Research Note Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
pubs.acs.org/IECR
Unusual Fabrication of Lightweight Injection-Molded Polypropylene Foams by Using Air as the Novel Foaming Agent Long Wang,*,† Yuta Hikima,† Masahiro Ohshima,*,† Atsushi Yusa,‡ Satoshi Yamamoto,‡ and Hideto Goto‡ †
Department of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan Technology Development Department, Maxell, Ltd., Kyoto 618-8525, Japan
‡
S Supporting Information *
ABSTRACT: We report a novel approach to prepare injection-molded foams using air as the foaming agent, which is realized through our newly developed foam injection molding technology. Microcellular polypropylene (PP) foams exhibiting cell size of ∼7 μm and cell density of 1.63 × 109 cells/cm3 were successfully obtained employing air foaming. Fourier transform infrared spectroscopy and rheological results illustrated that the molecular structures and melt properties of PP were greatly altered in air foaming, resembling a reactive foaming process. Furthermore, compared with nitrogen and carbon dioxide foaming, air foaming gave us a more flexible PP foam with finer cellular structure.
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to fabricate FIM foams.12,13 It has an injector valve and a gas venting unit for delivering low-pressure PBA directly from the gas cylinder and discharging excess gas from the molten polymer, respectively. The concentration of gas dissolved in the polymer could be well controlled in this developed FIM machine. To clearly differentiate from the conventional MuCell technology, very recently, our developed FIM machine has been further simplified by removing the injector valve.14 In brief, there is no pressurized system or injector valve in our newly developed FIM machine, and PBA is directly supplied from a gas cylinder and introduced into the molten polymer through a delivery hole and a pressure vessel. We have demonstrated that microcellular polypropylene (PP) foams could be prepared by using a very low-pressure gas, such as 4−6 MPa of N2 and 5 MPa of CO2.14 The flexibility of employing an extremely low-pressure gas such as the PBA provides the possibility of using air as a foaming agent for preparing plastic foams. To the best of our knowledge, no work has been carried out using air as the blowing agent, possibly resulting from the serious degradation of polymer when contacted with high-pressure air because of the existing oxygen (O2). Because purification has been performed prior to using CO2 and N2, the easy acquirement of air makes it attractive for preparing foamed products with highly economic benefits. Moreover, owing to the presence of O2, additional benefits such as modifying the polymer’s molecular structures and altering its melt properties might be simultaneously achieved. Previously, the controlled-rheology
ightweight polymeric foams are attractive for their wide range of applications in the fields of packaging, automobiles, acoustic absorbents, thermal and thermal insulators, as well as tissue engineering.1−3 Dating back to 1980s, the concept of microcellular foam was first advanced by Professor Nam P. Suh of MIT utilizing a supercritical fluid as the physical blowing agent (PBA) to prepare plastic foams aiming to reduce the material usage.4 Interestingly, the prepared microcellular foam usually illustrates extra merits, including improved toughness and impact strength,5,6 good thermal insulation,7 and excellent acoustic, shock, or vibration energy damping with respect to its solid counterpart.8,9 Since then, supercritical fluid (SCF) such as supercritical carbon dioxide (Sc-CO2) and nitrogen (Sc-N2) has been widely used as PBA in the microcellular foaming process, which is achieved through a specialized SCF pumping device. A typical SCF system is the commercial MuCell extrusion and injection molding technology.2 To reduce machine cost and simplify foaming process, however, several foaming technologies have been advanced with no need for SCF pump to prepare injection-molded foams. Shekisui Chemical developed a special gas-dosing approach by delivering nonsupercritical gas into pellets in a sealed hopper.10 Subsequently, the Institute of Plastic Processing (IKV) advocated a similar technology (ProFoam), which uses a high-pressure autoclave as the hopper and introduces gas into the hopper directly.11 Even though an expensive pressurized system is not needed in these foaming techniques, the content of foaming agent dissolved in the polymer is difficult to control, which might be the main issue constricting their further application. We have also developed a novel foam injection molding (FIM) technology demonstrating that pressurization of PBAs to a value far higher than their cylinder pressures is unrequired © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
December 24, 2017 January 23, 2018 February 28, 2018 February 28, 2018 DOI: 10.1021/acs.iecr.7b05331 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research
Figure 1. Variations of (a) the complex viscosity, η*, and (b) the storage modulus, G′, as a function of the frequency for various PP samples.
