1 Introduction

Environmental pollution is becoming one of the most serious problems facing mankind today. Air pollution control has aroused people’s fervent concern owing to the worsening smog problem [1, 2]. Conventional fibrous filtration materials including textiles, melt-blown nonwovens, needle-punched nonwovens and spun-bond nonwovens cannot meet the requirements of high filtration efficiency (FE) and energy saving in filtration industry [3,4,5]. Polypropylene (PP) melt-blown electret material has provided a new innovation method of high FE combining with low pressure drop (ΔP) due to abundant charges on the surface of fibers [6,7,8]. However, the charges decay with time in service or storage conditions, resulting in the reduction of FE [9].

Electrospun nanofiber membranes possess many advantages such as interconnected open pores, small pore size, high porosity and large surface-to-volume ratio, which make them promising candidates for air filtration. Al-Attabi [10], Wang [11] and Zhang [12] found that nanofiber membranes have superior FE owing to the nano-scale fiber diameter and small pore size, but they also exhibit high airflow resistance due to the compact structure [13].

In order to decrease airflow resistance while keep the high FE, an ideal filtration material is elaborately designed with a kind of gradient structure which provides an effective surface area inside filtration materials [14]. On the other hand, the structure facilitates the pressure gradient, improves FE and reduces the airflow resistance [15,16,17,18,19,20]. Micro/nano-fibers, nanoparticles and multi-layer hybrid structure are used for the designing of the gradient structure. Cheng et al. [21] introduced a PAN/PVA multi-level structure filtration material via multi-jet electrospinning process, and its FE was up to 99.996% and ΔP reached 418 Pa for the testing media particles size of 300 nm.

Improved works have been reported by Ding et al. [22, 23], who designed a fluffy material containing micro/nanofibers or multi-layer hybrid membranes using electrospinning technique and achieved the highest FE more than 99.99%. A special fluffy structure composed of larger cavities inside the hybrid filtration materials reduces the friction between the airflow and fibers greatly, resulting in the extremely low ΔP less than 110 Pa [24].

When raw materials using in air purification are marked with GREEN, they are always in a favorable pole in the selection list. Post-used polymer materials are one of them, and have been studied and evaluated for decades [25,26,27,28]. Recycled polyethylene terephthalate (R-PET) fiber can be made from post-used bottles and clothing. More significantly, fibers with unique curved structure can be easily produced thanks to the modern spinning technology, which reduce the cost of air filtration media [28,29,30]. The curved R-PET fibers exhibit a fluffy porous three-dimensional (3D) structure inside webs. However, special designed curved R-PET fiber matrix combining with nano-fiber fluffy materials is rarely reported.

In this research, we used micro R-PET fibers with different curved shape as the matrix fiber web, nano-scale biodegradable electrospun poly(lactic acid) (PLA) [31] membrane as the fluffy functional filtration layer, and designed a series of fluffy filtration materials. The filtration performance and mechanism of micro/nanofiber gradient structure filtration material controlled by different curved fibers were systemically studied and analyzed.

2 Experimental section

2.1 Materials

PLA (Mw = 1.6 × 105 g mol−1, 6202D) was produced by Natureworks LLC, USA. N, N-dimethylformamide (DMF, analytical grade) and dichloromethane (DCM, analytical grade) were purchased from Shanghai Chemical Reagents Co. Ltd., China.

R-PET webs were provided by Guangdong Jiangmen Yuexin Chemical Fiber Co. Ltd. The classification of R-PET substrates is shown in Supporting Information (Table S1). Here, R1.5/N-PET and R2.5/N-PET mean R-PET fibers with linear shaped and the fiber fineness are 1.5 denier and 2.5 denier, respectively. R/S-PET means R-PET fiber of 7 denier with S-shaped curve and R/Z-PET is R-PET of 7 denier with zigzag-shaped curve.

2.2 Preparation of nano-PLA/R-PET mats

PLA was dissolved in a mixed solvent of DCM and DMF at a volume ratio of 4 to 1 using Magnetic Stirring Setup HJ-4A (Changzhou Guohua Electric Appliance Co. Ltd., China) at 25 °C. The concentrations of PLA solutions were 8, 10, 12 and 14 wt%, respectively.

The electrospinning conditions to obtain various nanofibers membranes were shown in Support Information (Table S2). The nano-PLA membranes were prepared via ESF-Y1 Electrospinning Instrument [32] deposited on aluminum foil substrates or different R-PET webs.

