CN102658693B - Hot-pressing preparation method for aluminum-coated TPU (thermoplastic polyurethane) film resistance reduction microgrooves for airship skin - Google Patents

Abstract

The invention discloses a thermal embossing method for preparing Al-plated TPU film drag-reducing micro-grooves for airship skins, and relates to a method for preparing airship skin materials. The invention aims to solve the problems of poor weather resistance, large resistance and poor heat insulation of the skin. Preparation method: 1. Prepare a template made of Al; 2. Clean the template and apply a release agent; 3. Use vacuum evaporation technology to evaporate a layer of aluminum film on the surface of the TPU film; Under the condition of a pressure of 65-75N, embossing for 20-25 minutes, an Al-plated TPU film with drag-reducing micro-grooves is obtained. The invention obtains micro-grooves with high resolution and high aspect ratio structure, and the Al layer on the surface of the film can reflect solar radiation, increase weather resistance, and has simple process equipment and low cost. The invention is used for preparing a drag-reducing and weather-resistant skin material for an airship near space.

Description

Hot embossing preparation method of Al-plated TPU film drag-reducing micro-grooves for airship skin

technical field

The invention relates to a preparation method of an airship skin material.

Background technique

Near space can also be called near space or near space. At the same time, this altitude is also the area where the stratosphere is located, above the layer of natural phenomena such as wind, rain, thunder, and lightning, and belongs to the high security zone. After entering the 21st century, the value of the special field of near space has been paid more and more attention by countries all over the world, and various military powers have regarded it as a new field of military struggle in the 21st century. In recent years, the application research of near-space airship as a communication and navigation platform has attracted worldwide attention. In the process of developing near-space airships, the key technical issues involved are currently the focus of research by various countries. Since near-space airships are not low-altitude airships, many concepts of development, such as working environment, skin materials, energy, power propulsion, and other key technologies are completely different from low-altitude airships, and many aspects are facing great challenges. The four existing airship verification tests that have entered the near space have proved the feasibility and application value of the near space airship platform. During the application process, many key scientific problems have been raised to be solved urgently. Among them, how to design the skin surface microgroove The effective reduction of wind resistance by slots is one of the core issues to achieve the goal of long endurance of large near-space airships.

Inspired by nature, research has found that the frictional resistance on the surface of a moving object can be effectively reduced by properly laying certain grooves on the surface of the moving object. In recent years, a certain amount of research has been carried out on the drag reduction effect and mechanism of rigid surface grooves worldwide, and some engineering application numerical simulations and experimental verifications have been carried out. It is believed that the groove drag reduction technology is currently the most ideal surface drag reduction method.

At present, although the drag reduction technology of grooves has been applied to aircraft and ships, the research on the drag reduction of grooves for flexible substrates such as skins has not been reported yet. Different from the drag reduction of grooves on the surface of general rigid surface high Reynolds number aircraft, the flow field on the surface of the near-space airship is mainly thin and low-speed gas, which has low Reynolds number operation characteristics; at the same time, the near-space airship is a type of typical flexible structure. Under the action of wind load, large local deformation is easy to occur, and the grooves on the surface of the capsule are also flexible micro-folds, which are deformable under the action of the external flow field and are easy to couple with the external flow field. Large adjacent space airships are surrounded by flexible skin materials, and their resistance is mainly composed of pressure difference resistance and hull surface friction resistance. Since the airship has a larger surface area, reducing the surface frictional resistance can greatly reduce the total resistance of the hull. Therefore, it is very important to study the preparation process of micro-grooves on the surface of skin materials.

The research on micro-groove preparation methods in domestic and foreign literature can be divided into: micro-machining (such as micro-engraving, micro-forging, etc.), laser etching, soft etching technology and hot embossing methods. For example, Zhang Deyuan of Beihang University and others used pretreated shark skin as a micro-replication template, and used the soft molding technology in the soft engraving process to prepare a silicone rubber elastic negative template, and grafted water-based epoxy resin and polyacrylamide The copolymer is used as the base material, and the elastic female template is replicated and molded to form a composite drag-reducing shark skin with nano long-chain drag-reducing interface and realistic micro-groove morphology. The preparation method of the groove has not been reported in the relevant literature.

TPU film is one of the existing stratospheric airship skin weather-resistant layer materials. Although it has a certain aging resistance, it still cannot achieve the ideal effect that people hope for when used in harsh environments.