against ω for PP-5 and PP-6 in the double logarithmic plots were reduced to 1.10 and 0.54, respectively, revealing a much smaller value for PP-6 sample. Interestingly, as Figure 1b shows, with decreased frequency a progressively increased G′ was observed for PP-6 at the low-frequency region, signifying that a network-like structure is generated for PP treating under high air pressure. Consequently, this demonstrated that processing of PP under high-pressure air would notably modify its molecular structures as well as the melt properties. It is known that great efforts have been paid to prepare controlled-rheology PP using the method of peroxide-degradation of PP with added peroxides during reactive extrusion.15−17 However, our target to prepare injection-molded foams using air provides extra benefits such as simultaneously changing the molecular structures and rheological properties of PP. This indicates that our subsequent foaming can be regarded as a reactive foaming process. It is known that the injection-molded foams consist of the solid skin layer (nonfoamed layer) and foamed core layer. Here we mainly focus on the cellular structure in the foamed core layer. Figure 2 shows the cell morphology in the inner region of
PP was usually obtained during reactive extrusion of PP by blending with appropriate peroxides after polymerization.15−17 In this study, our newly developed FIM machine (Figure S1) combined with core-back operation was applied to fabricate the widely used PP resin by using air as the PBA. In this novel FIM machine (Figure S1), the air introduced into the polymer was directly provided by a gas cylinder via a delivery hole without the use of any pressurized pump or injector valve. The gas pressure could be easily controlled by the reducing pressure regulator connected to the gas cylinder. The original mold thickness was 2 mm and the core-back distance was separately set to 2, 4, 6, and 8 mm, which allows the preparation of injection-molded foams with void fractions of 50, 67, 75, and 80%, respectively.14,22 To improve the foamability of PP, 0.5 wt % of crystal nucleating agent was added.14,18 Two different gas pressures including 5 and 6 MPa of air were used in the foaming experiments. The PP blends processed under atmospheric pressure and 5 and 6 MPa of air pressure were briefly referred to as PP-0, PP-5, and PP-6, respectively. First, we examined the effect of introduced air on the changes of chemical composition of PP. The injection-molded samples under various PBA pressures without mold-opening (to minimize foaming) were collected and prepared for materials’ properties measurement. As shown in Figure S2, compared with the neat isotactic polypropylene (iPP), a weak peak at ∼1747 cm−1 corresponding to the CO absorption was detected in the attenuated total reflectance−Fourier transform infrared spectroscopy (ATR-FTIR) spectrum of PP-5 sample.19 With an increase in air pressure, this extra band at 1747 cm−1 tended to be intensified for the PP-6 specimen, while this peak was not observed in the PP-0 sample processed under atmospheric pressure. This signified that the added air caused the PP matrix to be oxidized during melt processing, which was due to the existence of a higher concentration of O2 under high pressure than that under the atmospheric pressure. Figure 1 shows the frequency dependences of complex viscosity (η*) and storage modulus (G′) of various PP samples, as measured at 180 °C. As displayed in Figure 1a, it revealed that PP’s complex viscosity was reduced with the introduction of air, and the difference was more obvious at the lowfrequency zone. This substantial decrease in melt viscosity evidenced that the PP matrix was degraded during melt processing with introduced air. Additionally, compared with the PP-5 sample, complex viscosity of the PP-6 sample slightly increased. Figure 1b illustrates that the G′ of PP had decreased in the whole frequency range when the PBA was added. PP-0 sample exhibited a nonterminal flow characterized by G′ ∼ ω1 19 at angular frequency ω ≤ 0.1 rad/s due to the wide molecular weight distribution,20 whereas the slopes of G′
Figure 2. SEM micrographs of PP-5 foams prepared under the foaming temperature of (a,a′) 88, (b,b′) 86, and (c,c′) 83 °C at a fixed void fraction of 50%. (a−c) and (a′−c′) are taken from the views parallel and perpendicular to the core-back direction, respectively. The core-back direction is parallel to the horizon.
