Better scCO2 foaming of polypropylene via earlier crystallization with

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Better scCO2 Foaming of Polypropylene via Earlier Crystallization with the Addition of Composite Nucleating Agent Chenguang Yang,†,‡,§ Zhe Xing,† Mouhua Wang,† Quan Zhao,† Minglei Wang,†,‡ Maojiang Zhang,†,‡,§ and Guozhong Wu*,†,§ †

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Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Jialuo Road 2019, Jiading, Shanghai 201800, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § School of Physical Science and Technology, ShanghaiTech University, Shanghai 200031, China S Supporting Information *

ABSTRACT: A method of earlier crystallization was applied to prepare high-performance polypropylene (PP) foam using supercritical CO2 (scCO2) with the addition of a composite nucleating agent (CNA). Increased crystallization point and decreased crystal size of PP were obtained in the presence of CNA. Earlier crystallization induced cell formation in an early stage of foaming; this, combined with enhanced heterogeneous nucleation caused by microcrystal growth, resulted in well-defined cellular structure. The cell size of PP/CNA foam decreased from 87 to 30 μm, and the cell density was more than 20 times that of the neat PP foam. Moreover, the first compression stress of PP/CNA foam increased from 125 to 180 kPa compared to neat PP foam. We proposed that earlier crystallization promoted the emergence of a large number of microcrystals at the beginning of the foaming process, which effectively suppressed cell growth and prevented cell rupture and collapse. foaming process.6,19−21 However, the PP foams described above are difficult to recycle because of their poor cleanliness. The foaming behavior of polymers is strongly influenced by variations in the crystallinity,22−28 which thereby determine the cell morphology, expansion ratio, and crystallinity of the resultant foams. Crystallite-induced heterogeneous nucleation generally increases the cell density and decreases the cell size.6,12,20,29−31 The evolution of the cell structure with the crystallinity of saturated polymers has been systematically investigated, and the strong plasticization effect of compressed CO2 has been found to greatly increase the crystallization rate of polymers, leading to the development of crystallinity.22,23 Interestingly, the crystal domains that formed in situ supplied nucleation sites to enhance cell nucleation and acted as physical cross-linking sites to stabilize the cell structure.23,32 Herein, we report for the first time the preparation of high-performance PP foam plastics via an earlier crystallization method using scCO2. Significantly enhanced heterogeneous nucleation was obtained by earlier crystallization of PP using a composite nucleating agent (CNA). The obtained PP/CNA foam exhibited a large cell density and small cell size with high cleanliness and was easy to

1. INTRODUCTION As one of the most useful polymer materials, polypropylene (PP) foam has long been attractive owing to its high strength and modulus, good thermal stability, and superior chemical resistance; in addition, it is easily recycled and can be used in thermal and acoustic insulation and bearing applications.1,2 Owing to its low melt strength and melt elasticity and high crystallinity, it is difficult to produce PP foam with a higher cell density.3−6 Consequently, PP foams usually exhibit a large cell diameter, nonuniform cell size distribution, and low cell density, as well as poor mechanical properties. In recent years, considerable effort has been made to optimize the PP foaming process to decrease the cell size and increase the cell number.7−10 It was found that the addition of nanoparticles to PP could provide heterogeneous nucleation sites to increase the cell density, decrease the cell size, improve the cell uniformity, and simultaneously reinforce the polymeric matrix.6,11,12 Nanoparticles such as graphene,13 carbon nanotubes,14 nanoclay,15 hollow molecular sieves,12 and carbon nanofibers16 have been widely applied to obtain high-cell-density microcellular polymer foaming by supercritical carbon dioxide (scCO2). In the previous studies, well-defined microporous polymeric foams were also prepared by extensional stress inducing with the presence of nucleating agents (e.g., talc particles).17,18 Moreover, poly(tetrafluoroethylene) powder was used to improve the melt strength of PP and enhance cell nucleation during the © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

August 13, 2018 October 17, 2018 October 25, 2018 October 25, 2018 DOI: 10.1021/acs.iecr.8b03866 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

glass slides was placed on the stage in the chamber of the visualization system. The stage can be heated and controlled by a controller. The specimen was rapidly heated from 25 to 180 °C at 15 °C/min and then maintained at 180 °C for 10 min to remove its prior thermal history. Thereafter, the specimen was cooled at 10 °C/min and was maintained at 120 °C to observe the crystallization process. The morphology of the foams was observed using a scanning electron microscope (SEM; Zeiss MERLIN Compact 14184, Germany). Samples were immersed in liquid nitrogen for 2 min, fractured, and mounted on stubs. They were then sputter-coated with gold to prevent charging during the test. 2.5. Morphological Observation of the Foams. The microstructural morphology of the PP/CNA foams was characterized by measuring the cell density and average cell size. Image-Pro Plus software was used to analyze the SEM photographs. The average diameter D of the cells in the micrographs was calculated using eq 2.

recycle. In addition, the resultant PP/CNA foam exhibited unusually high tensile strength and tensile strain, and the compressive stress reached 180 kPa. This finding provides a new easy strategy for fabricating PP foam with well-defined cellular structure and outstanding mechanical properties.