Contents of the invention

The invention aims to solve the problems of poor weather resistance, high resistance and poor heat insulation of the existing stratospheric airship skin, and provides a hot embossing preparation method of Al-plated TPU film drag-reducing micro-grooves for the airship skin.

The hot embossing preparation method of the Al-plated TPU film drag-reducing micro-groove for the airship skin of the present invention is carried out according to the following steps:

1. Use the ultra-precision micromachining machine tool system to process the Al template with a "V" groove structure on the surface. The "V" groove forms alternate transparent grooves on the surface of the Al template. Each groove The width s is 90-110 μm, and the depth h is 90-110 μm;

2. Ultrasonic clean the Al template prepared in step 1 with acetone for 15-20 minutes, and after drying, apply three layers of film cleaning agent, then three layers of sealing agent, and finally three layers of water-based mold release agent , the coating time interval of each layer is 15-20min;

3. Using vacuum evaporation technology, an aluminum film with a thickness of 90-110 μm is evaporated on the surface of the TPU film to be imprinted to obtain an Al-coated TPU film;

4. Install and fix the Al template processed in step 2 on the indenter of the hot embossing machine, place the quartz glass substrate on the bearing table of the hot embossing machine, and spread the Al-plated TPU film obtained through step 3 On the quartz glass substrate, after preheating to 140-145°C, lower the pressure head, and after the Al template is in contact with the Al-plated TPU film, the pressure is raised to 65-75N. Under the condition of 75N, imprint for 20-25 minutes, stop heating, cool naturally, and hold the pressure for 2-2.5 hours, remove the pressure, lift the pressure head, and complete the heat treatment of Al-plated TPU film drag-reducing micro-grooves for airship skin. Imprint preparation.

The thermal embossing mechanism analysis of film material of the present invention is as follows:

Thermal embossing of film materials is based on thermal flow molding of polymers to achieve graphic replication. When the size of the template pattern is small and the periodicity is strong, the polymer is easy to transfer completely, and the template pattern can be well replicated on the polymer substrate; when the size of the template pattern is large, the transfer of the polymer is not complete and the intrinsic relaxation behavior will be in the Various special patterns appear in the patterned area and its adjacent areas. The mechanism can be described as: the template is pressed into the polymer, the polymer under the convex area of the template is pressed into the cavity between two adjacent convex areas, and rises along the side wall of the cavity, while the cavity Due to the extrusion of the fluid on both sides, the original polymer inside will protrude and deform upwards, forming two joints at the junction of the two fluids. As the imprinting time prolongs, the polymers on both sides will continue to flow into the cavity Extrusion, the original polymer continues to compress and rise, and finally the entire cavity is filled. After a period of heat balance, the template and the substrate are separated, and the patterned polymer is obtained. Figure 5 is a schematic diagram of the polymer filling mechanism in the thermal embossing process. The downward arrow represents the extrusion force applied by the template to the film, the upward arrow represents the extrusion force applied by the substrate to the film, and the middle arrow represents the film under pressure. Flow direction. If the pressure applied during the embossing process is large, after the polymer under the template protrusion flows into the cavity, due to the effect of surface tension, it will first form hillocks one by one. If the polymer is thick enough and the imprinting time is long enough, then The mountain bag will gradually fuse into one, achieving complete filling of the cavity.

The beneficial effects of the present invention are: there is no preparation method of micro-grooves on the surface of the flexible film so far. The present invention uses the thermoplastic polyurethane film (TPU) as the flexible surface of the airship skin weather-resistant layer adjacent to the space, and adopts the method of hot embossing with a hard template Composite micro-grooves on the surface of the flexible surface, realized micro-nano replication of structural patterns on flexible films at the micro-nano scale, obtained micro-grooves with high-resolution, high-aspect-ratio structures, and compared the shape retention capabilities of imprinted films Well, the change of elastic recovery is relatively small, and the numerical simulation results of groove drag reduction show that the TPU film with a micro-groove structure prepared by the present invention has a drag reduction effect, and a thickness of 90-110 μm is evaporated on the TPU film. Aluminum layer, the coating is soft and has almost the original elasticity of the film, the coating has good adhesion, the surface is uniform and smooth, has excellent wear resistance and flexibility, good high temperature and low temperature resistance, ozone aging resistance, oxygen aging resistance , light aging and weather aging, the loss of tensile strength value of Al-plated TPU film after 20 days of irradiation is 39.29%, which is greatly improved compared with the original TPU film, and the resilience is lower than the original film, and the shape retention of the embossed film The ability is very good, the change of elastic recovery is very small, the compression permanent deformation is relatively improved, it can reflect the energy of solar radiation well, it can play the role of heat insulation and cooling, reflect ultraviolet rays to protect the skin material, and improve the weather resistance of the skin material It plays an important role in further improving the load and prolonging the dwell time. It is a simple, convenient, direct, efficient, and low-cost method to improve the weather resistance and drag reduction performance of TPU films, and has great application prospects.

