Low temperature curing powder coating and method of preparation and method of coating a hydraulic support for mining

Abstract

The application provides a low-temperature curing powder coating, a preparation method thereof and a coating method of a mine hydraulic support. The steps comprise: providing a low-temperature curing powder coating; electrostatically spraying the low-temperature curing powder coating on the mine hydraulic support under a static spraying voltage of 60 KV-90 KV, and controlling the coating thickness to be 60-150 mu m; placing the mine hydraulic support after electrostatic spraying on a curing furnace coating line, and baking the mine hydraulic support by using infrared radiation and hot air convection, so that the low-temperature curing powder coating is cross-linked and cured on the mine hydraulic support. The coating method of the mine hydraulic support can solve the problem of poor corrosion resistance and wear resistance of the mine hydraulic support caused by coating failure, reduce the curing temperature of the low-temperature curing powder coating, and improve the throughput of the coating line.

Description

Technical Field

[0001] This invention relates to the field of coatings for hydraulic supports, and more specifically, to a low-temperature curing powder coating and its preparation method, and a coating method for mining hydraulic supports. Background Technology

[0002] Because underground mines are high-humidity environments containing corrosive media, the requirements for the coatings applied to hydraulic supports are extremely high. The coatings must possess excellent corrosion resistance, impact resistance, abrasion resistance, adhesion, and certain resistance to aging and water. Compared to traditional solvent-based coatings, powder coatings emit no volatile organic compounds, are environmentally friendly, achieve a utilization rate of over 95%, and offer superior coating performance. However, traditional powder coatings require high-temperature curing at 180℃ to 200℃, which is relatively high.

[0003] Hydraulic supports are large, heavy structural components with uneven wall thickness in different locations. Therefore, after coating with paint, the entire hydraulic support needs to be heated to approximately 200°C for high-temperature curing. The entire heating and heat preservation process requires a large amount of heat and is time-consuming, resulting in a low throughput of the coating line, leading to high production costs and low production efficiency. Furthermore, the inconsistent thickness of the steel plates in different locations causes varying heating rates. Therefore, to ensure all parts reach above 200°C, a baking time of 5 to 10 hours or more is required. This can lead to over-baking and yellowing in thinner areas, resulting in appearance quality issues. Conversely, shortening the baking time to avoid yellowing can cause incomplete curing of the coating in some areas, such as the inner support column recesses, leading to coating failure and affecting the corrosion resistance and service life of the mining hydraulic support.

[0004] Meanwhile, existing powder coatings rely on air adsorption to remove fine powder after crushing, which has limited effect on removing fine powder. When the content of fine powder is high, it will result in a low powder application rate during coating, a large amount of hazardous waste, and easy clogging of the coating gun, which greatly limits the production efficiency of coating mining hydraulic supports. Summary of the Invention

[0005] To address the problems of yellowing and failure of mining hydraulic supports due to prolonged high-temperature baking, and poor corrosion and wear resistance caused by incomplete coating curing, this invention aims to reduce the curing temperature of powder coatings and increase the throughput of coating lines. The technical solution adopted in this invention is: a method for preparing low-temperature curing powder coatings, comprising the following steps: Preparation of powder coating raw materials: Weigh the following raw materials according to the following weight parts: 50-65 parts carboxylated polyester resin, 4-6 parts curing agent, 0.4-0.6 parts curing accelerator, 5-10 parts pigment, 20-40 parts filler, 0.5-1 part leveling agent, 1-2 parts high-temperature yellowing resistant agent, and 0.5-1 part toughening agent; wherein, the curing agent is triglycidyl isocyanurate, the curing accelerator is methyl diethanolamine or triphenylphosphine derivative or benzyltrimethylammonium bromide, the leveling agent is polybutyl acrylate, the pigment is aluminum phosphate, the filler is titanium dioxide, the high-temperature yellowing resistant agent is a hindered phenolic primary antioxidant or a phosphite auxiliary antioxidant, and the toughening agent is CTBN modified epoxy or polyurethane elastomer; Melt-pulverization: After mixing the raw materials, melt-extrusion and rapid cooling are carried out. After pulverization, sieving and separation, the content of fine particles with a particle size of less than 10μm and the content of coarse particles with a particle size of greater than 80μm are controlled to be less than 10% respectively, to obtain low-temperature curing powder coating.

[0006] Based on the above, in the melting and pulverizing step, the raw materials are first mixed for 3 to 5 minutes at a speed of 1000 rpm to 1500 rpm. Then, a screw extruder is used to melt and extrude the mixed raw materials, controlling the temperature of the feeding zone to 80℃~100℃ and the temperature of the melt mixing zone to 100℃~120℃. The molten material extruded from the screw extruder die is then rapidly cooled at 10℃~20℃ and pressed into continuous sheets with a thickness of 1 mm~2 mm. Finally, the continuous thin film is coarsely crushed using a pulverizer, finely ground and classified using an air classifier, and sieved and separated using a cyclone separator and a rotary vibrating screen to obtain the low-temperature curing powder coating.

[0007] Based on the above, in the melting and pulverizing step, the continuous thin sheet is first coarsely pulverized using a pulverizer, and then finely ground and classified using an air classifier mill to obtain finely ground powder with a median particle size D50 of 30μm to 60μm; then the finely ground powder is sieved and separated sequentially using a cyclone separator and a rotary vibrating screen, controlling the content of fine particles with a particle size less than 10μm and the content of coarse particles with a particle size greater than 80μm in the finely ground powder to be less than 10% respectively, to obtain the low-temperature curing powder coating.

