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超臨界 CO 2發泡 熱塑性聚氨酯泡沫板的壓縮成型

日期：2024-06-28
聚合物 2021，13 ( 4 )，656；https://doi.org/10.3390/polym13040656
提交日期：2021 年 1 月 18 日 / 修訂日期：2021 年 2 月 16 日 / 接受日期：2021 年 2 月 19 日 / 發布日期：2021 年 2 月 22 日
 
 

Abstract

Expanded thermoplastic polyurethane (ETPU) beads were prepared by a supercritical CO2 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.

 

 

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 CO2 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/cm3, 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 CO2 was pumped in with a high-pressure pump (CRS, Anyang, Korea). In order to obtain supercritical CO2, 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 140–150 °C and 3.5–10.5 MPa for 2–15 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 CO2 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 (ρTPU, ρETPU) using an electronic densitometer (SD-200L, Vaughan, ON, Canada) then calculating the expansion ratio (Φ) using the following equation.
Φ = ρTPU/ρETPU
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 (Ho) 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/Ho × 100(%)

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 20–60 μm and the cell density is 108 cells/cm3, 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 μm and the cell density is 106 cells/cm3, 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 CO2 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 3c, 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 80–90 bar and 110 °C, medium-density foams characterized by expansion ratios of 4–10 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 CO2 foaming process.
Figure 3. Physical properties of ETPU beads prepared at different foaming temperatures and pressures in the supercritical CO2 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 8–15 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 2–3 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 6a is smooth and does not show irregular surface melting of the TPU, but that molded at 150 °C in Figure 6b 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 (Vg), 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 (ΦTheoretical value) from the pellet volume (Vp = 1), is shown in Figure 9, along with the measured data. It shows a similar correlation with the experimental expansion ratio (Φ) calculated using the measured densities before and after foaming, suggesting the theoretical expansion ratio calculated considering the two mutually complementary factors—cell diameter and cell density—correlates with the properties of the foam sheet.


采用超臨界 CO 2發泡工藝制備膨脹熱塑性聚氨酯 (ETPU) 珠粒，然后通過壓縮成型制成泡沫板。研究了發泡珠粒的泡孔結構對泡沫板性能的影響。發泡壓力越高，泡孔數量越多，泡孔尺寸越??；在固定壓力下，發泡溫度越高，粘度越低，泡孔數量越少，泡孔尺寸越大，從而增加了 ETPU 的膨脹率。雖然與蒸汽箱成型相比，可以保持 ETPU 珠粒泡孔結構的加工窗口非常有限，但通過控制壓縮壓力和溫度來實現珠粒表面的燒結，可以通過壓縮成型 ETPU 珠粒來生產泡沫板。泡沫板的性能受珠粒膨脹率的影響，膨脹率的增加會增加泡沫回彈性，降低硬度，并增加拉伸強度和斷裂伸長率。

關鍵字：

熱塑性聚氨酯;發泡珠粒;超臨界 CO 2發泡;發泡倍數;回彈性;硬度

 

圖形摘要" >

圖形概要

1. 簡介

聚合物泡沫 [ 1,2 ]廣泛用于輕質聚合物模塑產品。制造輕質聚合物模塑產品的典型工藝是 Mucell 工藝 [ 3,4 ]和珠粒泡沫工藝 [ 5,6 ] 。在 Mucell 工藝中，將化學發泡劑 [ 7,8 ]或物理發泡劑 [ 9,10,11 ]添加到聚合物熔體中，熔體在壓力下通過模頭或進入模具，并在擠出或注塑過程中冷卻。在珠粒泡沫工藝中，使用已經發泡的膨脹珠粒 [ 12,13 ]或含有可發泡的發泡劑的可膨脹珠粒 [ 14 ]來制備泡沫產品。發泡劑加入到聚合物顆粒中，可以在聚合過程中添加[ 15,16 ] ，也可以采用高溫高壓反應釜將超臨界流體狀態的發泡劑引入到聚合物顆粒中[ 17,18 ]，或者在擠出機中加入發泡劑到聚合物熔體中，通過控制冷卻條件制備可發泡或膨脹珠粒[ 19 ]。

