Polymer 2021, 13 (4), 656; https://doi.org/10.3390/polym13040656 Submission date: January 18, 2021/Revision date: February 16, 2021/Acceptance date: February 19, 2021/Release date: February 22, 2021 Abstract Expanded thermoplastic polyurethane (ETPU) beads were prepared by a supercritical CO 2 foaming process and compression molded to manufacture foam sheets. The effect of the cell structure of the foamed beads on the properties of the foam sheets was studied. Higher foaming pressure resulted in a greater number of cells and thus, smaller cell size, while increasing the foaming temperature at a fixed pressure lowered the viscosity to result in fewer cells and a larger cell size, increasing the expansion ratio of the ETPU. Although the processing window in which the cell structure of the ETPU beads can be maintained was very limited compared to that of steam chest molding, compression molding of ETPU beads to produce foam sheets was possible by controlling the compression pressure and temperature to obtain sintering of the bead surfaces. Properties of the foam sheets are influenced by the expansion ratio of the beads and the increase in the expansion ratio increased the foam resilience, decreased the hardness, and increased the tensile strength and elongation at break. Keywords: thermoplastic polyurethane ; expanded bead ; supercritical CO 2 foaming ; expansion ratio ; resilience ; hardness Graphical Abstract 1. Introduction Polymer foams [ 1 , 2 ] are widely used for light weight polymer molded products. Typical processes for making light weight polymer molded products are the Mucell process [ 3 , 4 ] and the bead foam process [ 5 , 6 ]. In the Mucell process, chemical foaming agents [ 7 , 8 ] or physical foaming agents [ 9 , 10 , 11 ] are added to the polymer melt and the melt is transferred through the die or into the mold under pressure and cooled in the extrusion or injection molding process. In the bead foam process, expanded beads [ 12 , 13 ] which have already been foamed or expandable beads [ 14 ] containing foaming agents which can be foamed are used to prepare foamed products. Incorporation of the foaming agent into the polymer pellet can be carried out by addition in the polymerization process [ 15 , 16 ], by using a high temperature and pressure autoclave to introduce the foaming agent in the supercritical fluid state to polymer pellets [ 17 , 18 ], or by adding the foaming agent to the polymer melt in the extruder and preparing expandable or expanded beads by controlling the cooling condition [ 19 ]. Expandable beads are used most widely in the case of polystyrene [ 20 ], and expanded beads are used in the case of polypropylene (expanded polypropylene, EPP) [ 21 ], polystyrene (expanded polystyrene, EPS) [ 22 ], and polyethylene (expanded polyethylene, EPE) [ 23 ]. Expanded bead foams are manufactured through a sintering process using foamed polymer beads, which have excellent insulation, heat resistance, impact resistance, and energy absorption. In particular, EPP is widely used for light weight automobile parts due to its mechanical properties, low thermal conduction, and shock absorption properties [ 24 , 25 ]. Recently, interest in expanded thermoplastic polyurethanes (ETPU), which can be used to prepare soft and flexible material and whose properties can easily be controlled in the polymerization process, is increasing [ 26 , 27 ]. These thermoplastic polyurethane foams are excellent flexible materials with high hardness, rebound resilience, excellent mechanical properties, and dynamic shock absorption. In manufacturing polymer foam molded products from expanded beads, especially in the case of EPP, steam chest molding is used [ 28 , 29 ], where high temperature steam is fed into the injection mold to physically sinter and bond the bead surfaces. The critical factor in this process is maintaining the cell structure of EPP while bonding the bead surfaces, thus the temperature of the steam, pressure, and residence time are important variables. Along with the research and development of expanded beads, research on steam chest molding of expanded beads has been reported [ 30 ]; the incorporation of hot air along with steam for uniform penetration of the steam to reduce the molding defects from steam variation has also been reported [ 31 ]. Steam chest molding is indisputably the best process for molding of expanded beads, but due to high equipment costs, its general applicability is limited and thus, research on diverse methods to fabricate molded foam products appears to be required. Compression molding was utilized in this study to diversify the methods for fabricating foam products, as it is the most typical and inexpensive fabrication method in polymer processing. Foam sheets were prepared from expanded thermoplastic polyurethane (ETPU) foamed under diverse supercritical CO 2 foaming conditions, and the effect of bead foam structure on the characteristics of the foam sheets was studied. 2. Materials and Methods The thermoplastic polyurethane used in this study was Dongsung Corp. (Busan, Korea) aromatic polyether thermoplastic polyurethane (TPU: 6175AP), having a melting point of 150 C, specific gravity of 1.055 g/cm 3 , and Shore A hardness of 78. A lab-designed autoclave (CRS, Anyang, Korea) was used for the foaming of TPU to prepare the ETPU beads. The autoclave was charged with 250 g distilled water, 100 g TPU, 6.70 g tricalcium phosphate (TCP, Sigma-Aldrich, Merck KGaA, Darmstadt, Germany) stabilizer, and 0.13 g sodium dodecylbenzenesulfonate (SDBS, Sigma-Aldrich, Merck KGaA, Darmstadt, Germany) dispersing agent, then CO 2 was pumped in with a high-pressure pump (CRS, Anyang, Korea). In order to obtain supercritical CO 2 , the temperature was set at 90, 100, 105, or 110 C and the pressure was set at 75, 80, or 90 bar; the TPU was kept in the autoclave for 30 min, then the pressure was quickly released to atmospheric pressure by opening a ball valve to prepare expanded TPU (ETPU) beads. To prepare TPU foam sheets, a mold cavity measuring 10 cm 10 cm 2.0 mm with a temperature control system was mounted on a compression molding machine (QMESYS, QM900A, Uiwang, Korea). Foam sheets were prepared by keeping 15 g ETPU charged mold at 140150 C and 3.510.5 MPa for 215 min to sinter the bead surfaces then quenching in water at 4 C. A schematic of the foaming process to prepare the ETPU beads and the foam sheet compression molding process is shown in Figure 1 . Figure 1. Schematic of the CO 2 assisted foaming process for preparing expanded thermoplastic polyurethane (ETPU) beads and the compression molding process. The water displacement method used to measure the density of all samples was according to ASTM-D792. The foam structure of the ETPU beads prepared under different temperature and pressure conditions was characterized by measuring the cell diameter (D) and the cell density (N) using micrographs obtained with a scanning electron microscope (Coxem EM-30, Daejeon, Korea). The expansion ratio was determined by measuring the density of the pellet before and after foaming ( TPU , ETPU ) using an electronic densitometer (SD-200L, Vaughan, ON, Canada) then calculating the expansion ratio () using the following equation. = TPU/ ETPU (1) Five ETPU foam sheet samples with sizes of 20 mm 90 mm 3 mm were prepared for tensile testing at the speed of 10 mm/min. The mechanical properties of the prepared ETPU foam sheets were evaluated by measuring the tensile strength, modulus, and elongation at break as a function of extension ratio using a tensile tester (Lloyd LR30K, Cleveland, OH, USA) and measuring the Shore A hardness using a Shore hardness tester (BS-392-A, Guangzhou Amittari Instruments Co., Ltd., Guangzhou, China). The rebound properties of the foam sheets were evaluated by dropping a 5 mm ball weighing 0.486 g from 41 cm height ( H o ) and measuring the height it rebounded ( H ) with a lab-made rebound tester; the ball rebound ratio was calculated according to the following equation. R = H / H o 100(%) (2) 3. Results and Discussion The SEM micrographs of ETPU prepared under different foaming temperatures and pressures are shown in Figure 2 . The cell diameter and density measured from Figure 