[0008] Based on the above, the acid value of the carboxylated polyester resin is 32 mgKOH / g to 40 mgKOH / g, and the glass transition temperature is 60℃ to 65℃.

[0009] The present invention also provides a low-temperature curing powder coating, which is prepared by the above-described preparation method.

[0010] Based on the above, the curing temperature of this low-temperature curing powder coating is 140℃.

[0011] The present invention also provides a method for coating a mining hydraulic support using the above-mentioned low-temperature curing powder coating, the steps of which include: Provides the aforementioned low-temperature curing powder coating; Electrostatic spraying: The low-temperature curing powder coating is electrostatically sprayed onto the mining hydraulic support under an electrostatic spraying voltage of 60 KV to 90 KV, and the coating thickness is controlled to be 60 μm to 150 μm. Crosslinking and curing: The mining hydraulic support after electrostatic spraying is placed on the coating line of the curing oven, and the mining hydraulic support is baked by infrared radiation and hot air convection to heat the surface of the mining hydraulic support to 130℃~150℃, so that the low-temperature curing powder coating is crosslinked and cured on the mining hydraulic support.

[0012] Based on the above, the electrostatic spraying step further includes, before spraying the low-temperature curing powder coating, first performing shot blasting and sandblasting to remove rust on the mining hydraulic support, and then placing the mining hydraulic support in a preheating furnace to preheat at a temperature of 80℃~110℃ for 10 min~20 min.

[0013] Based on the above, the crosslinking and curing step includes: setting a gas infrared radiator at the entrance of the curing oven coating line to preheat the thick-walled area of ​​the mining hydraulic support after electrostatic spraying, and then sending it to the curing oven coating line to bake at a temperature of 190℃~220℃ for 120 min~240 min to crosslink and cure the low-temperature curing powder coating on the mining hydraulic support.

[0014] Specifically, in the raw materials of the low-temperature curing powder coating provided by the present invention, carboxyl polyester resin serves as the film-forming matrix, and the carboxyl group (-COOH) at the end of its molecular chain serves as a reactive functional group. After undergoing a cross-linking reaction with the curing agent, it forms a film, providing the coating with basic physical and mechanical properties and weather resistance.

[0015] Specifically, the carboxyl polyester resin of this invention is prepared through the following steps: It is prepared by polycondensation reaction of a diacid and a diol under the action of a catalyst. The diacid includes terephthalic acid and isophthalic acid; the diol includes neopentyl glycol and cyclohexanediol; trimellitic anhydride is introduced simultaneously to introduce carboxyl groups and regulate the acid value; Esterification reaction stage: a polyol and an esterification catalyst are added to a reaction vessel and heated until the material melts; then, the polyacid is added sequentially, and the temperature is gradually increased to 180℃~240℃ under nitrogen protection to carry out the esterification polycondensation reaction; Acid desealing stage: the esterification reaction... After reaching the endpoint, an acid hydrolysate is added to the reaction system to carry out an end-capping reaction to adjust the terminal carboxyl functional group and the final acid value of the polyester resin. Vacuum polycondensation stage: After the end-capping reaction is completed, the reaction system is vacuumed to remove residual small molecules and unreacted monomers, while further promoting the completion of the polycondensation reaction and precisely controlling the molecular weight and distribution range of the polyester resin. Discharge and cooling molding: After vacuum polycondensation is completed, heating is stopped, the vacuum is released, and the molten polyester resin is discharged from the reactor and pressed into thin sheet solids using a cooling steel belt. The final physicochemical properties of the carboxyl polyester resin are: acid value 32 mgKOH / g~40 mgKOH / g, glass transition temperature 60℃~65℃, softening point 98℃~108℃, and melt viscosity at 200℃ 3000~6000 mPa·s.

[0016] As highly efficient nucleophilic catalysts, curing accelerators can significantly activate the reaction between epoxy groups and carboxyl groups. By lowering the activation energy of the curing reaction, the crosslinking reaction can be completed at lower temperatures or in shorter times. The mechanism of action is as follows: the catalyst molecule first undergoes a nucleophilic attack with the epoxy group, forming a highly reactive zwitterionic intermediate. This intermediate significantly lowers the activation energy barrier of the ring-opening addition reaction between the carboxyl and epoxy groups. It is precisely the intervention of this accelerator that alters the reaction kinetic pathway, enabling the carboxyl / epoxy crosslinking reaction, which originally required 180℃ to 200℃ to proceed fully, to be efficiently completed in a low-temperature range of 130℃ to 150℃.

[0017] The leveling agent, through its polybutyl acrylate active ingredient, does not passively precipitate out during the initial heating and melting stage of the powder coating. Instead, it exhibits a limited compatibility difference with carboxylated polyester resins, a thermodynamic property that drives its directional migration to the gas / liquid interface of the molten coating.

