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2477

ISSN 1229-9197 (print version)

ISSN 1875-0052 (electronic version)

Fibers and Polymers 2015, Vol.16, No.11, 2477-2490

End Use Performance Characterization of Unconventional Knitted Fabrics

Hafsa Jamshaid1,2

, Rajesh Mishra1*, and Jan Novak

3

1Technical University of Liberec, Faculty of Textile Engineering, Department of Material Engineering, Liberec 46117,

Czech Republic2Department of Fabric Manufacturing, Faculty of Textile Engineering, National Textile University,

Faisalabad 37610, Pakistan3Department of Vehicle and Engines, Faculty of Mechanical Engineering, Technical University of Liberec,

Liberec 46117, Czech Republic

(Received June 19, 2015; Revised September 2, 2015; Accepted September 5, 2015)

Abstract: This paper presents a study conducted on the mechnical and functional properties of unconventional knittedstructures. The raw materials selected for investigation were basalt single jersey and 1×1 rib, polypropylene single jersey and1×1 rib, polyester single jersey and 1×1 rib and jute single jersey and 1×1 rib. The mechnical properties like bursting strengthand shear strength were studied. The thermal properties of the fabrics i.e., resistance & conductivity were studied vis-a-visphysiological behavior. Electrical and accoustic properties were studied as well. On the basis of the results, the influence ofthe fabric structure on various property parameters were analyzed. The results indicate that effect of fiber and knitted fabricstructure on mechanical and functional properties are significant. It can be concluded that rib fabrics have overall betterproperties as compared to single jersey knitted fabrics. The aim of this study is to analysis the properties of unconventionalknitted fabric for using them in technical applications i.e. composites.

Keywords: Knitted fabric, Basalt, Bursting strength, Shear strength, Thermal resistance, Electrical resistance, Acousticproperties

Introduction

Life in today’s world cannot be imagined without the

knitted textile. Knitted products are very popular and

integral part of textile material. Knitting is the process of

forming fabric by inter-looping of yarn by using needles. It

is the one of the major methods of fabric production. There

are two types of knitting i.e. warp and weft knitting. Weft

knitted fabrics are vastly used for daily uses. For the

production perspectives, knitted fabrics are easier to produce

than woven fabrics. As it is known that yarn used for knit

fabric needs no preparations like warping and sizing, yarn

can directly feed on machine after receiving from spinning

mills. Knitted fabrics are made from different types of yarn,

of different yarn fineness, composition of the raw material

and different structures. Due to the exceptionally good

property of knits like excellent elasticity, light weight and

due to highly developed and economical production of

knitted fabrics a major segment of market is captured by

knitted products. The high flexibility and comfort of knitted

garments have significantly increased their application in

many other areas of human needs i.e. sports, medicine, geo

applications and safety. Knitted fabrics have received much

consideration in latest years in the textile composite field

due to their outstanding formability [1-3].

Loops are the main structural elements of knitting and

their shape and size influences appearance and properties of

knitted products. Technically a knitted loop consists of a

needle loop & a sinker loop. Stitch/loop length is the

fundamental unit which controls all the properties of weft

knitted fabrics. It is the length of yarn knitted into one stitch

in a weft knitted fabric. Stitch/loop length is theoretically a

single length of yarn which includes one needle loop & half

the length of yarn (half a sinker loop) between that needle

loop & the adjacent needle loops on either side of it. Stitch

length is the most important/decisive factor affecting the

course per unit length (CPU) and wales per unit width

(WPU) and also other variable such as tightness factor,

gauge factor, linear density, stitch density, thickness etc.

Both the physical and mechanical properties of knitted

fabrics are influenced by the structural parameters of the

fabrics and relaxation/finishing process. Mainly stitch length

& knit structure are the major factors that affect all the

dimensional, comfort, handle & other properties [4,5].

Single jersey/plain knitted fabric is one of the popular

knitted structures. It is the simplest and most economical

weft knitted structure to produce. 1×1 Rib knits have a well-

balanced structure. It has technical face of plain fabric on

both sides until stretched. Torsional moment in the yarn is

neutralized by the loops which are formed on both sides of

the knitted fabric alternatively. There is no curling at the

edges. Rib has the tendency to contract laterally improving

the elasticity. These characteristics make rib fabric feel

thicker. Elastic recovery of rib 1×1 structure is very high

along the entire width. When knitted into a relaxed state it

shrinks so much that only the shorter loops can be seen on

both sides [6].

Physical and mechanical properties of knitted fabrics are

very important in many ways. All the new applications of

knitted fabric in variouse industrial fields call for better*Corresponding author: [email protected]

DOI 10.1007/s12221-015-5466-8

2478 Fibers and Polymers 2015, Vol.16, No.11 Hafsa Jamshaid et al.

understanding of mechnical properties. Among these properties,

bursting strength and shear strength are extremely important.

