In recent years, with the development of industrialization and urbanization, solid pollutants which are dominated by fine particles floating in the air or water has become the serious environmental problem and are considered a great threat to global health [1–3]. And there has much research on the preparation of various air filtration materials through melt-blown, electrospinning and other technologies. Membrane technology has become an efficient and needful technology for air filtration. And the commercial filtration fibrous membrane is usually produced via melt-blown process with the capability of scalable production [4], and the commonly raw materials are polypropylene, polystyrene [5], polycarbonate [6], or polyethylene. For example, PP materials have been widely used in masks, air purifiers, protective clothing, and hygienic products and multiple layers of PP membrane are commonly used in the medical masks. And an ideal air filtration membrane should meet several requirements of high filtration efficient, low airflow resistance (pressure drop) and antibacterial characteristic [7]. However, traditionally fibrous filters show indisposed filtration efficiency for fine particles and cannot balance the contradiction between high filtration efficiency and low pressure drop [8]. Besides, the improper pore size and messy fiber diameter may also cause the indisposed filtration performance. In many cases, the filtration efficiency of the membrane is improved by using a relatively thick filter layer, which may increase the pressure drop. Therefore, the filter membrane should have high performance in order to achieve a high filtration efficiency without increasing the pressure drop. The membrane filters fine particle mainly through the diffusion, interception, inertial impaction, gravitational settling, and electrostatic attraction [9, 10]. And electret air filtration membrane shown a high filtering efficiency compared to normal membrane, due to the enhanced of the coulombic interaction between the fibers and the particulate matter [11, 12]. Zhang et al. prepared needle-punched electret air filters with high filtration efficiency (99.2%) and a unique 3D fluffy structure created by needle-punching technique providing high dust holding capacity (23.6 g/m2) and low filtration resistance (13.93 Pa) [13]. In addition, the filtration efficiency of the membrane also was improved by in-situ growth [14] or the loaded of the functional particle. For example, Cheng et al. constructed ultra-stable polypropylene melt-blown membrane with efficient and stable filtration of PM2.5 particles via in-situ growth of nano-metal-organic frameworks on the PP melt-blown membrane [15]. Wang and his co-worker prepared a uniform hierarchical porous structure hybrid membrane by dip coating chitosan/polyvinyl alcohol on non-woven fabric and then surface deposition of halloysite nanotubes for highly air filtration [16]. However, the above methods to improve filtration efficiency still face various problems in terms of mass production and durability, such as poor stability of functional particles on the membrane, the failure of electret and so on.
The multifunction of nanotechnology can provide help to overcome the problems of low filtration efficiency faced by air filtration membranes. Electrospinning nanofibrous membrane has many advantages, such as a large surface area, an adjustable porous structure, and a simple preparation process that make them an ideal candidate for filtration materials [17–19]. And electrospinning nanofibers have attracted increased attention in air filtration applications [20, 21]. Polyvinylidene fluoride (PVDF) is a fluoro-polymer with excellent hydrophobicity, which is widely used in filtration, biology, medical and other fields. Many studies have used electrospinning technology to prepare PVDF membrane for air filtration. Polyurethane (PU) has the advantages of high strength, high elasticity, wear resistance and the electrospinning PU membrane with outstanding breathability [22]. In addition, PP melt-blown nonwovens are widely used as filter materials because of their high surface area, high filtration efficiency, and low pressure drop [23].
In this paper, a novel polyurethane (PU)/polyvinylidene fluoride (PVDF)@ polypropylene (PP) multi-scale membranes were fabricated by a lamination process with outstanding water vapor transmission and excellent air filtration efficiency and pressure drop were fabricated to fully take advantage of electrospinning nanofibrous membranes and melt-blown membrane in air filtration applications, the multi-scale membranes were developed to improve and optimize their air filtration performances. And the multi-scale membranes not only have all the advantages of single-scaled membranes, but also have more flexibility in the choice of functional layers.