2 , and the expansion ratio determined from the density measurements of the pellet before and after foaming shown in Figure 3 , reflect the effect of the foaming temperature and pressure on these values. It can be seen in Figure 2 that under the temperature and pressure conditions used in this study, the ETPU foam has a closed cell structure. As can be seen in Figure 2 , when the pressure is low (75 bar), the cell is not fully developed and the walls between cells are thick, suggesting that the condition is not adequate for preparing ETPU. The cell size decreases with the rise in pressure and at 90 bar, the cell diameter is 2060 m and the cell density is 10 8 cells/cm 3 , allowing it to be classified as a fine cell foam [ 32 ], regardless of the temperature. In contrast, below 90 bar, the cell diameter is greater than 100 m and the cell density is 10 6 cells/cm 3 , representative of conventional cell foam. This is a result of more nuclei being formed in the TPU at higher pressures, where the same total amount of CO 2 is subsequently diffused and the expansion occurring therefrom forms relatively smaller cells. At a fixed pressure, a temperature increase decreases the viscosity of TPU and results in larger cells and lower cell density. The expansion ratio increases with the increase in the pressure and temperature of the foaming process, suggesting that it is more dependent on the cell size compared with cell density. As can be seen in Figure 3 c, the expansion ratio of most ETPU obtained in this study is generally below 4, characteristic of high-density foams. However, when the foaming is carried out at 8090 bar and 110 C, medium-density foams characterized by expansion ratios of 410 are obtained, and when the foaming is carried out at 90 bar and 110 C, the highest expansion ratio of 7 is obtained. The foam structure, which is dependent on the foaming conditions, will no doubt affect the properties of the foam sheets made from ETPU beads. Figure 2. SEM micrographs of ETPU prepared at different foaming temperatures and pressures in the supercritical CO 2 foaming process. Figure 3. Physical properties of ETPU beads prepared at different foaming temperatures and pressures in the supercritical CO 2 foaming process: ( a ) foam size; ( b ) foam density; ( c ) expansion ratio. The effect of the molding temperature on the structure of foam sheets prepared by a 15 min compression molding of ETPU beads at 105 C and 90 bar can be seen in Figure 4 . As can be seen, when compression molded at 140 C, the fabrication of foam sheets is not possible as sintering does not occur sufficiently, while at 150 C, melting of the surface of the beads occurs, suggesting that preparation of foam sheets by compression molding should be carried out in a narrow range of temperature slightly below 150 C, which is the melting point of TPU. The effect of molding time on the formation of the foam sheets at 145 and 150 C is shown in Figure 5 . At 145 C, sintering of the beads does not occur in 5 min as in the case of molding at 140 C, but occurs sufficiently in 815 min without deformation of the cells. At 150 C, foam sheets maintaining the bead structure are formed when the molding time is relatively short at 23 min; however, at longer molding times, deformation of the sheet surface can be seen contrary to those molded at 145 C. Surface and cross section SEM micrographs of the samples, prepared under the same conditions as in Figure 5 , are shown in Figure 6 . The surface of the foam sheet molded at 145 C in Figure 6 a is smooth and does not show irregular surface melting of the TPU, but that molded at 150 C in Figure 6 b shows irregular surface melting and consequently, destructive deformation of the surface. It seems that similar cell morphology was obtained between the core and the close-to-skin layer. Under both conditions, the interface between the beads becomes thicker with molding time, suggesting effective sintering of the bead surfaces. Although there is no deformation of the cell structure when molded at 145 C, cell deformation from the original ETPU occurs at 150 C with an increase in molding time due to melting, especially at the interface between beads where interfacial sintering occurs. Figure 4. Effect of molding temperature in the compression molding of ETPU beads at 3.5 MPa for 15 min. Figure 5. Effect of molding time in the compression molding of ETPU beads at 3.5 MPa, molding temperature: ( a ) 145 C; ( b ) 150 C. Figure 6. SEM micrographs of the surface and cross section of ETPU sheets molded at ( a ) 145 C and ( b ) 150 C. The effect of compression molding pressure on the sintering of beads is shown in Figure 7 . The interface between beads becomes thicker with the increase in pressure which may increase the physical properties of the foam sheets; however, destructive deformation of the foam surface and cell deformation near the interface can be seen as in the case of increasing molding times ( Figure 6 ). Thus, compression molding at 3.5 MPa appears to result in the best foam sheets. Based on these results, compression molding of ETPU foam sheets is possible, but when compared with steam chest injection molding, the temperature and pressure range at which cell deformation can be minimized is very limited and thus, precise control of the molding temperature and pressure is required. Figure 7. Effect of compression molding pressure on the sintering of ETPU beads at 145 C. The effect of cell structure on the properties of the foam sheets is studied by compression molding ETPU beads, prepared under different conditions and thus, having different cell structure, at 3.5 MPa and 145 C for 15 min, which is the molding condition where sintering of the beads occurs and deformation of the cell can be minimized. The SEM micrographs of foam sheets compression molded at 145 C for 15 min with ETPU having different cell diameter and cell density are shown in Figure 8 . In all ETPU, sintering through surface fusion was sufficient; when the ETPU foamed at low pressure and temperature is used, the interface formed by fusion of the beads is thicker and the cell structure is not deformed in the compression molding process. Figure 8. SEM micrographs of cross section of ETPU foam sheets made by compression molding at 3.5 MPa and 145 C for 15 min. The effect of the cell diameter, cell density, and expansion ratio on the rebound properties of the compression molded foam sheets is shown in Figure 9 . The ball rebound property is generally used to evaluate the resilience of foams. Unlike hardness, the ball rebound property reflects the instantaneous feel of the foam and when the foam has poor resilience or low energy absorption, it exhibits lower rebound. The rebound property is generally controlled by appropriate selection of the isocyanate and polyol used in the polymerization of the polyurethane. However, as can be seen in Figure 9 , even with a single polyurethane different ball, rebound properties can be obtained by compression molding ETPU of different cell structure, obtained by foaming TPU under different conditions. The ball rebound property is dependent on the expansion ratio and is higher in the case of foams having higher expansion ratios ( Figure 9 ), suggesting that medium-density foams have higher foam resilience and energy absorption compared with high-density foams. The foam sheet prepared with ETPU that foamed at 75 bar exhibits a relatively low ball rebound ( Figure 9 ), which appears to be due to the insufficient cell expansion in TPU at the low foaming pressure ( Figure 2 ). The theoretical expansion ratio that can be calculated from the cell volume ( V g ), which, in turn, can be calculated from the number of cells ( N ) and the cell diameter ( D ) in Figure 3 and the theoretical expansion ratio ( Theoretical value ) from the pellet volume ( V p = 1), is shown in Figure 9 , along with the measured data. It shows a similar correlation with the experimental expansion ratio () calculated using the measured densities before and after foaming, suggesting the theoretical expansion ratio calculated considering the two mutually complementary factorscell diameter and cell densitycorrelates with the properties of the foam sheet.