2. MATERIALS AND METHODS 2.1. Materials. Isotactic polypropylene T03 (iPP) with a density of 0.91 g/cm3 and a melt flow index of 3.1 g/10 min (230 °C/2.16 kg) was purchased from Sinopec Shanghai Chemical Co. The CNA HPN-20E was purchased from Milliken. Carbon dioxide with a purity of 99.95% was supplied by Xiangkun Special Gases of Shanghai. 2.2. Sample Preparation. The PP pellets and CNA were vacuum-dried at 60 °C for 4 h before they were mixed, and the addition amount of CNA was 0.3 wt %. A two-screw extruder (Thermo Haake PolyDrive 7, Germany) was used to mix them. PP/CNA sheets (20 cm × 20 cm) with a thickness of 1 mm were prepared by hot-pressing at the temperature 190 °C and 20 MPa, and a neat PP sheet was also prepared for comparison. 2.3. Foaming Process. PP sheet samples were placed in an autoclave, and the autoclave was pressurized with CO2 using a high-pressure liquid pump; the parameters of the foaming device have been described in the literature.5,33−37 The system was kept at the preset temperature and pressure for 4 h, and the foaming conditions are shown in Table 1. Then the vessel was

D=

temp (°C)

saturation pressure (MPa)

saturation time (h)

7−9

152

20

4

i a ρf = jjj ka + b −

depressurized and vented in less than 10 s. Finally, the sample was removed from the vessel and allowed to cool to room temperature. 2.4. Sample Characterization. A NETZSCH STA 449 F3 Jupiter differential scanning calorimeter (DSC) equipped with a data station was used to scan the melting transitions of the samples in aluminum pans. The samples were initially heated from 25 to 230 °C at 10 °C/min under an argon flow (20 mL/min), then cooled to 30 °C at 10 °C/min, and again heated to 230 °C at the same heating rate, 10 °C/min. The first heating process was performed to eliminate the thermal history of the unfoamed samples. The melt points and melting enthalpies of the samples were obtained from the analysis software (NETZSCH-Proteus-6). The degree of crystallization was calculated using eq 1. Xc(%) =

ΔHf × 100 ΔHf0

(2)

where ni is the number of cells with a perimeter-equivalent diameter of di. To ensure the accuracy of the average pore size measurement, i is greater than 200. The volume expansion ratio of each sample was calculated as the ratio of the density of the original sample, ρs, to the measured density of the foam sample, ρf. The densities (ρf) of the foam samples were determined from Archimedes’ law by weighing the polymer foam in water with a sinker using an electronic analytical balance (HANG-PING FA2104) and using eq 3 to calculate the density.

Table 1. Foaming Conditions of Neat PP and PP/CNA Samples sample weight (g)

∑ dini ∑ ni

yz zzρ c{ w

(3)

where a, b, and c are the weights of the specimen in air without the sinker, the totally immersed sinker, and the specimen immersed in water with the sinker, respectively, and ρw is the density of water. The cell density (N) was determined as the number of cells per unit volume of the foam, which was calculated using eq 4. iny N = jjj zzz kA{

3/2

ρs ρf

(4)

where n and A are the number of cells in the micrograph and the area of the micrograph (cm2), respectively. 2.6. Tensile and Compression Testing. Tensile tests of the unfoamed and foamed samples were conducted using a universal testing machine (Instron 5943, America). The stretching splines of unfoamed samples were molded into dumbbell shape by a mold according to GB/T1040.2-2006/ISO 527-2:1993, and the foam samples were cut into 2 mm × 4 mm × 25 mm pieces. All the specimens were measured at room temperature in accordance with ASTM D-638 at a speed of 50 mm/min. The compression test of the foams was performed using an MTS universal microtester equipped with a 50 N load cell. The maximum strain was set to 80%, and the compression rate was set to 0.5 mm/min. The samples were compressed twice.

