Gradient functional laminated wood resistant to ultra-low temperature alternating working conditions and preparation method thereof

Abstract

A gradient functional laminated wood resistant to ultra-low temperature alternating conditions and its preparation method are disclosed. This invention belongs to the technical field of cryogenic load-bearing and heat-insulating composite materials for LNG carriers. From the load-bearing side to the heat-insulating side, it comprises: a load-bearing surface protective layer, a gradient-reinforced pressure-bearing core layer, and a heat-insulating surface adapting layer. The gradient-reinforced pressure-bearing core layer is composed of fiber-reinforced resin-based composite material. From the load-bearing side to the heat-insulating side, the content of reinforcing fibers decreases, while the content of pressure-resistant hollow microspheres increases. This invention fundamentally overcomes the triple trade-off dilemma of "heat insulation-load-bearing-long-term creep," achieving ultimate optimization of performance on both sides through structural gradient decoupling. A cross-scale synergistic mechanism systematically solves the core problem of interface debonding during ultra-low temperature cycling. This invention increases compressive strength by 17% and reduces thermal conductivity by 27% at -163℃. After 400 temperature cycles, the compressive strength retention rate increases from 55% to 93%, while the thermal conductivity increase rate decreases from 45% to 7%.

Description

Technical Field

[0001] This invention belongs to the field of cryogenic load-bearing thermal insulation composite materials for LNG ships, specifically relating to a gradient functional laminated wood resistant to ultra-low temperature alternating conditions and its preparation method. Background Technology

[0002] LNG carriers' independent cargo tanks operate at ultra-low temperatures of -163°C for extended periods. The cargo tanks are externally covered with a layer of perlite or polyurethane foam insulation several hundred millimeters thick. Load-bearing insulation pads, installed between this insulation layer and the hull support steel structure, are the core load-bearing and thermal barrier components connecting the cryogenic cargo tanks to the ambient-temperature hull. Their typical service environment involves contact with the insulation layer's outer surface at approximately -155°C to -130°C on one side, and the hull steel structure at approximately -30°C to +15°C on the other. Although not directly immersed in the -163°C cargo, they withstand significant thickness-direction temperature gradients and axial compressive loads.

[0003] In engineering practice, this component must simultaneously meet the following core requirements: dual functions of ultra-high load-bearing capacity and efficient thermal insulation; long-term dimensional and mechanical stability under ultra-low temperature alternating conditions; and interface compatibility and service integrity with upstream and downstream components. In existing technologies, homogeneous laminated wood structures cannot reconcile the contradiction between thermal insulation and load-bearing capacity. Furthermore, due to the mismatch in thermal expansion coefficients between the matrix and reinforcement, interfacial debonding easily occurs during long-term temperature cycling, leading to a precipitous drop in performance. In addition, the problem of microspheres floating during mass production casting, resulting in uneven performance, has not been fundamentally resolved. Summary of the Invention

[0004] The purpose of this invention is to provide a long-term reliable gradient functional laminated wood for LNG ships under ultra-low temperature alternating conditions and its preparation method, so as to systematically overcome the core bottlenecks of the above-mentioned prior art, and in particular solve the fundamental problem of interface debonding and performance degradation of the pad block due to temperature cycling during decades of service.

[0005] The technical solution adopted in this invention is:

[0006] A gradient functional laminated wood resistant to ultra-low temperature alternating conditions comprises, along the thickness direction from the load-bearing side to the cold-insulating side: a load-bearing surface protective layer, a gradient reinforced pressure-bearing core layer, and a cold-insulating surface adapting layer; the gradient reinforced pressure-bearing core layer is composed of fiber-reinforced resin-based composite material, and along the thickness direction from the load-bearing side to the cold-insulating side, the content of reinforcing fibers decreases and the content of pressure-resistant hollow microspheres increases.

[0007] Furthermore, the pressure-resistant hollow microspheres are surface chemically modified to carry active functional groups that can participate in the matrix curing reaction and are embedded in the matrix network by chemical bonding; the active functional groups are amino or epoxy groups, and the resin matrix of the gradient-reinforced pressure-bearing core layer is a polyurethane-epoxy semi-interpenetrating polymer network matrix.

[0008] Furthermore, a gradient modulus interface transition layer composed of silane coupling agent and nano-toughening particles is provided between the load-bearing protective layer and the gradient reinforced pressure-bearing core layer, between the cold insulation adapting layer and the gradient reinforced pressure-bearing core layer, and between each fiber group inside the gradient reinforced pressure-bearing core layer. The gradient modulus interface transition layer divides each fiber group inside the gradient reinforced pressure-bearing core layer into a high load-bearing area, a gradient transition area and a high heat insulation area from the load-bearing side to the cold insulation side.

[0009] Furthermore, the gradient-reinforced pressure-bearing core layer is formed by hot pressing m groups of fiber-reinforced layers, each group containing k layers of multiaxial fiber cloth impregnated with modified composite impregnating material, where m is a positive integer from 2 to 10 and k is a positive integer from 3 to 10.

[0010] Furthermore, between each group of fiber reinforcement layers inside the gradient reinforced pressure-bearing core layer, there is an insoluble physical barrier layer formed by pre-curing to the gel point, and there is a micro-interface between the physical barrier layer and the adjacent fiber reinforcement layer due to pre-curing.

[0011] Furthermore, the polyurethane-epoxy semi-interpenetrating network matrix of the gradient-reinforced pressure-bearing core layer is prepared from the following components in parts by weight: 70-90 parts of polyether polyol, 10-30 parts of epoxidized bio-based polyol derived from non-edible vegetable oil, 25-55 parts of bisphenol A type epoxy resin, 10-20 parts of microencapsulated latent amine curing agent, 5-15 parts of reactive flame-retardant polyol, 0.5-2 parts of organosilicon surfactant, 0.2-1 parts of moisture-insensitive composite catalyst, and 60-90 parts of polymeric MDI.

