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thermoplastic polyurethane blending foams in the presence of supercritical N2
European Polymer Journal 116 (2019) 291–301
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Cellular morphology evolution in nanocellular poly (lactic acid)/ thermoplastic polyurethane blending foams in the presence of supercritical N2 ⁎
Zhongjie Qua,b, Dexian Yina,b, Hongfu Zhoua,b, , Xiangdong Wanga,b, Shan Zhaoc,d,
T
⁎⁎
a
School of Materials and Mechanical Engineering, Beijing Technology and Business University, 11 Fucheng Road, Beijing 100048, PR China Beijing Key Laboratory of Quality Evaluation Technology for Hygiene and Safety of Plastics, Beijing 100048, PR China c Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China d University of Chinese Academy of Sciences, Beijing 100085, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Poly (lactic acid) Thermoplastic polyurethane Nanofoams Complex cellular structure Supercritical N2
Currently, the formation of nanocells and complex cellular structure (CCS) in semi-crystalline polymer using supercritical fluids as blowing agent had become a big and newly developing challenge worldwide. In this paper, a facile methodology of polymer blending and batch foaming was proposed to fabricate CCS with large cells in micro-size and small cells in nano-size in semi-crystalline poly (lactic acid) (PLA) foams. Thermoplastic polyurethane (TPU) was introduced into PLA through melt mixing method to improve the viscoelasticity and foaming behaviors of PLA. Differential scanning calorimetry results showed that with the addition of TPU, the cold crystallization temperature of various PLA samples increased and their crystallinity kept unchanged almost. Compared with those of pure PLA, the complex viscosity, and storage modulus of PLA/TPU blends increased. Interestingly, with the increasing TPU content, the CCS appeared and became distinct in PLA/TPU blending foams as well as the thermal conductivity of various PLA foams decreased, respectively. The formation mechanism of CCS in nanocellular PLA/TPU blending foams was proposed and explained by schematic diagram. With the foaming temperature rising from 135 to 140 °C, a transition from nano-cells to micro-cells was also observed in PLA/TPU 5 blending foam, due to the decreased melt viscosity.
1. Introduction Recently, nanocellular polymer foams as a new class of frontier materials have been attracting a great amount of attention and interest in both industrial and academic fields [1–3]. Because of its cell size in the range of nanometers and the cell density greater than 1012 cells/ cm3, nanocellular polymer foams may offer some unique properties that are superior to common polymer foams, such as higher toughness, lower thermal conductivity and excellent electrical properties [3–5]. Therefore, nanocellular polymer foams could be used in numerous high value-added applications in the fields of membranes, sensors, catalysis, electromagnetic shielding, tissue engineering, and insulation materials [6–8]. Among numerous methods for preparing polymer nanofoams [9–14], batch foaming in autoclaves using supercritical fluids as a blowing agent has been proved a good one to fabricate nanocellular
polymer foams efficiently due to its high controllability. Many efforts have been made to optimize this batch foaming process for preparing nanocellular polymer foams with well cellular morphology and foaming properties, such as chain extension, nano-filling and induced crystallization [15–17]. The transition from microcellular to nanocellular poly (lactic acid) (PLA) foams by controlling viscosity, branching and crystallization in the presence of supercritical N2 was observed. The study showed that the combined branching and nucleation effect in coagentmodified PLA was helpful for generating nanocellular foams at temperatures close to its crystallization temperature [15]. PLA/organically modified layered silicate nanocomposites (PLACNs) were foamed by supercritical CO2. The results showed that PLACNs foams possessed high cell density and small cell size from microcellular to nanocellular, which were prepared at low foaming temperature and high CO2 pressure [16]. In our previous research, isothermal crystallization-induced method was employed to fabricate the nanocellular PLA foams using
⁎
Corresponding author at: School of Materials and Mechanical Engineering, Beijing Technology and Business University, 11 Fucheng Road, Beijing 100048, PR China. ⁎⁎ Corresponding author at: Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China. E-mail addresses: [email protected] (H. Zhou), [email protected] (S. Zhao). https://doi.org/10.1016/j.eurpolymj.2019.03.046 Received 29 December 2018; Received in revised form 21 March 2019; Accepted 22 March 2019 Available online 10 April 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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supercritical CO2 as a blowing agent [17]. The cell density and volume expansion ratio (VER) of nanocellular PLA foams were increased by the addition of hydroxyl-functionalized graphene and chain extension reaction, respectively. In recent years, the formation of complex cellular structure (CCS) in polymer foams has developed a research hotspot gradually, because it has the advantages of containing both large cells and small cells [18,19]. The large cells were mainly used to decrease the bulk density and the small cells were employed to improve the mechanical and thermal insulation properties [20]. From the aforementioned statement, it may be an interesting and challenging task to prepare polymer nanofoams with CCS, in which the large cells were in micro-size and small cells were in nano-size. Especially in semi-crystalline polymer, this unique cellular structure may be much difficult to obtain due to two reasons. One was that the crystalline region formed during the depressurization and cooling period would influence the cell nucleation and cell growth [15]. The other was that the formed crystalline region may impede the solubility and diffusion of blowing agent during foaming process [10]. So far as we know, no investigation was reported about this novel structure in semi-crystalline polymer foams and the facile methodology proposed in this paper was expected to fill this vacancy. It was worthy to mention that some literatures had reported the foaming behaviors of PLA/thermoplastic polyurethane (TPU) blends and their cellular structure evolution. The crystallization behavior and melt strength of PLA were improved by the addition of TPU, and TPU acted as a nucleating agent to decrease remarkably the cell size as well as to increase the cell density and VER of PLA/TPU foams [21]. Microcellular PLA/TPU foams were fabricated using supercritical CO2assisted batch foaming process due to the enhanced cell nucleation by the presence of the interfaces between PLA and TPU [22]. Biocompatible shape memory polymer blending foams composed of TPU and PLA were prepared by Song et al. [23] and the cells were able to contract uniformly when this TPU/PLA blending foams were recovered with heat. Barmouz et al. [24] studied the microcellular foaming process of PLA/TPU blends and the results showed that combination of lower gas pressure and higher foaming temperature was helpful for obtaining foams with higher VER. Although some studies have reported PLA/TPU blending foams, very few literatures focused on nanocellular PLA/TPU blending foams with CCS. In this study, PLA/TPU blends were foamed to fabricate the unique cellular structure stated above using N2 as a physical blowing agent. The introduction of TPU into PLA matrix may offer some obvious merits. First, the melt strength of PLA may be improved by the addition of TPU. Secondly, the interfaces between PLA and TPU due to their immiscibility could be served as cell nucleation points to increase cell density. Thirdly, the difference of both N2 affinity and melt viscosity in PLA and TPU may be conducive to the generation of CCS. The thermal properties, rheological properties, and foaming performance of pure PLA and PLA/TPU blends were investigated systematically. Meanwhile, the effect of blending ratio, foaming temperature on the cellular structural evolution of pure PLA foam and PLA/TPU foams was studied in detail. Finally, the possible formation schematic of CCS in PLA/TPU blending foams was presented and explained.
