Triboelectric Effect of Polytetrafluoroethylene Fibers to Improve the Filtration Performance of Air-Purified Materials

Transcription

1 Triboelectric Effect of Polytetrafluoroethylene Fibers to Improve the Filtration Performance of Air-Purified Materials Shang Zhu, Yukang Xu, Chen Huang, Xiangyu Jin Donghua University, Songjiang Campus, Shanghai, Shanghai CHINA Correspondence to: Xiangyu Jin ABSTRACT The effect of triboelectric behavior of polytetrafluoroethylene (PTFE) on the filtration performance of common air-purified filters was studied. Variations of triboelectric charge density on PTFE films were analyzed by four typical parameters (cycle, applied load, contact area and velocity). The results demonstrated that the addition of PTFE fibers could significantly improve the filtration property of air-purified materials, owning to the accumulation of triboelectric static charge on the fibers. INTRODUCTION Triboelectric effect refers to the phenomenon that an electron transfer occurs between two dissimilar polymeric materials by contacting or sliding [1]. It has been applied widely in air-purification to improve the performance of filters [2-8]. As reported in the literature [8], fine particles in the tail gas from cars could be removed effectively (approximately 95.5%) under the high static electric field in the self-powered triboelectric filter. Smith et al analyzed the filtration performance of filters made from polypropylene/wool blended fibers and found that penetration of sodium chloride aerosol decreased from 6.90% to 0.14%, when the proportion of polypropylene fibers increased to 50 weight percent [3]. Tsai et al compared the difference in filtration characteristics of melt-blown fabrics with static charges from two different methods (corona and triboelectric effect) and found that the filtration efficiency of materials with triboelectric charges was much higher than that of fabrics with corona static charges [6]. Although the capability of filters to capture fine particles with submicron scale diameter could be modified by triboelectric charge significantly, at present most air-purified materials are generally manufactured using melt blown or needlepunching processes with a single type of polymeric fibers, including polypropylene (PP), polyethylene terephthalate (PET) or polyphenylene sulfide (PPS). Even though the filtration quality of these filtration materials could be modified by static charges from an external high voltage source, the service life of the filters is limited due to the dissipation of charges over time. To improve the filtration performance and working time, polytetrafluoroethylene (PTFE), having the strongest electronegative property to attract electrons, was studied as a potential raw material for air-filters [9, 10]. The low frictional coefficient, high volume resistance and water repellency of PTFE could help the charge remain on the surface for a long time, lengthening the service life of air-purified materials. However, the high mass density ( 1.70g/cm 3 ) of PTFE fibers generally leads to the filters with compact internal structure, which increases the airflow resistance during working. Aiming at modifying the contradiction between the filtration performance air flow resistance, five fibers in the triboelectric series [11-14], including polyamide (PA), cotton, PPS, PET, PP, were selected and then processed into needle-punching materials to investigate the characteristics of quality of triboelectric charge when sliding against PTFE. Next, the filtration performances of various needle-punching materials made of fibers with or without PTFE fibers were analyzed comparatively. EXPERIMENTAL SECTION Materials PTFE and cotton fibers were purchased from Shanghai LINFLON New Material Technology Co., Ltd and Shandong Aorong Garments Co., Ltd, respectively. PA, PPS, PET and PP fibers were all obtained from Shandong Textile Technology Co., Ltd. The PTFE sintered film (Figure 1) with a thickness of 46μm was manufactured from PTFE split-film fibers [15]. General characteristics of six different fibers are shown in Table I. Each value is the average of ten different test points. Journal of Engineered Fibers and Fabrics 60

