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Influence of different process and material parameters on chemical foaming of fluorinated ethylene propylene copolymers.

INTRODUCTION

Though physical foaming of fluorinated ethylene propylene copolymers, e.g., for coaxial cables, is state-of-the-art in many manufacturing processes today, rather no fundamental knowledge is available about its foaming by means of a chemical blowing agent. Thus, the focus of this article will be on the effect of different process and material parameters on the chemical foaming of fluorinated ethylene propylene copolymers (FEP) during extrusion. A major advantage of the chemical foaming process is that big technical changes with respect to the extrusion of compact polymers are not necessary. In contrast to the foaming by means of physical blowing agents, no extensive technical arrangements, as e.g., for the supply of gas into the melt, are needed.

In this article, the focus is on FEP as they exhibit some properties which make them appropriate for very special applications. Besides their ability to sustain high thermal loading conditions, they possess an excellent chemical resistance, high continuous use temperatures, outstanding ageing, and weathering stability along with appropriate mechanical properties. Very important for the application as polymer foam is their high flame resistance combined with a very low smoke emission and, therefore, the compounding of an additional flame retardant is not needed. However, fluoropolymers are quite expensive and the processing is more complex compared with other polymers used for foaming, such as polypropylene (PP) or polystyrene, not only because of the higher manufacturing temperatures but also due to the tendency to release hydrofluoric acid. The difficulties in processing might be one reason why not many investigations on fluoropolymer melts have been published.

Several studies exist, which describe the influence of process and material parameters on the foaming behavior and the resulting cellular structure of different commodity polymers, as e.g., for PP [1-4]. On the basis of this knowledge, it is interesting from a scientific point of view in which way the experiences available can be transferred to the foaming performance of FEP. Therefore, the objective of this study is to collect fundamental knowledge about the correlations between different process and material parameters and the chemical foaming behavior of FEP.

Various factors influence the porosity and the cellular structure resulting from the foaming process. Temperature and throughput rate during extrusion as well as material parameters such as viscosity strongly affect the foaming behavior. As the polymer melt undergoes pronounced elongational deformations during foaming, porosity and cell size highly depend on the melt viscosity. As this material function is strongly temperature-dependent, the temperature has an essential influence on the viscosity of the melt during the foam formation. On the one hand, the processing temperature should be high enough to enable bubble growth in the polymer melt, on the other hand, high foaming temperatures reduce the viscosity of the polymer and decrease the melt strength, which may result in a collapse of the cellular structure because of a rupture of the cell walls resulting in cell coalescence during foaming. Under such conditions, foams with an inhomogeneous cellular structure and a very low porosity may result, which make them unsuitable for many applications. Polymers with a low molar mass can be foamed even at lower extrusion temperatures, whereas more viscous polymers need higher processing temperatures to enable the growth of bubbles. The nucleation of cells is strongly influenced by the rate and the height of the pressure drop at the end of the die. It could be proven in several investigations that a higher processing pressure during foam extrusion leads to a reduction of cell diameter and an augmentation of the cell numbers per volume. It was found, that not only the magnitude but also the rate of the pressure drop was decisive (e.g. in Refs. 3, 5, and 6).

To achieve a maximum density reduction of the foam and a fine homogeneous cellular structure, the processing conditions as well as the material parameters of the polymer have to be optimized.

MATERIALS AND METHODS

Materials

To obtain a fundamental knowledge about the correlation between foaming behavior and material parameters for FEP, three commercial FEP, named FEP 1, FEP 2, and FEP 3, with different viscosities were investigated. The influence of changing extrusion rate or temperature during the foam extrusion was mainly studied for FEP 2, which is of an intermediate viscosity. The density at room temperature is 2.15 g/[cm.sup.3] for all three FEP, and the melting temperature is 260[degrees]C.

