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Abstract： In this paper， the manufacturing of high-efficiency air filter paper is reported. The air filter paper was produced using ultra-fine fibers and wateroat fibers mercerized by alkali， using an electrospinning apparatus with multiple rings. The high efficiency air filter paper has an antibacterial effect after adding a chitosan-copper complex which is harmless to humans. As a result of the measurement， the filtering efficiency of the air filter paper is approximately 99.998% and its antibacterial efficiency is approximately 99.5%.
Keywords： ultra-fine fiber; wateroat （Zizania latifolia） fiber; chitosan-copper complex; air filter paper
1 Introduction
With the development of science and technology， the standard of the requirements for the environment is rising. The rapid development of modern science and industry imposes the need for a higher level of air filtering. The industries including electronic， medicine， chemical， biological industries， and food processing， require miniaturization， high precision， purification， quality， and an indoor environment of higher reliability. These requirements impose raises of need for high efficiency air filter papers. Therefore， manufacturing high efficiency filters that can fully satisfy the requirements of consumers is very important[1-2].
The technology of the electrospinning-made ultra-fine fibers is developing rapidly. It is possible to manufacture high efficiency air filter papers using ultra-fine fibers[3-6]. The chitosan extracted from crab shell， and its derivatives provide antibacterial properties to the paper without harming human body[7-9].
In order to manufacture air filter papers with a 99.998% filtering efficiency and the antibacterial function， we conducted research on making the air filter paper by mixing the electrospinning-made ultra-fine fibers as the main filtering material and wateroat fibers mercerized by alkali as frame fibers and then adding a chitosan-copper complex， which is a natural sterilizer and harmless to the human body.
2 Experimental
2.1 Raw materials and equipments
2.1.1 Raw materials
Wateroat （Zizania latifolia）， acetate cellulose （substitution degree of 2.7）， acetone （AR）， chitosan （degree of deacetylation 95%， self-prepared）， copper sulphate （CuSO4， AR）， polyvinyl alcohol （PVA， average degree of polymerization of 1500）， sodium hydroxide （NaOH， AR）， and acetic acid （CH3COOH， AR）. 2.1.2 Experimental equipment
Hollander beater， table paper machine （F200） and air permeability tester （made by experimental equipment factory of Academy of Science， D P R Korea）， scanning electron microscope （SEM， QUANTA 200）， tensile strength tester （ZL-300A）， pore tester （HAD-KJ-10）， and a laser particle counter （PMS400-LASASP 100）.
2.2 Preparation of experimental materials
2.2.1 Preparation of wateroat fiber mercerized by alkali
Wateroat fibers mercerized by alkali are thinner and longer than wateroat fibers; therefore， they are suitable for manufacturing high efficiency filter paper.
Mercerized wateroat fibers were manufactured in the following steps： wateroat→cutting→cooking→washing→screening→bleaching→drying→mercerization→fraction→mercerized wateroat fibers.
The wateroat was cut into a length of 3～5 cm for cooking. Cooking condition was as follows： NaOH 15%， temperature 125℃， time 2.5 h， and the ratio of solid to liquid was 1∶3.5.
The wateroat fiber pulp was bleached in two stages and NaClO was used as a bleaching agent. The specifics of the first stage are as follows： pulp consistency 5%， temperature 55℃， time 3 h， and available chlorine 1.5%. The specifics of the second stage are as follows： pulp consistency 5%， temperature 55℃， time 2 h， and available chlorine 1%.
The bleached pulp was dried for mercerization. The conditions for mercerization are as follows： NaOH consistency 20%， time 10 min， and temperature 20℃. The parenchyma cell was separated from the pulp using an inclined screen.
2.2.2 Preparation of ultra-fine fibers
Using acetate cellulose as a main raw material for ultra-fine fibers， the acetate cellulose was dissolved in 8 wt%～10 wt% acetone and it was used as a spinning solution. The ultra-fine fibers were fabricated via a needleless electrospinning apparatus with multiple rings that we developed. Copper wire of 1 mm diameter was used for the multiple rings to make the diameter of 8 cm， and the distance between rings was 4 cm.