可發性珠粒最廣泛用于聚苯乙烯[ 20 ]，發泡珠粒則用于聚丙烯（發泡聚丙烯，EPP）[ 21 ]、聚苯乙烯（發泡聚苯乙烯，EPS）[ 22 ]和聚乙烯（發泡聚乙烯，EPE）[ 23 ]。發泡珠粒泡沫采用燒結工藝使用發泡聚合物珠粒制成，具有優異的絕緣性、耐熱性、抗沖擊性和能量吸收性。尤其是，EPP因其機械性能、低導熱性和減震性能而廣泛用于輕質汽車部件[ 24,25 ]。最近，人們對發泡熱塑性聚氨酯（ ETPU）的興趣日益濃厚，因為它可用于制備柔軟而有彈性的材料，并且在聚合過程中其性能易于控制[ 26,27 ]。這些熱塑性聚氨酯泡沫是優異的柔性材料，具有高硬度、回彈性、優異的機械性能和動態減震性。在由發泡珠粒（尤其是 EPP）制造聚合物泡沫成型產品時，通常使用蒸汽槽成型[ 28 , 29 ]，其中將高溫蒸汽送入注塑模具，以物理方式燒結和粘合珠粒表面。此過程中的關鍵因素是在粘合珠粒表面的同時保持 EPP 的泡孔結構，因此蒸汽的溫度、壓力和停留時間是重要變量。隨著發泡珠粒的研究和開發，關于發泡珠粒蒸汽槽成型的研究也已有報道[ 30 ]；還報道了將熱空氣與蒸汽結合以使蒸汽均勻滲透，以減少因蒸汽變化而導致的成型缺陷[ 31 ]。蒸汽槽成型無疑是成型發泡珠粒的最佳工藝，但由于設備成本高，其普遍適用性有限，因此似乎需要研究制造成型泡沫產品的多種方法。

本研究采用壓縮成型來豐富泡沫產品制造方法，因為它是聚合物加工中最典型且最便宜的制造方法。在不同的超臨界 CO 2發泡條件下，由發泡熱塑性聚氨酯 (ETPU) 制備泡沫板，并研究了珠狀泡沫結構對泡沫板特性的影響。

2。材料和方法

本研究中使用的熱塑性聚氨酯是 Dongsung Corp.（韓國釜山）芳香族聚醚熱塑性聚氨酯 (TPU: 6175AP)，其熔點為 150°C，比重為 1.055 g/cm3 ，肖氏 A 硬度為 78。實驗室設計的高壓釜（CRS，韓國安養）用于 TPU 發泡以制備 ETPU 珠粒。高壓釜中加入 250 g 蒸餾水、100 g TPU、6.70 g 磷酸三鈣（TCP，Sigma-Aldrich，Merck KGaA，德國達姆施塔特）穩定劑和 0.13 g 十二烷基苯磺酸鈉（SDBS，Sigma-Aldrich，Merck KGaA，德國達姆施塔特）分散劑，然后用高壓泵（CRS，韓國安養）泵入CO2 。為了獲得超臨界 CO2 ，將溫度設定為 90、100、105 或 110 °C，將壓力設定為 75、80 或 90 bar；將 TPU 保持在高壓釜中 30 分鐘，然后打開球閥快速將壓力釋放到大氣壓力以制備膨脹 TPU (ETPU) 珠粒。為了制備 TPU 泡沫片，將帶有溫度控制系統的尺寸為 10 厘米×10 厘米×2.0 毫米的模腔安裝在壓縮成型機（QMESYS，QM900A，韓國義王）上。通過將 15 克 ETPU 充電模具保持在 140–150 °C 和 3.5–10.5 MPa 下 2–15 分鐘來燒結珠粒表面，然后在 4 °C 的水中淬火來制備泡沫片。圖1顯示了制備ETPU珠粒的發泡工藝和泡沫板壓縮成型工藝的示意圖。

圖 1.用于制備發泡熱塑性聚氨酯 (ETPU) 珠粒 的 CO 2輔助發泡工藝和壓縮成型工藝示意圖。

所有樣品的密度測量均采用 ASTM-D792 的水置換法。使用掃描電子顯微鏡（Coxem EM-30，韓國大田）獲得的顯微照片測量泡孔直徑 (D) 和泡孔密度 (N)，從而表征在不同溫度和壓力條件下制備的 ETPU 珠粒的泡沫結構。膨脹率是通過使用電子密度計（SD-200L，加拿大安大略省沃恩）測量發泡前后顆粒的密度 ( ρ TPU，ρ ETPU ) 來確定的，然后使用以下公式計算膨脹率 (Φ)。

