE), linear low density polyethylene (LLDPE or mLLDPE), low density polyethylene (LDPE), ultra low density polyethylene (ULDPE) and polyethylene elastomer (POE or POP).

[0014] Preferably, the melt index of the metallocene linear polyethylene is 0.5 to 3 g / 10 min.

[0015] Preferably, the monomers of the metallocene linear polyethylene include ethylene and one or more of butene, hexene and octene.

[0016] Preferably, the melt index is the melt index under the test conditions of 190° C. and 2.16 KG.

[0017] Preferably, the melt rolling rate is measured according to the national standard GB / T 3682-2000.

[0018] Preferably, the toner is an organic toner.

[0019] Furthermore, the organic toner includes one or more of phthalocyanine blue, green 7, red 254 and yellow 180.

[0020] Preferably, the polyethylene rotomolded product further comprises the following raw materials in parts by weight: 0.1 to 0.5 parts of antioxidant, 0.05 to 0.3 parts of light stabilizer, and 0.1 to 0.4 parts of processing aid.

[0021] Preferably, the antioxidant includes hindered phenol antioxidants and / or phosphite antioxidants; the light stabilizer includes one or more of benzotriazole, benzophenone and triazine ultraviolet light absorbers; the processing aid includes one or more of polyethylene wax, fatty acid ester compounds, fatty acid amides and organosilicon compounds.

[0022] Further, the antioxidant includes one or more of tetrakis[β-(3.5-di-tert-butyl, 4-hydroxyphenyl) propionate] pentaerythritol ester, β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate, 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, dioctadecyl pentaerythritol diphosphite and tris(2.4-di-tert-butylphenyl) phosphite.

[0023] Further, the light stabilizer includes one or more of 2'-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole, 2-(2H-benzotriazole-2-yl)-6-(1-methyl-1-phenylethyl)-4-(1,1,3,3-tetramethylbutyl)phenol and 2-hydroxy-4-n-octyloxybenzophenone.

[0024] Furthermore, the processing aid includes one or more of polyethylene wax, zinc stearate, calcium stearate, erucic acid amide and silicone powder.

[0025] In a second aspect, the present invention provides a method for preparing the polyethylene rotomolded product, comprising the following steps: S1: Mix all raw materials to make mixed powder; S2: placing the mixed powder in a rotational molding mold, performing rotational molding, and cooling to obtain a polyethylene rotational molding product.

[0026] Preferably, in step S2, during the rotational molding process, the maximum temperature in the mold is controlled to be 180-220°C.

[0027] Preferably, step S1 specifically includes: S1.1: mixing the metallocene linear polyethylene and a portion of the high melt index polyethylene to prepare a masterbatch, and then mixing it with the remaining raw materials to obtain a mixture; S1.2: The mixture is melt-extruded, granulated and ground to obtain a mixed powder.

[0028] Preferably, in step S1.2, the melt extrusion is carried out in a twin-screw extruder with a length-to-diameter ratio of 30 to 40:1, the extrusion temperature is 50 to 220° C., and the screw speed is 300 to 500 r / min.

[0029] Preferably, in step S1, in the mixed powder, the proportion of particles with a diameter less than 100 mesh is less than 20%.

[0030] Compared with the prior art, the present invention has the following advantages: The present invention uses a metallocene linear polyethylene with a relatively low melt index and a melt flow rate ratio of less than 18, and mixes it with polyethylene with a relatively high melt index, which can effectively reduce the warping deformation of polyethylene rotomolded products induced by color powder while ensuring processing performance. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the embodiments.

[0032] Overall embodiment A polyethylene rotomolding product with low warpage deformation comprises the following raw materials in parts by weight: 5-30 parts of a metallocene linear polyethylene with a melt flow rate ratio of less than 18, 70-95 parts of a high melt index polyethylene with a melt index higher than the metallocene linear polyethylene, and 0.02-0.2 parts of a color powder; the melt flow rate ratio is the ratio of the melt rolling rates at 190°C when the nominal loads are 21.6kg and 2.16kg, respectively. In the above polyethylene rotomolding product, a metallocene linear polyethylene with a relatively low melt index and a melt flow rate ratio of less than 18 is used in combination with a polyethylene with a relatively high melt index, which can effectively reduce the warpage deformation of the polyethylene rotomolding product induced by the color powder while ensuring the processing performance.

