Process for the preparation of crosslinkable polyethylene insulation

Abstract

The application relates to the field of polyethylene cable materials, and discloses a preparation method of cross-linkable polyethylene insulating material. The method comprises the following steps: granulating high-pressure polyethylene and an antioxidant to obtain granules with an average particle size of 4-4.5 mm and a thickness of 1.0-3.0 mm; introducing the granules into a drum mixer under the protection of inert gas for mixing, and spraying liquid cross-linking agent at the same time to obtain a mixture; continuing to perform immersion treatment on the mixture in the drum mixer under the protection of inert gas; and the time of the immersion treatment is 6-8 h. The preparation method of the cross-linkable polyethylene insulating material can shorten the post-absorption time, realize uniform dispersion of the cross-linking agent in the inner and outer layers of the cross-linkable polyethylene insulating material granules, and obtain the insulating material without yellowing; in addition, the preparation method can improve production efficiency and reduce resource waste.

Description

Technical Field

[0001] This invention relates to the field of polyethylene cable materials, and more specifically to a method for preparing cross-linkable polyethylene insulation. Background Technology

[0002] In power transmission and distribution systems, cross-linked polyethylene (XLPE) insulated cables are widely used in power, construction, mining, metallurgy, petroleum, chemical, and transportation industries. With the rapid development of my country's power industry, the demand for XLPE cables is growing rapidly. For XLPE cables below 35 kV, the cross-linkable polyethylene insulation material is mostly produced using the DCP injection process. This process involves melting and extruding DCP into polyethylene during the extrusion of the insulation material. Therefore, the extrusion temperature must be strictly controlled to prevent pre-crosslinking, and long-cycle production is not feasible. For XLPE cables of 110 kV and above, the cross-linkable polyethylene insulation material is mostly produced using the DCP post-absorption process. While this process can reduce pre-crosslinking, the DCP post-absorption process takes a long time, resulting in resource waste.

[0003] CN101182377A discloses a post-absorption process for cable materials. The impregnation temperature used in this process is 50-90℃, and the impregnation time is 10-30 hours, which requires a relatively long impregnation time.

[0004] CN102276901A discloses a method for preparing ultra-clean cross-linkable polyethylene insulation material for high-voltage cables. In this method, the post-absorption temperature is 60-90℃, the absorption time is 3-20 hours, and the impregnation temperature is 80℃ with an impregnation time of 10 hours.

[0005] CN102357941A discloses a method for producing high-grade chemically cross-linked polyethylene cable insulation material. In the subsequent absorption process, the hot soaking temperature is 70-90℃ and the hot soaking time is 4-15 hours. When the wetting temperature is 80℃, the wetting time is 6 hours.

[0006] CN101817949A discloses a method for preparing cross-linkable polyethylene insulated cable material. The method involves dissolving an antioxidant in a cross-linking agent DCP to form a liquid composite additive, and then performing constant-temperature diffusion for 10-40 hours within a temperature range of 60-100℃.

[0007] Therefore, it can be seen that existing methods for preparing cross-linkable polyethylene insulation materials either involve a long absorption time after the cable insulation material is applied, wasting energy and reducing production efficiency, or a high impregnation temperature, leading to yellowing of the insulation particles. Furthermore, none of the aforementioned existing technologies address the issue of whether the cross-linking agent is evenly dispersed inside and outside the particles in the insulation material. This uniform dispersion of the cross-linking agent is particularly important in high-grade cable insulation materials of 110 kV and above. On the one hand, if the cross-linking agent is not dispersed inside the particles, it will quickly precipitate during raw material storage, resulting in losses during cable extrusion. On the other hand, it will cause uneven dispersion of the cross-linking agent during extrusion, leading to the formation of large micropores in the particles during vulcanization in the vulcanization pipe, causing electrical breakdown and affecting the cable's service life. Summary of the Invention

[0008] The purpose of this invention is to overcome the problems of existing technologies that cannot simultaneously achieve a short post-absorption time, uniform dispersion of the crosslinking agent, and yellowing of the insulation material. This invention provides a method for preparing crosslinkable polyethylene insulation material. This method shortens the post-absorption time while achieving uniform dispersion of the crosslinking agent within the crosslinkable polyethylene insulation material particles, and the resulting insulation material does not yellow.

