Transparent Polyurethane Nanofiber Air Filter for High-Efficiency PM2.5 Capture
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Fine particulate matter (PM) has seriously affected human life, such as affecting human health, climate, and ecological environment. Recently, many researchers use electrospinning to prepare nanofiber air filters for effective removal of fine particle matter. However, electrospinning of the polymer fibers onto the window screen uniformly is only achieved in the laboratory, and the realization of industrialization is still very challenging. Here, we report an electrospinning method using a rotating bead spinneret for large-scale electrospinning of thermoplastic polyurethane (TPU) onto conductive mesh with high productivity of 1000 m2/day. By changing the concentration of TPU in the polymer solution, PM2.5 removal efficiency of nanofiber-based air filter can be up to 99.654% with good optical transparency of 60%, and the contact angle and the ventilation rate of the nanofiber-based air filter is 128.5° and 3480 mm/s, respectively. After 10 times of filtration, the removal efficiency is only reduced by 1.6%. This transparent air filter based on TPU nanofibers has excellent filtration efficiency and ventilation rate, which can effectively ensure indoor air quality of the residential buildings.
KeywordsElectrospinning PM2.5 Nanofibers Air filter Removal efficiency
Fine particulate matter (PM) is composed of various solid fine particles and droplets with up to hundreds of chemical components. PM is mainly composed of three major chemical substances, including water-soluble ions, carbon-containing compounds, and other inorganic compounds [1, 2, 3, 4, 5]. PM is mainly from the burning of fossil fuels and garbages, and it is rich in toxic substances and harmful particulate matter [1, 3, 4, 5, 6]. According to the size of the particle diameter, PM is mainly divided into PM2.5 and PM10, which means that the aerodynamic diameter of the particles is less than 2.5 μm and 10 μm. PM10 stays in the air from a few minutes to a few hours with a limited travel distance; however, PM2.5 has a long residence time in the atmosphere and can last from several days to several weeks [2, 5]. Even if PM2.5 falls to the ground, it is easy to be blown back into the air by the wind. Through the process of breathing, PM2.5 can enter the body and accumulate in the trachea or the lung, which will negatively affect the human health [7, 8, 9]. PM2.5 also has a major impact on the climate and the ecological environment, such as affecting the rainfall process [10, 11, 12, 13, 14]. In the past 10 years, PM2.5 air pollution is becoming more and more serious, especially in some developing countries such as China and India [4, 15]. In daily life, people at those countries often encounter severe haze weather. For this reason, it is very necessary to take some protection against PM2.5.
At present, the protection measures to the severe haze are mainly focused on the outdoor personal protection, such as wearing professional dust masks, which can effectively filter the particle matter [16, 17]. The indoor personal protection, such as ventilation systems and air purifier are expensive, complicated to install and requiring replacement for the filter elements . The indoor air filters generally provide air protection for commercial building, due to the high cost of pumping systems for active air exchange. Recently, there are two transparent air filters for residential buildings by windows passive ventilation come into the vision of consumer . One is porous membrane filter, but the porosity of this filter is very low, which means high ventilation cannot be achieved. Another one is nanofiber air filter, which porosity can reach 70% and can achieve high ventilation. Some laboratories have prepared a variety of window screens to protect the quality of indoor air with nanofiber. For instance, Chen et al.  reported an air filter prepared using electrospun TPU polymer; TPU nanofiber air filter is very effective for removing PM2.5 (98.92%) with very low-pressure drop (10 Pa). Khalid et al.  reported a nanofiber window screen made by direct blowing technology, which has good optical transparency (80%) and high PM2.5 filtration efficiency (99%). Liu et al.  prepared a transparent air filter by electrospinning, which achieved high ventilation and high PM2.5 filtration efficiency (> 95.0%). However, this research was developed in laboratories and the research of the industrial process of nanofiber filter is little.
In recent years, electrospinning technology has received extensive attention due to its low energy consumption, simple operation, and environmentally friendly methods for preparing nanofibers [20, 21]. Nanofiber membranes prepared by electrospinning has high porosity, micro-nano channel interconnects, and high specific surface area [22, 23, 24, 25, 26, 27, 28, 29]. Recently, our team developed a TPU nanofiber air filter that can be mass-produced using a spinning bead spinneret [30, 31]. This air filter has very high thermal stability, good optical transparency of 60%, high PM2.5 removal efficiency of 99.654%, long lifetime, low airflow resistance (ventilation rate 3348 mm/s), and light weight.
