Prediction of thermal and moisture comfort properties of polyester filament plain knitted fabric

Excerpt: Water vapour permeability (WVP) is defined as the water vapour transmitted through the fabric of a given area in 24 hours time at a specific distance from water

Abstract

At present polyester knitted fabric is increasingly used in sports apparels. The main objectives of this study are to study the influence of fabric structural factors on moisture comfort properties to develop predictive models for moisture comfort properties from the fabric structural factors and to decide the structural factors for optimum moisture comfort for wearing in extreme environmental conditions. So polyester filament yarn is knitted in plain structure with independent variation in filament denier, fabric tightness factor and the yarn denier and tested for its wicking time, water vapour permeability, air permeability, water absorbency and drying time. It is concluded from the study that the density is positively correlated with wicking time, the absorbency and drying time but negatively correlated with WVP and air permeability; Thickness is positively correlated with wicking time, absorbency and drying time but negatively correlated with WVP and air permeability ; Filament denier is positively correlated wicking time, WVP, air permeability but negatively correlated with the moisture absorbency and drying time. In this study the extent of influence of individual parameters on the individual moisture comfort properties is discussed. The quadratic regression equations are also predicated for all the moisture comfort properties and validated. Based on this study the structural parameters for optimum moisture comfort for different climatic conditions for single jersey polyester knitted fabrics are discussed

Keywords - tightness factor, filament denier, yarn denier, wicking time, air permeability, water vapor permeability, absorbency, drying time

Introduction

The perception of thermal comfort is evaluated subjectively and it is the mental satisfaction with the environment in terms of temperature [1]. A clothing system which is suitable in one climate and activity level may not be suitable in another because the clothing thermal insulation required is different in each environment [2]. The lower vapor pressure gradient, higher wind speed and thinner clothing can decrease the insulating ability of the clothing material [3]. The convective heat loss due to fabric air permeability from knitted fabric is more important than conductive heat loss [4].

Literatre survey

In many studies on knitted fabrics [5, 6, 7, 8, 9]. It was found that the most influential factor affecting the thermal resistance, air permeability and water vapor permeability was fabric tightness factor, thickness fabric areal density.

A cooling vest containing phase change materials can help in maintaining the skin temperature of the covered parts at a comfortable level [10]. There is no influence in thermal comfort and thermal perception due to the application of phase change materials on bed mattress even though there is a slight reduction in skin temperature [11].

Skin temperature and moisture content of microclimate are strongly stimulated by the level of physical effort. The fabric type has a significant effect on moisture of skin [12, 13, and 14]. Higher amount of sweat is produced during active sports, so the sweat has to be absorbed, dried quicker and the body must be cooled by the sportswear worn. [15]. Polyester sportswear shows better performance with comfortable skin temperature, moisture and comfort sensation [16, 17, 18, 19]. Socks made from synthetic fibers were slightly more comfortable than the predominantly cotton[20].

Thermo-physiological comfort depends upon heat and moisture transport through the fabric, where moisture can be in the form of vapor and liquid [21, 22, 23, 24]. Wetting and wicking are two sequential processes for liquid transfer through a porous structure [25]. Non circular cross-sections of filament enhance the wicking ability by providing more surfaces to wet as compared to circular fibres [26, 27, 28]. Micro fibre polyester fabric show excellent drying rate, wicking and the absorbency [29]

The review of literature shows that a lot of research work has already been carried out in cotton knitted fabrics on thermal and moisture comfort properties. Mainly the convective and evaporative heat transfer of fabric affects the thermal and moisture comfort properties than conductive heat transfer since the thermal conductivity and density polyester fabric is very low. Presently the use of polyester knitted fabric is increasing worldwide. Even though some isolated research work has already been carried out in polyester knitted fabric the specific predictive equations for thermal and moisture comfort properties and specific solutions for extreme climatic conditions in using for polyester plain knitted fabric for clothing are not found in the literature. Human performance is expected to improve under comfortable conditions.

