1 Introduction

Effective capture of aerosols and respiratory comfort is crucial for medical protective materials [1]. However, high filtration efficiency always accompanies with high filtration resistance [2]. It is a challenge for fiber filtration materials to have both high filtration efficiency and low filtration resistance performance.

Micro-sized nanofibers have aerodynamic slip phenomenon, which can effectively intercept the particles, reduce the medium pressure reduction, and significantly enhance the filtration performance of materials [3, 4]. Knudsen (Kn) was used to express the importance of the molecular motion of air molecules on the fiber surface to the whole flow field [5]. The slip effect increased with increase of the Kn [6, 7]. The slip effect of air flow could be improved by reducing the fiber diameter [8,9,10] and increasing the non-uniformity of the reinforced fiber filler [11]. The heterogeneity of the fiber membrane could be effectively improved by collecting the particles-fibers with different diameters into fiber layers to achieve multi-dimensional mixing in the 2D layer [12], 13. Zhao et al. [14] introduced silicon hydroxyl groups on the surface of PAN, and mixed the hydrophilic PAN-SiO2 fiber and hydrophobic PVDF fiber by conjugate electrospinning to obtain PVDF/PAN-SiO2 fiber membrane. The PAN-SiO2 fiber has super hydrophilic behavior, which effectively drives moisture vapor transmission rate (MVTR). Meanwhile, the PVDF hydrophobic fiber can prevent the formation of capillary water and accelerate the transfer of water vapor from high concentration area to low concentration area. Zhang et al. [15] obtained a highly integrated PSU/PAN/PA6 air filter by constructing different polymer systems through sequential electrospinning. The air flow resistance could be effectively reduced and the humidity transmission could be accelerated by physical method, which can decrease the efficiency attenuation caused by electret failure.

However, fiber filtration materials greatly rely on electrostatic interactions to supplement aerosols. The static electricity stored on the material surface through the electret method will decay with the water vapor generated during respiration. It will significantly reduce filtering efficiency and lead to ineffective protection. Therefore, the supplementation and regeneration of static electricity play a positive role in extending the service life of masks [16]. Triboelectric nanogenerators (TENG) are a promising candidate in the field of energy conversion [17, 18]. The mechanical energy generated by the friction of the mask layer during the breathing process can be converted into electrical energy by TENG [19, 20].

Polytetrafluoroethylene (PTFE) has excellent thermal stability, chemical stability, low surface friction, and electrical insulation [21,22,23]. Its excellent physical and chemical properties make it suitable for industrial filtration, triboelectric nanogenerator (TENG) and air purification [24,25,26,27]. But, PTFE is usually decomposed before melting, which makes it unable to be used in common thermoplastic polymer fiber processing technologies, such as melt spinning and extrusion [28]. In addition, the extreme chemical stability of PTFE makes it insoluble in any organic solvent, and it is not suitable for phase conversion processing processes, such as wet or dry spinning [29]. Lotion electrospinning makes it possible to fabricate nanofibers with PTFE which is always dispersed in water. As a commonly used water-soluble electrospun polymer, PVA is an effective carrier of PTFE lotion [30].

In this study, PTFE/PVA-PVDF electrospun nanofiber membrane (HENM) was fabricated by conjugated electrospinning. HENM had a fluffy structure consisting of PTFE/PVA and PVDF fibers with significantly different diameters beneficial for reducing filtration resistance. HENM had excellent triboelectric properties due to the addition of PTFE with super electronegativity. Static electricity was generated by friction between HENM and the PP melt-blown cloth, which improved the filtration performance caused by static electricity dissipation caused by water vapor during the breathing process. It has broad application prospects in extending the service life of high-efficiency filter materials.

2 Experimental

2.1 Materials

PVDF (Solef 6020/1001) was provided by Wuxi United Hengzhou Chemical Co., Ltd. PTFE (Teflon DISP 30) is a milky white aqueous PTFE dispersion stabilized with a non-ionic surfactant provided by Dongguan Yingde Plastic Co., Ltd. The solid content of PTFE emulsion is 60%. PVA (2699) was purchased from Shanghai Kaiyuan Chemical Technology Co., Ltd. DMF and isopropanol (C3H8O) were provided by Shanghai Macklin Biochemical Co., Ltd. Polypropylene melt-blown cloth (25 g/m2) was obtained from Suzhou Doro New Material Technology Co., Ltd. Distilled water was made by the laboratory. All reagents used in this work were analytical reagents (A.R.) and were used as received without any further purification.

2.2 Fabrication

The 8 wt% PVA solution was obtained by dissolving 8 g PVA in 92 g distilled water and heating in a water bath for 4 h at 95 °C. The 60 wt% PTFE emulsion was mixed with 8wt% PVA to obtain PTFE/PVA electrospun solution.

