Preparation of microcellular injection-molded foams using different

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Materials and Interfaces

Preparation of microcellular injection-molded foams using different types of low-pressure gas via a new foam injection molding technology Long Wang, Yuma Wakatsuki, Yuta Hikima, Masahiro Ohshima, Atsushi Yusa, Hiromasa Uezono, and Akihiro Naitou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03330 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Industrial & Engineering Chemistry Research

Preparation of microcellular injection-molded foams using different types of low-pressure gas via a new foam injection molding technology Long Wang,†, Yuma Wakatsuki,† Yuta Hikima,† Masahiro Ohshima,†, Atsushi Yusa,‡ Hiromasa Uezono,§ Akihiro Naitou§ †Department

of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan

‡Technology

Development Department, Hitachi Maxell, Ltd., Osaka 567-8567, Japan

§Injection

Molding Division, Japan Steel Works, Ltd., Hiroshima, 736-8602, Japan

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ABSTRACT A new foam injection molding (FIM) machine was recently developed with a physical blowing agent directly delivered from its gas cylinder, while the supercritical fluid pumping unit is a must in the conventional FIM process. Herein, different types of gas including nitrogen (N2), helium (He), carbon dioxide (CO2), and argon (Ar) were used as the physical blowing agent for preparing polypropylene (PP) foams with a low pressure less than 5 MPa. It revealed that CO2 and Ar endowed PP foams with higher cell densities, smaller cell sizes, and larger expansion ratios. In contrast, the poorest cellular structure was obtained for PP foams with He. Moreover, the effect of gas type on mechanical properties of the resulting foams was investigated. Our findings demonstrated that the final cellular structures and mechanical performances of PP foams could be tailored by controlling the type of foaming agent, and this process could certainly be applied for other polymers.

Keywords: Low-pressure gas, Argon, Helium, Polypropylene foams, Foam injection molding

1. INTRODUCTION Microcellular polymeric foams are a field of growing interest both in academia and industry.13 Such foams are usually obtained by using a supercritical fluid such as supercritical nitrogen or supercritical carbon dioxide as a physical blowing agent. Compared with solid polymers, polymeric foams always exhibit elevated energy absorption,4 a higher impact strength and toughness,5,6 a higher strength-to-weight ratio7 and better sound absorption as well as thermal insulation performances.810 These excellent merits can afford microcellular foams wide applications in areas such as packaging materials, sporting equipment, automobile and aerospace products, thermal insulators, and acoustic absorbents.2,3 Moreover,


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the applications of microcellular foams can be extended to new areas such as catalyst carriers, tissue engineering scaffolds, separation, and gas capture as well as storage.11,12 Polymeric foams can be prepared by batch foaming, extrusion foaming, bead foaming, and foam injection molding techniques.13,13 Among these processing methods, foam injection molding (FIM) has been regarded as an efficient and cost-effective approach for producing foam products with threedimensional complex geometries and high dimensional accuracy.14 Additionally, compared with the regular solid injection molding process, FIM products usually exhibit fewer residual marks, shorter molding cycles, less product shrinkage, higher dimensional stability, and a reduced weight.14,15 Reports also have demonstrated that the injection-molded foams possess improved toughness5,1618 as well as higher acoustic and thermal insulation10,19,20 with respect to their solid counterparts. To fabricate injectionmolded foams, the widely-used technique is the commercialized MuCell injection molding technology, which was developed based on the Massachusetts Institute of Technology (MIT) studies.14,21 The MuCell molding configuration is mainly composed of a supercritical fluid pumping system for pressurizing a physical blowing agent and a specifically designed screw.14,21 In the MuCell injection molding process, either nitrogen (N2) or carbon dioxide (CO2) is employed as a physical blowing agent, and it is first pressurized to a level in the range of 15 to 25 MPa by the supercritical fluid pumping system. Then, the pressurized gas is delivered into the polymer melt through an exclusive injector valve in a supercritical state. To simplify the foaming process, we have developed a new FIM technology named resilient and innovative cellular foam injection molding (RIC-FIM) and revealed that the pressurization of a gas such as N2 or CO2 was not a must to obtain the microcellular foam products.22,23 In the first version of our FIM,2224 the gas concentration dissolved in the polymeric melt is governed via the combination of an injector valve and a gas venting unit, which can directly introduce low-pressure gas from the gas cylinder and discharge excess blowing agent from the molten polymer, respectively. Very recently, we have 3 ACS Paragon Plus Environment