Supercritical foaming is a process technology that uses supercritical carbon dioxide and nitrogen instead of organic foaming agents to foam at a certain pressure and temperature. As a physical foaming technology, supercritical foaming is one of the most effective methods for producing microporous plastics, and the pore size of the plastic produced is usually between 0.1-10 m. According to different processes, supercritical foaming can be divided into supercritical bead foaming process and supercritical sheet foaming process, among which supercritical bead foaming is further divided into extrusion foaming, kettle foaming, etc. In terms of foaming agents, supercritical foaming agents mainly consist of carbon dioxide and nitrogen. This is because carbon dioxide and nitrogen have stable properties, are environmentally friendly, inexpensive, and have mild reactions that are easy to control. Nowadays, with the promotion of the national dual carbon target, the required panels for construction and infrastructure are developing towards lightweight, green, and functional. As an important way to achieve lightweight polymer materials, foaming technology plays a crucial role. Supercritical fluid foaming polymer is a safe and green process technology, which meets the rapidly developing emerging industries and high-quality living needs with excellent insulation, mechanical properties, and low-carbon environmental protection characteristics. Its market prospects are quite promising. Among them, Linde Company, as a gas company specializing in the production of foaming agents such as nitrogen and carbon dioxide, provides high-purity foaming nitrogen, carbon dioxide and related pressurization and stabilization solutions, flow control equipment, customized gas supply for physical foaming enterprises, and ensures the stability of customer gas supply and the reliability of product purity. The companys main product is PRESS The high-efficiency liquid carbon dioxide pressurization device, with multiple advantageous functions, is widely used in fields such as supercritical physical foaming, insulation materials, mattresses, shoe materials, kitchen sponges, etc; Reliable and continuous supply of high-pressure supercooled liquid carbon dioxide; Automatic shutdown, with no energy consumption during shutdown; Gas assisted drive, no power supply required; Quality flow automatically adapts to customer needs; Flexible backup design to achieve maximum availability; Carbon dioxide comes from standard vacuum insulated tanks; Independent single tube, lower installation cost than circular pipeline; Low noise level; Optional device for temperature regulation. Image: High pressure kettle foaming process flow Source: official account @ plastic library network/intrusion and deletion The foam formed by high-pressure reactor will then be transported to the shoe material factory for preliminary cutting using a laser cutting machine to obtain a rough midsole. By using a customized roughing machine and relying on the grinding wheel on top for fine polishing, a basic and accurate three-dimensional shape can be obtained. Finally, the rubber sheet of the outsole is molded and placed underneath, compressed and heated, and softened and bonded together to obtain a complete midsole and outsole. 3) Injection molding+supercritical foaming+compression molding This method, also known as embryo membrane foaming, is used for the nitrogen midsole of Lightstrike Pro and C202 GT. The structure of an injection molding machine is similar to that of an extruder. The raw materials are heated and softened, and like a syringe, they are injected into the midsole mold through holes for heating and molding. In the case of chemical foaming, the foaming agent mixed in the raw materials will decompose at high temperatures to produce gas, thereby carrying out foaming. But in supercritical foaming, there is no foaming agent in the injection molding step, so a cold embryo without foaming is obtained. Then, dozens of cold embryos are placed on a bracket at once and sent to an autoclave for foaming. Therefore, the injection molding foam and one-step Mucell foam molding here are different. It should be noted that in the step of loading into the high-pressure reactor, there are two options: one is to expose the semi-finished product that has been injected and place it on a rack, which has a relatively large degree of freedom and requires precise calculation and control of the foaming ratio to ensure that the finished product size matches the design value and improve the yield rate; The second is to place the semi-finished product into the midsole mold. When foaming and expanding, it is limited by the mold to the size of the finished product. However, if the limitation is excessive, it will lead to excessive compression of the midsole and a decrease in elasticity. Supercritical fluid foaming applied in the food industry Supercritical CO2 can be selectively extracted for different molecules by adjusting temperature and pressure, and its aromatic components are not easily oxidized and destroyed. Thanks to this, the research and application of supercritical fluid technology in the extraction of effective ingredients of hops, the extraction of natural essence from fruits and vegetables, the extraction of animal and vegetable fats from animals and plants, the removal of caffeine from tea, the removal of cholesterol from milk fat and other aspects have made great progress. As early as 1993, a seabuckthorn oil industrial extraction device was built in China, successfully filling the industrial gap of SFE technology. In 1996, Tsinghua University built a pilot plant for hop extract SFE, which increased the utilization rate of hops from 25% to around 90%. At present, many domestic enterprises have used supercritical technology to extract natural spices, oils, etc., which has the advantages of heavy metal removal, compatibility, quantifiability, and fast flavor release, and is very popular among consumers. Figure: Interior view of the workshop of Jiangsu Yuesheng Factory Source: official account @ Plastic Vision Changzhou Jinwei Chemical Complete Equipment Co., Ltd. is a high-tech manufacturer dedicated to the research, development and manufacturing of plastic extrusion molding equipment. The companys products include complete sets of biodegradable plastic modification equipment, whose typical applications are alloy blending of fully degradable plastics such as PLA, PBAT, PBS, PPC, PCL, TPS and PHA, starch filling modification, bamboo and wood powder filling modification, mineral powder filling modification, etc. Figure: Complete set of equipment for modification of biodegradable plastics Source: official account @ Biodegradable Materials Research Institute/Taian Zenith Mechanical Equipment Co., Ltd. is a production enterprise specializing in research, development and manufacturing of high-pressure equipment. The main equipment includes various new material equipment such as supercritical foaming equipment and new powder material equipment. The SL1000 supercritical foaming equipment - mass production machine is its star product, mainly used for large-scale production in factories. The advantage of this equipment is that it can use nitrogen and carbon dioxide for foaming; The temperature uniformity can reach within 1.5 ℃; Its foamable materials include various polymer elastomers such as EVA, TPU, PE, TPEE, PP, ABS, PEBAX, etc., mainly used in the fields of supercritical nitrogen and carbon dioxide foaming of sports shoe materials, boards, sheets, and new materials. Currently, due to the excellent performance of supercritical foaming materials prepared by supercritical foaming technology, they are widely used in shoe materials, packaging, wind power, 5G communication and other fields. With the expansion of the application fields of supercritical foaming materials, the demand for supercritical foaming technology will continue to be released, and the market development prospects are broad. However, it should still be noted that due to the characteristics of clean and environmentally friendly, stable performance, and fine cell structure of supercritical foaming technology, it is more difficult in terms of formulation, equipment, and foaming agent compared to traditional chemical foaming technology. In the future, supercritical physical foaming is expected to serve as a platform technology that connects upstream high-performance materials with downstream application scenarios. The market space is vast and the development speed is promising.

