Polyethylene/polypropylene bicomponent spunbond air filtration materials containing magnesium stearate for efficient fine particle capture†

Jinxin Liu, a b Haifeng Zhang,c Hugh Gong,b Xing Zhang,a Yuxiao Wang,a Xiangyu Jina

*

a Key Laboratory of Textile Science & Technology, Ministry of Education, College of

Textiles, Donghua University, No. 2999 North Renmin Road, Shanghai 201620, China* Correspondence: jinxy@dhu.edu.cn (Xiangyu Jin)b School of Materials, The University of Manchester, Oxford Road, Manchester, M13

9PL, UKc College of Textile and Clothing, Nantong University, Nantong 226019, Jiangsu,

China

† Electronic supplementary information (ESI) available.

Abstract Particulate matter (PM) poses a threat to people’s living environment. The fresh air

ventilation system could filter particulate matter and play an important role in indoor

air quality. Therefore, a high filtration efficiency material with low pressure drop is

required to prolong service life and reduce energy consumption. However,

maintaining long-term storage of charge in electret materials remains a challenge.

Herein, we report a novel sheath/core bicomponent spunbond (BCS) electret materials

with low pressure drop and desirable charge stability using

polyethylene/polypropylene (PE/PP) as the matrix polymer and Magnesium stearate

(MgSt) as the charge enhancer. Benefiting from the three-dimensional (3D) fluffy

structure created by spunbond technique and through-air reinforcement, the resulting

electret materials exhibit low pressure drop of 37.92 Pa, excellent dust holding

capacity of 10.87 g m-2, and relatively high filtration efficiency of 98.94%. Moreover,

due to the introduction of MgSt, filtration efficiency only decreased by 4.1% in 90

days. The successful fabrication of PE/PP BCS materials with MgSt would not only

provide a promising candidate medium for particle capture, but also develop a new

approach for the design of fresh air filtration materials.

IntroductionWith the development of urbanization, particulate matter (PM) pollution has become

an increasingly serious problem.1-3 Many toxicological and epidemiological studies

have reported that particulate matter pollution poses a serious hazard to human

health.4-6 People spend an average of 86.9% of their time indoors,7 so indoor air

quality is very important. Controlling and reducing pollution sources is critical to

solving problems, but it is a long-term process related to policy development and

industry upgrading.8 To date, the use of air filter is the simplest and most effective

method to protect people from air pollution damage.9-10

Fibrous filtration materials are widely used in air filter due to their beneficial

characteristics, such as energy conservation, cost saving and ease of replacement.11-12

The particle capture is mainly based on five main filtration mechanism including

interception, physical sieving, inertial separation, diffusion, and electrostatic

attraction.13-15 Conventional filtration materials rely mainly on physical sieving effects

so that the pore size of materials must be reduced. Although this is able to achieve a

relatively high filtration efficiency, the pressure drop is super high, causing energy

consumption and noise pollution.16

Electret filtration materials are a kind of fibrous filter that is made up of dielectric

materials. The dielectric materials develop a quasi-permanent electrical charge when

they are subjected to electric field and corona charging treatment.17-18 The electret

filtration materials are able to improve the ability to capture particles without

increasing pressure drop by making full use of electrostatic attraction.19-20 Many

scholars have modified the polymer to improve the stability of charge by adding

additives, such as SiO2,20 barium titanate,21-22 boehmite,23 and boron nitride

nanosheets.24 Previous efforts mainly focused on higher filtration efficiency, ignoring

the low air resistance. Polyethylene/polypropylene (PE/PP) sheath/core bicomponent material is a material

that is configured from bicomponent fibers, wherein one polymer (PP) is surrounded

by another (PE).25 After through-air bonding, the effect of low resistance and high

dust holding capacity could be achieved.26 The material is a suitable electret filtration

material and is often treated by corona charging technology to further increase

filtration efficiency.27 However, the charge of electret filtration materials will escape

and cause a decrease in filtration efficiency.28 In order to improve the stability of

charge in the bicomponent materials, we introduce magnesium stearate (MgSt)

particles into the polymer as nucleating agents to improve the crystal structure and

electret performance. The challenge, therefore, is to construct a low air resistance,

high filtration efficiency electret filtration material capable of stable charge storage

capacity.In this study, we report a novel methodology for creating PE/PP sheath/core

bicomponent filtration materials containing MgSt nucleating agent by bicomponent

spunbond technique, through-air bonding, and corona charging treatment. Our PE/PP

BCS exhibit low air resistance, high efficiency, and excellent charge stability.

