Preparation method of porous thermoplastic polyurethane fiber and vamp

Abstract

The invention discloses a porous thermoplastic polyurethane fiber preparation method and a vamp, and the method comprises the following steps: S10: placing thermoplastic polyurethane fibers in an autoclave, and carrying out saturation treatment by adopting supercritical carbon dioxide fluid at a preset saturation temperature and saturation pressure; s20, performing two-stage pressure relief treatment on the autoclave; in the first-stage pressure relief process, reducing the pressure in the high-pressure kettle from saturated pressure to intermediate pressure at a first pressure relief rate; in the second-stage pressure relief process, reducing the pressure in the high-pressure kettle from the middle pressure to 0.1 MPa at a second pressure relief rate; wherein the second pressure relief rate is 1 / 1000-1 / 200 of the first pressure relief rate, the middle pressure is 30%-50% of the saturation pressure, and second-stage pressing is conducted immediately after first-stage pressure relief is completed; and S30, cooling the thermoplastic polyurethane fiber subjected to pressure relief treatment until the temperature is lower than the glass transition temperature of the thermoplastic polyurethane fiber, and shaping to obtain the porous thermoplastic polyurethane fiber. After the fibers prepared by the preparation method are applied to vamps, the vamps can be well balanced in light weight, air permeability and structural strength.

Description

Technical Field

[0001] This invention relates to the field of thermoplastic polyurethane fiber preparation technology, specifically to a method for preparing multi-porous thermoplastic polyurethane fiber and a shoe upper. Background Technology

[0002] In modern sportswear manufacturing, the performance of upper materials plays a decisive role in the comfort, support, and durability of the final product. Traditional upper materials, such as leather or multi-layered composite fabrics, while providing good support, often suffer from being heavy and having poor breathability, which can easily lead to stuffy and fatigued feet for the wearer.

[0003] In recent years, one-piece woven uppers have gained popularity due to their lightweight and snug fit. These uppers are typically woven from high-strength polymer yarns (such as TPU and polyester). However, current technology still faces an irreconcilable contradiction: to ensure sufficient support strength, thicker or multi-strand solid yarns must be used, which increases the weight of the upper and reduces breathability; conversely, pursuing lightweight and breathability sacrifices the structural stability and protection of the upper. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned defects or problems in the prior art and to provide a method for preparing multi-porous thermoplastic polyurethane fiber and a shoe upper, which enables the shoe upper made of the fiber to achieve a good balance between lightweight breathability and structural strength.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] Technical Solution 1: A method for preparing multi-porous thermoplastic polyurethane fiber, comprising the following steps: S10: placing the thermoplastic polyurethane fiber in an autoclave and saturating it with supercritical carbon dioxide fluid at a preset saturation temperature and saturation pressure; S20: performing a two-stage depressurization process on the autoclave; during the first stage depressurization, reducing the pressure inside the autoclave from the saturation pressure to an intermediate pressure at a first depressurization rate; during the second stage depressurization, reducing the pressure inside the autoclave from the intermediate pressure to 0.1 MPa at a second depressurization rate; wherein the second depressurization rate is 1 / 1000 to 1 / 200 of the first depressurization rate, the intermediate pressure is 30% to 50% of the saturation pressure, and the second stage depressurization is performed immediately after the first stage depressurization is completed; S30: cooling the depressurized thermoplastic polyurethane fiber to below its glass transition temperature for shaping to obtain the multi-porous thermoplastic polyurethane fiber.

[0007] Technical Solution 2 based on Technical Solution 1: In step S10, the saturation temperature is 125℃ to 135℃, and the saturation pressure is 19MPa to 21MPa.

[0008] Technical Solution 3 based on Technical Solution 1: In step S10, the saturation treatment time at the saturation temperature and saturation pressure is 45 minutes to 60 minutes.

[0009] Technical Solution 4 based on Technical Solution 1: In step S20, the first pressure relief rate is 12MPa / s, and the intermediate pressure is 8MPa.

[0010] Technical Solution 5 based on Technical Solution 1: The second pressure relief rate is 1.5 MPa / min.

[0011] Technical Solution Six based on Technical Solution One: In step S20, the duration of the first stage of depressurization process is less than or equal to 1 second, and the duration of the second stage of depressurization process is 5 to 15 minutes.

[0012] Technical solution seven based on technical solution one: In step S30, the fiber is cooled to below 40°C within 30 seconds.

[0013] Technical solution eight based on technical solution one: also includes a pretreatment step: before step S10, the thermoplastic polyurethane fiber is dried in a vacuum environment at 80°C for 4 hours.

[0014] Technical Solution Nine based on Technical Solution One: The thermoplastic polyurethane fiber is prepared from polyester thermoplastic polyurethane, wherein the polyester thermoplastic polyurethane has a Shore hardness of 90A and a melt index of 30-40g / 10min under test conditions of 190℃ and 2.16kg.

[0015] In addition, the present invention also provides technical solution ten: a shoe upper, at least partially made of multiporous thermoplastic polyurethane fiber prepared by the preparation method of multiporous thermoplastic polyurethane fiber as described in any one of technical solutions one to nine.

[0016] As can be seen from the above description of the present invention, compared with the prior art, the present invention has the following beneficial effects:

[0017] Technical solution one provides a method for preparing multi-porous thermoplastic polyurethane fibers, which includes three core steps: saturation treatment, two-stage depressurization treatment, and cooling and shaping. First, the saturation treatment step is to fully and uniformly dissolve supercritical carbon dioxide fluid into the thermoplastic polyurethane fiber matrix at a preset saturation temperature and saturation pressure.