PP foams with a constant void fraction of 50%, which was fabricated by our newly developed FIM machine with the combination of core-back operation. It should be noted that the prepared samples were taken from the middle of the foamed specimen.15,18 In general, the obtained cells were slightly deformed and oriented along the core-back direction, resulting from the extensional force induced during the mold-opening operation. In contrast, relatively spherical cells were obtained in the view perpendicular to the mold-opening direction, which B
DOI: 10.1021/acs.iecr.7b05331 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research was similar to our previous work.21−23 As Figure 2 shows, microcellular injection-molded PP foams were successfully prepared using a low-pressure air as the foaming agent, which was realized by our new FIM technology. It is worth mentioning that in a typical MuCell foaming process the usually used gas pressure is as high as 24 MPa, while a much lower gas pressure (5 MPa) is used here. To the best of our knowledge, this is the first study of using air as a blowing agent in the FIM process, which is also scarce in the other microcellular foaming technology. Moreover, according to Figure 2, cellular structures of the prepared PP foams were greatly improved by reducing the foaming temperature. For better comparison, subsequently, we will mainly focus the changes of cell morphology viewed in the perpendicular direction and assume that bubble is largely spherical to get the detailed cell information.21−23 By calculation,14 the cell density of PP foams with a void fraction of 50% fabricated at the foaming temperature of 88, 86, and 83 °C was 5.75 × 107, 1.31 × 108, and 4.04 × 108 cells/cm3, respectively. This evidenced that the foaming temperature significantly affected the cell structure of PP foams, which is attributed to the proceeding crystallization as well as the increase in melt strength with a decline in melt temperature.18,22,23 Thus controlling foaming temperature is a viable approach to tune the final cellular morphology of FIM foams. Correspondingly, the reduced foaming temperature was concurrent with a drop of cell size, reaching a small value of ∼12 μm for the samples prepared at the lowest foaming temperature of 83 °C. To examine the controllability of PBA content dissolved in the polymer melt, we have further increased the delivering pressure to 6 MPa to prepare the FIM foams. Because the actual gas content is difficult to detect, gas pressure is used here, which is a good indicator to evaluate the gas concentration dissolved in the molten polymer. Figure 3 presents the effect of
Figure S4a illustrates that the cell density of PP foams increased with the enhancement of gas pressure in the investigated void fraction range. For instance, with a slight increase of 1 MPa of air pressure, cell density of a 50% void fraction PP foams was enhanced from 4.04 × 108 cells/cm3 for PP-5 to 1.63 × 109 cells/cm3 for PP-6. Similarly, cell sizes (Figure S4b) of the obtained PP foams were correspondingly reduced with an enhancement of gas pressure, demonstrating a tiny cell size of ∼7 μm for the PP-6 sample. This demonstrates that the air concentration dissolved in the molten polymer can be regulated by the delivering pressure and simultaneously influence the final cellular structures. Additionally, with an increase in void fraction from 50 to 80%, cell density of the various PP foams decreased, which was concurrent with enlarged cell sizes. This was owing to the further growth and coalescence of bubbles because of the large mold-open distance.14 Our previous work14 exhibited that the 80% void fraction PP foams could not be achieved by using a 5 MPa N2 as the PBA. Herein, however, we have demonstrated that the 80% void fraction PP foams could be obtained by using air at the same pressure of 5 MPa. This suggests that in our newly developed FIM machine the use of air might be a better foaming agent than N2 in terms of the highest void fraction and fine cellular structures. To further clarify this, cell parameters of PP foams prepared using both air and N2 at the 5 MPa are presented in Figure 4. The optimum foaming results are presented under each void fraction, and the results of CO2 foaming are also displayed. It seems that for the PP materials used here, air foaming exhibits the finest cellular structures while N2 foaming has the worst cell morphologies. Interestingly, the optimum foaming temperature in air foaming was the