Balsa wood staged control micro-carbonization treatment method for wind power blade

Abstract

The invention provides staged control micro-carbonization treatment for balsa wood of a wind power blade. The process sequentially comprises the steps of preheating and drying, pulse pre-carbonization, controlled atmosphere main carbonization, stable cooling and interface stabilization. A compact carbonization layer is formed on the surface layer through depth-limited micro-carbonization, the layer depth growth rate is used as process master control, and the feedback of the thickness-direction water content gradient and segmented cooling are combined, so that the matching of heat transfer and reaction is realized, and moisture prevention and mechanical maintenance are considered; wettability and bonding strength of contact with resin adhesive are optimized through plasma activation, moisture absorption expansion and shrinkage are remarkably reduced, and interlayer shearing and long-term service reliability are improved.

Description

Technical Field

[0001] This invention relates to the field of wood carbonization technology, specifically to a method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades. Background Technology

[0002] Wind turbine blades commonly employ a sandwich structure to balance stiffness, strength, and weight. In manufacturing, pre-processed balsa wood core material and dry fiber fabric are laid together in a mold, and then vacuum infusion (resin is injected and cured under negative pressure) forms a composite sandwich of fiber / resin skin and balsa wood core. This process has become the mainstream method in large blade manufacturing because it improves molding quality and reduces weight.

[0003] Balsa wood is a natural porous fibrous material. Its cell cavities and pits form a capillary network, and its inherent hydrophilic and hygroscopic properties cause it to periodically absorb and dehydrate in high-humidity, salt-spray, and temperature-humidity cyclical service environments such as offshore wind power plants. This triggers a microscale "capillary pump" effect and volume expansion and contraction. Under long-term cyclic loading, the core-resin interface is prone to adhesive fatigue, microcrack propagation, and localized delamination, thus weakening interlaminar shear strength and overall stiffness. While traditional heat modification or carbonization treatments can reduce the polar groups and moisture absorption rate of wood, balsa wood has thin cell walls and low heat capacity, making it extremely sensitive to temperature windows: high temperatures easily lead to cell wall thermal degradation and strength reduction, while low temperatures make it difficult to achieve stable moisture-proof effects. Besides heat treatment, surface hydrophobic coatings or conventional resin impregnation often fail due to uneven penetration and microcracks caused by the complex internal pores of balsa wood, the combined effects of injection or curing stress, and service vibration.

[0004] Therefore, how to effectively reduce the hygroscopicity and moisture-induced swelling and shrinkage effect of balsa wood in the service environment of wind turbine blades while maintaining its cellular structure and mechanical properties, thereby improving the interfacial stability and interlaminar shear strength between it and the structural adhesive layer and extending the service life of the sandwich structure, is an urgent technical problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned problems existing in the prior art and provide a method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades. By controlling the micro-carbonization process parameters in stages, the hygroscopicity and moisture-induced expansion and contraction effect of balsa wood can be precisely controlled while maintaining the integrity of the balsa wood cell structure and mechanical properties. This improves the interfacial bonding stability with the fiber layer and the interlayer shear strength, thereby extending the service life of the sandwich structure of wind turbine blades.

[0006] To achieve the above objectives, the present invention provides a method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades, comprising the following steps: S1 Preheating and Drying: Under controlled convection heat transfer conditions, the wood boards or strips are heated at 40–95°C and dehumidified to reduce the moisture content inside the boards to 6.0–9.0 wt%. S2 selective pre-carbonization: 1 to 3 pulsed isothermal treatments at 100 to 150°C, each lasting 8 to 25 minutes; S3 main carbonization: constant temperature for 15-60 min in a controlled atmosphere of 150-205℃, oxygen volume fraction ≤2.0%, and water vapor partial pressure 0.5-1.5 kPa; S4 stable cooling: Cool down to 80-90℃ at a rate of 2-6℃ / min, hold at this temperature for 10-30 minutes, and then allow to cool naturally. S5 Interface Stabilization: Adjusts the surface free energy of the bonded surface to 28-36 mN / m.

[0007] By adopting the above technical solution, a staged, depth-limited surface carbonization treatment method is proposed to address the problem of balsa wood's susceptibility to moisture absorption and expansion / contraction during wind turbine blade service. Through precise control of temperature, humidity, and oxygen content, the carbonization reaction occurs only within the micron-level range of the wood's surface, forming a dense and stable moisture-proof carbonized layer while maintaining the original strength and toughness of the internal structure, thus achieving a balance between moisture resistance and mechanical properties.

[0008] The process begins with preheating and drying at a lower temperature. By slowly increasing the temperature and maintaining continuous ventilation, the free and bound water inside the balsa wood is gradually removed, preventing cell wall cracking or internal stress buildup caused by rapid moisture evaporation. This stage ensures even moisture distribution within the wood during subsequent heating, minimizing structural damage.

[0009] This is followed by a selective pre-carbonization stage. At a temperature range of approximately 100 to 150°C, the hemicellulose in the balsa wood surface undergoes mild pyrolysis, resulting in the removal of some polar hydroxyl groups and reducing the surface's hydrophilicity without disrupting the overall fiber network structure. This stage provides the chemical basis for the subsequent formation of a stable carbonized layer.