Table 1 Experimental formula of various PLA samples (pure PLA and PLA/TPU blends). Sample Name
PLA (wt.%)
TPU (wt.%)
Pure PLA PLA/TPU PLA/TPU PLA/TPU PLA/TPU
100 95 90 85 80
0 5 10 15 20
5 10 15 20
2.2. Preparation of various PLA samples Prior to mixing, PLA and TPU were placed in a drying oven at 80 °C for 12 h to remove the moisture. According to the formula shown in Table 1, PLA and TPU were blended in a Haake internal mixer at 180 °C, with the rotor speed of 60 rpm for 15 min. Subsequently, the resultant samples were pressed into sheets by compression molding method at 180 °C and 10 MPa for 8 min and then cooled to room temperature to obtain the sheet samples with the thickness of 1 mm. According to the preparation method stated above, at least three samples in each category of various PLA samples were prepared and then the small sheet with 1 cm × 1 cm (length × width) was cut in the center of each sample for further characterization and foaming process. 2.3. Foaming process of pure PLA and PLA/TPU blends Various PLA foams were prepared by the batch foaming method in a stainless steel autoclave with 250 mL in volume (a detailed schematic was described in our previous literature [17]) using supercritical N2 as a blowing agent. First, the autoclave was heated to the saturation temperature of 180 °C. Then, various PLA sheet samples were placed and saturated with N2 at 180 °C at 15 MPa for 30 min. Subsequently, the pressure cell was cooled with a spray of water (at 10 °C) within 6 s, as monitored by a thermocouple. The cooling conditions were chosen through a series of preliminary experiments. The pressure was released to allow the samples to foam at designed foaming temperature in 2 s. Finally, the foaming samples were quenched to room temperature with water to freeze the foam morphology. 2.4. Characterizations 2.4.1. Differential scanning calorimetry (DSC) The crystallization and melting behaviors of various PLA samples were investigated using DSC (Q20, TA, USA) under a nitrogen atmosphere. Various PLA samples about 5–10 mg was first heated to 180 °C rapidly and kept for 5 min to remove thermal history. Subsequently, these samples were cooled down to −60 °C and then reheated to 180 °C at a cooling/heating rate of 10 °C/min. The crystallinity (χc) of these samples was determined using the following Eq. (1):
χc(PLA) =
ΔHm(PLA) − ΔHcc(PLA) ΔH0m(PLA) × w(PLA)
× 100%
(1)
where ΔHm(PLA) and ΔHcc(PLA) are the melting enthalpy and the cold crystallization enthalpy of PLA, respectively, as well as ΔH0m(PLA) is the melting enthalpy of 100% crystalline PLA that is 93.6 J/g [25], w(PLA) is the weight fraction of PLA in the PLA/TPU blends.
2. Experimental part 2.1. Materials
2.4.2. Rheological properties Melt rheological properties of pure PLA, pure TPU and PLA/TPU blends were investigated in the oscillatory mode using ARES rheometer equipped with parallel plates (20 mm in diameter with a gap of 1.0 mm). The angular frequency (ɷ) was swept from 0.01 to 100 rad/s at 180 °C and the maximum strain was fixed at 1% to confirm that these conditions were within the linear viscoelastic region under nitrogen. The complex viscosity (η*), storage modulus (G′), loss factor (tan δ),
PLA (2003D) with the D-isomer content of 4.3% was provided by Natureworks Co. Ltd, USA. It has a density of 1.24 g/cm3 and melt flow rate (MFR) of 3.2 g/10 min (190 °C, 2.16 kg). TPU (ES55A10 WH000) was provided by BASF SE, with MFR of 6.58 g/10 min (190 °C/2.16 kg).
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and loss modulus (G′′) were measured at various ɷ, respectively.
This should be because PLA molecular chains did not have enough time to crystallization during the cooling process [27]. In Fig. 1b and Table 2, the Tcc of PLA/TPU blends increased significantly compared with that of pure PLA. The increment in Tcc implied that the ability of cold crystallization of PLA was decreased by the addition of TPU, probably because the formation of hydrogen bond between PLA and TPU restricted the movement of the PLA molecular chain [28,29]. Moreover, the changes in TPU content had no effect on the Tcc of PLA/TPU blends. Compared with that of pure PLA, the cold crystallization peak of PLA/TPU blends became wide. The reason for this phenomenon may be that the size distribution of spherulites became widened during the crystallization of PLA, which could be observed in the POM results. It could be observed in Table 2 that compared with that of pure PLA, the Tm of PLA/TPU blends increased slightly. With the content of TPU increasing, the χc of various PLA samples kept nearly unchanged, which meant that the addition of TPU affected hardly the χc of PLA. This phenomenon could be ascribed to two aspects. On one hand, the introducing TPU molecular chains restricted the PLA molecular chain mobility and thus restrained crystallization ability of PLA, owing to the hydrogen bond effect discussed above [28]. On the other hand, TPU could be used as the nucleating agent to promote the crystallization of PLA [30]. Based on these two reasons, the χc of PLA did not represented obvious changes, with the increasing concentration of TPU.
2.4.3. Polarized optical microscope (POM) Crystal morphology of three PLA samples (pure PLA, PLA/TPU 10, PLA/TPU 20) was observed by POM (BX-51, Olympus, Japan). These PLA samples were heated from room temperature to 200 °C at a rate of 30 °C/min, kept for 5 min to eliminate thermal history, and then cooled down at a rate of 30 °C/min to 115 °C and maintained for 40 min to observe the changes of crystal morphology. 2.4.4. Scanning electron microscope (SEM) The fractured surface morphology of various PLA samples and their foaming samples was investigated by a SEM (FEI, Quanta FEG) at an acceleration voltage of 10 kV. Before observations, the surfaces of these samples were sputter coated with Au to prevent build-up of electrostatic charge during observations. The number-average particle diameter (D) of TPU dispersion phase was determined by Nano Measurer 1.2 and at least 100 particles of TPU dispersion phase were analyzed per PLA/TPU blend. The D of TPU dispersion phase and its particle density (N0) were calculated by the following equations [26]: n
⎛ ∑ Di ⎞ D= ⎜ i= 1 ⎟ ⎝ n ⎠
(2)
3/2 nM2
⎞ N0 = ⎛ ⎝ A ⎠ ⎜
⎟
3.2. POM observation
(3)
where n was the particle number of TPU dispersion phase, M is the magnification factor, and A was the area of SEM image.
The crystal morphology of three PLA samples (pure PLA, PLA/TPU 10, PLA/TPU 20) was observed by POM and the effect of TPU addition on the crystal morphology of PLA/TPU blends was further confirmed. Fig. 2 reveals the changes in the spherulitic morphology of three PLA samples of isothermal crystallization at 105 °C for various time (10 min, 20 min, 30 min and 40 min). As expected, PLA as a kind of semi-crystalline polymer exhibited a typical spherulitic morphology and all its spherulites showed a typical black extinction structure. With the increasing crystallization time, the spherulite size of three PLA samples increased gradually. The spherulite size of pure PLA was much larger than that of the other two PLA samples under the same crystallization time. The reason for this could be explained by the interaction between PLA and TPU restricting the mobility of PLA molecular chains and resulting in the decrement in the growth rate of spherulites [28]. However, the spherulite number of PLA/TPU blends was slightly more than that of pure PLA under the same crystallization time. This should be because the addition of TPU as nucleating agent led to an increment in the nucleation density of PLA [30]. Moreover, it could be observed in POM images of crystallization for 40 min that the spherulite size distribution of PLA/TPU blends became widened and the corresponding graphs of spherulite size distribution were shown in the bottom of Fig. 2. Some spherulites with uneven size in PLA/TPU blends were marked by red circle in Fig. 2. As stated above in DSC results, when the incorporation of TPU into PLA matrix, the χc of PLA did not change obviously. This should be attributed to combined actions of the increment in spherulite number and the decrement in spherulite size.
2.4.5. Foaming properties The VER of various PLA foaming samples was calculated by the Eq. (4):
Φ=
ρf ρp
(4)
where Φ, ρf and ρp are the VER, bulk densities of the pre-foam and postfoam samples, respectively. The ρf and ρp were measured by the water displacement method according to ISO 1183-1987. The cell size was analyzed using an image analysis tool (UTHSCSA Image Tool), and the cell density (N1) was calculated using the following Eq. (5): 2 3/2
nM ⎞ N1 = ⎛ ⎝ A ⎠ ⎜
⎟
Φ
(5)
where n is the cell number in the SEM micrograph and a minimum of 200 cells were analyzed for each sample, A is the area of the micrograph in cm2 as well as M is the magnification factor. 2.4.6. Thermal insulation performance The thermal conductivity of various PLA samples and their foaming samples was measured by the LFA467 and the test temperature was kept at 25 °C. At least three samples were measured and the average value was recorded.
