Microcellular morphology evolution of polystyrene/thermoplastic polyurethane blends in the presence of supercritical CO 2

Abstract

In this article, a facile melt blending and solid batch foaming approach was proposed to prepare microcellular polystyrene/thermoplastic polyurethane (PS/TPU) blending foams with supercritical carbon dioxide (CO₂). Compared with those of pure PS and pure TPU, an interesting phenomenon about the enhanced complex viscosity and storage modulus, as well as decreased loss factor of PS/TPU blends, was found. The solubility of CO₂ in the PS/TPU blends was enhanced, owing to the CO₂ solubilization effects of TPU. An interesting bimodal cell structure (BCS) was observed in the PS/TPU blending foams with the TPU content of 10, 15, and 20%. Consequently, a significant conclusion could be speculated that the generation of BCS in the PS/TPU blending system depended on not only the viscosity and morphology of the polymer blends but also the solubility and diffusivity of the CO₂ as well as the type of cell nucleation. The thermal insulation property of PS foam was improved by the introduction of TPU.

Keywords

Polystyrene thermoplastic polyurethane foams bimodal cell structure

Introduction

Polymer blending as one of the cheapest and facile methods of tailoring polymeric materials has been widely applied in the industry, which has gained great attention for decades.^1

-4 Polymer blending could balance the properties of different polymeric components, improve the processability, reduce the cost of developing a new polymer, and eliminate the weakness on some properties of one polymeric component.^5,6

Because of the aforementioned advantages, polymer blends were often employed in polymer foaming process.^7
-9 Several merits were found in the polymer blending foam due to the introduction of foreign polymeric component, such as improving the crystallization behaviors and rheological properties, enhancing the solubility of blowing agent and the number of heterogeneous cell nucleation sites, and controlling the cellular morphology.^10

-14

An interesting phenomenon was found in our previous research that the interface between poly (lactic acid) (PLA) and high-density polyethylene (HDPE) in the PLA/HDPE blends could be acted as heterogeneous crystallization nucleation sites to increase the crystallinity of PLA.¹⁰ The introduction of polytetrafluoroethylene into thermoplastic polyurethane (TPU) could increase the storage modulus (G′) of TPU significantly at low frequency, which was helpful for increasing the foamability of TPU.¹¹ An open-cell foam of polypropylene/copolymer of ethylene–propylene–diene–monomer (PP/EPDM) blends was fabricated successfully due to the enhanced solubility of CO₂ in EPDM.¹² The cell density of PLA/poly (ethylene terephthalate glycol modified) (PLA/PETG) blending foams increased nearly exponential with the increasing compatibilizer content because PETG dispersion phase could serve as efficient heterogeneous nucleation sites.¹³ The blending ratio of the polymer blend was an important parameter to affect the cellular morphology. The PP/polystyrene (PP/PS) blending foams using supercritical CO₂ as the physical blowing agent with the blending ratio of 90:10 and 80:20 have the average cell diameter of 0.4 and 0.7 µm as well as the cell density of 8.3 × 10¹¹ and 6.4 × 10¹¹ cells/cm³.¹⁴

PS as a typical amorphous polymer with high modulus, excellent dimensional stability, and thermal insulation advantages is often employed to generate microcellular foams.^15
-17 However, PS possesses low solubility of CO₂. By considering the merit of TPU in high solubility of CO₂ and low viscosity, it is significant and worthy to prepare PS/TPU blends with enhanced solubility of CO₂ and adjust the cellular morphology in their foams. Meanwhile, the interfaces between PS and TPU may be expected to serve as cell nucleation sites to increase the cell density of PS foams and decrease the cell size of PS foams, respectively.

In this study, pure PS and PS/TPU blends were foamed using CO₂ as the physical blowing agent for creating microcellular foams. The rheological properties, solubility and diffusivity of CO₂, and microcellular foaming behavior of pure PS and PS/TPU blends were investigated. Simultaneously, the effect of blending ratio, foaming temperature, and foaming pressure on the cellular structure evolution of pure PS foam and PS/TPU blending foams was studied. Finally, the thermal insulation properties of pure PS, pure TPU and PS/TPU blends and their foams were also studied.

