A composite heat-retaining heating system for industrial equipment surfaces

Abstract

The present application relates to the technical field of industrial equipment heat preservation and heating, and particularly relates to a composite heat preservation and heating system for the surface of industrial equipment. The system comprises a carbon fiber heating layer, an intermediate heat preservation layer and a carbon fiber protection layer; the intermediate heat preservation layer is aerogel composite felt cloth, hydroxyl groups are introduced on the surface of glass fiber through plasma surface activation treatment, so that the aerogel and the fiber are chemically bonded through Si-O-Si covalent bond; vacuum assisted impregnation method is used to uniformly fill silica sol inside the fiber felt, and the nano-porous structure is reserved after supercritical CO2 drying. The present application realizes the heat preservation performance of low thermal conductivity at low density, overcomes the defects of weak interface bonding of the existing aerogel, difficult to balance density and thermal conductivity, insufficient flexibility and connection reliability of metal resistance wire, and improves the whole life cycle heat preservation reliability of industrial pipelines and other equipment in severe cold environment.

Description

Technical Field

[0001] This invention relates to the field of industrial equipment insulation and heating technology, and in particular to a composite insulation and heating system for the surface of industrial equipment. Background Technology

[0002] In industries such as petroleum, chemical, and power, heat tracing and insulation of pipeline equipment are key technical aspects for ensuring production safety, preventing media from solidifying or reaching excessive viscosity, and avoiding pipeline freezing and blockage. Especially in low-temperature environments during winter or long-distance transportation conditions, equipping pipelines with reliable heating and insulation systems to maintain them within a suitable temperature range is an important technical means to ensure the stable operation of industrial plants around the clock.

[0003] Currently, heat tracing and insulation systems used in industrial pipelines typically employ a composite structure of "electric heating element + insulation layer + protective layer." The electric heating element often uses metal resistance wire (such as self-regulating heating cables and constant power heating cables), while the insulation layer commonly uses materials such as rock wool, aluminum silicate fiber felt, or glass wool. The outer protective layer is usually galvanized iron or aluminum sheet. Aerogel, due to its extremely low thermal conductivity and good flexibility, has gained attention in recent years and is considered one of the most promising industrial insulation materials.

[0004] However, existing aerogel composite insulation systems for industrial pipelines have the following technical shortcomings that urgently need to be addressed: Existing aerogel composite felts are typically made by immersing glass fiber felt in silica sol, followed by gelation and drying before direct use. Glass fiber has a smooth surface and strong chemical inertness, allowing the aerogel to adhere to the fiber surface solely through physical binding. However, in actual industrial pipeline use, the pipeline experiences periodic temperature fluctuations (typically -20℃ to 200℃) during the transport of high-temperature media and maintenance shutdowns. Simultaneously, the axial thermal expansion and contraction of the pipeline exerts repeated tensile and compressive stresses on the insulation layer. This temperature-mechanical composite cycle applies repeated shear stress at the fiber-aerogel interface. After hundreds to thousands of cycles, microcracks appear at the interface and gradually propagate, causing the aerogel to pulverize and detach from the fiber surface, resulting in a significant decrease in insulation performance.

[0005] Meanwhile, to meet the requirements of industrial pipelines for insulation materials to be flexible (to adapt to pipe bends, valves, and other irregularly shaped parts) and lightweight, existing technologies often require the use of high-density materials (>250kg / m³). 3 Aerogel felts can be used, but this leads to an increased contribution of solid-state thermal conductivity, resulting in a high thermal conductivity (typically >0.018 W / m·K), making it difficult to achieve the synergistic optimization of "low density and low thermal conductivity". If the density is reduced, the mechanical properties of the felt deteriorate, and the interfacial bonding becomes more fragile.

[0006] Furthermore, the connection points between the metal resistance wire (such as nickel-chromium alloy wire, copper wire, etc.) used as the heating element and the external conductors are usually welded or crimped. Under long-term thermal cycling and pipeline vibration (medium turbulence, pressure pulsation, and mechanical equipment vibration transmission), the connection points are prone to increased contact resistance, localized overheating, and even faults such as loose connections and open circuits, seriously affecting the service life and safety of the heat tracing system. At the same time, the metal resistance wire generates a certain amount of electromagnetic interference during operation, which may adversely affect the signal acquisition of nearby instruments.

[0007] Therefore, the present invention aims to provide a composite thermal insulation and heating system for the surface of industrial equipment to solve the problems existing in the prior art, such as aerogel interface debonding, contradiction between density and thermal conductivity, poor flexibility of metal resistance wire and low connection reliability. Summary of the Invention

[0008] In view of this, the present invention provides a composite thermal insulation and heating system for the surface of industrial equipment, which at least partially solves the problems existing in the prior art.

[0009] According to one aspect of the present invention, a composite thermal insulation and heating system for the surface of industrial equipment is provided, comprising, arranged sequentially from the inside out: The carbon fiber heating layer includes a carbon fiber filament heating element. The two ends of the carbon fiber heating layer are provided with electrode lead-out structures. The electrode lead-out structures include two parallel tin-plated copper wires embedded in the carbon fiber filaments. The tin-plated copper wires and the carbon fiber are filled with high-temperature resistant conductive adhesive. The intermediate insulation layer is an aerogel composite felt, which is composed of glass fiber needle-punched felt and silica aerogel, and the preparation method includes the following steps: Step a: Perform plasma surface activation treatment on the glass fiber needle-punched felt to introduce hydroxyl groups on the glass fiber surface, enhance the chemical bonding between the fiber and the silica aerogel, and improve the hydrophilicity of the fiber surface; Step b: Use vacuum-assisted impregnation to fill the interior of the treated glass fiber needled felt with silica sol, and obtain wet gel composite felt through gelation. Step c: The wet gel composite felt is aged and then immersed in an ethanol solution of hexamethyldisilazane for in-situ hydrophobic modification. Step d: The hydrophobically modified wet gel composite felt is dried with supercritical CO2 to obtain aerogel composite felt; Carbon fiber protective layer: The carbon fiber protective layer covers the outer surface of the intermediate insulation layer to protect the intermediate insulation layer and provide structural strength and weather resistance; The composite insulation and heating system is installed on the outer surface of industrial equipment for heat tracing and insulation.

[0010] Furthermore, in the carbon fiber heating layer: the carbon fiber filament heating element is a 12K large bundle of carbon fiber filaments, and the heating element is covered with an insulating protective layer; the position where the tin-plated copper wire passes through the carbon fiber protective layer is provided with a sealing structure. The carbon fiber heating layer, intermediate insulation layer, and carbon fiber protective layer are bonded together by high-temperature pressure-sensitive adhesive or hot-pressed composite.

[0011] Furthermore, step a includes: Step a.1: Place the glass fiber needle-punched felt in the vacuum chamber of the plasma treatment machine and evacuate it to 10~50 Pa; Step a.2: Introduce oxygen to a chamber pressure of 50~100Pa and treat for 5 minutes at a radio frequency power of 200W to introduce hydroxyl groups onto the glass fiber surface.

[0012] Furthermore, in the aerogel composite felt of the intermediate insulation layer, the raw materials for preparing silica aerogel include, by mass parts: 100 parts of tetraethyl orthosilicate; 60-100 parts ethanol; 20-60 parts deionized water; 3-8 parts of hexamethyldisilazane; 0.1 to 0.5 parts of acidic catalyst; 0.2 to 1.0 parts of alkaline catalyst.

[0013] Furthermore, the raw materials for preparing silica aerogel include, by mass parts: 100 parts of tetraethyl orthosilicate; 80 parts ethanol; 40 parts deionized water; 5 parts of hexamethyldisilazane; 0.3 parts of acidic catalyst; 0.5 parts of alkaline catalyst; The acidic catalyst is 0.1 mol / L hydrochloric acid, and the basic catalyst is 25% ammonia water by mass.

[0014] Furthermore, in the preparation method of the aerogel composite felt of the intermediate insulation layer, the vacuum-assisted impregnation method includes the following sub-steps: Step b.1: Place the plasma-activated glass fiber needled felt in a vacuum-assisted impregnation tank, evacuate to below -0.095 MPa, and maintain for 5 to 15 minutes; Step b.2: Inject the sol and break the vacuum, soak for 20-40 minutes, remove and roll to squeeze out excess sol; Step b.3: Repeat steps b.1 and b.2 at least once.

[0015] Furthermore, in the preparation method of the aerogel composite felt of the intermediate insulation layer: The sol corresponding to silica aerogel is prepared by the following method: Tetraethyl orthosilicate, ethanol and deionized water are mixed, and an acidic catalyst is added first. Acidic hydrolysis is carried out under the conditions of pH 2~3 and temperature 40~60℃. Then, an alkaline catalyst is added, the pH is adjusted to 7-8, and alkaline polycondensation is carried out to obtain silica sol. The aging process is as follows: the wet gel composite felt obtained after vacuum-assisted gradient impregnation is immersed in the aging solution and aged at 40~60℃ for 12~36 hours; the aging solution is composed of ethanol and tetraethyl orthosilicate, wherein the volume ratio of ethanol to tetraethyl orthosilicate is (8~12):1. The in-situ hydrophobic modification steps are as follows: the aged wet gel composite felt is immersed in an ethanol solution of hexamethyldisilazane, the mass fraction of hexamethyldisilazane in the ethanol solution is 3~8%, the immersion temperature is 50~70℃, and the immersion time is 4~8 hours.

