A high strength welding method for fiber reinforced thermoplastic polyurethane composites

Abstract

The application provides a high-strength welding method of fiber-reinforced thermoplastic polyurethane composite material, and belongs to the technical field of polyurethane composite material welding, and the specific method is as follows: 2-ethyl-2-(hydroxymethyl)-1,3-propanediol is mixed with a polymer of bis(isocyanatomethyl)benzene to prepare a welding reinforcing agent; the back surfaces of two pieces of fiber-reinforced thermoplastic polyurethane composite material to be welded are coated with the welding reinforcing agent and are adhered to reciprocating rollers; finally, under the conditions that the welding machine frequency is 26-28 MHz, the gas source pressure is 0.3 Mpa-0.5 Mpa, the welding voltage is 220-240 V, the welding current is 0.45-1.7 A, and the fusion time is 8-25 s, the welding is completed after shaping. The welding method significantly improves the indexes and performance strength of the fiber after welding, and can be used in the production of flexible composite material products such as airships, flexible ocean oil and gas and chemical storage capsules or flexible high-pressure oxygen cabins.

Description

Technical Field

[0001] This invention belongs to the field of polyurethane composite material welding technology, specifically relating to a high-strength welding method for high-strength organic fiber reinforced thermoplastic polyurethane composite materials. Background Technology

[0002] The selection, preparation, and composite of skin materials have a crucial impact on the final performance of flexible aircraft. The assessment and prediction of the structural strength of airship materials has become a key issue in this field. Currently, in the experiments and applications of near-space airships, it has been found that the bulk strength of materials used to assess the airship's safety factor or pressure resistance is unreliable, failing to provide accurate and true structural pressure values, leading to accidental explosions during ground or flight tests.

[0003] Airship heat sealing technology is one of the core technologies in airship skin processing. Good sealing performance, high mechanical properties, easy welding operation, and high production efficiency are important characteristics of high-quality heat sealing technology. Currently, almost all airship outer skins are made by heat sealing and welding multiple composite materials. Therefore, the rational utilization of existing heat sealing technologies is crucial for airship skin processing. The skin material consists of a load-bearing layer, a protective layer, and an airtight layer, possessing advantages such as high strength, high airtightness, lightweight, and durability.

[0004] Radio frequency welding, also known as dielectric welding, is a welding method that utilizes the dielectric hysteresis loss of polar plastics under a high-frequency electric field to generate internal heat. In plastic high-frequency welding, high-frequency energy is applied to the connection area of ​​the materials, causing the polar molecules to vibrate frequently under the high-frequency electric field, generating heat. This causes the plastic materials between the electrodes to simultaneously reach a melting state, thus achieving the purpose of welding the two materials together. In a rapidly alternating electric field, polar groups attempt to align with the direction of the electric field, resulting in intermolecular friction and heat generation.

[0005] The principle behind this process is as follows: A dielectric (insulator) is placed between two parallel electrode plates, and a high-frequency voltage is applied. The molecules in the dielectric form dipoles with equal amounts of positive and negative charges at both ends. Initially, the dipoles are irregularly arranged, but the applied voltage causes attraction between the positively and negatively charged dipoles and the oppositely charged electrodes, causing the dipoles to rotate and align regularly along the electric field lines. Because the high-frequency voltage periodically changes the polarity, the polarity of the electrodes changes periodically, resulting in periodic rotation of the dipole orientation. This causes the dielectric molecules to collide and generate frictional heat, which is then used for bonding. Since the electrodes are cold, and the plastic conducts some heat away when heated, the plastic temperature is highest at the center and lowest on both sides and the four sides. This is ideal for plastic welding, providing a fast, efficient, uniformly heated, and simple welding method. The final weld strength is equivalent to that of the base material. Because of these advantages, high-frequency welding is widely used in plastics used in the automotive, medical device, stationery, inflatable items, large and medium-sized parts, and home appliance industries.

