Mechanical Properties of Microcellular and Nanocellular

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Mechanical Properties of Microcellular and Nanocellular Thermoplastic Polyurethane Nanocomposite Foams Created using Supercritical Carbon Dioxide Shu-Kai Yeh, Yu-Che Liu, Chien-Chia Chu, Kung-Chin Chang, and Sea-Fue Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00942 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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Industrial & Engineering Chemistry Research

Mechanical Properties of Microcellular and Nanocellular Thermoplastic Polyurethane Nanocomposite Foams Created using Supercritical Carbon Dioxide Shu-Kai Yeh1*, Yu-Che Liu2, Chien-Chia Chu3, Kung-Chin Chang4 and Sea-Fue Wang5 1. Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan. 106 2. Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan, 106 3. Industrial Technology Research Institute., Institute of Materials Research, Hsinchu, Taiwan, 310 4. Taiwan Textile Research Institute, Tucheng, Taipei, Taiwan, 236 5. Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan, 106 Corresponding Author: Dr. Shu-Kai Yeh, [email protected] Abstract In this study, the mechanical properties of sub-microcellular or nanocellular thermoplastic polyurethane (TPU) nanocomposite foams were investigated via batch foaming using CO2 as the blowing agent. Cloisite® 30B nanoclay (clay 30B) was as the nucleation agent. Adding clay 30B and foaming at 60°C resulted in a nanocellular foam. A cell size of 450 nm and a cell density of 1011 cells/cm3 were obtained. The relative density of the foam was within the range 0.9–0.95. The modulus of the foamed samples was found to be proportional to their relative density regardless of their structure (microcellular/sub-microcellular). The results indicated that the 1

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modulus of the foamed samples with a cell size larger than 400 nm decreased with foam density. However, to our surprise, although adding only 1 wt% of nanoclay reduced the foam density, an increase in the modulus was observed. In addition, the cells became small and uniform. Keywords: foam, polyurethane, nanocomposites, mechanical properties, nanocellular Introduction Polymer foams could reduce the weight and thus lessen the cost of materials. In addition, the high strength- or modulus-to-density ratio and low thermal conductivity give foam versatile value-added properties. Typically, foam can be classified into three categories: conventional foam, microcellular foam, and nanocellular foam. Conventional foam is defined as having a cell size larger than 10 µm and a cell density less than 109 cells/cm3. Microcellular foam is defined as having a cell size less than 10 µm and a cell density higher than 109 cells/cm3.1 Nanocellular foam is defined as having a cell size of 10–1000 nm and a cell density between 1010 and 1014 cells/cm3.2 In a typical foaming process, the physical blowing agent is dissolved in the polymer using high pressure. Thermal instability is created by either pressure quenching or temperature quenching to trigger the cell nucleation process. Then the cells start to grow and/or coalesce until the polymer cools down below its glass transition temperature (Tg). Among the different blowing agents, CO2 seems to be a promising material that helps create a fine cell structure because of its high diffusivity in most polymers. MuCell® processes are well known to produce microcellular foam by injection molding using CO2 as the blowing agent. Nevertheless, it is very challenging to generate nanocellular structures whose cell size is less than 1 µm. By 2


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nature, it is difficult to produce nanocellular foam because it needs an ultrahigh cell nucleation rate and fast cooling to prevent cell coalescence. One of the earliest studies of nanocellular polymeric foam was conducted by Krause et al.3-6 It was found that foaming polymers film of high Tg such as polyetherimide (PEI) and polyethersulfone (PES) using CO2 could result in a nanocellular structure with cell sizes as low as 200 nm. Yokoyama et al. generated a nanocellular monolith film by grafting CO2-philic polymer blocks of low Tg together with polystyrene blocks.7-9 The average cell size was approximately 20–30 nm and the cell density was approximately 9 × 1010 cells/cm2. Nevertheless, the mechanical properties of the nanoporous thin films were seldom reported with the exception of reference.8 Ohshima et al. studied the possibility of foaming polypropylene (PP) / rubber or elastomer, polystyrene (PS) / polymethyl methacrylate (PMMA), and polyether ether ketone (PEEK) / PEI blends to generate nanocellular structures. The critical issue was to disperse the rubber, or PMMA, or PEI domain in nanometer scale, and the foaming phenomena was confined to the dispersed phase. In this way, nanocellular foam was obtained by batch foaming.10-15 In 2009, a patent application proposed by Thiagarajan et al. from SABIC Innovative Plastics reported for the first time the physical limits of nanocellular foams using theory and equations.2 Forest et al. applied appropriate heat transfer models and found that the thermal conductivity of nanocellular foams reduces significantly when the cell size is less than 100 nm.16 Costeux et al. published a series of reports on the fabrication of nanocellular structures.17-21 These reports encouraged the foam research community. Researchers 3



