Indi an 10urnal of Fibre & Textile Research Vol. 26, December 200 I, pp. 398-402

Relationship between tensile properties of fibres and nonwoven fabrics

P C Pate l & V K Kotharia

Department of Tex ti le Technology, Indian Institute of Technology, New De lhi 11 00 16. Indi a

Received 21 June 2000; revised received and accepted 16 Ocrober 2000

The stress-stra in behaviour of spunbonded needle-punched fabri c, spun bonded heat-sealed fabric and staple fi bre nee­dle-punched fabric has been studied using wide-width tensile test method. The stress-strain behaviour of constituent fi bres of these fabrics has also been studied and the structural parameters of nonwoven fabrics evaluated as these are the main two fac tors which inOuence the mechanical properties of nonwoven fabrics. The fibre network theory has been used to predict the stress-strain behaviour of fab ric using the fibre data and structural parameters of nonwoven fabrics. It is observed that there is good agreement between theoretical and experimental values in case o f heat-sealed spunbonded nonwoven fab ric. In case of needle-punched fabrics, the stress-strain curve of the staple fibre fabric shows major deviati on from the ex perimental curve while the stress-strain curves fo r spunbonded needle-punched fabrics show substanti al deviation from the experimen­tal curves. The slippage of fibres is a dominating fac tor in the deformation of needle-punched nonwoven fabrics in general and staple fibre fa brics in particular and hence both the modulus and breaking stress are found to be much lower than the theoretical values. The structure of nonwoven fabri cs is the most important fac tor affec ting the tensi le behaviour of these fabrics.

Keywords: Heat-sealed fabric, Needle-punched fabric, Nonwoven fabri c, Stress-strain behavi our, Tensile properti es

1 Introduction Nonwoven fabrics are extensively used in different

applications due to their excellent properties. These fabrics for applications like geotextiles are manufac­tured from fibres like polypropylene and polyester using manufacturing processes like heat sealing (thermal bonding) and needle punching (mechanical bonding). The formation of the structure of these fab­rics depends on the fibre arrangement in the web, fi­bre properties and bonding method used in the pro­duction of fabric. When this structure is subjected to tensile load, the deformation of the fabric, viz. the extension of fabric in the direction of applied load and the contraction of fabric in the transverse direction, takes place. In case of nonwoven fabrics, fibre prop­erties and fabric structure mainly affect the fabric properties. The extension of the fabric is attributed to the rearrangement of fibre geometry and fibre exten­sion. If tensile properties of fibres and details of fabric structure are known, it should be possible to relate them to the tensile properties of the fabric.

Backer and Petterson 1 developed the fibre web theory to relate fibre modulus and web geometry to the fabric modulus of thin nonwoven structures .

"To whom all the correspondence shoul d be addressed. Phone: 659 1407; Fax: 09 1-0 11-6862037; E-mail : ko thari@textile. iitd.erneLin

Hearle and Stevenson2. 3 developed fibre network the­

ory for the prediction of tensile properties of non­woven fabrics. Hearle and Newton4 modified thi s the­ory to account for the fibre slippage through binder and theoretically determined the stress-strain curves of several bonded fibre nonwoven fa brics. They dem­onstrated the ex tent of applicability of theory in re la­tion to the amount of binder materi al and the fibre curl. Hearle and SultanS studied the influence of fibre type, fibre fin eness and staple length on the tensile behaviour of some needle-punched fabrics. In thi s work, the stress-strai n behavi our of nonwoven fabrics of different structures and polymer types has been studied to relate the tensile properties of fibres and fabric structure.

