High filtration efficiency fluffy material: nano-fiber constructing gradient structure on recycled curved PET micro-fibers web
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High efficiency filtration materials focus on electreted melt-blown polypropylene nonwoven and nano-scale fibers membrane in industry. However, the charge decay and the balance of efficiency and resistance are still facing challenges on research and practical applications. In this research, combined with traditional electrospinning and carding process, a simple and new fluffy gradient filtration material was designed by constructing nano-scale poly(lactic acid) (nano-PLA) fiber membranes on recycled three-dimensional curved poly(ethylene terephthalate) (R-PET) micro-fibers webs. Influences of basis weights of nano-PLA membranes and curved types of R-PET fibers on the filtration performance and the dust-holding capacity of R-PET/nano-PLA mats were investigated. Besides, the filtration mechanism of R-PET/nano-PLA mats was systemically studied. Results show that R-PET with zigzag curve (R/Z-PET) possesses the highest filtration efficiency of 99.992% comparing with 99.517% of linear R-PET, the pressure drop fall to 201.11 Pa, and a quality factor level up from 0.024 to 0.047 Pa−1. The pressure raising rate of the R/Z-PET/nano-PLA mat is the slowest. R/Z-PET fibers with a folding angle of 90° benefit to decrease the friction between fiber and particles when the location angle of zigzag fiber is 45° along the airflow direction, which decrease the pressure drop at high filtration efficiency. Surface residual charges contribute some effect on the overall filtration performance of the R-PET/nano-PLA mat. A new way of simple curved micro-PET fiber combining with nano-scale fibers provides an industrial available path to mass production of high efficiency filtration material serving for air pollution control without electret process.
KeywordsPoly(lactic acid) Zigzag curve Fluffy structure Recycled poly(ethylene terephthalate) Air filtration
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 .
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 , Wang  and Zhang  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 .
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 . 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.  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 .
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)  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
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  deposited on aluminum foil substrates or different R-PET webs.
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 . 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
3.2 Preparation and characterization of the nano-PLA fiber and R-PET/nano-PLA mats
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.
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 , while the fluffy structure filters can reduce resistance.
QF values of R-PET/nano-PLA mats with different curved structures of R-PET fiber
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.
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].
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.
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 , which results in the higher dust-holding capacity and FE (Fig. 4).
3.7 Filtration mechanism of R-PET/nano-PLA mats
Fitting of ΔP with different materials in windward (from Fig. 12)
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 . The Z-fiber and 3D gradient material create a “slip effect” of airflow, which promotes the penetration of airflow . 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.
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.
This study was funded by the National Natural Science Foundation of China–Youth Foundation (No. 5110620) and Guangdong Science and Technology Department (No. 2016A010103008).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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