TPU supercritical foaming material and method for fabricating midsoles in footwear

Abstract

The present invention relates to the technical field of foaming materials. Disclosed are a TPU supercritical foaming material and method for fabricating midsoles in footwear. According to the key points of the technical solution, the foaming material comprises the following raw materials in parts by weight: (1) 40-80 parts of polyol; (2) 20-50 parts of isocyanate; (3) 4-12 parts of a chain extender; (4) 0.1-30 parts of a modifier; and (5) a supercritical gas. The method for fabricating footwear midsoles from said raw materials comprises the following steps: S1, mixing, in the described parts by weight, the polyol, the isocyanate, and the chain extender, and compounding the resulting mixture in a twin-screw extruder to obtain a polymer melt; S2, preparing a supercritical gas by means of a gas inlet system, and then injecting the supercritical gas and the polymer melt into a single-screw extruder to prepare a molten mixture containing the supercritical gas; and S3, injecting the molten mixture into a mold, leaving it to stand, and then opening the mold to obtain the product.

Description

A TPU supercritical foaming material and method for preparing shoe midsoles Technical Field

[0001] The present invention relates to the technical field of foaming materials, and more particularly to a TPU supercritical foaming material and a method for preparing a shoe midsole. Background Art

[0002] Polyurethane foam materials are generally classified as thermosetting polyurethane foam and thermoplastic polyurethane foam based on the characteristics of the raw materials and the molecular structure of the end product. Traditional thermosetting polyurethane foaming typically involves the chemical reaction of polyols and isocyanates, while introducing water as a blowing agent to produce the foam. However, these materials and methods often result in open-cell foam structures, such as sponges. Due to their relatively poor physical and mechanical properties, thermosetting polyurethane materials have not been widely used in high-performance sports shoe midsoles.

[0003] Thermoplastic polyurethane elastomer (TPU) is currently used in traditional sports shoe midsoles due to its good processability. An existing invention patent application, publication number CN105884998A, discloses a method for preparing a foamed thermoplastic polyurethane elastomer material. This technical route requires two extrusions. The product produced by this method has a specific gravity above 0.165g / cc and a morphology of foamed beads, which are then molded to form the midsole. An existing invention patent application, publication number CN102276785A, discloses a low-density foamed thermoplastic polyurethane elastomer and its preparation process. The sample is prepared using polyols, chain extenders, isocyanates, and azo foaming agents. The sample density prepared by this method is above 0.26g / cc, and a chemical foaming agent is used for the foaming process.

[0004] Existing supercritical foaming midsole solutions primarily fall into four categories: 1. First, TPU beads are prepared, then compression molded; 2. The raw materials are blended and injection molded / cast to produce a small embryo, which is then foamed in a supercritical autoclave; 3. After extruding a sheet, supercritical foaming is performed, and the sheet is cut into the desired shape to produce the midsole; 4. Supercritical fluid is added during the injection molding process, and the mold is opened to produce the product. For these solutions, when TPU pellets are used for injection molding and foaming, the foamed product, affected by oriented internal stress, can exhibit problems such as inconsistent horizontal and vertical magnification, warping, and low yield. Alternatively, cutting techniques can result in significant waste and low efficiency. Technical issues

[0005] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a TPU supercritical foaming material and method for preparing shoe midsoles, which does not go through the step of preparing TPU particles, avoids the adverse effects of the internal stress of TPU particles, and does not cause waste of raw materials caused by cutting. Technical Solutions

[0006] To achieve the above object, the present invention provides the following technical solution: a TPU supercritical foaming material for preparing a shoe midsole, the material comprising the following raw materials in parts by weight:

[0007] (1) 40-80 parts of polyol;

[0008] (2) 20-50 parts of isocyanate;

[0009] (3) 4-12 parts of chain extender;

[0010] (4) 0.1-30 parts of modifier;

[0011] (5) Supercritical gas.

[0012] Furthermore, the polyol is a bifunctional polyol, including but not limited to a mixture of one or more of polyester polyol, polyether polyol, polycaprolactone, polycarbonate, and bio-based diol.