The invention is used for preparing a drag-reducing and weather-resistant skin material for an airship near space.

Description of drawings

Figure 1 is a schematic diagram of the structure of the airship skin material; where 1 represents the weather-resistant layer, 2 represents the load-bearing layer, 3 represents the barrier layer, and 4 represents the heat-sealing layer;

Fig. 2 is the sample assembly diagram of the hot embossing process of the present invention, wherein 5 represents the template made of Al, 6 represents the TPU film, and 7 represents the quartz glass substrate;

Fig. 3 is the template made of Al, TPU thin film and quartz glass substrate schematic diagram in hot pressing process of the present invention;

Fig. 4 is a schematic cross-sectional view of a TPU film with "V" grooves on its surface obtained by thermocompression molding in the present invention;

Figure 5 is a schematic diagram of the polymer filling mechanism in the hot embossing process;

Fig. 6 is the photo (observation from above) of the prepared Al template surface of embodiment one;

Fig. 7 is the photo (side observation) of the prepared Al template surface prepared by embodiment one;

Fig. 8 is the photo (observation from above) of the surface of the Al-plated TPU film with drag-reducing micro-groove structure prepared by embodiment one;

Fig. 9 is the photo (side observation) of the Al-plated TPU film surface with drag-reducing micro-groove structure prepared by embodiment one;

Figure 10 is the three-dimensional morphology (observation from above) of the Al-plated TPU film with drag-reducing micro-groove structure prepared in Example 1 after being placed for 2 months;

Figure 11 is the three-dimensional morphology (side observation) of the Al-plated TPU film with the drag-reducing micro-groove structure prepared in Example 1 after being placed for 2 months;

Figure 12 is an AFM image of the TPU film surface;

Fig. 13 is the AFM figure of the TPU film surface after artificial aging 240h;

Fig. 14 is the AFM figure of the TPU film surface after artificial aging 480h;

Fig. 15 is the SEM picture of TPU film section;

Fig. 16 is the SEM photograph of the TPU film section of artificial aging 480h;

Figure 17 is a SEM photo of the side of the TPU film artificially aged for 480h;

Fig. 18 is the AFM picture of the surface of the Al-plated TPU film prepared in Example 1;

Figure 19 is an AFM image of the surface of the Al-plated TPU film after 240h of artificial aging;

Figure 20 is an AFM image of the surface of Al-plated TPU film after artificial aging for 480h;

Fig. 21 is the SEM photo of the surface of the Al-plated TPU film prepared in Example 1;

Figure 22 is an SEM photograph of the surface of an Al-plated TPU film artificially aged for 480 h;

Figure 23 is a relational diagram of the tensile strength and irradiation time of the film;

Fig. 24 is the relationship diagram of elastic modulus and irradiation time of film;

Figure 25 is an ultraviolet-visible-near-infrared transmission spectrum;

Fig. 26 is a solar reflectance spectrogram;

Fig. 27 is a three-dimensional model grid division diagram;

Fig. 28 is the cloud diagram of the cross-sectional velocity in the flow direction when the incoming flow velocity is 10m/s;

Figure 29 is a velocity cloud map near the trench;

Figure 30 is a smooth flat surface velocity cloud diagram;

Fig. 31 is a cloud diagram of vortex intensity distribution in the flow direction cross section;

Figure 32 is a cloud diagram of eddy intensity distribution near the trench;

Figure 33 is the z-direction shear stress near the trench.

Detailed ways

The technical solution of the present invention is not limited to the specific embodiments listed below, but also includes any combination of the specific embodiments.

