Weiyin Su,Lanfeng Hui,Xiaoyan Ma,Lingyuan Wang,Yu Wang

Tianjin Key Lab of Pulp&Paper,Tianjin University of Science and Technology,Tianjin,300457,China

Abstract:Electrospinning technology has become a research hotspot because of its advantages,such as simple operation,low cost,large specific surface area,high porosity,and good fiber continuity.Here,a new type of composite nanoair filter paper was prepared using electrospinning technology.To improve the practicability of air filter base paper,phenolic resin was used as a curing agent to improve the strength.The results show that the electrospun nanoair filter paper with air filter paper as the receiving substrate and polyvinyl alcohol(PVA)solution as the spinning solution excellently performed in all aspects.The influence of the thickness(spinning time)of the PVA nanofiber membrane on the micromorphology,physical properties,and filtration performance of the electrospun nanoair filter paper was analyzed.According to the ISO 5011-2014 standard,the initial resistance,filtration efficiency,mean pore size,and dust capacity of the electrospun nanoair filter paper were 77.3 Pa,99.9941%,3.50μm,and 146 g/m2,respectively,when the spinning time was 15 min.

Keywords:electrospinning technology;electrospun nanoair filter paper;polyvinyl alcohol(PVA);initial resistance;filtration efficiency

1 lntroduction

Rapid industrial and continuous economic development has resulted in a significant increase in demand and access to air.As a result,air pollution has become a severe problem for human health[1-2].Fine particles(PM2.5)in the environment are harmful to human health[3],resulting in children's atopic dermatitis[4],and lung cancer[5].Therefore,it is necessary to mitigate the amount of PM2.5 indoors.One of the most effective methods is to improve the filtration efficiency and accuracy of air filter paper.Hence,it is important to prepare advanced air filter paper with high filtration efficiency and relatively low initial resistance.

An ancient technology(electrospinning),produced in 1897,was invented by Rayleigh[6].In this process,the polymer solution(a certain proportion of polymer and solvent)is injected into a spinneret;high-voltage static electricity is applied to the solution,and the jet is ejected from the surface of the Taylor cone.After a straight-line movement,it bends in the jet,and the solvent volatilizes in the jet,becoming disordered,and the form of the alignment is solidified in the receiving device[7-8].To date,electrospinning devices have been greatly modified,such as side-by-side[9],coaxial[10],hollow tube[11],rotary cone[12],and needleless electrospinning devices[13].Factors that influence the electrospinning process are classified into three main groups:solution,process,and ambient parameters[14-15].Recently,electrospinning has been widely considered and valued by society as an important technology for preparing nanofibers.Its outstanding advantage of application in many aspects has achieved great results.For instance,in the medical field,it has attracted the attention of many scholars and medical technology researchers and has been applied in drug control[16-18]and wound repair[19-21].Vasitaa et al[22]prepared a polyaniline nanofiber sensor doped with camphor sulfonic acid in a low-cost,low energy consumption,and fast-response sensor preparation process.In addition, electrospinning technology has broad application prospects in the fields of nanowires,nanocables,and other electronic devices[23-24].

Electrospinning solutions have wide selectivity for materials;however,most of the polymers that can be used for solution spinning are mainly organic solvents.Owing to the characteristics of volatile,soluble,flammable,and toxic organic solvents,they can easily harm the environment and human body.The macromolecular chain of the water-soluble polyvinyl alcohol(PVA)is connected to a large number of hydrophilic hydroxyl groups,and its properties are determined by its polymer and alcoholysis degree.The interaction between the large hydroxyl groups in the side chain of PVA renders it a semicrystalline polymer.This is because the interaction between the hydroxyl groups makes it difficult for PVA to form hydrogen bonds with water to dissolve;it needs to be heated and stirred in hot water to dissolve.The excellent hydrophilicity,gas barrier,acid and alkali resistance,organic solvent resistance,biological safety,and filmforming properties of PVA have attracted considerable attention to the application of PVA in electrospinning technology[25-26].

In this study,PVA was chosen as the spinning solution,and PVA nanofibers were deposited on the surface of filter paper via electrospinning to form a composite nanoair filter paper.To improve the physical strength of the composite nanoair filter paper,the air filter base paper was impregnated and solidified with phenolic resin and electrospun.The electrospun nanoair filter paper exhibits good filtration performance and stable strength and has high practical application value and great development potential.