2 Fabrication of Various Filters The blended fibers consisting of PTFE fibers with various ratios by weight (0%, 10%, 20%, 30%, 40%, 50% and 100%, respectively) and another dissimilar one selected from the mentioned above fibers (PA, cotton, PPS, PET or PP) were prepared. FIGURE 1. Image and morphology of the common PTFE sintered film. TABLE I. Common characteristics of six various fibers. Fiber Cross section Diameter /μm Length /mm Linear density /dtex Friction coefficient Initial modulus /cn dtex-1 Breaking Strength /cn Breaking elongation /% Volume resistance /106Ω cm PA round Cotton lumen PPS round PET round PP round PTFE irregular >10 7 Blend fibers with the same total mass for each sample were carded with a fine opening apparatus to decrease the degree of entanglement of fibers and then were fed into the carding machine (BG218, Jiaonan Defeng Machinery Manufacturing Co., Ltd). The acquired web was processed into a consolidated felt that became the core part of common air-purified materials using a needle-punching machine (WFC- 100, Jiaonan Defeng Machinery Manufacturing Co., Ltd). The depth of penetration, puncture density and the frequency of needle punching were 12mm, 400needle/cm 2 and 660r/min, respectively. Under the same process parameters, degrees of orientation and arrangement of fibers in the nonwoven materials were identical. MEASUREMENT METHODS Scanning Electronic Microscopy (SEM, TM3000, Hitachi) was employed to analyze the morphology of materials. A thin Au film was deposited onto the surfaces of nonwoven materials with the ion sputtering instrument at 5 Pa and 10 ma for 20s. A pore size analyzer (CFP-1100-AI, Porous Materials Incorporated) was operated to obtain pore diameter distributions. A circular specimen with a diameter of 3cm was required to avoid the tolerance caused by the anisotropic nonwoven materials. Triboelectric Schematic In this study, the sliding friction between PTFE sintered films and nonwoven materials was generated using the Martindale abrasion testing equipment (YG401E, Ningbo Textile Instrument Factory). The device is consisted of two main parts, a grinding head and a sample holder. Journal of Engineered Fibers and Fabrics 61

3 FIGURE 2. Triboelectric series and schematic of charge formation between dissimilar materials Generally, PTFE sintered films were fixed on the sample holder and nonwoven materials were used to cover the grinding head. The triboelectric static charge on the PTFE films was measured using a Faraday pail consisting of two metal containers bonded with three insulation pads connected to an electrometer. Then charge density could be calculated from the ratio of the triboelectric charge and the contact area [16]. Each test was carried out three times. The schematic of the sliding friction and formation of triboelectric static charge is shown in Figure 2. RESULTS AND DISCUSSION Characteristics of Various Needle - Punching Materials The characteristics and distributions of pore size of six different nonwoven materials with only one component are displayed in Figure 3. Except PTFE needle-punching materials, the other five materials all exhibited an expanded structure with high average pore diameter ranging from to 89.66μm. The low average pore diameter (24.35μm) of PTFE felts was due to the internal compact entanglement of flat fibers. Thickness and mass density of needle-punching materials with various proportions of PTFE fibers are shown in Table II, and each value was the average from ten different points. Although the thickness of the needle-punched felts made of 100 wt% PTFE fibers is the lowest (1.22mm), the unit weight was the highest (732.14g/m 2 ). This phenomenon was caused by the flat or irregular cross-sectional shape and high density ( 1.70g/cm 3 ) of PTFE fibers, resulting in the compact entanglement of fibers in the felts. This is consistent with the morphology characteristics exhibited in Figure 3. As for other materials, there were no significant relationships between thickness and area weight. Dependence of Charge Density on Sliding Parameters According to previous literature [16-23], the magnitude of triboelectric charge per unit area between dissimilar materials by sliding or contacting could be affected significantly by four factors, namely number of cycles, applied load, contact area and sliding velocity. Therefore, effects of these parameters on triboelectric charge density were studied. Journal of Engineered Fibers and Fabrics 62

4 FIGURE 3. Six various nonwoven materials and the corresponding distributions of pore diameters. TABLE II. Thickness and mass density of various needle-punching materials. Content of Thickness/mm Unit weight/g m -2 PTFE/wt% PA Cotton PPS PET PP PA Cotton PPS PET PP Journal of Engineered Fibers and Fabrics 63

