A method for producing a polyurethane foam

Abstract

The application provides a preparation method of a polyurethane foaming material. The preparation method comprises the following steps: firstly, pre-polymerization of polytetrahydrofuran ether glycol and hexamethylene diisocyanate; then, mixing, extruding, and granulating of polyhexalactone dihydric alcohol and a catalyst to obtain polyurethane granules. The granules are dried and injection molded into a small embryo to be foamed. The small embryo is immersed in supercritical carbon dioxide, nitrogen is introduced for auxiliary foaming, and gradient pressure relief control is implemented in the foaming process. After foaming, curing treatment is performed to obtain the polyurethane foaming material. The preparation method is beneficial to improving the cell uniformity of the polyurethane foaming material and improving the performance stability of the material after rebound, compression, and aging.

Description

Technical Field

[0001] This invention relates to the field of polyurethane foam material preparation technology, and specifically to a method for preparing polyurethane foam material. Background Technology

[0002] In athletic shoe midsoles must provide cushioning while maintaining good rebound performance during use. Simultaneously, the midsole also needs to balance weight control and durability. Therefore, using foamed elastomer materials to prepare shoe midsoles has become a common technical approach in this field. Current shoe midsole foaming processes typically involve first obtaining elastomer particles or forming a pre-formed component to be foamed, then impregnating it with high-pressure gas, followed by heating and foaming, and finally shaping it after demolding. However, current high-resilience polyurethane midsole materials for shoes still suffer from poor cell uniformity and performance stability. Summary of the Invention

[0003] The purpose of this invention is to overcome the above-mentioned defects or problems in the prior art and provide a method for preparing polyurethane foam material to improve the problems of poor cell uniformity and poor performance stability of existing high-resilience shoe polyurethane midsole materials.

[0004] The applicant discovered that during the foaming process of high-resilience polyurethane midsole materials, poor cell uniformity is not solely caused by a single foaming parameter, but is related to the internal state of the material, gas diffusion nucleation, and changes in the stress on the cell walls. If general-purpose granules or ordinary preforms directly enter the impregnation and foaming stages, the distribution of soft and hard segments, the degree of crystallinity, and the moisture content may differ in different regions of the preform, resulting in variations in viscoelastic response and melt strength. These differences lead to inconsistent diffusion rates and solubility of carbon dioxide in different regions. During subsequent heating or depressurization, different regions reach gas supersaturation at different times, with some areas nucleating and expanding first. These early-expanding cells alter the stress state and gas concentration in surrounding areas, causing insufficient nucleation or excessive cell growth in adjacent regions, further increasing the likelihood of cell collapse or localized collapse. The applicant also found that poor performance stability is related to the retention of the cell structure after formation. When the pressure is released too quickly, the cell wall will be subjected to a large internal and external pressure difference. When there is insufficient curing after foaming, the residual stress in the cell wall is difficult to be released in time, and the gas will continue to migrate between the cells. Smaller cells are prone to merging into larger cells or collapsing, making the material more prone to insufficient rebound after repeated compression and thermal aging, resulting in increased compression deformation and hardness fluctuations.

[0005] At least one embodiment discloses a method for preparing a polyurethane foam material, the method comprising: S1: reacting polytetrahydrofuran ether diol with hexamethylene diisocyanate at 70-80 degrees Celsius for 2-3 hours to obtain a prepolymer; S2: cooling the prepolymer to 45-55 degrees Celsius, adding polycaprolactone diol and a catalyst for compounding, extrusion granulation, and obtaining polyurethane granules; S3: drying the polyurethane granules at 80-100 degrees Celsius for 2-6 hours, and then injection molding them into preforms to be foamed; S4: subjecting the preforms to foaming at 18-25 MPa and 35-55 degrees Celsius. S5: The preform impregnated with supercritical carbon dioxide for 0.5-2 hours to achieve a carbon dioxide impregnation rate of 3-6% by mass; S6: The preform after supercritical carbon dioxide impregnation is transferred to a foaming device, and nitrogen gas is introduced at 100-130 degrees Celsius and 3-8 MPa for assisted foaming. Gradient pressure relief control is implemented at a rate of 0.05-0.2 MPa per second during the foaming process to obtain a foamed material preform; S7: The foamed material preform is cured at 20-30 degrees Celsius for 18-30 hours, or at 45-55 degrees Celsius for 6-10 hours to obtain polyurethane foam material.

