Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a

Transcription

1 Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a Caroline Vachon * and Richard Gendron Industrial Materials Institute, National Research Council Canada, 75, de Mortagne Blvd., Boucherville, Qc, Canada, J4B 6Y4 ABSTRACT Mixtures of blowing agents are becoming widely used in the industry either for economical reasons or for achieving better control of processing conditions. Despite the fact that they are commonly used for foaming, the literature is fairly scarce on that particular subject and the fundamentals are not very well understood. This work studies the effect of blending carbon dioxide and 1,1,1,2- tetrafluoroethane (HFC-134a) in polystyrene. Ultrasonic monitoring and online rheology provided information on the solubility and plasticizing effect of the gases. Results show that, on an equivalent molar basis, HFC-134a is slightly more soluble than CO 2 and is a more effective plasticizer. Moreover, HFC-134a generates foam samples with a higher nucleation density than CO 2 using similar processing conditions. Blending the two gases generates nucleate cell densities, which are intermediate to the pure gases but do not follow a logadditivity rule. It is hypothesized that blending gases affect their mutual diffusion coefficients, which in turn, largely dictates the final foam morphology. INTRODUCTION Blends of physical foaming agents (PFA) have been specifically developed in order to replace conventional CFCs. Reasons for blending may be numerous. For cases when carbon dioxide is used as a co-blowing agent, the incentives may be economical, environmental, and politically correct image driven (1). Practices based on the use of mixtures of physical foaming agents may also be justified by the increased production rate achieved through blending of PFAs acting as inflating gases with PFAs having a stabilizing role (2). Although considerable amount of work has been done in that area, very little information is being made public in the literature. Blends of HCFC-22 and HCFC-142b This paper is a revised version of the paper given at Foams 2002, Third International Conference on Thermoplastic Foam Processing & Technology, Houston, Texas, October 22-23, The Copyright is held by the Society of Plastics Engineers (SPE). * Author to whom correspondence should be addressed. Cellular Polymers, Vol. 22, No. 2,

2 Caroline Vachon and Richard Gendron have been formulated in order to replace CFC-11 (3). Unfortunately, the usefulness of this formulation is limited since HCFCs will also have to be completely banned by 2010 due to their ozone depleting potential. Most of the current efforts are aimed at using environmentally-friendly gases such as carbon dioxide and nitrogen, volatile hydrocarbons (butane and pentane) and HFCs, which are not currently restricted. HFC-134a was reported to be a suitable replacement for the production of expanded polystyrene insulation boards (4) and blends of HFC-134a and HFC-365mfc have also been studied (5). Another relevant study on mixtures of PFAs was based on blends of carbon dioxide and 2-ethyl hexanol (2-EH) as foaming agents for extruded polystyrene (6). Low densities and appropriate cell sizes were reported and, in some cases, intriguing bimodal cell size distributions were noted. 2-EH was regarded as a processing aid, since its high normal boiling point, 182 o C, would make the alcohol liquid under phase separation at temperatures in the range of o C. However, 2-EH was found to be an efficient plasticizer for PS, and it was suggested that the presence of 2-EH could slow down the CO 2 diffusion rate. This work describes the results of foaming polystyrene with blends of carbon dioxide and HFC-134a. Carbon dioxide and 1,1,1,2-tetrafluoroethane (HFC- 134a) are both targeted as promising alternative blowing agents since there is no current environmental concern related to their use. The present work studies the effect of blending these gases on foam morphology, processing and possible synergistic effects. EXPERIMENTAL Extrusion was carried out on a Leistritz 50 mm counter-rotating twin screw extruder equipped with an instrumented slit die (pressure, temperature and ultrasonic probes) as described elsewhere (7). A gear pump was placed at the die exit to control the pressure inside the die. HFC-134a was directly injected in the extruder barrel using a preparative chromatography pump. For carbon dioxide, gas was injected directly from the cylinder. However, a high-pressure pump (Applied Separations) was occasionally used to inject CO 2 above 830 psi. The nominal flow rate of polymer was maintained at 10 kg/hour. Crystal polystyrene (Nova 1301, MFI = 3.5) was used. HFC-134a was kindly provided by Atofina and carbon dioxide was obtained from Praxair. Three sets of experiments have been conducted in the present study: (1) Determination of the plasticization behavior for each type of PFA, at various levels set between concentrations of 0-14 mol%, using both rheological 76 Cellular Polymers, Vol. 22, No. 2, 2003