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advanced the FIM technology by removing the exclusive injector valve to make the technology totally different from the regular MuCell foaming technology,25 which is called RIC-II FIM or simple optimized foam injection molding technology (SOFIT). The low-pressure gas is directly delivered from a gas cylinder through a delivery hole in the advanced version.25,26 More importantly, the low-pressure gas (approximately 57 MPa) is continuously supplied from the gas cylinder and can maintain the stability of the gas concentration dissolved in the polymer melt in our advanced FIM technique, while in the MuCell foaming technology, the high-pressure blowing agent (usually approximately 1530 MPa) is intermittently introduced into the polymer melt. It clearly demonstrated that the pressure of the gas used could be as low as 1 MPa to produce the microcellular injection-molded foams in the advanced version of our FIM technique.25,26 Regarding polymer foamation, many studies have focused on the effects of the polymer, additives, nanoparticles, and processing conditions on the cellular structures and physical properties of the obtained foams.7,16,23,2731 However, only a few studies have been dedicated to clarify the effect of the gas type on the foams,3234 and even fewer studies have investigated the foaming behavior with different physical blowing agents in the FIM process.35 Kim et al.32,33 compared N2 with CO2 for the foaming behavior of thermoplastic polyolefin and thermoplastic vulcanizates in the batch and extrusion foaming processes and concluded that smaller cell sizes were achieved for the N2 foaming process, while larger expansion ratios were obtained in the CO2 foaming process. Ishikawa et al.35 studied the foaming behaviors of polypropylene (PP) in a MuCell process via a visual observation window by using N2 and CO2 as the foaming agents. They reported that PP foams with much higher cell numbers and smaller cell sizes were obtained in the N2 foaming process compared with the use of CO2. In this study, we studied the effect of the gas type on the foamability of polypropylene via the advanced RIC-FIM technique or SOFIT (Simple optimized foam injection molding technology). To obtain microcellular foams with different expansion ratios, the FIM was conducted with a core-back 4 ACS Paragon Plus Environment

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operation. Different gas types, including N2, helium (He), CO2, and argon (Ar), were used as the foaming agents, and their gas-delivery pressures were kept at as low as 4.8 MPa to produce the injection-molded foams. Besides the effect of the gas type, the effects of the expansion ratio and dwelling time on the final cellular structures of PP foams were investigated. Finally, the effects of the gas type on the thermal behavior and mechanical properties of the resulting foams were discussed.

2. EXPERIMENTAL SECTION 2.1. Materials High tacticity isotactic polypropylene (iPP), model F133A, was supplied from the Prime Polymer Co., Ltd., Tokyo, Japan and was used as received. Its weight-average molecular mass (Mw) and melt flow rate (MFR) were 379 kg/mol and 3.0 g/10 min (230 C/2.16 kg), respectively. The 1,3:2,4 bis-O-(4methylbenzyliden)-D-sorbitol gelling agent (Gel-all MD, New Japan Chemical), which is an efficient crystal nucleating agent as well as a foaming agent for PP,36 was used to enhance the foaming behavior of iPP, and 0.5 wt% of MD was used here. Four different types of high purity (higher than 0.99) gas, including N2, He, CO2, and Ar, were used as the physical foaming agents, which were provided by the Showa Denko Gas Products Co., Ltd., Kanagawa, Japan.

