Indian Journal of Fibre & Textile Research Vol. 3 1 , September 2006, pp. 4 1 5-42 1

Heat and mass transfer properties of 2-yarn fleece knitted fabrics

Sinem Gunesoglu & Binnaz Merica

Texti le Engineering Department, Uludag University, 1 6059 Bursa, Turkey

Received 14 March 2005; revised received alld accepted 1 2 September 2005

Heat and mass transfer propert ies of 2-yarn fleece fabrics of four different compositions have been studied by related measurements, such as thermal conductivity, water vapour permeabi l i ty , air permeabi l i ty and wicking abil i ty . It is observed that the raising process, used to produce typical commercially available fleece fabrics, is very effect ive on heat transfer properties; the effect of fibre lype is rather strong on mass transfer properties of fleece fabrics.

Keywords: Air permeabi l i ty, Fleece fabric, Heat transfer property, Mass transfer property, Thermal conductivity, Water vapour permeabi l i ty, Wicking

IPC Code: Int . Cl .8 D04B 1 3/00, GO I N33/36

1 Introduction Men wear cloths for social necessi ty and to protect

themselves from outdoor effects; nevertheless prefer garments which make them feel comfortable. Comfort i s a complex subject and i t is very difficult to make an exact definit ion. Holl ies and Fourt l stated that the comfort involves thermal and non-thermal components and the broader aspects of comfort are wearing s i tuations, such as working, non-critical or critical conditions.2 Hes3 expressed comfort as the complex effect of purely mechani cal properti es and heat/moisture transfer properties of textile fabrics and garments.

When garments are worn, the heat and moisture, d iffused from the skin , get exchanged by several transfer mechanisms. These mechanisms are s imply thermo-physiological regulators that i nclude the heat and mass transport through the texti le layers to keep the energetic balance (thermal energy) between human body and environment.4 These two concepts are considered to characterize the thermo­physiological comfort.5 Heat transport through the fabric concerns heat flux that determines the warmth i nsulation, whi le mass transport concerns moisture­vapour, air and l iquid moisture transmission by determining water vapour permeabi l i ty , air permeabi l i ty and wicking of garments respectively .

The mechanisms of heat transfer through textile fabrics may i nvolve conduction through air and fibres, radiation and convection within the fabric. It i s stated

"To whom all the correspondence should be addressed. E-mai l : binnaz@uludag.edu.tr l sinem@uludag.edu.tr

that the mechanisms of heat transfer through textile fabrics depend mainly on thermal conduction and radiation; however, the portion of heat flow transferred by radiation does not exceed 20% of the total heat f1ow.6.? Hence, the thermal conductiv i ty i s the dominant property to determi ne the heat transfer through fabrics and garments.

Meanwh ile, l iquid and moisture transfer mechanisms i nclude vapour diffusion in the void space and moisture sorption by the fibres, evaporation and capil lary effects . Water vapour moves through textiles as a result of water vapour concentration differences, whereas fibres absorb water vapour due to their chemical composition. Hence, when fabrics have s imilar structures but differ i n fibre type, i t i ndicates difference i n water vapour permeabil i ty . 8

The water vapour permeabi li ty of clothi ng materials i s a critical property for clothi ng systems that maintain the thermal equi l ibrium for the wearer. Cloth ing materials wi th high vapour permeabi l i ty al low the human body to take advantage of its abi l i ty to provide cooli ng due to sweat production and evaporation. H igh water vapour permeabi l i ty is also important to prevent or min imize water buildup i n clothing which often causes the wearer to feel uncomfortable.8- lo

Water bui ldup means the condensation in the garment, and when it occurs the heat released from the body to the environment i s enhanced through the i ncreased heat conduction of the l iquid water. I n severely low temperature environments, l ike at h igh alti tudes, the condensation is not only a discomfort factor but also a significant survival factor, s ince i t causes change of hypothermia due to the low thermal

4 1 6 INDIAN 1 . FIBRE TEXT. RES . , SEPTEMBER 2006

resistance of wet clothes; hence water vapour permeabi l i ty becomes critical cloth ing factor in cold cli mates. I I Previous research showed that both water vapour concentration and temperature gradient are determinant factors i n the water vapour transfer rate. " · 1 2 Studies I.l also supported the importance of hygroscopicity. It has been found that the actual total water vapour transport rates differed greatly depending on the hygroscopicity of fibres due to the fact that the hydroph i l ic fibres retain water molecules and can even swell to reduce the porosity of the fabrics. On the other hand, Yoo et at. 12 showed i n their studies that the clothing factors such as fabric and garment openings have great influence because moisture vapour can pass through openings.