[0010] The third stage is the main carbonization stage. Controlled carbonization takes place in a low-oxygen or trace water vapor environment within the range of 150 to 205°C. By precisely controlling the temperature, oxygen concentration, and reaction time, the carbonization reaction is limited to a depth of approximately 3 to 10 micrometers in the surface layer of the wood. The dense carbonized layer formed in this stage significantly reduces surface porosity and blocks capillary channels for moisture, while the internal cellulose and lignin structure of the wood remains intact, and its strength and toughness are largely unaffected.

[0011] After carbonization is completed, the material enters a stable cooling phase. Thermal stress is gradually released through segmented cooling to prevent cracks or peeling between the carbonized layer and the uncarbonized substrate due to temperature gradients, thereby ensuring the integrity and adhesion of the carbonized layer.

[0012] To further improve interfacial bonding performance, surface stabilization treatment can be optionally implemented. At this stage, slight physical or chemical modifications can be made to better match the carbonized layer surface with the resin system, thereby improving the wettability and adhesion of the adhesive layer.

[0013] This treatment creates a dense moisture-proof layer on the surface of the balsa wood, significantly reducing the rate of moisture ingress and egress and slowing down internal humidity changes. This effectively reduces dimensional changes and interfacial fatigue caused by moisture absorption expansion and contraction. It also results in high retention of compressive and shear strength, strong interlayer bonding, and significantly improved overall dimensional stability and service life.

[0014] As a further improvement of the present invention, the S3 main carbonization step uses the surface carbonization layer depth growth rate g as the main process control variable, which is achieved by adjusting the cross-sectional average linear velocity of the process gas to 0.15–0.35 m / s and / or controlling the convective heat transfer coefficient h to 18–45 W·m. -2 ·K -1 The g is controlled within the range of 0.05 to 0.25 μm / min.

[0015] The surface carbonization layer depth growth rate g is obtained by obtaining the carbonization layer depths d1 and d2 at two time points and calculating g = (d2 - d1) / (t2 - t1). Simple methods for obtaining d include: direct cross-sectional microscopic measurement (observing the color / structure interface between the carbonization layer and the matrix on a cross-section and measuring the thickness); sandpaper removal method using a micrometer or balance (grinding to the bare substrate under constant pressure and number of rotations, then converting the removed thickness or mass to d); surface resistivity or sheet resistance method (thin carbon layers are conductive while wood is insulating; first establish an Rs–d calibration and then use electrical measurements to infer d); ATR-FTIR characteristic peak ratio method (using the near-surface spectral ratio to calibrate the relationship between d and the standard); and indirect contact angle / surface energy method (converting d based on the correlation between the static drop contact angle or surface free energy and d).

[0016] This invention employs a handheld colorimeter as the primary method for on-site measurement of g. First, a set of samples with different isothermal times are prepared under the same material and atmosphere conditions. The reference thickness d is obtained using either the grinding method or the cross-sectional method, and the surface L* value is simultaneously measured. A linear or logarithmic model is used to establish a dL* calibration curve. After mass production, during the S3 isothermal process, the corresponding L* is rapidly measured at two fixed points t1 and t2. d1 and d2 are calculated from the calibration curve and substituted into g=(d2-d1) / (t2-t1) to calculate the growth rate.

[0017] As a further improvement of the present invention, when the surface carbonization layer depth growth rate g deviates from the target range of 0.05 to 0.25 μm / min, a univariate correction is performed sequentially according to the following steps: a) First, adjust the linear velocity of the process gas, and adjust the ratio of the gas volume flow rate Q to the equivalent cross-sectional area A of the flow channel u=Q / A within the range of 0.15~0.35m / s. The single adjustment step size is Δu=0.02~0.05m / s, and keep the static pressure deviation of the cavity not exceeding ±100Pa. After each linear velocity adjustment, g is remeasured at intervals of 3 to 5 minutes. If g still fails to return to the target range after three consecutive adjustments, proceed to step b. b) Fine-tune the isothermal time, using 2% to 5% of the current isothermal duration T as a single step to extend or shorten the isothermal time. The single adjustment range shall not exceed ±10%·T, and the cumulative adjustment range shall not exceed ±20%·T. If g still does not enter the target range after the above time adjustment, the constant temperature will be further fine-tuned, with the temperature adjustment range not exceeding ±3℃, or the stable cooling stage will be entered in advance.

[0018] During the carbonization process, even if the atmosphere composition and temperature conditions are within a suitable range, localized thermal imbalances can still occur on the balsa wood surface if the convective heat transfer intensity and the heat flux distribution on the surface layer are not coordinated. On the one hand, excessive heat flux will cause rapid pyrolysis of the surface layer, resulting in a dense, brittle carbon layer with fine cracks. On the other hand, insufficient heat transfer will hinder internal heat conduction, leading to insufficient carbonization layer depth, resulting in batch-to-batch differences in layer depth and fluctuations in mechanical properties.

[0019] In this method, the growth rate *g* of the carbonized layer depth is used as the core parameter for process control. By adjusting the matching relationship between the gas flow linear velocity and the convective heat transfer coefficient, the heat transfer rate is coordinated with the pyrolysis reaction rate of the wood cell wall. When the layer depth growth rate is too high, the gas linear velocity is appropriately reduced to weaken the convective heat transfer intensity, thereby reducing the surface heat flux; when the layer depth growth rate is too low, the isothermal time is appropriately extended or the temperature is slightly increased to allow heat transfer to penetrate into the inner layer. Through this sequential adjustment mechanism, a dynamic balance is achieved among heat transfer, reaction, and carbonized layer formation.