Specific Embodiment 1: In this embodiment, the hot embossing preparation method of Al-plated TPU film drag-reducing micro-grooves for airship skin is carried out according to the following steps:

1. Use the ultra-precision micromachining machine tool system to process the Al template with a "V" groove structure on the surface. The "V" groove forms alternate transparent grooves on the surface of the Al template. Each groove The width s is 90-110 μm, and the depth h is 90-110 μm;

2. Ultrasonic clean the Al template prepared in step 1 with acetone for 15-20 minutes, and after drying, apply three layers of film cleaning agent, then three layers of sealing agent, and finally three layers of water-based mold release agent , the coating time interval of each layer is 15-20min;

3. Using vacuum evaporation technology, an aluminum film with a thickness of 90-110 μm is evaporated on the surface of the TPU film to be imprinted to obtain an Al-coated TPU film;

4. Install and fix the Al template processed in step 2 on the indenter of the hot embossing machine, place the quartz glass substrate on the bearing table of the hot embossing machine, and spread the Al-plated TPU film obtained through step 3 On the quartz glass substrate, after preheating to 140-145°C, lower the pressure head, and after the Al template is in contact with the Al-plated TPU film, the pressure is raised to 65-75N. Under the condition of 75N, imprint for 20-25 minutes, stop heating, cool naturally, and hold the pressure for 2-2.5 hours, remove the pressure, lift the pressure head, and complete the heat treatment of Al-plated TPU film drag-reducing micro-grooves for airship skin. Imprint preparation.

The Al template with a "V" groove structure on the surface of the present invention is processed by the ultra-precision micromachining machine tool system of Harbin Institute of Technology; the embossing instrument adopts a hot embossing machine produced by Suss MicroTechnology Company in Germany. The film cleaning agent, sealing agent and water-based mold release agent used in this embodiment are all commercially available products.

So far, there is no preparation method for micro-grooves on the surface of flexible films. In this embodiment, thermoplastic polyurethane film (TPU), the weather-resistant layer material of the airship skin near space, is used as the flexible surface, and the method of hot embossing with a hard template is used to replicate the surface of the flexible surface. Micro-grooves realize the micro-nano replication of structural patterns on flexible films at the micro-nano scale, and obtain micro-grooves with high-resolution and high aspect ratio structures. The imprinted film has better shape retention and elastic recovery changes. Small, groove drag reduction numerical simulation results show that the TPU film with a micro-groove structure prepared in this embodiment has a drag reduction effect, and an aluminum layer with a thickness of 90-110 μm is evaporated on the TPU film, and the coating Soft and has almost the original elasticity of the film, good coating adhesion, uniform and smooth surface, excellent wear resistance and flexibility, good high temperature and low temperature resistance, ozone aging, oxygen aging, light aging and climate Aging, the loss of tensile strength value of Al-plated TPU film after 20 days of irradiation is 39.29%, which is greatly improved compared with the original TPU film, and the resilience is lower than the original film. The shape retention ability of the embossed film is very good, and the elasticity The recovery change is very small, the compression permanent deformation is relatively improved, and it can reflect the energy of solar radiation very well. It can play the role of heat insulation and cooling, reflect ultraviolet rays to protect the skin material, and improve the weather resistance of the skin material. In order to further improve It is a simple, convenient, direct, high-efficiency, and low-cost method to improve the weather resistance and drag reduction performance of TPU films, and has great application prospects.

Embodiment 2: The difference between this embodiment and Embodiment 1 is that in step 1, the length of the template made of Al is 50-52 mm, the width is 50-52 mm, and the thickness is 5-8 mm. Others are the same as in the first embodiment.

Embodiment 3: This embodiment differs from Embodiment 1 or Embodiment 2 in that: in step 1, the width s of each groove is 95-105 μm, and the depth h is 95-105 μm. Others are the same as in the first or second embodiment.

Embodiment 4: This embodiment is different from Embodiment 1 to Embodiment 3 in that: the thickness of the TPU film in Step 3 is 25-200 μm. Others are the same as those in the first to third specific embodiments.

Embodiment 5: This embodiment is different from Embodiment 1 to Embodiment 3 in that: the thickness of the TPU film in Step 3 is 90-110 μm. Others are the same as those in the first to third specific embodiments.

Embodiment 6: The difference between this embodiment and one of Embodiments 1 to 5 is that in step 4, after the template made of Al contacts with the TPU film, the voltage is increased to 70N. Others are the same as one of the specific embodiments 1 to 5.

Embodiment 7: This embodiment is different from Embodiment 1 to Embodiment 6 in that: in step 4, the embossing is carried out for 25 minutes at a temperature of 140° C. Others are the same as one of the specific embodiments 1 to 6.

Embodiment 8: This embodiment is different from Embodiment 1 to Embodiment 7 in that: In Step 4, the embossing is carried out for 20 minutes at a temperature of 145° C. Others are the same as one of the specific embodiments 1 to 7.