[0009] To achieve the above objectives, the present invention provides a method for preparing a crosslinkable polyethylene insulation material, the method comprising:

[0010] (1) High-density polyethylene (HDPE) is granulated with an antioxidant to obtain granules; the average particle size of the granules is 4-4.5 mm, and the thickness is 1.0-3.0 mm; wherein the melt flow rate of the HDPE at 190℃ and 2.16 kg load is 1.95-2.3 g / 10 min, and the density is 0.92-0.925 g / cm³. 3 The number of fish eyes smaller than 0.3mm is less than 2500 / m 2 The number of fisheyes larger than 0.3mm is less than 5 per meter. 2 ;

[0011] (2) The granules are introduced into a rotary drum mixer for mixing, and a liquid crosslinking agent is sprayed at the same time to obtain a mixture; the mixing conditions include: mixing temperature of 80-95℃, mixing pressure of 0.5-1.5MPa; the spraying conditions include: spraying rate of 300-370g / min, and average diameter of spray droplets of 20-30 micrometers;

[0012] (3) The mixture is further impregnated in the rotary drum mixer for 6-8 hours;

[0013] Steps (2) and (3) are carried out under the protection of an inert gas.

[0014] Through the above technical solution, the preparation method of crosslinkable polyethylene insulation material provided by the present invention can shorten the post-absorption time, achieve uniform dispersion of crosslinking agent in the inner and outer layers of crosslinkable polyethylene insulation material particles, and the obtained insulation material does not turn yellow; in addition, the preparation method provided by the present invention can also improve production efficiency and reduce resource waste. Attached Figure Description

[0015] Figure 1 The curves show the heating and cooling temperatures of the outer layer of the crosslinkable polyethylene insulation material prepared in Example 1 during the DSC heating-melting and cooling-crystallization process. Detailed Implementation

[0016] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0017] This invention provides a method for preparing crosslinkable polyethylene insulation material, the method comprising:

[0018] (1) High-density polyethylene (HDPE) is granulated with an antioxidant to obtain granules; the average particle size of the granules is 4-4.5 mm, and the thickness is 1.0-3.0 mm; wherein the melt flow rate of the HDPE at 190℃ and 2.16 kg load is 1.95-2.3 g / 10 min, and the density is 0.92-0.925 g / cm³. 3 The number of fish eyes smaller than 0.3mm is less than 2500 / m 2 The number of fisheyes larger than 0.3mm is less than 5 per meter. 2 ;

[0019] (2) The granules are introduced into a rotary drum mixer for mixing, and a liquid crosslinking agent is sprayed at the same time to obtain a mixture; the mixing conditions include: mixing temperature of 80-95℃, mixing pressure of 0.5-1.5MPa; the spraying conditions include: spraying rate of 300-370g / min, and average diameter of spray droplets of 20-30 micrometers;

[0020] (3) The mixture is further impregnated in the rotary drum mixer for 6-8 hours;

[0021] Steps (2) and (3) are carried out under the protection of an inert gas.

[0022] During their research, the inventors of this invention discovered that existing methods for preparing crosslinkable polyethylene insulation materials either suffer from long absorption times, wasting energy and reducing production efficiency, or high impregnation temperatures, leading to yellowing of the insulation particles. Furthermore, existing methods fail to recognize the problem of uneven dispersion of the crosslinking agent within and outside the insulation particles. Uniform dispersion of the crosslinking agent is particularly crucial in high-grade cable insulation materials of 110 kV and above. On the one hand, if the crosslinking agent is not dispersed within the particles, it will quickly precipitate during raw material storage, resulting in losses during cable extrusion. On the other hand, uneven dispersion of the crosslinking agent during extrusion causes the particles to form large micropores during vulcanization in the vulcanization pipe, leading to electrical breakdown and affecting cable lifespan.