For the chemical foam extrusion process, the blowing agent as a powder or masterbatch is mixed with the polymer prior to the extrusion. Because of the high processing temperature, the chemical blowing agent decomposes inside the extruder releasing gaseous products. The generated gas is solved into the polymer melt as a result of the high processing pressures and causes the foaming of the extrudate at the exit of the tool. Therefore, the processing temperature in the extruder and the selected chemical blowing agent have to be matched. If the processing temperature is too low, an incomplete decomposition of the chemical blowing agent results, leading to fluctuations in foam quality. A decomposition temperature of the blowing agent, which is significantly lower than the processing temperature may cause a leakage of gas through the funnel tube of the extruder or the formation of big cavities in the extruded foam. Consequently, a chemical blowing agent adapted to the high processing temperatures of FEP around 350[degrees]C had to be found. The selected magnesium hydroxide is generally used as a flame retardant for polymers as it is known to release water at temperatures in the range of 300[degrees]C. Because of the melting temperature of FEP of 260[degrees]C, it was chosen as chemical blowing agent for the foam extrusion process. The average particle size of the powder after the mixing step is smaller than 1 [micro]m.

Rheological Measurements in Shear

Rheological experiments in oscillatory shear to characterize the three different FEP were conducted, using the constant strain rheometer "ARES" (Advanced Rheometric Expansion System) manufactured by Rheometric Scientific. The measurements were performed by means of a plate-plate geometry of 25 mm in diameter and a gap of 1.9 mm at a temperature of 330[degrees]C under nitrogen atmosphere. Cylindrical samples with a diameter of 25 mm and a thickness of 2 mm were prepared from granules by compression moulding in an evacuated hot press at a temperature of 300[degrees]C. Dynamic-mechanical experiments were performed over an angular frequency range of 0.01-100 [s.sup.-1] with a deformation amplitude of 1%.

Thermogravimetric Analysis

Analyses to determine the decomposition behavior of the selected chemical blowing agent magnesium hydroxide were carried out, using a 2950 Thermogravimetric Analyzer manufactured by TA Instruments. The loss of weight of a small amount of magnesium hydroxide was detected during a temperature ramp from room temperature to 350[degrees]C with a heating rate of 10 K/min in air and nitrogen atmosphere.

Foaming

For the analysis of the foaming behavior of FEP, experiments at different conditions were performed at a laboratory scale. For that purpose, an apparatus based on a modified capillary rheometer had been constructed. The principle of the foaming unit is sketched in Fig. 1. A detailed description of the apparatus is given by Stange and Munstedt [2]. Because of the tendency of fully fluorinated thermoplastic melts, like those of FEP, to release small amounts of hydrofluoric acid during processing, the apparatus had to be equipped with special corrosion resistant components. The die applied for all foaming experiments performed had a diameter of 1 mm and a length of 15 mm. The essential difference between the laboratory foaming apparatus and a foaming extruder consists in the absence of an attached mixing unit. Consequently, the FEP has to be compounded with the chemical blowing agent by an additional step in an internal mixer. To avoid a decomposition of the magnesium hydroxide during the compounding, the mixing is done at the lowest possible temperature of 270[degrees]C. For all the foaming experiments a concentration of 10 wt% of magnesium hydroxide was chosen. The prepared mixture of polymer and blowing agent was fed into the cylinder of the foaming apparatus and heated to a temperature of 350[degrees]C. During the polymer melting and the decomposition process of the chemical foaming agent, a shut-off needle avoids the escape of gas from the cylinder (cf. Fig. 1). For dissolving the released gas into the molten polymer, a pressure in the range of 150 bar inside the cylinder was applied by driving down the piston. After 5 min at the feeding temperature, in case of a lower extrusion temperature, the apparatus was cooled down at an approximate cooling rate of 2 K/min. Over the period of temperature change, the pressure inside the cylinder was kept constant by adjusting the piston to keep the gas dissolved in the melt. At a constant foaming temperature, which was varied in the experiments between 290 and 370[degrees]C, the sealing was removed from the capillary die and the foam was extruded with a constant throughput rate by operating the piston with a defined velocity. For foaming temperatures higher than 350[degrees]C, the feeding of the material as well as the melting was carried out at the final extrusion temperature to avoid material degradation because of the otherwise additionally necessary heating to high temperatures.