Electrospinning was performed at a voltage of 35 kV and the spinning distance was 17 cm.
2.2.3 Preparation of chitosan-copper complex
Refined chitosan with 95% deacetylation degree was made from crab shells[11]. It was solved in 0.5% acetate solution. Then， 0.35 mol CuSO4 was added in 1 mol chitosan units and it underwent a reaction for 2 h with stirring. The pH value of the reaction medium was adjusted to 6.5. The product of chitosan-copper complex powder was separated from the solution after the reaction， multiple washes with distilled water， and drying. The chitosan-copper complex was then solved in an acetic acid solution with a pH value of 2 at 90℃ and used as an antibacterial agent.
2.2.4 Manufacture of high-efficiency air filter paper
The air filter paper manufactured using only ultra-fine fibers has very poor strength. If natural fibers are mixed with ultra-fine fibers， the strength of the air filter paper can be increased.
Ultra-fine fiber was beaten by the free-beating method in the Hollander beater. After beating， its average length was between 1.5 mm and 2.5 mm.
Mercerized wateroat fibers were defibered and mixed with ultra-fine fibers. By changing the mixing ratio of ultra-fine fibers and mercerized wateroat fibers， sheets were manufactured for testing using a table paper machine. Sheets were dehydrated using a vacuum dehydrator and dried using an experimental cylinder dryer. Then， PVA solution was sprayed on the sheets to increase the strength of the air filter paper. Finally， the solution of chitosan-copper complex was sprayed to add antibacterial properties to the air filter paper.
2.3 Analysis and measurement
Mercerized wateroat fibers and ultra-fine fibers were examined in terms of SEM， pore diameter， and permeability of the filter paper. These were tested according to the ISO 1924.2， ISO5636/1， and EN868-3 standards， respectively.
3 Results and discussion
3.1 Characteristics of mercerized wateroat fibers
The morphology of mercerized wateroat fibers before and after the fraction process was investigated using a digital microscope. The results are shown in Fig.1. As shown in Fig.1， there only existed mercerized wateroat fibers in the raw material after the fraction process. It is shown that the mercerized wateroat fibers manufactured in the above-mentioned process can be used as raw material for high-efficiency filter paper.
The diameters of mercerized wateroat fibers were investigated using SEM images. As shown in Fig.2， its diameter is 2～4 mm and length is 0.8～1.6 mm.
3.2 Characteristics of ultra-fine fibers
The diameter of ultra-fine fibers was investigated using SEM images. As shown in Fig.3， the diameter of the ultra-fine fiber is 0.3～0.7 mm and its length is 5～10 mm.
3.3 Analysis of the chitosan-copper complex
The high-efficiency air filters used in medicine should be such that they do not permit microorganisms in the air to infiltrate. It is possible that the high-efficiency air filter paper may be infected by germs in air during use. To prevent this phenomena， the high-efficiency air filter paper should have antibacterial properties. Chitosan-copper complex can be used as the antibacterial agent[11]. The Fourier-transform infrared FT-IR spectra of the synthesized chitosan-copper complex and chitosan are shown in Fig.4. The peak at environs of 450 cm-1 reveals that the compound has a coordination bond of copper and nitrogen. The peaks at 400～500 cm-1 reveal that the chitosan-copper complex is successfully synthesized.
3.4 Characteristics of the high-efficiency air filter paper
3.4.1 Air permeability of air filter paper with different ultra-fine fiber content
The air permeability of air filter paper with different ultra-fine fiber content is shown in Table 1. An increase in the content of ultra-fine fibers led to a decrease in the air permeability of the air filter paper. This is because the increase in the content of ultra-fine fibers leads to a decrease in the porosity of the air filter paper.
