Sorption and Swelling Measurements of CO2 and N2 on Polyol for

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Sorption and Swelling Measurements of CO2 and N2 on Polyol for Their Use As Blowing Agents in a New PU Foaming Process Device Tobias Fieback,†,* Walter Michaeli,‡ Simon Latz,‡ and María E. Mondejar§ †

Ruhr-University Bochum, Universit€atsstrasse 150, 44780 Bochum, Germany Institut f€ur Kunststoffverarbeitung, RWTH Aachen, 52062 Aachen, Germany § University of Valladolid, Paseo del Cauce, 59, 47011 Valladolid, Spain. ‡

ABSTRACT: From an industrial point of view, polymeric foams are of great importance because of their good mechanical and thermal properties and their low weight. Among them, polyurethane foams are the most versatile and, thus, the most consumed. Traditional methods for polyurethane foaming have the disadvantages of negative spin-offs due to chemical foaming reactions or use of flammable gases. A new foaming process device has been developed to overcome these drawbacks by performing physical foaming with safe blowing gases. In relation to this setup, the sorption of a polyol provided by Bayer MaterialScience, in the presence of pure N2 and pure CO2, was examined at three isotherms at 293, 303, and 313 K and at pressures up to 6 MPa. Swelling measurements for CO2 were also performed at 293 K at pressures up to 5.6 MPa. Both sorption and swelling measuring methods were based on the magnetic suspension balance. Sorption data were obtained by means of a very accurate gravimetric method, whereas swelling data were acquired visually by using an optical pressurized cell. Results for N2 sorption on polyol showed that no appreciable sorption occurred for this gas. However, CO2 sorption was observed to be significant, and thus, the swelling of the polyol sample was measured to correct the CO2 sorption values. Corrected sorption results for CO2 on the polyol yielded a maximum uptake of 617 mg/g at 293 K and 5.4 MPa with an increase in the sample volume of about 47%. Measuring data showed that CO2 sorption increased with pressure and decreased with temperature.

1. INTRODUCTION Currently, polymeric foams are widely used in worldwide industry because of their excellent mechanical and thermal properties such as light weight, high strength/weight ratio, and great insulating abilities.1 Among the different foamed polymers, polyurethane foams are the most extensively consumed because of their versatility, excellent final product properties, ease of production, and wide range of applications.24 Thus, the range of applications of polyurethane foams includes flexible foams for furniture, molded foams for transport facilities components, sealants, and insulation.46 There are three foaming technologies: chemical, mechanical, and physical. In chemical foaming, gas produced during a chemical reaction between some added components acts as the blowing agent. This reaction is usually carried out by mixing isocyanate with water, obtaining CO2 as a product. In physical foaming, the gas resulting from a phase transformation of an added liquid component is the blowing agent. In mechanical foaming, a gas, commonly nitrogen or air, is dispersed into the starting components.7,8 Chemical foaming is the most developed technology in the industry because of its wide range of applications and ease of implementation. However, the possibility of urea group formation because of the water content, which can change the mechanical properties of the final product, makes it necessary to monitor any chemical changes in the sample. The mechanical foaming technique presents the disadvantage of a limited application because of the low solubility for blowing agents in the starting mixture.9 Physical foaming, as the third foaming technology in use, has the r 2011 American Chemical Society

disadvantage of the high flammability of the blowing agents commonly used (pentanes). In the late 1950s, chlorofluorocarbons (CFCs) began to be used as physical blowing agents. These substances led to lowdensity foams with good mechanical properties and low thermal conductivity. However, because of the potential damage to stratospheric ozone that could be cause by their use, they were banned by the Montreal Protocol of 1987.2,4 After that, CFCs were replaced by hydrocarbons (HCs) and hydrochlorofluorocarbons (HCFCs), but the problem, as mentioned above, remains in the flammability of these gases. As new research approaches, liquid CO2 injection10 and CO2 sorption on the starting mixture are being investigated. Following the second line for using CO2 as a blowing agent, a new foaming process has been developed by the Institut f€ur Kunststoffverarbeitung (IKV).11 This setup has the advantage of reaching higher CO2 contents in the reaction mixture than current methods. However, its most important feature is the precise control of the foaming process by which the blowing gas remains in the mixture without dissolving prematurely. This makes it possible to wait for the ideal foaming viscosity level of the mixture and initiate the foaming process at the opportune moment during polymerization. The experimental device developed is shown in Figure 1.