[0018] Specifically, at the interface, the leveling agent forms a monolayer. Through a weak anchoring effect with the polar groups of the matrix resin, it not only reduces surface tension but, more importantly, regulates the viscosity gradient between the resin and pigments / fillers in the molten state, eliminating localized high-viscosity areas caused by pigment / filler aggregation. Before the viscosity rises sharply due to the crosslinking reaction driven by the curing accelerator, the leveling agent ensures the melt fully expands, thus forming a mirror-smooth coating even in a short time during low-temperature curing. This effectively suppresses the problem of insufficient flowability caused by premature crosslinking. This allows the powder to form a continuous, smooth coating during high-temperature curing, thereby avoiding appearance defects such as pinholes and orange peel caused by surface tension imbalance or poor dispersion.

[0019] Because the curing accelerator speeds up the reaction rate, the exothermic reaction at low temperatures is relatively concentrated, and the baking time is long, which places higher demands on the thermo-oxidative stability of the resin. The high-temperature anti-yellowing agent in this invention does not act alone; it works synergistically with the main antioxidant: on the one hand, it acts as a chain terminator to capture alkyl radicals, and on the other hand, it acts as an auxiliary stabilizer to decompose hydrogen peroxide.

[0020] Most importantly, the anti-yellowing agent selected in this invention can effectively quench trace amounts of side-reaction free radicals that may be generated by the curing accelerator during the catalytic cycle, preventing the active center of the catalyst from initiating dehydrogenation of the resin skeleton. This synergistic effect ensures that even when the carboxyl polyester resin is in an activated reaction state within a continuous baking window of 130°C to 150°C, the coating will not generate carbonyl or other chromophores due to thermal oxidation, thereby guaranteeing the whiteness and color durability of the low-temperature cured coating.

[0021] The toughening agent selected in this invention is uniformly dispersed in the crosslinking network of carboxylated polyester resin in the form of microparticles, forming an "island structure". During the crosslinking reaction initiated by the curing accelerator, the presence of the toughening agent phase region fine-tunes the local crosslinking shrinkage stress. When the coating is subjected to external impact, the toughening agent particles act as stress concentration points, inducing crazes, but this is only the initial stage; the deeper synergistic mechanism lies in the fact that the active groups grafted on the toughening agent surface form weak chemical bonds or strong physical entanglements with the carboxylated polyester resin matrix. This interfacial bonding force causes the crazes to be bound and terminated during their expansion by the crosslinking network, thereby absorbing a large amount of fracture energy while preventing the penetration of microcracks. This synergistic design balances the contradiction between the increased cohesion caused by low-temperature curing and the coating's flexibility requirements, achieving a unity of high hardness and bending resistance.

[0022] The present invention utilizes a combination of titanium dioxide and aluminum phosphate to construct an "active-passive" dual protection system. Titanium dioxide provides a dense physical barrier layer to the coating, delaying moisture penetration. Aluminum phosphate, as an active anti-rust pigment, exhibits a key synergistic effect with the hydrolysis behavior of carboxyl polyester resin: when ambient moisture permeates the coating's micropores, trace amounts of carboxyl groups in the resin create a localized weakly acidic microenvironment. This precisely promotes the controlled hydrolysis of aluminum phosphate, continuously releasing phosphate ions (PO4). 3- These phosphate ions penetrate to the metal substrate interface and react with iron ions to form an insoluble iron phosphate passivation film. This passivation film has a strong hydrogen bond adsorption effect with the polar ester groups in the carboxylated polyester resin.

[0023] This three-in-one interface structure of resin-passivation film-substrate effectively blocks the formation of electrochemical corrosion circuits. Compared with simple physical shielding, it significantly improves the coating's salt spray resistance and flash rust resistance under low-temperature curing conditions.

[0024] Therefore, this invention has outstanding substantive features and significant progress compared to the prior art. Specifically, the coating method and low-temperature curing powder coating for mining hydraulic supports provided by this invention, on the one hand, by reasonably matching the types and weight ratios of various low-temperature curing powder coating raw materials, and adopting melting, crushing and sieving processes that match the coating, can reduce the curing temperature window of the powder coating to 140°C. This not only shortens the time required for the workpiece to rise from room temperature to 140°C, reducing energy consumption and significantly improving the production cycle of the complete coating of hydraulic supports, but also avoids problems such as yellowing, aging and embrittlement of the coating caused by over-baking of the surface and thin-walled areas of the mining hydraulic support workpiece.

[0025] On the other hand, by controlling the particle size range of the low-temperature curing powder coating and using infrared radiation and hot air convection to bake the mining hydraulic support, the thicker areas of the mining hydraulic support can be preheated and baked in advance. This allows the surface temperature of the mining hydraulic support to reach the curing temperature of the low-temperature curing powder coating of 140°C, ensuring that the low-temperature curing powder coating can be cured and cross-linked uniformly. This allows the coating to maintain optimal mechanical properties, appearance, and long-term weather resistance, significantly improving the overall quality and reliability of the product. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0027] The endpoints and any values ​​of the ranges disclosed in this invention 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 in this invention.

[0028] In this invention, unless otherwise specified and / or stated, all numerical values ​​involving component amounts are "by weight". Unless otherwise specified, the terminology used in this invention are common terms in the relevant field. Unless otherwise specified, the preparation processes, testing methods, etc., used in the various embodiments are conventional methods well known to those skilled in the art, and the raw materials and equipment used can be obtained from publicly available commercial sources.

