Method for identifying supercritical physical foaming shoe material

Abstract

The invention relates to the technical field of shoe material detection and identification, in particular to an identification method of a supercritical physical foaming shoe material. According to the identification method of the supercritical physical foaming shoe material, a full-process identification system of foaming performance verification, fine judgment of microtopography and confirmation of chemical components is constructed, the core function difference of the material is quantitatively evaluated through the foaming performance, and the essential characteristics of a foam structure are revealed by virtue of observation of the microtopography; finally, a foaming process source is locked through chemical composition analysis, a complete identification method from performance evaluation to a micromechanism and then to component tracing is formed, the limitation of a single detection means in the prior art is solved, collaborative identification of the supercritical physical foaming shoe material is achieved, operation is convenient and fast, and the accuracy rate is high.

Description

Technical Field

[0001] This invention relates to the field of footwear material testing and identification technology, specifically to a method for identifying supercritical physical foamed footwear materials. Background Technology

[0002] The midsole is the component between the outsole and insole of footwear products, and it has a significant impact on athletic performance, therefore it needs to have good cushioning, rebound, and stability. Common midsole materials include polyurethane (PU), polyvinyl chloride (PVC), thermoplastic elastomers, and ethylene-vinyl acetate copolymer (EVA), which are generally achieved through foaming technology to obtain excellent shock absorption and comfort.

[0003] Chemically foamed shoe materials are made by foaming organic foaming agents (mainly azodicarbonamide (AC) and N,N-dinitrospentamethylenetetramine (H)) through thermal decomposition, resulting in materials with large pore size, low pore density, poor morphology, and uneven pore distribution, which seriously affects the wearing experience. Furthermore, due to the potential hazards of foaming agents and their byproducts, chemically foamed shoe materials pose certain risks to human health and environmental protection during use.

[0004] In recent years, supercritical physical foaming materials, prepared using supercritical fluids as foaming agents, have been widely used in the field of running shoe materials. Supercritical physical foaming materials are produced by mixing a supercritical gas (such as carbon dioxide CO2 or nitrogen N2) with a matrix to form a homogeneous system. The gas is then rapidly released under pressure to achieve a supersaturated state, forming bubble nuclei. Because supercritical fluids lack a gas / liquid interface and can lower the glass transition temperature of polymers, the supercritical swelling foaming method offers unparalleled advantages over traditional foaming methods in preparing microporous materials. The excellent diffusion properties of supercritical fluids allow for uniform dispersion within the polymer matrix, resulting in uniformly distributed, small-sized pore structures with superior lightweight, resilience, shock absorption, compression set resistance, and low-temperature performance.

[0005] Compared to chemically foamed shoe materials, supercritical physical foaming materials have a finer cell structure and superior stability, while also meeting food safety standards, making them a green and pollution-free environmentally friendly material. However, supercritical physical foamed shoe soles and chemically foamed shoe soles have a similar overall appearance, making them difficult for ordinary consumers to distinguish. Currently, there is no effective testing technology to differentiate between the two, leading to the phenomenon of chemically foamed shoe materials being passed off as supercritical physical foamed shoe materials in the market. This seriously disrupts the shoe material market order and infringes on consumer rights.

[0006] Accurately and efficiently identifying supercritical physical foaming shoe materials, ensuring material quality, and preventing fraudulent practices such as false labeling and substandard products have become urgent technical problems to be solved in the field of shoe material testing. Existing single-method testing (such as infrared spectroscopy or density testing) is easily affected by the complexity of materials, making accurate identification impossible. Furthermore, there is currently no publicly available scientific identification method. Therefore, developing a multi-technology collaborative identification method is of great significance. Summary of the Invention

[0007] The purpose of this invention is to provide a method for identifying supercritical physical foamed shoe materials. This method identifies the materials based on their foaming performance, microstructure, and chemical composition, overcoming the limitations of single detection methods in existing technologies. It achieves synergistic identification of supercritical physical foamed shoe materials, is convenient to operate, and has a high accuracy rate.

