Grantee Research Project Results
2004 Progress Report: Cellulosic Carbon Fiber Precursors from Ionic Liquid Solutions
EPA Grant Number: R831658Title: Cellulosic Carbon Fiber Precursors from Ionic Liquid Solutions
Investigators: Collier, John R. , Rials, Timothy G. , Petrovan, Simioan
Institution: University of Tennessee
EPA Project Officer: Aja, Hayley
Project Period: June 1, 2004 through May 31, 2007 (Extended to May 31, 2008)
Project Period Covered by this Report: June 1, 2004 through May 31, 2005
Project Amount: $350,000
RFA: Technology for a Sustainable Environment (2003) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , Sustainable and Healthy Communities
Objective:
The central objective of this research project is to develop a better understanding of the relationship between the morphology of cellulose fibers spun from an ionic liquid (IL) and the properties of carbon fibers produced from these cellulose fibers. ILs are considered a relatively new class of compounds that have gained attention because of their unique properties as green solvents, determined by their almost zero vapor pressure (Brennecke, 2005; Rodgers, 2005a, 2005b; Swatloski, et al., 2002). The renewable abundance of a biodegradable polymer, such as cellulose, and the “green” properties of ionic liquids as solvents, have created an interest in the development of new cellulosic materials that are processed in an environmentally benign (i.e., friendly, “green”) way. A wide range of ILs have been used to dissolve cellulose, but the best results are obtained using 1-butyl-3-methylimidazolium chloride ([C4mim]Cl), because its anion is a strong hydrogen bond acceptor (Swatloski, et al., 2002). When cellulose is dissolved in a nonderivitizing solvent like [C4mim]Cl, the chloride ion disrupts and breaks the intermolecular hydrogen-bonding of the cellulose hydroxyl groups. The high chloride activity and concentration [C4mim]Cl allows for the dissolution of larger amounts of cellulose at a faster rate when compared to traditional solvents. Ionic liquid solutions of cellulose can be processed by wet spinning to manufacture regenerated cellulosic fibers that are expected to show different properties, based on the different properties of these new solvents.
In this report, solutions preparation and their rheological properties are presented.
Progress Summary:
Experimental Part
Materials. Ionic liquid [C4mim]Cl (melting point 73°C), shown in Figure 1, and dissolving pulp of 640 degrees of polymerization (DP) were used to prepare solutions of different concentrations of cellulose. Propyl gallate was added (1% on dry pulp) to prevent cellulose degradation in the course of solution preparation and rheological characterization.
Figure 1. 1-Butyl-3-Methylimidazolium Chloride ([C4mim]Cl)
Apparatus. Rheological measurements were done on an ARES (Advanced Rheometric Expansion System, TA Instruments) rheometer, using parallel plate geometry. Dynamic frequency sweep mode of operation was used to measure and plot complex viscosity and dynamic moduli (storage (G') and loss (G") modulus, respectively) versus angular velocity at different temperatures. The linear viscoelastic region was tested by performing a dynamic strain sweep test.
Technique. IL solutions (in small amounts, needed for rheological characterization on ARES rheometer) of different cellulose concentration were prepared by the following technique. IL solid powder was melted in a beaker on a water bath. Dissolving pulp and propyl gallate were added and manually agitated with a metal spatula until a clear yellow solution was obtained. All solutions were stored in glass vials until used for rheological measurements.
Dynamic rheological measurements were performed on the ARES instrument using the parallel plate geometry (d=20 mm at a gap of 1 mm). To avoid water uptake by the sample while running the experiment, the edge of the specimen between the plates was covered with a thin layer of viscosity standard silicon oil (29.1 Pa s at 25°C).
Results and Discussion
One of the dynamic train sweep tests is presented in Figure 2, showing a constant viscosity for the entire range of strain. Similar results were obtained for other concentrations. A strain of 1 percent was chosen for all other tests. In Figures 3 to 7, viscosity curves are shown for concentrations of 3, 7, 10, 12, and 15 percent cellulose in the solution and at different temperatures, obtained by running the dynamic frequency sweep tests. Multiple runs were performed to check the experimental errors, and error bars are shown in Figure 7, for viscosity curves of 15 percent solutions. For 3 percent solution, only curves at 80 and 90°C are shown, the results at higher temperatures being below the lower limit of the torque range (0.02 g cm). The same is valid for 7 percent solution at 110°C. The viscosity curves show a shear thinning behavior at all temperatures and concentrations. The higher the concentration, the higher is the viscosity. Increasing the testing temperature brings about a decrease in viscosity. A quasi-Newtonian behavior is noticed, mainly at low angular velocities and lower temperatures.
