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thermoplastic amide elastomer (TPAE) composite foams with supercritical carbon dioxide and their mechanical properties
Journal of Manufacturing Processes 48 (2019) 127–136
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Fabrication of thermoplastic polyurethane (TPU) / thermoplastic amide elastomer (TPAE) composite foams with supercritical carbon dioxide and their mechanical properties
T
Chun-Ta Yua, Chiu-Chun Laib,*, Fu-Ming Wanga, Lung-Chang Liuc, Wen-Chung Liangc, Chih-Lang Wuc, Jen-Chun Chiuc, Hsin-Chu Liuc, Ho-Ting Hsiaoc, Chien-Ming Chenc a b c
Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan, ROC Department of Textile Engineering, Chinese Culture University, Taipei, 11114, Taiwan, ROC Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, 30011, Taiwan, ROC
ARTICLE INFO
ABSTRACT
Keywords: Thermoplastic polyurethane Thermoplastic amide elastomer Supercritical carbon dioxide Mechanical properties Foam
The low resilience and poor elongation characteristics of thermoplastic polyurethane (TPU) foams make it unsuitable for applications such as high-end sports shoes. In this work the incorporation of thermoplastic amide elastomer (TPAE) into TPU foam has been investigated to meet address this shortcoming. Five TPU/TPAE composite pellets were first prepared using varying combinations of TPU, compatibilizer additive, antioxidant, processing aid, and UV stabilizer. Out of these studies, the pellet with the most hardness was foamed with supercritical CO2, to obtain the TPU/TPAE composite foams. The TPU/TPAE composite foams consisting of aminized polyether polyol, nylon 6 (as the hard segment), and Jeffamine D400 (as the soft segment) exhibit excellent resilience, high elongation, good hardness, high tensile strength, and smaller cell size. Experimental results demonstrated that introduction of TPAE into the TPU composite foam efficiently improves their mechanical performance.
1. Introduction Thermoplastic polyurethane (TPU) prepared by the copolymerization of diisocyanate, polyol, and chain extender is a well-known elastomer with miscellaneous applications (e.g. surgical meshes [1], 3D printing [2], tissue engineering scaffolds [3], cable jacketing [4], electrolytes [5], wastewater filtration [6], gas barrier films [7], textiles [8], and so on) and has attracted much attention because of its high resilience [9], good processibility [10,11], high abrasion [12], excellent mechanical properties [13,14], and high chemical resistance [15], being the promising candidate for replacement of polyvinyl chloride (PVC) [16]. In recent years, TPU foam with high sound, heat, and electrical insulation [17], excellent cushioning capability [18,19], and
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good stiffness [20,21] has been extensively used for automobiles [22], biomedical materials [23], shoes [24], electrical packages [25], sporting goods [26], structural materials [27], building usage [28], and so forth. When the current TPU foams are utilized for high-end sports shoes, however, their mechanical properties are unsatisfactory [29] due to low resilience and poor elongation. Since thermoplastic amide elastomer (TPAE) consisting of rigid polyamide (i.e. hard segment) and flexible polyether (i.e. soft segment) [30] is a renowned functional copolymer with extraordinary mechanical characteristics such as excellent elasticity [31], good ductility [32] as well as high toughness [33,34] and its foam possesses versatile utilizations (e.g. footwear [35], clothing [36], medical use [37], etc.), we have tried to further improve the mechanical performances of TPU foam with the incorporation of
Corresponding author. E-mail address: [email protected] (C.-C. Lai).
https://doi.org/10.1016/j.jmapro.2019.09.022 Received 2 January 2018; Received in revised form 12 September 2019; Accepted 27 September 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Chemical structures of TPU, SMA 500, Irganox 1010, CHDADE, and Tinuvin 320.
Fig. 2. Manufacturing procedure of lab-made TPAE pellets. Fig. 3. Manufacturing procedure of lab-made TPU composite foam and TPU/TPAE composite foams with supercritical CO2 foaming apparatus.
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Fig. 4. Synthetic mechanism of TPU composite foam and TPU/TPAE composite foams with supercritical CO2.
represent that introduction of TPAE into TPU composite foam can further enhance the hardness, resilience, elongation, tensile strength, and density of TPU composite foam, demonstrating lab-made TPU/ TPAE composite foams are highly potential cushion elastomers with high mechanical performances.
Table 1 Prescription of lab-made TPAE pellets. Material
TPAE TPAE TPAE TPAE TPAE TPAE
pellet pellet pellet pellet pellet pellet
I II III IV V VI
APEPO (phr)
PA6 (phr)
PTMEG1000 (phr)
Jeffamine D2000 (phr)
Jeffamine D400 (phr)
100 100 100 100 100 100
70 60 70 60 70 60
30 40 – – – –
– – 30 40 – –
– – – – 30 40
2. Experimental 2.1. Materials and methods TPU (weight-average molecular weight (Mw) = 116,500; numberaverage molecular weight (Mn) = 75,100; polydispersity index (PDI ; Mw/Mn ratio) = 1.55 ; Fig. 1), compatibilizer (SMA 500; Fig. 1), antioxidant (Irganox 1010; Fig. 1), processing aid (CHDADE; Fig. 1), UV stabilizer (Tinuvin 320; Fig. 1), aminized polyether polyol (APEPO; Mw = 105,800; Mn = 65,300; PDI = 1.62; Fig. 2), Nylon 6 (PA6; Mw = 286,500; Mn = 189,200; PDI = 1.51; Fig. 2), PTMEG-1000 (Fig. 2), and Jeffamine D2000/D400 (Fig. 2) were acquired from Greco Co., Chembridge Co., Ciba Co., BASF Co., Ciba Co., Echo Chemical Co., NanYa Plastics Co., Union Chemical Co., and Uni-onward Co., respectively. All the materials in this study were used without further purification. The scanning electron microscope (SEM) results, thickness, molecular weight data (viz. Mw, Mn, and PDI), and intrinsic viscosity (I.V.) were examined with a SEM (Hitachi 4100), a pachymeter
TPAE as a reinforcing phase. In this paper, thermoplastic amide elastomers (TPAEs) have been manufactured with aminized polyether polyol (APEPO), hard segment (i.e. nylon 6), and soft segment (i.e. PTMEG, Jeffamine D2000 or Jeffamine D400). After blending TPAEs with TPU and additives (i.e. compatibilizer, antioxidant, processing aid, and UV stabilizer), TPU/ TPAE composite pellets have been obtained. We have prepared TPU/ TPAE composite foams with supercritical carbon dioxide (CO2) and investigated their mechanical characteristics. Experimental results
Table 2 Mechanical properties of lab-made TPAE pellets. Material
TPAE TPAE TPAE TPAE TPAE TPAE a
pellet pellet pellet pellet pellet pellet
I II III IV V VI
I.V. (dL/g)
Mw
Mn
PDI
Tensile strengtha (Kgf/cm2)
Elongationa (%)
Hardnessa (Shore D)
1.073 1.032 0.818 0.781 0.866 0.843
163,800 152,700 128,500 117,400 145,200 136,300
105,700 96,600 79,300 70,700 98,100 94,000
1.55 1.58 1.62 1.66 1.48 1.45
503 363 411 346 595 528
586 624 484 510 604 635
52 48 51 47 58 56
The value was measured with an average of five samples.