3.3. Rheological properties 3. Results and discussion The rheological properties of pure PLA and pure TPU, PLA/TPU blends were measured by a rotational rheometer in oscillating shear mode. The viscous and elastic properties of polymer could be obtained simultaneously to describe the foaming properties of polymer [31]. Fig. 3a–c shows the η*, G′ and tan δ of pure PLA and pure TPU, PLA/ TPU blends as a function of ɷ at 180 °C, respectively. In Fig. 3a, the η* of pure PLA and pure TPU, PLA/TPU blends decreased as ɷ increased, which was consistent with the shear thinning phenomenon of the polymer. An interesting behavior was found that the η* of PLA/TPU blends was higher than those of pure PLA and pure TPU in the low
3.1. Crystallization and melting behaviors Fig. 1 shows the DSC curves of various PLA samples at the cooling (a) and heating (b) rates of 10 °C/min. The corresponding thermal properties parameters gained from the DSC curves, including cold crystallization temperature (Tcc), ΔHcc, melting temperature (Tm), ΔHm, and χc were collected in Table 2. It could be seen from Fig. 1a that there were no crystallization peaks in the cooling curve of various PLA samples. 293
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Fig. 1. DSC curves of various PLA samples at cooling (a) and heating (b) rate of 10 °C/min.
frequency region. In addition, the η* of PLA/TPU blends enhanced as the TPU content increased. This may be because the interfacial interactions between PLA phase and TPU phase were strengthened with the hydrogen bond effect [28]. The increased η* of PLA/TPU blends was beneficial to prevent cell rupture during cell growth stage [32]. In general, the G′ is related to the foaming properties of the polymer and the high G′ could provide better foaming performance during the foaming process [33]. Fig. 3b shows the relationship between G′ and ɷ in pure PLA, pure TPU, and PLA/TPU blends. With the increment in ɷ, the G′ of pure PLA and pure TPU, PLA/TPU blends increased gradually. This may be since the relaxation time of molecular chains was far greater than the deformation time of the system, with the increasing ω [30]. The G′ of PLA/TPU blends was higher than those of pure PLA and pure TPU in the low frequency region, suggesting that PLA/TPU blends had much larger elasticity with the increasing TPU content. It should be attributed to the interfacial interactions and entanglement between PLA and TPU [34]. The effect of TPU content on the viscoelasticity of PLA/TPU blends could be reflected by the tan δ-ɷ curves, in which tan δ was defined as the ratio of G'' to G′. In general, the bigger tan δ means the greater viscosity, while the smaller tan δ indicates the greater elasticity [30]. In Fig. 3c, the tan δ of PLA/TPU blends reduced significantly as the content of TPU increased in the low frequency region, indicating the elasticity of PLA/TPU blend was significantly improved, favorable for the cell growth [35]. The compatibility of polymer blends could be characterized by Han plot (that is G' versus G“) [36]. If a binary blend is miscible, the same slope is observed between the blend composition and pure component. Otherwise, it would be considered to be phase-separated [37]. It could be found in Fig. 3d that the slope of Han plots for various PLA samples exhibits different slopes, as well as their slopes decreased with increasing TPU content. This indicated that there exists phase separation in PLA/TPU blends and the compatibility between PLA and TPU became poor gradually [38].
3.4. Dispersion phase morphology In general, the study on the phase morphology of polymer blends could provide useful information on their mechanical and foaming properties [39]. The SEM images for the cryo-fractured surface of various PLA samples as well as the D and N0 of TPU dispersion phase in PLA/TPU blends were represented in Fig. 4 and Table 3, respectively. In Fig. 4a, smooth and flat fractured morphology in pure PLA exhibited representative brittle fracture [28]. With the introduction of TPU into PLA matrix, it could be easily observed that the sea-island structure appeared and the roughness increased evidently in the fractured surface of PLA/TPU blends, compared with that in pure PLA. TPU dispersion phase was spherically dispersed in the PLA matrix. As shown in Table 3, the D of TPU dispersion phase increased from 0.36 ± 0.09 to 0.95 ± 0.35 μm and its N0 decreased from 4.2 × 1011 to 0.9 × 1011 particles/cm3, with the TPU content increasing from 5 to 20 wt%. In Fig. 4d and e, some clear interfaces between TPU dispersion phase and PLA matrix could be observed, which were marked with yellow circles. Besides that, some voids (marked with red circle in Fig. 4c–e) in the fractured surface of PLA/TPU blends were also observed, because the TPU dispersion phase was pulled out from PLA matrix during the brittle fracture process. The presence of interfaces and voids indicated that the compatibility between PLA matrix and TPU dispersion phase was not good, which was consistent with the results of Han plots. The interfaces could be expected to act as the cell nucleation sites during foaming process [38,40]. 3.5. Foaming properties Figs. 5 and 6 represented the cellular morphology and cell size distribution of various PLA foams at the foaming temperature of 130 °C, respectively. The corresponding foaming parameters of various PLA foams were summarized in Table 4. In Fig. 5a, pure PLA foam exhibits a relatively incomplete cellular
Table 2 Thermal properties of various PLA samples. Sample Name
Tcc/°C
Pure PLA PLA/TPU PLA/TPU PLA/TPU PLA/TPU
117.7 127.2 127.3 127.6 128.0
5 10 15 20
± ± ± ± ±
0.1 0.0 0.1 0.2 0.0
ΔHcc/(J/g)
Tm/°C
20.0 ± 0.2 8.1 ± 0.1 6.5 ± 0.1 4.1 ± 0.0 3.3 ± 0.2
148.3 149.9 149.8 149.5 149.6
294
± ± ± ± ±
0.1 0.1 0.2 0.1 0.1
ΔHm/(J/g)
χc/%
22.7 ± 0.3 11.1 ± 0.0 9.0 ± 0.1 7.1 ± 0.1 5.3 ± 0.2
2.8 3.4 3.0 3.8 2.7
± ± ± ± ±
0.1 0.0 0.2 0.1 0.3
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Fig. 2. POM images of three PLA samples isothermally crystallized at 115 °C for various time (10 min, 20 min, 30 min and 40 min): (a) pure PLA, (b) PLA/TPU 10, (c) PLA/TPU 20.
scattered around large cells, in which the small cells and large cells were marked by orange arrows and blue arrows, respectively. It could be noted in Table 4 that with the content of TPU from 10 to 20 wt%, the large cell density of PLA/TPU blending foams decreased from 4.9 × 1011 to 2.5 × 1011 cells/cm3 and their cell size increased from 1000 ± 180 to 1400 ± 360 nm. This should be ascribed to the decrement in the number of TPU dispersion phase and the increment in the size of TPU dispersion phase (in Table 3), respectively. The corresponding small cell density of PLA/TPU blending foams decreased from 2.1 × 1012 to 0.8 × 1012 cells/cm3, due to the decreasing content of PLA in PLA/TPU blends. The corresponding small cell size of PLA/TPU blending foams was changed hardly, due to the unchanged viscoelasticity of PLA melt. In Figs. 5 and 6, with the content of TPU, the CCS in PLA/TPU blending foams became obvious, their large cell size distribution became wide and their small cell size distribution changed slightly. As shown in Table 4, as the content of TPU increased, the VER
morphology due to its low melt strength. As shown in Fig. 5b, after the TPU was introduced into PLA, the cellular morphology of PLA/TPU blending foams became complete and better gradually, owing to the increment in the G′. It could be found in Table 4 that nanocellular morphology generated in the PLA/TPU 5 blending foam with the cell size of 515 ± 198 nm and the cell density of 3.4 × 1012 cells/cm3, respectively. The reason for this could be explained by two aspects. On one hand, the interfaces between PLA and TPU could be acted as the cell nucleation points, which could decrease the energy barrier for cell nucleation and promote the number of cell nucleation sites [21,22,41]. On the other hand, the increment in η* of PLA/TPU 5 blend would restrict the cell growth, prevent cell coalescence and stabilize the cellular structure [42]. With the increment of TPU content, an interesting behavior was found that CCS appeared in the PLA/TPU 10 blending foams with the small cell size about 403 ± 152 nm in nanoscale and the large cell size around 1.0 μm in microscale. The small cells were 295
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Fig. 3. Dynamic shear rheological properties of pure PLA and pure TPU, PLA/TPU blends: (a) η*, (b) G′, (c) tan δ, (d) Han plot.