φ = ρ TPU / ρ ETPU

（1）

制備 5 個尺寸為 20 mm × 90 mm × 3 mm 的 ETPU 泡沫片材樣品，以 10 mm/min 的速度進行拉伸試驗。通過使用拉伸試驗機（Lloyd LR30K，美國俄亥俄州克利夫蘭）測量拉伸強度、模量和斷裂伸長率與伸長率的關系，并使用肖氏硬度計（BS-392-A，廣州安妙儀器有限公司，中國廣州）測量肖氏 A 硬度，來評估制備的 ETPU 泡沫片材的力學性能。通過使用實驗室制造的回彈儀，將一個重 0.486 g 的 5 mm 球從 41 cm 的高度落下（H o），并測量其反彈的高度（H），來評估泡沫片材的回彈性能；球的回彈率根據以下公式計算。

R = H / H o × 100(%)

（2）

3。結果與討論

圖2顯示了在不同發泡溫度和壓力下制備的ETPU的SEM顯微照片。圖2測得的泡孔直徑和密度以及圖3所示的通過測量發泡前后顆粒的密度確定的膨脹比反映了發泡溫度和壓力對這些值的影響。從圖2可以看出，在本研究采用的溫度和壓力條件下，ETPU泡沫具有閉孔結構。從圖2可以看出，當壓力較低（75 bar）時，泡孔未充分發育，泡孔間壁較厚，表明該條件不適合制備ETPU。泡孔尺寸隨著壓力的升高而減小，在90 bar時，泡孔直徑為20~60μm，泡孔密度為108個泡孔/cm3 ，無論溫度如何，它都可以歸類為細泡孔泡沫[ 32 ]。相反，在 90 bar 以下，泡孔直徑大于 100 μm，泡孔密度為 10 6泡孔/cm 3，是傳統泡孔泡沫的代表。這是因為在較高壓力下 TPU 中形成了更多的核，其中相同總量的 CO 2隨后擴散，由此發生的膨脹形成相對較小的泡孔。在固定壓力下，溫度升高會降低 TPU 的粘度，從而導致泡孔變大和泡孔密度降低。膨脹率隨著發泡過程的壓力和溫度的升高而增加，這表明它更多地取決于泡孔大小而不是泡孔密度。從圖 3c中可以看出，本研究中獲得的大多數 ETPU 的膨脹率一般低于 4，這是高密度泡沫的特征。然而，當發泡溫度為 80–90 bar 和 110 °C 時，可獲得中等密度泡沫，其膨脹率為 4–10；當發泡溫度為 90 bar 和 110 °C 時，可獲得最高膨脹率為 7。泡沫結構取決于發泡條件，這無疑會影響由 ETPU 珠粒制成的泡沫片材的性能。

圖2.超臨界CO 2發泡工藝 中不同發泡溫度和壓力下制備的ETPU的SEM顯微照片。

圖3.超臨界CO 2發泡工藝 中不同發泡溫度和壓力下制備的ETPU珠粒的物理性質：( a )泡沫尺寸；( b )泡沫密度；( c )膨脹比。

圖 4顯示了成型溫度對泡沫板結構的影響。泡沫板是在 105 °C 和 90 bar 條件下對 ETPU 珠粒進行 15 分鐘壓縮成型而形成的。從圖中可以看出，在 140 °C 下壓縮成型時，由于燒結不充分，所以無法制造泡沫板；而在 150 °C 下，珠粒表面會熔化，這表明，通過壓縮成型制備泡沫板應在略低于 150 °C（TPU 的熔點）的一個較窄的溫度范圍內進行。圖5顯示了成型時間對 145 和 150 °C 下泡沫板形成的影響。在 145 °C 下，珠粒的燒結不會像在 140 °C 下成型那樣在 5 分鐘內發生，但會在 8-15 分鐘內充分發生，并且不會發生泡孔變形。在 150 °C 下，當成型時間相對較短（2-3 分鐘）時，可以形成保持珠粒結構的泡沫片材；然而，在較長的成型時間下，可以看到與在 145 °C 下成型的片材相反的片材表面的變形。圖 6顯示了在與圖 5相同的條件下制備的樣品的表面和橫截面 SEM 顯微照片。圖 6a中在 145 °C 下成型的泡沫片材表面光滑，沒有顯示 TPU 的不規則表面熔融，但圖 6b中在 150 °C 下成型的泡沫片材顯示出不規則的表面熔融，因此導致表面的破壞性變形。在芯層和貼近皮膚的層之間似乎獲得了相似的細胞形態。在兩種條件下，珠粒之間的界面都會隨著成型時間的增加而變厚，表明珠粒表面進行了有效燒結。雖然在145°C下成型時泡孔結構沒有變形，但是由于熔融，在150°C時原始ETPU的泡孔會發生變形，并且成型時間會延長，特別是在發生界面燒結的珠粒之間的界面處。