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(TENCOR P-10), a gel permeation chromatography (GPC) instrument in toluene at 25℃(Waters Alliance GPC V2000), and a rheometer in toluene at 25℃ (MCR500), respectively. In addition, the tensile strength, elongation, hardness, resilience, density, and average cell size were measured by a universal testing machine (JTM-UTS210A) following ASTM D638 standard, a universal testing machine (JTM-UTS210A) following ASTM D412 standard, a durometer (PCE-DDD 10) following ASTM D2240 standard, a resiliometer (JIA-913B) at 25℃ following ASTM D2632 standard, a gravitometer (MH-300E) following ASTM D792 standard, and a SEM (Hitachi 4100) following ASTM D3576 standard, respectively. The manufacturing procedures of lab-made TPAE pellets, TPU composite foam, and TPU/TPAE composite foams are displayed in Fig. 2 and Fig. 3, respectively, and their details were described in Section 2.2 and 2.3. Moreover, the synthetic mechanism of TPU composite foam and TPU/TPAE composite foams with supercritical CO2 was illustrated in Fig. 4. 2.2. Manufacturing of TPAE pellets APEPO, hard segment (i.e. PA6), and soft segment (i.e. PTMEG1000, Jeffamine D2000 or Jeffamine D400) in anhydrous dimethylformamide (DMF) were mechanically stirred under solvent reflux at 155℃ for 12 h. The solution was then cooled to room temperature (25℃) and the polymer was precipitated by addition of a methanol/ethyl ether solution (4/1; volume ratio). The yellow precipitated solid was collected and dried at 160℃ and 0.02 Kgf/cm2, producing TPAE I-VI as shown in Fig. 2. Their prescription and mechanical properties are indicated in Tables 1 and 2 and Fig. 5 (a)-(c), respectively. 2.3. Manufacturing of TPU composite pellet and TPU/TPAE composite pellets TPU was compounded with SMA 500, Irganox 1010, CHDADE, and Tinuvin 320 at 190℃ and 200 r.p.m. by a twin-screw extruder (Brabender PL2100), obtaining TPU composite pellet. With the identical procedure, TPU/TPAE composite pellets (i.e. TPU/TPAE composite pellet A–E) were acquired by the introduction of TPAE pellet V because TPAE pellet V possesses highest tensile strength and largest hardness among lab-made TPAE pellets. The prescription and mechanical properties of TPU composite pellet and TPU/TPAE composite pellets are shown in Table 3 and 4 and Fig. 6 (a)-(d), respectively.
Fig. 5. (a) Tensile strength, (b) elongation, and (c) hardness of lab-made TPAE pellets.
Table 3 Prescription of lab-made TPU composite pellet and TPU/TPAE composite pellets. Material TPU composite pellet TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite
pellet pellet pellet pellet pellet
A B C D E
TPU (phr)
TPAE V (phr)
SMA 500 (phr)
Irganox 1010 (phr)
CHDADE (phr)
Tinuvin 320 (phr)
100 90 80 70 60 50
– 10 20 30 40 50
– 3 3 3 3 3
1 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 1 1
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Table 4 Mechanical properties of lab-made TPU composite pellet and TPU/TPAE composite pellets. Material
TPU composite pellet TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite a
pellet pellet pellet pellet pellet
A B C D E
Hardnessa (Shore A)
Resiliencea (%)
Elongationa (%)
Tensile strengtha (Kgf/cm2)
Density (g/cm3)
83 84 85 86 87 88
34 37 38 39 40 41
600 656 712 776 838 892
370 254 287 325 358 396
1.11 1.14 1.17 1.20 1.23 1.26
The value was measured with an average of five samples.