5 Due to the strong ability to attract electrons, excellent water repellency and low frictional coefficient, PTFE fibers were considered as the ideal negative electrode in the contacting electrification system. However, it was difficult to measure the triboelectric charge magnitude of single PTFE splitfilm fiber because of its submicron scale and wide distribution of fiber diameters. To solve the above defect, the PTFE sintered film was employed to quantitatively explore the triboelectric effect between PTFE and dissimilar materials such as PA, cotton, PPS, PET and PP. As shown in Figure 4a, triboelectric charge density on PTFE sintered films increased gradually with the number of cycles and then reached a maximum. Further increase the sliding cycles resulted in a slight decrease of triboelectric charge per unit area. These variations could be explained by the low thermal conductivity of raw materials. Thus, frictional heat in the contact interface could not transfer immediately, resulting in an increase in probability of contact between molecular chains of two insulator materials as well as the magnitude of triboelectric static charge [21,24]. After sliding against PPS materials, triboelectric charge density on PTFE films was significantly lower than that on PTFE films contacted by the other materials. This phenomenon might be due to the fact that PPS has the lowest volume resistance (3.2 10^6Ω cm). This means that triboelectric electrons could transfer from the PTFE sintered film to PPS materials or ground easily during friction, which decreased the charge magnitude [3]. When the number of cycles was 30, the triboelectric charge per unit area reached the maximum (- 14.9μC/cm 2 ). Triboelectric charge density on PTFE films contacting with PA nonwoven materials exhibited the highest value (-28.0μC/cm 2 ) after 70 frictional cycles, which could be attributed to the poor ability of PA to attract electrons [12, 13]. The effects of contact pressure on charge density were analyzed; results are presented in Figure 4b. When the applied loads increased from 1.9N to 10.3N, triboelectric charge densities on PTFE films increased slowly to approximately μC/cm 2 and then dropped slightly. This variation could be due to the fact that when the contact distance of two dissimilar polymeric materials reached cm or less, electrons transferred from the material with the lower work function to the one with higher work function [25, 26]. The contact distance between PTFE sintered films and nonwoven materials decreased under higher applied loads, resulting in an increase of the actual contact area and electron mobility. Under a load of 7.7N, the charge magnitude per unit area on the surfaces of all PTFE sintered films reach a plateau. Specifically, when rubbed with PA, triboelectric static charge density was maximized (-32.3μC/cm 2 ). When sliding against PPS, the PTFE sintered film displayed the lowest triboelectric charge density, as shown in Figure 4b to Figure 4d, mainly due to the low volume resistance. Dependence of triboelectric charge on the interface area in the PTFE film/nonwoven material pairs was also studied (Figure 4c). When PA or PP nonwoven material was selected to cover the grinding head, the static charge density on PTFE film showed a gradual increase, with the increase of contact area from 8.6 to 28.5cm 2. When the contact area was 13.0cm 2, triboelectric charge per unit area (-32.3μC/cm 2 ) was higher than those resulting from rubbing against other needle-punching materials. When the grinding head was covered with cotton, PPS or PET materials, triboelectric charge density on the PTFE film slowly decreased initially, and then slightly increased. Journal of Engineered Fibers and Fabrics 64

6 FIGURE 4. Electric charge density of PTFE films after rubbing, a-number of cycles with the contact area of 21.6cm 2 under 7.7N at 5.2m/min, b-load with the contact area of 21.6cm 2 and 70 cycles at 5.2m/min, c-contact area with 70 cycles under 7.7N at 5.2m/min, d-friction velocity with 70 cycles and contact area of 21.6cm 2 under 7.7N. Although triboelectric charge densities increased at a contact area of 28.5cm 2 when PTFE sintered films rubbed against cotton, PPS or PET nonwoven materials, there was no significant difference when the contact area was 8.6cm 2. The above curves suggest that triboelectric charge density had no significant change versus contact area, except when rubbed with PA materials. According to previous studies [19, 27], it is reasonable to believe that the relative sliding velocity between PTFE films and nonwoven materials could be an important influence on triboelectric charge density. As displayed in Figure 4d, when the sliding velocity between two dissimilar materials ranged from 2.9 to 6.6m/min, the surface charge densities on all PTFE films showed similar trend of variation and reached maxima at a velocity of 5.2m/min. The surface triboelectric charge density had the highest value (-29.6μC/cm 2 ) at 5.2 m/min when PTFE sintered film was rubbed against PA materials. The relation between sliding velocity and charge density indicated that the velocity of 5.2m/min was ideal for the transfer and accumulation of triboelectric charges. This might be due to the fact that the interface between two contact dissimilar polymeric materials did not have enough time to accumulate necessary energy for electrons to transfer under high sliding velocity. Morphology of PTFE Films Figure 5 shows the morphology of PTFE sintered films sliding against various nonwoven materials under different applied loads. It is evident that the applied loads affect the degree of surface smoothness of PTFE sintered films. The size, as well as the number of microgrooves on PTFE films increased remarkably when friction loads increased from 1.9 to 10.3N. What s more, when the contact load was 10.3N, the smooth PTFE films were damaged and formed a few macroscopic fractures or bumps. Journal of Engineered Fibers and Fabrics 65