[0020] After being controlled in this way, the formation rate of the carbonized layer tends to be stable, the thermal gradient between the surface and the inner layer is gentle, the cell wall degradation reaction is more uniform, the occurrence of overburned or uncarbonized areas is significantly reduced, and the consistency of layer depth and structural integrity are improved.

[0021] As a further improvement of the present invention, in the S3 main carbonization stage, the flexural strength retention rate is ≥85% as the process control threshold. When the measured flexural strength retention rate is lower than the threshold, the main carbonization stage is terminated and the process proceeds to the S4 stable cooling step.

[0022] Setting a bending strength retention rate threshold of ≥85% ensures that the carbonization process achieves surface-level densification while preventing excessive pyrolysis of balsa wood thin-walled cells that could lead to structural embrittlement. When the detected value falls below this threshold, the isothermal treatment is terminated promptly, initiating a cooling phase. This effectively prevents a sudden drop in material mechanical properties, achieving a balance between reduced hygroscopicity and maintained bending and shear strength, thus improving the structural safety and reliability of balsa wood cores in the long-term service of wind turbine blades.

[0023] As a further improvement of the present invention, the final moisture content of the balsa wood after the S4 stabilization cooling is 5.0-7.5 wt%, and the volatile matter mass fraction is 70-82 wt%.

[0024] As a further improvement of the present invention, in the S4 stabilization cooling step, after the temperature is reduced to 80-90°C at the main carbonization temperature, it is held isothermally for 10-30 minutes; then it is reduced to 40-50°C at a rate of 2-6°C / min and then naturally cooled.

[0025] This stable cooling process, through step-by-step temperature control, can effectively alleviate the thermal stress concentration generated during the main carbonization stage and prevent the balsa wood cell structure from becoming brittle and cracking due to sudden cooling. The isothermal holding at 80-90℃ can promote the uniform diffusion of hemicellulose degradation products in the cell wall and reduce microscopic defects. The subsequent gradient cooling process further reduces the moisture content gradient difference, enabling the wood boards or strips to maintain dimensional stability during the natural cooling stage, ultimately improving the interfacial bonding strength between the carbonized layer and the substrate and its long-term resistance to damp heat.

[0026] As a further improvement of the present invention, the S5 interface stabilization adopts low-power plasma activation at atmospheric pressure, with air or oxygen as the process gas. The activation conditions are: plasma effective power 60-100W, distance from nozzle to plate surface 8-20mm, scanning line speed 0.10-0.30m / s, and single-sided processing time 20-45s.

[0027] As a further improvement of the present invention, after the preheating and drying in S1 is completed, the thickness-direction moisture content gradient ΔW of the wood board or wood strip is measured. When ΔW > 2.0wt%, reduce the heating rate by 10-20% or extend the S2 pulse count by one.

[0028] By introducing a dynamic detection and feedback control mechanism for the thickness-oriented moisture content gradient ΔW during the staged micro-carbonization process, this invention achieves adaptive matching control between the internal humidity field and carbonization reaction rate of balsa wood, thereby significantly improving carbonization uniformity, mechanical properties, and energy efficiency. When ΔW is greater than 2.0 wt%, the system automatically reduces the heating rate by 10–20% or extends the pre-carbonization pulse time to suppress excessively rapid surface reaction. After this control, the porosity distribution of the carbon layer is more continuous, the degree of cell wall carbonization is consistent, local embrittlement and stress concentration are avoided, and the bending strength and dimensional stability of the boards or strips are improved.

[0029] In addition, the ineffective heating stage is reduced by ΔW feedback regulation, which shortens the carbonization cycle, reduces unit energy consumption, and ensures the consistency between samples in the furnace.

[0030] The present invention, by adopting the above technical solution, has the following beneficial effects: 1. By implementing a phased, limited-depth surface micro-carbonization treatment under a controlled atmosphere, a dense and stable moisture-proof carbonized layer is formed on the surface of balsa wood. This effectively blocks capillary channels in cell cavities and pits, significantly reducing the rate of moisture adsorption and migration. The treated balsa wood exhibits significantly reduced moisture content fluctuations in humid and hot environments, and its dimensional stability is significantly improved, fundamentally inhibiting structural deformation and interfacial fatigue caused by moisture absorption expansion and shrinkage.

[0031] 2. The micro-dense layer formed after surface carbonization makes the pore size distribution of balsa wood more uniform, and moisture is less likely to accumulate at the interface. After interface stabilization treatment, the surface free energy is maintained in a reasonable range of 28-36 mN / m, which can improve the wettability of the resin adhesive and avoid interface embrittlement caused by excessive penetration. The peel strength and interlaminar shear strength of the bonding interface are significantly improved under long-term humid heat cycling.

[0032] 3. This invention uses the surface carbonization layer depth growth rate g as the core control parameter. By monitoring g and adjusting the gas linear velocity, heat transfer coefficient and isothermal time in conjunction, the heat transfer rate is matched with the cell wall pyrolysis reaction rate, ensuring that the carbonization layer depth is stable within the range of 3 to 10 μm. This effectively avoids the problems of over-burning, under-burning and uneven layer depth, making the carbonization layer structure dense and continuous and firmly attached, thus balancing the moisture-proof performance of the balsa wood surface with its internal mechanical properties.