[0008] The method for identifying supercritical physical foamed shoe materials described in this invention involves identifying the foamed shoe material sample to be tested according to the following steps: (1) Identification of foaming performance: The specific surface area and ball rebound rate of the foamed shoe material sample to be tested were measured; When the specific surface area of ​​the foamed shoe material sample to be tested is ≤1m² 2 If the ball rebound rate is less than 50%, it is identified as a non-supercritical physical foaming shoe material. When the specific surface area of ​​the foamed shoe material sample to be tested is ≥5m² 2 When the ball rebound rate is ≥50% and the ball weight is / g, it is identified as meeting the characteristics of supercritical physical foaming shoe material and can directly proceed to the chemical composition identification step. When the specific surface area of ​​the foamed shoe material sample to be tested is greater than 1m² 2 / g, less than 5m 2 When the ball rebound rate is ≥50% and the ball weight is / g, proceed to the microstructure identification step; (2) Microscopic morphology identification: The cell density and pore size distribution of the cross section of the foamed shoe material sample to be tested were measured; When the cell density of the cross-section of the foamed shoe material sample to be tested is ≤10 7 pcs / cm 3 If the pore size distribution is bimodal or multimodal, it is identified as a non-supercritical physical foaming shoe material. When the cell density of the cross-section of the foamed shoe material sample to be tested is >10 7 pcs / cm 3 When the pore size distribution exhibits a unimodal normal distribution, it is identified as conforming to the characteristics of supercritical physical foaming shoe materials, and proceeds to the chemical composition identification step. (3) Chemical composition identification: Test the formamide content of the foamed shoe material sample to be tested; If formamide is not detected in the foamed shoe material sample to be tested, it is identified as a supercritical physical foamed shoe material; otherwise, it is a non-supercritical physical foamed shoe material.

[0009] In step (1), the specific surface area of ​​the foamed shoe material sample to be tested is measured using a BET specific surface area meter.

[0010] Preferably, the BET surface area analyzer has a measurement range of 0.01-10. 4 m 2 / g.

[0011] Specifically, the specific surface area test method is as follows: the foamed shoe material sample to be tested is crushed to a particle size of <1mm, vacuum dried at 60℃ for 12h, 0.5-1.0g of dried sample is weighed and loaded into a sample tube, and a nitrogen adsorption-desorption experiment is carried out at a liquid nitrogen temperature of -196℃. Data from at least 20 adsorption points are collected, and the specific surface area of ​​the foamed shoe material sample to be tested is calculated using the BET equation.

[0012] In step (1), the ball rebound rate of the foamed shoe material sample to be tested is tested according to ISO 8307 standard.

[0013] Specifically, the ball rebound rate test method is as follows: A steel ball with a diameter of 16±0.5mm is dropped freely from a height of 500mm to impact the surface of the foamed shoe material sample to be tested, and the ball rebound rate is calculated based on the rebound height.

[0014] Supercritical physical foaming of shoe materials utilizes a supercritical gas (such as carbon dioxide CO2 or nitrogen N2) mixed with a matrix to form a homogeneous system. By rapidly releasing pressure, the gas is brought to a supersaturated state, forming bubble nuclei. The excellent diffusivity of supercritical fluids allows for uniform dispersion within the polymer matrix, resulting in a uniformly distributed, small-sized cell structure. Therefore, supercritical physical foaming shoe materials exhibit uniform color, no visible bubble agglomeration or color layering, and regularly spherical, uniformly distributed cells with a large specific surface area and high resilience. In contrast, chemically foamed shoe materials are made by foaming with gases generated from the thermal decomposition of organic foaming agents. Uneven heating during the foaming process can lead to mottled coloring, such as localized yellowing or whitening. Significant differences in bubble size are common, often accompanied by the collapse of large cells or the aggregation of small cells. They also have a smaller specific surface area and lower resilience.