Objectives

The main objectives of this study are to study the influence of fabric structural factors such as filament denier, tightness factor and the yarn denier on moisture comfort properties of 100% polyester filament knitted fabric. Secondly, to develop predictive models for moisture comfort properties from the fabric structural factors and validate the same. Third, to decide the structural factors for optimum moisture comfort with 100% polyester filament knitted single jersey for wearing in extreme environmental conditions

Experimental

Method

Twenty seven polyester weft-knitted fabrics in plain structure were produced with variations in its structural parameters; (Filament denier, fabric tightness factor and yarn denier.) They were scoured and heat set them to ensure their dimensional stability and tested for their thermal and moisture comfort properties. (Water vapour permeability, air permeability, water absorbency, drying time and wicking time). The influence of the fabric structural parameters on its properties were analysed and also developed were the regression equations for each of the thermal and moisture comfort properties from its structural parameters and validated the same

Materials

Three different yarn linear densities (80,155 and 235 denier) with three different filament component linear densities (0.5, 1.4 and 4.5 denier) were selected in polyester material for knitting in plain structure with three levels (low, medium and high) of tightness factor (1.21, 1.34 and 1.49) in a knitting machine (Mesdan, S.P.A. Italy) of 10 cm diameter, 9 needles per cm (22 needles/inch) and with a single feeder. The knitted fabric samples were scoured in a bath with 1:20 of material liquor ratio, 1 g/l of wetting agent, 2 g/l of sodium carbonate (Na2Co3), and 2 g/l of detergent at 70°C for 30 min and washed at room temperature for 10 minutes and then neutralized with 1% of acetic acid at 55°C for 30 min. The scoured fabric samples were heat set at 190°C for 30 seconds with 20 % stretch to ensure their dimensional stability.

Nomenclature used

```
AP: Air Permeability (cm/sec)
WVP: Water Vapour Permeability (g/m2/24/hr)
GSM: Grams per Square Meter (Areal density)
TF: Tightness Factor of the fabric = (Yarn Denier)½/ 3/ knitted loop length (mm) = (Tex) ½ / knitted loop length in mm
Denier: Unit of linear density which is the weight in gm per 9000m length of yarn or filament
Tex: Unit of linear density which is the weight in gm per 1000m length of yarn or filament
Filament: A very fine continuous length of fibre used for making yarn
Yarn : A twisted assembly of filaments
Course: horizontal knitting loop line
Wale: vertical knitting loop line
Weft knitted: As fabric assembly produced by interloping of yarns travelling in horizontal direction

```

Testing of fabric properties

All the 27 fabric samples under study were tested for dimensional thermal and moisture comfort properties under standard atmospheric conditions of 65% relative humidity and 27°C. Ten readings were taken for calculating the average for all the properties.

Dimensional properties:

The fabric mass per unit area was measured by weighing the fabric of 100cm2 area and calculated in grams per square meter as per standard ASTMD 3776/D 3776M - 09a; the fabric thickness was measured by the SDL thickness gauge at a pressure of 100 Pa according to ASTM D1777-96. The fabric density was calculated by dividing fabric weight per unit area by its thickness. The loop length was calculated by measuring the course length of yarn in a course of 50 wales and dividing by 50. The tightness factor indicates the relative tightness and looseness of a structure which is equal to square root of yarn denier count divided by 3 and loop length (mm). The loops/inch2 (6.3 cm2 )was calculated using magnifying counting glass as per standard ASTMD-3887:1996

Water vapour permeability:

Water vapour permeability (WVP) is defined as the water vapour transmitted through the fabric of a given area in 24 hours time at a specific distance from water. It was measured using SDL Shirley water vapour permeability tester M-261 as per ASTM standard E96-80. In the ASTM method E 96-80 the air gap above the water surface is 19mm and an air velocity of 2.8 m/s is used over the surface of the fabric.

Air permeability

Air permeability (AP) is the volume of air allowed through unit area fabric at a particular pressure drop across the fabric. It was measured using Mesdanlab instrument at a pressure drop of 100 Pa as per ASTM D737-96 standard.

Water absorbency

Water absorbency was measured by allowing the water drop by drop from 10 mm height on a 5cm*5cm fabric sample. It was calculated as the weight (grams) of water absorbed by one gram weight of fabric.

Drying rate

Drying rate was measured as per AATCC199 standard moisture analysis method, expressed as the time required in minutes to reach 20% residual water content at human skin temperature of 33 deg C and 65% RH.