Particularly, the effects of additives on the crystalline grain size, pore structure, charge

storage ability, and filtration efficiency of the materials are thoroughly investigated.

The filtration performance of the PE/PP BCS materials, including the loading test

performance and the decay test of up to 90 days, are further studied.

ExperimentalMaterialsPolyethylene (PE) pellets with a melt flow index (MFI) of 20 g (10 min)-1 were

provided by PetroChina Co., Ltd. (Fushun, Liaoning, China). Polypropylene pellets

with MFI 30 g (10 min)-1 were supplied by Sinopec Zhenhai Refining & Chemical

Company (Ningbo, Zhejiang, China). MgSt powders with MFI of 400 g (10 min)-1

were purchased from Sinopharm Chemical Reagen Co., Ltd. (Shanghai, China).Preparation of polymer masterbatchThe PP pellets and MgSt powders should be mixed according to a suitable ratio to

pelletize (Figure S1†). The PP pellets and MgSt were heated and melted by a twin-

screw extruder and extruded into strips, which were then cooled in a water bath. After

being blown dry by the wind, it is cut into the masterbatch. As shown in Figure S2†,

we can clearly observe the photographs and SEM images of polymer pellets, MgSt,

and modified masterbatch.Fabrication of PE/PP BCS materialsA 50 cm wide bicomponent spunbond research line at Nonwoven Research and

Development Center of Donghua University (Shanghai, China) was used to produce

the PE/PP (1:1, v/v) sheath/core bicomponent spunbond materials. The bicomponent

spunbond process includes polymer melting, extrusion spinning, drawing, and web

formation (Figure 1a), and processing parameters are shown in Table S1†.29 Then the

fiber web was strengthened and reinforced by through-air bonding. A suction box

located below the web promotes the passage of heated air (Figure 1b). Finally, the

corona charging treatment was applied to further improve the filtration efficiency by

electrostatic attraction effects (Figure 1c). Unless otherwise noted, the base weight of

the spunbond materials was 120 g m-2.

Figure 1. Schematic diagram of the (a) bicomponent spunbond process, (b) through-air reinforcement, and (c)

corona charging treatment.

The melted sheath component of PE and core component of PP were evenly divided

by the spinning pack, and finally merge to form a sheath/core bicomponent fiber at

spinneret. The schematic diagram of fabrication of bicomponent spunbond fiber in a

single hole at spinneret was shown in Figure 2. Due to the particularity of the

sheath/core materials, we fabricated four different types of materials by adding MgSt

to different components, including one control group PE/PP (Figure 2a). The

materials in which MgSt were added to the sheath component of PE component and

core component of PP were denoted as M-PE/PP (Figure 2b) and PE/PP-M (Figure 2c), respectively. The material in which both sheath component of PE component and

core component of PP were added with MgSt was named M-PE/PP-M (Figure 2d).

The PE/PP-M materials containing various contents of MgSt were denoted as PE/PP-

M-δ (where δ is the content of MgSt in the bicomponent spunbond materials).

Figure 2. Schematic diagram of fabrication of different types of bicomponent spunbond fiber in a single hole at

spinneret. (a) PE/PP fiber, (b) M-PE/PP fiber, (c) PE/PP-M fiber, and (d) M-PE/PP-M fiber.

Corona charging and charge property testsThe corona charging apparatus is composed of a high voltage power supplier (DW-

P503-2AODE, Tianjin Dongwen High-voltage Power Supply Co., Ltd., Tianjin,

China), an array of electrode needles and a grounded copper plate (Figure 1c). After

applying a high potential between the needles and the copper plate electrode, ions

were generated and deposited on the PE/PP BCS sample when corona discharge took

place. In this work, all the prepared materials were corona charged before testing

filtration performance. The applied voltage, the charging distance between the needles

and the copper plate electrode, and the charging time were 18 kV, 30 mm and 120 s,

respectively. The surface potential of the charged BCS materials was measured by a

non-contacting electrostatic voltmeter (TREK-542A-2-CE, TREK Inc., USA). The

distance between the probe and the material to be tested was 1.5 cm, and each sample

was tested in 5 different positions. Each test results takes an absolute value. The test

process was carried out under constant conditions of a temperature of 25 ± 2℃ and

relative humidity of 45 ± 5 %.

