Foaming of thin films of a fluorinated ethylene propylene copolymer using supercritical carbon dioxide

J. of Supercritical Fluids 49 (2009) 103–110

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Foaming of thin films of a fluorinated ethylene propylene copolymer using supercritical carbon dioxide Larissa Zirkel, Martin Jakob, Helmut Münstedt ∗ Institute of Polymer Materials, University Erlangen-Nürnberg, Martensstr. 7, D-91058 Erlangen, Germany

a r t i c l e

i n f o

Article history: Received 12 August 2008 Received in revised form 18 November 2008 Accepted 19 November 2008 Keywords: Fluorinated ethylene propylene copolymer (FEP) Foaming Supercritical carbon dioxide Film Processing parameters

a b s t r a c t The foaming of films of a fluorinated ethylene propylene copolymer (FEP) using supercritical carbon dioxide (scCO2 ) was investigated. For this purpose, a one-step foaming process was applied to the films inside a pressure vessel at temperatures in excess of 200 ◦ C and pressures up to 30 MPa. The films prepared show a homogeneous cellular structure with thin compact polymer layers on both sides. The foaming behaviour of the FEP films is determined in dependence on the exposure time of the polymer to the scCO2 , the processing temperature and pressure as well as on the pressure drop rate during the foaming process. The resulting density and the foam morphology can be related to the pressure level during the saturation process. The size of the cells generated mainly depends on the foaming temperature, whereas their number per volume is determined by the magnitude of the applied pressure. Furthermore, the pressure drop rate influences the size of the cells in a way that significantly fewer but larger bubbles are found for longer times of depressurizing. This finding can be explained by nucleation rates becoming the smaller the slower the rate of pressure decrease. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Supercritical fluids exhibit some very special properties related to both, the liquid and the gaseous state, as e.g. densities typical of liquids along with gas-like high diffusion rates. This makes them excellent processing agents for a multiplicity of even large-scale applications like the use of supercritical carbon dioxide (scCO2 ) as agent for extracting caffeine from coffee or tea as well as biological products from medicinal herbs or spices [1]. Other materials related domains for scCO2 reported during the last years are the generation of polymer nanoparticles, filaments, e.g. [2,3], and (microcellular) foams [4–20]. Besides its low critical values (Tcrit ∼ 31.1 ◦ C and pcrit ∼ 7.4 MPa) the benefits of CO2 are that it is cheap, chemically inert, non-toxic and fire-proof. The fact that it is not toxic and evaporates without residues makes it a qualified alternative to many organic solvents or blowing agents in polymer processing. Furthermore, the ability of scCO2 to plasticize polymers is advantageous for many processes as lower operation temperatures are required. All these properties of scCO2 can be exploited for the foaming of polymers. A large number of studies were published on this subject during the last 20 years, mostly related to the foaming of amorphous polymers. Their focus was mainly on the relation

∗ Corresponding author. Tel.: +49 9131 852 8593; fax: +49 9131 852 8321. E-mail address: [email protected] (H. Münstedt). 0896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.11.013

between the processing conditions of the foaming process on the one side and the foaming behaviour and properties of the foams, on the other. Systematic investigations on the influence of the process parameters saturation time, temperature, and pressure were performed for polystyrene (PS) [4–7], polymethylmethacrylate (PMMA) [8,9], polyurethane (PU) [10], cellulose acetate (CA) [5], and poly(ethylene terephthalate) (PET) [11–14]. Other papers deal with high temperature thermoplastics like polyetherimide (PEI) [15], polysulfone (PSU) [16], polyethersulfone (PES) [16,17], polyphenylsulfone (PPSU) [17] and polyetheretherketone (PEEK) [15] as well as with polymer blends based on PS [18–20]. It could be concluded from the experiments, that inhomogeneities of the cellular structure were prevented by longer saturation times [8]. Higher processing pressures resulted in a reduction of the cell diameter [4,5,8,10] and higher foaming temperatures gave rise to larger cells [4,5,8,10]. However, some investigations also point at the reduction of the cell diameter with increasing temperature [21,22]. In principle, the published studies agree, that the size and the size distribution of the pores generated in the polymer can systematically be influenced by a small variation of the process parameters mentioned. Two methods are reported to generate microcellular polymer foams. The first consists of a saturation process with a supercritical fluid in a pressure chamber, followed by an external heat treatment effecting the foaming of the soaked sample (two-step process). The second directly combines the treatment with a supercritical fluid at the final foaming temperature with the foaming process

