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Merits of the addition of PTFE micropowder in scCO2 foaming of polypropylene: ultra-high cell density, high tensile strength and good sound insulation Chenguang Yang, Zhe Xing, Mouhua Wang, Quan Zhao, and Guo-Zhong Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04644 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Industrial & Engineering Chemistry Research

Merits of the addition of PTFE micropowder in scCO2 foaming of polypropylene: ultra-high cell density, high tensile strength and good sound insulation Chenguang Yang,†,‡,§ Zhe Xing,† Mouhua Wang,† Quan Zhao,† and Guozhong Wu*,†,§ †

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Jialuo Road 2019, Jiading, Shanghai 201800, China ‡University of China Academy of Sciences, Beijing, 100049, China §School of Physical Science and Technology, ShanghaiTech University, Shanghai, 200031, China *Corresponding author: Wu Guozhong Mailing address: P.O. Box 800-204, Shanghai 201800, China Tel/Fax: +86-21-39194531/+86-21-39195118. Email: [email protected] ORCID ID: 0000-0003-3814-2074

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

Abstract Ultra-high-cell-density polypropylene foam was prepared by supercritical CO2 foaming in the presence of porous polytetrafluoroethylene (PTFE) micropowder. The voids of PTFE microparticles were filled with scCO2, causing them to split into multiple granules due to the force of its expansion during the pressure release, resulting in the formation of a large number of nucleation sites. The cell density of this foam reached 1010～1011 cells/cm3, which was 2 to 5 orders of magnitude higher than pristine PP foam. We proposed that the nanoscale granules resulting from the splitting of PTFE microparticles, and the growth of nucleated small cells generated the local strain field variation in the multiple-phase system were responsible for the considerably increased nucleation number of PP foaming. Additionally, the tensile strength and sound absorption property of PP/PTFE foam were largely enhanced and the preparation of ultra-high-cell-density PP/PTFE foam was easily controlled over a wider foaming temperature window. Keywords: PP foam, PTFE microparticle, cell density, scCO2 foaming, tensile and sound absorption property


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1. Introduction Polypropylene (PP) foam is normally prepared with a wide range of cell size distributions and a low cell density, which mainly affects the mechanical properties and thus the suitable applications of the material 1, 2. Since high cell-density PP foam has a higher modulus, tensile strength, and chemical resistance, it can be used in heat- and sound-insulations as well as bearing applications 3-8. Due to its renowned merits, high cell-density PP foam has also attracted much attention for use in other applications, including electronic packaging, automotive, ship building, and aerospace industries 3-7, 9-12

. Owing to the very-low weak melt strength, melt elasticity and high crystallinity, it

is difficult for linear PP to foam with higher cell density. Consequently, foamed PP usually has large cell size and non-uniform cell distribution and thus not good mechanical property 1, 2. It was found that the addition of nanoparticles could enhance the heterogeneous nucleation of PP during the foaming process 13-17, which was in favor of the formation of high cell density

18-20

. Zhai et al.

14

attempted to improve the cell

density of homo polypropylene foam by adding nanoclay as a nucleation agent. They found that the addition of clay facilitated an increase in cell density from 105 to 108 cells/cm3. Many approaches have been tested to increase the nucleation density, improve the foaming processability of PP and enhance the tensile property of the foam

13, 14

.

Graphene, carbon nanotubes, and carbon nanofibers have been added to prepare highcell-density polymer foams by scCO2 foaming. However, the costs of these nanoparticles are high, making it difficult to use them for the high-volume production 3 ACS Paragon Plus Environment


of polymer foams. More recently, virgin polytetrafluoroethylene (PTFE) powder was used to improve the melt strength of polymer 21-24. PTFE has also attracted attention for its high dispersion property and thermal stability

