Abstract. Concept. Introduction

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1 FOAM INJECTION-MOLDING PROCESS DESIGNED TO PRODUCE SUB-MICRON CELLS Stéphane Costeux, Hyunwoo Kim, Devin Foether, The Dow Chemical Company, Midland, MI, U.S.A. Abstract Significant progress has been made in recent years towards the production of low density foams with cell size around 100 nm. However, the process commonly used is batch foaming with high pressure CO 2, which is not easily scalable and ill-suited for the production of larger specimen with controlled dimensions necessary for reliable property testing. A new approach to generate sub-microcellular foams with expansion ratio up to 4 by a modified injectionmolding process is presented. Homogeneous polymer/co 2 mixtures produced by an extrusion foaming line are injected under controlled pressure into a variable thickness mold, which can then be opened at a controlled speed to adjust cell morphology. Foams with cell size below 500 nm were made by this process. Introduction In the past 10 years, significant effort has been dedicated to the development of polymer systems and processes capable of producing nanocellular foams. [1,2] These foams offer the promise of unique properties [3] (thermal [4,5], mechanical [6,7], acoustic [8], dielectric [9] etc.). The process used is almost exclusively batch foaming, in which a polymer sample is saturated with CO 2 in an autoclave under medium to high pressure (5 to 40 MPa) at relatively low temperature (typically -30 C to 50 C) and foamed either during pressure release (1-step), or during a secondary heat treatment (2-step). Foams with cells below 100 nm and expansion ratios up to 7 (600% expansion or 0.15 relative density) have been produced by such process. [10] Unfortunately, due to saturation times that dramatically increase with thickness, to limitations in autoclave dimension and pressure limitations, and to uncontrolled shape due to free foam expansion, this process remains a research tool with limited commercial prospects. Continuous extrusion of nanocellular foams has been demonstrated at a small pilot scale, [11] but significant barriers remain for large scale continuous production. Foam injection molding, which has found commercial applications for microcellular foams, has not yet been successfully applied to the production of nanocellular foams. A recent literature review [2] indicates that the minimum cell size reported by this process in greater than 10 µm, and limited to 10-20% expansion (expansion ratio <1.2). Therefore, there is a need to understand the limitations of traditional injection molding and to develop equipment that capable of producing nanocellular foams. Concept Conditions to produce nanocellular foams from homogeneous polymer systems by a 1-step process generally require: [1] Saturation temperature and pressure such that CO 2 concentration exceeds a minimum value (about wt% for PMMA and other acrylic polymers). Temperature such that the polymer saturated with CO 2 is above its T g, so that expansion is possible during pressure drop. In PMMA, the effect of CO 2 plasticization produce significant Tg reduction. [12] Rapid pressure drop to allow generation of a large number of nuclei (10 14 per cm 3 or higher) without diffusion-induced coalescence. [13] Saturation conditions around 35 C and 30MPa have proven effective for PMMA to achieve cell densities in excess of /cm 3 and expansion ratios of 4 to 7. [10,14] These conditions were also realized in continuous extrusion, by solving the problems of the addition of a large excess of CO 2 and of extrusion below the T g of the pure polymer (but above that of the polymer saturated with CO 2). [11,15] Therefore, we target similar conditions for an injection molding process, in which an extrusion foaming line is used for preparation on the cooled, pressurized polymer/co 2 mixture. The foamable mixture then needs to be transferred to a mold and without significant loss of pressure, in order to avoid uncontrolled nucleation. This can be achieved in a variable volume mold with controlled counter-pressure. SPE ANTEC Anaheim 2017 / 2460

