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Mechanical Characterisation of Through-air Bonded Nonwoven

StructuresAmit Rawal*, Stepan Lomov**, Thanh Ngo**,

Ignaas Verpoest**and Jozef Vankerrebrouck***

*Current Address: The University of Bolton, Deane Road, Bolton BL3 5AB, UK**Department of Metallurgy and Material Engineering, Katholieke Universiteit,Leuven, Kasteelpark Arenberg, 44, B-3001 Leuven, Belgium

***Libeltex bvba, Marialoopsteenweg 51, 8760 Meulebeke, Belgium


Contents

• Introduction • Objectives• Experimental Work• Micromechanical Model for Initial Tensile

Behaviour of Through-air Bonded Structures• Results• Conclusions• Potential Outputs


Introduction


Definition of Nonwovens*

A fabric consisting of an assembly of textile fibres (oriented in one direction or in a random manner) held together (1) by mechanical interlocking; (2) by fusing of thermoplastic fibres, or (3) by bonding with a rubber, starch, glue, casein, latex, or a cellulose derivative or synthetic resin.

*www.nonwovens.com


Classification of Nonwovens


Importance of Mechanical Properties in Nonwoven Structures

Thermal bonded structures undergo various modes of deformation during their end-use performance. For example, geotextiles when placed under the soil exhibit high levels of tensile and compressive modes of deformation. Characterisation of tension, shear, compressional and bending resistance is needed for accurate prediction of fabric draping during the formation of a three-dimensional (3D) shaped composite part for many automotive applications.


Objectives


The main objective of the research work was to characterise the mechanical properties of thermal bonded nonwoven structures namely, tension, bending, shear and compression, including anisotropy in the properties, along with the fibre orientation distribution in the fabric. A simple micromechanical model has also been proposed to investigate the initial tensile behaviour of thermal bonded structures.


Experimental Work


Production of Through-air Bonded Structures

C

R1R2 D

Web

Properties TB1 TB2 Type of Fibres PET

(Bicomponent) PET (Normal)

PET (Bicomponent)

PET (Normal)

Proportion (%) 50 50 50 50 Linear Density (dtex)

2.2 3.3 4.4 12

Computed Diameter (µm)

14.20 17.40 20.15 35.03

Average Length (mm)

48.83 58.38 47.80 61.20


Fibres Stress-Strain Curve

0

1

2

3

4

5

6

7

0 2 4 6 8 10

Strain (%)

Str

ess

(cN

/tex)

2.2 dtex

3.3 dtex

4.4 dtex

12 dtex


Properties of Through-air Bonded Nonwovens

0.43 ± 0.003828.46 ± 0.031TB20.44 ± 0.001231.09 ± 0.082TB1

Thickness (mm)Mass per unit area (g/m2)

Fabric Sample IDs


Measurement of Mechanical Properties• Tension: Fabric strips of 20x15 cm were tested

on an Instron tensile testing machine under uniaxial loading in a “grab” test. Poisson’s ratio was also determined by measuring the contraction in the centre of the specimen relative to the strain in the test direction.

• Shear: Picture frame was mounted on Instrontensile testing machine.

• Bending: KES-F B2 (Bending Tester) was used and the fabric was bent between the curvature of –2.5 and 2.5 cm-1. Three cycles were repeated for each sample in the machine (0°) and cross-machine (90°) directions.

• Compression: KES-F B3 (Compression Tester) was used and the pressure was increased gradually up to 50 g/cm2.


Measurement of Fibre Orientation Angles

YZ

Machine Direction

z

y

XZ

x

Sectioning of fabric samples


Measurement of fibre Orientation Distribution (Cont.)

’T

’M

b

a

X’

Y’

Z’

px’

py’

pz’

Scheme for computing the fibre orientation angles

2D image analysis software provides the values of minor and major axes along with an in-plane fibre orientation angle M´

)/arccos( ab cT


Ambiguities, Errors and Corrections in 2D Image Analysis

• Ambiguity: There are two possible values for fibre in the in-plane orientation angle ( ) as the fibres with orientations and + 180 have identical cross-sections.