Embodiment 9: This embodiment is different from Embodiment 1 to Embodiment 8 in that: the holding time in step 4 is 2 hours. Others are the same as one of the specific embodiments 1 to 8.

Embodiment 10: The difference between this embodiment and Embodiments 1 to 9 is that a Si template or a Ni template with a "V" groove structure on the surface is used instead of an Al template with a "V" groove structure on the surface. . Others are the same as one of the specific embodiments 1 to 9.

Adopt the following examples to verify the beneficial effects of the present invention:

Embodiment one:

In this embodiment, the hot embossing preparation method of the Al-plated TPU film drag-reducing micro-groove for airship skin is carried out according to the following steps:

1. Use the ultra-precision micromachining machine tool system to process the Al template with a "V" groove structure on the surface. The "V" groove forms alternate transparent grooves on the surface of the Al template. Each groove The width s is 100 μm, and the depth h is 100 μm; the length of the Al template is 50 mm, the width is 50 mm, and the thickness is 6 mm; the model of the Al material used for the Al template is Al-LY12;

2. Ultrasonic clean the Al template prepared in step 1 with acetone for 20 minutes. After drying, apply three layers of film cleaning agent, then three layers of sealing agent, and finally three layers of water-based mold release agent. The coating time interval of one layer is 20min;

3. Using vacuum evaporation technology, evaporate an aluminum film with a thickness of 10 μm on the surface of the TPU film to be imprinted to obtain an Al-coated TPU film;

4. Install and fix the Al template processed in step 2 on the indenter of the hot embossing machine, place the quartz glass substrate on the bearing table of the hot embossing machine, and spread the Al-plated TPU film obtained through step 3 On the quartz glass substrate, after preheating to 140°C, lower the pressure head, and after the Al template is in contact with the Al-coated TPU film, the pressure is raised to 70N, and the temperature is 140°C, the pressure is 70N, and the pressure is 25min. , stop heating, cool naturally, and hold the pressure for 2 hours, remove the pressure, lift the indenter, and complete the thermal embossing preparation of Al-plated TPU film drag-reducing micro-grooves for the airship skin.

The raw materials used in this example are all commercially available products, and the film cleaning agent, sealing agent and water-based mold release agent used in step 2 are products produced and sold by Alphabi International Trade (Shanghai) Co., Ltd. The physical properties of the thermoplastic polyurethane film (TPU film) used in this embodiment are shown in Table 1.

Table 1 TPU film physical properties table

Using a three-dimensional optical microscopic observation system [Keyence Corporation, digital microscope (digital microscope), model VHX-600] to characterize the micro-groove morphology on the surface of the template and the embossed skin, Figure 8 is an example of a The photo (observation from the top) of the prepared Al-plated TPU film surface with the drag-reducing micro-groove structure; observation). It can be seen from the figure that the grooves on the surface of the TPU film are obvious, the shape and size of the groove stripes are well matched with the embossing template, and the three-dimensional shape is uniform. Discussion on the resilience of the film after embossing micro-grooves:

In order to analyze the resilience characteristics of the film after embossing micro-grooves, the Al-plated TPU film with a better micro-groove structure prepared in Example 1 was placed for 60 days, and then the same part of the embossed film was observed. The recovery changes of the three-dimensional structure of the film are used to study the conformal ability of the thin film groove. Figure 10 is the three-dimensional morphology (observed from above) of the Al-plated TPU film with the drag-reducing micro-groove structure prepared in Example 1 after being placed for 2 months; Figure 11 is the anti-drag micro-groove prepared in Example 1 The three-dimensional morphology (side observation) of the Al-plated TPU film with a groove structure after being placed for 2 months. It can be seen from the figure that the shape retention ability of the embossed film is very good, the elastic recovery changes very little, and the size of the microgroove changes with time. The rebound change data are shown in Table 2.

Table 2 Springback changes of microgrooves over time

Get the Al-plated TPU film (hereinafter referred to as the Al-plated TPU film) and the TPU film (hereinafter referred to as the TPU film) without Al plated with the drag-reducing micro-groove structure prepared in Example 1, using the U.S. Atlas (Atlas) Ci3000+ xenon lamp climate aging tester conducts artificial climate aging experiments on films. By changing the temperature, humidity and spraying, the xenon lamp is used to simulate natural climate and full-spectrum artificial sunlight to test the aging resistance of Al-coated TPU films and TPU films. The set experimental parameters are: irradiance intensity 90W/m ² , humidity 10%, wind speed 32m/s, no spraying.