2.4. Manufacturing of TPU composite foam and TPU/TPAE composite foams with supercritical CO2 The TPU composite pellet and TPU/TPAE composite pellets were extrusion-molded to be thin sheets (length: 80 mm; width: 80 mm; thickness: 1 mm) by a single-screw extruder (Toshiba SE-90C) at 200℃ and 100 r.p.m. as shown in Fig. 3. After the thin sheets were put into the supercritical CO2 foaming apparatus with a high-pressure thermostat batch autoclave of l L made by Jeoou Rong Industrial Co., we turned on the supercritical CO2 generator and operated at 155 Kgf/cm2 and 38.5℃. Afterwards, the entrance valve for supercritical CO2 was opened and the supercritical CO2 was infused into the foaming apparatus (Stage I of Fig. 4). Then the operating conditions were kept for 30 min (Stage II of Fig. 4), 281 Kgf/cm2 and 150℃ for 1 h (Stage III of Fig. 4), and 352 Kgf/cm2 and 80℃ for 30 min (Stage IV of Fig. 4). Consequently, we released the pressure to ambient pressure (1.03 Kgf/cm2) and cooled the temperature to ambient temperature (25℃) (Stage V of Fig. 4). After cutting with a foam cutter (OSM-2100), the TPU composite foam was cut into a 80 mm diameter disc with a thickness of 4 mm. While the TPU/TPAE composite foams A–E had the same diameter of 80 mm, their thickness varied as 3.8 mm, 3.5 mm, 2.9 mm, 2.5 mm and 2.0 mm, respectively. The mechanical properties of TPU composite foam and TPU/TPAE composite foams were displayed in Table 5 and Fig. 7 (a)-(d). The decrease in thickness of the discs across the different samples that were produced results from the increase for the blending amount of TPAE V, which is a rigid material with high tensile strength and large hardness. In case of TPU/ TPAE composite foam E, hence, there is a chemical limit on the maximum expansion of the disc thickness. Furthermore, the pressure-composition isotherm (PCI) was investigated with a specimen of TPU/TPAE composite foam E (length: 50 mm; width: 10 mm; thickness: 2.0 mm) by a gas sorption analyzer (PCTPro) at 38.5℃ (30 min), 80℃ (1 h), and 150℃ (30 min) under distinct pressure, eventually obtaining the concentrations of supercritical CO2 in TPU/TPAE composite foam E (Fig. 8). 3. Results and discussion As manifested in Table 4, the TPU composite pellet cannot be used directly for high-end sports shoes due to its low resilience (i.e. 34%) and high density (1.11 g/cm3). In order to further improve its resilience as well as density and explore the mechanical properties’ relationship with the pellet as well as the foam, the supercritical CO2 foaming procedure has been utilized for the TPU composite pellet. After foaming, as shown in Table 4 and 5, all the mechanical properties of TPU composite pellet dropped except the resilience. This result originates from that the formation of micro-pores destroys the mechanical structure of TPU composite pellet and raises its deformation volume,
Fig. 6. (a) Hardness, (b) resilience, (c) elongation, and (d) tensile strength of lab-made TPU composite pellet and TPU/TPAE composite pellets. 131
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Table 5 Mechanical properties of lab-made TPU composite foam and TPU/TPAE composite foams. Material
TPU composite foam TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite a b c
foam foam foam foam foam
A B C D E
Hardnessa (Shore A)
Resiliencea (%)
Elongationa (%)
Tensile strengtha (Kgf/cm2)
Density (g/cm3)
Average cell sizeb (μm)
Foaming ratioc
45 46 47 48 49 50
41 42 43 44 45 46
200 224 294 372 446 521
105 78 80 87 98 113
0.27 0.30 0.35 0.42 0.51 0.61
110 ± 5 60 ± 5 50 ± 5 30 ± 5 20 ± 5 15 ± 5
4.1 3.8 3.4 2.9 2.4 2.0
The value was measured with an average of five samples. The value was determined by calculating the average diameter of the cells with five samples in the field of view for SEM results. Foaming ratio (FR) was defined as FR = o , where δo and δe represented the density of material for original state and experimental state, respectively. e
thus reducing the hardness, elongation, tensile strength, and density but enhancing the resilience. Moreover, TPU composite foam prepared with supercritical CO2 exhibits comparable foaming ratio and superior visual surface appearance to those prepared with other blowing agents (e.g. cyclopentane [38], nitrogen (N2) [39], and hydrofluorocarbons (HFC) [40,41], etc.), elucidating that the supercritical CO2 foaming process is practical and effective for the expansion of TPU composite pellet due to low critical point (critical temperature: 31.7℃; critical pressure: 75 Kgf/cm2), high diffusivity, eco-compatibility, non-inflammability, high equilibrium concentration, and low toxicity of supercritical CO2. Although TPU composite foam exhibits acceptable mechanical performances, its resilience and elongation cannot completely meet the requirements of commercial high-end sports shoes [42,43] (i.e. hardness≧45 (Shore A); resilience≧45%; elongation≧500%; tensile strength≧100 Kgf/cm2; density≦0.7 g/cm3). The elongation value is an important factor for the evaluation of wearing comfortability of sports shoes [44,45] because high elongation corresponds to high flexibility and good ductility. In order to further improve the mechanical properties of TPU composite foam, we have tried to manufacture TPAE I-VI with different blending amounts of PTMEG-1000, Jeffamine D2000, and Jeffamine D400 as manifested in Fig. 2 and Table 1 and 2. For increasing concentrations of PTMEG-1000, the I.V., Mw, Mn, tensile strength, and hardness decreased, while the elongation and PDI increased because PA6 is a hard polyamide and PTMEG-1000 is a soft long-chain polyether. Therefore, TPAE II possesses higher flexibility and ductility than TPAE I. The similar result can be observed in case of Jeffamine D2000 (i.e. TPAE III and IV) and Jeffamine D400 (i.e. TPAE V and VI). Among all the lab-made TPAEs (i.e. TPAE I-VI), TPAE V exhibits the highest tensile strength (595 Kgf/cm2), the best hardness (58 (Shore D)), moderate elongation (604%), and adequate I.V. (0.866 dL/g), proving that Jeffamine D400 is a more appropriate material for the soft segment of TPAE than PTMEG-1000 and Jeffamine D2000. Additionally, TPAE VI has a lower tensile strength and hardness but higher elongation than TPAE V because TPAE VI has lower amounts of PA6 (i.e. hard segment) and higher amounts of Jeffamine D400 (i.e. soft segment) than TPAE V. The processing feasibility of TPAE highly depends on its flowability, as