圖 4. 成型溫度對 ETPU 珠粒在 3.5 MPa 壓力下壓縮成型 15 分鐘的影響。

圖 5. ETPU 珠粒在 3.5 MPa 壓力下壓縮成型時成型時間的影響，成型溫度：( a ) 145°C；( b ) 150°C。

圖 6.在（ a）145°C 和（b）150°C 下成型的 ETPU 片材表面和橫截面的 SEM 顯微照片。

模壓成型壓力對珠粒燒結的影響如圖7所示。隨著壓力的增加，珠粒之間的界面變厚，這可能會提高泡沫片材的物理性能；但是，隨著成型時間的增加，可以看到泡沫表面的破壞性變形和界面附近的泡孔變形（圖6）。因此，在3.5 MPa下模壓成型似乎可以得到最好的泡沫片材。根據這些結果，ETPU泡沫片材的模壓成型是可能的，但是與蒸汽箱注塑成型相比，可以最小化泡孔變形的溫度和壓力范圍非常有限，因此需要精確控制成型溫度和壓力。

圖 7. 壓縮成型壓力對 145°C 下 ETPU 珠粒燒結的影響。

通過對在不同條件下制備的具有不同泡孔結構的 ETPU 珠粒進行模壓成型，在 3.5 MPa 和 145 °C 的溫度下模壓 15 分鐘，研究了泡孔結構對泡沫板性能的影響，在此成型條件下，珠粒會發生燒結，并且泡孔的變形最小。圖 8 顯示了使用具有不同泡孔直徑和泡孔密度的 ETPU 在 145 °C 下模壓 15 分鐘后泡沫板的 SEM 顯微照片。在所有 ETPU 中，通過表面融合進行燒結已經很充分；當使用在低壓和低溫下發泡的 ETPU 時，珠粒融合形成的界面更厚，并且泡孔結構在模壓成型過程中不會變形。

圖 8. 在 3.5 MPa 壓力和 145 °C 溫度下壓縮成型 15 分鐘后制成的 ETPU 泡沫片橫截面的 SEM 顯微照片。

圖9顯示了泡孔直徑、泡孔密度和發泡倍率對模壓成型泡沫片回彈性能的影響。球回彈性能通常用于評價泡沫的回彈性能。與硬度不同，球回彈性能反映泡沫的瞬時手感，當泡沫的回彈性能差或能量吸收低時，其回彈性能也較低?；貜椥阅芡ǔＭㄟ^選擇聚氨酯聚合中所用的異氰酸酯和多元醇來控制。但是，如圖9所示，即使使用單一聚氨酯不同的球，也可以通過模壓成型具有不同泡孔結構的ETPU（由在不同條件下發泡TPU獲得）來獲得回彈性能。球回彈性能取決于發泡倍率，發泡倍率越高，泡沫的回彈性能越高（圖9），這表明與高密度泡沫相比，中密度泡沫具有更高的泡沫回彈性能和能量吸收能。在 75 bar 下發泡的 ETPU 泡沫板表現出相對較低的球回彈率（圖 9），這似乎是由于低發泡壓力下 TPU 的泡孔膨脹不足造成的（圖 2）。理論發泡率可由泡孔體積（V g ）計算得出，而泡孔體積又可由圖 3中的泡孔數（N）和泡孔直徑（D）計算得出，理論發泡率（Φ理論值）可由顆粒體積（V p = 1）計算得出，圖 9中顯示了該數據以及測量數據。它與使用發泡前后測量的密度計算出的實驗發泡率（Φ）顯示出相似的相關性，這表明考慮兩個相互補充的因素——泡孔直徑和泡孔密度——計算出的理論發泡率與泡沫板的性能相關。