Received: January 10, 2011 Accepted: May 3, 2011 Revised: May 3, 2011 Published: May 03, 2011 7631

dx.doi.org/10.1021/ie2000538 | Ind. Eng. Chem. Res. 2011, 50, 7631–7636

Industrial & Engineering Chemistry Research

Figure 1. Controlled physical foaming device.2

This discontinuous unit separates the polymerization process from the foaming process by means of a prereaction chamber, which ensures a controlled foaming and avoids spraying of the reagents as they leave the mixing head. Figure 2 shows a diagram of the principle of its operation. For the polymerization process, both polyol and isocyanate are injected simultaneously into the prereaction chamber through a high-pressure mixing head under the presence of the blowing gas (in this case, N2 or CO2 atmosphere) so that the gas is absorbed in the reaction mixture. Once the mixture has reached the desired increase of viscosity, it is forced into the mold by means of a hydraulic piston. There, a pressure drop takes place, forcing the sorbed gas to desorb so that bubbles are generated. The higher viscosity avoids a collapse of the bubbles and an escape of the blowing agent. The most important parameters in this process are the prereaction time and the discharge pressure.2 As a contribution to the development of this new foaming device, sorption for several physical blowing agents is being investigated. Because of the safety and low cost of N2 and CO2, sorption measurements of both gases on a new hard foaming system are presented in this work to study their potential use as blowing agents in this process.

2. EXPERIMENTAL DETAILS As mentioned above, sorption values for CO2 and N2 on a polyol component of a hard foam mixture are presented in this article. Swelling measurements for CO2 on the polyol were also performed to determine the increase in the sample volume due to gas sorption, necessary to correct the buoyancy effect in sorption values. 2.1. Starting Materials. For both sorption and swelling measurements, a formulation of polyol provided by Bayer MaterialScience AG was used. This component, together with isocyanate Desmodur CD-S, forms a mixture that has a water content of approximately 0.1% and that is specially prepared for physical foaming. The nitrogen and carbon dioxide used to perform the measurements had purities of 99.999% and 99.9%, respectively, and were supplied by Linde Gas. 2.2. Sorption Equilibrium Measurement Principle. The static gravimetric method for sorption measurements used in this work is based on the sorption measuring apparatus with magnetic suspension balance (MSB) developed by Dreisbach and L€osch.12 The schematic setup of the installation is shown in Figure 3. This installation consists of a thermostatted measuring cell. Inside is a container with the desired absorbent sample. This container is directly connected to a magnetic suspension

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Figure 2. Principle of the device for the controlled physical foaming of reactive materials.11

Figure 3. Schematic setup of the gravimetric installation used for sorption measurements on polyol.

coupling, consisting of a free-floating permanent magnet inside the measuring cell and a controlled electromagnet outside, hanging under a very accurate balance. Because of the coupling, the mass of the sample is transmitted without direct contact with the high-resolution balance. This design makes it possible to work at high pressures in a wide temperature range. Thus, inside the measuring cell, temperatures between 233 and 473 K and pressures up to 20 MPa can be reached. The resolution of the balance of 1 μg allows the reading of very accurate small changes in the weight of the sample. Sorption equilibria on a specified sample can be measured with this method with a measuring uncertainty of about 4%. When the cell is pressurized, the gas is partly sorbed in the sample after a stabilization time. This sorption causes a mass increase of the sample, which is weighed with the magnetic suspension balance. By combining this increase in weight measured by the balance and the buoyancy effect acting on both the sample and the sample container, the gas-sorbed mass can be calculated. The prepared sample of polyol was loaded into the balance and activated at 320 K for 12 h under vacuum. The regenerated sample was then used for the sorption equilibrium measurements. These 7632

dx.doi.org/10.1021/ie2000538 |Ind. Eng. Chem. Res. 2011, 50, 7631–7636


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Figure 4. Sorption isotherms for N2 on the polyol: (2) 293, (9) 303, and (b) 313 K. Figure 5. Polyol sample in the sample container inside the measuring cell at 0.1 MPa and 293 K.