3.4.2 Pore diameter of air filter paper with different ultra-fine fiber content
The pore diameter of air filter paper with different ultra-fine fiber content is shown in Table 2.
As shown in Table 2， the pore diameter of the air filter paper decreases with an increase in the ultra-fine fiber content. It reveals that ultra-fine fibers play an important role in the formation of small pores in air filter paper.
3.4.3 The strength of air filter paper with different PVA dosage
To increase the strength of air filter paper， a series of comparative experiments were performed using the PVA solution， and the results are shown in Table 3.
As shown in Table 3， when PVA dosage increases， the strength of the air filter paper also increases.
As PVA dosage increases， the air permeability of the air filter paper decreases and the flowing resistance increases. When PVA dosage is 0.6%， its mechanical strength increases to two-fold that of the original air filter paper; simultaneously， changes to the diameter and air permeability are slight. Thus the PVA dosage was determined to be 0.6%.
3.4.4 Filtering efficiency of air filter paper
The filtering efficiency of the air filter paper was measured using the DOP method with dioctyl phthalate[12-13]. The filtering efficiency of air filter paper was measured by fixing the base weight of the air filter paper to be 90 g/m2 and change the content of ultra-fine fibers in the air filter paper. When the content of ultra-fine fibers increases， the filtering efficiency also increases， but the change is not as significant as expected. When the content of ultra-fine fibers is 75%， the filtering efficiency of the air filter paper is approximately 99.998%. 3.4.5 Antibacterial effect of the chitosan-copper complex
Chitosan and chitosan-copper complexes have high antibacterial function. These materials have antibacterial capacities because their molecules contain —NH2. Generally， materials that comprise a cell nucleus have negative charge. However， —NH2 has positive charge and can combine with the materials of a cell nucleus and palsies their function. Therefore， the cells stop growing[11，14].
Different amount of chitosan-copper complex solution was sprayed on the high-efficiency air filter paper. After drying， the barley malt culture fluid was added on the papers. Then， the paper was placed in a 30℃ atmosphere and inoculated naturally for 5 days. Finally， the research on the antibacterial efficiency of high-efficiency air filter paper was carried out by measuring the areas contaminated and not contaminated by germs. Non-antibacterial paper is used as a control sample. The results are shown in Table 4.
As shown in Table 4， the antibacterial effect of air filter paper increases with the increasing amount of the chitosan-copper complex sprayed on the air filter paper. When the amount of chitosan-copper complex sprayed on the air filter paper is 0.025% by weight， the antibacterial effect reaches its maximum potential and antibacterial efficiency is observed to be approximately 99.5%. When the amount of the chitosan-copper complex is increased to more than 0.025% by weight， the antibacterial effect does not increase further.
As shown Fig.5， when the amount of chitosan-copper complex was 0.025%， no strain growth was observed （Fig.5（a））， whereas well-developed strains were observed in the absence of chitosan-copper complex （Fig.5（b））， indicating the excellent antibacterial efficiency of the chitosan-copper complex.
3.4.6 Technical characteristics of the high-efficiency air filter paper
The technical characteristics of the high-efficiency air filter paper with ultra-fine fiber of 75 wt% is shown in Table 5.
As shown in Table 5， the technical characteristics of the high-efficiency air filter paper are observed to be similar to that of the HEPA filter.
4 Conclusions
The high-efficiency filter paper can be manufactured with the wateroat fibers mercerized by alkali and ultra-fine fibers， which are produced through needleless electrospinning using multiple rings. Especially， the results have proved that mercerized wateroat fibers can be used to manufacturing high-efficiency air filter paper. In addition， the technology that prevents contamination of high-efficiency filter paper from germs spread in air has been established by sprafing chitosan-copper complex on the air filter paper. References
[1] Liu L H. The development and applications of air filter[J]. Filter & Separator， 2000， 10（4）： 8-18.
[2] Zhao H. Filter paper on based ultra fine glass fiber[J]. Paper and Papermaking， 2004， 3（2）： 56-59.