[0006] Polymers with a certain dipole moment (typically above 0.5 Debye) align their polar molecules in the opposite direction to the electric field under the influence of a high-frequency electric field, resulting in dipole polarization. This means that positive charges in the molecules align with the negative pole, and negative charges align with the positive pole. If the direction of the electric field changes, the direction of charge movement also changes. As the frequency of the applied electric field increases, the orientation electrodes lag behind the change in electric field, resulting in a phase difference. This phase lag indicates that overcoming the resistance generated by molecular interactions is necessary to achieve dipole polarization, and overcoming this resistance requires energy. This energy consumption is usually released as heat, with a portion of the electric field energy being converted into heat. In a high-frequency electric field, the dichotomous orientation of the dipoles causes frequent vibrations and friction of the molecular chains, heating up the plastic until it melts, and then bonding together under the pressure of the welding electrodes. Summary of the Invention

[0007] To address the challenges of achieving high weld strength and performance requirements for fiber-reinforced thermoplastic polyurethane composites used in airship skin fabrication, where traditional welding techniques struggle to meet these requirements, this invention provides a high-strength welding method for fiber-reinforced thermoplastic polyurethane composites by improving and optimizing the welding method and incorporating a welding reinforcing agent. The method specifically includes the following steps:

[0008] ① Dilute 2-ethyl-2-(hydroxymethyl)-1,3-propanediol with solvent, and then slowly add the polymer of bis(isocyanate methyl)benzene under stirring to prepare a welding strengthening agent;

[0009] ② Apply welding reinforcing agent to the back of the two pieces of fiber-reinforced thermoplastic polyurethane composite material to be welded. After the solvent evaporates, gently stick the two pieces of fiber-reinforced thermoplastic polyurethane composite material to be welded together and roll them back and forth.

[0010] ③ Under the conditions of welding machine frequency of 26~28MHz, gas source pressure of 0.3Mpa~0.5Mpa, welding voltage of 220~240V, and welding current of 0.45~1.7A, welding is carried out for 8~25s, and the shape is set for 8~12s after welding to obtain the welded fiber-reinforced thermoplastic polyurethane composite material.

[0011] The welding reinforcing agent, calculated based on the contact area of ​​the welding materials, has a coating amount of 10–100 mg / cm². 2 .

[0012] The solvent is acetone or ethyl acetate.

[0013] In the fiber-reinforced thermoplastic polyurethane composite material, the polyurethane is Huntsman IROGRANA 85P4394 thermoplastic polyurethane, and the fiber is Vectran fiber.

[0014] The method for preparing the fiber-reinforced thermoplastic polyurethane composite material is as follows:

[0015] (1) Place Huntsman IROGRANA 85P 4394 thermoplastic polyurethane film and Vectran fiber in the first unwinding and second unwinding positions of the laminating machine, respectively; add TPU adhesive to the impregnation tank of the laminating machine, turn on the drying tunnel and supply air at a temperature of 40-60°C.

[0016] (2) Turn on the laminating machine and, under the conditions of 80-180℃, pressure 0.1-0.5MPa, and speed 1-12m / min, allow Huntsman IROGRANA 85P 4394 thermoplastic polyurethane film and Vectran fiber material to pass through the surface treatment unit, coating unit and laminating unit at a uniform speed in a flat state to achieve lamination and winding, and obtain self-made fiber-reinforced thermoplastic polyurethane composite material.

[0017] The mass ratio of the polymer of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol to bis(isocyanate methyl)benzene is (1-3):1, and the solid content in the entire solution is 10-40%.

[0018] Beneficial effects

[0019] This invention employs high-frequency welding technology, and by using a combination of high-frequency welding conditions and welding reinforcing agents, the welding strength of fiber-reinforced thermoplastic polyurethane composite materials is greatly improved.

[0020] In the early stages of this invention, under different high-frequency welding process parameters and welding reinforcing agents, Vectran polyarylate fibers were used as reinforcement and combined with a thermoplastic polyurethane matrix to form a composite material. Uniaxial tensile tests were conducted to investigate the effect of different welding processes on tensile strength, and peel tests were performed to investigate the effect of different welding processes on peel strength. Furthermore, a 100-cycle test was used to simulate the influence of the environment on the fiber-reinforced thermoplastic polyurethane composite material, further examining the change in uniaxial tensile strength after cyclic loading. Scanning electron microscopy was used to analyze the interfacial morphology between the fiber and resin under different process parameters and with and without welding reinforcing agents.