Vertical wicking time

Vertical wicking time is defined as the time (in minutes) required for the fabric to transport water by wicking vertically for 15 cm height. It was tested by freely mounting the fabric strip vertically in a beaker containing water touching the bottom end as per AATCC197 standard.

Statistical analysis

The analysis of data of all properties against the structural parameters was done using MINI TAB 14 (Minitab Inc., State College, Pennsylvania) Statistic software.

Results

The properties of knitted fabrics are given in Table 1

Regression equations

Equations of the form y = k+ax1+bx2+cx3+dx12+ex22+fx32+gx1x2+hx1x3+I x2x3 were formed to predict the moisture comfort properties from the fabric structural parameters and the actual prediction equations are given below.

Wicking time (min):y5 = 19.0671+1.34966x1-62.415x2-40.698x3+0.176255x12+81.8841x22+88.7879x32-3.89928x1x2+2.42225x1x3+77.7219x2x3----------------------- (1) Water vapour permeability (gm/m2 /24 hrs):y1 = 912.616-216.525x1+2996.08x2+995.428x3+30.1294x12-1775.03x22-2844.59x32+88.6224x1x2++31.0965x1x3-7467.41x2x3; ----------------------------------------------------- (2) Air permeability (cm/sec):y2 = 750.809-29.7355x1-581.931x2-2682.93x3++3.81650x12+1189.40x22 4363.04x32 -52.10367x1x2+138.906x1x3-1493.44x2x3; -------------------- (3) Absorbency (gm/gm):y3= -0.66978+0.397588x1-3.38988x2 +8.73068x3-0.05805x12-5.52209x22 -24.0682x32 +0.440677x1x2-1.20577x1x3+ .0112x2x3 ; --------------------------------------------------------- (4) Drying time (min):y4 = -34.3768-10.6578x1+87.8114x2+207.072x3+1.45498x12 +2.57717x22-204.625x32-7.32385x1x2+17.4155x1x3 +40.6647x2x3 ; -------------------------------------------------- (5)

Where x1 is filament denier; x2 is thickness and x3 is density which are independent variables. The regression coefficients for wicking time, (R2 =0.929; p<.001) WVP, (R2=0.861; p<.001) air permeability, (R2=0.948; p<.001) absorbency (R2=0.951; p<.001) and drying time, (R2 =0.983; p<.001) are significant. Linear regression equations of the form: y = k+ax1+bx2+cx3 were also formed to predict the moisture comfort properties from the structural parameters and the actual prediction equations are given below. WVP (gm/m2/24 hours):y6 = +1710.18 +1.37049x1-338.688x2-2316.24x3-------------------------------------------- (6) AP (cm/sec):y7 = 379.49+2.71199x1-97.8042x2 -727.693x3- ---------------------------------------------------------( 7) Absorbency (g/g):y8= -1.02117-0.03008x1+2.03618x2 +6.64069x3 --------------------------------------------------------(8) Drying time (min):y9 = -37.6968-1.65653x1+82.5096x2 +169.4x3 -----------------------------------------------------------(9) Wicking time (min):y10 = -6.20179+1.22122x1+16.4378x2+39.9683x3----------------- (10) The regression coefficients for WVP, (R2=0.724; p<.001), air permeability, (R2=0.849; p<.001), absorbency, (R2=0.851; p=.001), drying time, (R2 =0.963; p<.001), and wicking time (R2 =0.898; p<.001) are significant.

Error analysis of regression equations for validation

Regression equations were tested for their validity by calculating the R2 (square of correlation coefficient) values between the actual and predicted values of moisture comfort properties of fresh samples produced in the same route as that of study samples; The root mean square deviations (RMSD) of predicted values actual values were also calculated and are given in Table 2 .Considering the comparatively high degree of correlation (R2 values) between the observed and predicted values and lower absolute RMSD (root mean square deviation) values it was concluded that quadratic equations for prediction of all properties can give more accuracy. The scatter plots of actual and predicted quadratic equations for moisture comfort properties are given in Fig. 3 to 7.