[0018] Secondly, the two-stage depressurization process includes a first-stage depressurization and a second-stage depressurization. The first-stage depressurization process, which rapidly reduces the pressure from saturation to an intermediate pressure at an extremely high initial depressurization rate, aims to achieve highly efficient cell nucleation. This process utilizes the nucleation principle of conventional supercritical fluid foaming, inducing a large number of densely distributed microporous nuclei within the fiber matrix through a sudden and significant thermodynamic shock. The synergistic setting of the initial depressurization rate and the intermediate pressure allows the desired microporous nucleus structure to form within the fiber. The initial depressurization rate is set to an extremely high value, its physical function being to induce a sudden and significant thermodynamic supersaturation in the system. The intermediate pressure is set at a still relatively high level, its function being to immediately stabilize the system after high-density nucleation, preventing the formed microporous nuclei from prematurely and uncontrollably expanding and merging due to the continued pressure drop.

[0019] After the first stage of depressurization is completed, the second stage of depressurization begins immediately. This stage employs a significantly lower depressurization rate than the first stage, slowly reducing the pressure from the intermediate pressure to atmospheric pressure. During this second stage, the slow depressurization provides ample time for cell growth. During this period, existing cell nuclei grow, and some adjacent cells merge to form larger cells. Simultaneously, throughout the second stage of depressurization, the external ambient pressure continuously decreases, while the internal pressure of the cells remains relatively higher due to the lag in gas diffusion. This creates a continuous internal and external pressure difference extending from the inside of the cells to the outside. Physically, this pressure difference manifests as a continuous tensile stress on the polymer thin wall encapsulating the cells, constituting the mechanical force driving the cells to break through the polymer wall. Ultimately, this forms interconnected channels within the fiber and open pores on the surface, transforming the fiber from an impermeable closed-cell structure to an open-cell structure that allows air convection, thus achieving excellent air permeability. The second depressurization rate is set to an extremely slow value that is much lower than the first depressurization rate, thus providing sufficient time for the bubble nuclei to grow slowly through gas diffusion and allowing some bubbles to undergo Ostwald ripening and merge into larger bubbles, while maintaining the internal and external pressure difference during the stage.

[0020] Finally, after depressurization, the fibers remain in a high-temperature viscoelastic state, where the resulting cell structure is unstable. Therefore, the fibers are rapidly cooled below their glass transition temperature and shaped. At this temperature, the mobility of the polymer chains is frozen, and the entire system transforms into a rigid glassy state. The multi-scale cell structure formed during the depressurization process is solidified, ensuring the performance of the final product.

[0021] In summary, the preparation method of multi-porous thermoplastic polyurethane fiber provided in this solution offers the following steps: Step S10 provides a uniform material basis and potential energy reserve for foaming; Step S20, through its unique two-stage decompression procedure, achieves precise decoupling and control of the two physical processes of cell nucleation and opening, which is the core of forming a lightweight and breathable composite structure; Step S30 ensures the final stability of the structure. These three steps are indispensable and together constitute a complete technical solution from preparation and molding to curing, successfully creating a complex porous structure inside the fiber that is not present in traditional solid fibers. This systematically solves the contradiction between lightweight breathability and high structural strength in existing thermoplastic polyurethane fiber materials when preparing shoe uppers.

[0022] In technical solution two, the temperature and pressure during the saturation step are further limited to the ranges of 125℃ to 135℃ and 19MPa to 21MPa, respectively. Within this temperature range, the thermoplastic polyurethane is in a highly elastic state. In this state, the polymer chains have sufficient mobility, providing the necessary free volume for the penetration and diffusion of carbon dioxide molecules, thereby significantly improving the dissolution rate and solubility of carbon dioxide. If the temperature is too low, the polymer is in a glassy or semi-crystalline state, and the chain segment movement is restricted, which will hinder the dissolution of carbon dioxide and lead to insufficient saturation. If the temperature is too high, the polymer will soften excessively or even melt, causing the fiber to lose its original shape and mechanical strength. At the same time, the pressure of 19MPa to 21MPa is much higher than the supercritical pressure point of carbon dioxide. The high pressure can force more carbon dioxide molecules to dissolve into the polymer matrix, reserving sufficient gas source and potential energy for the subsequent foaming process. Therefore, the specific temperature and pressure parameters work together to ensure that the thermoplastic polyurethane fibers are saturated to the maximum extent and uniformly by the supercritical carbon dioxide fluid while maintaining their own shape and strength.

[0023] In technical solution three, the saturation treatment time is limited to 45 to 60 minutes. The diffusion of supercritical fluids is a time-consuming process. If the saturation treatment time is insufficient, carbon dioxide will mainly accumulate in the surface region of the fiber, while the concentration in the core region will be relatively low. This uneven concentration distribution will lead to uneven cell nucleation during the subsequent first-stage depressurization process, resulting in a fiber cell structure that is either sparse on the outside and dense on the inside, or has a difference between the inside and outside, leading to inconsistent and uncontrollable product performance. By setting the saturation treatment time to 45 to 60 minutes, it is ensured that carbon dioxide can penetrate the entire cross-section of the fiber and reach a dissolution equilibrium.