2 The last essential step is to achieve rapid pressure release in the mold. [16] Guidelines from batch foaming indicate that pressure drop rate around 1 GPa/s should provide sufficient driving force for high nucleation rates. Ideally such rates need to be achieved within the closed, variable volume mold; alternatively, fast rates could be produced by rapid opening of the mold, which may be easier from an engineering standpoint. In the following, we describe a first iteration in the design of such equipment, and the effect on engineering variables on the resultant foam characteristics. Materials Experimental The polymer system chosen is a miscible polymethylmethacrylate (PMMA) / styrene acrylonitrile copolymer (SAN) blend (60/40 w/w), which was previously shown to yield homogeneous, 100 nm cell nanofoams with expansion ratio around 4.0. [17] Resins details are in Table 1. Carbon dioxide was added to the blend at 20 wt%. Table 1. Polymer resins characteristics and closing of the outer mold. Oil pressure (max 1200 psi) is controlled in the outer mold cylinder and the absolute vertical position of the outer mold is measured. At the maximum oil flow rate, the system is designed for an outer mold displacement speed of 100 cm per second (40 in/s). A second pneumatic system controls the displacement of the inner mold, responsible for the variation of the mold cavity volume. Maximum oil pressure in the corresponding cylinder is also 1200 psi, and should theoretically allow displacement speed up to 150 cm/s. Speed is nonetheless constrained by the limited course of the inner mold inside the outer mold. A feedback loop allows for accurate position of both the inner and outer mold. Pressure transducers provide real-time data on the pressure into the mold cavity. The extrusion foam line brings the polymer/co 2 mixture through a channel underneath the mold. A gate open during filling of the mold cavity and closes when the desired volume of foamable mixture has been injected. Two sets of mold cavity are available, providing maximum mold cavity dimensions of 10 x 10 x 2.5 cm and 15 x 15 x 2.5 cm respectively (described as 4 x 4 inch and 6 x 6 inch molds in the following). Copolymers comonomer Mw Mw/Mn Tg wt% kg/mol ( C) SAN16L 16 wt% AN MMA-EA9* 8.9 wt% EA * Plexiglas VM-100 (Arkema) contains ethyl acrylate Extrusion equipment Inner mold cylinder Outer mold cylinder The extrusion foaming line (described in a previous publication [11] ) utilizes an extruder equipped with a 1 diameter screw (L/D=24). The 3 zones of the extruder were set to 120 C, 160 C and 180 C. Polymer dry blend was fed at 5 lb/hr. CO 2 was introduced with a dual cylinder ISCO 100HLX continuous pump, and injected in a rotary mixer downstream of the extruder set to 175 C. At the conditions present in the mixer, CO 2 concentration exceeds solubility in the blend. The polymer/co 2 mixture then travels through a cooling apparatus with 35 min residence time. This brings the temperature down to the desired die inlet temperature (50-60 C), while maintaining a pressure sufficient for CO 2 to be fully soluble in the homogeneous blend. Variable volume mold system A schematic of the injection molding system in shown in Figure 1. A first pneumatic system controls the opening Inner mold Figure 1. Schematic of foam injection molding process with variable volume cavity Injection molding protocol Outer mold Gate Foam Line To maximize opening speed, the protocol shown in Figure 2 is followed. With the outer mold and gate closed, the inner mold is positioned flush with the outer mold, to reduce the cavity volume to zero (step 1). Next, the gate is SPE ANTEC Anaheim 2017 / 2461

3 opened, and the polymer/co 2 mixture flows in (2). The inner mold is slowly retracted to accommodate the volume of the foamable mixture (3), while pressure transducers monitor the pressure experienced by the mixture. The inner mold displacement is thus controlled to ensure the pressure remains above a threshold that maintains CO 2 dissolved (typically similar to the pressure from the extrusion foaming line if the temperature is also similar). When the desired volume fills the cavity (4), the gate is closed and the mixture left to equilibrate. Mold opening testing For accurate control, it is preferable to operate a range where opening speed varies linearly with oil flow rate. (here defined as % Open Speed, i.e. the percentage of the maximum possible oil flow rate). Figure 3 shows that, for both inner and outer mold, the actual velocity of the mold is initially linear with the % Open Speed (oil flow rate) before is levels off. The limiting velocity values being different, in order to have matching velocities for both mold during rapid opening, one has to operate at a fraction of the maximum speed. We chose a set point range of 40-70% for the outer mold, and 30-40% for the inner mold, which yields a maximum actual opening speed of 70 cm/s (27 inch/s). This allows full mold opening in about 50 ms ( dp/dt ~1 GPa.s). Figure 3. Actual mold displacement speed as a function of oil flow rate (as % of flow rate for maximum opening speed) Foam characterization Figure 2. Schematic of foam injection molding process with variable volume cavity Rapid mold opening occurs in two steps: first, the outer mold is raised with the inner mold stationary until both are flush (5); then the inner mold is set in motion at a speed matching that of the outer mold, so that both mold remain flush until fully open. The sample thus expands in the three directions, with the possibility to be constrained in the vertical direction. Foam produced were characterized by their density ρ foam measured by Archimedes method. The porosity or void fraction of the foams were determined by p(%) = 100(1 ρ foam ρ polymer ) For cell size measurements, foams were cryofractured and imaged by a scanning electron microscope. A minimum of 200 cells were measured from the images using the software ImageJ (NIH) and the diameters were averaged to obtain the average cell size. SPE ANTEC Anaheim 2017 / 2462