[0021] (1) By comparing uniaxial tension and uniaxial cyclic tension, it can be seen that the tensile strength of the spline decreases after cyclic tension. This indicates that cyclic tension reduces the tensile strength of the material. This may be because the material is damaged as a whole after cyclic tension, which leads to a decrease in tensile strength.

[0022] (2) The tensile strength of the material was significantly improved after adding the welding reinforcing agent prepared in this invention. This may be because the addition of the welding reinforcing agent improves the bonding between the fiber and the resin, thereby increasing the tensile strength of the fiber-reinforced thermoplastic polyurethane composite. Excessive addition of the welding reinforcing agent leads to poor adhesion between the fiber and the polyurethane, thus reducing its tensile strength. The peel strength results were similar to the tensile strength results. Electron microscopy scans also fully confirm this conclusion.

[0023] The welding strengthening agent described in this invention has the advantages of low preparation cost, no damage to fibers, and significant improvement in the welding strength of fiber-reinforced thermoplastic polyurethane composites, making it highly practical. Attached Figure Description

[0024] Figure 1 Clamping and fracture modes of uniaxial tensile splines;

[0025] Figure 2 : Uniaxial tensile force-displacement curves of the materials in the examples and comparative examples;

[0026] Figure 3 Force-displacement curves of the materials in the examples and comparative examples after 100 cycles of treatment;

[0027] Figure 4 : Schematic diagram of spline stripping;

[0028] Figure 5 Peeling force-displacement curves of the six parallel splines in Example 1;

[0029] Figure 6 Scanning electron microscope (SEM) images of the cross-section of the welded parts of the materials in the examples and comparative examples. Detailed Implementation

[0030] In this embodiment, the polyurethane used in the fiber-reinforced thermoplastic polyurethane composite material is Huntsman IROGRANA 85P4394 thermoplastic polyurethane (TPU), and the Vectran fiber is purchased from Kuraray Corporation of Japan. The airship skin material is prepared using these two raw materials, specifically a Vectran fiber-reinforced thermoplastic polyurethane composite material, and its preparation method is as follows:

[0031] (1) Place Huntsman IROGRANA 85P 4394 thermoplastic polyurethane film and Vectran fiber in the first unwinding and second unwinding positions of the laminating machine, respectively; add Yantai TS-9015A polyurethane adhesive into the impregnation tank of the laminating machine, turn on the drying tunnel air supply, and the air supply temperature is 40-60℃.

[0032] (2) Turn on the laminating machine and, under the conditions of 80-180℃, pressure 0.1-0.5MPa, and speed 1-12m / min, allow Huntsman IROGRANA 85P 4394 thermoplastic polyurethane film and Vectran fiber material to pass through the surface treatment unit, coating unit and laminating unit at a uniform speed in a flat state to achieve lamination and winding, and obtain the self-made fiber-reinforced thermoplastic polyurethane composite material.

[0033] The experimental instruments included a computer-controlled electronic universal testing machine (CMT4304), manufactured by MTECH Industrial Systems (China) Co., Ltd.; a scanning electron microscope (JSM-6510LV), manufactured by Nippon Eiichiro Co., Ltd.; a micrometer (0-25mm), manufactured by Harbin Measuring & Cutting Tool Group; and a vernier caliper (0-300mm), manufactured by Harbin Measuring & Cutting Tool Group.

[0034] Example 1

[0035] This embodiment provides a high-strength welding method for Vectran fiber-reinforced thermoplastic polyurethane composites, specifically including the following steps:

[0036] 1. Before welding, inspect the welding machine to ensure that the electrical system is normal, sensitive and reliable. Ensure that the welding machine is used in a dry indoor environment with little dust and no corrosive gases. The indoor temperature should be 25℃ and the relative humidity should not exceed 85%.

[0037] 2. After checking that all parts are normal, turn on the welding machine control switch and preheat for 10-15 minutes.

[0038] 3. Place 2-ethyl-2-(hydroxymethyl)-1,3-propanediol in a mixing tank, dilute with acetone, and then slowly add the polymer of bis(isocyanate methyl)benzene while stirring to prepare a welding strengthening agent. The mass ratio of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol to the polymer of bis(isocyanate methyl)benzene is 1:1, and the solid content of the entire solution is 20%.