[0016] Furthermore, in the preparation method of the aerogel composite felt of the intermediate insulation layer: The conditions for supercritical CO2 drying are: temperature 40~50℃, pressure 10~15MPa, supercritical state maintained for 2~4 hours, and pressure reduction rate of 0.03~0.08MPa / min; Furthermore, after supercritical CO2 drying, a hot-pressing shaping step is also included: the dried aerogel composite felt is hot-pressed at 140~160℃ and 0.005~0.02MPa for 5~15 minutes.

[0017] Furthermore, the areal density of glass fiber needle-punched mat is 400~700 g / m². 2 The fiber diameter is 5~15μm, and the glass fiber needle-punched felt has a gradient density structure: the fiber density of the upper and lower surface layers is higher than that of the middle layer.

[0018] Furthermore, it also includes a temperature controller, which is electrically connected to the electrode lead-out structure of the carbon fiber heating layer, and is used to control the power supply or de-energization of the carbon fiber heating layer based on the temperature signal on the surface of the industrial equipment.

[0019] The technical solution of the present invention has at least the following beneficial effects: First, in the preparation method of the intermediate insulation layer of this invention, "plasma surface activation treatment is performed on glass fiber needle-punched felt to introduce hydroxyl groups." This treatment generates active functional groups on the originally chemically inert glass fiber surface. In the subsequent aging step, the hydroxyl groups on the fiber surface undergo a co-condensation reaction with the silanol groups of silica sol to generate Si-O-Si covalent bonds, thereby constructing a molecular-scale chemical bonding interface between the aerogel network and the fiber reinforcement. Compared with the weak bonding method of aerogels in the prior art, which relies only on physical friction and mechanical encapsulation to adhere to the fiber surface, it is extremely sensitive to the interfacial shear stress caused by temperature fluctuations (-20℃~200℃) and thermal expansion and contraction of industrial pipelines, and is prone to microcrack propagation, powdering and detachment. This invention improves the interfacial peel strength through chemical anchoring, so that the insulation layer maintains structural integrity and stable insulation performance after hundreds to thousands of thermal-mechanical composite cycles. This overcomes the technical defect of existing aerogel felt fabrics, which suffer from a weak interfacial bonding mechanism that leads to a continuous decline in thermal insulation performance during service, and systematically improves the reliability of industrial equipment insulation and heating systems throughout their entire life cycle.

[0020] Furthermore, this invention enhances the hydrophilicity of the fiber surface by introducing hydroxyl groups during plasma surface activation treatment of the glass fiber needle-punched felt. In the subsequent process of "filling the interior of the treated glass fiber needle-punched felt with silica sol using a vacuum-assisted impregnation method," the increased hydrophilicity of the fiber surface and the pressure difference driven by the vacuum-vacuum transition create a progressive synergistic effect. Regarding the mechanism of action, in existing atmospheric pressure impregnation processes, due to the hydrophobicity of the glass fiber surface and the high resistance to sol spreading, the flow of the sol in the fiber gaps mainly relies on capillary effects. The uneven pore size distribution of the fiber felt leads to the sol preferentially filling large pores, while nanoscale pores are difficult to fully penetrate, ultimately resulting in polarization defects such as an overly dense surface and internal voids. This invention significantly reduces the spreading energy barrier of the sol on the fiber surface through hydrophilic modification, enabling the sol to rapidly wet the fiber surface and penetrate into the micropores with lower flow resistance under the pressure difference driven by the vacuum transition. The pre-removal of gas from the fiber gaps by the vacuum environment further eliminates the limitation of gas resistance on the sol penetration depth. The synergistic effect of hydrophilic spreading and vacuum driving further enhances the sol-filling rate throughout the thickness of the fiber felt, and the pore size distribution of the nanoframework remains highly uniform along the thickness direction after gelation. This uniform filling mechanism lays a crucial foundation for preform uniformity in subsequent supercritical CO2 drying, which fully preserves the nanoporous structure and achieves extremely low thermal conductivity at low density, overcoming the filling polarization defects caused by insufficient wettability in existing impregnation processes.

[0021] Simultaneously, this invention achieves a simultaneous reduction in the density and thermal conductivity of the insulation layer through the progressive synergy of two technical features: "filling the interior of glass fiber needle-punched felt with silica sol using a vacuum-assisted impregnation method" and "supercritical CO2 drying of the hydrophobically modified wet gel composite felt." Specifically, vacuum-assisted impregnation utilizes the pressure difference generated by the vacuuming and devastating process to drive the sol to uniformly fill the fiber felt along its entire thickness, eliminating the gradient defects of excessive surface density and internal voids caused by uneven capillary flow in existing atmospheric pressure impregnation processes, thus laying a uniform wet gel preform foundation for low density. Furthermore, supercritical CO2 drying utilizes its physical properties of having no gas-liquid interface and zero surface tension, protecting the nanoscale pore network in the wet gel preform from capillary pressure damage during solvent removal, thus preserving the high porosity structure of the wet gel stage. In contrast, existing atmospheric pressure evaporation drying generates capillary contraction forces of up to several megapascals due to solvent surface tension, forcing the nanopore walls to collapse and the framework to densify, resulting in a final material density that is forced to increase to 250 kg / m³. 3 The above, and the thermal conductivity remains high due to the enhanced thermal conduction of the solid. This invention, through a synergistic mechanism of "uniform filling - complete retention," achieves a thermal conductivity of 180~220 kg / m³. 3 Achieving thermal conductivity ≤0.015W / m·K at low bulk density overcomes the structural contradiction of difficulty in achieving both low density and low thermal conductivity in existing technologies, providing greater engineering margin for lightweight design of industrial pipeline insulation systems.

[0022] Furthermore, this invention employs a "carbon fiber filament heating element" as the heating element, and sets an electrode lead-out structure at its end consisting of "two parallel tin-plated copper wires pre-embedded in the carbon fiber filament and filled with high-temperature resistant conductive adhesive." Regarding the heating element itself, the carbon fiber filament is a fibrous, flexible conductive material. Its inherent flexibility allows it to adapt to the bending and thermal expansion and contraction deformation of industrial pipeline surfaces, avoiding the risk of fatigue fracture of metal resistance wires due to rigid constraints under repeated strain. Simultaneously, in the energized state, the graphite microcrystalline structure within the carbon fiber induces violent vibrations of the carbon atom lattice under electric field excitation. The carbon atoms collide and rub against each other, efficiently converting electrical energy into heat energy. Moreover, the pure resistance heating of the carbon fiber does not generate an alternating magnetic field, fundamentally eliminating the potential interference of electromagnetic radiation generated by energized metal wires on the signal acquisition of nearby instruments. Regarding the electrical connection structure, tin-plated copper wires are pre-embedded parallel to the interior of carbon fiber filaments, and filled with high-temperature conductive adhesive. This transforms the traditional "point contact" or "line contact" of welding or crimping into a large-area "body contact," and solidifies the wire-carbon fiber-insulation layer into a mechanically continuous whole. Compared to existing metal resistance wires that use soldering or mechanical crimping, which are prone to gradually increasing contact resistance, local overheating, and even broken connections under the combined effects of pipeline vibration and thermal cycling due to thermal expansion mismatch of heterogeneous materials and fretting wear, the integrated pre-embedded connection structure of this invention fundamentally eliminates stress concentration points and microscopic relative displacement space at heterogeneous interfaces, ensuring stable contact resistance of the power supply circuit during long-term service. Thus, it systematically solves the multi-mode failure problem of the heating element body and electrical connection links under harsh industrial conditions, achieving synergistic long-term reliability of electrical connection and structural integrity. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Fig. 1 This is an exploded structural diagram of a composite thermal insulation and heating system for the surface of industrial equipment according to an embodiment of this application; Fig. 2 The images show the microstructure of the fiber-aerogel interface in this application, with scale bars of 50 μm. Wherein: a—interface microstructure of the comparative example before 800 cycles of thermal-mechanical composite, b—interface microstructure of the comparative example after 800 cycles of thermal-mechanical composite, c—interface microstructure of Example 5 of the present invention before 800 cycles of thermal-mechanical composite, d—interface microstructure of Example 5 of the present invention after 800 cycles of thermal-mechanical composite. Fig. 3 This is a schematic diagram of the composite thermal insulation and heating system used in an industrial pipeline insulation application scenario, according to another embodiment of this application. Detailed Implementation

[0025] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0026] It should be noted that, in the absence of conflict, the following embodiments and features can be combined with each other; and, based on the embodiments of this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.