Thermal, electrical and accoustic properties are also important

for industrial applications.

Basalt fiber is one type of high performance inorganic

fibers which are made from natural basalt. These fibers are

100 % natural and inert. Basalt products have no toxic

reaction with air or water, are non-combustible and explosion

proof. Basalt fiber is cheaper than carbon fibers, and exhibits

a higher strength thanglass ones [7].

Polypropylene is a thermoplastic and low-cost polymer

with versatile applications. It is light weight, its density is

lowest among all synthetic fibers. PP yarn has very good

insulation properties. It is recyclable and ecologically friendly

[8]. Polyester (PET) has a highly important place among

synthetic fibers. The importance of PET stems from both the

indispensible features it adds to products (high tenacity,

resistance to sunlight, etc.) and its widespread, easy and

cheap production technology [9].

Jute is known as the golden fiber. It is a long, soft, shiny

vegetable fiber that can be spun into coarse, strong threads.

It is one of the most affordable natural fibers and is second

only to cotton in amount produced. Jute fiber is 100 % bio-

degradable and recyclable and thus environmentally friendly.

It has good tensile strength, low extensibility, and ensures

better breathability of fabrics, good insulating and antistatic

properties, as well as having low thermal conductivity and a

moderate moisture regain [10].

There are not many studies on unconventional knitted

fabrics from basalt, jute etc. Knitting of high performance

yarn is very difficult due to its brittle nature. In this study an

attempt has been made to analyze the mechanical and

functional properties of single and double jersey structures

made out of basalt, polypropylene, polyester and jute yarns.

Methodology

Materials

The polyester (PET) and jute yarns used in this study were

available commercially. Polypropylene yarn was taken from

company Synthetic (Pakistan). The basalt yarn was received

from company Kamenny Vek (KV) (Russia).

All the samples were knitted on Stoll flat knitting

machine. All the fabric samples were prepared from yarn of

similar linear densities, courses per cm and loop length. The

thickness of the fabrics varied with the loop length and

course count. Since the wale count per cm usually depends

on the machine settings, so a constant range 4.33 was

maintained.

Methods

After knitting, the fabric samples were taken from the

machine and were laid on smooth and flat surface for free

relaxation under standard atmospheric conditions at 20±2 oC

and relative humidity of 65±2 % for about 24 hours. The

conditioned samples were tested for physical, mechanical

and functional properties.

Fabric physical properties were tested since they would

influence functional properties. Areal density/GSM of samples

were tested by taking test samples with the help of GSM

cutter & weighing balance (electronic).

Thickness

The fabric thickness was obtained using thickness meter

(Mesdan Lab). Measurement is done at different positions.

the probe with a disc delivers a pressure of 1 kP over an area

of 20 cm2, then the thickness is obtained in mm. Ten

readings were obtained and an average was statistically

computed.

Stitch Length

The stitch length was measured by unraveling number of

courses over 10 needles and measuring the total length of

yarn and then dividing the total length by 10 to get the stitch

length. Ten measurements were repeated to get the average

stitch length.

Bursting Strength

The bursting strength is an important property especially

in knitted fabrics. The fabric bursting strength was tested

according to BS3424 using a ball-burst strength tester [11].

The instrument consists of a polished steel ball that has a

diameter of 25.2±0.005 mm. The conditioned fabric specimen

is placed tension-free in the ring clamp of the device. The

polished steel ball is then pushed through the specimen until

it ruptures. The bursting strength is determined as the force

applied to the ball at the instant of fabric rupture. Bursting

strength measurement was performed on Testometric

M350-10CT. Test was carried out at 10 different places per

sample. The schematic of bursting strength tester is shown

in Figure 1.

Shear Behavior

The macro-level fabric deformation is associated with in-

plane shear. Shear properties of all fabrics were measured on

tensile testing machine using picture frame method. It is an

effective way for characterizing intra-ply shear property of

fabrics. A tensile force is applied at the crosshead mounting.

The rig is joined at each corner such that its sides can rotate

and the interior angle between adjacent sides can change.

The initially square frame thus becomes of rhomboid (or

diamond) shape. Material inside the rig is subjected to pure

shear deformation kinematics. The force required to deform

the material is recorded at the crosshead mounting as a

function of crosshead displacement.

The frame is extended along diagonally opposing corners

using simple tensile testing equipment as shown in Figure 2.