2.2. Foam injection molding with core-back operation A 35-ton clamping force electric injection molding machine (J35AD-AD30H, Japan Steel Work, Ltd. Hiroshima, Japan) with a 22-mm screw diameter was used to prepare the foamed samples. Different from the conventional FIM machine equipped with MuCell technology, there is no injector valve and supercritical pumping system in the RIC-FIM machine, and low-pressure gas can be directly supplied from its gas cylinder and introduced into the molten polymer through a delivery hole, which was described in detail in our previous studies.25,26 The mold consisted of a rectangular cavity, and the dimensions were



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70 mm  50 mm  2 mm. To produce different expansion ratios of specimens, the FIM experiments were conducted with a core-back operation. Consequently, 2-, 3-, 4-, and 5-fold expansions of PP foams were acquired by shifting a portion of the moveable part of the mold from 0 to 2, 4, 6, and 8 mm, respectively.7,25 To study the influence of the gas type on the cellular structures of PP, the gas pressure used was the same for all of the blowing agents, and ultra-low-pressure gas of approximately 4.8 MPa was used for each case. Hereafter, the PP foams prepared using N2, He, CO2, and Ar as the foaming agents were notated by N2-foams, He-foams, CO2-foams, and Ar-foams, respectively. The pressure and temperature of the polymer in the mold cavity are monitored in situ using a data device (Mold Marshaling system EPD-001, Futaba, Chibea-ken, Japan).36,37 The detailed variables for the FIM process are listed in Table 1.

Table 1. Processing parameters used in the RIC-FIM process. Parameters

Values

Melt temperature (C)

200

Injection speed (mm/s)

70

Injection pressure (MPa)

140

Packing pressure (MPa)

30

Gas pressure (MPa)

4.8

Back pressure (MPa)

8

Mold temperature (C)

40

Core-back distance (mm)

2, 4, 6, 8

Dwelling time (s)

6.28.0

Core-back rate (mm/s)

20

Shot size (mm)

45

2.3. Foam characterization To measure the cell size and cell density, the microstructures of foam samples were investigated using a scanning electron microscope (Tiny-SEM Mighty-8, Technex, Japan) operating at an accelerating voltage of 17 kV. A small slice used for the morphological observation was cut from the center of the injection-molded article and cryogenically fractured in liquid nitrogen. Prior to observation, the fractured 6 ACS Paragon Plus Environment

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surface of the prepared sample was gold-coated using a VPS-020 Quick Coater (ULVAC KIKO, Ltd., Japan). Then, SEM images were analyzed using Image J software (National Institutes of Health, US), and the cell density, N0, was calculated from the following expression:38 n N 0  ( )3/2 A

(1)

where n is the number of cells in the selected micrograph and A is the area of the micrograph. Consequently, the average cell diameter is obtained from the SEM micrographs assuming that the cell shape is spherical.37,42

2.4. Open cell content A gas pycnometer (AccuPycII, Shimadzu, Kyoto, Japan) was used to measure the open cell content (OCC) of the foam samples. The specimen volume measured by the pycnometer precludes the sample’s open pore volume, and thus, the OCC was acquired by:

OCC = (1 

Vmeasure ) 100% V polymer

(2)

where Vmeasure is the volume measured by the gas pycnometer and Vpolymer is the overall volume of the measured sample.

2.5. Sorption measurements The solubilities of different gases in the PP matrix were obtained using a simple gravimetric method.3941 According to the gravimetric approach, a specimen was measured using an electronic balance before and after sorption in a pressure vessel to calculate the solubility of the gas in the polymer. The weighed PP specimen was loaded in a high-pressure autoclave and immersed in different gases at 200 C and 4.8 MPa for more than 1 hour. Then, the solubilities of various gases in PP were calculated, and the measurement was repeated at least three times for each gas. 7 ACS Paragon Plus Environment


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2.6. Differential scanning calorimetry analysis The thermal behaviors of the solid and foam samples were investigated by using differential scanning calorimetry (DSC 7020, Hitachi High-Tech Science Corporation, Tokyo, Japan) under a nitrogen atmosphere. Each specimen was heated from 30 to 200 C at a heating rate of 10 C/min, and then, the melting thermogram was obtained. The sample weight was approximately 57 mg and was selected from the center region of the injection-molded part. The area and temperature of the endothermic peak were taken as the heat of fusion (f) and melting peak temperature (Tp), respectively. Then, the crystallinity (Xc) of the injection-molded sample was calculated using the following expression:

