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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.

超臨界流體既有氣體壓縮或膨脹性質，又有液體流動性和熱傳導性能，也可通過壓力調節流體的性質。 由于CO2的臨界溫度比較低（364.2K），臨界壓力也不高（7.28MPa），無毒無臭無公害，所以在實際操作中常使用CO2超臨界流體。 其發泡過程可以分為4個階段，分別為： (1) 使超臨界流體進入聚合物基體中，達到 飽和狀態， 形成聚合物/氣體均相體系 ; (2)溫度驟升或壓力驟降使均相體系內的氣體達到過飽和狀態，即熱力學不穩定狀態，從而 引起氣泡成核 ; (3)氣體快速擴散入氣泡核中， 泡孔逐漸長大 ; (4)快速降溫， 完成泡孔結構的定型 。 發泡原理示意圖 超臨界發泡技術是近20年快速發展起來的制備微孔塑料最有效方法，這里的微孔結構類似 爆米花結構 。近年來，在各大領域都有了廣泛的應用。 超臨界發泡成型技術是一種物理發泡成型技術，同時也是一種微孔發泡成型技術，通常可以將 孔徑控制在0.1-10m，泡孔密度一般為10 9 ～10 15 個/cm 3 。 （1）當材料中的泡孔小于該材料零件的內部缺陷時，材料的強度并不會因為泡孔的存在而降低； （2）微孔的存在使得材料中的裂紋尖端被鈍化，防止裂紋在應力作用下的擴展，從而改善材料的機械性能。 微孔塑料不僅具有一般發泡材料的一些獨特性能，而且與傳統發泡材料相比還具有優秀的機械性能，孔的存在減少了相同體積下材料的用量， 可以減少塑料制件的重量和節省材料，表現出高性價比如材料沖擊強度和耐疲勞性的5倍，密度降低5%-90%。 主要應用案例 鞋材 超臨界發泡成型技術逐漸成為鞋材領域的 黑科技 勢力，慢慢的推向市場，應用超臨界發泡工藝的 TPU鞋材 回頭率高達99%，屬實是王者中的王者 ！ 經過加壓加熱預處理后 ， 原來5毫米大小的顆粒可以像爆米花一樣膨脹 。在這個過程中，內含微型密閉氣泡的橢圓形微球的 體積將增大10倍 。 這些密閉氣泡能夠賦予發泡微球以優異的彈性和需要的回彈效果， 防震效果極佳 。 交通工具 超臨界發泡材料應用于汽車內飾、軌道交通等領域，有其獨特的優勢： 1） 無VOC，無任何異味，徹底解決氣味問題； 2） 輕量化，密度可低至30Kg/m3，可為整車減重; 3） 輕質高強，綜合力學性能優于傳統發泡材料； 4） 非交聯，可再回收利用； 5） 優異的隔熱、減震、防水、隔音性能。 超臨界發泡材料在汽車上的應用 新能源電池 超臨界發泡POE應用于新能源電池隔熱墊片，主要起到補償裝配公差和隔熱緩沖的作用。同時還具有質量輕，密度低，蠕變性能好，耐化學腐蝕，耐電壓擊穿，熱穩定性能好等優良性能。 超臨界發泡POE應用于新能源電池隔熱墊片 5G行業應用 超臨界發泡PP應用于5G天線罩，其高強度，滿足了抗風要求以及滿足戶外10年以上的抗光氧老化要求，表面不掛水，表面具有類似荷葉表面的超疏水層。 超臨界發泡PP應用于5G天線罩 日常消費 超臨界發泡TPE應用于瑜伽墊 近年來，隨著我國風電機組技術的不斷提高和風電裝機規模的穩步增長，直接帶來了成本的降低。昔日昂貴的能源，但是如今變成許多地方價格最低的一種新能源。我國也在2020年到2022年取消風電行業補貼。相信在未來超臨界發泡材料將會應用到更多領域！

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 ? or nitrogen N ? as physical foaming agents. CO ? and N ? 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 ? as a foaming agent, CO ? has higher solubility and diffusion rate in polymers. Adding fillers can control the pore structure and is suitable for large-scale production. Using N ? 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 ? 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.