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keep continue to seeking ways of creating nanocellular structures. Recently, Costeux published a review article that summarized the current research progress of CO2-blown nanocellular foam.22 According to Costeux, most of the research in this context was focused on creating nanofoams using a batch foaming method, except for the work of Costeux and Foether 21 and that of Sandler and coworkers.23 In their patent, Sandler and coworkers provided detailed process variables and experimental data.23 Okolieocha also reviewed the applications of mass-scale production methods such as injection molding and extrusion for nanocellular foams.24 Both one-step and two-step foaming methods are applied for creating nanocellular foam. In comparison to two-step foaming, one-step foaming saturated the polymer in a pressure vessel above its Tg. When the CO2 pressure is removed, the polymer is immediately foamed. Nevertheless, the foaming temperature cannot be precisely controlled. Also, the temperature of the pressure vessel is significantly reduced due to the Joule–Thomson effect.25 Most studies have focused on generating nanocellular structures. However, the mechanical properties of nanocellular foam have been difficult to determine since nanocellular foam is mostly fabricated in the form of pellets or thin films. Nevertheless, some researchers have made efforts in this context. Miller et al.26, 27 and Notario et al.28 investigated the mechanical properties of PEI and PMMA nanocellular foam and found that nanocellular foam possessed a much higher impact strength and toughness than microcellular foam of similar or lower density. In this study, TPU, a typical block copolymer, was chosen for foaming. Because of its excellent engineering characteristics such as anti-abrasion, outstanding low-temperature performance, high elasticity, and transparency, it has many 4


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applications in automobiles, footwear, sporting goods, medical devices, and electronics.29 Like other block copolymers, TPU combines the excellent characteristics of soft segments and hard segments, and it is interesting to note that the solubility of CO2 in the soft segments is significantly higher than in the hard segments.30 In addition, the CO–N group present in the structure may increase the free volume and thus the solubility of CO2.31 This characteristic may provide a route for creating nanocellular foams. However, the diversified choices of soft and hard segments make it difficult to systematically study TPU foam. At the moment, there are few studies on foaming TPU using CO2,30, 32-46 and only two references mention submicron or nanocellular foam.34, 44 To create a nanocellular structure with a high cell density, it is critical to increase the cell nucleation rate and solidify the cell structure at the early stage of the foaming process. Using a nucleation agent could be helpful in achieving such a high cell density and low cell size. Nano-sized filler particles are preferred in creating nanocellular foam. Because micro-filler particles are larger than the diameter of the cells, it could tear them up. In addition, well-dispersed nanoparticles would improve the strength of the polymer and compensate for the loss of strength due to foaming. However, it is usually very challenging to disperse nanoparticles well in the polymer matrix. In order to investigate the effect of nanoparticles, nanoclay was used as nucleation agents, and we investigated the microstructure and mechanical properties of the resulting foam. Experiments TPU pellets, Kuotane 320, provided by Kuo Ching Chemical Co. Ltd., is a polyester-based TPU with a Shore A hardness of 85. Nanoclay, Cloisite® 30B, was 5