X c (%) 

H f H 0f  f

100%

(3)

where f and H 0f are the weight fraction of iPP and the standard melting enthalpy of iPP, respectively. The f of iPP is 99.5% and the H 0f is 207 J/g for the -phase of iPP.16

2.7. Mechanical properties A universal testing instrument (Autograph AGS-1kN, Shimadzu, Japan) was used to investigate the compression properties of the injection-molded specimens. Cubic-shaped samples with a side length of 10 mm, cut from the middle of the injection-molded parts, were used for compression tests. A crosshead speed of 1 mm/min was used to conduct the tests, and at least five specimens were measured for each condition.

3. RESULTS AND DISCUSSION 3.1. Influence of the gas type and core-back timing on cell structures


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Typically, injection-molded foams consist of a thin solid layer (skin layer) and a large portion of core layer (foam area).17,42 Herein, we mainly focused on the cellular morphology in the core layer, and the SEM images at core regions of PP foams prepared by using different gases as the physical blowing agent are shown in Figure 1. Foams were observed from the direction perpendicular to the core-back direction and produced at a fixed gas pressure of 4.8 MPa. As displayed in Figure 1, microcellular PP foams with a 2-fold expansion ratio (void fraction of 50%) were successfully fabricated by using N2, He, CO2, and Ar as the foaming agents, which was realized via our advanced RIC-FIM technique. As far as we are concerned, this is the first study of using low-pressure helium and argon as the physical blowing agents for the FIM process, which potentially broadens the range of types of foaming agents for the foaming industry.

Figure 1. SEM micrographs of the cross-sections of PP foams prepared by using (a) nitrogen (b) helium, (c) carbon dioxide, and (d) argon as the foaming agent, respectively. The expansion ratio was fixed at 2-fold, and all of the images were taken from the view perpendicular to the core-back direction. The left column of time denotes the core-back timing (dwelling time).



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As shown in Figure 1, relatively spherical cells were obtained for the 2-fold PP foams viewed vertical to the mold-opening direction, which was in accordance with earlier work.16,37 Compared with the N2foams (Figure 1a), relatively larger cells were obtained when He was used as the foaming agent (Figure 1b). These variations in cellular structures were largely attributed to the solubility difference between N2 and He in PP, which will be discussed later. Moreover, as shown in Figure 1c, much finer cellular morphologies were observed in CO2-foams. This possibly resulted from the relatively high content of CO2 dissolved in the polymer at the same pressure as well as the enhanced crystallization process for PP.4345 Interestingly, it was demonstrated that using Ar as the foaming agent also produced fine cellular morphologies for PP foams (Figure 1d) in our FIM machine, manifesting that Ar could be an efficient blowing agent in preparing microcellular plastic foams.

Figure 2. (a) Cell density and (b) average cell diameter of PP foams prepared using different foaming agents as a function of the dwelling time.

To further investigate the effect of the gas type on the cellular structures of PP, the cellular parameters of foams prepared with various foaming agents were calculated. Figure 2 displays the changes in the cell density and average cell diameter as a function of the dwelling time (core-back timing). Generally, N2 and He foams exhibited lower cell densities and larger cell sizes, while CO2 and Ar foams gave us much 10 ACS Paragon Plus Environment