Supercritical foaming molding is a physical foaming molding technology, as well as a microporous foaming molding technology, which replaces organic foaming agents with supercritical carbon dioxide or nitrogen gas and performs foaming under certain pressure and temperature. This process has no residual formamide, is odorless and non-toxic, provides long-term protection for both consumers and producers, does not use crosslinking agents, can be melted and recycled, and is environmentally friendly. We have previously sorted out the shoe material midsole supercritical foam materials, including EVA, TPU, TPE, PEBA, etc. Some of them are formed by compression molding supercritical foam, while others are formed by kettle pressing supercritical foam. And today we are organizing the foam forming of supercritical sheet materials. The characteristics of supercritical fluid sheet foaming technology include: Fully utilizing the fast diffusion rate and high solubility of supercritical carbon dioxide in polymers. During foaming, the polymer is in a semi-solid state, and the melting zone allows for the growth of pores, while the unmelted zone provides melt strength and maintains the pore structure. Extremely fast pressure relief induces the formation of extremely high nucleation rates, ensuring the formation of bubble structures with micro nano size and high pore density. The following is an analysis of the foaming performance of TPU, TPEE, and PEBA supercritical sheets. 1、 TPU foam TPU (thermoplastic polyurethane elastomer) is a polymer material made by reacting and polymerizing diisocyanate molecules such as diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) with high molecular weight polyols and low molecular weight polyols (chain extenders). Schematic diagram of TPU molecular chain structure Using board foam, with a size of 2 * 1.3 meters and a density of around 0.15, its biggest feature is its rebound performance. Although it is not as eye-catching as PEBA, its compression performance is excellent, with a temperature measurement within 30% and a stable rebound performance of over 65%. 2、 TPEE foam TPEE is the abbreviation for thermoplastic polyester elastomer, which is a type of linear block copolymer containing PBT (polybutylene terephthalate) polyester hard segments and aliphatic polyester or polyether soft segments. Figure TPEE molecular structure formula After foaming, the density of the board is generally between 0.11-0.3, and the hardness of the main material is between 25D-50D. Different application types choose raw materials with different hardness. The rebound of TPEE is higher than that of TPU, at over 70%. 3、 PEBA foam Thermoplastic polyamide elastomer, also known as nylon elastomer, abbreviated as TPAE in English, is essentially a linear block copolymer composed of polyamide hard segments and polyether or polyester soft segments. It has the characteristics of high tensile strength, good elastic recovery, high low-temperature impact strength, and excellent low-temperature resistance. There are various types of TPAE based on the different components of hard and soft segments. At present, the vast majority of nylon elastomers on the market are polyetheramide block copolymers, also known as poly (ether block amide) and abbreviated as PEBA. Therefore, PEBA and nylon elastomers are common names. Figure: General formula of PEBA structure The biggest feature of PEBA is its lightness and elasticity. The normal density is 0.07-0.8, but it can also be made into 0.09-0.10. The rebound performance of the ball is very excellent, reaching 73% -75%. 4、 Application TPU, TPEE, and PEBA are three thermoplastic materials that can be thermoplastic processed and reused. Applied to shoe materials, it provides the possibility of whole shoe recycling and is environmentally friendly. Using physical foaming of the board instead of chemical foaming, it is non-toxic, odorless, slip resistant, wear-resistant, and scratch resistant.

超临界流体既有气体压缩或膨胀性质，又有液体流动性和热传导性能，也可通过压力调节流体的性质。 由于CO2的临界温度比较低（364.2K），临界压力也不高（7.28MPa），无毒无臭无公害，所以在实际操作中常使用CO2超临界流体。 其发泡过程可以分为4个阶段，分别为： (1) 使超临界流体进入聚合物基体中，达到 饱和状态， 形成聚合物/气体均相体系 ; (2)温度骤升或压力骤降使均相体系内的气体达到过饱和状态，即热力学不稳定状态，从而 引起气泡成核 ; (3)气体快速扩散入气泡核中， 泡孔逐渐长大 ; (4)快速降温， 完成泡孔结构的定型 。 发泡原理示意图 超临界发泡技术是近20年快速发展起来的制备微孔塑料最有效方法，这里的微孔结构类似 爆米花结构 。近年来，在各大领域都有了广泛的应用。 超临界发泡成型技术是一种物理发泡成型技术，同时也是一种微孔发泡成型技术，通常可以将 孔径控制在0.1-10m，泡孔密度一般为10 9 ～10 15 个/cm 3 。 （1）当材料中的泡孔小于该材料零件的内部缺陷时，材料的强度并不会因为泡孔的存在而降低； （2）微孔的存在使得材料中的裂纹尖端被钝化，防止裂纹在应力作用下的扩展，从而改善材料的机械性能。 微孔塑料不仅具有一般发泡材料的一些独特性能，而且与传统发泡材料相比还具有优秀的机械性能，孔的存在减少了相同体积下材料的用量， 可以减少塑料制件的重量和节省材料，表现出高性价比如材料冲击强度和耐疲劳性的5倍，密度降低5%-90%。 主要应用案例 鞋材 超临界发泡成型技术逐渐成为鞋材领域的 黑科技 势力，慢慢的推向市场，应用超临界发泡工艺的 TPU鞋材 回头率高达99%，属实是王者中的王者 ！ 经过加压加热预处理后 ， 原来5毫米大小的颗粒可以像爆米花一样膨胀 。在这个过程中，内含微型密闭气泡的椭圆形微球的 体积将增大10倍 。 这些密闭气泡能够赋予发泡微球以优异的弹性和需要的回弹效果， 防震效果极佳 。 交通工具 超临界发泡材料应用于汽车内饰、轨道交通等领域，有其独特的优势： 1） 无VOC，无任何异味，彻底解决气味问题； 2） 轻量化，密度可低至30Kg/m3，可为整车减重; 3） 轻质高强，综合力学性能优于传统发泡材料； 4） 非交联，可再回收利用； 5） 优异的隔热、减震、防水、隔音性能。 超临界发泡材料在汽车上的应用 新能源电池 超临界发泡POE应用于新能源电池隔热垫片，主要起到补偿装配公差和隔热缓冲的作用。同时还具有质量轻，密度低，蠕变性能好，耐化学腐蚀，耐电压击穿，热稳定性能好等优良性能。 超临界发泡POE应用于新能源电池隔热垫片 5G行业应用 超临界发泡PP应用于5G天线罩，其高强度，满足了抗风要求以及满足户外10年以上的抗光氧老化要求，表面不挂水，表面具有类似荷叶表面的超疏水层。 超临界发泡PP应用于5G天线罩 日常消费 超临界发泡TPE应用于瑜伽垫 近年来，随着我国风电机组技术的不断提高和风电装机规模的稳步增长，直接带来了成本的降低。昔日昂贵的能源，但是如今变成许多地方价格最低的一种新能源。我国也在2020年到2022年取消风电行业补贴。相信在未来超临界发泡材料将会应用到更多领域！

Compared to the once mainstream mechanical structure, the current midsole technology of sports shoes focuses more on foam. Even Nike has abandoned the home technology air cushion in its latest flagship basketball shoes and instead played with ZoomX. What is supercritical foaming? Why is it so favored? 1、 Principle In the foaming process of polymer materials, it can be divided into two categories based on the different production mechanisms of bubbles: chemical foaming and physical foaming. Supercritical foaming is a type of physical foaming, also known as supercritical fluid microcellular foaming. The physical properties of supercritical fluids are between those of liquids and gases, and under certain temperature and pressure, they possess both gas and liquid properties. They can diffuse easily like gases and infiltrate easily like liquids. The basic principle of supercritical foaming is to utilize the dependence of gas solubility in polymers on pressure and temperature, so that the polymer mixture system after supersaturation with supercritical fluid enters a thermodynamically unstable state during the cooling process, inducing the formation of gas nuclei and obtaining microporous structures. Chemical foaming is a traditional and mature foaming process that involves heating a chemical foaming agent added to a polymer material to decompose and release gas, resulting in foaming. The entire foaming process is similar to the fermentation of pastry, and is still widely used in the production of ordinary midsoles. 