2 Experimental

2.1 Preparation of PVAsolution and phenolic resin

PVA powder(Mw=44.05)purchased from Aladdin Industrial Corporation was added to deionized water and stirred at 80℃for 5-6 h at a ratio of 1:9(w/w)to prepare a spinning solution with a mass concentration of 10%.Phenolic resin(58%)obtained from Shandong Wanhao Paper Group Co.,Ltd.was added to anhydrous methanol,and the concentration was diluted to 7%.Owing to the excellent volatility of anhydrous methanol,phenolic resin is best prepared prior to the experiment to avoid the volatility of anhydrous methanol when not used in the experiment.

2.2 Impregnation and solidification

Phenolic resin(7%)was poured into glassware,and the air filter base paper was immersed in it for 30 s.Subsequently, the sample was removed, and quantitative filter paper was used to absorb the residual phenolic resin.Finally,the sample was cured in an electric blast drying oven (DKM610C,Yamato Company,Japan)for 10 min.

2.3 Process of electrospinning nanoair filter paper

The PVA solution was injected into a 10 mL syringe.A 22#stainless steel needle was used for the injection.Its outer and inner diameters were 0.72 and 0.41μm,respectively.An electrospinning machine (PD30,Yunfan Instrument Co.,Ltd.,China)was used for spinning at a spinning voltage of 21 kV,and the injection speed was set at 0.56 mL/h.The receiving substrate(filter paper)adhered to the rotating drum.The distance between the needle and receiving device was 15 cm,and the rotation speed of the roller was 500 r/min.PVA nanofibers were deposited on the filter papers.The samples were removed from the roller and dried in a 60℃vacuum drying oven(DKM610C,Yamato,Japan)for 3-5 min.

2.4 Characterization

2.4.1Scanning electron microscopy(SEM)analysis

After the samples were sputter-coated with gold,the surface and cross-sections of different filter paper samples were observed using a SEM(JSM-IT300LV,Hitachi,Japan)at an accelerating voltage of 15 kV.

2.4.2Physical performance analysis

The influence of the thickness of the PVA membrane on the performance of the air filter paper was investigated.The thickness of the filter paper was tested using a thickness tester(L&W Company,Sweden).The tensile and bursting strengths of the filter paper were tested using a tensile strength tester(L&W Company,Sweden)and bursting strength tester(L&W Company,Sweden).

2.4.3Air permeability test

Air permeability is an important indicator of the permeability of the filter media.To a certain extent,it reflects the porosity of the filter paper and the fluency of the gas molecules through the filter paper.The filter paper was placed in the fixed value circle of the digital air permeability meter(YG461E,Ningbo Textile Factory,China)and pressed with the compression ring and automatically entered the test state after calibration.Under the specified pressure difference of 200 Pa,the airflow through a given area of samples at a certain time was measured,and the air permeability was calculated.

2.4.4Pore size test

A Porolux 100 FM capillary flow pore size analyzer(Promet Company,Belgium)was used for the pore size analysis.A circular paper with a diameter of 25 mm was fully wetted with isopropyl alcohol.The sample was pressed under the instrument,and high-purity nitrogen gas was passed through one side of the paper to the other side.The mean value of the gap between the fibers in the filter paper was obtained.

2.4.5PALAS MFP3000 filter material test system analyses

The results obtained from the PALAS MFP3000 filter material test system (Germany)show the initial resistance,filtration efficiency,and dust capacity of the filter paper.The dust used in the test was A2 standard dust(Shanghai Ruibei Trade Co.,Ltd.,China).The end pressure difference was 2000 Pa,dust concentration 1000 mg/m3,rated gas flow rate 66 L/min,sample area 100 m2,and surface velocity of the sample 11.1 cm/s.

3 Results and discussion

3.1 Influence of the thickness of electrospun nanofiber membrane on air filter base paper

The factory-produced air filter base paper was solidified to obtain air filter paper,and two filter papers were used as the receiving substrate to prepare electrospun nanoair filter base paper and electrospun nanoair filter paper,respectively.Under the same experimental conditions of electrospinning,as shown in Fig.1,after 9,15,30,and 45 min of electrospinning,the grammage and thickness of the electrospun nanoair filter base paper slowly and evenly increased.SEM was used to observe the morphology of the composite,as shown in Fig.2.The number of PVA nanofibers on the surface of the filter paper was different because of the different spinning times and represented a gradually increasing trend.The number of PVA nanofibers on the surface of the filter paper after 9 min of spinning was the lowest and that after 45 min was the highest.However,the vertical and horizontal distributions of the PVA nanofibers remained uniform.When the spinning time exceeded 45 min,the PVA nanofiber layer on the surface of the air filter paper was visible,and even the PVA nanofibers on the upper layer could be observed in air with the naked eye because of the weak adhesion with the surface.As shown in the crosssection of the electrospun nanoair filter base paper in Fig.3,the lower layer was the fiber of the original filter paper,and the diameter of the fiber was large.The upper layer of the electrospun PVA nanofiber was extremely fine.This is because PVA exhibits good adhesiveness.Qiu et al[27]studied the importance of hydrogen bonding when PVA was bound to other surfaces,which proved that PVA nanofibers had not entered the filter paper;however,they were closely bonded with the lower layer of fibers,evenly covering the surface of the filter paper to form a layer of PVA nanofiber film.