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higher cell densities with smaller cell sizes. For example, at the dwelling time of 6.8 s, the cell densities of N2, He, CO2, and Ar foams were 7.39  107, 4.36  107, 5.56  108, and 2.69  108 cells/cm3, respectively, and the cell sizes were 28, 32, 14, and 18 m, respectively. In summary, it was revealed that, in terms of the cell density, CO2-foams  Ar-foams  N2-foams  He-foams, which signified that the cellular structures of PP foams can be controlled by tuning the type of foaming agent in the injection molding process. Moreover, the effect of the dwelling time on the cell parameters was investigated. The dwelling time, i.e., core-back timing, is the time duration between the completion of melt-injection and start of the moldopening operation. With the increase of the dwelling time, the melt temperature of the polymer in the mold cavity becomes lower due to the cooling effect of a low-temperature metal mold, which leads to the enhancement of the melt viscosity and promotion of the crystallization process.16,36,37 As illustrated in Figure 2, the cell densities of all of the foams were increased and the cell sizes were decreased with the prolongation of the dwelling time, which clearly indicated that an improvement in cellular structures could be made by increasing the dwelling time. For example, by extending the dwelling time from 6.0 to 6.8 s, the cell density of CO2-foams was enhanced from 1.93  108 to 5.68  108 cells/cm3, which was in agreement with the reduction of the cell size from 19 to 13 m. This improvement in the cellular structure was attributed to the changes in the crystal morphology and/or crystallinity of the polymer when modifying the dwelling time, and the promotion effect of crystallization on the foaming process has been well-reported.4547 In addition, the adjustment of the dwelling time was crucial for reinforcing the melt strength, which could bring out restricted cell growth and less cell coalescence, thereby reducing the cell size and narrowing the cell size distribution.16,37,48

3.2. Effects of the gas type and expansion ratio on cellular structures



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In order to further investigate the foamability of PP with various foaming agents, we prepared injection-molded foams with different expansion ratios by changing the mold-opening distance during the core-back operation. Figure 3 displays the SEM images at the center of PP foams with a 3-fold expansion ratio. The optimal cellular structures were selected from the foams with different core-back timing for each case. As shown in Figure 3, better cellular morphologies were obtained in CO2- and Ar-foams, while relatively larger and irregular cells were observed in N2- and He-foams. Moreover, similar to the microstructures at a 2-fold expansion ratio, CO2-foams exhibited the finest cellular structures, while Hefoams produced the worst cell morphologies.

Figure 3. SEM micrographs of the 3-fold PP foams prepared by using (a) nitrogen, (b) helium, (c) carbon dioxide, and (d) argon as the foaming agent, respectively.

Figure 4 shows the cell densities and the average cell diameters of PP foamed with different gases. Specifically, the cell densities of the 3-fold N2-, He-, CO2-, and Ar-foams were 9.42  106, 3.07  106, 3.77  107, and 1.70  107 cells/cm3, respectively (Figure 4a). It was found that the cell density of PP 12 ACS Paragon Plus Environment

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foams was enhanced approximately 12 times by changing the gas type from helium to carbon dioxide, which was attributed to the increase of the degree of thermodynamic instability in the CO2 foaming process caused by its higher dissolved gas concentration in the polymer. Similarly, foams with smaller cell diameters and narrower cell size distributions were achieved in the CO2 foaming process compared to He-foams (Figure 4b).

Figure 4. (a) Cell density and (b) average cell diameter of PP foams at a 3-fold expansion ratio. A, B, C, and D denote PP foams prepared by using nitrogen, helium, carbon dioxide, and argon as the foaming agent, respectively.

Figure 5 shows SEM images of the PP foams at higher expansion ratios of 4- and 5-fold prepared by using CO2 and Ar as the foaming agents. However, PP foams with a 4-fold expansion ratio (void fraction of 75%) could not be achieved by either N2 or He. This revealed that, in comparison with N2 and He, higher foamabilities were acquired by employing CO2 and Ar as the blowing agents at the same gas pressure. Figure 6 displays the resulting cell densities and average cell diameters of PP foams fabricated by CO2 and Ar. Generally, CO2-foams exhibited higher cell densities and smaller cell sizes than Ar-foams at the same gas pressure. In addition, with the increase of the expansion ratio, the cell densities of both the CO2 and Ar foams decreased, and the cell sizes increased, which was due to the further cell growth during the core-back operation. This was in agreement with our previous findings.7,17,37 Moreover, the 13 ACS Paragon Plus Environment


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enhanced gas content dissolved in the polymer might also benefit the crystallization process (can be demonstrated from the following thermal behavior) and thus promoted the heterogeneous cell nucleation process, which would occur around the increased amounts of nucleated crystals.4446

Figure 5. SEM micrographs of the (a) 4-fold CO2-foam, (b) 5-fold CO2-foam, (c) 4-fold Ar-foam, and (d) 5-fold Ar-foam.