Fig. 4. SEM images for the cryo-fractured surface of various PLA samples: (a) Pure PLA, (b) PLA/TPU 5, (c) PLA/TPU 10, (d) PLA/TPU 15, (e) PLA/TPU 20. 296
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the solubility and diffusion coefficient in TPU were higher than these in PLA (see Table S1 in Supporting Information) as well as the viscosity of TPU was lower than that of PLA (see Fig. 3), the cells grew and coalesced easily, resulting in the formation of large cells in TPU phase. Moreover, the physical blowing agent absorption by PLA would elevate the χc of PLA and play an opposing role against foam expansion through the heating process [24], leading to the formation of small cells in PLA phase. The effect of foaming temperature on the cellular morphology of PLA/TPU 5 blending foam was also investigated. In general, increasing the foaming temperature would lead to the decreased melt viscosity of polymer, which would have a significant effect on the cell size. The low melt viscosities would promote cell growth and cell coalescence, resulting in the large cell size, low cell density, and high VER [43]. Therefore, it could be seen obviously in Fig. 8 and Table 5 that when the foaming temperature increased from 130 to 145 °C, the cell size of PLA/TPU 5 blending foam increased from 515 ± 198 to
Table 3 D and N0 of TPU dispersion phase in PLA/TPU blends. Sample Name
D (µm)
PLA/TPU PLA/TPU PLA/TPU PLA/TPU
0.36 0.72 0.83 0.95
5 10 15 20
± ± ± ±
N0 (particles/cm3) 0.09 0.29 0.30 0.35
4.2 × 1011 2.3 × 1011 1.8 × 1011 0.9 × 1011
of various PLA foams enhanced gradually, due to the increment in G′ in the low frequency range. To better understand the formation process of CCS in PLA/TPU blending foams, a possible mechanism was presented in Fig. 7. In the batch foaming process, the foaming system composed of PLA/TPU blend and gas was formed after the sorption of N2 under pressure. Once the pressure was released suddenly, the system was destabilized, the cell nucleation would occur in the TPU phase and PLA phase. Because
Fig. 5. SEM images of cellular morphology for various PLA foams with the foaming temperature at 130 °C: (a) Pure PLA, (b) PLA/TPU 5, (c) PLA/TPU 10, (d) PLA/ TPU 15, (e) PLA/TPU 20. 297
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Fig. 6. Cell size distribution of PLA/TPU blending foams: (a) PLA/TPU 5; (b) PLA/TPU 10; (c) PLA/TPU 15; (d) PLA/TPU 20.
1409 ± 481 nm and their cell density decreased from 3.4 × 1012 to 2.5 × 1011 cells/cm3 as well as their VER enhanced from 1.2 ± 0.16 to 1.7 ± 0.14 times, gradually and respectively.
0.073 ± 0.03 W (m K)−1. The decrement in the thermal conductivity looked to be dominated by a reduction in foam density, i.e. a reduction in solid phase conduction by the foam matrix [45].
3.6. Thermal insulation properties
4. Conclusion
The thermal insulation properties of various PLA samples and their foaming samples were tested as well as the corresponding thermal conductivity was shown in Table 6. The thermal conductivity of pure PLA was 0.178 ± 0.04 W (m K)−1. The thermal conductivity of PLA/ TPU blends was almost unchanged after the addition of TPU. As expected, the thermal conductivity of pure PLA reduced from 0.178 ± 0.04 to 0.152 ± 0.09 W (m K)−1 after it was foamed by supercritical N2 [44]. This indicated that the thermal insulation performance of pure PLA was improved by foaming. In addition, the thermal conductivity of PLA/TPU blending foams decreased significantly with the content of TPU increasing. When the TPU content reached 20 wt%, the thermal conductivity of PLA/TPU blending foam decreased to
In this study, PLA/TPU blending foams with CCS in micro- and nanocellular size were fabricated successfully via batch foaming method using N2 as a blowing agent. DSC results showed that the addition of TPU had a significant effect on the Tcc, but hardly any changes in the χc of PLA samples. The POM images displayed that the spherulite number of PLA increased and its spherulite size decreased with the increasing content of TPU. Compared with those of pure PLA, the η* and G′ of PLA/TPU blends increased. TPU dispersed in the PLA matrix as particles and its size increased gradually, with the increment in TPU content. The influence of blending ratio and foaming temperature on the foaming behaviors of PLA/TPU blends was investigated systematically.
Table 4 Foaming parameters of pure PLA foam and PLA/TPU blending foams. Sample Name
VER
Pure PLA PLA/TPU PLA/TPU PLA/TPU PLA/TPU
1.1 1.2 1.4 1.5 1.7
5 10 15 20
± ± ± ± ±
0.18 0.15 0.20 0.23 0.17
Small cell density (cells/cm3)
Small cell size (nm)
Large cell density (cells/cm3)
Large cell size (nm)
— 3.4 × 1012 2.1 × 1012 1.4 × 1012 0.8 × 1012
— 515 403 396 442
— — 4.9 × 1011 4.1 × 1011 2.5 × 1011
— — 1000 ± 180 1200 ± 190 1400 ± 360
298
± ± ± ±
198 152 132 189
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Fig. 7. Possible formation schematic of CCS in PLA/TPU blending foams.
Fig. 8. SEM images of the fractured surfaces of PLA/TPU 5 blending foams at different foaming temperature: (a) 130 °C; (b) 135 °C; (c) 140 °C; (d) 145 °C.
140 °C, a transition of nano-cells to micro-cells was observed in PLA/ TPU 5 blending foam. The thermal conductivity of PLA/TPU blending foams reduced significantly with the TPU content increasing.
As the TPU concentration increased, the nanocellular structure in various PLA foams became complete gradually. Very interestingly, when the TPU content was more than 5 wt%, the CCS appeared in PLA/TPU blending foams. With the foaming temperature rising from 135 to 299
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Table 5 Foaming parameters of PLA/TPU 5 blending foams at different foaming temperature. Foaming Temperature
VER
130 °C 135 °C 140 °C 145 °C
1.2 1.3 1.6 1.7
± ± ± ±
0.16 0.09 0.17 0.14
Cell density (cells/cm3)
Cell size (nm)
3.4 × 1012 1.0 × 1012 4.8 × 1011 2.5 × 1011
515 ± 198 830 ± 269 1110 ± 389 1409 ± 481
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Table 6 Thermal conductivity of various PLA samples and their foams. Sample Name
Pure PLA PLA/TPU PLA/TPU PLA/TPU PLA/TPU
5 10 15 20
Thermal conductivity (W (m K)−1) Unfoamed samples
Foams
0.178 0.172 0.175 0.174 0.173
0.152 0.126 0.102 0.088 0.073
± ± ± ± ±
0.04 0.02 0.08 0.04 0.03
± ± ± ± ±
0.09 0.06 0.05 0.07 0.03
Acknowledgements This study was funded by the National Natural Science Foundation of China (51703004 and 51673004) and the National Key Research and Development Program of China (2016YFB0302203 and 2016YFB0302205). Conflict of interest The authors declare that they have no competing interests. Data availability statement The raw data related to this manuscript would be made available on request. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.03.046. References [1] P. Muanchan, H. Ito, Nanocellular foams confined within PS microfibers obtained by CO2 batch foaming process, Microsyst. Technol. 24 (2018) 655–662, https://doi. org/10.1007/s00542-017-3396-7. [2] S. Liu, J. Duvigneau, G.J. Vancso, Nanocellular polymer foams as promising high performance thermal insulation materials, Eur. Polym. J. 65 (2015) 33–45, https:// doi.org/10.1016/j.eurpolymj.2015.01.039. [3] B. Notario, J. Pinto, E. Solorzano, J.A. De Saja, M. Dumon, M. Rodriguez-Perez, Experimental validation of the Knudsen effect in nanocellular polymeric foams, Polymer 56 (2015) 57–67, https://doi.org/10.1016/j.polymer.2014.10.006. [4] D. Miller, V. Kumar, Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide II. Tensile and impact properties, Polymer 52 (2011) 2910–2919, https://doi.org/10.1016/j.polymer.2011.04.049. [5] S. Costeux, L. Zhu, Low density thermoplastic nanofoams nucleated by nanoparticles, Polymer 54 (2013) 2785–2795, https://doi.org/10.1016/j.polymer.2013. 03.052. [6] J. Pinto, M. Dumon, M. Pedros, J. Reglero, M.A. Rodriguez-Perez, Nanocellular CO2 foaming of PMMA assisted by block copolymer nanostructuration, Chem. Eng. J. 243 (2014) 428–435, https://doi.org/10.1016/j.cej.2014.01.021. [7] C. Forest, P. Chaumont, P. Cassagnau, B. Swoboda, P. Sonntag, Polymer nano-foams for insulating applications prepared from CO2 foaming, Prog. Polym. Sci. 41 (2015) 122–145, https://doi.org/10.1016/j.progpolymsci.2014.07.001. [8] G. Yang, X. Liu, V. Lipik, Evaluation of silica aerogel-reinforced polyurethane foams for footwear applications, J. Mater. Sci. 53 (2018) 1–10, https://doi.org/10.1007/ s10853-018-2244-1. [9] L. Li, X. Shen, S.W. Hong, R.C. Hayward, T.P. Russell, Fabrication of co-continuous nanostructured and porous polymer membranes: spinodal decomposition of homopolymer and random copolymer blends, Angew. Chem. 124 (2012)