25

, and it also shows promise as a

nucleation agent for polymer foams. Herein, we present a novel and effective method for preparing of ultra-high-cell-density PP foam by scCO2 foaming, with the addition of low molecular weight PTFE microparticles containing many nanoscale voids (Figure 1). The cell density was easily controlled by varying the amount of PTFE micropowder added to the PP. The cellular morphologies (cell size, cell density and cell size distribution), tensile strength, and acoustic property of the neat PP and PP/PTFE foam materials were investigated in detail. The effect of PTFE microparticles on the foaming behaviors of the PP/PTFE, in particular cell density, were analyzed. In addition, it was found that an enhanced acoustic property and higher tensile property of this foam was obtained compared to neat PP foam. It is believed that this work can provide a more effective strategy for fabricating PP foam with controlled cellular structure, high cell density and tensile strength, and outstanding noise reduced coefficient (NRC).

2. Experimental Procedures 2.1 Materials PPH T03 (pellets, isotactic) with a density of 0.91 g/cm3. It displays a melt flow index (MFI) of 3.0 g/10 min (230 °C/2.16 kg) and was purchased from Sinopec Shanghai Chemical Co. Low molecular weight PTFE micropowder F481 (particle size: 2～4 μm) was purchased from Jia Shan Senga technology Co., Ltd. Carbon dioxide with a purity of 99.95% was supplied by Xiangkun Special Gases of Shanghai. 4 ACS Paragon Plus Environment

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2.2 Sample preparation The isotactic PP (iPP) pellets and PTFE micropowder were vacuum dried at 60 ℃ for 4 h before they were mixed. A series of blends of PP with PTFE micropowder with contents of 0.5, 3.0, and 5.0 wt % were made at 190 ℃ using a single screw melt compouder (Thermo Haake PolyDrive). PP/PTFE sheets with 1 mm thickness were prepared by hot pressing. These samples were coded as PP/PTFE(0.5), PP/PTFE(3.0), and PP/PTFE(5.0), respectively. 2.3. Foaming process The parameters of the foaming device was previously described in the literature 7, 26, 27

. PP/PTFE sheet sample (4～5 g) was placed in an autoclave and the autoclave was

pressurized with CO2 using a high-pressure liquid pump. And then the autoclave was heated to preset temperature. The system was kept at the preset temperature and pressure for 2 h 7, 28. Then the vessel was depressurized and venting CO2 in less than 10 s. After that, the sample was removed from the vessel and allowed to cool to room temperature. 2.4. Foam characterization The morphology of the PP/PTFE foams was examined by a scanning electron microscope (Zeiss MERLIN Compact 14184). Samples were immersed in liquid nitrogen for 2 min, fractured, and mounted on stubs. 2.5 Morphology observation of the Foam Microstructure morphology of the PP/PTFE foams was characterized by measuring the cell density and average cell size. Image Pro-plus software was used to 5 ACS Paragon Plus Environment


analyze the SEM photographs. The average diameter of the cells in the micrographs, D, was calculated using Eq. (1). D=

∑ di ni ∑ ni

,

(1)

where ni is the number of cells with a perimeter-equivalent diameter of di . In order to ensure the accuracy of the average pore size measurement, 𝑖 is greater than 200. The densities of foamed specimens, 𝜌𝑓 , were determined by deducing the Archimedes Law involving weighing polymer foam in water with a sinker using an electronic analytical balance (HANG-PING FA2104), calculated using Eq. (2). 𝜌𝑓 = (

a a+b-c

) ρw

,

(2)

where a, b and c are the weight of the specimen in air without sinker, the totally immersed sinker and the specimen immersed in water with sinker, respectively, and ρw is the density of water. The volume expansion ratio of the foamed PP, Rv , was the ratio of the bulk density of pristine PP (ρs ) to that of the foamed PP (ρf ), calculated as follows: ρ

Rv = ρs ,

(3)

f

The cell density (Nf ) was determined by the number of cells per unit volume of the foam, calculated using Eq. (4). Nf =(

nM2 A

3⁄2

)

, (4)

where n, A and M are the number of cells in the micrograph, the magnification of micrograph, and the area of micrograph (cm2), respectively 29. The cell density, N0 , defined as the number of cells per unit volume of the unfoamed sample, was calculated by 6 ACS Paragon Plus Environment

