1. Introduction

Upon decades, the air pollution from the industrial development has widely been accepted as one of the greatest challenge that the humanity should untangle as soon as possible. Emissions of contaminated dust from industrial plants pose a great threaten to the survival of both human and ecosystem on the Earth. Indeed, nano-sized dust particles are floating on the atmosphere with numerous pollutants attached to the surface due to its high specific surface area[1-3].

The problem of air pollution on Earth has been a crucial issue that humanity must address urgently. The emission of particulate matter from various industrial operations poses a significant threat to both human health and the ecosystem[4,5]. These particles, due to their large specific surface area, tend to accumulate various pollutants while suspended in the Earth's atmosphere, leading to numerous environmental challenges[6-8]. One effective approach to mitigate air pollution is the utilization of energy-efficient air filters for capturing airborne pollutants. However, the high-temperature discharge of hot air from most industrial facilities necessitates dust filters with exceptional heat resistance, making it challenging to employ polymer- based adhesive air filters[9,10].

Air filters play a critical role in various applications, with differing material standards and performance requirements. The selection of air filters depends on the size and nature of the targeted dust particles, resulting in classifications such as bag filters, electric dust collection filters, and ceramic filters[11-14]. Extensive research has been conducted to enhance the performance of these air filters. In a study by Rogozinski et al., a non-woven fabric-based dust collection filter bag was employed at a wood-based furniture factory to filter out particles larger than tens of nanometers[15]. The study demonstrated remarkable performance, achieving close to 100% efficiency for particles measuring 10 μm or larger. Additionally, the filter exhibited exceptional stability, maintaining its efficiency for a duration of 272 days.

Further advancements in the performance of dust collection filters were investigated by Park et al. and they focused on studying the characteristics of enhancing filter performance through dust precharging[16]. These research endeavors contribute to the ongoing efforts in improving the efficacy and efficiency of air filters, addressing the diverse challenges posed by airborne pollutants in various industrial and environmental settings.

Air filters are available in various types of materials, including polyethylene, polyester, ceramic, polyimide, and PTFE[17-20]. Liu et al. conducted an air-spinning method on windows coated with metal, utilizing PAN, PVP, PS, PVA, and PP, to achieve high-performance adsorption of particulate matter (PM_x, particle size < x μm)[21]. Their study presented an analysis of filter performance based on the coating. Remarkable performance was observed in PM_2.5 and PM_10-2.5 dust separation, with each material demonstrating unique characteristics. Notably, PAN exhibited performance close to 100% efficiency. Chen et al. reported on the characteristics of filter performance, including filter running time, resistivity, and dust particle size, for a variety of filter materials[22]. While numerous studies have explored the performance, operational conditions, and processes of dust collection filters, there remains a limited amount of research on highly heat-resistant dust collection filters suitable for operation in high-temperature conditions.

In this study, we present a novel method for manufacturing a high-temperature dust collection filter by bonding thin e-PTFE to the surface using polysulfone and PPS fabric. Our aim is to develop a filter with exceptional durability, even under operating conditions of approximately 180 to 200 degrees Celsius. The proposed filter material exhibits excellent heat resistance and can be thermally compressed at high temperatures. Consequently, we determined the optimal conditions to address challenges related to impregnation rate reduction, adhesion, and air permeability of PPS fabric based on the amount of polysulfone added. By subjecting the PPS fabric and PTFE to compression at high temperatures, we achieved a strong bond between the two materials. Notably, the resulting filter can operate seamlessly under conditions of approximately 200°C or higher, making it highly suitable for use as a dust collection filter in industries that emit high-temperature particulate matter.

2. Materials and Methods

2.1. Materials

Polysulfone (PSf, M_n ~22,000) was purchased from Sigma-Aldrich. Extended polytetrafluoroethylene (ePTFE) and polyphenylene sulfide fabric (PPS fabric) was supplied by Hyupjin Chemical and tetrahydrofuran (THF, 99.9%) was purchased from Daejung Chemicals. All the chemicals were used directly without any further purification process.

2.2. Preparation of PPS/ePTFE dust filter membrane using the adhesive PSf

To prepare the PSf solution, different weight percentages (5, 7.5, and 10 wt%) were obtained by dissolving the appropriate amount of PSf in THF. The stirring process lasted for 3 hours until the solution achieved a homogeneous consistency. Subsequently, the PSf solution was poured into a glass petri dish, and a 5 × 5 cm² piece of PPS fabric was dip-coated in the solution for varying dipping times of 5 to 30 min. Following the coating step, the fabric was dried to remove any remaining solvent residue. This was achieved by hanging the fabric in a conventional oven at 50°C for 12 hours. Next, the PSf-coated PPS fabric was carefully placed on top of the ePTFE membrane, ensuring the membrane was sandwiched between two aluminum foils to prevent excessive PSf solution leakage. To create the final PPS/ePTFE dust filter membrane, the sandwiched films were pressed at 180°C for 10 minutes under a pressure of 10 MPa. Finally, the aluminum foil was removed from the resulting membrane, completing the preparation process.