(1)

where ΔHf is the melting enthalpy measured in the heating experiments, and ΔHf0 is the theoretical enthalpy of 100% crystalline PP, 207.1 J/g.38 X-ray diffraction (XRD) patterns were collected by a Bruker D8 Discover apparatus (Bruker, Germany) with Cu Kα as the radiation source (λ = 1.5418 Å). It was operated at 40 kV and 40 mA, and the scanning range was 10−70° at a scanning rate of 2°/min. In addition, an in situ crystallization visualization system (Axio Imager M2m, Carl Zeiss, Germany) was employed to investigate the isothermal crystallization behavior of the PP and PP/CNA samples. A thin-film specimen sandwiched between

3. RESULTS AND DISCUSSION 3.1. Effect of the CNA on the Crystallization of Unfoamed PP. Figure 1a shows the melting and cooling B

DOI: 10.1021/acs.iecr.8b03866 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. (a) DSC heating and cooling curves of neat PP and PP/CNA. (b) Crystallization curves of neat PP and PP/CNA with time at a cooling rate of 10 °C/min. (c) XRD curves of neat PP and PP/CNA. (d and d′) Mechanical properties of neat PP and PP/CNA. Metalloscope micrographs of (e) neat PP and (f) PP/CNA at 130 °C.

curves of the neat PP and PP/CNA samples. The melting point of PP/CNA increased by approximately 0.7 °C compared with that of neat PP. Interestingly, the crystallization temperature increased by approximately 7.0 °C compared with that of neat PP. Moreover, to qualitatively investigate the crystal nucleation ability of the CNA on iPP, the crystallization periods of iPP with and without the CNA were also measured. Figure 1b shows the crystallization process of neat PP and PP/CNA over time. The crystallization time advanced by nearly 1 min at a cooling rate of 10 °C/min. Typical XRD patterns of neat PP and PP/CNA are presented in Figure 1c. The intensity of the diffraction peak of PP/CNA at 2θ = 17.1°, which is the (0 4 0) plane, is significantly enhanced compared with that of neat PP, and the crystallinity of PP/CNA from Figure 1a increased greatly, from 37.7% to 43.3%. Figure S1 shows the thermogravimetric curves of unfoamed neat PP and PP/CNA samples. It can be seen that the starting decomposition temperature of PP/CNA shifted to the lower temperature side. This is attributed to the loss of auxiliary component in CNA.39 The SEM micrographs of fracture surfaces of neat PP and PP/CNA samples are shown in Figure S2. There was great difference of the morphologies, which was caused by the change of crystallization. All the results described above show that the addition of the CNA significantly increased the crystallization temperature of PP, improved the crystallinity, and greatly enhanced the crystallization rate owing to the heterogeneous nucleation induced by the CNA. The added CNA can trigger the formation of more crystallization sites during the crystallization process, resulting in a large number of microcrystals. There is a slight left shift of the diffraction peaks of PP/CNA sample compared to the neat PP. The reason may be the lattice deformation induced by CNA. Figure 1d and d′ show the mechanical properties of neat PP and PP/CNA samples. The tensile strain of neat PP was only approximately 20%, whereas the PP/CNA sample exhibited an ultrahigh strain during the tensile process, which reached 170%.

An increase of impact strength of PP/CNA was obtained. The results indicated that the small crystal size, change of crystallization, and high crystallinity promoted interfacial adhesion between crystal planes and endowed PP/CNA with good mechanical properties.40 The introduction of the CNA induced the formation of microcrystals at a higher temperature, and the size of the microcrystals (Figure 1f) was much smaller than that of neat PP (Figure 1e). Hence, we believe that the enhanced heterogeneous nucleation combined with earlier crystallization of PP/CNA induced the appearance of a large number of microcrystals, which might be beneficial in the subsequent foaming process. 3.2. Morphology and Properties of Neat PP and PP/ CNA Foams. Figure 2a illustrates how the microcrystals acted to suppress cell growth during the foaming process. Foaming is known to be a rapid process for cells growing quickly in few seconds, depending on the thermophysical and rheological properties of polymer/CO2 mixtures.41 The increased crystallization temperature induced the earlier appearance of a large number of microcrystals, which might occur at the same time with the starting of cell growth during the foaming process. We believe that the change of crystallization has great influence on the cellular structure of PP/CNA foam. The cell morphologies of neat PP and PP/CNA foams prepared at 152 °C and 20 MPa are shown in Figure 2b and c, respectively. Clearly, the cell size of the PP/CNA foam decreased greatly. Many cells in the neat iPP foam were cracked, collapsed, and consolidated; they also exhibited poor cell continuity. In contrast, the cells in the PP/CNA foam were roughly polygonal and exhibited good cocontinuous cellular structure with a small cell size. The cell structure parameters are very important for polymer foam materials because they are closely related to the mechanical properties. The average diameter, cell density, and cell distribution of the cellular structure of the neat PP and PP/CNA foams are summarized in Figure 2d and e, respectively. Compared to that of the neat PP foam, the cell size of the PP/CNA C