Optimum structural parameters for moisture comfort in extreme climates

Hot and dry climate

Under hot and dry humidity condition, the perspiration is dissipated quickly due to higher vapour pressure gradient and the microclimate reduces. So the clothing must retain at least a minimum amount of moisture so that the body's excess heat is dissipated slowly by evaporative cooling to give a comfortable microclimate to the wearer. In hot and dry humidity conditions (>40degC and <30% relative humidity) the fabric should allow for high water vapour transmission in active conditions in order to reduce the thermal strain through the buffering effect [30], otherwise sweat will accumulate on the skin and cause discomfort. The quick absorption keeps the skin dry and comfortable but the slow release of moisture give a buffering effect which increases the evaporative heat loss and therefore is best suited for hot and dry conditions [7]. The fabric structure must be open to facilitate ventilation under hot and dry climatic conditions [14, 31]. The optimum solution to meet this requirement in polyester plain knitted fabric of 0.213g/cc density and 0.56mm thickness and filament denier of 4.5 which will give the WVP g/m2/day of 1100, AP cm/sec of 190, absorbency gm/gm of 1.1, drying time(min) of 35, and a wicking time(min) of 17.

Hot and humid climate

Under tropical climates of higher temperature and higher humidity (>40 degree C and >75% relative humidity) the low vapour pressure gradient between the skin and the atmosphere reduces the rate of vapour diffusion [21, 31]. Under this condition the fabric must have to wick the sweat quickly from the skin and move away to the outer layers so that the clinging of the clothing with the skin is reduced. Higher air permeability for better ventilation, lower moisture absorbency quicker wicking and drying time is preferred to relieve the skin from stickiness. Highest air permeability with the shortest drying time and lowest water absorption are best suited for hot and humid conditions [7]. The optimum solution to meet this requirement in polyester plain knitted fabric is of 0.191g/cc density and 0.35mm thickness and filament denier of 4.5 which will give the WVP ( g/m2/day) of 1170, AP (cm/sec) of 220, absorbency gm/ gm of 0.9, drying time(min) of 5, and a wicking time(min) of 12.

Cold climate

Under cold conditions, trapped moisture in between the skin and the clothing significantly reduces the insulating capacity leading to a chilling sensation, decreases in body temperature eventually the possibility of hyperthermia, [32]. So the fabric have to prevent the accumulation of sweat on the skin surface by allowing it to pass from to the outside environment or at least or to the outer layer of clothing [33] Insulation of clothing is the main factor in cold weather. The fabric must have low moisture absorbency for better insulation. Higher physical activity will increase body's core temperature and produces insensible perspiration. Under this condition the fabric must have good WVP to transmit the insensible perspiration, lower air permeability [14] and lower the moisture absorbency to reduce the heat loss from the body. The optimum solution to meet this requirement in polyester plain knitted fabric of 0.324 g/cc density , 0.35mm thickness and filament denier of 0.5 which will give the WVP g/m2/day of 840, AP cm/sec of 110, absorbency g/ g of 1.9, drying time (min) of 45, and a wicking time(min) of 13.

Influence of fabric structural parameters on moisture comfort

Water vapour permeability

Water vapour permeability (WVP) is defined as the rate of water vapour transmitted through the fabric of a given area at a specific distance from water. The transmission of vaporous moisture occurs generally due to the moisture vapour gradient and the diffusion coefficient of the medium. The analysis of variance in moisture and thermal comfort properties in this study reflects the variance in WVP is due to density by 62.32% (F=37.60, p=.000) thickness by 11.92%(F=7.193, p=.013) and filament denier by 0.04%.(F=0.027, p=.871) Under constant vapor gradient with standard testing conditions WVP is negatively correlated with density and thickness of the fabric placed in between the two ends. Higher density leads to lower inter yarn porosity which gives lesser space for air passage. Increase in the inter yarn surface area, results in an increase in air drag resistance which also leads to lower AP and WVP. This is in confirmation with the previous research findings [17, 34]. Decreases in filament denier and diameter results in reduced inter yarn porosity which gives lesser space for air passage. This is also in confirmation with the previous research findings [7, 35]. AP and WVP are positively correlated as the AP indicates the amount of space through which water vapour can diffuse from one space to another [27].

