Investigation of Porosity and Air Permeability Values of Plain Knitted Fabrics

Transcription

1 . Tugrul Ogulaa, Serin Mavruz The Universiy of ukurova, Deparmen of Texile Engineering, 00 Adana, Turkey Invesigaion of Porosiy and Air Permeabiliy Values of Plain Knied Fabrics Absrac The air permeabiliy of a fabric is defined as he amoun of air passed over a surface under a cerain pressure difference in a uni ime. This value has significance wih respec o he usage area. Since knied fabrics have a loop srucure, hey have more pores han woven fabrics; herefore, in general, he air permeabiliy of knied fabrics is higher han ha of woven fabrics of he same weigh. An experimen o deermine he air permeabiliy is very imporan as i defines he properies of keeping warm, proecion agains he wind, breahabiliy ec. of knied fabrics used as clohing. In his sudy, i has been aemped o esablish a heoreical model for he porosiy and prediced air permeabiliy of plain knied fabrics. A heoreical model was creaed o predic he porosiy and air permeabiliy of a knied srucure depending on he geomerical parameers, such as he courses per cm, wales per cm, sich lengh, fabric hickness, yarn coun, diameer of yarn and fiber densiy. For his purpose, a heoreical model of porous sysems based on D Arcy s law was used, he validiy of which was confirmed by experimenal resuls using 00% coon plain knied fabrics produced from ring and compac yarns of differen yarn number linear densiy and ighness. Key words: knied srucure, porosiy, air permeabiliy, geomeric modelling. Designaions used A - cross-secional area of pore, cm A - fabric area esed, cm c - number of courses per cenimeer d h - hydraulic diameer of a pore, cm d p - pore diameer, cm d y - yarn diameer, cm f - fricion coefficien m - number of pores per square n - coefficien indicaing he flow regime Q - oal flow rae of he air, m /s - air permeabiliy, mm/s e - eynolds number r p - pore radius, cm r y - yarn radius, cm - fabric hickness, cm U m - mean air flow velociy, m/s w - number of wales per cenimeer r - air densiy, kg/m r y - yarn densiy, g/cc e - rae of void area v - kinemaic viscosiy of he air, m /s h - dynamic viscosiy of he air, Pa.s DP - pressure drop, Pa l - coefficien of laminar and urbulen flow Inroducion Kniing is he process of forming fabric by inerloping yarn in a series of conneced loops using needles. Kni fabrics provide ousanding comfor qualiies and have long been preferred in many ypes of clohing. In addiion o comfor impared by he exensible looped srucure, knis also provide lighweigh warmh, wrinkle resisance, and ease of care []. Such knied srucures have a more open characer when compared o oher exile fabric srucures, such as woven and braided []. Air permeabiliy is ofen used in evaluaing and comparing he breahabiliy of various fabrics (coaed and uncoaed) for such end uses as raincoas, ens and uniform shirings. I helps evaluae he performance of parachues sails, vacuum cleaners, air bags, sail cloh and indusrial filer fabrics [ - ]. Air permeabiliy is defined as he volume of air in liers which is passed hrough 00 cm (0 cm 0 cm) of he fabric in one minue a a pressure difference of 0 mm head of waer []. Due o he manner in which yarns and fabrics are consruced, a large proporion of he oal volume occupied by a fabric is usually airspace. The disribuion of his airspace influences a number of imporan fabric properies such as warmh and proecion agains wind and rain in clohing, and he efficiency of filraion in indusrial clohs. Air permeabiliy is an imporan facor in he comfor of a fabric as i plays a role in ransporing moisure vapour from he skin o he ouside amosphere. The assumpion is ha vapour ravels mainly hrough fabric spaces by diffusion in air from one side of he fabric o he oher []. The porosiy of a knied srucure will influence is physical properies, such as he bulk densiy, moisure absorbency, mass ransfer and hermal conduciviy []. Air flow hrough exiles is mainly affeced by he pore characerisics of fabrics. I is quie clear ha pore dimension and disribuion is a funcion of fabric geomery. The yarn diameer, surface formaion echniques and he number of loop