[0039] 4. Place the back sides of the two fiber-reinforced thermoplastic polyurethane composite materials to be welded at a welding contact area of ​​30 mg / cm². 2 Apply a uniform amount of welding reinforcing agent by brushing. After the acetone evaporates, bond the two fiber-reinforced thermoplastic polyurethane composite materials together and use a pressure roller to press them back and forth to increase the bonding strength.

[0040] 5. After the welding machine preheats, welding can begin. Adjust the welding machine frequency to 27MHz and the working voltage to 230V. Place the skin material to be welded in the welding tank, adjust the gas source pressure to 0.4Mpa, the welding current to 1.26A, and the welding time to 16s. After welding, set the shape for 10s to obtain the welded product.

[0041] Example 2

[0042] This embodiment provides a high-strength welding method for Vectran fiber-reinforced thermoplastic polyurethane composites, specifically including the following steps:

[0043] 1. Before welding, inspect the welding machine to ensure that the electrical system is normal, sensitive and reliable. Ensure that the welding machine is used in a dry indoor environment with little dust and no corrosive gases. The indoor temperature should be 25℃ and the relative humidity should not exceed 85%.

[0044] 2. After checking that all parts are normal, turn on the welding machine control switch and preheat for 10-15 minutes.

[0045] 3. Place 2-ethyl-2-(hydroxymethyl)-1,3-propanediol in a mixing tank, dilute with ethyl acetate, and then slowly add the polymer of bis(isocyanate methyl)benzene while stirring to prepare a welding strengthening agent. The mass ratio of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol to the polymer of bis(isocyanate methyl)benzene is 2:1, and the solid content of the entire solution is 10%.

[0046] 4. Place the back sides of the two fiber-reinforced thermoplastic polyurethane composite materials to be welded at a welding contact area of ​​10 mg / cm². 2 Apply the welding reinforcing agent evenly by brushing it on. After the ethyl acetate evaporates, bond the two fiber-reinforced thermoplastic polyurethane composite materials together and use a pressure roller to press them back and forth to increase the bonding strength.

[0047] 5. After the welding machine preheats, welding can begin. Adjust the welding machine frequency to 26MHz and the working voltage to 220V. Place the skin material to be welded in the welding tank, adjust the gas source pressure to 0.5Mpa, the welding current to 1.7A, the welding time to 8s, and the shaping time to 12s after welding to obtain the welded product.

[0048] Example 3

[0049] This embodiment provides a high-strength welding method for Vectran fiber-reinforced thermoplastic polyurethane composites, specifically including the following steps:

[0050] 1. Before welding, inspect the welding machine to ensure that the electrical system is normal, sensitive and reliable. Ensure that the welding machine is used in a dry indoor environment with little dust and no corrosive gases. The indoor temperature should be 25℃ and the relative humidity should not exceed 85%.

[0051] 2. After checking that all parts are normal, turn on the welding machine control switch and preheat for 10-15 minutes.

[0052] 3. Place 2-ethyl-2-(hydroxymethyl)-1,3-propanediol in a mixing tank, dilute with acetone, and then slowly add the polymer of bis(isocyanate methyl)benzene while stirring to prepare a welding strengthening agent; wherein, the mass ratio of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol to the polymer of bis(isocyanate methyl)benzene is 3:1, and the solid content in the whole solution is 40%.

[0053] 4. Place the back sides of the two fiber-reinforced thermoplastic polyurethane composite materials to be welded at a welding contact area of ​​100 mg / cm². 2 Apply a uniform amount of welding reinforcing agent by brushing. After the acetone evaporates, bond the two fiber-reinforced thermoplastic polyurethane composite materials together and use a pressure roller to press them back and forth to increase the bonding strength.

[0054] 5. After the welding machine preheats, welding can begin. Adjust the welding machine frequency to 28MHz and the working voltage to 240V. Place the skin material to be welded in the welding tank, adjust the gas source pressure to 0.3Mpa, the welding current to 0.45A, the welding time to 25s, and the setting time to 8s after welding to obtain the welded product.