[0013] Furthermore, the isocyanate is a difunctional isocyanate, including but not limited to aromatic, aliphatic or alicyclic isocyanates.

[0014] Furthermore, the isocyanate is a difunctional isocyanate, including but not limited to a mixture of one or more of diphenylmethane diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, xylylene diisocyanate, tetramethyl-m-xylylene diisocyanate, lysine diisocyanate, p-phenylene diisocyanate, and cyclohexane-1,4-diisocyanate.

[0015] Furthermore, the molar ratio of isocyanate functional group (-NCO) to alcohol hydroxyl functional group (-OH) is (0.7-1.5):1.

[0016] Furthermore, the chain extender is a difunctional alcohol, including but not limited to a mixture of one or more of ethylene glycol, 1,2-propylene glycol, 1,4-butanediol, and 1,6-hexanediol.

[0017] Furthermore, the modifier includes but is not limited to one or more of a masterbatch, a coupling agent, a toughening agent, and a filler.

[0018] Furthermore, the supercritical gas is a mixture of two or more of carbon dioxide, nitrogen, air, and water vapor.

[0019] Furthermore, a method for preparing a TPU supercritical foam material using the above raw materials comprises the following steps:

[0020] S1, mixing the above-mentioned polyol, isocyanate, and chain extender in parts by weight and then putting them into a twin-screw extruder for blending to obtain a polymer melt;

[0021] S2. preparing a supercritical gas through an air intake system, and injecting the supercritical gas and a polymer melt into a single-screw extruder to prepare a molten mixture containing the supercritical gas;

[0022] S3. Inject the molten mixture into a mold, let it stand, and open the mold to obtain the product.

[0023] Furthermore, a modifier may be added in step S1, and the modifier includes but is not limited to one or more of a masterbatch, a coupling agent, a toughening agent, and a filler.

[0024] Furthermore, the mold working parameters in step S3 are a pressure of 5-20 MPa, a temperature of 30-150° C., and a standing temperature of 30 seconds to 2 hours. Beneficial effects

[0025] Since the method of the present invention does not include the preparation of TPU particles, the components and raw materials of the present invention are blended and reacted in a twin-screw extruder, and a polymer with a predetermined hardness and resilience is obtained by bulk polymerization, thereby avoiding the internal stress of TPU particles. The present invention uses two or more supercritical mixed gases to avoid the defects of a single supercritical fluid having limited solubility and diffusivity in the polymer. The use of a supercritical mixed gas can increase the solubility of the supercritical fluid in the polymer, and the foaming ratio of the prepared product is increased, and the specific gravity is lower. The shoe midsole made of the TPU supercritical foam material prepared by the present invention has a specific gravity of 0.06-0.2 g / cc and a hardness range of 10-50 Asker C. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG1 is a graph showing the relationship between torque and time when TPU of various embodiments of the present invention and comparative examples are kneaded in a Haake rheometer. In the graph, the blue curve is Example 1; the green curve is Example 2; and the red curve is Example 3.

[0027] FIG2 is an SEM image of the foamed midsole of each embodiment and comparative example of the present invention, in which a: embodiment 1; b: embodiment 2; c: embodiment 3; d: comparative example 1; and e: comparative example 2. Modes for Carrying Out the Invention

[0028] The present invention is further described in detail below with reference to the examples.

[0029] Example 1

[0030] 60 parts of polytetramethylene ether glycol, 6 parts of 1,4-butanediol, and 32 parts of MDI-100, with a hard segment content of 38% and an NCO coefficient of 1.01, are mixed and put into a twin-screw extruder, and blended at 180°C to obtain a polymer melt; the air intake system is set to a CO2:N2 ratio of 10:90, and supercritical gas and polymer melt are injected into a single-screw extruder to prepare a molten mixture containing supercritical gas; the mixture is then injected into a mold at 40°C, maintained at pressure for 30 minutes, and then the mold is opened and the pressure is released to obtain a midsole product.