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triggered by a fast depressurization step inside the autoclave (onestep process). In this paper, a one-step process for the foaming of semicrystalline thin FEP (fluorinated ethylene propylene copolymer) films by means of scCO2 is presented. One reason for the foaming of FEP with scCO2 as a foaming agent is that fully fluorinated polymers tend to release traces of hydrofluoric acid in the molten state, e.g. during a conventional extrusion foaming process, q.v. [23], which requires special corrosion resistant equipments. The other reason is that from foaming with scCO2 a wider variety of the foam morphology can be expected. Foaming by means of scCO2 has not been investigated for FEP in literature, so far. Many of the studies published were performed directly with polymer pellets. With some exceptions, e.g. [10,14], the foaming of polymer films was not closely examined. One motivation behind foaming thin films of FEP is their promising application as ferroelectric films for electromechanical transducers, e.g. as membranes for audio systems. Due to a charging process of the foamed films in an electric field these nonpolar polymers are able to exhibit piezoelectric properties. In contrast to the so far common ferroelectrets based on polypropylene (PP), FEP combines a higher thermal and temporal charge stability with appropriate dielectric properties, which makes it promising for novel applications with special requirements [24]. Besides charging, the piezoelectric performance of the films is strongly dependent on the morphology of the foams generated. Therefore, it is of practical and scientific interest to investigate the dependence of the foam morphology on processing parameters.

of different pressure drop rates, various restrictors with a length of 4.5 mm and diameters of 1 mm, 2 mm and 4 mm were attached to the gas outlet of the valve. The outlet of the pneumatic valve had a diameter of 6 mm. The density of the foamed films was determined from samples with a diameter of 26 mm using the buoyancy method. The porosity Vf is related to the density of the foam f and the unfoamed polymer 0 according to

2. Materials and methods

The number of nucleation sites in the unfoamed polymer film N0 can be calculated from the volume density of the cells Nf and the volume fraction Vf of the bubbles as (cf. Eq. (1)):

2.1. Materials A commercial semi-crystalline FEP of a viscosity of ca. 2000 Pa s at 330 ◦ C was chosen. The density at room temperature is 2.15 g/cm3 and the melting temperature 255 ◦ C. Thin films of 250 ␮m in thickness were prepared by means of a cast-film extrusion process. For foaming, disk-like samples with a diameter of about 60 mm were cut out from the extruded film. To avoid shrinkage of the films during the foaming process at elevated temperatures, they were annealed for 10 min at 250 ◦ C under a load of 10 bar in a hot press. The crystallinity of the FEP film after this treatment is ca. 28%. This value was calculated from differential scanning calorimetry (DSC) experiments using the heat of fusion of the 100% crystalline polymer of 88 J/g [25]. 2.2. Foaming of FEP films The film samples prepared in the described way were inserted into the autoclave. The whole setup was heated to the final foaming temperatures between 200 ◦ C and 240 ◦ C by means of a heating jacket. After reaching a constant temperature, the autoclave was loaded with compressed CO2 up to the selected foaming pressure, which was varied in the experiments between 10 MPa and 30 MPa. Due to the exceptional properties of scCO2 it is capable of penetrating the polymer film until a saturation with CO2 is achieved. The saturation time of the films started after loading the autoclave with gas. As the liquid CO2 filled into the apparatus was of low temperature, it took about 15 min until an equilibrium processing temperature was reached. For the foaming process, a supersaturation of the polymer film with CO2 is required. This state can be achieved by either reducing the pressure or increasing the temperature. For the current investigation, the method of a pressure release was selected. It was performed at the end of the saturation time varied in the experiments between 10 min and 120 min, by means of a pneumatic valve which allowed depressurizing the autoclave from 30 MPa to ambient condition in about 2 s. For the investigation