indicated by the I.V. value, whose qualified level is lower than 1 dL/g experimentally [46,47]. Hence, we have blended TPU composite pellet with diverse mixing amounts of TPAE pellet V for manufacturing TPU/ TPAE composite pellet A-E. As depicted in Table 3 and 4, the hardness, resilience, elongation, and density of TPU/TPAE composite pellet A are higher than those of TPU composite pellet, indicating that introduction of 10 phr (i.e. parts per hundreds of resin) TPAE pellet V into TPU composite pellet can further improve the mechanical performances of TPU composite pellet except the tensile strength owing to the rigidity of PA6, the flexibility of Jeffamine D400, and the compatibilizing capability of SMA 500 (Fig. 10 (a) and (b)). As shown in Fig. 11, the hydrogen bond takes place between the anhydride group of SMA 500 and the carboxyl groups of TPU and TPAE while strong Van der Waals forces produce high intermolecular attractions between the alkyl backbones of SMA 500 and the long alkyl chains of TPAE and TPU. The generation of hydrogen bonds and strong Van der Waals forces may pragmatically combine TPU and TPAE to procure TPU/TPAE composite pellet A, improving the hardness, resilience, elongation, and density. Furthermore, the mechanical properties of TPU/TPAE composite pellets were enhanced with increasing amounts of TPAE pellet V and no phase separation can be observed in Fig. 10 (b)-(f). While the blending amount of TPAE pellet V reaches 50 phr, TPU/TPAE composite pellet E, whose hardness, resilience, elongation, tensile strength, and density are 88 (Shore A), 41%, 892%, 396 Kgf/cm2, and 1.26 g/cm3, respectively, has a higher tensile strength than TPU composite pellet. The similar result can also be found in the case of TPU composite foam and TPU/TPAE composite foam A-E as manifested in Table 5. Introduction of TPAE into the TPU composite foam can further improve all the mechanical properties of TPU composite foam except tensile strength and foaming ratio because the hardness, resilience, elongation, and average cell sizes of all the lab-made TPU/TPAE composite foams (i.e. TPU/TPAE composite foam A–E) are superior to those of TPU composite foam. In the case of lab-made TPU/TPAE composite foams, the tensile strength increases with the blending amounts of TPAE pellet V. The tensile strength of TPU/TPAE composite foam E is higher than that of TPU composite foam while the amount of TPAE achieves 50 phr.
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Fig. 8. Pressure-composition isotherm (PCI) of CO2 in TPU/TPAE composite foam E.
However, incorporation of TPAE pellet V into TPU composite foam reduces the foaming ratio and average cell size, demonstrating that TPAE pellet V is a stiff elastomer with low foaming capability and may exhibit high concentration of supercritical CO2 during the mixing/diffusion (Stage II), diffusion (Stage III), and thermodynamic instability stages (Stage IV) of foaming procedure (Fig. 4). As illustrated in Fig. 8, the concentration of supercritical CO2 in TPU/TPAE composite foam E for Stage II (at 38.5℃ under 155 Kgf/cm2), Stage III (at 150℃ under 281 Kgf/cm2), and Stage IV (at 80℃ under 352 Kgf/cm2) are approximately 2.7 wt.%, 4.5 wt.%, and 5.7 wt.%, respectively. Furthermore, we have also discovered that the concentration of supercritical CO2 in TPU/TPAE composite foam E heightens with the processing temperature when the processing pressure is lower than 75 Kgf/cm2 (i.e. the critical pressure of supercritical CO2) since the viscosity of CO2 decreases with the increase of processing temperature. Nevertheless, the opposite result can be observed when the processing pressure is higher than 75 Kgf/cm2 because the viscosity of supercritical CO2 increases with the processing temperature. Therefore, the foaming ratio and average cell size dropped with the raise for blending amounts of TPAE pellet V in the case of TPU/TPAE composite foam A–E as shown in Table 5 and Fig. 9 (b)-(f). Among all the lab-made TPU/TPAE composite foams, TPU/TPAE composite foam E, whose hardness, resilience, elongation, tensile strength, density, average cell size, and foaming ratio reached 50 (Shore A), 46%, 521%, 113 Kgf/cm2, 0.61 g/cm3, 15 μm, and 2.0, respectively, possesses the best mechanical properties, fitting the minimum requirements for high-end sports shoes. 4. Conclusions By using a supercritical carbon dioxide foaming process, thermoplastic polyurethane/ thermoplastic amide elastomer composite foams exhibiting excellent resilience, high elongation, good hardness, high tensile strength, and tiny cell size have been manufactured with thermoplastic polyurethane and thermoplastic amide elastomer, consisting of aminized polyether polyol, nylon 6, and Jeffamine D400. Experimental results demonstrated that introduction of thermoplastic amide elastomer into the thermoplastic polyurethane
Fig. 7. (a) Hardness, (b) resilience, (c) elongation, and (d) tensile strength of lab-made TPU composite foam and TPU/TPAE composite foams.
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Fig. 9. SEM results of (a) TPU composite foam, (b) TPU/TPAE composite foam A, (c) TPU/TPAE composite foam B, (d) TPU/TPAE composite foam C, and (e) TPU/ TPAE composite foam D, and (f) TPU/TPAE composite foam E.
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Fig. 10. SEM results of (a) TPU composite pellet, (b) TPU/TPAE composite pellet A, (c) TPU/TPAE composite pellet B, (d) TPU/TPAE composite pellet C, and (e) TPU/TPAE composite pellet D, and (f) TPU/TPAE composite pellet E.
composite foam efficiently improves the mechanical performance. In the near future, we will further apply thermoplastic polyurethane/ thermoplastic amide elastomer composite foams for various cushion utilizations such as shoes, packages, automobiles, sporting goods, aerospace, and so on. Declaration of Competing Interest No conflict of interest can occur since we have no financial, commercial, legal, or professional relationship with other organizations, or with the people working with them, that could influence our research. References
Fig. 11. Compatible mechanism of SMA 500 with TPU and TPAE.