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purchased from Southern Clay Products Inc. The nanoparticles and TPU were pre-dried in a vacuum oven at 80°C for 8 h to remove moisture and then compounded using a Thermo Haake MiniLab II conical twin-screw micro-compounder. The screw speed and temperature were fixed at 80 rpm (1.3 s−1) and 190°C, respectively. It was found that a screw speed higher than 100 rpm would cause severe degradation and the TPU would darken. Therefore, the screw speed was set at 80 rpm to prevent degradation during compounding. After five minutes compounding, the extruded polymers were quenched in ice water at the exit to prevent thermal degradation. Neat TPU was also compounded before foaming or mechanical testing in order to duplicate the process history. Since both TPU and clay are hydrophilic, the compounded pellets were again dried in a vacuum oven at 80°C for 8 h before foaming or injection molding to remove moisture. The drying time was determined using a Shimadzu MOC63u moisture analyzer. In particular, the TPU/clay 30B nanocomposite pellets were collected and injection molded into a tensile testing specimen (ISO 527 Type 5A) using a Thermo Haake MiniJet II micro-injection molding machine. The density of the TPU was 1.2 g/cm3. A broad melting peak was observed between 100 and 150°C. Our samples were injection molded at a barrel temperature of 190°C. The injection pressure was 45 MPa and the holding pressure was set at 20 MPa. All samples were allowed to settle for at least 48 h before testing. The mechanical properties of the TPU and TPU nanocomposites were tested using a Tinius Olsen H5K5 universal testing machine equipped with pneumatic grips and rubber grip pads to prevent sample slippage. The pressure of the grip was set at 6 kg/cm2. The rate of testing was set at 50 mm/min. The strain was calculated by the displacement of the grip from the original gauge length of 6


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44 mm. At least five samples were tested for each experiment. The morphology of samples was characterized using transmission electron microscopy (TEM, Hitachi H-7100). TEM samples were prepared using a Leica cryo-microtome. To determine the solubility of CO2 in TPU, the samples were immersed in CO2 at 40°C for 3 to 72 h. After the pressure release, the samples were carefully weighed using a microbalance to estimate the solubility of CO2 in the polymers. It was found that the TPU pellets could reach equilibrium CO2 content within 6 h. The detailed information is given elsewhere.34 Two different methods of obtaining foamed samples were used. In the first method, the compounded polymer pellets were immersed in CO2 in high-pressure inline filters (Swagelok SS-2F-05) for foaming. The length and diameter of the filters were 40 mm and 14.3 mm, respectively, and the thickness of the filter wall was 2 mm. The pressure was set at 13.79 MPa (2000 psi) for 6 h and the temperature was varied from 60–80 °C. The samples were foamed right after the pressure was released (one-step foaming). The density of the foam was determined by the Archimedes’ principle. Since CO2 may diffuse through foam and thus decrease the volume of the foam, the samples were allowed to settle for 48 h before measuring the density. In the second method, the injection-molded mechanical testing specimen of TPU nanocomposite foam was made by putting two tensile testing samples in a mold (see Supplementary Information Figure S1.) The depth of the mold was 4 mm. Metal sheets of different thicknesses were inserted into the mold to control the thickness or relative density of the foamed samples. The relative density refers to the density of the foam divided by that of the solid. In our case, a metal sheet 1.72 mm thick was used to set the relative density of the foam to approximately 0.9. It is worth noting that the 7



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samples grew in only one direction during the foaming process instead of growing unrestrictedly. The samples were inserted into the mold and immersed in a high-pressure thermostat chamber supplied by Jeoou Rong Industrial Co. Ltd. (Kaohsiung, Taiwan) for batch foaming. The volume of the foaming vessel was 1 L. Again, the saturation pressure was fixed at 13.79 MPa. Samples were saturated at 50, 60, 70, or 80°C. After 24 h of immersion, the samples were foamed by fast pressure release. Theoretically, there are an infinite number of combinations of polyol and isocyanates for TPU. This makes it challenging to determine the time required to reach CO2 equilibrium. Although TPU membranes have been widely used in gas separation and the permeability (diffusivity × solubility) of CO2 through TPU films has been extensively studied, it was found that the permeability of CO2 in different TPU membranes is quite different.47-53 Many factors such as the type and molecular weight of soft segments, the ratio of different soft segments, and the type of hard segments affect the permeability of CO2 in TPU membranes. Therefore, it is challenging to estimate the time required to reach CO2 saturation in TPU specimens. Based on the limited information, the time required to reach equilibrium in this study was estimated. The diffusion coefficient of CO2 in TPU was estimated to be 10−6 cm2/s at 31.1°C, 30