The air permeabi l i ty of a fabric can influence its comfort behaviour in several ways . In the first case, an air permeable material is, in general , l ikely to be permeable to water in either the vapour or the l iquid phase. Thus, the moisture vapour permeabi l i ty and the l iquid moisture transmission are normal ly closely related to

' air permeabi l ity. In the second case, the

thermal resistance of a fabric is strongly dependent on the enclosed sti l l air and this factor is , i n turn, i nfluenced by the fabric structure, as also is observed for .the air permeabil ity . Zhang et al. 14 stated that the air and water vapours pass through a fabric i n differept ways . A l l textile fibres, i rrespective of their chemical composi tion, are i mpermeable to air and therefore the passage of air through a fabric can only take place through space between the fibres and between the yarns.

The interactions of yarns and fabrics with l iquids depend on the chemical nature of the fibre content, liquid properties such as surface tension, viscosity and density, and character of the l iquid flow. The flow of liquid moisture through the textiles (wicking) i s caused b y the fibre-l iquid molecular attraction at the surface of fibres, which is determined mainly by surface tension and effective capil lary pore distribution, fibres d iameter and pathways. Wicking with in a fabric occurs through (perpendicular) and along (paral lel) the fabric plane and it is a . I . .

b d 1 5 1 6 s lmu taneous occurrence on a persplflng 0 y . .

The present study i s aimed at investigating the heat and mass transfer properties, cons ideri ng thermal conductivity, water vapour permeabi l i ty , wicking and ai r permeabi l i ty of fleecy knitted fabrics of four di fferent composi tions. The fabrics have relat ively high mass and thickness and are widely used as an

outdoor garment for active and sportswear. The study is carried out to investigate the comfort characteristics of active wear fabrics and garments over the past number of years. The sportswear sector in textile i ndustry has expanded on the global basis and the manufacturers and wearers want to point out their comfort performance beside the aesthetic appeals . The effects of composition of 2-yarn fleecy knitted fabrics and the role of fibre type and rais ing treatment on the heat and mass transfer properties of the fabrics have also been studied. Experimental devices, methods used and stati stical contribution of each factor to these parameters have been discussed using an analysis of variance (ANOY A). The results are evaluated at 5% signi ficance level .

2 Materials and Methods

2. 1 Materials

Single s ided fleecy fabrics of four different compositions were kni tted on a 32/1E 22 circular knitt ing machine. Each fabric type was kni tted with the same course and wale counts and was separated in accordance with the yarn type i n both ground and loop. The first letter i n fabric codes (CC, CP, PP and PC) stands for the back of the fabric which touches the skin and the loop yarn in back was Ne 1 0/ 1 in all cases while the second letter stands for the face of the fabric and the ground yarn in face was Ne 20/ I . All the fabrics were rinsed and then half of each was subjected to rais ing to produce common fleecy kni tted fabric. The R in fabric codes (CCR, PPR, CPR and PCR) represents the raised fabric, e .g . CPR is the raised fleecy fabric with 1 00% cotton back and 87: 1 3 · polyester-cotton face. The constructional detai ls of fabrics are given i n Table 1 .