couns per uni area are he main facors affecing he porosiy of exiles. The porosiy of a fabric is conneced wih cerain of is imporan feaures, such as air permeabiliy, waer permeabiliy, dyeing properies ec. [, 0]. Mos of he previous sudies invesigaed he relaionship beween he air permeabiliy and srucural characerisics of plain knied fabrics. For example, Oinuma (0) showed ha he sich lengh, porosiy and air permeabiliy increase and he hermal reaining propery decreases for dry relaxed coon rib knied fabrics []. Marmarali (00) invesigaed he air permeabiliy of coon/spandex single jersey fabrics wihin heir dimensional and physical properies and compared resuls wih fabrics knied from coon alone. I proved ha he air permeabiliy of fabrics conaining spandex was lower []. Karaguzel (00) calculaed values of pore size and pore volume for plain knied fabrics. Those of he pore size were measured wih image analysis and fluid exrusion procedures. I was found ha here was a noiceable difference beween he esimaed and measured values []. On he basis of compuer image analysis, he surface porosiy of knied fabrics was evaluaed for plain double-layered and lining knied fabrics by Wilbik-Halgas e al. (00). I was found ha air permeabiliy, conrary o waer vapour permeabiliy, is a funcion of he hickness and surface porosiy of knied fabrics []. Benloufa e al. (00) invesigaed mehods of deermining jersey porosiy, which proved ha geomery modelling is he mos suiable and easies mehod of deermining poros- Ogulaa. T., Mavruz S.; Invesigaion of Porosiy and Air Permeabiliy Values of Plain Knied Fabrics. FIBES & TEXTILES in Easern Europe 00, Vol., No. () pp. -.

2 Figure. Sich diagram of a plain knied srucure. iy []. The hermal properies and hermal behaviour of cellulose exile fabrics (air permeabiliy, porosiy) were invesigaed by Sankovic e al. (00), in which hey found ha air permeabiliy and hea ransfer hrough fabrics is closely relaed o boh he capillary srucure and surface characerisics of yarns []. Dias and Delkumburewae, (00) creaed a heoreical model o predic he porosiy of a knied srucure. I was deermined ha porosiy depended on fabric parameers and relaxaion progression []. Mavruz and Ogulaa, (00) invesigaed air permeabiliy of coon knied fabrics. Before manufacuring he fabrics he equaion of regression was used o predic air permeabiliy, depending on some fabric properies []. Esablishing a more complex heory o express air permeabiliy relaed o all fabric parameers will have difficulies. To simplify he maer, cerain imporan parameers such as he pore of he fabric Figure. epresenaion of a plain knied fabric. were aken ino accoun in he calculaion of air permeabiliy. Three facors are mainly considered ha are relaed o he pores in fabrics. ) ross-secional area of each pore, ) Deph of each pore or he hickness of he fabric and ) The number of pores per uni area or he number of courses and wales per uni area. In his work, hese parameers are considered o develop a heoreical model for porosiy and air permeabiliy. Theoreical Model A knied fabric consiss of one or more looped yarns. Plain knied fabric, which is illusraed in Figure, is one srucure of knied fabric. In his paper, we consider plain knied fabrics which are commonly used in many apparel applicaions. W and represen wale and course spacing, whereas w and c correspond o he number of wales per cm and number of courses per cm, respecively []. (Wales per cm: he number of visible loops per uni lengh in cm measured along a course. ourses per cm: he number of visible loops per uni lengh in cm measured along a wale. Sich lengh: he lengh of yarn in a knied loop). When considering fluid flow hrough exiles, he shape arrangemen and size disribuion of voids hrough which he fluid flows are of grea imporance. The fabric hickness and differenial pressure beween he wo surfaces of a fabric are he oher dominan facors ha affec permeabiliy. The pressure gradien hrough a exile is a funcion of he viscosiy, densiy, rae of fluid flow and porosiy, as in he case of flow hrough a pipe []. The dependence of he fricion coefficien, f, on he eynolds Number, e, for laminar and urbulen flow is described by he