CharacterizationThe surface and cross-section morphology of BCS materials was examined by

scanning electron microscope (SEM) (TM3000, Hitachi Ltd., Japan) after being

sputter-coated with gold. The fiber diameter was determined by measuring 100 fibers

according to the SEM images, with the help of the software Nano Measurer 1.2.5.Energy dispersive X-ray mapping images were obtained by Aztec analysis system

installed on a high performance scanning electron microscope (MIRA3, TESCAN,

Inc., Czech Republic).The crystalline structure was measured by an X-ray diffractometer (D/max-2550 PC,

Rigaku Corp., Japan). The scanning range was 5°- 40° with a rate of 4°/min and a step

length of 0.02.The pore size and pore distribution of BCS materials were characterized by a bubble-

point test utilizing a capillary flow porometer (CFP-1100AI, Porous Materials Inc.,

USA).The charge storage stability was investigated by thermally stimulated discharge (TSD)

techniques. Open circuit TSD measurements were performed in a linear heating

system with a heating rate of 3 min℃ -1, including a temperature-controlled oven, an

electrometer (Model 6514, Keithley) and a data processing computer.Filtration performance was measured by an automated filter tester (TSI 8130, TSI

Inc., USA), which could generate sodium chloride (NaCl) aerosol with different flow

rate (Figure S3†). The mass median diameter of aerosol was 0.26 μm, and the

geometric standard deviation of the particles was less than 1.83. NaCl aerosol

particles were delivered through the BCS materials, which were clamped by a filter

holder with an effective area of 100 cm2. An electron-laser particle photometer was

used to measure the concentration of the aerosol particles in the upstream (Cu) and

downstream (Cd) respectively. Filtration efficiency η was calculated as follows:η = (1 - Cd / Cu) × 100% (1)

Unless otherwise noted, the filtration performance of the materials was tested under

the industrial standard air flow rate of 32 L min-1. We recorded the time when the

pressure drop of samples reached 1000 Pa at a high air flow rate of 85 L min -1 under

dynamic loading filtration test. And the weight before and after the loading test

reflected their dust holding capacity.

Results and discussionBCS filtration materials designWe designed the BCS materials based on three criteria (1) the filtration materials must

have low air resistance and high filtration efficiency, (2) the charge of filtration

materials must have good stability, (3) the filtration materials must have a three-

dimensional (3D) fluffy structure to accommodate dust. The first requirement was

satisfied by a spunbond technique and corona charging treatment26. For the second

criterion, MgSt was introduced to improve the stability of the electret charge. Stearate

has a certain improvement in filtration performance in melt-blown filter materials,30-31

but to date, we have not found reports in the spunbond materials. To satisfy the third

criterion—the formation of 3D fluffy structure—the obtained unbonded fiber web

materials were bonded by through-air bonding at 135 for 2 min. The principle of℃

the bonding process is based on the fact that the different components of the

bicomponent fibers have different melting points.32 Effect of MgSt doping on properties of BCS filtration materialsAs shown in Figure 3a, the representative SEM images indicated that the fibers in the

spunbond materials were randomly distributed. From the cross-sectional morphology

of fiber, we can clearly observe two different components of the sheath (PE) and core

(PP) (Figure 3b). During the through-air bonding process, the heated air went through

the fiber web, causing the sheath component of PE to melt and bond while the core

component of PP didn’t melt and remain stable structure. Under those circumstances,

every fiber crossover point in the web was potentially a bond site, which was called

“point-to-point” adhesion, and the whole material formed a three-dimensional fluffy

structure. With the doping of MgSt, there is no agglomeration on the surface and

cross-section of bicomponent materials (Figure 3c-3e). This can be explained by the

fact that the MgSt as a mixture has two melting point of 85.38 and 117.8 (℃ Figure S4†), whereas the temperature of the spinning pack is 260 . The MgSt has been℃

completely melting dispersed into the polymer. In addition, there was no significant

difference in the average diameters of the PE/PP, M-PE/PP, PE/PP-M, and M-PE/PP-

M BCS fibers, which were 14.13, 14.4, 14.08, and 15.03 μm, respectively (Figure 3f). Elemental mapping is a powerful technique for characterizing the distribution of

elements in materials.33-34 Energy dispersive X-ray analysis (Figure 4) also confirmed

the uniform distribution of MgSt in BCS fibers through the detection of C elements

and Mg elements. Naturally, we did not observe a similar distribution in the control

group PE/PP.