Vf = 1 −

f

(1)

0

2.3. Characterization of the foam morphology The cellular structures of the foamed FEP films were visualized by scanning electron microscopy. For that purpose, the samples were cut by means of a razor blade and sputter coated with gold to make them conductive. For the quantitative evaluation of the foam morphologies, a manual method was applied to determine the dimensions, the size distributions and the numbers of the cells. The given diameters of the cells are representative values. They were calculated from the cell areas assuming an ideally round shape of the bubbles. To determine the density of the cells, the number of bubbles n in a selected area A was counted. From these numbers the volume density Nf follows as: Nf =

N0 =

 n 3/2

(2)

A

Nf 1 − Vf

= Nf ·

0 f

(3)

3. Results and discussion 3.1. Determination of the saturation time For a reproducible manufacturing of FEP foams with a homogeneous cellular structure it is essential to determine the saturation time of the films. Usually, the saturation time is evaluated by a gravimetric method, e.g. [4], measuring the mass uptake of gas for polymer samples held for different times at a constant pressure and temperature. However, this method could not be applied for the FEP samples because of the high pressures and temperatures required. For this reason, foaming experiments were performed to determine the saturation of the FEP films at a temperature of 230 ◦ C and a pressure of 30 MPa. A saturation can be assumed if the foam density does not change as a function of loading time. The results for the foaming experiments performed for saturation times varying between 10 min and 120 min are shown in Fig. 1. Within the accuracy of the measurements, no significant reduction of the foam density at times exceeding 30 min can be observed. In contrast, a saturation time of 10 min results in a distinctly higher foam density.1 Fig. 2 gives the cross sections of the two FEP films foamed after 30 min and 90 min, respectively. The visual impression does not reveal considerable differences in the size and homogeneity of the cellular structures, which underlines the finding of a saturation at loading times higher than 30 min. For example, Goel and Beckman [8] found for PMMA for process times shorter than the saturation time significantly fewer cells. This finding was related to a reduced rate of

1 It cannot be decided, however, if the increased density of this foam is due to an incomplete saturation of the film or to the fact that the final foaming temperature of 230 ◦ C might not have been attained again after the loading with liquid CO2 .

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Fig. 1. Densities of FEP films foamed after different saturation times at a temperature of T = 230 ◦ C, a pressure drop of p = 30 MPa, and the maximum applied pressure drop rate (FEP (23 ◦ C) = 2.15 g/cm3 ).

nucleation resulting from the lower amount of CO2 absorbed by the polymer. For these shorter times, they also expected an inhomogeneous cellular structure due to a concentration gradient across the sample thickness. As no such effects can be recognized from the two images of the FEP foams in Fig. 2, it is assumed, that the films are completely saturated at times exceeding 30 min, indicating that the diffusion of scCO2 under these conditions is very fast in FEP. To make sure, that all the foaming experiments on FEP are performed in an entirely saturated state, a saturation time of 90 min was selected. 3.2. Variation of the foaming temperature One of the most effective parameters to control the porosity and the cellular structure of foams is the temperature. It strongly affects the solubility and diffusion of the process gas in the polymer as well as the stiffness of the polymer matrix during foaming. Experiments were performed at different temperatures varying from 200 ◦ C to 240 ◦ C at a constant processing pressure of 30 MPa. The densities and porosities of the foamed FEP films are plotted in Fig. 3 versus the applied foaming temperatures. With increasing processing temperature, a steady decrease of the foam density can be observed. The value at 200 ◦ C corresponds to the density of the unfoamed polymer, which indicates, that no remarkable foaming took place at this low temperature. Rising the processing temperature by 10–210 ◦ C generates a lower density corresponding to a considerable porosity of 43%. However, the foam produced consists of discrete polymer layers representing an inadequate morphology. A suitable homogeneous cellular structure could only be achieved for temperatures exceeding 220 ◦ C. A maximum porosity of 73% was found for the highest temperature of 240 ◦ C. A further reduction of the foam density by a still higher temperature was not possible due to a strong