[0027] It should be noted that various aspects of embodiments within the scope of the appended claims are described below. It will be apparent that the aspects described herein can be embodied in a wide variety of forms, and any particular structure and / or function described herein is merely illustrative. Based on this disclosure, those skilled in the art will understand that one aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number of aspects set forth herein can be used to implement the device and / or practice the method. Additionally, this device and / or method can be implemented using structures and / or functionalities other than one or more of the aspects set forth herein.

[0028] As one embodiment of the present invention, such as Figs. 1 to 3 As shown, a composite insulation and heating system for the surface of industrial equipment is provided. This composite insulation and heating system is installed on the outer surface of industrial pipelines, storage tanks or other industrial equipment for heat tracing and insulation of the equipment.

[0029] Specifically, the composite insulation and heating system includes, from the inside out, the following components arranged sequentially: I. Carbon fiber heating layer 1 The carbon fiber heating layer 1 includes a carbon fiber filament heating element. Electrode lead-out structures 10 are provided at both ends of the carbon fiber heating layer 1. The electrode lead-out structures 10 include two parallel tin-plated copper wires embedded in the carbon fiber filaments. High-temperature resistant conductive adhesive is filled between the tin-plated copper wires and the carbon fiber.

[0030] In carbon fiber heating layer 1: the carbon fiber heating element is a 12K large-tow carbon fiber filament, and the heating element is covered with an insulating protective layer. A sealing structure is provided where the tin-plated copper wire passes through the carbon fiber protective layer 3.

[0031] The carbon fiber heating layer 1 serves as the active heat source for this system, and its structural design fully considers the special operating conditions required for heat tracing and insulation of industrial pipelines. The 12K large-tow carbon fiber filaments consist of 12,000 monofilaments, each with a diameter of approximately 7 μm, exhibiting excellent flexibility and weavability. The heating element can be fabricated into a flexible heating cloth with a thickness of 0.5~1.5 mm and a sheet resistivity of 3~10 Ω / m. 2 The required heating power can be generated under a 12~24V DC power supply. Carbon fiber filaments are woven into a cloth-like heating element using plain or twill weave, ensuring uniform distribution of the heating element in a two-dimensional plane and avoiding the problem of excessively high local heat flux density that occurs with serpentine arrangements of metal resistance wires. In electrode lead-out structure 10 (… Fig. 1 (This is just a schematic diagram of the orientation of the electrode lead-out structure 10). Two tin-plated copper wires are pre-embedded in parallel inside the carbon fiber filament braided structure. The wires are embedded during the braiding process, rather than being surface-welded after the fabric is formed.

[0032] The high-temperature conductive adhesive filling the space between the conductor and the carbon fiber is a single-component or two-component existing silicone conductive silver paste, with a long-term temperature resistance of ≥200℃. After curing, the conductive adhesive forms a flexible conductive bonding layer, transforming the electrical connection between the conductor and the carbon fiber from the "point contact" of traditional welding to a large-area "body contact," with a contact resistance of <0.1Ω, and no contact resistance drift occurs under repeated bending and vibration conditions.

[0033] The insulating protective layer is made of 0.05~0.2mm thick polyimide film or silicone rubber coating with a dielectric strength ≥3kV, providing electrical insulation protection while maintaining the overall flexibility of the heating element. The sealing structure uses potting compound to seal the wire outlet, preventing moisture or corrosive gases that may be present in the industrial environment from seeping into the heating layer through the wire gaps.

[0034] 12K large-tow carbon fiber filaments are inherently fibrous, flexible, and conductive materials with a breaking elongation of approximately 1.5% to 2.0%. They can withstand bending radii of ≤5mm without breaking. This intrinsic flexibility allows the heating element to adapt to the bending and deformation of industrial pipe surfaces and the thermal expansion and contraction of the pipes, avoiding the fracture failure of metal resistance wires due to rigid constraints and low fatigue limits under repeated strain. In terms of the electrothermal conversion mechanism, carbon fibers are composed of graphite microcrystals aligned along the fiber axis. Under the influence of an electric field, the migration of charge carriers between the graphite microcrystal layers induces high-frequency vibrations in the carbon atom lattice. The collisions and friction between carbon atoms release electrical energy in the form of Joule heat. Because carbon fibers are a purely resistive load, the current flows along the fiber axis when energized, without generating an alternating magnetic field. Therefore, there is no electromagnetic radiation interference during operation, and it will not adversely affect the signal acquisition of nearby instruments and other electronic equipment.

[0035] In terms of the electrode lead-out structure, the pre-embedded wires combined with the high-temperature conductive adhesive filling method fundamentally avoids the micro-cracks that occur at the welding interface when traditional metal resistance wires are led out using soldering or mechanical crimping, leading to increased contact resistance and causing local overheating or even open circuit failure. This invention pre-embeds the wires in a carbon fiber braided structure and fills them with conductive adhesive. The elastic modulus of the cured conductive adhesive is much lower than that of the solder, and it can absorb the difference in thermal expansion through its own elastic deformation, allowing the contact resistance to remain at its initial value after hundreds of temperature cycles.

[0036] The aforementioned carbon fiber heating layer can be manufactured in the following manner: First, multiple bundles of 12K carbon fiber filaments are used as warp yarns, and a tin-plated copper wire is pre-embedded on the inner side of each of the two edges along the width direction of the heating element. The two wires extend continuously along the length direction of the heating element (i.e., the axial direction of the pipe) and remain parallel without crossing. The wires are only arranged in the side area, and no wires are laid in the heating area inside the heating layer.

[0037] Then, multiple bundles of carbon fiber filaments are used as weft yarns, repeatedly weaving and plain-weave between the two side conductors. The weft carbon fiber filaments traverse the entire width of the heating element, with the two ends of each bundle adhering to the sidewalls of the two side conductors respectively. At the same time, all warp carbon fiber filaments are pressed together from top to bottom, so that all warp carbon fiber filaments inside the heating layer are physically interconnected through the weft carbon fiber filaments. This forms a current conduction path: the current is input from one side conductor, conducted along the sidewall adhering point to the side warp carbon fiber, then evenly distributed through the laterally arranged weft carbon fiber to all warp carbon fibers inside the heating layer, and finally converges to the other side conductor, realizing synchronous energization and uniform surface heating of the entire heating element.

[0038] After weaving, high-temperature conductive adhesive is injected using a vacuum potting method. This allows the adhesive to fully penetrate the gaps between the copper wires and the side carbon fibers, the interlacing pores of the warp and weft carbon fibers, and the gaps on the fiber surface. After curing, a continuous flexible conductive network is formed at the interlacing points of the carbon fibers and between the wires and carbon fibers. Finally, an insulating protective layer is wrapped around the surface of the heating element, leaving only the terminals of two wires exposed. The wire exit points are sealed with potting adhesive.

[0039] II. Intermediate Insulation Layer 2 The intermediate insulation layer 2 is an aerogel composite felt, which is composed of glass fiber needle-punched felt and silica aerogel.

[0040] Specifically, the areal density of fiberglass needle-punched mat is 400~700 g / m². 2 The fiber diameter is 5~15μm. Preferably, the glass fiber needle-punched felt has a gradient density structure: the fiber density of the upper and lower surface layers is higher than that of the middle layer.

[0041] In industrial pipeline insulation systems, the outer layer of aerogel felt needs to withstand the bonding pressure with adjacent layers, bending stress, and potential abrasion, requiring high surface strength and flatness. The middle layer primarily serves as an insulation layer to block heat flow, and its solid-state heat conduction should be minimized while maintaining structural integrity. Therefore, the aforementioned glass fiber needle-punched felt employs a gradient density structure: the surface layer uses finer fibers (5~8μm) combined with a higher areal density (200~250g / m²). 2 The finer fibers provide a larger specific surface area and a denser fiber network, which is beneficial for forming a smooth and flat felt surface, and provides sufficient local compressive strength when combined with the heating layer and the protective layer; the intermediate layer uses coarser fibers (10~15μm) with a lower areal density (150~200g / m²). 2 The fewer overlap points between coarse fibers and the longer solid conduction path effectively reduce the contribution of solid heat conduction. This gradient structure is strongly coupled with the subsequent vacuum-assisted impregnation process: if atmospheric pressure impregnation is used, the sol cannot overcome the flow resistance of the high-density area on the surface, resulting in insufficient filling of the intermediate layer; however, the vacuum-assisted impregnation of this invention can remove the gas from each layer by evacuating the vacuum, and when the vacuum is broken, the pressure difference drives the sol to penetrate the layer with resistance difference, achieving uniform filling of the entire thickness.

[0042] Specifically, in the aerogel composite felt of the intermediate insulation layer 2, the raw materials for preparing silica aerogel include, by mass parts: 100 parts of tetraethyl orthosilicate; 60-100 parts ethanol; 20-60 parts deionized water; 3-8 parts of hexamethyldisilazane; 0.1 to 0.5 parts of acidic catalyst, wherein the acidic catalyst is 0.1 mol / L hydrochloric acid; 0.2~1.0 parts of alkaline catalyst, which is ammonia water with a mass fraction of 25%.