[0033] 4. By setting a stable cooling stage and implementing segmented cooling after isothermal holding at 80–90℃, the thermal stress concentration formed during the main carbonization stage is significantly alleviated, preventing cracks or delamination between the carbonized layer and the matrix. This cooling method also promotes the uniform diffusion of hemicellulose degradation products, further improving the microstructural integrity and dimensional stability of the material.

[0034] 5. During the preheating and drying stage, dynamic detection of the moisture content gradient ΔW is introduced. When ΔW > 2.0 wt%, the heating rate is automatically adjusted or the pre-carbonization pulse time is extended to match the humidity gradient with the carbonization rate, avoiding excessively rapid surface reaction and internal heat stagnation. This mechanism improves carbonization uniformity and energy utilization efficiency, shortens the process cycle, and reduces energy consumption. Detailed Implementation

[0035] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0036] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0039] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0040] Example 1: To facilitate process evaluation, the following release criteria are set: flexural strength retention rate not less than 85%; microlayer depth controlled at 5–7 μm; surface free energy maintained at 28–36 mN / m (of which the polar component accounts for 20–40%). Any process that does not meet any of these criteria is considered to require readjustment of the process.

[0041] The balsa wood staged controlled micro-carbonization treatment method for wind turbine blades disclosed in this embodiment includes the following steps: S1 preheating and drying: A density of 0.13 g / cm³ was selected. 3 The balsa wood planks, with uniform growth rings and no knots or rot, are 1000mm × 100mm × 10mm in size. Before processing, they are left to stand for 48 hours at 20℃ and (50±5)% relative humidity to balance the initial moisture content.

[0042] Drying was carried out in a controlled convective heat transfer chamber, with an inlet dry-bulb temperature of 70°C and a return air temperature of approximately 60°C. The convective heat transfer coefficient was calibrated to be 22–26 W·m⁻¹. -2 ·K -1 The temperature was increased to 70℃ at a rate of 1.4℃ / min and held at that temperature for 80 minutes, reducing the moisture content inside the plate to 6.9wt%.

[0043] After drying, sawdust was collected from three points: the surface layer, the middle layer, and the back layer (drill bit φ6mm, feed ≤1mm / s, depths 1.0mm for the surface layer, 5.0mm for the middle layer, and 1.0mm for the back layer; samples were sealed in aluminum foil bags after <10s and weighed within 2 minutes). The measured moisture contents were 8.0wt% for the surface layer, 6.7wt% for the middle layer, and 6.4wt% for the back layer, with a thickness gradient ΔW = 1.6wt%.

[0044] S2 selective pre-carbonization: Two pulsed isothermal programs were used: 120℃×12min; 140℃×10min. Between the two pulses, the temperature was increased slowly at 1.0℃ / min, with a 2-min isothermal buffer. A slightly humid, low-oxygen environment was maintained within the chamber (oxygen volume fraction <3.0%, water vapor volume fraction 2–6%, monitored online by a dew point sensor with fluctuations ±0.2℃), achieving gentle, selective pre-carbonization without damaging the cell cavity structure.

[0045] S3 primary carbonization: The target constant temperature is 180℃. Before entering the section, the chamber is purged with nitrogen for 3 minutes to ensure an oxygen volume fraction ≤2.0%; saturated steam is injected to maintain a water vapor volume fraction of 1-3%, and the online dew point is controlled at -15 to -10℃, with an oxygen volume fraction ≤2.0% (oxygen electrode accuracy ±0.1%V / V). The equivalent cross-section of the channel is set to A=0.045m. 2 Volumetric flow rate Q = 0.54 m³ 3 / min, the average linear velocity of the cross section u=Q / A=0.20m / s; the convective heat transfer coefficient under this condition is h=24~28W·m -2 ·K -1 .

[0046] Using a handheld colorimeter and the L* method (with prior regression established using 2, 5, and 8 μm depth standard samples), the layer depth was converted online and the growth rate g was calculated. g was measured at 10, 18, and 26 minutes, respectively, at 0.20, 0.19, and 0.19 μm / min, remaining stable within the target window of 0.05–0.25 μm / min, without requiring corrections for linear velocity, isothermal time, or set temperature. The total isothermal time was 32 minutes.

[0047] The flexural strength retention rate of the three-point bending test strips sampled at the end of the isothermal stage was 90%–92%, which is higher than the control threshold. After the section was removed, a cross-section was taken at 200 mm from the end of the plate, and the average depth of the carbonized layer on the surface was measured to be 6.0 μm under a microscope.

[0048] S4 stabilizes cooling: A two-stage cooling process was adopted. First, the temperature was lowered to 85°C at a rate of 3°C / min and held isothermally for 20 minutes to relieve stress; then, the temperature was lowered to 45°C at a rate of 4°C / min and allowed to cool naturally.

[0049] After standing for 2 hours after being taken out of the furnace, the final moisture content was measured to be 6.4 wt%, and the volatile matter content was 75 wt%.