[FIGURE 1 OMITTED]

The density of the produced FEP foams was measured using the buoyancy method. The porosities achieved, which correspond to the gas volumes in the foams [V.sub.f] were calculated from the densities of the foam [[rho].sub.f] and the unfoamed polymer [[rho].sub.0] according to

[V.sub.f] = 1 - [[rho].sub.f]/[[rho].sub.0]. (1)

RESULTS AND DISCUSSION

Analysis of the Decomposition Behavior of the Blowing Agent

For the chemical foam extrusion, a blowing agent adapted to the properties of the polymer has to be selected. Depending on the chemical structure and the content of the agent used, different kinds ([H.sub.2]O, C[O.sub.2], etc.) and amounts of gases are released. One blowing agent appropriate to the high processing temperature of FEP is magnesium hydroxide releasing [H.sub.2]O. For the processing of the extrusion foaming, information about the decomposition behavior of the chemical blowing agent is essential. For this purpose, a thermo-gravimetric analysis was performed. The results for the heating of magnesium hydroxide in air atmosphere are shown in Fig. 2. The decomposition of magnesium hydroxide begins at a temperature in the range of 280[degrees]C and accelerates at temperatures exceeding 300[degrees]C. Another increase of decomposition velocity can be observed at about 340[degrees]C. A similar measurement on magnesium hydroxide was performed using nitrogen as atmospheric gas. Comparing the decomposition behavior in air and in nitrogen atmosphere, no significant differences could be found, which indicates that the observed loss of weight can actually be referred to an intrinsic decomposition of the magnesium hydroxide and not to an oxygen-controlled process. According to the found decomposition behavior of the chemical blowing agent, a feeding temperature of 350[degrees]C was determined. Foaming experiments at different feeding temperatures and times indicated a complete decomposition of the magnesium hydroxide after 5 min at 350[degrees]C. Therefore, these feeding conditions were selected for all foaming experiments with an extrusion temperature of 350[degrees]C or lower.

[FIGURE 2 OMITTED]

Variation of Extrusion Rate

[FIGURE 3 OMITTED]

To determine the influence of the rate of pressure drop during extrusion on the foaming behavior of FEP, the throughput rate through the die is varied at a constant foaming temperature of 350[degrees]C. The apparent shear rates D in the die (with a radius R) can be calculated from the throughput rates Q according to

D = 4Q/[[pi][R.sup.3]] (2)

with

Q = [pi][r.sup.2][v.sub.p], (3)

[v.sub.p] is the velocity of the piston moving in the barrel with the radius r.

[FIGURE 4 OMITTED]

In Fig. 3 the density and the resulting porosity of the foamed FEP 2 strands are plotted as a function of the apparent shear rates. A nearly linear increase of the porosity with shear rate is found. Figure 4 shows the respective microscopic images of the three FEP strands fractured in liquid nitrogen. The increase in porosity becomes apparent by the significant growth of cell numbers. In addition, the diameter of the bubbles decreases considerably with increasing extrusion rate. Such a kind of correlation between the rate of pressure drop and the foaming behavior has also been reported for other polymers, e.g. for PP [3].

The growing porosity with increasing throughput rate can be referred to several factors. When the molten material exits the die and the pressure exerted on the melt is reduced, the solubility of the dissolved gas is lessened and a supersaturation occurs. Because of the thermodynamic instability, cell nucleation takes place and a certain amount of bubbles is formed. At a high throughput rate, the dissolved gas is abruptly released into the melt. As in this case the time for the diffusion of the gas is very short, because of the cooling and solidification of the foam, a high amount of small cells nucleates. However, a slow pressure drop during the foaming process first initiates the formation of a small amount of bubbles. From a certain cell diameter on, the growth of the bubbles by diffusion is more probable from an energetic point of view than the formation of new ones. Moreover, it could be observed from the experiments that there is still some influence on the foamed strand by the heat irradiated by the die. The longer times for diffusion, referred to the slower temperature drop caused by the lower extrusion velocity, may give rise to cell coalescence and, therefore, an increased escape of gas because of an enhanced diffusion of the blowing agent through the ruptured cells of the foam. Consequently, a foamed strand with significantly less and bigger bubbles results as can be seen in the two first images of Fig. 4. At further increased extrusion velocities, a pronounced melt fracture of the produced strand was observed. The cellular structure of the foams collapsed and the skin was partially ruptured, which was followed by a very high loss of gas. Hence, the maximum applicable apparent shear rate of 2300 [s.sup.-1] was selected for further investigations on the influence of foaming temperature and melt viscosity on the foaming behaviour of FEP.