[0029] Specifically, to reduce the curing temperature of powder coatings and ensure uniform curing and cross-linking, thereby maintaining optimal mechanical properties, appearance, and long-term weather resistance, this invention employs the following principle: the synergistic cross-linking reaction between carboxylated polyester resin and curing accelerator alters the reaction kinetic pathway, allowing the powder to complete the process efficiently at a lower temperature. The limited compatibility difference between the leveling agent and the carboxylated polyester resin, a thermodynamic property that drives its directional migration to the gas / liquid interface of the molten coating, regulates the viscosity gradient between the resin and pigments / fillers in the molten state, and eliminates localized high-viscosity areas caused by pigment / filler aggregation.

[0030] Meanwhile, the added high-temperature anti-yellowing agent can effectively quench trace amounts of side reaction free radicals that may be generated by the curing accelerator during the catalytic cycle, preventing the active center of the catalyst from initiating dehydrogenation of the resin skeleton. The active groups grafted onto the toughening agent surface form weak chemical bonds or strong physical entanglements with the carboxylated polyester resin matrix. Phosphate ions in the pigments and fillers penetrate to the metal substrate interface, react with iron ions to form an insoluble iron phosphate passivation film, and this passivation film has a strong hydrogen bond adsorption effect with the polar ester groups in the carboxylated polyester resin.

[0031] The technical solution of the present invention will be further described in detail below through specific embodiments. Example 1

[0032] This embodiment provides a low-temperature curing powder coating, comprising the following raw materials in parts by weight: 55 parts carboxylated polyester resin, 5 parts curing agent, 0.5 parts curing accelerator, 8 parts pigment, 30 parts filler, 0.6 parts leveling agent, 1.5 parts high-temperature anti-yellowing agent, and 0.8 parts toughening agent.

[0033] In this embodiment, the curing agent is triglycidyl isocyanurate. The curing accelerator is methyl diethanolamine. The leveling agent is polybutyl acrylate. The pigment is aluminum phosphate. The filler is titanium dioxide. The high-temperature anti-yellowing agent is a sterically hindered phenolic primary antioxidant. The toughening agent is a polyurethane elastomer.

[0034] This embodiment also provides a method for preparing a low-temperature curing powder coating, the steps of which include: Mixing: Weigh each raw material according to the above weight proportions, then place each weighed raw material in a mixer and mix for 5 minutes at a speed of 1000 rpm to obtain a mixture; Melt extrusion and tableting: The mixture is placed in a screw extruder for melt extrusion, the temperature of the feeding zone is controlled at 100°C and the temperature of the melt mixing zone is controlled at 120°C, and the molten material extruded from the screw extruder die is rapidly cooled at 10°C and then pressed into a continuous sheet with a thickness of 2 mm.

[0035] Crushing and classifying mill: First, the continuous thin sheet is coarsely crushed using a crusher, and then finely ground and classified using an air classifier mill to obtain finely ground powder with a median particle size D50 of 30μm to 60μm.

[0036] Removal of fine and coarse powder: The finely ground powder is sieved and separated sequentially using a cyclone separator and a rotary vibrating screen to control the content of fine particles with a particle size of less than 10 μm and the content of coarse particles with a particle size of greater than 80 μm in the finely ground powder to be less than 10% respectively, so as to obtain a low-temperature curing powder coating.

[0037] When mixing the raw materials, the principle of "larger components first, then smaller ones; solid components first, then liquid components" is followed to prevent liquid components from adhering to the inner wall of the mixer. At the same time, by controlling the rotation speed and mixing time, it is important to avoid uneven mixing due to too short a time, while too long a time may cause localized softening or clumping of the materials due to frictional heat, which is particularly detrimental to sensitive components such as curing accelerators.

[0038] The molten material extruded from the die immediately passes through a cooling roller, which is filled with circulating cooling water at a temperature controlled between 10°C and 20°C. This rapid cooling freezes the already formed uniform microstructure and minimizes any possible subsequent reactions, ensuring the product's storage stability.

[0039] Subsequently, by adjusting the rotation speed of the classifying wheel, the median particle size D50 of the powder was precisely controlled to be 30μm to 60μm, which is the ideal range for electrostatic spraying. Simultaneously, controlling the content of fine particles smaller than 10μm to within 10% significantly improves powder flowability, reduces dust generation during spraying, avoids spray gun clogging during electrostatic spraying, and improves coating thickness uniformity and appearance, reducing orange peel effect. Controlling the content of coarse particles larger than 80μm to within 10% reduces the overall particle size range, increases the powder application rate and powder utilization, improves film thickness uniformity during vertical surface spraying, and ensures a smooth, particle-free, and impurity-free coating surface.

[0040] Specifically, when using a cyclone separator and a rotary vibrating screen to sieve and separate the finely ground powder, two layers of planar filter screens are installed inside the rotary vibrating screen. The upper layer is a 160-200 mesh filter screen, and the lower layer is a 900-1000 mesh filter screen. A finished product outlet with a valve is added to the side wall of the rotary vibrating screen located between the upper and lower filter screens, and a push-pull device is installed between the upper and lower filter screens. This allows the 160-200 mesh filter screen to sieve coarse particles larger than 80 μm onto the uppermost layer of the rotary vibrating screen, while allowing fine particles smaller than 10 μm to pass through to the lowermost layer. Particles between 10 μm and 80 μm are retained between the upper and lower filter screens, thus achieving control over the particle size. Example 2

[0041] This embodiment provides a low-temperature curing powder coating, comprising the following raw materials in parts by weight: 50 parts carboxylated polyester resin, 4 parts curing agent, 0.4 parts curing accelerator, 5 parts pigment, 20 parts filler, 0.5 parts leveling agent, 1 part high-temperature anti-yellowing agent, and 0.5 parts toughening agent.