Shear test was performed on TIRA (LaborTech s.r.o., Opava,

End Use Performance of Knitted Fabrics Fibers and Polymers 2015, Vol.16, No.11 2479

Czech Republic) with a crosshead speed of 10 mm/min. For

each topology 10 measurements were taken. The specimens

were then subjected to a 20 mm vertical displacement, which

corresponded to a shear angle of 36.2 o. This was the largest

shear angle that could be accommodated in all of the

experiments without the onset of buckling. During an in-

plane test, because of interactions between bending and

deformation within the fabric plane, such as tension and in-

plane shear, an out of plane bending deformation, buckling

will occur. As the shear stress increases, a critical stage is

reached where the fabric starts to deform out of its plane,

buckling as a result.

Figure 1. Schematic diagram of the experimental set up to measure bursting strength.

Figure 2. Picture frame test rig.

Figure 3. Fabric orientations (a) force applied at 45 o to wale and course directions, (b) force applied parallel to wale and course

directions, and (c) actual sample.


Force displacement data was gathered. Along with this,

empty frame data was collected so that it could be subtracted

from the raw data to give only the response of the material.

The orientation of the fabric when mounted in the picture

frame rig was a further consideration. The fabric could either

be cut so that the force was applied at 45 o to the wale and

course directions or parallel to them. However for fabric

sample when force is applied parralel to the wale and course

directions, buckling occurred at a much later stage. The

picture frame and the direction of applied force are shown in

Figures 2, 3 respectively.

In the picture frame shear test, the specimen is bolted to a

stiff four bar linkage system. A tensile load is applied

(vertically), which also causes a horizontal compressive

force because of the linkage system. As a result, a state of

shear (at 45 o to loading directions) is created in the test

specimen [13,14].

Air Permeability

Air permeability is defined as volume of air flow rate per

unit area of fabric when there is a specified differential

pressure across two faces of the fabric. The air permeability

of the samples were analyzed by using the FX 3300 air

permeability tester 111 according to standard EN ISO9237.

The principle of FX 3300 air permeability instrument

depends on the measurement of air flow passing through the

fabric at a certain pressure gradient ∆p. In this instrument

any part of the fabric can be placed between the sensing

circular clamps (discs) without the garment destruction. The

measurement was performed at a constant drop of 200 Pa

(20 cm2 test area) in the standard atmosphere [15]. The air

permeability tester is shown in Figure 4.

Thermal Properties

Measurement of the thermal insulation properties of the

fabrics was done by means of TCI developed by C-Therm

which is a device for conveniently measuring the thermal

conductivity of small sample by using the MTPS (modified

transient plane source) method. The principle of the apparatus

(TCI) is based on conductors in series with respect to the

direction of heat flow. The ratio of the temperature drop

across the conductors is equal to the ratio of their thermal

resistance. Thus, if the temperature drop across a material of

known thermal resistance (standard resistance) and across a

test specimen in series is measured, the thermal resistance of

the test specimen can be evaluated. Contrary to other devices,

TCI can measure the thermal conductivity of materials in the

states of solid, liquid, powder and mixed state. In addition, it

can measure thermal conductivity using only one side. The

TCI consists of a sensor, power control device, and computer

software. A spiral-type heating source is located at the center

of the sensor, and heat is generated at the center. The heat

that has been generated enters the material through the

sensor during which a voltage decrease occurs rapidly at the

heating source, and the thermal conductivity is calculated

through the voltage decrease data. The thermal properties of

the sample material are inversely proportional to the rate of

increase in the sensor voltage. The thermal conductivity is

calculated through the voltage drop data [16,17]. The

thermal measurement set up is shown in Figure 5.

Figure 4. Schematic diagram of the experimental set up to measure air permeability.

Figure 5. Schematic diagram of the experimental set up to measure thermal properties.


Electrical Properties

The electrical volume and surface resistivity of the samples

were measured according to the standard ASTM D257-07

(2007) at 100 V, temperature 22.3 oC and relative humidity

40.7 %. Measurements were recorded after 60 s from the

moment of placing the electrodes on the textile sample.

Volume resistivity was measured by applying a voltage

potential across opposite sides of the sample and measuring

the resultant current through the sample

Electrical resistance measurement was done on a measuring

device (ohmmeter) 4339B High Resistance Meter [18,19].

Volume resistivity ρv (Ω·cm) was calculated from the following

relation:

(1)

where, RV (Ω) is volume resistance reading, t is thickness of

the fabric (cm), and S is the surface area of the electrodes

(cm2).

Surface resistivity is measured by applying a voltage

potential between two electrodes of specified configuration

that are in contact with the same side of a material under test.

Surface resistivity ρs (Ω) was calculated from relation:

(2)

where RS (Ω) is the surface resistance reading, R1 is the

outer radius of the center electrode (m), and R2 is the inner

radius of the outer ring electrode (m). The experimental set

up is shown in Figure 6.

Acoustic Properties

As knitted fabrics are highly porous in nature, the

performance of these fabrics were also analyzed for acoustic

absorption. The acoustic impedance of a material is its most

basic acoustic property.