[3] Wang X， Lin T， Wang X G. Scaling up the production rate of nanofibers by needleless electro spinning from multiple ring[J]. Fibers and Polymers， 2014，15（5）： 961-965.
[4] Kostakova E， Meszaros L， Gregr J. Composite nanofibers produced by modified needleless electrospinning[J]. Materials Letters， 2009， 63： 2419-2422.
[5] LV X L， Zhang B， Zhang X G， et al. Recent Development of Electrospun Polymer Nanofibers[J]. Chemistry and Adhesion， 2014， 36（5）： 364-368.
[6] Wang X， Wang X G， Lin T. Electric field analysis of spinneret design for needleless electrospinning of nanofibers[J]. J Mater Res， 2012， 27（23）： 3013-3019.
[7] Zhuang X P， Li Z， Liu X F， et al. Progress in study on antibacterial fiber of chitosan/celluolse[J]. Chemical Industry and Engineering Progress， 2002， 21（5）： 310-313.
[8] Qin C Q， Li H R， Qi X， et al. Water-solubility of chitosan and its antimicrobial activity[J]. Carbohydrate Polymers， 2006， 63： 367-374.
[9] Qin C Q. Antimicrobial Activity of Chitosan in vitro[J]. Journal of Xiaogan University， 2005， 25（6）： 5-8.
[10] Xing S N. Test Methods of HEPA Filter and their Application[J]. Chinese Medical Equipment Joural， 2005， 26（1）： 29-31.
[11] Liu B Y， Wang J， Yao S W. The use of chitin-copper complex in antibacterium paper[J]. China Pulp & Paper Industry， 2004， 25（4）： 43-45.
[12] Nie X L， Shi M， Chen Y H. The Discussion of the Result of HEPA Air-filter Examining[J]. Light Industry Machinery， 2006， 24（2）： 168-170.
[13] Cao G Q. Discussion On New Testing System to Measure the Performance of High Efficiency Air Filters[J]. CC&AC， 2005（3）： 25-30.
[14] Yang F. Antibacterial Agent and Its Application in Antibacterial Paper[J]. China Pulp & Paper， 2006， 25（8）： 51-55. .
Keywords： ultra-fine fiber; wateroat （Zizania latifolia） fiber; chitosan-copper complex; air filter paper
1 Introduction
With the development of science and technology， the standard of the requirements for the environment is rising. The rapid development of modern science and industry imposes the need for a higher level of air filtering. The industries including electronic， medicine， chemical， biological industries， and food processing， require miniaturization， high precision， purification， quality， and an indoor environment of higher reliability. These requirements impose raises of need for high efficiency air filter papers. Therefore， manufacturing high efficiency filters that can fully satisfy the requirements of consumers is very important[1-2].
The technology of the electrospinning-made ultra-fine fibers is developing rapidly. It is possible to manufacture high efficiency air filter papers using ultra-fine fibers[3-6]. The chitosan extracted from crab shell， and its derivatives provide antibacterial properties to the paper without harming human body[7-9].
In order to manufacture air filter papers with a 99.998% filtering efficiency and the antibacterial function， we conducted research on making the air filter paper by mixing the electrospinning-made ultra-fine fibers as the main filtering material and wateroat fibers mercerized by alkali as frame fibers and then adding a chitosan-copper complex， which is a natural sterilizer and harmless to the human body.
2 Experimental
2.1 Raw materials and equipments
2.1.1 Raw materials
Wateroat （Zizania latifolia）， acetate cellulose （substitution degree of 2.7）， acetone （AR）， chitosan （degree of deacetylation 95%， self-prepared）， copper sulphate （CuSO4， AR）， polyvinyl alcohol （PVA， average degree of polymerization of 1500）， sodium hydroxide （NaOH， AR）， and acetic acid （CH3COOH， AR）. 2.1.2 Experimental equipment
Hollander beater， table paper machine （F200） and air permeability tester （made by experimental equipment factory of Academy of Science， D P R Korea）， scanning electron microscope （SEM， QUANTA 200）， tensile strength tester （ZL-300A）， pore tester （HAD-KJ-10）， and a laser particle counter （PMS400-LASASP 100）.