The 15 wt% PVDF electrospun solution was prepared by dissolving 15 g PVDF in 85 g DMF solvent and stirring at 80 °C for 4 h.

The components of hybrid electrospinning nanofiber membranes (HENM) are shown in Table1. The PTFE/PVA and PVDF solutions were independently injected into the double-needle electrospinning equipment. Electrospinning was carried out at 25 kV (applied voltage), 15 cm (working distance) and 1 mL/h (injection rate). The PTFE/PVA fibers and the PVDF fibers were uniformly mixed and collected on a drum wrapped with PP melt-blown cloth and dried in a blast drying oven at 35 °C for 12 h.

Table 1 Component of PTFE/PVA-PVDF electrospun solution
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3 Characterization

3.1 SEM

The fiber membranes were prayed with gold by ion sputtering instrument (JFC-1600, Japan Electronics Corporation). The morphology of fiber membranes was observed by field emission scanning electron microscope (Apreo S LoVac, Thermo Fisher Scientific).

3.2 FTIR

The surface groups of fiber membrane were characterized by Fourier transform infrared spectroscopy (FTIR) (TENSOR, Brooke spectrometers). The fiber membranes were tested by total reflection method, and the scanning wavelength range was 4000 ~ 400 cm−1.

3.3 TG&DSC

The thermal properties of the fiber membrane were tested by synchronous thermal analyzer (STA 449 F5 Jupiter, Netzsch AG). The 10 mg sample was placed in a crucible and increased from room temperature to 800 ℃ at a heating rate of 10 ℃/min in nitrogen atmosphere. The weight loss process, residual weight percentage and heat flux of the fiber during heat treatment were measured.

3.4 Triboelectric Properties

The friction force stroked the fiber membrane material and collects the output electrical signal. PP non-woven fabric (outer layer of mask) was selected as friction material. The test current was at pA level, the sampling frequency was 10000 Hz, and the time interval was 0.0001 s.

3.5 Initial Filtration Performance

The filtration efficiency and filtration resistance of fiber membrane to 0.26 ± 0.04 μm salt particles under 85 L/ ± 2 L/min gas flow rate were tested by using automatic filter material tester (SLGLY-2626, Bei**g Institute of labor protection) according to GB19083-2010 Technical requirements of medical protective masks. Static electricity of HENM was removed by fumigating with the steam of isopropanol and 40% hydrochloric acid.

3.6 Filtration Fatigue of Particulate Matter

According to the standard of GB19083-2010 (Technical requirements for Medical protective masks), the loading test was used for each cycle of 6 min. The requirements for fatigue failure were the filtration efficiency to non-oily particles less than 95% and the filtration resistance higher than 343.2 Pa. The influence of cycle times on particle filtration efficiency, filtration resistance and the service life were analyzed.

3.7 Air Permeability

Fabric permeability reflected the ability of gas molecules pass through the fabric, which directly reflected the wearing comfort of the mask. According to standard of GB/T 545–1997 (determination of air permeability of textiles and fabrics), the automatic fabric air permeability tester YG(b)461E was used to test air permeability. The test area is 20cm2, and the pressure difference was 200 Pa.

3.8 Moisture Permeability

According to standard of GB/T12704.1–2009 (Test method for moisture permeability of textile fabrics-part 1: moisture absorption method), the moisture permeability tester YG(B)216 T was used to test the moisture permeability of the fiber membrane. Anhydrous calcium chloride particles were dried at 160 °C for 3 h. The breathable box temperature was set to 38 °C ± 2 °C, and the humidity was set to 90% ± 2%. The nanofiber membranes were placed on a moisture-permeable cup containing 35 g of anhydrous calcium chloride. The moisture-permeable cup was put into the test box, and it was taken out and weighed per hour. Repeat the above weighing steps until the difference in weight of the saturable cup was stable.

$$WVT = \frac{{(\Delta m{ - }\Delta m^{^{\prime}} )}}{{A \cdot {\text{t}}}}$$

where WVT—Moisture permeability, g/ (m2·24 h); Δm—The weight difference between two experiments of same specimen, /g; Δm′—The weight difference between two experiments of blank specimen, /g; A—Effective test area, /m2; T—The experiment time, /h.

