PTFE Emulsion Foam Coating Finishing of Needled Pre-oxidized PAN/PPS Composite Filters Bin YU and Xiao-ming ZHAO *

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1 2017 3rd International Conference on Green Materials and Environmental Engineering (GMEE 2017) ISBN: PTFE Emulsion Foam Coating Finishing of Needled Pre-oxidized PAN/PPS Composite Filters Bin YU and Xiao-ming ZHAO * School of Textile, Tianjin Polytechnic University, Tianjin , China *Corresponding author Keywords: Foam coating, Composite filter, Pore size, Filtration efficiency. Abstract. In order to improve the filtration properties of needled pre-oxidized PAN/PPS composite filters, a series of composite filters were prepared through foam coating. The morphology, porosity, air permeability, pore size and filtration properties of filers were evaluated. The results showed that with increasing of PTFE emulsion containing in coating solution, the porosity, air permeability, pore size and pressure drop of filters decreased. When the ratio of PTFE emulsion above 10 %, the pressure drop increased more significantly with the increase of ratio of PTFE emulsion, compared with the ratio of PTFE emulsion below 10 %. The filtration efficiency of filters increased with the increase of ratio of PTFE emulsion. For particles with a diameter of 2.5 µm, the filtration of filters finished by foam coating was as high as %. Introduction Suspended particles that are small enough to be inhaled have increasingly become a great concern in the world because of a range of reasons such as population growth, rapid development of industry, over-quick construction of urbanization and some other increased human activities [1, 2]. Due to advantages of low cost, low energy consumption, simple construction for filter regeneration and high filtration efficiency, fiber filters are widely used to remove airborne particles [3, 4], especially for hot gas filtration. Nowadays, most filters for hot gas filtration are made of polyimide (PI)[5], polytetrafluoroethylene (PTFE)[6], polyphenylene sulfide (PPS)[7], glass[8] and metal fibers[9]. Pre-oxidized PAN fibers are the intermediate products in the production of commercial carbon fibers based on PAN [10]. Pre-oxidization of PAN fibers is carried out in an air atmosphere between 180 and 300 o C. During the pre-oxidization process, fibers undergo structural transformations because of dehydrogenation, cyclization, oxidation reactions and crosslinking of molecular chains, which endows OPAN fibers with excellent thermal stability [11]. In the process of preparation of high-performance carbon fiber, some pre-oxidized PAN fibers are discarded as a result of low cost. In this research, foam coating method was applied for improving the filtration performances of needled pre-oxidized PAN/PPS composite filters. Materials and Methods Materials PTFE emulsion (solid content 60±2 %, particle size µm) was purchased from Guangzhou Songbai Chemical Co., ltd. Melamino-formaldehyde (MF) resin emulsion (solid content 50±2 %, curing temperature o C) was provided by Jinan Qianlai Environmental Protection Technology Co., ltd. Sodium dodecyl sulfate and hydroxyethyl cellulose, chemically pure, were purchased from Tianjin Institute of fine Chemicals. Needled pre-oxidized PAN/PPS composite filters were prepared in Wedo (Tianjin) Ltd. The sample properties are listed in table