2.3. Characterization

In the characterization process, the PPS filter was affixed onto the ePTFE membrane using the Hydraulic Lab Press (HLP-C12H, Hantech, South Korea). Chemical bonds were identified using Fourier transform infrared spectroscopy (FT-IR, IRAffinity-1S, Shimadzu, Japan). The morphology of the filter membranes was examined using field emission scanning electron microscopy (FE-SEM, MAIA 3 LM, Tescan, Czech Republic) with an accelerated voltage of 10 kV. The thermal degradation behavior of the filter membranes was analyzed by thermogravimetric analysis (TGA, TGA Q500, TA instruments, USA) under N₂ atmosphere at a scan rate of 20°C min^-1, up to 800°C. The thermal behavior of the samples was studied using differential scanning calorimetry (DSC, DSC Q20, TA instruments, USA) from 0 to 200°C at a scan rate of 20°C min^-1, with the second heating data used for the plot. Peel resistance of adhesion tests were performed using a universal testing machine (UTM, WL2100, Withlab, South Korea) with a 20 N weighed load cell, at a rate of 254 mm min^-1, and the test was performed at Korea Polymer Testing & Research Institute (KOPTRI).

3. Results and Discussion

Scheme 1 describes the synthesis procedure of the PSf@PPS/ePTFE dust filter membrane using the dip-coating and hot pressing methods. Fig. 1a presents the chemical structures of PPS fabric, ePTFE film, and the thermally stable PSf adhesive. Despite the high electronegativity of the F atom, the C-F bond in PTFE remains highly stable, limiting its interaction with other molecules. However, due to the chemical similarity be-tween the PPS and PSf backbones, they exhibit a strong interaction during the dipping process. The ePTFE support exhibited characteristic peaks of the C-F bond at 1213 and 1153 cm^-1, while the C-O and S=O stretching vibrations appeared at 1242 and 1149 cm^-1 for PSf, respectively, as shown in Fig. 1b[23-25]. Furthermore, the characteristic C-S bond at 1091 cm^-1 of the PPS fabric was clearly observed[26]. Following the impregnation of the PPS fabric with PSf polymeric adhesive using 5%, 7.5%, and 10% wt. PSf dipping solutions, evident C-O and S=O stretching vibrations were observed (Fig. 1c). Remarkably, with increasing PSf dipping solution concentration (x value), the relative peak intensity of the PSf polymer generally increased, indicating the successful dip-coating of PPS fabric with the PSf polymeric adhesive. Therefore, the PSf solution concentration is a crucial factor in the dip-coating process. After hot-pressing at 180°C, all characteristic peaks of PPS, PSf, and ePTFE were clearly observed, confirming the successful impregnation and pressing ability of PSf adhesive[27].

PSf loading through dip-coating was investigated using FE-SEM, with two key parameters controlled: i) dipping time and ii) concentration of the PSf solution[28]. The untreated PPS fabric (Fig. 2a and b) exhibited a uniform thickness of approximately 15 μm, with individual PPS fibers showing no apparent interconnection, maintaining their distinct physical properties due to their twisted arrangement. After dip-coating for 5 minutes using a 5 wt% PSf solution, the thickness of the PPS fibers increased by approximately 0.5 μm (Fig. 2c and d). This suggests that the polysulfone solution effectively wetted the fibers, and the polymer remained in a solid form after drying. Subsequently, after dip-coating for 30 minutes with the same solution, the thickness significantly increased to 17 μm, and small chunks of PPS polymer were observed. However, the interstitial voids within the fabric were preserved, and there was only minimal blocking. The increased coating time led to better coverage of the fabric fiber surfaces, resulting in significantly enhanced fiber interconnection[29]. For attaching the PPS fabric to the ePTFE support, it was observed that a 5-minute dipcoating time did not provide sufficient PSf impregnation, which could compromise adhesion. Consequently, to ensure a secure adhesion, the coating time was fixed at 30 minutes for all subsequent processes. As depicted in Fig. 3, visual observation did not reveal apparent distinctions among the PSf@PPS-x samples as the dipping time ranged from 5 to 30 minutes. Nevertheless, upon manual bending, it became evident that the coated fabric exhibited greater rigidity compared to the pristine PPS. Notably, the PSf@ PPS-10 fabric exhibited the highest hardness and displayed limited flexibility when bent[30].