[0015] Therefore, by identifying foaming performance, this invention can exclude samples that clearly do not conform to the characteristics of supercritical physical foamed shoe materials, thereby reducing identification costs and improving identification efficiency. For samples that fully conform to the characteristics of supercritical physical foamed shoe materials, chemical composition identification is still required to avoid the risk of misjudgment. For samples with foaming performance between supercritical and non-supercritical physical foamed shoe materials, due to the current improvement in counterfeit technology, they may be chemically foamed products, chemically and supercritically physically foamed products, or supercritical physical foamed shoe materials of average quality. For such samples, this invention further identifies them through microscopic morphology.

[0016] In step (2), after the foamed shoe material sample to be tested is quenched with liquid nitrogen, the fracture surface is first treated with gold sputtering using an ion sputtering instrument, and then the cell density and pore size distribution of the cross section of the foamed shoe material sample to be tested are observed by scanning electron microscopy.

[0017] Preferably, the resolution of the scanning electron microscope is ≤5nm.

[0018] Preferably, when the fracture surface is treated with gold sputtering using an ion sputtering instrument, the coating thickness is 5-20 nm.

[0019] Specifically, the microstructure testing method is as follows: The foamed shoe material sample to be tested is immersed in liquid nitrogen for 5-10 minutes, quenched, and the fresh cross-section is exposed. The fracture surface is then treated with gold sputtering using an ion sputtering instrument. The cross-section of the foamed shoe material sample is then observed using a scanning electron microscope. Five fields of view are randomly selected as counting areas. The cell density is calculated based on the counting area and the number of cells in that area. Cell density (cells / cm³) is... 2 = Number of bubbles (cells) / Counting area (cm²) 2 Simultaneously, the diameter of more than 100 bubbles in each field of view is measured, and a histogram of pore size distribution is plotted to identify whether it is a unimodal distribution.

[0020] This invention uses microscopic morphology testing to further identify samples with foaming performance within the critical range. Because supercritical physical foaming materials have a uniformly distributed and tiny cell structure, they generally exhibit a unimodal distribution and high cell density. Based on this microscopic morphology characteristic, non-supercritical physical foaming samples can be excluded, reducing the cost and time of chemical composition testing. To prevent some shoe materials from mimicking supercritical physical foaming materials through process optimization or additives, thus creating a deceptive microscopic morphology, chemical composition testing is still necessary to ensure the accuracy of the identification results. During microscopic morphology identification, observing the consistency of cell morphology can also aid in identifying whether a material is supercritical physical foaming. Supercritical physical foaming materials have regularly round cells, while chemically foamed materials often exhibit irregular polygonal or collapsed cells.

[0021] In step (3), the formamide content of the foamed shoe material sample to be tested was determined by gas chromatography-mass spectrometry (GC-MS).

[0022] The gas chromatography-mass spectrometry (GC-MS) test conditions are as follows: Chromatographic column: VF-WAXms column, specifications 30m × 0.25mm × 0.25μm; Temperature program: Initial temperature 50℃, hold time 2 min, increase to 200℃ at a rate of 30℃ / min, hold for 5 min; Carrier gas: Helium, purity ≥99.999% Flow rate: 1 mL / min; Inlet temperature: 250℃; Injection method: splitless injection; Injection volume: 1.0 μL; Interface temperature: 280℃; Ion source: EI; Measurement mode: Ion scan (SIM); Qualitative ions (m / z): 45, 44, 29 (abundance ratio: 100:28:25); Quantitative ion count (m / z): 45; Solvent delay time: 7 min.

[0023] Specifically, the formamide content test method is as follows: After removing the dense layer on the surface of the foamed shoe material sample to be tested, take the internal foam material and cut it into small pieces with a side length not exceeding 3mm. Weigh a total of 0.5g of sample and put it into a reaction bottle. Add 10ml of methanol using a pipette and seal the bottle. Shake and extract in a constant temperature water bath shaker for 90min. Cool to room temperature and filter with an organic filter membrane. Perform qualitative and quantitative detection on the filtrate using gas chromatography-mass spectrometry. The presence of formamide in the sample is determined by comparing the relative abundance of the corresponding qualitative ions in the chromatogram of the gas chromatography-mass spectrometry test with the chromatogram of a formamide standard solution with a similar concentration.