[0042] The carboxylated polyester resin has an acid value of 40 mgKOH / g and a glass transition temperature of 65℃.

[0043] The curing agent is triglycidyl isocyanurate. The curing accelerator is a triphenylphosphine derivative. The leveling agent is polybutyl acrylate. The pigment is aluminum phosphate. The filler is titanium dioxide. The high-temperature anti-yellowing agent is a phosphite-based auxiliary antioxidant. The toughening agent is CTBN-modified epoxy.

[0044] This embodiment also provides a method for preparing a low-temperature curing powder coating, the steps of which include: Mixing: Weigh each raw material according to the above weight proportions, then place each weighed raw material in a mixer and mix for 3 minutes at a speed of 1500 rpm to obtain a mixture.

[0045] Melt extrusion and tableting: The mixture is placed in a screw extruder for melt extrusion, the temperature of the feeding zone is controlled at 80°C and the temperature of the melt mixing zone is controlled at 100°C, and the molten material extruded from the screw extruder die is rapidly cooled at 20°C and then pressed into a continuous sheet with a thickness of 1 mm. Crushing and classifying mill: First, the continuous thin sheet is coarsely crushed using a crusher, and then finely ground and classified using an air classifier mill to obtain finely ground powder with a median particle size D50 of 30μm to 60μm.

[0046] Removal of fine and coarse powder: The finely ground powder is sieved and separated sequentially using a cyclone separator and a rotary vibrating screen to control the content of fine particles with a particle size of less than 10 μm and the content of coarse particles with a particle size of greater than 80 μm in the finely ground powder to be less than 10% respectively, so as to obtain a low-temperature curing powder coating. Example 3

[0047] This embodiment provides a low-temperature curing powder coating, comprising the following raw materials in parts by weight: 65 parts carboxylated polyester resin, 6 parts curing agent, 0.6 parts curing accelerator, 10 parts pigment, 40 parts filler, 1 part leveling agent, 2 parts high-temperature anti-yellowing agent, and 1 part toughening agent.

[0048] The carboxylated polyester resin has an acid value of 32 mgKOH / g and a glass transition temperature of 60℃.

[0049] The curing agent is triglycidyl isocyanurate. The curing accelerator is benzyltrimethylammonium bromide. The leveling agent is polybutyl acrylate. The pigment is aluminum phosphate. The filler is titanium dioxide. The high-temperature anti-yellowing agent is a phosphite-based auxiliary antioxidant. The toughening agent is a polyurethane elastomer.

[0050] This embodiment also provides a method for preparing a low-temperature curing powder coating, the steps of which include: Mixing: Weigh each raw material according to the weight parts, then place the weighed raw materials in a mixer and mix for 4 minutes at a speed of 1200 rpm to obtain a mixture.

[0051] Melt extrusion and tableting: The mixture is placed in a screw extruder for melt extrusion, the temperature of the feeding zone is controlled at 90°C and the temperature of the melt mixing zone is controlled at 110°C, and the molten material extruded from the screw extruder die is rapidly cooled at 15°C and then pressed into a continuous sheet with a thickness of 1 mm.

[0052] Crushing and classifying mill: First, the continuous thin sheet is coarsely crushed using a crusher, and then finely ground and classified using an air classifier mill to obtain finely ground powder with a median particle size D50 of 30μm to 60μm.

[0053] Removal of fine and coarse powder: The finely ground powder is sieved and separated sequentially using a cyclone separator and a rotary vibrating screen to control the content of fine particles with a particle size of less than 10 μm and the content of coarse particles with a particle size of greater than 80 μm in the finely ground powder to be less than 10% respectively, so as to obtain a low-temperature curing powder coating. Example 4

[0054] This embodiment provides a method for coating a mining hydraulic support using the low-temperature curing powder coating from Embodiment 1, the steps of which include: A low-temperature curing powder coating as described in Example 1 is provided.

[0055] Electrostatic spraying: First, the mining hydraulic support is shot blasted to remove rust. Then, the mining hydraulic support is placed in a preheating furnace and preheated at 110°C for 10 minutes. The low-temperature curing powder coating is electrostatically sprayed onto the mining hydraulic support using a high-voltage electrostatic spray gun at an electrostatic spraying voltage of 90 KV, and the coating thickness is controlled to be 150 μm.

[0056] Crosslinking and curing: A gas infrared radiator is installed at the entrance of the curing oven coating line to preheat the thick-walled area of ​​the mining hydraulic support after electrostatic spraying, and then it is sent to the curing oven coating line and baked at 220°C for 120 minutes to heat the surface of the mining hydraulic support to 140°C, thereby crosslinking and curing the low-temperature curing powder coating on the mining hydraulic support.

[0057] In order to compare with the method for coating mining hydraulic supports provided by this invention, and to prove that this invention integrates low-temperature curing powder coatings, coating preparation methods, and special steps during coating, it finally solves the problem of poor corrosion resistance and wear resistance of mining hydraulic supports due to coating failure, the following comparative test was designed.