[0024] In technical solution four, the first depressurization rate is specified as 12 MPa / s, and the intermediate pressure is precisely set at 8 MPa. The extremely high depressurization rate of 12 MPa / s enables the formation of a large thermodynamic supersaturation within the system in a very short time. This allows the system's free energy to be primarily used for the formation of an extremely large number of new phase interfaces, i.e., micropore nuclei, rather than being consumed in the growth of existing pore nuclei. This quantified rate ensures the efficient achievement of the goal of high-density nucleation during the first stage of depressurization. Simultaneously, precisely setting the depressurization endpoint at the intermediate pressure plateau of 8 MPa immediately stabilizes the system. This pressure value is still high enough to effectively inhibit significant size growth of the already formed micropore nuclei, thus temporarily maintaining the pore growth process at an appropriate state for the second stage of processing.

[0025] In technical solution five, the depressurization rate in the second stage is specified as 1.5 MPa / min. Based on the high-density microporous cores formed in the first stage, the slow depressurization process of 1.5 MPa / min provides two necessary conditions for the controlled growth and opening of the cells: sufficient time and a continuous driving force. First, the slow depressurization process gives the cell cores ample time to grow through gas diffusion, and also allows some of the gas in the smaller cells to dissolve and diffuse into adjacent larger cells, thus forming larger cells. Second, as the external pressure continuously decreases, while the internal pressure of the cells remains at a high level due to the hysteresis effect of gas diffusion, a persistent and sufficiently large internal and external pressure difference is created. This pressure difference applies a stable tensile stress to the polymer thin wall encapsulating the cells. When this stress exceeds the yield strength of the material, it drives the cells to break through the fiber surface or the thin wall between adjacent cells, thereby forming surface openings and internal through-channels.

[0026] In technical solution six, the first-stage depressurization time is limited to less than or equal to 1 second, and the second-stage depressurization time is limited to 5 to 15 minutes. The first-stage depressurization time of less than or equal to 1 second ensures the instantaneous nature of the pressure drop process, physically guaranteeing that the system's energy is preferentially used for nucleation rather than growth, a prerequisite for achieving high-density microporous nuclei. The second-stage depressurization time of 5 to 15 minutes provides a specific and operable time window for the controlled growth, merging, and final opening of the cells. This time length is sufficient for the internal and external pressure difference to take effect, driving cell wall breakage and forming effective air-permeable channels, while also avoiding excessively long process cycles.

[0027] In technical solution seven, step S30 involves rapidly cooling the fiber to below 40°C within 30 seconds. Upon completion of pressure relief, the thermoplastic polyurethane fiber remains in a high-temperature, highly elastic state above its glass transition temperature, and the polymer chain segments still possess high mobility. The resulting cell structure, especially the thin-walled and open-cell structure, is unstable under surface tension and internal stress. If cooled slowly, the cells will collapse, shrink, or deform due to gas escape and polymer relaxation, leading to a decrease in porosity and the disappearance of the open-cell structure. By forcibly and rapidly lowering the fiber temperature below its glass transition temperature within 30 seconds, the mobility of the polymer chain segments is instantly fixed, causing the fiber to rapidly transform from a highly elastic state to a rigid glassy state. This process effectively and permanently fixes the complex multi-scale open-cell structure formed in step S20, ensuring the structural integrity and performance stability of the final product.

[0028] In technical solution eight, vacuum drying effectively removes adsorbed moisture from the thermoplastic polyurethane raw material, eliminating a key interfering factor for the subsequent supercritical carbon dioxide foaming process. Thermoplastic polyurethane materials are hygroscopic. If moisture is present within the fibers during the foaming process, it will vaporize into water vapor at the high temperature of S10, acting as an uncontrollable physical foaming agent. The foaming behavior of water vapor is completely different from that of supercritical carbon dioxide; it forms cells of uneven size and irregular shape, severely interfering with the precisely controlled carbon dioxide nucleation and growth process under two-stage pressure relief, resulting in a chaotic cell structure, degraded performance, and poor batch-to-batch consistency in the final product.

[0029] In technical solution nine, a Shore hardness of 90A indicates that the TPU material possesses high rigidity and mechanical strength. During the foaming process, especially in the second depressurization stage where the internal and external pressure differences are significant, this inherent material strength provides sufficient support for the growing cells, forming stable cell walls and preventing premature rupture or collapse. A melt index range of 30-40 g / 10 min signifies that the material has a suitable viscosity at saturation temperature. Excessive viscosity hinders carbon dioxide dissolution and cell growth; insufficient viscosity leads to thin cell walls and insufficient strength, resulting in collapse. The selected MFI range ensures both effective carbon dioxide diffusion and smooth cell growth, while also guaranteeing sufficient cell wall strength to maintain its structure until cooling and setting.

[0030] Technical solution ten provides a shoe upper, at least a portion of which is made of multi-porous thermoplastic polyurethane fiber prepared by the above-described method, thereby improving the contradiction between lightweight breathability and high structural strength in shoe uppers made of thermoplastic polyurethane fiber in the prior art. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are preferred embodiments of the present invention and should not be considered as excluding other embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0032] In the claims and description of this invention, the terms "comprising," "having," and variations thereof are used to mean "including but not limited to."

[0033] This invention relates to a shoe upper, which is at least partially made of porous thermoplastic polyurethane fiber prepared by the method of the present invention. Specifically, the porous thermoplastic polyurethane fiber can be used as the main functional yarn and directly woven into a three-dimensional shoe upper structure through an integral molding weaving technology, such as computerized flat knitting process.