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N0 =Nf ×Rv , (5) 2.6. Tensile testing Tensile tests of the foams were carried out using a universal testing machine (Instron 5943). The foam samples were cut into 2 mm × 4 mm × 22 mm, and the specimens were measured at room temperature in accordance with ASTM D-638 at a speed of 50 mm/min. 2.7 Sound absorption coefficient measurement An impedance tube device (BSWA SW260, BSWA Technology Co., Ltd) was used to measure the change of sound absorption coefficient following the ISO standard (ISO10534-2) based on the transfer function method. The absorption coefficient is defined as the ratio of the acoustic energy absorbed by a sample (Iincident - Ireflected) to the incident acoustic energy (Iincident) as a function of frequency. Six measurements were performed for each sample in the frequency range from 20 to 2100 Hz with two 1/4 inch microphones.

3. Results and discussion 3.1. Effect of addition amount of low molecular weight PTFE micropowder on the scCO2 foaming behavior of iPP. The cell morphologies and cell size distributions of neat PP and PP/PTFE foams prepared at 154 °C and 20 MPa are shown in Figure 2. It can be clearly seen that the cell size of the foamed samples decreases as the PTFE loading increases. Moreover, the cell size distribution becomes narrower than the foams of long-chain branched polypropylene (LCB-PP) containing 3 wt.% PTFE nanofibers 23. This indicates that low 7 ACS Paragon Plus Environment


molecular weight PTFE micropowder has an excellent dispersion stability in PP

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30

,

allowing it to increase the nucleation number while providing a uniform distribution and a lower cell size. The SEM micrographs of fractured surface of the PP/PTFE samples are shown in Figure S2. In addition, the presence of PTFE microparticles suppressed the cell growth 31, which induced a uniform cell size in the PP/PTFE foams. Cell density is a very important parameter for polymer foam materials. A change in cell density can greatly influence the potential applications of a foam material. Cell density has been widely used to study cell nucleation during the foaming process. The average diameter and cell density of the cellular structure of the PP/PTFE foam materials are summarized in Figure 3. Smaller cell sizes within the cellular structures were obtained for the PP/PTFE foams compared to the neat PP foam. Compared to other research for PP foams

21, 32

, the cell size declined significantly (from more than

70 μm to 7.6 μm) as the loading of PTFE micropowder increased. Of particular interest are the remarkably high cell densities (1010 cells/cm3) of the PP/PTFE foams and the controllable cell density by varying the amount of PTFE micropowder added. The cell densities were 2 to 5 orders of magnitude higher than those of PP foams previously reported in the literature (105～108 cells/cm3)

20, 21, 23

. The reason for this is vividly

shown in Figure 4 (a). Many voids in nanosacle size (50～150 nm) can be seen in the low molecular weight PTFE microparticles (Figure 1). CO2 can enter into these voids at the preset pressure and temperature (154 °C and 20 MPa). As CO2 diffuses into the mixed phases and reaches a saturated equilibrium state, the void channels of the PTFE microparticles are also filled with pressurized CO2. The pressure drops rapidly when 8 ACS Paragon Plus Environment

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the saturated CO2 is released from the void channels of the PTFE microparticles, causing nucleation. The low molecular weight PTFE microparticles were split into many nanoscale granules by the force of CO2 expansion, and the nanoscale granules decreased the energy barrier to cell nucleation

33

. The appearance of these nucleation

sites then triggered the generation of cells around them via explosive dispersion and nucleation