2、 Characteristics Compared to traditional chemical foaming, supercritical foaming technology has significant advantages in achieving both lightweight midsole and high elasticity. Supercritical foaming can create small and uniform bubbles, making the density of the midsole lower, thereby reducing the weight of the shoe and reducing the burden on the users feet. Supercritical foaming can evenly distribute bubbles in the midsole material, forming a cushioning structure that can effectively absorb impacts, provide better energy feedback, reduce joint and muscle pressure, and improve compression resistance, making the midsole more durable. In addition, supercritical foaming does not produce any harmful gases to the environment or human body like chemical foaming, and has the advantage of being environmentally friendly and clean. However, supercritical foaming requires much higher equipment and process control than chemical foaming, resulting in higher production costs. Chemical foaming - poor uniformity of cell structure: the average cell size is large and varies, and the cell structure is less rounded Physical foaming - excellent uniformity of cell structure: the average cell size is small and the consistency is high, and the cell structure is relatively round and delicate 3、 Type There are currently four main raw materials for supercritical foaming: EVA TPU、TPEE、PEBA。 EVA (ethylene vinyl acetate) is the earliest, most mature, and widely used foam shoe material. Its various properties are relatively ordinary, but its durability index is relatively backward. After long-term wear, it is easy to be flattened, thereby reducing rebound and cushioning performance. TPU (thermoplastic polyurethane) has excellent rebound performance and strong durability, but it is relatively heavy and will generate heat after long-term pressure deformation. TPU is divided into aromatic and aliphatic types. Overall, aliphatic compounds have better performance than aromatic compounds. TPEE (thermoplastic polyester) can be regarded as an upgraded version of TPU, with little difference in rebound, durability and other indicators compared to TPU, but significant improvement in lightweight indicators. PEBA (polyether block amide), also known as nylon elastomer, has the highest level of resilience, lightweight performance, and durability performance compared to TPU and TPEE, making it a standard midsole for top sports shoes. PEBA has a high cost, and it is common to use PEBA mixed with EVA, TPU and other materials for foaming to reduce costs. Supercritical foaming can be divided into sheet, injection molding, embryo mold, and bead foaming processes. Boost is bead foaming, while ZoomX is sheet foaming. Supercritical foaming generally uses carbon dioxide CO ₂ or nitrogen N ₂ as physical foaming agents. CO ₂ and N ₂ have stable chemical properties, low prices, are non-toxic and harmless, and the critical pressure and temperature are easy to reach. They have strong solubility and diffusion ability for polymers and are easy to control. But there are still differences between the two. Using CO ₂ as a foaming agent, CO ₂ has higher solubility and diffusion rate in polymers. Adding fillers can control the pore structure and is suitable for large-scale production. Using N ₂ as a foaming agent, the pore size of the bubbles in the foam is smaller, the density of the bubbles is high, and they are fine and uniform. The diffusion of N ₂ is slow, the formed foam is not easy to collapse, and the structure is good, which is more conducive to the foaming of elastic materials. 4、 Application Cases 1. Adidas: Boost+Lightstrike PRO Boost is a midsole technology developed in collaboration between Adidas and BASF, a German chemical company, in 2007 and applied to sports shoes in 2013. It is commonly known as popcorn and is mainly composed of supercritical foam TPU, which has excellent elasticity and cushioning effects. The first pair of Boost technology running shoes, Energy Boost, caused a sensation in the entire shoe material field as soon as it was launched. In the 2014 Berlin Marathon, Kimmel broke the world record wearing Boost shoes and became the first person to run in 2 hours and 3 minutes. In 2020, Adidas launched the Lightstrike PRO midsole, which is currently Adidas most responsive midsole material. It can absorb vibrations and generate thrust back to the foot during long-distance running. There is no official announcement about the foam material of Lightstrike PRO, and the online discussion mainly focuses on supercritical foaming using TPEE (thermoplastic polyester elastomer) as the raw material. At present, Adidas only has three main midsole technologies, Lightstrike, Boost, and Lightstrike PRO, corresponding to low, medium, and high-end product lines respectively, all of which are supercritical foaming. 2. Nike: ZoomX In 2017, Nike launched the ZoomX midsole, which is made of PeBax substrate through supercritical foaming technology. The ZoomX foam cushioning system adopts innovative design, providing excellent energy return function, converting each step of impact into the next step of energy. Lighter, softer, and more responsive, specifically designed to provide excellent energy return (up to 85% energy return), maximizing speed. In 2019, Kipchog wore the Nike ZoomX Vaporfly Next% and crossed the finish line in 1 hour, 59 minutes, and 40 seconds, marking the first time in human history that a marathon ran under 2 hours. The latest Nike G.T. Cut 3 is Nikes first use of ZoomX on basketball shoes. 3. Skechers: HYPER BURST+HYPER BURST PRO At the end of 2018, SKECHERS launched the HYPER BURST midsole technology, which uses SKECHERS exclusive special formula EVA to create an ultra lightweight, highly resilient, and more durable midsole through supercritical foaming. It can help runners fully absorb landing kinetic energy while running, and provide feedback as forward propulsion kinetic energy when starting, continuously providing high support and shock absorption energy for runners for a long time. Since 2019, all shoes in the SKECHERS Performance series have been equipped with a HYPER BURST midsole. In 2022, SKECHERS launched the HYPER BURST PRO midsole, which uses TPU as the substrate for supercritical foaming, achieving ultimate lightweight and softness, providing more elasticity, and offering runners durable high shock absorption and cushioning energy. 4. New Balance: FuelCell In 2017, New Balance launched FuelCell, which is made of TPU as the raw material for supercritical injection foaming. It is placed in the lower layer of a typical midsole and has a resilient bullet that quickly responds to the foots impact, allowing runners to fully unleash their explosive power. In 2019, New Balance launched the innovative FuelCell high-speed kinetic technology, which was changed to a supercritical nitrogen foam made of EVA+TPU mixture and applied to the entire midsole. The first application of FuelCell 5280 racing marathon running shoes, as well as subsequent FuelCell Rebel series running shoes and FuelCell Propel series running shoes, all use FuelCell supercritical nitrogen foam midsole, which is the best foam material in NBs energy return performance to date. 5. 