[0006] In the above design, S1 and S2 first undergo a prepolymerization reaction, followed by mixing, extrusion, and granulation after cooling. This ensures that the reaction products of the polyol component and isocyanate form a relatively uniform material state before impregnation. The dispersion of the modified components and catalyst into the polyurethane matrix during mixing reduces differences in material composition and viscoelastic response, making the diffusion and dissolution of carbon dioxide in the preform easier and more consistent. Furthermore, S3 reduces preform moisture fluctuations through drying and controls the shape and size of the preform through injection molding, resulting in more stable gas transfer distances and heating conditions when subsequent gases enter the preform.

[0007] In the above design, S4 first introduces carbon dioxide into the embryo to be foamed, creating a gas-impregnated state required for subsequent nucleation. S5 then uses nitrogen-assisted foaming to drive cell expansion, with gradient pressure relief control implemented during the foaming process. Because the gas first forms a relatively stable impregnation state inside the embryo, and the cells then expand with nitrogen assistance, the pressure is released more gradually, making it easier to control changes in gas concentration and internal / external pressure difference during cell growth. Therefore, this method helps reduce localized over-foaming and under-foaming, as well as reducing cell coagulation and pore coarsening.

[0008] In the above design, S6 continues the curing process after the foamed material preform is formed. During the curing process, the residual internal stress of the cell walls and polyurethane matrix can be released under relatively stable temperature conditions, and the gas migration and viscoelastic relaxation that are still occurring after foaming will gradually weaken. As a result, the obtained polyurethane foam material is less prone to structural collapse and performance fluctuations after repeated compression or thermal aging, which helps to improve resilience and improve the performance stability after compression deformation and aging.

[0009] In the preparation method disclosed in at least one embodiment, preferably, in the raw materials for obtaining the polyurethane granules, by weight, there are 62-68 parts of polytetrahydrofuran ether diol, 18-22 parts of hexamethylene diisocyanate, and 5-8 parts of polycaprolactone diol.

[0010] In the above design, polytetrahydrofuran ether diol is mainly used to provide elastic segments, hexamethylene diisocyanate is used to form the polyurethane reactive backbone, and polycaprolactone diol is used to adjust the mechanical properties and processing conditions of the material. The above ratio range is beneficial for balancing the prepolymer reaction activity and also for balancing granule formability and elastic recovery after foaming, thereby reducing differences in cell growth caused by materials that are too hard or too soft, and also reducing differences in cell growth caused by uneven local reactions.

[0011] In the preparation method disclosed in at least one embodiment, preferably, the catalyst used in S2 is stannous octoate, and the amount of stannous octoate added is 0.02-0.2 parts by mass.

[0012] In the above design, when the amount of catalyst is within the above range, the reaction rate and material viscosity of the prepolymer are relatively stable in the subsequent mixing process. This can reduce the impact of excessively fast or insufficient local reaction on the uniformity of the particles and provide a more stable material state for subsequent gas impregnation and cell nucleation.

[0013] In the preparation method disclosed in at least one embodiment, preferably, boron nitride nanosheets are added in S2 as a pore-regulating agent, and the amount of boron nitride nanosheets added is 0.2-0.8 parts by mass.

[0014] In the above design, boron nitride nanosheets dispersed in the polyurethane matrix can promote heterogeneous nucleation and improve local heat transfer. When the dosage is within the above range, it can increase the number of nucleation sites while reducing the possibility of excessive agglomeration of additives, thereby benefiting cell refinement and cell structure stability.

[0015] In the preparation method disclosed in at least one embodiment, preferably, the preform to be foamed is formed by injection molding, with an injection temperature of 160-190 degrees Celsius and a mold temperature of 30-60 degrees Celsius.

[0016] In the above design, when the injection temperature and mold temperature are within the aforementioned range, the polyurethane granules can be fully plasticized and form preforms with relatively stable size and shape in the mold. Because the preforms have a more consistent shape and internal density, the diffusion distance and impregnation depth during the carbon dioxide impregnation stage are more likely to remain consistent, thereby reducing cell unevenness caused by local density differences in the preforms.