2.3 Characterization

The viscosity of PLA spinning solutions was measured using a DV2TLVTJ0 rotational viscometer (Brookfield, USA). The conductivities of PLA solutions were tested on a CON510 Conductivity Meter (Singapore EUTECH Company, Singapore). The surface tensions of solutions were tested using the maximum bubble method [33]. For all testing, the final results were average values based on five parallel results and error analyses were considered.

The morphology of the fibers was observed using Quanta 200 Environmental Scanning Electron Microscope (Netherlands FEI Co., Netherlands) and Axioskop 40 POL Polarizing Microscope (Carl Zeiss, Germany). The fiber diameters were analyzed with Digimizer software.

Basis weights of PLA electrospun membrane on R-PET substances were measured according to ISO 9073-1 and the deviations were controlled.

The permeability of samples was characterized using FX3300 Fabric Breathability Tester (Switzerland TEXTEST, Switzerland). The tested sample area was 20 cm2 and an applied pressure was 200 Pa. All result values were average of three parallel results.

The pore sizes and their distributions of samples were analyzed on a CFP-1200AP Capillary Flow Porometer (PMI Co., USA).

The surface voltages of filtration materials were measured on a SIMCO FMX-003 Electrostatic Field meter (SIMCO Co., Japan) at standard environment.

The aerosol filtration properties of samples were evaluated using a TSI 8130 Automatic Filtration Material Tester (TSI Co., USA). Sodium chloride (NaCl) aerosol with a mass median diameter of 260 nm was used. The flow rate was 32 L min−1 or the airflow velocity was 5.3 cm s−1. The valid testing area of samples was 100 cm2.

3 Results and discussion

3.1 Structure design of the R-PET/nano-PLA mats

The structure of fluffy filtration materials is presented in Fig. 1. Here, the R-PET/nano-PLA mats are composed of micro-R/Z-PET fiber web and nano-PLA fiber membrane. R/Z-PET web serves as the receiving substrate, and an interpenetrating micro/nanofibers gradient layer is formed between micro-R/Z-PET web and nano-PLA membrane. Curved R-PET and micro/nanofibers layer establish a 3D loosing structure which benefit to high FE and low ΔP.

Fig. 1
figure 1

Structure of 3D R/Z-PET/nano-PLA fluffy filtration material

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3.2 Preparation and characterization of the nano-PLA fiber and R-PET/nano-PLA mats

The morphology and average fiber diameter of electrospun nanofiber are affected by solution viscosity, solution conductivity, applied voltage, distance from tip to collector, surrounding temperature and humidity, and flow rate of solution [34]. An appropriate formation of Taylor cone is important for the achievement of continuous and stable electrospinning process [15]. The viscosity, conductivity, and surface tension of the PLA solution at various concentrations are shown in Fig. S1 (Supporting Information). The morphologies of nano-PLA fibers are given in Fig. 2.

Fig. 2
figure 2

SEM images of electrospun PLA fibers with PLA concentrations of a 8 wt%, b 10 wt%, c 12 wt%, and d 14 wt%, respectively

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According to Fig. 2, the morphologies of nano-PLA fibers transform from beads to continuous fibers with the increasing concentration of PLA solutions. High solution viscosity always results in large fiber diameter [35, 36]. The voltage, tip-to-collector distance and spinning rate also influence the morphology of nanofibers (Supporting Information, Figs. S2, S3 and S4). The nano-PLA fiber diameters are 300 ± 70 nm, 740 ± 200 nm, 770 ± 170 nm, and 1300 ± 300 nm with PLA concentrations of 8, 10, 12, and 14 wt%, respectively, when the spinning voltage is 15 kV, the tip-to-collector distance of 8 cm, and the spinning rate of 3.2 mL h−1. The nano-PLA fiber preparing from the PLA solution of 12 wt% possesses uniform fiber diameter and narrow fiber diameter distribution. Then, the PLA solution of 12 wt% was used to prepare R-PET/nano-PLA mats in this research.

To further investigated structure of prepared materials, the pore size distribution of filtration materials was studied. The pore size distribution of R-PET web and R/Z-PET/nano-PLA mat exhibits well-developed peak centering and the pore size range vary from 1.2 to 3.4 and from 1.3 to 5 μm, respectively (Fig. 3). Due to the wide pore size distribution of R/Z-PET web (range from 11.0 to 350.0 μm), the pore size distribution of R/Z-PET/nano-PLA mat become widening, which are determined by the shapes of the receiving substrate [37].