7 FIGURE 5. SEM images of PTFE sintered films after rubbing against different nonwoven materials under various loads at number of cycles of 70, contact area of 21.6cm2 and velocity of 5.2m/min. Journal of Engineered Fibers and Fabrics 66

8 Combining the results from Figure 4b and Figure 5, it is reasonable to conclude that a large number of microgrooves on surface of PTFE sintered films improved the capacity of storing charge, causing an increase of triboelectric charge density. The main reason behind the formation of these microgrooves is the low initial modulus and poor heat-transfer properties of PTFE [20, 21]. During rubbing, surface deformation of PTFE sintered films occurred. Such deformation improved the effective contact area between films and nonwoven materials, facilitating the increase of the surface triboelectric charge density, which led to a conclusion with the data in Figure 4c. Filtration Quality Factor of Various Air-Purified Materials In order to explore the effects of triboelectric charge on filtration performance of air-purified materials, a variety of needle-punched materials made of one or two types of fibers (with or without PTFE split-film fibers) were prepared. However, it was difficult to manufacture various nonwoven materials with the same thickness and mass per unit area. Therefore, in this article, the quality factor of filtration (QF) was employed to evaluate the filtration performance of air-purified materials to eliminate the tolerance caused by the thickness and the surface density. Equation (1) was developed to calculate the value of QF [28]. QQFF = llllll PP (1) P is the penetration of sodium chloride aerosol and p is the pressure drop, Pa. Penetration (%) and pressure drop (Pa) of nonwoven materials were collected from three samples using the automatic filtration tester at 85L/min. Values of QF of various air-purified materials with different proportions of PTFE splitfilm fibers are shown in Figure 6. In general, QF increased with an increase of PTFE fibers. After reaching the peak at 40 or 50wt%, a sharp decrease was observed. When PTFE split-film fibers mixed with PA, cotton or PPS fibers, QF reached maximum when the ratio of PTFE fibers was 40 weight percent. The maximum values for PA, cotton and PPS were (28.4%, 30.3Pa), (24.1%, 19.9Pa), (87.5%, 15.4Pa), respectively. Other nonwoven materials made of the blend fibers (50wt% PET/50wt% PTFE or 50wt% PP/50wt% PTFE) reached maxima at (75.6%, 12.7Pa) and (63.5%, 14.7Pa), respectively. Average pore diameters of air-purified materials with a proportion of PTFE fibers were higher than that of materials made of single type of fibers, except for PA and PP materials. The mean diameters of pores in cotton, PPS and PET nonwoven materials were 48.60, and 65.09μm, respectively. When PTFE fibers were added into these nonwoven materials with proper ratios by weights, although the corresponding average pore diameters increased to 50.94, and 77.32μm, respectively, values of QF of materials could be increased effectively (300%- 600%) (Figure 6). Based on the results, it is reasonable to believe that the triboelectric static charge caused by the contact and sliding behaviors of the dissimilar fibers could improve the filtration performance. Despite a slight decrease of pore diameter, filtration properties of PA (or PP)/PTFE material could be modified significantly. In addition, although the irregular PTFE fibers with their high electronegativity could enhance the filtration efficiency of air-purified filters [29, 30], the QF of 100%PTFE materials is much less than those of fiber compounds. These experimental results confirmed that the addition of PTFE fibers could improve the filtration performance of airpurified materials by the formation of triboelectric static charges resulting from the contacting or sliding friction between dissimilar fibers. As for the fact that QF of the cotton/ptfe material is higher than the QF values of the others, although the volume resistance of cotton fibers is lower than many other fibers, cotton fibers have flat or irregular cross sections, which is conducive to the capacity to capture fine particles [29, 31]. With regard to the dependence of QF on time, several different air-purification materials were further analyzed (Figure 6). As time increases, no significant differences in QF were found in samples containing a single type of fibers. For blended materials, the values of QF of needle-punched felts made from PTFE fibers and a second one (PA, PPS, PET or PP) exhibited a slight decrease (approximately 5%- 10%) over 29 days, which could be explained by the outstanding water repellency of polymeric fibers facilitating the storage triboelectric static charges. QF of the nonwoven felts consisting of PTFE fibers and cotton decreased substantially from (24.1%, 19.9Pa) to (43.9%, 16.0Pa) at a ratio of 27.9 percent. This is due to the excellent absorption property of cotton fibers. The relationship between QF and time implies that triboelectric charge remains in the blended polymeric materials and maintains the outstanding filtration performance of blended airpurified materials for a long time [2]. Journal of Engineered Fibers and Fabrics 67