Impedance, defined as the ratio of the pressure to the

volume displacement at a given surface in a sound-transmitting

medium, is usually a frequency-dependent number. Acoustic

property was measured by using two-microphone impedance

tube according to ASTM E1050-08. The following measurement

methods are divided according to the size of evaluated

samples. The device is used to determine the sound absorption

coefficient (SAC) of circular samples with a diameter of

100 mm for the frequency range from 50-1600 Hz and

29 mm for the determination of sound absorption coefficientρv

RvS

t--------=

ρs

Rs2π

ln R2/R1( )----------------------=

Figure 6. (a) Resistivity measuring unit and (b) schematic diagram of the experimental set up to measure electrical resistance.

Figure 7. (a) Acoustic measuring unit and (b) schematic diagram of impedance tube.

Table 1. Properties of fibers and yarns

Properties Basalt Polypropylene Polyester Jute

Diameter of fibres

(micron)

12 34 22 18

No. of filaments 890 300 900 -

Linear density of yarn

(tex)

295 292 250 296

TPM (twists/m) 120 36 24 180

Tensile Force (N) 161.59 109.21 81.45 48.93

Tensile elongation (%) 4.75 28.65 30.28 2.848

Tenacity (N/tex) 0.55 0.37 0.33 0.17

E (MPa) 3390 767.24 611.37 1415.7


(SAC) of circular samples for the frequency range of 500-

6400 Hz. The lower working frequency is due to the accuracy

of the signal processing equipment. The upper working

frequency has been chosen to avoid the occurrence of non-

plane wave mode propagation. This method is based on the

evaluation of the sound absorptive properties of materials at

normal incidence sound waves [20-22]. A schematic is

shown in Figure 7.

The construction parameters of knitted fabrics developed,

physical properties, knitting notations as well as the real

fabric sample images are given in Tables 1 to 4.

Results and Discussion

Physical Properties

Areal density/GSM depends on knit structure, yarn fineness

& dimensional properties of knitted fabrics. Rib fabric has

more GSM than plain fabric. Knit structure of single jersey

consumes less yarn in loop formation compared to rib fabric

due to structure of fabric. Fabric thickness depends on knit

structure, loop shape, compactness of structure & relative

closeness of the loops (stitch density). Thickness can significantly

vary for high yarn diameter, low yarn twist, lower lateral

compression force etc. Rib fabrics have higher thickness

than their plain counter-parts produced from same yarn and

knitting machine parameters as shown in Figure 8. It is

because the double jersey knitted fabrics are composed of

two layers which are formed by two needle beds in the knitting

process. Since double jersey fabrics are manufactured with two

needle beds, two layers are formed. Therefore, the fabric

generally has a higher areal mass and thickness. A rib fabric

has one wale line of plain knit stitch to the front followed by

another wale line of plain knit stitch to the back. Each strip

of these wale lines would curl either in the convex mode or

in the concave mode depending on the direction of stitching.

The extent of curling of each strip of wale lines in rib would

be higher than plain knit and hence it has higher thickness.

Table 2. (a) Construction parameters of fabrics

Sample code Structure Fiber Courses (cm) Wales (cm) Stitch length (mm) Stitch density (cm-2)

S1 Plain B 3.54 4.33 13.34 15.33

S2 Plain PP 3.93 4.33 11.23 17.02

S3 Plain PET 4.72 4.33 10.89 20.44

S4 Plain J 3.93 4.33 10.96 17.02

S5 Rib B 5.512 4.33 13.96 23.86

S6 Rib PP 5.91 4.33 12.96 25.59

S7 Rib PET 6.29 4.33 12.96 27.24

S8 Rib J 5.91 4.33 12.96 25.59

Table 2. (b) Measured physical properties of fabrics

Sample code Thickness (mm)Areal density (g/m2)

Measured

Areal density (g/m2)

Calculated

Fabric density

(g/cm3)Tightness factor

S1 1.25 550 589.12 0.4393 13.20

S2 1.87 560 548.39 0.2998 15.53

S3 2.43 507 557.63 0.2086 14.51

S4 2.20 500 551.76 0.2273 15.70

S5 4.47 1000 984.09 0.2237 12.30

S6 4.08 950 948.97 0.2328 13.46

S7 4.33 850 866.40 0.1963 12.45

S8 4.74 915 958.46 0.1932 13.55

Table 3. Knitting notations and used structures

Structure Notataion diagram Fabric

Plain/

Single jersey

Rib/

Double jersey


Mechanical Properties

The measured mechanical properties of samples e.g.

bursting and shear strength are given in Table 5.

Bursting Strength

Bursting strength is a measurement that shows how much

force an object can take before it ruptures. It is one of the

most important mechanical properties of knitted fabrics.

