Cellular Thermoplastic Polyurethane Thin Film: Preparation, Elasticity

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Cellular Thermoplastic Polyurethane (TPU) Thin Film: Preparation, Elasticity, and Thermal Insulation Performance Chengbiao Ge, and Wentao Zhai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05037 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Industrial & Engineering Chemistry Research

Cellular Thermoplastic Polyurethane (TPU) Thin Film: Preparation, Elasticity, and Thermal Insulation Performance Chengbiao Ge †,‡ and Wentao Zhai *,†

†

Ningbo Key Lab of Polymer Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang Province 315201, China

‡

University of Chinese Academy of Sciences, Beijing 100049, China

E-mail address: [email protected] * Corresponding author E-mail address: [email protected]; Tel.: +86 0574 86685256 (Wentao Zhai).

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ABSTRACT Thermoplastic polyurethane (TPU) has excellent extensibility, high abrasion resistance, good elastic resilience and biocompatibility, and the fabrication of cellular TPU thin film by an environmental friendly method is attractive in both the academic and industrial communities. In this work, by a novel constrained surface diffusion foaming method, the cellular TPU thin films with thickness of 10-40 µm were prepared using CO2 as the physical blowing agent for the first time. The TPU thin film was sandwiched by two polyimide (PI) films via compression molding. The PI films reduced the gas escape, which ensured the nucleated bubbles grew steadily and then produced cellular TPU thin film with special structure, i.e., the microcellular structure within the thin film and the micro/nanocellular bubbles on the surface of TPU thin film by the physical foaming for the first time. Furthermore, Our morphological observations showed that the foam morphology in the cross-section can be easily changed by adjusting the processing parameters. This interesting structure endowed the TPU thin film with improved elasticity, and good thermal insulation performance. The hysteresis loss decreased by 21% (from 51.6% to 40.7%), and the thermal conductivity reduced by 37% (from 0.257 W·(m·K)-1 to 0.162 W·(m·K)-1).

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1. INTRODUCTION Cellular structure has been introduced into polymer matrix and their significant roles have fully been exhibited such as in elastic conductor,1 thermal insulation,2 electrode,3 electromagnetic interference shielding,4 and sound insulation field.5 However, the aforementioned materials are bulk systems, there is considerable and growing interest in preparing cellular film for use in some special areas. For example, by reducing the material thickness to micron level, the as-prepared polymer films have been recently used in some special fields including semiconductor,6 implantable neural prosthetic device,7 biomedicine,8 microelectronics,9 etc., and play a very good role due to their unique characteristics, i.e., cell structure, low thickness and flexibility. However, the multi-step processes, the organic solvent or the use of additive is most commonly adopted in the fabrication of the above-mentioned materials, of which environment-unfriendly nature might adversely affect their widespread applications. The physical foaming process, as an efficient technique for preparing cellular materials, shows great advantages in terms of its environment-friend, cost-effective, and simple features.10,11 For example carbon dioxide, nitrogen, water vapor and other blowing agents in the batch foaming, foam extrusion, and foam injection moulding, they have been used to fabricate polymeric foams, including polypropylene, polycarbonate, polyethylene terephthalate, polylactic acid, and other bulk foams.12-16 However, the preparation of cellular polymer film by conventional foaming methods still faces technical limitations, due to the rapid diffusion of blowing agents during foaming process. Therefore, the key to prepare cellular film is to keep sufficient 3