Complex viscosity of IL solutions of different concentrations at 90°C, as compared with the viscosity of three melt blowing grade PPs and a lyocell solution (cellulose dissolved in N-methyl morpholine oxide) prepared from the same dissolving pulp, is presented in Figure 8. It is evident that the viscosity of IL solutions is comparable with melt blowing grades PPs and lyocell solution on almost the whole range of angular velocities. This would suggest that IL solutions of cellulose could be processed by both spinning and melt or solution blowing, for manufacturing fibers or nonwoven products.
Figure 2. Dynamic Strain Sweep Test for 10% Ionic Liquid Solution at 90°C
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Figure 3. Complex Viscosity of 3% Ionic Liquid Solution |
Figure 4. Complex Viscosity of 7% Ionic Liquid Solution |
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Figure 5. Complex Viscosity of 10% Ionic Liquid Solution |
Figure 6. Complex Viscosity of 12% Ionic Liquid Solution |
Viscoelastic properties of the IL solutions are displayed in Figures 9 to 17, where dynamic moduli are plotted against angular velocity. Both concentration and temperature are important parameters for solutions viscoelasticity. At lower concentrations (3, 7, and even 10%), solutions behave as viscous liquids, loss modulus (G") being higher in value than the storage modulus (G'). But at higher concentrations (see Figures 12 and 13) and high deformation rates, the IL solutions behave as viscoelastic liquids. The cross-over point is shifted to lower angular velocities, extending the viscoelastic domain, as the concentration increases. Temperature shows a different effect on the viscoelastic properties of IL solutions, as seen in Figures 14-17.
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Figure 7. Complex Viscosity of 15% Ionic Liquid Solution |
Figure 8. Ionic Liquid Solutions Viscosity as Compared With Melt Blowing Grades (MBG) PP and a Lyocell Solution |
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Figure 9. Dynamic Moduli for 3% Ionic Liquid Solution at 90°C |
Figure 10. Dynamic Moduli for 7% Ionic Liquid Solution at 90°C |
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Figure 11. Dynamic Moduli for 10% Ionic Liquid Solution 90°C |
Figure 12. Dynamic Moduli for 12% Ionic Liquid Solution 90°C |
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Figure 13. Dynamic Moduli for 15% Ionic Liquid Solution 90°C |
Figure 14. Dynamic Moduli for 15% Ionic Liquid Solution 80°C |
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Figure 15. Dynamic Moduli for 15% Ionic Liquid Solution 90°C |
Figure 16. Dynamic Moduli for 15% Ionic Liquid Solution 100°C |
At lower temperatures and low angular velocities, the solutions behave as viscous liquids, whereas at high angular velocities there is a viscoelastic domain. At 110°C, the 15 percent solution behaves as a viscous liquid on the entire range of angular velocities, as seen in Figure 17. The reciprocal of the cross-over point angular velocity has units of time and is an indication of the relaxation time of the macromolecular chains from the solution, an important parameter of the fiber-forming process.
Figure 17. Dynamic Moduli for 15% Ionic Liquid Solution 110°C
The effect of temperature on the shear viscosity of any fluid can be better quantified by shifting the viscosity curves at different temperatures on a reference temperature viscosity curve to generate master curves and shift factors, from which activation energy for flow can be calculated.
In the following, the method of reduced variables (Bird, et al., 1987) will be used to develop master curves by plotting the reduced complex viscosity η*r vs. reduced angular velocity ωr.
The shift factors are defined by the following equation
, (1)
where η0(T) and η0(T0)are zero-shear-rate viscosities of the solutions at temperature T and reference temperature T0, respectively. Then, the reduced variables can be calculated with equations (2) and (3):
(2)
. (3)
Zero-shear-rate viscosities of the IL solutions of different concentrations and at different temperatures were determined by fitting the experimental data with a Carreau model given by equation (4):
, (4)
where λ is a time constant and η is the power-law exponent. Carreau parameters are given for 7, 10, 12, and 15 percent solutions in Figures 4-7.