[0050] S5 Interface Stabilization: Low-power plasma activation treatment at atmospheric pressure was employed. The process gas was air, with an effective power of 75W. The distance between the nozzle and the plate surface was 12mm, the scanning linear velocity was 0.22m / s, and the single-sided treatment time was 30s.

[0051] After treatment, the surface free energy was calculated to be 32 mN / m (polar component approximately 29%) using the Owens–Wendt method under standard conditions with a water / diiodomethane contact angle combination.

[0052] Under low-oxygen conditions with a certain amount of water vapor, the hemicellulose component first undergoes pyrolysis and dehydroxylation reactions, resulting in the removal of low-molecular-weight products. This reduces the number of hydrophilic groups in the wood, improving dimensional stability. Simultaneously, maintaining a moderate water vapor partial pressure inhibits oxidative combustion reactions, reducing the degree of breakage in cellulose and lignin, thus significantly slowing down strength loss. Compared to conventional dry-air carbonization processes, this process achieves a balanced reaction of "micro-carbonization—weak oxidation—low damage" at 180–185 °C by controlling the oxygen volume fraction to ≤2.0% and the water vapor volume fraction to 1–3%.

[0053] By controlling the surface carbonization layer depth to 5–7 μm and using the layer depth growth rate g as the main control variable, the carbonization process can be kept stable, thus achieving the optimal trade-off between improving surface wettability and maintaining flexural strength.

[0054] Example 2: A density of 0.13 g / cm³ was selected. 3 The balsa wood planks, with uniform growth rings and free of knots and decay, measure 1000mm × 100mm × 10mm. Before processing, they are left to stand for 48 hours at 20℃ and (50±5)% relative humidity to balance the initial moisture content. To facilitate process sampling, three monitoring samples are prepared for each batch, and three untreated control samples are prepared as benchmarks for mechanical and interface parameters.

[0055] The balsa wood staged controlled micro-carbonization treatment method for wind turbine blades disclosed in this embodiment includes the following steps: S1 preheating and drying: Drying was carried out in a controlled convective heat transfer chamber, with an inlet dry-bulb temperature of 70°C and a return air temperature of approximately 60°C. The convective heat transfer coefficient was calibrated to be 22–26 W·m⁻¹. -2 ·K -1 The temperature was increased to 70℃ at a rate of 1.4℃ / min and held for 80 minutes to reduce the moisture content of the board to 6.9wt% (drying method). After drying, φ6mm sawdust was drilled at three points: surface layer, middle layer, and back layer, and the moisture content was measured to be 8.0wt% / 6.7wt% / 6.4wt%, with a thickness gradient of ΔW=1.6wt%.

[0056] S2 selective pre-carbonization: Two pulsed isothermal programs were used: 120℃×12min; 140℃×10min. Between the two pulses, the temperature was increased slowly at 1.0℃ / min, with a 2-min isothermal buffer. A slightly humid, low-oxygen environment was maintained within the chamber (oxygen volume fraction <3.0%, water vapor volume fraction 2–6%, monitored online by a dew point sensor with fluctuations ±0.2℃), achieving gentle, selective pre-carbonization without damaging the cell cavity structure.

[0057] S3 primary carbonization: The main carbonization was carried out at a constant temperature of 185℃. The chamber was first purged with nitrogen for 3 minutes to reduce the oxygen volume fraction to ≤2.0%. Then, saturated steam was injected to maintain the water vapor volume fraction at 1-3%. The online dew point was controlled at -15 to -10℃, and the oxygen volume fraction was ≤2.0% (oxygen electrode accuracy ±0.1%V / V).

[0058] During the isothermal period, the growth rate g of the surface carbonization layer depth was used as the main control variable. The airflow cross-section A = 0.045m. 2 The initial volumetric flow rate Q = 0.64 m³ / s. 3 / min, average linear velocity of cross section u=0.24m / s, convective heat transfer coefficient h=24~28W·m -2 ·K -1 .

[0059] The depth of the carbonization layer was monitored in real time using a pre-calibrated handheld colorimeter with the L* method. At 10 min and 20 min after furnace start-up, g was 0.28 μm / min and 0.27 μm / min, respectively, which is higher than the target range (0.05~0.25 μm / min).

[0060] Following the principle of single-variable correction, first adjust the linear velocity: reduce Q by 0.08m. 3 The velocity was adjusted by Δu / min to reduce u to 0.21 m / s (static pressure fluctuation ±60 Pa). After 4 minutes, g stabilized at 0.23 μm / min. During the isothermal period, if g locally rose to 0.26 μm / min again, a small adjustment Δu = 0.02 m / s was made, reducing u to 0.19 m / s, and g returned to 0.22 μm / min. If g still did not enter the target range after adjusting the linear velocity, the isothermal time was first finely adjusted by ±10% according to the single-variable principle. If the target was still not met, the isothermal temperature was then finely adjusted by ±3℃ to prevent the accumulation of thermal stress.

[0061] The total constant temperature time was 36 minutes. During this period, the flexural strength retention rate was tested by the rapid three-point bending method of small test strips and found to be 88% to 90%, which was higher than the control threshold of 85%. The main carbonization was then completed.

[0062] After exiting the section, the average thickness of the surface carbonized layer was measured to be 6.1 μm using a microscope.