2.2 Methods

Traditional ly , most of the measurements in fabric thermal properties were conducted in equi l ibrium state, analyzing easi ly measured properties, such as thermal conducti vity and resistance. Thermal conductivity is a fami liar term appl ied to materials that conduct heat and it is defined as the heat flux divided by the temperature gradient, where heat (Wnf I KI ) is transferred by conduction. The human body i s an i nterior source of heat and the heat transfer mechanism through fabrics from the skin has been studied for a long time.6.7. 1 7. 1 8 Different methods have been developed to determine thermal conductivity of fabrics; Hes and Dolezal developed thei r own

GUNESOGLU & MERIC: HEAT AND MASS TRANSFER PROPERTIES OF 2-YARN FLEECE KNITTED FABRICS 4 1 7

Table I-Constructional properties of the fleecy fabrics

Fabric Yarn type Fabric mass code g/m2

CC Cotton back ( loop), 302.7 CCR Cotton face (ground) 273.8

CP Cotton back(loop) , 340 CPR 87/1 3polyester/cotton face (ground) 328.6

PP 87/ 1 3 polyester /cotton back(loop), 3 1 3 PPR 87/ 1 3 polyester /cotton face (ground) 295.5

PC 87/ 1 3 polyester /cotton back (loop), 32 1 .8 PCR Cotton face (ground) 324

C-Cotton, P-87/1 3 Polyester/ cotton and R-Raised fabric.

measuring device (Alambeta) which i s used in this study for thermal conduct iv i ty measurements.3 ,7

This i nstrument s imulates, to some extent, the heat flow q (W/m2) from human skin to a fabric during a short in i tial contact. The main feature i s a sensor that measures the thermal drop between the surfaces of a very thi n non-metal l i c . plate using a multiple differential microthermocouple. Thi s sensor is 0,2 mm thick and on contact with a subject at a different temperature, i t reaches the max imum heat flow qmax (W/m2) in 0.2s. Thus, it s imulates the human skin which is - 0.5 mm thick and whose neutron ends, located in the middle, also take 0. 1 -0.3 s to reach qmax, as the heat begins to flow through the contact subject. Having reached thi s peak value, the heat flow decreases and then stabi l izes wi th in 3- 1 5s (ref. 1 7) . The typical t ime dependence of th i s heat flow is given in Fig. 1 .

The thermal drop detected by the heat flow sensor and the i nfluence of thermal capacity is automatically compensated by a computer. Thermal conductivi ty A (W/mK) i s derived from the steady state heat flow q(O,oo) (W/m\ taking place at the end of the transient phase (Fig. 1 ) as given by the following equation:

A q(O,oo) = -(tl - to ) Iz

. . . ( 1 )

where h (mm) i s the thickness; and (tl - to) (K), the temperature gradient.

For the measurement of water vapour permeabil ity wvp (%), another i nstrument called Permetest was used. The working principle of this instrument i nvolves the measurement of heat flow caused by the evaporation of water passi ng through the tested spec imen. S ince every gram of water needs 2 .5 J to be

Wales /cm

-N E ::-­::::

1 0

1 0

1 0

1 0

qmnx T

Courses /cm

1 6

1 6

1 6

1 6

Thickness Fabric density mm g/m'

1 . 19 0.254 1 .67 0. 1 64

1 .27 0.268 1 .84 0. 179

1 . 1 0 0.285 1 .36 0.2 1 7

1 .22 0.263 1 .78 0. 1 82

q (0 ,,110 q (0,00)

Fig. I-Typical t ime dependence of heat flow

evaporated (to pass from the l iquid state to water vapour), thi s method is considered very sensit ive. The heart of the Permetest is sensed on an ultra thin direct heat flow sensor (0. 1 5 mm), whose thermal inertia is s imi lar to that of the human skin. The relative water vapour permeabi l i ty given by thi s i nstrument is defined as the ratio of the heat loss measured by the sensor with the sample to that measured without the sample.

The wicking characteristics of fabrics were determined i n both vertical Wv (mm) and horizontal w" (mm) directions to s imulate the real wearing conditions, as described by Mogahzy et al. 1 9 and Wong et a11 6• For the measurements, a 2 .5 x 20 cm ( I x 8 i nch) long fabric strip was lowered into the dist i l led water. The depth of fabric i mmersion in the reservoir was 1 0 mm and the distance (mm) travelled by water along the fabric was determined 2, 5, 1 0, I S , 20, 25 and 30 min after the i mmersion. In the vertical wicking experiments, the strip was hanged and i mmersed in a vertical d irection, meanwhile in the horizontal the strip was laid on a platform. The effect of gravity on the movement of water along the fabric has also been studied for thi s experiment.

