Experimental and theoretical investigation of water-jet triboelectric charging of meltblown nonwoven fabrics

Abstract

The water-jet triboelectric charging technique has been widely used in China to improve the filtration efficiency of meltblown nonwoven fabric. However, there are few reports on research about water-jet triboelectric charging in the literature. In this paper, influences of the conductivity and pH of water on the filtration efficiency of meltblown nonwoven fabrics were investigated. The surface charge potential and its distribution of water-jet triboelectric charged meltblown nonwoven fabrics were characterized from microscopic and macroscopic aspects by using a bio atomic force microscope and electrostatic voltmeter. The charge stability was analyzed by using the thermally stimulated discharge technique. Meltblown nonwoven fabrics were water-jet triboelectric charged repeatedly in order to explore the charging mechanism. It is found that meltblown nonwoven fabrics with high filtration efficiency were developed by water-jet triboelectric charging treatment and the electrical conductivity of water had more influence than the pH on the filtration efficiency of triboelectric charged meltblown nonwoven fabrics. In addition, a possible mechanism of water-jet triboelectric charging is proposed based on experiments and characterizations.

Keywords

Triboelectric charging filtration meltblown nonwoven fabric surface potential

Corona charging treatment is widely used to improve the filtration efficiency of meltblown nonwoven fabrics while keeping a low pressure drop just by letting meltblown nonwoven fabrics go through a high-voltage electric field continuously before they are rolled into sheets.¹ One of the disadvantages of the corona charging process is that the surface charge of obtained meltblown nonwoven fabrics is susceptible to moistures in the environment and will decay gradually.²

The severe acute respiratory syndrome coronavirus-2 induced COVID-19 pandemic that began in 2019 has caused drastic changes in the world.^3,4 Since the outbreak of COVID-19, face masks have attracted great attention worldwide and the hydro charging technique, which is developed by the 3M Company,^5,6 has been widely used to improve the filtration performance of meltblown nonwoven fabric, which is the key layer of face masks. However, there are few reports on research about hydro charging of meltblown nonwoven fabrics in the literature. In 1999, a paper concerning the efficiency of electrostatic charging of water-sprayed fibers during the melt-blowing process was published. It was found that the charging efficiency was not affected by the water-spraying process if the amount of spraying was controlled.⁷ In 2014, a paper in Korean was published about meltblown nonwoven fabric filters for medical masks by using an online hydro charging system.⁸ The effects of the basis weight of meltblown nonwoven fabrics and die-to-collector distance on the filtration performance of meltblown nonwoven fabrics were investigated, while there was no deep discussion about the charge origin of the hydro charging treatment. Pan et al.⁹ reported a novel electret polypropylene/electret masterbatch composite meltblown nonwoven with enhanced charge stability using water electret charging. The micro mechanism of triboelectric initiation between a single fiber and deionized water was proposed, but it can only explain why the charge stability of water electret charging nonwovens is less affected by temperature and humidity than that of traditional corona charging nonwovens.

Water electret charging or hydro charging is one kind of triboelectric charging method. In order to attract more readers, we use the words ‘water-jet triboelectric charging’ to characterize the charging treatment. Meltblown nonwoven fabrics with high filtration efficiency were prepared in our lab by using simulating water-jet triboelectric charging equipment. Influences of the conductivity of water and drying temperature on the filtration efficiency of meltblown nonwoven fabrics were investigated.¹⁰ It is found that both negative and positive charges appeared on a single fiber of meltblown nonwoven fabric after water-jet triboelectric charging treatment, which is more desirable as a filter.

Based on the above background and our previous study, in this paper, influences of the conductivity and pH of water on the filtration efficiency of meltblown nonwoven fabrics were investigated. The surface charge potential and charge stability of triboelectric charged meltblown nonwoven fabrics were investigated by using atomic force microscope (AFM) and thermally stimulated discharge (TSD) techniques. Meltblown nonwoven fabrics were water-jet triboelectric charged repeatedly with the aim to uncover the charging mechanism.

Material and methods

Triboelectric charging experiment

A simulating water-jet triboelectric charging equipment reported in our previous paper¹⁰ was used to study the influences of the conductivity and pH of water on the filtration efficiency of polypropylene meltblown nonwoven fabrics. Water with different conductivity and pH was sprayed out of nozzles of a portable sprayer and continuously impinged on meltblown nonwoven fabrics for a certain time. Then the wet meltblown nonwoven fabrics were dried in an oven for 2 minutes under 110°C to obtain water-jet triboelectric charged meltblown nonwoven fabrics. Polypropylene meltblown nonwoven fabrics was prepared by Shanghai Zhenpu Medical Apparatus Co., Ltd, by blending with 2.5% water charging electret additives (provided by Qingdao Fundchem Co., Ltd), with the basis weight, fiber diameter, and web thickness of 25 g/m², 3.0 µm, and 0.18 mm, respectively.