2.2 Preparation of experimental materials
2.2.1 Preparation of wateroat fiber mercerized by alkali
Wateroat fibers mercerized by alkali are thinner and longer than wateroat fibers; therefore， they are suitable for manufacturing high efficiency filter paper.
Mercerized wateroat fibers were manufactured in the following steps： wateroat→cutting→cooking→washing→screening→bleaching→drying→mercerization→fraction→mercerized wateroat fibers.
The wateroat was cut into a length of 3～5 cm for cooking. Cooking condition was as follows： NaOH 15%， temperature 125℃， time 2.5 h， and the ratio of solid to liquid was 1∶3.5.
The wateroat fiber pulp was bleached in two stages and NaClO was used as a bleaching agent. The specifics of the first stage are as follows： pulp consistency 5%， temperature 55℃， time 3 h， and available chlorine 1.5%. The specifics of the second stage are as follows： pulp consistency 5%， temperature 55℃， time 2 h， and available chlorine 1%.
The bleached pulp was dried for mercerization. The conditions for mercerization are as follows： NaOH consistency 20%， time 10 min， and temperature 20℃. The parenchyma cell was separated from the pulp using an inclined screen.
2.2.2 Preparation of ultra-fine fibers
Using acetate cellulose as a main raw material for ultra-fine fibers， the acetate cellulose was dissolved in 8 wt%～10 wt% acetone and it was used as a spinning solution. The ultra-fine fibers were fabricated via a needleless electrospinning apparatus with multiple rings that we developed. Copper wire of 1 mm diameter was used for the multiple rings to make the diameter of 8 cm， and the distance between rings was 4 cm.
Electrospinning was performed at a voltage of 35 kV and the spinning distance was 17 cm.
2.2.3 Preparation of chitosan-copper complex
Refined chitosan with 95% deacetylation degree was made from crab shells[11]. It was solved in 0.5% acetate solution. Then， 0.35 mol CuSO4 was added in 1 mol chitosan units and it underwent a reaction for 2 h with stirring. The pH value of the reaction medium was adjusted to 6.5. The product of chitosan-copper complex powder was separated from the solution after the reaction， multiple washes with distilled water， and drying. The chitosan-copper complex was then solved in an acetic acid solution with a pH value of 2 at 90℃ and used as an antibacterial agent.
2.2.4 Manufacture of high-efficiency air filter paper
The air filter paper manufactured using only ultra-fine fibers has very poor strength. If natural fibers are mixed with ultra-fine fibers， the strength of the air filter paper can be increased.
Ultra-fine fiber was beaten by the free-beating method in the Hollander beater. After beating， its average length was between 1.5 mm and 2.5 mm.
Mercerized wateroat fibers were defibered and mixed with ultra-fine fibers. By changing the mixing ratio of ultra-fine fibers and mercerized wateroat fibers， sheets were manufactured for testing using a table paper machine. Sheets were dehydrated using a vacuum dehydrator and dried using an experimental cylinder dryer. Then， PVA solution was sprayed on the sheets to increase the strength of the air filter paper. Finally， the solution of chitosan-copper complex was sprayed to add antibacterial properties to the air filter paper.
2.3 Analysis and measurement
Mercerized wateroat fibers and ultra-fine fibers were examined in terms of SEM， pore diameter， and permeability of the filter paper. These were tested according to the ISO 1924.2， ISO5636/1， and EN868-3 standards， respectively.
3 Results and discussion
3.1 Characteristics of mercerized wateroat fibers
The morphology of mercerized wateroat fibers before and after the fraction process was investigated using a digital microscope. The results are shown in Fig.1. As shown in Fig.1， there only existed mercerized wateroat fibers in the raw material after the fraction process. It is shown that the mercerized wateroat fibers manufactured in the above-mentioned process can be used as raw material for high-efficiency filter paper.