Fig. 3
figure 3

The pore size distribution curves of R-PET web, nano-PLA membrane and R/Z-PET/nano-PLA mat (the basis weight of nano-PLA membrane is 9 ± 0.1 g m−2)

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3.3 Filtration properties of R-PET/nano-PLA mats with different curved structures of R-PET fibers

Filters combined with fine fiber always show high FE, but increase in the airflow resistance [38], while the fluffy structure filters can reduce resistance.

As shown in Fig. 4, FE of R1.5/N-PET/nano-PLA, R2.5/N-PET/nano-PLA, R/S-PET/nano-PLA and R/Z-PET/nano-PLA mats are 99.986, 99.517, 99.949 and 99.992%, respectively, and corresponding ΔP are 253.33 Pa, 226.11 Pa, 198.27 Pa and 201.11 Pa, respectively. The combined effect of fiber size and shape need to be considered for curved fibers as filtration materials. The fineness of fiber will advantage to FE, while the fiber shape to change the airflow mechanism.

Fig. 4
figure 4

FE and ΔP of filtration materials changed with fiber fineness and curved shape of R-PET fiber in R-PET/nano-PLA mats (the basis weight of nano-PLA membrane is 9 ± 0.1 g m−2)

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The quality factor (QF) can be used to evaluate the filtration capacity of filtration material by Eq. 1 [16].

$$ QF = \frac{ - \ln P}{\Delta p} $$
(1)

where P is the particle penetration (%) and Δp is the pressure drop (Pa).

QF is a parameter on the energy expenditure in filtration, and influences by particle size and filtration velocity, regardless of thickness. In terms of FE or airflow resistance, a high QF usually shows a superior filtration performance [39]. Here, QF was used to evaluate the influence of curved shapes of R-PET on filtration performance and values are listed in Table 1. The R/Z-PET/nano-PLA mat shows the maximum QF, which means this mat is provided with the optimal comprehensive filtration performance.

Table 1 QF values of R-PET/nano-PLA mats with different curved structures of R-PET fiber
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3.4 Filtration properties of R/Z-PET/nano-PLA mats with different basis weight of nano-PLA membranes

In order to optimize filtration performance of the R/Z-PET/nano-PLA mat, the fluffy filtration materials were fabricated with various basis weight of nano-PLA membranes including 4 ± 0.2, 5 ± 0.1, 6 ± 0.2, 7 ± 0.2, 8 ± 0.1 and 9 ± 0.1 g m−2, respectively.

When the basis weight of nano-PLA membrane increased from 4 to 9 g m−2, the average pore size of nano-PLA membrane decreases from 2.15 to 1.62 μm, and R/Z-PET/nano-PLA mat from 3.17 to 1.99 μm, respectively (Fig. 5), but all are smaller than that of R/Z-PET web (138.12 μm). Meanwhile, the breathability of R/Z-PET/nano-PLA mat decreases from 98.95 to 39.17 mm s−1. The increasing basis weight of the nano-PLA membrane enhanced the entanglement percentage between fibers in per unit area [40, 41]. At the same basis weight, the average pore size of R/Z-PET/nano-PLA mat decreases comparing with that of the nano-PLA membrane, and this is the result of modification of R/Z-PET web on the pore structure of mat.

Fig. 5
figure 5

Average pore size of nano-PLA membrane, R/Z-PET/nano-PLA mat, and breathability of R/Z-PET/nano-PLA mat varied with basis weights of nano-PLA membranes

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As shown in Fig. 6a, b, the FE and ΔP of R/Z-PET/nano-PLA mats elevate with the increasing basis weight of nano-PLA membrane. As the loading time extended, the corresponding FE and ΔP accordingly increase at the same time. FE of R/Z-PET/nano-PLA mat with the low (4 g m−2) basis weight of nano-PLA membrane increases rapidly at first, and then undergoes a gradual change after the filtration time exceeds 2 min (Fig. 6a). Rising tendency of FE decreases with weight of nano-PLA membrane increasing. This phenomenon could be essentially ascribed to gradual accumulation of the trapped particles in pore channels inside the filtration material, and step by step, the particles block the channels with increasing filtration loading time, which reduces the pore size inside the filtration material [13]. This tendency is especially obvious for filtration material with a low initial FE value. With the increasing loading time, the following of cake filtration mechanism of the mat results in the increase of ΔP (Fig. 6b) [42]. As shown in Fig. 6c, the growth rate of ΔP of R/Z-PET/nano-PLA mat with increasing basis weight of nano-PLA shows a slow tendency at the beginning and then increase. This result is consistent with the trend of the average pore size of R/Z-PET/nano-PLA mat (Fig. 5).