19

. During this process, the PTFE microparticles are divided into smaller

granules, with each smaller granule acting as a nucleation site during the foaming process. SEM images of the original PTFE microparticles and PTFE microparticles present in PP foams are shown in Figure 4 (b) and (c). It can be clearly seen that the PTFE microparticles in the foam are much smaller than the original PTFE microparticles, which confirms that original PTFE microparticles are split into many smaller nanoscale granules during the foaming process. The obtained nanoparticles largely increases the number of nucleation sites at the starting foaming, leading to high cell density. Furthermore, the existence of these PTFE microparticles in PP restricted the cell growth, which combined with the enhanced heterogeneous nucleation, resulted in a decrease in cell size and an increase in cell density of PP/PTFE foams. 3.1.1 Acoustic property As the environmentally-friendly or recyclable material, PP foam has attracted more attention to improve the acoustic property, it shows the promise as the substitute of polyurethane foam, which is commonly used as sound insulators 34. Figure 5 shows the absorption coefficient of neat PP foam and PP/PTFE foams at the frequency spectrum of 20-2000 Hz. This foam shows a visible absorption peak over a wide range 9 ACS Paragon Plus Environment


of 900-1600 Hz, where human sensitivity to noise is high

35

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. The PP/PTFE foams

showed higher absorption capacity than the neat PP foam in the tested frequency spectrum and the foams demonstrated an increasing absorption coefficient with an increase of PTFE content in PP. There is a high absorption coefficient in a wider frequency spectrum of 600 - 1800 Hz for PP/PTFE foam comparing to other polymer foams investigated

36, 37

. The high cell density of PP/PTFE foam indicated more cell

walls existing, which largely increased the vibration, dissipation of the cell walls and the number of sound wave reflections, led to an increase in the absorption coefficient 38-41

. In addition, the increased number of air interlayers between inside cavities also

enhanced the absorption efficiency of PP foam 36. In order to further analyze the sound absorption efficiency, the noise reduction coefficient (NRC, arithmetic mean of absorption coefficient at 250, 500, 1000, and 2000 Hz) is generally used to indicate the ability of a material to absorb sound

42, 43

. Figure 5(b) shows the values of noise

reduction coefficient of the neat PP and PP/PTFE foams. It can be observed that with the increase of PTFE micropowder percentage, the value of NRC also increases. The PP/PTFE(5.0) foam possessed the maximum NRC of 0.59, which was almost twice that of the neat PP foam, and the value of NRC was very close to the commonly used acoustic polyurethane (PU) foam 43. 3.1.2 Tensile property The stress-strain curves for the neat PP foam and the PP/PTFE foams prepared at 154 °C and 20 MPa are shown in Figure 6. The stress strength of the PP/PTFE foam was improved significantly from 6.1 MPa to 9.7 MPa, and the tensile strains of 10 ACS Paragon Plus Environment

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PP/PTFE(0.5), PP/PTFE(3.0) and PP/PTFE(5.0) foams also increased substantially, which reached up to 280 %. For polymer foams, small cell size and high cell density are favorable in terms of mechanical property 44, 45. During the tensile process, the cells were stretched from circle shape to elliptical, neighboring cell walls began to connect to each other, which increased the tensile strain and stress 46. These results indicate that high-cell density foam materials can greatly increase the tensile stress and tensile strain at break, which is preferable for potential applications 47. 3.2 Controlling of foaming behavior of PP/PTFE composite 3.2.1 Effect of saturation pressure on foaming behavior of PP/PTFE(3.0) Figure 7 shows the influence of foaming pressure on the cell morphology of PP/PTFE(3.0) at 154 ℃. It can be seen that all the samples were foamed regardless of the pressure (10, 15, 20, or 25 MPa). However, the sample was not foamed well at 10 MPa, with some non-foamed regions (yellow dotted circles) as shown in Figure 7 (a). It was because CO2 cannot saturate PP/PTFE(3.0) completely at 10 MPa, making it difficult for PP/PTFE to foam well and the non-foamed regions might be the crystalline domains, which suppressed the cell growth, resulting in small cell size of PP/PTFE(3.0) foam 15. The visible non-foamed regions reduced as the saturation pressure increasing and the remaining small non-foamed areas were filled with tiny holes as shown in Figure 7 (c' and d'). The possible reason was that some amount of CO2 could saturate PP matrix before foaming at higher saturation pressure. Interestingly, the existence of PTFE nanoscale granules could decrease the crystal growth rate and thereby facilitated the CO2 foaming of PP/PTFE(3.0) 15. In the multiple-phase (PP, PTFE, and CO2) system, 11 ACS Paragon Plus Environment


the high matrix stiffness restrained the cell growth, which combined with the enhanced heterogeneous nucleation, led to an obvious decrease in cell size and an increase in cell densities of PP/PTFE(3.0) foams