Fig.1 Analysis of the grammage and thickness ofelectrospun nanoair filter base paper at different spinning times

Fig.2 SEMimages of electrospun nanoair filter base paper at different spinning times

Fig.3 Cross-section SEMimage of electrospun nanoair filter base paper

Using electrospinning technology, PVA was deposited on the air filter base paper in the form of nanofibers,which improved its performance and increased its potential for real-life application.Subsequently, the filtration performance of the electrospun nanoair filter base paper was tested.Table 1 shows the filtration performance indices of the air filter base paper for comparison.

Table 1 Filtration performance indices of air filter base paper

As shown in Fig.4,the air permeability and initial resistance of the electrospun nanoair filter base paper were analyzed.To a certain extent,the air permeability of the filter paper reflected the porosity and fluency of the gas molecules passing through the filter paper[28].The air permeability of the electrospun nanoair filter base paper constantly decreased,which indicated that when the airflow passed through the interior of the paper,the collision effect between the gas molecules and fiber reduced the movement speed of the airflow.Therefore,the energy consumed by the movement of the gas molecules reduced,and the number of gas molecules passing through the electrospun nanoair filter base paper per unit time also reduced,leading to an increase in its initial resistance.Fig.5 shows that the mean pore size of the electrospun nanoair filter base paper continuously decreased.As shown in Fig.2 and Fig.3,the fibers of the composite nanoair filter base paper were laminated.The fiber pores of the air filter base paper were divided into several smaller pores by PVA nanofibers,which decreased the pore size.The smaller the pore size,the smaller the particle size of the filterable material,and the greater the filtration efficiency.When the spinning time was 45 min,the mean pore size of the electrospun nanoair filter base paper was 18%that of the air filter base paper.The filtration efficiency increased to 99.9965%,which was closely related to the improvement in the filtration accuracy.To prevent the particles from being trapped in the filter paper to block the pores,the smaller the pore size,the more solid particles could pass.Conversely,there was a reason for the decrease in air permeability[29-30].The decrease in pore size increased the collision probability of gas molecules and fibers,when the air flowed through the filter paper,and reduced the airflow rate.Fig.6 shows the change in the dust capacity of the electrospun nanoair filter base paper.The dust capacity of the electrospun nanoair filter base paper did not continuously increase;however,it reached its maximum at 15 min.This is because the dust capacity was significantly related to the change in the initial resistance and mean pore size.When the dust particles passed through the fiber layer,the smaller the pores,the stronger the interception and adsorption of dust particles by the filter paper,and the larger the amount of dust stored on the surface of the filter paper[31].A PALAS MFP3000 filter material test system was used in this experiment,and the end pressure difference was 2000 Pa.The large initial resistance reduced the dust flow in the test system and affected the dust capacity.

Fig.4 Analysis of air permeability and initial resistance of electrospun nanoair filter base paper at different spinning times

Fig.5 Analysis of mean pore size and filtration efficiency of electrospun nanoair filter base paper at different spinning times

Fig.6 Analysis of the dust capacity of electrospun nanoair filter base paper at different spinning times

3.2 Study on the strength of composite nanoair filter paper

Although the filtration performances of the electrospun nanoair filter base paper were significantly improved,inadequate physical properties hindered its usefulness in practical applications.To enhance the strength of the paper,the electrospun nanoair filter base paper was solidified to obtain a cured electrospun nanoair filter base paper.The results showed that its filtration performance was unsatisfactory.Table 2 compares the filtration performances of the air filter base paper,electrospun nanoair filter base paper(electrospinning for 15 min),and cured electrospun nanoair filter base paper.After impregnation and solidification,it was observed that the filtration performance of the cured electrospun nanoair filter base paper was considerably similar to that of the air filter base paper,which indicated that the electrospinning experiment did not play a role.To investigate this,the fiber morphology of the cured electrospun nanoair filter base paper(electrospinning for 15 min)is shown in Fig.7.The PVA nanofibers covering the upper layer of the original fibers changed.After solidification,the fibers exhibited a crosslinking phenomenon,which rendered the fibers extremely fine,and the gap between the fibers increased.Finally,the filtration performance of the cured electrospun nanoair filter base paper was not as good as before.Zhang et al[32]studied the crosslinking phenomenon of PVA and glyoxal and demonstrated the crosslinking characteristics of PVA.