[0012] Furthermore, the protective layer of the bearing surface is a dense epoxy-modified polyurethane matrix, in which organic modified nano-layered silicate barrier filler and conductive carbon black composite are uniformly dispersed.

[0013] Furthermore, the cold-insulating surface adapter layer is a dense polyurethane-epoxy semi-interpenetrating network matrix modified with nanoporous silica thermal insulation particles.

[0014] A method for preparing graded functional laminated wood resistant to ultra-low temperature alternating conditions includes the following steps:

[0015] S1. Raw material pretreatment: Hollow microspheres are chemically grafted with silane coupling agents containing amino or epoxy active functional groups; multiaxial fiber cloth is subjected to low-temperature plasma surface activation treatment, and the allowable resting time from treatment to impregnation is controlled to not exceed 4 hours.

[0016] S2. Preparation of modified composite impregnating material: According to the formula, the components of the semi-interpenetrating network matrix, the halogen-free flame retardant system and the pressure-resistant hollow microspheres modified by surface chemical grafting are mixed and dispersed by vacuum to obtain multiple groups of gradient impregnating materials with different microsphere volume fractions, and the viscosity of each group of impregnating materials at the processing temperature is adjusted to fall into the matching window.

[0017] S3. Gradient Laying and Impregnation: Coat the bottom surface of the mold with a composite interface treatment liquid and heat it to surface dry at 60±5℃; pour the protective layer of the bearing surface; lay the fiber cloth in groups from the bearing side to the cold insulation side and pour the corresponding impregnation material. After each group of layup is poured, immediately pre-cur it at 60±5℃ until the layer reaches the gel point to form a physical barrier layer; then coat it with the composite interface treatment liquid and semi-cur it; after all layup is completed, pour the cold insulation surface adapter layer.

[0018] S4. Stepwise gradient curing and controllable stress relief: First stage: 55~65℃, pressure 1~2MPa, constant temperature for 30 minutes; Second stage: 85~95℃, pressure 3~5MPa, constant temperature for 45 minutes; Third stage: 115~125℃, pressure 2~3MPa, constant temperature for 30 minutes; After curing, the temperature is gradually reduced at a rate of 3~5℃ / min, and the temperature is maintained at 80℃ and 40℃ for 30 minutes each, while maintaining contact pressure throughout the process;

[0019] S5. Post-demolding treatment: After demolding, precision machining is performed, and the exposed end faces are sealed a second time.

[0020] Furthermore, in S2, the preparation process of the surface chemically grafted modified pressure-resistant hollow microspheres is as follows: vacuum drying borosilicate hollow microspheres; adding them to an ethanol solution containing 2% by mass of γ-aminopropyltriethoxysilane, stirring at a constant temperature, filtering, and then vacuum drying to obtain grafted modified microspheres with active amino groups on the surface.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] This invention breaks through the triple trade-off dilemma of "insulation-load-long-term creep" in terms of physical essence, and achieves ultimate optimization of performance on both sides through structural gradient decoupling; the cross-scale collaborative mechanism systematically solves the core pain point of interface debonding at ultra-low temperature temperature cycling. This invention improves the compressive strength at -163℃ by 17% and reduces the thermal conductivity by 27%; after 400 temperature cycles, the compressive strength retention rate increases from 55% to 93%, an increase of 38 percentage points; and the thermal conductivity increase rate decreases from 45% to 7%, a reduction of 38 percentage points.

[0023] Meanwhile, the polyurethane-epoxy semi-interpenetrating network matrix formulation of the present invention does not contain soluble chlorides. Combined with the dense structure of the protective layer on the bearing surface and the secondary sealing treatment of the end face, it effectively blocks the precipitation of chloride ions, reducing the amount of chloride ions to 0.15 ppm, which is far lower than the 6.2 ppm of the existing homogenization scheme. This significantly reduces the risk of stress corrosion cracking of the hull steel structure and meets the stringent corrosion resistance requirements of LNG ship materials.

[0024] Furthermore, this invention constructs a reliability verification framework and mass production adaptability for full life cycle service, solidifies the process window quantification, and achieves a process capability index CPK≥1.67, meeting the requirement of 30 years of maintenance-free service. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the laminated wood three-dimensional structure of the present invention;

[0026] Figure 2 This is a schematic diagram of the cross-sectional structure of the laminated wood of the present invention;

[0027] Figure 3 This is a schematic diagram of the preparation method of laminated wood in this invention;

[0028] Figure 4 This is a comparison chart of the compressive strength retention rate of Embodiment 1 and Comparative Examples 1-4 after multiple ultra-low temperature cycles.

[0029] Figure 5 This is a comparison graph showing the change in thermal conductivity of Embodiment 1 and Comparative Examples 1-4 as a function of the number of temperature cycles.

[0030] Figure 6 This is a comparison chart of the long-term compression creep curves of Example 1 of the present invention and Comparative Examples 1-4 at -163°C;

[0031] Figure 7 These are scanning electron microscope images of the microsphere-matrix interface region of the gradient-reinforced pressure-bearing core layer in Embodiment 1 of the present invention;

[0032] Figure 8 This is a comparison of the Fourier transform infrared spectra of the surface chemically grafted hollow microspheres and the unmodified microspheres used in Example 1 of this invention.

[0033] Among them: 10, bearing surface protective layer; 20, gradient reinforced pressure-bearing core layer; 21, high load-bearing area; 22, gradient transition area; 23, high heat insulation area; 30, cold insulation surface adapting layer; 40, gradient modulus interface transition layer. Detailed Implementation

[0034] To better understand the purpose, structure, and function of this invention, the invention will be described in further detail below with reference to the accompanying drawings.

