Development of a Simplified Foam Injection Molding Technique and

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Development of a Simplified Foam Injection Molding Technique and its Application to the Production of High Void Fraction Polypropylene Foams Long Wang, Yuta Hikima, Masahiro Ohshima, Atsushi Yusa, Satoshi Yamamoto, and Hideto Goto Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03382 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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Industrial & Engineering Chemistry Research

Development of a Simplified Foam Injection Molding Technique and its Application to the Production of High Void Fraction Polypropylene Foams Long Wang,† Yuta Hikima,† Masahiro Ohshima,*,† Atsushi Yusa,‡ Satoshi Yamamoto,‡ Hideto Goto,‡ †

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

‡

Technology Development Department, Hitachi Maxell, Ltd., Kyoto 618-8525, Japan

ABSTRACT We previously developed a resilient and innovative cellular foam injection molding (RIC-FIM) technology to illustrate that pressurization of the physical blowing agent (PBA), such as carbon dioxide (CO2), to the supercritical state is unnecessary for preparing microcellular foams. Herein, our developed FIM machine is further simplified by using the previously auxiliary venting unit as the only PBA delivery unit. Subsequently, high void fraction polypropylene (PP) foams were successfully obtained using this novel FIM machine with 4−6 MPa of nitrogen (N2). The results showed that an increase in the vessel pressure reduced the PP foam cell size and enhanced the cell density as well as the void fraction, signifying that the concentration of dissolved gas was controlled by the pressure vessel. Furthermore, compared with the N2 foaming process, CO2 foaming revealed a finer cellular structure for PP. This simplified FIM technology holds great potential for the industrial manufacturing of lightweight injection-molded products with better economic benefits. Keywords: Low-pressure fluid, foam injection molding machine, microcellular foam, polypropylene

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1. INTRODUCTION The microcellular foaming technology was developed in the late 1980s, and it was first pioneered by Professor Suh et al. of the Massachusetts Institute of Technology (MIT).1,2 The original impetus behind this invention was to reduce the amount of material used with little or no compromise to the mechanical properties of the product.1−3 Additionally, many other benefits, such as an excellent energy absorption,4 an elevated impact strength and toughness,2,5 and good thermal and acoustical properties,6−8 were achieved in microcellular plastic foams. These excellent features make polymer foams attractive for several applications, including use in automobiles, packaging, sporting equipment, and construction industries.9,10 The first commercial application of the microcellular technology was the MuCell extrusion and injection molding processes,10,11 which was licensed by Trexel, Inc. and developed based on the MIT studies.3,11 The MuCell process configuration mainly consisted of a specifically configured screw and a metering system for the supercritical fluid.3,10,11 In its foaming process, supercritical carbon dioxide (sc-CO2) or nitrogen (sc-N2) was used as the physical blowing agent (PBA). Typically, the PBA supplied by a cylinder was first pressurized through a specialized SCF pump system; then, the SCF was delivered into a molten polymer through an exclusive injector valve to form a single-phase polymer/gas solution prior to extrusion or injection. With the great commercial success of the MuCell technology, various microcellular foam injection molding (FIM) technologies, such as the Optifoam system from the Institute of Plastics Processing (IKV)12 and the Ergocell process licensed by Demag,10 were successively developed. These modified technologies aimed to improve the delivery efficiency of PBA and to reduce the machine cost with varying degree of success.10 However, a pressurized system was still required in these FIM technologies for increasing the pressure of N2 or CO2 far higher than its gas cylinder pressure, which inevitably increased the operation and machine costs because of the expensive SCF pumping system. To further reduce the cost, industrial researchers and university scientists have been striving to develop foaming techniques without using the SCF system to prepare microcellular injection-molded foams. For instance, 2 ACS Paragon Plus Environment