DOI: 10.1021/acs.iecr.8b03866 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 2. (a) Schematic of the restriction of cell growth by microcrystals during the foaming process. Cell morphologies of (b) neat PP and (c) PP/ CNA foams. Average cell diameter, cell density (d), and cell size distribution (e) of neat PP and PP/CNA foams. (f) Tensile stress−strain and (g) compressive stress−strain curves of PP and PP/CNA foams. All the obtained samples were saturated at 20 MPa and foamed at 152 °C for 10 s.

foam decreased significantly (from 87 to 30 μm), and the corresponding cell density increased from 3.6 × 107 cells/cm3 to 7.3 × 108 cells/cm3, which was more than 20 times that of the neat PP foam. Moreover, the PP/CNA foam exhibited a uniform cell size distribution compared to the neat PP foam, as indicated in Figure 2e. All these results indicated that earlier crystallization promoted the formation of well-defined cellular structure. The crystallization temperature increased by 7 °C compared with that of neat PP, which accelerated cell formation during the rapid foaming process. During the cell growth process, the earlier appearance of microcrystals limited cell growth, in combination with the changes in local stress caused by the growth of microcrystals,42 which tended to trigger the generation of cell sites around the original cell and thus enhance heterogeneous nucleation. Furthermore, it should be noted that enhanced heterogeneous nucleation consumed more CO2, which also suppressed the cell growth.31,41,43 The mechanical properties of PP foams are very important for their application. Figure 2f compares the tensile stress−strain curves of neat PP with those of PP/CNA saturated and then foamed at 20 MPa and 152 °C. A much higher tensile stress and elongation at break are obtained for the PP/CNA foam, indicating that the well-defined and cocontinuous cells help toughen the PP/CNA sample. The compressive behavior of the neat PP and PP/CNA foams is also studied by examining the compressive stress as a function of strain, as shown in Figure 2g. During the compression process, the samples may undergo three phases, i.e., linear elastic process at low stress, long collapse plateau at high loading of stress, and material densification process. The maximum stress of the first compression of

PP/CNA is approximately 180 kPa, which is much higher than that of the neat PP foam. Under a compressive strain of nearly 77%, PP/CNA undergoes a permanent strain of only approximately 25%, being much lower than that of the neat PP foam (36%), and the second compression of PP/CNA foam also possesses a higher compressive stress than neat PP foam. It can be seen that the difference in compression curves mainly happened in the long collapse plateau phase and material densification process. PP/CNA foam possessed well continuous polygonal cell morphology, exhibiting outstanding compression resistance. However, the cell rupture and coalesce in the neat PP foam resulted in higher permanent strain during the compression process, which exhibited lower compressive stress. Additionally, the crystallization changes of the foams were also studied in detail, as shown in Figure 3a, b, and c. The foamed samples exhibit similar melting and cooling curves, and double melt peaks appear in Figure 3a. The melting point and crystallization temperature, as well as the crystallinity, are higher than those of the unfoamed samples in Figure 1a. The reason is that the strong plasticization effect of compressed CO2 greatly increases the crystallization rate of the polymer, which leads to development of the crystallinity.22,23 The PP/CNA foam crystallized earlier than the neat PP foam (Figure 3b), and the intensity of the diffraction peaks of the PP/CNA foam (Figure 3c) were much higher than those of the neat PP foam. The result is a synergistic effect of nucleating agent and plasticization of CO2. Both the curves of foamed samples show the typical diffraction pattern for PP. There were two new peaks appearing at 2θ values of 9.3° and 28.7°, corresponding to the crystalline hexagonal/ trigonal induced by the plasticization of saturated CO2. In the D

DOI: 10.1021/acs.iecr.8b03866 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 3. (a) DSC heating and cooling curves of neat PP and PP/CNA foams. (b) Crystallization curves of neat PP and PP/CNA foams with time at a cooling rate of 10 °C/min. (c) XRD curves of neat PP and PP/CNA foams. SEM micrographs of cellular structure of neat PP (d) and PP/CNA foams (e, f).