[0017] In the preparation method disclosed in at least one embodiment, preferably, the supercritical carbon dioxide impregnation pressure is 20-22 MPa, the temperature is 40-50 degrees Celsius, the impregnation time is 45-90 minutes, and the carbon dioxide impregnation rate is 4-5% by mass.

[0018] In the above design, the impregnation pressure affects the amount of carbon dioxide dissolved and the diffusion depth in the polyurethane preform, while the impregnation temperature and impregnation time also affect the amount of carbon dioxide dissolved and the diffusion depth in the polyurethane preform. The above ranges are conducive to achieving a more sufficient and uniform impregnation state of carbon dioxide inside the preform, thereby avoiding insufficient local gas leading to a low number of nucleation sites, and also avoiding excessive impregnation leading to subsequent rapid local expansion.

[0019] In the preparation method disclosed in at least one embodiment, preferably, the foaming temperature for nitrogen-assisted foaming is 110-130 degrees Celsius, the nitrogen pressure is 4-8 MPa, and the foaming time is 5-20 minutes.

[0020] In the above design, the foaming temperature determines the chain segment mobility and cell wall deformation capacity of the polyurethane matrix, while the nitrogen pressure and foaming time affect the duration of cell expansion. The above ranges are beneficial for allowing the cells to continue expanding after carbon dioxide impregnation, while avoiding excessive stretching of the cell walls due to excessively high temperature or pressure, and also preventing insufficient foaming due to insufficient pressure.

[0021] In the preparation method disclosed in at least one embodiment, preferably, the pressure relief rate of the gradient pressure relief control is 0.08-0.12 MPa per second.

[0022] In the above design, when the depressurization rate is within the specified range, the pressure difference between the inside and outside of the bubble can be gradually released, and the bubble wall has sufficient time to undergo viscoelastic adjustment with pressure changes. Compared with one-time depressurization, gradient depressurization makes the time relationship between bubble expansion and structural shaping more gradual, thus helping to reduce the risk of bubble rupture caused by rapid depressurization, and also helping to reduce the risk of bubble coalescence and local collapse, thereby improving the uniformity of the bubble structure and the dimensional stability of the foamed material preform.

[0023] In the preparation method disclosed in at least one embodiment, preferably, the foamed material preform is cured at 20-25 degrees Celsius for 22-26 hours, or cured at 45-50 degrees Celsius for 7-9 hours.

[0024] In the above design, room temperature curing is beneficial for releasing residual stress after foaming under lower thermal disturbance conditions, while heating curing is beneficial for promoting the stability of cell walls and polyurethane matrix in a shorter time. Through the above curing conditions, gas migration that is not yet fully balanced after foaming can be gradually reduced, and the chain segment relaxation and dimensional recovery processes will also tend to stabilize, thereby further improving the resilience of the obtained polyurethane foam material and improving its retention of compression performance and post-aging performance.

[0025] In summary, this invention improves material uniformity through prepolymerization and modified granulation, ensuring a relatively stable composition and viscoelastic state for the preform before entering the impregnation stage. Supercritical carbon dioxide impregnation and nitrogen-assisted foaming allow gas to first enter the preform and create nucleation conditions, subsequently driving further cell expansion. Gradient depressurization and curing treatment smooth out pressure changes on the cell walls and gradually release internal stress. Therefore, this invention improves cell uniformity and enhances the performance stability of polyurethane foam materials by addressing the gas diffusion, cell nucleation, cell expansion, and structure retention processes. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments are briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic flowchart of the preparation method of polyurethane foam material according to an embodiment of the present invention. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0029] In the claims, description and accompanying drawings of this invention, the terms "comprising," "having," and variations thereof are used to mean "including but not limited to."

[0030] This invention relates to a method for preparing polyurethane foam material, which mainly includes the following steps: S1: Polytetrahydrofuran ether diol and hexamethylene diisocyanate are reacted at 70-80 degrees Celsius for 2-3 hours to obtain a prepolymer.

[0031] S2: After cooling the prepolymer to 45-55 degrees Celsius, polycaprolactone diol and catalyst are added for compounding, extrusion and granulation to obtain polyurethane granules.