[0055] Comparative Example 1

[0056] This embodiment provides a welding method for Vectran fiber reinforced thermoplastic polyurethane composite materials, specifically including the following steps:

[0057] 1. Before welding, inspect the welding machine to ensure that the electrical system is normal, sensitive and reliable. Ensure that the welding machine is used in a dry indoor environment with little dust and no corrosive gases. The indoor temperature should be 25℃ and the relative humidity should not exceed 85%.

[0058] 2. After checking that all parts are normal, turn on the welding machine control switch and preheat for 10-15 minutes.

[0059] 3. After the welding machine preheats, welding can begin. Adjust the welding machine frequency to 27MHz, place the two pieces of fiber-reinforced thermoplastic polyurethane composite material to be welded in the welding tank, adjust the gas source pressure to 0.4MPa, welding voltage to 230V, welding current to 1.53A, welding time to 30s, and set for 10s after welding to obtain the welded product.

[0060] The welding test method is as follows:

[0061] ① Uniaxial tensile test

[0062] Uniaxial tensile testing is a common method for testing mechanical properties. This test measures important parameters of a material, such as strength, elastic modulus, plastic deformation, and fracture toughness, by applying a uniaxial tensile force. In the test, five parallel specimens are selected for each group of tests and subjected to tensile stress at a rate of 100 mm / min. The experimental materials include... Figure 1 The sample is clamped and fixed in the manner shown in the left figure. The sample is clamped in the fixture and stretched along the long axis of the testing machine. Figure 1 The image on the right shows one type of fracture.

[0063] Calculations show that the average tensile strengths of Examples 1-3 and the comparative examples are 662 N / cm, 652 N / cm, 629 N / cm, and 575 N / cm, respectively. It can be seen that the method of the examples significantly increases the tensile strength of the material under the same conditions.

[0064] ② Uniaxial cyclic loading test

[0065] During operation, the skin material needs to maintain its internal and external pressure difference to resist external environmental factors such as temperature, wind, rain, and snow. Simultaneously, thermal expansion and contraction due to diurnal temperature variations, changes in wind speed, and lightning strikes will cause changes in the airship skin over a certain period. Therefore, the harsh working environment requires the skin material to possess fatigue resistance. Thus, the stress variation characteristics of the skin material under normal operating conditions can be essentially considered as stress variation under cyclic loading.

[0066] Cyclic loading tests, by simulating the cyclic loading process under actual working conditions, can more realistically reflect the performance of materials in harsh environments. The experiment was also conducted using a computer-controlled electronic universal testing machine, with the specimen fixed on two clamps for testing. The maximum tensile force was 1 / 4 of the skin body strength, and the minimum tensile force was 1 / 40 of the skin body strength, with a safety factor of 4. In this experiment, 100 uniaxial tensile cycles were performed. At the end of each cycle, the material was subjected to another uniaxial tensile test to study its performance characteristics.

[0067] The welding materials prepared in the examples and comparative examples were subjected to cyclic uniaxial tensile testing. Five parallel specimens were then selected from each group for testing, and the average tensile strength of the five parallel specimens was taken as the tensile result. Calculations showed that the tensile strengths of Examples 1-3 and the comparative example were 646 N / cm, 631 N / cm, 608 N / cm, and 566 N / cm, respectively. See details below. Figure 3 .

[0068] ③ Peeling test

[0069] The peel test of the skin material is used to test the degree of adhesion between the fibers and thermoplastic polyurethane in the airship skin material. This peel test is performed according to ISO / DIS 2411:2016(E) standard. During the test, the sample is clamped at both ends with fixtures, the testing machine is started, and peeling is stopped when the displacement reaches 100 mm. Figure 4 As shown in the figure. Six parallel spline samples were selected for each group of experiments.

[0070] Example 1 exhibits the best peel strength. Figure 5 The force-displacement curves for the peeling experiments of six parallel spline materials in Example 1 are shown. During the peeling process, the experimental curves exhibited continuous oscillations as the fixture displacement increased. At a power of 400W, the midpoint values ​​of the six curves were approximately the same: 59.25N, 53.91N, 46.09N, 52.81N, 48.94N, and 47.78N. Because the experimental curves oscillated frequently and were difficult to discern on a single graph, the experimental curves obtained from the six parallel spline materials were presented separately. Similarly, the peeling strengths of Examples 1-3 and the comparative example were calculated to be 27N / cm, 26N / cm, 25N / cm, and 20N / cm, respectively.