[0058] Comparative Example 1 (Process Missing: No Rapid Cooling Treatment) Comparative Example 1 provides a method for coating a mining hydraulic support. The difference from Example 4 is that in this comparative example, the molten material extruded from the screw extruder die is directly pressed into a continuous sheet with a thickness of 2 mm without quenching. This aims to verify the indispensability of quenching in this invention.

[0059] Comparative Example 2 (Process Missing: No Particle Size Classification Control) Comparative Example 2 provides a method for coating a mining hydraulic support. Unlike Example 4, this comparative example omits the step of sieving and separating the finely ground powder using a cyclone separator and a rotary vibrating screen. It also does not limit the content of fine particles smaller than 10 μm and coarse particles larger than 80 μm in the finely ground powder to be less than 10%, respectively. This aims to verify the indispensability of particle size classification control in this invention.

[0060] Comparative Example 3 (Coating process missing: no infrared radiation preheating) Comparative Example 3 provides a method for coating a mining hydraulic support. The difference from Example 4 is that the step of installing a gas-fired infrared radiator at the entrance of the curing oven coating line is omitted in this comparative example. Instead, the mining hydraulic support is directly fed into the curing oven coating line and baked at 220°C for 120 minutes. This aims to verify the indispensability of infrared radiation preheating in this invention.

[0061] Comparative Example 4 (Curing Accelerator Type Replacement) The difference between Comparative Example 4 and Example 4 is that the curing accelerator with a specific structure in this invention is replaced with an equal amount of conventional industrial-grade 2-methylimidazole. All other components and preparation process parameters remain the same as in Example 4. This aims to verify the irreplaceable role of the specific accelerator of this invention in reducing activation energy and maintaining storage stability.

[0062] Comparative Example 5 (Toughening Agent Dosage Exceeds the Limit): This Comparative Example 5 differs from Example 4 in that the amount of toughening agent added was adjusted. Its dosage was significantly increased from the 3%-8% range of the total resin content limited by this invention to 15%. All other components and preparation process parameters remained consistent with Example 4. The aim was to verify the negative impact of exceeding the upper limit of toughening agent dosage on coating hardness, density, and corrosion resistance.

[0063] Comparative Example 6 (Component Synergistic Defect: Lack of Aluminum Phosphate Active Rust Inhibiting Pigment) The difference between Comparative Example 6 and Example 4 is that aluminum phosphate was omitted from the formulation, and replaced with an equal volume of barium sulfate filler to maintain the pigment-to-binder ratio. All other components (including titanium dioxide) and process parameters remained the same as in Example 4. The aim was to verify the synergistic corrosion protection mechanism of "active passivation-physical shielding" between aluminum phosphate, the resin matrix, and the corrosive medium.

[0064] Comparative Example 7 (Extrusion temperature deviates from the optimal window) The difference between Comparative Example 7 and Example 4 is that the melt extrusion temperature of the twin-screw extruder was reduced from 100°C–110°C as defined in this invention to 80°C–85°C. This aims to verify the impact of insufficient extrusion temperature on the appearance and performance of the coating due to poor resin wetting and dispersion of the filler and uneven local pre-crosslinking.

[0065] The coatings of the mining hydraulic support after coating in Example 4, as well as the coatings in Comparative Examples 1 to 7, were tested for adhesion, pencil hardness, impact resistance under normal impact, bending test, gloss, resistance to neutral salt spray, resistance to artificial weathering, and resistance to yellowing. Specific results are shown in Table 1. Adhesion was tested using the cross-cut test (0 or 1 being the best). For yellowing resistance, two methods were used: one method involving baking at 140℃ for 30 minutes to simulate the slowest heating condition (shortest baking time and temperature), and another method involving baking at 220℃ for 240 minutes to simulate both temperature and time. The difference between the two methods was measured using a colorimeter to determine the color difference.

[0066] The reference standards for adhesion are GB / T9286-2021, pencil hardness is GB / T6739, impact resistance (positive impact) is GB / T1732-2020, bending test is GB / T6742-2007, gloss is GB / T9754-2007, neutral salt spray resistance is GB / T1771-2007, and artificial weathering resistance is GB / T1865-2009.

[0067] Table 1. Coating performance of low-temperature curing powder coatings Adhesion ≤0 level Level 0 Level 1 Level 1 Level 3 Level 1 Level 0 Level 1 Level 2 Pencil hardness ≥H 2H H H 3B H F 2H H Impact resistance (normal impact) 50cm Passed, no cracks Failed, cracked Failed, cracked Failed, cracked Failed, microcracks Passed, no cracks Passed, no cracks Failed, cracked Bending test 3mm Passed, no cracks Passed, no cracks Failed, cracked Failed, cracked Failed, cracked Passed, no cracks Passed, no cracks Failed, cracked luster ≥60° 81° 56° 62° 40° 72° 85° 80° 55° Resistance to neutral salt spray 500h 600h 300h, single-sided erosion width 2.3mm 480h, single-sided erosion width 2.1mm 240h, single-sided erosion width 2.6mm 450h, single-sided erosion width 2.3mm 360h, single-sided erosion width 2.8mm 200h, single-sided erosion width 2.2mm 400h, single-sided erosion width 2.5mm Resistant to artificial climate aging 500h 600h 400h, color change level 3 600h 300 hours, Level 3 light loss 550h, Level 2 light loss 600 hours, Level 2 light loss 500 hours, Level 3 light loss 450 hours, Level 3 light loss Yellowing resistance Color difference value ≤ 3 2.6 2.4 2.5 1.3 4.2 2.7 2.5 2.9 As shown in Table 1, in Example 4, the unidirectional corrosion spread width at the scratched area after 500 hours of neutral salt spray resistance is ≤2mm, and there are no abnormalities such as blistering, rusting, cracking, or peeling in the unscratched area. After 500 hours of artificial weathering resistance, the discoloration is ≤2 levels, the gloss loss is ≤2 levels, and there are no abnormalities such as powdering, blistering, cracking, or peeling.