[0035] The present invention specifically relates to a support system for independent cargo tanks of LNG / LPG carriers—namely, a load-bearing insulation pad installed between the insulation layer of the cargo tank and the steel structure of the hull support—and its preparation method.

[0036] This invention abandons the paradigm of "homogeneous single material" and innovatively proposes a cross-scale synergistic solution of "structural gradient functional decoupling + matrix molecular stabilization + interfacial chemical bridging".

[0037] The first level is structural gradient decoupling: a gradient reinforced pressure-bearing core layer is constructed along the thickness direction of the pad with decreasing reinforcing fiber content and increasing pressure-resistant hollow microsphere content. The "thermal insulation" function is prioritized on the cold insulation side, and the "load-bearing and creep resistance" function is prioritized on the load-bearing side, thus satisfying the ultimate performance requirements of both sides in a physical sense.

[0038] The second level—matrix molecular stabilization—uses a polyurethane-epoxy semi-interpenetrating network designed with reaction kinetics matching as the matrix. The epoxy component constrains the thermal motion of the polyurethane chain segments at the molecular chain level, significantly reducing the matrix's coefficient of thermal expansion and making it closer to the reinforcing fibers, thereby reducing the generation of thermal stress at the heterogeneous interface from the root.

[0039] The third level – interfacial chemical bridging: At the interface of the key functional layer, an interfacial transition layer composed of silane coupling agent and nano-toughening particles is used to establish a chemical covalent bond bridge between the matrix and the fiber / microsphere, and to mitigate the interfacial stress concentration through modulus gradient, providing an enhanced defense against residual thermal stress.

[0040] There is a synergistic dependency among the three levels mentioned above: the structural gradient determines the spatial roles of thermal insulation and load-bearing, matrix molecular stabilization reduces the amplitude of interfacial stress caused by temperature differences from the source, and interfacial chemical bridging increases the interfacial resistance to the load-bearing threshold of weakened stress. The combined effect of these three factors enables the pad to maintain a high level of performance under the coupled effects of ultra-long-term temperature cycling and marine environment, an effect that cannot be achieved by any single means.

[0041] like Figure 1 , Figure 2 As shown, the present invention provides a gradient functional laminated wood for LNG ships that is resistant to ultra-low temperature alternating conditions. It is an integrated molding structure, which includes, along the thickness direction from the load-bearing side to the cold insulation side, the following components in sequence: a load-bearing surface protective layer 10, a gradient reinforced pressure-bearing core layer 20, and a cold insulation surface adapter layer 30. The gradient reinforced pressure-bearing core layer 20 is composed of fiber-reinforced resin-based composite material. Along the thickness direction from the load-bearing side to the cold insulation side, the content of reinforcing fiber decreases and the content of pressure-resistant hollow microspheres increases.

[0042] The pressure-resistant hollow microspheres undergo surface chemical modification to carry active functional groups that can participate in the matrix curing reaction, and are embedded into the matrix network via chemical bonding. The active functional groups are amino or epoxy groups, with an isostatic compressive strength ≥80 MPa and a true density of 0.15~0.45 g / cm³. 3 The resin matrix of the gradient-reinforced pressure-bearing core layer 20 is a polyurethane-epoxy semi-interpenetrating polymer network matrix.

[0043] Between the bearing surface protective layer 10 and the gradient reinforced pressure-bearing core layer 20, between the cold insulation surface adapter layer 30 and the gradient reinforced pressure-bearing core layer 20, and between each fiber group inside the gradient reinforced pressure-bearing core layer 20, a gradient modulus interface transition layer 40 composed of silane coupling agent and nano-toughening particles is provided. The thickness of the gradient modulus interface transition layer 40 is 100~500nm, and the nano-toughening particles are core-shell rubber nanoparticles.

[0044] The gradient-reinforced load-bearing core layer 20 is formed by hot-pressing m sets of fiber-reinforced layers, where m is a positive integer from 2 to 10. Each set contains k layers of multi-axial fiber cloth impregnated with modified composite impregnating material, where k is a positive integer from 3 to 10. The m sets are arranged sequentially from the load-bearing side to the cold-insulating side along the thickness direction, and can be divided into a high load-bearing zone 21, a gradient transition zone 22, and a high thermal insulation zone 23 according to the stepwise increase in the volume fraction of microspheres.

[0045] A gradient modulus interface transition layer 40 is provided between the load-bearing protective layer 10 and the high load-bearing area 21, between the high load-bearing area 21 and the gradient transition area 22, between the gradient transition area 22 and the high insulation area 23, and between the high insulation area 23 and the cold insulation surface adapter layer 30.

[0046] Along the thickness direction, the volume fraction of surface-modified pressure-resistant hollow microspheres in each group of impregnating materials increases in a stepwise manner, with an increase of 3 vol% to 8 vol per group.

[0047] Between each group of fiber reinforcement layers inside the gradient reinforced pressure-bearing core layer 20, there is an insoluble physical barrier layer formed by pre-curing to the gel point. There is a micro interface formed by pre-curing between the physical barrier layer and the adjacent fiber reinforcement layer. The physical barrier layer is used to block the mutual solubility convection of the interlayer impregnating material and the migration of microspheres. The viscosity of the impregnating material used in the gradient reinforced pressure-bearing core layer is 1.0~2.2 Pa·s at 25°C.