Experiments

Materials

PS (158 K) was bought from BASF Industry Co. Ltd (Nanjing, China), with a melt flow rate of 3.0 g/10 min (200°C/5.0 kg), according to ASTM D1238, and its density is 1.05 g/cm³. TPU (ES55A10 WH000) was provided by BASF SE, with a melt flow rate of 6.6 g/10 min (190°C/2.16 kg) and the density is 1.20 g/cm³.

Preparation of PS/TPU blends

In the first stage, TPU and PS were placed in a drying oven at 80°C for 12 h to remove moisture. PS and TPU were mixed with different blending ratios in a Haake internal mixer at 170°C, with the mixing time of 8 min and the mixing speed of 60 r/min. The experimental formula is presented in Table 1. Subsequently, the resultant PS, TPU, and PS/TPU blends were shaped into sheets with a thickness of about 1 mm by compression molding at 170°C and 10 MPa for 8 min and then cooled to room temperature to obtain the sheet samples for further characterizations and foaming process.

Table 1.

Experimental formula of PS and PS/TPU blends.

Sample name	PS (wt%)	TPU (wt%)
Pure PS	100	0
PS/TPU 5%	95	5
PS/TPU 10%	90	10
PS/TPU 15%	85	15
PS/TPU 20%	80	20
Pure TPU	0	100

PS: polystyrene; TPU: thermoplastic polyurethane.

Foaming process of pure PS and PS/TPU blends

Pure PS foams and PS/TPU blending foams were prepared via a batch foaming process using CO₂ as the physical blowing agent. CO₂ was introduced into the high-pressure vessel (its volume is 250 mL and a detailed schematic was described in our previous literature¹⁸) and circulated for 2 min to drive off the air. Subsequently, the obtained pure PS and PS/TPU sheets with a size of 20 × 20 × 1 mm³ were placed in an autoclave after the system temperature was raised to the designed value (60, 65, 70, and 75°C). Then, CO₂ was injected into the high-pressure vessel, adjusting the system pressure at 5.0, 10.0, 15.0, and 20.0 MPa, respectively. After CO₂ dissolving and diffusing in the polymer melt matrix for 4 h, the pressure of the high-pressure vessel dropped by the release of CO₂ in approximately 10 MPa/s, which provided a driving force for cell nucleation and growth to obtain the foaming samples. Finally, the foaming samples were then quickly removed and cooled in the ambient temperature in about 7 s.

Characterizations

Rheological measurement

The rheological properties of pure PS, pure TPU, and PS/TPU blends were measured in the oscillatory mode at 190°C using ARES rheometer (MARS Ⅲ, TA) equipped with 20-mm diameter parallel plates at a gap of 1 mm. The range of angular frequency (ω) was set as 0.01–100 rad/s, and the maximum strain was fixed at 0.5%, to confirm that these conditions were within the linear viscoelastic region under nitrogen atmosphere. The complex viscosity (η*), G′, and loss factor (tanδ) of various samples were measured at various ω.

CO₂ solubility and diffusivity in pure PS, pure TPU, and PS/TPU blends

The CO₂ solubility and diffusivity of pure PS, pure TPU, and PS/TPU blends have been tested at 70°C and 15 MPa. The measurement method was reported in previous reference in detail.¹⁹ The instant sample weight (M _t) during the desorbed process and the desorbed time (t _d) is made into a curve of M _t as a function of ${t_{d}}^{1 / 2}$ , as well as the intercept (M _gas,0) and the slope (k) of the curve are obtained after fitting. The diffusion coefficient could be expressed as follows:

D_{d} = π {(\frac{k l}{4 M_{gas,0}})}^{2}

where D _d is the diffusion coefficient, l is the sample thickness, and M _gas,0 is the solubility.

Scanning electron microscope

Pure PS and PS/TPU blends were cryogenically fractured under liquid nitrogen, and their fractured cross sections were sputtered with gold. Scanning electron microscope (SEM; FEI Quanta FEG, USA) was used to observe the morphology of the unfoamed samples and foaming samples, at an acceleration voltage of 10 kV. The number-average particle diameter (D) of TPU dispersion phase in various PS/TPU blends was measured by Nano Measurer 1.2, and at least 100 particles of TPU dispersion phase were selected for each sample. The D of TPU dispersion phase and its particle density (N) were calculated using the following equations:¹⁴

D = \frac{\sum_{i = 1}^{n} D_{i}}{n}

N = {(\frac{n M^{2}}{A})}^{3 / 2}

where n is the particle number of TPU dispersion phase, M is the magnification factor, and A is the area of SEM image, respectively.