4 Results and Discussion

4.1 Fiber Membranes Morphology

The morphologies of PTFE/PVA-PVDF fiber membranes are shown in Fig. 1. As shown in Fig. 1a, b, it was observed that PVA fibers show smoothness. But the surface of PTFE/PVA fiber is pretty rough, besides the diameter is increased compared with PVA fibers. It is caused by the self-assembly of PTFE nanoparticles on the surface of PVA fibers. In Fig. 1c–f, it was observed that the fibers with two sizes represent PTFE/PVA fiber and PVDF fiber, respectively, which indicating that the multi-scale mixed fiber membrane in the layer was successfully fabricated by conjugate electrospinning. The surface roughness of PTFE/PVA fiber increased with the increase of PTFE/PVA solid ratio.

Fig. 1
figure 1

SEM images of fiber membranes. a PVA fiber membrane; b PTFE/PVA fiber membrane; c HENM-2; d HENM-3; e HENM-4; f HENM-5

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4.2 Chemical Structure Analysis of Fiber Membranes

The infrared spectrum of the PTFE/PVA-PVDF fiber membranes is shown in Fig. 2. Strong absorption peaks in the broadband of 3599–3060 cm−1 were associated with O–H bond stretching in PVA [24]. However, 2918–1, 2853–1, and 1464 cm−1 correspond to CH2 asymmetric vibration, symmetrical vibration, and deformed torsion in PVDF and PVA, respectively. Among them, the intensity of the HENM peak is higher at 2918 cm−1 due to the superposition of symmetric C-H vibrations in PVA here [21]. 1275 and 1175 cm−1 correspond to the asymmetric stretching vibrations and symmetric stretching vibrations of CF2 in PVDF and PTFE, respectively [25, 30]. 971 cm−1 is attributed to the asymmetric stretching vibrations of the C-O bond in PVA [21]. The presence of characteristic peaks of O–H, C–O, and CF2 indicates that the HENM fiber membrane contains both PVA, PTFE and PVDF.

Fig. 2
figure 2

FTIR spectra of fiber membranes

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4.3 Thermal Analysis of Fiber Membrane

There are three major weight drops in TG curve, which relating to PVA decomposition (at 281–388 ℃), PVDF decomposition (at 310–550 ℃) and PTFE decomposition (at 515–610 ℃) [31, 32]. The final residue content increased from 30.6% to 35.1% with increase of the solid content ratio between PTFE and PVA. It is speculated that the residue is composed of vinylidene fluoride monomer, carbon [33] in PVA and tetrafluoroethylene monomer[34]. The thermal effect enthalpy change of DSC reaction materials in the heating process shows the process of thermal deformation, melting and thermal decomposition in the polymer. PVDF melting occurs at 173 ℃ and PTFE melting occurs at 340 ℃ [35], and the curve corresponds to TG. The results show that the material contains PVA, PTFE and PVDF, and its thermal stability can meet the temperature requirements of medical protective human body (Fig. 3).

Fig. 3
figure 3

TG and DSC of fiber membranes

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4.4 Triboelectric Properties

The working style of triboelectric nanogenerators is shown in Fig. 4a. The open-circuit voltage (Voc), short-circuit current (Isc), and transferred charge (Qsc) was about 58 V, 789nA and 23 nC as shown in Fig. 4b–d. The charge generated by friction between PTFE/PVA-PVDF nanofiber membrane and PP meltblown cloth could compensate for the static electricity loss during the breathing process. It will provide a positive effect on maintaining the high filtration of the material.

Fig. 4
figure 4

Triboelectric properties of PTFE/PVA-PVDF

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4.5 The Initial Filtration Analysis

The initial filtration efficiency of the PTFE/PVA-PVDF fiber membrane is shown in Fig. 5. The filtration performance of the fiber membrane is firstly decreased and then increased with increase of PTFE content. The filtration efficiency of PTFE/PVA-PVDF fiber membrane eliminated charge with isopropanol is only 43.92–66.15%, while the filtration efficiency of pristine fiber membrane is as high as 95.11–99.68%. The reason is that the voltage applied by electrospinning is not only used for fiber drafting but also helps to generate a large amount of positive charge and is in situ injected into the whole system of multi-component solution during electrospinning. At the same time, PVDF and PTFE are rich in C-F bonds. Due to the strong electronegativity of fluorine atoms, they can produce rich polarized dipoles under the strong action of high voltage and form volume charge with the solidification of nanofibers. Moreover, due to the different electron transport capabilities of PVDF and PTFE, the charge at the interface will accumulate into interfacial polarized charge in situ [36]. The addition of PTFE organic electret can greatly improve the electrification capacity of the fiber, enhance the electrostatic capture effect of the fiber in the particle filtration, and improve the filtration efficiency. However, the filtration resistance of the fiber membrane was not affected before and after the charge was eliminated.