[0043] The above-mentioned raw material ratio range differs from existing aerogel preparation technologies in the following key ways: First, in terms of composition, this invention explicitly selects hexamethyldisilazane (HMDS) as the in-situ hydrophobic modifier, rather than trimethylchlorosilane (TMCS) or post-treatment spray-on hydrophobic agents commonly used in existing technologies. TMCS generates highly corrosive HCl gas during the hydrophobic modification reaction, posing a corrosion risk to the fiber substrate and equipment, and the reaction rate is too fast to control; the modification byproduct of HMDS is neutral NH3 gas, which has no corrosive effect on equipment and fiber substrate, and the modification process is more gentle and uniform.

[0044] Furthermore, the overall characteristic of the aforementioned raw material ratio range lies in the adoption of a "high water content, moderate alcohol content" ratio strategy, which significantly distinguishes it from existing aerogel preparation technologies. Existing technologies typically control the molar ratio of water to silicon at a relatively low 3:1. This is because it is generally believed that excessive water content leads to excessively rapid sol hydrolysis, premature aggregation of silicate monomers before impregnation, and reduced impregnation penetration and gel uniformity. This invention overcomes this limitation by increasing the molar ratio of water to silicon to approximately 4:1 to 5:1 (corresponding to 20 to 60 parts of deionized water), while simultaneously adjusting the sol viscosity to a suitable range with 60 to 100 parts of ethanol. The technical advantages of this ratio strategy are: the high water content allows for more thorough hydrolysis of tetraethyl orthosilicate and more complete generation of silicate monomers. In the subsequent alkaline polycondensation stage, this facilitates the formation of a nanonetwork with a narrower particle size distribution and a more sparse and uniform framework, resulting in higher porosity and lower thermal conductivity for the same solid volume. The reason why this invention can use such a high water ratio without premature coagulation is due to the synergistic effect of the subsequent vacuum-assisted impregnation process and aging reinforcement step: vacuum-assisted impregnation uses pressure difference to drive the sol to quickly enter the interior of the fiber felt and distribute it evenly before coagulation, which greatly shortens the residence time of the sol in the tank; after the aging solution is soaked in the gel, a second TEOS grafting reinforcement is performed to make up for the possible insufficient strength of local nodes in the high water content skeleton.

[0045] Preferably, the raw materials for preparing silica aerogel include, by mass parts: 100 parts of tetraethyl orthosilicate; 80 parts ethanol; 40 parts deionized water; 5 parts of hexamethyldisilazane; 0.3 parts of acidic catalyst; 0.5 parts of alkaline catalyst.

[0046] This preferred ratio corresponds to a water to silicon molar ratio of approximately 4:1 and an alcohol to silicon molar ratio of approximately 8:1 (including ethanol generated from the hydrolysis of TEOS). Compared to existing technologies, this ratio is based on the following three synergistic considerations: First, the selection of water content (40 parts). Increasing the water-to-silicon molar ratio from the conventional 3:1 to 4:1 allows for more thorough TEOS hydrolysis, resulting in more uniform primary particle size and a sparser framework during alkaline polycondensation, which is beneficial for achieving higher porosity and lower thermal conductivity. If the water content is too low, hydrolysis will be insufficient, the framework will be too densely cross-linked, and the porosity will be limited; if the water content is too high, the sol stability will decrease, and premature coagulation will easily occur before impregnation. This invention effectively mitigates the risk of coagulation caused by high water content through rapid filling in subsequent vacuum-assisted impregnation and secondary reinforcement with aging solution.

[0047] Second, the selection of ethanol content (80 parts). An ethanol-to-silicon ratio of 8:1 is within a suitable range that balances sol viscosity, impregnation flowability, and skeletal structural stability. This allows the sol to smoothly penetrate the full thickness of the fiber mat under vacuum assistance while ensuring the gel skeleton has sufficient strength to resist drying shrinkage. If the ethanol content is too low, the sol viscosity will be too high, significantly increasing the penetration resistance in the fiber gaps and leading to uneven impregnation and filling. If the ethanol content is too high, the gel skeleton will be too sparse, exacerbating shrinkage during the drying stage, which is detrimental to maintaining low density and dimensional stability.

[0048] Third, the selection of the dosage of HMDS (5 parts) and acid-base catalysts (0.3 / 0.5 parts). The dosage of HMDS ensures sufficient hydrophobic modification of the pore walls while avoiding the residue of by-products caused by excessive use; the ratio of acid-base catalysts precisely corresponds to the pH control requirements of the acid-base two-step catalytic method, ensuring sufficient acid hydrolysis and uniform alkaline polycondensation.

[0049] Specifically, the preparation method of aerogel composite felt includes the following steps: Step a: Perform plasma surface activation treatment on the glass fiber needled felt. This introduces hydroxyl groups onto the glass fiber surface, enhancing the chemical bonding between the fiber and the silica aerogel, and improving the hydrophilicity of the fiber surface.

[0050] The core purpose of this step is to fundamentally change the chemical inertness of the glass fiber surface, laying the interfacial chemical foundation for the synergistic effect of all subsequent processes. Glass fibers (mainly composed of SiO2, Al2O3, etc.) originally have only a very small amount of silanol groups on their surface, resulting in low surface energy, poor wettability to water and polar sols, and a contact angle of approximately 90°, exhibiting a hydrophobic state. In actual use in industrial pipelines, if the aerogel adheres to the fiber surface solely through physical shrinkage and friction after curing, the interface will gradually peel off under the repeated thermal expansion mismatch stress caused by temperature cycling.

[0051] Plasma surface activation uses oxygen as the working gas to generate high-energy oxygen plasma (including O· radicals and O2) under radio frequency electric field excitation. + Plasma etching bombards the fiber surface, breaking the Si-O-Si bonds and allowing them to recombine with oxygen free radicals. This introduces a large number of highly hydrophilic active hydroxyl (-OH) functional groups onto the fiber surface. This significantly increases the surface energy and wettability of the fiber, allowing water droplets to spread rapidly rather than form beads, macroscopically manifested as a sharp decrease in the contact angle. Simultaneously, plasma etching increases the surface roughness of the fiber, further increasing the effective contact area. This process does not introduce any liquid chemical reagents or add heterogeneous components, thus preventing the formation of an additional thermally conductive layer at the interface.

[0052] Specifically, step a includes: Step a.1: Place the glass fiber needle-punched felt in the vacuum chamber of the plasma treatment machine and evacuate it to 10~50 Pa.

[0053] Step a.2: Introduce oxygen to a chamber pressure of 50~100Pa and treat for 5 minutes at a radio frequency power of 200W to introduce hydroxyl groups onto the glass fiber surface.

[0054] A vacuum level of 10–50 Pa ensures minimal residual air within the chamber, preventing nitrogen and other impurities from generating nitrogen-containing functional groups during discharge that could interfere with the hydroxyl group introduction effect. An oxygen pressure of 50–100 Pa within this vacuum range allows for stable glow discharge, achieving an oxygen plasma density of 102. 1 ~10 11 cm -3 The intensity of the treatment is sufficient to induce adequate functionalization on the fiber surface. Simultaneously, the 200W RF power treatment intensity, within 5 minutes, can drastically reduce the contact angle from approximately 90° to approximately 20°, completely transforming the fiber surface from a hydrophobic to a hydrophilic state without causing thermal damage to the fiber bulk structure or a decrease in strength due to excessive power or prolonged treatment. After treatment, the hydroxyl groups introduced onto the fiber surface do not merely possess physical adsorption properties but are covalently bonded to the silicon atoms on the fiber surface, exhibiting chemical stability and durability.

[0055] Specifically, the sol corresponding to silica aerogel is prepared by the following method: Step a.3: Mix tetraethyl orthosilicate, ethanol and deionized water, add an acidic catalyst first, and carry out acidic hydrolysis under the conditions of pH 2~3 and temperature 40~60℃.

[0056] Step a.4: Add an alkaline catalyst, adjust the pH to 7-8, and carry out alkaline polycondensation to obtain silica sol.

[0057] The aforementioned acid-base two-step catalytic method is a key precursor step in constructing a uniform nanoframework in this invention. During the acidic hydrolysis stage, at pH 2-3, the hydrolysis of tetraethyl orthosilicate follows an electrophilic substitution mechanism, with H3O... + The attack on the oxygen atom of the ethoxy group causes the Si-OC2H5 bond to break and generate a silanol group (Si-OH). The hydrolysis rate is greater than the polymerization rate, and the system mainly exists in the form of silica monomer Si(OH)4, with almost no Si-O-Si polymerization. A temperature of 40-60℃ ensures sufficient hydrolysis while avoiding excessive solvent evaporation due to high temperatures. This stage ensures that the hydrolysis products are uniformly dispersed in the solvent in monomer form, providing uniform structural units for subsequent polymerization.