[0063] S4 stabilizes cooling: A two-stage cooling process was adopted. First, the temperature was lowered to 85°C at a rate of 3°C / min and held isothermally for 20 minutes to relieve stress; then, the temperature was lowered to 45°C at a rate of 4°C / min and allowed to cool naturally.

[0064] After standing for 2 hours after being taken out of the furnace, the final moisture content was measured to be 6.4 wt%, and the volatile matter content was 75 wt%.

[0065] S5 Interface Stabilization: Low-power plasma activation treatment at atmospheric pressure was employed. The process gas was air, with an effective power of 80W. The distance between the nozzle and the plate surface was 12mm, the scanning linear velocity was 0.20m / s, and the single-sided treatment time was 30s.

[0066] After treatment, the surface free energy was measured to be 31 mN / m under standard experimental conditions, with the polar component accounting for approximately 28%.

[0067] Example 3 The difference between this embodiment and Embodiment 1 is that in the S4 stabilization cooling step, the temperature is first lowered to 85°C at a rate of 3°C / min and held isothermally for 20 minutes to relieve stress; then it is naturally cooled to room temperature. After being removed from the furnace and left to stand for 2 hours, the final moisture content is 6.0 wt% and the volatile matter content is 74 wt%.

[0068] Example 4: The balsa wood staged controlled micro-carbonization treatment method for wind turbine blades disclosed in this example includes the following steps: S1 preheating and drying: A density of 0.13 g / cm³ was selected. 3 A 1000mm×100mm×10mm balsa wood board was pre-equilibrated for 48 hours (20℃, relative humidity 50±5%). The temperature was increased at 1.5℃ / min within a controlled convection heat exchange chamber, with an inlet dry-bulb temperature of 70℃ and a return air temperature of approximately 60℃. The convection heat transfer coefficient was 22–26 W·m⁻². -2 ·K -1 The temperature was maintained at 80 min until the moisture content inside the board decreased to 7.1 wt%. After drying, φ6 mm sawdust samples were drilled from the surface / middle / back layers and measured to be 8.6 / 6.5 / 6.0 wt%, with a thickness gradient ΔW = 2.6 wt%. This embodiment does not reduce the subsequent heating rate or increase the number of pulses based on this, keeping the subsequent program consistent with the conventional settings.

[0069] S2 selective pre-carbonization: Two pulsed isothermal cycles were employed: 120℃×12min; 140℃×10min. Between the two pulses, the temperature was gradually increased at a rate of 1.0℃ / min with a 2min isothermal buffer. During the cycle, a slightly humid and low-oxygen environment was maintained (oxygen volume fraction <3.0%, water vapor volume fraction 2-6%, monitored online by a dew point sensor, with fluctuations within ±0.2℃).

[0070] S3 primary carbonization: The target constant temperature is 185℃. Before entering the section, the chamber is purged with nitrogen for 3 minutes to ensure an oxygen volume fraction ≤2.0%; saturated steam is injected to maintain a water vapor volume fraction of 1-3%, and the online dew point is controlled at -15 to -10℃, with an oxygen volume fraction ≤2.0% (oxygen electrode accuracy ±0.1%V / V). The equivalent cross-section of the channel is A=0.045m², the initial volumetric flow rate is Q=0.64m³ / min, and the average linear velocity of the cross-section is u≈0.24m / s; the convective heat transfer coefficient under this condition is 24-28 W·m³. -2 ·K -1 .

[0071] Using a handheld colorimeter and the L* method (with regression analysis of 2, 5, and 8 μm standard samples), the layer depth was converted online and the growth rate g was calculated. After 10 and 18 minutes of constant temperature measurement, g was found to be 0.27 and 0.26 μm / min, respectively, which is too high. Following the single-variable correction procedure, the linear velocity was adjusted first, and Q was reduced by 0.08 m. 3 / min (Δu≈0.03m / s, static pressure fluctuation in the cavity ±60Pa), g was measured again after 4 minutes, g=0.23μm / min; g was measured again after 26 minutes, g=0.22μm / min, and it stably entered the target range of 0.05~0.25μm / min, without the need for further adjustment of the isothermal time or temperature. The total isothermal time was 34 minutes.

[0072] Three-point bending test strips were randomly sampled at the end of the isothermal stage, and the flexural strength retention rate was 86%–88%, which is higher than the control threshold. After the section was removed, a cross-section was taken at 200 mm from the end of the plate, and the average depth of the surface carbonized layer was measured to be 5.9 μm.

[0073] S4 stabilizes cooling: First, the temperature was lowered to 85°C at a rate of 3°C / min and held isothermally for 20 minutes; then, it was lowered to 45°C at a rate of 4°C / min and allowed to cool naturally to room temperature. After standing for 2 hours after removal from the furnace, the final moisture content was 6.5 wt% and the volatile matter content was 74 wt%.

[0074] S5 Interface Stabilization: Atmospheric pressure low-power plasma activation: Air was used as the process gas, with an effective power of 80W. The nozzle distance from the plate surface was 12mm, and the scanning linear velocity was 0.20m / s. Single-sided treatment lasted 30s, and both sides were treated sequentially. After treatment, the surface free energy calculated using the water / diiodomethane contact angle was 31mN / m (polar component approximately 28%), falling within the 28–36mN / m window. Measurements were taken every 200mm along the plate length, with a deviation ≤±1.5mN / m.