[0023] The preparation method provided by this invention involves granulating specific high-density polyethylene (LDPE) with an antioxidant to obtain granules with a specific average particle size and thickness. Under inert gas protection and specific mixing and spraying conditions, the granules are sprayed and impregnated with a liquid crosslinking agent. This method achieves a short post-absorption (impregnation) time while ensuring that the crosslinking agent is uniformly dispersed inside and outside the crosslinkable polyethylene insulating material particles, and the resulting insulating material does not yellow.

[0024] According to some embodiments of the present invention, preferably, in step (1), the melting point of the antioxidant is lower than the granulation temperature, which is beneficial to achieving uniform dispersion of the antioxidant in LDPE. To further shorten the post-absorption time of the crosslinking agent and further improve the uniformity of the crosslinking agent dispersion in the insulation material, and to ensure that the insulation material does not yellow, preferably, the antioxidant is at least one of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (CAS: 6683-19-8), ethyl 2,2'-thiobis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] (CAS: 41484-35-9), 4,4'-thiobis(2-methyl-6-tert-butylphenol) (CAS: 96-66-2), and 2,2'-thiobis(4-methyl-6-tert-butylphenol) (CAS: 90-66-4), preferably 2,2'-thiobis(4-methyl-6-tert-butylphenol), and more preferably, the antioxidant has a melting point of 82-85°C. The antioxidant has a melting point that meets the above conditions. A low melting point is beneficial for improving the uniformity of crosslinking agent dispersion in the insulation material and for increasing the service life of the cable.

[0025] According to some embodiments of the present invention, in step (1), there are no particular limitations on the equipment and conditions for granulation. Conventional equipment and conditions in the art can be used, as long as high-pressure polyethylene and antioxidant can be granulated to obtain granules that meet the above-mentioned average particle size and thickness. Preferably, the granulation temperature is 150-180°C.

[0026] According to some embodiments of the present invention, preferably, in step (1), the weight ratio of high-pressure polyethylene to antioxidant is 100:(0.1-0.4). Using the above preferred embodiments helps to reduce impurities in the insulating material and ensures the uniformity of crosslinking agent dispersion during the impregnation time, making the produced insulating material more suitable for the production of insulating materials above 110 kV.

[0027] According to some embodiments of the present invention, preferably, in step (2), the crosslinking agent is dicumyl peroxide (DCP).

[0028] According to some embodiments of the present invention, preferably, in step (2), the temperature of the crosslinking agent introduced into the rotary drum mixer is 39-65°C. At the above temperature, the crosslinking agent can melt into a liquid state. The liquid crosslinking agent can be provided by heating the crosslinking agent to the above temperature, or it can be provided in the form of a crosslinking agent solution; there are no particular limitations on this, and both can achieve the inventive purpose of the present invention to a certain extent. In order to improve the utilization rate of the crosslinking agent, reduce costs, and further improve the uniformity of the crosslinking agent dispersion inside and outside the insulating material, preferably, the liquid crosslinking agent is provided by heating the crosslinking agent to the above temperature.

[0029] According to some embodiments of the present invention, in step (2), spraying with a liquid crosslinking agent helps to reduce impurities in the crosslinking agent and ensure its cleanliness. To improve the product quality of the insulation material and further enhance the uniformity of the crosslinking agent's dispersion inside and outside the insulation material, preferably, in step (2), the liquid crosslinking agent is filtered before spraying to remove impurities.

[0030] According to some embodiments of the present invention, preferably, in step (2), the weight ratio of the granules to the crosslinking agent is 100:1.5-2.5. Adopting the above preferred embodiments is beneficial for ensuring the crosslinking degree of the insulation material meets the requirements, while also preventing excessive addition of the crosslinking agent from causing excessively large micropores during the cable crosslinking process, leading to cable breakdown and reducing the cable's service life.

[0031] According to some embodiments of the present invention, in step (2), the temperature of the granules is 70-80°C. Using the above embodiments is beneficial for improving the absorption effect and utilization rate of the crosslinking agent, and reducing waste. Any temperature control method well known to those skilled in the art can be used, and there are no particular limitations, as long as the temperature of the granules meets the above range. For example, the granulated granules can be preheated in a rotary drum mixer for 30 minutes to ensure that the temperature of the granules meets the above range.