amount of blowing agent inside the polymeric material. Constrained surface diffusion of CO2 provides a solution to solve this problem.9,17,18 The poly(methyl methacrylate) (PMMA) was tightly sandwiched between two smooth metal plates, and then it was foamed under supercritical condition (34.5 MPa and 80 °C) for producing cellular film with well-defined cell structure and thickness of about 100 µm. Also, improving supercritical condition is another method to prepare cellular film, that is 200 °C and 30 MPa, cellular fluorinated ethylene propylene copolymer (FEP) film with about 300 µm thickness can be fabricated.19 However, the high saturation pressure, the unfoamed skin layer, the special equipment, the films lacking deformability, and the film thickness higher than 50 µm which limits their applications in microelectronics and other fields, indicate that these techniques noted above need to be improved. Thermoplastic polyurethane (TPU), a type of elastomer, shows potential for the preparation of cellular film because of its excellent extensibility, good elastic resilience, and excellent biocompatibility.20-23 Already reported some foaming techniques including extrusion foaming, foam injection molding, and batch foaming, have been successfully used to fabricate TPU cellular materials,24-27 and the corresponding products have been widely used in packaging, thermal insulation, sound insulation, tissue engineering and other fields.28 It is well known that TPU is a block copolymer composed of hard and soft segments. Because of the difference of polarity between molecular structures, micro-phase separation is happened in the TPU matrix and then leads to form hard domain and soft domain.29,30 Under the influence of strong intermolecular force, there are crystalline areas in hard domain. However, 4


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unlike the chemical crosslinking structure of rubber, the hard domains are physically cross-linked.31 Therefore, thermal history, annealing, processing condition, saturation pressure, saturation temperature, etc., will affect the crystallization behavior of hard domains, which will influence the morphology of foamed material.27 Hossieny et al.27 used twin-screw extruder to destroy the hydrogen bond structure within hard domain to form wide distribution of hard domain, meanwhile adopted high-pressure gas to promote the molecular chain movement resulting in the formation of more crystalline areas. They found that in the batch foaming process, with the aid of improved crystallization behavior, the energy barrier for the cell nucleation could be significantly reduced, and thus higher cell density was presented relative to neat TPU specimen. Similarly, the introduction of fillers into TPU matrix can also serve as nucleation sites to effectively promote the formation of the cells. For example, by introducing fiber, nano-clay and montmorillonite, the cell density of TPU composite foam increased by more than twice times compared to neat TPU foam through extrusion foaming, foam injection molding, or batch foaming process.25,28,32 In addition, strain energy within TPU matrix can also effectively improve the foamability. Wang et al.33 endowed TPU with strain energy in the foaming process, resulting in high expansion ratio. In the above-mentioned foaming process, these improved methods did improve the foamability of TPU. However, it is impossible to avoid the fact that unfoamed skin layers always exist due to rapid cooling and gas escape. Since both upper and lower surfaces have unfoamed skin layer with more than 10 µm thickness, the 10-40 µm thick cellular TPU thin film can not be prepared through the 5



existing eco-friendly physical foaming technique. The use of water and carbon dioxide as coblowing agents or the addition of cross-linking agent in the injection foaming process can lower the thickness of the skin layer.34,35 However, this still does not prepare thin cellular film, because the narrow mold cavity is a challenge for the flow of polymer melt and the fabrication of mold. So far, there are no reports related to this subject. Herein, we report for the first time a simple method to fabricate cellular TPU thin film with thickness of about 10-40 µm. By sandwiching TPU film between two PI films, the escape of CO2 from TPU film is reduced and then the cell morphologies within this layer can be easily controlled by changing the processing parameters. Interestingly, different with the presence of skin layer,17 the prepared cellular TPU thin films using this technology have special structure on the surface, i.e., presence of numerous cells. In the last part, the influences of cell structures on the tensile elasticity, thermal conductivity of the TPU thin film are discussed. 2. EXPERIMENTAL 2.1. Materials. TPU pellets (380A) were purchased from Austin, Co., China. The Kapton® polyimide (PI) film was supplied by DuPont with a thickness of about 45 µm. According to the information provided by the manufacturer, it is derived from the polymerization of aromatic dianhydride and aromatic diamine. The CO2 as the physical blowing agent was obtained from Ningbo Wanli Gas, Co., China with 99.9% purity. 6