and we assumed that CO2 would diffuse through a slab 2.05 mm thick. The

saturation time was calculated via the one-dimensional diffusion model and the time of saturation was approximately 2.1 h.54 Thus, 24 h is more than enough for CO2 saturation. Because CO2 diffused out of the sample and caused foam shrinkage, the density of the foam changed slightly. The foamed samples were allowed to settle for 8


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48 h before measuring the mechanical properties and density. The density of the foam was determined by the Archimedes’ principle. The morphology of the foam was characterized using a Hitachi S-3000H scanning electronic microscope (SEM). Samples were immersed in liquid nitrogen for 3 min and cryo-fractured for observation. The average cell size was estimated by Image J software. The cell density was estimated using the following equation:55 ܰ௙ = (

௡ெ మ ଷ/ଶ ) ஺

ఘ೛

×( )

(1)

ఘ೑

where Nf is the cell density (cell/cm3), N is the number of cells in the SEM image, M is the magnification, A is the actual measuring area of the SEM image (cm2), ߩ௣ is the density of the solid, and ߩ௙ is the density of the foam. Results and Discussion The TPU nanocomposites were foamed at 70°C and 13.79 MPa using a one-step method. SEM images of the foams and the cell size distributions are shown in Figure 1. It was found that adding 1 wt% clay 30B resulted in the finest cell size and the highest cell density among the samples, and average cell size was as low as 920 nm. Standard deviation of cell size is a good index of cell size uniformity. The average cell sizes of TPU with and without nanoclay were 0.92±0.25 µm and 1.15±0.47 µm, respectively. Adding 1 wt% clay 30B reduced the cell size standard deviation from 0.47 µm to 0.25 µm. The uniform cell size distribution and fine cell size could be an indication of good nanoparticle dispersion because several studies showed that good dispersion of nanoparticles led to fine and uniform cell size distribution.56 By nature, clay 30B is compatible with TPU and could be exfoliated by compounding.57 On the other hand, the cell density increased from 1.46 × 1011 cells/cm3 to 2.94 × 1011 9



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cells/cm3. It was doubled in the presence of only 1 wt% clay 30B. The foam density of TPU with and without clay 30B is 0.94 and 0.89 g/cm3, respectively.

(a) Neat TPU

(b) TPU-1wt%Clay 30B

Figure 1. Cell morphology and size distribution of TPU pellets with and without nanoclay foamed at 70°C and 13.79 MPa. The TEM images also confirmed the good dispersion of clay 30B in TPU. TEM images of samples containing 1, 3, and 5 wt% of clay 30B are shown in Figures 2 (a), (b), Figures 2 (c), (d) and Figures 2 (e), (f), respectively. Nanoclay platelets were intercalated into the polymer matrix and exfoliated single platelets were observed. Adding well-dispersed clay 30B in TPU thus created submicron or nanocellular structures.

10


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 2. TEM pictures of TPU containing (a, b) 1, (c, d) 3, and (e, f) 5 wt% clay 30B. The magnification of (b), (d), and (f) is 100,000×. The results of TPU nanocomposites foamed at various temperatures are listed in Table 1. As can be seen from Table 1, the foaming temperature was very effective in decreasing the cell size (see Supplementary Information Figure S2). The addition of 1 wt% clay 30B also helped decrease the cell size. Adding 1 wt% clay 30B decreased the cell size by at least 12%. Also the standard deviation of cell size decreased significantly in the presence of clay 30B. However, the cell-size reduction effect was not apparent when the clay loading level was above 1 wt%. This could be because the 11