Fig. 6
figure 6

a FE, b ΔP of R/Z-PET/nano-PLA mat under continuous loading time and c the slope tendency of the ΔP varied with the basis weight of nano-PLA membranes

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Additionally, from Fig. 7, we found that, as the basis weight of nano-PLA membrane increasing, particles intercepted in the surface of filtration material increase, and this is the results of the densifying of fibers in nano-PLA membrane and narrowing of pore size in membrane.

Fig. 7
figure 7

SEM images of R/Z-PET/nano-PLA fluffy filtration material under continue loading time

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Aerosol particles can be classified as mono-dispersed and polydispersed samples according to particles size in filtration. The penetration capacity of particles is related to the nano-PLA membrane weight or thickness. Manipulation of this equation gives as follows [43],

$$ \ln p = - \alpha X $$
(2)

where P is the penetration of aerosol particles through the filtration material, α is the filtration index, and X is the basis weight of nano-fiber membrane. The curve of penetration of aerosol versus basis weight of nano-PLA membrane conform to the polydisperse distribution of the testing NaCl particles (Fig. 8). The QF of thicker filtration materials decrease steadily when polydisperse aerosols are studied, thus the value of QF shows no big significant [44].

Fig. 8
figure 8

Illustration of nano-PLA membrane weight efficiency of R/Z-PET/nano-PLA fluffy filtration material

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3.5 Residual charge on nano-PLA membrane and its influence on filtration properties of R/Z-PET/nano-PLA mat

During electrospinning process, a high voltage electric field forms between the tip of needle and the collection layer, thus polarized charges resident on the surface of fiber, which cannot dissipate immediately. The residual charges enhance the temporary filtration ability [45, 46].

Tendency of charge dissipation depends on properties of fiber materials itself such as conductivity and hydrophobic or hydrophilic. For hydrophobic nano-PLA membrane, the surface voltage decreases from 2.9 to 0.35 kV under an equilibrium condition for up to 145 days (Fig. 9a). The filtration performance of R/Z-PET/nano-PLA mat before and after electret elimination treatment (EET-R/Z-PET/nano-PLA mat) is studied and results show the FE of EET-R/Z-PET/nano-PLA mat drops from 99.992 to 99.985%, but the ΔP slightly increases from 201.11 to 205.14 Pa (Fig. 9b). This means the surface residual charge contributes some effect on the overall filtration performance of the R/Z-PET/nano-PLA mat.

Fig. 9
figure 9

Surface voltage decayed with storage time (a) and filtration performance before and after electret elimination treatment (b) of R/Z-PET/nano-PLA mat (the basis weight of nano-PLA membrane is 9 ± 0.1 g m−2)

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The QF (0.043 Pa−1) of EET-R/Z-PET/nano-PLA shows the highest value as compared with those of other R-PET/nano-PLA mat (Table 1) except the R/Z-PET/nano-PLA mat. This further indicates that the excellent filtration performance of R/Z-PET/nano-PLA mat benefit from its zigzag curved structure of R-PET fiber.

3.6 The dust-holding capacity of optimal R/Z-PET/nano-PLA mat

R/Z-PET/nano-PLA mats with different weights of nano-PLA membrane were prepared and their filtration properties were evaluated. The highest efficiency filtration material (> 99.992%) with a ΔP of 201.11 Pa achieved.

Considering the basis requirement of filtration, we further investigated the dust-holding capacity of R/Z-PET/nano-PLA mat under loading time of 5, 15 and 30 min, respectively (Fig. 10), and competed that of R2.5-PET/nano-PLA mat under loading time of 30 min.

Fig. 10
figure 10

SEM images and dust-holding capacity of R/Z-PET/nano-PLA mat under different loading time (the basis weight of nano-PLA membrane is 9 ± 0.1 g m−2)

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Results show that the numbers of captured particles obvious increase on the surface of R/Z-PET/nano-PLA mat with prolonging loading time. The intercepted particles limit the pore size inside the filtration material, resulting in an improvement of FE of material itself. As a consequence, the dust-holding capacity is enhance from 3.8 to 4.7 times. The dust-holding capacity of R/Z-PET/nano-PLA mat (1.0 g m−2) was improved comparing with that of R2.5/N-PET/nano-PLA mat (0.9 g m−2) after loading for 30 min. The fluffy 3D gradient structure in R/Z-PET/nano-PLA mat provides more space for the particles and extends the effective time for the particles diffusion inside the R/Z-PET layer [47], which results in the higher dust-holding capacity and FE (Fig. 4).