[0075] Comparative Example 1: This comparative example discloses a method for carbonizing balsa wood, comprising the following steps: S1: Select a density of 0.13 g / cm³ 31000mm×100mm×10mm balsamic wood panels were pre-equilibrated for 48 hours (20℃, relative humidity 50±5%). A conventional box-type hot air circulating oven was used, with an inlet air temperature of 60℃ and natural return air. The temperature was increased to 60℃ at approximately 1.0℃ / min and held for 120 minutes. The moisture content inside the panels was measured to be 7.8wt%. Samples taken from the surface / middle / back layers showed moisture contents of 8.9 / 7.0 / 6.5wt%, with a thickness gradient ΔW = 2.4wt%.

[0076] S2: The process is carried out in a dry, hot air environment within the same box-type oven: set the temperature to 200℃ and maintain it for 60 minutes. The oven is under normal pressure with natural air replacement, and the oxygen volume fraction is close to the ambient value (approximately 20-21%); the fan is only used for internal circulation.

[0077] After the constant temperature period, small test strips were taken for rapid three-point bending tests, and the bending strength retention rate was 74% to 79%. When a cross-section was taken at 200 mm from the end of the plate, the average depth of the surface carbonization layer was 14.2 μm, ranging from 8.5 to 22.6 μm; microcracks penetrating into the earlywood were visible in some areas.

[0078] S3: After heating is turned off, allow the furnace to cool naturally to room temperature. After standing for 2 hours after being removed from the furnace, the final moisture content was measured to be 4.8 wt%, and the volatile matter content was 66 wt%.

[0079] Comparative Example 2: This comparative example discloses a method for carbonizing balsa wood, comprising the following steps: S1 preheating and drying: The process was conducted in a controlled convective heat transfer chamber, with an inlet dry-bulb temperature of 70°C and a return air temperature of approximately 60°C. The convective heat transfer coefficient was 22–26 W·m⁻¹. -2 ·K -1 The temperature was increased to 70℃ at a rate of 1.4℃ / min and held at that temperature for 85 minutes, reducing the moisture content inside the plate to 7.0wt%. The moisture contents of the surface layer, middle layer, and back layer were 7.9wt%, 6.6wt%, and 6.3wt%, respectively, with a thickness gradient ΔW = 1.6wt%, which is within the optimal range, requiring no adjustment of the heating rate or the number of pulses.

[0080] S2 selective pre-carbonization: Two pulsed isothermal programs were used: 120℃×12min; 140℃×10min. Between the two pulses, the temperature was increased slowly at 1.0℃ / min, with a 2min isothermal buffer. During the phase, a slightly humid low-oxygen environment was maintained with an oxygen volume fraction of <3.0% and a vapor partial pressure of ≈0.3kPa. The sample surface remained a uniform light brown color, with no carbonization bands appearing.

[0081] S3 primary carbonization: The target constant temperature is 185℃. Before entering the section, the chamber is purged with nitrogen for 3 minutes to ensure that the oxygen volume fraction is ≤2.0%; then saturated steam is injected to maintain the water vapor partial pressure at 0.9~1.1kPa.

[0082] The equivalent cross-section of the channel is A = 0.045 m², the volumetric flow rate is Q = 0.68 m³ / min, the average linear velocity of the cross-section is u ≈ 0.25 m / s, and the convective heat transfer coefficient is h = 27–31 W·m³. -2 ·K -1 .

[0083] The carbonization layer depth was calculated using the L* method with a handheld colorimeter. The g values ​​measured at 10, 18, and 26 minutes were 0.29, 0.31, and 0.32 μm / min, respectively, which were consistently outside the target range (0.05–0.25 μm / min).

[0084] This comparative example did not undergo any corrections to the linear velocity, isothermal time, or temperature, maintaining the original parameters for 35 minutes of isothermal control. The flexural strength retention rate of small test strips sampled during the isothermal phase was 80%–82%, significantly lower than that of Example 1 (approximately 90%). After the main carbonization was completed, the average thickness of the surface carbonized layer, measured under a cross-section microscope, was 9.8 μm, indicating a significantly thicker layer with high dispersion, and some surface layers showed light-colored microcracks.

[0085] S4 stabilizes cooling: The temperature was lowered to 85°C at a rate of 3°C / min and held isothermally for 20 min; then lowered to 45°C at a rate of 4°C / min and allowed to cool naturally to room temperature. After standing for 2 hours after removal from the furnace, the final moisture content was measured to be 6.2 wt% and the volatile matter content to be 74 wt%.

[0086] S5 Interface Stabilization: Activation was performed using low-power plasma at atmospheric pressure, with air as the process gas. The effective power was 80W, the nozzle distance from the plate surface was 12mm, the scanning linear velocity was 0.20m / s, and the single-sided processing time was 30s. The surface free energy after treatment was 30mN / m (polar component approximately 27%), which is within the target range.

[0087] Performance testing: (1) Dimensional stability of moisture absorption / desorption cycles (moisture-induced expansion and contraction): After equilibrating the samples at (23±2)℃ and (50±5)%RH for 48 hours, they were placed in a controlled humidity chamber and cycled in the sequence of "RH 30%→85%→30%", maintaining each humidity point for 24 hours. At each steady-state point, the thickness and width were measured using a dial gauge, and the maximum and minimum values ​​of each cycle were recorded. The thickness change rate TS (%) and the width change rate WS (%) were calculated as "peak-valley difference / initial size × 100%" for the first and third cycles, respectively. At least 5 samples were collected in each direction, and the average value was taken.