[0032] S3: After drying the polyurethane granules at 80-100 degrees Celsius for 2-6 hours, the granules are injection molded into foamed preforms.

[0033] S4: The embryo to be foamed is impregnated with supercritical carbon dioxide for 0.5-2 hours at 18-25 MPa and 35-55 degrees Celsius, so that the carbon dioxide impregnation rate is 3-6% by mass.

[0034] S5: The preform impregnated with supercritical carbon dioxide is transferred into a foaming device, where nitrogen gas is introduced at 100-130 degrees Celsius and 3-8 MPa for assisted foaming. Gradient pressure relief control is implemented at a rate of 0.05-0.2 MPa per second during the foaming process to obtain a foamed material preform.

[0035] S6: The foamed material preform is cured at 20-30 degrees Celsius for 18-30 hours, or at 45-55 degrees Celsius for 6-10 hours, to obtain polyurethane foamed material.

[0036] The above six steps constitute a continuous preparation method. Specifically, S1 to S4 are used to obtain a relatively uniform preform to be foamed and to allow carbon dioxide to enter the preform more uniformly; S5 is used to further expand the pores with nitrogen assistance and to reduce the risk of pore rupture and collapse through gradient pressure relief; S6 is used to release residual stress after foaming and to stabilize the material structure. Each step is further explained below.

[0037] In S1, a jacketed mechanically stirred reactor can be used for the prepolymerization reaction. The reactor can be equipped with a temperature probe, a nitrogen protection port, and a vacuum degassing port. To reduce the influence of moisture on the isocyanate reaction, polytetrahydrofuran ether glycol and polycaprolactone diol can be pre-dehydrated at 90-100 degrees Celsius for 1-2 hours. After adding a predetermined amount of polytetrahydrofuran ether glycol to the reactor, a predetermined amount of hexamethylene diisocyanate is added. Under nitrogen protection, the reaction system is heated to 70-80 degrees Celsius and maintained for 2-3 hours to allow the two to undergo a prepolymerization reaction. In the raw materials for obtaining this polyurethane granules, the amount of polytetrahydrofuran ether glycol added can be 62-68 parts by mass, and the amount of hexamethylene diisocyanate added can be 18-22 parts. During the reaction, the stirring speed can be controlled at 200-400 rpm. When the viscosity of the system increases significantly and the material is uniformly dispersed, the prepolymer is obtained.

[0038] In step S2, the prepolymer obtained in step S1 is cooled to 45-55 degrees Celsius, and then a predetermined amount of polycaprolactone diol and catalyst are added. In the raw materials used to obtain the polyurethane granules, the amount of polycaprolactone diol added, by mass, can be 5-8 parts. The catalyst can be stannous octoate, and its addition amount can be 0.02-0.2 parts. To improve the nucleation state during subsequent foaming, a cell-regulating agent can also be added in this step. The cell-regulating agent can be boron nitride nanosheets, and its addition amount can be 0.2-0.8 parts. In actual operation, boron nitride nanosheets can be dispersed in polycaprolactone diol first, and then added to the prepolymer together with stannous octoate to ensure that the cell-regulating agent is more uniformly distributed in the polyurethane matrix. The resulting material is then fed into a co-rotating twin-screw extruder for compounding and extrusion. The temperature of the front section of the barrel can be controlled at 120-135 degrees Celsius, and the temperature of the middle and rear sections can be controlled at 140-160 degrees Celsius. After cooling, stretching, and pelletizing, the extrudate yields polyurethane granules with relatively uniform particle size.

[0039] In step S3, the polyurethane granules obtained in step S2 can be placed in a dehumidifying drying oven and dried at 80-100 degrees Celsius for 2-6 hours to reduce the moisture content of the granules. The dried granules are then injection molded into preforms for foaming using an injection molding machine. The injection temperature can be controlled at 160-190 degrees Celsius, and the mold temperature at 30-60 degrees Celsius. In actual operation, the mold temperature can be stabilized at the preset value first, and then the granules are plasticized by the injection molding machine barrel and injected into the predetermined mold cavity. After cooling and demolding, the preforms for foaming are obtained. The shape of the preforms for foaming can be pre-designed according to the target midsole structure, and its thickness and volume can be set in conjunction with the subsequent foaming ratio.