361 : QU! KFORM+ENRG-X 361 launches Q-Bullet Technology midsole QU in 2020! KFORM。 At first, Q-Ball Ultra running shoes were made of EVA+TPU composite supercritical foam, which later transformed into pure EVA supercritical foam. The Flyburn ST midsole technology Q-Cube uses EVA supercritical foam, which is lightweight and soft elastic, with excellent cushioning and energy feedback of 85%, providing continuous racing assistance. In 2022, 361 will launch a new EVA supercritical foam material ENRG-X, along with QU! The difference between KFORM is that the foot feels more resilient and elastic, with optimized hardness, specific gravity, and elasticity. The landing sensation has a short cushioning compression time and quick rebound. The running shoes used include Chiqing and E-tough, while basketball shoes have AG3. 6. Jordan: Pro Jordan In 2020, the PRO rebound technology was released, as well as new products equipped with this technology midsole - Jordan Dry Kung Fu Shoes and the first carbon plate racing shoe, the Flying Shadow PB. Breaking through the performance limitations of the original EVA material, advanced TPE (styrene based thermoplastic elastomer) sheets are selected and manufactured through supercritical foaming technology. The material density is controlled within the range of 0.10-0.16g/cm3, while ensuring 37% high compression performance and energy feedback of over 80%. The latest Pan PRO in 2023 uses PEBA material, which has excellent lightweight, cushioning, and rebound performance. The Flying Shadow Plaid (flagship professional marathon running shoe) weighs about 199g per size 41. 7. Puma: NITIO+NITIRO ELITE PUMA launched the NITRO liquid nitrogen foam midsole in 2021, which uses EVA mixed with TPU for supercritical foaming. It has the characteristics of lightweight and high cushioning, and the rebound force when stepped on is very solid, combining stability and comfort. The TEVERIS NITRO nitrogen walking shoes with this midsole feature extremely lightweight, stable foot feel, and efficient cushioning ability. FUSION NTRO is the first basketball shoe equipped with NTRO Foam technology, providing ample cushioning protection with a full foot configuration. In 2022, we upgraded and launched the Nitro ELITE Elite Edition nitrogen technology midsole, which is made of EVA mixed with PEBA for supercritical foaming. Compared with the Nitro nitrogen midsole, it is 31% lighter, with buffering and rebound increased by 18% and 5.7% respectively. It is used in the elite racing PUMA Nitro Elite Racer professional running shoes. 8. ANTA: Nitrogen Technology ANTA Nitrogen Technology was released in 2021, which is a new generation of supercritical nitrogen foaming midsole technology that can endow products with rebound, lightweight, and long-lasting performance. It was the first to introduce C202 GT professional racing marathon running shoes and Thompsons 7th generation signature basketball shoe KT7. The C202 GT is equipped with a double-layer NITROSPEED NUC nitrogen speed technology midsole, which is foamed with PEBA embryo mold. The upper midsole has a hardness of 40 degrees (soft and comfortable, lightweight rebound), and the lower midsole has a hardness of 50 degrees (cushioning and shock absorption, improving stability), with a layer of 3D carbon plate sandwiched in the middle. The NITROSPEED NUC nitrogen speed technology has a midsole density of 0.09g/cm , an energy recovery rate of 86.8%, a durability improvement of 30%, and an overall cushioning performance improvement of 26%. 9. Pick: Ultra lightweight state pole Peak uses P4U (STF material) and various TPE materials to mix and foam, creating the State Pole midsole technology, and establishing the Peak State Pole series product matrix, which has achieved great commercial success. Among them, the ultra lightweight state extreme midsole used in the state extreme 3.0GT running shoe is made of supercritical nitrogen foam, with a density only half of the normal state extreme midsole density, reducing weight by 54%. It is ultra lightweight to wear, easy to run without pressure, and has stronger elasticity and more sensitive response, making up for the shortcomings of the state extreme foot feeling that is soft but not surging enough. 10. Xtep: ACE cushioning technology, power nest The main cases of Xteps supercritical foam shoes are the Power Nest series and XTEP ACE cushioning technology. XTEP ACE cushioning technology: The worlds first PISA supercritical foaming technology provides powerful energy feedback to runners, giving them a strong sense of rebound every step. The rebound rate is as high as 85%, far exceeding ordinary materials and more than 28% higher than ETPU materials. The Power Nest series includes PB, X, X2, etc., which are foamed with TPU, nylon elastomers, and other materials. X-Dynamical FOAM PB Power Nest PB: Made using the supercritical foaming process of nylon particles, it has a density 50% lighter than ETPU and a rebound force of over 80%. X-Dynamical FOAM Power Nest X2: In 2019, Xtep developed an improved version based on pure TPU foam for the midsole material of Power Nest X, which is also the current flagship version of Xtep and has stronger rebound than Power Nest X. 11. Hongxing Erke: Jiong Technology Jiong Technology is a supercritical foam midsole technology released by Hongxing Erke in 2022. Based on the demand of fans to improve the rebound performance of artificial muscle technology midsoles, Hongxing Erke continuously upgrades and evolves midsole materials, increasing elasticity by 23%. The upgraded sole rebound performance is close to 80%, which can effectively alleviate fatigue in the calf and foot joints during running. At the same time, increasing investment in research and development, solving the problem of shoe weight gain by trying out good formula materials, and achieving a 2.6% reduction in weight compared to the previous generation of running shoes while ensuring high resilience. 12. Victory: NitroLite In 2021, Shengli launched NitroLite supercritical nitrogen foaming midsole technology, which uses high-performance EVA material for foaming, and the foam hardness is more in line with the requirements of badminton sports. The first S99ELITE to use NitroLite set a new record for lightweight badminton shoes, while also boasting excellent rebound and durability. The A970NitroLite, launched in 2023, also features a NitroLite midsole, achieving performance upgrades on top of the A970ACEs versatile design. Antonsen, Li Zijia, and Prannoy are all wearing A970NitroLite now. 5、 Conclusion It has to be said that in recent years, the progress of midsole technology in sports shoes has been rapid. For consumers, the biggest feeling is that they feel much more comfortable wearing sports shoes now than before.

