THERMAL AND CHEMICAL STABILITY OF THE POLYIMIDE NANOFIBER COMPOSITES

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1 THERMAL AND CHEMICAL STABILITY OF THE POLYIMIDE NANOFIBER COMPOSITES 1 Michal KOMÁREK, 2 Petr SYSEL, 1 Petr ŠIDLOF, 1 Jakub HRŮZA, 1 Pavel HRABÁK 1 Technical University of Liberec, Studentská 2, Liberec, Czech Republic, michal.komarek@tul.cz 2 Institute of Chemical Technology, Technická 5, Prague 6, Czech Republic, petr.sysel@vscht.cz Abstract Polyimide nanofibers are utilized especially in the applications demanding high temperature and chemical stability. The application of polyimide nanofibers as the catalyst carriers for the exhaust gases purification has been studied extensively at the Technical university of Liberec. The exhaust gases cleaning process usually operates at the temperatures above 200 C and the composition of the combustion gases can be considerably chemically aggressive depending on the burned matter. In the tested application extremely low ph combustion gases are prevalent. The polyimide nanofibers were prepared by the electrostatic spinning of the precursor solution in the organic solvent on the industrial scale spinning line NS4S1600U. The nanofiber layers were imidized by the thermal sequence treatment and the morphology was evaluated by the scanning electron microscopy. The composite structures were prepared by mechanical bonding of the polyimide nanofiber layer into the polyterafluorethylene felt to ensure the required mechanical properties. The thermal and chemical stabilities were evaluated by the series of the laboratory and the in situ testing methods. The combination of the testing methods provides the comprehensive information about the behavior of the prepared materials in the thermal and thermal/chemical combined straining. Keywords: polyimide, nanofibers, stability, composite materials 1. INTRODUCTION Polyimides are the group of specialty polymers significant because of their high heat-resistant properties. The basic properties, synthesis mechanisms and use was outlined by Hergenrother in [1]. Polyimides are widely used in adhesives, dielectrics, photoresists, membrane materials for separation, and others. Polyimides are used in a diverse range of applications, including the aerospace, defense, and optoelectronics, they are also used in liquid crystal alignments, composites, electroluminescent devices, polymer electrolyte fuel cells, polymer memories, fiber optics, etc. [2]. Polymer nanofiber preparation for the various applications is described in several publications. Usually the laboratory needle system is used. In [3] a method of nano-paper production from a commercial P84 NT polyimide using an electrospinning and fiber heat fusing approach is described. Arai et al. in [4] describe the study focused on the preparation of nonbeaded ultrafine uniform nanofibers with a narrow fiber diameter distribution from fluorinated polyimide by electrospinning. The paper shows the possibility of the non-beaded ultrafine uniform nanofibers within a narrow nano-range (27±5 nm) preparation, which is amongst the smallest nanofiber diameters prepared by the electrostatic spinning technique. Chemically modified polyimide nanofibers preparation was published by Tamura et al. [5] Their paper discusses the nanofibers electrospun from the five-membered ring sulfonated polyimide containing the fluorinated group. The material showed an ultrafine and uniform non-beaded structure with diameters ranging from 80 to 160 nm. Industrial scale preparation method of the polyimide nanofibers by the Nanospider TM method was published by Jirsák et al. in [6]. This method allows the multiple polymer jet formation, thus the productivity increase, but requires the more precise spinning parameters definition. In the work of Jirsák et al., the three types of the polyimides were synthetized and their nanofiber forming ability via needle-less electrospinning method was experimentally evaluated. It was found that the