Measurements

The conductivity of water used for the water-jet triboelectric charging experiment was measured by using a conductometer (FE30K, FiveEasy Plus).

The filtration efficiency of meltblown nonwoven fabrics was established by using an automated filter tester (TSI 8130, TSI Inc., USA). Sodium chloride was used to generate aerosol particles using an atomizing air pump with a mass median diameter of 0.26 mm and a geometric standard deviation of less than 1.83. The air flow rate was 85 L/min and the valid test area was 100 cm². Each sample was tested five times to ensure accuracy. An electron-laser particle photometer was used to measure the concentration of aerosol particles in the upstream (C_u) and downstream (C_d) of the sample, respectively. Filtration efficiency (η) is calculated as follows:

η = (1 - C_{d} / C_{u}) \times 100 %

(1)

An electrostatic voltmeter (Model 542, TREK Inc., USA) was used to determine the surface charge potential of meltblown nonwoven fabrics with the size of 12 cm × 12 cm, which were laid on an insulating desk. The values of surface charge potential at different point above the meltblown nonwoven fabrics were recorded one by one when the sensor, which was held by hand, was moving over meltblown nonwoven fabrics 15 mm away. In order to characterize the charge potential distribution, each sample was tested 25 times and the distance between every adjacent measuring point was about 2 cm.

A Dimension FastScan Bio AFM (Bruker Co., USA) was used to measure the surface potential of meltblown nonwoven fabrics. The model of the conducting probe was OMCL-AC240TM-R3 with the spring constant and resonance range of 2 N/m and 2 V, respectively. The lift-off distance of the probe was 200 nm.

The open circuit TSD measurements of meltblown nonwoven fabrics were carried out in a system consisting of a temperature controlled oven with a linear heating rate of 3°C/min, an electrometer (Model6517B, Keithley Instruments Inc., Ohio, USA), and a data processing computer. The linear temperature range of the TSD test was 30–150°C. The charge trap calculated was from TSD spectra using the following equation

E = \frac{k {T_{m}}^{2} I (T_{m})}{\int_{T_{m}}^{\infty} I (T) d T}

(2)

where E denotes the charge trap, k is the Boltzmann constant, T_m and I(T_m) are the peak temperature and electric current corresponding to T_m, respectively, and I(T) is the current density.

The stored electrical charge calculated was from TSD spectra using the following equation¹¹

Q = \frac{1}{β} \int_{T_{0}}^{T_{\infty}} I (T) d T

(3)

where Q denotes the stored charge density and β is the heating rate.

Wide-angle X-ray diffraction (WAXD) was measured using an X-ray diffractometer (Rigaku International Corporation, Japan) in continuous mode to determine the degree of crystallinity and the crystal structure of meltblown nonwoven fabrics.

Results and discussion

Influences of the electrical conductivity of water on the filtration efficiency of meltblown nonwoven fabrics

The electrical conductivity of distilled water used widely in the study of solid–liquid contact electrification is only 0.18 μS/cm (the resistivity is 18.2 MΩ cm), while the conductivity of tap water is in the range of 125–1250 μS/cm.¹² Hence, in this paper, the influence of the electrical conductivity of water on the effect of water-jet triboelectric charging was studied firstly. Four kinds of water, namely purified water, deionized water, 50 ppm Na₂CO₃ aqueous solution, and tap water, with the electrical conductivities of 3.0, 20.8, 130.0, and 460.3 μS/cm, respectively, were used for water-jet triboelectric charging treatment, while keeping other experimental conditions the same. The effects of the electrical conductivity of water on the filtration efficiency of the obtained meltblown nonwoven fabrics are shown in Table 1. In addition, the pH of water was measured and is also shown in Table 1.

Table 1.