[0024] This invention, through component analysis of supercritical physical foaming shoe materials and chemical foaming shoe materials, found that chemical foaming shoe materials leave organic residues, while supercritical foaming shoe materials, being physically foamed and without any added chemical additives, leave no organic residues. Chemical foaming shoe materials generally use azodicarbonamide (AC) or N,N-dinitrospentamethylenetetramine (H) foaming agents. Analysis of these two chemical foaming agents revealed that their thermal decomposition produces some gaseous and solid residues. However, the gaseous residues are difficult to analyze and identify, and some gases are similar to those in supercritical foaming. Therefore, analyzing the solid residues is an effective way to identify chemical foaming shoe materials. Further analysis of the molecular structure of AC and H foaming agents and the solid residues after thermal decomposition revealed that both contain formamide residues. Therefore, formamide testing can be used to distinguish between supercritical physical foaming shoe materials and chemical foaming shoe materials.

[0025] Compared with the prior art, the present invention has the following beneficial effects: This invention achieves multi-dimensional and precise identification of supercritical physical foamed shoe materials by constructing a complete identification system encompassing "foaming performance verification—microscopic morphology precise judgment—chemical composition confirmation." This method quantifies the differences in core materials' functions through foaming performance evaluation, reveals the essential characteristics of cell structure through microscopic morphology observation, and finally pinpoints the origin of the foaming process through chemical composition analysis. This forms a complete identification method from performance evaluation to microscopic mechanism and component traceability. This layered and progressive technical architecture not only improves detection efficiency but also eliminates the risk of misjudgment from a single dimension through multi-technology collaboration, solving the industry problems of strong appearance imitation and one-sided detection indicators in existing technologies. Simultaneously, the system integrates standardized processes, significantly enhancing the reliability and traceability of identification results. This provides core technical support for regulating the shoe material market and promoting the industrialization of green foaming technology, and has significant strategic importance for improving the quality control level of the shoe manufacturing industry and promoting industrial upgrading. Attached Figure Description

[0026] Figure 1 This is a scanning electron microscope image of sample C in Example 2 of the present invention; Figure 2 This is a histogram of the pore size distribution of sample C in Example 2 of the present invention; Figure 3 This is a GC-MS test image of sample A in Example 3 of the present invention; Figure 4 This is a GC-MS test image of sample G in Example 3 of the present invention. Detailed Implementation

[0027] The present invention will now be described in detail through specific embodiments, but the present invention is not limited to these embodiments.

[0028] The embodiment uses eight commercially available foamed shoe materials as test materials and identifies them according to the identification method of the present invention. The eight foamed shoe materials are as follows: Sample A: Supercritical foamed TPU (thermoplastic polyurethane elastomer) shoe midsole; Sample B: Supercritical foamed PEBA (nylon elastomer) shoe midsole; Sample C: Supercritical foamed TPEE (thermoplastic polyester elastomer) shoe midsole; Sample D: Supercritical foamed EVA (ethylene vinyl acetate copolymer) shoe midsole; Sample E: Chemically foamed (AC foaming agent) EVA shoe midsole; Sample F: Chemically foamed (H foaming agent) EVA shoe midsole; Sample G: Hybrid foamed TPU shoe midsole; Sample H: Chemically foamed TPEE shoe midsole.

[0029] Example 1 The foaming performance of the above 8 foamed shoe material samples was identified using the following method: The specific surface area of ​​the foamed shoe material samples was tested using a BET surface area analyzer, the measurement range of which was 0.01-10. 4 m 2 / g, crush the foamed shoe material sample to be tested to a particle size <1mm, vacuum dry at 60℃ for 12h, weigh 0.5-1.0g of the dried sample and put it into a sample tube, and carry out a nitrogen adsorption-desorption experiment at a liquid nitrogen temperature of -196℃, collect data from at least 20 adsorption points, and calculate the specific surface area of ​​the foamed shoe material sample to be tested using the BET equation. To test the ball rebound rate of the foamed shoe material sample according to ISO 8307 standard, a steel ball with a diameter of 16±0.5mm was dropped freely from a height of 500mm to impact the surface of the foamed shoe material sample, and the ball rebound rate was calculated based on the rebound height.