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Fig. 3. Densities and porosities of FEP films foamed at different temperatures after a saturation time of ts = 90 min at a pressure drop of p = 30 MPa and the maximum applied pressure drop rate.

deformation of the sample. The finding of decreasing foam density with rising temperature confirms the results found for other polymers foamed by means of scCO2 like PET [14] and PS [18]. The SEM-images in Fig. 4 display the cellular structures of FEP films foamed at 220 ◦ C, 225 ◦ C, 230 ◦ C and 235 ◦ C. Comparing the four morphologies, a continuous increase in cell diameter with rising temperature can be recognized. Moreover, the cellular structure at 235 ◦ C exhibits an increased number of cell wall fractures which can be explained by the lower stiffness of the polymer matrix at the higher temperature and, thus, the reduced resistance of the bubble walls to deformation. The anisotropy of the cells at 235 ◦ C can be related to the geometry of the sample as the resistance against bubble growth in the radial direction of the film is higher than perpendicular to it. This results in an elongation of the cells perpendicular to the plane of the film. The quantitative analysis of the foam morphology is plotted in Fig. 5 for the cellular structures at temperatures exceeding 220 ◦ C. The error bars for the mean cell diameters do not describe the accuracy of the measurement but represent the broadness of the distribution of the cell sizes. The increase of cell sizes observed from the SEM-images is found for the whole temperature range, starting with a diameter of 14 ␮m at 220 ◦ C to 33 ␮m at 240 ◦ C. Simultaneously, the cell size distribution becomes broader which can be related to the increasing cell coalescence with decreasing matrix stiffness. Concurrently with the growth of the cell size, the number of cells significantly decreases with rising temperature. The behaviour observed can be related to the solubility and the diffusion of the processing gas in the polymer, which both strongly depend on the temperature. For the solubility S S = S0 exp

 −H  S

RT

(4)

Fig. 2. Scanning electron micrographs of the cross sections of FEP films foamed at T = 230 ◦ C, a pressure drop of p = 30 MPa, and the maximum applied pressure drop rate after a saturation time of (a) 30 min and (b) 90 min.

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Fig. 4. Scanning electron micrographs of the cross sections of FEP films foamed after a saturation time of ts = 90 min at a pressure drop of p = 30 MPa and the maximum applied pressure drop rate at a temperature of (a) 220 ◦ C, (b) 225 ◦ C, (c) 230 ◦ C, and (d) 235 ◦ C.

is valid, where S0 is the solubility extrapolated to an infinite temperature and HS is the enthalpy of solubility. For the diffusion coefficient D the relationship D = D0 exp

 −E  D

RT

(5)

holds, where D0 is the diffusion coefficient extrapolated to an infinite temperature and ED is the activation energy of diffusion. As known from other polymers, the solubility of CO2 normally decreases with rising temperature as a result of negative values for the enthalpy of solution of the gas in the polymer (cf. Eq. (4)). Therefore, less CO2 is absorbed in the polymer at higher temperatures which provides fewer nucleation sites. A smaller number of nuclei are generated which due to the enhanced diffusion are able to grow in a comparatively larger material domain and finally result in bigger bubbles. This tendency is enforced by the advanced diffusion of the processing gas inside the polymer with rising temperature contributing to the growth of bubbles already nucleated. Both effects result in the increased growth of a smaller number of bubbles during foaming at higher temperatures. These observations correspond to the results found for other polymers like PMMA [8], PS [4,5], and PU [10] which are amorphous polymers. In contrast, Baldwin et