9 FIGURE 6. Filtration properties of PTFE blended filter materials.Ⅰ- QF, Ⅱ- decay of QF (solid sphere-materials with the highest QF, hollow sphere-materials with one component). a-pa/ptfe, b-cotton/ptfe, c-pps/ptfe, d-pet/ptfe, e-pp/ptfe Journal of Engineered Fibers and Fabrics 68

10 FIGURE 7. Morphology of five various needle-punching materials and corresponding distributions of pore diameters. a-60wt%pa/40wt%ptfe, b-60wt%cotton/40wt%ptfe, c-60wt%pps/40wt%ptfe, d-50wt%pet/50wt%ptfe and e-50wt%pp/50wt%ptfe CONCLUSION Triboelectric charge densities of PTFE sintered films sliding against five various nonwoven materials (PA, cotton, PPS, PET and PP) were analyzed in this study. Based on a series of frictional experiments, it was found that the maximum charge density was -32.3μC/cm 2 under optimal sliding parameters. The experimental results proved that adding PTFE fibers with proper proportions by weight could improve the filtration performance of common air-purified materials significantly. Due to the excellent water repellency of PTFE fibers, triboelectric static charge remains for a long time, effectively extending the service life of filters. Journal of Engineered Fibers and Fabrics 69

11 ACKNOWLEDGEMENT This study was finished with help from Shanghai Linflon New Materials Technology Co., Ltd and was funded by the Shanghai Economic and Information Technology Commission (HUCXY ), Chen Guang project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (NO. 14CG34), Fundamental Research Funds for the Central Universities (NO D3-15), and the Doctoral Fund of Ministry of Education of China (CUSF-DH- D ). REFERENCES [1] Greason, W.D. et al, Triboelectric charging between polytetrafluoroethylene and metals, IEEE Transactions on Industry Applications, 40, 2004, [2] Stenhouse, J.I.T., Gradon, L. and Marijnissen, J.C.M., Electret filters, production and properties; proceedings of the international workshop on electret filters, production and properties, Delft University Press, January 29 and [3] Smith, P.A. et al, Generation of triboelectric charge in textile fibre mixtures, and their use as air filters, Journal of electrostatics, 21, 1988, [4] Krucińska, I. et al, Investigation of blended fibre filtering materials, International Journal of Occupational Safety and Ergonomics, 3, 1997, [5] Brown, R.C., "Blended-fibre filter material, U.S. Patent, , [6] Tsai, P.P., Schreuder-Gibson, H., and Gibson, P., Different electrostatic methods for making electret filters, Journal of Electrostatics, 54, 2002, [7] Krucinska, I., The influence of technological parameters on the filtration efficiency of electret needled non-woven fabrics, Journal of electrostatics, 56, 2002, [8] Han, C.B. et al, Removal of Particulate Matter Emissions from a Vehicle Using a Self-Powered Triboelectric Filter, ACS nano, 9, 2015, [9] Yang, J. et al, Triboelectrification-based organic film nanogenerator for acoustic energy harvesting and self-powered active acoustic sensing, ACS nano, 8, 2014, [10] Zhang, X.