Foaming properties

The volume expansion ratio (VER) of pure PS foam and PS/TPU blending foams was calculated by the following equation:

Φ = \frac{ρ_{f}}{ρ_{p}}

where Φ, ρ_f , and ρ_p are the VER, bulk densities of the pre-foam and post-foam samples, respectively, which were measured a density balance (Sartorius, Goettingen, Germany).

The cell size was measured using an image analysis tool (Image-Pro Plus) based on the SEM micrographs, and at least 200 cells were collected for each sample to gain the average cell size. The cell nucleation density (N₀ ) was calculated by the following equation:²⁰

N_{0} = {(\frac{n M^{2}}{A})}^{3 / 2} Φ

where n is the cell number in the SEM micrograph, M is the magnification factor, and A is the area of the micrograph (cm²), respectively.

Thermal conductivity tests

The thermal conductivity of pure PS, pure TPU and PS/TPU blends and their foams was measured by a laser thermal conductivity analyzer (LFA467, NETZSCH Scientific Instruments Co., Ltd, Germany), and the testing temperature was kept at 25°C. The thermal conductivity of each foaming sample was gained based on the mean of three values.

Results and discussion

Rheological properties of pure PS, pure TPU, and PS/TPU blends

Rheological properties play an important role in investigating the viscoelastic properties of the polymer.²¹ The η*, G′, and tan δ of pure PS, pure TPU, and various PS/TPU blends as a function of ω, as well as Cole–Cole plots of PS/TPU blends (i.e. η″ as a function of η′ curves, where η″ = G′/ω, η′ = G″/ω, and the G″ is the loss moduli) are represented in Figure 1.

Figure 1.

Dynamic shear rheological properties of pure PS, pure TPU, and PS/TPU blends as a function of ω: (a) η*, (b) G′, (c) tan δ, and (d) Cole–Cole plot.

The η* of pure PS, pure TPU, and PS/TPU blends as a function of ω is shown in Figure 1(a). The η* of all the samples decreased with the increasing ω, which exhibited the shear-thinning phenomenon of the polymer. An interesting phenomenon was observed that the η* of PS/TPU blends was slightly higher than that of pure PS and pure TPU, which was beneficial for preventing of cell rupture during the cell growth process.²² Similar interesting phenomenon about η* was observed in our previous research of PLA/low-density polyethylene and other research, which may be due to the presence of simple physical entanglement between the two polymer molecular chains.^23,24

Generally, the G′ is related to the melt elasticity of the polymer. The relationships between the G′ of pure PS, pure TPU, PS/TPU blends, and ω are displayed in Figure 1(b). The G′ curves of pure PS, pure TPU, and PS/TPU blends increased with the increment of ω. This may be because the molecular chains could rearrange at the low frequency and they had less time to relax at the high frequency.²⁵ The G′ of pure PS was much higher than that of pure TPU, suggesting that the melt elasticity of PS was better than that of TPU. After TPU was introduced into PS, an interesting effect was found that the G′ of PS/TPU blends were higher than that of pure PS and pure TPU. The increment of melt elasticity could be attributed to the relaxation process of dispersed phase droplets when slightly sheared.²⁶ Meanwhile, the improvement of the G′ indicated the enhancement of the foamability.²⁷

Figure 1(c) depicted the tan δ curves of pure PS, pure TPU, and PS/TPU blends as a function of ω. Tan δ is the ratio between the G″ and the G′, which is also an indication of foamability of the polymer. Similarly, an interesting phenomenon could be observed in Figure 1(c) that the tan δ of PS/TPU blends was lower than those of pure PS and pure TPU. The decrease in the tan δ of PS/TPU blends indicated that the melt response was accelerated and the melt elasticity was increased.²⁴

The miscibility of the two phases in polymer blend could be characterized by the Cole–Cole plot. When phase separation occurred in the polymer blending system, a tail or two circular arcs would appear in its Cole–Cole plot.^28,29 The Cole–Cole plots of PS/TPU blends are represented in Figure 1(d). It could be seen from Figure 1(d) that after TPU was introduced into the PS matrix, the right end of the Cole–Cole plot of all the PS/TPU blends showed different degrees of warpage. With the increase of TPU content, the degree of warpage in the Cole–Cole plots of PS/TPU blends increased, indicating that there were two relaxation mechanisms in the PS/TPU blends.³⁰ In other words, it reflected that the phase separation occurred in the PS/TPU blends.