Effects of the conductivity of water on the filtration efficiency of meltblown nonwoven fabrics

Water	Conductivity (μS/cm)	pH	Filtration efficiency (%)
Purified water	3.0	7.1	92.4
Deionized water	20.8	8.2	91.1
50 ppm Na₂CO₃ solution	130.0	10.1	85.5
Tap water	460.3	7.2	60.4

It can be found from Table 1 that meltblown nonwoven fabrics with filtration efficiency as high as 92.4% were obtained after being charged by using water with the conductivity as low as 3.0 μS/cm, while the filtration efficiency of the original sample was only 33.4%, suggesting water-jet triboelectric charging treatment makes an important contribution to the filtration efficiency of meltblown nonwoven fabrics by making full use of the electrostatic attraction filtration mechanism. In addition, the filtration efficiency of meltblown nonwoven fabrics decreased with the increase of the conductivity of water significantly, suggesting more charges were generated on the fiber surface of meltblown nonwoven fabrics when water with low conductivity was used, consistent with the findings in the study of solid–liquid contact electrification that solutes in the solution affect the surface charge generated during the solid–liquid liquid–solid contact electrification.¹³ In addition, it seems that the electrical conductivity of water had more influence than pH on the filtration efficiency of water-jet triboelectric charged meltblown nonwoven fabrics, whose filtration efficiency decreased from 92.4% to 60.4% when purified water and tap water with similar pH values were used, respectively.

Influence of water pH on the filtration efficiency of meltblown nonwoven fabrics

It is reported that the pH of the solution affects the surface charge generated during the solid–liquid contact electrification.¹⁴ Hence, it is necessary to study the effects of water pH on the filtration efficiency of water-jet triboelectric charged meltblown nonwoven fabrics. In this paper, water with pH values of 3 and 11 was prepared by regulating purified water with HCl and NaOH solutions, respectively. The filtration efficiencies of meltblown nonwoven fabrics after water-jet triboelectric charging treatment with different pH values of water are shown in Table 2. In comparison, the filtration efficiency of meltblown nonwoven fabrics triboelectric charged by using purified water is listed as well. In order to exclude its influence, the conductivity of water was measured and is also shown in Table 2.

Table 2.

Effects of pH values of water on the filtration efficiency of meltblown nonwoven fabrics

Conductivity (μs/cm)	pH	Filtration efficiency (%)
3.9	3.0	89.1
3.0	7.1	92.4
18.0	11.0	78.1

It can be seen from Table 2 that meltblown nonwoven fabrics triboelectric charged by using neutral water exhibited the highest filtration efficiency, suggesting more charges were generated on the fiber surface of meltblown nonwoven fabrics when neutral water was used, consistent with the findings in the study of the influences of the electrical conductivity of water on the filtration efficiency of meltblown nonwoven fabrics that solutes in the solution affected the surface charge generated during the solid–liquid liquid–solid contact electrification.

Surface charge potential of meltblown nonwoven fabrics from microscopic and macroscopic aspects

In order to verify the contribution of static charges to the filtration efficiency of meltblown nonwoven fabrics, the surface charge potentials of meltblown nonwoven fabrics water-jet triboelectric charged by using water with different pH values were measured by using a non-contacting voltmeter, and the results are shown in Figure 1, which was plotted by using Origin software with the surface charge potential as the vertical z-axis against position of the measuring point as the horizontal x-axis and y-axis, respectively. The smallest surface charge potential of each sample is taken as the origin of the z-axis.

Figure 1.

Distribution diagram of the surface charge potential of triboelectric charged meltblown nonwoven fabrics under water with different pH values. (a) pH = 3, (b) pH = 7 and (c) pH = 11.

As shown in Figure 1, negative and positive surface charge potential appeared randomly on meltblown nonwoven fabrics water-jet triboelectric charged by using water with different pH values. However, in other research on triboelectrification, only one kind of surface charge can be achieved, that is, negative or positive.¹⁵

To further characterize the surface charge potential of water-jet triboelectric charged meltblown nonwoven fabrics from the microscopic aspect, meltblown nonwoven fabrics water-jet triboelectric charged with neutral water was measured by using the AFM and the surface charge potential is as shown in Figure 2.

Figure 2.

Surface charge potential distribution of water-jet triboelectric charged meltblown nonwoven fabrics.

It can be found from Figure 2 that negative and positive charge potential appeared simultaneously on water-jet triboelectric charged meltblown nonwoven fabrics. Since the mean fiber diameter of meltblown nonwoven fabrics studied in this paper is in the range of 1–4 μm, it can be concluded that positive and negative surface charge potential appeared simultaneously on a single fiber of triboelectric charged meltblown nonwoven fabrics, which is totally different from the charges widely reported in the study of triboelectrification and is consistent with the results shown in Figure 1. During the filtration process, neutral particles are polarized by the strong electrostatic field acting around the surface of negatively and positively charged fibers and are thereby attracted by the fibers.¹⁶ Therefore, water-jet triboelectric charging of meltblown nonwoven fabrics is an ideal method to prepare air filters by making full use of the electrostatic attraction filtration mechanism. On the other hand, the surface charge potential of meltblown nonwoven fabrics measured by using an electrostatic voltmeter cannot reflect the net charges of meltblown nonwoven fabrics and cannot be used directly to evaluate the filtration efficiency of charged filters.

