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Production of Engineering Plastics Foams by Supercritical CO2 Ming-Tsai Liang* and Chang-Ming Wang Department of Chemical Engineering, I-Shou University, Taiwan 84008, Republic of China
The foams of poly(ethylene terephthalate) (PET) and polycarbonate (PC) are prepared by rapid depressurization of CO2-saturated molten resin. It is found that the foamable temperature and pressure for PET and PC are nearly identical, and the attainable expansion ratio of the produced foam is generally less than 10. The effect of saturation temperature on the nucleation is mainly a result of the competition between the viscosity effect and the solubility effect. However, the effect of saturation pressure on the nucleation is twofold: the degree of supersaturation and the rate of depressurization. The effect of lowering the depressurization rate and changing the depressurization pattern on the polymeric foams is also investigated in this work. By careful observation of the microstructure, it is concluded that the foaming process is dominated by the nucleation kinetics. This study provides several alternative techniques for controlling the microstructure and the expansion ratio of PET and PC foams. Introduction During the past decade, the application of supercritical fluid in producing polymer foam has been widely studied for its ability to produce microcellular polymer with very homogeneous cell structure.1-8 CO2 is the most popular blowing gas in this research area because of its nontoxicity, nonflammablility, low cost, and high environmental acceptance. On the basis of the path to induce thermodynamic instability to initiate the foaming process, the application of supercritical fluid in producing polymeric foam can principally be categorized as temperature-induced, solvent-induced, and pressureinduced phase separations. Among them, foaming by pressure induction is the most successful and potential technology for its fast rate of phase change without pressure gradient. However, in temperature- and solventinduced processes, the temperature gradient and diffusion barrier need to be carefully considered during scaleup. In this work, we utilize the principle of pressureinduced phase separation to obtain poly(ethylene terephthalate) (PET) and polycarbonate (PC) foams by depressurizing molten polymer solution. Polymeric foams with cell sizes of less than 10 µm and cell densities of greater than 108 cm-3 are called microcellular. They possess superior mechanical properties because the cell size is smaller than the flaw. To obtain microcellular foams with high void ratios, the cell density has to be greatly increased through extended supersaturation. In a highly supersaturated solution, a large amount of cell nucleus is homogeneously and abruptly created and rapidly grown within the mother phase. The supersaturation is then consumed by the growth of the nucleus. The growth of the cell may be interrupted by crystallization or by cooling to its glass transition temperature, Tg. In free expansion, the cell size of the cooled foams can be controlled by several parameters. However, the number of parameters affecting the cell size is far beyond that considered in experimental studies. Researchers have tried to model * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 886-7-657-7711 ext. 3413. Fax: 8867-657-8945.
the foaming process by combining the growth kinetics of a single constrained cell and the classical nucleation kinetics. Yet, no simple method is available for the pressure-induced foaming process.2,3,6,7 Although foaming by injection molding and by extrusion is now commercially available without a theoretical basis, investigation by experiments is still a practicable way toward industrial application. Most of the applications and studies of foaming with supercritical CO2 focused on low-Tg polymers. When dealing with high-Tg polymers, engineers commonly try to modify the polymer by blending with other polymers to lower the Tg. Because PC and PET possess high Tg and are highly crystalline polymers, they are seldom foamed by the pressure-induced process because of the inevitable crystallization or glass transition during the foaming process. Thus far, microcellular PET may be produced only by the temperature-induced process in the laboratory.9 The main objective of this study is to extend the CO2 foaming process to the high-Tg polymer. Also, the effects of processing conditions on the expansion ratio and cell morphology are investigated.