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Fabrication of thermoplastic polyurethane (TPU) / thermoplastic amide elastomer (TPAE) composite foams with supercritical carbon dioxide and their mechanical properties
T
Chun-Ta Yua, Chiu-Chun Laib,*, Fu-Ming Wanga, Lung-Chang Liuc, Wen-Chung Liangc, Chih-Lang Wuc, Jen-Chun Chiuc, Hsin-Chu Liuc, Ho-Ting Hsiaoc, Chien-Ming Chenc a b c
Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan, ROC Department of Textile Engineering, Chinese Culture University, Taipei, 11114, Taiwan, ROC Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, 30011, Taiwan, ROC
ARTICLE INFO
ABSTRACT
Keywords: Thermoplastic polyurethane Thermoplastic amide elastomer Supercritical carbon dioxide Mechanical properties Foam
The low resilience and poor elongation characteristics of thermoplastic polyurethane (TPU) foams make it unsuitable for applications such as high-end sports shoes. In this work the incorporation of thermoplastic amide elastomer (TPAE) into TPU foam has been investigated to meet address this shortcoming. Five TPU/TPAE composite pellets were first prepared using varying combinations of TPU, compatibilizer additive, antioxidant, processing aid, and UV stabilizer. Out of these studies, the pellet with the most hardness was foamed with supercritical CO2, to obtain the TPU/TPAE composite foams. The TPU/TPAE composite foams consisting of aminized polyether polyol, nylon 6 (as the hard segment), and Jeffamine D400 (as the soft segment) exhibit excellent resilience, high elongation, good hardness, high tensile strength, and smaller cell size. Experimental results demonstrated that introduction of TPAE into the TPU composite foam efficiently improves their mechanical performance.
1. Introduction Thermoplastic polyurethane (TPU) prepared by the copolymerization of diisocyanate, polyol, and chain extender is a well-known elastomer with miscellaneous applications (e.g. surgical meshes [1], 3D printing [2], tissue engineering scaffolds [3], cable jacketing [4], electrolytes [5], wastewater filtration [6], gas barrier films [7], textiles [8], and so on) and has attracted much attention because of its high resilience [9], good processibility [10,11], high abrasion [12], excellent mechanical properties [13,14], and high chemical resistance [15], being the promising candidate for replacement of polyvinyl chloride (PVC) [16]. In recent years, TPU foam with high sound, heat, and electrical insulation [17], excellent cushioning capability [18,19], and
⁎
good stiffness [20,21] has been extensively used for automobiles [22], biomedical materials [23], shoes [24], electrical packages [25], sporting goods [26], structural materials [27], building usage [28], and so forth. When the current TPU foams are utilized for high-end sports shoes, however, their mechanical properties are unsatisfactory [29] due to low resilience and poor elongation. Since thermoplastic amide elastomer (TPAE) consisting of rigid polyamide (i.e. hard segment) and flexible polyether (i.e. soft segment) [30] is a renowned functional copolymer with extraordinary mechanical characteristics such as excellent elasticity [31], good ductility [32] as well as high toughness [33,34] and its foam possesses versatile utilizations (e.g. footwear [35], clothing [36], medical use [37], etc.), we have tried to further improve the mechanical performances of TPU foam with the incorporation of
Corresponding author. E-mail address: [email protected] (C.-C. Lai).
https://doi.org/10.1016/j.jmapro.2019.09.022 Received 2 January 2018; Received in revised form 12 September 2019; Accepted 27 September 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Chemical structures of TPU, SMA 500, Irganox 1010, CHDADE, and Tinuvin 320.
Fig. 2. Manufacturing procedure of lab-made TPAE pellets. Fig. 3. Manufacturing procedure of lab-made TPU composite foam and TPU/TPAE composite foams with supercritical CO2 foaming apparatus.
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Fig. 4. Synthetic mechanism of TPU composite foam and TPU/TPAE composite foams with supercritical CO2.
represent that introduction of TPAE into TPU composite foam can further enhance the hardness, resilience, elongation, tensile strength, and density of TPU composite foam, demonstrating lab-made TPU/ TPAE composite foams are highly potential cushion elastomers with high mechanical performances.
Table 1 Prescription of lab-made TPAE pellets. Material
TPAE TPAE TPAE TPAE TPAE TPAE
pellet pellet pellet pellet pellet pellet
I II III IV V VI
APEPO (phr)
PA6 (phr)
PTMEG1000 (phr)
Jeffamine D2000 (phr)
Jeffamine D400 (phr)
100 100 100 100 100 100
70 60 70 60 70 60
30 40 – – – –
– – 30 40 – –
– – – – 30 40
2. Experimental 2.1. Materials and methods TPU (weight-average molecular weight (Mw) = 116,500; numberaverage molecular weight (Mn) = 75,100; polydispersity index (PDI ; Mw/Mn ratio) = 1.55 ; Fig. 1), compatibilizer (SMA 500; Fig. 1), antioxidant (Irganox 1010; Fig. 1), processing aid (CHDADE; Fig. 1), UV stabilizer (Tinuvin 320; Fig. 1), aminized polyether polyol (APEPO; Mw = 105,800; Mn = 65,300; PDI = 1.62; Fig. 2), Nylon 6 (PA6; Mw = 286,500; Mn = 189,200; PDI = 1.51; Fig. 2), PTMEG-1000 (Fig. 2), and Jeffamine D2000/D400 (Fig. 2) were acquired from Greco Co., Chembridge Co., Ciba Co., BASF Co., Ciba Co., Echo Chemical Co., NanYa Plastics Co., Union Chemical Co., and Uni-onward Co., respectively. All the materials in this study were used without further purification. The scanning electron microscope (SEM) results, thickness, molecular weight data (viz. Mw, Mn, and PDI), and intrinsic viscosity (I.V.) were examined with a SEM (Hitachi 4100), a pachymeter
TPAE as a reinforcing phase. In this paper, thermoplastic amide elastomers (TPAEs) have been manufactured with aminized polyether polyol (APEPO), hard segment (i.e. nylon 6), and soft segment (i.e. PTMEG, Jeffamine D2000 or Jeffamine D400). After blending TPAEs with TPU and additives (i.e. compatibilizer, antioxidant, processing aid, and UV stabilizer), TPU/ TPAE composite pellets have been obtained. We have prepared TPU/ TPAE composite foams with supercritical carbon dioxide (CO2) and investigated their mechanical characteristics. Experimental results
Table 2 Mechanical properties of lab-made TPAE pellets. Material
TPAE TPAE TPAE TPAE TPAE TPAE a
pellet pellet pellet pellet pellet pellet
I II III IV V VI
I.V. (dL/g)
Mw
Mn
PDI
Tensile strengtha (Kgf/cm2)
Elongationa (%)
Hardnessa (Shore D)
1.073 1.032 0.818 0.781 0.866 0.843
163,800 152,700 128,500 117,400 145,200 136,300
105,700 96,600 79,300 70,700 98,100 94,000
1.55 1.58 1.62 1.66 1.48 1.45
503 363 411 346 595 528
586 624 484 510 604 635
52 48 51 47 58 56
The value was measured with an average of five samples.