寬闊的高速公路上，炫麗的汽車快速奔馳; 蔚藍的海洋上，優雅的渡輪穩健的航行; 知名的國際品牌，如智慧手機、運動鞋、網路數位科技產業，甚至是原本排碳大戸的重工業，已紛紛開始思考如何減碳、使用數位轉型、綠電能源、綠色製程或再生能源及可回收材料等。 越來越多跨國企業，除了已制定了淨零碳排的目標，從設計、製造、回收，更要求供應鏈配合，提供綠電採購比例，或提供零碳產品等配套。 顯見ESG已成為衡量一個企業是否永續性的指標。 而在這減碳浪潮中，整個供應鏈正在產生改變，我們準備好如何應對了嗎? 以塑膠產業來說，我們從供應鏈的原料端視角，綠色製程可從使用環保材料、生質材料、及可回收材料開始著手。大東樹脂長期深耕綠色技術，從鞋用接著劑的環保水性化開始，到環保塑料TPU樹脂，更研發許多高附加價值的特殊規格，如不黃變脂肪族TPU、生質TPU、輕量化長纖維增強複合材料等等，這些都是屬于綠色環保產品，易于回收利用。 這些環保材料的性能，隨著技術的進步，性能也越來越穩定，逐漸可取代傳統的PVC與其他不易回收的塑膠材料。 從應用端來看，在汽車產業- 近期，TPU薄膜也被應用到比較高端的領域，那就是隱形車衣膜行業。 隱形車衣膜又稱之為汽車漆面保護膜。 汽車貼膜根據不同需求可分為不同的種類，最常見的分類為隔熱膜、玻璃盾甲膜、漆面保護膜和改色膜。 前兩者屬于車窗膜、后兩者屬于車身貼膜，而TPU主要的應用領域為玻璃盾甲膜與漆面保護膜。 早期和中低端車衣膜使用PVC和PU作為基材, 隨著環保趨勢與新材料的精進，高端車款的車衣膜偏好使用優異的耐UV照射及不黃變脂肪族的TPU材料，而TPU隱形車衣則是使用TPU基材層，通過精密涂佈防護的表面功能性涂層和膠水、貼合離保膜製成。簡單來說，所有的隱形車衣膜都是由涂層、基材和膠層這些材料複合而成。 汽車領域越來越多的TPU薄膜被用于漆面保護，防刮保護、防碎石等。汽車工業正在向綠色環保，輕量化，舒適安全的方向發展，聚氨酯材料正在逐步替代部分金屬及傳統橡塑部件，現今聚氨酯用量已成為衡量轎車質量的重要標誌之一。 綠色材料的未來趨勢，朝改性與複合式技術發展 TPU材料本身性能優異，應用范圍廣泛，是各行各業對于環保材料需求的最佳選擇，但其本身特性仍存在一些不足之處，為解決此問題，大東樹脂研發團隊根據不同領域的要求，必須朝改性TPU、複合式TPU技術的方式來開發適合客戶的產品。例如: TPU與某些高分子材料共混改性，成為TPU特殊複合材料，能夠改善其他材料的彈性、耐磨性、抗沖擊性等。 長纖維增強熱塑性聚氨酯TPU複合材料就是很好的例子，透過複合式技術，結合兩項原材料的優點，具高剛性、耐熱性佳，尺寸穩定性優，適用性變更廣。 未來，TPU與其他材料的複合式技術與創新的改性技術，將成為全球TPU技術的重要發展趨勢并將成為綠色供應鏈的環保材料中，高性能、穩固的隱形保護。