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Shekisui Chemical advanced a simple gas-dosing technique using a sealed hopper to deliver nonsupercritical gas into pellets.13 Later, a similar technology (ProFoam) was developed by IKV,14 which employed a high-pressure autoclave as the hopper and introduced gas into the raw materials prior to entering the barrel of the injection molding machine. This absorption and control of the PBA resembles that of the typical bead foaming process, which needs an extra-long time for the PBA to diffuse into the plastic pellets. Additionally, Shikuma et al. initiated a gas-laden approach by producing gas-laden pellets through a gas cylinder and an autoclave, which were completely separated from the injection molding machine.15 This was a simple method of preparing FIM products, but it was challenging to regulate the concentration of PBA dissolved in the polymer and somewhat time consuming to preprepare the gas-laden polymeric pellets. Recently, we introduced a novel FIM technology named resilient and innovative cellular foam injection molding, i.e., RIC-FIM I, at FOAMS2015.16 This developed RIC-FIM I process demonstrated that the pressurization of N2 or CO2 from its gas cylinder was unnecessary for obtaining injectionmolded foams.16,17 The foamed products showed that microcellular polypropylene (PP) foams with cell sizes lower than 25 µm could be obtained using either 8 MPa N2 or 5 MPa CO2. The PBA was directly introduced from a gas cylinder into the molten polymer through an injection valve, which was controlled through a specifically designed operation sequence and screw configuration.17 To control the content of PBA dissolved in the polymer, a subsidiary vent hole with a venting vessel was installed in the middle of the barrel. Through this assistant vent hole, excess gas (i.e., residual PBA that exists as a gas in the polymer melt) can be discharged from the barrel into the outside atmosphere. Alternatively, the vent hole can also be utilized to introduce additional PBA into the molten polymer when the polymer is not saturated.16,17 To distinctly differentiate the new process from the conventional MuCell technology, our RIC-FIM technology was further upgraded and simplified to deliver the PBA solely from the vent hole while removing the injector valve. This newly developed FIM technology, namely, RIC-FIM II, was applied 3 ACS Paragon Plus Environment


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to the foaming of commonly used PP resins, which are more attractive in the foaming industry than other thermoplastics such as polyethylene and polystyrene.18 Moreover, due to limited void fraction in the regular FIM process, an FIM technique with core-back operation was used here, which greatly enlarged the void fraction of the injection-molded products.19−21 To check the flexibility of our developed FIM process, N2 with various delivery pressures was directly introduced from a gas cylinder and used for the foaming process. The use of CO2 as the PBA to control the cell morphology was also comparatively investigated. 2. DESIGN OF THE NOVEL INJECTION-MOLDING MACHINE To better illustrate the subsequent FIC-FIM II technology, the basic design principles of our previous version of FIM are first briefly introduced. Figure 1 shows a schematic diagram of the first version of the RIC-FIM machine. Different from the conventional microcellular FIM technology, there was no supercritical fluid pumping unit in our developed FIM machine.16 As shown in Figure 1, the injector valve for PBA delivery was directedly connected to a gas cylinder, which enabled the PBA to be dosed into the molten polymer without pumping. This was achieved by the design of a special screw and operation of the screw rotation, which was detailed in our previous work.16,17 Briefly, the amount of injected gas was regulated by the injector valve opening time and the set point of the reducing pressure regulator on the cylinder (Figure 1). For instance, the gas cylinder pressures of N2 and CO2 at room temperature are approximately 12 and 6 MPa, respectively. Using a reducing pressure regulator, the pressure of the supplied PBA could be controlled, and the normally used pressure range was 7−10 MPa for N2 and 5−6 MPa for CO2, respectively. Moreover, as shown in Figure 1, an auxiliary vent hole with a venting vessel was installed between the injector valve and the nozzle. Through this venting hole, excess gas that remained un-dissolved in the molten polymer could be released into the atmosphere, or extra PBA could be supplemented into the polymer melt when the polymer was not saturated.16,17