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2.2. Preparation of TPU sandwich structure film. Before the PI/TPU/PI sandwich film was prepared, these received TPU pellets were dried at 60 °C in a vacuum oven for 5 h in order to remove the moisture, and then a thermal compression process was carried out. Typically, 5 g TPU pellets were dispersed onto 20 cm × 20 cm PI film, further a same PI film was placed on these TPU pellets. After that, they were loaded into a hydraulic press machine to heat at 190 °C for 3 min, and then the PI/TPU/PI film was formed under 15 MPa after 3 min. Through this process, the TPU layer with a thickness of about 30 µm could be obtained. Notly, 2 g TPU pellets were used in the hot compression process under 20 MPa, the sandwich film with 10 µm thick TPU layer could be received. 2.3. Preparation of cellular TPU thin film. The PI/TPU/PI sandwich film was sealed in a stainless steel chamber flushing with low pressure CO2 for 2 min. After that, the film was saturated for 12 h at room temperature and desired pressure to ensure the equilibrium adsorption of CO2. Once the saturation process was completed, the chamber was depressurized at rate of 0.5 MPa/s, and then the saturated sample was transferred within 30 s to a hot oil bath with polydimethylsiloxane as the heating medium at a fixed temperature for 10 s to foam. The sample after foaming process was placed into an ice bath to fix the cell structure. Fortunately, during the foaming process, two PI films were automatically separated from the foamed TPU thin film. 2.4. Characterizations. In order to reveal the barrier properties of the TPU film and the PI film, gas 7



permeability measurements were carried out by Labthink VAC-V2 gas permeability tester using a differential pressure method.36 The measurement condition was under 25 °C with the relative humidity (RH) of 50%, and the CO2 with the purity of 99.99% was used. The bonding force between PI film and TPU film was determined by using a universal testing machine (Instron 5567) according to ASTM D1876 and the reference.37 The distance between two clamps was 50 mm, and the peel rate was 100 mm/min. The CO2 desorption was measured according to the reference.38 The TPU film, the PI film or the PI/TPU/PI film were placed into a high-pressure chamber, and then the sample after saturation was removed from the chamber and its mass loss as function of time via a digital balance was recorded at room temperature. The weight percentage of CO2 content (Mgas,t) at time t was calculated by the sample weight (Mt) at time t and the initial sample weight (Mi, before the measurement), according to the eq 1. Mgas,t =

Mt -Mi × 100 Mi

(1)

For the microstructure observations of foams, a scanning electron microscope (SEM; Zeisss EVO18) analysis was used. These specimens were prepared by using a knife with a sharp edge, and then coated with gold. The accelerating voltage of the scanning electron microscope was 20 kV. The cell density was determined in eq 2:39 nM 2 N0 = A

3/2

× φ (2) 8


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where n is the number of cells in the SEM images, M is the magnification factor, A is the area of the image (by cm2), and φ is the expansion ratio of the TPU foam calculated using eq 3 as follows:

φ =

ρ (3) ρf

where ρ and ρf are the densities of TPU and TPU foam, respectively. The thermal behavior of TPU films was recorded before and after saturation process, using differential scanning calorimeter (DSC) measurements. The melting temperature (Tm) and the enthalpy of fusion (∆Hm) were detected through the Diamond DSC (PerkinElmer), and the corresponding data was obtained from the first heating scan that ranged from 25 °C to 220 °C at a heating rate of 10 °C/min in a nitrogen environment. Before the measurement, the saturated TPU film was allowed to degas at room temperature for 48 h. Thermal conductivity of cellular TPU thin film and TPU thin film was measured via the LFA457 in which the test temperature was kept at 25 °C. Thermal conductivity of each specimen was acquired based on the mean of five values. The cyclic tensile test was implemented by using a universal testing machine (Instron 5567). The specimens with 50 mm length, 10 mm width, and 0.03 mm thickness, were carried out the test under strain rate of 25 mm/min at room temperature. In order to quantificationally evaluate the tensile elasticity of these specimens, the hysteresis and the residual strain after 100% deformation were compared. The hysteresis was calculated in eq 4: Hysteresis =

Wloading − Wunloading × 100% (4) Wloading 9



where Wloading and Wunloading based on the area under the loading path and the area under the unloading path, respectively. 3. RESULTS AND DISCUSSION 3.1. Preparation of cellular TPU thin film.