[0030] The identification criteria are: When the specific surface area of ​​the foamed shoe material sample to be tested is ≤1m² 2 If the ball rebound rate is less than 50%, it is identified as a non-supercritical physical foaming shoe material. When the specific surface area of ​​the foamed shoe material sample to be tested is ≥5m² 2 When the ball rebound rate is ≥50% and the ball weight is / g, it is identified as meeting the characteristics of supercritical physical foaming shoe material and can directly proceed to the chemical composition identification step. When the specific surface area of ​​the foamed shoe material sample to be tested is greater than 1m² 2 / g, less than 5m 2 When the ball rebound rate is ≥50% and the ball weight is / g, then proceed to the microscopic morphology identification step.

[0031] The test results and identification results are shown in Table 1.

[0032] Table 1. Foaming performance test results and identification results Example 2 The microstructure of samples C, G, and H was identified using the following method: The foamed shoe material sample to be tested was immersed in liquid nitrogen for 5-10 minutes, then quenched to expose a fresh cross-section. The fracture surface was then sputtered with gold to a thickness of 5-20 nm. The cross-section of the sample was then observed using a scanning electron microscope (SEM) with a resolution ≤5 nm. Five fields of view were randomly selected as counting areas. The cell density (cells / cm³) was calculated based on the area of ​​each field of view and the number of cells within that area. 2 = Number of bubbles (cells) / Counting area (cm²) 2Simultaneously, the diameter of more than 100 bubbles in each field of view is measured, and a histogram of pore size distribution is plotted to identify whether it is a unimodal distribution.

[0033] The identification criteria are: When the cell density of the cross-section of the foamed shoe material sample to be tested is ≤10 7 pcs / cm 3 If the pore size distribution is bimodal or multimodal, it is identified as a non-supercritical physical foaming shoe material. When the cell density of the cross-section of the foamed shoe material sample to be tested is >10 7 pcs / cm 3 Furthermore, if the pore size distribution exhibits a unimodal normal distribution, it is identified as conforming to the characteristics of supercritical physical foaming shoe materials, and proceeds to the chemical composition identification step.

[0034] Test results and identification results are as follows Figure 1-2 As shown in Table 2.

[0035] Table 2. Microscopic morphology test results and identification results The scanning electron microscope image of sample C (supercritical foamed TPEE) is shown below. Figure 1 As shown, from Figure 1 It can be seen that the pores are regularly round and uniformly distributed; the pore size distribution histogram of sample C (supercritical TPEE) is as follows. Figure 2 As shown, from Figure 2 It can be seen that its aperture distribution exhibits a single-peak distribution.

[0036] Example 3 The chemical composition of samples A, B, C, D, and G was identified using the following method: After removing the dense layer from the surface of the foamed shoe material sample, the internal foamed material was cut into small pieces with a side length not exceeding 3 mm. A total of 0.5 g of sample was weighed and placed into a reaction flask. 10 ml of methanol was added using a pipette, and the flask was sealed. The sample was extracted by shaking in a constant temperature water bath shaker for 90 min. After cooling to room temperature, the sample was filtered using an organic filter membrane. The filtrate was then subjected to qualitative and quantitative detection using GC-MS. The presence of formamide in the sample was determined by comparing the relative abundance of the corresponding qualitative ions in the GC-MS spectrum with those in the formamide standard solution spectrum. The formamide content could be calculated by combining the formamide standard curve.