[0071] ④ Scanning electron microscopy test

[0072] This paper uses a JSM-6510LV scanning electron microscope (SEM) manufactured by Nippon Electron Ltd. to test and observe the microscopic surface morphology of the welded areas and the cross-section of the skin material. The material was prepared into samples with a height of approximately 0.3 mm, with the cross-section facing outwards. These samples were fixed to the sample stage with conductive adhesive, sputtered with gold, and then photographed. The bonding between the fibers and resin in the cross-section was analyzed using SEM.

[0073] The electron microscopy results of Examples 1-3 and the comparative examples are shown in the figure. Figure 6 As can be seen from Comparative Example 1 in the figure, the bonding between the fiber and the resin is poor, and there are large gaps in the bottom layer, with a gap width of nearly 200 micrometers. Examples 1 to 3 have similar surface morphology, and the bonding between the fiber and the resin is good. However, there is delamination between the fiber and the resin in some places. It can be seen that the fiber and resin are well wetted in Examples 1 to 3, which is consistent with the tensile test results and peel test results.

Claims

1. A high-strength welding method for fiber-reinforced thermoplastic polyurethane composite materials, characterized in that, Includes the following steps: ① Dilute 2-ethyl-2-(hydroxymethyl)-1,3-propanediol with a solvent, then add a polymer of bis(isocyanate methyl)benzene under stirring to prepare a welding strengthening agent; the mass ratio of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol to the polymer of bis(isocyanate methyl)benzene is (1-3):1, and the solid content in the whole solution is 10%-40%; ② Apply the welding reinforcing agent to the back of the two pieces of fiber-reinforced thermoplastic polyurethane composite material to be welded. After the solvent evaporates, put the two pieces of fiber-reinforced thermoplastic polyurethane composite material to be welded together and roll them. ③ Under the conditions of welding machine frequency of 26~28MHz, gas source pressure of 0.3MPa~0.5MPa, welding voltage of 220~240V, and welding current of 0.45~1.7A, welding is carried out for 8~25s, and the shape is set for 8~12s after welding to obtain the welded fiber-reinforced thermoplastic polyurethane composite material.

2. The high-strength welding method for fiber-reinforced thermoplastic polyurethane composites according to claim 1, characterized in that: The welding reinforcing agent, calculated based on the contact area of ​​the welding materials, has a coating amount of 10–100 mg / cm². 2 .

3. The high-strength welding method for fiber-reinforced thermoplastic polyurethane composites according to claim 1, characterized in that: The solvent is acetone or ethyl acetate.

4. The high-strength welding method for fiber-reinforced thermoplastic polyurethane composites according to claim 1, characterized in that: In the fiber-reinforced thermoplastic polyurethane composite material, the polyurethane is Huntsman IROGRANA 85P 4394 thermoplastic polyurethane, and the fiber is Vectran fiber.

5. The high-strength welding method for fiber-reinforced thermoplastic polyurethane composites according to claim 4, characterized in that: The fiber-reinforced thermoplastic polyurethane composite material is prepared as follows: (1) Place Huntsman IROGRANA 85P 4394 thermoplastic polyurethane film and Vectran fiber in the first unwinding and second unwinding positions of the laminating machine, respectively; add Yantai TS-9015A polyurethane adhesive into the impregnation tank of the laminating machine, turn on the drying tunnel air supply, and the air supply temperature is 40-60℃. (2) Turn on the laminating machine and, under the conditions of 80-180℃, pressure 0.1-0.5MPa, and speed 1-12m / min, allow Huntsman IROGRANA 85P 4394 thermoplastic polyurethane film and Vectran fiber material to pass through the surface treatment unit, coating unit and laminating unit at a uniform speed in a flat state to achieve lamination and winding, and obtain self-made fiber-reinforced thermoplastic polyurethane composite material.

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