Covestro and molding partner Alpha bring out the best in high-performance footwear. Covestro as a professional TPU resin and HDI supplier provides high-performance HDI-based aliphatic ETPU materials to numerous leading manufacturers and brands of sports footwear. The leading manufacturer of compounds and foaming parts, Alpha High Tech Materials Co. Ltd. develops and produces elastomer-based foams and midsoles. Apart from its know-how in super-critical foaming processes, the company develops high-performance, moldable thermoplastic polymer foam beads as well as directly foamed midsoles. In addition, Alpha provides compounding masterbatches which can be added to TPU materials. The product can be used in a technique known as embryo foaming through injection molding, whereby TPU is first injected into a base mold which is subsequently introduced into a reactor and foamed into an actual-size model. This innovative technique enhances midsole production by improving consistency, reducing material waste, and allowing for design customization. Alpha has an excellent and close partnership with leading Chinese sporting shoe brands such as Li-Ning and Anta. With their joint expertise in chemicals and material sciences as well as in research on midsole materials for footwear and super-critical foaming technologies and equipment, the collaboration between Alpha and a number of leading universities, in particular in terms of research and development, is invaluable. The collaboration brings together the expertise of the partners in TPU material supply, compounding and foam molding techniques, and research. Combining the strengths of each partner, this synergy allows for the development of high-performance and sustainable aliphatic ETPU midsole solutions. Furthermore, these materials and solutions are instrumental in enhancing sustainability and the product portfolio of the sporting, leisure and footwear industry. Combining functionality, elegance and comfort. Aliphatic ETPU is a thermoplastic polyurethane known for its high performance. It is crucial in the footwear industry due to its durability, flexibility, and ability to provide superior midsole performance. In addition, vital properties such as super-high rebound, high tear strength, and low compression set are some of the major advantages aliphatic ETPUs offer manufacturers and consumers alike. The material boasts excellent durability, flexibility, shock absorption, and energy return, resulting in enhanced performance and comfort in sports and leisure footwear. Robust, elastic, flexible, scratch-resistant, and more sustainable, Desmopan CQ series aliphatic ETPU allows manufactures to create applications that integrate up to 38%* bio-based content, freedom of design, and the means to combine functionality and comfort to bring sleek and stylish footwear to athletes and consumers who choose to be sporty, fashionable or both. *Bio-based content refers to bio-carbon fraction measured by ASTM D6866. Reputed across the globe as a leading TPU supplier, Covestro provides high-performance and more sustainable materials to most of the famous sports shoe brands. High quality, innovation and Covestros commitment to sustainability have secured not only close and long-term relationships but also earned Covestro the trust of its partners across the industry. Wilson Chan Head of Business Development, TPU, APAC, Covestro Aliphatic ETPUs for sustainable solutions. Covestro is committed to sustainability, the circular economy, and driving eco-friendly solutions. Covestros aim is to continue developing products and solutions that push the boundaries of whats possible. The next generation of sustainable solutions will likely be mass-balanced ETPUs or even bio-based, depending on market needs. In contrast to materials such as PEBA and TPEE, which at present are widely used in midsole production, renewable raw materials used in the Covestro portfolio will not only help reduce the carbon footprint but, at the same time, maintain