Advanced Polymer Materials to Support NASA Technology Needs

This CAN proposal focuses on the utilizing the resources and knowhow in the research groups of Prof. Jason E. Bara (Dept. of Chemical & Biological Engineering) and Prof. Paul A. Rupar (Dept. of Chemistry & Biochemistry) at the University of Alabama to support NASA Marshall Space Flight Center (MSFC) and advance important thrust areas relating to additive manufacturing and life support. Profs. Bara and Rupar offer their expertise in polymer materials in three projects that have been identified and outlined in collaboration with Dr. Enrique Jackson and Dr. Eric Fox from NASA MSFC. Project 1 relates to the continued development of ionic polyimides as feedstocks for fused deposition modeling (FDM) based 3-D printing. Bara and Jackson have previously worked together on characterizing the thermal properties of a range of ionic polyimide compositions through CIF funding over the past year. This CAN proposal will facilitate scale up of material production, extrusion of filaments and printing of demonstration parts in an FDM- based 3-D printer as a viable new material for additive manufacturing during space travel and eventual Mars exploration. Project 2 relates to the use of these ionic polyimides in combination with NASA MSFC’s custom ionic liquids (ILs) which are able to remove CO₂ for atmosphere revitalization and life support. The goal is to stabilize these ILs within the solid ionic polyimide and produce pellets that can be used to remove CO₂ from the ambient air within a closed environment such as a space shuttle. Project 3 relates to developing feasible and beneficial uses for Bosch carbon which is a byproduct of the atmosphere revitalization process. It has been proposed that the Bosch carbon may be extremely beneficial within additive manufacturing technologies to support on-demand production of items and parts needed. Through extrusion, Bosch carbon can be directly incorporated into polymer filaments such as ABS or ionic polyimides. Another viable option is to employ 3-D printing to create molds/shells of needed objects and then use the Bosch carbon as a filler for thermosetting materials such as room-temperature vulcanizing (RTV) silicones, thus creating excellent composites that can make use of large amounts of Bosch carbon. The success of this project will benefit specific NASA objectives for space exploration relating to additive manufacturing and beneficial use of Bosch carbon, which might otherwise be considered a waste product. Furthermore, this work will benefit the research mission of the University of Alabama and further the value of existing intellectual property, including issued and pending U.S. patents covering the ionic polyimide material compositions.

The first proposed project relates to the use of ionic ultra-high performance (UHP) polymers for additive manufacturing. These ionic UHP polymers are based on known UHP materials including polyimides, polyamides and others. The ionic UHP polymers are also able to incorporate ILs into their structures as a means of improving processability and controlling/improving thermal, mechanical, and chemical properties.

Prof. Bara and Dr. Enrique Jackson (EM22) have collaborated through a CIF agreement (NNX17WA24P) relating to characterizing the thermal properties of these cationic (i.e., positively charged) UHP polymers in order to select the best candidate(s) for use in additive manufacturing processes. Based on these data, several materials with targeted thermal properties (e.g. appropriate melting point and excellent thermal stability) will make excellent filament feedstocks for fused-deposition modeling (FDM) in non-metallic 3-D printing applications. The goal of the work proposed in this CAN agreement is to now scale-up the synthesis of cationic UHP polymers and extrude the raw polymer and polymer-IL compositions into filaments using the 3Devo Advance extruder in Prof. Bara’s laboratory. Prof. Bara’s laboratory has the only extruder of this type in North America. First, the appropriate conditions and extruder parameters (temperature profile, feed rate, feed particle size) will be studied so as to learn how best to consistently produce filaments of 1.75 mm diameter (the typical size required for most 3-D printers). The filament feedstocks will then be used on 3-D printers available in Prof. Bara’s labs and at NASA MSFC to print demonstration parts for mechanical, thermal and/or electrical testing. The success of this scale-up, extrusion and printing will bring these first-generation ionic UHP polymers close to maturity and make them viable candidates for use in developing lightweight components for use in NASA missions. We would expect that follow-on work with NASA and/or commercialization efforts be devoted to printing specific part designs for applied testing, and formulation of customer-specific materials based application-specific target thermal and mechanical conditions. Furthermore, we would also expect to pursue thermosetting-based compositions of these ionic UHP polymers in additive manufacturing applications. We will rely on continued collaboration with Dr. Jackson for thermal analysis (DSC, TGA, TG-IR, etc.).

In parallel to the processing of cationic UHP polymers, Rupar will develop the synthesis of anionic UHPs. As with the cationic UHPs, the anionic UHPs are expected to be easily tuned and more processable compared to traditional UHPs. Importantly, the complementary charge of anionic UHPs can be combined with cationic UHPs to produce parts and devices with highly controlled properties.

One potential challenge that may be encountered during the course of this work could be working in larger reaction and processing vessels that will allow us to synthesize ionic UHP polymers at in batches of 1 kg or more which will be needed to generate sufficiently long runs of filament for 3-D printing. In this case, we will work with the AIME center on the UA campus and utilize the larger (20 L) chemical reactors available in that facility which will be more than sufficient for producing 1+ kg batches of ionic UHP polymer. We may also need to design and build new low-cost unit operations for washing, homogenizing and drying the raw polymer material prior to extrusion. It is anticipated that most of this work can be done using plastic vessels with inexpensive mechanical mixers as well as a with large lab oven with forced air convection. A detailed analysis of the best approaches for synthesizing and handling larger quantities of polymer will be carried out prior to the start of the work.