[0034] In this shoe upper structure, the multi-porous thermoplastic polyurethane fiber produced by this invention can be used alone or interwoven or blended with other conventional textile yarns, such as polyester, nylon, or spandex yarns, to achieve composite properties. By programming and controlling the weaving process, different organizational structures and fiber densities can be achieved in different areas of the shoe upper. For example, a tighter knit structure can be used in areas requiring stronger support and stability, such as the sides, heel, and lace eyelets; while a looser mesh knit structure can be used in areas requiring extreme breathability, such as the toe, tongue, and instep.

[0035] Because the fibers obtained by this invention have a permeable porous structure, when woven into shoe uppers, not only does the fabric structure itself have pores, but the individual fiber units also constitute microscopic air convection channels. Therefore, compared with shoe uppers made of traditional solid TPU fibers, the shoe uppers provided by the embodiments of this invention are significantly lighter, while their breathability is fundamentally and orders of magnitude improved, providing the wearer with a dry and comfortable foot environment. Furthermore, because the fiber retains the excellent mechanical strength and resilience of TPU material, the entire one-piece woven shoe upper can provide sufficient structural support without additional reinforcing components, thus solving the technical contradiction in the prior art of balancing lightweight breathability and structural strength.

[0036] The preparation method of the above-mentioned porous thermoplastic polyurethane fiber includes the following steps:

[0037] S10: The thermoplastic polyurethane fiber is placed in an autoclave and saturated with supercritical carbon dioxide fluid at a preset saturation temperature and saturation pressure.

[0038] S20: Perform a two-stage depressurization process on the high-pressure vessel; during the first stage of depressurization, reduce the pressure inside the high-pressure vessel from the saturation pressure to an intermediate pressure at a first depressurization rate; during the second stage of depressurization, reduce the pressure inside the high-pressure vessel from the intermediate pressure to 0.1 MPa at a second depressurization rate; wherein the second depressurization rate is 1 / 1000 to 1 / 200 of the first depressurization rate, the intermediate pressure is 30% to 50% of the saturation pressure, and the second stage of depressurization is performed immediately after the first stage of depressurization is completed;

[0039] S30: Cool the thermoplastic polyurethane fiber after pressure relief treatment to below its glass transition temperature for shaping to obtain the multi-porous thermoplastic polyurethane fiber.

[0040] The following is a detailed explanation of step S10.

[0041] The saturation treatment in step S10 is fundamental to the entire preparation method. Its purpose is to uniformly and saturately dissolve supercritical carbon dioxide as a physical foaming agent into the thermoplastic polyurethane fiber matrix, providing a uniform material basis and sufficient potential energy reserve for the subsequent depressurization foaming process. The specific implementation process of this step is as follows.

[0042] First, before commencing the saturation treatment, the thermoplastic polyurethane fibers used as raw materials must undergo rigorous pretreatment. Fiber spindles made of polyester-type thermoplastic polyurethane with a Shore hardness of 90A and a melt index of 30-40 g / 10 min at 190℃ and 2.16 kg were selected and placed in an industrial vacuum drying oven. The oven temperature was set to 80℃, and the vacuum pump was started, drying the fibers continuously for 4 hours under vacuum. This operation aims to completely remove moisture adsorbed by the polyurethane material due to its hygroscopic nature. The presence of moisture will vaporize in the subsequent high-temperature environment, forming uncontrollable water vapor bubbles, severely interfering with the carbon dioxide-dominated cell nucleation and growth process, leading to uneven cell structure and decreased performance in the final product.

[0043] After pretreatment, the dried thermoplastic polyurethane fiber spindles are removed from the drying oven and immediately fixed onto a dedicated stainless steel rack inside the autoclave to ensure the fibers spread out and make full contact with the carbon dioxide fluid. Subsequently, following the equipment operating procedures, the autoclave lid and flange surfaces are cleaned, and after confirming the sealing rings are intact, the seals are tightened to ensure the autoclave has the airtightness required for operation under high pressure.

[0044] After the autoclave is sealed, the jacket heating system or internal heating element of the autoclave is activated to begin heating the interior space. The target temperature inside the autoclave is set within the saturation temperature range of 125℃ to 135℃ using a temperature controller. The temperature control system needs to be highly accurate to ensure that the actual temperature fluctuation inside the autoclave remains within ±1℃ of the set value throughout the entire subsequent saturation treatment process.

[0045] Once the temperature inside the autoclave reaches and stabilizes at the set saturation temperature, the high-pressure carbon dioxide injection system connected to the autoclave is activated. This system typically includes a carbon dioxide storage tank, a refrigeration unit, and a high-pressure injection pump. The high-pressure pump pressurizes liquid carbon dioxide and injects it into the autoclave, which has reached saturation temperature, through pipelines. During the injection process, pressure changes inside the autoclave are monitored in real time by pressure sensors. The control system adjusts the operation of the injection pump based on pressure feedback until the pressure inside the autoclave rises steadily and eventually stabilizes within the preset saturation pressure range of 19 MPa to 21 MPa.