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Cellular morphology evolution in nanocellular poly (lactic acid)/ thermoplastic polyurethane blending foams in the presence of supercritical N2 ⁎
Zhongjie Qua,b, Dexian Yina,b, Hongfu Zhoua,b, , Xiangdong Wanga,b, Shan Zhaoc,d,
T
⁎⁎
a
School of Materials and Mechanical Engineering, Beijing Technology and Business University, 11 Fucheng Road, Beijing 100048, PR China Beijing Key Laboratory of Quality Evaluation Technology for Hygiene and Safety of Plastics, Beijing 100048, PR China c Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China d University of Chinese Academy of Sciences, Beijing 100085, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Poly (lactic acid) Thermoplastic polyurethane Nanofoams Complex cellular structure Supercritical N2
Currently, the formation of nanocells and complex cellular structure (CCS) in semi-crystalline polymer using supercritical fluids as blowing agent had become a big and newly developing challenge worldwide. In this paper, a facile methodology of polymer blending and batch foaming was proposed to fabricate CCS with large cells in micro-size and small cells in nano-size in semi-crystalline poly (lactic acid) (PLA) foams. Thermoplastic polyurethane (TPU) was introduced into PLA through melt mixing method to improve the viscoelasticity and foaming behaviors of PLA. Differential scanning calorimetry results showed that with the addition of TPU, the cold crystallization temperature of various PLA samples increased and their crystallinity kept unchanged almost. Compared with those of pure PLA, the complex viscosity, and storage modulus of PLA/TPU blends increased. Interestingly, with the increasing TPU content, the CCS appeared and became distinct in PLA/TPU blending foams as well as the thermal conductivity of various PLA foams decreased, respectively. The formation mechanism of CCS in nanocellular PLA/TPU blending foams was proposed and explained by schematic diagram. With the foaming temperature rising from 135 to 140 °C, a transition from nano-cells to micro-cells was also observed in PLA/TPU 5 blending foam, due to the decreased melt viscosity.
1. Introduction Recently, nanocellular polymer foams as a new class of frontier materials have been attracting a great amount of attention and interest in both industrial and academic fields [1–3]. Because of its cell size in the range of nanometers and the cell density greater than 1012 cells/ cm3, nanocellular polymer foams may offer some unique properties that are superior to common polymer foams, such as higher toughness, lower thermal conductivity and excellent electrical properties [3–5]. Therefore, nanocellular polymer foams could be used in numerous high value-added applications in the fields of membranes, sensors, catalysis, electromagnetic shielding, tissue engineering, and insulation materials [6–8]. Among numerous methods for preparing polymer nanofoams [9–14], batch foaming in autoclaves using supercritical fluids as a blowing agent has been proved a good one to fabricate nanocellular
polymer foams efficiently due to its high controllability. Many efforts have been made to optimize this batch foaming process for preparing nanocellular polymer foams with well cellular morphology and foaming properties, such as chain extension, nano-filling and induced crystallization [15–17]. The transition from microcellular to nanocellular poly (lactic acid) (PLA) foams by controlling viscosity, branching and crystallization in the presence of supercritical N2 was observed. The study showed that the combined branching and nucleation effect in coagentmodified PLA was helpful for generating nanocellular foams at temperatures close to its crystallization temperature [15]. PLA/organically modified layered silicate nanocomposites (PLACNs) were foamed by supercritical CO2. The results showed that PLACNs foams possessed high cell density and small cell size from microcellular to nanocellular, which were prepared at low foaming temperature and high CO2 pressure [16]. In our previous research, isothermal crystallization-induced method was employed to fabricate the nanocellular PLA foams using
⁎
Corresponding author at: School of Materials and Mechanical Engineering, Beijing Technology and Business University, 11 Fucheng Road, Beijing 100048, PR China. ⁎⁎ Corresponding author at: Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China. E-mail addresses: [email protected] (H. Zhou), [email protected] (S. Zhao). https://doi.org/10.1016/j.eurpolymj.2019.03.046 Received 29 December 2018; Received in revised form 21 March 2019; Accepted 22 March 2019 Available online 10 April 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.
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supercritical CO2 as a blowing agent [17]. The cell density and volume expansion ratio (VER) of nanocellular PLA foams were increased by the addition of hydroxyl-functionalized graphene and chain extension reaction, respectively. In recent years, the formation of complex cellular structure (CCS) in polymer foams has developed a research hotspot gradually, because it has the advantages of containing both large cells and small cells [18,19]. The large cells were mainly used to decrease the bulk density and the small cells were employed to improve the mechanical and thermal insulation properties [20]. From the aforementioned statement, it may be an interesting and challenging task to prepare polymer nanofoams with CCS, in which the large cells were in micro-size and small cells were in nano-size. Especially in semi-crystalline polymer, this unique cellular structure may be much difficult to obtain due to two reasons. One was that the crystalline region formed during the depressurization and cooling period would influence the cell nucleation and cell growth [15]. The other was that the formed crystalline region may impede the solubility and diffusion of blowing agent during foaming process [10]. So far as we know, no investigation was reported about this novel structure in semi-crystalline polymer foams and the facile methodology proposed in this paper was expected to fill this vacancy. It was worthy to mention that some literatures had reported the foaming behaviors of PLA/thermoplastic polyurethane (TPU) blends and their cellular structure evolution. The crystallization behavior and melt strength of PLA were improved by the addition of TPU, and TPU acted as a nucleating agent to decrease remarkably the cell size as well as to increase the cell density and VER of PLA/TPU foams [21]. Microcellular PLA/TPU foams were fabricated using supercritical CO2assisted batch foaming process due to the enhanced cell nucleation by the presence of the interfaces between PLA and TPU [22]. Biocompatible shape memory polymer blending foams composed of TPU and PLA were prepared by Song et al. [23] and the cells were able to contract uniformly when this TPU/PLA blending foams were recovered with heat. Barmouz et al. [24] studied the microcellular foaming process of PLA/TPU blends and the results showed that combination of lower gas pressure and higher foaming temperature was helpful for obtaining foams with higher VER. Although some studies have reported PLA/TPU blending foams, very few literatures focused on nanocellular PLA/TPU blending foams with CCS. In this study, PLA/TPU blends were foamed to fabricate the unique cellular structure stated above using N2 as a physical blowing agent. The introduction of TPU into PLA matrix may offer some obvious merits. First, the melt strength of PLA may be improved by the addition of TPU. Secondly, the interfaces between PLA and TPU due to their immiscibility could be served as cell nucleation points to increase cell density. Thirdly, the difference of both N2 affinity and melt viscosity in PLA and TPU may be conducive to the generation of CCS. The thermal properties, rheological properties, and foaming performance of pure PLA and PLA/TPU blends were investigated systematically. Meanwhile, the effect of blending ratio, foaming temperature on the cellular structural evolution of pure PLA foam and PLA/TPU foams was studied in detail. Finally, the possible formation schematic of CCS in PLA/TPU blending foams was presented and explained.