Figure 1. Schematic illustration for preparation of cellular TPU thin film. (a-b) the preparation of PI/TPU/PI sandwich film under thermal compression of 15 MPa at 190 °C for 3 min; (c) the gas saturation process under the compressed CO2, gas could diffuse into TPU thin film from both the surfaces and the thickness direction as pointed by arrows; (d-e) the two PI films were automatically separated from cellular TPU thin film during the foaming process and the final foamed thin film was obtained.

Different from the chemical foaming, solvent induced phase separation foaming, 10


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and reactive foaming, a typical feature of polymeric foams blown with the physical blowing agent is that the as-received bulk foams possess the skin layers, where no cell structures are presented, resulting from a high gas diffusivity.39,40 In the case of thin film, the rapid gas escape from the film surface prevents the generation of bubbles within thin film. In order to reduce the gas escape, as depicted in Figure 1, the sandwiched TPU thin film with thickness of about 30 µm was prepared under thermal compression. A solid state foaming process with a mild CO2 pressure of 2.5-4.5 MPa and foaming temperature of 90-120 °C was applied to foam the thin TPU films. The generated force due to CO2 fast escape during temperature-induced foaming process stripped off the adherent PI films, and then the cellular TPU thin films were prepared.

Figure 2. (a) the cross-section of PI/TPU/PI sandwich structure after compression molding process; (b) the measurement of the interface bonding force of PI/TPU/PI film; (c) CO2 permeability coefficient of the TPU and PI, 1 Barrer = 1 × 10-10 11



cm3·cm/cm2·s·cmHg. (d) the weight reduction of the gas-saturated films (Mgas,t) evolution as function of the square root of the desorption time (t1/2), the films were saturated with CO2 under pressure of 4.5 MPa and temperature of 25 °C for 12 h.

Our study showed that a formation of strong interface bonding between TPU film and PI films was critical to foam the TPU thin films. As indicated in Figure 2a, the obvious interface gap can not be observed in the sandwiched TPU. A further interface peel testing shows that the peel strength of TPU and PI films is 29.6 N, which demonstrates that a well-defined interface bonding was formed between TPU and PI films during the thermal compression. It is known that PI film exhibited a better gas barrier over TPU film, resulting from the presence of the dense packed chain structure within PI,41 which was proofed by a 24-time decrease in gas permeability coefficient of the former than the latter (shown in Figure 2c). PI film has good gas barrier and well-defined interface bonding with TPU film, which could reduce the escape of CO2 from the TPU layer in the foaming process and significantly improve the TPU's foamability. A visual test used to prove the obstruction of the PI film to CO2 escape was measured by a weight reduction process with time of the CO2 saturated TPU thin film and the sandwiched TPU thin film, and the corresponding result is shown in Figure 2d. It is obvious that the initial part of the curve has a clear linear relationship with the square root of the desorption time. According to the Fickian law, the slope of the linear curve can represent the gas diffusivity, and high slope means that the gas escapes quickly from the polymer matrix.38 As shown in Figure 2d, the linear curve of 12


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TPU thin film is more steeper than that of the sandwiched TPU thin film, which clearly demonstrates that the PI films hinder CO2 escape from TPU thin film. Similarly, the TPU film is also steeper than that of PI thin film, which again proved the PI film has good gas barrier property. It should be pointed out that the PI films can absorb gas too during CO2 saturation, but they could not foam at all at the selected foaming conditions.

Figure 3. SEM micrographs of the cross-section of the TPU thin films foamed (i) without and (ii) with constrained surface diffusion.

A comparative foaming result of TPU thin film without and with the sandwiched structure is indicated in Figure 3, where the thickness of TPU thin film was about 30 µm, the CO2 saturation pressure was 4.5 MPa, and the foaming temperature was 120 °C. There are no any bubbles being observed in TPU thin film, while the polygon shaped bubbles with sizes of about 8.2 µm are seen in the sandwiched TPU thin film. Furthermore, the thickness of the cellular TPU thin film is about 35.9 µm, and about three bubbles are presented across the thickness direction of it. These results suggest 13



that the sandwiched structure is helpful for generating tiny bubbles in TPU thin film. Obviously, this technique in this work has more efficiently constrained CO2 surface diffusion than that of those references,9,17 in which the unfoamed skin layer is obvious resulting from the incomplete contact between the polymer film and the barrier material. 3.2. Morphology of cellular TPU thin film and mechanism of nucleation.