3 Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a and ultrasonic methods. These measurements were conducted at 190 o C. (2) The degassing conditions, i.e. pressure and temperature for which the PS/PFA mixture undergoes phase separation, were evaluated for different compositions of CO 2 /HFC-134a, within a temperature range of o C, as listed in Table 1. (3) Finally, foaming experiments were conducted for the PFA concentrations listed in Table 2, at a melt temperature of 149 o C. Measurements of the degassing conditions were carried out as follows. First, a steady flow of the homogeneous single-phase mixtures was established at different fixed temperatures. The pressure was high enough to prevent bubble nucleation in the die. The melt temperature was monitored at the ultrasonic die and kept constant to within a degree. Then, in a second step, the pressure in the die was lowered rapidly by increasing steadily the speed of the gear pump. During these operations, the pressure and the ultrasonic parameters (attenuation Table 1 Blowing agent blend formulations used for degassing experiments CO 2 w t. % HFC 134a wt. % CO 2 m ol% HFC-134a mol% mol% total Table 2 Blend composition, density and morphological analysis results for foam extrusion trials. The foam extrusion was carried out at T=149 o C CO 2 mol% HFC-134a mol% mol% total ρ ( kg/m 3 ) d cell (µ m) β (nucleation sites/cm 3 ) open cell (%) x x x x x Cellular Polymers, Vol. 22, No. 2,

4 Caroline Vachon and Richard Gendron and sound velocity) were continuously monitored. The ultrasonic data provided the information on the onset of bubble nucleation as reported elsewhere (7). Foam samples were produced using a 2 mm strand die. Density was measured by water displacement and morphological parameters were obtained following the methodology reported by Moulinié et al. (8). Open cell content was determined with an Accupyc 1330 gas pycnometer. RESULTS AND DISCUSSION Plasticization Figure 1 shows the sound velocity in the polystyrene/carbon dioxide or polystyrene/hfc-134a mixtures, during extrusion, as a function of blowing agent concentration measured at constant temperature (T = 190 C). The data were taken at pressures ranging from 9 to 10.3 MPa where phase separation cannot occur (i.e. no gas bubbles are present at this stage). Reported velocity values are corrected for a given constant pressure (P = 8.3 MPa). Blowing agent concentrations are reported as mol%. They were calculated as follows based on the molecular weights of the gases (44.0 g/mol for CO 2 and g/mol for HFC-134a) and the repeat unit of the polymer (104.1 g/mol for PS). An example is given below for 3.2 wt.% CO 2 : mol% CO 2 mol CO2 = ( mol CO + mol PS) mol% 32. ( ) ( ) = = ( ) It can be seen that sound velocity decreases linearly with increasing concentration. The sound velocity is a function of the bulk modulus of the polymer, which is then sensitive to the plasticized state. As reported in the work of Gendron et al. (9), the decrease of sound velocity is proportional to the lowering of the glass transition temperature of the polymer due to the plasticizing effect of the dissolved gas. When both blowing agents are compared on an equivalent molar basis, HFC-134a has a greater plasticizing effect than carbon dioxide as seen by the steeper slope. Simultaneous but independent results were obtained from conventional rheological measurements using the in-line slit die (Figure 2). In agreement to what was observed with the ultrasonic sensors, at constant shear rate, shear stress gradually decreases as a function of gas concentration with lower values obtained for PS/HFC-134a. Typically an increase of 50% in molar concentration of the carbon dioxide would be required to match the viscosity decrease induced by a given amount of HFC-134a. 78 Cellular Polymers, Vol. 22, No. 2, 2003

5 Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a Figure 1 Sound velocity (homogeneous phase) of polystyrene-blowing agent mixtures as a function of concentration. Measurements were conducted at a melt temperature of 190 C. All data normalized at P = 8.3 MPa Figure 2 Semi-logarithmic plot of shear stress as a function of blowing agent concentration. Measurements were conducted at a melt temperature of 190 C Cellular Polymers, Vol. 22, No. 2,