[0032] According to some embodiments of the present invention, preferably, in step (2), the mixing speed is 20-40 revolutions per minute.

[0033] According to some embodiments of the present invention, steps (2) and (3) are carried out under inert gas protection, which can prevent the insulation material from oxidizing and discoloring due to excessively high temperature.

[0034] According to some embodiments of the present invention, preferably, the inert gas is selected from at least one of nitrogen, argon, neon and helium, and preferably nitrogen.

[0035] According to some embodiments of the present invention, preferably, step (3) further includes unloading the product obtained by the impregnation treatment, and before unloading, pressing the inert gas into a vacuum container connected to the rotary drum mixer for reuse.

[0036] According to some embodiments of the present invention, the impregnation treatment time is 6-8 hours, which is shorter than the post-absorption (impregnation treatment) time of the present invention compared with the prior art. It should be noted that the impregnation treatment time does not include the time spent spraying with a liquid crosslinking agent.

[0037] According to some embodiments of the present invention, the impregnation treatment is carried out at the mixing temperature and mixing pressure. By employing the above preferred embodiments, production efficiency can be improved and resource waste reduced, while ensuring that the crosslinking agent is uniformly dispersed inside and outside the insulating material particles.

[0038] The present invention will be described in detail below through embodiments. In the following embodiments and comparative examples,

[0039] The high-density polyethylene (LDPE) used was purchased from Yanshan Petrochemical, grade LD9202W. Its melt flow rate at 190℃ and 2.16 kg load was 2.1 g / 10 min, and its density was 0.9219 g / cm³. 3 Impurity detection was performed using a blown film method, with the number of fisheyes smaller than 0.3 mm being less than 2500 per m. 2 The number of fisheyes larger than 0.3mm is less than 5 per meter. 2 ;

[0040] The antioxidant 2,2'-thiobis(4-methyl-6-tert-butylphenol) used was purchased from Advantest, Inc. in the United States, with the brand name TBP-6 and a melting point of 82-85℃.

[0041] The crosslinking agent used, dicumyl peroxide (DCP), was purchased from AkzoNobel under the brand name PerkadoxBC.

[0042] The melt mass flow rate was tested in accordance with GB / T 3682.1-2018, and the test conditions included a temperature of 190℃ and a load of 2.16kg.

[0043] Example 1

[0044] (1) LDPE and antioxidant 2,2'-thiobis(4-methyl-6-tert-butylphenol) were granulated to obtain granules; the average particle size of the granules was 4.2 mm and the thickness was 1.5 mm; the granulation temperature was 165℃; the weight ratio of LDPE to antioxidant was 100:0.25.

[0045] (2) The granules are introduced into a rotary drum mixer under nitrogen protection and mixed while liquid DCP is sprayed to obtain a mixture; wherein, the temperature of the granules introduced into the rotary drum mixer is 75°C; the temperature of the DCP is 55°C; and the weight ratio of the granules to the DCP is 100:1.9.

[0046] The mixing conditions include: a mixing temperature of 80℃, a mixing pressure of 1.0MPa, and a drum mixer speed of 30 rpm.

[0047] The spraying conditions include: a spraying rate of 300 g / min and an average droplet diameter of 30 micrometers;

[0048] (3) The mixture is further impregnated in the rotary drum mixer under nitrogen protection for 8 hours.

[0049] (4) The product obtained by impregnation treatment is unloaded to obtain No. 1 insulating material. Before unloading, the nitrogen gas in the drum mixer is pressed into the vacuum container connected to the drum mixer. After new granules are added to the drum mixer, the nitrogen gas is pressed into the drum mixer for reuse.

[0050] Example 2

[0051] The method of Example 1 is the same as in Example 1, except that in step (2), the mixing temperature is 95°C; in step (3), the impregnation time is 6 hours; and the rest are the same as in Example 1, to obtain No. 2 insulating material.