al. [11] stated for a semi-crystalline PET a constant cell density for the temperatures investigated. Therefore, regarding the temperature dependence for the semi-crystalline FEP an amorphous-like foaming behaviour can be noticed. At a first glance, the lower solubility and the enhanced diffusion of CO2 cannot explain the reduction of the foam density observed with rising temperature (cf. Fig. 3). The lower solubility reduces the amount of gas for the foaming process and the improved diffusion could give rise to an increased loss of gas from the polymer film to the environment not available for foaming anymore. Another factor for foaming, however, is the stiffness of the FEP matrix as it determines the resistance of the polymer against bubble growth. With rising temperature, the stiffness of the polymer decreases, enabling an easier deformation of the matrix and, therefore, foaming even at lower gas pressures inside the film. Due to the increase of the foam porosity observed with rising temperature, this stiffness reduction has a big effect on the porosity increase of the FEP investigated. 3.3. Variation of the process pressure Besides the temperature, the height of the pressure drop is an important parameter to influence the foaming process. To get an insight into this dependency, experiments were performed at different process pressures between 9.6 MPa and 29.6 MPa at a constant temperature of 230 ◦ C. The results of the foam densities and porosities are plotted in Fig. 6. A considerable foaming of the FEP investigated could only be achieved for pressure drops higher than 15 MPa with an almost linear decrease of the foam density with increasing pressure. For pressures below 15 MPa, no homogeneous cellular foam could be obtained. The maximum porosity achieved for this FEP was 59% at 30 MPa. The linearity of the density reduction can be explained by means of Henry’s law, giving a direct proportionality between the hydrostatic pressure p and saturation concentration c: c = Sp =

Fig. 5. Cell diameters and cell densities of FEP films foamed at different temperatures after a saturation time of ts = 90 min at a pressure drop of p = 30 MPa and the maximum applied pressure drop rate.

mgas mpolymer

(6)

where S is the solubility coefficient. Therefore, higher pressures generate increased saturation concentrations of the processing gas

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Fig. 6. Densities and porosities of FEP films foamed at different pressure drops after a saturation time of ts = 90 min at a temperature of T = 230 ◦ C and the maximum applied pressure drop rate.

Fig. 8. Cell diameters and cell densities of FEP films foamed at different pressure drops after a saturation time of ts = 90 min at a temperature of T = 230 ◦ C and the maximum applied pressure drop rate.

in the polymer. The higher amount of CO2 absorbed in the polymer films increases the degree of supersaturation in the film after the pressure drop and, therefore, results in lower densities of the foams generated (cf. Fig. 6). Moreover, the free enthalpy G* necessary for the nucleation of cells is reduced by an increased pressure drop p according to

the cell size distribution with growing temperature (cf. Fig. 5), they are nearly constant for the different applied pressures, indicating that no significant change in the cellular structure of the foams takes place. However, the cell density in the FEP films notably increases to the threefold by rising the pressure drop from 15.7 MPa to 29.6 MPa (Fig. 8). But this growth is not uniform. After a distinct augmentation of cells between 15 MPa and 20 MPa, there is a plateau region up to 25.2 MPa. A further increase of the processing pressure leads to an increase of the cell numbers which is more pronounced than that at lower pressures. The observation of constant cell sizes is in contrast with the findings for other polymers like PMMA [8], PS [5,18], and PU [10] which show cell diameters significantly decreasing with pressure. The discrepancies between the results observed for these polymers and those found for the FEP investigated might be related to their differences in molecular structure. PMMA and PS are amorphous polymers whereas FEP is foamed in a partially crystalline state even at the highest temperature of 240 ◦ C. That these differences may influence the foaming behaviour becomes evident from literature. Baldwin et al. [12] foamed PET in its amorphous and semi-crystalline state. They verified a decreasing cell size with

∗ Ghom =

3 16bp

3p2

(7)

with  bp being the interfacial energy. For a qualitative insight into the relation between the free enthalpy and the pressure drop level the interfacial energy is considered to be constant. A reduction of the nucleation energy required results in an increase of the nucleation rate. For the FEP investigated this effect is illustrated by the cross sections of the foamed films in Fig. 7 which show that the number of cells significantly rises with the pressure drop. This observation is in agreement with other polymers [5,8,10,18]. However, the geometry and the size of the cells seem to remain the same. The analysis of the foam morphology shown in Fig. 8 confirms that the cell size obtained does not depend on the processing pressure applied. In contrast to the increasing width of

Fig. 7. Scanning electron micrographs of the cross sections of FEP films foamed after a saturation time of ts = 90 min at a temperature of T = 230 ◦ C, the maximum applied pressure drop rate, and a pressure drop of (a) 15.7 MPa, (b) 20.2 MPa, (c) 25.2 MPa, and (d) 29.6 MPa.