6 Caroline Vachon and Richard Gendron Degassing Conditions Carbon dioxide, HFC-134a and blends of the two blowing agents were prepared in order to obtain equivalent molar concentrations (Table 1). In most cases, the total molar concentration was around 7.3 mol% except for one particular blend, where abnormal feed rate fluctuations occurred during the experiments. On average, the total molar concentration for that particular formulation was 8.3 mol% throughout the degassing trials. Degassing or phase separation pressures inside the instrumented die were determined from the ultrasonic attenuation as reported in a previous work (7). Results are shown in Figure 3 where degassing pressures are plotted as a function of the melt temperature for the different polymer/blowing agent mixtures. In all cases, i.e. for each mixture ratio, the data follow a parabola. A similar behavior had been previously reported for the PS/HCFC 142b system by Sahnoune et al. (7) and was explained in terms of competitive factors such as gas solubility (high temperatures) and polymer viscosity (low temperatures). More recently, Tatibouët and Gendron (10) proposed a tentative explanation for this peculiar phenomenon. The extensional stress induced by the convergence flow at the die entrance would not relax completely under conditions when long relaxation times prevail (low temperature, polymer with large compliance value). This stretched state prevailing along the machine direction may favor low solubility conditions, similar to those experienced under low-pressure levels. A closer look at Figure 3 show that higher degassing pressures are noted for pure CO 2, in the range of 6-7 MPa. The degassing pressures for neat HFC-134a are roughly 1 MPa below those of carbon dioxide. Generally, increasing the relative proportion of HFC-134a in the blend decreases the corresponding degassing pressures. Degassing results for the blends containing 3.02 mol% CO mol% 134a diverge from the overall trend, with unusually high degassing pressures. As indicated previously, this set of experiments has been performed with a higher PFA molar fraction, i.e. 8.3 mol% versus 7.3 mol% for the other trials. This approximately 10% increase in the blowing agent concentration may explain the proportional raise of the measured degassing pressures. Only one data (obtained at the lowest temperature) has a degassing pressure that is intermediate to those obtained for the pure gases. For comparison, the solubility of carbon dioxide and HFC-134a in PS for temperatures in the o C range is illustrated in Figure 4, using data of Sato and coworkers (11,12). Under given pressure and temperature conditions, HFC-134a is slightly more soluble than CO 2. Extrapolating the solubility data of HFC-134a in the 5-6 MPa range, and comparing both solubility results of CO 2 and HFC- 134a to the degassing results obtained in the present work, a close agreement can be seen between the two sets of results, which would indicate that the onset 80 Cellular Polymers, Vol. 22, No. 2, 2003

7 Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a Figure 3 Degassing pressure as a function of temperature for equivalent molar blends. A curve for each data set has been plotted as a guide for the eye only Figure 4 Solubility data for CO 2 and HFC-134a in polystyrene. Data plotted from references (11, 12) Cellular Polymers, Vol. 22, No. 2,

8 Caroline Vachon and Richard Gendron of nucleation, in this case, is essentially related to the solubility function. It would be logical at first thought to assume that the pressure at which gas comes out of the polymer, at a set concentration should be in the neighborhood of the equilibrium pressure required to dissolve that same amount of gas in the first place. Although degassing pressure results are slightly different than those obtained from static measurements, they do provide quick and useful information on the solubility behavior of blowing agents. Foam Extrusion Foam samples were extruded at constant temperature (T melt = 149 C) using a 2 mm strand die (D 0 ). The diameter of the exiting foam (D) was measured with a digital caliper as a function of distance from the die. Care was taken to produce foam samples having blowing agent concentrations as close as possible to those studied during the degassing experiments (Table 2). Results are summarized in Figure 5. Despite small differences, all blowing agent blends have very similar expansion profiles. In both cases, the expansion is abrupt and maximum expansion is obtained around 10 mm. Slightly larger diameters of exiting foams are obtained for foams extruded with only CO 2. As the proportion of HFC-134a increases in the blend, diameters become smaller. In the case of the foam samples extruded with only HFC-134a, there is even some evidence of shrinkage as foam diameter tends to decrease slightly over distance. It should be emphasized that although foam samples were extruded at the same temperature, the plasticizing effect of each blowing agent is different. As previously demonstrated in Figure 1, HFC-134a has a greater plasticizing effect than CO 2, therefore a softer foam may lead to partial collapse and cell wall rupture. Moreover, since wall thickness is inversely proportional to the nucleation density, thinner wall will be prone to rupture, which should conduct to the formation of more open cells. As expected, measurements performed on the samples confirmed that the open-cell content increases with increasing proportion of HFC-134a in the blend (Table 2). The density and morphology of the foam samples were also determined. Formulations based on equivalent molar blends of blowing agent generated foams having very similar densities except for the foam samples made of 100% HFC-134a where shrinkage is likely responsible for the slight density increase (Table 2). Micrographs of the foam samples were obtained and analyzed in order to extract cell size distributions (Figure 6). A constant decrease in the cell sizes is seen with increasing proportion of HFC-134a in the blend. The global morphology of the foams is characterized by a narrow cell size distribution as seen by the steepness of the slope. A slightly wider distribution is noted for the blend containing nearly equal proportions of CO 2 82 Cellular Polymers, Vol. 22, No. 2, 2003