[0040] In S4, a supercritical fluid impregnation vessel with a pressure resistance of not less than 30 MPa can be used to treat the preforms to be foamed. The preforms are placed on a sample rack, maintaining spacing between them, and then the vessel is closed and carbon dioxide is introduced. The impregnation pressure can be controlled at 18-25 MPa, the impregnation temperature at 35-55 degrees Celsius, and the impregnation time at 0.5-2 hours. Preferably, the impregnation pressure can be controlled at 20-22 MPa, the impregnation temperature at 40-50 degrees Celsius, and the impregnation time at 45-90 minutes, so that the carbon dioxide impregnation rate reaches 4-5% by mass.

[0041] In step S5, the preform processed in step S4 is transferred to a foaming device for auxiliary foaming. The foaming device can have heating and ventilation functions, as well as programmed pressure control, and can be equipped with a heating chamber, nitrogen inlet, pressure sensor, and proportional pressure relief valve. After the preform is placed in the foaming device, it is first heated to the set foaming temperature, then nitrogen is introduced and a predetermined pressure is maintained. The nitrogen-assisted foaming temperature can be controlled between 100-130 degrees Celsius, and the nitrogen pressure can be controlled between 3-8 MPa. Preferably, the foaming temperature can be controlled between 110-130 degrees Celsius, the nitrogen pressure can be controlled between 4-8 MPa, and the foaming time can be controlled between 5-20 minutes. After the foaming stage, instead of a one-time rapid pressure relief, a gradient pressure relief is implemented through a proportional pressure relief valve or a programmed control valve. The gradient pressure relief rate can be controlled between 0.05-0.2 MPa per second. Preferably, the gradient pressure relief rate can be controlled between 0.08-0.12 MPa per second. In practice, the depressurization process can be set to two or three stages to gradually reduce the pressure to atmospheric pressure, thereby reducing the risk of cell coarsening, reducing the risk of cell merging and collapse, and improving cell uniformity.

[0042] In step S6, the foamed material blank obtained in step S5 is demolded and placed in a constant temperature room or oven for curing. Curing conditions can be 18-30 hours at 20-30 degrees Celsius, or 6-10 hours at 45-55 degrees Celsius. Preferably, room temperature curing conditions are 22-26 hours at 20-25 degrees Celsius, and accelerated curing conditions are 7-9 hours at 45-50 degrees Celsius. In actual operation, the demolded foamed material blanks can be stacked flat to avoid mutual compression and kept at a constant temperature for the predetermined time.

[0043] In the polyurethane foam material prepared by the above method, the material uniformity is improved through prepolymerization and modified granulation, giving the preform a relatively stable composition and viscoelastic state before entering the impregnation stage. Supercritical carbon dioxide impregnation and nitrogen-assisted foaming allow gas to first enter the preform and create nucleation conditions, then drive further cell expansion. Gradient depressurization and curing treatment smooth out pressure changes on the cell walls and gradually release internal stress. Therefore, this invention improves cell uniformity and enhances the performance stability of the polyurethane foam material by addressing the gas diffusion, cell nucleation, cell expansion, and structure retention processes.

[0044] To further illustrate the effects of the polyurethane foam material involved in this invention, the following examples and comparative examples are provided.

[0045] In the following examples and comparative examples, polytetrahydrofuran ether diol was purchased from BASF (China) Co., Ltd., brand name PolyTHF 2000; hexamethylene diisocyanate was purchased from Covestro Polymers (China) Co., Ltd., brand name DesmodurH; polycaprolactone diol was purchased from Perstorp Chemical Co., Ltd., brand name CAPA 2200; stannous octoate was purchased from Sinopharm Chemical Reagent Co., Ltd., brand name analytical grade; boron nitride nanosheets were purchased from Nanjing Xianfeng Nanomaterials Technology Co., Ltd., brand name XF224; carbon dioxide and nitrogen were both purchased from Air Liquide (China) Investment Co., Ltd., with carbon dioxide purity not less than 99.9% by volume and nitrogen purity not less than 99.99% by volume. The commercially available polyurethane elastomer granules used in Comparative Example 1 were purchased from BASF (China) Co., Ltd., brand name Elastollan 1185A10.