Table 1 Experimental formula of various PLA samples (pure PLA and PLA/TPU blends). Sample Name
PLA (wt.%)
TPU (wt.%)
Pure PLA PLA/TPU PLA/TPU PLA/TPU PLA/TPU
100 95 90 85 80
0 5 10 15 20
5 10 15 20
2.2. Preparation of various PLA samples Prior to mixing, PLA and TPU were placed in a drying oven at 80 °C for 12 h to remove the moisture. According to the formula shown in Table 1, PLA and TPU were blended in a Haake internal mixer at 180 °C, with the rotor speed of 60 rpm for 15 min. Subsequently, the resultant samples were pressed into sheets by compression molding method at 180 °C and 10 MPa for 8 min and then cooled to room temperature to obtain the sheet samples with the thickness of 1 mm. According to the preparation method stated above, at least three samples in each category of various PLA samples were prepared and then the small sheet with 1 cm × 1 cm (length × width) was cut in the center of each sample for further characterization and foaming process. 2.3. Foaming process of pure PLA and PLA/TPU blends Various PLA foams were prepared by the batch foaming method in a stainless steel autoclave with 250 mL in volume (a detailed schematic was described in our previous literature [17]) using supercritical N2 as a blowing agent. First, the autoclave was heated to the saturation temperature of 180 °C. Then, various PLA sheet samples were placed and saturated with N2 at 180 °C at 15 MPa for 30 min. Subsequently, the pressure cell was cooled with a spray of water (at 10 °C) within 6 s, as monitored by a thermocouple. The cooling conditions were chosen through a series of preliminary experiments. The pressure was released to allow the samples to foam at designed foaming temperature in 2 s. Finally, the foaming samples were quenched to room temperature with water to freeze the foam morphology. 2.4. Characterizations 2.4.1. Differential scanning calorimetry (DSC) The crystallization and melting behaviors of various PLA samples were investigated using DSC (Q20, TA, USA) under a nitrogen atmosphere. Various PLA samples about 5–10 mg was first heated to 180 °C rapidly and kept for 5 min to remove thermal history. Subsequently, these samples were cooled down to −60 °C and then reheated to 180 °C at a cooling/heating rate of 10 °C/min. The crystallinity (χc) of these samples was determined using the following Eq. (1):
χc(PLA) =
ΔHm(PLA) − ΔHcc(PLA) ΔH0m(PLA) × w(PLA)
× 100%
(1)
where ΔHm(PLA) and ΔHcc(PLA) are the melting enthalpy and the cold crystallization enthalpy of PLA, respectively, as well as ΔH0m(PLA) is the melting enthalpy of 100% crystalline PLA that is 93.6 J/g [25], w(PLA) is the weight fraction of PLA in the PLA/TPU blends.
2. Experimental part 2.1. Materials
2.4.2. Rheological properties Melt rheological properties of pure PLA, pure TPU and PLA/TPU blends were investigated in the oscillatory mode using ARES rheometer equipped with parallel plates (20 mm in diameter with a gap of 1.0 mm). The angular frequency (ɷ) was swept from 0.01 to 100 rad/s at 180 °C and the maximum strain was fixed at 1% to confirm that these conditions were within the linear viscoelastic region under nitrogen. The complex viscosity (η*), storage modulus (G′), loss factor (tan δ),
PLA (2003D) with the D-isomer content of 4.3% was provided by Natureworks Co. Ltd, USA. It has a density of 1.24 g/cm3 and melt flow rate (MFR) of 3.2 g/10 min (190 °C, 2.16 kg). TPU (ES55A10 WH000) was provided by BASF SE, with MFR of 6.58 g/10 min (190 °C/2.16 kg).
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and loss modulus (G′′) were measured at various ɷ, respectively.
This should be because PLA molecular chains did not have enough time to crystallization during the cooling process [27]. In Fig. 1b and Table 2, the Tcc of PLA/TPU blends increased significantly compared with that of pure PLA. The increment in Tcc implied that the ability of cold crystallization of PLA was decreased by the addition of TPU, probably because the formation of hydrogen bond between PLA and TPU restricted the movement of the PLA molecular chain [28,29]. Moreover, the changes in TPU content had no effect on the Tcc of PLA/TPU blends. Compared with that of pure PLA, the cold crystallization peak of PLA/TPU blends became wide. The reason for this phenomenon may be that the size distribution of spherulites became widened during the crystallization of PLA, which could be observed in the POM results. It could be observed in Table 2 that compared with that of pure PLA, the Tm of PLA/TPU blends increased slightly. With the content of TPU increasing, the χc of various PLA samples kept nearly unchanged, which meant that the addition of TPU affected hardly the χc of PLA. This phenomenon could be ascribed to two aspects. On one hand, the introducing TPU molecular chains restricted the PLA molecular chain mobility and thus restrained crystallization ability of PLA, owing to the hydrogen bond effect discussed above [28]. On the other hand, TPU could be used as the nucleating agent to promote the crystallization of PLA [30]. Based on these two reasons, the χc of PLA did not represented obvious changes, with the increasing concentration of TPU.
2.4.3. Polarized optical microscope (POM) Crystal morphology of three PLA samples (pure PLA, PLA/TPU 10, PLA/TPU 20) was observed by POM (BX-51, Olympus, Japan). These PLA samples were heated from room temperature to 200 °C at a rate of 30 °C/min, kept for 5 min to eliminate thermal history, and then cooled down at a rate of 30 °C/min to 115 °C and maintained for 40 min to observe the changes of crystal morphology. 2.4.4. Scanning electron microscope (SEM) The fractured surface morphology of various PLA samples and their foaming samples was investigated by a SEM (FEI, Quanta FEG) at an acceleration voltage of 10 kV. Before observations, the surfaces of these samples were sputter coated with Au to prevent build-up of electrostatic charge during observations. The number-average particle diameter (D) of TPU dispersion phase was determined by Nano Measurer 1.2 and at least 100 particles of TPU dispersion phase were analyzed per PLA/TPU blend. The D of TPU dispersion phase and its particle density (N0) were calculated by the following equations [26]: n
⎛ ∑ Di ⎞ D= ⎜ i= 1 ⎟ ⎝ n ⎠
(2)
3/2 nM2
⎞ N0 = ⎛ ⎝ A ⎠ ⎜
⎟
3.2. POM observation
(3)
where n was the particle number of TPU dispersion phase, M is the magnification factor, and A was the area of SEM image.
The crystal morphology of three PLA samples (pure PLA, PLA/TPU 10, PLA/TPU 20) was observed by POM and the effect of TPU addition on the crystal morphology of PLA/TPU blends was further confirmed. Fig. 2 reveals the changes in the spherulitic morphology of three PLA samples of isothermal crystallization at 105 °C for various time (10 min, 20 min, 30 min and 40 min). As expected, PLA as a kind of semi-crystalline polymer exhibited a typical spherulitic morphology and all its spherulites showed a typical black extinction structure. With the increasing crystallization time, the spherulite size of three PLA samples increased gradually. The spherulite size of pure PLA was much larger than that of the other two PLA samples under the same crystallization time. The reason for this could be explained by the interaction between PLA and TPU restricting the mobility of PLA molecular chains and resulting in the decrement in the growth rate of spherulites [28]. However, the spherulite number of PLA/TPU blends was slightly more than that of pure PLA under the same crystallization time. This should be because the addition of TPU as nucleating agent led to an increment in the nucleation density of PLA [30]. Moreover, it could be observed in POM images of crystallization for 40 min that the spherulite size distribution of PLA/TPU blends became widened and the corresponding graphs of spherulite size distribution were shown in the bottom of Fig. 2. Some spherulites with uneven size in PLA/TPU blends were marked by red circle in Fig. 2. As stated above in DSC results, when the incorporation of TPU into PLA matrix, the χc of PLA did not change obviously. This should be attributed to combined actions of the increment in spherulite number and the decrement in spherulite size.