2 Materials and Methods Polyester and polypropylene fabrics of the specifi­

cations as g iven in Table I were used. The stress­strain relationship of the fibres in these fabrics has been studied on the Instron tensile tester after prepar­ing the specimens of filaments/fibres removed care­fully from the respecti ve nonwoven fabrics. A total of 30 fil aments/fibres of 100 mm gauge length were tes ted and average stress-strain curve was obtained in each case. Fabric tensile tests were carried out using wide-width test method with 200 mm fabric width and 100mm gauge length using special jaws at a strain

PATEL & KOTHARI: TENSILE PROPERTIES OF FIBRES & NONWOVEN FABRICS 399

Table I-Fabric details

Fabric Type of fibre Linear density

dtex

AF Polypropylene filament 23

BF Polypropylene filament II

CF Polyester filament 6.1

DF Polyester filament 9.6

EF Polypropylene staple 4.4

rate of 10% / min . Ten samples were tested to obtain the average stress-strain curve for each type of non­woven fabric. During the tensile test, the width of the specimen at the mid position between the jaws was noted at the fixed time intervals corresponding to 10% fabric strain in the test direction. Average of these values at various strain levels was taken to calculate the Poisson's ratio.

Fabric structure was studied on projection micro­scope using optical method similar to that used by Hearle and Stevenson2

. Magnification of xl00 was used and a tracing paper having 25 mm diameter cir­cle marked in the centre of the paper was fitted on the screen of the projection microscope. Samples of 2 cm x 5 cm size were prepared from the nonwoven fabrics such that larger dimension being cut in the machine direction of the fabric. Each sample was carefully placed on a clean microscope glass slide and a few drops of n-decane were added to improve the fibre definition. On focusing, the fibre projections were clearly visible on the tracing paper through the screen tabl e. A position on the slide was then chosen at ran­dom and fibres passing through the circle were traced. Different layers of the fabric were progress ively fo­cused to examine fibre layers in turn and every time the fibres projections were traced on the tracing paper (Fig. 1). When a full traverse of the focu s through the thickness of the specimen appeared, a second position on the microscope slide was chosen at random and a new sheet of tracing paper was positioned. The same process was repeated again. A total of 50 such trac­ings were prepared in case of each nonwoven fabric. The angle of each fibre segment in the circle with rc­spect to longitudinal direction (L) was measured. The number of fibres in each 10° segment was counted and the orientation distribution function ¢(8 j ) , defined

Manufacturing/bonding Mass method g/m2

Spunbonded heat-sealed 200

Spunbonded needle-punched (surface 190 calendered)

Spun bonded needle-punched 170

Spun bonded needle-punched 220

Staple fibre needle-punched 200

L f{

t L

(b)

(0)

Fig. I--Measurement of structural parameters of non woven fab­rics (a) tracing of fibres on the projection microscope screen, and (b) angular orientation of fibres in relation to machine direction [fl-f9 - fibres and L -longitudinal direction]

as the ratio of number of fibres in a 10° angular inter­val to the total number of fibres for various mid­values of angular orientation, was obtained. The fibre curl factor (C), defined as the ratio of length of the fibre segment to the shortest distance between the segment ends, was also measured from the same tracing for each type of fabric.

3 Results and Discussion 3.1 Stress-strain Characteristics of Fibres

Average stress-strain curves of the fibres taken from the nonwoven fabrics are shown in Fig. 2. Table 2 shows that the polypropylene filaments (A r and Br) removed from spun bonded fabrics have higher extension-at-break and lower initial modulus. Polyestcr filaments (Cr and Dr) removed from needle­punched spunbondcd fabrics show relatively higher initial modulus and lower extension-at-break. Poly­propylene staple fibre (Er) shows the highest tenacity and low elongation as the fibres are fully drawn be­fore the formation of web from the staple fibres . In

400 INDIAN 1. FIBRE TEXT. RES ., DECEMBER 2001

~u~-------------------------------,

~

"'s l-Af

40 2-8f / x J- Cf ~ I ___ 2 4-0f

30 I z i /) s- Ef u /~;:;:''''4

- i ., /"," '" i / ./P·· '" 20 ~ . / .11 ~ . ;?

"V ~/

00 20 40 60 80 100 120 140

Strain, 0/0

Fig. 2-Stress-strain behaviour of fibres

Fibre

Ar

Br

Cr

Dr

Er

Table 2-Tensile properties of fibres

Fibre linear Tensile density strength

dtex cN

23 23 .7

11 31.6

6.1 17.2

9.6 26.8

4.4 16.9

Tenacity cN/tex

10.7

28.4

28.1

27 .6

37.8

Extension­at-break

%

127

76

31

64

30

Table 3--Tensile properties of fabrics (machine direction)

Fabric Mass Tensile Tenacity Extension-g/m2 strength cN/ lex at-break

kN/m %

AF 200 9.7 4.7 80

BF 190 9.4 4.9 132

CF 170 8.7 5.0 61

OF 220 15.0 6.4 53

EF 200 14.5 7. 1 75

particular, the filaments removed from spunbonded heat-sealed fabric show very high extension.

3.2 Stress-strain Behaviour of Nonwoven Fabrics Table 3 gives the main tensile test results for differ­

ent fabrics based on wide-width tensile test in the ma­chine direction (along the length) of the fabrics . A ver­age stress-strain curves of various nonwoven fabrics have been compared and the results are shown in Fig. 3.

x 6 .. -z

",- 4

'" ~ ~

Vl

20 40 60 SO 100 120 140

Strain, %

Fig. 3--Slress-strain behaviour of fabrics

Spunbonded heat-sealed fabric (AF) shows much higher initial modulus but offers very little resistance after 20% extension, eventually leading to the failure of the specimen. However, the need le-punched spun­bonded polypropylene fabric (BF) shows lower initial modulus and higher extension-at-break.

The above results show distinct differences bet­ween the stress-strain behaviour of nonwovens of dif­ferent structures. The stress-strain curves show the striking similarity between the heat-sealed spun bon­ded fabric AF and the fibre Ar. However, needle­punched fabrics show lower initial modulus even when the initial modulus of fibres is high. This may be owing to the high degree of reorientation and slip­page of individual fibres during the tensile load appli­cation on these fabrics. This effect is very prominent in case of staple fibre needle-punched fabric if one compares the shapes of stress-strain curves of fibre Er (Fig. 2) and fabric EF(Fig. 3).

The high modulus of the heat-sealed fabrics can be attributed to the rigid bonds formed as a result of fu­sion of fibres at the crossing points in the fabric. There is a gradual decrease in the slope of the curve right upto rupture and the stress value drops sharply at the rupture. The cause of fabric failure is fibre break­age with few fibres slipping in the rupture zone of broken specimen.

In case of needle-punched spunbonded fabric (B F), the observations during the test show that the fibres are free to move after the initial breakage of weak bonds formed due to the surface calendering and the fabric behaves like a simple needle-punched structure for all practical purposes .

Polyester spun bonded need le-punched fabrics (CF and DF) give higher strength with lower extension-at­break compared to polypropylene spunbonded fabrics (AF and BF). The results also indicate that the fabric

PATEL & KOTHARI : TENSILE PROPERTIES OF FIBRES & NONWOVEN FABRICS 401

structure can alter the stress-strain behaviour signifi­cantly. Fibres Cr and Dr have nearly the same tenacity but the extension of Dr is nearly two times that of the Cr. The needle punching process produces fabric OF which has higher breaking stress and lower breaking elongation .

The needle-punched nonwoven made from staple fibres (EF) shows the lowest initial modulus and higher strength than other fabrics. As the load in­creases initially, the reorientation and slippage of fi­bres occur without offering much resistance to exten­sion. The structure develops a tension because of the presence of vertical pegs formed by the needling pro­cess, and jamming of the structure occurs after the initial phase, giving rise to a distinct increase in fabric modulus. The slippage of the fibres dominates during the test, giving higher extension-at-break.

3.3 Prediction of Fabric Properties The theoretical stress-strain relationship of fabric

was obtained using the following equations based on the fibre network theory3 :

er= (l/C) {(l+e)2 .cos28t +(l-v.e)2.sin28. } 112 - 1 . .. (1)

qr= J(er) (2) QL = qr cos 8. . ¢( 8.) cos 8. (3)

where e is the fabric strain; er, the fibre strain; C, the curl factor; 8. , the fibre angle to the stress direction; v, the fabric Poisson's ratio; qr , the fibre stress; ¢( 8.), the fibre orientation distribution function; and QL, the fabric stress.