[0088] (2) Water absorption rate and water absorption in 2 hours (%): The five sides, excluding the exposed surface, were sealed with wax. The sample was placed horizontally with the exposed surface in contact with the surface of settled deionized water; it was removed after 15 min and 120 min, the surface water was wiped off, and the sample was weighed quickly. The "water absorption rate at 15 min (%)" and "water absorption rate at 2 h (%)" were calculated based on the dry weight. The "average water absorption rate (15–120 min, % / h)" was calculated as (2 h mass increment - 15 min mass increment) / 1.75 h. For each group with n ≥ 5, the average value was taken.

[0089] (3) Shear strength τ (MPa) of the adhesive interface: Leaf samples were prepared by vacuum infusion using a resin adhesive formulation for leaf blades. Curing and post-curing were performed according to the adhesive technical specifications. The samples were divided into two groups: ① a room temperature group (23℃ / 50%RH) tested directly; ② a damp heat group tested after aging for 72 hours at (50℃ / 95%RH). Tensile shearing was performed using a standard universal tensile testing machine to determine τ; damp heat retention rate = τ after damp heat / τ at room temperature × 100%. For each group, n≥6, and the average value was taken.

[0090] The test results are shown in Table 1 below.

[0091] Table 1:

[0092] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades, characterized in that, Includes the following steps: S1 Preheating and Drying: Under controlled convection heat transfer conditions, the wood boards or strips are heated at 40–95°C and dehumidified to reduce the moisture content inside the boards to 6.0–9.0 wt%. S2 selective pre-carbonization: 1 to 3 pulsed isothermal treatments at 100 to 150°C, each lasting 8 to 25 minutes; S3 main carbonization: constant temperature for 15-60 min in a controlled atmosphere of 150-205℃, oxygen volume fraction ≤2.0%, and water vapor volume fraction 1-3%; S4 stable cooling: Cool down to 80-90℃ at a rate of 2-6℃ / min, hold at this temperature for 10-30 minutes, and then allow to cool naturally. S5 Interface Stabilization: Adjusts the surface free energy of the bonded surface to 28-36 mN / m.

2. The method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades according to claim 1, characterized in that, The S3 main carbonization step uses the surface carbonization layer depth growth rate g as the main process control variable, which is achieved by adjusting the cross-sectional average linear velocity of the process gas to 0.15–0.35 m / s and / or controlling the convective heat transfer coefficient h to 18–45 W·m. -2 ·K -1 The g is controlled within the range of 0.05 to 0.25 μm / min.

3. The method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades according to claim 2, characterized in that, When the growth rate g of the surface carbonization layer depth deviates from the target range of 0.05–0.25 μm / min, univariate correction is performed sequentially according to the following steps: a) First, adjust the linear velocity of the process gas, and adjust the ratio of the gas volume flow rate Q to the equivalent cross-sectional area A of the flow channel u=Q / A within the range of 0.15~0.35m / s. The single adjustment step size is Δu=0.02~0.05m / s, and keep the static pressure deviation of the cavity not exceeding ±100Pa. After each linear velocity adjustment, g is remeasured at intervals of 3 to 5 minutes. If g still fails to return to the target range after three consecutive adjustments, proceed to step b. b) Fine-tune the isothermal time, using 2% to 5% of the current isothermal duration T as a single step to extend or shorten the isothermal time. The single adjustment range shall not exceed ±10%·T, and the cumulative adjustment range shall not exceed ±20%·T. If g still does not enter the target range after the above time adjustment, the constant temperature will be further fine-tuned, with the temperature adjustment range not exceeding ±3℃, or the stable cooling stage will be entered in advance.

4. The method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades according to claim 1, characterized in that, In the S3 main carbonization stage, the flexural strength retention rate is ≥85% as the process control threshold. When the measured flexural strength retention rate is lower than this threshold, the main carbonization stage is terminated and the process proceeds to the S4 stabilization and cooling step.

5. The method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades according to claim 1, characterized in that, After the S4 stabilization cooling process, the final moisture content of the balsa wood was 5.0–7.5 wt%, and the volatile matter content was 70–82 wt%.

6. The method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades according to claim 1, characterized in that, In the S4 stabilization cooling step, after the temperature is reduced to 80-90°C at the main carbonization temperature, it is held isothermally for 10-30 minutes; then it is reduced to 40-50°C at a rate of 2-6°C / min and then naturally cooled.

7. The method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades according to claim 1, characterized in that, The S5 interface stabilization adopts low-power plasma activation at atmospheric pressure, with air as the process gas. The activation conditions are: effective plasma power 60-100W, distance from the nozzle to the plate surface 8-20mm, scanning line speed 0.10-0.30m / s, and single-sided processing time 20-45s.

8. The method for staged controlled micro-carbonization treatment of balsa wood for wind turbine blades according to claim 1, characterized in that, After the preheating and drying in step S1 is completed, the thickness-wise moisture content gradient ΔW of the wood board or strip is measured. When ΔW > 2.0 wt%, reduce the heating rate by 10-20%, and / or add one pulse isothermal treatment in step S2.

Images