Figure 3. Morphology and diameter of different types BCS materials. SEM images of (a) the surface of PE/PP

BCS materials, (b)cross-section of PE/PP BCS materials. SEM images of surface and cross section of (c)

M-PE/PP, (d) PE/PP-M (e) M-PE/PP-M, and (f) fiber diameter.

Figure 4. Energy-dispersive X-ray mapping images of (a) C , (b) Mg in PE/PP. (c) C, (d) Mg in M-PE/PP. (e) C, (f)

Mg in PE/PP-M. (g) C, (h) Mg in M-PE/PP-M.

Figure S5† revealed that the pore size and pore size distribution of all samples were

identical in the range of 4–46 μm, with a well-developed peak centered at around 11

μm. The result demonstrated that the pore size distribution of all samples is uniformly

similar. It can be seen from SEM images, diameter chart and pore size distribution,

the introduction of MgSt did not affect the appearance and structure of the BCS

materials. Moreover, we did not find any impact on the equipment during the

production process.The filtration performance of four different BCS materials was also systematically

investigated by using the charge neutralized sodium chloride (NaCl) particles. Since

small airborne pollutants were difficult to control manually, let alone control them at

different flow rates, NaCl aerosols were generally used in the filtration industry and

scientific research papers to detect filtration efficiency.35-36 Before corona charging,

the filtration efficiencies of the as-prepared PE/PP, M-PE/PP, PE/PP-M, and

M-PE/PP-M BCS materials were 38.73%, 37.77%, 37.76%, and 38.33%, while

corresponding pressure drops were 22.17, 21.69, 22.42, and 22.48 Pa, respectively

(Figure 5a). There was usually a trade-off parameter of quality factor (QF) between

filtration efficiency and pressure drop, evaluating comprehensively the filtration

performance of a given filtration medium. The QF of a filtration medium is defined

by the following formula:QF = -ln(1-η)/ΔP (2)

Where η and ΔP represented the filtration efficiency and pressure drop, respectively.37-

39 This equation fairly indicated that large QF value means high filtration efficiency

and/or low pressure drop. An excellent filtration material should have good filtration

efficiency and a large QF value.40 As illustrated in Figure 5b, the QF of the four

materials is basically at the same low level. This is because NaCl particles were

mainly captured by physical sieving and interception at this stage. Since the pore size

and fiber diameter of four samples were basically the same, the difference in their

filtration performance was not obvious. According to the previous report, PE/PP BCS

material was reported to be a promising electret material.26 After charging treatment, it

can rely on electrostatic forces to capture fine particles. In order to improve filtration

efficiency and QF value, we also carried out a corona charging treatment of all as-

prepared samples. As shown in Figure 5c, after corona charging the filtration

efficiencies of four kinds of samples increased significantly (88.27%, 86.61%,

91.32%, and 89.26%) compared with that of as-prepared uncharged samples, while

the pressure drop remained constant. This can be ascribed that the charged BCS fibers

relied on electrostatic attraction to capture particles, especially fine particles.41

Moreover, in the absence of dielectric breakdown, corona charging did not transform

the structure of material, thus the pressure drop remained almost the same.42 It was

worth noting that the increase in filtration efficiency of the PE/PP-M was the largest,

which indicated that the introduction of MgSt to core component is optimal for

improving filtration efficiency. Their QF value (PE/PP of 0.096 Pa -1, M-PE/PP of

0.090 Pa-1, PE/PP-M of 0.11 Pa-1, M-PE/PP-M of 0.099 Pa-1) also confirmed the result

(Figure 5d).

Figure 5. (a) Filtration performance and (b) quality factor of different kinds of samples before corona charging. (c)

Filtration performance and (d) quality factor of different kinds of samples after corona charging.