Charge storage stability of water-jet triboelectric charged meltblown nonwoven fabrics

In order to uncover the nature of charge carriers and their transfer mechanism, meltblown nonwoven fabrics water-jet triboelectric charged with purified water and tap water, as shown in Table 1, were investigated by using TSD current measurement, which is often applied to study the stable behavior of charge storage.¹⁷ For comparison, the original meltblown nonwoven fabrics was measured as well. The charge trap and stored electric charges of three samples were calculated according to Equations (2) and (3). The original meltblown nonwoven fabrics and meltblown nonwoven fabrics water-jet triboelectric charged with tap water and purified water are named samples 1–3, respectively, as shown in Table 3.

Table 3.

Charge trap and stored electric charges of meltblown nonwoven fabrics

Samples	Filtration efficiency (%)	E (eV)	Q (pC)
1	37.5	1.26	134.3
2	60.4	1.10	1763.7
3	92.4	1.15	4121.3

It is found from Table 3 that the charge trap of the three samples was similar, suggesting meltblown nonwoven fabrics composited with the same electret additives exhibited a strong charge trapping ability. On the other hand, the stored electric charge of meltblown nonwoven fabrics water-jet triboelectric charged by using purified water was much higher than that of meltblown nonwoven fabrics triboelectric charged with tap water, consistent with the filtration performance, as shown in Table 1.

Influences of repeated water-jet triboelectric charging treatment on the filtration efficiency and crystal structure of meltblown nonwoven fabrics

It is reported that the number of charges on the fibers is significantly reduced after isopropyl alcohol immersion.¹⁸ In order to uncover the water-jet triboelectric charging mechanism, meltblown nonwoven fabrics were water-jet triboelectric charged repeatedly, which were conditioned by isopropyl alcohol vapor to remove the charge effect before the next water-jet triboelectric charging treatment. The filtration efficiency of meltblown nonwoven fabrics triboelectric charged repeatedly different times is shown in Table 4.

Table 4.

Filtration efficiency of meltblown nonwoven fabrics water-jet triboelectric charged repeatedly different times

Times	Filtration efficiency (%）
1	94.4
2	92.6
3	91.0
4	75.5
5	74.1

It can be found from Table 4 that the filtration efficiency of meltblown nonwoven fabrics decreased obviously after four cycles of water-jet triboelectric charging treatment. In order to rule out the influence of physical damage to the meltblown nonwoven fabrics due to the repeated water-jet triboelectric charging treatment, the filtration efficiency of meltblown nonwoven fabrics water-jet triboelectric charged repeatedly four times was measured after being fumed by isopropyl alcohol vapor to remove the charge effect. It is found that the filtration efficiency was 31.9%, similar to the original one, the filtration efficiency of which was 33.4%. Therefore, it is believed that the filtration efficiency decrease of meltblown nonwoven fabrics after water-jet triboelectric charging treatment repeated four times was due to the worsening water-jet triboelectric charging effect.

It is reported that the charge storage ability of polypropylene is influenced by its crystal structures.¹⁹ Hence, the crystal structure of meltblown nonwoven fabrics water-jet triboelectric charged repeatedly three and five times was measured, respectively. For comparison, WAXD of the original sample was measured as well, and the results are shown in Figure 3.

Figure 3.

Wide-angle X-ray diffraction of meltblown nonwoven fabrics water-jet triboelectric charged repeatedly different times.

As reported in the literature, diffraction peaks at every single curve of WAXD correspond to the lattice planes (110), (040), and (130), respectively, and are all characteristic of the α phase with monoclinic configuration for polypropylene. Diffraction peaks at 16.0° and 21.0° corresponding to the lattice planes (300) and (301) are characteristic of the β phase.²⁰ It can be established from Figure 3 that the diffraction peaks were in good agreement with the α and β crystals, which were helpful to provide more space charge traps for polypropylene meltblown nonwoven fabrics. However, peaks at 14° and 16° of meltblown nonwoven fabrics water-jet triboelectric charged repeatedly five times were split into two peaks, respectively, suggesting that another phase appeared. It is reported that isopropyl alcohol vapor can diffuse on the polypropylene surface and may infiltrate into the chain segments or the lattice of polypropylene, causing the swelling of the polypropylene configuration.²¹ In this study, it is believed that another phase appeared in polypropylene meltblown nonwoven fabrics because of the repeated isopropyl alcohol fuming treatment, which was not helpful to attract charges, and its filtration efficiency decreased consequently.