Utilize the atomic force microscope (AFM) to test the TPU film irradiated for different times. Figure 12 is the AFM picture of the surface of the TPU film; Figure 13 is the AFM picture of the surface of the TPU film after 240h of artificial aging; Figure 14 is the picture of the surface of the TPU film after 480h of artificial aging AFM image of the TPU film surface.

The change of surface roughness Ra (nm) of TPU film before and after xenon lamp irradiation is shown in Table 3. It can be seen that before irradiation, the surface of the sample is relatively smooth and has few surface defects; The prolongation of the film surface roughness gradually increases, and the surface defects gradually increase.

Table 3 Changes in surface roughness Ra (nm) of TPU film without Al coating before and after irradiation

Using a scanning electron microscope (SEM) to observe the tensile fracture surface morphology of the TPU film before and after xenon lamp irradiation aging, the results are as follows: Figure 15 is the SEM photo of the TPU film section; Figure 16 is the SEM photo of the TPU film section artificially aged for 480h ; Fig. 17 is the SEM photograph of the TPU film side of artificial aging 480h.

It can be seen from the figure that the tensile fracture of the unirradiated polyurethane film (TPU) is tearing, and the bonding force is relatively uniform everywhere. With the increase of irradiation time, the surface shape of the tensile fracture of the polyurethane film The surface gradually becomes smoother, the tear shape gradually becomes less, the surface brittleness increases, and the tensile damage is all on the irradiated side of the film.

Utilize the atomic force microscope (AFM) to test the Al-coated TPU film irradiated for different times. Figure 18 is an AFM image of the surface of the Al-coated TPU film prepared in Example 1. It can be concluded from the figure that the surface roughness is 8.16 , the coating on the surface of the film is relatively smooth; Figure 19 is the AFM image of the surface of the Al-plated TPU film after 240 hours of artificial aging; Figure 20 is the AFM image of the surface of the Al-plated TPU film after 480 hours of artificial aging; it can be seen from the figure It can be seen that after the start of irradiation, the surface roughness of the film increases slowly with the increase of irradiation time, indicating that the smooth structure of the film surface suffers little damage, and the surface defects do not increase significantly.

The change of surface roughness Ra (nm) of Al-plated TPU film before and after xenon lamp irradiation is shown in Table 4:

Table 4 Changes in surface roughness Ra (nm) of Al-plated TPU films before and after irradiation

Use scanning electron microscope (SEM) to observe the tensile fracture surface morphology of Al-plated TPU film before and after xenon lamp irradiation aging, the results are as follows: Figure 21 is the SEM photo of the surface of the unirradiated Al-plated TPU film; Figure 22 is The SEM photo of the surface of the Al-coated TPU film artificially aged for 480 hours; it can be seen from the figure that the surface morphology changes after the film is coated with aluminum before and after irradiation. Destruction, only slight cracks can be seen, the surface of the aluminum coating is still very smooth, and can still adhere to the surface of the film to protect the film from weathering and aging.

Tensile performance test:

When testing the tensile properties of thermoplastic polyurethane (TPU) film samples and Al-coated TPU films before and after aging by the American Atlas (Atlas) Ci3000+ xenon lamp, the samples were all cut by a film precision cutting machine, and the size was 300× 40mm, 5 samples in each group, select effective data and take the average value for processing.

Figure 23 is a graph showing the relationship between tensile strength and irradiation time of the film, where Represents Al-plated TPU film, Represents the TPU film; the relationship between the elastic modulus and the irradiation time of the film in Figure 24, Represents Al-plated TPU film, Stands for TPU film. It can be seen from Figure 23 and Figure 24 that the tensile properties of the unirradiated original polyurethane film are: the tensile strength is 81.80MPa, the breaking strain is 497.15%, and the elastic modulus is about 50.97MPa; while the irradiation time reaches 10d, The tensile strength of the polyurethane film decreased obviously, and the loss reached 35.23%. It can be seen that the performance of the polyurethane film was greatly damaged by the xenon lamp irradiation. Afterwards, as the irradiation time increased, the tensile strength value of the polyurethane film continued to decrease. When the irradiation time was 20 days The loss reaches 49.26%, which shows that xenon lamp irradiation has a great influence on the mechanical properties of the film, and its toughness changes obviously. However, when the surface of the film is coated with Al, the decline in mechanical properties is relieved after irradiation, and the loss of tensile strength value after 480 hours of Al plating is 39.29%, indicating that the weathering resistance of the film surface is significantly improved after Al coating.