Dispersion phase morphology of pure PS and PS/TPU blends

The morphology of cryofractured surface of pure PS and PS/TPU blends was observed using SEM. All the SEM images with the scale bar of 30 µm were taken at the same magnification 5000×. As illustrated in Figure 2(a), pure PS showed a typical morphology of fracture surface of an amorphous polymer with no visible plastic deformation, indicating that the pure PS fractured in a brittle mode.³¹ With the introduction of TPU into the PS matrix, sea island structure appeared in the PS/TPU blends and the interface between the PS phase and the TPU phase became distinct, indicating that the miscibility between the PS phase and the TPU phase was poor in the PS/TPU blends. This was consistent with the analysis of Cole–Cole plots in rheological properties. The morphology of TPU dispersion phase in the PS/TPU blends was an irregular circle. As presented in Table 2, the D of TPU dispersion phase increased from 0.3 µm to 2.0 µm and its N decreased from 3.6 × 10¹⁰ particles/cm³ to 0.9 × 10¹⁰ particles/cm³, with the TPU content increasing from 5% to 20%. The interfaces between PS and TPU may be expected to act as the heterogeneous nucleation sites for cell nucleation in the subsequent foaming process.

Figure 2.

SEM images for the cryofractured surface of pure PS and PS/TPU blends: (a) pure PS, (b) PS/TPU 5%, (c) PS/TPU 10%, (d) PS/TPU 15%, and (e) PS/TPU 20%.

Table 2.

D and N of TPU dispersion phase in PS/TPU blends.

Sample name	D (µm)	N ₀ (particles/cm³)
PS/TPU 5%	0.3	3.6 × 10¹⁰
PS/TPU 10%	0.5	2.2 × 10¹⁰
PS/TPU 15%	1.6	1.4 × 10¹⁰
PS/TPU 20%	2.0	0.9 × 10¹⁰

PS: polystyrene; TPU: thermoplastic polyurethane.

Solubility and diffusivity of CO₂ in pure PS, pure TPU, and PS/TPU blends

The solubility and diffusivity of CO₂ in pure PS, pure TPU, and PS/TPU blends were characterized to further understand its role in the foaming stage, which could also provide guidance for optimizing the polymer foaming processing condition and regulating cellular structure.³²

The measured solubility and diffusivity of CO₂ in pure PS, pure TPU, and PS/TPU blends are shown in Figure 3. It could be observed in Figure 3 that the CO₂ solubility in TPU was higher than that in PS, which indicated that TPU had a higher affinity for CO₂ than PS, thus it caused enhanced cell nucleation in the TPU phase.¹⁶ It could be also found that the CO₂ solubility in the PS/TPU blends was higher than that of pure PS. With the increasing content of TPU, the solubility of PS/TPU blends increased slightly. It could be explained by the two aspects. One was that the solubility of CO₂ in pure TPU was higher than that in pure PS. The other was that the CO₂ may enrich in the interfaces between the PS phase and the TPU phase due to the more free volume in the interfaces.³³

Figure 3.

The solubility and diffusivity of pure PS, pure TPU, and PS/TPU blends.

The diffusivity is an important parameter reflecting the gas diffusion rate in the polymer. It could be observed in Figure 3 that the diffusivity of CO₂ in pure TPU was much higher than that in pure PS. Compared with that in pure PS, the diffusivity of CO₂ in PS/TPU blends was slightly increased, which could be attributed to two aspects. One was that the diffusivity of CO₂ in TPU was very high. The other was the presence of the interfaces between PS and TPU, which could act as the diffusion channel of CO₂ in the PS/TPU blends.

Cellular morphology evolution of pure PS foam and PS/TPU blending foams

The microcellular morphology, cell size distribution, and foaming parameters of pure PS and PS/TPU blending foams with the foaming temperature of 70°C and the foaming pressure of 15 MPa are displayed in Figures 4 and 5 and Table 3, respectively. As presented in Table 3, the cell size and cell nucleation density of pure PS foam were 2.0 µm and 6.5 × 10¹⁰ nuclei/cm³, respectively.

Figure 4.