[0046] Once the two key parameters, temperature and pressure, within the autoclave reach and stabilize at the preset saturation conditions, the saturation treatment step officially begins. This isothermal and pressure-controlled phase needs to last 45 to 60 minutes. During this period, supercritical carbon dioxide molecules continuously penetrate and diffuse into the amorphous regions of the thermoplastic polyurethane fibers. This is a physical dissolution process that gradually penetrates from the fiber surface to its core. A sufficiently long saturation time is crucial to ensuring that carbon dioxide molecules can diffuse to the very center of the fiber and that the dissolved carbon dioxide concentration across the entire cross-section of the fiber tends to be uniform and balanced. Throughout the saturation period, the autoclave's temperature and pressure control system must operate continuously to compensate for potential heat loss due to environmental conduction and pressure fluctuations caused by minor system leaks, ensuring constant saturation conditions.

[0047] Once the set duration of 45 to 60 minutes has been reached, the saturation treatment in step S10 is complete. At this point, it can be assumed that the fiber matrix has stored sufficient carbon dioxide as a physical foaming agent for subsequent foaming, and the entire system is in a uniform, high-potential thermodynamic state, providing ideal initial conditions for the subsequent two-stage depressurization treatment in step S20.

[0048] The following provides a detailed explanation of step S20.

[0049] Step S20 is the core process step for forming the final multi-scale cell structure. This step uses a precisely programmed depressurization procedure divided into two physically distinct stages to separately govern the nucleation and growth / opening processes of the cells.

[0050] The goal of the first stage of pressure relief is to form a massive, high-density number of microporous nuclei within the fibers. This stage is initiated immediately after the saturation treatment. The autoclave's control system instantly opens a large-diameter rapid pressure relief valve, such as a pneumatic or electric ball valve. This valve is set to fully open within one second or less, allowing the high-pressure gas inside the autoclave to escape instantly. The autoclave's control system monitors the pressure inside the autoclave in real time using a high-frequency pressure sensor. When the pressure is precisely reduced from the saturation pressure of 19 MPa to 21 MPa to an intermediate pressure of 8 MPa at a first pressure relief rate of up to 12 MPa / s, the control system immediately closes the rapid pressure relief valve. The entire process is strictly controlled to last less than one second. This process creates intense thermodynamic disturbances and extremely high supersaturation within the fibers, causing the system's free energy to be primarily used to form new phase interfaces, i.e., to generate a vast number of microporous nuclei, rather than being consumed in the growth of existing pores. The pressure relief endpoint was set at an intermediate pressure of 8 MPa, rather than dropping directly to atmospheric pressure. The purpose was to stabilize the system immediately after high-density nucleation was completed, preventing the formed microporous nuclei from expanding and merging prematurely and uncontrollably due to the continuous and rapid pressure drop, thereby decoupling the nucleation and growth processes in time.

[0051] After the first stage of depressurization is completed, the control system immediately switches to the second stage of depressurization without any delay. At this time, the rapid depressurization valve remains closed, while the system activates the precision back pressure regulating valve or needle valve on another exhaust line to begin the second stage of slow depressurization. The pressure control mode in this stage is completely different from that in the first stage, and its goal is to provide a controlled environment for the growth, merging, and opening of the formed micropores. The control system linearly and slowly reduces the pressure inside the vessel from the intermediate pressure plateau of 8 MPa at a constant second depressurization rate of 1.5 MPa / min, according to a preset program.

[0052] During this slow depressurization process, several key physical changes occur within the fiber. First, the slow depressurization provides ample time for carbon dioxide molecules dissolved in the polymer matrix to diffuse into the formed microporous cores, driving their gradual growth. Second, due to the Ostwald ripening effect, gas in some smaller cells tends to redissolve and diffuse into adjacent larger cells, causing further growth of the larger cells. Most importantly, throughout the entire second stage of depressurization, while the external environmental pressure continues to decrease, the gas pressure inside the cells remains at a relatively higher level due to the lag in diffusion. This persistent internal and external pressure difference applies a stable tensile stress to the polymer thin walls enveloping the cells in a high-temperature, high-elasticity state. When this stress exceeds the material's yield strength at that temperature, it leads to the rupture of the cell walls. For cells near the fiber surface, their outer thin walls rupture first, forming open surface pores; for adjacent cells within the fiber, the septa between them rupture, forming interconnected internal channels.

[0053] The entire second-stage depressurization process, reducing pressure from 8 MPa to nearly atmospheric pressure (0.1 MPa), lasted for a set period of 5 to 15 minutes based on a rate of 1.5 MPa / min. This time window ensures the transition from a closed-cell to an open-cell structure. When the pressure inside the vessel finally reaches 0.1 MPa, the depressurization process in step S20 is complete. At this point, a complex multi-scale porous structure with both high-density micropores and surface / through-pore macropores has formed inside the fiber. However, this structure is still thermodynamically unstable and requires immediate cooling and shaping in step S30.

[0054] The following provides a detailed explanation of step S30.

[0055] The core purpose of step S30 is to physically solidify the multi-scale pore structure formed in step S20, which is still in a thermodynamically unstable state, through rapid cooling, thereby obtaining a final product with stable structure and consistent performance.