9 Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a Figure 5 Strand expansion ratio for foam samples extruded at constant temperature, T = 149 o C Figure 6 Cell size distribution of foams samples extruded at constant temperature Cellular Polymers, Vol. 22, No. 2,

10 Caroline Vachon and Richard Gendron and HFC-134a. The general trend seen in the cell sizes and even degassing pressures would suggest that gas molecules evolve as a whole and do not form independent clusters exclusively comprised of one type of gas. For a constant amount of gas molecules dissolved in the polymer melt (7.2 mol%), a higher number of small bubbles are produced with HFC-134a. This feature is also reflected in the measurement of β, the nucleate cell density, which represents the number of nucleation sites per cm 3 of unfoamed polymer. When comparing pure CO 2 and HFC-134a, β is almost 1000 more important for HFC-134a (Table 2). In other terms, for a fixed amount of blowing agent, and in absence of significant cell coalescence, the quantity of gas molecules is 1000 times less important in each cell for HFC-134a. The nucleate cell density (β) was plotted as a function of relative proportion of each blowing agent in the blend (close symbols, Figure 7). Micrographs of the foams are also shown in the same figure. Correction of the nucleate cell density for those Figure 7 Nucleate cell density (β) as a function of relative proportion of HFC-134a in the blend. Experimental data points are represented by close symbols, while open symbols correspond to results corrected on a basis of equally set molar concentration (details can be found in the text). Micrographs of the resulting foams made from equivalent molar concentration of blowing agent are also included (same magnification for all) 84 Cellular Polymers, Vol. 22, No. 2, 2003

11 Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a blends that deviate from the nominal concentration of 7.2 mol% total, especially the mixture made of 2.40 mol% CO 2 and 5.15 mol% 134a, has also been attempted. The correction performed, based on the assumption that nucleation density is an exponential function of the PFA concentration, is represented by the open symbols in Figure 7. The large difference in the nucleation densities reported in Figure 7 between the foams obtained with the pure PFAs can be explained by the differences in their diffusion coefficients. Once nucleation is launched, one must consider the competition that exists between subsequent nucleation from still-dissolved PFA molecules and cell growth of existing cells. This latter is closely related to the diffusion of the gas molecules into existing adjacent nucleation sites (13), while the former originates from the favorable conditions finally met in case of lower PFA molecule concentrations. Huge differences between the diffusion coefficients of HFC-134a and that of carbon dioxide in PS have been reported (14), with that of HFC-134a being a thousand times smaller and this, even if the PS matrix experiences more plasticization with HFC-134a. In that case, the extra free volume generated through dissolution of HFC-134a does not necessarily increase the mobility of the solvent molecules, as indicated by the large difference in their diffusion coefficients. When considering the formulations having a constant amount of gas molecules, the higher diffusion of CO 2 would likely contribute to a fast migration of PFA molecules toward existing nucleation sites, and a rapid expansion of the resulting foam. Because of its higher solubility, HFC-134a has a lower propensity to phase-separate. And because of its smaller diffusion coefficients, HFC-134a molecules are less prone to feed existing bubbles within the timeframe of the gradual pressure reduction, and the still-dissolved PFA would favor additional nucleation to take place, under milder conditions. A simple log-additivity rule is illustrated by a dotted line in Figure 7. Another relationship is hypothesized and is represented by a reverse-s-shape curve that was drawn very close to the experimental and corrected data points. Should bubble growth be driven by the slow diffusion of gas molecules, these results would indicate that small amounts of HFC-134a greatly affect the overall nucleation of CO 2. In the CO 2 -rich blend, nucleation is significantly enhanced; at the opposite end, a small amount of CO 2 in a HFC-134a-rich blend drops the nucleation density drastically. It has been reported by Ettouney and Majeed (15) for the permeability of mixtures of gases in membranes that presence of faster permeating species enhances permeation rates of slower species, while reduction in the faster species permeability is caused by presence of the slower permeating species. In the present case, even though there exists a slight difference in the solubility, the difference in the permeability of the two PFAs is largely dictated by their diffusion coefficient. Respective permeability coefficients of both Cellular Polymers, Vol. 22, No. 2,