[0033] In some specific embodiments, the high melt index polyethylene has a melt index of 5 to 20 g / 10 min and a density of 0.920 to 0.960 g / cm 3 ; The melt index is the melt index under the test conditions of 190°C and 2.16KG.

[0034] In some specific embodiments, the high melt index polyethylene includes one or more of high density polyethylene (HDPE), linear low density polyethylene (LLDPE or mLLDPE), low density polyethylene (LDPE), ultra low density polyethylene (ULDPE) and polyethylene elastomer (POE or POP).

[0035] In some specific embodiments, the melt index of the metallocene linear polyethylene is 0.5-3 g / 10 min; the melt index is the melt index under the test conditions of 190° C. and 2.16 KG.

[0036] In some specific embodiments, the monomers of the metallocene linear polyethylene include ethylene and further include one or more of butene, hexene and octene.

[0037] In some specific embodiments, the melt rolling rate is measured according to the national standard GB / T 3682-2000.

[0038] In some specific embodiments, the toner is an organic toner including one or more of Phthalocyanine Blue, Green 7, Red 254 and Yellow 180.

[0039] In some specific embodiments, the polyethylene rotomolded product further comprises the following raw materials in parts by weight: 0.1 to 0.5 parts of antioxidant, 0.05 to 0.3 parts of light stabilizer, and 0.1 to 0.4 parts of processing aid. Among them: The antioxidant includes a hindered phenol antioxidant and / or a phosphite antioxidant, and can be specifically selected from one or more of tetrakis[β-(3.5-di-tert-butyl, 4-hydroxyphenyl) propionate] pentaerythritol ester (antioxidant 1010), β-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate octadecyl alcohol ester, 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, dioctadecyl pentaerythritol diphosphite and tris(2.4-di-tert-butylphenyl) phosphite (antioxidant 168); The light stabilizer includes one or more of benzotriazole, benzophenone and triazine ultraviolet light absorbers, and specifically can be selected from 2'-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole (ultraviolet absorber UV326), 2-(2H-benzotriazole-2-yl)-6-(1-methyl-1-phenylethyl)-4-(1,1,3,3-tetramethylbutyl)phenol and 2-hydroxy-4-n-octyloxybenzophenone. The processing aid includes one or more of polyethylene wax, fatty acid ester compounds, fatty acid amides and organosilicon compounds, and can be specifically selected from one or more of polyethylene wax, zinc stearate, calcium stearate, erucamide and silicone powder.

[0040] A method for preparing the above-mentioned polyethylene rotomolded product comprises the following steps: S1: Mix all raw materials to make mixed powder; S2: placing the mixed powder in a rotational molding mold, performing rotational molding, and cooling to obtain a polyethylene rotational molding product.

[0041] In some specific embodiments, in step S1, in the mixed powder, the proportion of particles with a diameter less than 100 mesh is less than 20%.

[0042] In some specific embodiments, in step S2, during the rotational molding process, the maximum temperature in the mold is controlled to be 200-220°C.

[0043] In some specific implementations, step S1 specifically includes: S1.1: mixing the metallocene linear polyethylene and a portion of the high melt index polyethylene to prepare a masterbatch, and then mixing it with the remaining raw materials to obtain a mixture; S1.2: Place the mixture in a twin-screw extruder with a length-to-diameter ratio of 30 to 40:1, set the extrusion temperature to 50 to 220°C, and the screw speed to 300 to 500 r / min, perform melt extrusion, granulation, and grinding to obtain a mixed powder. Specific embodiments The present invention is described below by specific examples. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. Without departing from the spirit and scope of the inventive concept, changes and advantages that can be thought of by those skilled in the art are included in the present invention, and the attached claims and any equivalents thereof are the protection scope of the present invention.