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For the RIC-FIM I machine, an injector valve was still required to introduce the foaming agent, which was somewhat similar to the conventional FIM technology, with the main difference being the state of the injected gas. To completely distinguish the new method from the regular MuCell technology and further reduce the machine cost, we proposed the removal of the injector valve from the RIC-FIM I machine and used the previously auxiliary venting vessel as the dominant gas-dosing component. Figure 2 illustrates a schematic diagram of the new version of our developed RIC-FIM machine (RIC-FIM II). As displayed in Figure 2, the major modifications compared to the original technology was the removal of the injector valve and the installation of an automatic pellet feeder. In this upgraded version, the PBA is directly provided from a gas cylinder and introduced into the polymer melt via a delivery hole in a pressure vessel. Thus, the auxiliary vent hole and venting vessel in the RIC-FIM I machine were separately transformed into a delivery hole and pressure vessel in the new FIM machine. The concentration of PBA dissolved in polymer melt can be adjusted by regulating the pressure in the pressure vessel. The pressure of the vessel is manipulated by adjusting a reducing valve on the gas cylinder (Figure 2), which is kept within a specified range of 5−7 MPa for N2 and 5−6 MPa for CO2 depending on the foam void fraction (expansion ratio) and cell morphology needed for the specific purpose. As illustrated in Figure 2, the screw is basically composed of four configuration zones, as designed for the previous RIC-FIM I machine.16,17 Zone A is the plasticization zone where polymer granules are heated and plasticized. Zone B was the mixing zone for polymer/gas blending in the preceding RIC-FIM I machine, while it mainly acts as the first compression zone in the newly developed FIM machine. Zone C, which is the polymer-starved zone and new gas/polymer mixing zone, plays the most important role of a stable PBA delivery. A polymer starvation was intentionally created in zone C in the design of the screw geometry, which created a deeper space where the PBA could constantly contact the molten polymer and dissolve into the polymer before injection. Control of the pellets supply rate by the automatic pellet feeder is also important to keep the stable polymer-starved condition. In addition, the 5 ACS Paragon Plus Environment


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polymer pressure in zone C is controlled by the pressure of the pressure vessel. In Zone D, the molten polymer with dissolved gas is again compressed, which produces a single-phase gas/polymer solution in preparation for the subsequent injection process. 3. EXPERIMENTAL SECTION 3.1. Materials High tacticity isotactic polypropylene (iPP, F133A) with a weight-average molecular mass of 379 kg/mol and a melt flow rate (230 °C/2.16 kg) of 3.0 g/10 min was provided by the Prime Polymer Co., Ltd., Tokyo, Japan. Due to the low melt strength of the linear iPP resin and the resultant poor foamability,2,19−22 a crystal nucleating agent, 1,3:2,4 bis-O-(4-methylbenzyliden)-D-sorbitol gelling agent (Gel-all MD, New Japan Chemical Co., Ltd., Kyoto, Japan), was used as a bubble nucleating agent to improve the foaming performance of iPP, and 0.5 wt % of MD was used in this study. For simplicity, the iPP/MD blend with 0.5 wt % MD was named PP in the following discussion. N2 with 0.999 purity and CO2 with 0.99 purity, which were supplied from the Showa Denko Gas Products Co., Ltd., Kanagawa, Japan, were independently used as the physical blowing agent. 3.2. Sample preparation Foaming experiments were carried out on a 35-ton clamping force electric injection molding machine with a screw diameter of 22 mm (J35AD-AD30H, Japan Steel Work, Ltd. Hiroshima, Japan). The maximum screw speed, injection capacity, and injection pressure of this developed FIM were 500 min-1, 30 cm3, and 200 MPa, respectively. The plasticizing capacity was estimated to be approximately 10.5 kg/h. Excluding the injector valve unit and its operation procedure, the machine setup and operating procedure of this new FIM technology was the same as those of the previous RIC-FIM I machine.16,17 The mold consisted of a rectangular cavity (70 mm × 50 mm) with an initial thickness of 2 mm, which was able to increase to 10 mm. To obtain a high degree of void fractions, the RIC-FIM II machine was operated in combination with a core-back molding technique.19−22 For instance, 50, 67, and 6 ACS Paragon Plus Environment