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increasing saturation pressure for the amorphous PET, whereas the dimensions for the semi-crystalline material did not change in their experiments. The results were explained by the assumption that the viscoelastic behaviour of the semi-crystalline polymer matrix and, particularly, the relaxation modulus which was found to be a strong function of the crystallinity determines the bubble growth mechanism. Concerning the cell densities, a continuous increase with rising pressure drop was reported for different amorphous polymers [8,12,18], whereas a constant value was stated for semi-crystalline PET [12]. The different behaviour of amorphous and semicrystalline materials was related to their diverse nucleation mechanisms taking place during the foaming process. In contrast to the pure homogeneous nucleation of cells in amorphous polymers in absence of nucleation agents, Baldwin et al. [11] assumed that the crystallites inside the semi-crystalline PET could act as heterogeneous nucleation sites. While the homogeneous nucleation strongly depends on the height of the pressure drop performed (cf. Eq. (7)), the heterogeneous one is determined by the number of nucleation sites existent in the material and, therefore, is relatively weakly dependent on the processing pressure as the crystallinity is not much influenced by pressures of the magnitude applied. Since the experiments presented for FEP were performed at a temperature lower than the melting point, the polymer is in a partially molten state. Therefore, the non-molten fraction of crystallites inside the FEP film can act as sites for a heterogeneous nucleation. As these experiments were performed at the same foaming temperature the number of the non-molten crystallites (the heterogeneous nucleation sites) remains constant. Thus, the plateau in Fig. 8 can be explained by a complete activation of all these heterogeneous nucleation sites. The distinct augmentation of the cell density at higher pressures points to a homogeneous nucleation process which strongly depends on the pressure level (cf. Eq. (7)). This finding means that besides the activation of the existing heterogeneous sites, the additional homogeneous nucleation contributes to a further increase of the number of nuclei within the FEP film. The initial increase of the cell density is based on just one single point at the pressure of 15.7 MPa. However, the very low number of cells is evidenced by the cellular structure of the foam shown in Fig. 7a. As an explanation for the low value an incomplete initiation of the heterogeneous nucleation sites at the low processing pressure might be assumed. The constant cell sizes at rising pressure indicate that the geometry of the cells generated only depends on the viscoelastic properties of the polymer matrix strongly influenced by the foaming temperature. The cell density, however, is determined by the height of the applied pressure drop.

Fig. 9. Densities and porosities of FEP films foamed at different valve diameters, i.e. pressure drop rates after a saturation time of ts = 90 min at a temperature of T = 230 ◦ C and a pressure drop of p = 30 MPa.

Another important factor for the foaming behaviour of polymers is the rate of pressure drop during foaming [26]. To realize different pressure drop rates, restrictors with diameters of 1 mm, 2 mm and 4 mm were attached to the pneumatic valve. The original diameter of the pneumatic valve was 6 mm. The measurements were performed at a foaming temperature of 230 ◦ C and a constant processing pressure of 30 MPa. The results obtained for the FEP foam densities are plotted in Fig. 9 versus the valve diameters used. An increase in valve diameter corresponds to a rise in pressure drop rate. From Fig. 9 distinct reductions of the foam density with increasing valve diameter, i.e. with growing pressure drop rate, can be recognized.2 For the 4 mm restrictor and the pneumatic valve