[0058] During the alkaline condensation stage, ammonia is added to adjust the pH to 7-8, OH... - Extracting a proton from the silanol group, making Si-O - The increased nucleophilicity leads to rapid condensation reactions between monomers, forming primary particles linked by siloxane bonds.

[0059] Due to the high monomer dispersion during the acidic hydrolysis stage, alkaline polycondensation can be completed rapidly within 30 seconds, resulting in primary particles with a narrow particle size distribution range of 2–5 nm. This high uniformity in particle size is the chemical basis for maintaining structural consistency in the thickness direction of the subsequent gel framework. If a one-step catalytic method (acid catalysis only or alkaline catalysis only) is used, hydrolysis and polycondensation occur simultaneously, resulting in a wide primary particle size distribution (2–20 nm). Consequently, the particles in the final framework are randomly packed, the pores are unevenly distributed, and the thermal conductivity is difficult to reduce to below 0.015 W / m·K.

[0060] Step b: The silica sol is filled into the treated glass fiber needled felt using a vacuum-assisted impregnation method, and the wet gel composite felt is obtained after gelation.

[0061] In this step, vacuum-assisted impregnation and the hydrophilicity of the fiber surface obtained in step a produce a progressive synergistic effect. The plasma activation in step a reduced the fiber surface contact angle from approximately 90° to approximately 20°, significantly increasing the spreadability of the sol on the fiber surface. During the vacuum breaking process, the pressure difference between the external atmospheric pressure and the residual pressure inside the container (approximately 0.095 MPa) drives the sol to penetrate into the fiber felt, while the hydrophilic modification allows the sol to rapidly spread and wet the fiber upon contact with the fiber surface, penetrating the nanoscale fiber gaps with lower flow resistance. Simultaneously, the vacuum environment pre-emptively eliminates air from the pores of the fiber felt, removing the limitation of air resistance on the sol penetration depth. The synergy of these two factors further improves the sol filling rate.

[0062] Specifically, the vacuum-assisted impregnation method includes the following sub-steps: Step b.1: Place the plasma-activated glass fiber needled felt in a vacuum-assisted impregnation tank, evacuate to below -0.095MPa (i.e., the vacuum degree is not lower than 0.095MPa), and maintain for 5 to 15 minutes.

[0063] The purpose of maintaining the vacuum for 5-15 minutes in step b.1 above is to fully remove the air inside the fiber felt. Especially for fiber felts with a gradient density structure, the air removal rate of the dense surface layer is relatively slow, so the vacuum time needs to be appropriately extended to 10-15 minutes to ensure that each density layer reaches the required vacuum level.

[0064] Step b.2: Inject the sol and break the vacuum, soak for 20-40 minutes, then remove and roll to squeeze out excess sol.

[0065] In step b.2, soaking for 20-40 minutes after breaking the vacuum provides sufficient time for the sol to fully penetrate and wet the fiber gaps. The soaking time is positively correlated with the thickness of the fiber felt.

[0066] Step b.3: Repeat steps b.1 and b.2 at least once.

[0067] The core reason for setting up a second repeated impregnation in step b.3 is that, during the first impregnation, although the synergistic effect of vacuum and hydrophilic modification significantly improves the filling rate, a small number of tiny bubbles or weak points in the near-surface area of ​​the fiber felt will still remain due to differences in concentration gradient and flow path length in the initial stage of sol injection. After rolling, although some of the sol in the fiber felt is squeezed out, the fiber gaps are still wetted by the sol. The second vacuuming and breaking can utilize the residual sol that has already wetted the fibers and fresh sol to fill the gaps left by the first impregnation, thereby further improving the volume filling rate. This "double-filling" mechanism is particularly crucial in gradient density structures. Without the second impregnation, defects such as sol enrichment or voids are prone to occur at the interface between the high-density area on the surface of the fiber felt and the low-density area in the middle layer.

[0068] Step c: The wet gel composite felt is aged and then immersed in an ethanol solution of hexamethyldisilazane for in-situ hydrophobic modification.

[0069] The aging process is as follows: Step c.1: Immerse the wet gel composite felt obtained after vacuum-assisted gradient impregnation into an aging solution and age it at 40~60℃ for 12~36 hours. The aging solution consists of ethanol and tetraethyl orthosilicate, wherein the volume ratio of ethanol to tetraethyl orthosilicate is (8~12):1.

[0070] The aging step plays a dual role in the aerogel preparation process: "reinforcing the framework" and "forming chemical bonding interfaces." In the wet gel stage, the primary particles formed by alkaline condensation are connected into a three-dimensional network via Si-O-Si bonds. However, a large number of unreacted silanol groups (Si-OH) and some incompletely connected network nodes still exist, and the framework strength is insufficient to withstand the stress of the drying stage. Immersing the wet gel mat in a mixed aging solution composed of ethanol and TEOS allows the TEOS molecules in the solution to penetrate into the nanopores of the gel network. At a temperature of 40–60°C, they hydrolyze at the network nodes and condense with adjacent silanol groups, thickening and strengthening the originally weak "neck" connections. Ethanol serves as both a solvent and diluent for TEOS, and a volume ratio of (8–12):1 ensures a suitable TEOS concentration, providing sufficient reinforcing monomers without causing excessive polymerization and clogging of the pores due to excessive TEOS concentration. An aging temperature of 40–60°C provides the thermodynamic conditions required for the polycondensation reaction, but does not exceed the boiling point of ethanol (78.3°C) to avoid excessive solvent evaporation leading to gel cracking. An aging time window of 12–36 hours corresponds to the balance between target skeleton strength and production efficiency; thicker felts require longer aging times to ensure TEOS penetrates to the central layer.

[0071] More importantly, the aging stage is also the stage of chemical bonding formation at the fiber-aerogel interface. The hydroxyl groups introduced on the fiber surface in step a adsorb onto the silanol groups of the silica monomers in the sol via hydrogen bonds after impregnation. Under the aging temperature (40-60℃) and the catalytic condensation environment of TEOS in the aging solution, the hydrogen bonds between the Si-OH groups on the fiber surface and the silanol groups undergo dehydration condensation, transforming into stable Si-O-Si covalent bonds. This chemical bonding formation allows the aerogel network to adhere to the fiber surface through chemical anchoring rather than physical friction, improving the interfacial peel strength. The two-stage synergy of plasma activation introducing hydroxyl groups and the aging step forming Si-O-Si covalent bonds has a clear temporal progression: activation precedes bonding, and both are indispensable.

[0072] The in-situ hydrophobic modification steps are as follows: Step c.2: Immerse the aged wet gel composite felt in an ethanol solution of hexamethyldisilazane, where the mass fraction of hexamethyldisilazane in the ethanol solution is 3-8%, the immersion temperature is 50-70℃, and the immersion time is 4-8 hours.

[0073] In-situ hydrophobic modification is a key pretreatment step to ensure that the nanoframework does not collapse during the drying stage. The pore walls of the wet gel are densely covered with silanol groups, which exhibit strong hydrogen bonding adsorption between themselves and with the solvent ethanol.

[0074] If drying is carried out directly, whether using atmospheric pressure evaporation or supercritical drying, the trace amounts of water or solvent remaining on the pore wall surface after solvent removal will form menisci between the pore walls due to the hydrophilicity of the silanol groups. The capillary tension of the menisci can reach several megapascals, which is enough to cause irreversible collapse and densification of the nanoscale pore walls.

[0075] The hydrophobic modification reaction of hexamethyldisilazane (HMDS) is as follows: 2≡Si-OH+(CH3)3Si-NH-Si(CH3)3→2≡Si-O-Si(CH3)3+NH3↑. The reaction replaces the hydrophilic silanol groups on the pore wall surface with hydrophobic trimethylsilyl groups (-Si(CH3)3), changing the contact angle from a hydrophilic state of about 20° to a hydrophobic state of 130°~150°.

[0076] The HMDS concentration in the ethanol solution is set at 3-8% (i.e., 3-8 wt%): if the concentration is too low, the modification is insufficient, and residual silanol groups remain on the pore walls; if the concentration is higher than 8 wt%, the reaction is too vigorous, and the rapid generation of NH3 in the pores may lead to local pressure accumulation and microcracks in the gel. The soaking temperature is 50-70℃, slightly higher than room temperature, to moderately increase the reaction rate, and the soaking time is 4-8 hours to ensure that HMDS fully penetrates the entire thickness of the felt and reacts completely with the silanol groups on the pore walls. In-situ hydrophobic modification differs from simple surface spraying hydrophobic treatment: the latter only forms a hydrophobic film on the surface of the felt, while the internal pore walls remain hydrophilic and will still collapse when drying; while this invention achieves overall hydrophobic modification from the inside out by soaking the HMDS in the solution to penetrate all pore wall surfaces of the full-thickness gel.

[0077] Step d: The hydrophobically modified wet gel composite felt is dried with supercritical CO2 to obtain aerogel composite felt.