The master curve for the complex viscosity of 15 IL solution is given in Figure 18. Coefficient of determination (R2 = 0.99691) shows that the shifting procedure is very accurate.
The temperature dependence of αT often is found to be an “Arrhenius dependence” of the following form:
, (5)
where ΔEα is the activation energy of flow and R is the universal constant.
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Figure 18. Complex Viscosity Master Curve of 15 Ionic Liquid Solution |
Figure 19. Arrhenius Plot for the Shift Factors of 15 Ionic Liquid Solution Master Curve |
Similar master curves were generated for other solutions and activation energies, calculated from the Arrhenius plots, are presented in Table 1.
Table 1. Activation Energies for Flow of Ionic Liquid Solutions of Different Concentrations
Concentration of ionic liquid solution, % |
Activation energy for shear flow, Kcal/mol |
Coefficient of determination for Arrhenius plot |
7 |
43.215 |
0.97971 |
10 |
21.991 |
0.99713 |
12 |
29.089 |
0.99835 |
15 |
26.774 |
0.99691 |
Activation energies for flow of the IL solutions prepared from DP 670 dissolving pulp are higher than the corresponding values for some of the common industrial polymers melts (Bird, et al., 1987).
Conclusions
Cellulosic solutions of different concentrations in an IL solvent were prepared by dissolution of DP 670 dissolving pulp in 1-butyl-3-methylimidazolium chloride. Rheological characteristics—complex viscosity and dynamic moduli—were determined at different temperatures. These solutions behave as viscous or viscoelastic liquids, depending on the concentration and temperature. At high concentrations, low temperatures, and high angular velocities, the IL solutions of cellulose feature a viscoelastic character. Diluted solutions or concentrated ones at low angular velocities behave as viscous liquids.
Master curves for the complex viscosity were generated and activation energies were calculated from the Arrhenius plots of the shift factors. The activation energy for shear flow of cellulose solutions in IL is higher than that of some polymeric melts, such as low-density or high-density polyethylenes.
IL cellulose solutions have rheological characteristics similar with some melt blowing grade polypropylene melts or lyocell solutions of comparable concentration, suggesting feasible processing procedures, such as spinning or melt blowing, for the manufacture of fiber or nonwoven products.
Future Activities:
No future activities were reported by the investigators.
References:
Brennecke JF. Physical properties of ionic liquids. Presented at the CCR NICHE Conference on Ionic Liquids: Background, State-of-the-Art, and Applications, South Bend, IN, February 20-21, 2005.
Rodgers RD. Solvent strength of ionic liquids. Presented at the CCR NICHE Conference on Ionic Liquids: Background, State-of-the-Art, and Applications, South Bend, IN, February 20-21, 2005a.
Rodgers RD. Polymer chemistry in ionic liquids. Presented at the CCR NICHE Conference on Ionic Liquids: Background, State-of-the-Art, and Applications, South Bend, IN, February 20-21, 2005b.
Swatloski RP, Spear SK, Holbrey JD, Rogers RD. Dissolution of cellulose with ionic liquids. Journal of the American Chemical Society 2002;124(18):4974-4975.
Bird RB, Curtiss CF, Armstrong RC, Hassager O. Dynamics of polymeric liquids. Volume 1: Fluid Mechanics, 2nd ed. New York, NY: Wiley, 1987.
Journal Articles:
No journal articles submitted with this report: View all 4 publications for this projectSupplemental Keywords:
sustainable environment, pollution prevention, automotive industry, carbon fibers, catalysts, cellulose, clean manufacturing, elongational flow spinning process, ionic liquids,, RFA, Scientific Discipline, INTERNATIONAL COOPERATION, TREATMENT/CONTROL, Sustainable Industry/Business, Chemical Engineering, Sustainable Environment, Environmental Chemistry, cleaner production/pollution prevention, Technology, Technology for Sustainable Environment, Chemistry and Materials Science, pollution prevention, Environmental Engineering, carbon fibers, clean technologies, cleaner production, automotive industry, automotive components, catalysts, clean manufacturing, elongational flow spinning process, ionic liquids, celluloseProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.