2.4.5. Foaming properties The VER of various PLA foaming samples was calculated by the Eq. (4):
Φ=
ρf ρp
(4)
where Φ, ρf and ρp are the VER, bulk densities of the pre-foam and postfoam samples, respectively. The ρf and ρp were measured by the water displacement method according to ISO 1183-1987. The cell size was analyzed using an image analysis tool (UTHSCSA Image Tool), and the cell density (N1) was calculated using the following Eq. (5): 2 3/2
nM ⎞ N1 = ⎛ ⎝ A ⎠ ⎜
⎟
Φ
(5)
where n is the cell number in the SEM micrograph and a minimum of 200 cells were analyzed for each sample, A is the area of the micrograph in cm2 as well as M is the magnification factor. 2.4.6. Thermal insulation performance The thermal conductivity of various PLA samples and their foaming samples was measured by the LFA467 and the test temperature was kept at 25 °C. At least three samples were measured and the average value was recorded.
3.3. Rheological properties 3. Results and discussion The rheological properties of pure PLA and pure TPU, PLA/TPU blends were measured by a rotational rheometer in oscillating shear mode. The viscous and elastic properties of polymer could be obtained simultaneously to describe the foaming properties of polymer [31]. Fig. 3a–c shows the η*, G′ and tan δ of pure PLA and pure TPU, PLA/ TPU blends as a function of ɷ at 180 °C, respectively. In Fig. 3a, the η* of pure PLA and pure TPU, PLA/TPU blends decreased as ɷ increased, which was consistent with the shear thinning phenomenon of the polymer. An interesting behavior was found that the η* of PLA/TPU blends was higher than those of pure PLA and pure TPU in the low
3.1. Crystallization and melting behaviors Fig. 1 shows the DSC curves of various PLA samples at the cooling (a) and heating (b) rates of 10 °C/min. The corresponding thermal properties parameters gained from the DSC curves, including cold crystallization temperature (Tcc), ΔHcc, melting temperature (Tm), ΔHm, and χc were collected in Table 2. It could be seen from Fig. 1a that there were no crystallization peaks in the cooling curve of various PLA samples. 293
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Fig. 1. DSC curves of various PLA samples at cooling (a) and heating (b) rate of 10 °C/min.
frequency region. In addition, the η* of PLA/TPU blends enhanced as the TPU content increased. This may be because the interfacial interactions between PLA phase and TPU phase were strengthened with the hydrogen bond effect [28]. The increased η* of PLA/TPU blends was beneficial to prevent cell rupture during cell growth stage [32]. In general, the G′ is related to the foaming properties of the polymer and the high G′ could provide better foaming performance during the foaming process [33]. Fig. 3b shows the relationship between G′ and ɷ in pure PLA, pure TPU, and PLA/TPU blends. With the increment in ɷ, the G′ of pure PLA and pure TPU, PLA/TPU blends increased gradually. This may be since the relaxation time of molecular chains was far greater than the deformation time of the system, with the increasing ω [30]. The G′ of PLA/TPU blends was higher than those of pure PLA and pure TPU in the low frequency region, suggesting that PLA/TPU blends had much larger elasticity with the increasing TPU content. It should be attributed to the interfacial interactions and entanglement between PLA and TPU [34]. The effect of TPU content on the viscoelasticity of PLA/TPU blends could be reflected by the tan δ-ɷ curves, in which tan δ was defined as the ratio of G'' to G′. In general, the bigger tan δ means the greater viscosity, while the smaller tan δ indicates the greater elasticity [30]. In Fig. 3c, the tan δ of PLA/TPU blends reduced significantly as the content of TPU increased in the low frequency region, indicating the elasticity of PLA/TPU blend was significantly improved, favorable for the cell growth [35]. The compatibility of polymer blends could be characterized by Han plot (that is G' versus G“) [36]. If a binary blend is miscible, the same slope is observed between the blend composition and pure component. Otherwise, it would be considered to be phase-separated [37]. It could be found in Fig. 3d that the slope of Han plots for various PLA samples exhibits different slopes, as well as their slopes decreased with increasing TPU content. This indicated that there exists phase separation in PLA/TPU blends and the compatibility between PLA and TPU became poor gradually [38].
3.4. Dispersion phase morphology In general, the study on the phase morphology of polymer blends could provide useful information on their mechanical and foaming properties [39]. The SEM images for the cryo-fractured surface of various PLA samples as well as the D and N0 of TPU dispersion phase in PLA/TPU blends were represented in Fig. 4 and Table 3, respectively. In Fig. 4a, smooth and flat fractured morphology in pure PLA exhibited representative brittle fracture [28]. With the introduction of TPU into PLA matrix, it could be easily observed that the sea-island structure appeared and the roughness increased evidently in the fractured surface of PLA/TPU blends, compared with that in pure PLA. TPU dispersion phase was spherically dispersed in the PLA matrix. As shown in Table 3, the D of TPU dispersion phase increased from 0.36 ± 0.09 to 0.95 ± 0.35 μm and its N0 decreased from 4.2 × 1011 to 0.9 × 1011 particles/cm3, with the TPU content increasing from 5 to 20 wt%. In Fig. 4d and e, some clear interfaces between TPU dispersion phase and PLA matrix could be observed, which were marked with yellow circles. Besides that, some voids (marked with red circle in Fig. 4c–e) in the fractured surface of PLA/TPU blends were also observed, because the TPU dispersion phase was pulled out from PLA matrix during the brittle fracture process. The presence of interfaces and voids indicated that the compatibility between PLA matrix and TPU dispersion phase was not good, which was consistent with the results of Han plots. The interfaces could be expected to act as the cell nucleation sites during foaming process [38,40]. 3.5. Foaming properties Figs. 5 and 6 represented the cellular morphology and cell size distribution of various PLA foams at the foaming temperature of 130 °C, respectively. The corresponding foaming parameters of various PLA foams were summarized in Table 4. In Fig. 5a, pure PLA foam exhibits a relatively incomplete cellular
Table 2 Thermal properties of various PLA samples. Sample Name
Tcc/°C
Pure PLA PLA/TPU PLA/TPU PLA/TPU PLA/TPU
117.7 127.2 127.3 127.6 128.0
5 10 15 20
± ± ± ± ±
0.1 0.0 0.1 0.2 0.0
ΔHcc/(J/g)
Tm/°C
20.0 ± 0.2 8.1 ± 0.1 6.5 ± 0.1 4.1 ± 0.0 3.3 ± 0.2
148.3 149.9 149.8 149.5 149.6
294
± ± ± ± ±
0.1 0.1 0.2 0.1 0.1
ΔHm/(J/g)
χc/%
22.7 ± 0.3 11.1 ± 0.0 9.0 ± 0.1 7.1 ± 0.1 5.3 ± 0.2
2.8 3.4 3.0 3.8 2.7
± ± ± ± ±
0.1 0.0 0.2 0.1 0.3
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Fig. 2. POM images of three PLA samples isothermally crystallized at 115 °C for various time (10 min, 20 min, 30 min and 40 min): (a) pure PLA, (b) PLA/TPU 10, (c) PLA/TPU 20.