The ionic UHP polymers developed for FDM-based 3-D printing are also extremely useful for use as supports for ILs with (2-trifluoromethyl)pyrrolide anions developed by NASA MSFC that selectively react with CO₂. Here, we propose to work with NASA to blend ILs with ionic UHP polymers to create highly stable particles/pellets/fibers that will remove/recover both moisture and CO₂. Ionic UHP polymers serve as excellent supports to hold ILs within a solid matrix place, while the IL retains its reactivity/function. This work will study determine the maximum loading of IL into the ionic UHP polymers and then extrude the composite into filaments which will then be pelletized. The absorptive properties of these pellets for CO₂ and water vapor will then be tested in a simulated crewed spacecraft cabin environment.

We will also work with NASA to better understand the nature of the chemical reactions driving CO₂ and moisture absorption and analyze ways in which the IL and/or ionic UHP polymer can be tailored to further enhance these behaviors. We would expect that follow-on work with NASA be devoted to long-term testing in an actual environment and further research into improving IL and polymer properties and performance, minimization of regeneration energies and design of continuously operating systems. We would expect that such technologies might also be of interest to the Department of Defense for use in submarines and other closed environments requiring life support atmosphere revitalization. One potential challenge associated with this work may be slightly different (stronger or weaker) reactivity of the (2-trifluoromethyl)pyrrolide anion when present in the polymer matrix compared to the liquid phase. However, such issues can typically be addressed through process design considerations (adsorbent mass, adsorbent surface area, adsorbent shape, air circulation/flow rates, etc.) to achieve the desired removal rates of CO₂ and moisture from the cabin atmosphere.

The Bosch Process is under consideration by NASA as a method to remove CO₂ from the atmosphere of space habitats. In the Bosch Process, CO₂ and H₂ react in the presence of an iron catalyst to produce H₂O and graphitic carbon (Bosch carbon). The Bosch carbon builds up and fouls the catalyst. Project 3 will examine this Bosch carbon as a filler in FDM thermoplastic filaments and in molded thermosets. Using Bosch carbon as a filler for plastics will allow NASA to enhance space-based 3D printing capabilities by potentially improving the properties of 3D printed materials and reducing the quantity of raw polymers necessary to fabricate devices and parts. Carbon, typically as carbon black (CB), is widely used as a filler for polymers in industrial and consumer products (e.g. car tires). In addition to cost savings through reduction of polymer weight percentage in the final material, the inclusion of CB often improves the material properties (e.g. better abrasion resistance).

Although the use of CB as a filler for polymers is well known, Bosch carbon does have different properties than CB and may require modification/treatment to maximize its performance. In the case of CB, surface chemistry and particle size play a large roll in how it behaves as a filler. As such, simple modifications to Bosch carbon, such as washing with dilute hydrogen peroxide solutions, exposure to an oxygen plasma, or pulverization to reduce particle size, will be examined during Project 3.

In the initial stages of Project 3, we will perform a detailed characterization of the Bosch carbon provided by NASA. Analytical techniques that will be used include a combination of powder XRD (to estimate graphitic content), Raman spectroscopy (also to estimate graphitic content), sieve analysis (to measure particle size), contact angle measurements (to estimate surface energy), and SEM (to estimate particle size and particle morphology). These techniques will allow us to understand the composition of the Bosch carbon and monitor how the Bosch carbon is altered though various treatments. ABS will be combined with Bosch carbon through mechanical mixing and extruded into filaments using the 3DEvo advance extruder in Prof. Bara’s laboratories. 

The Bosch carbon loading will be varied and the impact on the printing properties of the ABS will be examined. A potential difficulty of using Bosch carbon as a filler is that ABS is known to become brittle at higher carbon loadings (i.e. > 20%). Therefore, a focus of this research will be to probe the maximum Bosch carbon content in ABS while maintaining suitable mechanical properties. Follow-up work will include testing of Bosch carbon in other thermoplastics, such as PLA, PEEK, and the ionic UHP polymers described in Projects 1 and 2 above. The ionic UHP polymers are especially promising for this project as they are likely to interact favorably with Bosch carbon that has been treated to be more hydrophilic (e.g., treated with hydrogen peroxide or oxygen plasma).

In addition to exploring Bosch carbon in thermoplastics, we will look at the incorporation of Bosch carbon as filler in thermosets. 

Although FDM is not typically performed with thermosets, NASA’s desire for FDM-based 3-D printing capabilities can still be utilized by first printing reusable molds for the thermosets, a practice widely used in device prototyping. Room temperature vulcanizing (RTV) silicone will be used as a model thermoset as its behavior with CB filler is widely understood. In addition, the elastomeric properties of RTV silicone are complimentary to those of ABS and thus will allow for the production of flexible/compressible/deformable parts not possible with ABS (e.g. gaskets). The envisioned process works by first 3-D printing molds from ABS, then filling these molds with the RTV silicone + CB mixture, which is allowed to cure times on the order of minutes to hours). The cured RTV silicone + CB part is then cleanly removed from the ABS mold, and the mold can be reused many times. In addition to RTV silicone, other Bosch carbon filled resins, such epoxies, can also be examined in the future. 3-D printing and the subsequent mechanical testing of filaments, printed parts, and cast thermosets, will be performed using mechanical testing equipment (e.g., Instron) available at both NASA and UA. Samples will also be provided to NASA for thermal analysis.

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