Therefore, we further explored the effect varying concentration on the filtration

performance when the MgSt were introduced into the core component (PE/PP-M). As

demonstrated in Figure 6a, the filtration efficiencies of the PE/PP-M BCS materials

with various MgSt concentrations with 0, 0.2, 0.4, 0.6, 0.8 and 1 wt% were 88.27%,

89.47%, 91.32%, 93.02%, 92.98%, and 93.17%, while the pressure drops were 22.34,

22.77, 22.5, 22.56, 23.24, and 23.04 Pa, respectively. It was clearly seen that the

filtration efficiency revealed a progressive rise and afterward achieved a metastable

value when the concentration exceeded 0.6 wt%, indicating that there was a saturation

value for the content of MgSt. The QF value of all samples doping MgSt were

calculated, which also revealed a trend of rising first and then stabilizing (Figure 6b),

therefore the PE/PP-M-0.6 would be carried out in the following study. Figure S6† presented the cross-section SEM images of PE/PP BCS fibers containing various

MgSt concentrations, and we still did not observe the agglomeration of MgSt.

Figure 6. (a) Filtration efficiency, pressure drop, and (b) QF value of PE/PP-M BCS materials with various MgSt

concentrations.

Charge and crystallization properties / The mechanism of improving the filtration efficiency To investigate the mechanism by which MgSt can increase filtration efficiency, we

tested the surface potential for PE/PP and PE/PP-M-0.6 samples. As revealed in

Figure S7, the surface potential of PE/PP sample was 3.57 kV. In contrast, the surface

potential of PE/PP-M-0.6 sample was 4.78 kV. This demonstrated that the filtration

efficiency of PE/PP-M-0.6 sample was higher than that of PE/PP because the former

has a higher charge amount. Figure 7a illustrated the process of corona charging and how the charges were

captured by the materials. When a high positive voltage was applied to the needle

electrode, the electric field around the needle tip would be distorted. The distorted

electric field caused neutral molecules in the air to be ionized out of H+, NO+, and

NO2+ ions.43 Due to the large potential difference between the high voltage electrode

and the ground electrode, these positive ions move toward the ground electrode.

Eventually, positive ions were deposited on the electret material to form surface

charges (Figure 7b) and trapped by traps inside the material to form space charges.44

More importantly, benefiting from the special structure of bicomponent fibers, space

charges could be stored at the interface between the sheath component of PE and the

core component of PP (Figure 7c). Structural defects and impurity defects of the

polymer material itself are also important storage locations for space charges (Figure 7d).45 In addition, as revealed in Figure 7e, when an electrical current passed through

the dielectric, the Maxwell-Wagner effect was generated at both end faces of the

crystal granules, causing the accumulation of opposite charges to form polarization

charges, which also becomes an interfacial polarization.18 The thermally stimulated

discharge (TSD) current measurements were also applied to study charge trap

parameters which were closely related to the stable behavior of charge storage.46 The

linear temperature range of the TSD test was 25-200 . As demonstrated in ℃ Figure 7f, the intense peaks in the low-temperature region of 25-90 for PE/PP samples℃

were displacement current caused by the release of charges in the shallow deeps. In

the temperature range of 150-180 , the first peak was attributed to the relaxation of℃

the polarization charges, because a negative current could be generated when the

polarization charges were depolarized under positive corona charging.47 The cause of

the second current peak could be contributed by the release of charge in the deep

shallows. It was observed that the current peak intensity of the PE/PP-M-0.6 in this

region is stronger than PE/PP, indicating that the introduction of MgSt allowed the

material to store more charges. Interestingly, we noticed that the position of the peak

of PE/PP-M-0.6 shifted slightly toward higher temperature, which demonstrated that

the trap energy band converted to a higher energy band.48 The result illustrated that

MgSt was beneficial to the stability of charge.

Figure 7. (a). The schematic diagram of trapping and detrapping process in energy band after positive corona

charging. EC is the bottom of conduction band, and EF is the Fermi level. (b) surface charges. Space charges

stored in (c) the interface between sheath and core component, and (d) defects of polymer. (e) Polarization charges

generated by Maxwell-Wagner effect. (f) Thermally stimulated discharge current spectra of the PE/PP and PE/PP-

M-0.6 samples.