[0045] In the following examples and comparative examples, the melt flow rate ratio and melt index involved are measured under the following conditions: (1) Melt flow rate ratio: According to the national standard GB / T 3682-2000, the melt rolling rate is measured at a test temperature of 190°C and nominal loads of 21.6kg and 2.16kg respectively; the ratio between the melt rolling rates measured under the two nominal loads is the "melt flow rate ratio".

[0046] (2) Melt index (melt index): Measured under 190℃ and 2.16KG test conditions.

[0047] Example 1 A polyethylene rotomolding product is prepared according to the following steps: S1: Preparation of masterbatch Weigh 70 parts by weight of high melt index polyethylene R735U (Zhenhai Refining and Chemical, density 0.935 g / cm 3 , melt index is 7g / 10min) and 30 parts of metallocene linear polyethylene Exceed 1018RA (Exxon, USA, melt flow rate ratio is 16.5, melt index is 1.0g / 10min), and co-extrude to obtain a masterbatch.

[0048] S2: Preparation of mixed powder According to weight, 30 parts of the masterbatch prepared in step S1 and 70 parts of high melt index polyethylene HDPE8920 (Fujian Refining and Chemical Industry, density 0.955 g / cm 3, melt fat is 20g / 10min), 0.1 part of phthalocyanine blue (BASF, Germany), 0.05 part of antioxidant 1010, 0.1 part of antioxidant 168, 0.3 part of ultraviolet absorber UV326 and 0.1 part of polyethylene wax, put into a twin-screw extruder with a length-to-diameter ratio of 35:1, set the speed to 400r / min, the extrusion temperature to 200℃, carry out melt extrusion, granulation and grinding to obtain a mixed powder, of which the particle size less than 100 mesh accounts for 12%.

[0049] S3: Roto-molding The rotomolding mold is a cubic iron mold with a length of 30 cm. The mixed powder prepared in step S2 is put into the rotomolding mold with a feeding amount of 2.1 kg. During the rotomolding process, the maximum temperature in the mold is controlled to be 210° C. After the rotomolded product is separated from the mold, it is air-cooled and then cut into discs with a diameter of 15 cm using a circular electric hot cutter. The discs are placed in a 100° C. oven for heat treatment for 2 h to eliminate the stress generated during cutting, and then the oven is closed and naturally cooled to room temperature.

[0050] Example 2 A polyethylene rotomolding product is prepared according to the following steps: S1: Preparation of masterbatch Weigh 87.5 parts of high melt index polyethylene R735U (Zhenhai Refining and Chemical, density 0.935 g / cm 3 , melt index is 7g / 10min) and 12.5 parts of metallocene linear polyethylene Exceed 1018RA (Exxon, USA, melt flow rate ratio is 16.5, melt index is 1.0g / 10min), and co-extrude to obtain a masterbatch.

[0051] S2: Preparation of mixed powder According to weight, 40 parts of the masterbatch prepared in step S1 and 60 parts of high melt index polyethylene HDPE8920 (Fujian Refining and Chemical Industry, density 0.955 g / cm 3 , melt fat is 20g / 10min), 0.02 parts of phthalocyanine blue (BASF, Germany), 0.05 parts of antioxidant 1010, 0.1 parts of antioxidant 168, 0.3 parts of ultraviolet absorber UV326 and 0.1 parts of polyethylene wax, put into a twin-screw extruder with a length-to-diameter ratio of 35:1, set the speed to 400r / min, the extrusion temperature to 200℃, carry out melt extrusion, granulation and grinding to obtain a mixed powder, of which the particle size less than 100 mesh accounts for 12%.

[0052] S3: Roto-molding The rotomolding mold is a cubic iron mold with a length of 30 cm. The mixed powder prepared in step S2 is put into the rotomolding mold with a feeding amount of 2.1 kg. During the rotomolding process, the maximum temperature in the mold is controlled to be 210° C. After the rotomolded product is separated from the mold, it is air-cooled and then cut into discs with a diameter of 15 cm using a circular electric hot cutter. The discs are placed in a 100° C. oven for heat treatment for 2 hours to eliminate the stress generated during cutting, and then the oven is closed and naturally cooled to room temperature.