Table 1. Sorption of N2 on Polyol p (MPa)

muptake (g/g)

standard deviation (g/g)

T = 293 K 0.00

0.0000

0.0001

2.01

0.0006

0.0796

4.03 6.03

0.0013 0.0009

0.1593 0.2388

0.00

0.0000

0.0001

2.04

0.0004

0.0781

4.04

0.0000

0.1543

6.01

0.0001

0.2300

0.00

0.0000

0.0001

2.03

0.0005

0.0751

4.04 6.04

0.0002 0.0002

0.1495 0.2237

T = 303 K

T = 313 K

sorption uptake values were calculated according to the equation muptake ¼

mA mBAL mSCþS þ FVSCþS ¼ mS mS

ð1Þ

where muptake is the mass of gas absorbed in the sample per unit mass of the sample; mA is the mass of gas absorbed in the sample; mBAL corresponds to the signal of the balance at the measuring position; mSCþS and VSCþS are the calculated mass and volume, respectively, of the sample container and the sample; mS is the mass of the sample; and F is the density of the gas for the working pressure and temperature. 2.3. Swelling Measurement Principle. The installation used to perform the swelling measurements on the polyol was based on the same principle as the setup used to perform sorption measurements. The particularity of this device is that the cell has a glassy window and the sample container is also made of glass so that the increase in volume of the sample due to gas sorption can be observed from outside. Because of a scale marked on the sample container, the increase in volume can be determined by visual observation. Thereby, the swelling data presented in this work were taken visually by observation until the sample volume for each pressure step stabilized.

Figure 6. Polyol sample in the sample container inside the measuring cell at 5.6 MPa and 293 K.

3. RESULTS AND DISCUSSION Sorption measurements for N2 and CO2 on the polyol were performed at three isotherms at 293, 303, and 313 K at pressures up to 6 MPa. CO2-induced swelling of the polyol was measured at 293 K at pressures up to 5.6 MPa in order to determine the increase in the sample volume. Thus, the sorption results for CO2 were corrected by taking into account the new sample volume in the buoyancy effect term. 3.1. N2 Sorption Isotherms. In a first series, the sorption behavior of pure N2 on the polyol was measured. The sorption isotherms of N2 on the polyol up to pressures of 6 MPa are given in Figure 4. Numerical sorption values for nitrogen together with their associated standard uncertainty are reported in Table 1. The density values of nitrogen for each sorption point were obtained from FLUIDCAL software, which calculates thermodynamic properties through the equation of state for nitrogen reported by Span et al.13 The mass uptake of nitrogen on the polyol at the three isotherms is almost zero, which means that nitrogen is not easily absorbed on the polyol. Negative values observed for sorption 7633



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Table 3. Langmuir Isotherm Parameters temperature (K)

S

β

293

1.38718

0.08559

303

1.07566

0.07009

313

0.97259

0.06263

Figure 7. Sorption isotherms for CO2 on the polyol: (2) 293, (9) 303, and (b) 313 K. Open symbols are sorption uptake values before being corrected with swelling measurements.

Table 2. Sorption of CO2 on Polyol p (MPa)

muptake (g/g)

muptake corrected (g/g)

0.00

0.0000

0.0000

1.98

0.0879

0.1420

3.99 5.42

0.2061 0.2771

0.3725 0.6172

T = 293 K

Figure 8. Fitted Langmuir isotherms with the CO2 mass uptake experimental values: (2) 293, (9) 303, and (b) 313 K.

Table 4. Uncertainty of the Influence Magnitudes

T = 303 K 0.00

0.0000

0.0000

1.96

0.0621

0.1061

3.83

0.1318

0.2485

6.04

0.2002

0.4960

0.00

0.0000

0.0000

1.98

0.0547

0.0973

4.05 6.17

0.1144 0.1659

0.2316 0.4301

T = 313 K

results are within the measuring uncertainty of sorption so that they can be considered as well as null sorption. 3.2. CO2-Induced Swelling. Swelling of the polyol induced by CO2 was measured at four different pressures to correct the influence of the buoyancy effect on the sorption data. As the increase in the volume of a polymer is almost independent of pressure and temperature and depends only on the amount of gas sorbed in it and follows a linear trend, swelling was measured only at a temperature of 293 K up to pressures of 5.6 MPa. Figures 5 and 6, respectively, show the polyol sample inside the measuring cell before the swelling measurements and once the stabilization for the last pressure step was reached. The change in the volume, of about 47%, can be clearly observed. The sample volumes at 293 K for the different pressure steps were fitted to the linear regression expression VPolyolþCO2 ¼ ð1 þ 1:6983muptake ÞVPolyol 3