?????? = Nπ6???3五???=否???6德3

（3）

φ理論值 = (??????+??????） /??????= 1 +??????= 1 + ???π6???3???理論 價值=（五頁+五???）/五頁=1+五???=1+否???6德3

（4）

圖9. 泡沫結構對ETPU泡沫片球回彈的影響。

圖 10顯示了制備的泡沫板的肖氏 A 硬度。與球回彈性能相反，由于泡孔直徑小、泡孔數量少，膨脹率低的泡沫相對較硬；另一方面，膨脹率較高的泡沫硬度低，因此較軟。在低壓和 75 bar 和 90 °C 的溫度下用 ETPU 發泡制備的泡沫板，泡孔膨脹不完全，其硬度值與未發泡的 TPU 板相似。這似乎是由于發泡條件不充分導致大部分 ETPU 未發泡。隨著膨脹率超過 4，硬度幾乎沒有變化，表明在中等密度泡沫板（膨脹率 ≥ 4）的情況下，由泡孔數量和大小決定的膨脹率不會影響泡沫板的硬度，而在高密度泡沫（膨脹率 < 4）的情況下則會影響。

圖 10. 泡沫結構對 ETPU 泡沫片肖氏 A 硬度的影響。

泡沫板的拉伸性能如圖11所示。拉伸強度和斷裂伸長率隨發泡率的增加而增加，但模量降低。由ETPU珠粒在相對較低的壓力和溫度下發泡制成的泡沫板具有較厚的泡孔壁，因此發泡率較低，由于未發泡部分的模量較高，施加的拉伸力很容易在珠粒之間的燒結界面處破裂。由發泡珠粒制成的泡沫板的拉伸強度取決于珠粒之間燒結界面的失效和珠粒內部泡孔的失效。當壓縮成型條件不合適且燒結不充分時，珠粒界面處的失效預計會導致非常差的拉伸強度，但在適當條件下加工的泡沫板的機械性能預計取決于界面或泡孔的失效，具體取決于泡孔結構。圖 12顯示了本研究中在最佳壓縮成型條件下加工的泡沫板的拉伸試驗斷裂表面的橫截面 SEM 顯微照片，可以看出，界面和孔隙處都會發生失效，其相對程度取決于所用的 ETPU。由低發泡率珠粒制成的泡沫板會在珠粒-珠粒界面處失效，而由于發泡壓力和溫度較高而具有較高發泡率的泡沫板會在孔隙處失效，從而產生較高的拉伸強度和斷裂伸長率。也就是說，高發泡率珠粒中的閉孔結構吸收了拉伸試驗中的能量，而不會在界面處失效，直到最終失效的是孔隙而不是界面。根據這些結果，似乎使用中密度泡沫珠粒而不是高密度泡沫珠粒更有利于通過壓縮成型制成的泡沫板具有足夠的機械強度。

圖 11. ETPU 泡沫片的機械性能：( a ) 拉伸強度；( b ) 斷裂伸長率；( c ) 楊氏模量。

圖 12. 拉伸試驗中 ETPU 泡沫板斷裂表面的 SEM 顯微照片。

4。結論

研究了發泡壓力和溫度對發泡TPU泡孔形成的影響，以及將制備好的EPTU模壓成型為泡沫片材的可能性。研究了發泡珠粒結構對模壓成型泡沫片材性能的影響，得出以下結論。

本研究中使用的 TPU 在 75-90 bar 和 90-110 °C 下發泡時呈現封閉的泡孔結構，并且可以制備具有細泡孔和/或常規泡孔的多種泡沫結構的 ETPU 珠粒。在較高的發泡壓力下，會形成更多的核，泡孔尺寸會減小，從而導致更高的膨脹率，而在固定壓力下發泡溫度的升高會影響粘彈性，從而增加泡孔尺寸并減少泡孔數量，從而降低膨脹率。已經證實了壓縮成型 ETPU 以制備泡沫板的可能性，但在燒結 ETPU 珠粒期間，EPTU 中的泡孔結構不會變形，從而獲得泡沫板的加工窗口非常狹窄。ETPU 的膨脹率會影響泡沫板的性能，膨脹率越高，硬度越低，因此回彈性越高。發達的泡孔結構還有助于提高機械性能，例如拉伸強度和斷裂伸長率。

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