[0037] The gas chromatography-mass spectrometry (GC-MS) test conditions are as follows: Chromatographic column: VF-WAXms column, specifications 30m × 0.25mm × 0.25μm; Temperature program: Initial temperature 50℃, hold time 2 min, increase to 200℃ at a rate of 30℃ / min, hold for 5 min; Carrier gas: Helium, purity ≥99.999% Flow rate: 1 mL / min; Inlet temperature: 250℃; Injection method: splitless injection; Injection volume: 1.0 μL; Interface temperature: 280℃; Ion source: EI; Measurement mode: Ion scan (SIM); Qualitative ions (m / z): 45, 44, 29 (abundance ratio: 100:28:25); Quantitative ion count (m / z): 45; Solvent delay time: 7 min.

[0038] The identification criteria are: If formamide is not detected in the foamed shoe material sample to be tested, it is identified as a supercritical physical foamed shoe material; otherwise, it is a non-supercritical physical foamed shoe material.

[0039] The test results and identification results are shown in Table 3.

[0040] Table 3. Results of Chemical Composition Tests and Identification As can be seen from Table 3, no formamide was detected in samples A-D, while the formamide content in sample G was 100 mg / kg. Therefore, samples A-D were identified as supercritical physical foaming shoe materials, and sample G was identified as non-supercritical physical foaming shoe materials.

[0041] The GC-MS test results for sample A (supercritical expanded TPU) are shown below. Figure 3 As shown, the GC-MS test pattern of sample G (hybrid foamed TPU) is as follows. Figure 4 As shown, from Figure 3 and Figure 4 The comparison shows that Figure 4 There is a distinct peak representing formamide.

[0042] As can be seen from the above identification results, the present invention has greatly improved the detection efficiency and reduced the detection cost by constructing a full-process identification system of "foaming performance verification - microscopic morphology judgment - chemical composition confirmation". This identification method, which is simple to difficult and progressively advanced, has been implemented. When identifying common foamed shoe materials on the market, the identification results are accurate, eliminating the risk of misjudgment from a single dimension and solving the industry problems of strong appearance imitation and one-sided detection indicators in the existing technology.

Claims

1. A method for identifying supercritical physical foamed shoe materials, characterized in that: The foamed shoe material samples to be tested are identified according to the following steps: (1) Identification of foaming performance: The specific surface area and ball rebound rate of the foamed shoe material sample to be tested were measured; When the specific surface area of ​​the foamed shoe material sample to be tested is ≤1m² 2 If the ball rebound rate is less than 50%, it is identified as a non-supercritical physical foaming shoe material. When the specific surface area of ​​the foamed shoe material sample to be tested is ≥5m² 2 When the ball rebound rate is ≥50% and the ball weight is / g, it is identified as meeting the characteristics of supercritical physical foaming shoe material and can directly proceed to the chemical composition identification step. When the specific surface area of ​​the foamed shoe material sample to be tested is greater than 1m² 2 / g, less than 5m 2 When the ball rebound rate is ≥50% and the ball weight is / g, proceed to the microstructure identification step; (2) Microscopic morphology identification: The cell density and pore size distribution of the cross section of the foamed shoe material sample to be tested were measured; When the cell density of the cross-section of the foamed shoe material sample to be tested is ≤10 7 pcs / cm 3 If the pore size distribution is bimodal or multimodal, it is identified as a non-supercritical physical foaming shoe material. When the cell density of the cross-section of the foamed shoe material sample to be tested is >10 7 pcs / cm 3 When the pore size distribution exhibits a unimodal normal distribution, it is identified as conforming to the characteristics of supercritical physical foaming shoe materials, and proceeds to the chemical composition identification step. (3) Chemical composition identification: Test the formamide content of the foamed shoe material sample to be tested; When formamide is not detected in the foamed shoe material sample to be tested, it is identified as a supercritical physical foamed shoe material. Otherwise, it is a non-supercritical physical foaming shoe material.

2. The method for identifying supercritical physical foamed shoe materials according to claim 1, characterized in that: In step (1), the specific surface area of ​​the foamed shoe material sample to be tested is measured using a BET specific surface area meter.