3.7 Filtration mechanism of R-PET/nano-PLA mats

For sub-micron aerosols, the filtration mechanism of filter primarily is a diffusion process. The particles leave the flowing streamline, move randomly in air, and are finally captured through the Brownian motion. Another capture mechanism is the interception for 100 nm and other larger aerosols [12, 48, 49]. Figure 11 exhibits the schematic of airflow in R-PET substrate. For linear fiber (Fig. 11a), high percentage airflow is an even dent resistance instead of retarding flowing when the air run down meeting the surface of fiber. Thus, the filter often shows a high resistance.

Fig. 11
figure 11

Schematic mechanism of airflows through linear (a), horizontally placed zigzag (b), zigzag fiber 45° orienting along the air flow direction and the effective functional surface illustrated from zigzag (c) and horizontally placed S shape (d) fiber

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For curved fiber, an interpenetrating micro/nanofibers gradient fiber layer could impose a pressure gradient that enhance the FE and reduce ΔP of filtration material [20]. More significantly, the zigzag fiber (Fig. 11b) is beneficial to the “slip effect” of air on the surface of fiber. The computational equation of its effective functional surface (Fig. 11c) is as follows,

$$ d = L \cdot \cos \left( {\frac{\pi - \theta }{ 2}} \right) = L \cdot \sin \frac{\theta }{2}\left( {0 < \theta \le 90^\circ } \right) $$
(3)

where d is the effective functional surface of zigzag fiber, L is the length of folding angle of zigzag fiber, and θ is folding angle of zigzag fiber. The effective functional surface of zigzag fiber shows the biggest value when the folding angle is at 90°. If the zigzag fiber orient at 45° along the airflow direction (Fig. 11c), then half of fibers can posit at the same direction to the airflow, which means the friction of air on the surface of fiber will be at the minimum value. This tendency will not happen if the fiber curved in S shape. On the other hand, the stabilization of zigzag is more than that of the S-curved R-PET fiber. Though the S-shaped R-PET fiber (Fig. 11d) also has curved angle, this angle easily deforms [50] because of the unstable curved of fiber and transforms into linear shape that reduces the “slip effect” of airflow, and thus decreases the performance of filtration materials.

Figure 12 demonstrates the FE and ΔP of nano-PLA membrane or R-PER web placed in windward. The growth rate of ΔP of R/Z-PET/nano-PLA mat shows the slowest increase tendency as slop values in Table 2. This is the result that half of air resistance on the surface of R/Z-PET fiber changes from dent resistance to slip resistance.

Fig. 12
figure 12

FE and ΔP of different filtration mats charge with loading time, and the mats are placed in windward (the basis weight of nano-PLA membrane is 9 ± 0.1 g m−2)

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Table 2 Fitting of ΔP with different materials in windward (from Fig. 12)
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The R/Z-PET web in fluffy 3D R/Z-PET/nano-PLA mat provides more airflow reflecting effect, and then changes the airflow direction at the surface of zigzag fiber, which extends the retention time of airflow through the whole filtration material [51]. The Z-fiber and 3D gradient material create a “slip effect” of airflow, which promotes the penetration of airflow [39]. Furthermore, the zigzag fiber will improve functional surface of material for filtration function, which increases the chance of an inertial collision between the nanofiber and the airborne particles on a Brownian diffusion mechanism, resulting in an increased interception probability of particles for filtration materials. FE of R/Z-PET/nano-PLA mat is up to 99.992% and the ΔP is 201.11 Pa. More significantly, the fluffy structure of R/Z-PET can extremely decrease the growth rate of ΔP. The environmental-friendly R/Z-PET/nano-PLA mats exhibit prospect in air pollution control applications.

4 Conclusions

Biodegradable PLA and post-used PET bottles were used as raw materials for electrospun nano-PLA fiber membrane and direct industrial produced R-PET fiber web. An efficient integrated 3D fluffy R/Z-PET/nano-PLA mats using for high efficacy air filtration material were designed to reduce ΔP. Optimal design of R/Z-PET/nano-PLA mat shows a high FE of 99.992%, a low ΔP of 201.11 Pa, and the QF value of 0.047 Pa−1. The residual charges on the surface of nano-PLA membrane show some contributed to filtration performance of mats. The zigzag curved fluffy structure R-PET fibers with a folding angle of 90° provide more chance for particle travelling inside the filtration material and decrease the friction between fiber and particles when the location angle of R-PET fiber is 45° along the airflow direction. Otherwise, the fluffy structures significantly decrease the growth rate of ΔP and enhance the dust-holding capacity of mat. Therefore, the environmental-friendly R/Z-PET/nano-PLA mat establishes a novel design of air filtration material as a powerful strategy would present in various applications ranging from industrial security to environmental governance.