[0053] Example 3 A polyethylene rotomolding product is prepared according to the following steps: S1: Preparation of masterbatch Weigh 40 parts by weight of high melt index polyethylene R735U (Zhenhai Refining and Chemical, density 0.935 g / cm 3 , melt index is 7g / 10min) and 60 parts of metallocene linear polyethylene UL0300 (PetroChina Dushanzi Petrochemical, melt flow rate ratio is 17.6, melt index is 3.0g / 10min), and co-extruded to obtain a masterbatch.

[0054] S2: Preparation of mixed powder According to weight, 50 parts of the masterbatch prepared in step S1, 50 parts of high melt index polyethylene HDPE8920 (Fujian Refining, density 0.955 g / cm 3 , melt fat is 20g / 10min), 0.2 parts of phthalocyanine blue (BASF, Germany), 0.05 parts of antioxidant 1010, 0.1 parts of antioxidant 168, 0.3 parts of ultraviolet absorber UV326 and 0.1 parts of polyethylene wax, put into a twin-screw extruder with a length-to-diameter ratio of 35:1, set the speed to 400r / min, the extrusion temperature to 200℃, carry out melt extrusion, granulation and grinding to obtain a mixed powder, of which the particle size less than 100 mesh accounts for 12%.

[0055] S3: Roto-molding The rotomolding mold is a cubic iron mold with a length of 30 cm. The mixed powder prepared in step S2 is put into the rotomolding mold with a feeding amount of 2.1 kg. During the rotomolding process, the maximum temperature in the mold is controlled to be 210° C. After the rotomolded product is separated from the mold, it is air-cooled and then cut into discs with a diameter of 15 cm using a circular electric hot cutter. The discs are placed in a 100° C. oven for heat treatment for 2 hours to eliminate the stress generated during cutting, and then the oven is closed and naturally cooled to room temperature.

[0056] Comparative Example 1 Comparative Example 1 does not use metallocene linear polyethylene. Specifically, the steps for preparing polyethylene rotomolding products in this comparative example are as follows: S1: preparing mixed powder According to weight, weigh 100 parts of high melt index polyethylene HDPE8920 (Fujian Refining, density 0.955g / cm 3 , melt fat is 20g / 10min), 0.1 part of phthalocyanine blue (BASF, Germany), 0.05 part of antioxidant 1010, 0.1 part of antioxidant 168, 0.3 part of ultraviolet absorber UV326 and 0.1 part of polyethylene wax, put into a twin-screw extruder with a length-to-diameter ratio of 35:1, set the speed to 400r / min, the extrusion temperature to 200℃, carry out melt extrusion, granulation and grinding to obtain a mixed powder, of which the particle size less than 100 mesh accounts for 12%.

[0057] S2: Roto-molding The rotomolding mold is a cubic iron mold with a length of 30 cm. The mixed powder prepared in step S1 is put into the rotomolding mold with a feeding amount of 2.1 kg. During the rotomolding process, the maximum temperature in the mold is controlled to be 210°C. After the rotomolded product is separated from the mold, it is air-cooled and then cut into discs with a diameter of 15 cm using a circular electric hot cutter. The discs are placed in a 100°C oven for heat treatment for 2 hours to eliminate the stress generated during cutting, and then the oven is closed and naturally cooled to room temperature.

[0058] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that: in Comparative Example 2, the metallocene linear polyethylene Exceed 1018RA is replaced with another metallocene linear polyethylene with a relatively high melt flow rate; the rest of the raw materials and steps are the same as those in Example 1. Specifically, the steps for preparing the polyethylene rotomolded product in this comparative example are as follows: S1: Preparation of masterbatch Weigh 70 parts by weight of high melt index polyethylene R735U (Zhenhai Refining and Chemical, density 0.935 g / cm 3 , melt index is 7g / 10min) and 30 parts of metallocene linear polyethylene SP0510 (Japan Priman, melt flow rate ratio is 19.2, melt index is 1.1g / 10min), and co-extrude to obtain a masterbatch.