[0048] The polyurethane-epoxy semi-interpenetrating network matrix of the gradient-reinforced pressure-bearing core layer 20 is prepared from the following components in parts by weight: 70-90 parts of polyether polyol, 10-30 parts of epoxidized bio-based polyol derived from non-edible vegetable oil, 25-55 parts of bisphenol A type epoxy resin, 10-20 parts of microencapsulated latent amine curing agent, 5-15 parts of reactive flame-retardant polyol, 0.5-2 parts of organosilicon surfactant, 0.2-1 parts of moisture-insensitive composite catalyst, and 60-90 parts of polymeric MDI.

[0049] The protective layer 10 on the bearing surface is a dense epoxy-modified polyurethane matrix, in which organic modified nano-layered silicate barrier filler and conductive carbon black composite are uniformly dispersed, and the layer thickness is 0.5~2mm.

[0050] The cold insulation surface adapter layer 30 is a dense polyurethane-epoxy semi-interpenetrating network matrix modified with nanoporous silica thermal insulation particles, with a layer thickness of 1~3mm.

[0051] The laminated wood has a nominal compressive strength ≥180MPa, interlaminar shear strength ≥35MPa, and thermal conductivity ≤0.13W / (m·K) at -163℃; after 400 temperature cycles from -163℃ to room temperature, the compressive strength retention rate is ≥90%, and the thermal conductivity increase rate is ≤10%; the compression creep deformation is ≤0.5% after 1500 hours at -163℃ and 80% compressive strength load.

[0052] like Figure 3 As shown, a method for preparing LNG marine graded functional laminated wood resistant to ultra-low temperature alternating conditions includes the following steps:

[0053] S1. Raw material pretreatment: Hollow microspheres are chemically grafted with silane coupling agents containing amino or epoxy active functional groups; multiaxial fiber cloth is subjected to low-temperature plasma surface activation treatment, and the allowable resting time from treatment to impregnation is controlled to not exceed 4 hours.

[0054] S2. Preparation of modified composite impregnating material: According to the formula, the components of the semi-interpenetrating network matrix, the halogen-free flame retardant system and the pressure-resistant hollow microspheres modified by surface chemical grafting are mixed and dispersed by vacuum to obtain multiple groups of gradient impregnating materials with different microsphere volume fractions, and the viscosity of each group of impregnating materials at the processing temperature is adjusted to fall into the matching window.

[0055] The viscosity matching window is 1.0~2.2 Pa·s at 25℃. Fine-tuning is achieved by adding or reducing the amount of silicone surfactant to ensure sufficient interlayer penetration of the impregnating material without significant microsphere floating. Fine-tuning is done by adding or reducing 0.5~2 parts of silicone surfactant. Optionally, 0.3~0.8 wt% of fumed silica or polyamide wax thixotropic agent can be added to each group of impregnating materials to achieve a viscosity ≥8000 mPa·s at low shear rates (static state) and a viscosity maintained at 1.2~2.0 Pa·s at high shear rates (roller impregnation), further enhancing the microsphere spatial locking effect. After standing for 30 minutes, the volume fraction deviation between the top and bottom microspheres is ≤1 percentage point.

[0056] S3. Gradient Laying and Impregnation: Coat the bottom surface of the mold with a composite interface treatment liquid and heat at 60℃ for 10 minutes until surface dry; pour the protective layer 10 mixture (a semi-interpenetrating network matrix containing 5wt% organic modified montmorillonite and 1wt% conductive carbon black). Coat the second interface treatment liquid and semi-cur under the same conditions to form the first gradient modulus interface transition layer 40. Then, lay the fiber cloth in groups from the load-bearing side to the cold insulation side and pour the corresponding volume fraction of impregnating material. After each group of impregnating material is laid and poured, immediately pre-curing treatment is carried out at 60±5℃ for 10~15 minutes. The intersection point (gel point) of the storage modulus G' and loss modulus G" of the layer is monitored by rheological testing to ensure that the layer reaches above the gel point (G'≥G"), forming an insoluble and infusible physical barrier layer. There is a micro-interface formed by pre-curing between this physical barrier layer and the adjacent fiber reinforcement layer, which completely blocks the mutual solubility convection and microsphere migration between the impregnating material of the subsequent pouring layer and this layer. After pre-curing, a composite interface treatment liquid is applied and semi-cured under the same conditions to form the next gradient modulus interface transition layer 40, and then the next set of layups is performed. After all layups are completed, an interface treatment liquid is applied to the top layer and semi-cured to form the last gradient modulus interface transition layer 40. Then, the cold insulation surface adapter layer 30 mixture (a semi-interpenetrating network matrix containing 15wt% nano silica aerogel powder) is poured and leveled.

[0057] S4. Stepwise Gradient Curing and Controlled Stress Relief: A stepwise gradient curing procedure is adopted: The first stage is 55-65℃, pressure 1-2MPa, held for 30 minutes, allowing the polyurethane network to initially form while the epoxy component remains essentially inert; the second stage is 85-95℃, pressure 3-5MPa, held for 45 minutes, triggering the release of the microcapsule-end-curing agent, and the epoxy component begins to crosslink and interpenetrate within the polyurethane network; the third stage is 115-125℃, pressure 2-3MPa, held for 30 minutes, completing the full development of the semi-interpenetrating network structure and stress relaxation. After curing, the temperature is gradually reduced at a rate of 3-5℃ / min, with insulation platforms set at 80℃ and 40℃ respectively, maintaining contact pressure throughout the process, and releasing pressure after cooling to room temperature.

[0058] S5. Post-demolding treatment: After demolding, precision grinding is performed, and the exposed end face is sealed a second time.