with a diameter of 6 mm there is hardly a difference of the values. This might be explained by the assumption that under the experimental conditions given pressure drop rates are reached which are high enough to give the maximum porosity attainable. The foam morphologies for the different pressure drop rates are displayed in Fig. 10. Increasing the rate of pressure drop during foaming results in decreasing cell diameters simultaneously enhancing the numbers of cells. The quantitative analysis of the foam morphology is given in Fig. 11 showing an increase of the cell density with growing pressure drop rate. The mean cell diameter approaches a plateau value for valve diameters larger than 2 mm. Moreover, the constant cell size distribution for these three values indicates, that no differences between the cellular structures of these foams exist, i.e. no structural changes like cell coalescence occurred. The observed foaming behaviour of FEP correlates with studies on PS [4] and PET [13] using the procedure with supercritical CO2 . A detailed investigation on the influence of the pressure drop rate on the cell nucleation was performed by Park et al. [26] for an extrusion foaming of a high impact polystyrene (HIPS). They referred the findings to a competition between the cell nucleation and the diffusion related growth of existing cells inside the polymer. Because of the time dependence of the diffusion, lower pressure drop rates and, therefore, longer foaming times enable the absorbed gas to cover longer distances in the polymer matrix. Thus, due to energetic reasons, the gas tends to diffuse into already existing bubbles instead of nucleating new ones. Moreover, gas that has contributed to the cell growth cannot be used for further nucleation. If the gas concentration in the material surrounding the cells is lower than required for nucleation, no additional cells will be generated. In contrast, at high pressure drop rates, a stronger thermodynamic instability due to the increased supersaturation of the polymer film with gas is generated, resulting in a higher number of cells nucleated. Therefore, the pressure drop rate not only influences the diffusion behaviour of the gas but also the nucleation time and, thus, the nucleation rate. In case of the FEP at the lowest pressure drop rate the absorbed gas has mainly been used for cell growth instead of nucleation, resulting in the larger size of a lower amount of cells. Another effect could be that at a longer time period for diffusion more gas molecules are able to leave the film without contributing to the foaming process, which explains the higher densities at lower pressure drop rates observed in Fig. 9.

2 The given valve diameter, however, does not represent any well-defined physical number as, for example, the depressurization time would be. Because of the high

rates of the pressure drops for the valves with diameters of 4 mm and 6 mm, an exact time could not be determined.

3.4. Variation of the pressure drop rate

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Fig. 10. Scanning electron micrographs of the cross sections of FEP films foamed after a saturation time of ts = 90 min at a temperature of T = 230 ◦ C and a pressure drop of p = 30 MPa with a valve diameter of (a) 1 mm, (b) 2 mm, (c) 4 mm, and (d) 6 mm.

Fig. 11. Cell diameters and cell densities of FEP films foamed at different pressure drop rates after a saturation time of ts = 90 min at a temperature of T = 230 ◦ C and a pressure drop of p = 30 MPa.

4. Conclusions The intention of these investigations was to get an insight into the effect of different processing parameters on the foaming of thin FEP films by means of scCO2 . By the experiments performed, porosities of up to 73% and mean cell diameters from 14 ␮m to 39 ␮m could be obtained for FEP films of 250 ␮m initial thickness. From the results interesting conclusions with respect to the mechanisms taking place during the foaming of FEP with scCO2 can be drawn. The fact, that the saturation of the FEP films with CO2 seems to be terminated within a time shorter than the observation period of 30–120 min indicates that the diffusion of the densified CO2 into the FEP film under the experimental conditions was very fast. This can be concluded from the results that no differences in the foam morphologies for the various saturation times chosen could be detected. A distinct effect of the processing temperature on the foamability of FEP could be found. A significant density reduction of the foams was achieved by increasing the temperature. This result gives a hint to the dominating role of the stiffness of the polymer showing a decrease with rising temperature. The acceleration of the diffu-