Experiments and Materials The PC used was from Mitsubishi, Uplion S200 (MW ) 28 000-30 000), and the PET was of industrial grade and was purchased from Far-East Co. with an intrinsic viscosity of 0.8. Industrial-grade CO2 (>99.5%) is precooled and pumped into a saturation tank preloaded with polymer granules, which are about 1 mm in diameter and 2.5 mm in length. The saturation tank with an internal volume of 150 mL (Swagelok SS-316L50DF4-150) is equipped with a heating jacket and a temperature controller. The pressure was adjusted through a back-pressure regulator. To fully dissolve CO2 into the resin, their granules were immersed in the saturation tank for 30 min. The pressure was then released immediately by opening a valve attached to the tank to induce the phase instability. This causes CO2 to be released from the polymer’s matrix and produces foamy granules. During depressurization, the temper-
10.1021/ie000062y CCC: $19.00 © 2000 American Chemical Society Published on Web 10/27/2000
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Figure 1. Effect of saturation pressure on the expansion ratio of PC foams.
ature of the foaming granules decreases because of the expansion of CO2 gas and the release of CO2 gas from the polymer’s matrix. Right after the depressurization, the CO2 source tank is also opened to further cool the foamed samples in order to solidify and stabilize the foam structure. Because the depressurization theoretically follows the isentropic path, the temperature during the foaming process could be calculated from the initial state and left as an uncontrolled parameter in this work. Longer saturation times of more than 30 min up to 2 h were also evaluated in terms of the expansion ratio and its microstructure. No significant difference was observed. Therefore, the saturation time is set to 30 min in this work. The pressure was released by various procedures: (1) directly by a ball valve (Swagelok SS83KF4); (2) directly by a needle valve (Swagelok SS3NBM4); (3) by first depressurization to 6 MPa and then to ambient pressure in 30 s. The first method is to obtain polymeric foam produced by very fast depressurization, and the second is to examine the influence of the depressurization rate. The third method is to examine the nucleation kinetics of the foaming process. The produced polymeric foams were collected and weighed to calculate their expansion ratios (the ratio of polymer density to foam density, FP/Ff). The microstructure of the foams was inspected by scanning electron microscopy (SEM). The cell size and density were then carefully measured by examining the SEM photographs. To examine the nucleation kinetics directly, the cell density, Ncell, was multiplied by the expansion ratio to obtain the nucleus density, Nnucl. The PET foams prepared by slow depressurization were only inspected by an optical microscope. Results and Discussion Effect of Saturation Pressure and Saturation Temperature. According to Henry’s law, the CO2 solubility linearly depends on the saturation pressure and exponentially relates to 1/T. Therefore, depressurization by a ball valve is used, the saturation pressure linearly affects the expansion ratio of PC foams as shown in Figure 1. It is also found in Figure 1 that the expansion ratio of PC foams saturated at 553 K is smaller than that at 523 K over the entire range of investigated pressure. This is due to the high CO2
Figure 2. Effect of saturation temperature on the expansion ratio of PC foams.
Figure 3. Effect of saturation temperature on the expansion ratio of PET foams.
solubility at low temperature. However, a wide range of temperature leads to a complicated result. Figure 2 shows the effect of the saturation temperature on the expansion ratio of PC foams, which are prepared by depressurization by a ball valve. A maximum expansion ratio is observed along the saturation temperature course at constant saturation pressure. It is presumed that the melt is too viscous to nucleate and to grow the bubbles at low temperature. Increasing the temperature would reduce the viscosity and prompt the rate of cell nucleation and growth. Once the viscosity of molten resin does not retard the cell nucleation and growth, the expansion ratio simply decreases with the temperature in accordance with the decrease of the CO2 solubility.10 It is concluded that the foaming process is constrained by the melt’s viscosity at lower temperature and limited by the CO2 solubility at higher temperature. The maximum expansion ratio along the temperature course is also found in foaming PET as shown in Figure 3. However, the effect of saturation pressure on the PET foams shows very differently from that of PC foams. This is due to different nucleation kinetics. Figure 4 shows the change of the expansion ratio of PET foams with the saturation pressure, which are saturated at
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Figure 5. Effect of saturation pressure on the cell size and the nucleus density of PC foams when saturated at 523 K. Figure 4. Effect of saturation pressure on the expansion ratio of PET foams.