Knitted fabrics are exposed to multi-axial forces not only

during their dry and wet processing in the industry but also

during their end-use. Due to their distinct structural features,

tensile and tear strength testing as applicable to woven

fabrics, is not suitable for the knitted fabrics. Therefore,

bursting strength test of knitted fabrics is conducted to assess

the fabric’s ability to withstand multiaxial stresses without

breaking off.

Fabric strength largely depends upon yarn strength. Effect

of different factors on the bursting strength of knitted fabrics

have been studied in the past and it is known that the

bursting strength increases by increasing the constituent yarn

tenacity, decreasing knitting stitch length and increasing

knitted fabric areal density (GSM). It has also been described

Figure 8. (a) Areal density and (b) thickness of fabrics.

Table 4. Images of the samples used

Basalt Polypropylene Polyester Jute

Plain knitted

1×1 Rib knitted

Table 5. Measured bursting strength and shear strength of samples

Sample

codeStructure Fiber

Bursting

strength (N)

Shear

strength

(N)

Shear rigidity

G (MPa)

S1 Plain B 1251 18.82 0.0028

S2 Plain PP 3058.3 36.75 0.0054

S3 Plain PET 1890 24.55 0.0036

S4 Plain J 547.5 -

S5 Rib B 1366 16.46 0.0024

S6 Rib PP 4058.5 20.39 0.0030

S7 Rib PET 2229.75 17.52 0.0026

S8 Rib J 720 26.51 0.0039


that the effect of different variables on the bursting strength

is not linear [23-25].

Single jersey knitted fabrics have lower bursting strength

with all yarn types as compared to corresponding 1×1 rib

fabric. It is mainly because of the structure and areal density.

Rib fabric has higher areal density, so bursting strength

increases due to increase in number of loops per unit length

which ultimately bear the multidirectional forces.

It is clear from the Figure 9 that bursting strength of PP

fabric in both single jersey and double jersey is highest

followed by Polyester fabric. Basalt fabric shows lower

bursting strength although basalt yarns are the strongest in

terms of tensile strength and modulus. In fact, basalt is a

high performance, brittle fiber and knitting is not easy due to

relatively higher stiffness and lower extensibility of these

yarns. It often results in fiber/yarn breakage during knitting

especially when yarn is forced to bend into loops. The

degree of fiber damage during knitting in case of basalt is

more as compared to knitting of other yarns [26]. PP fabric

has high strength and highly extensible yarn which leads to

highest bursting strength followed by polyester. It is due to

good integration of fibers and uniform fiber arrangement

with better twist which leads to effective exploitation of

fiber strength. When load is applied to the fabric, the yarn

moves until jamming occurs. After the yarn movement is

completed, the yarn elongates until it breaks which is according

to studies of many researchers [27]. Tightness factor and

areal density of PP fabric is also more than PET fabrics. Jute

is a staple yarn and has lowest strength and elongation at

break so it exibits lowest bursting strength among all

samples investigated.

Shear Property

From all the mechanical properties, shear behavior of

knitted fabrics are least understood, as they are difficult to

investigate. The shear mechanisms influence the draping and

pliability of a fabric and determine the ability of the material

to conform to a 3-D surface such as contour of the covered

surfaces [28]. The ability of a fabric to conform to a double-

curvature surface depends mainly on the in-plane shear

behavior. Shear rigidity is the resistance of fabric to angular

deformation. Although the application of textiles expanded

in industrial use and composites areas involving formation

and molding operations but still understanding of shear

behavior is very limited. The shear characteristics of fabrics

can be analyzed in a number of ways ranging from mathematical

models, which predict the material behavior based on fiber

geometry and friction coefficients, to mechanical testing,

which directly measures the materials bulk resistance to

shear deformation [28-30].

Picture frame shear tests are state of the art for determining

the shear force vs. shear angle behaviour for in-plane

deformation of most technical textiles. The shear rigidity

can be calculated from the tensile properties of the fabric in a

bias (45 o) direction based on the inter-relations of the in-

plane properties of the frame.

Rib fabric has lower shear strength compared to single

jersey fabric due to decrease of tightness factor and fabric

density (number of loops per unit area) i.e. increase of stich

length, leading to an decrease in inter fiber and yarn pressure

[12,30].

Jute based knitted fabrics show highest shear strength/

shear resistance followed by PP fabrics. As jute is a staple

fiber yarn and it has high level of hairniness, so it benefits

Figure 9. (a) Bursting strength and (b) shear strength.

Figure 10. Deformation kinematics of picture frame.


from higher inter-fiber and inter-yarn friction. It has also

highest tightness factor, an increase in the tightness factor

(tightness of structure) results in an increase in inner friction

and compression. Thus shear parameters are improved. PP

yarn has also high level of twist thus leading to more inter

fiber friction and also high tightness factor following jute

fabrics.