SEM images of the fracture surfaces of pure PS and PS/TPU blending foams: (a) pure PS, (b) PS/TPU 5%, (c) PS/TPU 10%, (d) PS/TPU 15%, and (e) PS/TPU 20%.

Figure 5.

Cell number frequency distribution of pure PS foam and PS/TPU blending foams: (a) pure PS, (b) PS/TPU 5%, (c) PS/TPU 10%, (d) PS/TPU 15%, and (e) PS/TPU 20%.

Table 3.

The foaming parameters of pure PS foam and PS/TPU blending foams.

Sample name	VER	Cell nucleation density (nuclei/cm³)	Small cell size (μm)	Large cell size (μm)
Pure PS	1.3	6.5 × 10¹⁰	2.0	—
PS/TPU 5%	1.4	8.5 × 10¹⁰	1.8	—
PS/TPU 10%	1.5	4.4 × 10¹⁰	1.7	3.9
PS/TPU 15%	1.5	3.0 × 10¹⁰	1.4	4.5
PS/TPU 20%	1.6	1.3 × 10¹⁰	1.3	6.8

PS: polystyrene; TPU: thermoplastic polyurethane; VER: volume expansion ratio.

After TPU was added into PS, the cell size of PS/TPU 5% blending foam decreased from 2.0 µm to 1.8 µm, and its cell nucleation density increased from 6.5 × 10¹⁰ nuclei/cm³ to 8.5 × 10¹⁰ nuclei/cm³. This was because the interfaces between PS and TPU could be served as the heterogeneous cell nucleation points.^34,35 It could be observed in Figures 4, 6, and 7 that TPU was attached physically on the cell walls, which is marked with red arrows.

Figure 6.

SEM images for the fracture surfaces of PS/TPU 5% blending foams at different foaming pressure: (a) 5 MPa, (b) 10 MPa, (c) 15 MPa, and (d) 20 MPa.

Figure 7.

SEM images of the fracture surfaces of PS/TPU 5% blending foams at different foaming temperature: (a) 60°C, (b) 65°C, (c) 70°C, and (d) 75°C.

In general, polymer foaming goes through four stages: (1) immersing stage, (2) cell nucleation, (3) cell growth, and (4) cell stabilization.³⁶ According to classical nucleation theory,³⁷ it could be learnt that the appearance of the interfaces between PS and TPU in the PS/TPU 5% blends would lead to a decrease in the free energy of the cell nucleation. As a result, the cell nucleation in PS/TPU 5% blend would favor over heterogeneous cell nucleation because of its lower energy barrier, resulting in the increment in cell nucleation density.¹⁵

A very interesting phenomenon was observed in the SEM images of PS/TPU blends (Figure 4(c) to (e)) that bimodal cell structure (BCS) appeared when the content of TPU was more than 5%. BCS had the advantages of both large and small cell structures. Large cells could be employed to decrease the bulk density, and small cells could be used to enhance the mechanical and thermal insulation properties.³⁸

Moreover, with the content of TPU increasing from 10% to 20%, the large cell size increased obviously from 3.9 µm to 6.8 µm. Meanwhile, some TPU particles were embraced at a large cell in the blending foams, indicating that the large cells should be nucleated in the interfaces between PS and TPU. The small cells were scattered around large cells with the cell size decreasing slightly from 1.7 µm to 1.3 µm, which did not show significant changes compared with the cell size of pure PS foam. Therefore, it could be speculated from the aforementioned discussion and analysis that large cells should be originated in the interfaces between PS and TPU, while small foams should be generated in the PS matrix. With the increasing content of TPU, the VER of pure PS foam and PS/TPU blending foam had a very slight change. This may be because the low foaming temperature was negative to the large growth of cells in the solid foaming process.³⁹ It could be observed in Figures 4 and 5 that with the increasing content of TPU, the BCS of PS/TPU blending foams became obvious, their large cell size distribution became wide, and their large cell number frequency decreased gradually.