[0059] S2: Preparation of mixed powder According to weight, 30 parts of the masterbatch prepared in step S1 and 70 parts of high melt index polyethylene HDPE8920 (Fujian Refining and Chemical Industry, density 0.955 g / cm 3, melt fat is 20g / 10min), 0.1 part of phthalocyanine blue (BASF, Germany), 0.05 part of antioxidant 1010, 0.1 part of antioxidant 168, 0.3 part of ultraviolet absorber UV326 and 0.1 part of polyethylene wax, put into a twin-screw extruder with a length-to-diameter ratio of 35:1, set the speed to 400r / min, the extrusion temperature to 200℃, carry out melt extrusion, granulation and grinding to obtain a mixed powder, of which the particle size less than 100 mesh accounts for 12%.

[0060] S3: Roto-molding The rotomolding mold is a cubic iron mold with a length of 30 cm. The mixed powder prepared in step S2 is put into the rotomolding mold with a feeding amount of 2.1 kg. During the rotomolding process, the maximum temperature in the mold is controlled to be 210° C. After the rotomolded product is separated from the mold, it is air-cooled and then cut into discs with a diameter of 15 cm using a circular electric hot cutter. The discs are placed in a 100° C. oven for heat treatment for 2 hours to eliminate the stress generated during cutting, and then the oven is closed and naturally cooled to room temperature.

[0061] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that: in Comparative Example 3, the metallocene linear polyethylene Exceed 1018RA is replaced with another linear polyethylene with a low melt index; the rest of the raw materials and steps are the same as those in Example 1. Specifically, the steps for preparing the polyethylene rotomolded product in this comparative example are as follows: S1: Preparation of masterbatch Weigh 70 parts by weight of high melt index polyethylene R735U (Zhenhai Refining and Chemical, density 0.935 g / cm 3 , melt index is 7g / 10min) and 30 parts of linear polyethylene LLD7042 (Zhenhai Refining and Chemical, melt flow rate ratio is 27.6, melt index is 2.0g / 10min), and co-extrude to obtain a masterbatch.

[0062] S2: Preparation of mixed powder According to weight, 30 parts of the masterbatch prepared in step S1 and 70 parts of high melt index polyethylene HDPE8920 (Fujian Refining and Chemical Industry, density 0.955 g / cm 3 , melt fat is 20g / 10min), 0.1 part of phthalocyanine blue (BASF, Germany), 0.05 part of antioxidant 1010, 0.1 part of antioxidant 168, 0.3 part of ultraviolet absorber UV326 and 0.1 part of polyethylene wax, put into a twin-screw extruder with a length-to-diameter ratio of 35:1, set the speed to 400r / min, the extrusion temperature to 200℃, carry out melt extrusion, granulation and grinding to obtain a mixed powder, of which the particle size less than 100 mesh accounts for 12%.

[0063] S3: Roto-molding The rotomolding mold is a cubic iron mold with a length of 30 cm. The mixed powder prepared in step S2 is put into the rotomolding mold with a feeding amount of 2.1 kg. During the rotomolding process, the maximum temperature in the mold is controlled to be 210° C. After the rotomolded product is separated from the mold, it is air-cooled and then cut into discs with a diameter of 15 cm using a circular electric hot cutter. The discs are placed in a 100° C. oven for heat treatment for 2 hours to eliminate the stress generated during cutting, and then the oven is closed and naturally cooled to room temperature.

[0064] Test example: Warpage deformation test Take the polyethylene rotomolded products (discs) prepared in each embodiment and comparative example, draw a cross on the disc, and place the disc on a horizontal table, press any point on the cross, measure the height of the point from the table, and calculate the average height of these 8 points, which is the warpage height of the rotomolded disc. The raw material formulas of each embodiment and comparative example and the measured warpage height of the rotomolded disc are shown in Tables 1 and 2.

[0065] Table 1 Raw material formula and warpage deformation degree of Examples 1 to 3 Table 2 Raw material formula and warpage deformation degree of comparative examples 1 to 3 Test result analysis and conclusion: (1) Comparison of Examples 1 to 3 with Comparative Example 1 shows that the warpage of polyethylene rotomoulded products can be reduced by introducing the metallocene linear polyethylene of the present invention.