4 1 8 INDIAN J . FIBRE TEXT. RES . • SEPTEMBER 2006

The air permeabil i ty of fabrics airl} (Llm2s) was measured according to EN ISO 9237 Standard Test Method by means of WIRA Air Permeabil i ty tester.20

The measurement area was 5 .08 cm2 and the results were derived in Llm2 s by multiplying the values read at a scale of the instrument with a constant K as [ 1 1 (0.000508 x 60)] .

All the measurements were completed i n laboratory environment at about 24°C and 55% RH and repeated for four times. The average of four measurements was taken for each parameter.

3 Results and Discussion

3.1 Thermal Conductivity

Figure 2 shows the thermal conduct ivi ty of the fabrics measured by Alambeta with the mean and coefficient of variation of four types of fabric measurements . During the measurements, the fabrics were placed so as to keep them in contact with the sensor from the backside which touches the skin during wearing.

The rate of heat flow between the skin and the fabric is strongly determined by the fabric property, which is so called thermal inertia. Ukponmwan l 8

defined thermal inerti a as the product of thermal conductivity and a combination of thermal conductivity of the fibre substance and that of the air contained within the fabric . He stated that the fabric density and specific heat of fibre substance determine the rate of heat transfer into the fabric along with thermal inertia. The results first reveal that the fleece fabrics with cotton back (CC or CP) have sl ightly higher thermal conductivity . Although the thermal conductivity for polyester (0. 1 4 1 W/mK)2 1 i s approximately three times lower than that for cotton (0.46 1 W/mK) 2 1 , the PP, PC and CP fabrics have higher fabric density than CC which only consists of 1 00% cotton; the result ing thermal conductiv i ty of all the fabrics studied remains s imi lar.

The raising process shows great i nfluence on thermal conductivity of fleece kni tted fabrics which definitely reduces for each fabric construction. The raising led hairy structure forming inside the fabric that entraps more air and i ncreases air gap between skin and fabric, which reduces thermal conductivity of fabric due to lower thermal conductivity of air (0.026 W/mK) 2 1 as compared to that of fibres. The ANOV A reveals that the change of fibre type at s ides of the tested fleece fabrics is non-significant on thermal conductivity after rais ing and hence it is

C'c ceR

IT CCR

PP PPR PC peR Fabric

I CP erR I

i

Fabric code

CC

CCR

PP

PPR

PC

PCR

CP

CPR

Fig. 2-Thermal conductivity of fabrics

PP PPR PC PCR Fabric

I I I

Fabric code

CC

CCR

PP

PPR

PC

PCR

CP

CPR

A. .W/mK i Mean CV O/O 1 value

(xIO') I 55.93 1.43

47. 68 2.48

53.58 1.77 i 47.20 1 .93 I 5 1 . 5 3 135 I 45.55 4.45 I 55. 13 2.32 ,

47.20 2. 1 0 i I

Wvp.%

Mean CV% value 1 7.62 5.75

1 7.75 1 .8 1

19.79 5.01

19.70 4.06

1 8. 75 1 2. 70

18. 18 5.46

18.88 3 . 1 9

1 7.05 1 .36

Fig. 3-Water vapour permeabi l i ty of fabrics

concluded that the heat transfer property of typical raised fleece knitted fabrics is strongly related to fabric constructional parameters; the effect of fibre type is negl igible.

3.2 Water Vapour Permeability

Figure 3 shows the water vapour permeabil i ty , measured by Permetest, of fleece fabrics with the mean and coefficient of variation of four types of fabric measurements.

These results i ndicate the effect of fibre type on moisture vapour management of fleece fabrics. PP fabrics, which contain too much poorly absorbing polyester fibres, give the h ighest water vapour permeabi l i ty value, whereas CC have the lowest and they were defined as significantly different by the ANOV A. On the other hand, PC and CP fabrics have the same level of water vapour permeabil i ty . Although the rais ing certainly affected the physical characteristics l ike thickness, surface d imensions, roughness, and openings of backside of fleecy fabrics , i t i s found that the contribution of rais ing was not s ign ificant on water vapour permeabil ity. It i s therefore assumed that . the fibre type and hygroscopic i ty are chemical characteristics of fabrics and thus concluded that the chemical characteristics of typical fleecy kni tted fabrics change their behaviour in terms of water vapour transfer more than the physical characteristics.