Variation of Foaming Temperature

Besides the rate of pressure drop, the foaming temperature is a decisive factor for the foaming behavior. It essentially influences not only the viscosity of the polymer melt during formation of the cellular structure but also the stabilization of the foam. Investigations on the foaming behavior of FEP 2 were performed at temperatures between 290 and 370[degrees]C and an apparent shear rate of 2300 [s.sup.-1]. The resulting densities and porosities obtained for the foamed strands are plotted in Fig. 5. For a temperature change from 290 to 300[degrees]C a pronounced increase of porosity results, that level off at higher temperatures. For temperatures exceeding 320[degrees]C a plateau is obtained, where there is no significant change of porosity with temperature. For temperatures between 320 and 370[degrees]C the resulting porosities vary from 54 to 56% on their average. Following the increase of porosity with temperature there seems to be a slight decrease for 370[degrees]C. The highest values can be recognized between 340 and 360[degrees]C. Considering the accuracy of the results, however, the differences in porosity for the temperatures from 320 to 370[degrees]C are not significant. The increase of foam porosity with temperature can be referred to the reduction of melt viscosity. At lower temperatures, higher melt viscosities implicate increased melt strengths. The cell walls can withstand larger stresses before rupture occurs, which prevents cell coalescence and, therefore, loss of gas to the environment. On the other hand, the viscosity of the melt has to be low enough to enable growth and formation of bubbles. For a small bubble radius, the surface tension is dominant and shrinkage of the bubbles is energetically more favorable than their growth. During cell formation, a critical radius has to be exceeded to allow a stable bubble growth as the free energy is reduced with increasing radius. For a high melt viscosity, the nucleation rate is lower and the energy for bubble formation is increased. As a result, only few cells develop and a considerable part of the gas is lost to the environment without contributing to the foaming of the FEP melt because of its higher melt strength, which increases the resistance against bubble growth. In contrast, very high temperatures induce low melt viscosities and, therefore, a reduction in melt strength. Hence, collapse of the cellular structure with cell coalescence results. An optimum in porosity can be observed for temperatures about 350[degrees]C, where the melt strength is low enough to allow bubble formation and growth but also high enough to withstand cell extension without rupture of cell walls.

[FIGURE 5 OMITTED]

An impression of the strong influence of the foaming temperature on the resulting cellular structure is given in Fig. 6, which shows the corresponding microscopic images of the cross sections of four FEP foams for different foaming temperatures. Comparing these pictures, it can be recognized that the cellular structure obtained strongly differs by changing the extrusion temperature of the foaming process. The strand foamed at the lowest temperature of 290[degrees]C exhibits a ruptured foam skin, which probably caused escape of gas during extrusion and correlates with the lower resulting porosity in Fig. 5. The cellular structure is very inhomogeneous. Large sectors of unfoamed material as well as few and partly very big bubbles can be observed. Because of the high melt viscosity, only few bubbles are able to grow and small bubbles tend to shrink. For most applications such a cellular structure will not be appropriate. An increase of the extrusion temperature to 320[degrees]C yields a reduction of unfoamed sectors. Thereby, a more homogeneous cellular structure and an increased number of cells can be observed. For a temperature of 350[degrees]C a very fine bubble structure uniformly distributed over the whole cross section of the strand developed. The foam extruded at 370[degrees]C shows a homogeneous cellular structure with a bigger bubble size than that for 350[degrees]C. Besides the influence on melt viscosity, the foaming temperature is decisive for the stabilization of the foam structure. Higher processing temperatures induce longer times for the fixation of the foam and, therefore, promote bubble growth which may also lead to a rupture of cell walls and cell coalescence. For the characterized FEP 2 of medium viscosity, there seems to be an optimum concerning porosity and foam structure resulting from melt viscosity and foam stabilization at temperatures in the range of 350[degrees]C. Most of the foams produced exhibit larger cells in the middle of the strand. Because of the temperature distribution in the foam after exiting the die, there is a longer period of time for diffusion and bubble growth in the center than in the outer skirts.