Figure 4. SEM images of the as-prepared cellular TPU thin films, the films were saturated under 4.5 MPa and then were foamed at 120 °C for 10 s. (a) illustration of 14


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the cellular structure distribution on/within the foamed thin films based on SEM observation; (b)-(e), the cellular structure distribution in the cross-section (b) and on the surface (c-e) of the cellular TPU thin film with thickness of 33.3 µm; the cellular structure distribution within the cross-section (f-g) and on the surface (h-j) of the cellular TPU thin film with thickness of 12.8 µm.

Figure 4 further shows the cell morphologies of cellular TPU thin films, where the saturation pressure was 4.5 MPa, and the foaming temperature was 120 °C. It is obvious that the as-prepared cellular TPU thin film exhibits unique cell morphologies, where both the film surface and the cross-section have a large amount of well dispersed tiny bubbles. In the cross-section of film, the average cell size is 8.0 µm and the cell density is 1.1 × 109 cells/cm3; while in the surface area, the average cell size decreases to 0.4 µm. Furthermore, these bubbles are the closed structure, which is a typical characteristic of polymer foams being fabricated by the physical blowing agent. A further decrease of TPU film thickness was carried out with aim to elaborate the advantage of the proposed foaming technology in fabrication of cellular thin film. As shown in Figures 4f-j, a cellular TPU thin film with thickness of 12.8 µm was prepared, of which the closed-cell bubbles with uniform dispersion and various cell sizes were dispersed in the cross-sections and film surfaces.

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Figure 5. DSC thermogram of TPU specimens: (i) original TPU; (ii) TPU after saturating process.

Figure 5 shows the DSC thermographs of the TPU film before and after saturating process under 4.5 MPa saturation pressure, associated with a melting peak of about 190 °C. This melting peak is considered as the melting of hard segment micro-crystalline area.42 Before saturating process, Tm and ∆Hm are 188.9 °C and 1.24 J/g, respectively, followed by the increased value after the saturating process. They are 190.9 °C and 1.81 J/g, respectively. This result indicates that CO2 can facilitate the chains to move, improving the perfection of hard segment domain.43 Meanwhile, this phenomenon illustrates the crystalline area always exists in neat TPU matrix whether before or after saturating process. It has been reported that TPU's crystallization area has a significant contribution to improving the foaming ability, due to its heterogeneous nucleation effect.27 Consequently, we believe that within TPU matrix the homogeneous nucleation and the heterogeneous nucleation exist. 16


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3.3. Effect of saturation pressure on the cross-section morphology.

Figure 6. The cross-section of TPU thin films foamed at 120 °C with different saturation pressures: (a) 2.5 MPa; (b) 3.5 MPa; (c) 4.5 MPa. (d) the cell density and the average cell size of cellular TPU thin film under different saturation pressures.

The technique mentioned above to obtain the foamed thin film with different thicknesses is successful under 4.5 MPa saturation pressure. However, it is unclear if this technique can effectively form the cellular TPU thin film in a wide processing window. Based on this reason, the saturation pressure, here, has been changed from 4.5 MPa to 2.5 MPa. Accordingly, the morphology of the cross-section of the foamed TPU thin film under different saturation pressures was investigated by scanning electron microscopy (SEM). These SEM images shown in Figure 6 exhibit that these formed TPU foams possess well-defined cell structure, uniform cell distribution and 17