2 Thickness [mm] Air permeability [mm/s] Table 1. Properties of needled pre-oxidized PAN/PPS composite filters. Breaking strength in machine direction [N] Breaking strength in cross direction [N] Breaking elongation in machine direction [%] Breaking elongation in cross direction [%] Process of PTFE Foam Coating Finishing A certain amount of PTFE emulsion, MF resin emulsion with a weight percentage of 6 %, Sodium dodecyl sulfate with a weight percentage of 2 %, hydroxyethyl cellulose with a weight percentage of 1 % and pure water with the rest of weight percentage were placed in an beaker, then, carry out mechanical stirring, 800 r/min, 3 min and 2000 r/min, 7 min. As a result, the coating solution foam ratio is about 1:3. The coating solution was coated on the composite filters, then heat in the oven for 5 mins at 90 o C, and 3 mins at 160 o C. The composite filters finished with coating solution containing PTFE emulsion with mass fractions of 0 %, 5 %, 10 %, 15 % and 20 % were labeled as 1#, 2#, 3#, 4# and 5#, respectively. Characterization of Composite Filters The morphology of composite filters was obtained using scanning electron microscopy (SEM, TM-3030, Japan). The porosities of composite filters were tested with full automatic true density analyzer (Ultra PYC 1200E, America). Air permeability of the samples was evaluated with a digital air permeability tester (YG 461H, China). Pore size of composite filters was measured using a pore size tester (TOPAS PSM-165, Germany). Samples were placed into the sample holder with a circular opening of diameter 11 mm. The testing liquid was Torpor supplied by TOPAS. To obtain pore size information, two measurements (without and with the testing liquid) were carried out. Pore size distributions were calculated automatically with PSM Win software (TOPAS). The filtration efficiency and pressure drop of various composite filters were evaluated with an automated filter tester (LZC-K1, Huada Filter Technology Co., Ltd., China). The testing area of the composite filter was 100 cm 2, and the samples were tested at a flow rate of 32 L/min. Results and Discussions SEM The SEM images of composite filters finished by foam coating with different percentages of PTFE emulsion are shown in Figure 1. It can been seen from the figure that the needled composite filters consist of interconnecting individual fibers forming a porous network structure, which contributes to the passing of gas fluid and interception of suspended particles. Some attachments forming membrane come out on the surface of filters finished by coating, which makes the interspace between individual fibers decrease. PTFE emulsion was coated uniformly on the composite filters surface, then, through the process of pre-drying and baking, the PTFE resin solidified and formed membrane. On the other hand, it also can been seen that with the increase of PTFE emulsion content in the coating solution, more PTFE membrane attachments occurred on the surface of composite filters, and the filter surface become more compact, which contributes to the decrease of porosity and improve the filtration efficiency of composite filters. Figure 1. SEM images of composite filters 193

3 Porosity and Air Permeability Table 2. Thickness, porosity and air permeability of composite filters. Samples 1# 2# 3# 4# 5# Thickness [mm] Porosity [%] Air permeability [mm/s] The thickness, porosity and air permeability of composite filters finished by foam coating are shown in Table 2. It can been seen from the values of thickness that the thickness of composite filters coating with various weight percentage of PTFE emulsion are around 2 mm, having no significant changes. After foam coating, PTFE and MF emulsion were deposited on the filter surface, and the pore between fibers filled with them, making the porosity of composite filters decreased. With increasing of PTFE content, the porosity of filters decreased generally. Air permeability is a primary parameter for filters, better air permeability, the suspend particles through the filter easily, less particles are trapped, as a result of a lower filtration efficiency [12]. The air permeability dates indicated that the air permeability of composite filters decreased obviously after foam coating. Comparing to the primitive sample, the air permeability of 5# decreased from mm/s to 91.6 mm/s, decreased %. In addition, the air permeability of filters decreases with the increase of the weight percentage of PTFE emulsion in coating solution. Pore Size Table 3. Pore size of composite filters. Samples 1# 2# 3# 4# 5# Minimum pore size [µm] Maximum pore size [µm] Mean pore size [µm] The method of capillary flow was applied to evaluate the pore size and pore size distributions of the composite filters. Minimum pore size, maximum pore size and mean pore size of samples were listed in Table 3. It was clearly that the pore size decreased with an increase in the ratio of PTFE emulsion within coating solution. Comparing to the 1#, the mean pore size of the 2#, 3#, 4# and 5# decreased 11.70%, 31.96%, 49.71% and 60.58%, respectively. The reason was that with an increase in the ratio of PTFE emulsion, the PTFE membrane attachment on the filter surface increased, as a result of pore size decreasing. This also can be verified by the variation of air permeability listed in Table 2. Air permeability of composite filters is a function of the pore size present on the filters. As the pore size decreases, the amount of the air passing through the filters also decreases [13], which will improve the filtration efficiency of filters. Filtration Properties The most important parameters to estimate in filter medium are filtration efficiency and pressure drop [14]. Figure 3 illustrated the filtration performance of composite filters tested. As can been seen from the figure, the filtration efficiency of composite filters varies with the size of particles. Compared with particles with a diameter of 0.3 µm and 0.5 µm, the composite filters show more excellent filtration efficiencies for particles with a diameter of 1.0 µm and 2.5 µm. For particles with a diameter of 0.3 µm, 0.5 µm, 1.0 µm and 2.5 µm, the filtration efficiencies of 5# are 37.40%, 42.09%, 96.73% and 99.72%, respectively. On the other hand, with increasing weight percentage of PTFE emulsion, the filtration efficiency and pressure drop of the filters increased. For particles with a diameter of 0.3 µm, the filtration of 2#, 3#, 4# and 5# increase 31.40%, 39.56%, 52.27% and 76.45%, compared to 1#. When the ratio of PTFE emulsion above 10%, the pressure drop increased more significantly with the increase of ratio of PTFE, compared to the ratio of PTFE emulsion below 10%. Higher ratio of PTFE emulsion, more membrane attachments deposited on the surface or filters, making the porosity and pore size smaller. As a result, the suspend particles were captured easily by the filters obtaining high filtration efficiency. However, the air permeability of filters decreased as a result of increasing of pressure drop. 194