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80% void fraction PP foams were obtained by moving a portion of the mold from 0 to 2, 4, and 8 mm, respectively.21 Additionally, a data logging system (Mold Marshaling system EPD-001, Futaba Corp., Chiba Prefecture, Japan) was used to monitor the real temperature and pressure of the injected polymer in the cavity.17,19 The operating parameters of the FIM experiments are summarized in Table 1. 3.3. Foam characterization To investigate the microstructure, a small slice cut from the middle of the FIM product was cryogenically fractured and coated with gold using a VPS-020 Quick Coater (ULVAC KIKO, Ltd., Japan). The cellular morphology was then examined using scanning electron microscopy (Tiny-SEM Mighty-8, Technex Lab Co., Ltd., Tokyo, Japan). The obtained images were further analyzed using ImageJ software, which was developed by the National Institutes of Health, US. The average cell size (d) was calculated from the SEM micrographs using Eq. 1:23

d =

∑dn ∑n i

(1)

i

i

where ni is the number of bubbles with a diameter of di, assuming that the bubble shape is spherical. The cell density (N0) was then obtained by:23 n N 0 = ( ) 3/ 2 A

(2)

where n is the number of bubbles in the selected SEM image and A is the area of the image. 3.4. Open cell content The open cell content of the foamed samples was calculated using the following equation:22

Open cell content =

Vpolymer − Vmeasure Vpolymer

×100%

(3)

where Vpolymer is the total volume of the foamed sample and Vmeasure is the volume measured by the gas pycnometer, which excludes the specimen’s open cell volume. The volume was measured using a gas pycnometer (AccuPycII, Shimadzu, Kyoto, Japan). 7 ACS Paragon Plus Environment


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4. RESULTS AND DISCUSSION 4.1. Effect of the delivery pressure on the cellular structure Figure 3 shows the cell morphologies in the core region of PP foams with a fixed void fraction of 50%, which were made using the newly developed FIM machine with core-back operation. SEM images were taken from views both perpendicular and parallel to the core-back direction, and the foams in Figure 3 were fabricated under various delivery pressures using N2 as the PBA. As shown in Figure 3a, it was revealed that fine cellular structures of injection-molded foams were achieved from our new RICFIM II technology, even when using a very low delivery pressure of approximately 4 MPa, while the commonly used value of the pressuring gas in the MuCell technology was as high as 24 MPa.19,22 This observation indicated the feasibility of only using a vent hole as the gas-dosing point and the possibility of using low-pressure non-supercritical gas as a PBA. To check the controllability of the gas concentration dissolved in the molten polymer, different delivery pressures of N2 were used to prepare FIM foams. As shown in Figure 3, it was revealed that with an increase in the delivery pressure, smaller cell sizes and finer cellular structures were achieved in the prepared PP foams, which was related to the increased concentration of N2 dissolved in the PP melt. This signified that the content of PBA dissolved in the polymer could be controlled by the delivery pressure regulator, and meanwhile, the content greatly affected the final foaming process. To quantitatively compare the differences in the cellular structures among the various samples, the cell density and average cell size were used. It should be mentioned that the cells of the FIM samples were not perfectly uniform and were sometimes deformed, and therefore, it was somewhat inappropriate to use Eqs. 1 and 2 to acquire the cell information. Thus, the calculated cell densities and average cell sizes were mainly used as reference values in the following analysis, as usually done by researchers in the analysis of injection-molded foams.19,22,24−26 Figure 4 presents the change in the cell density and average cell diameter as a function of the delivery pressure for the PP foams. The plot shows that the cell density of the injection-molded foams increased with an increase in delivery pressure. Specifically, 8 ACS Paragon Plus Environment