foamed film thickness of about 35 µm. Obviously, the unfoamed skin layer can not be found, indicating that two PI layers can still constrain the gas surface diffusion under very low saturation pressure. In comparison with the cellular TPU thin film saturated under 2.5 MPa, these under 3.5 MPa saturation pressure present smaller average cell size and larger cell density. Additionally, the corresponding morphological information based on Figures 6a-c was shown in Figure 6d. When the saturation pressure increases, the cell density increases from 5.8 × 108 cells/cm3 to 8.9 × 108 cells/cm3 and 1.1 × 109 cells/cm3, and the average cell size decreases from 11.5 µm to 9.7 µm and then to 8.0 µm, respectively. The cell density and the average cell size have obvious dependence on the saturation pressure. This is related to the fact that the saturation pressure stands for the amount of CO2 used to the cell nucleation, resulting in small cell size and large cell density under high saturation pressure.17,44 The expansion ratio and the average cell wall thickness also show a significant correlation with the saturation pressure. The expansion ratio is 1.8 under 2.5 MPa, and increases to 2.1 under 3.5 MPa, and then increases to 2.7 under 4.5 MPa. The average cell wall thickness decreases from 3.9 µm to 2.9 µm, and then decreased to 1.3 µm with the increased saturation pressure.

3.4. Effect of foaming temperature on the cross-section morphology.

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Figure 7. SEM images of the cross-section of the TPU foams saturated under 2.5 MPa with different foaming temperatures: (a) 90 °C; (b) 100 °C; (c) 110 °C. (d) the cell density and the average cell size of cellular TPU thin film under different foaming temperatures.

The foaming temperature is another effective factor to control the foam morphology, including cell size and cell density. Usually, high foaming temperature can effectively improve the foaming ability of the material,15,45,46 but the over normal foaming temperature could cause cell collapse or structural damage.21 Figure 7 shows the cross-section morphology of the foamed TPU film originated from different foaming temperatures, i.e., 90-110 °C, and the corresponding cell density and cell size according to these morphological images. As expected, along with the increased foaming temperature, the foaming ability of the material improves, meanwhile 19



foamed TPU films with different morphologies can be easily obtained. When the foaming temperature is 90 °C, few cells appear in Figure 7a. As the foaming temperature increases to 100 °C and then to 110 °C, the number of cells in the cross-section increases. But at 120 °C foaming temperature, the number of cells in the cross-section decreases due to the presence of large cell (Figure 6a). For example, from 90 °C to 120 °C, the cell density increases from 2.7 × 108 cells/cm3 to 7.1 × 108 cells/cm3, to 8.4 × 108 cells/cm3, and then decreases to 5.8 × 108 cells/cm3. Similarly, the expansion ratio increases from 1.1 to 1.6, to 2.0, and then decreases to 1.8. Although the foamed elastomer thin film is first reported in this work, the similar variation of cell density has been reported in the foaming process of poly(methyl methacrylate) (PMMA) film.17 It is well known that the low foaming temperature inhibits the nucleation of cells.47 Consequently, with the increased temperature from 90 °C to 110 °C, the improved nucleation ability of the material leads to larger cell density. But when the temperature increases, the strength of the cell wall in the process of cell formation decreases, leading to increased cell coalescence and then reduced cell density proved by the presence of huge cells.18,39 For example, as the foaming temperature increases, the size of the material's cells increases from 5.1 µm to 7.7 µm, to 9.4 µm, and then to 11.5 µm.

3.5. Thermal insulation performance and elasticity.

Table 1. The thermal conductivity of cellular TPU thin film under different foaming 20


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conditions. Samples TPU TPU-2.5 TPU-3.5 TPU-4.5 TPU-110

Thermal conductivity (W·(m·K)-1) 0.257 0.187 0.170 0.162 0.179

In order to investigate the thermal insulation performance of these as-prepared foamed films, the thermal conductivity of them was tested and then presented in Table 1. The samples prepared under 2.5 MPa saturation pressure and 120 °C foaming temperature were labelled as TPU-2.5. Following the same principle, the samples prepared under 3.5 MPa and 4.5 MPa saturation pressure were labelled as TPU-3.5 and TPU-4.5, respectively. As for TPU-110, it was prepared under 2.5 MPa saturation pressure and 110 °C foaming temperature. As expected, the thermal conductivity of the material is significantly reduced after introducing cells, down from 0.257 W·(m·K)-1 to 0.187 W·(m·K)-1, even to 0.162 W·(m·K)-1. This indicates that the thermal insulation property of TPU film is improved through foaming technique, resulting from the presence of cells within polymer matrix.10,48,49 Moreover, the thermal conductivity of foamed films decreases gradually from TPU-2.5 to TPU-4.5, although the variation is very small. That is 0.187 W·(m·K)-1 for TPU-2.5, 0.179 W·(m·K)-1 for TPU-110, 0.170 W·(m·K)-1 for TPU-3.5, and 0.162 W·(m·K)-1 for TPU-4.5. This phenomenon might be due to the increased expansion ratio and the disappearance of the surface defect.48,50 As shown in Figures 6a and 7c which are the SEM images of the TPU-2.5 and TPU-110, respectively, the large cell structure on the 21