[0078] The conditions for supercritical CO2 drying are: temperature 40~50℃, pressure 10~15MPa, maintaining the supercritical state for 2~4 hours, and pressure reduction rate of 0.03~0.08MPa / min.

[0079] There is a progressive synergy between supercritical CO2 drying and the hydrophobicity of the pore walls obtained in step c. The core advantage of supercritical CO2 drying lies in the fact that when the temperature of CO2 exceeds 31.1℃ and the pressure exceeds 7.38MPa, it enters a supercritical state. At this point, CO2 possesses both the diffusivity of a gas and the solubility of a liquid, but the gas-liquid interface disappears, and the surface tension is zero. After the solvent ethanol in the wet gel channels is replaced by liquid CO2, the CO2 transforms from a liquid state to a supercritical state through heating and pressurization, and then slowly depressurizes from the supercritical state to a gaseous state for discharge. The entire process lacks a gas-liquid interface, therefore the capillary pressure is zero. The nanopore walls are not subject to meniscus capillary contraction forces, the framework can completely retain the pore structure of the wet gel stage, and the porosity remains stable.

[0080] The synergistic effect of supercritical drying and in-situ hydrophobic modification in step c is reflected in the following: During the circulation of supercritical CO2, CO2 molecules diffuse in the nanopores. Although the surface tension is zero in the supercritical state, if the surface of the pore wall is still hydrophilic when the CO2 gas is discharged from the drying vessel under reduced pressure, the adsorbed small amount of water will condense into local droplets or liquid films due to pressure and temperature changes during the depressurization stage, producing a micro-capillary effect.

[0081] The hydrophobic modification in step c ensures that the pore walls remain highly hydrophobic throughout the drying process (including the depressurization stage), preventing water molecules from adhering to and condensing on the pore walls, thus completely eliminating any potential capillary effect from residual moisture during depressurization. Furthermore, the NH3 released during the HMDS modification reaction reacts with CO2 and residual hydrochloric acid in a supercritical CO2 environment to form ammonium carbonate, which is soluble in supercritical CO2 and discharged with the CO2 circulation. This prevents the accumulation and blockage of insoluble salts in the drying vessel pipes, further improving drying efficiency and long-term equipment stability.

[0082] The critical point of CO2 is 31.1℃ and 7.38MPa. When the pressure drops from the supercritical state to below the critical point, if the depressurization rate is too fast (e.g., >0.1MPa / min), the phase transition of CO2 from the supercritical state to the gaseous state is too rapid, which may generate instantaneous density and pressure gradients in the pores, impacting the nanoframework. A slow depressurization of 0.03~0.08MPa / min allows CO2 sufficient time to gradually escape from the nanopores through molecular diffusion, maintaining a dynamic balance between the pressure inside the micropores and the external environment, thus preventing the generation of microcracks and local collapse.

[0083] Furthermore, after supercritical CO2 drying, a hot-pressing shaping step is also included: Step e: Hot-press the dried aerogel composite felt at 140~160℃ and 0.005~0.02MPa for 5~15 minutes.

[0084] The aerogel composite felt obtained through supercritical drying is fluffy after drying, but the internal skeleton still contains nanoscale structural micro-stresses, which are residual stresses that are difficult to completely eliminate even after slow pressure reduction. If used directly, these micro-stresses will gradually release during subsequent lamination, installation, and use, leading to dimensional instability of the felt and the accumulation and propagation of microcracks. The temperature (140~160℃) of the hot-pressing shaping step is set above the glass transition temperature (approximately 120℃) of the silica aerogel skeleton. Near and above the glass transition temperature, the three-dimensional network skeleton of the aerogel has a certain molecular chain segment mobility and rheological properties. Applying a slight pressure of 0.005~0.02MPa at this temperature is sufficient to fully release the residual stress in the skeleton through conformational adjustment of the molecular chain segments, without compressing and destroying the nanoporous structure due to excessive pressure. After hot pressing for 5~15 minutes, the skeleton is gradually cooled to room temperature, and after new expansion and stress release, it is re-fixed. This treatment not only does not damage the nanoporous structure, but also significantly improves the dimensional stability and resistance to warping deformation of the felt during use due to the elimination of residual stress. The hot-pressing temperature should not exceed 180℃, otherwise the organic methyl hydrophobic groups in the aerogel skeleton may undergo thermal decomposition, affecting the final hydrophobic properties.

[0085] III. Carbon fiber protective layer 3 Carbon fiber protective layer 3 covers the outer surface of intermediate insulation layer 2, serving to protect intermediate insulation layer 2 and provide structural strength and weather resistance.

[0086] The carbon fiber heating layer 1, the intermediate insulation layer 2, and the carbon fiber protective layer 3 are bonded together by high-temperature pressure-sensitive adhesive or hot-pressed composite.

[0087] The carbon fiber protective layer 3 is made of plain-weave carbon fiber cloth with a thickness of 0.3~1.0mm and an areal density of 200~300g / m³. 2 Carbon fiber cloth uses PAN-based or pitch-based carbon fiber as raw material, with a single filament tensile strength ≥3.5GPa and an elastic modulus ≥230GPa. While providing high specific strength and high specific modulus, its density is only 1.7~1.8g / cm³. 3This will not significantly increase the overall weight of the system. The protective layer has three functions: First, physical protection. The high strength and high modulus of the carbon fiber cloth effectively resist the punctures, scratches, and impacts that the outer surface of the insulation layer may suffer during the installation and use of industrial pipelines, preventing local damage and powdering of the internal aerogel composite felt. Second, structural integrity enhancement. After the carbon fiber cloth is bonded to the intermediate insulation layer 2 and the carbon fiber heating layer 1 with high-temperature pressure-sensitive adhesive or hot-pressed, it forms an integrated composite structure similar to a sandwich structure. The carbon fiber cloth, as the outer panel, bears the main bending and tensile stress, while the aerogel composite felt, as the core layer, bears the insulation and shear transmission. The overall bending stiffness of the system is significantly better than that of a single-layer aerogel composite felt. Third, weather resistance and flame retardancy. The carbon fiber itself can withstand temperatures of ≥400℃ in air, has a high oxygen index, and does not melt or drip. In extreme situations such as fires, the protective layer can provide an effective fire barrier and delay the spread of fire. The three layers are bonded together by high-temperature pressure-sensitive adhesive or hot-pressing. The high-temperature pressure-sensitive adhesive can be silicone or acrylic type, with a temperature resistance of ≥200℃, ensuring reliable bonding while maintaining overall flexibility and bendability.

[0088] IV. Temperature Control and System Integration The temperature controller is electrically connected to the electrode lead-out structure 10 of the carbon fiber heating layer 1. A temperature sensor (such as an NTC thermistor or thermocouple) is attached to the surface of the industrial equipment or embedded inside the intermediate insulation layer 2 to collect temperature signals from the equipment surface or insulation layer in real time and feed them back to the temperature controller. The temperature controller has a preset target temperature range (set according to the process requirements of the conveying medium, such as 20℃~80℃ or higher). When the temperature is below the set lower limit, the temperature controller connects the power supply circuit of the carbon fiber heating layer 1, and the carbon fiber heating element begins heating. When the temperature reaches the set upper limit, the temperature controller disconnects the power supply, and heating stops. This closed-loop temperature control logic ensures the heat tracing and insulation of industrial pipelines and other equipment in frigid environments, maintaining a constant temperature and preventing overheating from adversely affecting the equipment or medium. The carbon fiber heating layer 1 is powered by a local 12V or 24V DC power supply (or can be powered from an industrial power source via a transformer), with a heating power density of 50~200W / m². 2 It can be matched and designed according to the heat dissipation load of the pipeline and the extreme low temperature conditions.

[0089] Examples 1-8 below were all carried out according to the preparation steps (steps a-e) described in the above instructions, with the only difference being the selection of raw material ratios and / or process parameters.

[0090] I. Preparation Parameter Table for Examples Table 1. Raw material ratios for each embodiment (by mass parts)

[0091] Table 2 Main process parameters for each embodiment

[0092] Table 3. Specific parameters of fiber felt structure selection and gradient density for each embodiment.

[0093] In the table, "—" indicates that it is not applicable (uniform density fiber felt has no distinction between surface and middle layers). The gradient density example consists of three layers: upper surface layer + middle layer + lower surface layer. The parameters of the upper and lower surface layers are the same, and the thickness ratio of the three layers is approximately 25%:50%:25%.

[0094] II. Performance Test Results The following performance tests were performed on the aerogel composite felt fabrics prepared in the above embodiments: density, thermal conductivity (25℃), density deviation in the thickness direction, and hydrophobic contact angle. A comparative example was also set up (an aerogel composite felt fabric prepared using conventional methods of atmospheric pressure impregnation, plasma-free activation, and atmospheric pressure drying). The test results are shown in Table 4.