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the cell densities of the 50% void fraction PP foams prepared at 4, 5, and 6 MPa of N2 were 4.76 × 107, 2.10 × 108 and 5.84 × 108 cells/cm3, respectively. These values indicate that the cell density of the PP foam increased nearly 12 times with the increase of the gas pressure from 4 to 6 MPa, which was attributed to the enhanced cell nucleation mainly caused by the increased degree of thermodynamic instability.27,28 According to the classical cell nucleation theory,28−30 the homogeneous and heterogeneous bubble nucleation rates were strongly influenced by the super-saturation level of the blowing agent, which was closely related to the concentration of gas dissolved in the polymer. With an increase in delivery pressure, the gas concentration dissolved in the molten polymer increased, and thus, a higher degree of thermodynamic instability was produced,27,28 which promoted cell nucleation and led to the resultant enhanced cell density. In addition, with the increase of gas content, the surface tension might fall;28 this would reduce the free energy barrier for cell nucleation and thus again benefit the foaming process to some extent. The enhanced PBA concentration might have also favored the crystallization process and thereby possibly expedited heterogeneous cell nucleation, which would occur around the increased amounts of nucleated crystals.31,32 As displayed in Figure 4b, the cell sizes of the PP foams decreased with rising delivery pressure, and the smallest cell size of approximately 10 µm was obtained at a relatively high pressure of 6 MPa, which was also an effect of the increased gas content caused by the higher delivery pressure. To further compare the effects of the different delivery pressures, we examined the injectionmolded foams with the highest void fraction for each of the various gas pressures, as shown in Figure 5. The optimum cellular structures in the core region observed perpendicular to the core-back direction are presented for each gas pressure. It was found that the highest void fractions of the PP foams under 4, 5, and 6 MPa of N2 were 66 (3-fold), 75 (4-fold), and 80% (5-fold), respectively. This increase in the maximum void fraction with increasing gas pressure additionally verified the flexibility of using the vent hole to control the gas concentration dissolved in the polymer melt. Moreover, as shown in Figure 5c, a microcellular FIM foam with a void fraction as high as 80% was achieved for the PP foam 9 ACS Paragon Plus Environment


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prepared under 6 MPa of N2. Considering the low void fractions of conventional FIM products (5−30%),24,25,33,34 this achieved void fraction is very high, and it is also larger than most of the reported values for FIM with mold-opening operation.24,26,35,36 4.2. Influence of the void fraction on the cell morphology Figure 6 shows the cell morphologies in the core regions of the foamed PP as a function of the void fraction. The foams were prepared at a fixed delivery pressure of 6 MPa and are viewed perpendicular to the core-back direction. It is shown that various void fractions of microcellular injection-molded foams could be easily obtained by changing the core-back distance in the newly developed FIM machine. Additionally, with rising void fraction, the cell sizes of the foamed samples were enlarged, which was due to the further cell growth and cell coalescence caused by the high extensional force applied during the core-back process.20−22 Figure 7 illustrates the effects of the void fraction on the cell density and average cell size of the PP foams. The cell densities of the foamed samples decreased with increasing void fraction, as shown in Figure 7a. For instance, the cell densities of the PP foams decreased from 5.84 × 108 to 3.01 × 107 cells/cm3 upon increasing the void fraction from 50 to 80%. Correspondingly, the average cell sizes of the PP foams increased with increasing void fraction, and a relatively large cell size of approximately 50 µm was obtained in the PP foam with a high void fraction of 80%. Even though the cell density and cell size of the 80% void fraction foamed PP were inferior to the 50% void fraction foams, the cell density was still higher or comparable to that of the PP foam from the regular FIM process with a low void fraction.25,34,37 This was mainly ascribed to the enhanced foaming process caused by the added MD, which was verified in our previous research.22 4.3. Effects of the delivery pressure and void fraction on the open cell content Figure 8a shows the open cell content (OCC) of the PP foams prepared at a fixed void fraction of 50% as a function of the delivery pressure. To calculate the OCC, the volume of solid skin layer was excluded from the calculation of OCC and the reported values of OCC is only for the foamed layer.19,22 10 ACS Paragon Plus Environment