12 Caroline Vachon and Richard Gendron carbon dioxide and HFC-134a, when they are mixed, may tend to come to close average numbers with their difference being significantly reduced. This could explain the quasi plateau proposed for intermediate concentrations in Figure 7. This observation is quite interesting since it would mean that overall nucleation could be controlled through adequate mixtures of gases having different permeability coefficients. In addition, since interaction between the two PFAs is assumed to affect diffusivity and be responsible for the behavior illustrated in Figure 7, it could also be hypothesized that solubility, as represented by the degassing curves of Figure 3, could be sensitive to the interactions between the HFC-134a and CO 2 molecules, which could lead to deviations such as those displayed in this Figure 3. CONCLUSION Results reported in this work show that at equivalent molar concentration, HFC-134a has a greater plasticizing power than CO 2. Degassing experiments conducted on the pure gases and blends showed that a higher pressure is required to maintain CO 2 dissolved in the polymer, which is in agreement with the solubility data. Although all formulations based on an equivalent amount of gas (mol%) generated foams of comparable bulk density, the nucleate cell density (β) was increased with increasing proportion of HFC-134a in the blend. This feature has been explained in terms of differences in gas diffusivity. While lower diffusion rates might favor local nucleation, higher diffusion rates would favor cell growth mechanisms driven by migration of molecules towards existing cells. Moreover, it has been proposed that nucleation using blends of blowing agents does not follow a log-additivity rule. Hence, small quantities of one gas were found to significantly alter the cell nucleation density. REFERENCE 1. C. Eaton, Proceedings of Foam Conference 96, Somerset, New Jersey, December (1996) J. L. Throne, Proceedings of Foam Conference 96, Somerset, New Jersey, December (1996) X. Fanichet, E. Kuhn and P. Schindler, J. Cell. Plast., 30, (1994) A. Albouy, J.-D. Roux, D. Mouton and J. Wu, Cell. Polym., 17, (1998) L. Zipfel and P. Dournel, J. Cell. Plast., 38, (2002) L. E. Daigneault and R. Gendron, J. Cell. Plast., 37, (2001) Cellular Polymers, Vol. 22, No. 2, 2003

13 Foaming Polystyrene with Mixtures of Carbon Dioxide and HFC-134a 7. A. Sahnoune, J. Tatibouët, R. Gendron, A. Hamel and L. Piché, J. Cell. Plast., 37, (2001) P. Moulinié, L, E. Daigneault, C. Woelfle and R. Gendron, SPE Antec Conference, Orlando, USA (2000). 9. R. Gendron, M. Huneault, J. Tatibouët and C. Vachon, Cell. Polym., 21, (2002) J. Tatibouët and R. Gendron, SPE Antec Conference, Nashville, USA (2003). 11. Y. Sato, T. Iketani, S. Takishima and H. Masuoka, Polym. Eng. Sci., 40, (2000) Y. Sato, M. Yurugi, K. Fujiwara, S. Takishima and H. Masuoka, Fluid Phase Equil., 125, (1996) C. B. Park, D. F. Baldwin and N. P. Suh, Polym. Eng. Sci., 35, (1995) Z. Zhang and P. Handa, J. Polym. Sci.: Part B: Polym. Phys., 36, (1998) H. Ettouney and U. Majeed, J. Membr. Sci., 135, (1997) Cellular Polymers, Vol. 22, No. 2,

14 Caroline Vachon and Richard Gendron 88 Cellular Polymers, Vol. 22, No. 2, 2003