Theoretical Modeling of Shear Behavior

A geometric model is explained in Figure 10.

Direct measurement of axial load and shear angle is

possible through this following relationship.

(3)

Shear force (Fs) is determined by the axial force (Fx)

frame rig length (L) and the frame angle (ϕ). Meanwhile

frame angle can be determined directly from cross head

displacement (d). Shear angle (γ) can be obtained from

frame angle by using the following equations (2) and (3).

(4)

(5)

It was a challenge to maintain uniform rate of loading on

the tensile testing device. Therefore, uniform displacement

method was selected. In order to ensure that the frictional

effects of the clamping device and picture frame do not

infulence the uniform displacement, the linear fit curve was

plotted between time and displacement as shown in Figure

11.

The correlation of theoretical model and picture frame test

isshown in Figure 12.

The correlation is very good in the 95 % confidence

interval.

Functional Properties

The fabric functional properties e.g. air permeability,

thermal conductivity, thermal resistance and electrical

resistance are given in Table 6.

Air Permeability

Permeability is an important material property, its knowledge

is required in various flow simulations while processing

composites. Air permeability is one of the important factors

on which thermal properties depend. Generally, air per-

meability depends on the material of constitutive yarns and

structural parameters(type of knit structure, type of yarn

(spun or filament), yarn size (linear density), twist factor in

the yarn, loop density (wales and courses) and thickness) of

Fs

Fs

2cosϕ---------------=

ϕ cos1– L 2 d+

2L------------------=

γπ

2--- 2ϕ–=

Figure 11. Linear fit model between time and shear displacement

(10 mm/min).

Figure 12. Correlation of theoretical model and picture frame test

of shear strength.

Table 6. Measured air permeability, thermal conductivity, thermal resistance and electrical resistance of samples

Sample

code

Air permeability

(l/m2/s)

Thermal conductivity

(W/mK)

Thermal resistance

(km2/W)

Electrical

resistance (ohm)

Surface rersistivity

ρs (Ω)

Volume resistivity

ρv (Ω·cm)

S1 8635 0.064 0.02 2.89E+11 2.89E+12 4.42E+12

S2 3034 0.065 0.029 1.99E+11 1.99E+12 1.73E+12

S3 1150 0.053 0.046 1.47E+10 1.47E+11 1.94E+11

S4 2932 0.053 0.042 2.38E+11 2.38E+12 1.92E+12

S5 6140 0.049 0.092 5.21E+11 5.20E+12 5.76E+12

S6 2597 0.059 0.069 3.87E+11 3.87E+12 2.62E+12

S7 567.7 0.055 0.079 2.75E+10 2.75E+11 1.75E+11

S8 2922 0.051 0.092 3.86E+11 3.86E+12 5.01E+12


the fabric. Air permeability which is mainly a fabric

transport property is more sensistive to fabric structure.

Fabric structure is an important factor affecting the comfort

properties. Fabrics with more pores or bigger sizes of pore,

potentially allow more air movement through the fabric

which results in a cooler feeling for the wearer. The results

of air permeability are shown in Figure 13.

It can be seen from Figure 13 that Rib fabrics have lower

air permeability than single jersey fabric in all fiber com-

positions. As thickness increases air permeability decreases

[31].

It is clear that highest air permeability is in basalt fabrics

followed by jute and polypropylene fabrics. Basalt is a

compact yarn because of high twist level. Basalt fabric has

high stich length and low stich density. Loose knitted fbrics

are more permeable to air. On the contrary, jute is staple yarn

having more irregularities in its structures so more inter-yarn

porosity and inter-fiber porosity [32,33]. Polyester is a bulky

yarn, resulting in a better cover of the fabric and is therefore

less permeable to air.

Thermal Properties

Thermal properties are shown in Figure 14.

Thermal conductivity depends on material intrinsic behavior.

However, thermal resistance depends mainly on geometry or

thickness of fabric. As thickness increases thermal resistance

increases.

Rib fabric has lower thermal conductivity than single

jersey fabric due to tighness factor. In all samples the thermal

conductivity of structures having PP yarn is higher due to

higher thermal conductivity of PP fiber.

Model for Thermal Conductivity

Further the thermal conductivity was calculated based on

two phase model of prorous systems. The limit model is

shown in Figure 15.

The thermal conductivity of parallel arrangement λhP

(higher limit) is equal to,

(6)

For serial arrangements, thermal conductivity λhS (lower

limit) is defined as,

(7)

Actual composition of a fibers and air phases can be

presented by linear combination of parallel and series

structures. The compromise is to compute the mean thermal

conductivity of fibrous structure λh as arithmetic mean

between upper and lower limit.