UV spectrum analysis:

Under the irradiation of sunlight, heat continuously accumulates on the surface of the irradiated object, which will increase the surface temperature and increase energy consumption. It can be seen from the solar radiation spectrum that 95% of the solar radiation energy is in the visible light region and near-infrared light region (400-2500nm), according to the relationship: α+ρ+τ=1

Among them, α—absorption rate, the ratio of the radiation energy absorbed by the surface of the object to the radiation energy incident on the surface of the object; ρ——reflectivity, the ratio of the radiation energy reflected by the surface of the object to the radiation energy incident on the surface of the object; τ——Transmittance, the ratio of the radiant energy passing through the object to the radiant energy incident on the surface of the object.

In order to improve the anti-radiation and heat-insulating ability of the film on the surface of the airship, only by increasing the reflectivity ρ of the film can the surface of the film absorb less energy, that is, when α is low, the temperature of the film will not rise too high. This is the property of the heat reflective coating. primary ability. In addition, the heat reflective film must also have the ability to re-emit the absorbed energy.

At the same time, the intensity of ultraviolet rays in the adjacent space is very high, so the transmittance and reflection ability of the film to ultraviolet rays are also the focus of our investigation.

Figure 25 is the ultraviolet-visible-near-infrared transmission spectrum; where a represents the TPU film, and b represents the TPU film with a thickness of 10 μm deposited on an aluminum layer; Figure 26 is the solar reflectance spectrum; where a represents the TPU film , b represents a TPU film with a thickness of 10 μm deposited on an aluminum layer. It can be seen from the figure that although the ultraviolet transmittance of the film after aluminum plating has almost no obvious change, the visible-near infrared transmittance is the same as that of the original film It is much lower than that, indicating that the aluminum coating prevents the transmission of visible-near-infrared light well; Reflecting the energy of solar radiation can play a role in heat insulation and cooling, and reflect ultraviolet rays to protect the skin material.

After being protected by aluminum plating, the mass loss of the film material after irradiation becomes smaller, and its ability to resist ultraviolet light is greatly enhanced.

The metallized film not only prevents the material from deforming due to outgassing and severe temperature difference changes, but also prevents microcracks in the space environment due to moisture absorption, and prevents deformation and damage due to radiation such as atomic oxygen, thereby ensuring the structural strength and stability of the material. and service life. Moreover, the aluminized film has a high reflection coefficient, can reflect heat away, and has good protective properties such as light radiation reflection and barrier. It has good absorption and reflection properties for ultraviolet rays and good reflection properties for visible light, which can effectively reflect and block the effects of light and heat, and improve the protective and insulating properties of materials. The coating is soft and has almost the original elasticity of the film, the coating has good adhesion, the surface is uniform and smooth, has excellent wear resistance and flexibility, good high temperature and low temperature resistance, ozone aging resistance, oxygen aging, light aging and weather aging. And the rebound elasticity is lower than that of the original film, and the compression set is relatively improved.

Numerical simulation of trench drag reduction:

There are two ways to place the groove surface, that is, flow direction and vertical flow direction, and the flow direction groove is more conducive to drag reduction. The flow field of a symmetrical V-shaped groove is analyzed. The height and spacing of the grooves in the model are both 0.1mm, the span of the flow field area is 1mm, the flow direction of the flow field is 3mm, and the distance between the groove surface and the flat surface is 3mm. The grid division is shown in Figure 27.

The inlet of the flow field is a velocity inlet, the incoming flow velocity is 10m/s, and the outlet is a free outlet; the left and right boundaries of the flow field are defined as symmetrical boundaries. The atmospheric density is taken as 0.08803kg/m ³ , and the air viscosity coefficient is 1.4126×10 ^-5 kg/m×s. Using the RNGκ-ε turbulent kinetic energy equation to solve, the results are as follows:

Figure 28 is the velocity cloud diagram of the flow cross section when the incoming flow velocity is 10m/s. The upper end of the figure is a smooth flat surface, and the lower end is a grooved surface. Figure 28 shows that the wall of the flow field has an obvious boundary layer. When the flow field is close to the wall The velocities are relatively small; Fig. 29 and Fig. 30 show the velocity contours of the groove wall and the flat wall respectively. From the two figures, it can be seen that there is a big difference between the surface velocity distribution of the groove and the flat surface, and the external high-speed fluid directly flows from the groove The low-velocity fluid on the surface of the groove flows up, avoiding the energy loss caused by a large area of contact with the plate surface; at the same time, the velocity gradient in the y direction at the bottom of the groove is smaller than that at the peak of the groove, so that the entire groove surface has a higher Small frictional resistance becomes possible;