ð2Þ

where V PolyolþCO 2 is the sample volume (cm ) once the swelling is stabilized, V Polyol is the volume (cm3 ) of the

magnitude

units

standard uncertainty

u(mBAL)

g

0.000140

u(mSCþS) u(VSCþS)

g cm3

0.000121 0.047866

u(ΔVS)

cm3

0.005600

u(F)

g/cm3

0.03F

u(mS)

g

0.000241

original sample, and m uptake is the amount of gas sorbed (g CO 2/gPolyol). By applying this volume correction to the buoyancy force term in eq 1, sorption values for CO2 on the polyol were corrected. 3.3. CO2 Sorption Isotherms. To complete this work, the sorption behavior of CO2 on the polyol was studied. The experimental values were then corrected with the new sample volume given in eq 2. The three sorption isotherms obtained for CO2 up to pressures of 6 MPa, along with the uncorrected experimental values, are shown in Figure 7. The sorption results are collected in Table 2. CO2 densities were obtained from FLUIDCAL software through the equation of state for CO2 reported by Span and Wagner.14 Even if the Langmuir model is stated for adsorption, the behavior of CO2 can be well described by a Langmuir isotherm, the expression of which is given by the equation n¼

Sp 1 þ βp

ð3Þ

where p is the pressure (MPa), n is the amount absorbed (mmol g1), and S and β are constants to be fitted. The parameters of the model for the three experimental isotherms were fitted and are given in Table 3. Experimental values with the fitted Langmuir isotherms are shown in Figure 8. 7634



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Table 5. Mass Uptake Uncertainty Budget magnitude

units

standard uncertainty

∂muptake/∂Xi

u(mBAL)

g

0.000140

1/mS

0.716481

u(mSCþS)

g

0.000121

1/mS

0.716481

u(V(SCþS)corr)

cm3

0.048192

F/mS

0.716481F

u(F)

g/cm3

0.03F

VSCþS/mS

2.09628F

u(mS)

g

0.000241

muptake/mS

0.716481muptake

total standard uncertainty (g/g)

u(muptake)

0.000133 þ 0.0717F þ 0.00017muptake

total expanded uncertainty (g/g)

U(muptake) (k = 2)

0.000265 þ 0.1435F þ 0.00035muptake

The mass uptake of carbon dioxide reached a maximum of about 0.617 gCO2/gPolyol at 293 K (5.4 MPa). The uncorrected value of this maximum was 0.277 gCO2/gPolyol. This means that the CO2 swelling study was indispensable to correct the sorption values because the first sorption data were almost half the corrected values. When the temperature is increased, this maximum decreases gradually, so that the sorption at 313 K is about 0.430 gCO2/gPolyol. The uptake of CO2 increases with pressure and decreases with temperature. 3.4. Mass Uptake Measuring Uncertainty. The measuring uncertainty for the sorption values presented in this work was determined. Starting from eq 1 as the propagation law, the measuring uncertainty of the sorbed mass uptake can be calculated according to the equation sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2 ðmBAL Þ þ u2 ðmSCþS Þ þ VSCþS 2 u2 ðFÞ uðmuptake Þ ¼ mS þ

muptake F uðVSCþS Þ þ uðmS Þ mS mS

ð4Þ

From the calibration certificate of the balance and the sorption measurements performed, each term of eq 4 was calculated. These values are reported in Table 4. Thus, the uncertainty budget for the sorbed mass uptake was evaluated. The results are listed in Table 5. The total expanded uncertainty (coverage factor k = 2) was estimated to be 0.000265 þ 0.1435F þ 0.00035muptake. Therefore, the measuring uncertainty for the maximum mass uptake was approximately 11.45 mgCO2/gPolyol, which corresponds to 3.8% of the sorption value.

4. CONCLUSIONS The sorption of CO2 and N2 at 293, 303, and 313 K within the pressure range of 06 MPa was studied on a new polyol provided by Bayer MaterialScience AG. These measurements contribute to the development of a new device for physical foaming using safe gases as blowing agents. The N2 experimental values showed that there was no appreciable sorption of this gas in the polyol for the working ranges of temperature and pressure. However, the CO2 results yielded that a considerable sorption took place for this gas. Because the calculated sorption values were found to depend directly on the sample volume, the swelling of the polyol was studied to correct any effect on the final data. These swelling measurements showed that the increase in the sample volume was high enough to be an important influence to take into account. Thus, the CO2 sorption results were corrected for the swelling effects, and the final sorption data were almost doubled.