[0056] When step S20 is completed, although the pressure inside the reactor has dropped to atmospheric pressure, the thermoplastic polyurethane fiber itself is still in a high-temperature state close to 125°C to 135°C. At this temperature, the polyurethane material is far above its glass transition temperature (Tg) and is in a highly elastic viscous flow state. In this state, the long-chain molecular segments of the polymer have high mobility, and the entire fiber matrix exhibits rubber-like flexibility and plasticity. Therefore, the newly formed complex cell structure, especially the extremely thin cell walls formed by stretching and the edges of surface openings, is extremely unstable under the action of polymer internal stress and surface tension. If slow natural cooling is carried out at this time, the residual gas inside the cells will continue to escape, and the polymer chains will relax and rearrange. This will cause the formed cells to collapse, shrink, and deform, and may even cause the already opened surface pores to close again, ultimately failing to achieve the expected low density and high permeability.

[0057] To avoid damage to the aforementioned structure, rapid forced cooling, also known as quenching, is necessary. Specifically, when the pressure inside the autoclave reaches 0.1 MPa, the autoclave's heating system is automatically shut down immediately, while the cooling medium circulation system is simultaneously activated. This system, through valve switching, rapidly circulates pre-prepared cooling water at approximately 10°C into the autoclave's jacket. The large volume of low-temperature cooling water flowing at high speed within the jacket efficiently exchanges heat with the high-temperature autoclave body, causing a rapid drop in temperature inside the autoclave.

[0058] The goal of this cooling process is to rapidly reduce the temperature of the thermoplastic polyurethane fibers inside the autoclave from their current high temperature to below their glass transition temperature within 30 seconds, specifically below 40°C. When the fiber temperature is rapidly cooled below its glass transition temperature, the mobility of the polymer chain segments is instantly fixed, and the entire material transforms from a flexible, highly elastic state to a rigid glassy state. This physical state transition permanently fixes all microstructures formed during the pressure relief process, including micropores, macropores, through-holes, and surface openings, preventing further shrinkage or deformation.

[0059] Through this strictly controlled rapid cooling and shaping process, the multi-scale open-cell structure precisely constructed in step S20 is completely preserved. Once the temperature inside the autoclave stabilizes below 40°C, the entire preparation process is complete. At this point, the pressure in the autoclave can be released, the lid opened, and the fiber spindles removed from the rack, yielding the finished multi-cellular thermoplastic polyurethane fiber product with a stable porous structure, lightweight properties, and high air permeability.

[0060] To further illustrate the technical solutions involved in this invention, the following embodiments and comparative examples are provided. It should be noted that the following embodiments and comparative examples do not constitute a limitation of this invention.

[0061] To make the description of the present invention clearer and more reproducible, the main raw materials used in the following embodiments and comparative examples are as follows:

[0062] Thermoplastic polyurethane (TPU) chips: Elastollan series polyester TPU manufactured by BASF (Germany), specifically Elastollan 1190A. This TPU material has a Shore hardness of 90A and a melt flow index (MFI) of 35 g / 10 min under test conditions of 190℃ and 2.16 kg, and its performance parameters meet the requirements of technical solution nine of this invention.

[0063] Supercritical fluid foaming medium: Industrial pure carbon dioxide (CO2) with a purity ≥99.9% is used.

[0064] The aforementioned TPU chips were melt-extruded using a single-screw extruder at 185-205°C, and then melt-spun into 250D TPU precursor fibers. These precursor fibers served as the starting material for all subsequent foaming experiments. The precursor fibers had a solid structure and an initial density of approximately 1.20 g / cm³. 3 .

[0065] The properties of the prepared fiber samples were characterized using the following methods:

[0066] Fiber density test: According to GB / T 1033.1-2008 "Determination of density of non-foamed plastics - Part 1: Impregnation method, liquid pyrometer method and titration method", the apparent density of the fiber was determined by the impregnation method (Method A).

[0067] Monofilament breaking strength test: According to GB / T 3916-2013 "Determination of breaking strength and elongation at break of single yarn of textiles", the breaking strength of the fiber is tested on a single yarn strength tester.

[0068] Fabric air permeability test: The prepared fiber samples are knitted into plain knitted fabrics of standard size using the same process parameters (same knitting density and structure). Then, according to GB / T 5453-1997 "Textiles - Determination of air permeability of fabrics", the air permeability of the fabric is tested under a pressure difference of 100Pa.

[0069] Example 1

[0070] This embodiment aims to illustrate the preferred implementation process of the technical solution of the present invention.

[0071] A method for preparing multi-porous thermoplastic polyurethane fiber, the specific steps of which are as follows:

[0072] (1) Pretreatment: Dry the 250D TPU precursor fiber in a vacuum environment at 80℃ for 4 hours.

[0073] (2) Saturation treatment (step S10): Place the dried TPU precursor in a 5L autoclave and seal the autoclave. Raise the autoclave temperature to 130℃, then inject CO2 and pressurize to 20MPa. Maintain this saturation temperature and pressure for 60 minutes.

[0074] (3) Two-stage pressure relief treatment (step S20): In the first stage, the pressure inside the vessel is instantly reduced from 20MPa to an intermediate pressure of 8MPa within 1 second; in the second stage, the pressure inside the vessel is immediately reduced slowly and linearly from 8MPa to 0.1MPa at a constant rate of 1.5MPa / min.

[0075] (4) Cooling and shaping (step S30): After the pressure drops to 0.1 MPa, cooling water is immediately introduced into the reactor jacket to reduce the temperature inside the reactor to below 40°C within 30 seconds.

[0076] The sample was removed, and the multi-porous thermoplastic polyurethane fiber of Example 1 was obtained.

[0077] Example 2

[0078] This embodiment aims to illustrate the implementation effect of the technical solution of the present invention within the lower limit of the claimed parameter range.