the performance properties of conventional aliphatic TPUs. The Desmopan CQ range thus not only offers a reduced carbon footprint but also allows manufacturers to enhance their product portfolio without compromising on quality or performance Key benefits Thermoplastic Polyurethane Tough, comfortable: Comfortable fit, flexible, strong, durable, freedom for fashionable design. Reliable supply of raw materials: As a leading manufacturer of TPU resin and HDI, Covestro ensures the reliable supply of high-performance HDI-based aliphatic ETPU materials. Sustainable: Bio-aliphatic ETPUs provide more sustainable options with up to 38%* bio-based content Strong partnerships: Reliable, committed to fostering collaboration with innovative partners.

宽阔的高速公路上，炫丽的汽车快速奔驰; 蔚蓝的海洋上，优雅的渡轮稳健的航行; 知名的国际品牌，如智慧手机、运动鞋、网路数位科技产业，甚至是原本排碳大戸的重工业，已纷纷开始思考如何减碳、使用数位转型、绿电能源、绿色製程或再生能源及可回收材料等。 越来越多跨国企业，除了已制定了淨零碳排的目标，从设计、製造、回收，更要求供应链配合，提供绿电採购比例，或提供零碳产品等配套。 显见ESG已成为衡量一个企业是否永续性的指标。 而在这减碳浪潮中，整个供应链正在产生改变，我们准备好如何应对了吗? 以塑胶产业来说，我们从供应链的原料端视角，绿色製程可从使用环保材料、生质材料、及可回收材料开始着手。大东树脂长期深耕绿色技术，从鞋用接着剂的环保水性化开始，到环保塑料TPU树脂，更研发许多高附加价值的特殊规格，如不黄变脂肪族TPU、生质TPU、轻量化长纤维增强複合材料等等，这些都是属于绿色环保产品，易于回收利用。 这些环保材料的性能，随着技术的进步，性能也越来越稳定，逐渐可取代传统的PVC与其他不易回收的塑胶材料。 从应用端来看，在汽车产业- 近期，TPU薄膜也被应用到比较高端的领域，那就是隐形车衣膜行业。 隐形车衣膜又称之为汽车漆面保护膜。 汽车贴膜根据不同需求可分为不同的种类，最常见的分类为隔热膜、玻璃盾甲膜、漆面保护膜和改色膜。 前两者属于车窗膜、后两者属于车身贴膜，而TPU主要的应用领域为玻璃盾甲膜与漆面保护膜。 早期和中低端车衣膜使用PVC和PU作为基材, 随着环保趋势与新材料的精进，高端车款的车衣膜偏好使用优异的耐UV照射及不黄变脂肪族的TPU材料，而TPU隐形车衣则是使用TPU基材层，通过精密涂佈防护的表面功能性涂层和胶水、贴合离保膜製成。简单来说，所有的隐形车衣膜都是由涂层、基材和胶层这些材料複合而成。 汽车领域越来越多的TPU薄膜被用于漆面保护，防刮保护、防碎石等。汽车工业正在向绿色环保，轻量化，舒适安全的方向发展，聚氨酯材料正在逐步替代部分金属及传统橡塑部件，现今聚氨酯用量已成为衡量轿车质量的重要标誌之一。 绿色材料的未来趋势，朝改性与複合式技术发展 TPU材料本身性能优异，应用范围广泛，是各行各业对于环保材料需求的最佳选择，但其本身特性仍存在一些不足之处，为解决此问题，大东树脂研发团队根据不同领域的要求，必须朝改性TPU、複合式TPU技术的方式来开发适合客户的产品。例如: TPU与某些高分子材料共混改性，成为TPU特殊複合材料，能够改善其他材料的弹性、耐磨性、抗冲击性等。 长纤维增强热塑性聚氨酯TPU複合材料就是很好的例子，透过複合式技术，结合两项原材料的优点，具高刚性、耐热性佳，尺寸稳定性优，适用性变更广。 未来，TPU与其他材料的複合式技术与创新的改性技术，将成为全球TPU技术的重要发展趋势并将成为绿色供应链的环保材料中，高性能、稳固的隐形保护。

超临界发泡成型是一种物理发泡成型技术，同时也是一种微孔发泡成型技术，将超临界状态的二氧化碳或氮气替代有机发泡剂，在一定压力、温度下进行发泡的工艺技术。这种工艺不存在甲酰胺的残留，无味无毒，对消费者和生产者都有长久的保护，没有使用交联剂，可以熔融回收，绿色环保。前面我们整理过鞋材中底超临界发泡材料，包括EVA、ETPU、TPU、TPE、PEBA等等，其中有的用模压超临界发泡成型，也有的用釜压超临界发泡成型。而今天我们所整理的是超临界片材发泡成型。超临界流体片材发泡技术的特点有：充分利用了超临界二氧化碳在聚合物中的较快的扩散速率和较大的溶解度。发泡时聚合物处于半固态，熔融区使得泡孔可以生长，而未熔融区则提供了熔体强度，维持了泡孔结构。极快速泄压，诱导形成极高的成核速率，确保形成具有微纳米尺寸、高孔密度的泡孔结构。下面是TPU、TPEE、PEBA的超临界片材发泡性能分析。 一、TPU发泡TPU（热塑性聚氨酯弹性体）由二苯甲烷二异氰酸酯（MDI）或甲苯二异氰酸酯（TDI）等二异氰酸酯类分子和大分子多元醇、低分子多元醇（扩链剂）共同反应聚合而成的高分子材料。图 TPU分子链结构示意图采用板材发泡，大小2*1.3米，密度在0.15左右，最大的特点是它的回弹性能，虽然没有PEBA亮眼，但是它的压缩性能十分优秀，带温测在30%以内，回弹性能稳定保持在65%以上。 二、TPEE发泡 TPEE是热塑性聚酯弹性体的英文简称，是一类含有PBT(聚对苯二甲酸丁二醇酯)聚酯硬段和脂肪族聚酯或聚醚软段的线型嵌段共聚物。 TPEE分子结构式 发泡后板材密度普遍在0.11-0.3之间，本体母料的硬度在25D-50D内，不同的应用类型选择不同硬度的原料。TPEE的回弹比TPU的更高，在70%以上。 三、PEBA发泡 热塑性聚酰胺弹性体，又叫尼龙弹性体，英文缩写是TPAE，其本质是由聚酰胺硬段和聚醚或聚酯软段组成的线性嵌段共聚物，具有拉伸强度高、弹性恢复性好、低温抗冲击强度高、耐低温性优异等特点。 根据硬段和软段组分的不同，TPAE有比较多的种类。目前市面上绝大部分的尼龙弹性体为聚醚酰胺嵌段共聚物，英文名为poly(ether-block-amide)，英文缩写为PEBA。因此PEBA、尼龙弹性体是常见的叫法。 PEBA最大的特点是又轻又弹，正常的密度是0.07-.08，也可以做成0.09-0.10.落球回弹性能十分优异，能达到73%-75%。 四、应用 ETPU、TPU、TPEE、PEBA四种热塑性材料可以热塑加工，重复使用。应用于鞋材，为整鞋回收提供了可能性，绿色环保。采用板材物理发泡，而不是化学发泡，无毒、无味、止滑、耐磨、抗刮。

ETPU是一种TPU材料经超临界发泡后形成的珠粒，因外形酷似爆米花，所以用ETPU做的运动鞋中底被叫做爆米花中底，它具有优异的回弹性、极低的压缩永久形变、良好的穿着体验、加工过程环保高效等优点。 阿迪达斯是最早应用爆米花鞋中底的运动品牌，其boost系列鞋子一经推出即火爆市场。由于ETPU中底的优异性能表现，后面也吸引了越来越多的运动品牌在运动鞋上采用ETPU中底。 但随着时间的推移，人们发现，洁白的ETPU中底穿久了一定会发黄，并且是无法清洗的那种，在一定程度上影响了穿着体验。 为什么ETPU中底穿久了会变黄？这得从TPU的分子结构说起。 TPU是热塑性聚氨酯弹性体的简称，英文是Thermoplastic polyurethanes，是一种由二异氰酸酯、扩链剂、多元醇组成的多相嵌段共聚物。 如上图，TPU根据二异氰酸酯的种类不同，主要分为芳香族TPU和脂肪族TPU，两者的性能差异主要集中在耐黄变性上。 芳香族TPU常用组分：二苯甲烷二异氰酸酯（MDI） 脂肪族TPU常见组分：六亚甲基二异氰酸酯（HDI） 日常生活中用到的TPU大部分都是芳香族TPU，它之所以容易黄变，是因为MDI上的苯环结构，如下图： MDI在两个苯基之间有亚甲基桥，这个亚甲基在紫外线的环境下很容易被氧化，从而失去亚甲基桥上的氢，如下图： 这种化学反应在两个芳环之间形成共轭体系，称为醌亚胺。尽管这种氧化反应对于TPU的物理性能影响较小，但是会导致TPU不可逆转的泛黄。 因为ETPU是采用常见的芳香族TPU进行发泡制备，所以发黄就在所难免。怎么解决这个问题呢？采用脂肪族TPU就是个不错的办法。 脂肪族TPU，又叫无苯TPU，下图可以看到，它的二异氰酸酯分子结构是直线型，相对芳香族TPU来说，耐黄变性大幅提高。 脂肪族TPU硬度范围很广，和普通TPU一样，以40D和45D两个硬度等级为例，发泡后分别对应45C和50C的中底，具有密度低、弹性好、极佳的耐黄变性能、优异的抗撕裂性能等特性。 脂肪族TPU发泡参数，源自函夏科技 得益于脂肪族TPU优异的耐黄变性能，现已有运动品牌将其应用到鞋中底材料中，比如安踏为汤普森推出的全新战靴KT7 ，其中底就是采用了脂肪族TPU，结合超临界发泡工艺所制备。