关键词: 静电辅助, 空气过滤, PM2.5, PVDF, 静电喷射, 织物基滤料

Abstract:

With the rapid development of industry, air pollution has gradually become a serious threat to public health. Fine particulate matter, such as PM  2.5 , is the main component of air pollutants and also the main carrier of other types of air pollutants. The efficient filtration technology of indoor fine particulate matter has received increasing attention. The traditional filtration technology mainly includes mechanical filtration and electrostatic assisted filtration. The improvement of mechanical filtration efficiency based on physical interception effect mainly depends on the increase in the thickness and fiber density of filters, but provokes the elevation of filtration resistance and energy consumption. Thereby, it is difficult to achieve high filtration efficiency while maintaining the low pressure drop. The electrostatic assisted filtration system includes electrostatic precipitator and electret filtration system. The removal of particulate matter depends on the electrostatic interaction between the fine particulate matter and filter material, which can greatly reduce the resistance to air. However, the traditional electrostatic precipitator shows low efficiency in capturing fine particulate matter, and the electret filtration system has limited capability to carry sufficient charges for long time serving, and the decay in charges often occurs. Therefore, the development of long term, stable, highly efficient and low resistant filtration systems has long been an obstacle in the field of air filtration. Our team has developed an electrostatic assisted filtration system in the early stage. Using a high voltage power supply, the filtration system maintains stable charge loading during the whole serving life and establishes a strong electrostatic field which is able to charge fine particles and remove them through electrostatic interaction. This system can overcome the above mentioned disadvantages of the traditional electrostatic filtration systems. However, the charging efficiency is still limited. The difficulty in further elevation of the filter charging capacity thwarts the improvement of the filtration efficiency. 

To combine the electrical conductivity and storage capabilities, the current study first constructed a polydopamine binding layer and a silver conductive layer by in situ chemical reaction on the super loose woven polyethylene terephthalate (PET) substrate, and then constructed the micro nano structure polyvinylidene fluoride (PVDF) electrical storage layer by using electrostatic spraying technology. This study successfully prepared a super loose electrostatic loaded filter system by controlling the adhesion amount and structure of PVDF on the substrate through different spraying parameters, while retained the super loose woven structure of the substrate fabric. We systematically characterized the basic properties of the PVDF attached filters, including surface morphology, thickness, mechanical properties, electrical properties and air permeability, and investigated the filtration performance and the particle distribution with different electrostatic voltage supplies. The filtration mechanism was deduced with the simulation of the electric fields. 

The results proved that electrostatic spraying concentration and spraying duration could adjust the loading amount of PVDF and their accumulation morphology. The filters coated with 3 % PVDF for 3 min showed uniform high loading of PVDF without fibrous structure. The PVDF amount exerted limited influence on the thickness, surface specific resistance, breaking strength, air permeability and pressure drop of the filters, but displayed significant effects on the filtration efficiency and quality factor of PM  2.5 . By raising the PVDF amount from 0 to 19.2 μgcm 2, the filtration efficiency could be enhanced from 42.5 % to 67 % at a low voltage (10 kV), and from 67.02 % to 94.29 % at a high voltage (40 kV), while still maintained an ultra low pressure drop of 6.7 Pa. Hence, the quality factor was able to reach 0.43 Pa  -1 . The active electrostatic assisted filtration system loaded with PVDF could not only achieve a high filtration efficiency at an ultra low pressure drop, but also reduce the amount of dust accumulation on the filter surface. The possible reason might be the existence of electrostatic repulsion. 

The design of this system was based on the combination of electrical conductivity and storage. PVDF electrical storage layer was constructed on conductive substrates to incorporate the advantages of the conductive and the electrical storage materials. This design could achieve further enhancement in filtration performance by establishing more stable, stronger electrostatic field with larger coverage area. 

In this work, we further improved the performance of woven filters with active electrostatic charging by loading PVDF. This work might promote the development of ultra low resistance woven filters, and provide inspiration for the future development of the core filter design with low carbon emission. This filter system has the potential to be applied for vehicle air cleaning, air conditioner and central ventilation system. Meanwhile, via the observation of dust accumulation and electric field simulation analysis, the filtration mechanism of the active electrostatic assisted air filtration system was revealed, which provided a theoretical foundation for the further development of filtration systems with high efficiency and low resistance.

Key words: electrostatic assistant, air filtration, PM2.5, PVDF, electrostatic spraying, fabric filter