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clay 30B platelets were not dispersed well when the clay content was higher than 1 wt%. The lowest cell size that could be reached in the batch foaming experiments was approximately 450 nm when the TPU nanocomposites were saturated and foamed at 13.79 MPa and 50°C. Table 1. Cell size, cell density, and relative density of TPU clay 30B nanocomposite foamed by one-step foaming. The foaming conditions were 13.79 MPa and various temperatures. The numbers in the parentheses are standard deviations. Temperature (°C)

Clay content

Cell size Cell density (cells/cm3)

Relative density

(wt%)

(µm)

0

2.06 (0.74)

7.84 × 1010

0.58

1

1.83 (0.64)

1.16 × 1011

0.61

3

1.80 (0.58)

1.45 × 1011

0.6

5

1.62 (0.53)

1.76 × 1011

0.55

0

1.15 (0.47)

1.46 × 1011

0.74

1

0.92 (0.25)

2.94 × 1011

0.78

3

1.05 (0.27)

3.04 × 1011

0.76

5

1.01 (0.28)

3.49 × 1011

0.74

0

1.00 (0.37)

1.69 × 1011

0.83

1

0.75 (0.02)

2.39 × 1011

0.88

3

0.64 (0.15)

4.44 × 1011

0.81

5

0.63 (0.22)

5.49 × 1011

0.76

0

0.58 (0.38)

2.39 × 1010

0.97

80

70

60

50

12


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1

0.45 (0.11)

1.08 × 1011

0.96

3

0.45 (0.13)

1.79 × 1011

0.94

5

0.45 (0.12)

1.06 × 1011

0.94

The cell morphology of the foam is shown in Figure 3, and the cell density of the foam was on the order of 1010 to 1011 cells/cm3. However, the effect of foaming temperature on the cell density is not clear. Adding 1 wt% clay 30B increased the cell density significantly, but the effect of clay 30B on the cell density was not apparent when the loading level was above 1 wt% (also, see Supplementary Information Figure S3). As expected, the relative density of TPU decreased with the foaming temperature because the cell density remained of the order of 1010 to 1011 cells/cm3 and the cell size decreased with increasing foaming temperature. A previous study on PS nanocomposite foam showed that the cell density increased with decreasing foaming temperature because the high viscosity of the polymer matrix prevented cell growth and cell coalescence.58 Similar trends were not observed in this study. The reason could be complicated. As compared to amorphous PS, TPU is a semi-crystallized polymer. Crystallized soft or hard segments may either act as nucleating agent or retard the cell growth.59 Another interesting question is whether an increase in the blowing agent solubility affects the cell density and thus decreases the density of TPU foams. Note that the solubility of CO2 in Kuotane 320 TPU at 40˚C is around 12–14 wt%.34 In some polymers such as PEI and polysulfone, it is possible to generate a nanocellular foam with a cell density of 1012–1015 cells/cm3.26, 60 In this study TPU foamed right after the pressure release, thus making it challenging to measure the solubility. 13



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However, it is well-known that the gas solubility in polymers is temperature dependent and follows an Arrhenius equation.61 The CO2 solubility in polymers decreases exponentially with the increasing temperature. Therefore, the CO2 solubility in TPU at 50, 60, 70, and 80˚C is expected to be considerably lower than 12–14 wt% and an increase in the cell density may not be achieved. On the other hand, in our previous study, TPU was saturated at 40˚C and foamed at 150˚C for only 5 s (two-step foaming process). The results showed that the cell density was of the order of 1011 cells/cm3.34 It is known that CO2 may diffuse out of a sample during two-step foaming.22 To avoid this, TPU and the TPU/clay 30B nanocomposite pellets were foamed at 27.58 MPa (4000 psi) and 50–80˚C by one-step foaming. Nevertheless, the cell density was still of the order of 1011 cells/cm3. These results can be found elsewhere.62 Even though the previous studies used different TPUs, all of them reported the upper limit of the cell density of TPU foam to be around 1011 cells/cm3.30, 38, 45, 46 We also obtained similar cell density in our follow up studies.59, 63 We believe that the hard and soft segments play important roles in the nucleation. However, this needs further investigation.