The diameters of mercerized wateroat fibers were investigated using SEM images. As shown in Fig.2， its diameter is 2～4 mm and length is 0.8～1.6 mm.
3.2 Characteristics of ultra-fine fibers
The diameter of ultra-fine fibers was investigated using SEM images. As shown in Fig.3， the diameter of the ultra-fine fiber is 0.3～0.7 mm and its length is 5～10 mm.
3.3 Analysis of the chitosan-copper complex
The high-efficiency air filters used in medicine should be such that they do not permit microorganisms in the air to infiltrate. It is possible that the high-efficiency air filter paper may be infected by germs in air during use. To prevent this phenomena， the high-efficiency air filter paper should have antibacterial properties. Chitosan-copper complex can be used as the antibacterial agent[11]. The Fourier-transform infrared FT-IR spectra of the synthesized chitosan-copper complex and chitosan are shown in Fig.4. The peak at environs of 450 cm-1 reveals that the compound has a coordination bond of copper and nitrogen. The peaks at 400～500 cm-1 reveal that the chitosan-copper complex is successfully synthesized.
3.4 Characteristics of the high-efficiency air filter paper
3.4.1 Air permeability of air filter paper with different ultra-fine fiber content
The air permeability of air filter paper with different ultra-fine fiber content is shown in Table 1. An increase in the content of ultra-fine fibers led to a decrease in the air permeability of the air filter paper. This is because the increase in the content of ultra-fine fibers leads to a decrease in the porosity of the air filter paper.
3.4.2 Pore diameter of air filter paper with different ultra-fine fiber content
The pore diameter of air filter paper with different ultra-fine fiber content is shown in Table 2.
As shown in Table 2， the pore diameter of the air filter paper decreases with an increase in the ultra-fine fiber content. It reveals that ultra-fine fibers play an important role in the formation of small pores in air filter paper.
3.4.3 The strength of air filter paper with different PVA dosage
To increase the strength of air filter paper， a series of comparative experiments were performed using the PVA solution， and the results are shown in Table 3.
As shown in Table 3， when PVA dosage increases， the strength of the air filter paper also increases.
As PVA dosage increases， the air permeability of the air filter paper decreases and the flowing resistance increases. When PVA dosage is 0.6%， its mechanical strength increases to two-fold that of the original air filter paper; simultaneously， changes to the diameter and air permeability are slight. Thus the PVA dosage was determined to be 0.6%.
3.4.4 Filtering efficiency of air filter paper
The filtering efficiency of the air filter paper was measured using the DOP method with dioctyl phthalate[12-13]. The filtering efficiency of air filter paper was measured by fixing the base weight of the air filter paper to be 90 g/m2 and change the content of ultra-fine fibers in the air filter paper. When the content of ultra-fine fibers increases， the filtering efficiency also increases， but the change is not as significant as expected. When the content of ultra-fine fibers is 75%， the filtering efficiency of the air filter paper is approximately 99.998%. 3.4.5 Antibacterial effect of the chitosan-copper complex
Chitosan and chitosan-copper complexes have high antibacterial function. These materials have antibacterial capacities because their molecules contain —NH2. Generally， materials that comprise a cell nucleus have negative charge. However， —NH2 has positive charge and can combine with the materials of a cell nucleus and palsies their function. Therefore， the cells stop growing[11，14].
Different amount of chitosan-copper complex solution was sprayed on the high-efficiency air filter paper. After drying， the barley malt culture fluid was added on the papers. Then， the paper was placed in a 30℃ atmosphere and inoculated naturally for 5 days. Finally， the research on the antibacterial efficiency of high-efficiency air filter paper was carried out by measuring the areas contaminated and not contaminated by germs. Non-antibacterial paper is used as a control sample. The results are shown in Table 4.
As shown in Table 4， the antibacterial effect of air filter paper increases with the increasing amount of the chitosan-copper complex sprayed on the air filter paper. When the amount of chitosan-copper complex sprayed on the air filter paper is 0.025% by weight， the antibacterial effect reaches its maximum potential and antibacterial efficiency is observed to be approximately 99.5%. When the amount of the chitosan-copper complex is increased to more than 0.025% by weight， the antibacterial effect does not increase further.
As shown Fig.5， when the amount of chitosan-copper complex was 0.025%， no strain growth was observed （Fig.5（a））， whereas well-developed strains were observed in the absence of chitosan-copper complex （Fig.5（b））， indicating the excellent antibacterial efficiency of the chitosan-copper complex.
3.4.6 Technical characteristics of the high-efficiency air filter paper
The technical characteristics of the high-efficiency air filter paper with ultra-fine fiber of 75 wt% is shown in Table 5.
As shown in Table 5， the technical characteristics of the high-efficiency air filter paper are observed to be similar to that of the HEPA filter.
4 Conclusions
The high-efficiency filter paper can be manufactured with the wateroat fibers mercerized by alkali and ultra-fine fibers， which are produced through needleless electrospinning using multiple rings. Especially， the results have proved that mercerized wateroat fibers can be used to manufacturing high-efficiency air filter paper. In addition， the technology that prevents contamination of high-efficiency filter paper from germs spread in air has been established by sprafing chitosan-copper complex on the air filter paper. References
[1] Liu L H. The development and applications of air filter[J]. Filter & Separator， 2000， 10（4）： 8-18.
[2] Zhao H. Filter paper on based ultra fine glass fiber[J]. Paper and Papermaking， 2004， 3（2）： 56-59.
[3] Wang X， Lin T， Wang X G. Scaling up the production rate of nanofibers by needleless electro spinning from multiple ring[J]. Fibers and Polymers， 2014，15（5）： 961-965.
[4] Kostakova E， Meszaros L， Gregr J. Composite nanofibers produced by modified needleless electrospinning[J]. Materials Letters， 2009， 63： 2419-2422.
[5] LV X L， Zhang B， Zhang X G， et al. Recent Development of Electrospun Polymer Nanofibers[J]. Chemistry and Adhesion， 2014， 36（5）： 364-368.
[6] Wang X， Wang X G， Lin T. Electric field analysis of spinneret design for needleless electrospinning of nanofibers[J]. J Mater Res， 2012， 27（23）： 3013-3019.
[7] Zhuang X P， Li Z， Liu X F， et al. Progress in study on antibacterial fiber of chitosan/celluolse[J]. Chemical Industry and Engineering Progress， 2002， 21（5）： 310-313.
[8] Qin C Q， Li H R， Qi X， et al. Water-solubility of chitosan and its antimicrobial activity[J]. Carbohydrate Polymers， 2006， 63： 367-374.
[9] Qin C Q. Antimicrobial Activity of Chitosan in vitro[J]. Journal of Xiaogan University， 2005， 25（6）： 5-8.
[10] Xing S N. Test Methods of HEPA Filter and their Application[J]. Chinese Medical Equipment Joural， 2005， 26（1）： 29-31.
[11] Liu B Y， Wang J， Yao S W. The use of chitin-copper complex in antibacterium paper[J]. China Pulp & Paper Industry， 2004， 25（4）： 43-45.
[12] Nie X L， Shi M， Chen Y H. The Discussion of the Result of HEPA Air-filter Examining[J]. Light Industry Machinery， 2006， 24（2）： 168-170.
[13] Cao G Q. Discussion On New Testing System to Measure the Performance of High Efficiency Air Filters[J]. CC&AC， 2005（3）： 25-30.
[14] Yang F. Antibacterial Agent and Its Application in Antibacterial Paper[J]. China Pulp & Paper， 2006， 25（8）： 51-55. .