Fig. 5
figure 5

Initial filtration efficiency of PTFE/PVA-PVDF fiber membrane

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4.6 Filtration Fatigue Analysis

After 24 cycles of loading and filtration, the particle filtration efficiency and filtration resistance fatigue of the PTFE/PVA-PVDF fiber membrane are shown in Fig. 6. The unelectrified PTFE/PVA-PVDF fiber membrane is filtered 24 times, once every 6 min, and contains about 120 mg of salt particles in total. The filtration efficiency is always higher than 95% and gradually approaches 100%. The filtration resistance is maintained at 90-140 Pa, although slightly increased, the increase is less than 10 Pa. The results show that the fiber membrane not only has a higher specific surface area and dust capacity, but also the filtration may verify from fiber deep filtration to fiber surface filter cake filtration, and the accumulation of particles will result in increase in resistance.

Fig. 6
figure 6

Effect of cycle loading times on fiber membrane filtration efficiency and filtration resistance

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The initial filtration efficiency of the electrified PTFE/PVA-PVDF fiber membrane was varied from 40 to 70%. However, after 4 times of filtration, the filtration efficiency increased sharply to more than 95% and gradually approaches 100%. This was attributed to the PTFE is an excellent triboelectric material. In the filtration process, friction collision occurred between particles and fiber, resulting in an electrostatic charge on the fiber surface. In the first four cycles of filtration, since the filtration resistance did not change significantly, the substantial improvement in filtration efficiency was mainly attributed to the triboelectric action rather than the filter cake effect. In addition, the filtration resistance of the fiber membrane was basically the same before and after electricity elimination.

The results show that the filtration efficiency of the PTFE/PVA-PVDF fiber membrane gradually increases to more than 95%, and the resistance is always maintained at a lower level under the action of electrostatic filtration in the early stage and filter cake filtration in the later stage, which meets the protection requirements. Its service life can ensure continuous cycle filtration 24 times in 144 min under 85L/min wind speed, and filter 120 mg salt particles with high efficiency and low resistance.

4.7 Air Permeability Analysis

The air permeability of the PTFE/PVA-PVDF fiber membrane is verified from 262.88 to 370.70 mm/s, as shown in Fig. 7, which is higher than that of the PVDF fiber membrane, indicating the improvement of the fluffy degree of the fiber membrane. The variation trend of air permeability of fiber membrane is opposite to that of filtration resistance (Fig. 8).

Fig. 7
figure 7

Air permeability of PTFE/PVA-PVDF fiber membrane

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Fig. 8
figure 8

Moisture permeability of PTFE/PVA-PVDF fiber membrane

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4.8 Moisture Permeability Analysis

The moisture permeability of PTFE/PVA-PVDF fiber membrane is as high as 7721–8471 g/ (m2·24 h), which is nearly consistent with the variation of air permeability and related to the variation of pore structure. In addition, since PVA contains a large number of strong hydrophilic group hydroxyl groups, it is easy to capture water molecules, and the fiber membrane has a fluffy and porous structure, which is conducive to the diffusion of water molecules, resulting in about 30% higher moisture permeability of HENM than that of PVDF fiber membrane (6000 g/m2·24 h). Although PVA contains hydrophobic group acetate, its steric hindrance hinders the formation of hydrogen bonds between macromolecules or macromolecules themselves and promotes water solubility. However, the 2699 type PVA is used in this paper, the degree of polymerization is as high as 2600, the degree of alcoholysis is 99%, the content of residual acetate is only less than 0.2%, and the hydrogen bonds in macromolecules hinder water dissolution so that it can only be dissolved in water at 95 ℃. Therefore, the PTFE/PVA-PVDF fiber membrane will not dissolve at the experimental temperature of 38 ± 2 ℃, which meets the normal temperature conditions of medical protection and will not be destroyed.

5 Conclusion

The charge decay of an electret mask was detrimental to long-lasting protection against harmful airborne particles. Here, an efficient and durable air filter that can continuously replenish electrostatic charges in a self-charging manner has been developed. In this paper, hybrid electrospinning nanofiber membrane (HENM) was prepared through mixing PTFE/PVA nanofibers with PVDF nanofibers by conjugate electrospinning to improve the filtering performance and breathing comfortability of the mask. Utilizing electrostatic filtration enhanced by collision friction generation, PTFE/PVA-PVDF nanofiber membranes demonstrated filtration efficiencies approaching 100%, it remained stable even in multiple cycle tests, far better than conventional mechanical filtration. The structure with multi-scale diameter and high porosity effectively improves the air permeability and moisture permeability of PTFE/PVA-PVDF fiber membrane. The air permeability was 262.88–370.70 mm/s, and the moisture permeability was as high as 7721-8471 g/ (m2·24 h). This work provided a promising strategy for efficient and durable PM capture that benefited from its prolonged electrostatic adsorption efficacy.