[0095] Table 4 Comparison of performance test results of each embodiment and comparative example with existing technology

[0096] The comparative example used was a commercially available glass fiber aerogel mat (Zhongning Technology Co., Ltd., AG-ST-GF series) prepared by atmospheric pressure drying. This product was prepared by atmospheric pressure impregnation and drying processes, without plasma surface activation treatment, and underwent cold-heat-mechanical composite cycle testing under the same conditions as the embodiments of this application. The original performance parameters of this comparative example before cycling were obtained by actual measurement.

[0097] The parameter ranges listed in the existing conventional technology (reference range) are a comprehensive summary of the parameters of existing glass fiber aerogel mat products with atmospheric pressure drying process and published literature. They do not represent a single test value for a specific product and are used to compare and illustrate the differences between the glass fiber aerogel mat of this application and existing related products.

[0098] The tests for "thermal conductivity after 800 cycles of thermal-mechanical composite cycling" and "increase in thermal conductivity" in the table above were conducted by performing material-level accelerated composite cycling tests on the aerogel composite felt samples prepared in each embodiment and comparative example according to the following method, and then measuring their thermal conductivity and calculating the increase: (1) Test equipment: A high and low temperature alternating test chamber (single chamber method) is used, with temperature control accuracy ≤ ±1℃ and temperature uniformity ≤ ±2℃. The test chamber is equipped with fixed-space compression clamps, which can apply controllable compression displacement to the sample in the thickness direction.

[0099] (2) Temperature cycling conditions: The cycling temperature range is -20℃ to 200℃, simulating the typical operating temperature range of an industrial pipeline between winter shutdown and high-temperature medium transportation. The heating and cooling rate is set to 5℃ / min. A single temperature cycle is as follows: from room temperature to 200℃ at 5℃ / min → hold at a constant temperature for 15 minutes → decrease to -20℃ at 5℃ / min → hold at a constant temperature for 15 minutes → increase to 200℃ at 5℃ / min. This constitutes one complete temperature cycle, which is repeated 800 times.

[0100] (3) Mechanical Alternating Superposition: Mechanical tension-compression cycles are synchronously superimposed in each temperature cycle to simulate the alternating stress on the insulation layer caused by the axial thermal expansion and contraction of the pipeline due to temperature changes. Specifically, in the high-temperature stage (200℃ isothermal range) of each temperature cycle, a tensile strain of 0.5%~1.0% in the length direction is applied to the sample through a tension fixture (simulating the thermal expansion of the pipeline and stretching the insulation layer); in the cooling stage, as the temperature drops to -20℃, the tensile strain is gradually released (simulating the cooling contraction of the pipeline and the insulation layer returning to a free state); the tensile strain is applied and released again in the next cycle. The application and release rate of tensile strain is ≤0.5mm / min to ensure that the sample is not subjected to instantaneous impact. 800 mechanical tension-release cycles are completed synchronously throughout the entire 800 temperature cycles.

[0101] (4) Experimental Principle: This accelerated composite cycle test simulates the periodic temperature change stress and axial alternating stress caused by the thermal expansion and contraction of the pipeline simultaneously experienced by the material during industrial pipeline use. In each transport-stop cycle, the temperature fluctuates between -20℃ and 200℃ while the pipeline undergoes thermal expansion and contraction deformation. Both of these factors act synchronously on the fiber-aerogel interface, applying repeated shear stress. This test, through the synchronous superposition of temperature cycling and mechanical alternation, can achieve a high-fidelity simulation of the actual service conditions of industrial pipelines in the laboratory, fully exposing the degree of interfacial bonding degradation of the material.

[0102] (5) Interface integrity evaluation method: After 800 cycles of cold-heat-mechanical composite cycling, the sample was removed from the test chamber and the microstructure of the fiber-aerogel interface was observed using a scanning electron microscope. At the same time, the surface of the felt was inspected with the naked eye and at 10x magnification to check for visible aerogel powdering or detachment particles. Interface integrity was described qualitatively in two levels: "interface integrity" (the fiber-aerogel interface is tightly bonded, with no obvious cracks or debonding gaps, and no obvious powdering or detachment particles on the surface) or "interface debonding" (obvious cracks or gaps where the aerogel peels off from the fiber surface exist at the interface, and obvious powdering or detachment particles are present on the surface).

[0103] (6) Timing of thermal conductivity measurement: After completing the 800th cold-heat-mechanical composite cycle, the sample is taken out of the test chamber and left to stand for 24 hours under the environmental conditions of (23±2)℃ and (50±5)% relative humidity to measure the thermal conductivity.

[0104] Depend on Fig. 2 It can be seen that the interface of the comparative example before the cycle (Figure a) is relatively intact with no obvious defects; after 800 cycles of heating and cooling... After mechanical composite cycling (Figure b), the interfacial bonding failed, and the aerogel and fiber showed significant separation, with voids appearing at the interface. The aerogel also exhibited pulverization and detachment. In Example 5 of this invention, the interfacial bonding was uniform and dense before cycling (Figure c); after cycling (Figure d), the fiber and aerogel interface remained tightly bonded, with virtually no interfacial delamination, no crack formation, and virtually no aerogel pulverization or detachment, maintaining good structural integrity.

[0105] As can be seen from the test data in Table 4, the aerogel composite felt fabrics prepared in each embodiment of the present invention are significantly superior to the comparative examples and existing conventional technologies in the following aspects: 1. Synergistic optimization of density and thermal conductivity The densities of Examples 1-8 are all between 185 and 210 kg / m³. 3 Within the range, it is significantly lower than the comparative example's 265 kg / m³. 3 and existing conventional technologies of 200~280kg / m 3 The reference range is 0.0125~0.0148 W / m·K, which is much lower than the 0.0230 W / m·K of the comparative example and the 0.019~0.025 W / m·K reference range of existing conventional technologies. This indicates that the present invention achieves a lower thermal conductivity at a lower density through the synergy of a high water content ratio strategy and a vacuum-assisted impregnation-supercritical drying process, overcoming the structural contradiction of difficulty in achieving both low density and low thermal conductivity in existing technologies. Among them, Example 5 (preferred ratio + intermediate process parameters) has the best overall performance, with a density of 190 kg / m³. 3 Thermal conductivity 0.0125 W / m·K.

[0106] 2. Uniformity of filling in the thickness direction The thickness-direction density deviations of Examples 1-8 were all between 3.5% and 4.8%, significantly lower than the 18.5% of the comparative example and the 15%-25% reference range of existing conventional technologies. This is attributed to the synergistic effect of plasma activation imparting hydrophilicity to the fiber surface and vacuum-assisted impregnation, enabling the sol to uniformly fill the fiber felt throughout its thickness direction. This eliminates the surface over-density and internal void polarization defects caused by insufficient wetting and uneven capillary flow in traditional atmospheric pressure impregnation. Among them, the density deviations of Examples (3, 4, 5, 8) using gradient density fiber felt were generally lower than those of the examples using uniform density fiber felt, indicating that the coupling of the gradient structure and vacuum-assisted impregnation further improved the filling uniformity.

[0107] 3. Hydrophobic properties All embodiments achieved contact angles exceeding 130°, demonstrating the sufficiency of in-situ hydrophobic modification. Examples 4 and 2 exhibited the highest contact angles (150° and 148°, respectively), corresponding to the combination of the upper limit of HMDS concentration (8 wt%) and a relatively high modification temperature. Example 5's 142° was sufficient to meet the requirements for drying protection and hydrophobicity during service, while avoiding the costs and byproduct handling burdens associated with excessive HMDS use.

[0108] 4. Long-term service stability under combined thermal and mechanical cycles After 800 cycles of thermal cycling (each cycle simultaneously superimposed with a mechanical stretching-release cycle), the thermal conductivity of the comparative example increased significantly from 0.0230 W / m·K to 0.0325 W / m·K, an increase of 41.3%. Furthermore, interface integrity evaluation showed significant debonding and pulverization at the fiber-aerogel interface. This verifies the defect mentioned in the background art, where physically bonded interfaces are prone to interface degradation and thermal insulation performance decline under the combined temperature and mechanical stress of industrial pipelines. In contrast, the thermal conductivity increase in each embodiment of the present invention was only 7.7%~11.5%, and the interface integrity evaluation was "interface intact," indicating no obvious cracks or pulverization at the interface. Examples 8 (7.7% increase) and 5 (8.0% increase) showed the best performance. This fully demonstrates that the Si-O-Si chemically bonded interface formed by the synergistic plasma activation and aging steps of the present invention can effectively resist the interfacial shear stress and alternating tensile stress caused by temperature-mechanical combined cycles, fundamentally preventing the thermal insulation performance decline caused by aerogel detachment. Under the harsh conditions of simultaneous application of temperature cycling and mechanical alternation, the absolute value of the increase is still controlled at a low level, further demonstrating that the chemical bonding interface of the present invention remains stable under the synchronous alternating stress of actual transportation and shutdown in industrial pipelines.