The influence of foaming pressure on the cell structure of PS/TPU 5% blending foams was investigated by varying foaming pressure at the foaming temperature of 65°C. The SEM images and foaming parameters of PS/TPU 5% blending foams at different foaming pressures are shown in Figure 6 and Table 4, respectively. It could be observed clearly that when the foaming pressure was 5 MPa, there were no cells in the PS/TPU 5% blending foams and their VER was 1, due to the very low foaming pressure and depressurization rate. When the foaming pressure reach 10 MPa, the cell nucleation density of PS/TPU 5% blending foams was 4.8 × 10¹⁰ nuclei/cm³ and their cell size was 2.3 µm. As the foaming pressure increased, the cell nucleation density increased as well as the cell size and the area of unfoamed region decreased gradually. When the foaming pressure increased to 20 MPa, the cell nucleation density of PS/TPU 5% blending foams increased to 1.0 × 10¹¹ nuclei/cm³, and their cell size decreased to 1.5 µm. This reason for the phenomenon may be that the increase in saturation pressure led to a higher solubility of CO₂ in the PS/TPU 5% blend, and thus higher homogeneous nucleation.^40,41 As a result, the cell nucleation density of PS/TPU 5% blending foams increased and their cell size decreased, which is demonstrated in Figure 6(b) to (d) and Table 4.

Table 4.

The foaming parameters of PS/TPU 5% blending foams at different foaming pressure.

Foaming pressure (MPa)	VER	Cell nucleation density (nuclei/cm³)	Cell size (μm)
5	1.0	—	—
10	1.2	4.8 × 10¹⁰	2.3
15	1.2	8.1 × 10¹⁰	1.9
20	1.3	1.0 × 10¹ ¹	1.5

PS: polystyrene; TPU: thermoplastic polyurethane; VER: volume expansion ratio.

The effect of foaming temperature on the cellular morphology of PS/TPU 5% blending foam was also investigated at the foaming pressure of 20 MPa. As presented in Table 5, when the foaming temperature was 60°C, the cell nucleation density of PS/TPU 5% blending foam was 1.5 × 10¹¹ nuclei/cm³, its cell size was 1.3 µm, and its VER was 1.2 times. In general, the solubility of CO₂ in the polymer was associated with the change in temperature. Higher temperature would lead to lower CO₂ absorbed, and thus lower homogeneous nucleation.⁴² On the other hand, the viscoelastic properties of a polymer are related to the temperature.⁴³ As the foaming temperature increased, the viscosity of the polymer blends would become low and the cell size would become large. Therefore, it is obvious that when the foaming temperature increased from 65°C to 75°C, the cell nucleation density of PS/TPU 5% blending foam decreased from 1.0 × 10¹¹ nuclei/cm³ to 0.8 × 10¹¹ nuclei/cm³, its cell size increased from 1.5 µm to 1.7 µm, and its VER enhanced from 1.3 to 1.6. It could be found in Figure 8 that the foaming temperature had little effect on the cell size distribution of PS/TPU 5% blending foams.

Figure 8.

Cell number frequency distribution of PS/TPU 5% blending foams at different foaming temperature: (a) 60°C, (b) 65°C, (c) 70°C, and (d) 75°C.

Table 5.

The foaming parameters of PS/TPU 5% blending foams at different foaming temperature.

Foaming temperature (°C)	VER	Cell nucleation density (nuclei/cm³)	Cell size (μm)
60	1.2	1.5 × 10¹¹	1.3
65	1.3	1.0 × 10¹¹	1.5
70	1.5	0.9 × 10¹ ¹	1.6
75	1.6	0.8 × 10¹¹	1.7

PS: polystyrene; TPU: thermoplastic polyurethane; VER: volume expansion ratio.

Foaming mechanism on the pure PS foam and PS/TPU blending foams

Figure 9 shows the solid batch foaming mechanism of pure PS and PS/TPU blends at the foaming temperature of 70°C and the foaming pressure of 15 MPa. The cells in pure PS were only generated through homogeneous cell nucleation, while the cells in PS/TPU blends should be generated through both homogeneous and heterogeneous cell nucleation. When PS/TPU blends were foamed, not only the viscosity and morphology of polymer blends but also solubility and diffusivity of CO₂ as well as the type of cell nucleation determines the final cellular structure. When the content of TPU in the PS/TPU blend was 5%, the size of TPU dispersion phase was smaller than/close to the size of cells formed through homogeneous cell nucleation, monoporous cell structure (MCS) would generate. However, with the content of TPU more than 5%, the size of TPU dispersion phase became larger. The cell nucleation was enhanced at the interfaces between the PS and the TPU due to the low-energy barrier, according to the classical nucleation theory.^16,44 Due to the higher diffusivity of CO₂ and lower viscosity in TPU, the cells generated in the interface regions grew faster and become larger than those nucleated in the PS matrix. As the foaming temperature decreased, the cell structure almost stabilized. TPU in the large cells formed a particle, resulting from the wettability of TPU against PS.⁴⁵ Consequently, BCS was created, in which TPU particle was embraced at a larger cell.