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(TENCOR P-10), a gel permeation chromatography (GPC) instrument in toluene at 25℃(Waters Alliance GPC V2000), and a rheometer in toluene at 25℃ (MCR500), respectively. In addition, the tensile strength, elongation, hardness, resilience, density, and average cell size were measured by a universal testing machine (JTM-UTS210A) following ASTM D638 standard, a universal testing machine (JTM-UTS210A) following ASTM D412 standard, a durometer (PCE-DDD 10) following ASTM D2240 standard, a resiliometer (JIA-913B) at 25℃ following ASTM D2632 standard, a gravitometer (MH-300E) following ASTM D792 standard, and a SEM (Hitachi 4100) following ASTM D3576 standard, respectively. The manufacturing procedures of lab-made TPAE pellets, TPU composite foam, and TPU/TPAE composite foams are displayed in Fig. 2 and Fig. 3, respectively, and their details were described in Section 2.2 and 2.3. Moreover, the synthetic mechanism of TPU composite foam and TPU/TPAE composite foams with supercritical CO2 was illustrated in Fig. 4. 2.2. Manufacturing of TPAE pellets APEPO, hard segment (i.e. PA6), and soft segment (i.e. PTMEG1000, Jeffamine D2000 or Jeffamine D400) in anhydrous dimethylformamide (DMF) were mechanically stirred under solvent reflux at 155℃ for 12 h. The solution was then cooled to room temperature (25℃) and the polymer was precipitated by addition of a methanol/ethyl ether solution (4/1; volume ratio). The yellow precipitated solid was collected and dried at 160℃ and 0.02 Kgf/cm2, producing TPAE I-VI as shown in Fig. 2. Their prescription and mechanical properties are indicated in Tables 1 and 2 and Fig. 5 (a)-(c), respectively. 2.3. Manufacturing of TPU composite pellet and TPU/TPAE composite pellets TPU was compounded with SMA 500, Irganox 1010, CHDADE, and Tinuvin 320 at 190℃ and 200 r.p.m. by a twin-screw extruder (Brabender PL2100), obtaining TPU composite pellet. With the identical procedure, TPU/TPAE composite pellets (i.e. TPU/TPAE composite pellet A–E) were acquired by the introduction of TPAE pellet V because TPAE pellet V possesses highest tensile strength and largest hardness among lab-made TPAE pellets. The prescription and mechanical properties of TPU composite pellet and TPU/TPAE composite pellets are shown in Table 3 and 4 and Fig. 6 (a)-(d), respectively.
Fig. 5. (a) Tensile strength, (b) elongation, and (c) hardness of lab-made TPAE pellets.
Table 3 Prescription of lab-made TPU composite pellet and TPU/TPAE composite pellets. Material TPU composite pellet TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite
pellet pellet pellet pellet pellet
A B C D E
TPU (phr)
TPAE V (phr)
SMA 500 (phr)
Irganox 1010 (phr)
CHDADE (phr)
Tinuvin 320 (phr)
100 90 80 70 60 50
– 10 20 30 40 50
– 3 3 3 3 3
1 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 1 1
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Table 4 Mechanical properties of lab-made TPU composite pellet and TPU/TPAE composite pellets. Material
TPU composite pellet TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite a
pellet pellet pellet pellet pellet
A B C D E
Hardnessa (Shore A)
Resiliencea (%)
Elongationa (%)
Tensile strengtha (Kgf/cm2)
Density (g/cm3)
83 84 85 86 87 88
34 37 38 39 40 41
600 656 712 776 838 892
370 254 287 325 358 396
1.11 1.14 1.17 1.20 1.23 1.26
The value was measured with an average of five samples.