Blasius equaion: f = l. e -n () where λ is he coefficien of laminar or urbulen flow, and n is a coefficien indicaing he flow regime. Laminar flow: λ =, n = Turbulen flow: λ = 0., n = 0. The ype of flow depends on he eynolds number. The eynolds number represens he raio of he ineria force o he viscous force []. The eynolds number is calculaed as follows: d m h e = U. () ν where U m is he mean flow velociy, d h he hydraulic diameer of a pore, and v is he kinemaic viscosiy of he air [, ]. The pressure drop of he flow hrough a duc over he hickness of he fabric is relaed o he fricion facor f hrough D Arcy s formulaion. U m D P = f r () d h where is he hickness of he fabric, and r is he air densiy []. Knied fabric has a porous srucure. For his reason, he air velociy in pores mus be aken ino consideraion. U U = m () e where U is he air velociy hrough pores, and e is rae of he void area (porosiy) []. Figure shows he pore wihin a loop. Our heoreical model was creaed by considering one repeaing uni cell of a knied srucure. By deermining he course (c) per cm, wale (w) per cm, hickness (), yarn diameer (d y ) and loop lengh (l), he porosiy can be shown as follows []: Yarn volume e = () Toal volume Finally, pd l Yarn volume = y pd = y l Toal volume = c w = cw () () pd y e = () lcw The air velociy hrough pores of he fabric does no usually have a high value. Therefore, he fluid flow in he pores is laminar. According o kineic heory, if he eynolds number is below 0, he flow in he ube is laminar [, ]. For his reason, he mean air velociy hrough one pore can be expressed as: d h U m = DP () h The flow rae of air for a fabric of porous maerial, Q, becomes: FIBES & TEXTILES in Easern Europe 00, Vol., No. ()

3 Q = m. A. U (0) where m is he number of pores, and A is he cross-secional area of he pore [] A = p d p () where, dp is he pore diameer (In his sudy, he loops are assumed o be composed of ideal yarns which are circular in cross-secion and have a consan diameer hroughou heir lengh. Yarn deformaion a he crossover poins is omied, hence i is acceped ha dp = dh). Thus, equaion (0) can be wrien as follows: Q = m p d p 00 DP (m/s) () e h The value of air permeabiliy () is calculaed according o he following equaion = Q () A where A is he fabric area esed []. n Maerials and mehods In order o compare he values of air permeabiliy from heoreical modelling and hose from experimens of air permeabiliy, we produced (eigheen) knied samples wih various kniing parameers. The yarns were made of coon fibers, and he bonding ype of each sample was plain. Plain knied fabrics were knied wih hree differen ighnesses (slack, medium and igh) on a circular kniing machine using ring and compac yarns of., and ex. () Figure. epresenaion of yarn in cm of fabric; ry - yarn radius, rp - pore radius, ry - yarn densiy (Pierce used a value of 0.0 g/cc for coon yarn), T - yarn linear densiy in ex.. ex ypes, we repeaed his measuremen for en (0) samples, hus obaining a body of ( 0 =) 0 measuremens. Furhermore, he loop lengh, wales per cm, courses per cm, hickness and weigh of he fabrics were measured according o he relevan sandards [0 - ]. ex ex ing T ppp r yy 0 0 l S l S The knied samples were condiioned for hours in amospheric condiions of 0 ± emperaure and ± % relaive humidiy before he ess were performed. The air permeabiliy of he samples in mm/s was measured according o he mehod specified by Sandard TS EN ISO [], using a Texes FX 00 air permeabiliy eser. The measuremens were performed a a consan pressure drop of 00 Pa (0 cm es area). For each one of he fabric dp is calculaed from Figure (yarn represenaion) and Figure. ry = ry Volume of free space (cm) in of fabric = = - Slpry () cm Area of free space (cm) in - Slpr () cm of fabric = Sl pryy Area of open space wihin - Slpr () one loop = Sl pryy SS As menioned beforehand, if he area occupied by a pore is ransformed o ha of a circle, he pore radius (rp) can be wrien as equaion []. yy pr () rp = - SlSlpr pps S As indicaed by equaion, when he sich densiy, sich lengh or yarn diameer increases, pore size values decrease. The equaions developed in his sudy will be used o predic he pore size and air permeabiliy of plain knied fabrics. FIBES & TEXTILES in Easern Europe 00, Vol., No. () ompac Volume of yarn in cm = Slpry () Figure. Microscopic views of he yarns. Table. Fabric properies and experimenal air permeabiliy values. Sample number Yarn ype Yarn : ing number, : ompac ex Loop lengh, cm ourse coun per cm Wale coun per cm Thickness, cm Weigh, g/m Experimenal air permeabiliy, mm/s

4 0 Figure. Microscopic views of he knied srucure. Fabric properies (yarn number, loop lengh, course coun per cm, wale coun per cm, hickness, weigh) and he (experimenal) air permeabiliy values measured are presened in Table. I can be seen ha he fabrics differed in erms of he yarn linear densiies, yarn ype, courses per cm and loop lengh. The hickness of he fabrics varied wih he loop lengh and course coun. Since he wale coun per cm usually depends on he machine seings, a consan range from - should be mainained. According o Table, he fabric wih he lowes course coun per cm and yarn number in ex has he highes air permeabiliy values. Therefore, he rising loop lengh resuled in a looser surface on he fabric, hereby increasing he air permeabiliy. In order o predic air permeabiliy values for he knied fabrics, he following were used: values of he porosiy, he radius of pores and yarns, and he flow rae calculaed using he newly developed equaions expressed in he heoreical model. Moreover, we observed microscopic views of he yarns and pores in he knied fabrics from picures aken using an image analysis device (Figure and ). Sample numbers were marked on he picures in Figure. In Figure i can be seen ha as he yarn coun decreased, yarn hairiness values fell in general. Furhermore, microscopic views revealed ha he surface of knied samples produced from hin yarns (boh ring and compac yarns) is highly nonuniform wih deformaions (especially in loose fabrics) because of he bigger pore dimensions (Figure ). As is widely known, he compac spinning mehod forms a differen yarn srucure. The mos eviden properies of hese yarns are heir high breaking srengh, high elongaion and low hairiness. Moreover, oher yarn properies such as yarn unevenness, hin/ hick places ec. are comparable o hose of convenional ring yarn. As he surface of compac yarns is smooher and less hairy, fabrics produced from such yarns canno block air, hus indicaing high air permeabiliy. However, he difference beween compac and convenional ring yarns is no oo grea. In order o simplify he heoreical calculaions, loops are assumed o be composed of ideal yarns bu wih grea variabiliy in pore size disribuion. Thus samples produced wih and ex yarns have pore dimensions which are highly variable across he fabric. I is difficul o esimae he porosiy of fabrics by calculaing he pore dimensions Air permeabiliy, mm/s n esuls and discussion experimenal and yarn diameer. As is known, yarns in he srucure of he fabric do no have a smooh surface nor a solid consrucion. There is also a lo of empiness in he yarns. onsequenly, in order o obain closer resuls, he value of he pore diameer of knied fabrics produced from and ex yarns was increased by %, and hese new values were used for calculaions. omparing he heoreical model mehod (Equaion ) and ha using experimenal air permeabiliy values, we obained he resul shown in Figure (see page 0). The mach is close, which is also indicaed by he high values of correlaion coefficien,, obained from he saisical analysis. Figure shows correlaion plos beween he prediced and measured values (x axis-measured values, y axis-prediced values). The values in Figure indicae ha he correlaion for fabrics produced from. ex yarn is superior o ha for fab- heoreical 0 Sample no Figure. Air permeabiliy values according o he experimenal and heoreical models. FIBES & TEXTILES in Easern Europe 00, Vol., No. ()

5 Figure. orrelaion plos for prediced and measured values. Figure. elaionship beween prediced air permeabiliy values and pore size. rics produced from. and. ex yarns. Air permeabiliy in knied fabrics is relaed o pore size. As discussed earlier, variabiliy in pore size affecs permeabiliy values. Moreover, as demonsraed in Figure, here is a near posiive linear relaionship beween pore size and prediced air permeabiliy values. Thus, he higher he pore sizes, he higher he air permeabiliy. onclusion An experimenal sudy was carried ou o develop a heoreical model o predic air permeabiliy values for knied fabrics. The heoreical model predics he value of he air permeabiliy using he pore size and some fabric properies before manufacuring. D Arcy s formulaion was used o esablish an equaion expressing he relaionship beween he air permeabiliy of knied fabrics and fabric srucure parameers. According o he experimenal resuls, he fabric wih he lowes course coun per cm and yarn number in ex has he highes air permeabiliy values. Moreover, increasing he loop lengh produced a looser surface in he fabric and increased air permeabiliy. As he yarn ges hinner and he pores beween loops ge larger, he air permeabiliy will increase accordingly. According o some formulaions, when he sich densiy, sich lengh or yarn diameer increase, pore size values decreases. FIBES & TEXTILES in Easern Europe 00, Vol., No. () Due o he differences beween ideal and real geomery and he random variaion of he fabric srucure, here are no exac dependences beween experimenal air permeabiliy and prediced air permeabiliy values. However, he closeness of he resuls of predicions based on calculaed values from he heoreical model and experimenal values show ha our model can be successfully used for he predicion of he air permeabiliy of knied fabrics ( = 0.). This model is simple and efficien. Permeabiliy and porosiy are srongly relaed o each oher. If a fabric has very high porosiy, i can be assumed ha i is permeable. I was also found ha here is a near posiive linear relaionship beween pore size and air permeabiliy values ( = 0.), hence could be assumed ha he model developed is applicable for predicing he air permeabiliy of plain knied fabrics produced wih differen fiber ypes. eferences. Tou N. A.; An Invesigaion of Arcing in Two Srucure Wef Kni Fabrics, MSc Thesis, Norh arolina Sae Universiy, Texile & Apparel Technology & Managemen, 00.. Dias T., Delkumburewae G. B.; Fibers and Polymers, Vol., No. (00) pp. -.. Air Permeabiliy Tes Mehod & Explanaory Noes, Ogulaa. T.; Journal of Texile and Apparel, Technology and Managemen, Volume, Issue (00) pp Tokarska M.; Fibres & Texiles in Easern Europe, Vol., No. (), 00 pp Ogulaa. T, Koc E.; Associaion for he Advancemen of Modelling & Simulaion Techniques in Enerprises, Vol. 0, No., (00) pp. -.. TS EN ISO, Texiles-Deerminaion of he Permeabiliy of Fabrics o Air, Turkish Sandards Insiuion, Ankara,.. Karaguzel B.; haracerizaion and ole of Porosiy in Knied Fabrics, MSc Thesis, Norh arolina Sae Universiy, Deparmen of Texile Engineering, hemisry and Science, 00.. ay A., Vassiliadis S., angoussi M., Tarakcioglu I.; Inernaional Journal of Signal Processing, Vol., Number, 00 pp ay A., Tarakcioglu I.; Journal of he Texile Insiue, Vol., Issue, (00) pp Oinuma., Journal of he Texile Machinery Sociey of Japan, Vol., No., 0, pp. -.. Marmarali A. B.; Texile esearch Journal, () 00, pp. -.. Wilbik-Halgas B., Danych., Wiecek B., Kowalski K.; Fibres & Texiles in Easern Europe, Vol., No. () 00, pp Benloufa S., Fayala F., heikhrouhou M., Ben Nasrallah, S., AUTEX esearch Journal, Vol., No., 00 pp. -.. Sankovic S. B., Popovic D., Poparic G. B.; Polymer Tesing, Vol., No., 00 pp. -.. Mavruz S., Ogulaa. T.; Teksil ve Konfeksiyon, Vol., No.. 00, pp. -.. Booh J. E., Texile Mahemaics, The Texile Insiue, ISBN 0 00 X, p.,.. Xu G., Wang F.; Journal of Indusrial Texiles, Vol., No:, 00 pp. -.. Holman J. P.; Hea Transfer, Sevenh Ediion, McGraw-Hill Book ompany,. 0. TS EN 0, Texiles - Knied fabrics - Deerminaion of Sich Lengh and Yarn Linear Densiy in Wef Knied Fabrics, Turkish Sandards Insiuion, Ankara, 00.. TS EN, Texiles - Knied fabrics - Deerminaion of Number of Siches per Uni Lengh and Uni Area, Turkish Sandards Insiuion, Ankara, 00.. TS EN ISO 0, Texiles-Deerminaion of Thickness of Texiles and Texile Producs, Turkish Sandards Insiuion, Ankara,.. TS, Deerminaion of Mass per Uni Lengh and Mass per Uni Area, Turkish Sandards Insiuion, Ankara,. eceived..00 eviewed.0.00