Figure 6. (a) Cell density and (b) average cell diameter of the 4- and 5-fold PP foams formed by using CO2 and Ar as the physical blowing agents.

3.3. Gas solubility 14 ACS Paragon Plus Environment

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The gravimetric method was used to obtain the solubilities of different gases in the PP resin.39,40 To simulate the injection molding process, the same melt temperature, such as 200 C, and gas pressure, such as 4.8 MPa, were used in the measurement. As it is difficult to avoid the diffusion loss of dissolved gas in the polymer during measurement, it should be noted that the measured solubilities are less than the real values.3941 Figure 7 shows the solubilities of various gases in PP resin. Due to the use of a low pressure of 4.8 MPa, the overall solubility was very low for each case. The data of He was not shown here because the solubility of He in polymer is extremely low and could not be detected at such a low pressure. As displayed in Figure 7, the solubilities of different gases in PP follows the order CO2 > Ar  N2  He. Interestingly, it was found that the order of gas solubilities corresponds to the above-mentioned order of cell densities of PP foams using different types of gases. Since the processing conditions were kept the same except for the type of physical blowing agent, the gas type was the main driving force that differentiated the final cellular structures. Simply, a higher physical blowing agent concentration in the polymer could increase the cell (bubble) nucleation. According to the classical cell nucleation theory,4951 the increase of gas content would enhance the nucleation rate by increasing the number of gas molecules per unit surface area for heterogeneous nucleation and/or the number of gas molecules per unit volume for homogeneous nucleation. Additionally, higher gas concentration would promote the crystallization process and lead to a higher cell nucleation rate through the nucleating effect of the existing crystals.45,46



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Figure 7. The measured solubilities of nitrogen, carbon dioxide, and argon in polypropylene at a gas pressure of 4.8 MPa.

3.4. Thermal behavior The first melting curves of the core regions of injection-molded foams are shown in Figure 8. For comparison, the melting curve of the solid (non-foamed) injection-molded specimen is also measured and displayed in Figure 8a. As shown in Figure 8, a single peak of the -phase of PP was observed for each sample, which was attributed to the addition of MD since it was found to be beneficial for the formation of the -phase of PP.36 Table 2 lists the DSC melting results of the solid and foam injection-molded PP with different expansion ratios. It was found that the melting peak temperature (Tp) values of all the foam samples were larger than that of the solid specimen, indicating that the integrity of iPP crystals is improved after foaming. Furthermore, with an increasing expansion ratio, an increase of Tp was detected for PP foams, which was ascribed to the larger extensional force applied during the core-back operation.37 For example, the Tp values were 167.89, 168.05, 168.35, and 168.49 C for the 2-fold, 3-fold, 4-fold, and 5fold CO2-foams, respectively. In addition, it is worth mentioning that the changes in Tp for PP foams fabricated with different gases are as follows: CO2 > Ar  N2  He, which is the same as the above solubility results of different gases in PP.


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Figure 8. The first heating curves of PP foams with different expansion ratios prepared by using (a) nitrogen, (b) helium, (c) carbon dioxide, and (d) argon as the foaming agent, respectively.

The crystallinity (Xc) values of solid and foamed PP samples are also illustrated in Table 2. It can be seen that the Xc of foamed PP was higher than that of the solid specimen, indicating that the added gases facilitate the mobility of PP chains and the growth of PP crystals under the current processing conditions. For example, the Xc values of the solid specimen, N2-foams, He-foams, CO2-foams, and Ar-foams with 2-fold expansion ratios were 44.0%, 46.6%, 46.3%, 48.6%, and 48.3%, respectively. Compared with the N2- and He-foams, the relatively higher crystallinities achieved in CO2- and Ar-foams were due to their higher contents of gases dissolved in the PP. It is known that a high crystallinity is favorable for the enhancement of the mechanical properties of the injection-molded specimen, especially with regards to the stiffness and strength. In addition, the Xc values of foamed PP specimens were largely increased with an increase of the expansion ratio, which was consistent with our previous results.42 For example, the Xc values for the 2-fold, 3-fold, 4-fold, and 5-fold CO2-foams were 48.6%, 49.5%, 50.0%, and 51.6%, respectively, which was due to the formation of higher extensional force during the core-back operation.