523, 533, and 543 K, respectively. It is observed that the expansion ratio of the PET foams is not linearly related to the saturation pressure as in the foaming PC resin. Also, no significant change of the expansion ratio in foaming PET is found if the resins are saturated at 543 K over the entire range of investigated pressure and if they are saturated at 523 and 533 K for pressures under 28.4 MPa. The expansion ratio, however, abruptly increases for pressures above 28.4 MPa and at 523 and 533 K. If the dissolved CO2 gas is totally consumed to expand the melt resin, the slope of the curves in Figure 4 must exponentially relate to the activation energy of CO2 dissolution. The difference of the slope implies a different polymer structure. The interaction between CO2 and PET was studied by Dorouiani et al.11 They observed the crystallization of amorphous PET when immersed in CO2 at 5.5 MPa and 24 °C due to the increase of the segmental motion of the polymer and also due to the increase of the melting point by increasing the pressure. It is, therefore, presumed that the polymeric matrix is not totally amorphous for CO2 pressures under 28.4 MPa, although the saturation temperature of 523 or 533 K is already above PET’s normal melting point. It is well-known that the dissolved CO2 acts not only as an active molecule to induce the crystallization but also as a diluent to reduce the polymer’s melting point. Because the amount of dissolved CO2 increases with the saturation pressure, PET molecules in the crystalline matrix may melt again when the CO2 pressure increases to 28.4 MPa. Because the CO2 solubility is limited when saturated at 543 K, to melt the PET, we need to extend the saturation pressure over the investigated pressure of 32.0 MPa. Therefore, the abrupt increase of the expansion ratio at 543 K could not be observed in this study. This unusual behavior of the PET-CO2 binary system is very similar to the so-called retrograde vitrification observed in the PMMA-CO2 system.12 Because polypropylene (PP) is also a highly crystalline polymer, it is expected that the abrupt increase of the expansion ratio can also be observed in foaming PP. In a prior study, the abnormal increase of the expansion ratio of PP foams was observed and explained by the crystallization due to the cooling effect after depressurization.5 However, in foaming PET the abrupt in-
crease of the expansion ratio is due to the change of the polymer’s structure. According to the classical nucleation theory, the nucleus density can be expressed as
Nnucl ) Ncell
()
FP ) Ff
I dP ) ∫0tI dt ) ∫PP dP/dt f
i
I0 -dP/dt
∫PP exp i
f
{
-
}
∆G* dP (1) kT
where ∆G* is the energy barrier for the nucleation, Pi and Pf are the saturation pressure and the nucleation pressure where the nucleation starts, and I0, which depends only on temperature, is the prefactor for the nucleation rate expression I. With the assumptions of the spherical nucleus and Henry’s law for the CO2 solubility, the energy barrier for the nucleation can be derived as
∆G* )
σ3 16π 2 2 3k T (ln Pf - ln Pi)2
(2)
where σ is the interfacial energy of the nucleus. On the assumption of isothermal foaming process, one can theoretically obtain a large amount of nucleus by either decreasing the depressurization rate -dP/dt or enlarging the pressure ratio Pf/Pi. Because it is very difficult to control the nucleation pressure Pf accurately, the remaining feasible ways to increase the nucleus density is increasing the saturation pressure and/or decreasing the depressurization rate. In this work, we examine the effects of the saturation pressure and temperature, the depressurization rate, and the postpressure after depressurization on the expansion ratio and also on the microstructure of PET foams. On the assumption that the nucleation pressure and the depressurization rate are constants, the nucleus density should increase with the saturation pressure. Figure 5 illustrates the cell size and the nucleus density of PC foams, whose expansion ratios are shown in Figure 1 as solid diamonds and which will vary with the saturation pressure. In this paper, the symbols with error bars illustrated in the figures represent the mean diameter and 1 standard deviation of the cell. When Figures 1 and 5 are compared, it is observed that the increase of the expansion ratio is contributed mainly
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Figure 6. Effect of saturation pressure on the cell size and the nucleus density of PET foams when saturated at 533 K.