[0052] Example 3

[0053] The method of Example 1 is the same as in Example 1, except that in step (2), the mixing temperature is 90°C, the mixing pressure is 0.5MPa, and the spraying rate is 370g / min; in step (3), the impregnation time is 7h; the rest are the same as in Example 1, and 3# insulation material is obtained.

[0054] Comparative Example 1

[0055] The method of Example 1 is the same as that of Example 1, except that in step (2), the mixing temperature is 90°C; in step (3), the impregnation time is 6h; and steps (2) and (3) are not carried out under nitrogen protection (that is, in the presence of air, the same below), and the rest are the same as that of Example 1, and D1# insulation material is obtained.

[0056] Comparative Example 2

[0057] The method of Example 1 is the same as in Example 1, except that in step (2), the mixing temperature is 70°C; in step (3), the impregnation time is 16h; and steps (2) and (3) are not carried out under nitrogen protection. The rest are the same as in Example 1, and D2# insulation material is obtained.

[0058] Comparative Example 3

[0059] The method of Comparative Example 2 is the same, except that the impregnation time in step (3) is 6 hours, while the rest is the same as in Comparative Example 2, and D3# insulation material is obtained.

[0060] Comparative Example 4

[0061] The method of Example 1 was followed, except that the LDPE (melt mass flow rate of 0.78 g / 10 min at 190°C and 2.16 kg load) used was purchased from Daqing Petrochemical Company, grade 2426F. All other aspects were the same as in Example 1, and D4# insulation material was obtained.

[0062] The appearance colors of the insulating materials prepared in the examples and comparative examples are shown in Table 1. The crosslinking enthalpy values ​​of the inner and outer layers of the insulating material particles prepared in the examples and comparative examples were detected by DSC method, and the results are shown in Table 1.

[0063] This invention provides, by way of example, the DSC heating and cooling crystallization process of the crosslinkable polyethylene insulation outer layer prepared in Example 1, as shown in the following figure. Figure 1As shown, after spraying DCP onto granulated LDPE and antioxidant, the resulting mixture is impregnated at a certain temperature for a certain time. During this process, the molecular structural units of the particles rearrange, forming a more ordered crystal structure. Therefore, the insulating material melts during the DSC heating process; this temperature is called the recrystallization temperature. Different impregnation temperatures result in different recrystallization structural units. The higher the impregnation temperature, the larger the volume of the continuously arranged structural units, and thus the higher the recrystallization temperature. Since the temperature corresponding to a DCP half-life of 1 minute is 173℃, DCP decomposes rapidly above 173℃ during the gradual heating of DSC, and polyethylene undergoes cross-linking. A cross-linking exothermic peak appears during the DSC melting and heating process. The DCP content is characterized by the magnitude of the cross-linking enthalpy; the higher the enthalpy, the higher the DCP content.

[0064] from Figure 1 It can be seen that during the DSC heating process, the insulating material undergoes phase changes of melting and cross-linking. 88.0℃ is the melting temperature of polyethylene molecules during recrystallization (i.e., the recrystallization temperature), and 186.34℃ is the cross-linking temperature after DCP decomposition. The cross-linking process is exothermic, releasing 13.5 J / g, which is the cross-linking enthalpy. Its magnitude can be used as a characterization of the DCP content; the higher the DCP content, the higher the cross-linking enthalpy. If the cross-linking enthalpy values ​​of the inner and outer layers of the insulating material are the same or comparable, it indicates that the cross-linking agent is uniformly dispersed inside and outside the insulating material particles.

[0065] Table 1

[0066]

[0067] The results above show that the higher the mixing temperature of insulating materials 1, 2, and 3, the higher the recrystallization temperature. The cross-linking enthalpy values ​​of the inner and outer layers of the particles are similar, indicating that DCP is relatively uniformly dispersed throughout the particles. Furthermore, the colors of insulating materials 1, 2, and 3 are not yellow.

[0068] Although the recrystallization temperature of D1# insulation material is relatively high and the cross-linking enthalpy values ​​of the inner and outer layers of the particles are similar, the surface of the particles has already turned yellow.