lowest, while that in N2 foaming was the largest. Specifically, the optimum foaming temperatures in air, CO2, and N2 foaming for fabricating 50% void fraction PP foams were 83, 89, and 91 °C, respectively. Owing to the degradation of PP during air foaming and the resultant decline in melt viscosity, the foaming temperature should be correspondingly reduced to reach an approximate melt viscosity for foaming. Thereby, the optimum foaming temperature in air foaming was lower than those of the CO2 and N2 foaming. In addition, compared with N2 foaming, the stronger plasticization effect of CO2 lead to a reduction of optimum foaming temperature in CO2 foaming.26,27 Factors that promoted cellular structures of polymeric foams can be roughly divided into the crystallization-dominant and melt-strength-dominant promoted effects. In our previous work, the crystallization-dominant promoted effect such as adding nucleating agent and blending with cellulose nanofibers always produced PP with finer cellular structure and higher open cell content (OCC).18,21,22 The fine cell morphology and high OCC are mainly attributed to the notably enhanced cell nucleation caused by expedited crystallization and concurrent thinner cell walls. In contrast, in the melt-strength-dominantpromoted process, including introducing long-chain branches always gives PP a finer cell morphology but a lower OCC, which mainly results from the high melt strength that stabilizes bubbles and prevents bubbles from coalescence in the cell growth stage.23 Because the real foaming process is very complex and many factors such as surface tension, pressure drop, system pressure, and system temperature are combined together to affect the final cellular structures, the present classification of promoted-factor might provide a simple way to evaluate the variation of cell morphologies macroscopically.
Figure 3. SEM micrographs of the PP-5 (a−c) and PP-6 (d−f) foams with void fractions of (a,d) 50, (b,e) 67, and (c,f) 80%. The SEM images are taken from the view perpendicular to the core-back direction.
gas pressure on the cell morphology of PP foams prepared under various void fraction. The optimum cellular structures were selected for each condition. The enhanced gas pressures endowed the prepared PP foams with finer cellular structures and smaller bubble sizes, which was attributed to the introduction of a greater degree of thermodynamic instability with enhanced gas content.24,25 In addition, the above rheological results indicated that improved melt property was achieved in PP at a higher gas pressure, which would additionally benefit the foaming process. C
DOI: 10.1021/acs.iecr.7b05331 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Research Note
Industrial & Engineering Chemistry Research
Figure 4. Variations of (a) cell density, (b) average cell diameter, and (c) open cell content for PP foams prepared by using various PBA under the constant delivery pressure of 5 MPa.
According to Table S2, PP-5 and PP-6 samples show an equivalent or even higher crystallization temperature and crystallinity than those of the PP-0 sample under various nonisothermal crystallization conditions. This promotion effect in crystallization was related to the enhanced mobility of short molecular chains produced during the air degradation of PP.28,29 Because the component of air mainly consists of N2, it is reasonable to assume that the pure gas effect on the crystallization and plasticization of PP might be equal for air and N2 at the same pressure. Thus the improved cellular structures in air foaming were largely attributed to its lower foaming temperature, which would result in preceding crystallization prior to the initiation of foaming. This was realized by its low melt viscosity caused by the air-degradation of PP, while in N2 foaming the relatively high viscosity makes it difficult to reduce excessive melt temperature because too-high melt viscosity would impair the foam expansion. Consequently, compared with N2 foaming, the promoted cellular structure in PP foams is mainly ascribed to the crystallization-dominant promoted factor. On the contrary, much higher OCC was obtained in air foaming samples, which was due to the increased cell nucleation induced by more crystals and accompanied thinner cell walls. This enhanced OCC was also an indication of crystallization-dominant promoted process.18,21,22 Moreover, similar enhanced cellular structures were observed in air foaming with respect to CO2 foaming at void fractions