4 (a. filtration efficiency) (b. pressure drop) Figure 3. Filtration properties of composite filters with various PTFE weight percentages. Conclusions A series of composite filters were prepared through foam coating with solution containing different weight percentage of PTFE emulsion. The morphology, porosity, air permeability, pore size and filtration properties of filers were evaluated. When the ratio of PTFE emulsion above 10%, the pressure drop increases more significantly with the increase of ratio of PTFE, compared to the ratio of PTFE emulsion below 10%. The filtration efficiency of filters increased with the increase of ratio of PTFE emulsion. For particles with a diameter of 2.5 µm, the filtration of filters finished by foam coating was as high as 99.72%. This was because with increasing of PTFE emulsion containing in coating solution, the air permeability and pore size of filters decreased. The coating pre-oxidized PAN/PPS composite filters are promised to suing for hot gas filtration. References [1] Banihashemi Tehrani SM, Moosavi A, Sadrhosseini H. Filtration of aerosol particles by cylindrical fibers within a parallel and staggered array, J. Microsystem Technologies. 22(2016) [2] Li W, Shen S, Li H. Study and optimization of the filtration performance of multi fiber filter, J. Adv Powder Technol. 27(2016) [3] Huang H, Wang K, Zhao H. Numerical study of pressure drop and diffusional collection efficiency of several typical noncircular fibers in filtration, J. Powder Techno. 292(2016) [4] Zhang S, Liu H, Yin X, Yu J, Ding B. Anti-deformed Polyacrylonitrile/Polysulfone Composite Membrane with Binary Structures for Effective Air Filtration, J. Acs Appl Mater Inter. 8(2016) [5] Wang Q, Bai Y, Xie J, Jiang Q, Qiu Y. Synthesis and filtration properties of polyimide nanofiber membrane/carbon woven fabric sandwiched hot gas filters for removal of PM 2.5 particles, J. Powder Technol. 292(2016) [6] Jaworek A, Krupa A, Czech T. Modern electrostatic devices and methods for exhaust gas cleaning: A brief review, J. J Electrostat. 65(2007) [7] Yang B, Zheng D, Shen Y, Qiu Y, Li B, Zeng Y, et al. Influencing factors on low-temperature denox performance of Mn La Ce Ni Ox/PPS catalytic filters applied for cement kiln, J. J Ind Eng Chem. 24(2015)

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