Figure 9.

Schematic diagram for the cellular morphology evolution of pure PS foam and PS/TPU blending foams.

Thermal insulation performance

Microcellular foam had a promising prospect in the fields of thermal insulation material.⁴⁵ The thermal insulation properties of pure PS, pure TPU, and PS/TPU blends and their foams are shown in Figure 10. It could be seen that the thermal conductivity of pure TPU (0.257 ± 0.07 W/mK) was higher than that of pure PS (0.180 ± 0.002 W/mK). The thermal conductivity of PS/TPU blends increased after the addition of TPU, owing to higher thermal conductivity of TPU. As expected, the thermal conductivity of pure PS reduced from 0.180 ± 0.002 W/mK to 0.136 ± 0.001 W/mK after it was foamed. After TPU was added into PS, the thermal conductivity of PS/TPU 5% blending foam decreased to a minimum value of 0.098 ± 0.006 W/mK. As the content of TPU further increased, the thermal conductivity of PS/TPU blending foams increased slightly. This phenomenon could be explained by two aspects. On the one hand, increasing VER would result in a decrement in thermal conductivity for PS/TPU blending foams.⁴⁶ On the other hand, the higher thermal conductivity of pure TPU would enhance thermal conductivity of PS/TPU blending foams. Because the changes in VER were very tiny, the higher thermal conductivity of TPU had a predominant effect on the thermal conductivity of PS/TPU blending foams.

Figure 10.

Thermal conductivity of pure PS, pure TPU, and PS/TPU blends and their foams.

Conclusions

In this article, a facile melt blending and solid batch foaming approach was proposed to prepare microcellular PS/TPU blending foams. Compared with those of pure PS and pure TPU, the η*, G′, and tan δ of PS/TPU blends were improved in rheological test results. SEM images of unfoamed samples displayed that the miscibility between PS and TPU was poor and typical sea island structure appeared in PS/TPU blends. The solubility of CO₂ in the PS/TPU blends increased with the increasing content of TPU, due to the CO₂ solubilization effect of TPU.

MCS exhibited in the pure PS foam and PS/TPU 5% blending foam. A very interesting phenomenon (BCS) was found in the PS/TPU blending foam with the TPU content of 10, 15, and 20%. The appearance of BCS in the PS/TPU blending system relied on not only the viscosity and morphology of polymer blends but also the solubility and diffusivity of CO₂ as well as the type of cell nucleation. As the foaming pressure increased, the cell nucleation density of pure PS foam and PS/TPU blending foam increased largely. The foaming temperature had a small effect on the foaming parameters of pure PS foam and PS/TPU blending foams in the solid batch foaming process. The introduction of TPU into PS was beneficial to improve the thermal insulation property of pure PS foam.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was funded by the National Key Research and Development Program of China (2016YFB0302203 and 2016YFB0302205).

ORCID iD

Hongfu Zhou

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Microcellular morphology evolution of polystyrene/thermoplastic polyurethane blends in the presence of supercritical CO 2

Abstract

Keywords

Introduction

Experiments

Materials

Preparation of PS/TPU blends

Foaming process of pure PS and PS/TPU blends

Characterizations

Rheological measurement

CO2 solubility and diffusivity in pure PS, pure TPU, and PS/TPU blends

Scanning electron microscope

Foaming properties

Thermal conductivity tests

Results and discussion

Rheological properties of pure PS, pure TPU, and PS/TPU blends

Dispersion phase morphology of pure PS and PS/TPU blends

Solubility and diffusivity of CO2 in pure PS, pure TPU, and PS/TPU blends

Cellular morphology evolution of pure PS foam and PS/TPU blending foams

Foaming mechanism on the pure PS foam and PS/TPU blending foams

Thermal insulation performance

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

CO₂ solubility and diffusivity in pure PS, pure TPU, and PS/TPU blends

Solubility and diffusivity of CO₂ in pure PS, pure TPU, and PS/TPU blends