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It was observed that the OCC of the foamed sample increased with increasing delivery pressure, resulting from the thinner cell walls of high delivery pressure sample. This was due to the occurrence of a greater extent of cell nucleation in the high delivery pressure sample that was promoted by the higher gas concentration, which was consistent with our previous research.27 This increased cell nucleation would result in a decrease in the thickness of the cell walls while keeping the same void fraction, thereby initiating cell opening more easily. Moreover, changes in the OCC for the FIM samples at a fixed delivery pressure of 6 MPa under various void fractions are displayed in Figure 8b. It was demonstrated that the OCC of the PP foams notably increased with the enhancement of the void fraction, reaching a highest OCC of approximately 72% for the foams prepared at a void fraction of 80%. During the mold-opening manipulation, bubbles were deformed and then stretched along the coreback direction,19−21 and with the increase in the void fraction, the cells were further extended, and meanwhile, the thickness of the cell walls was reduced, resulting in a higher OCC. 4.4. Effect of the gas type on the cellular structure Since N2 and CO2 are two commonly used PBAs in the microcellular foaming technology, we further studied the foaming behavior of PP using CO2 as the PBA in our newly developed FIM machine. Figure 9 shows the cell morphologies of the PP foams with various void fractions prepared under a delivery pressure of 5 MPa. As discussed above, the highest void fraction for the foams prepared at the same pressure of N2 was only 75% (4-fold), while a higher void fraction reaching 80% (5-folds) was achieved by using CO2. This signified that a better foaming performance was achieved in the PP foams by utilizing CO2 as the PBA at the same delivery pressure, which will be discussed later. Generally, Figure 9 reveals that the bubble sizes of the PP foams increased with increasing void fraction, which is similar to the abovementioned variations in the cell size with N2 foaming. Figure 10 illustrates the cell density and average cell size of the PP foams prepared by using CO2 as the PBA as a function of the void fraction. For comparison, the results of N2 foaming under the same delivery pressure are also presented. The optimum foaming results were selected for each void fraction. 11 ACS Paragon Plus Environment


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As a result of this study, a microcellular injection-molded foams with a cell size of 15−25 µm and a cell density ranging of 4.8 × 107− 2.4 × 108 cells/cm3 were successfully prepared using CO2 as the PBA. The cell size of the PP foams slightly increased with increasing void fraction in the CO2 foaming method, while the change was more severe in the N2 foaming process. Interestingly, as shown in Figure 10a, compared with the N2 foaming process, using CO2 produced a higher cell density for the PP sample at the same delivery pressure and void fraction. This promotion of the cell density was more substantial for the PP foams fabricated with a higher void fraction. Moreover, regarding the N2 delivery, 80% void fraction injection-molded foams were difficult to prepare. This might be due to the solubility difference between N2 and CO2 in the PP resins.38 It is well known that CO2 exhibits a much higher solubility than N2 in PP melts under the same pressure and temperature.38,39 A larger void fraction might need a high concentration of PBA, and thus, the use of CO2 was favorable for preparing high void fraction FIM foams. Likewise, the cell sizes of the PP foams prepared using CO2 were smaller than those from N2, and the distinction in cell size was more obvious for the high void fraction PP foams. Figure 11 presents the effect of the gas type on the open cell content of the FIM foams with various void fractions. In the abovementioned N2 foaming process, an increase in the delivery pressure and the resultant rise in the concentration of N2 dissolved in the polymer reduced the cell size and increased the OCC in the prepared PP foams. However, unlike the effect of the delivery pressure on the OCC, the samples prepared using CO2, which has a higher solubility in PP than N2, exhibited smaller cell sizes but a lower OCC than the samples prepared using N2. This phenomenon suggested that there are other factor besides the gas solubility that differentiate the effects of CO2 and N2 in the foaming behavior of PP. Intensive studies have examined the plasticization and crystallization behaviors of various polymers, such as polylactide, poly(ethylene terephthalate), and PP, under a high gas pressure and found that a strong plasticization effect was observed when the polymer was contacted with CO2.40−46 More importantly, the addition of CO2 usually exhibited an enhanced crystallization process and plasticization effect compared to that of N2 addition.40,47 Considering the same delivery pressure and other main 12 ACS Paragon Plus Environment