Figure 7. Effect of saturation pressure on the cell size and the nucleus density of PET foams when saturated at 543 K.
from the nucleus density rather than from the cell size. Obviously, the nucleation kinetics dominates the PC foaming process, and the classical nucleation theory is consistent with the experimental results qualitatively. Figures 6 and 7 illustrate that the cell size and the nucleus density of PET foams vary with the saturation pressure. It is observed that all of the cells are smaller than 50 µm and averaged at about 20 µm over the entire range of pressure. Again, foaming PET is primarily dominated by the nucleation kinetics rather than by the growth kinetics. Although the PET is likely foamed from different states, we do not obtain any significant conclusion about its nucleation mechanism at this stage. Further examination on the microstructure and theoretical study on the growth kinetics are recommended in modeling the foaming process. The change of the cell size and the nucleus density of PET foams in different saturated temperatures is depicted in Figure 8. It is, once again, observed that the cell size is roughly constant, and the variation of the nucleus density with temperature roughly follows the trend of the expansion ratio shown in Figure 3. It is confirmed that the nucleation kinetics determines the microstructure of the PET foams. According to eq 1, the graph of nucleus density Nnucl versus saturation pressure Pi will be an increasing and concave upward curve if all other parameters are held.
Figure 8. Effect of saturation temperature on the cell size and the nucleus density of PET foams when saturated at 30.0 MPa.
Figures 5 and 7 seem to confirm this. However, Figure 6 is not intrinsically consistent with the theoretical prediction. If the degree of supersaturation is held, the nucleus density is directly inverse proportional to -dP/ dT. Because we used the same ball valve to release the pressure, the depressurization rate is roughly proportional to the square root of the pressure difference between the chamber and the ambient. Because the ambient pressure in our case is 1 atm, the nucleus density is, therefore, roughly proportional to xPi. This is consistent with the observation in Figure 6 but not in Figures 5 and 7. If Figure 6 is transformed into dNnucl/dPi versus 1/(ln Pi)2, it shows an increasing function. Yet, it is a decreasing function when Figures 5 and 7 are transformed. It seems that the effect of the saturation pressure on the nucleation is twofold: the supersaturation and the depressurization rate. However, the nucleus density measured in this work is based on a single SEM photograph. Because the sample size is very insufficient, misleading the kinetics is very possible when interpreting the experimental data, especially when differentiation or integration of the data is inevitable. Further experimental studies will be necessary to confirm the conclusion. Effect of the Depressurization Rate and Depressurization Process. Because the foaming process is dominated by the nucleation kinetics, it is worth investigating the feasibility of controlling the nucleation. Instead of depressurizing directly to ambient pressure, the saturated molten resin is depressurized to 6 MPa and then released to ambient. It is observed that the expansion ratio by the two-step depressurization is less than that of one-step depressurization, as shown in Figure 3. With two-step depressurization, the cell size decreases but the nucleus density is rarely affected. This implies that the degree of supersaturation is equivalent in both cases. It is presumed that the postpressure after depressurization is not the nucleation pressure, and the nucleation pressure in our case must be higher than 6 MPa. When the pressure is released by a needle valve, the depressurization rate -dP/dt is definitely lowered. According to eq 1, the nucleus density of the foams should increase if the degree of supersaturation is held. However, the experimental results are not consistent with the theoretical prediction. It is observed that the expansion ratios of PET foams decrease when a needle valve is used. The microstructure observed from an optical
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microscope shows that the diameter of the cell is normally in the range of 50-80 µm, which is significantly larger than those prepared by fast depressurization, as shown in Figure 8. This implies that the PET foams prepared by rapid depressurization have higher nucleus densities. It is presumed that the nucleation pressure increases when decreasing the depressurization rate. A greater degree of supersaturation can, therefore, be obtained by increasing the rate of inducing the thermodynamic instability. Conclusion This work provides an alternative technique to foam PC and PET. The effect of operation parameters in the foaming process is also investigated by inspecting the expansion ratio and the microstructure of the produced foams. In the case of free expansion, the nucleation kinetics primarily dominates the foaming process, and the growth kinetics contributes a minor effect. The experimental observation is explained by the classical nucleation theory and concluded as follows: 1. The expansion ratio increases with the saturation pressure and shows a maximum along with the saturation temperature course in foaming PC and PET. 2. The saturation pressure could affect the foaming process twofold: the degree of obtainable supersaturation and the depressurization rate. 3. By two-step depressurization, the cell growth is limited but the cell nucleation is rarely affected. 4. The postpressure after depressurization is not the nucleation pressure, which is higher than 6 MPa in this study. 5. The degree of supersaturation increases with the saturation pressure and the depressurization rate. 6. The increase of the depressurization rate can increase the nucleation rate because of lower nucleation pressure. Acknowledgment Financial support from National Science Council, R.O.C., under Grant NSC 89-2216-E-214-004 is gratefully acknowledged. Literature Cited (1) Goel, S. K.; Beckman, E. J. Nucleation and Growth in Microcellular Materials: Supercritical CO2 as Foaming Agent. AIChE J. 1995, 41, 357-367.