Fig. 4 displays FE-SEM images of PSf@PPS/ ePTFE-x samples with a dipping time of 30 minutes, which were created by hot-pressing the PSf@PPS fabric onto the ePTFE surface. The FE-SEM image of neat PPS/ePTFE could not be obtained due to the absence of the polymeric adhesive, resulting in film detachment. As shown in Fig. 4a and b, even with the presence of PTFE, the fiber structure was still observable on the surface, and there was an evident increase in connectivity between the fibers compared to the situation before PTFE attachment[31]. This indicates that PSf acts as an adhesive, effectively connecting the film and the fabric. When the PPS fabric was dipped into a 7.5 wt% PSf solution, the fiber visibility on the PTFE surface decreased compared to the 5 wt% PSf solution, suggesting better attachment of PTFE to the PSf/PPS fabric (Fig. 4c and d). However, in the cross-sectional image, there was no significant difference between PSf@PPS/ePTFE-7.5 and PSf@PPS/ ePTFE-5.

For PSf@PPS/ePTFE-10, PTFE exhibited complete attachment to the PSf/PPS fabric, and the fiber visibility was minimal due to the pore filling of the PPS fabric with the PSf adhesive(Fig. 4e and f). The excess of PSf in the PPS fabric induces the clumping lumps of the membrane. This implies that the adhesion performance improves with the higher content of PSf. However, in the cross-sections, classification was unclear due to the overflowing PSf adhesive after hot-pressing at 180°C. Additional low magnification FE-SEM images in Fig. 5 reveal that PSf@PPS/ ePTFE-5 displayed the most porous structure compared to PSf@PPS/ePTFE-7.5 and PSf@PPS/ePTFE-10, suggesting that PSf@PPS/ePTFE-5 is the most suitable option as a dust filter membrane. The cross-sectional FE-SEM images at low magnification confirm that only the PSf@PPS/ePTFE-5 filter membrane maintains the porous fabric morphology. As illustrated in Fig. 6, the dust filter membranes exhibited a closely adhered ePTFE support, and notably, the cross-section of the PSf@PPS/ePTFE-5 dust filter membrane displayed excellent cohesion without any notable defects.

Fig. 7 presents the TGA and DSC curves of the samples, elucidating the thermal behavior and stability of the dust filter under demanding operational conditions. Remarkably, even at 400°C, neither the PSf, PPS fabric, nor the ePTFE membrane displayed any signs of thermal degradation, suggesting exceptional stability for the PSf@PPS/ePTFE-x composite membrane under high-temperature conditions, as depicted in Fig. 7a (approximately up to 160°C). Notably, the PSf@PPS demonstrated delayed thermal degradation compared to pristine PPS, providing effective prevention of PPS fabric melting at elevated temperatures. In Fig. 7c, the PPS fabric exhibited a melting temperature (T_m) at 21°C, attributed to its rubbery polymeric coating, which, unfortunately, remains undisclosed due to company confidentiality. However, the observation of the glass transition temperature (T_g) at 184°C for PSf confirmed the successful coating on the PPS fabric[32]. It is noteworthy that the glassy PSf not only contributes to the thermal stability of the PPS fabric by forming a blend but also functions as a polymeric adhesive during the hot press procedure under high-temperature conditions. This combination of properties further enhances the overall stability and performance of the PSf@PPS/ePTFE-x dust filter membrane[33].

The peel resistance of the PSf@PPS/ePTFE-x dust filter membrane was assessed following the guidelines outlined in the ASTM D1876 standard test method for peel resistance of adhesives. The test results, presented in Table 1, indicate that the PSf@PPS/ePTFE-10 sample achieved the highest maximum load compared to the other samples. However, for the purpose of dust filtration, the peel resistance requirements are not as demanding. The dust filter membrane can effectively serve as a dust collecting filter by simply adhering well to the membrane with a weak force, maintaining its performance even under tough, high-temperature conditions without compromising air permeability.

4. Conclusion

In conclusion, this study successfully developed a PSf@PPS/ePTFE composite dust filter membrane with excellent thermal stability and adhesion properties for high-temperature conditions. The FT-IR analysis confirmed the successful impregnation of PSf adhesive onto the PPS fabric and its interaction with the ePTFE support. FE-SEM images revealed that increasing the PSf dipping solution concentration resulted in better fiber interconnection and enhanced adhesion between the PPS fabric and ePTFE surface. Among the PSf@ PPS/ePTFE-x samples, PSf@PPS/ePTFE-5 exhibited the most suitable porous structure for a dust filter membrane application. Thermal analysis demonstrated the exceptional stability of the PSf@PPS/ ePTFE-x composite membrane even at elevated temperatures. The delayed thermal degradation of PSf effectively prevented the melting of PPS fabric, maintaining the filter's integrity. Peel resistance tests indicated that, despite achieving the highest maximum load, the PSf@ PPS/ePTFE-10 sample's superior peel resistance may not be necessary for efficient dust filtration. Overall, the PSf@PPS/ePTFE composite dust filter membrane offers promising potential for various industrial applications that require reliable filtration capabilities under harsh operating conditions. The successful synthesis and comprehensive characterization of this composite membrane contribute to the advancement of high-performance filtration materials for demanding environments. Further studies and optimizations can be pursued to explore its practical applications and potential scalability in industrial settings.