[0046] Example 1 62 parts of polytetrahydrofuran ether diol and 18 parts of hexamethylene diisocyanate were weighed and added to a jacketed mechanically stirred reactor. The mixture was reacted at 70°C for 2 hours under nitrogen protection to obtain a prepolymer. The prepolymer was cooled to 45°C, and 5 parts of polycaprolactone diol were added. Separately, 0.2 parts of boron nitride nanosheets were pre-dispersed in the polycaprolactone diol, and 0.02 parts of stannous octoate were added, followed by the addition of these to the prepolymer. The resulting material was fed into a co-rotating twin-screw extruder for compounding and granulation. The extruder's front section temperature was set to 120°C, the middle section temperature to 140°C, and the rear section temperature to 150°C, yielding polyurethane granules. The granules were dried at 80°C for 2 hours and then injection molded into preforms for foaming. The injection temperature was set to 160°C, and the mold temperature to 30°C. The preform to be foamed was placed in a supercritical fluid impregnation vessel and impregnated with carbon dioxide at 20 MPa and 40°C for 45 minutes, achieving a carbon dioxide impregnation rate of 4% by mass. After impregnation, the preform was transferred to a programmed pressure-controlled foaming device, where nitrogen gas was introduced at 110°C and 4 MPa for assisted foaming for 5 minutes. A two-stage gradient depressurization process was used during foaming, with the average depressurization rate controlled at 0.08 MPa per second. Finally, the resulting foamed material preform was cured at 22°C for 22 hours to obtain a polyurethane foam material sample.

[0047] Example 2 65 parts of polytetrahydrofuran ether diol and 20 parts of hexamethylene diisocyanate were weighed and added to a jacketed mechanically stirred reactor. The mixture was reacted at 75°C for 2.5 hours under nitrogen protection to obtain a prepolymer. The prepolymer was cooled to 50°C, and 7 parts of polycaprolactone diol were added. Separately, 0.5 parts of boron nitride nanosheets were pre-dispersed in the polycaprolactone diol, and 0.08 parts of stannous octoate were added, followed by the addition of these to the prepolymer. The resulting material was fed into a co-rotating twin-screw extruder for compounding and granulation. The extruder's front section temperature was set to 125°C, the middle section temperature to 145°C, and the rear section temperature to 155°C, yielding polyurethane granules. The granules were dried at 90°C for 4 hours and then injection molded into preforms for foaming. The injection temperature was set to 175°C, and the mold temperature to 45°C. The preform to be foamed was placed in a supercritical fluid impregnation vessel and impregnated with carbon dioxide at 21 MPa and 45°C for 60 minutes, achieving a carbon dioxide impregnation rate of 4.5% by mass. After impregnation, the preform was transferred to a programmed pressure-controlled foaming device, where nitrogen gas was introduced at 120°C and 6 MPa for assisted foaming for 10 minutes. A two-stage gradient depressurization process was used during foaming, with the average depressurization rate controlled at 0.10 MPa per second. Finally, the resulting foamed material preform was cured at 24°C for 24 hours to obtain a polyurethane foam material sample.

[0048] Example 3 68 parts of polytetrahydrofuran ether diol and 22 parts of hexamethylene diisocyanate were weighed and added to a jacketed mechanically stirred reactor. The mixture was reacted at 80°C for 3 hours under nitrogen protection to obtain a prepolymer. The prepolymer was cooled to 55°C, and 8 parts of polycaprolactone diol were added. Separately, 0.8 parts of boron nitride nanosheets were pre-dispersed in the polycaprolactone diol, and 0.2 parts of stannous octoate were added, followed by the addition to the prepolymer. The resulting material was fed into a co-rotating twin-screw extruder for compounding and granulation. The extruder's front section temperature was set to 135°C, the middle section temperature to 150°C, and the rear section temperature to 160°C, yielding polyurethane granules. The granules were dried at 100°C for 6 hours and then injection molded into preforms for foaming. The injection temperature was set to 190°C, and the mold temperature to 60°C. The preform to be foamed was placed in a supercritical fluid impregnation vessel and impregnated with carbon dioxide at 22 MPa and 50°C for 90 minutes, achieving a carbon dioxide impregnation rate of 5% by mass. After impregnation, the preform was transferred to a programmed pressure-controlled foaming device, where nitrogen gas was introduced at 130°C and 8 MPa for assisted foaming for 20 minutes. A two-stage gradient depressurization process was used during foaming, with the average depressurization rate controlled at 0.12 MPa per second. Finally, the resulting foamed material preform was cured at 50°C for 9 hours to obtain a polyurethane foam material sample.

