A two-step method for the preparation of an embedded rtv coating based on a gradient diluent

Abstract

The present application relates to the technical field of power transmission line external insulation, and provides a two-step embedded RTV coating preparation method based on gradient diluent.The present application analyzes the main influencing factors of coating curing shrinkage from the perspective of RTV coating formula, and determines that the large amount of diluent added is the reason for the high coating curing shrinkage rate.Based on this, the present application proposes a two-step embedded RTV coating preparation method based on gradient diluent, which realizes the gradient control of coating curing shrinkage by using RTV coating with different diluent contents in the two-step preparation process, thereby significantly improving the surface flatness and interface bonding performance.The embedded RTV coating prepared by the method of the present application has no depression on the surface, the bonding interface is continuous and dense, and has excellent adhesion and long-term operation stability.Furthermore, the method has the characteristics of strong universality, good process compatibility and easy industrialization promotion.

Description

Technical Field

[0001] This invention relates to the field of external insulation technology for power transmission lines, and in particular to a two-step method for preparing embedded RTV coatings based on gradient diluents. Background Technology

[0002] Room temperature vulcanizing (RTV) silicone rubber coating is one of the most widely used external insulation methods for preventing flashover in power systems. By coating ceramic or glass insulators with an RTV coating, the high mechanical strength of ceramic / glass insulators and the excellent flashover protection performance of composite insulators can be combined.

[0003] However, RTV coatings are inherently organic materials, and aging and performance degradation are unavoidable during service. In particular, when the coating peels off and the hydrophilic substrate is exposed, it leads to an irreversible and significant decrease in the flashover voltage on the insulation surface, thus creating operational risks and significantly increasing maintenance costs. To address this, researchers have recently proposed embedded RTV coating structures. These structures involve creating patterned grooves on the insulator surface and embedding the RTV coating within them, using mechanical interlocking to increase the interfacial bonding area, thereby improving coating adhesion and durability.

[0004] Despite the advantages of embedded RTV structures in terms of coating adhesion, new technical challenges remain in their actual fabrication. Existing commercially available RTV anti-flashover coatings typically exhibit a volume shrinkage rate of approximately 40% during curing. When using the traditional one-step direct dip-coating method, noticeable indentations form on the surface after curing, and these indentations are prone to accumulating dirt and water during operation, reducing insulation performance. To reduce surface indentations, researchers have proposed a two-step fabrication strategy: first, multiple scraping coats are applied to fill the grooves, followed by a surface dip-coating. However, this two-step method only partially improves surface smoothness and cannot completely eliminate indentations caused by shrinkage. Furthermore, due to the potential for insufficient curing at the interlayer interface during the two-step fabrication process, weak bonding areas may form, leading to poor adhesion between the surface RTV and the embedded portion, affecting overall adhesion and service life. Summary of the Invention

[0005] In view of this, the present invention provides a two-step embedded RTV coating preparation method based on gradient diluents. The inventors discovered that the diluent content is the main influencing factor on coating curing shrinkage, and based on this discovery, proposed the method of the present invention: by using RTV coatings with different diluent contents in the two-step preparation process, gradient control of coating curing shrinkage is achieved, thereby significantly improving surface smoothness and interfacial bonding performance.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: A two-step method for preparing embedded RTV coatings based on gradient diluents includes the following steps: (1) A patterned groove on the surface of a substrate is filled with an RTV coating with a low diluent content, and an embedded layer is formed after curing; the curing shrinkage rate of the RTV coating with a low diluent content is ≤5%; (2) Using an RTV coating with a high diluent content, a surface coating is prepared on the surface of the embedded layer by dip coating to obtain an embedded RTV coating; the viscosity of the RTV coating with a high diluent content is ≤80cSt.

[0007] Preferably, the diluent content in the low-diluent RTV coating is greater than 0 and less than or equal to 10 phr; the diluent content in the high-diluent RTV coating is 80 to 200 phr.

[0008] Preferably, the curing time is 5 to 7 hours.

[0009] Preferably, the method for determining the diluent content in the low-diluent-content RTV coating and the high-diluent-content RTV coating includes: testing the curing shrinkage rate and viscosity of RTV coatings with different diluent contents, obtaining a quantitative relationship between diluent content and curing shrinkage rate and a quantitative relationship between diluent content and viscosity, and determining the diluent content in the low-diluent-content RTV coating and the high-diluent-content RTV coating respectively based on the quantitative relationship between diluent content and curing shrinkage rate and the quantitative relationship between diluent content and viscosity.

[0010] Preferably, when testing the curing shrinkage rate, the curing conditions include: a curing temperature of 25°C, a relative humidity of 50%RH, and a curing time of 72 h.

[0011] Preferably, the components of the low-diluent-content RTV coating and the high-diluent-content RTV coating independently include a base adhesive, fillers, and additives, wherein the additives include a diluent.

[0012] Preferably, the base adhesive is one or more of polydimethylsiloxane, silicon-modified fluoropolymer, fluorosilicone rubber, bismaleimide resin, and methylphenyl silicone resin; the filler includes reinforcing filler and functional filler; the reinforcing filler includes one or more of fumed silica and calcium carbonate; the functional filler includes one or more of flame retardant, self-cleaning filler, dielectric filler, and aging-resistant filler; and the diluent is petroleum ether.

[0013] Preferably, the additives further include one or more of adhesion promoters, hydrophobic promoters, crosslinking agents, catalysts, and pigments.

[0014] Preferably, the low-diluent-content RTV coating comprises the following components: hydroxyl-terminated polydimethylsiloxane 95-105 phr, fumed silica 19-21 phr, alumina trihydrate 9.5-10.5 phr, crosslinking agent 2.85-3.15 phr, catalyst 0.95-1.05 phr, pigment 1.9-2.1, and diluent 4-6 phr.

[0015] Preferably, the high diluent content RTV coating comprises the following components: hydroxyl-terminated polydimethylsiloxane 95~105 phr, fumed silica 19~21 phr, alumina trihydrate 9.5~10.5 phr, crosslinking agent 2.85~3.15 phr, catalyst 0.95~1.05 phr, pigment 1.9~2.1, and diluent 90~110 phr.

[0016] This invention provides a two-step embedded RTV coating preparation method based on gradient diluent, comprising the following steps: (1) filling patterned grooves on the surface of a substrate with an RTV coating of low diluent content, and forming an embedded layer after curing; the curing shrinkage rate of the RTV coating of low diluent content is ≤5%; (2) preparing a surface coating on the surface of the embedded layer by dip coating with an RTV coating of high diluent content to obtain an embedded RTV coating; the viscosity of the RTV coating of high diluent content is ≤80cSt. To address the issue of depressions and delamination caused by the high shrinkage rate of commercially available RTV coatings, this invention innovatively introduces a gradient dilution strategy in a two-step preparation process: The first step uses RTV coating with a low diluent content to fill the depressions. This low-diluent-content RTV coating has high viscosity, low flowability, and minimal curing shrinkage, ensuring a strong mechanically interlocking interface in the embedded layer. The second step uses RTV coating with a high diluent content for surface dipping, giving the surface excellent leveling properties. This macroscopically eliminates depressions and microscopically achieves a continuous interlayer transition, fundamentally solving the technical problems of shrinkage depressions in one-step methods and poor bonding in traditional two-step methods. The RTV coating prepared by this invention has a depression-free surface, a continuous and dense bonding interface, and exhibits excellent adhesion and long-term operational stability.

[0017] This invention achieves a match between the embedded layer and the surface layer in terms of adhesion and volume shrinkage without changing the existing RTV coating system. The method is characterized by strong versatility, good process compatibility, and ease of industrialization. Attached Figure Description

[0018] Figure 1 This is a process flow diagram of the two-step embedded RTV coating preparation method in Example 1; Figure 2 This is a schematic diagram of the curing shrinkage rate test method; Figure 3Photographs of the RTV coating prepared in Example 1, where (a) is the first step filling and (b) is the second step filling; Figure 4 The images show photos of patterned grooves filled once and twice with commercially available RTV paint, where (a) shows one filling and (b) shows two fillings. Figure 5 This is a schematic diagram comparing the traditional method and the method of the present invention. Detailed Implementation

[0019] This invention provides a two-step method for preparing embedded RTV coatings based on gradient diluents, comprising the following steps: (1) A patterned groove on the surface of a substrate is filled with an RTV coating with a low diluent content, and an embedded layer is formed after curing; the curing shrinkage rate of the RTV coating with a low diluent content is ≤5%; (2) Using an RTV coating with a high diluent content, a surface coating is prepared on the surface of the embedded layer by dip coating to obtain an embedded RTV coating; the viscosity of the RTV coating with a high diluent content is ≤80cSt.

[0020] This invention uses an RTV coating with low diluent content to fill patterned grooves on the surface of a substrate, forming an embedded layer. In this invention, the curing shrinkage rate of the RTV coating with low diluent content is ≤5% (shrinkage rate after curing for 72 hours at 25°C and 50% RH), preferably 2~4%.

[0021] In a specific embodiment of the present invention, the method for determining the diluent content in the low-diluent-content RTV coating and the high-diluent-content RTV coating preferably includes: testing the curing shrinkage rate and viscosity of RTV coatings with different diluent contents, obtaining a quantitative relationship between diluent content and curing shrinkage rate and a quantitative relationship between diluent content and viscosity, and determining the diluent content in the low-diluent-content RTV coating and the high-diluent-content RTV coating respectively based on the quantitative relationship between diluent content and curing shrinkage rate and the quantitative relationship between diluent content and viscosity.

[0022] In this invention, when testing the curing shrinkage rate, the curing conditions include: a curing temperature of 25°C, a relative humidity of 50%RH, and a curing time of 72 h.

[0023] In this invention, when testing the curing shrinkage rate, the size of the test mold is preferably 30 mm × 100 mm × 10 mm, and a groove is provided in the center of the test mold, the size of which is preferably 10 mm × 60 mm × 1 mm. During the test, the RTV coating is preferably filled into the groove and leveled, and then cured. After curing, the distance d between the upper surface of the RTV coating in the groove and the bottom surface of the mold is measured, and then the shrinkage rate s is calculated according to Equation 1: Formula 1.

[0024] In this invention, the diluent content in the low-diluent RTV coating is preferably greater than 0 and less than or equal to 10 phr, more preferably 4 to 6 phr, and specifically 5 phr.

[0025] In this invention, the components of the low-diluent RTV coating preferably include a base adhesive, fillers, and additives; the base adhesive is preferably one or more of polydimethylsiloxane, silicon-modified fluoropolymer, fluorosilicone rubber, bismaleimide resin, and methylphenyl silicone resin; in specific embodiments of this invention, a compound system of multiple base adhesives can be used to achieve different viscosities and crosslinking characteristics. The filler preferably includes reinforcing filler and functional filler; the reinforcing filler preferably includes one or more of fumed silica and calcium carbonate; in specific embodiments of the present invention, reinforcing fillers of different particle sizes can be compounded and used, and silane coupling agents can be used to modify the surface of the reinforcing filler to improve dispersibility and interfacial bonding; the functional filler preferably includes one or more of flame retardants, self-cleaning functional fillers, dielectric functional fillers and aging-resistant functional fillers; the flame retardant preferably includes one or more of inorganic flame retardants and organic flame retardants; the inorganic flame retardant preferably includes one or more of alumina trihydrate, antimony trioxide and magnesium oxide; the organic flame retardant is preferably decabromodiphenyl ethane; the present invention does not have special requirements for the self-cleaning functional filler, dielectric functional filler and aging-resistant functional filler, and those well known to those skilled in the art can be used, such as titanium dioxide.

[0026] In this invention, the additives include a diluent, preferably petroleum ether; the additives may also include one or more of adhesion promoters, hydrophobic promoters, crosslinking agents, catalysts, and pigments; the adhesion promoter is preferably a silane coupling agent, and the hydrophobic promoter can be a low-molecular-weight silicone oil; this invention does not have special requirements for the crosslinking agent, catalyst, and pigment, and those well known to those skilled in the art can be used.

[0027] As a specific embodiment of the present invention, the RTV coating with low diluent content may include the following components: hydroxyl-terminated polydimethylsiloxane 95~105 phr, preferably 100 phr, fumed silica 19~21 phr, preferably 20 phr, alumina trihydrate 9.5~10.5 phr, preferably 10 phr, crosslinking agent 2.85~3.15 phr, preferably 3 phr, catalyst 0.95~1.05 phr, preferably 1 phr, pigment 1.9~2.1 phr, preferably 2 phr, and diluent 4~6 phr, preferably 5 phr.

[0028] In this invention, the RTV coating with low diluent content has high viscosity, low fluidity, and low curing shrinkage. In the first step, the RTV coating with low diluent content is used to fill the patterned grooves on the surface of the substrate, which can ensure the adhesion and flatness of the embedded layer and form a strong mechanical locking interface.

[0029] In a specific embodiment of the present invention, it is preferable to apply RTV coating with low diluent content to the patterned grooves, and then cure it for 5 to 7 hours, preferably 6 hours, before preparing the surface coating; the curing temperature is preferably 25°C, and the humidity is preferably 50%RH.

[0030] After forming the embedded layer, this invention uses an RTV coating with a high diluent content to prepare a surface coating on the surface of the embedded layer by dip coating, thereby obtaining an embedded RTV coating. In this invention, the viscosity of the RTV coating with a high diluent content is preferably ≤80 cSt (viscosity test temperature is 25°C), preferably 60~75 cSt; the diluent content in the RTV coating with a high diluent content is preferably 80~200 phr, more preferably 90~110 phr, specifically 100 phr; the other components of the RTV coating with a high diluent content are the same as those of the RTV coating with a low diluent content, and will not be described again here.

[0031] As a specific embodiment of the present invention, the high diluent content RTV coating comprises the following components: hydroxyl-terminated polydimethylsiloxane 95-105 phr, preferably 100 phr, fumed silica 19-21 phr, preferably 20 phr, alumina trihydrate 9.5-10.5 phr, preferably 10 phr, crosslinking agent 2.85-3.15 phr, preferably 3 phr, catalyst 0.95-1.05 phr, preferably 1 phr, pigment 1.9-2.1 phr, preferably 2 phr, and diluent 90-110 phr, preferably 100 phr.

[0032] The present invention does not have any special requirements for the dip coating method, and any method known to those skilled in the art can be used. In a specific embodiment of the present invention, the dip coating operation method is as follows: the test piece is buried in the coating, and then the excess coating is drained out.

[0033] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0034] Example 1 The process flow diagram for preparing the two-step embedded RTV coating in Example 1 is as follows: Figure 1 As shown, the specific steps are as follows: 1. Formulate RTV coatings containing gradient diluent concentrations. The RTV coating formulation used in this invention is as follows (unit: phr): Hydroxyl-terminated polydimethylsiloxane 100, fumed silica 20, alumina trihydrate 10, crosslinking agent (methyltrimethoxysilane) 3, catalyst (dibutyltin dilaurate) 1, pigment (iron oxide) 2, diluent (petroleum ether) 30~200 adjustable.

[0035] The preparation method of the coating is as follows: hydroxyl-terminated polydimethylsiloxane, fumed silica, alumina trihydrate, pigment, and diluent are mixed, dispersed, ground, stirred, and vacuum degassed. Then, crosslinking agent and catalyst are added, and the mixture is packaged for later use.

[0036] By adjusting the amount of thinner, the flowability and curing shrinkage characteristics of RTV coatings can be significantly altered. This invention sets thinner concentration gradients of 5, 10, 20, 30, 50, 100, and 200 phr. The corresponding viscosities (tested at 25°C) are as follows: 230.4 cSt at 30 phr, 107.6 cSt at 50 phr, 73.5 cSt at 100 phr, and 64.4 cSt at 200 phr. When the thinner content is below 20 phr, the coating is paste-like and lacks self-leveling properties.

[0037] 2. Quantitative relationship between diluent content and coating curing shrinkage rate. The coating was cured for 72 hours at 25°C and 50% RH. The curing shrinkage rate was measured using a custom-made 30 mm × 100 mm × 10 mm epoxy substrate mold. Figure 2This is a schematic diagram of the curing shrinkage rate test method. A 10 mm × 60 mm × 1 mm groove is designed in the center of the mold. The RTV coating is scraped, filled, and leveled. After curing, the distance between the upper surface of the RTV and the bottom surface of the substrate is measured. d Calculate the shrinkage rate according to the following formula. s : The test results for viscosity and shrinkage are shown in Table 1.

[0038] Table 1. Shrinkage rate variation with different diluent contents

[0039] The experimental results in Table 1 show that the curing shrinkage rate of RTV coatings increases significantly with the increase of diluent content.

[0040] 3. Preparation of two-step embedded RTV coating Based on the above principles, an embedded RTV coating was prepared using a two-step method by controlling the content of diluent in different steps: Step 1: Fill the patterned grooves with RTV coating with low diluent content (5 phr) to ensure high viscosity, low flowability, and low curing shrinkage of the embedded layer, forming a strong mechanical interlocking interface.

[0041] Step 2: After the embedded layer has cured for 6 hours (curing temperature is 25℃, humidity is 50%RH), the surface coating is prepared by dip coating process using RTV coating with high diluent content (100 phr) to achieve good leveling and smooth appearance of the surface.

[0042] Figure 3 Photographs of the RTV coating prepared in Example 1, where (a) shows the first step of filling and (b) shows the second step of filling. According to... Figure 3 As can be seen, the embedded RTV coating prepared by the method of the present invention has a smooth surface without depressions and a dense interface, which significantly improves the overall adhesion and long-term service stability of the coating.

[0043] Comparative Example 1 An embedded RTV coating was prepared using commercially available RTV coatings. The commercially available RTV coatings consist of hydroxyl-terminated polydimethylsiloxane, fumed silica, alumina trihydrate, crosslinking agent, catalyst, pigment, diluent, etc., with a viscosity of 107.6 cSt and a curing shrinkage rate of 27.4%.

[0044] Commercially available RTV coatings were used to fill patterned grooves on the substrate surface. The grooves were filled by multiple scraping processes (curing was performed at 25°C and 50%RH after each filling, for a total of two fillings). Then, a surface coating was prepared by dip coating. Figure 4Photos showing the patterned grooves filled once and twice with commercially available RTV paint. According to... Figure 4 It can be seen that the commercially available RTV coating has a large curing shrinkage rate. After two fillings, the groove is still not completely filled, and there are many depressions and poor flatness.

[0045] Figure 5 This is a comparative diagram of the conventional method and the method of the present invention. Figure 5 It can be seen that traditional methods require multiple scraping and filling of grooves, while this application only requires one scraping and filling, making the preparation process simpler.

[0046] Using a thickness gauge, the height difference between the coating in the grooved area and the coating in the non-grooved area in the RTV coatings obtained in Example 1 and Comparative Example 1 was measured (grooving depth 1 mm). The test results are shown in Table 2.

[0047] Table 2 Test Results

[0048] The test results in Table 2 show that the height difference between the grooved and non-grooved areas of the RTV coating prepared by the present invention is much smaller than that of the traditional method. This indicates that the RTV coating prepared by the method of the present invention can fully fill the groove and has almost no defects such as undercoating or void filling. At the same time, the smaller height difference means that the coating function (such as insulation and sealing) is more consistent and can effectively reduce the risk of functional failure caused by uneven coating.