[0109] In addition, the composite insulation and heating system provided by this invention can be widely used in the heat tracing and insulation of steam pipelines, process medium pipelines, water supply and drainage pipelines and cryogenic pipelines in the fields of petroleum, chemical, power and municipal engineering. It effectively solves problems such as pipeline freezing, abnormal medium viscosity and crystallization, and has outstanding technical advantages and broad industrial application prospects.

[0110] like Fig. 3 As shown, this system is customized into an arc-shaped covering or a cylindrical integral structure according to the pipe shape, and is sequentially attached to the outer wall of the pipe from the inside out: the carbon fiber heating layer 1, as a planar heat source, is directly attached to the pipe surface to achieve uniform heat tracing in the entire circumference, and its electrode lead-out structure is led out along the pipe axis and connected to an external temperature controller; the middle insulation layer 2 tightly wraps the heating layer, and the aerogel composite felt cloth has an extremely low thermal conductivity, which greatly reduces the heat loss of the pipe to the outside, and achieves the dual effect of heat tracing and cold insulation; the carbon fiber protective layer 3, as the outermost layer, provides mechanical protection, weather protection and corrosion protection for the overall structure. The three-layer structure is composite molded to form an integrated flexible covering, which can adapt to the winding and wrapping installation of pipes of different diameters.

[0111] Compared with existing pipeline heat tracing systems (metal heat tracing tape + rock wool / aluminum silicate insulation layer + metal protective layer), this invention has the following outstanding advantages: Superior heating method: It adopts carbon fiber surface uniform heating, without defects such as local overheating and uneven temperature, higher electrothermal conversion efficiency, no electromagnetic interference, stable and reliable electrode connection, no power attenuation, no poor contact, and no risk of open circuit during long-term operation. Superior thermal insulation performance: The thermal conductivity of the intermediate aerogel composite insulation layer is significantly lower than that of traditional insulation materials. It is thinner and lighter while maintaining the same insulation effect. The thermal conductivity of the intermediate aerogel composite insulation layer is as low as 0.0125 W / m·K, while that of traditional rock wool is typically around 0.04 W / m·K. Under the condition of achieving the same insulation effect (i.e., the same thermal resistance), the thickness of the intermediate insulation layer of this invention is only 1 / 4 to 1 / 3 of that of a traditional rock wool layer. Taking a typical oil pipeline heating condition as an example, a traditional rock wool layer requires a thickness of 90 mm, while the aerogel composite felt of this invention only needs 20-30 mm to achieve the same insulation performance. Furthermore, through plasma activation and chemical bonding reinforcement, it does not pulverize, detach, or experience a decrease in insulation performance after long-term hot and cold cycling. Enhanced protection and lifespan: The outer carbon fiber protective layer has high tensile strength, is resistant to acid, alkali, and salt spray corrosion, does not rust, and does not oxidize. It is suitable for outdoor, underground, chemical, and other highly corrosive and complex working conditions. The overall service life of the system is significantly longer than that of traditional metal protective layer solutions.

[0112] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A composite heat insulation and heating system for the surface of industrial equipment, characterized in that, Including those set from the inside out: The carbon fiber heating layer includes a carbon fiber filament heating element. The two ends of the carbon fiber heating layer are provided with electrode lead-out structures. The electrode lead-out structures include two parallel tin-plated copper wires embedded in the carbon fiber filaments. The tin-plated copper wires and the carbon fiber are filled with high-temperature resistant conductive adhesive. The intermediate insulation layer is an aerogel composite felt, which is composed of glass fiber needle-punched felt and silica aerogel, and the preparation method includes the following steps: Step a: Perform plasma surface activation treatment on the glass fiber needle-punched felt to introduce hydroxyl groups on the glass fiber surface, enhance the chemical bonding between the fiber and the silica aerogel, and improve the hydrophilicity of the fiber surface; Step b: Use vacuum-assisted impregnation to fill the interior of the treated glass fiber needled felt with silica sol, and obtain wet gel composite felt through gelation. Step c: The wet gel composite felt is aged and then immersed in an ethanol solution of hexamethyldisilazane for in-situ hydrophobic modification. Step d: The hydrophobically modified wet gel composite felt is dried with supercritical CO2 to obtain aerogel composite felt; A carbon fiber protective layer covers the outer surface of the intermediate insulation layer, serving to protect the intermediate insulation layer and provide structural strength and weather resistance; The composite heat preservation and heating system is installed on the outer surface of industrial equipment for heat tracing and insulation.

2. The composite heat preservation and heating system according to claim 1, characterized in that, In the carbon fiber heating layer: the carbon fiber filament heating element is a 12K large bundle carbon fiber filament, and the heating element is covered with an insulating protective layer; the position where the tin-plated copper wire passes through the carbon fiber protective layer is provided with a sealing structure; The carbon fiber heating layer, intermediate insulation layer, and carbon fiber protective layer are bonded together by high-temperature pressure-sensitive adhesive or hot-pressed composite.

3. The composite heat preservation and heating system according to claim 1, characterized in that, Step a includes: Step a.1: Place the glass fiber needle-punched felt in the vacuum chamber of the plasma treatment machine and evacuate it to 10~50 Pa; Step a.2: Introduce oxygen to a chamber pressure of 50~100Pa and treat for 5 minutes at a radio frequency power of 200W to introduce hydroxyl groups onto the glass fiber surface.

4. The composite heat preservation and heating system according to claim 1, characterized in that, In the aerogel composite felt of the intermediate insulation layer, the raw materials for preparing the silica aerogel include, by mass parts: 100 parts of tetraethyl orthosilicate; 60-100 parts ethanol; 20-60 parts deionized water; 3-8 parts of hexamethyldisilazane; 0.1 to 0.5 parts of acidic catalyst; 0.2 to 1.0 parts of alkaline catalyst.

5. The composite heat preservation and heating system according to claim 4, characterized in that, The raw materials for preparing the silica aerogel include, by mass parts: 100 parts of tetraethyl orthosilicate; 80 parts ethanol; 40 parts deionized water; 5 parts of hexamethyldisilazane; 0.3 parts of acidic catalyst; 0.5 parts of alkaline catalyst; The acidic catalyst is 0.1 mol / L hydrochloric acid, and the alkaline catalyst is 25% ammonia water by mass.

6. The composite heat preservation and heating system according to claim 1, characterized in that, In the preparation method of the aerogel composite felt of the intermediate insulation layer, the vacuum-assisted impregnation method includes the following sub-steps: Step b.1: Place the plasma-activated glass fiber needled felt in a vacuum-assisted impregnation tank, evacuate to below -0.095 MPa, and maintain for 5 to 15 minutes; Step b.2: Inject the sol and break the vacuum, soak for 20-40 minutes, remove and roll to squeeze out excess sol; Step b.3: Repeat steps b.1 and b.2 at least once.

7. The composite heat preservation and heating system according to claim 5, characterized in that, In the preparation method of the aerogel composite felt of the intermediate insulation layer: The sol corresponding to the silica aerogel is prepared by the following method: Tetraethyl orthosilicate, ethanol and deionized water are mixed, and the acidic catalyst is added first. Acidic hydrolysis is carried out under the conditions of pH 2~3 and temperature 40~60℃. Then, the alkaline catalyst is added, the pH is adjusted to 7-8, and alkaline polycondensation is carried out to obtain the silica sol. The aging step is as follows: the wet gel composite felt obtained after vacuum-assisted gradient impregnation is immersed in an aging solution and aged at 40~60℃ for 12~36 hours; the aging solution is composed of ethanol and tetraethyl orthosilicate, wherein the volume ratio of ethanol to tetraethyl orthosilicate is 8~12:

1. The in-situ hydrophobic modification step is as follows: the aged wet gel composite felt is immersed in an ethanol solution of hexamethyldisilazane, wherein the mass fraction of hexamethyldisilazane in the ethanol solution is 3~8%, the immersion temperature is 50~70℃, and the immersion time is 4~8 hours.

8. The composite heat preservation and heating system according to claim 1, characterized in that, In the preparation method of the aerogel composite felt of the intermediate insulation layer: The conditions for supercritical CO2 drying are: temperature 40~50℃, pressure 10~15MPa, supercritical state maintained for 2~4 hours, and pressure reduction rate of 0.03~0.08MPa / min. Furthermore, after supercritical CO2 drying, a hot-pressing shaping step is also included: the dried aerogel composite felt is hot-pressed at 140~160℃ and 0.005~0.02MPa for 5~15 minutes.

9. The composite heat preservation and heating system according to claim 1, characterized in that, The areal density of the glass fiber needle-punched felt is 400~700 g / m³. 2 The fiber diameter is 5~15μm, and the glass fiber needle-punched felt has a gradient density structure: the fiber density of the upper and lower surface layers is higher than that of the middle layer.

10. The composite heat preservation and heating system according to claim 1, characterized in that, It also includes a temperature controller, which is electrically connected to the electrode lead-out structure of the carbon fiber heating layer, and is used to control the power supply or de-energization of the carbon fiber heating layer according to the temperature signal on the surface of the industrial equipment.

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