Enhancing the processability of plastics using carbon dioxide and water

27 November 2017
Silas Owusu-Nkwantabisah, Alan J. Lesser, and Claudia Staudt
The synergy obtained by combining supercritical carbon dioxide and superheated water enables polyethersulfone to be processed at significantly lower temperatures than in conventional processes.

Polyethersulfone (PES) is notable for its use in a variety of high-performance applications (e.g., in the automotive, aerospace, biomedical, and water-purification industries). This wide application space is largely due to the polymer's high heat resistance, flame retardance, high mechanical strength, and antifouling properties.

Unfortunately, these properties also pose a challenge in terms of PES processing, since they make necessary the use of excessive temperatures and toxic solvents. Such conditions increase the production costs associated with PES and can degrade the material's properties. Moreover, the residual toxic and flammable solvents in PES products are hazardous.

To circumvent the use of high processing temperatures for PES, we have investigated the use of a melt-extrusion process that can be carried out at temperatures as low as 90°C below those used in conventional methods.1 In our first approach, we coextruded the PES with small fractions of a surfactant (calcium stearate) to reduce its melt viscosity. The incorporated surfactant also significantly increased the material's flame retardance. However, surfactant additives are not desirable in some applications (e.g., ultrafiltration and fuel cells). To overcome these issues, we have more recently proposed the use of supercritical carbon dioxide (scCO2)—which is environmentally safe and can reduce the processing temperature of many thermoplastics2—as a promising alternative. However, when used alone, scCO2 typically produces closed-cell foams, which are not suitable for water-purification processes. Therefore, to further improve the polymer processability and to readily achieve closed- and open-cell foams, we have explored the use of scCO2 and superheated water.

Known to be soluble in many polymers, scCO2 effectively plasticizes the polymer and reduces its melt viscosity.2 When first introduced, scCO2 elevates the pressure and temperature of the polymer, causing it to swell. A valve is then opened to allow the CO2 to exit the pressure vessel, thus causing the polymer to foam, in accordance with classical nucleation theory.2 In contrast, superheated water (shH2O, i.e., water above 100°C and at elevated pressures) exhibits a less polar character. Thus, shH2O achieves favorable interaction with the less polar PES. Consequently, scCO2/shH2O comedia could enhance the polymer's processability by inducing a greater plasticization (compared with scCO2 alone) of the high-glass-transition-temperature polymer.

To explore the processability of PES using this approach, we foamed solid samples with scCO2 alone, and with scCO2/shH2O comedia.3 In all experiments, we used a constant CO2 saturation pressure (276 bar) and varied the temperature (from around 145 to 250°C). We found that scCO2 alone could not produce a PES foam at 165°C—see Figure 1(a)—but that treatment with the scCO2/shH2O comedia produced a uniform PES foam—see Figure 1(b)—with 52% porosity.3 This result provides a clear indication of the superior plasticization effect of the scCO2/shH2O combination.


(a) Illustration showing the results obtained in polyethersulfone (PES) foamed with supercritical carbon dioxide (scCO2), and with comedia of scCO2/superheated water (shH2O). Photographs show PES in the form of (b) a solid block (after treatment with scCO2) and (c) foam (after treatment with scCO2/shH2O comedia).

Further effects on the material properties that arise due to the scCO2/shH2O comedia are shown in Figure 2. Our results for the temperature-dependent porosities of the PES foams, after treatment with the various media, show that successful foaming was achieved using the scCO2/shH2O comedia at 85°C below the 225°C glass transition temperature (Tg) of PES. Interestingly, at higher temperatures (>180°C)—i.e., under which scCO2 is able to produce PES foams—the porosities achieved in samples processed using scCO2/shH2O were significantly higher (by ~25%). We also found that, for temperatures below the Tg of PES, a combination of argon and shH2O produced foams with lower porosities than the scCO2/shH2O comedia. Thus, there is likely a favorable interaction between scCO2 and shH2O that enhances the processability of PES.


Porosity vs. foaming temperature in PES processed with scCO2(triangles) and scCO2/shH2O (squares).

Finally, to investigate the morphological differences of the PES foams, we analyzed scanning electron micrographs of their cross-sections (see Figure 3). The images show that scCO2 alone produced closed cells and a low cell density. In contrast, and in addition to the enhanced processability, scCO2/shH2O produced an open-cell morphology. To investigate the continuity of the open pores obtained using scCO2/shH2O, we tested the water permeability of a 1mm-thick disc-shaped membrane cut out of the foamed PES sample with a cylindrical geometry. We recorded water fluxes of over 600L/m2hbar for the membranes. Such water fluxes, comparable to those of commercial polymeric membranes, indicate that scCO2/shH2O is promising for producing filtration and purification membranes.


Scanning electron micrographs of cross sections of PES foamed with (a) scCO2and (b) scCO2/shH2O. Scales are 50μm.

In summary, we have found that the use of scCO2/shH2O comedia offers a cost-effective and green approach to enhancing the processability of PES via plasticization. Our approach enables processing to be carried out at significantly lower temperatures compared with conventional methods, thus limiting the release of hazardous byproducts. Using the scCO2/shH2O comedia, we achieved remarkable PES foaming at temperatures as low as 85°C below its Tg.3 Furthermore, the benign constituents of the comedia—as well as the open-cell morphology that it allows—are promising for biomedical and purification applications. In the next stage of our work, we will focus on applying the scCO2/shH2O comedia to other thermoplastics. We will also investigate the use of scCO2 with other solutions.


Authors

Silas Owusu-Nkwantabisah
Kodak Research Laboratories

Silas Owusu-Nkwantabisah received his PhD in chemistry from the University of Maine. He joined Kodak as a materials scientist in 2015, after completing his postdoctoral research with Alan J. Lesser at the University of Massachusetts Amherst. He currently serves as a referee for two polymer journals. His interests include scCO2processing, surface chemistry, stereolithography, and stimuli-responsive polymers.

Alan J. Lesser
University of Massachusetts Amherst

Alan J. Lesser is a professor of polymer science and engineering, and an expert in polymers and composite processing using scCO2. He has produced several peer-reviewed articles, patents, and conference proceedings. He is the editor-in-chief of a number of journals (Polymer Engineering & Science, Polymer Composites, and Journal of Vinyl and Additive Technology) and is a member of the advisory board of the Journal of Applied Polymer Science. He has chaired and co-chaired Society of Plastics Engineers events, including ANTEC, on several occasions.

Claudia Staudt
BASF

Claudia Staudt received her PhD in chemistry from the University of Heidelberg, Germany. After completing her postdoctoral research at the University of Texas at Austin, she held a professorship for nearly 10 years. In 2011, she joined BASF, where she led a global platform on membrane materials. Since 2016, she has been responsible for the China Market Research Group and Network for Asian Open Research in Shanghai.


References

  1. S. Owusu-Nkwantabisah, C. Staudt and A. J. Lesser, Improving the flame retardancy and melt processability of polyethersulfone using low molecular weight additives, J. Appl. Polym. Sci. 133, pp. 43525, 2016.

  2. S. P. Nawalade, F. Picchioni and L. P. B. M. Janssen, Supercritical carbon dioxide as a green solvent for processing polymer melts: processing aspects and applications, Prog. Polym. Sci. 31, pp. 19-43, 2006.

  3. S. Owusu-Nkwantabisah, C. Staudt and A. J. Lesser, Synergy of supercritical CO2 and superheated H2O for enhanced processability of polyethersulfone towards open cell foams, Polym. Eng. Sci., 2017.

DOI:  10.2417/spepro.006976