3. The method for identifying supercritical physical foamed shoe materials according to claim 2, characterized in that: The specific surface area test method is as follows: the foamed shoe material sample to be tested is crushed to a particle size of <1mm, vacuum dried at 60℃ for 12h, 0.5-1.0g of dried sample is weighed and loaded into a sample tube, and nitrogen adsorption-desorption experiment is carried out at liquid nitrogen temperature of -196℃. Data from at least 20 adsorption points are collected, and the specific surface area of ​​the foamed shoe material sample to be tested is calculated using the BET equation.

4. The method for identifying supercritical physical foamed shoe materials according to claim 1, characterized in that: In step (1), the ball rebound rate of the foamed shoe material sample to be tested is tested according to ISO 8307 standard.

5. The method for identifying supercritical physical foamed shoe materials according to claim 4, characterized in that: The ball rebound rate test method is as follows: A steel ball with a diameter of 16±0.5mm is dropped freely from a height of 500mm to impact the surface of the foamed shoe material sample to be tested, and the ball rebound rate is calculated based on the rebound height.

6. The method for identifying supercritical physical foamed shoe materials according to claim 1, characterized in that: In step (2), the foamed shoe material sample to be tested is quenched with liquid nitrogen and then sprayed with gold. The cell density and pore size distribution of the cross section of the foamed shoe material sample to be tested are then measured by scanning electron microscopy.

7. The method for identifying supercritical physical foamed shoe materials according to claim 6, characterized in that: The microstructure testing method is as follows: The foamed shoe material sample to be tested is immersed in liquid nitrogen for 5-10 minutes, quenched, and the fresh cross-section is exposed. The fracture surface is treated with gold sputtering using an ion sputtering instrument. Then, the cross-section of the foamed shoe material sample to be tested is observed using a scanning electron microscope. Five fields of view are randomly selected as counting areas. The cell density is calculated based on the counting area and the number of cells in the counting area. At the same time, the diameter of more than 100 cells in each field of view is measured, and a histogram of cell diameter distribution is plotted to identify whether it is a unimodal distribution.

8. The method for identifying supercritical physical foamed shoe materials according to claim 1, characterized in that: In step (3), the formamide content of the foamed shoe material sample to be tested is determined by gas chromatography-mass spectrometry.

9. The method for identifying supercritical physical foamed shoe materials according to claim 8, characterized in that: The formamide content test method is as follows: After removing the dense layer on the surface of the foamed shoe material sample to be tested, take the internal foam material and cut it into small pieces with a side length of no more than 3 mm. Weigh a total of 0.5 g of sample and put it into a reaction bottle. Add 10 ml of methanol using a pipette and seal the bottle. Shake and extract in a constant temperature water bath shaker for 90 min. Cool to room temperature and filter with an organic filter membrane. Perform qualitative and quantitative detection on the filtrate using gas chromatography-mass spectrometry. The presence of formamide in the sample is determined by comparing the relative abundance of the corresponding qualitative ions in the chromatogram of the gas chromatography-mass spectrometry test with the chromatogram of a formamide standard solution with a similar concentration.

10. The method for identifying supercritical physical foamed shoe materials according to claim 8, characterized in that: The gas chromatography-mass spectrometry (GC-MS) test conditions are as follows: Chromatographic column: VF-WAXms column, specifications 30m × 0.25mm × 0.25μm; Programmed temperature rise: initial temperature 50℃, hold time 2 min, rise to 200℃ at a rate of 30℃ / min, hold for 5 min; Carrier gas: Helium, purity ≥ 99.999%; Flow rate: 1 mL / min; Inlet temperature: 250℃; Injection method: splitless injection; Injection volume: 1.0 μL; Interface temperature: 280℃; Ion source: EI; Measurement mode: Ion scan (SIM); Qualitative ions (m / z): 45, 44, 29 (abundance ratio: 100:28:25); Quantitative ion count (m / z): 45; Solvent delay time: 7 min.

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