Table 2. DSC results of solid and foamed PP samples with different expansion ratios. Sample

Expansion ratio

Solid



Heat of fusion (Hf, J/g) 91.12

Melting peak temperature (Tp, C) 165.44


Crystallinity (Xc) 44.0


N2-foams N2-foams He-foams He-foams CO2-foams CO2-foams CO2-foams CO2-foams Ar-foams Ar-foams Ar-foams Ar-foams

2-fold 3-fold 2-fold 3-fold 2-fold 3-fold 4-fold 5-fold 2-fold 3-fold 4-fold 5-fold

96.51 99.55 95.91 98.41 100.53 102.44 103.52 106.77 99.88 100.84 102.28 104.76

166.65 167.45 166.54 167.03 167.89 168.05 168.35 168.49 167.69 167.83 168.08 168.19

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46.6 48.1 46.3 47.5 48.6 49.5 50.0 51.6 48.3 48.7 49.4 50.6

3.5. Analysis of the open cell content Figure 9 reveals the changes in the open cell content against the type of gas used for foaming PP at different expansion ratios. To evaluate the open cell content (OCC), the solid skin layer was removed and not considered in the OCC calculation. As shown in Figure 9, with an increase in the expansion ratio, the OCC of PP foams increased for all of the conditions. This result could be interpreted as follows: the bubbles tend to be stretched along the mold-opening direction in the FIM process, and with the increase of the expansion ratio (mold-opening distance), the cell size was enlarged and the cell wall become thinner, which could easily cause cell rupture and cell coalescence, thus producing foams with a higher OCC. Taking Ar-foams as an example, the OCCs of PP foams at 2-, 3-, 4- and 5-fold expansion ratios were 3.7%, 28.7%, 68.0%, and 72.3%, respectively. Moreover, when comparing the OCCs of foams prepared with different gases, the OCCs follow the order of N2-foams  He-foams  Ar-foams  CO2-foams. The relatively higher OCCs in N2- and He foams were ascribed to their large cells and non-uniform cellular distribution. The lower OCCs in Ar- and CO2-foams were possibly caused by their promoted crystallization process, which would reinforce the melt strength and thus stabilize bubbles preventing cell coalescence in the cell growth stage.25,44 Since a low gas pressure (4.8 MPa) was used here, the effect on the crystallization process was much weaker than the effect of adding nucleating agents and nanoparticles, while the addition of nucleating agents and nanoparticles always rendered polymeric foams with a higher OCC.17,42,52 This was due to the substantial increase of cell nucleation sites in the presence of a nucleating 18 ACS Paragon Plus Environment

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agent, which would easily cause cell rupture in the cell growth stage due to the limited content of dissolved gas.

Figure 9. Change of the open cell contents for PP foams prepared with different types of foaming agents.

3.6. Mechanical properties The compressive tests of various products were investigated to study the effect of the gas type on the mechanical properties of injection-molded PP foams. Figure 10 shows the dependencies of the compressive strength and elastic modulus of PP foams on the expansion ratio when using different foaming gases. The elastic modulus is calculated from the slope of the elastic area of the stressstrain curve. As presented in Figure 10a, the compressive strengths of all of the samples obviously decreased with the increase of the expansion ratio, which is attributed to the enlarged cell sizes and the increased volume of voids. In addition, the type of foaming agent affected the compressive strength of PP foams. The compressive strengths of 3-fold N2-, He-, Ar-, and CO2-foams were 2.5, 2.8, 3.9, and 3.5 MPa, respectively, which exhibited a 40% increase by changing the foaming gas from He to CO2. This improvement in compressive strength was ascribed to the smaller cell sizes and higher crystallinity in CO2-foams, which is in agreement with the previous reports.17,53,54 Moreover, the lower open cell content 19 ACS Paragon Plus Environment


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of CO2-foams also helped to increase the compressive strength. When comparing the compressive strengths of N2-foams and He-foams, He-foams exhibited a higher compressive strength. From the above discussion, it is known that the average cell sizes of 2-fold N2- and He-foams were 28 and 32 m, respectively, while the OCCs of corresponding 2-fold N2- and He-foams were 19.7% and 6.5%, respectively. Therefore, compared with the N2-foams, the higher compressive strength achieved in Hefoams was mainly caused by the lower OCC, while the effect of the cell size was relatively smaller.