λhP Pλa 1 P–( )λf+=

λhs

λaλf

Pλf 1 P–( )λa+-----------------------------------=

Figure 13. Air permeability of fabric samples.

Figure 14. (a) Thermal conductivity and (b) thermal resistance.

Figure 15. Limit arrangement of polymeric phase (gray) and air

phase (white) in conductivity model.


(8)

The parallel/series structure gives a first hand prediction

and gives reasonable prediction accuracy for practical

application due to its simplicity. A correlation between

theoretical calculation and actual measurement of thermal

conductivity is shown in Figure 16.

Thermal Resistance

Fabric thickness is one of the most important factors

determining thermal comfort. It is good insulator because

the air that is located between loops retain the heat, but most

knitwears allow air permeability at the same time, which

gives a pleasant feeling when wearing knitted garments [34].

It is observed that thermal resistance value of rib knitted

fabrics is higher than plain knitted fabrics. This is because of

higher thickness and low air permeability of rib knitted

fabrics. Basalt and jute rib knitted fabrics have highest

thermal resistance among all samples. Compactness of yarn

structure leads to higher packing density of yarn and lower

resistance. However, high areal density leads to more

thermal resistance of basalt and jute knitted fabric.

Among single jersey samples, the polyester fabric shows

highest thermal resistance followed by jute. As polyester has

low air permeability, least stitch length and high stitch

density so more retention of air would lead to increased

thermal resistance. Jute has good thermal resistance due to

two reasons. One, jute has smaller diameter of fibers so by

decreasing the diameter, the no. of fibers increase for same

fineness and thus increases the air pockets within the yarn.

Its specific heat value is high. Second, due to staple yarn it

has more hairiness in fabric thus increasing content of air

pores between loops of yarns. Physical clogging of air will

lead to increase in thermal insulation

Electrical Resistance

Fabrics are typically porous media and can be treated as

mixture of fiber and air, The main feature of knitted products

is that they are more or less porous materials. The high

resistance values of the fabrics are mainly determined by the

presence of air in them. i.e. free spaces between the loop,

which specifically affects the density of knitwear. In addition,

the characteristics and composition of the yarn of knits also

has a significant impact on its electrical behavior. Air is a

bad conductor of electric current showing resistivity from

1.3E+16 to 3.3E+16 at 20 oC.

Volume resistivity and surface resistivity of materials

depend mainly on fabric thickness and electrode dimensions.

Because samples do not contain any additional surface

modification, surface resistance (which indirectly describes

material’s ability to conduct an electric current on the

surface) should not be significantly different for the single

and double jersey fabrics. On the other hand, volume

resistance (which describes ability to conduct an electric

current in mass of sample) is a better descriptor of material

bulk electrical behavior. Dependence between volume and

surface resistance is shown in Figure 17. Volume resistance

is increasing with increasing surface resistance. Corresponding

coefficient of determination R2=0.752 indicates a good

quality of fit.

For the same voltage level, the rib knitted structures were

observed to allow the lowest electrical current to pass. This

was due to the higher areal density and thickness of rib

fabrics which can entrap more air and offer more resistance

to current.

Basalt and jute knitted fabrics show higher electrical

λh

λhP λhS+

2--------------------=

Figure 16. Correlation between theoretical model and experimental

measurement of thermal conductivity.

Figure 17. (a) Electrical resistance and (b) dependence between volume resistance and surface resistance.


resistance although the corresponding fibers have lower

electrical resistivity than polyester and polypropylene. Thus

it is evident that fabric porosity and tighness plays most

important role in deciding the electrical behavior of such

knitted fabrics.

Static electricity is the collection of electrically charged

particles on the surface of a material. Various materials

either give up electrons and become positive(+) in charge or

have the tendency of attracting electrons and becoming

negative(−) in charge. Some materials cause or create more

static electricity than others. Static electricity can be generated

on insulating materials (layers) surface by the certain

specific ways which are, charging by electrostatic induction,

charging by friction, charging by ionizing radiation etc. The

high resistivity of polymers results in accumulation of large

amount of electrostatic charges. Antistatic formulations of

polymers possess much reduced resistivity so that electrostatic

charge may be dissipated by leakage currents [35-38]. Basalt

is a compact yarn and jute is staple yarn and also highly twisted

as compared to PP and PET. Both of the later are filaments

with very light twist so they become more flattened, the fibers

in the yarn spreads out in the fabric resulting in smaller inter

fiber pores and higher conductivity for static charge.

Acoustic Properties

Now-a-days much importance is given worldwide to the

acoustical environment. Fibrous, porous and other kinds of

materials have been widely used as sound absorptive materials.