[0031] Example 2

[0032] 60 parts of polytetramethylene ether glycol, 8 parts of 1,4-butanediol, and 38 parts of MDI-100, with a hard segment content of 43% and an NCO coefficient of 1.01, are mixed and put into a twin-screw extruder, and blended at 190°C to obtain a polymer melt. The air intake system is set to a CO2:N2 ratio of 10:90, and supercritical gas and polymer melt are injected into a single-screw extruder to prepare a molten mixture containing supercritical gas. The mixture is then injected into a mold at 40°C, maintained at pressure for 25 minutes, and then the mold is opened and the pressure is released to obtain a midsole product.

[0033] Example 3

[0034] 60 parts of polytetramethylene ether glycol, 10 parts of 1,4-butanediol, and 44 parts of MDI-100, with a hard segment content of 47% and an NCO coefficient of 1.01, are mixed and put into a twin-screw extruder, and blended at 200°C to obtain a polymer melt; the air intake system is set to a CO2:N2 ratio of 10:90, and supercritical gas and polymer melt are injected into a single-screw extruder to prepare a molten mixture containing supercritical gas; the mixture is then injected into a mold at 40°C, maintained at pressure for 20 minutes, and then the mold is opened and the pressure is released to obtain a midsole product.

[0035] Comparative Example 1: A TPU two-step autoclave foaming midsole was prepared according to the invention patent application publication number CN 108081652 A.

[0036] Comparative Example 2: A midsole was obtained by cutting, polishing, and secondary molding the TPU sheet foam material according to the invention patent application publication number CN110524781A.

[0037] Performance testing

[0038] The hardness, rebound and specific gravity of the embodiments of the present invention and the comparative examples were tested using the following test methods:

[0039] Hardness: tested according to ASTM D2240;

[0040] Rebound: Tested according to DIN 53512;

[0041] Specific gravity: tested according to ISO 845-2006;

[0042] The test results are shown in Table 1.

[0043] Table 1 Performance test results of products of various embodiments and comparative examples Name Hardness (Asker C) Rebound (%) Specific gravity (g / cc) Example 1 29740.08 Example 2 36750.10 Example 3 45700.12 Comparative example 1 35700.13 Comparative example 2 40700.15

[0044] As shown in Table 1, the shoe midsoles prepared in the examples of the present invention have adjustable hardness ranging from 20 to 50 Asker C, a low specific gravity not exceeding 0.12 g / cc, and good resilience. In contrast, the shoe midsoles of Comparative Examples 1 and 2 have high specific gravity and poor resilience.

[0045] The intermediate polymer melts of various embodiments of the present invention were tested using a Haake rheometer, with the temperatures of Zones 1, 2, and 3 set at 120°C, 120°C, and 120°C, respectively, and the rotation speed set at 60 rpm. The results are shown in Figure 1. The different torque peaks in Figure 1 represent the different hardnesses of the materials, and the different times at which the torque decreases represent the melting rates, which in turn reflect the different melting points. Figure 1 shows that the intermediate products obtained from Examples 1-3, with increasing amounts of the MDI hard segment component added to the raw materials, exhibit increasing hardness. This increase in hard segment content requires more energy to melt the material, leading to a corresponding increase in the melting point. Adjusting the MDI hard segment content in the raw materials allows for flexible adjustment of product hardness and expands the product's applicability. The sample from Example 1 exhibits lower torque and the lowest hardness initially. While the samples from Examples 2 and 3 exhibit similar initial torques, Example 2 exhibits an earlier inflection point and a lower melting temperature than Example 3. This indicates that the hardness and melting point of the samples from Examples 1, 2, and 3 increase sequentially, consistent with the expectation that hardness can be adjusted by adjusting the MDI hard segment content in the raw materials.

[0046] As can be seen from Figure 2, the TPU supercritical foaming material of the present invention is used to prepare the shoe midsole, and the foam structure is uniform and dense, which is significantly better than the TPU midsole prepared by the two-step method in Comparative Example 1, and comparable to the plate foaming and cutting midsole in Comparative Example 2; although the plate foaming and cutting midsole can reduce the problem of oriented internal stress, the production steps require first making the foamed plate, and then cutting the plate into the required sole shape, and secondary molding and shaping. The plate cutting greatly increases the production cost, wastes the cutting scraps, and has high cutting machinery costs; the shoe midsole of the present invention has similar performance to the product of Comparative Document 2, avoids the waste of material caused by cutting, does not require cutting equipment, and greatly reduces the production cost.