Fig.7 SEMimage of the cured electrospun nanoair filter base paper

Therefore,another method was to use air filter paper as the receiving substrate to obtain electrospun nanoair filter paper through electrospinning.The performance of the electrospun nanoair filter base paper and electrospun nanoair filter paper were tested and compared(Table 2).After curing,the phenolic resin and fibers were crosslinked[33].A stable threedimensional network structure was formed inside,resulting in the enhancement of the physical properties of the air filter base paper.The changes in the physical properties and filtration performance of the air filter base paper and air filter paper are shown in Table 3.The amount of glue on the air filter paper was 20.21%,the paper thickness increased by 1μm,and the tensile and bursting strengths increased approximately 1.5 times that of the air filter base paper.After impregnating and curing,compared with the air filter base paper,the air permeability increased by 4.2%,mean pore size increased by 23.3%,initial resistance decreased by 6.3%,and filtration efficiency decreased by 0.267 percentage points.However,the dust capacity practically did not change.This is because after the air filter base paper was impregnated and solidified by phenolic resin,a series of procedures were performed,such as impregnation,penetration,and film formation.Thereafter,it completely adhered to the fiber and fiber junction.The combination of the fiber and the layers was tighter,which reduced the resistance between the layers,leading to an increase in the pores between the fibers and an increase in air permeability.The corresponding leakage filtration efficiency reduced[34].

Table 2 Comparison of air filter base paper,electrospun nanoair filter base paper,and cured electrospun nanoair filter base paper

The electrospun nanoair filter paper was obtained with an air filter paper.The results show that the filtration performance of the filter paper remarkably changed.As shown in Fig.8,the overall air permeability of the electrospun nanoair filter paper was better than that of the electrospun nanoair filter base paper.Compared with the mean pore size of air filter base paper and air filter paper in Table 3,the mean pore size of electrospun nanoair filter base paper and electrospun nanoair filter paper was reduced,and the longer the spinning time,the smaller the mean pore size.It can be seen from the figure that the filtration efficiency of electrospun nanoair filter paper prepared at 15,30,and 45 min exceeded 99.99%.Compared with electrospun nanoair filter base paper,the initial resistance and dust capacity of electrospun nanoair filter paper were increased.The larger the dust capacity,the better the usability of the filter paper.These changes were related to the three-dimensional structure of the air filter paper,permeation,and membrane of the phenolic resin and are consistent with the previous analysis of filter paper changes.Another reason could be that many warped fibers were observed on the surface of the air filter base paper.After curing,the rough surface of the filter paper was smoothened.The rougher the surface of the filter paper,the larger the binding area between the fibers and the better the binding effect between the PVA nanofibers and filter paper.Summarily,the filtration indices were influenced by each other.When the spinning time was 15 min,the air permeability of the electrospun nanoair filter paper was 268 L/(m2·s),mean pore size 3.50μm,initial resistance 77.3 Pa,filtration efficiency 99.9941%,and dust capacity 146 g/m2.Under these conditions,the filter paper exhibited the best performance and a high practical value.

Table 3 Performance indices of air filter base paper and air filter paper

Fig.8 Characteristic changes of electrospun nanoair filter base paper and electrospun nanoair filter paper

4 Conclusions

Here,the thickness(spinning time)of the polyvinyl alcohol(PVA)nanofiber membrane on the filtration performance of an electrospun nanoair filter base paper was studied.The results showed that there was a balance between the filtration efficiency and initial resistance.Concomitantly,the method of impregnating and curing the air filter base paper prior to electrospinning greatly improved the physical properties of the electrospun nanoair filter paper.The electrospun nanoair filter paper prepared by spinning for 15 min exhibited outstanding features with a filtration efficiency of 99.9941% and an initial resistance of 77.3 Pa.Its mean pore size was 3.50μm.The tensile and bursting strengths of the air filter paper were 16.0 N·m/g and 1.73 kPa/(m2·g),respectively,which increased its potential for practical applications.To better meet the world's requirements for air quality in future,this new type of composite nanoair filter paper possesses great potential with further development.