[0059] Example 1

[0060] See Figure 1 and Figure 2 This embodiment provides a long-term reliable gradient functional laminated wood for LNG ships operating under ultra-low temperature alternating conditions. The laminated wood body is a one-piece molded structure, consisting of the following layers along its thickness from the bottom surface in contact with the ship's hull support steel structure to the top surface in contact with the cargo tank insulation layer: a load-bearing protective layer 10, a gradient-reinforced pressure-bearing core layer 20, and a cold-insulating adapting layer 30. The gradient-reinforced pressure-bearing core layer 20 is divided into a high load-bearing zone 21, a gradient transition zone 22, and a high insulation zone 23 from bottom to top. The volume fraction of microspheres increases in a stepped manner from the load-bearing side to the cold-insulating side, while the fiber volume fraction decreases accordingly. Gradient modulus interface transition layers 40 are provided between the load-bearing protective layer 10 and the gradient-reinforced pressure-bearing core layer 20, between the cold-insulating adapting layer 30 and the gradient-reinforced pressure-bearing core layer 20, and between the fiber groups within the gradient-reinforced pressure-bearing core layer 20.

[0061] Gradient-reinforced load-bearing core layer 20 consists of 30 layers with a density of 800 g / m² 2 The materials are made of biaxial glass fiber cloth laminated and hot-pressed, and are divided into 5 groups (m=5), with 6 layers in each group (k=6). From the load-bearing side to the cold-insulating side, the volume fraction of surface chemically grafted modified hollow microspheres in each group of impregnating materials is as follows: 5%, 9%, 13%, 17%, and 21%, with an increasing step of 4%. Among them, the first two groups (5% and 9%) constitute the high load-bearing zone 21, the third group (13%) constitutes the gradient transition zone 22, and the last two groups (17% and 21%) constitute the high thermal insulation zone 23.

[0062] The polyurethane-epoxy semi-interpenetrating network matrix formulation, by weight, consists of: 80 parts polyether polyol, 20 parts epoxy soybean oil-based bio-polyol, 30 parts bisphenol A type epoxy resin E-51, 12 parts microencapsulated latent amine curing agent, 8 parts reactive phosphorus-containing flame-retardant polyol, 1.2 parts organosilicon surfactant, 0.5 parts moisture-insensitive composite catalyst, and 70 parts polymeric MDI. The total addition amount of the halogen-free intumescent flame-retardant system (ammonium polyphosphate / pentaerythritol / melamine mass ratio 3:1:2) is 22 parts.

[0063] The moisture-insensitive composite catalyst is composed of triethylenediamine and dibutyltin dilaurate in a mass ratio of 3:1, and is pre-mixed with 4A molecular sieve (added at 5% of the total catalyst mass). The microcapsule-capsulated latent curing agent is prepared by in-situ polymerization using dicyandiamide as the core material and polymethyl methacrylate as the wall material. The microcapsule particle size D50 = 15 μm, and the wall material melt triggering temperature is 85 ± 5 °C.

[0064] The preparation process of surface chemical grafting modified hollow microspheres is as follows: borosilicate hollow microspheres (isostatic compressive strength of about 80 MPa) are vacuum dried at 120℃ for 2 hours; an ethanol solution containing 2% by mass of γ-aminopropyltriethoxysilane (KH-550) is added, and the mixture is stirred at 60℃ for 3 hours. After filtration, the microspheres are vacuum dried at 100℃ to obtain grafted modified microspheres with active amino groups on the surface.

[0065] The treatment solution for the gradient modulus interface transition layer 40 was prepared by hydrolyzing γ-glycidoxypropyltrimethoxysilane (KH-560) in an aqueous acetic acid solution at pH=4 for 1 hour, adding 0.5% by mass of core-shell rubber nanoparticles (particle size 50~100nm), and ultrasonically dispersing for 30 minutes.

[0066] Preparation method:

[0067] S1. Raw material pretreatment: Hollow microspheres are surface chemically grafted and modified according to the above process; Biaxial glass fibers are arranged in a low-temperature plasma treatment equipment and treated with 120W power for 45 seconds in an argon / nitrogen mixed atmosphere; After treatment, the fiber cloth is stored under dry nitrogen protection and enters the impregnation process within 2 hours.

[0068] S2. Preparation of Modified Composite Impregnating Material: According to the formula, all components except polymeric MDI and the corresponding proportions of grafted modified microspheres are stirred at 2500 rpm for 20 minutes under vacuum. Before use, they are mixed with polymeric MDI in the specified proportion. Five groups of impregnating materials with microsphere volume fractions of 5%, 9%, 13%, 17%, and 21% are prepared respectively. The viscosity values ​​of each group are measured at 25℃ in the range of 1.2~1.8 Pa·s.

[0069] S3. Gradient Laying and Impregnation: Coat the bottom surface of the mold with a composite interface treatment liquid and heat at 60℃ for 10 minutes until surface dry; pour the protective layer 10 mixture (a semi-interpenetrating network matrix containing 5wt% organic modified montmorillonite and 1wt% conductive carbon black). After coating with a second interface treatment liquid and semi-curing under the same conditions, the first gradient modulus interface transition layer 40 is formed. Then, in the order from the bearing side to the cold insulation side, fiber cloth is laid in groups and impregnated with the corresponding volume fraction of impregnating material is poured. After each group of impregnating material is laid and poured, it is immediately pre-cured at 60±5℃ for 10~15 minutes. The intersection point (gel point) of the storage modulus G' and loss modulus G" of the layer is monitored by rheological testing. The layer reaches above the gel point (G'≥G"), forming an insoluble and infusible physical barrier layer. There is a micro-interface formed by pre-curing between this physical barrier layer and the adjacent fiber reinforcement layer, which completely blocks the mutual solubility convection and microsphere migration between the impregnating material of the subsequent pouring layer and this layer. After pre-curing, a composite interface treatment liquid is applied and semi-cured under the same conditions to form the next gradient modulus interface transition layer 40, and then the next set of layups is performed. After all layups are completed, an interface treatment liquid is applied to the top layer and semi-cured to form the last gradient modulus interface transition layer 40. Then, the cold insulation surface adapter layer 30 mixture (a semi-interpenetrating network matrix containing 15wt% nano silica aerogel powder) is poured and leveled.