Acoustic absorption is the transformation of sound energy to

thermal energy due to friction between the moving air

particles and the material itself. Textile materials i.e. porous

materials which reduce the acoustic energy while the sound

wave passes through, are referred as efficient acoustic

absorbing materials. When the sound wave strikes on a

textile material, some amount of the sound will be reflected,

some amount will be transmitted through and some will be

absorbed by the material. The reflection, transmission and

absorption of sound are influenced by the type of fiber and

structural characteristics of the fabric. These acoustical

characteristics of textile or porous material are always

represented by Sound Absorption Coefficient (SAC). SAC

varies from zero (0) to one (1). Sound absorption performance is

a function of frequency and is performed generally with the

increase in frequency. Performance improves with the

increase in thickness.

The various factors affecting the sound absorption coefficient

of the fabrics are thickness, density, porosity and air flow

resistance [39]. Among these factors, the air flow resistance

is the major contributing parameter in various materials. The

air flow resistance can yield good absorption regardless of

the thickness of the fabrics. Fabric air permeability is very

important parameter for thermal and acoustical insulation of

textiles. Higher air permeability results in higher sound

transmission, therefore less sound insulation.

From Figure 18, it can be observed that with the increase

of frequency, the sound absorption coefficient (SAC) of all

the samples increases. As is known that with the increase of

frequencies, larger is the sound energy which accelerates the

air vibration inside fabrics and creates the opportunities of

friction between air and pore walls. This process causes

sound energies to be converted into heat energies and

gradually diminishes the sound effect. This is indicated as an

increase in SAC. It is evident from the results that rib fabrics

have high SAC than single jersey fabrics. At low frequency

levels, Polyester rib fabric has highest value followed by PP

rib fabric. Polyester plain knitted fabric has relatively higher

SAC value in both low and high frequencies compared to

basalt rib knitted fabric. As basalt fabrics have highest air

permeability i.e. less air flow resistance than all other samples,

their sound absorption capacities are not good.

At higher frequency also Polyester rib fabric has highest

value followed by jute rib fabric. The reason for better SAC

of polyester fabrics may be finer fibers which resulst in

higher number of fibers per unit weight of the material. This

leads to higher total fiber surface area, and greater possibilities

for a sound wave to interact with the fibers and ultimately

dissipate inside the structure. Moreover, Polyester knitted

fabrics have highest stitch density and highest air flow

Figure 18. Sound absorption coefficient (SAC) of fabrics with

(a) low frequency range 100-1600 Hz and (b) high frequency

range 500-6400 Hz.


resistance than all other fabrics. Basalt knitted fabric shows

lowest SAC values in low frequencies.

Theoretical Calculation of SAC (Sound Absorption

Coefficient)

The sound absorption coefficient was further calculated

based on the theoretical model proposed by previous

researchers.

The Sound Absorption Coefficient (SAC) is a measure of

how much sound is absorbed by a particular material, and is

derived from the measured Sound Absorption Coefficients

[39]. The SAC was determined using the following formula

(equation (9)).

(9)

A very good correlation of this model was found with

measured values of SAC. Results are shown in Figure 19.

Conclusion

The influence of knitted structure and fiber type on fabric

properties is investigated. Some of the important conclusions of

this research are:

1. Bursting strength of all rib knitted fabrics are higher than

single jersey fabrics with simillar yarn type and machine

parameters. PP knitted fabrics have highest values followed

by Polyester fabrics.

2. Shear strength of rib knitted fabrics are lower than single

jersey knitted fabrics. Jute knitted fabric has highest shear

strength and rigidity followed by PP fabrics.

3. Air permeability of rib knitted fabrics is lower than single

jersey knitted fabric. Basalt fabrics have highest values

followed by jute fabrics.

4. Thermal conductivity of rib knitted fabrics is lower than

single jersey knitted fabrics. PP fabrics have highest

values.

5. Thermal resistances of rib knitted fabrics are higher than

single jersey knitted fabrics. Basalt fabrics have highest

values followed by jute fabrics.

6. Electrical resistances of rib knitted fabrics are higher than

single jersey knitted fabricr. Basalt fabrics have highest

values followed by jute fabrics.

7. Sound absorption coefficients of rib knitted fabrics are

higher than single jersey knitted fabrics. Polyester fabrics

have highest values.

It can be concluded that rib fabrics have overall better

properties as compared to single jersey knitted fabrics.

Acknowledgement

The authors would like to thank Nazer & Co. Karachi,

Pakistan for produdcing samples on Stoll flat knitting machine.

Also this work was supported by project No. L1213 of

program NPU in Czech Republic.

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SACα250Hz α500Hz α1000Hz α2000Hz+ + +

4----------------------------------------------------------------------------=

Figure 19. Calculated SAC vs measured SAC for lower frequencies

(2000 Hz).


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