2.4. Manufacturing of TPU composite foam and TPU/TPAE composite foams with supercritical CO2 The TPU composite pellet and TPU/TPAE composite pellets were extrusion-molded to be thin sheets (length: 80 mm; width: 80 mm; thickness: 1 mm) by a single-screw extruder (Toshiba SE-90C) at 200℃ and 100 r.p.m. as shown in Fig. 3. After the thin sheets were put into the supercritical CO2 foaming apparatus with a high-pressure thermostat batch autoclave of l L made by Jeoou Rong Industrial Co., we turned on the supercritical CO2 generator and operated at 155 Kgf/cm2 and 38.5℃. Afterwards, the entrance valve for supercritical CO2 was opened and the supercritical CO2 was infused into the foaming apparatus (Stage I of Fig. 4). Then the operating conditions were kept for 30 min (Stage II of Fig. 4), 281 Kgf/cm2 and 150℃ for 1 h (Stage III of Fig. 4), and 352 Kgf/cm2 and 80℃ for 30 min (Stage IV of Fig. 4). Consequently, we released the pressure to ambient pressure (1.03 Kgf/cm2) and cooled the temperature to ambient temperature (25℃) (Stage V of Fig. 4). After cutting with a foam cutter (OSM-2100), the TPU composite foam was cut into a 80 mm diameter disc with a thickness of 4 mm. While the TPU/TPAE composite foams A–E had the same diameter of 80 mm, their thickness varied as 3.8 mm, 3.5 mm, 2.9 mm, 2.5 mm and 2.0 mm, respectively. The mechanical properties of TPU composite foam and TPU/TPAE composite foams were displayed in Table 5 and Fig. 7 (a)-(d). The decrease in thickness of the discs across the different samples that were produced results from the increase for the blending amount of TPAE V, which is a rigid material with high tensile strength and large hardness. In case of TPU/ TPAE composite foam E, hence, there is a chemical limit on the maximum expansion of the disc thickness. Furthermore, the pressure-composition isotherm (PCI) was investigated with a specimen of TPU/TPAE composite foam E (length: 50 mm; width: 10 mm; thickness: 2.0 mm) by a gas sorption analyzer (PCTPro) at 38.5℃ (30 min), 80℃ (1 h), and 150℃ (30 min) under distinct pressure, eventually obtaining the concentrations of supercritical CO2 in TPU/TPAE composite foam E (Fig. 8). 3. Results and discussion As manifested in Table 4, the TPU composite pellet cannot be used directly for high-end sports shoes due to its low resilience (i.e. 34%) and high density (1.11 g/cm3). In order to further improve its resilience as well as density and explore the mechanical properties’ relationship with the pellet as well as the foam, the supercritical CO2 foaming procedure has been utilized for the TPU composite pellet. After foaming, as shown in Table 4 and 5, all the mechanical properties of TPU composite pellet dropped except the resilience. This result originates from that the formation of micro-pores destroys the mechanical structure of TPU composite pellet and raises its deformation volume,
Fig. 6. (a) Hardness, (b) resilience, (c) elongation, and (d) tensile strength of lab-made TPU composite pellet and TPU/TPAE composite pellets. 131
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Table 5 Mechanical properties of lab-made TPU composite foam and TPU/TPAE composite foams. Material
TPU composite foam TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite TPU/TPAE composite a b c
foam foam foam foam foam
A B C D E
Hardnessa (Shore A)
Resiliencea (%)
Elongationa (%)
Tensile strengtha (Kgf/cm2)
Density (g/cm3)
Average cell sizeb (μm)
Foaming ratioc
45 46 47 48 49 50
41 42 43 44 45 46
200 224 294 372 446 521
105 78 80 87 98 113
0.27 0.30 0.35 0.42 0.51 0.61
110 ± 5 60 ± 5 50 ± 5 30 ± 5 20 ± 5 15 ± 5
4.1 3.8 3.4 2.9 2.4 2.0
The value was measured with an average of five samples. The value was determined by calculating the average diameter of the cells with five samples in the field of view for SEM results. Foaming ratio (FR) was defined as FR = o , where δo and δe represented the density of material for original state and experimental state, respectively. e
thus reducing the hardness, elongation, tensile strength, and density but enhancing the resilience. Moreover, TPU composite foam prepared with supercritical CO2 exhibits comparable foaming ratio and superior visual surface appearance to those prepared with other blowing agents (e.g. cyclopentane [38], nitrogen (N2) [39], and hydrofluorocarbons (HFC) [40,41], etc.), elucidating that the supercritical CO2 foaming process is practical and effective for the expansion of TPU composite pellet due to low critical point (critical temperature: 31.7℃; critical pressure: 75 Kgf/cm2), high diffusivity, eco-compatibility, non-inflammability, high equilibrium concentration, and low toxicity of supercritical CO2. Although TPU composite foam exhibits acceptable mechanical performances, its resilience and elongation cannot completely meet the requirements of commercial high-end sports shoes [42,43] (i.e. hardness≧45 (Shore A); resilience≧45%; elongation≧500%; tensile strength≧100 Kgf/cm2; density≦0.7 g/cm3). The elongation value is an important factor for the evaluation of wearing comfortability of sports shoes [44,45] because high elongation corresponds to high flexibility and good ductility. In order to further improve the mechanical properties of TPU composite foam, we have tried to manufacture TPAE I-VI with different blending amounts of PTMEG-1000, Jeffamine D2000, and Jeffamine D400 as manifested in Fig. 2 and Table 1 and 2. For increasing concentrations of PTMEG-1000, the I.V., Mw, Mn, tensile strength, and hardness decreased, while the elongation and PDI increased because PA6 is a hard polyamide and PTMEG-1000 is a soft long-chain polyether. Therefore, TPAE II possesses higher flexibility and ductility than TPAE I. The similar result can be observed in case of Jeffamine D2000 (i.e. TPAE III and IV) and Jeffamine D400 (i.e. TPAE V and VI). Among all the lab-made TPAEs (i.e. TPAE I-VI), TPAE V exhibits the highest tensile strength (595 Kgf/cm2), the best hardness (58 (Shore D)), moderate elongation (604%), and adequate I.V. (0.866 dL/g), proving that Jeffamine D400 is a more appropriate material for the soft segment of TPAE than PTMEG-1000 and Jeffamine D2000. Additionally, TPAE VI has a lower tensile strength and hardness but higher elongation than TPAE V because TPAE VI has lower amounts of PA6 (i.e. hard segment) and higher amounts of Jeffamine D400 (i.e. soft segment) than TPAE V. The processing feasibility of TPAE highly depends on its flowability, as