[0068] Meanwhile, in Embodiment 4 of this invention, a gas infrared radiator is installed at the entrance of the curing oven coating line to preheat the thicker parts. According to the oven temperature tracking, the surface temperature of the thicker parts of the 6-ton workpiece is above 140°C for 35 minutes, which meets the requirement of a curing window of 140°C. The coating is smooth, with a gloss of 81°, and the adhesion test is grade 0, which is qualified.

[0069] In contrast, the coating formed by the paint provided in Comparative Example 1 exhibited orange peel-like texture and reduced gloss on the support surface during use, with the gloss level decreasing from 81° to 56°, and pitting also appeared. Analysis suggests that the paint powder in Comparative Example 1, due to the lack of rapid cooling, failed to dissipate the accumulated heat within the material, leading to a continuous increase in system temperature. This caused partial pre-crosslinking of the resin and curing agent, an irreversible reaction that resulted in poor flowability during use. DSC analysis showed that the enthalpy of heat released during baking and curing of the powder in Example 1 was 24 J / g, while the enthalpy of heat released by the powder in Comparative Example 1 was 18.29 J / g. The reduced heat absorption during the subsequent baking and curing process indicates that pre-curing of the powder occurred.

[0070] In Comparative Example 2, due to the lack of particle size classification, problems such as low powder application rate in a single coat, easy gun clogging, and severe orange peel occurred during the coating process. By using a laser particle size analyzer to detect the powder particle size in Comparative Example 2, it was found that powder smaller than 10μm accounted for 15.6%, powder between 10μm and 80μm accounted for 65.8%, and powder larger than 80μm accounted for 18.6%. The uneven particle size distribution was unfavorable for powder application. Furthermore, excessive fine powder easily caused gun clogging, uneven powder distribution, and dust pollution, while excessive coarse powder resulted in severe orange peel and poor appearance quality.

[0071] Electrostatic spraying was performed on test panels under the same process parameters, and various data of the powder were tested during spraying. The powder had a single-pass coating rate of 46%, and most of the powder fell off. Large-diameter particles fell off because their mass was greater than their charge, while small-diameter particles floated more due to the relationship between mass and charge. Uneven coating thickness was also observed, with local deviations exceeding 40 μm and severe orange peel texture on the surface. In contrast, when spraying the powder from Example 1 under the same conditions, the single-pass coating rate reached 68%, and the local coating thickness deviation was within 20 μm.

[0072] In Comparative Example 3, without infrared preheating, the surface temperature of the thicker parts of the 6-ton workpiece exceeded 120℃ for 20 minutes according to furnace temperature tracking, failing to meet the coating curing window requirement of 140℃. This resulted in granular powder coating, a low gloss of 55°, and an adhesion test grade of 3, which was unacceptable.

[0073] In Comparative Example 4, when the accelerator was replaced with conventional 2-methylimidazole instead of the specific catalyst of this invention, although the gloss and appearance were still acceptable, the impact resistance and flexural properties decreased. Analysis suggests that the conventional catalyst did not reduce the activation energy sufficiently or had poor selectivity, resulting in uneven crosslinking density of the coating at 140°C and deterioration in yellowing resistance (ΔE=4.2). This indicates that the conventional catalyst is prone to side reactions under low-temperature long-term baking.

[0074] In Comparative Example 5, the toughening agent was excessive. After the toughening agent content was increased to 15%, the coating exhibited excellent toughness, but the pencil hardness decreased from 2H to F, and the neutral salt spray resistance time shortened from 600h to 360h. This indicates that the excessive toughening agent formed an overly large "island structure" phase region, which disrupted the continuous density of the cross-linked network, providing a channel for water vapor penetration and sacrificing the core anti-corrosion performance.

[0075] In Comparative Example 6, aluminum phosphate was absent. After removing aluminum phosphate, the initial physical properties (hardness, impact) of the coating did not change significantly, but its salt spray resistance dropped sharply, with severe corrosion occurring after only 200 hours. This verifies the mechanism by which aluminum phosphate and the weakly acidic microenvironment generated by resin hydrolysis synergistically form an iron phosphate passivation film. The absence of this component results in the coating only having the physical shielding effect of titanium dioxide, lacking active corrosion inhibition capabilities, and failing to meet the stringent corrosion protection requirements of mining supports.

[0076] In Comparative Example 7, the extrusion temperature was too low, ranging from 80°C to 85°C. This resulted in the resin failing to fully melt and wet the pigments and fillers, leading to the presence of undispersed filler agglomerates in the system. These defects became crack initiation points under bending stress and impact, causing the impact and bending tests to fail, and resulting in a coating with low gloss and a rough appearance.