[0079] A method for preparing multi-porous thermoplastic polyurethane fiber, the specific steps of which are as follows:

[0080] (1) Pretreatment: Same as in Example 1.

[0081] (2) Saturation treatment (step S10): Raise the temperature of the reactor to 125°C, then inject CO2 and pressurize to 19 MPa. Maintain constant temperature and pressure under these conditions for 45 minutes.

[0082] (3) Two-stage pressure relief treatment (step S20): In the first stage, the pressure inside the vessel is instantly reduced from 19MPa to an intermediate pressure of 7.6MPa (40% of the saturation pressure) within 1 second; in the second stage, the pressure inside the vessel is immediately reduced slowly and linearly from 7.6MPa to 0.1MPa at a constant rate of 1.9MPa / min.

[0083] (4) Cooling and shaping (step S30): Same as in Example 1.

[0084] The sample was removed, and the multi-porous thermoplastic polyurethane fiber of Example 2 was obtained.

[0085] Example 3

[0086] This embodiment aims to illustrate the implementation effect of the technical solution of the present invention within the upper limit of the claimed parameter range.

[0087] A method for preparing multi-porous thermoplastic polyurethane fiber, the specific steps of which are as follows:

[0088] (1) Pretreatment: Same as in Example 1.

[0089] (2) Saturation treatment (step S10): Raise the temperature of the reactor to 135°C, then inject CO2 and pressurize to 21 MPa. Maintain constant temperature and pressure under these conditions for 60 minutes.

[0090] (3) Two-stage pressure relief treatment (step S20): In the first stage, the pressure inside the vessel is instantly reduced from 21MPa to an intermediate pressure of 8.4MPa (40% of the saturation pressure) within 1 second; in the second stage, the pressure inside the vessel is immediately and slowly reduced linearly from 8.4MPa to 0.1MPa at a constant rate of 1.0MPa / min.

[0091] (4) Cooling and shaping (step S30): Same as in Example 1.

[0092] The sample was removed, and the multi-porous thermoplastic polyurethane fiber of Example 3 was obtained.

[0093] Comparative Example 1

[0094] This comparative example aims to illustrate the effects obtained by using a conventional, single-stage rapid depressurization supercritical foaming method.

[0095] (1) Pretreatment: Same as in Example 1.

[0096] (2) Saturation treatment: Same as in Example 1, i.e., saturate at 130°C and 20MPa for 60 minutes.

[0097] (3) Single-stage pressure relief: After saturation, the pressure inside the vessel is reduced directly from 20MPa to 0.1MPa within 1 second using a fast pressure relief valve. There is no intermediate pressure plateau or slow pressure relief in the second stage during this process.

[0098] (4) Cooling and shaping: Same as in Example 1.

[0099] The sample was removed to obtain the foamed thermoplastic polyurethane fiber of Comparative Example 1.

[0100] Comparative Example 2

[0101] This comparative example aims to illustrate the effect obtained by using a single-stage slow-decompression supercritical foaming method, in order to compare it with the present invention.

[0102] (1) Pretreatment: Same as in Example 1.

[0103] (2) Saturation treatment: Same as in Example 1, i.e., saturate at 130°C and 20MPa for 60 minutes.

[0104] (3) Single-stage pressure relief treatment: After saturation, the pressure inside the vessel is directly and slowly reduced from 20MPa to 0.1MPa at a constant rate of 1.5MPa / min through the back pressure regulating valve. There is no rapid pressure relief in the first stage and no intermediate pressure plateau in this process.

[0105] (4) Cooling and shaping: Same as in Example 1.

[0106] The sample was removed to obtain the foamed thermoplastic polyurethane fiber of Comparative Example 2.

[0107] Comparative Example 3

[0108] This comparative example is intended to illustrate the effect of pressure relief treatment when the intermediate pressure is lower than the range required by the present invention (30% to 50%).

[0109] (1) Pretreatment: Same as in Example 1.

[0110] (2) Saturation treatment: Same as in Example 1, i.e., saturate at 130°C and 20MPa for 60 minutes.

[0111] (3) Two-stage pressure relief treatment: In the first stage, the pressure inside the vessel is instantly reduced from 20MPa to the intermediate pressure of 4MPa (20% of the saturation pressure) within 1 second; in the second stage, the pressure inside the vessel is immediately reduced slowly and linearly from 4MPa to 0.1MPa at a constant rate of 1.5MPa / min.

[0112] (4) Cooling and shaping: Same as in Example 1.

[0113] The sample was removed, and the foamed thermoplastic polyurethane fiber of Comparative Example 3 was obtained.

[0114] The fiber samples obtained in Examples 1-3 and Comparative Examples 1-3, as well as the untreated TPU filaments, were subjected to performance tests, and the results are summarized in the table below.

[0115]

[0116] Examples 1, 2, and 3 all successfully reduced fiber density from 1.20 g / cm³. 3 Significantly reduced to 0.21 g / cm³ 3 The following results in excellent lightweight performance. At the same time, its monofilament breaking strength remains above 2.1 cN / dtex, retaining more than 60% of the mechanical properties of the original filament, which is sufficient to meet the strength requirements of applications such as shoe uppers.