[0070] S4. Stepwise Gradient Curing and Controlled Stress Relief: Place the mold in a hot press equipped with a multi-zone independent temperature control system and cure according to the following procedure: First stage: 60℃, pressure 1.5MPa, constant temperature for 30 minutes; Second stage: 90℃, pressure 4MPa, constant temperature for 45 minutes; Third stage: 120℃, pressure 2.5MPa, constant temperature for 30 minutes. After curing, cool down at a rate of 4℃ / min, hold at 80℃ and 40℃ for 30 minutes each, maintain a contact pressure of 1MPa throughout the process, and release pressure after cooling to room temperature.

[0071] S5. Post-demolding treatment: After demolding, precision grinding is performed to ensure parallelism and flatness; the exposed end faces are coated with a secondary interface treatment liquid and cured at 60℃ to seal the holes.

[0072] Example 2 (Lower Limit Verification)

[0073] The preparation method is the same as in Example 1, except that the gradient-reinforced load-bearing core layer consists of two groups (m=2) of fiber reinforcement layers, each group having three layers (k=3), for a total of six layers; the volume fraction of microspheres is 5% from the load-bearing side to 10% from the cold-insulating side, with an increasing step of 5%. The resulting laminated wood has a nominal compressive strength of 196 MPa at -163℃, and retains 90% of its compressive strength after 400 temperature cycles.

[0074] Example 3 (Upper Range Verification)

[0075] The preparation method is the same as in Example 1, except that the gradient-reinforced load-bearing core layer consists of 10 groups (m=10) of fiber reinforcement layers, with 10 layers in each group (k=10), for a total of 100 layers; the volume fraction of microspheres increases uniformly from the load-bearing side to the cold-insulating side in increments of 3% to 8%. The resulting laminated wood has a nominal compressive strength of 210 MPa at -163℃, and retains 92% of its compressive strength after 400 temperature cycles.

[0076] Example 4 (Verification of the lower limit of the formulation range)

[0077] The preparation method is the same as in Example 1, except that the polyurethane-epoxy semi-interpenetrating network matrix is ​​formulated with 70 parts polyether polyol, 10 parts bio-based polyol, 25 parts epoxy resin, 10 parts latent curing agent, and 60 parts polymeric MDI. The resulting laminated wood has a compressive strength of 185 MPa at -163°C and retains 90% of its strength after 400 temperature cycles.

[0078] Example 5 (Verification of the upper limit of the formula range)

[0079] The preparation method is the same as in Example 1, except that the polyurethane-epoxy semi-interpenetrating network matrix is ​​formulated with 90 parts polyether polyol, 30 parts bio-based polyol, 55 parts epoxy resin, 20 parts latent curing agent, and 90 parts polymeric MDI. The resulting laminated wood has a compressive strength of 195 MPa at -163°C and a retention rate of 91% after 400 temperature cycles.

[0080] Example 6 (KH-560 epoxy silane grafted microspheres)

[0081] The preparation method was the same as in Example 1, except that the surface chemical grafting was carried out using an ethanol solution containing 2% by mass of γ-glycidoxypropyltrimethoxysilane (KH-560), and the grafted modified microspheres with active epoxy groups on the surface were prepared under the same conditions. The resulting laminated wood retained 91% of its compressive strength after 400 temperature cycles.

[0082] The above embodiments demonstrate that, within the numerical range defined by the present invention, graded functional laminated wood that meets the performance index requirements can be obtained through conventional process adjustments.

[0083] Comparative Example 1 (Existing Homogenization Scheme)

[0084] The method employs a typical homogeneous glass fiber reinforced polyurethane microsphere-filled laminated wood solution found in existing technologies. The matrix is ​​pure polyurethane, the microspheres are not surface modified, and the fiber cloth is a homogeneous layup without gradient structure or interface transition layer.

[0085] Comparative Example 2 (gradient structure + pure polyurethane matrix, without IPN)

[0086] Referring to the gradient structure and preparation method of Example 1, the difference lies in that: the matrix uses a pure polyurethane system, without adding bisphenol A type epoxy resin and latent curing agent; the formulation consists of 80 parts polyether polyol, 20 parts bio-based polyol, 8 parts reactive flame-retardant polyol, 1.2 parts organosilicon surfactant, 0.5 parts catalyst, and 70 parts polymeric MDI. The resulting laminated wood has a compressive strength of 165 MPa at -163℃, and after 400 temperature cycles, the compressive strength retention rate drops sharply to 62%, while the thermal conductivity increases by 35%.

[0087] Comparative Example 3 (IPN matrix + unmodified microspheres, no chemical bonding)

[0088] The preparation method was the same as in Example 1, except that the hollow microspheres were not surface-modified by chemical grafting; untreated borosilicate hollow microspheres were used directly. The resulting laminated wood retained only 72% of its compressive strength after 400 temperature cycles, and SEM cross-sectional observation revealed obvious debonding cracks at the microsphere-matrix interface.

[0089] Comparative Example 4 (structural gradient only, no IPN and no chemical bonding)

[0090] Referring to the gradient structure of Example 1, but using a pure polyurethane system as in Comparative Example 2 as the matrix, and without surface treatment of the microspheres as in Comparative Example 3, the resulting laminated wood had an initial compressive strength of 160 MPa. After 400 temperature cycles, the compressive strength retention rate was only 35%, and macroscopic interlaminar cracking appeared after approximately 250 temperature cycles. Testing showed that the compressive strength retention rate of this comparative example after 250 temperature cycles was approximately 52%, while the compressive strength retention rate of Example 1 was still higher than 95% after the same number of cycles, further highlighting the significant performance advantages of the three-layer synergistic mechanism of this invention during the mid-term service stage.