Figure 31 shows that vortices are generated on both the flat surface and the groove surface of this example; Figure 32 shows the distribution cloud diagram of vortex intensity near the groove, which shows that the vortex intensity is relatively large near the V-shaped tip, and the flow is more complicated;

Figure 33 shows that the shear stress mainly occurs near the tip of the V-groove because the velocity gradient varies more along the y-direction here than at the bottom of the groove.

The numerical simulation results of drag reduction by grooves show that the TPU film with micro-groove structure on the surface prepared by the present invention has a drag reduction effect.

Claims (10)

1. The hot embossing preparation method of Al-plated TPU film drag-reducing micro-groove for airship skin is characterized in that the hot-press printing preparation method of Al-plated TPU film drag-reducing micro-groove for airship skin is carried out according to the following steps : 1. Use the ultra-precision micromachining machine tool system to process the Al template with a "V" groove structure on the surface. The "V" groove forms alternate transparent grooves on the surface of the Al template. Each groove The width s is 90-110 μm, and the depth h is 90-110 μm; 2. Ultrasonic clean the Al template prepared in step 1 with acetone for 15-20 minutes, and after drying, apply three layers of film cleaning agent, then three layers of sealing agent, and finally three layers of water-based mold release agent , the coating time interval of each layer is 15-20min; 3. Using vacuum evaporation technology, an aluminum film with a thickness of 90-110 μm is evaporated on the surface of the TPU film to be imprinted to obtain an Al-coated TPU film; 4. Install and fix the Al template processed in step 2 on the indenter of the hot embossing machine, place the quartz glass substrate on the bearing table of the hot embossing machine, and spread the Al-plated TPU film obtained through step 3 On the quartz glass substrate, after preheating to 140-145°C, lower the pressure head, and after the Al template is in contact with the Al-plated TPU film, the pressure is raised to 65-75N. Under the condition of 75N, imprint for 20-25 minutes, stop heating, cool naturally, and hold the pressure for 2-2.5 hours, remove the pressure, lift the pressure head, and complete the heat treatment of Al-plated TPU film drag-reducing micro-grooves for airship skin. Imprint preparation. 2. the hot embossing preparation method of Al-plated TPU film drag-reducing microgroove for airship skin according to claim 1, it is characterized in that the length of Al template in step 1 is 50～52mm, and width is 50～ 52mm, thickness 5-8mm. 3. The thermal embossing preparation method of Al-plated TPU film drag-reducing micro-grooves for airship skin according to claim 1 or 2, characterized in that in step 1, the width s of each groove is 95-105 μm , The depth h is 95-105 μm. 4. The thermal embossing preparation method of Al-plated TPU film drag-reducing micro-grooves for airship skin according to claim 3, characterized in that the thickness of the TPU film in step 3 is 25-200 μm. 5. The thermal embossing preparation method of Al-plated TPU film drag-reducing micro-grooves for airship skin according to claim 3, characterized in that the thickness of the TPU film in step 3 is 90-110 μm. 6. The hot embossing preparation method of the Al-plated TPU film drag-reducing micro-groove for the airship skin according to claim 4, characterized in that in step 4, the pressure is increased to 70N after the Al-made template and the TPU film are contacted. 7. The hot embossing preparation method of the Al-plated TPU film drag-reducing micro-groove for the airship skin according to claim 4, 5 or 6, characterized in that in step 4, under the condition that the temperature is 140°C, pressing Print 25min. 8. According to claim 4, 5 or 6, the hot embossing preparation method of Al-plated TPU film drag-reducing micro-groove for airship skin is characterized in that in step 4, under the condition that the temperature is 145 ° C, pressing Print 20min. 9. The hot embossing preparation method of the Al-plated TPU film drag-reducing micro-groove for airship skin according to claim 7, characterized in that the holding time in step 4 is 2h. 10. The thermal embossing preparation method of Al-plated TPU film drag-reducing micro-grooves for airship skin as claimed in claim 1, characterized in that the Si template or Ni template with a "V" groove structure on the surface is used instead An Al template with a "V"-shaped groove structure on the surface.