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processing conditions used for the CO2 and N2 foaming processes, the lower OCC in the FIM foams prepared with CO2 might be due to the promoted crystallization process. Compared with N2 foaming, this promoted crystallization process would increase the melt strength of the PP matrix in CO2 foaming.48−50 Subsequently, the increase in melt strength would stabilize the cells and prevent their coalescence,18,19 which finally caused a lower OCC in the CO2-foamed sample. Since 80% void fraction foams could not be obtained using 5 MPa of N2, the gas pressure was increased to 6 MPa to prepare the 80% void fraction PP foams, and the results are displayed in Figure 12. The foams were obtained under various foaming temperatures. The foaming temperature corresponded to the molten polymer temperature in the mold cavity when the core-back operation was commenced19,22. When CO2 was used as the PBA, the core-back timing was delayed to decreasing the foaming temperature and obtain a fine cell structure, which was ascribed to the stronger plasticization effect of CO2 than N2.39,40,47 As shown in Figure 12a, the PP foams prepared at 103 °C exhibited a fibrillary structure, which might be due to the relatively low melt strength and large degree of extensional deformation of the cell wall at the high void fraction. By reducing the foaming temperature, the cell morphologies of the FIM foams were improved (Figure 12b), but a relatively large cell size with many small pores in the cell walls was still obtained using N2. In contrast, much smaller cell sizes were observed for the PP foams prepared using CO2 as the PBA, as displayed in Figure 12c and d, which were attributed to the increased gas concentration dissolved in the polymer as well as to the possibly promoted crystallization process. These results revealed that in our new FIM machine, CO2 had a better foamability for PP resins than that of N2, while in the conventional MuCell machine, N2 usually produced a smaller cell size;27,39 further studies should be conducted in the future. As a consequence, we verified the flexibility of further simplifying our RIC-FIM technology by using the venting unit as a PBA delivery unit to produce microcellular foams and demonstrated that high void fraction PP foams could be obtained by both low-pressure N2 and CO2, which holds great potential for the wide application of injection-molded products with low cost and easy operation. 13 ACS Paragon Plus Environment


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5. CONCLUSION In this study, our new microcellular FIM technology (RIC-FIM) was further simplified to reduce the machine cost while still maintaining the foaming performance and producing fine cellular structures. The key feature of this novel and viable technology is that it does not need a high-pressure pump or injector valve, as the injector valve was still required in the previous RIC-FIM I machine. In this newly developed RIC-FIM II machine, the PBA could be delivered directly from a gas cylinder to the molten polymer through the delivery vessel and hole. The controllability of the cell morphology of PP foams was comparatively investigated by using CO2 and N2 as the PBA. We revealed that the delivery of PBA through the vessel and hole could produce high void fraction PP foams with either CO2 or N2. Specifically, with an increase in the delivery pressure, the cell size of the PP foams decreased and the cell density correspondingly increased, which illustrated that the concentration of PBA dissolved in the molten polymer was well controlled by the vessel pressure. In addition, it was determined that to achieve a high void fraction, the delivery pressure should be increased to a certain level. By fixing the delivery pressure, it was unveiled that samples with a higher void fraction were always concurrent with larger cell sizes and lower cell densities as well as higher OCCs. Interestingly, compared with N2 foaming, using CO2 produced finer cellular structures in the PP foams together with lower OCCs, which were attributed to their differing solubilities in PP and the resultant difference in crystallization. Consequently, we demonstrated the flexibility of using the new FIM technology for producing microcellular foams, and this simplified FIM machine offers a viable and promising method for fabricating lightweight injection-molded products with reduced cost and easy manipulation. This technology will be applicable not only to PP but also polyamide 6 (PA6) and other thermoplastic polymers.