16, 17

. Cell size and cell density of the foamed

PP/PTFE(3.0) were further performed to obtain more convincing evidence of the foaming behaviors at different pressure in Figure S3, which were consistent with the discussion above. Figure 8 shows the changes of tensile stress at break and the noise reduction coefficient (NRC) of foamed PP/PTFE(3.0). It is observed that tensile stress decreases as the saturation pressure increasing, but the tensile stress of PP/PTFE(3.0) foams prepared at different pressures are still higher than neat PP foam (6.1 MPa). The change in tensile strength is related to the foam density 46, and the volume density of this foams is shown in Figure S5. It was observed that volume density decreased as the saturation pressure increasing, which was similar to the change of tensile stress. The existence of many non-foamed crystalline regions will lead to a relatively higher tensile strength 48. The noise reduction coefficient (NRC) of PP/PTFE(3.0) foams increases with the foaming pressure increasing as shown in Figure 8(b). High saturation pressure also implies a higher pressure release rate during the foaming process, which increases the expansion ratio of foams and leads to a higher absorption coefficient of the foam 36. At pressures of 17.5, 20, and 25 MPa, there is a relative good acoustic property for PP/PTFE(3.0) foams, which may be the potential substitute of polyurethane foams. 3.2.2 Effect of saturation temperature on foaming behavior of PP/PTFE(3.0) Figure 9 shows the influence of saturation temperature on the cell morphology of 12 ACS Paragon Plus Environment

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PP/PTFE(3.0) at 20 MPa. The samples were not foamed completely at 147 and 150 °C, with many non-foamed regions observed especially at 147 °C. Although a very small amount of CO2 could be dissolved in the PP/PTFE matrix before melting, but the temperatures were low for PP/PTFE samples to melt, hence CO2 could not saturate the mixed phases of PP/PTFE(3.0) samples well during the saturation period. Figure S6 shows the changes of cell density and average diameter of foamed PP/PTFE(3.0) at different foaming temperatures under a pressure of 20 MPa. The foams possessed higher cell density and smaller cell size at 153, 156, and 159 °C. It was indicated that the preset foaming conditions were suitable for the PP/PTFE(3.0) samples to foam. Still some non-foamed regions were generated in the prepared samples, and some of these regions were crystalline domains as shown in Figure 9 (yellow dotted circles). Many tiny cells formed in the non-foamed regions indicated that the crystalline domains might suppress the cell size and lead to a nonuniform cell structure in the foams 15. An increase in foaming temperature tended to enhance the cell growth rate, resulting in an increase expansion ratio 15, 49. Figure 10 shows the change of tensile stress and noise reduction coefficient (NRC) of foamed PP/PTFE(3.0). It can be seen that the tensile stress decreased from 27 MPa to 8.4 MPa as the foaming temperature increasing. Inversely, there is an increase in NRC of the foams, it is apparent at 153~156 ℃. Both the changes are closely related to the parameters of morphologies of the foamed samples, for instance, volume density. The change of volume density is shown in Figure S7. It was found that the volume density distribution performed an approximate “S” shape under different foaming temperatures. Lower volume density meant a higher expansion ratio, 13 ACS Paragon Plus Environment