[FIGURE 6 OMITTED]

The effect of temperature variation on the foaming behavior of FEP is in contrast to the correlations found for other polymers, like e.g. PP [7]. For PP a rise in porosity by lowering the processing temperature could be observed because of the increase of melt viscosity and the reduction of the diffusion coefficient as well as the shortening of the time until solidification.

Influence of Adding a Nucleation Agent

It could be shown that the resulting cellular structure is strongly influenced by the rate of pressure drop and the processing temperature during the extrusion. Besides the regulation of these two factors, nucleating agents can be applied to achieve a very fine-cellular structure by the facilitation of bubble formation. In contrast to a self-nucleation, small solid particles are compounded into the melt to decrease the necessary formation energy. The achieved energy reduction depends on the wetting behavior, the amount, and the geometry of the particle. A high amount of homogeneously distributed small particles of low wettability are beneficial for nucleation [8, 9]. Most successful nucleation agents also release a small amount of gas. Consequently, no extra energy for cell nucleation will be required for foaming as the gas will diffuse from the melt into the voids given by the particles.

In the case of a chemical foaming process, the solid rests of the blowing agent, which remain after decomposition, can act as implements for nucleating cell growth, themselves. However, the influence of an additional inorganic particle on the foaming behavior was investigated by adding calcium fluoride particles in amounts of 5 and 10 wt% to the FEP 2 melt containing the chemical blowing agent. Foam extrusion was performed at a temperature of 350[degrees]C with an apparent shear rate of 2300 [s.sup.-1]. Comparing the first two microscopic images in Fig. 7, no significant difference between the cellular structures of the strands obtained without and with 5 wt% of calcium fluoride can be recognized. However, a further improvement of the foam structure can be observed by compounding an amount of 10 wt% of calcium fluoride into the FEP melt. There is an obvious increase in cell quantity and homogeneity accompanied by a simultaneous reduction of the cell diameter. The nucleating effect of the calcium fluoride is demonstrated in Fig. 7d showing a section of Fig. 7c in a higher magnification. The calcium fluoride particles, which were responsible for cell nucleation, can distinctly be recognized at the bottom of the bubbles.

The nucleation of cells around particles in a FEP melt is relieved compared with many other polymers because of its wetting behaviour, which is known to be very low. The high amount of calcium fluoride necessary for obtaining an improvement in cellular structure might probably be referred to the nucleation behavior of the chemical foaming agent itself. However, a significant effect of the bigger calcium fluoride particles can be found for a concentration of 10 wt%, which indicates an improved nucleating effect for these particles compared with the magnesium hydroxide. This might be related on one hand to the bigger size of the weakly wetted calcium fluoride particles, which create larger voids than the very fine magnesium hydroxide. On the other hand it could be proven in experiments for which solely calcium fluoride was compounded into the melt and afterwards extruded that this filler itself releases small amounts of gas around its particles, reacting with traces of hydrofluoric acid present in the FEP melt. Combined with the larger size of the particles, quite big voids form, which enable a stable growth by diffusion of the blowing gas without the need of additional energy for bubble formation.

[FIGURE 7 OMITTED]

Variation of the Melt Viscosity

As the polymer melt undergoes a pronounced biaxial elongational deformation during foaming, the resulting porosity and cell size strongly depend on the elongational viscosity of the melt. As the linear FEP used are not strain hardening, their zero-shear viscosities can be regarded as a qualitative measure to assess the flow behaviour. Three FEP with different viscosities were investigated to determine the effect of this parameter on their foaming behavior. The results of the dynamic-mechanical experiments in oscillatory shear for a temperature of 330[degrees]C are plotted in Fig. 8 as the magnitude of the complex shear viscosities as functions of the applied angular frequencies. The zero-shear viscosities can be determined from the plateau of the curve at lower angular frequencies to ~20,000 Pa s for FEP 1, 6000 Pa s for FEP 2, and 2300 Pa s for FEP 3. This difference in zero-shear viscosity can be explained by diversities in the molar masses of the three FEP. Except for the level of the functions, all three FEP show comparable viscoelastic behavior.