The maximum uptake for CO2 on the polyol within the working range was about 617 mg g1. The sorption isotherms indicated that CO2 sorption increases with pressure and decreases with temperature. The measuring uncertainty for the mass uptake values presented in this work was also evaluated, and its estimated value was 4% for the maximum mass uptake.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Research project 15807 N of the Forschungsvereinigung Kunststoffverarbeitung was sponsored as part of the “industrielle Gemeinschaftsforschung und -entwicklung (IGF)” by the German Bundesministerium f€ur Wirtschaft und Technologie (BMWi) due to an enactment of the German Bundestag through the AiF. M.E.M. thanks the Programa Nacional de Formacion de Profesorado Universitario (FPU) of the Spanish Ministry of Science and Innovation for the financial support of her work in Bochum, Germany. ’ NOMENCLATURE m = mass, g or g/g p = pressure, MPa T = temperature, K u = standard uncertainty U = expanded uncertainty V = volume, cm3 ΔV = increase in volume induced by swelling, cm3 xi = mass uptake uncertainty of component i F = density, g/cm3 Subscripts

A = sorbed BAL = balance reading corr = value corrected with swelling effect S = sample SCþC = sample and sample container uptake = mass uptake

’ REFERENCES (1) Klempner, D.; Sendijarevic, V. Polymeric Foams and Foam Technology; Hanser Publications: Munich, Germany, 2004. (2) Singh, S. N. Blowing Agents for Polyurethane Foams; Rapra Review Reports; Rapra Technology Ltd.: Shrewsbury, Shropshire, U.K., 2001; Vol. 12, No. 10. (3) Vanacker, P. Weltmarkt der Kunststoffe: Polyurethane (PUR) Weltweiter Verbrauch steigt. Kunststoffe 2007, 10 (97), 142–148. 7635



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(4) IPCC/TEAP Special Report. Safeguarding the ozone layer and the global climate system Presented at UNFCCC/SBSTA 22, Bonn, Germany, May 19, 2005. (5) Uhlig, K. Polyurethan Taschenbuch; Carl Hanser Verlag: Munich, Germany, 1998. (6) Schloz, D. R. H.W.P. Entwicklung der M€arkte f€ur Schaumkuntstoffe. In Tagungsband 21. Fachtagung Schaumkunststoffe; Fachverband f€ur Schaumkunststoffe und Polyurethane e.V.: Frankfurt am Main, Germany, 2008; Vol. 11, pp 1819. (7) Ebnesajjad, S. Fluoroplastics: Melt Processible Fluoropolymers; Plastics Design Library: Norwich, NY, 2003; Vol. 2. (8) Lepkes, R. Polyurethan—Werkstoff mit vielen Gesichtern; Verlag Moderne Industrie: Landsberg am Lech, 1993. (9) Br€uninghaus, V.; Sulzbach, H. M. Neue verfahrenstechnische L€osung zur Rohdichtereduktion von Weichsch€aumen. Kunststoffberater 1998, 43 (10), 34–40. (10) Koot, W.; Maas, W. P. M. Physical properties of flexible polyurethane “liquid CO2” slabstock foaming mixtures. J. Cell. Plast. 2002, 38, 69–89. (11) Michaeli, W.; Gr€onlund, O.; Meyer, F.; Latz, S. Further advances in the PU Foaming Process with CO2 as the blowing agent. FAPU Eur. Polyurethane J. 2010, 2, 34–36. (12) Dreisbach, F.; L€osch, H. W. Magnetic suspension balance for simultaneous measurement of a sample and the density of the measuring fluid. J. Therm. Anal. 2000, 62, 515–521. (13) Span, R.; Lemmon, E. W.; Jacobsen, R. T.; Wagner, W.; Yokozeki, A. A reference quality thermodynamic property formulation for nitrogen. J. Phys. Chem. Ref. Data 2000, 29 (6), 1361–1433. (14) Span, R.; Wagner, W. A new equation of state for carbon dioxide covering the fluid region from the triple-point temperature to 1100 K at pressures up to 800 MPa. J. Phys. Chem. Ref. Data 1996, 25, 1509–1596.

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Sorption and Swelling Measurements of CO2 and N2 on Polyol for

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