sion as well as the decrease of the solubility of CO2 in the FEP with rising foaming temperature result in an increase of the cell diameter simultaneously reducing their number per volume unit. The magnitude of the pressure drop is a suitable variable to control the foam and cell density whereas it does not affect the size of the bubbles generated. The foam density decreasing with increasing pressure drop can be referred to both, the higher solubility of CO2 in the FEP film at higher processing pressures described by Henry’s law and the enhanced nucleation due to a reduction of the free enthalpy necessary. The nucleation could be separated into two regimes: the pressure-independent heterogeneous nucleation due to non-molten crystallites in the polymer and an additional homogeneous one initiated at higher pressures. The constant size of the bubbles generated for various pressures at a fixed temperature again underlines the role of the stiffness of the matrix which does not change much with pressure. The foaming behaviour of FEP for different pressure drop rates can be related to the competition between nucleation and bubble growth at a given amount of CO2 absorbed in the polymer. A faster pressure drop increases the thermodynamic instability due to a supersaturation of the polymer film with gas, resulting in an intensified nucleation rate combined with a reduced possibility for diffusion. The experiments demonstrate that the process parameters during the foaming by means of scCO2 significantly influence the foaming behaviour of the FEP investigated. Using these results the foam morphologies can be optimized within certain limits. The density can be adjusted by all the parameters investigated. To tune the cell size, the foaming temperature and the pressure drop rate are well suited and the cell density can be enhanced by reducing the temperature and raising the processing pressure as well as increasing the pressure drop rate. Although many similarities between the foaming behaviour of the FEP investigated and other polymers could be stated also some differences were found if compared to literature. In contrast to the results published for PET, the FEP investigated partially behaved like amorphous polymers, although it is semi-crystalline. Thus, the cell density decreasing with temperature for FEP was found to correlate with that of PMMA, PS and PU and disagreed with

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the temperature-independent value stated for the semi-crystalline PET. Regarding the pressure drop dependence the nucleation could be divided into a heterogeneous regime, reported for PET and a homogeneous one common for amorphous polymers. This foaming behaviour observed might be referred to the partially molten state of the FEP processed at temperatures slightly below its melting point. However, the melting temperature of PET is comparable and the experiments published in literature were performed in the same temperature range. Therefore, no general dependence of the foaming behaviour on the process parameters temperature and magnitude of pressure drop seem to be valid for semi-crystalline polymers, but there must exist an additional influence of the inherent properties of the material due to its specific chemical structure. However, the effect of the pressure drop rate seems to be similar for the two different polymers. This can be explained by the fact that in case of this parameter thermodynamic processes dominate the foaming behaviour, but not material properties. Acknowledgements The authors are indebted to Dr. Detlef Freitag from the High Pressure Laboratory of the University Erlangen-Nürnberg for his technical support of the experiments and to their project partners at the Institute of Applied Condensed-Matter Physics of the University Potsdam, Prof. Reimund Gerhard, Dr. Michael Wegener and Werner Wirges for stimulating ideas and discussions. Part of this work was sponsored by the German Research Foundation (DFG). The supply of the polymer material by the Dyneon GmbH & Co.KG, Burgkirchen, is gratefully acknowledged. References [1] G. Brunner, Gas Extraction: An Introduction to Fundamentals of Supercritical Fluids and the Application to Separation Processes, Springer, Darmstadt, 1994. [2] J.L. Fulton, G.S. Deverman, C.R. Yonker, J.W. Grate, J. De Young, J.B. McClain, Thin fluoropolymer films and nanoparticle coatings from the rapid expansion of supercritical carbon dioxide solutions with electrostatic collection, Polymer 44 (2003) 3627–3632. [3] D.W. Matson, J.L. Fulton, R.C. Petersen, R.D. Smith, Rapid expansion of supercritical fluid solutions: solute formation of powders, thin films, and fibers, Industrial & Engineering Chemical Research 26 (1987) 2298–2306. [4] K.A. Arora, A.J. Lesser, T.J. McCarthy, Preparation and characterization of microcellular polystyrene foams processed in supercritical carbon dioxide, Macromolecules 31 (1998) 4614–4620. [5] E. Reverchon, S. Cardea, Production of controlled polymeric foams by supercritical CO2 , Journal of Supercritical Fluids 40 (2006) 144–152. [6] V. Kumar, N.P. Suh, A process for making microcellular thermoplastic parts, Polymer Engineering and Science 30 (1990) 1323–1329. [7] N.S. Ramesh, D.H. Rasmussen, G.A. Campbell, Numerical and experimental studies of bubble growth during the microcellular foaming process, Polymer Engineering and Science 31 (1991) 1657–1664.

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