surface of the material could lead to high thermal conductivity. TPU-4.5 has a higher expansion ratio relative to that of TPU-3.5 due to the high saturation pressure, resulting in a lower thermal conductivity.

Figure 8. Cyclic tensile curve of cellular TPU thin film.

The elastic property of the material is usually evaluated by the hysteresis and residual strain caused by the cyclic process.29 The low value represents the material has very good elastic recovery, conversely, that means the material in the deformation process causes obvious energy loss which is unfavorable to the material elasticity.31 In Figure 8, sample cases under different saturation pressures are chosen to implement the loading-unloading test under 100% strain, and the loading and unloading paths in the cycle are presented. Obviously, during the cycle, loading and unloading paths are not overlap, indicating that the hysteresis does generate during the cyclic deformation. 22


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Furthermore, it observed that the cellular TPU thin film with high expansion ratio has lower strength at 100% strain than those with more unfoamed region.

Figure 9.

(a) hysteresis and (b) residual strain of cellular TPU thin films prepared

under different foaming conditions.

Figure 9 compares the hysteresis and the residual strain of TPU film and foamed TPU film prepared under different foaming conditions after cyclic tensile test. It should be noted that all the foamed samples show better recovery proved by the reduction of hysteresis from 51.6% to 40.7% relative to the TPU film. These phenomena suggest the cellular structure is benefit for the improvement of elastic properties of TPU thin film, which could result from the positive effect of cells in the recovery process.51 In addition, from Figure 9a, it can be observed that the hysteresis from the TPU-2.5 to the TPU-4.5 presents a gradual reduction behavior, i.e., from 48.4% to 43.0%, to 41.2% and then to 40.7%, this may originate from the defect of the large cells within foamed material in the deformation process.52 Due to the same 23



reason, the residual strain decreases from 12.1% to 9.5%, to 8.7%, to 7.7%, and then to 6.9%. In conclusion, the cells improve material elasticity. 4. CONCLUSIONS In this study, we were challenging to fabricate the cellular TPU thin film with thickness of about 10-40 µm by a novel constrained surface diffusion foaming method using CO2 as the physical blowing agent. The sandwiched PI/TPU/PI films were prepared under thermal compression, and a strong interface bonding with peel strength of 29.6 N was generated between TPU and PI film. During the solid state foaming process, the PI films were used to reduce the gas escape, which ensured the nucleated bubbles grew steadily, leading to the formation of microcellular structure within the TPU thin film. The influences of cellular structure on the tensile elasticity and thermal insulation of TPU thin film were investigated in this study too. It showed that the presence of cellular structure facilitated the improvement of elasticity. Cellular TPU thin film had a lower thermal conductivity of 0.162 W·(m·K)-1 vs. 0.257 W·(m·K)-1 of the unfoamed counterpart, resulting in the foamed TPU thin film with the improved thermal insulation property.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. 24


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ACKNOWLEDGEMENTS The work was supported by National Natural Science Foundation of China (No. 51573202), Provincial Science and Technology Project of Guangdong Province (2016B090918013) and STS Project of Fujian Academy of Sciences (2016T3025).

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Table of Contents：：

Cellular Thermoplastic Polyurethane (TPU) Thin Film: Preparation, Elasticity, and Thermal Insulation Performance Chengbiao Ge †,‡ and Wentao Zhai *,†

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Cellular Thermoplastic Polyurethane Thin Film: Preparation, Elasticity

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