[0069] The impregnation times for D2# and D3# insulating materials differed at the same mixing temperature (lower than that of this invention). Although D2# insulating material had a higher recrystallization temperature and comparable crosslinking enthalpy values ​​between the inner and outer layers of the particles, its impregnation time was too long. Although D3# insulating material had a shorter impregnation time, the crosslinking enthalpy values ​​between the inner and outer layers of its particles differed significantly, indicating that DCP was not evenly dispersed throughout the particles, with a lower content in the inner layer and DCP not diffusing into the particle interior, but rather concentrated in the outer layer.

[0070] Comparative Example 4 uses LDPE outside the scope of this invention, which has a lower melt flow rate, higher crystallinity, slower diffusion rate of DCP within the insulating material particles, and a larger difference in crosslinking enthalpy between the inner and outer layers. Comparing Example 1 and Comparative Example 4, it is clear that only by using the specific LDPE defined in this invention can a shorter post-absorption time be achieved while ensuring uniform dispersion of the crosslinking agent within and outside the crosslinkable polyethylene insulating material particles, and the resulting insulating material does not yellow.

[0071] Therefore, the preparation method of crosslinkable polyethylene insulation material provided by the present invention can shorten the post-absorption time, achieve uniform dispersion of the crosslinking agent inside and outside the crosslinkable polyethylene insulation material particles, and the resulting insulation material does not turn yellow.

[0072] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing a cross-linkable polyethylene insulation material, characterized in that, The method includes: (1) High-density polyethylene is granulated with an antioxidant to obtain granules; the average particle size of the granules is 4-4.5 mm, and the thickness is 1.0-3.0 mm; wherein the melt flow rate of the high-density polyethylene at 190℃ and 2.16 kg load is 1.95-2.3 g / 10 min, and the density is 0.92-0.925 g / cm³. 3 The number of fish eyes smaller than 0.3mm is less than 2500 / m 2 The number of fisheyes larger than 0.3mm is less than 5 per meter. 2 ; (2) The granules are introduced into a rotary drum mixer for mixing, and a liquid crosslinking agent is sprayed at the same time to obtain a mixture; the temperature of the crosslinking agent introduced into the rotary drum mixer is 39-65℃; the mixing conditions include: mixing temperature of 80-95℃, mixing pressure of 0.5-1.5MPa, and mixing speed of 20-40 rpm; the spraying conditions include: spraying rate of 300-370g / min, and average diameter of spray droplets of 20-30 micrometers. (3) The mixture is further impregnated in the rotary drum mixer for 6-8 hours; Steps (2) and (3) are carried out under the protection of an inert gas.

2. The preparation method according to claim 1, wherein, In step (1), the antioxidant is at least one of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], ethyl 2,2'-thiobis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 4,4'-thiobis(2-methyl-6-tert-butylphenol) and 2,2'-thiobis(4-methyl-6-tert-butylphenol).

3. The preparation method according to claim 1, wherein, In step (1), the antioxidant is 2,2'-thiobis(4-methyl-6-tert-butylphenol).

4. The preparation method according to claim 1, wherein, In step (1), the antioxidant has a melting point of 82-85℃.

5. The preparation method according to claim 1 or 2, wherein, In step (1), the granulation temperature is 150-180℃.

6. The preparation method according to any one of claims 1-4, wherein, In step (1), the weight ratio of high-density polyethylene to antioxidant is 100:(0.1-0.4).

7. The preparation method according to any one of claims 1-4, wherein, In step (2), the crosslinking agent is dicumyl peroxide.

8. The preparation method according to any one of claims 1-4, wherein, In step (2), the weight ratio of the granules to the crosslinking agent is 100:1.5-2.

5.

9. The preparation method according to any one of claims 1-4, wherein, The inert gas is selected from at least one of nitrogen, argon, neon and helium.

10. The preparation method according to any one of claims 1-4, wherein, The inert gas is nitrogen.

11. The preparation method according to any one of claims 1-4, wherein, Step (3) also includes unloading the product obtained from the impregnation treatment, and before unloading, pressing the inert gas into a vacuum container connected to the rotary drum mixer for reuse.

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