[0049] Comparative Example 1 The only difference between Comparative Example 1 and Example 2 is that the prepolymerization reaction and modified compounding granulation described in S1 and S2 were not performed. Instead, Elastollan 1185A10 granules from BASF (China) Co., Ltd. were used directly. These granules were dried at 90 degrees Celsius for 4 hours and then injection molded into preforms to be foamed. All other impregnation conditions, auxiliary foaming conditions, gradient depressurization conditions, and curing conditions were the same as in Example 2.

[0050] Comparative Example 2 The only difference between Comparative Example 2 and Example 2 is that nitrogen gas was not introduced for auxiliary foaming during the foaming stage; instead, only the small embryos impregnated with supercritical carbon dioxide were heated and foamed. The remaining raw material system, granulation conditions, impregnation conditions, gradient depressurization conditions, and curing conditions were the same as in Example 2.

[0051] Comparative Example 3 The only difference between Comparative Example 3 and Example 2 is that, after foaming, gradient depressurization control was not implemented; instead, the pressure was directly depressurized to atmospheric pressure in one go. The remaining raw material system, granulation conditions, impregnation conditions, nitrogen-assisted foaming conditions, and curing conditions were the same as in Example 2.

[0052] Performance tests were conducted on the samples obtained from Examples 1 to 3 and Comparative Examples 1 to 3. The test items and methods are mainly as follows: In the average cell size test, a sample from the middle section was cut along the thickness direction of the base, and the cell size was measured according to GB / T 12811-1991 "Test Method for Average Cell Size of Rigid Foamed Plastics". For the polyurethane foam material obtained in this invention, the sample cross-section can first be subjected to liquid nitrogen embrittlement and gold sputtering treatment, and then the cell morphology image can be obtained using a scanning electron microscope. Three fields of view were selected for each sample, and the cell size was statistically analyzed at the same magnification, and the average value was taken as the average cell size.

[0053] The apparent density test was conducted according to GB / T 6343-2009 "Determination of Apparent Density of Foamed Plastics and Rubber". The length, width, and thickness of the specimens were measured according to GB / T 6342-1996 "Determination of Linear Dimensions of Foamed Plastics and Rubber". Five specimens were tested in each group, and the average value was taken as the apparent density of that group.

[0054] The rebound rate test was conducted according to GB / T 6670-2008 "Determination of Rebound Performance of Flexible Foam Polymer Materials by Falling Ball Method". Five samples were tested for each group, and the average value was taken as the rebound rate of that group.

[0055] Compression set tests were conducted according to GB / T 6669-2008 "Determination of Compression Set of Flexible Foamed Polymer Materials". All samples in each group underwent the same compression, compression time, test temperature, and recovery time. The compression set was calculated after the tests.

[0056] The constant load impact fatigue test was conducted according to GB / T 18941-2003 "Determination of Constant Load Impact Fatigue of Porous Polymer Elastic Materials". After the test, the thickness loss rate of the sample was recorded to evaluate the material's structural retention ability after repeated impact compression. In the aging resistance test, each group of samples was subjected to hot air aging treatment according to GB / T 3512-2014 "Accelerated Aging and Heat Resistance Test of Vulcanized Rubber or Thermoplastic Rubber". The indentation hardness before and after aging was measured according to GB / T 10807-2006 "Determination of Hardness of Flexible Foam Polymer Materials (Indentation Method)" and the change rate of indentation hardness after aging was calculated.