Air permeability

The analysis of variance in moisture and thermal comfort properties in this study reflects the variance in AP is due to density by 54.85% (F=71.029, p=.000) thickness by 8.86 %(F=11.479, p=.003) and filament denier by 1.56 %.(F=2.027, p=.168) Air permeability is negatively correlated with density and thickness. With an increase in pore size, there is an increase in AP. Fabrics with lesser densities show higher air permeability. Higher density leads reduced inter yarn porosity which gives lesser space for air passage which leads to lower AP and WVP. This is in confirmation with the previous research findings [7, 36, 37, 39]. Decreases in filament denier and diameter results in reduced inter yarn porosity which gives lesser space for air passage.

Water absorbency

The analysis of variance in moisture and thermal comfort properties in this study reflects the variance in water absorbency is due to density by 41.43 %(F=54.22, p=.000) thickness by 34.85 %(F=45.609, p=.000) and filament denier by 17.31%.(F=2.28, p=.145) The water absorbency is positively correlated with thickness and density but negatively correlated with filament denier. This is due to the available yarn surface area to hold the water. Primarily water is absorbed by fibre due the presence of water absorbing OH groups and its accessibility for contact with water. Secondly, it is loosely attached on the fibre surface contours due to capillary pressure and surface configuration. In polyester mainly the second possibility prevails. Extreme tightness reduces the absorbency due to higher physical barrier for water entry. Moderate tightness absorbs the water easily and holds it. Low tightness even though have access for water absorbency, it doesn't hold the water. Micro denier polyester shows excellent moisture transfer properties such as wicking, absorbency and drying rate [29].

Drying time

The analysis of variance in moisture and thermal comfort properties in this study reflects the variance in drying time is due to density by 25.36%,(F=153.04, p=.000) thickness by 53.82%(F=324.83, p=.000) and filament denier by 4.97%.(F=30.01, p=.000) The drying time is positively correlated with thickness and density but negatively correlated with filament denier.

Wicking time

The analysis of variance in moisture and thermal comfort properties in this study reflects the variance in wicking time is due to density by 17.75%(F=35.89, p=.000) thickness by 26.87%(F=54.31, p=.000) and filament denier by 33.99%.(F=68.71, p=.000). Liquid transport is characterized by the concentration gradient and capillary forces .Smaller capillary size results in higher wicking force. But under constant concentration gradient with standard testing conditions the capillary size and wicking time is positively correlated with thickness, density and filament denier. In loose fabrics due to good water absorbency supply driven wicking force is generated which results in shorter wicking time to reach the wicking height.capillary wicking, which is determined mainly by effective capillary pore distribution, pathways and surface tension [26] and it is the natural flow of a water in a absorbent substance, determined by capillary forces and its velocity go after Lucas -Washburn kinetics equation [38]. In micro denier polyester, because of its smaller capillary size results in higher capillary pressure and consequent higher rate of wicking [29]. Smaller capillary size results in higher wicking pressure and in shorter wicking time to reach the wicking height

Conclusions

The structural variables viz. density thickness and filament denier influences all the moisture comfort properties to varying degrees; The fabric density influences WVP by 62.32 %, AP by 54.85%, water absorbency by 41.43%, drying time by 25.36%, and wicking time by 17.75%; Thickness influences wicking time by 26.87% , WVP by11.9% ,AP by 8.86%, water absorbency by 34.85%, and drying time by 53.82%,; filament denier influences wicking time by 33.99%, WVP by 0.04% AP by 1.56%, water absorbency by 1.73% and drying time by 4.97%. The quadratic regression equations are developed from the structural parameters for all the moisture comfort properties and error analysis is done and the equations are found valid. The density is positively correlated with wicking time, the absorbency and drying time but negatively correlated with WVP and air permeability; Thickness is positively correlated with wicking time, absorbency and drying time but negatively correlated with WVP and air permeability; Filament denier is positively correlated wicking time, WVP, air permeability but negatively correlated with the moisture absorbency and drying time. The most suitable structural parameters for optimum moisture comfort in plain polyester knitted fabrics are a) Density of 0.213 g/cc , thickness of 0.56mm and filament denier of 4.5 for hot and dry humidity conditions b) Density of 0.191 g/cc , thickness of 0.35mm and filament denier of 4.5 for hot and high humidity conditions c) Density of 0.324 g/cc , thickness of 0.35mm and filament denier of 0.5 for cold climatic conditions.

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Author Details

R. Varadaraju, J. Srinivasan