scattered around large cells, in which the small cells and large cells were marked by orange arrows and blue arrows, respectively. It could be noted in Table 4 that with the content of TPU from 10 to 20 wt%, the large cell density of PLA/TPU blending foams decreased from 4.9 × 1011 to 2.5 × 1011 cells/cm3 and their cell size increased from 1000 ± 180 to 1400 ± 360 nm. This should be ascribed to the decrement in the number of TPU dispersion phase and the increment in the size of TPU dispersion phase (in Table 3), respectively. The corresponding small cell density of PLA/TPU blending foams decreased from 2.1 × 1012 to 0.8 × 1012 cells/cm3, due to the decreasing content of PLA in PLA/TPU blends. The corresponding small cell size of PLA/TPU blending foams was changed hardly, due to the unchanged viscoelasticity of PLA melt. In Figs. 5 and 6, with the content of TPU, the CCS in PLA/TPU blending foams became obvious, their large cell size distribution became wide and their small cell size distribution changed slightly. As shown in Table 4, as the content of TPU increased, the VER
morphology due to its low melt strength. As shown in Fig. 5b, after the TPU was introduced into PLA, the cellular morphology of PLA/TPU blending foams became complete and better gradually, owing to the increment in the G′. It could be found in Table 4 that nanocellular morphology generated in the PLA/TPU 5 blending foam with the cell size of 515 ± 198 nm and the cell density of 3.4 × 1012 cells/cm3, respectively. The reason for this could be explained by two aspects. On one hand, the interfaces between PLA and TPU could be acted as the cell nucleation points, which could decrease the energy barrier for cell nucleation and promote the number of cell nucleation sites [21,22,41]. On the other hand, the increment in η* of PLA/TPU 5 blend would restrict the cell growth, prevent cell coalescence and stabilize the cellular structure [42]. With the increment of TPU content, an interesting behavior was found that CCS appeared in the PLA/TPU 10 blending foams with the small cell size about 403 ± 152 nm in nanoscale and the large cell size around 1.0 μm in microscale. The small cells were 295
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Fig. 3. Dynamic shear rheological properties of pure PLA and pure TPU, PLA/TPU blends: (a) η*, (b) G′, (c) tan δ, (d) Han plot.
Fig. 4. SEM images for the cryo-fractured surface of various PLA samples: (a) Pure PLA, (b) PLA/TPU 5, (c) PLA/TPU 10, (d) PLA/TPU 15, (e) PLA/TPU 20. 296
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the solubility and diffusion coefficient in TPU were higher than these in PLA (see Table S1 in Supporting Information) as well as the viscosity of TPU was lower than that of PLA (see Fig. 3), the cells grew and coalesced easily, resulting in the formation of large cells in TPU phase. Moreover, the physical blowing agent absorption by PLA would elevate the χc of PLA and play an opposing role against foam expansion through the heating process [24], leading to the formation of small cells in PLA phase. The effect of foaming temperature on the cellular morphology of PLA/TPU 5 blending foam was also investigated. In general, increasing the foaming temperature would lead to the decreased melt viscosity of polymer, which would have a significant effect on the cell size. The low melt viscosities would promote cell growth and cell coalescence, resulting in the large cell size, low cell density, and high VER [43]. Therefore, it could be seen obviously in Fig. 8 and Table 5 that when the foaming temperature increased from 130 to 145 °C, the cell size of PLA/TPU 5 blending foam increased from 515 ± 198 to
Table 3 D and N0 of TPU dispersion phase in PLA/TPU blends. Sample Name
D (µm)
PLA/TPU PLA/TPU PLA/TPU PLA/TPU
0.36 0.72 0.83 0.95
5 10 15 20
± ± ± ±
N0 (particles/cm3) 0.09 0.29 0.30 0.35
4.2 × 1011 2.3 × 1011 1.8 × 1011 0.9 × 1011
of various PLA foams enhanced gradually, due to the increment in G′ in the low frequency range. To better understand the formation process of CCS in PLA/TPU blending foams, a possible mechanism was presented in Fig. 7. In the batch foaming process, the foaming system composed of PLA/TPU blend and gas was formed after the sorption of N2 under pressure. Once the pressure was released suddenly, the system was destabilized, the cell nucleation would occur in the TPU phase and PLA phase. Because
Fig. 5. SEM images of cellular morphology for various PLA foams with the foaming temperature at 130 °C: (a) Pure PLA, (b) PLA/TPU 5, (c) PLA/TPU 10, (d) PLA/ TPU 15, (e) PLA/TPU 20. 297
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Fig. 6. Cell size distribution of PLA/TPU blending foams: (a) PLA/TPU 5; (b) PLA/TPU 10; (c) PLA/TPU 15; (d) PLA/TPU 20.
1409 ± 481 nm and their cell density decreased from 3.4 × 1012 to 2.5 × 1011 cells/cm3 as well as their VER enhanced from 1.2 ± 0.16 to 1.7 ± 0.14 times, gradually and respectively.
0.073 ± 0.03 W (m K)−1. The decrement in the thermal conductivity looked to be dominated by a reduction in foam density, i.e. a reduction in solid phase conduction by the foam matrix [45].
3.6. Thermal insulation properties
4. Conclusion
The thermal insulation properties of various PLA samples and their foaming samples were tested as well as the corresponding thermal conductivity was shown in Table 6. The thermal conductivity of pure PLA was 0.178 ± 0.04 W (m K)−1. The thermal conductivity of PLA/ TPU blends was almost unchanged after the addition of TPU. As expected, the thermal conductivity of pure PLA reduced from 0.178 ± 0.04 to 0.152 ± 0.09 W (m K)−1 after it was foamed by supercritical N2 [44]. This indicated that the thermal insulation performance of pure PLA was improved by foaming. In addition, the thermal conductivity of PLA/TPU blending foams decreased significantly with the content of TPU increasing. When the TPU content reached 20 wt%, the thermal conductivity of PLA/TPU blending foam decreased to
In this study, PLA/TPU blending foams with CCS in micro- and nanocellular size were fabricated successfully via batch foaming method using N2 as a blowing agent. DSC results showed that the addition of TPU had a significant effect on the Tcc, but hardly any changes in the χc of PLA samples. The POM images displayed that the spherulite number of PLA increased and its spherulite size decreased with the increasing content of TPU. Compared with those of pure PLA, the η* and G′ of PLA/TPU blends increased. TPU dispersed in the PLA matrix as particles and its size increased gradually, with the increment in TPU content. The influence of blending ratio and foaming temperature on the foaming behaviors of PLA/TPU blends was investigated systematically.
Table 4 Foaming parameters of pure PLA foam and PLA/TPU blending foams. Sample Name
VER
Pure PLA PLA/TPU PLA/TPU PLA/TPU PLA/TPU
1.1 1.2 1.4 1.5 1.7
5 10 15 20
± ± ± ± ±
0.18 0.15 0.20 0.23 0.17
Small cell density (cells/cm3)
Small cell size (nm)
Large cell density (cells/cm3)
Large cell size (nm)
— 3.4 × 1012 2.1 × 1012 1.4 × 1012 0.8 × 1012
— 515 403 396 442
— — 4.9 × 1011 4.1 × 1011 2.5 × 1011
— — 1000 ± 180 1200 ± 190 1400 ± 360
298
± ± ± ±
198 152 132 189
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Fig. 7. Possible formation schematic of CCS in PLA/TPU blending foams.
Fig. 8. SEM images of the fractured surfaces of PLA/TPU 5 blending foams at different foaming temperature: (a) 130 °C; (b) 135 °C; (c) 140 °C; (d) 145 °C.
140 °C, a transition of nano-cells to micro-cells was observed in PLA/ TPU 5 blending foam. The thermal conductivity of PLA/TPU blending foams reduced significantly with the TPU content increasing.
As the TPU concentration increased, the nanocellular structure in various PLA foams became complete gradually. Very interestingly, when the TPU content was more than 5 wt%, the CCS appeared in PLA/TPU blending foams. With the foaming temperature rising from 135 to 299
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Table 5 Foaming parameters of PLA/TPU 5 blending foams at different foaming temperature. Foaming Temperature
VER
130 °C 135 °C 140 °C 145 °C
1.2 1.3 1.6 1.7
± ± ± ±
0.16 0.09 0.17 0.14
Cell density (cells/cm3)
Cell size (nm)
3.4 × 1012 1.0 × 1012 4.8 × 1011 2.5 × 1011
515 ± 198 830 ± 269 1110 ± 389 1409 ± 481
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Table 6 Thermal conductivity of various PLA samples and their foams. Sample Name
Pure PLA PLA/TPU PLA/TPU PLA/TPU PLA/TPU
5 10 15 20
Thermal conductivity (W (m K)−1) Unfoamed samples
Foams
0.178 0.172 0.175 0.174 0.173
0.152 0.126 0.102 0.088 0.073
± ± ± ± ±
0.04 0.02 0.08 0.04 0.03
± ± ± ± ±
0.09 0.06 0.05 0.07 0.03
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