4 Results Role of key control parameters A full factorial design of experiments (DOE) was conducted, in which: Mold temperature was varied between 60 and 70 C Mold cavity pressure was set between 1100 and 5100 psi by controlling the inner mold oil pressure The % opening speed (oil flow rate) set point for the outer mold was varied between 40 and 70%. The inner mold oil flow rate was set to match the outer mold speed during opening. Foams were produced with the 60/40 PMMA/SAN blend with 20wt% CO 2, using the 4 inch mold system. Figure 4 shows the result of the DOE analyzed with JMP statistical software (SAS Institute Inc.). The most sensitive parameter is the mold pressure, which has a significant positive effect on cell size. This is expected, as high mold pressures allow solubilization of higher CO 2 level and larger pressure drops, both favorable to high cell nucleation. Conversely, foam with small cells are more difficult to produce with low density, which results in lower porosity at high pressures. The effect of mold opening speed is similar, albeit less pronounced. Over the range explored (60 to 70 C), the role of temperature is modest and at the limit of statistical significance. Nonetheless, lower temperatures are expected to be advantageous due to higher CO 2 solubility. Figure 4 shows SEM images for two foams produced at low and high pressure, with cell size around 10 µm and 0.8 µm and expansion ratio of 5 and 4 (i.e. 80% and 75% porosity), respectively. In view of ref. [2], such low density foams with cell in this size range is unprecedented. Parameter optimization for cell size reduction Additional experiments were conducted to validate the best conditions to produce foams with cells below 1 µm. In the first set of experiments, mold temperature is controlled at 62 C and the opening speed at 62 cm/s (60 ms for full opening). As shown in Figure 5, at this temperature, the pressure needs to exceed 4000 psi to maintain 20 wt% CO 2 dissolved. Figure 5. CO 2 solubility map in the polymer blend. Isosolubility line corresponding to 20 wt% CO 2 loading is shown by the dashed line. Numbers refer to foam samples of Table 2. Figure 4. Influence of key control parameters (opening speed and temperature and pressure in the mold cavity) on cell size and void fraction of the foam. Foams in Table 2 were produced by injecting the polymer/co 2 mixture in the mold at high pressure (~5000 psi) to avoid premature foaming, followed by an adjustment of the pressure in the mold. For Foam 1, the mold pressure was reduced to 2450 psi. For Foam 3, it was reduced to 4780 psi had cells in excess of 10 µm. Pressure was not reduced for Foams 2 and 4. Table 2 shows that foams produced at pressures above 4000 psi had cells below 1 µm and porosities close to 75% (expansion ratio slightly below 4.0). In contrast, Foam 1 had cell size above 10 µm, due to early CO 2 nucleation when pressure was decreased in the mold below the saturation limit. Note that SPE ANTEC Anaheim 2017 / 2463

5 due to expansion in 3 directions, foams made in the 4 x 4 inch mold expanded to about 6 x 6 x 0.6. Table 2. Foams produced at 62 C, with rapid mold opening (62 cm/s) Foam # Mold T ( C) Mold P (psi) Porosity (%) Avg. cell size (nm) In the second set of experiments, temperature in the extrusion foaming line and in the mold was reduced to 54 C, which is expected to provide some minor improvement in cell size. More importantly, it reduced the saturation pressure for CO 2 to 3300 psi. Foams #5 to 9 were then produced at various mold equilibrium pressures, as shown in Table 3. Table 3. Foams made at 54 C, with rapid mold opening Foam # Mold T C Mold P (psi) Porosity (%) Avg. cell size (nm) * * Same as Foam 9, post-expanded in hot bath, 60 C for 3 min Foam #5 was made at a pressure very close to the CO 2 saturation pressure, and this resulted in a cell size average in excess of 4 µm. Upon increasing the pressure to 5150 psi, average cell size decreased to 450 nm with porosity close to 72% (expansion ratio of 3.6). Further increase of the pressure to 5880 psi (Foam #9) did not produce an improvement in cell size. This is understandable as, unlike batch foaming, pressure increase in injection molding does not lead to higher CO 2 levels. Therefore, during depressurization, nucleation only starts once the pressure decreases