The crystalline structure of the two samples was investigated using X-ray diffraction

and crystallite size was also calculated. As shown in Figure 8a, the characteristic

peaks of polypropylene of PE/PP and PE/PP-M-0.6 were consistently presented at 14,

17, and 18.6 degrees, corresponding to the lattice planes (110, 040, and 130) of α-PP.49

It could be inferred that the introduction of MgSt did not change the crystalline form

of polypropylene. The crystallite size and other crystal parameters were listed in

Table S2†. Crystallite size (D) was calculated using Scherrer’s formula:50

D= 0.89 λβ cosθ

(3)

where λ is the X-ray wavelength (1.5418 Å), β is the full-width at half-maximum

(FWHM) of the XRD peak in radians, and θ is the Bragg angle.Figure 8b revealed the crystallite sizes at lattice planes (110, 040, and 130) and their

average values. The average crystallite size of PE/PP-M-0.6 was 72.6 Å, which was

smaller than that of PE/PP samples (86.3 Å), suggesting that more fine grains were

formed due to the introduction of MgSt. Figure 8c showed a schematic diagram of

crystallization process of PE/PP and PE/PP-M-0.6, from which we could clearly

observe the introduction of MgSt changed the crystalline structure of the polymer. For

PE/PP samples, nucleation was first produced by the movement of molecular

segments and the slowly crystallized to form a large spherical size. However, for

PE/PP-M-0.6 samples, MgSt was equivalent to a nucleating agent, which crystallized

rapidly during melt cooling and collided with each other before forming large

spherulites. Compared with the two, the crystal structure of PE/PP-M-0.6 samples

decreased in the average crystallite size and an increased in the number of crystal

granules due to the action of nucleating agent, which resulted in an increase of the

crystal/amorphous interface.51 This undoubtedly improved the storage of space

charges and generated polarized charges through the Maxwell-Wagner effect.

Figure 8. The crystalline structure of PE/PP and PE/PP-M-0.6 nonwovens. (a) XRD spectra, (b) crystallite size and

(c) nucleation processes of PE/PP and PE/PP-M-0.6 nonwovens.

Air filtration performance evaluationThe values of filtration efficiency undoubtedly varied according to the basis weight of

samples. Figure 9a showed the filtration performance of the PE/PP-M-0.6 samples

with stepwise incremental gram weight of 40, 80, 120, 160, and 200 g m-2 were

55.92%, 76.2%, 93.02%, 96.53%, and 98.94%, meanwhile the corresponding pressure

drops were 8.45, 14.93, 22.21, 27.63, and 37.92 Pa, respectively. These results

indicated that a synchronous increase upon improving the basis weight. It could be

clearly seen that the filtration efficiencies revealed a sharp rise and then reached a

relatively high value level (> 96%) when the basis weight exceeded 160 g m-2, while

maintaining a relatively low pressure drop of 27.63 Pa, which was unachievable for

the traditional fresh air filtration materials. The QF values of materials fluctuated

slightly, but all remained at a desirable level above 0.0964 Pa-1 (Figure 9b). To

compare with mainstream materials, we summarized the filtration performance of

common filter materials in Table S3†. The air flow through the fiber material was

critical to the filtration performance. Based on complex application conditions for the

filters, the filtration performance versus air flow rates (20-100 L min-1) was

systematically measured using PE/PP-M-0.6 BCS material with a basis weight of 200

g m-2. As showed in Figure 9c, with the increase of air flow rate, the filtration

efficiency decreased laggardly and finally remained at 92.06% when the air flow was

under the highest rate of 100 L min-1. This result could be attributed to the reduced

retention time of particles in the BCS materials caused by enhancive air flow rate,

which directly reduced the possibility for particles to collide on the fibers through

Brownian diffusion.36, 52 Moreover, we observed that there was almost a linear

relationship between pressure drop and air flow, which was consistent with Darcy’s

law for viscous resistance.53 The slope of the linear fit of pressure drop versus air flow

rate was only 0.81, which was much smaller than other fiber materials (e.g., 1.49 for

PSU/TiO2, 3.33 for PAN/PU),54-55 indicating that it had remarkable air permeability in

practical applications. The long-term stability of filtration efficiency was investigated via a 90-day tracking

test. The overall profile of filtration efficiency decay versus time presented a

continuous declining trend, especially in the first 30 days (Figure 9d). In the initial 30

days, the decrements of the filtration efficiencies of PE/PP-M-0.6 and PE/PP were