GUNESOGLU & MERIC: HEAT AND MASS TRANSFER PROPERTIES OF 2-YARN FLEECE KNITTED FABRICS 4 1 9

3.3 Air Permeability

Air permeabil i ty values of the fabrics are given i n Fig.4. The results show a l imited s imilarity with water vapour permeabil ity; PP fabrics are the most permeable to air as well as water vapour and CC fabrics are the least. Among the fleece fabrics, which have different yarn compositions in their both sides, the PC fabrics, backside of which touches the skin and contains polyester fibre, have higher air permeabi lity than CP, as expected. The raising reduces air permeabi l i ty of fleece fabrics because of the increased fabric thickness by air gap; the contribution of both yarn types at the fabric sides and raising was significant according to the ANOV A. It i s found that the fleece fabrics which have polyester­cotton back (PP and PC) show significantly higher air permeability than the remainders, before and after raiSing.

The relationship between air permeabi lity and water vapour permeabi li ty of tested fleece fabrics has also been studied. As expected, a l inear relationship was observed but it was not strong; the correlation coefficient was about 0.34. Hence, when mass transfer property of fleece fabrics was concerned, there was less dependence between their behaviour in terms of water vapour transfer and air transfer.

3.4 Wicking Ability

Figure 5 shows the distance travel led by water along the fabric i n horizontal and vertical directions with dependence on time. The longer the distance, the higher the fabric has wicking capabi l i ty . S ince the capil lary spaces in the fabrics are not uniform and water may penetrate i nto some capillaries before the others, it is assumed that the water spreading would not be a continuous front as stated by Wong et 01. 1 6 i n their paper. Hence, the wicking distance was determined not considering the end of wet fabric region but by considering average of the longest and the shortest wicking distance s ince the prior measurement. S ince all the measurements were made in standard atmospheric conditions, it was accepted that the water evaporation was simi lar and thus was not an issue to consider for al l cases . The wicking distance was measured along the fabric face as well as the interior and the difference was not significant; hence the results presented here show the wicking distance along the face sides of fleece fabrics.

The results show the effect of gravity on the water flow along the fabric. As pointed by Wong et 01. 16, during vertical upward wicking, the gravity affects the

400 N� 350 � 300 "" ::- 250 'il � 200 � I SO � 100 '" 50 '';: 0

250

200

1 50

1 00

1 50

100

50

CC

Fabric Airp ,Urn s code Mean CV%

value CC 230.75 1 0.70

CCR 202.32 2.34

PP 359.25 10.44

PPR 265.20 4.72

PC 3 1 4.41 3.98

PCR 279.42 1 0.34

CP 247. 1 5 4.98 CCR PP PPR PC peR CP CPR

(a)

Fabric CPR 226.92 4 . 1 7

Fig. 4 - Air permeabi l ity of fabrics

I -+- CC __ CCR -.- PP __ PPR ____ CP I . -+- CPR -+-PC � PCR . � f � O �-=�----�----�--�----,-----.---�

5 1 0 1 5

Timc (min) 20 25 30

Fig. 5-Wicking measurements of fabrics (a) horizontal direction and (b) vertical direction

flow of l iquid. At the onset of absorption in a vertical capil lary system, the absorbed liquid is relatively close to the l iquid source and the effect of gravity can be neglected in this situation. However, at a longer period of time or wicking distance, gravity plays an i ncreas ingly i mportant role. If the upward distance travelled by the liquid becomes long enough, the capil lary action i s balanced by gravi ty, i . e . by the weight of the raised l iquid. The effect of gravity becomes i mportant when wicking period exceeds approximately 5 min for the tested fabrics; the d istance that water travelled was lower in vertical direction than that in horizontal. The wicking measurements represent lower standard deviation especially after 5 min due to gravity effect in vertical direction (Fig. 5) .