Figure 4. Schematic diagram showing the effect of crystals on the structural evolution of the inner region of (a) neat PP and (b) PP/CNA composites. For clarity, the size of each symbol is not proportional to the real size.

dissolved into the melt PP matrix. For the saturation time of 2 h, some nonfoamed regions can be seen in Figure 5a. This indicates that the melt PP matrix was not fully saturated by CO2 in 2 h and the dissolved CO2 was insufficient for foaming of all nucleation sites, resulting in a smaller cell size and nonfoamed regions. On the contrary, for the saturation time 6 h, the dissolved CO2 was sufficient to support the cell growth, and the resulting foam possessed a bigger cell size, as seen in Figure 5c. From the previous discussion, we ascribe the well-defined cellular structure of the PP/CNA foam, with a small cell size and high cell density, to the early crystallization and the limitation of cell growth by microcrystals as well as enhanced heterogeneous nucleation. We believe that the uniform appearance of microcrystals was a key factor in obtaining good foams. To observe the crystallization behavior of the PP matrix, an in situ crystallization visualization system was employed to observe the crystallization process of the neat PP and PP/CNA samples. Figure 6 shows the evolution of the crystal morphology of the neat PP and PP/CNA samples during the isothermal crystallization process at 120 °C. Clearly, the number of nucleated crystal sites was greatly increased and the crystal size was significantly reduced by introducing the CNA. Moreover, the crystallization rate was dramatically increased by the presence of the CNA. All these phenomena confirmed that the CNA was very effective in enhancing crystallization, and the above findings were consistent with the

amorphous region, cells grew easily, and the cells became large. In contrast, the crystalline regions contained some small cells because of the limited crystal growth (Figure 3d, e, and f), as illustrated schematically in Figure 4. However, cell nucleation sites might appear between microcrystals in the PP/CNA phases early in the foaming process, as shown in Figure 4b. Hence, the cell growth rate was limited by adjacent microcrystals. Moreover, the new heterogeneous nucleation sites near crystalline region competed for the limited amount of CO2, which further suppressed the cell growth.41 In addition, the appearance of microcrystals can change the local stress and cause many more nucleation sites to appear, which, in combination with the limitation of cell growth by microcrystals in cells, resulted in a decrease in the cell size and an increase in the cell density of the PP/CNA foam. The cellular structure of PP foam can also be influenced by the saturation time, and it is necessary to gain the optimum saturation time, being important for the practical application. Figure 5 shows the cell morphologies of different PP/CNA foams prepared at different saturation times. It is clearly seen that the cell size increased as the saturation time increased from 2 to 6 h, and all the foams exhibited a good cell size uniformity. Increased amount of dissolved CO2 in PP means a larger amount of CO2 used for the cell growth during the foaming process. With an increase of saturation time, more CO2 was continuously E

DOI: 10.1021/acs.iecr.8b03866 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 5. SEM micrographs and cell size distribution of PP/CNA foams prepared at 152 °C and 20 MPa with different saturation time: (a) 2, (b) 4, and (c) 6 h.

Figure 6. Evolution of crystal morphology of neat PP and PP/CNA composites during isothermal crystallization at 120 °C.

prepare ideal foams may make it possible to break through the bottleneck in current PP foaming technology and broaden the application range of PP.

DSC results. Notably, whereas neat PP possesses a common spherical crystal structure, PP/CNA shows many microcrystals during the early stages of crystallization. The above results further demonstrate that the CNA plays a key role and can greatly improve the cellular structure of PP foam.

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ASSOCIATED CONTENT

S Supporting Information *

4. CONCLUSIONS High-performance PP foams with well-defined cellular structure were prepared using scCO2 by an early crystallization method. It was found that the presence of the CNA can remarkably increase the crystallization temperature and significantly decrease the crystal size of PP. The large number microcrystals resulting from the presence of the CNA could greatly enhance heterogeneous nucleation and increase the cell density considerably. The earlier crystallization started when foaming began; in combination with the appearance of microcrystals, it strongly limited cell growth and prevented cell rupture and collapse. Additionally, the resultant PP/CNA foam exhibited unusually high tensile strength and tensile strain; the compressive stress reached 180 kPa, and the permanent strain of the PP/CNA foam was only approximately 25%. Thus, the discovery of the earlier crystallization method to

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b03866. Discussion of materials thermal stability of unfoamed neat PP, PP/CNA, and foamed PP/CNA samples (Figure S1); morphologies of fracture surfaces of neat PP and PP/ CNA (Figure S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Mailing address: P.O. Box 800-204, Shanghai 201800, China. Tel./Fax: +86-21-39194531/+86-21-39195118. E-mail: [email protected]. ORCID

Guozhong Wu: 0000-0003-3814-2074 F

DOI: 10.1021/acs.iecr.8b03866 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by Science Challenge Project (TZ2018004), the National Natural Science Foundation of China (No.11079048), and the Strategic Priority Research Program of the Chinese Academy of Science with Grants (XDA 21080101).

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DOI: 10.1021/acs.iecr.8b03866 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX