翟雯 韓世嬌 范偉思 蘆路路 李源 蔣秋冉
摘要:為攻克空氣濾材高濾效低壓降難以兼顧的瓶頸,進(jìn)一步提高濾材的過濾性能,在導(dǎo)電機(jī)織濾材的研究基礎(chǔ)上增附儲(chǔ)電層。以聚對(duì)苯二甲酸乙二酯(PET)機(jī)織結(jié)構(gòu)織物為基底,利用化學(xué)原位反應(yīng)構(gòu)筑結(jié)合層與導(dǎo)電層,而后采用靜電噴涂技術(shù)構(gòu)筑具有微米顆粒結(jié)構(gòu)的聚偏氟乙烯(PVDF)儲(chǔ)電層,最終獲得具有超疏松結(jié)構(gòu)導(dǎo)儲(chǔ)結(jié)合特性的靜電負(fù)載功能濾材。研究靜電噴涂參數(shù)對(duì)濾材表觀形貌、厚度、力學(xué)性能、電學(xué)性能、透氣性的影響,并在有源靜電負(fù)載條件下研究濾材的過濾性能以及積塵分布情況,基于電場(chǎng)模擬分析過濾機(jī)理。結(jié)果表明:隨PVDF噴涂溶液質(zhì)量分?jǐn)?shù)提升和噴涂時(shí)間延長(zhǎng),在保證無細(xì)絲產(chǎn)生的條件下,PVDF附著量可在0~19.2 μg/cm2之間調(diào)整;3%噴涂質(zhì)量分?jǐn)?shù)、3 min的噴涂時(shí)間為最佳參數(shù);濾材厚度、表面比電阻、頂破強(qiáng)力、透氣率不隨PVDF附著量變化而有顯著變化;濾材壓降也無明顯變化,可維持在超低值(6.7 Pa),但對(duì)PM2.5的濾效隨PVDF附著量增加和電壓的提高而提升,可達(dá)94.29%,品質(zhì)因子高達(dá)0.43 Pa-1;靜電輔助下,積塵量減少,可推測(cè)靜電排斥為除塵機(jī)理之一,有利于提升過濾性能,延長(zhǎng)濾料使用壽命。
關(guān)鍵詞:靜電輔助;空氣過濾;PM2.5;PVDF;靜電噴射;織物基濾料
中圖分類號(hào):TS105.5
文獻(xiàn)標(biāo)志碼:A
文章編號(hào):1009-265X(2023)02-0112-10
隨著現(xiàn)代工業(yè)的進(jìn)步和社會(huì)的發(fā)展,空氣污染愈發(fā)嚴(yán)重,由此引起的環(huán)境問題和造成的健康危害不容忽視。相較大尺度顆粒物,空氣動(dòng)力學(xué)當(dāng)量直徑不大于2.5 μm的細(xì)顆粒物(PM2.5)極易隨呼吸進(jìn)入肺部,沉積在肺泡,甚至滲入血液,引發(fā)呼吸系統(tǒng)及心血管系統(tǒng)疾?。?],因此對(duì)PM2.5的高效過濾成為控制PM2.5危害的主要手段,也是近年來的研究熱點(diǎn)。
傳統(tǒng)空氣過濾技術(shù)以機(jī)械過濾為主,包括慣性效應(yīng)、重力效應(yīng)、擴(kuò)散效應(yīng)和攔截效應(yīng)。其過濾效率直接受到濾材纖維堆積密度及厚度的影響,通常濾阻較大,無法滿足高通量、低耗能的過濾需求。而基于靜電作用的過濾技術(shù),因其顯著增強(qiáng)的過濾效果而備受關(guān)注,常見類型包括駐極體過濾器、靜電除塵器(Electrostatic precipitator,ESP),以及在二者基礎(chǔ)上發(fā)展而來的有源靜電輔助空氣過濾器。駐極體過濾器主要依靠預(yù)充電荷的靜電吸附作用去除空氣中的細(xì)顆粒物。由于存儲(chǔ)電荷有限,且會(huì)隨使用而衰減,其過濾效率的穩(wěn)定性不佳。駐極體濾材結(jié)構(gòu)多為靜電紡納米纖維或非織造結(jié)構(gòu),強(qiáng)度有限,且一般結(jié)構(gòu)致密,濾阻較大。ESP體系通過在平行金屬極板上負(fù)載高壓靜電,產(chǎn)生極板間強(qiáng)電場(chǎng),將荷電的細(xì)顆粒物吸附到板上,但該技術(shù)不僅對(duì)亞微米級(jí)細(xì)顆粒物的單次收集效率較低,而且易發(fā)生二次揚(yáng)塵。有源靜電輔助空氣過濾器將ESP的有源特性和駐極濾材的紡織材料基底特性相結(jié)合,可分為兩類,一種是將可極化的電絕緣材料置于通電支架形成的電場(chǎng)中極化,產(chǎn)生極化電場(chǎng)[2-7];另一種是將高壓靜電直接連通至導(dǎo)電濾料上,在濾料周圍產(chǎn)生靜電場(chǎng),從而實(shí)現(xiàn)對(duì)細(xì)顆粒物的過濾效果[8-10]。有源靜電輔助過濾器,因具有積極供電的設(shè)計(jì),濾材所負(fù)電荷不會(huì)因逸散而減損。本團(tuán)隊(duì)前期研究已在織物基底上利用化學(xué)原位反應(yīng)制備了導(dǎo)電層,并探究了導(dǎo)電層相關(guān)參數(shù)對(duì)過濾效率的提升效果影響規(guī)律。然而,僅具有電荷傳導(dǎo)功能的有源靜電負(fù)載濾材對(duì)電荷的存貯能力有限。為進(jìn)一步提升濾材過濾效果,本文在團(tuán)隊(duì)前期研究成果及結(jié)論的基礎(chǔ)上,在導(dǎo)電層上增附儲(chǔ)電層,并集中對(duì)儲(chǔ)電層的設(shè)計(jì)進(jìn)行討論研究。通過采用導(dǎo)儲(chǔ)結(jié)合的設(shè)計(jì)理念,在超疏松機(jī)織聚對(duì)苯二甲酸乙二酯(滌綸,PET)基材上構(gòu)建結(jié)合層、導(dǎo)電層、儲(chǔ)電層的多層結(jié)構(gòu),開發(fā)一種新型導(dǎo)儲(chǔ)復(fù)合型靜電輔助空氣濾材,將充電電場(chǎng)和極化電場(chǎng)兩個(gè)獨(dú)立部分結(jié)合,集導(dǎo)電濾料與電介質(zhì)濾料的優(yōu)勢(shì)于一體。導(dǎo)儲(chǔ)復(fù)合型濾料的導(dǎo)電層既能起到直接構(gòu)建靜電場(chǎng)的作用,又可以為上層的儲(chǔ)電層(電介質(zhì)層)提供極化靜電場(chǎng),產(chǎn)生靜電響應(yīng),進(jìn)一步增強(qiáng)整體電場(chǎng)的作用范圍及強(qiáng)度,有利于細(xì)顆粒物的靜電捕獲。在諸多電介質(zhì)聚合物中,聚偏氟乙烯(PVDF)具有優(yōu)良的機(jī)械強(qiáng)度、化學(xué)穩(wěn)定性和熱穩(wěn)定性[11],其結(jié)構(gòu)內(nèi)氟原子的存在,賦予PVDF極強(qiáng)的電偶極矩,因而PVDF及其共聚物是具有最高介電常數(shù)和電活性響應(yīng)的聚合物家族[12],這也是本文選用PVDF構(gòu)建儲(chǔ)電層的原因。此外,鑒于電場(chǎng)的復(fù)合可顯著提升過濾效率,本文所開發(fā)濾材摒棄傳統(tǒng)緊密的非織造結(jié)構(gòu),采用疏松的機(jī)織結(jié)構(gòu),不僅其超大的孔隙可輔助實(shí)現(xiàn)超低濾阻,且機(jī)織材料具有比非織造結(jié)構(gòu)更優(yōu)的機(jī)械性能。本文通過場(chǎng)發(fā)射掃描電鏡(SEM)、厚度測(cè)試、頂破實(shí)驗(yàn)、表面比電阻測(cè)試、透氣率測(cè)試等表征方法對(duì)復(fù)合濾材形貌及性能進(jìn)行分析,并采用靜電負(fù)載空氣過濾測(cè)試體系對(duì)濾材的PM2.5過濾性能進(jìn)行測(cè)試,最后通過COMSOL Multiphysics軟件對(duì)復(fù)合濾材的負(fù)載特征電場(chǎng)進(jìn)行模擬,為濾材的過濾行為提供理論解釋。
1實(shí)驗(yàn)
1.1實(shí)驗(yàn)材料與儀器
實(shí)驗(yàn)材料:PET平紋組織機(jī)織基材(平方米質(zhì)量55 g/m2,嘉興益泰樂電子有限公司);鹽酸(AR,昆山晶科微電子材料有限公司);多巴胺鹽酸鹽(DA·HCl,北京百靈威科技有限公司);PVDF(FR904,上海三愛富新材料股份有限公司);超細(xì)試驗(yàn)粉塵(ISO 12103-1,A1, 美國PTI公司);硝酸銀、葡萄糖、N,N-二甲基甲酰胺(DMF)、丙酮(AC)等藥品,均為AR,由上海凌峰化學(xué)試劑有限公司提供。
實(shí)驗(yàn)儀器:微量注射推泵(LSP01-1A型,保定蘭格恒流泵有限公司);高壓靜電源(TD2202型,大連泰斯曼有限公司);場(chǎng)發(fā)射掃描電子顯微鏡(FlexSEM 1000型,日立高新技術(shù)公司),透氣性測(cè)試儀(YG461型,溫州方圓儀器有限公司),織物厚度測(cè)試儀(YG141型,常州新紡檢測(cè)儀器設(shè)備有限公司),電子織物強(qiáng)力儀(HD026NE型,上海三思實(shí)驗(yàn)儀器有限公司),雙顯示屏數(shù)字萬用表(34450A型,安捷倫科技有限公司);粉塵氣溶膠發(fā)生器(RBG 1000型,德國帕剌斯儀器公司);粒子計(jì)數(shù)器(Fidas Frog型,德國帕剌斯儀器公司);數(shù)字微壓計(jì)(9565P型,美國TSI公司)。
1.2濾材制備
將PET織物用丙酮浸漬洗滌20 min,洗滌烘干后,浸沒在30 ℃的多巴胺水溶液(2 g/L, pH 8.5)中處理1 h,干燥后,將已建立結(jié)合層的織物置于硝酸銀處理液(10 g/L, pH 11)中繼續(xù)處理0.5 h。隨后將葡萄糖溶液(20 g/L)作為還原劑滴加入處理液中,以在織物上進(jìn)一步構(gòu)建金屬Ag導(dǎo)電層。PVDF分別以1%~6%質(zhì)量分?jǐn)?shù)置于DMF/AC混合溶液(質(zhì)量比4∶1)中,在70 °C下以240 r/min的速度攪拌溶解10 h。溶解后溶液采用靜電噴射技術(shù)噴涂在固定于接收板的織物基底上(推速1.0 mL/h,電壓15 kV,接收距離9 cm)。經(jīng)過烘干后,稱量噴涂 PVDF前后的濾材,并計(jì)算增重。
1.3性能測(cè)試
1.3.1形貌表征
測(cè)試樣品尺寸為0.5 mm × 0.5 mm,表面噴金10 s后通過SEM以10 kV的加速電壓在300倍和450倍放大倍數(shù)下觀察表面形貌。
1.3.2厚度測(cè)試
厚度測(cè)試參照標(biāo)準(zhǔn)GBT 3820—1997《紡織品和紡織制品厚度的測(cè)定標(biāo)準(zhǔn)》,選擇加壓重塊為50 cN,連續(xù)加壓10 s,在每種樣品的5個(gè)不同位置測(cè)試厚度,如有異常值則剔除重新測(cè)試,計(jì)算平均值。
1.3.3頂破強(qiáng)力測(cè)試
參考標(biāo)準(zhǔn)GBT 19976—2005《紡織品頂破強(qiáng)力的測(cè)定鋼球法》,測(cè)試樣品為直徑6.0 cm的圓形,將試樣固定在內(nèi)徑為2.5 cm的夾布圓環(huán)內(nèi)。以100 mm/min的下落速度和5.0 cm的初始間隔測(cè)量濾料的頂破強(qiáng)力,每種樣品測(cè)試5個(gè)試樣。
1.3.4透氣性測(cè)試
透氣性測(cè)試參照標(biāo)準(zhǔn)EN ISO 9237-1995《的紡織品纖維織物透氣性測(cè)定》,試樣面積為20 cm2,試樣壓差選擇25 Pa,每種樣品分別選取三塊試樣,每份試樣測(cè)試5個(gè)不同位置計(jì)算透氣率平均值及標(biāo)準(zhǔn)差。
1.3.5表面比電阻測(cè)試
參照標(biāo)準(zhǔn)GBT22042—2008《服裝防靜電性能表面電阻率試驗(yàn)方法》,試樣尺寸為60 mm×30 mm,將樣品進(jìn)行預(yù)調(diào)濕后,在標(biāo)準(zhǔn)溫、濕度環(huán)境中使用數(shù)字萬用表測(cè)試表面電阻。每種樣品測(cè)試5個(gè)試樣,在每個(gè)試樣上選取3個(gè)不同位置。表面比電阻根據(jù)式(1)計(jì)算:
Rs=ρs×LW(1)
式中:ρs是表面電阻率,Ω;L、W分別是試樣寬度和長(zhǎng)度,mm。
1.4空氣過濾性能測(cè)試
依據(jù)標(biāo)準(zhǔn)EN779—2002《一般通風(fēng)用空氣顆粒
過濾器-過濾性能測(cè)定》和GB/T 6165—2008《高效空氣過濾器性能試驗(yàn)方法效率和阻力》,設(shè)計(jì)并搭建空氣過濾測(cè)試體系,如圖1所示。將試驗(yàn)粉塵裝載于粉塵氣溶膠發(fā)生器中,經(jīng)過氣泵分散,隨著潔凈空氣通過變速風(fēng)機(jī)吸入管道,形成細(xì)顆粒物濃度穩(wěn)定的測(cè)試氣流(1.0 mg/m3)。在測(cè)試段,通過高壓靜電源向復(fù)合濾材供給正極靜電(0~40 kV)。過濾過程中,在濾材前后各15 cm距離處通過粒子計(jì)數(shù)器測(cè)定PM2.5濃度,并由數(shù)字微壓計(jì)測(cè)量濾材前后壓降,根據(jù)式(2)、式(3)計(jì)算得出過濾效率E及品質(zhì)因子QF,以綜合表征濾料的整體過濾性能。
E/%=1-cc0×100(2)
式中:C0、C分別為管道內(nèi)過濾前后空氣中的PM2.5濃度,mg/m3。
QF=-ln(1-E)ΔP(3)
式中:E為過濾效率;ΔP為壓降,Pa。
1.5COMSOL模擬及靜電場(chǎng)分析
本文使用COMSOL Multiphysics 5.5軟件對(duì)復(fù)合濾材在不同靜電負(fù)載情況下的電勢(shì)、電場(chǎng)強(qiáng)度等電學(xué)特性進(jìn)行模擬及計(jì)算。研究采用了靜電場(chǎng)分析方法,即濾材所形成的電場(chǎng)正比于表面負(fù)載的電壓。為進(jìn)一步明晰電場(chǎng)在濾材織造結(jié)構(gòu)內(nèi)的分布,仿真幾何模型選取局部四根交織紗線作為模擬研究對(duì)象,主體包括紗線、過濾管道及空氣介質(zhì)(計(jì)算區(qū)域尺寸簡(jiǎn)化為直徑20 cm、高度60 cm的圓柱體),根據(jù)實(shí)際情況設(shè)置域內(nèi)電荷守恒及紗線終端負(fù)載不同電壓、管道接地等邊界條件,以便后續(xù)進(jìn)行參數(shù)化掃描。構(gòu)建物理場(chǎng)控制網(wǎng)格,根據(jù)計(jì)算需要選擇細(xì)化四面體網(wǎng)格結(jié)構(gòu)進(jìn)行研究。
2結(jié)果與討論
2.1復(fù)合濾材形貌特征分析
由圖2所示,本研究所構(gòu)筑濾材是由直徑約60 μm的單絲以平紋結(jié)構(gòu)織造而成的織物作基底。濾材孔洞約65 μm,遠(yuǎn)高于常規(guī)尺度纖維無紡布濾材(3~50 μm)[13-14]及靜電紡結(jié)構(gòu)濾材(2~10 μm)[15],且濾材僅為單層,這種超疏松和超薄設(shè)計(jì),為超低壓降的過濾效果奠定了結(jié)構(gòu)基礎(chǔ)。
靜電噴涂PVDF溶液的質(zhì)量分?jǐn)?shù)一定程度上影響濾材表面形態(tài)。如圖2(a)—(c)所示,單絲基底表面光滑,噴涂質(zhì)量分?jǐn)?shù)1%、2%的PVDF溶液后,表面形成薄膜,在交織點(diǎn)處易脫落。PVDF質(zhì)量分?jǐn)?shù)提至3%后,單絲表面覆膜完整,并出現(xiàn)幾百納米至幾微米的均勻分布的顆粒狀凸起(見圖2(d)),提高至4%后,纖維表面覆膜增厚,且部分顆粒狀凸起連塊(見圖2(e))。繼續(xù)增加PVDF質(zhì)量分?jǐn)?shù)至5%~6%,單絲表面覆層均勻度進(jìn)一步降低,且在單絲間的孔洞出現(xiàn)絲狀、片狀甚至粒狀物質(zhì)(見圖2(f)—(g))。由以上結(jié)果可知3%為PVDF靜電噴涂最佳質(zhì)量分?jǐn)?shù)參數(shù)。通過調(diào)整噴涂時(shí)間,同樣可調(diào)控纖維表面形態(tài)。經(jīng)過1 min噴涂,PVDF以小顆粒形式少量附著于單絲表面(見圖3(a))。隨噴涂時(shí)間延長(zhǎng)至2~3 min,PVDF顆粒在單絲表面的排布更密集(見圖3(b)-(c)),但顆粒尺寸并未如質(zhì)量分?jǐn)?shù)提升時(shí)一樣增大,且未出現(xiàn)細(xì)絲狀物。當(dāng)噴涂時(shí)間增加到4 min(見圖3(d)),PVDF顆粒因覆疊而出現(xiàn)團(tuán)聚,最終導(dǎo)致PVDF顆粒尺寸及尺寸不勻率增大,且易脫落,同時(shí)單絲之間出現(xiàn)少量細(xì)絲,因此噴涂時(shí)間需設(shè)置為4 min以下。調(diào)節(jié)噴涂時(shí)間可控制PVDF在織物上的附著量。由圖3(e)顯示,PVDF的附著量隨處理時(shí)間延長(zhǎng)顯示出線性增加趨勢(shì),從6.4 μg/cm2提升至19.2 μg/cm2,增加速率大致為6.4 μg/(cm2·min)。不同PVDF附著量的濾材厚度基本在0.08 mm (見圖4),無顯著差異,證明PVDF的附著極薄,不會(huì)對(duì)濾材厚度產(chǎn)生宏觀影響。
2.2復(fù)合濾材力學(xué)性能分析
在使用中,濾材承受來自空氣流體的單向壓力,其頂破強(qiáng)力影響濾材的可用性及其壽命。如圖5所示,復(fù)合濾材雖具有超疏松結(jié)構(gòu),但因其特殊的機(jī)織結(jié)構(gòu),濾材的頂破強(qiáng)力高達(dá)420 N,相較常規(guī)非織造濾料(50~400 N)[16]提高了數(shù)倍。由此結(jié)果可推斷,本研究所開發(fā)的機(jī)織結(jié)構(gòu)濾材,可承受更高風(fēng)速的過濾,也可提供更長(zhǎng)的服務(wù)期。此外,濾材的頂破強(qiáng)力并未隨PVDF的附著量發(fā)生改變,證明該靜電噴涂工藝過程既不會(huì)破壞濾材基礎(chǔ)結(jié)構(gòu),或造成任何化學(xué)降解,所附層的PVDF也不會(huì)為濾材的力學(xué)性能提供額外的增強(qiáng)。
2.3復(fù)合濾材電學(xué)性能分析
鑒于本研究中高壓靜電需通過濾材導(dǎo)電層加載至濾材上,且濾材的儲(chǔ)電層也需要傳導(dǎo)至濾材上的高壓電場(chǎng)來極化。因此,濾材的導(dǎo)電性能對(duì)其后續(xù)過濾性能是十分重要的。但表面噴涂的PVDF材料本體導(dǎo)電性較差,需要對(duì)噴涂處理后的濾材進(jìn)行導(dǎo)電性的評(píng)價(jià),以調(diào)控噴涂工藝,從而在儲(chǔ)電和導(dǎo)電之間達(dá)到應(yīng)用所需的合理設(shè)計(jì)。未處理的PET織造基底的表面比電阻過高,無法測(cè)得。但構(gòu)建導(dǎo)電層后,濾材的表面比電阻可降至僅15.56 Ω/m2(見圖6)。當(dāng)進(jìn)一步構(gòu)建PVDF儲(chǔ)電層時(shí),表面比電阻隨PVDF附著量的提升而逐步提高,但提高幅度很小。少量的PVDF附著(6.4 μg/cm2)并未引起濾材表面比電阻的顯著變化(15.72 Ω/m2)。當(dāng)PVDF附著量提升至12.8 μg/cm2時(shí)和19.2 μg/cm2時(shí),濾材表面比電阻升高至16.06 Ω/m2和16.29 Ω/m2,相較未噴涂濾材,變化率僅為3.21%和4.69%。此結(jié)果證明了,即便附加非導(dǎo)電材料PVDF,復(fù)合濾材仍維持其優(yōu)異的導(dǎo)電性,并不會(huì)影響高壓靜電在織物上的負(fù)載。
2.4復(fù)合濾材的空氣過濾性能
2.4.1復(fù)合濾材的透氣性能
濾材的透氣性能直接影響濾材的濾阻及能耗。良好的透氣性有利于降低風(fēng)機(jī)的能耗,從而實(shí)現(xiàn)節(jié)能減排的目的。在25 Pa壓力下,未經(jīng)噴涂處理的濾材,其透氣率為1128.45 mm/s(見圖7),是普通非織造濾材的1~2倍[14]。靜電噴涂6.4~19.2 μg/cm2 PVDF涂層后,復(fù)合濾材的透氣率最低降至1096.01 mm/s,最大變化率僅2.87%,且經(jīng)過統(tǒng)計(jì)分析,與未噴涂處理樣品間并無顯著差異。該結(jié)果證明噴涂處理并未明顯影響處理后濾材的透氣性,這主要是因?yàn)闉V材基底本身的結(jié)構(gòu)極其疏松,基礎(chǔ)透氣率較高,且PVDF噴涂顆粒層超薄,使得基材良好的透氣性能得以保留。
2.4.2復(fù)合濾材的過濾性能
與無儲(chǔ)電層的導(dǎo)電濾材相比,不同附著量PVDF處理后濾材的過濾性能如圖8所示。圖8(a)顯示了不同樣品在加載0~40 kV電壓時(shí)的過濾效率。在電壓未負(fù)載時(shí),有無PVDF覆層的濾材顯示出較為接近且較低的過濾效率。無PVDF覆層濾材的過濾效率僅為21.47%,而覆層濾材的過濾效率隨PVDF附著量的增加,從24.66%微量提升至26.53%。此時(shí)的過濾效果僅依靠機(jī)械過濾。當(dāng)10 kV的高壓靜電加載于濾材上時(shí),無覆層和低附著量濾材的濾效分別提高至42.5%和44%,而中、高附著量濾材的過濾效率可提升至55%和67%,分別提高了2至3倍,由此可證明,即便加載了較低的電壓,只要濾材附著足量的PVDF,便可通過極化電場(chǎng)的附加作用,提升過濾效果。隨著電壓的升高,有無覆層濾材的過濾效率也逐步提升。在40 kV電壓加載下,中、高附著量濾材的過濾效率已較為接近,可達(dá)88.00%和94.29%,對(duì)比無電壓過濾效果,提高率可達(dá)62.68%和67.76%,而無覆層和低附著量濾材的過濾效率也較為接近,但僅有67.02%和73.91%,提高幅度為45.55%和49.25%。圖8(b)顯示了不同濾材在10 cm/s風(fēng)速下的壓降,鑒于各濾材在加載不同電壓時(shí),其壓降并無變化,在此僅顯示平均結(jié)果。有無PVDF覆層濾材的壓降都較為接近,約6.5~6.7 Pa,而常規(guī)非織造濾材的壓降在20~300 Pa[14, 17],是本研究中濾材壓降的3~50倍。超低的濾阻主要得益于濾材超疏松的機(jī)織結(jié)構(gòu),這一結(jié)果與透氣率結(jié)果一致。品質(zhì)因子由過濾效率和壓降計(jì)算得出,正比于濾效而反比于壓降,可綜合表達(dá)濾材的過濾效果。如圖9(c)所示,當(dāng)負(fù)載電壓為0 kV時(shí),因?yàn)V材濾效較低,所有樣品的品質(zhì)因子僅為0.036~0.046 Pa-1。隨著負(fù)載電壓逐步提升至40 kV,無覆層濾材的品質(zhì)因子提升至0.166 Pa-1,增加了3.59倍,而PVDF覆層的濾材品質(zhì)因子可達(dá)0.207、0.321和0.427 Pa-1,分別提升了3.75、6.26和8.29倍。這一結(jié)果充分證明,不堵塞織物孔隙的PVDF覆層,可大幅提升濾材的綜合過濾性能。
為了進(jìn)一步明確過濾性能提升的原因,本研究使用COMSOL Multiphysics軟件對(duì)PVDF覆層的濾材在管道中的電場(chǎng)強(qiáng)度分布進(jìn)行了仿真模擬研究。如圖9(a)所示,隨著施加電壓的升高,濾材產(chǎn)生的電場(chǎng)覆蓋范圍逐步擴(kuò)大,且越接近濾材平面處的電場(chǎng)強(qiáng)度越高。圖9(b)-(c) 顯示了電場(chǎng)強(qiáng)度最大值隨施加電壓和PVDF附著量的變化規(guī)律。電場(chǎng)強(qiáng)度最大值與施加電壓成正比線性關(guān)系,這符合電場(chǎng)與電壓成正比的規(guī)律。PVDF附著量的提升也會(huì)引起電場(chǎng)強(qiáng)度最大值近似線性的提升。這一結(jié)果說明,濾材所產(chǎn)生的電場(chǎng)范圍及強(qiáng)度可通過施加電壓及PVDF附著量進(jìn)行線性調(diào)控,向?yàn)V材靠近的細(xì)顆粒物所受電場(chǎng)力也會(huì)隨著距離的縮短和供給電壓的提升而增加。
2.4.3復(fù)合濾材表面細(xì)顆粒物沉積特點(diǎn)分析
圖10展示了PVDF覆層濾材過濾前后及電壓施加前后表面積塵狀態(tài)。附著量為19.2 μg/cm2的濾材在濾前表面僅有均勻排布的PVDF凸起。在無高壓靜電負(fù)載條件下,經(jīng)過5 h的機(jī)械過濾,被直接攔截的細(xì)顆粒物松散堆積在單絲表面,部分聚集的顆粒物形成較大塊狀物(見圖10(b)),在單絲交織點(diǎn)縫隙易堆積較多顆粒。在負(fù)載20 kV電壓過濾5 h后,濾材上細(xì)顆粒物的量明顯減少(見圖10(c))。值得關(guān)注的是,相較無電壓負(fù)載時(shí),電壓負(fù)載濾材的濾效提升了2.55倍。計(jì)算可知,經(jīng)過5 h過濾,電壓負(fù)載條件下從空氣中所清除的細(xì)顆粒物總量是無電壓條件下的3.55倍,但濾材表面積塵量情況卻正相反。由此可推斷,本研究所開發(fā)有源靜電負(fù)載過濾體系的過濾機(jī)理并不僅是機(jī)械過濾或靜電吸附。
分析可知,電壓負(fù)載為0 kV時(shí),濾材的過濾效果僅依靠機(jī)械過濾作用,即慣性效應(yīng)、重力效應(yīng)、擴(kuò)散效應(yīng)和攔截效應(yīng)。但超疏松的機(jī)織濾材結(jié)構(gòu)所能產(chǎn)生的機(jī)械過濾作用有限,因此濾效較低,且濾材表面積塵量不多。因孔徑超大,即便經(jīng)過5 h的長(zhǎng)時(shí)間過濾,濾材表面依然無法形成完整的濾餅層,孔洞結(jié)構(gòu)依然可見。當(dāng)高壓靜電負(fù)載于濾材上,以濾材為中心,在其周圍形成了棗核形態(tài)電場(chǎng)(見圖9(a))。進(jìn)入電場(chǎng)范圍內(nèi)的顆粒物將依據(jù)荷電極性的不同而受到不同作用。負(fù)載相同電荷的顆粒物,會(huì)被濾材排斥,而負(fù)載異性電荷的顆粒物或產(chǎn)生鏡像電荷的顆粒物會(huì)被濾材捕獲,從而獲得更高效的過濾效果。但濾材上所負(fù)載為正極高壓靜電,在電場(chǎng)覆蓋范圍內(nèi)甚至周邊空間,為缺電子空間,因此顆粒物更傾向表現(xiàn)為正電,從而受到靜電排斥力而遠(yuǎn)離濾材表面。最終,雖然濾效在加載電壓時(shí)有顯著提高,但是濾材上的積塵量并未增加,反而有所降低。這不僅將大幅降低濾材的污染速度,延長(zhǎng)濾材的清理周期,同時(shí)可減少過濾過程的總體能耗。
對(duì)濾材局部四根紗線結(jié)構(gòu)的電場(chǎng)負(fù)載進(jìn)行模擬后得知,鑒于濾材中單絲具有良好的導(dǎo)電性,每根單絲所負(fù)載電壓在各處強(qiáng)度基本相同,電場(chǎng)強(qiáng)度在最靠近單絲處最強(qiáng)(見圖11(a))。在單絲交織點(diǎn)上,兩根單絲所產(chǎn)生電場(chǎng)疊加,起到增強(qiáng)效果(見圖11(b)),細(xì)顆粒物在交織點(diǎn)所受靜電力更強(qiáng),進(jìn)一步降低了在交織點(diǎn)處的堆積程度(見圖10(c))。
3結(jié)論
本文采用表面原位化學(xué)改性及靜電噴涂技術(shù),在構(gòu)建了結(jié)合層、導(dǎo)電層的超疏松機(jī)織結(jié)構(gòu)PET基底上進(jìn)一步構(gòu)建儲(chǔ)電層,形成多層功能層,實(shí)現(xiàn)靜電負(fù)載功能濾材的構(gòu)筑,分析了PVDF噴涂工藝對(duì)濾材本體性能和負(fù)載靜電下過濾性能的影響,得到結(jié)論如下:
a)靜電噴涂PVDF溶液質(zhì)量分?jǐn)?shù)為3%,噴涂時(shí)長(zhǎng)為3 min時(shí),織物表面可均勻附著顆粒狀PVDF涂層,無成絲堵塞現(xiàn)象。
b)PVDF附著量在0~19.2 μg/cm2的范圍內(nèi),濾材的厚度、頂破強(qiáng)度、表面比電阻、透氣性無顯著變化。
c)過濾過程中濾材壓降不隨PVDF附著量而改變,維持在超低值6.7 Pa,但濾效和品質(zhì)因子隨之增加,在附著量為19.2 μg/cm2,負(fù)載電壓為40 kV時(shí),對(duì)PM2.5的過濾效率可達(dá)94.29%,品質(zhì)因子達(dá)到0.43 Pa-1。
d)根據(jù)積塵減少的現(xiàn)象和電場(chǎng)模擬的結(jié)果,可推斷,靜電排斥可能是過濾作用方式之一,可大幅降低濾材污染速度,減少運(yùn)行時(shí)壓降與耗能,延長(zhǎng)濾材單次使用時(shí)長(zhǎng)。
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Preparation of static-electrical assistant PET/PVDF woven structural filter with ultralow pressure drop and its air filtration properties
ZHAI Wen, HAN Shijiao, FAN Weisi, LU Lulu, LI Yuan, JIANG Qiuran
(College of Textiles, Donghua University, Shanghai 201620, China)
Abstract: With the rapid development of industry, air pollution has gradually become a serious threat to public health. Fine particulate matter, such as PM2.5, is the main component of air pollutants and also the main carrier of other types of air pollutants. The efficient filtration technology of indoor fine particulate matter has received increasing attention. The traditional filtration technology mainly includes mechanical filtration and electrostatic assisted filtration. The improvement of mechanical filtration efficiency based on physical interception effect mainly depends on the increase in the thickness and fiber density of filters, but provokes the elevation of filtration resistance and energy consumption. Thereby, it is difficult to achieve high filtration efficiency while maintaining the low pressure drop. The electrostatic assisted filtration system includes electrostatic precipitator and electret filtration system. The removal of particulate matter depends on the electrostatic interaction between the fine particulate matter and filter material, which can greatly reduce the resistance to air. However, the traditional electrostatic precipitator shows low efficiency in capturing fine particulate matter, and the electret filtration system has limited capability to carry sufficient charges for long-time serving, and the decay in charges often occurs. Therefore, the development of long-term, stable, highly efficient and low-resistant filtration systems has long been an obstacle in the field of air filtration. Our team has developed an electrostatic assisted filtration system in the early stage. Using a high voltage power supply, the filtration system maintains stable charge loading during the whole serving life and establishes a strong electrostatic field which is able to charge fine particles and remove them through electrostatic interaction. This system can overcome the above-mentioned disadvantages of the traditional electrostatic filtration systems. However, the charging efficiency is still limited. The difficulty in further elevation of the filter charging capacity thwarts the improvement of the filtration efficiency.
To combine the electrical conductivity and storage capabilities, the current study first constructed a polydopamine binding layer and a silver conductive layer by in-situ chemical reaction on the super-loose woven polyethylene terephthalate (PET) substrate, and then constructed the micro-nano structure polyvinylidene fluoride (PVDF) electrical storage layer by using electrostatic spraying technology. This study successfully prepared a super-loose electrostatic loaded filter system by controlling the adhesion amount and structure of PVDF on the substrate through different spraying parameters, while retained the super-loose woven structure of the substrate fabric. We systematically characterized the basic properties of the PVDF attached filters, including surface morphology, thickness, mechanical properties, electrical properties and air permeability, and investigated the filtration performance and the particle distribution with different electrostatic voltage supplies. The filtration mechanism was deduced with the simulation of the electric fields.
The results proved that electrostatic spraying concentration and spraying duration could adjust the loading amount of PVDF and their accumulation morphology. The filters coated with 3 % PVDF for 3 min showed uniform high-loading of PVDF without fibrous structure. The PVDF amount exerted limited influence on the thickness, surface specific resistance, breaking strength, air permeability and pressure drop of the filters, but displayed significant effects on the filtration efficiency and quality factor of PM2.5. By raising the PVDF amount from 0 to 19.2 μg/cm2, the filtration efficiency could be enhanced from 42.5 % to 67 % at a low voltage (10 kV), and from 67.02 % to 94.29 % at a high voltage (40 kV), while still maintained an ultra-low pressure drop of 6.7 Pa. Hence, the quality factor was able to reach 0.43 Pa-1. The active electrostatic assisted filtration system loaded with PVDF could not only achieve a high filtration efficiency at an ultra-low pressure drop, but also reduce the amount of dust accumulation on the filter surface. The possible reason might be the existence of electrostatic repulsion.
The design of this system was based on the combination of electrical conductivity and storage. PVDF electrical storage layer was constructed on conductive substrates to incorporate the advantages of the conductive and the electrical storage materials. This design could achieve further enhancement in filtration performance by establishing more stable, stronger electrostatic field with larger coverage area.
In this work, we further improved the performance of woven filters with active electrostatic charging by loading PVDF. This work might promote the development of ultra-low resistance woven filters, and provide inspiration for the future development of the core filter design with low-carbon emission. This filter system has the potential to be applied for vehicle air cleaning, air conditioner and central ventilation system. Meanwhile, via the observation of dust accumulation and electric field simulation analysis, the filtration mechanism of the active electrostatic assisted air filtration system was revealed, which provided a theoretical foundation for the further development of filtration systems with high efficiency and low resistance.
Keywords: electrostatic assistant; air filtration; PM2.5; PVDF; electrostatic spraying; fabric filter
收稿日期:20220507
網(wǎng)絡(luò)出版日期:20220816
基金項(xiàng)目:中央高校基本科研業(yè)務(wù)費(fèi)專項(xiàng)資金資助項(xiàng)目(2232022D-13)
作者簡(jiǎn)介:翟雯(1998—),女,山東煙臺(tái)人,碩士研究生,主要從事靜電輔助空氣濾材方面的研究。
通信作者:蔣秋冉,E-mail:jj@dhu.edu.cn