[0049] In summary, this invention, from the perspective of RTV coating formulation, systematically analyzed the main influencing factors of coating curing shrinkage and determined that the excessive addition of diluent is the reason for the persistently high coating curing shrinkage rate. Furthermore, this invention proposes a two-step embedded RTV coating preparation method based on gradient diluents. This method achieves gradient control of coating curing shrinkage by using RTV coatings with different diluent contents in the two preparation steps, thereby significantly improving surface smoothness and interfacial bonding performance. The embedded RTV coating prepared by this method has a depression-free surface, a continuous and dense interfacial structure, and exhibits excellent adhesion and long-term operational stability.

[0050] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A two-step embedded RTV coating preparation method based on gradient diluent, characterized in that, Includes the following steps: (1) A patterned groove on the surface of a substrate is filled with an RTV coating with a low diluent content, and an embedded layer is formed after curing; the curing shrinkage rate of the RTV coating with a low diluent content is ≤5%; (2) Using RTV coating with high diluent content, a surface coating is prepared on the surface of the embedded layer by dip coating method to obtain embedded RTV coating; The viscosity of the RTV coating with high diluent content is ≤80 cSt.

2. The two-step embedded RTV coating preparation method based on gradient diluent according to claim 1, characterized in that, The low-diluent-content RTV coating has a diluent content greater than 0 and less than or equal to 10 phr; the high-diluent-content RTV coating has a diluent content of 80 to 200 phr.

3. The two-step embedded RTV coating preparation method based on gradient diluent according to claim 1, characterized in that, The curing time is 5-7 hours.

4. The two-step embedded RTV coating preparation method based on gradient diluent according to claim 1, characterized in that, The method for determining the diluent content in the low-diluent-content RTV coating and the high-diluent-content RTV coating includes: testing the curing shrinkage rate and viscosity of RTV coatings with different diluent contents, obtaining the quantitative relationship between diluent content and curing shrinkage rate and the quantitative relationship between diluent content and viscosity, and determining the diluent content in the low-diluent-content RTV coating and the high-diluent-content RTV coating respectively based on the quantitative relationship between diluent content and curing shrinkage rate and the quantitative relationship between diluent content and viscosity.

5. The two-step embedded RTV coating preparation method based on gradient diluent according to claim 4, characterized in that, When testing the curing shrinkage rate, the curing conditions included: a curing temperature of 25°C, a relative humidity of 50%RH, and a curing time of 72 h.

6. The two-step embedded RTV coating preparation method based on gradient diluent according to claim 1, characterized in that, The components of the low-diluent-content RTV coating and the high-diluent-content RTV coating independently include a base adhesive, fillers, and additives, the additives including diluents.

7. The two-step embedded RTV coating preparation method based on gradient diluent according to claim 6, characterized in that, The base adhesive is one or more of polydimethylsiloxane, silicon-modified fluoropolymer, fluorosilicone rubber, bismaleimide resin, and methylphenyl silicone resin; the filler includes reinforcing filler and functional filler; the reinforcing filler includes one or more of fumed silica and calcium carbonate; the functional filler includes one or more of flame retardant, self-cleaning filler, dielectric filler, and aging resistant filler; the diluent is petroleum ether.

8. The two-step embedded RTV coating preparation method based on gradient diluent according to claim 6, characterized in that, The additives also include one or more of adhesion promoters, hydrophobic promoters, crosslinking agents, catalysts, and pigments.

9. The two-step embedded RTV coating preparation method based on gradient diluent according to claim 1 or 6, characterized in that, The RTV coating with low diluent content comprises the following components: hydroxyl-terminated polydimethylsiloxane 95-105 phr, fumed silica 19-21 phr, alumina trihydrate 9.5-10.5 phr, crosslinking agent 2.85-3.15 phr, catalyst 0.95-1.05 phr, pigment 1.9-2.1 phr, and diluent 4-6 phr.

10. The two-step embedded RTV coating preparation method based on gradient diluent according to claim 1 or 6, characterized in that, The RTV coating with high diluent content comprises the following components: hydroxyl-terminated polydimethylsiloxane 95~105 phr, fumed silica 19~21 phr, alumina trihydrate 9.5~10.5 phr, crosslinking agent 2.85~3.15 phr, catalyst 0.95~1.05 phr, pigment 1.9~2.1 phr, and diluent 90~110 phr.

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