AUTHOR INFORMATION Corresponding Author 14 ACS Paragon Plus Environment

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*E-mail: [email protected] (Masahiro Ohshima). 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

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). REFERENCES (1) Martini, J. E.; Waldman, F. A.; Suh, N. P. The Production and Analysis of Microcellular Thermoplastic Foams. SPE ANTEC Tech. Pap. 1982, 43, 674–676. (2) Doroudiani, S.; Park, C. B.; Kortschot, M. T. Processing and Characterization of Microcellular Foamed High Density Polyethylene/Isotactic Polypropylene Blends. Polym. Eng. Sci. 1998, 38, 1205−1215. (3) Suh, N. P. Impact of Microcellular Plastics on Industrial Practice and Academic Research. Macromol. Symp. 2003, 201, 187–202. (4) Cui, L.; Kiernan, S.; Gilchrist, M. D. Designing the Energy Absorption Capacity of Functionally Graded Foam Materials. Mater. Sci. Eng. A 2009, 507, 215–225. (5) Bao, J.; Junior, A. S.; Weng, G.; Wang, J.; Fang, Y.; Hu, G. H. Tensile and Impact Properties of Microcellular Isotactic Polypropylene (PP) Foams Obtained by Supercritical Carbon Dioxide. J. Supercrit. Fluids 2016, 111, 63–73.



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Figure Captions Figure 1. Schematic diagram of the first version of the resilient and innovative cellular foam injection molding (RIC-FIM I) machine.


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Figure 2. Schematic diagram of the newly developed resilient & innovative cellular foam injection molding (RIC-FIM II). Figure 3. SEM micrographs of PP foams prepared under delivery pressure of (a, a′) 4 MPa, (b, b′) 5 MPa, and (c, c′) 6 MPa at a fixed void fraction of 50%. Images (a−c) and (a′−c′) were taken from the views parallel and perpendicular to the core-back direction, respectively. Figure 4. Variations in the (a) cell density and (b) average cell diameter of the PP foams prepared at a fixed void fraction of 50% as a function of the delivery pressure. Figure 5. SEM micrographs of PP foams obtained at delivery pressures of (a, a′) 4 MPa, (b, b′) 5 MPa, and (c, c′) 6 MPa with maximum void fractions of 66, 75, and 80%, respectively. Images (a−c) and (a′−c′) were taken from the views parallel and perpendicular to the core-back direction, respectively. Figure 6. SEM images of the core-layer cross-sections of PP foams prepared at a fixed delivery pressure of 6 MPa at void fractions of (a) 50%, (b) 67%, (c) 75%, and (d) 80%. Figure 7. Variations in the (a) cell density and (b) average cell diameter for the PP foams prepared under a delivery pressure of 6 MPa as a function of the void fraction. Figure 8. Change in the open cell content for the PP foams prepared under a constant gas pressure of 6 MPa as functions of (a) the delivery pressure and (b) the void fraction. Figure 9. SEM micrographs of the core layer cross-sections of PP foams prepared at a fixed delivery pressure of 5 MPa at void fractions of (a) 50%, (b) 67%, (c) 75%, and (d) 80%. Figure 10. Variations in the (a) cell density and (b) average cell diameter for PP foams prepared under a delivery pressure of 5 MPa as a function of the void fraction. Figure 11. Open cell content of the PP foams prepared under a constant delivery pressure of 5 MPa as a function of the void fraction. Figure 12. SEM micrographs of PP foams with a void fraction of 80% prepared at foaming temperatures of (a) 103 °C and (b) 101 °C under 6 MPa of N2 and at (c) 99 °C and (d) 97 °C under 5 MPa of CO2. 21 ACS Paragon Plus Environment


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Figure 1


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Figure 3


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Figure 5


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Figure 7


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Figure 9


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Figure 11


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Table Captions

Table 1. Processing variables used for the foam injection molding process. Parameters

Values 60, 190, 220, 230, 210, 200, 200, 200, 200

Barrel temperature from hopper to barrel (°C) Injection speed (mm/s)

200

Injection pressure (MPa)

160

Holding pressure (MPa)

30

Pressure of gas cylinder (MPa)

12 (N2), 6 (CO2)

Pressure of delivery vessel (MPa)

4, 5, and 6 (N2), 5 (CO2)

Screw back pressure (MPa)

11

Mold temperature (°C)

40

Cooling water temperature for mold (°C)

30

Core-back distance (mm)

2, 4, 6, 8

Core-back timing (s)

6.2−8.0

Core-back rate (mm/s)

20

Shot size (mm)

45


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For Table of Contents only


Development of a Simplified Foam Injection Molding Technique and

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