-S. et al, High-performance triboelectric nanogenerator with enhanced energy density based on single-step fluorocarbon plasma treatment, Nano Energy, 4, 2014, [11] Lee, J.H. et al, Control of Skin Potential by Triboelectrification with Ferroelectric Polymers, Advanced materials, 27, 2015, [12] Wang, Z.L., Triboelectric nanogenerators as new energy technology for self-powered systems and as active mechanical and chemical sensors, ACS nano, 7, 2013, [13] Diaz, A.F. and Felix-Navarro, R.M., A semiquantitative tribo-electric series for polymeric materials: the influence of chemical structure and properties, Journal of Electrostatics, 62, 2004, [14] Wen-Hao, H. et al, Investigation of nonwoven carding process with the application of static electricity to various fibres and process parameters, Fibres and Textiles in Eastern Europe, 15, 2007, 76. [15] Xu, Y., Huang, C. and Jin, X., A comparative study of characteristics of polytetrafluoroethylene fibers manufactured by various processes, Journal of Applied Polymer Science, 133, [16] Soh, S. et al, Contact de-electrification of electrostatically charged polymers, Journal of the American Chemical Society, 134, 2012, [17] Lowell, J. and Rose-Innes, A.C., Contact electrification, Advances in Physics, 29, 1980, [18] Greason, W.D., Triboelectrification of wood with PTFE, Journal of Electrostatics, 71, 2013, [19] Liu, L. et al, Effect of relative rubbing speed on the tribo electrification of continuous filament yarn by stainless steel pins, Journal of the Textile Institute, 102, 2011, [20] Ohara, K., Relationship between frictional electrification and molecular motion of polymers, Journal of Electrostatics, 9, 1980, [21] Ohara, K., Temperature and friction speed dependence of frictional electrification between polymer films. Contribution of molecular motion of polymers to frictional electrification, Journal of Electrostatics, 4, 1978, [22] Greason, W.D., Investigation of a test methodology for triboelectrification, Journal of Electrostatics, 49, 2000, [23] Liu, L., Seyam, A.M. and Oxenham, W., Frictional electrification on polymeric flat surfaces, Journal of Engineered fibers and Fabrics, 8, Journal of Engineered Fibers and Fabrics 70

12 [24] J Jian, L. et al, Triboelectrification electrostatic potential of MC nylon 6 under point contact dry sliding, Tribology letters, 36, 2009, [25] Lee, L.-H., Dual mechanism for metalpolymer contact electrification, Journal of electrostatics, 32, 1994, [26] Hogue, M.D. et al, Two-phase equilibrium model of insulator insulator contact charging with electrostatic potential, Journal of electrostatics, 63, 2005, [27] Chang, Y.-P., Chiou, Y.-C. and Lee, R.-T., Tribo-electrification mechanisms for dissimilar metal pairs in dry severe wear process: Part I. Effect of speed, Wear, 264, 2008, [28] Huang, S.-H. et al, Factors affecting filter penetration and quality factor of particulate respirators, Aerosol and Air Quality Research, 13, 2013, [29] Baumgartner, H., Loffler, F. and Umhauer, H., Deep-bed electret filters: the determination of single fiber charge and collection efficiency, IEEE transactions on electrical insulation, 1986, [30] Lathrache, R., Fissan, H.J. and Neumann, S., Deposition of submicron particles on electrically charged fibers, Journal of Aerosol Science, 17, 1986, [31] Lathrache, R. and Fissan, H., Fractional penetrations for electrostatically charged fibrous filters in the submicron particle size range, Particle & Particle Systems Characterization, 3, 1986, AUTHORS ADDRESSES Xiangyu Jin Shang Zhu Yukang Xu Chen Huang Donghua University Songjiang Campus 2999 North Renmin Road Shanghai, Shanghai CHINA Journal of Engineered Fibers and Fabrics 71