[0047] The steps of the present invention do not include preparing TPU particles. The components and raw materials of the present invention are blended and reacted in a twin-screw extruder, and a polymer with predetermined hardness and resilience is obtained by bulk polymerization, thereby avoiding the internal stress of the TPU particles, thereby reducing the deformation of the prepared shoe midsole and improving the yield rate of the product. The present invention adopts two or more supercritical mixed gases to avoid the defects of limited solubility and diffusivity of a single supercritical fluid in the polymer. The use of the supercritical mixed gas can increase the solubility of the supercritical fluid in the polymer, thereby increasing the foaming ratio of the prepared product and lowering the specific gravity. The shoe midsole made of the TPU supercritical foaming material prepared by the present invention has a specific gravity of 0.06-0.2, a hardness range of 10-50, good resilience, excellent performance, and does not require conventional cutting technology, thereby reducing waste and saving costs.

[0048] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. After reading this specification, those skilled in the art may make non-creative modifications to this embodiment as needed. However, as long as such modifications are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A TPU supercritical foaming material for preparing a shoe midsole, characterized in that, The material contains the following raw materials in parts by weight: (1) 40 - 80 parts of polyol; (2) 20 - 50 parts of isocyanate; (3) 4 - 12 parts of chain extender; (4) 0.1 - 30 parts of modifier; (5) Supercritical gas.

2. The TPU supercritical foaming material for preparing a shoe midsole according to claim 1, wherein, The polyol is a bifunctional polyol, including but not limited to one or a mixture of more than one of polyester polyol, polyether polyol, polycaprolactone, polycarbonate, and bio - based diol.

3. The TPU supercritical foaming material for preparing a shoe midsole according to claim 2, wherein The isocyanate is a bifunctional isocyanate, including but not limited to aromatic, aliphatic, or cycloaliphatic isocyanates.

4. The TPU supercritical foaming material for preparing a shoe midsole according to claim 1, wherein The isocyanate is a bifunctional isocyanate, including but not limited to one or a mixture of more than one of diphenylmethane diisocyanate, toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane - 4,4'-diisocyanate, xylylene diisocyanate, tetramethylxylylene diisocyanate, lysine diisocyanate, p - phenylene diisocyanate, cyclohexane - 1,4 - diisocyanate.

5. The TPU supercritical foaming material for preparing a shoe midsole according to claim 3, wherein The molar ratio of the isocyanate functional group - NCO to the alcohol hydroxyl functional group - OH is (0.7 - 1.5):

1.

6. The TPU supercritical foaming material for preparing a shoe midsole according to claim 1, characterized in that, The chain extender is a bifunctional alcohol, including but not limited to one or a mixture of more than one of ethylene glycol, 1,2 - propanediol, 1,4 - butanediol, 1,6 - hexanediol.

7. The TPU supercritical foaming material for preparing a shoe midsole according to claim 1, wherein, The modifier includes but not limited to one or a mixture of more than one of masterbatch, coupling agent, toughening agent, filler.

8. The TPU supercritical foaming material for preparing a shoe midsole according to claim 1, characterized in that, The supercritical gas is a mixed gas of two or more of carbon dioxide, nitrogen, air, and water vapor.

9. The preparation method of the TPU supercritical foaming material according to any one of claims 1-8, characterized in that, It includes the following steps: S1. Mix the polyol, isocyanate, and chain extender according to the parts by weight and put them into a twin - screw extruder for blending to obtain a polymer melt; S2. Prepare supercritical gas through an air intake system, and inject the supercritical gas and the polymer melt into a single - screw extruder to prepare a molten mixture containing supercritical gas; S3. Inject the molten mixture into a mold, let it stand, and open the mold to obtain the product.

10. The preparation method according to claim 9, characterized in that, The working parameters of the mold in step S3 are a pressure of 5 - 20 MPa and standing for 30 s - 2 h at 30 - 150 °C.

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