2 PAAs based on ODPA and ODA having the solution concentration in the range 21.5 to 26 wt% were successfully electrospun using a needless electrospinning method. The average fiber diameter increased with the PAA solution concentration from 140 to 470 nm. The important part of the polyimide nanofiber preparation is the imidization process usually utilizing the thermal [7] or chemical initialization. The presented paper describes the research activities performed in order to evaluate the stability of the polyimide nanofibrous material, prepared by means postulated in [6], for the application in the hot exhaust gases environment. 2. MATERIAL PREPARATION Polyimide polymer was synthesized from 4,4-Oxydiphthalic anhydride (ODPA) and 4,4-Oxydianiline (ODA) in an aprotic polar solvent N,N-dimethylformamide. Details of the synthesis process can be found in [6]. The solution of PAA in the DMF was electrospun using an industrial scale spinning line NS4S1600U. The production of the nanofiber flat sheet layer is illustrated in the fig. 1 left. The morphology of the PAA nanofibers was evaluated using the Nova NanoSEM electron microscope. The typical morphology is shown in the fig. 1 (right). Fig. 1 Production of the PAA nanofiber sheet (left) The SEM of the PAA nanofibrous layer (right). The nanofibrous layer was thermally imidized at 60 C for 1h, 100 C /1 h, 150 C /1 h, 200 C /2 h, and 250 C /1 h and the material was inspected by the SEM again to detect possible morphology changes. Because the PI nanofibers do not exhibit the sufficient mechanical stability, it is necessary to incorporate the nanofibrous layer into the carrier substrate. During the preparation of the composite filtration material PI nanofiber layers were enclosed into the polytetrafluorethylene (PTFE) needle punched felt forming the sandwich composite structure. The surface of this composite material was laminated with the eptfe membrane to prevent the penetration of the dust into the body of the filtration material. The significant difference between the diameters of the PTFE felt and PI nanofibers can be noticed on the fig. 2 showing the SEM of the composite material cross-section.

3 Fig. 2 SEM picture of the composite nanofibrous structure assembly 3. MATERIAL TESTING The applicability of the filtration material in the real application involves the assessment of the operating life. Multiple stage testing was proceeded in order to gather important information about the effect of various wearing factors. 3.1 Laboratory testing of the thermal stability The initial testing comprised the Dynamic thermogravimetric measurements (TGA) in nitrogen using a TG- 750 Stanton-Redcroft (heating rate 10 Cmin 1 ) [6].This test showed the 5% loss of the nanofiber layer weight at 408 C. Because the thermogravimetric analysis doesn t evidence the morphology changes, which have significant effect on the material functionality, the thermal straining of the nanofibrous material at the constant temperatures for the constant time was performed. The testing proceeded in the standard laboratory resistant heating furnace LMH11/12 from the LAC Ltd Company. Samples were heated in the air environment. After each testing the possible change of the nanofiber morphology was observed using the SEM microscopy. Comparison of two tests is shown in the fig. 3. The testing results confirmed the stability of the material without structural changes up to 265 C. In the designed application the operation temperatures given by the technology should not exceed the 250 C (in the peak) and prevalently the working temperature is approximately 230 C. Therefore the material has the potential to survive the thermal load in the application. Fig. 3 SEM pictures of the PI nanofibrous layer after the thermal treatment. Thermal load 265 C 32h (left), b) 300 C 32h (right)

4 3.2 Laboratory testing of the mechanical stability In the real application the material is subjected to the mechanical wearing. The mechanical straining comes mainly from the pulse-jet cleaning of the filter media, where the material undergoes the reverse pressure pulse MPa for the several milliseconds whenever the pressure drop reaches defined value. The testing machine shown on the fig. 4 allows evaluation of the material mechanical stability. During the test the air with defined temperature, flow velocity and concentration of dust particles flows through the filter material. When the defined value of the pressure drop increase is reached, the material is cleaned by the reverse pressure pulse. Cycles repeat until the desired number of cycles is reached, or the material is damaged. The test can simulate years of the material lifetime in several hour testing procedure. Fig. 4 Apparatus for the testing of the combined thermal-mechanical loading of the filtration material Applied testing parameters are shown in the tab. 1 and the test results are recorded in the tab. 2. The analysis of the results proves that the material survived cleaning cycles without damage and that the lifetime filtration efficiency doesn t decline under %. Tab. 1. Parameters of the mechanical stability testing Parameter Value Unit Standard VDI/DIN 3926, ASTM D , ISO11057 Temperature C Flow velocity 1 m 3 /hod Reverse pulse pressure 0.6 MPa Sample size 100 cm 2 Tab. 2. The test results Number of the cycles Pressure drop on the beginning of the cycle Δpres (Pa) Amount of the residual particles after cleaning Δmres (g/m 2 ) Cycle time tcycle (sec) Aging: Test process Amount of the elusive particles: mabs (mg) Initial test 0 cleaning pulses Final test 0.2 Test time ttest (min) Dust concentration behind sample: c2 (mg/m 3 ) Filtration efficiency: Ef (%)