(a)

(b)

14


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(c) (d) Figure 3. Cell morphologies of TPU nanocomposite foam reinforced with (a) 0, (b) 1, (c) 3, and (d) 5 wt% of clay 30B. Samples were saturated and foamed at 13.79 MPa and 50˚C. The role of nanoclay as a nucleation agent is also very interesting. In this study, a slight increase was observed in the cell density. An increase of less than five times was observed when 1 wt% clay 30B was used. Similar increase was observed for a PS/exfoliated clay system 64. However, in the same study, a PMMA/intercalated clay system showed an increase of almost 100 times in its cell density. Thus, the increase in the cell density depends not only on the type of nanoparticles and their surface modification but also on the polymer matrix. Based on the assumption that one particle nucleates one cell, the potential nucleation density of 1 wt% exfoliated clay can be estimated using equation (2):65 ே௨௖௟௘௔௡௧௦ ௖௠య

=

௪ ఘ೎೚೘

ఘ೛ ௏೛

(2)

where w is the weight fraction of clay 30B in the composites and ߩ௣ is the density of the nanoclay, which was estimated to be 2.83 g/cm3.66 ߩ௖௢௠ is the density of the composite. Vp is the volume of individual exfoliated clay platelets. The clay platelets were assumed to be disks with a diameter of 150 nm and a thickness of 1 nm. The estimated potential nucleation density was 1.36 × 1014 cells / cm3. Thus, the 15



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nucleating efficiency of clay 30B was low as compared to that of TPU. Increasing the clay loading level can increase the cell density. However, clay dispersion is an important issue. In addition, since CO2 does not dissolve in clay, the solubility of CO2 decreased with the clay content.34 The decrease in the CO2 solubility might negate the effect of increasing the clay content. Therefore, the cell density showed only a slight increase with an increase in the clay content. It can be concluded from the above results that a low foaming temperature and 1 wt% nanoclay helps decrease the cell size to less than 500 nm. Adding more clay 30B does not further decrease the cell size. Similar results have not been reported in the literature. The above study was based on one foamed pellet. Only the cell morphology is reported, but the physical properties of the foam are unknown. To prove that the nanocomposite is not only useful in improving the cell morphology, the mechanical properties of the foam were investigated. It is known that adding nanoparticles significantly increases the mechanical properties of the polymer matrix. The Young’s modulus, yield strength, tensile strength, and elongation at break of unfoamed TPU are listed in Table 2. The modulus and yield strength increased with increasing clay content. When the clay 30B content increased from 0 to 5 wt%, the Young’s modulus increased from 28.89 MPa to 35.24 MPa and the yield strength increased from 2.27 MPa to 2.87 MPa. It is apparent, therefore, that adding clay 30B reinforced the mechanical properties of TPU. On the other hand, the elongation at break decreased from 768% to 495% and the maximum strength decreased from 26.27 MPa to 11.84 MPa at 5 wt% clay loading.

16


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The density of the foamed TPU dumbbell-shaped sample ranged from 1.09 to 1.18 g/cm3, which translates into a relative density of 0.91 to 0.97 because the density of solid TPU is 1.2 g/cm3. Only samples foamed at 60–80°C were collected in the foaming experiment. Samples foamed at 50°C were unable to fill the mold and were twisted. Also, because increasing the clay loading above 1 wt% was not helpful in decreasing the cell size, the clay 30B loading level was fixed at 1 wt% in the dumbbell-shaped sample foaming experiment. The sample was cryo-fractured and the cell morphology was observed using SEM (see Supplementary Information Figure S4). Most of the cells remained spherical and cells less than 1 µm in size were observed. The results indicated that the cells were not squeezed or collapsed during foaming. Again, adding nanoclay 30B reduced the cell size by at least 8%. It was found that an average cell size less than 1 µm was observed when samples were saturated and foamed at 60°C. The smallest average cell size obtained was 650 nm. The critical foaming temperature to create nanocellular foam is consistent with the pellet foaming results. However, the cell density of the dumbbell-shaped foam was on the order of 1010 cells/cm3, which is only one-tenth that of the pellet foam (see Supplementary Information Table S1). It is suspected that some cells collapsed during foaming because the foam was confined in the mold and grew one-dimensionally. Table 2. Mechanical properties of TPU reinforced with different amounts of clay 30B. The numbers in the parentheses are standard deviations.