3.1% and 6.75%, respectively (Table S4†). In the subsequent 30-60 days, the

attenuation of filtration efficiency was greatly reduced by only 0.81% and 1.23%,

respectively (Table S4†). Ultimately, the change of their filtration efficiency was not

obvious and tended to be stable in the region of 60-90 days. Throughout the whole

process, the filtration efficiency of PE/PP-M-0.6 sample decreased from initial

98.94% to 94.9% by 4.1%. In contrast, the attenuation of PE/PP sample was more

significantly attenuated from 93.92% to 86.06% by 8.37%. The results illustrated that

PE/PP-M-0.6 samples possessed superior stability of filtration efficiency.To further provide insight into the mechanism of filtration efficiency decay, we

synchronously performed the evaluation of charge decay of BCS materials for 90 days

via the measurement of surface potential decay. As illustrated in Figure S8†, the

tendency of the potential decay is roughly the same as the filtration efficiency, which

indicates that the attenuation of the filtration efficiency is mainly caused by the escape

of the charge. Table S5† demonstrated the decrements of surface potential, and the

main part of which was still in the first 30 days. These results revealed once again that

the introduction of MgSt into BCS materials was an effective method to prevent

attenuation of filtration performance.We also carried out the measurement of loading filtration with purpose of observing

the dynamic filtration performance. Figure 9e presented that the comparison of the

loading time between electrospun samples, melt blown samples and BCS PE/PP-M-

0.6 samples. The time required for pressure drop of electrospun, melt blown and BCS

PE/PP-M-0.6 were 20, 23.75 and 72.5 min, respectively. This meant that BCS PE/PP-

M-0.6 materials could reduce the replacement time and possessed better dynamic

filtration behavior than the conventional filter materials. Furthermore, the dust

holding capacity of electrospun and melt blown were 3.92 and 4.13 g m-2,

respectively, while BCS PE/PP-M-0.6 achieved a relatively high value of 10.87 g m-2

(Figure 9f). These results could be explained by the fact that the former two materials

mainly relied on the surface filtration mechanism to intercept the particles (Figure S9a† and S9b†), which made the particles easily accumulate on the surface, so the

pressure drop rose rapidly and the dust holding capacity was not high. In contrast, the

PE/PP-M-0.6 material benefits from the Three-dimensional structure formed by the

through-air bonding technique, which intercepted the particles in the internal cavity of

the material, formed the dendritic structure, and gradually closed the open channels56

(Figure S9c†). In addition, the fluffy structure of PE/PP-M-0.6 materials prolonged

the passage time of airborne particles through the filter media, and significantly

improved the possibility of collision between particles and the surface of fibers,

especially some fine particles.57 By comparing the dust holding capacity differences, it

was further proved that the filtration performance of PE/PP-M-0.6 BCS materials was

robust in prolonging service life and reducing energy consumption.

Figure 9. (a) Filtration efficiency and pressure drop, and (b) quality factor of PE/PP-M-0.6 with various basis

weights. (c) Filtration efficiency, pressure drop, (d) Filtration efficiency decay of PE/PP-M-0.6 and PE/PP samples.

ConclusionsIn summary, novel PE/PP BCS air filtration materials containing MgSt capable of

capturing fine particles were successfully fabricated via spunbond technique.

Through-air reinforcement enabled the materials to form a fluffy 3D structure

conducive to trapping particulate matter. Furthermore, by introducing MgSt to change

the crystalline structure of the polymer, it endowed the materials with improved

surface potential and reinforced charge storage stability. The PE/PP-M-0.6 BCS

materials with basis weight of 200 g m-2 presented a robust filtration efficiency of

98.94%, low pressure drop 37.92 Pa, and excellent dust holding capacity of 10.87 g

m-2. In particular, the materials exhibited highly satisfactory resistance to attenuation

of filtration efficiency, ensuring its long-term storage and usage. We envision that the

PE/PP BCS materials will be a candidate for a wide range of applications in the field

of air filtration.

Conflicts of interestThere are no conflicts to declare.

AcknowledgmentJinxin Liu was sponsored by the China Scholarship Council (CSC) for 1-year study at

The University of Manchester (File No. 201806630025). This work was supported by

the Key Technologies R&D Program of China (No.2015BAE01B01) and China

Textile Industry Association Applied Basic Research Project (J201703).

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