15,---------------------------------,

" .. z u

-'"

12

~ 6

Vl

20

Sample AF

---------;Fibrt

~ ___ .... Fabric (Theoretical)

Fa bric (Experimental)

60 80 100 120 It.O Strain, %

Fig. 4--Comparison of theoretical and experimental stress-strain behaviour of sample AF

The relationship between stress and strain of fibres removed from different fabrics was obtained from the average stress-strain curve of fibre . The theoretical and experimental stress-strain curves of different fab-

35r-------------------------------, Samp le BF

Fibre 30

25

'" '" .. L

Vl

Strain , %

Fa br ic ( Experime ntal)

100 120 140

Fig. 5--Comparison of theoretical and experimental stress-strain behaviour of sample BF

30.-----------------------------------, Sample (F

24

)(

~ 18 -z u

'" '" ! 12 Vl

6

20

Fi bre

30 40

Strain, '".

Fabric (Theoretical J

50 60 70

Fig. 6---Comparison of theoretical and experimental stress-strain behaviour of sample CF

402 INDIAN J. FIBRE TEXT. RES. , DECEMBER 2001

'" '" .. Vl

30,-------------------------~~---­Sample OF

24

12

Strain, %

f ibre

Fabric I Theoretical J

Fabric (Experimental J

60 70

Fig. 7--Comparison of theoretical and ex perimental stress-strain behaviour of sample DF

rics (AF, BF, CF, O F and EF) based on above equations have been plotted along with stress-strain curves of fibres for comparison (Figs. 4-8).

The theoretical stress-strain curve of spun bonded heat-sealed fabric (AF) shows good agreement with the experimental curve (Fig. 4). This is because the fabric is thin with rigid bonds and hence the network of web remains intact during the tensile test of the specimen. The orientation of the fibres in the network, measured in two dimensions, holds good during the test for this type of structure. Theoretical stress-strain curves of spunbonded needle-punched fabrics (Figs 5-7) and staple fibre needle-punched fabric (Fig. 8) show large deviations from the experimental curves.

The thick web of the fabric and loose frictional bonds in the structure of the needle-punched fabrics allow slippage of fibres on application of external load which changes the fabric structure substantially during the course of testing, resulting in lower initial modulus than predicted by the theory.

In case of needle-punched fabric made from carded web of staple fibres (EF)' there is even larger differ­ence between the theoretical and the experimental

50,---------------------------------

)(

'"

40

.::. 30 z u

-'" '" '" ~ 20

Sample EF

fabric I Theore ticoll

40 50 60 70 80 90 100

Stroin, %

Fig. &-Comparison of theoretical and experimental stress-strain behaviour of sample EF

stress-strain curves (Fig. 8) due to the higher slippage at fibre ends.

4 Conclusions Deformation in various nonwoven fabrics depends

on the type of structure formed by the bonding method. In case of heat-sealed nonwoven fabrics, ini­tial modulus is high but it decreases gradually with the increase in stress while in case of needle-punched fabrics, initial modulus is low but it increases with the further increase in load as the structure gets locked.

The prediction of stress-strain behaviour using the fibre network theory gives close approximation in case of heat-sealed fabrics. However, in case of nee­dle-punched fabrics, the predicted stress-strain char­acteristics differ substantially with the experimental results.

References 1 Backer S & Petterson DR, Text Res J, 30( 1960)704-71 1. 2 Hearle J W S & Stevenson P J, Text Res J, 33( 1963)877-888 . 3 Hearle J W S & Stevenson P J, Text Res J, 34(1964) 18 1- 19l. 4 Hear1e J W S & Newton A, Text Res J, 38( 1968)343-351 . 5 Hearle J W S & Sultan M A I, J Textlnsf, 59(1968)137-147.