[0057] The test results are as follows:

[0058] The results show that the average cell size of Examples 1 to 3 is significantly smaller than that of the comparative examples, indicating that the prepolymerization reaction, modified compounding and granulation, supercritical carbon dioxide impregnation, nitrogen-assisted foaming, and gradient pressure control of the present invention are beneficial for forming a finer cell structure. Examples 1 to 3 maintain a relatively similar apparent density, indicating a relatively stable degree of foaming. Examples 1 to 3 exhibit higher rebound rates, lower compression set, lower thickness loss after constant load impact fatigue, and lower indentation hardness change rate after aging, indicating that the obtained polyurethane foam materials have good performance in terms of rebound retention, compression recovery, structural retention after repeated compression, and performance retention after thermal aging. Among them, Example 2 has the best overall performance, indicating that the intermediate process conditions are more conducive to balancing cell uniformity and performance stability. Compared with Example 2, the test results of Comparative Example 1 were significantly worse, indicating that the prepolymerization reaction and modified compounding granulation play an important role in subsequent uniform foaming; the test results of Comparative Example 2 show that introducing nitrogen gas to assist foaming after supercritical carbon dioxide impregnation is beneficial to improving cell uniformity and enhancing rebound-related properties; the test results of Comparative Example 3 show that gradient pressure relief control can limit cell coarsening and reduce bubble formation and collapse, thereby helping to improve the performance stability of polyurethane foam materials.

[0059] 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 a polyurethane foam material, characterized in that, Includes the following steps: S1: Polytetrahydrofuran ether diol and hexamethylene diisocyanate are reacted at 70-80 degrees Celsius for 2-3 hours to obtain a prepolymer; S2: After cooling the prepolymer to 45-55 degrees Celsius, polycaprolactone diol and catalyst are added for compounding, extrusion and granulation to obtain polyurethane granules. S3: The polyurethane granules are dried at 80-100 degrees Celsius for 2-6 hours and then injection molded into small preforms to be foamed; S4: The embryo to be foamed is impregnated with supercritical carbon dioxide for 0.5-2 hours at 18-25 MPa and 35-55 degrees Celsius, so that the carbon dioxide impregnation rate is 3-6% by mass. S5: The small blank impregnated with supercritical carbon dioxide is transferred into a foaming device, and nitrogen gas is introduced for assisted foaming at 100-130 degrees Celsius and 3-8 MPa. Gradient pressure relief control is implemented at a rate of 0.05-0.2 MPa per second during the foaming process to obtain a foamed material blank. S6: The foamed material preform is cured at 20-30 degrees Celsius for 18-30 hours, or at 45-55 degrees Celsius for 6-10 hours, to obtain polyurethane foamed material.

2. The method for preparing polyurethane foam material according to claim 1, characterized in that, In the raw materials used to obtain the polyurethane granules, by weight, there are 62-68 parts of polytetrahydrofuran ether diol, 18-22 parts of hexamethylene diisocyanate, and 5-8 parts of polycaprolactone diol.

3. The method for preparing polyurethane foam material according to claim 1, characterized in that, The catalyst mentioned in S2 is stannous octoate, and the amount of stannous octoate added is 0.02-0.2 parts by mass.

4. The method for preparing polyurethane foam material according to claim 1, characterized in that, S2 also includes a cell-regulating agent, which is boron nitride nanosheets, and the amount of boron nitride nanosheets added is 0.2-0.8 parts by mass.

5. The method for preparing polyurethane foam material according to claim 1, characterized in that, The preform to be foamed in S3 is formed by injection molding, with an injection temperature of 160-190 degrees Celsius and a mold temperature of 30-60 degrees Celsius.

6. The method for preparing polyurethane foam material according to claim 1, characterized in that, The supercritical carbon dioxide impregnation described in S4 is carried out at a pressure of 20-22 MPa, a temperature of 40-50 degrees Celsius, an impregnation time of 45-90 minutes, and a carbon dioxide impregnation rate of 4-5% by mass.

7. The method for preparing polyurethane foam material according to claim 1, characterized in that, The nitrogen-assisted foaming process described in S5 has a foaming temperature of 110-130 degrees Celsius, a nitrogen pressure of 4-8 MPa, and a foaming time of 5-20 minutes.

8. The method for preparing polyurethane foam material according to claim 1, characterized in that, The pressure relief rate of the gradient pressure relief control described in S5 is 0.08-0.12 MPa per second.

9. The method for preparing polyurethane foam material according to claim 1, characterized in that, The foamed material preform described in S6 is cured at 20-25 degrees Celsius for 22-26 hours.

10. The method for preparing polyurethane foam material according to claim 1, characterized in that, The foamed material preform described in S6 is cured at 45-50 degrees Celsius for 7-9 hours.

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