[0066] (2) Comparison of Example 1 and Comparative Example 2 shows that when the melt flow rate ratio of the metallocene linear polyethylene used is too large, the effect of reducing the warpage of the polyethylene roto-molded product is relatively poor. The reason for this is that for metallocene linear polyethylene, the smaller the melt flow rate ratio, the narrower the molecular weight distribution, and the more uniform the distribution of comonomers (vinyl and other alkenyl groups, such as butenyl, hexenyl, octenyl) in the molecule, which can have a more significant hindering effect on the secondary crystallization of the comonomer during the slow cooling process of roto-molding.

[0067] (3) Comparison of Example 1 and Comparative Example 3 shows that: compared with other low melt index polyethylene, the use of metallocene linear polyethylene can reduce the degree of warpage deformation of polyethylene roto-molded products to a greater extent. The reason for this is that metallocene linear polyethylene has a lower melt flow rate ratio than other low melt index polyethylene, its molecular weight distribution is narrower, and the distribution of comonomers (vinyl and other alkenyl groups, such as butenyl, hexenyl, octenyl) in the molecule is more uniform, which is conducive to inhibiting secondary crystallization during the slow cooling process of roto-molding.

[0068] Unless otherwise defined, all technical terms and scientific terms used in the present invention have the same meanings as those commonly understood by ordinary technicians in the field to which the present disclosure belongs. Unless otherwise specified, the raw materials and equipment used in the present invention are conventional raw materials and equipment in the field and can be obtained from conventional commercial channels; unless otherwise specified, the methods used in the present invention are conventional methods in the field.

[0069] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent transformation made to the above embodiment based on the technical essence of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A polyethylene rotomolded product with low warpage deformation, characterized in that: The method comprises the following raw materials in parts by weight: 5-30 parts of a metallocene linear polyethylene having a melt flow rate ratio less than 18, 70-95 parts of a high melt index polyethylene having a melt index higher than that of the metallocene linear polyethylene, and 0.02-0.2 parts of a color powder; the melt flow rate ratio is the ratio of the melt rolling rates at 190° C. when the nominal loads are 21.6 kg and 2.16 kg respectively.

2. The polyethylene rotomolded product according to claim 1, characterized in that: The high melt index polyethylene has a melt index of 5-20 g / 10 min and a density of 0.920-0.960 g / cm 3 .

3. The polyethylene rotomolded product according to claim 1, characterized in that: The melt index of the metallocene linear polyethylene is 0.5-3 g / 10 min.

4. The polyethylene rotomolded product according to claim 2 or 3, characterized in that: The melt index is a melt index under the test conditions of 190° C. and 2.16 KG.

5. The polyethylene rotomolded product according to claim 1, characterized in that: The toner is an organic toner.

6. The polyethylene rotomolded product according to claim 1, characterized in that: The invention also comprises the following raw materials in parts by weight: 0.1 to 0.5 parts of antioxidant, 0.05 to 0.3 parts of light stabilizer and 0.1 to 0.4 parts of processing aid.

7. The polyethylene rotomolded product according to claim 6, characterized in that: The antioxidant includes hindered phenol antioxidants and / or phosphite antioxidants; the light stabilizer includes one or more of benzotriazole, benzophenone and triazine ultraviolet light absorbers; the processing aid includes one or more of polyethylene wax, fatty acid ester compounds, fatty acid amides and organosilicon compounds.

8. A method for preparing a polyethylene rotomolded product according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1: Mix all raw materials to make mixed powder; S2: placing the mixed powder in a rotational molding mold, performing rotational molding, and cooling to obtain a polyethylene rotational molding product.

9. The preparation method according to claim 8, characterized in that: In step S2, during the rotational molding process, the maximum temperature in the mold is controlled to be 180-220°C.

10. The preparation method according to claim 8, characterized in that: Step S1 specifically includes: S1.1: mixing the metallocene linear polyethylene and a portion of the high melt index polyethylene to prepare a masterbatch, and then mixing it with the remaining raw materials to obtain a mixture; S1.2: The mixture is melt-extruded, granulated and ground to obtain a mixed powder.

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