TPU

Young’s modulus

Maximum strength

Yield strength

Elongation at

(MPa)

(MPa)

(MPa)

break (%)

20.89 (1.07)

26.27 (1.73)

2.27 (0.04)

768 (10)

17



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TPU-1% clay

23.23 (2.40)

22.81 (0.78)

2.38 (0.18)

740 (22)

TPU-3% clay

30.33 (1.24)

17.35 (0.96)

2.81 (0.12)

637 (45)

TPU-5% clay

35.24 (5.23)

11.84 (0.77)

2.87 (0.43)

495 (49)

The mechanical properties of the dumbbell-shaped nanocomposite foam characterized (see Supplementary Information Table S2). In general, the TPU-clay 30B nanocomposites showed higher Young’s moduli and yield strengths than neat TPU, but the maximum strength and elongation at break tended to decrease in the presence of the nanoclay. According to the theory proposed by Gibson and Ashby, the Young’s modulus of a closed cell foam can be obtained from equation (3).67 ா∗ ாೞ

ఘ∗ ଶ

ఘ∗

≈ ߶ ଶ ቀ ቁ + (1 − ߶)( ) ఘೞ

(3)

ఘೞ

where E* is the modulus of the foam and Es is the modulus of the solid. ρ* and ρs are the density of the foam and solid, respectively. ߶ is the fraction of the foam struts, satisfying the following condition. ߩ∗ ( )≤߶≤1 ߩ௦ The tensile modulus and yield strength of the foamed and unfoamed TPU reinforced with and without 1 wt% clay 30B was plotted as a function of the foam density, and the results are shown in Figures 4(a) and 4(b), respectively. Only the data points marked by the circle represented solid material. Assuming ߶ to be 0.95, equation (3) was also plotted in Figure 4(a). As can be seen from Figure 4(a), the modulus of the foam increased with an increase in the density. However, the experimental value of the foam modulus was lower than that predicted by equation (3). This is not surprising. Jo et al. and Chen et al. reported similar observations for open cell 68 and closed cell foams, respectively.69 Jo et al. developed a new model for 18


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PMMA open cell foams 68. Chen et al. modified the model for PMMA nanocomposite closed cell foams using the following equations:69 ா∗ ாೞ

=ቀ

ଷ஺మೝ ିଶ஺యೝ

ଵି(ଵି஺ೝ

)య

ቁ ‫ܣ‬ସ௥ + ߚ(1 −

ଷ஺మೝ ିଶ஺యೝ

ଵି(ଵି஺ೝ )య

)

஺ೝ ଶ

భ

‫ܣ‬௥ = 1 − (1 − ߩ௥ )య

(4) (5)

where β can be calculated from the experimental data. By substituting the experimental values of ρr (0.9468) and E*/Es (0.5294), β was calculated to be 4.82. The results of Chen’s model are shown in Figure 4(a). It can be observed that the value of modulus obtained by us was higher than that predicted by Chen’s model because Chen’s model tends to underestimate the modulus at relative densities higher than 0.5.69 The modulus of our nanocomposite foam was between the values predicted by Gibson & Ashby’s and Chen’s models. It is very challenging to predict the foam modulus at high relative densities. High-density foams can have inter-cell distances or cell wall thicknesses comparable to or even higher than the cell size with a cell edge thickness of the order of the edge length (see Supplementary Information Figure S4). This violates the basic assumptions of the model.69 Our goal was not to develop a model which can predict the modulus of high density TPU nanocomposite foams. However, we have tried our best to explain the data based on the available models. Similar problems were encountered in measuring the yield strength of the TPU nanocomposite foam. The tensile yield strength of TPU and the TPU nanocomposite foam is shown in Figure 4(b). The yield strength predicted by Gibson and Ashby’s70 and Chen’s models69 were significantly lower than the experimental value because both the models assumed the relative foam density to be less than 0.7. The yield strength predicted by these models is not plotted in Figure 4(b) to avoid confusion. It is difficult to produce low-density foams for mechanical testing. Foam shrinkage 19