which indicated weak tensile strength and strong acoustic property. These interesting results indicate that this controlling of preparation of high cell density of PP foam can be easily obtained with high tensile and acoustic properties. 3.2.3 Qualitative foaming window (temperature and pressure) for PP/PTFE(3.0) The suitable pressure and temperature window for preparing ultra-high-celldensity foams was also studied in this work, as this is critical for the continuous preparation of PP foams. In general, a larger foaming window can facilitate the foaming process with high cell density. The matlab fitting diagram of the suitable foaming window for the preparation of ultra-high-cell-density PP/PTFE(3.0) is shown in Figure 11. It is seen that the foaming temperature window for PP/PTFE(3.0) is 8～9 °C, which is slightly wider than the foaming window for linear PP containing 4 wt.% PTFE nanofibers as this material has a foaming window of 7 °C 21. At the foaming conditions of fixed temperature (150 to 158 ℃) and pressure (15 to 25 MPa), the prepared foams possessed a high cell density and exhibited good mechanical properties. These results indicate that the addition of PTFE microparticles can largely broaden the foaming window of isotactic polypropylene (iPP), which will facilitate the fabrication of PP foam by scCO2 foaming.

4. Conclusions A controllable ultra-high-cell-density PP foam was prepared by the addition of low molecular weight PTFE micropowder to increase the nucleation sites. Highpressure scCO2 entered the voids of the PTFE microparticles, and the microparticles were split into many granules serving as nucleation sites, greatly increasing the cell 14 ACS Paragon Plus Environment

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density to 1010 cells/cm3. The tensile strength at break (from 6.1 MPa to 9.7 MPa), the elongation at break (from 120% to 280%) and acoustic property (NRC, from 0.33 to 0.59) were considerably improved comparing to the neat PP foam, as these properties depended on cell density, cell size and volume density. The preparation of high celldensity of PP/PTFE foam can be easily obtained under different foaming pressure and temperature, with high tensile strength and absorption coefficient. Finally, PP/PTFE blend possessed a broader foaming window, indicating a possible application in a continuous extrusion foaming process. Associated Content Supporting Information. DSC analysis of PP/PTFE samples (Figure S1); Distribution of PTFE particles in PP matrix(Figure S2); Cell size, cell density, cell size distribution and volume density of foamed PP/PTFE(3.0)(Figure S3, S4, S5, S6, S7); Discussion of complex viscosity of neat PP and PP/PTFE at different frequency (Figure S8). Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 11079048).



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Figures

Figure 1. (a) Low magnification and (b) high magnification SEM images of PTFE microparticles used in the PP foaming.

Figure 2. SEM micrographs, and cell size distributions of foamed (a) neat PP, (b) PP/PTFE(0.5), (c) PP/PTFE(3.0), and (d) PP/PTFE(5.0), all prepared at 154 °C and 20 MPa.



Figure 3. (a) Average cell diameters and (b) cell densities of the neat PP and PP/PTFE foams prepared at 154 °C and 20 MPa.

Figure 4. (a) Schematic of preparation high cell density PP foam by explosive nucleation of PTFE microparticles. SEM micrographs of (b) original PTFE microparticles and (c) foamed PP/PTFE (5.0) with the exploded PTFE microparticles circled in red.


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Figure 5. Absorption coefficient (a) and noise reduction coefficient (NRC) (b) for the foamed PP and PP/PTFE prepared at 154 ℃ and 20 MPa.

Figure 6. Strain-stress curves of the foamed PP/PTFE at 154 ℃ and 20 MPa.



Figure 7. Low- and high-magnification SEM micrographs of PP/PTFE(3.0) foam prepared at 154 ℃ and different pressure: 10 MPa (a) and (a'), 15 MPa (b) and (b'), 20 MPa (c) and (c'), and 25 MPa (d) and (d').

Figure 8. Tensile stress (MPa) and noise reduction coefficient (NRC) of PP/PTFE(3.0) foam prepared at 154 ℃ and different saturation pressure.


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Figure 9. SEM micrographs of PP/PTFE(3.0) foam prepared at 20 MPa and different temperature: 147 ℃ (a), 150 ℃ (b), 153 ℃ (c), 156 ℃ (d), and 159 ℃ (e).

Figure 10. Tensile stress (MPa), and NRC of PP/PTFE(3.0) foams prepared at 20 MPa and different temperature (147, 150, 153, 156, and 159 ℃).



Figure 11. A diagram of the matlab-simulated foaming window suitable for fabricating ultra-high-cell-density PP/PTFE(3.0).

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