(2) Goel, S. K.; Beckman, E. J. Generation of Microcellular Polymeric Foams Using Supercritical Carbon Dioxide. I: Effect of Pressure and Temperature on Nucleation. Polym. Eng. Sci. 1994, 34, 1137-1147. (3) Goel, S. K.; Beckman, E. J. Generation of Microcellular Polymeric Foams Using Supercritical Carbon Dioxide. II: Cell Growth and Skin Formation. Polym. Eng. Sci. 1994, 34, 11481156. (4) Park, C. B.; Behravesh, A. H.; Venter, R. D. Low-Density Microcellular Foam Processing in Extrusion Using CO2. Polym. Eng. Sci. 1998, 36, 1812-1823. (5) Liang, M. T.; Wang, C. M. Production of Very Low-Density Microcellular Polypropylene by Supercritical Carbon Dioxide. Proceedings of the 6th Meeting on Supercritical Fluids: Chemistry and Materials, Nottingham, United Kingdom, April 10-13, 1999; International Society for the Advancement of Supercritical Fluids, pp 151-156. (6) Ramesh, N. S.; Rasmussen, D. H.; Campbell, G. A. Numerical and Experimental Studies of Bubble Growth. Polym. Eng. Sci. 1991, 31, 1157-1664. (7) Ramesh, N. S.; Rasmussen, D. H.; Campbell, G. A. The Heterogeneous Nucleation of Microcelluar Foams by the Survival of Microvoids in Polymers Containing Low Glass Transition Particles. Part I: Mathematical Modeling and Numerical Simulation. Polym. Eng. Sci. 1994, 34, 1685-1697. (8) Ramesh, N. S.; Rasmussen, D. H.; Campbell, G. A. The Heterogeneous Nucleation of Microcellular Foams by the Survival of Microvoids in Polymers Containing Low Glass Transition Particles. Part II: Experimental Results and Discussion. Polym. Eng. Sci. 1994, 34, 1698-1706. (9) Baldwin, D. F.; Park, C. B.; Suh, N. P. A Microcellular Processing Study of Poly(Ethylene Terephthalate) in the Amorphous and Semicrystalline States. Part II: Cell Growth and Process Design. Polym. Eng. Sci. 1996, 36, 1446-1453. (10) Liang, M. T.; Tsai, C. X.; Wang, C. M. Production of Microcellular Polycarbonate by Supercritical CO2: Examination of Pressure and Temperature Effects. 5th International Symposium on Hydrothermal Reactions; Bordeaux, France, July 1999. (11) Dorouiani, S.; Park, C. B.; Kortschot, M. T. Effect of the Crystallinity and Morphology on the Microcellular Foam Structure of Semicrystalline Polymers. Polym. Eng. Sci. 1996, 36, 26452662. (12) Johnston, K. P.; Condo, P. D. Retrograde Virtrification of Polymers with Compressed Fluid Diluents: Experimental Confirmation. Macromolecules 1992, 25, 6730-6732.
Received for review January 18, 2000 Revised manuscript received July 12, 2000 Accepted July 15, 2000 IE000062Y