5 3.3 In situ testing of the combined thermo-chemical stability The chemical straining is caused especially by the low ph of the exhaust gasses caused by the presence of HCl/HF and SO2 content. Condensate from the exhaust gases can reach (depending on the burned matter) the value of ph = 0.7. Prepared materials were fixed in the sample holder and placed into the raw hot exhaust gasses duct in the incineration plant Termizo Inc. in Liberec, see fig. 5.The temperature of the exhaust gasses in the testing location ranges C and gasses still contain solid particulate and the acidic compounds. Fig. 5 Detail of the sample holder with inserted material before (left) and after (right) the testing After the defined testing period the small part of the sample vas cut from the material, disassembled and analyzed by the SEM. The analysis was complicated by the presence of the dust particulate captured in the composite material. In the normal operational state the dust cannot penetrate the material because of the membrane on the material surface. In this testing setup the raw exhaust gases were surrounding the material even from the unprotected side. Nevertheless the main purpose was to evaluate the stability of the composite and especially the nanofiber component. The macroscopic analysis did not register any change in the mechanical properties of the composite. Presence of the nanofiber component was proven in all the tested samples by the SEM. The fig. 6 shows the evidence of the nanofibrous structures in the tested samples after the one year testing period. Fig. 6 SEM pictures after the 12 month exposure of the material to the raw exhaust gas stream

6 4. CONCLUSIONS The article describes the preparation of the polyimide nanofiber layers, assembling of the composite filtration material and the methods of the material stability evaluation. PI nanofibers morphology was tested to be stable up to 265 C in the laboratory experimental evaluation. Samples of the composite filtration materials with the PI nanofibrous component exhibited long-term thermal and chemical stability, when they were exposed to the stream of the raw exhaust gases for period of 12 month without substantial observable degradation. Samples consisting of the PI nanofibers, PTFE carrier nonwoven felt and PTFE membrane meet also the thermal-mechanical requirements put on the filtration material used for the flue gas decontamination by the VDI 3926 international standard. The developed composite materials are superior to the most of the standart comercial products with the respect to the thermal and chemical stability, pressure drop and the regeneration ability of the filtration material. ACKNOWLEDGEMENT The research work was supported by the Ministry of Industry and Trade Czech republic. Project code FR-TI1/457. The paper was supported in part by the Project OP VaVpI Centre for Nanomaterials, Advanced Technologies and Innovation CZ.1.05/2.1.00/ and by the Project Development of Research Teams of R&D Projects at the Technical University of Liberec CZ.1.07/2.3.00/ LITERATURE [1] HEGENROTHER, P. M. The use, design, synthesis, and properties of high performance/high temperature polymers: An overview. High Performance Polymers 2003, 15, [2] LIAW, D.-J., WANG, K.-L., HUANG, Y.-C., LEE, K.-R., LAI, J.-Y., HA, C.-S. Advanced polyimide materials: Syntheses, physical properties and applications. Progress in Polymer Science 2012, 37, [3] LINGAIAH, S., SHIWAKUMAR, K. Electrospun high temperature polyimide nanopaper. European Polymer Journal. [4] ARAI, T., KAWAKAMI, H. Ultrafine electrospun nanofiber created from cross-linked polyimide solution. Polymer 2012, 53, [5] TAMURA, T., TAKEMORI, R., KAWAKAMI, H. Proton conductive properties of composite membranes containing uniaxially aligned ultrafine electrospun polyimide nanofiber. Journal of Power Sources 2012, 217, [6] JIRSÁK, O., SYSEL, P., SANETRNÍK, F., HRŮZA, J., CHALOUPEK, J. Polyamic Acid Nanofibers Produced by Needleless Electrospinning. Journal of Nanomaterials 2010, [7] SYSEL, P., KONEČNÁ, V., VOLKA, K. Structure-curing relation for polyamic acids. European Polymer Journal 1996, 32,