[0091] Performance testing

[0092] All mechanical property tests at -163℃ were conducted in a constant-temperature chamber. Samples were cooled to -163℃ at a rate of 3℃ / min and held at that temperature for 2 hours before testing. Compressive strength was measured according to GB / T1448-2005, and interlaminar shear strength according to GB / T1450.1-2005. Fixtures and sensors were all cryogenically compensated. Thermal conductivity was measured according to ISO22007-2:2015 transient planar heat source method, with the hot plate at 25℃ and the cold plate at -163℃. Flame retardancy was measured according to IMO FTPC Part 5, smoke density and toxicity according to IMO FTPC Part 2, and chloride ion emission according to IEC60754-1:2011 (immersion in 50℃ deionized water for 24 hours).

[0093] The temperature cycling regime was as follows: the temperature was lowered from room temperature to -163°C at a rate of 3°C / min, held for 2 hours, then raised to room temperature at a rate of 3°C / min, and held for 1 hour; this constituted one cycle. Creep testing was conducted at -163°C and 80% compressive strength load for 1500 hours.

[0094] The performance test results of each embodiment and comparative example are summarized in the table below.

[0095]

[0096] As shown in the table, the strength retention rate of Comparative Example 2 (without IPN only) decreased to 62% after 400 temperature cycles, a decrease of 31 percentage points compared to Example 1 (93%). This demonstrates that the molecular stabilization of the IPN matrix is ​​the core foundation for performance improvement—it reduces the generation of thermal stress at the heterogeneous interface from the source and contributes the main long-term stability. The retention rate of Comparative Example 3 (without chemical bonding only) was 79%, a decrease of 14 percentage points, demonstrating that chemical bonding interface bridging further improves the damage threshold of the interface and is an indispensable key defense for solidifying the IPN matrix gain. Comparative Example 4 lacked all three mechanisms, with a retention rate of only 35%, and macroscopic interlayer cracking appeared after about 250 temperature cycles. Combining the univariate comparison between Example 1 and Comparative Examples 2-4, the synergistic effect of the three-layer synergistic mechanism of "structural gradient + IPN matrix stabilization + chemical bonding interface bridging" in this invention is clearly demonstrated: the three are not simply superimposed, but exhibit a coupling amplification effect of "1+1+1>3", and the absence of any link leads to a significant deterioration in long-term temperature cycling performance. This synergistic effect cannot be reasonably predicted from existing technologies.

[0097] Figure 4 This is a comparison chart of the compressive strength retention rate of Example 1 and Comparative Examples 1-4 after multiple ultra-low temperature cycles.

[0098] Figure 5 This is a comparison graph showing the change in thermal conductivity of Example 1 and Comparative Examples 1-4 with the number of temperature cycles.

[0099] (Note: The difference in initial thermal conductivity is determined by a combination of factors: the molecular structure of the material determines its intrinsic thermal conductivity (such as the difference between metals and non-metals); the microstructure (porosity, anisotropy) alters the heat conduction path; experimental conditions (temperature fluctuations, contact thermal resistance, measurement methods) introduce errors; and formulation design (filler dispersion, flame retardant system, matrix composition) affects thermal conductivity by changing material density, crystallinity, etc. These factors are interrelated, leading to differences in the initial thermal conductivity of different samples.)

[0100] Figure 6 This is a comparison of the long-term compression creep curves of Example 1 and Comparative Examples 1-4 at -163°C.

[0101] Furthermore, the chloride ion release in Example 1 was only 0.15 ppm, a reduction of over 97% compared to 6.2 ppm in Comparative Example 1. This significant improvement stems from the fact that the polyurethane-epoxy semi-interpenetrating network matrix formulation of this invention does not contain soluble chlorides. Combined with the dense structure formed by the uniformly dispersed organic modified nano-layered silicate barrier filler in the protective layer of the bearing surface, and the secondary sealing treatment at the end face, the generation and release of chloride ions are synergistically blocked from both the source and the pathway, significantly reducing the risk of stress corrosion cracking in the ship's steel structure. This is a prominent technical advantage that existing homogeneous laminated wood solutions do not possess.

[0102] Microstructure and chemical bonding characterization

[0103] Figure 7 The image shows a SEM image of the microsphere-matrix interface after 400 temperature cycles in Example 1. The interface is tightly bonded, with no debonding or microcracks observed. In Comparative Example 3, at the same magnification, a distinct annular debonding gap was observed around the microspheres. Figure 8 Infrared spectral comparison showed that the microspheres modified by KH-550 grafting exhibited a higher spectral density at 1560 cm⁻¹. -1 An absorption peak for the NH bending vibration appears at 1090 cm⁻¹. -1 The presence of Si-O-Si characteristic peaks confirms that the silane coupling agent has been successfully chemically grafted onto the surface of the microspheres.