indicated by the I.V. value, whose qualified level is lower than 1 dL/g experimentally [46,47]. Hence, we have blended TPU composite pellet with diverse mixing amounts of TPAE pellet V for manufacturing TPU/ TPAE composite pellet A-E. As depicted in Table 3 and 4, the hardness, resilience, elongation, and density of TPU/TPAE composite pellet A are higher than those of TPU composite pellet, indicating that introduction of 10 phr (i.e. parts per hundreds of resin) TPAE pellet V into TPU composite pellet can further improve the mechanical performances of TPU composite pellet except the tensile strength owing to the rigidity of PA6, the flexibility of Jeffamine D400, and the compatibilizing capability of SMA 500 (Fig. 10 (a) and (b)). As shown in Fig. 11, the hydrogen bond takes place between the anhydride group of SMA 500 and the carboxyl groups of TPU and TPAE while strong Van der Waals forces produce high intermolecular attractions between the alkyl backbones of SMA 500 and the long alkyl chains of TPAE and TPU. The generation of hydrogen bonds and strong Van der Waals forces may pragmatically combine TPU and TPAE to procure TPU/TPAE composite pellet A, improving the hardness, resilience, elongation, and density. Furthermore, the mechanical properties of TPU/TPAE composite pellets were enhanced with increasing amounts of TPAE pellet V and no phase separation can be observed in Fig. 10 (b)-(f). While the blending amount of TPAE pellet V reaches 50 phr, TPU/TPAE composite pellet E, whose hardness, resilience, elongation, tensile strength, and density are 88 (Shore A), 41%, 892%, 396 Kgf/cm2, and 1.26 g/cm3, respectively, has a higher tensile strength than TPU composite pellet. The similar result can also be found in the case of TPU composite foam and TPU/TPAE composite foam A-E as manifested in Table 5. Introduction of TPAE into the TPU composite foam can further improve all the mechanical properties of TPU composite foam except tensile strength and foaming ratio because the hardness, resilience, elongation, and average cell sizes of all the lab-made TPU/TPAE composite foams (i.e. TPU/TPAE composite foam A–E) are superior to those of TPU composite foam. In the case of lab-made TPU/TPAE composite foams, the tensile strength increases with the blending amounts of TPAE pellet V. The tensile strength of TPU/TPAE composite foam E is higher than that of TPU composite foam while the amount of TPAE achieves 50 phr.
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Fig. 8. Pressure-composition isotherm (PCI) of CO2 in TPU/TPAE composite foam E.
However, incorporation of TPAE pellet V into TPU composite foam reduces the foaming ratio and average cell size, demonstrating that TPAE pellet V is a stiff elastomer with low foaming capability and may exhibit high concentration of supercritical CO2 during the mixing/diffusion (Stage II), diffusion (Stage III), and thermodynamic instability stages (Stage IV) of foaming procedure (Fig. 4). As illustrated in Fig. 8, the concentration of supercritical CO2 in TPU/TPAE composite foam E for Stage II (at 38.5℃ under 155 Kgf/cm2), Stage III (at 150℃ under 281 Kgf/cm2), and Stage IV (at 80℃ under 352 Kgf/cm2) are approximately 2.7 wt.%, 4.5 wt.%, and 5.7 wt.%, respectively. Furthermore, we have also discovered that the concentration of supercritical CO2 in TPU/TPAE composite foam E heightens with the processing temperature when the processing pressure is lower than 75 Kgf/cm2 (i.e. the critical pressure of supercritical CO2) since the viscosity of CO2 decreases with the increase of processing temperature. Nevertheless, the opposite result can be observed when the processing pressure is higher than 75 Kgf/cm2 because the viscosity of supercritical CO2 increases with the processing temperature. Therefore, the foaming ratio and average cell size dropped with the raise for blending amounts of TPAE pellet V in the case of TPU/TPAE composite foam A–E as shown in Table 5 and Fig. 9 (b)-(f). Among all the lab-made TPU/TPAE composite foams, TPU/TPAE composite foam E, whose hardness, resilience, elongation, tensile strength, density, average cell size, and foaming ratio reached 50 (Shore A), 46%, 521%, 113 Kgf/cm2, 0.61 g/cm3, 15 μm, and 2.0, respectively, possesses the best mechanical properties, fitting the minimum requirements for high-end sports shoes. 4. Conclusions By using a supercritical carbon dioxide foaming process, thermoplastic polyurethane/ thermoplastic amide elastomer composite foams exhibiting excellent resilience, high elongation, good hardness, high tensile strength, and tiny cell size have been manufactured with thermoplastic polyurethane and thermoplastic amide elastomer, consisting of aminized polyether polyol, nylon 6, and Jeffamine D400. Experimental results demonstrated that introduction of thermoplastic amide elastomer into the thermoplastic polyurethane
Fig. 7. (a) Hardness, (b) resilience, (c) elongation, and (d) tensile strength of lab-made TPU composite foam and TPU/TPAE composite foams.
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Fig. 9. SEM results of (a) TPU composite foam, (b) TPU/TPAE composite foam A, (c) TPU/TPAE composite foam B, (d) TPU/TPAE composite foam C, and (e) TPU/ TPAE composite foam D, and (f) TPU/TPAE composite foam E.
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Fig. 10. SEM results of (a) TPU composite pellet, (b) TPU/TPAE composite pellet A, (c) TPU/TPAE composite pellet B, (d) TPU/TPAE composite pellet C, and (e) TPU/TPAE composite pellet D, and (f) TPU/TPAE composite pellet E.
composite foam efficiently improves the mechanical performance. In the near future, we will further apply thermoplastic polyurethane/ thermoplastic amide elastomer composite foams for various cushion utilizations such as shoes, packages, automobiles, sporting goods, aerospace, and so on. Declaration of Competing Interest No conflict of interest can occur since we have no financial, commercial, legal, or professional relationship with other organizations, or with the people working with them, that could influence our research. References
Fig. 11. Compatible mechanism of SMA 500 with TPU and TPAE.
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