Figure 10. (a) Compressive strength and (b) elastic modulus of PP foams prepared by using different types of foaming agents.

As illustrated in Figure 10b, similar to the changes in compressive strength, the elastic modulus of PP foams prepared with different physical blowing agents decreased with the increase of the expansion ratio (void fraction), but the rate of decrease was smaller. Specifically, compared with the 2-fold Ar-foams, the percentages of decrease in the elastic moduli of 3-, 4- and 5-fold PP foams were 6.5%, 12.3%, and 55.8%, respectively, while the percentages of decrease in the compressive strengths of the corresponding 3-, 4- and 5-fold PP foams were 47.6%, 75.2% and 92.5%, respectively. This was due to the difference in the definition of the elastic modulus and compressive strength, which were separately acquired from the elastic region and plastic deformation region of the stressstrain curves.54 Additionally, it was clearly 20 ACS Paragon Plus Environment

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demonstrated that the compressive strength of the PP foams was changed by selecting different types of gas. The elastic moduli of 3-fold N2-, He-, Ar-, and CO2-foams were 64.5, 67.1, 82.4, and 73.5 MPa, respectively. Among all the gases used here, the CO2-foams always exhibited the highest elastic modulus, which resulted from their finer cellular structures, higher crystallinity, and lower OCC, while the N2foams had the lowest elastic modulus because of their relatively larger cell sizes, smaller crystallinity, and higher OCC. These findings clearly demonstrated that different types of low-pressure gas such as N2, He, Ar, and CO2 can be used as foaming agents in our advanced RIC-FIM to produce microcellular foams and indicated that the selection of different gases can greatly affect the final cellular structures as well as mechanical properties of injection-molded foams.

4. CONCLUSION In this work, different expansion ratios of microcellular PP foams were successfully fabricated by using various types of gases such as N2, He, CO2, and Ar as the physical foaming agents, which were achieved via our advanced RIC-FIM technique. With the use of a gas at low pressure, such as 4.8 MPa, the maximum expansion ratios of N2- and He-foams were approximately 3-fold (void fraction 75%), while the maximum expansion ratios of Ar- and CO2-foams can reach higher than 5-fold, which can be attributed to the solubility differences between the gas types. The foaming experiments displayed that PP foams with smaller cell sizes and higher cell densities were obtained in the Ar- and CO2-foams. In contrast, the N2- and He-foams exhibited relatively larger cell sizes and lower cell densities. The cell density follows the order of CO2-foams  Ar-foams  N2-foams  He-foams, signifying that the cellular structures of PP foams can be controlled by tuning the type of foaming agent. The compressive results manifested that CO2-foams exhibited the highest compressive strength and elastic modulus, whereas the use of N2 gave PP foams the lowest compressive strength and elastic modulus, which were caused by its poor cellular structures, low crystallinity, and high open cell content. As a consequence, the flexibility of using different low-pressure gases to produce microcellular foams via the RIC-FIM technology was clearly 21 ACS Paragon Plus Environment


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demonstrated, which opens up the range of applications of physical blowing agents, and they can be potentially applied in other foaming techniques.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Address: A4 Building, B1 Floor, Kyoto Univ. Katsura Campus, Nishikyo-ku, Kyoto 615-8510, Japan

ORCID Long Wang: 0000-0002-4519-2400

Masahiro Ohshima: 0000-0003-0870-5438

Author Contributions The manuscript was written with contributions from all of the authors. All authors have given approval of the final version of the manuscript.

Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS The research was conducted in association with the Grants-in-Aid for Scientific Research (B), Number: 26289289 of the Japan Society for the Promotion of Science (JSPS) and the Advanced Low Carbon Technology Research and Development Program (ALCA).


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