to the saturation pressure (~3300 psi), regardless of initial pressure. The main benefit of pressures that largely exceed the saturation pressure comes in ensuring CO 2 is fully dissolved, in increasing the pressure drop rate at the time supersaturation is reached, and in increasing the viscosity of the mixture. These factors help produce submicrocellular structures, but this falls short of the nanocellular foams with cells of 100 nm or less that could be made from the same blend by batch foaming. To increase expansion ratio, Foam #9 was repeated and, immediately after retrieval from the mold, it was immersed for 3 min into a water bath heated to 60 C. This technique, common for 2-step foaming, was previously applied with success to lower nanocellular foam density without significant increase in cell size. [10,14] The resulting foam (#10) indeed has an average cell size on 600 nm, similar to Foam #9, but higher porosity (78%) and expansion ratio (4.6). Figure 6 illustrates the improvement in cell size and expansion ratios achieved by our custom rapid opening injection foaming system compared to foams previously reported in the literature. [2] Diamond symbols include Foams #1 to 10 and foams from the DOE. Squares symbols represent foam described in the next section. Figure 6. Characteristics of foams produced from novel injection molding system. In-mold expansion As described in the experimental section, the equipment has the ability to perform as a core-back system, in which rapid opening of both inner and outer molds (steps 5 and 6 of Figure 2) is replaced by the displacement of the inner mold only inside the outer mold that remains closed). the 6 x 6 inch mold was filled with the polymer/co2 mixture by retracting the inner mold by 0.2 (5 mm) the mixture was equilibrated for 1 min at 5800 psi and 53.8 C. The inner mold was retracted by 0.42 (10.6 mm) at the maximum speed allowed within the outer mold (5 cm/s). SPE ANTEC Anaheim 2017 / 2464

6 Figure 7 shows the foam sample (left) in comparison with a sample from the rapid opening method (right). Dimensions are similar, but the former is more flat and does not require trimming. However, due to the lower rate of depressurization, cell size are larger than 1 µm. Acknowledgments We thank Warren Griffin, Dan Beaudoin and Dennis Lantz for installation and troubleshooting of the equipment. This material is based upon work supported by the U.S. Department of Energy under Award Number DE- EE Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. References Figure 7. Comparison of foams produced within the cavity vs by full rapid opening of both inner and outer molds Conclusions A custom injection foaming system was designed and built to meet high opening speed requirements expected to be necessary for the production on nanocellular foams. A dual pneumatic system allows filling of the mold with controlled pressure to avoid premature nucleation and for rapid opening of the mold up to 70 cm/s, corresponding to pressure drop rate up to 1 GPa/s. Foams were produced with this method using a PMMA/SAN blend with 20 wt% CO 2. Optimization of the temperature and pressure around 54 C and 5200 psi produced foams with cells below 500 nm and expansion ratio approaching 4. Expansion ratio was further increased by post expansion in a water bath. Despite being the finest cellular structure ever produced in injection molding, cells remain larger than for the same polymer system foamed in a batch foamer. Further reduction of temperature and increase in the CO 2 loading may bridge this gap in the future. 1. S. Costeux, J. Appl. Polym. Sci., 131, 41293/1-16 (2014). 2. C. Okolieocha, D. Raps et al., Eur. Polym. J., 75, (2015). 3. B. Notario, J. Pinto et al., Prog. Mater Sci., 78 79, (2016). 4. C. Forest, P. Chaumont et al., Prog. Polym. Sci., 41, (2015). 5. B. Notario, J. Pinto et al., Polymer, 56, (2015). 6. D. Miller and V. Kumar, Polymer, 52, (2011). 7. B. Notario, J. Pinto et al., Polymer, 63, (2015). 8. B. Notario, A. Ballesteros et al., Mater. Lett., 168, (2016). 9. X. Li, H. Zou et al., J. Appl. Polym. Sci., 132, (2015). 10. S. Costeux and L. Zhu, Polymer, 54, (2013). 11. S. Costeux and D. Foether, "Continuous Extrusion of Nanocellular Foam", SPE ANTEC Proc. (Orlando, FL, March 23-25, 2015). 12. P. D. Condo, I. C. Sanchez et al., Macromolecules, 25, (1992). 13. Z. Zhu, C. B. Park et al., "Challenges to the Formation of Nano-Cells in Foaming Processes", SPE ANTEC Proc. (Charlotte, N.C, May 7-11, 2006). 14. S. Costeux, I. Khan et al., J. Cell. Plast., 51, (2015). 15. S. Costeux, US patent 9,145,478, assigned to Dow Global Technologies LLC (2015) 16. S. Costeux, D. A. Beaudoin et al., PCT Patent Appl. WO , assigned to Dow Global Technologies LLC (2015) 17. S. Costeux, S. P. Bunker et al., J. Mater. Res., 28, (2013). SPE ANTEC Anaheim 2017 / 2465