[0117] Comparative Example 1 (single-stage rapid depressurization) also achieved a lower density (0.25 g / cm³). 3 It exhibits high strength (2.5 cN / dtex), but its key performance characteristic is extremely poor air permeability. Comparative Example 2 (single-stage slow pressure relief) and Comparative Example 3 (intermediate pressure too low) perform poorly in both density and strength, with the density only dropping to 0.5 g / cm³. 3 Furthermore, the fracture strength deteriorates significantly, dropping below 1.2 cN / dtex, rendering it unsuitable for use as a structural material. This demonstrates that only the two-stage pressure relief scheme of this invention can synergistically achieve a balance between extreme lightweighting and sufficient mechanical strength.

[0118] The fibers obtained in Examples 1, 2, and 3 produced fabrics with air permeability of 1250, 1180, and 1210 L / m, respectively. 2 •s, almost non-breathable with TPU filament fabric (<5L / m 2 Compared to the previous version (·s), it achieved a performance leap of over 200 times. Specifically, although Comparative Example 1 achieved weight reduction, its breathability was only 18L / m³. 2 The fiber in Comparative Example 1 is essentially no different from the original fiber. This is because the fiber in Comparative Example 1 has a uniform, fine closed-cell structure. These closed pores cannot form air convection pathways. The two-stage pressure relief process of this invention is a creative solution to the above bottleneck. The rapid pressure relief in the first stage to an intermediate pressure forms the microporous nucleus base required for weight reduction. The slow pressure relief in the second stage provides a controlled growth and opening environment for the pores, driving the pores to break through the fiber surface through a continuous internal and external pressure difference, forming macroscopic air convection channels. Furthermore, Comparative Example 2 lacks the rapid nucleation in the first stage, resulting in a deteriorated pore structure and unsatisfactory strength and permeability. The intermediate pressure setting in Comparative Example 3 is lower than the required range of this invention, causing uncontrolled growth and rupture of the pores during the first stage of pressure relief, destroying the basic structure required for the slow opening in the second stage, ultimately resulting in performance far inferior to the embodiments of this invention.

[0119] The foregoing description of the specifications and embodiments is intended to explain the scope of protection of this invention, but does not constitute a limitation on the scope of protection of this invention. Modifications, equivalent substitutions, or other improvements to the embodiments of this invention or a portion thereof that can be obtained by those skilled in the art through logical analysis, reasoning, or limited experimentation, based on the teachings of this invention or the foregoing embodiments, in conjunction with common knowledge, general technical knowledge, and / or existing technology, should all be included within the scope of protection of this invention.

Claims

1. A method for preparing multi-porous thermoplastic polyurethane fiber, characterized in that, Includes the following steps: S10: The thermoplastic polyurethane fiber is placed in an autoclave and saturated with supercritical carbon dioxide fluid at a preset saturation temperature and saturation pressure. S20: Perform a two-stage depressurization process on the high-pressure vessel; during the first stage of depressurization, reduce the pressure inside the high-pressure vessel from the saturation pressure to the intermediate pressure at a first depressurization rate; During the second stage of depressurization, the pressure inside the autoclave is reduced from the intermediate pressure to 0.1 MPa at a second depressurization rate; wherein the second depressurization rate is 1 / 1000 to 1 / 200 of the first depressurization rate, the intermediate pressure is 30% to 50% of the saturation pressure, and the second stage of depressurization is performed immediately after the first stage of depressurization is completed. S30: Cool the thermoplastic polyurethane fiber after pressure relief treatment to below its glass transition temperature for shaping to obtain the multi-porous thermoplastic polyurethane fiber.

2. The method for preparing a multi-porous thermoplastic polyurethane fiber as described in claim 1, characterized in that, In step S10, the saturation temperature is 125°C to 135°C, and the saturation pressure is 19 MPa to 21 MPa.

3. The method for preparing a multi-porous thermoplastic polyurethane fiber as described in claim 1, characterized in that, In step S10, the saturation treatment time at the saturation temperature and saturation pressure is 45 to 60 minutes.

4. The method for preparing a multi-porous thermoplastic polyurethane fiber as described in claim 1, characterized in that, In step S20, the first pressure relief rate is 12 MPa / s, and the intermediate pressure is 8 MPa.

5. The method for preparing a multi-porous thermoplastic polyurethane fiber as described in claim 1, characterized in that, The second pressure relief rate is 1.5 MPa / min.

6. The method for preparing a multi-porous thermoplastic polyurethane fiber as described in claim 1, characterized in that, In step S20, the duration of the first stage of depressurization is less than or equal to 1 second, and the duration of the second stage of depressurization is 5 to 15 minutes.

7. The method for preparing a multi-porous thermoplastic polyurethane fiber as described in claim 1, characterized in that, In step S30, the fiber is cooled to below 40°C within 30 seconds.

8. The method for preparing a multi-porous thermoplastic polyurethane fiber as described in claim 1, characterized in that, It also includes a pretreatment step: before step S10, the thermoplastic polyurethane fiber is dried in a vacuum environment at 80°C for 4 hours.

9. The method for preparing a multi-porous thermoplastic polyurethane fiber as described in claim 1, characterized in that, The thermoplastic polyurethane fiber is made of polyester-type thermoplastic polyurethane, which has a Shore hardness of 90A and a melt index of 30-40 g / 10 min under test conditions of 190℃ and 2.16 kg.

10. A shoe upper, characterized in that, It is made at least in part from the porous thermoplastic polyurethane fiber prepared by the method of preparing porous thermoplastic polyurethane fiber as described in any one of claims 1-9.

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