The resulting porosities for foaming the different FEP are plotted in Fig. 9 against the foaming temperatures during extrusion. Comparing the values, a very strong influence of the melt viscosity on the foaming behavior can be recognized. The highest porosities were accessed for all temperatures for the lowest viscous FEP 3. The resistance against bubble formation and, thus, the degree of foaming seems to increase with the strength of the polymer melt. This tendency cannot only be detected by decreasing the extrusion temperature, as already described, but also for an increase of the polymer viscosity. Therefore, very low values of about 10% of porosity can be found for the highest viscous FEP 1 even for the foaming at high temperatures of 330 and 350[degrees]C, respectively. Just an increase in processing temperature to 370[degrees]C reduces the viscosity and thus the strength of the FEP 1 melt to a state, at which a foaming with porosities comparable with the other two FEP is possible.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

The influence of the viscosity on the resulting foam structure can be recognized by a comparison of microscopic images of fractured cross sections of the different FEP foamed at 370[degrees]C in Fig. 10. It can be recognized from Fig. 9 that the porosities for the three FEP for this extrusion temperature are located in the same range, varying only between 52 and 55%. Therefore, the resulting cellular structures can just be related to the different viscosities of the FEP melts. For the highest viscous FEP 1, few and big bubbles as well as unfoamed areas in the outer skirts of the strand can be observed. The similar porosity of this strand, compared to the other FEP can be referred to the big size of the fewer cells. A higher quantity of finer cells developed for the medium viscous FEP 2. Unfoamed material can mainly be found at the rim area of the strand because of the faster cooling of this zone after exiting the die. Foaming of the lowest viscous FEP 3 at this temperature resulted in a very fine cellular structure distributed over the whole cross section. Compared with the higher viscous FEP 2 the development of cells even in the outer skirts of the foam is very pronounced.

[FIGURE 10 OMITTED]

Change of Geometry

By exchanging the circular nozzle by a rectangular slit die, the experimental foaming setup can also be used to produce differently shaped films of voided FEP that would be of interest for applications in which planar geometries of foamed FEP are needed. For the changed geometry, generally the same extrusion conditions as found for the capillary die can be applied. The effects of varying extrusion rates and foaming temperatures can be determined from the microscopic images presented in Fig. 11. They show cross sections of three films of FEP 2 foamed at different apparent shear rates of 17 and 67 [s.sup.-1] as well as different temperatures of 350 and 370[degrees]C. Despite the differences in processing, they all exhibit the same porosity value of about 30%. Because of the change of the foam profile, a new important factor decisive for the quality of these foams with respect to the grade of skin formation comes into play, as for some applications compact surface layers are required. For a temperature of 350[gegrees]C and an apparent shear rate of 67 [s.sup.-1] a foam structure with a skin covering all bubbles at the surface is generated. For the influence of a reduction of the extrusion rate and the increase of foaming temperature on the resulting cellular structure, similar dependencies can be found. In both cases, few and big cells developed in the center and the bubbles at the surfaces are not completely covered by polymer caused by breakthroughs of gas. The largest cells can be recognized in Fig. 11a for the foaming with an apparent shear rate of 17 [s.sup.-1]. Because of the nucleation of only a small number of bubbles combined with a longer cooling time as a result of the low extrusion rate, few big cells form in the center. The generation of the larger cells in the inner area at 370[degrees]C can be referred to the lower melt viscosity of the FEP 2 under this condition. Concerning the skin formation, the melt strength governed by its viscosity is the decisive factor. A higher temperature during the growing process of the bubbles given by a slower cooling rate for foaming because of a lower extrusion rate, for example, is responsible for a comparably lower melt viscosity, which supports the breakthrough of bubbles and the escape of gas to the environment. An optimum cellular structure can be achieved by adjusting the processing parameters extrusion rate and foaming temperature.