Assumption of the mechanism of water-jet triboelectric charging of meltblown nonwoven fabrics

Based on the above experiments and characterizations, a schematic diagram of the water-jet triboelectric charging mechanism is proposed, as shown in Figure 4, and can be explained as follows.

Figure 4.

Schematic diagram of the water-jet triboelectric charging mechanism. (a) Water-jet triboelectric charging and (b) drying.

The water droplets have been positively or negatively charged due to the friction with the pipeline and splashing and bubbling movement in the air before they contact with the meltblown nonwoven fabrics. They will be neutralized after receiving electrons or transferring electrons to fibers of the meltblown nonwoven fabrics during the water-jet triboelectric charging treatment. At the same time, neutral water droplets will lose the lone pair electrons from the oxygen atoms as well. Correspondingly, every single fiber of the meltblown nonwoven fabrics will be charged positively and negatively simultaneously, as shown in Figure 4(a). Then the wet water-jet triboelectric charged meltblown nonwoven fabrics are dried in hot air to drive away unstable charges and residual moisture on the surface of the fibers. Other surface charges move into the inner layer of fibers and stay in charge traps, thereby endowing meltblown nonwoven fabrics with excellent filtration performance.

It is believed that electret additives in meltblown nonwoven fabrics help to provide more charge traps, resulting in meltblown nonwoven fabrics with higher filtration efficiency. In contrast, the filtration efficiency of meltblown nonwoven fabrics will decrease if there exist fewer charge traps in meltblown nonwoven fabrics, such as meltblown nonwoven fabrics water-jet triboelectric charged repeatedly five times.

Based on the above mechanism, hydroxyl ions or hydrogen ions would adsorb on the fiber surface of meltblown nonwoven fabrics when water with pH values of 3 and 11 was used, and charges on the fiber surface would be screened during the water-jet triboelectric charging process. In the following drying process, hydroxyl ions or hydrogen ions would be driven away with the moisture, resulting in meltblown nonwoven fabrics with lower filtration efficiency, as shown in Table 2.

Conclusion

In summary, influences of the conductivity and pH of water on the filtration efficiency of meltblown nonwoven fabrics were investigated. The surface charge potential and charge stability of water-jet triboelectric charged meltblown nonwoven fabrics were explored by using an AFM and TSD. Influences of repeated triboelectric charging treatment on the filtration efficiency and crystal structure of meltblown nonwoven fabrics were studied as well. Meltblown nonwoven fabrics with high filtration efficiency were developed and the charging mechanism was explored for the first time. More details are summarized as follows.

The water-jet triboelectric charging is an ideal method to produce meltblown nonwoven fabrics with high filtration efficiency, which was decreased with the increase of the electrical conductivity of water significantly. Meltblown nonwoven fabrics water-jet triboelectric charged by using neutral water exhibited higher filtration efficiency in comparison with those triboelectric charged by using acid or basic water. Negative and positive charge potential appeared simultaneously on water-jet triboelectric charged meltblown nonwoven fabrics, while the surface charge potential of meltblown nonwoven fabrics measured by using the electrostatic voltmeter could not reflect the net charges of meltblown nonwoven fabrics and could not be used directly to evaluate the filtration efficiency of charged filters.

A possible mechanism of water-jet triboelectric charging is proposed as follows: during the water-jet triboelectric charging process, positive and negative charges generate simultaneously on the fiber surface of meltblown nonwoven fabrics because water droplets have been positively or negatively charged due to the friction with the pipeline and splashing and bubbling movement in the air before they contact with meltblown nonwoven fabrics. Then the wet water-jet triboelectric charged meltblown nonwoven fabrics are dried in hot air to drive away unstable charges and residual moisture on the surface of the fibers. Other surface charges move into the interior of the fibers and stay in charge traps, resulting in meltblown nonwoven fabrics with high filtration efficiency.

For future studies, other methods need to be developed to measure charges carried by water droplets during the water-jet triboelectric charging process in order to verify the water-jet electrification mechanism proposed in this study.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Hong Wang

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