[0104] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A graded functional laminated wood resistant to ultra-low temperature alternating conditions, characterized in that: Along the thickness direction from the load-bearing side to the cold-insulating side, it sequentially includes: a load-bearing surface protective layer (10), a gradient-reinforced pressure-bearing core layer (20), and a cold-insulating surface adapting layer (30); the gradient-reinforced pressure-bearing core layer (20) is composed of fiber-reinforced resin-based composite material, and along the thickness direction from the load-bearing side to the cold-insulating side, the content of reinforcing fibers decreases and the content of pressure-resistant hollow microspheres increases; the pressure-resistant hollow microspheres are borosilicate hollow microspheres. The pressure-resistant hollow microspheres are surface-modified with chemical agents to carry active functional groups that participate in the matrix curing reaction and are embedded in the matrix network by chemical bonding. The active functional groups are amino or epoxy groups, and the resin matrix of the gradient-reinforced pressure-bearing core layer (20) is a polyurethane-epoxy semi-interpenetrating polymer network matrix. The polyurethane-epoxy semi-interpenetrating polymer network matrix of the gradient-reinforced pressure-bearing core layer (20) is prepared from the following components in parts by weight: 70-90 parts of polyether polyol, 10-30 parts of epoxidized bio-based polyol derived from non-edible vegetable oil, 25-55 parts of bisphenol A type epoxy resin, 10-20 parts of latent amine curing agent with microcapsule end-capsulation, 5-15 parts of reactive flame-retardant polyol, 0.5-2 parts of organosilicon surfactant, 0.2-1 parts of moisture-insensitive composite catalyst, and 60-90 parts of polymeric MDI.

2. The graded functional laminated wood resistant to ultra-low temperature alternating conditions according to claim 1, characterized in that: Between the bearing surface protective layer (10) and the gradient reinforced pressure-bearing core layer (20), between the cold insulation surface adapter layer (30) and the gradient reinforced pressure-bearing core layer (20), and between each fiber group inside the gradient reinforced pressure-bearing core layer (20), a gradient modulus interface transition layer (40) composed of silane coupling agent and nano toughening particles is provided.

3. The graded functional laminated wood resistant to ultra-low temperature alternating conditions according to claim 1, characterized in that: The gradient-reinforced pressure-bearing core layer (20) is formed by hot pressing m groups of fiber reinforcement layers. Each group contains k layers of multiaxial fiber cloth impregnated with modified composite impregnating material. m is a positive integer from 2 to 10, and k is a positive integer from 3 to 10. The m groups are arranged sequentially from the load-bearing side to the cold-insulating side along the thickness direction, and are divided into a high load-bearing zone (21), a gradient transition zone (22), and a high heat insulation zone (23) according to the stepwise increase of the microsphere volume fraction.

4. The graded functional laminated wood resistant to ultra-low temperature alternating conditions according to claim 1, characterized in that: Between each group of fiber reinforcement layers inside the gradient reinforced pressure-bearing core layer (20), there is an insoluble physical barrier layer formed by pre-curing to the gel point. There is a micro-interface between the physical barrier layer and the adjacent fiber reinforcement layer due to pre-curing.

5. The graded functional laminated wood resistant to ultra-low temperature alternating conditions according to claim 1, characterized in that: The protective layer (10) on the bearing surface is a dense epoxy-modified polyurethane matrix, in which organic modified nano-layered silicate barrier filler and conductive carbon black composite are uniformly dispersed.

6. The graded functional laminated wood resistant to ultra-low temperature alternating conditions according to claim 1, characterized in that: The cold insulation surface adapter layer (30) is a dense polyurethane-epoxy semi-interpenetrating polymer network matrix modified with nanoporous silica thermal insulation particles.

7. A method for preparing graded functional laminated wood resistant to ultra-low temperature alternating conditions as described in any one of claims 1 to 6, characterized in that: Includes the following steps: S1. Raw material pretreatment: Hollow microspheres are chemically grafted with silane coupling agents containing amino or epoxy active functional groups; multiaxial fiber cloth is subjected to low-temperature plasma surface activation treatment, and the allowable resting time from treatment to impregnation is controlled to not exceed 4 hours. S2. Preparation of modified composite impregnating material: According to the formula, the components of the polyurethane-epoxy semi-interpenetrating polymer network matrix, the halogen-free flame retardant system and the pressure-resistant hollow microspheres modified by surface chemical grafting are mixed and dispersed by vacuum to obtain multiple groups of gradient impregnating materials with different microsphere volume fractions, and the viscosity of each group of impregnating materials at the processing temperature is adjusted to fall into the matching window. S3, Gradient Laying and Impregnation: Coat the bottom surface of the mold with composite interface treatment liquid and heat it to surface dry at 60±5℃; pour the protective layer of the bearing surface (10); lay the fiber cloth in groups from the bearing side to the cold insulation side and pour the corresponding impregnation material. After each group of layup is poured, immediately pre-cur it at 60±5℃ until the layer reaches the gel point to form a physical barrier layer; then coat the composite interface treatment liquid and semi-cur it; after all layup is completed, pour the cold insulation surface adapter layer (30). S4. Stepwise gradient curing and controllable stress relief: First stage: 55~65℃, pressure 1~2MPa, constant temperature for 30 minutes; Second stage: 85~95℃, pressure 3~5MPa, constant temperature for 45 minutes; Third stage: 115~125℃, pressure 2~3MPa, constant temperature for 30 minutes; After curing, the temperature is gradually reduced at a rate of 3~5℃ / min, and the temperature is maintained at 80℃ and 40℃ for 30 minutes each, while maintaining contact pressure throughout the process; S5. Post-demolding treatment: After demolding, precision machining is performed, and the exposed end faces are sealed a second time.

8. The method for preparing graded functional laminated wood resistant to ultra-low temperature alternating conditions according to claim 7, characterized in that: In S2, the preparation process of the surface chemically grafted modified pressure-resistant hollow microspheres is as follows: vacuum drying of borosilicate hollow microspheres; The mixture was added to an ethanol solution containing 2% γ-aminopropyltriethoxysilane by mass, stirred at a constant temperature, filtered, and then dried under vacuum to obtain grafted modified microspheres with active amino groups on the surface.

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