超臨界發泡成型是一種物理發泡成型技術，同時也是一種微孔發泡成型技術，將超臨界狀態的二氧化碳或氮氣替代有機發泡劑，在一定壓力、溫度下進行發泡的工藝技術。這種工藝不存在甲酰胺的殘留，無味無毒，對消費者和生產者都有長久的保護，沒有使用交聯劑，可以熔融回收，綠色環保。前面我們整理過鞋材中底超臨界發泡材料，包括EVA、ETPU、TPU、TPE、PEBA等等，其中有的用模壓超臨界發泡成型，也有的用釜壓超臨界發泡成型。而今天我們所整理的是超臨界片材發泡成型。超臨界流體片材發泡技術的特點有：充分利用了超臨界二氧化碳在聚合物中的較快的擴散速率和較大的溶解度。發泡時聚合物處于半固態，熔融區使得泡孔可以生長，而未熔融區則提供了熔體強度，維持了泡孔結構。極快速泄壓，誘導形成極高的成核速率，確保形成具有微納米尺寸、高孔密度的泡孔結構。下面是TPU、TPEE、PEBA的超臨界片材發泡性能分析。 一、TPU發泡TPU（熱塑性聚氨酯彈性體）由二苯甲烷二異氰酸酯（MDI）或甲苯二異氰酸酯（TDI）等二異氰酸酯類分子和大分子多元醇、低分子多元醇（擴鏈劑）共同反應聚合而成的高分子材料。圖 TPU分子鏈結構示意圖采用板材發泡，大小2*1.3米，密度在0.15左右，最大的特點是它的回彈性能，雖然沒有PEBA亮眼，但是它的壓縮性能十分優秀，帶溫測在30%以內，回彈性能穩定保持在65%以上。 二、TPEE發泡 TPEE是熱塑性聚酯彈性體的英文簡稱，是一類含有PBT(聚對苯二甲酸丁二醇酯)聚酯硬段和脂肪族聚酯或聚醚軟段的線型嵌段共聚物。 TPEE分子結構式 發泡后板材密度普遍在0.11-0.3之間，本體母料的硬度在25D-50D內，不同的應用類型選擇不同硬度的原料。TPEE的回彈比TPU的更高，在70%以上。 三、PEBA發泡 熱塑性聚酰胺彈性體，又叫尼龍彈性體，英文縮寫是TPAE，其本質是由聚酰胺硬段和聚醚或聚酯軟段組成的線性嵌段共聚物，具有拉伸強度高、彈性恢復性好、低溫抗沖擊強度高、耐低溫性優異等特點。 根據硬段和軟段組分的不同，TPAE有比較多的種類。目前市面上絕大部分的尼龍彈性體為聚醚酰胺嵌段共聚物，英文名為poly(ether-block-amide)，英文縮寫為PEBA。因此PEBA、尼龍彈性體是常見的叫法。 PEBA最大的特點是又輕又彈，正常的密度是0.07-.08，也可以做成0.09-0.10.落球回彈性能十分優異，能達到73%-75%。 四、應用 ETPU、TPU、TPEE、PEBA四種熱塑性材料可以熱塑加工，重復使用。應用于鞋材，為整鞋回收提供了可能性，綠色環保。采用板材物理發泡，而不是化學發泡，無毒、無味、止滑、耐磨、抗刮。

ETPU是一種TPU材料經超臨界發泡后形成的珠粒，因外形酷似爆米花，所以用ETPU做的運動鞋中底被叫做爆米花中底，它具有優異的回彈性、極低的壓縮永久形變、良好的穿著體驗、加工過程環保高效等優點。 阿迪達斯是最早應用爆米花鞋中底的運動品牌，其boost系列鞋子一經推出即火爆市場。由于ETPU中底的優異性能表現，后面也吸引了越來越多的運動品牌在運動鞋上采用ETPU中底。 但隨著時間的推移，人們發現，潔白的ETPU中底穿久了一定會發黃，并且是無法清洗的那種，在一定程度上影響了穿著體驗。 為什么ETPU中底穿久了會變黃？這得從TPU的分子結構說起。 TPU是熱塑性聚氨酯彈性體的簡稱，英文是Thermoplastic polyurethanes，是一種由二異氰酸酯、擴鏈劑、多元醇組成的多相嵌段共聚物。 如上圖，TPU根據二異氰酸酯的種類不同，主要分為芳香族TPU和脂肪族TPU，兩者的性能差異主要集中在耐黃變性上。 芳香族TPU常用組分：二苯甲烷二異氰酸酯（MDI） 脂肪族TPU常見組分：六亞甲基二異氰酸酯（HDI） 日常生活中用到的TPU大部分都是芳香族TPU，它之所以容易黃變，是因為MDI上的苯環結構，如下圖： MDI在兩個苯基之間有亞甲基橋，這個亞甲基在紫外線的環境下很容易被氧化，從而失去亞甲基橋上的氫，如下圖： 這種化學反應在兩個芳環之間形成共軛體系，稱為醌亞胺。盡管這種氧化反應對于TPU的物理性能影響較小，但是會導致TPU不可逆轉的泛黃。 因為ETPU是采用常見的芳香族TPU進行發泡制備，所以發黃就在所難免。怎么解決這個問題呢？采用脂肪族TPU就是個不錯的辦法。 脂肪族TPU，又叫無苯TPU，下圖可以看到，它的二異氰酸酯分子結構是直線型，相對芳香族TPU來說，耐黃變性大幅提高。 脂肪族TPU硬度范圍很廣，和普通TPU一樣，以40D和45D兩個硬度等級為例，發泡后分別對應45C和50C的中底，具有密度低、彈性好、極佳的耐黃變性能、優異的抗撕裂性能等特性。 脂肪族TPU發泡參數，源自函夏科技 得益于脂肪族TPU優異的耐黃變性能，現已有運動品牌將其應用到鞋中底材料中，比如安踏為湯普森推出的全新戰靴KT7 ，其中底就是采用了脂肪族TPU，結合超臨界發泡工藝所制備。