[0077] In summary, after determining the specific components and proportions of the low-temperature curing powder coating provided by this invention, further measures are needed to strictly control the particle size range of the low-temperature curing powder coating, perform rapid cooling during melt extrusion, and preheat areas with thicker walls during coating. These measures are essential to ultimately solve the problems of poor corrosion resistance and wear resistance caused by coating failure in mining hydraulic supports, reduce the curing temperature of the low-temperature curing powder coating, and increase the throughput of the coating line. The technical solution provided by this invention is not a simple sum of individual steps, but rather an inseparable technical whole formed by selecting specific accelerator chemical structures, strictly controlling particle size distribution ranges, compensating for the heat capacity of thick workpieces through infrared radiation preheating, and utilizing the electrochemical synergistic passivation effect of aluminum phosphate and the resin matrix. The absence or deviation of any single element will lead to significant deterioration in the coating's adhesion, corrosion resistance, or mechanical strength.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A method for preparing a low-temperature curing powder coating, comprising the following steps: Preparation of powder coating raw materials: Weigh the following raw materials according to the following weight parts: 50-65 parts carboxylated polyester resin, 4-6 parts curing agent, 0.4-0.6 parts curing accelerator, 5-10 parts pigment, 20-40 parts filler, 0.5-1 part leveling agent, 1-2 parts high-temperature yellowing resistant agent, and 0.5-1 part toughening agent; wherein, the curing agent is triglycidyl isocyanurate, the curing accelerator is methyl diethanolamine or triphenylphosphine derivative or benzyltrimethylammonium bromide, the leveling agent is polybutyl acrylate, the pigment is aluminum phosphate, the filler is titanium dioxide, the high-temperature yellowing resistant agent is a hindered phenolic primary antioxidant or a phosphite auxiliary antioxidant, and the toughening agent is CTBN modified epoxy or polyurethane elastomer; Melt-pulverization: After mixing the raw materials, melt-extrusion and rapid cooling are carried out. After pulverization, sieving and separation, the content of fine particles with a particle size of less than 10μm and the content of coarse particles with a particle size of greater than 80μm are controlled to be less than 10% respectively, to obtain low-temperature curing powder coating.

2. The method for preparing low-temperature curing powder coating according to claim 1, characterized in that: In the melting and pulverizing step, the raw materials are first mixed for 3 to 5 minutes at a speed of 1000 rpm to 1500 rpm. Then, a screw extruder is used to melt and extrude the mixed raw materials, controlling the temperature of the feeding zone to 80℃~100℃ and the temperature of the melt mixing zone to 100℃~120℃. The molten material extruded from the screw extruder die is then rapidly cooled at 10℃~20℃ and pressed into continuous sheets with a thickness of 1 mm~2 mm. Finally, the continuous thin film is coarsely crushed using a pulverizer, finely ground and classified using an air classifier, and sieved and separated using a cyclone separator and a rotary vibrating screen to obtain the low-temperature curing powder coating.

3. The method for preparing low-temperature curing powder coating according to claim 2, characterized in that: In the melting and pulverizing step, the continuous thin sheet is first coarsely pulverized using a pulverizer, and then finely ground and classified using an air classifier mill to obtain finely ground powder with a median particle size D50 of 30μm to 60μm; then the finely ground powder is sieved and separated sequentially using a cyclone separator and a rotary vibrating screen, controlling the content of fine particles with a particle size less than 10μm and the content of coarse particles with a particle size greater than 80μm in the finely ground powder to be less than 10% respectively, to obtain the low-temperature curing powder coating.

4. The method for preparing low-temperature curing powder coating according to claim 3, characterized in that: The carboxylated polyester resin has an acid value of 32 mgKOH / g to 40 mgKOH / g and a glass transition temperature of 60℃ to 65℃.

5. A low-temperature curing powder coating, characterized in that: The low-temperature curing powder coating is prepared by the preparation method described in any one of claims 1 to 4.

6. The low-temperature curing powder coating according to claim 5, characterized in that: The curing temperature of this low-temperature curing powder coating is 140℃.

7. A method for coating a mining hydraulic support with the low-temperature curing powder coating as described in claim 5 or 6, comprising the following steps: Provides the aforementioned low-temperature curing powder coating; Electrostatic spraying: The low-temperature curing powder coating is electrostatically sprayed onto the mining hydraulic support under an electrostatic spraying voltage of 60 KV to 90 KV, and the coating thickness is controlled to be 60 μm to 150 μm. Crosslinking and curing: The mining hydraulic support after electrostatic spraying is placed on the coating line of the curing oven, and the mining hydraulic support is baked by infrared radiation and hot air convection to heat the surface of the mining hydraulic support to 130℃~150℃, so that the low-temperature curing powder coating is crosslinked and cured on the mining hydraulic support.

8. The painting method for a mining hydraulic support according to claim 7, characterized in that: The electrostatic spraying step also includes first performing shot blasting and sandblasting to remove rust on the mining hydraulic support before spraying the low-temperature curing powder coating, and then placing the mining hydraulic support in a preheating furnace to preheat at a temperature of 80℃~110℃ for 10 min~20 min.

9. The painting method for a mining hydraulic support according to claim 8, characterized in that: The cross-linking and curing step includes: setting a gas infrared radiator at the entrance of the curing oven coating line to preheat the thick-walled area of ​​the mining hydraulic support after electrostatic spraying, and then sending it to the curing oven coating line to bake at a temperature of 190℃~220℃ for 120 min~240 min to cross-link and cure the low-temperature curing powder coating on the mining hydraulic support.