[FIGURE 11 OMITTED]

CONCLUSIONS

The intention of this work was to investigate the effect of different processing and material parameters on the chemical foaming behavior of FEP. For this purpose, a chemical blowing agent adapted to the high processing temperatures of FEP had to be found. Magnesium hydroxide, which releases small amounts of water at temperatures exceeding 280[degrees]C. could be proven to be appropriate. For 10 wt% magnesium hydroxide compounded to the FEP melt, porosities up to 56% were achieved.

From the results presented it can be concluded that especially the development of the cellular structure is strongly influenced by processing parameters like extrusion rate and foaming temperature. With rising throughput rate of the molten polymer, a nearly linear increase of the resulting porosities could be observed, which correlates with a growth of cell numbers as well as a decrease in their cell diameters. These findings can mainly be referred to the nucleation behavior of the bubbles because of the supersaturation of the melt with gas during the pressure drop at the exit of the capillary die. Besides the throughput rate or, respectively, the shear rate, the processing temperature strongly affects the resulting cellular structure. An increase in temperature is related to a reduction of the melt viscosity and following from that the melt strength becomes lower. As a result, the resistance against cell collapse decreases, some of the blowing gas is lost from the ruptured cells to the environment because of an enhanced diffusion rate, which leads to a lower porosity. In contrast, if the melt viscosity due to a lower processing temperature is higher, only a small number of cells is formed and the gas will diffuse into these few bubbles able to grow.

In addition to the optimization of the processing parameters, an enhancement of the number of cells combined with a reduction of cell dimension can be achieved by adding a small amount of fine solid particles. Though the residues of the decomposed chemical blowing agent are able to activate bubble formation, too, it could be proven for a concentration of 10 wt% calcium fluoride particles that the nucleation behavior is enhanced probably because of the larger sized voids they create.

As there occurs a biaxial elongational deformation during cell growth, the viscosity of the melt is of outstanding importance for the foam morphology, too. The decisive factor for the resulting porosity of FEP foam thereby seems to be its melt strength, which can be assumed to be related to the viscosity. It has to be low enough to enable the formation and growth of cells. A FEP with a high viscosity needs increased processing temperatures and gives foams of lower porosities. Because of the reduced melt strength, the best foam properties defined by a high porosity and a fine cellular structure could be found for the lowest viscous FEP.

The results of this investigation contribute to a deeper understanding of the foaming behavior of FEP with respect to its dependence on processing and material parameters for the chemical foaming process. An interesting question is how these laboratory results could be transferred to larger scale industrial processes.

ACKNOWLEDGMENTS

The authors would like to thank the Dyneon GmbH & Co. KG for providing the polymer material. Additionally they thank Jennifer Reiser and Michelle Malter for experimental assistance.

REFERENCES

1. J. Stange and H. Munstedt, J. Rheol., 50, 907 (2006).

2. J. Stange and H. Munstedt, J. Cell. Plast., 42, 445 (2006).

3. C.B. Park, D.F. Baldwin, and N.P. Suh, Polym. Eng. Sci., 35, 432 (1995).

4. C.B. Park and L.K. Cheung, Polym. Eng., 37, 1 (1997).

5. A.H. Behravesh, C.B. Park, L.K. Cheung, and R.D. Venter, J. Vinyl Addit. Technol., 2, 349 (1996).

6. A.H. Behravesh, C.B. Park, and R.D. Venter, Cell. Polym., 17, 309 (1998).

7. J. Stange: Einfluss rheologischer Eigenschaften auf das Schaumverhalten von Polypropylenen unterschiedlicher mole-kularer Struktur, Ph.D. Thesis, University of Erlangen-Nurnberg (2006).

8. C.B. Park, L.K. Cheung, and S.-W. Song, Cell. Polym., 17, 221 (1998).

9. J.H. Saunders, "Fundamentals of Foam Formation," in Handbook of Polymeric Foams and Foam Technology, edited by D. Klempner and K.C. Frisch, Hanser Publishers, Munich, 5-15 (1991).

Larissa Zirkel, Helmut Munstedt

Institute of Polymer Materials, University of Erlangen-Nurnberg, D-91058 Erlangen, Germany

Correspondence to: Helmut Munstedt: e-mail: polymer@ww.uni-erlangen.de

Contract grant sponsor: Deutsche Forschungsgemeinschaft
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Date:Nov 1, 2007
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