A strong correlation is observed between horizontal and vertical wicking abi l i ty of fleece fabrics; the correlation coefficient between the sum of wicking

420 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 2006

Table 2-ANOV A results for thermal conductivity, water vapour permeabi l i ty and air permeabi l i ty

Thermal conductivitl: Parameter F-ratio Probabi l i ty

(F-ratio)

Main Level

Fabric type 1 1 .63 0.0001

Raising 296. 1 7 0.0000

Interaction

Fabric type x raising 0. 1 7 I .S4 1 2

distances per unit t ime period of vertical and horizontal d irections respectively is about 0.89. The correlation between wicking abi lity and either water vapour permeabil i ty or air permeability of fleece fabrics has not been found.

The results also reveal that the fibre material is effective on wicking behaviour of fleece fabrics; the order of fleece fabrics from the best wickability to the worst i s derived as CC, CP, PP and PC respectively, showing that the fleece fabric with cotton back have better wickabilty. The raising does not change this order of fleece fabrics and it is derived as CCR, CPR, PPR and PCR from the best to the worst wickabil ity .

3.5 ANOV A Results

The ANOY A results were performed for thermal conductivity , water vapour permeabil i ty and air permeability of fleece fabrics to demonstrate the importance of each variable; the statistical analysis for the wicking behaviour was not performed. The contribution of the variables (fabric type and raising process) was determined using all experimental data and the results were evaluated based on the F-ratio and the probability of the F-ratio. The lower the probability of the F-ratio, the stronger is the contribution of variation and the more significant i s the variable. The result of variance analysis i s given in Table 2. Thus, the contribution of rais ing was highly significant in thermal conductivity and air permeabi lity of fleece fabrics. To define the exact classification of tested fabrics, a single-factor, completely randomized ANOYA model was developed, the variable of which is the fabric type and the SNK (Student-Newman-Keuls) range test was used to designate, which d iffers signifi cantly from others (Table 3) . The treatment levels were marked in accordance with the mean values and any level marked by same letter shows that they are not

Water vaQour Qermeabil i tl: Air Qermeabi l i tl: F-ratio Probabi l i ty F-ratio Probabi l ity

(F-ratio) (F-ratio)

5.96 0.0035 59.50 0.0000

0.76 0.3923 55. 1 7 0.0000

1 .73 O. I SSO 7.93 O.OOOS

Table 3-SNK ranking at 5% significance level after single factor ANOV A model

Fabric code Thermal Water vapour Air conductivity permeabi l i ty permeabi lity

CC a ab ef CP ab ab de PP b a a PC c ab b

CCR d ab f CPR d b ef PPR d a cd PCR d ab c

significantly different. The contribution of fabric type is found to be non-significant i n terms of only thermal conductivity after raising; raised fabrics have irregular contribution in air permeability and water vapour permeability.

4 Conclusions Raising process is found to be the only designative

factor on heat transfer properties of fleece fabrics . Heat transfer property of any material i s related with physical and chemical characteristics. When structural parameters (mass, thickness and density), surface properties as physical characteristics and fibre type, and hygroscopicity as chemical characteristics of fleece fabrics are considered, it is observed that the heat transfer properties of fleece fabrics are strongly defined by physical characteristics after raising. All raised fleece fabrics represent s imilar heat transfer properties; the effect of fibre type is found to be negligible.

On the other hand, the rais ing process does not change the importance of fibre type on wickabil ity, air permeabil ity and water vapour permeabi l ity of fleece fabrics. It is concluded that the chemical characteristics have greater importance on the mass transfer properties of fleece fabrics .

GUNESOGLU & MERIC: HEAT AND MASS TRANSFER PROPERTIES OF 2-YARN FLEECE KNITTED FABRICS 42 1

Acknowledgement The authors are thankful to Prof. Lubos Hes, The

Technical University of Liberec, The Czech Republic, for the valuable ass istance and to Mr. Abdulceli l Karayi lan, Fi stik Tekstil , Gaziantep, Turkey, for the help in sample manufacturing.

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