became more apparent and resulted in severe deformation when the samples were foamed one-dimensionally in a mold. It is interesting to note that the modulus and yield strength of the solid TPU nanocomposite are higher than those of the neat TPU. This is expected because the density of nanoclay is 2.83 g/cm3, which is more than twice that of TPU.66 Adding nanoclay increases the modulus of the composite but the density of the composite also increases. After foaming, the results were very different. The density of the foamed TPU nanocomposite was lower than that of the neat TPU. Based on the theory mentioned above, the modulus and yield strength of the foam should increase with increasing density. Nevertheless, in this case, the TPU nanocomposite foam exhibited a lower density but a higher modulus than the neat TPU. This is because 1 wt% exfoliated nanoclay not only acted as the nucleation agent during foaming, it also reinforced the mechanical properties of the nanocomposite foam. 28

24

Modulus (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

20

Neat TPU 1wt%-Clay 30B G&A Neat TPU G&A 1 wt%-Clay 30B Chen Neat TPU Chen 1 wt%-Clay 30B

Solid

16

12

8 1.08

1.1

1.12

1.14 1.16 Density (g/cm3)

1.18

1.2

1.22

(a) 20


Page 21 of 31

3

Solid

Neat TPU 1wt%-Clay30B Yield Strength (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

1

0 1.08

1.1

1.12

1.14 1.16 Density (g/cm3)

1.18

1.2

1.22

(b) Figure 4. (a) Modulus and (b) yield strength versus density of TPU and TPU nanocomposite foam. A similar modulus-reinforcing effect was reported by Fu and Naguib in PMMA-clay nanocomposite foam 71. However, the average cell size of the nanocomposite foam was not reported. The cell size of neat PMMA ranges from 8 to 12 µm and the cell density is on the order of 108 cells/cm3.72 The findings in this report prove that a similar reinforcing phenomenon not only happened in the microcellular foam but also that it occurred for cell sizes less than 1 µm. There are very few studies on the mechanical properties of submicron or nanocellular foams.27, 28

This study provides further information on the mechanical properties of

nanocellular foam, and the reinforcing effect of nanoclay was clearly observed. Supporting Information 21



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Page 22 of 31

The supporting information contains the figures of the foaming mold, cell size and cell density of TPU nanocomposite pellets and the morphology of dumbbell shaped TPU nanocomposites. The cell size, cell density and mechanical properties of dumbbell shaped TPU nanocomposite foam are listed in Tables. Conclusion Clay 30B, was compounded with TPU and foamed by batch foaming using CO2 as the blowing agent. The TEM images of the TPU clay 30B nanocomposites confirmed that clay 30B platelets were exfoliated in TPU. An average cell size of 450 nm was achieved by lowering the foaming temperature. The TPU nanocomposites were also injection molded into dumbbell-shaped sample and foamed one dimensionally by batch foaming. Again, nanocellular foam was obtained and the smallest average cell obtained was 650 nm. A comparison of the mechanical properties of foamed neat TPU and TPU nanocomposites showed that TPU nanocomposites possessed lower density but a higher modulus and yield strength. Nanoclay not only acts as a nucleation agent but also reinforces the mechanical properties of the foam. The cell uniformity also improved significantly by adding clay 30B, which was confirmed by the reduction in the standard deviation of cell size. The modulus and yield strength of the TPU nanocomposite foam decreased with the foam density and the foam modulus could be explained with the help of different models. Acknowledgements The authors were partly funded by Taiwan Textile Research Institute, Ministry of Economics, Taiwan, and partly funded by contract number MOST 102-2221-E-011-164-MY3 from the Ministry of Science and Technology, Taiwan, R.O.C. 22


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Mechanical Properties of Microcellular and Nanocellular

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