Materials and Instruments
Polymer TPU was obtained from Bayer Co., Ltd., Germany, with tear resistance, abrasion resistance, and UV protection; the substrate conductive mesh is provided by Qingdao Junada Technology Co., Ltd., China. The N,N-dimethylfomamide (DMF) and acetone were provided by the Tianjin Zhonghe Shengtai Chemical Co., Ltd. Scanning electron microscopy (SEM Feiner High Resolution Professional Edition Phenom Pro) is used to study the morphology of TPU fibers. An automatic filtration performance tester for evaluating filtration performance FX3300 Lab Air-IV was purchased from Shanghai Lippo Co., Ltd., China. AFC-131 is used to test ventilation rate purchased from Shanghai Huifen Electronic Technology Co., Ltd. Thermo Scientific Nicolet iS5 is used to measure infrared and analyze the functional groups of TPU fiber membranes. Theta optical contact angle meter was used to analyze the contact angle of TPU fiber film. The light transmittance was evaluated using a UV1901PC ultraviolet spectrophotometer and purchased from Shanghai Aoxiang Scientific Instrument Co., Ltd., China.
Preparation of Nanofibrous Membranes
Results and Discussion
Characterization of Morphology and Structures
Fourier Transform Infrared Spectrum Analysis
Filtration Efficiency Analysis
Ventilation Rate Analysis
Here, α is the nanofiber packing density, W is the basis weight of the nanofiber membrane, ρf is the density of nanomaterial, and Z is the nanofiber film thickness. The ventilation rate begins to decline is primarily owing to the addition of TPU nanofiber average diameters (Fig. 5b, c). As the concentration of TPU increases from 8 to 14 wt%, decreasing in the packing density of nanofibers leads to an increase in the distance between the nanofibers, which is beneficial to ventilation rate, even though the diameter of the nanofibers is increased (Fig. 5d). When the nanofiber membrane is made of a solution with a TPU concentration of 14 to 16 wt%, nanofiber diameter plays a crucial role in ventilation rate, and the associated ventilation rate drops slightly (Fig. 5e). When the TPU concentration increases to 10 wt%, the ventilation rate is up to 3480 mm/s, such a high ventilation rate is equivalent to a blank screen without a nanofiber membrane.
Contact Angle Analysis
Here, r is the surface roughness factor, which is the proportion of the actual area of the surface to the geometric projected area ( r ≥ 1), θ′ is the contact angle of the rough surface. As shown in Fig. 6h–i, with the TPU concentration increases, the diameter of the TPU nanofiber increases, and increased roughness of the surface of the nanofiber membrane, resulting in an increasingly low contact angle.
Transparency and Reproducibility Testing
In summary, we use a rotating bead spinneret for electrospinning to create a transparent air filter that can be produced in a large scale. By changing the concentration of TPU polymer in solution, not only significant PM2.5 removal efficiency (99.654%) is achieved, but also good optical transparency (60%) and ventilation rate (3480 mm/s) are achieved. In addition, by performing 10 cycles of filtration and gas venting tests on the TPU transparent air filter, the results showed that the filtration efficiency was only reduced by 1.6%, and the ventilation rate was changed very slowly and remained substantially unchanged. These results indicate that TPU nanofiber membranes prepared by electrospinning have many advantages such as good water repellency, good optical transparency, high ventilation rate, and high filtration performance, which can be used as filter materials in a lot of fields.
WL conducted the experiments and drew up the manuscript. YX and XL synthesized the nanomaterial and provided methodology to decipher the characteristics. XXW, HDZ, and MY played a role in review and editing. RS and YZL conceived and designed the research. All authors discussed the results and made comments on the manuscript. All authors read and approved the final manuscript.
This work was supported by the National Natural Science Foundation of China (51703102), the China Postdoctoral Science Foundation (2016 M592128), the Natural Science Foundation of Shandong Province (ZR2016AM22), the Shandong Medical and Health Technology Development Plan Project (2017WS033), the Postdoctoral Scientific Research Foundation of Qingdao (2017009), and the National Natural Science Foundation of 392 China (51703102 and 51973100).
The authors declare that they have no competing interests.
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