• Correction: Assuming the orientation distribution to be symmetrical, and the out-of-plane orientation angle is randomised by using the following equation.

M cM c

T´1 = (sign (RAND (0,1)-0.5)� T’�� °̄°®

�� !

11

0001

x

x

x

xsign

,,,

)(

M c


Ambiguities, Errors and Corrections in 2D Image Analysis (Cont.)

• Ambiguity: The probability of finding a fibre with defined orientation ( ) such that it has an elliptical cross-section on the sectioning plane.

• Correction: The “probability of intersection”between the fibre and the sectioning plane has been determined by dividing the fibre orientation distribution with cosine of the sectioning angle (Zak et al., 2001).

MT cc,

’

( , )

cos

XZi jXZ

ijcorrectedi

Q T MQ Tc c

Zak, G., Park, C. B. and Benhabib, B., Estimation of three-dimensional fibre-orientation distribution in short-fibre composites by a two-section method, Journal of Composite Materials, 35, 316-339 (2001).


Ambiguities, Errors and Corrections in 2D Image Analysis (Cont.)

• Error: Small errors in the measurement of elliptical axes for the fibres oriented nearly perpendicular ( ) to the sectioning plane can yield large errors in the measured value of (Mlekusch, 1999).

• Correction: Fitting a normal distribution to the measured values of in-plane fibre orientation angles .

T cT c

Mlekusch, B., Fibre-orientation in short-glass-fibre composites II: Quantitative measurements by image analysis, Composites Science and Technology, 59, 547-560 (1999).


Micro-mechanical Model for Initial Tensile

behaviour of Through-air Bonded Nonwoven

Structures


A simple micro-mechanical model has been developed to predict the anisotropy of the tensile properties of through-air bonded structures. The model is based on the averaging schemes of bond distribution and fibre orientation proposed by Pan et al.(1993), Komori and Makashima(1977, 1978) and Lee and Lee (1985).


Assumptions

• Each fibre segment is straight before loading, i.e. local fibre crimp is neglected.

• The constituent fibres of same type have identical properties and are elastic.

• Bending, torsion and compression deformations of the fibre segments and compliance of bond are neglected.

• Fibres do not slip past each other.


Calculation of Number of Contacts

Assuming the centre of fibreB ( ) is brought into contact with the surface of fibre A ( ), then the parallelepiped consisting of two rhombuses of length l and height 2D, where D is the diameter of the fibre.

MT ,MT cc,

FMTMT sin2),,,( 2Dlv cc

where V is the total volumeP is the probability that fibre A will contact with fibre B

is the angle between the two neighbouring axes of fibres A and B

VDl

PFsin2 2

Contact of Fibre A with Fibre B (Komori and Makashima, 1977)

)cos(sinsincoscoscos MMTTTTF c�c�c

1. Komori, T., and Makishima, K., Numbers of fibre-to-fibre Contacts in General fibre Assemblies, Textile Research Journal, 47, (1977).

F


Calculation of Real Number of Contacts

Total number of contact places in whole fibrous assembly (m)

Real contact between the fibresVIlDN

VIlNDN

m222 2)1(2 �

³³ : SS

MMTTMTT00

),(sin),( dJdI

³³ cccc:ccc SS

MTMTMTMTFTMT00

sin),(),,,(sin),( ddJ

VIDL

VIlDNm

n222

2

where N is the total number of fibres L is the total length in the volume V

n is the real number of contacts between the fibres

(as N>>1)

1 contact = 2 contact places


Formulation of Micromechanical Model

Consider a tension test in the direction D with respect to the machine direction. Here, volume ( ) of the sample, having a unit cross-sectional area (AD=1) that is limited by two planes normal to the test direction. According to Lee and Lee (1985),

Projections of a fibre length between two bonds

is the directional parameter indicating the length projections of the fibres on the test direction.

is the projection of the average length of the fibre between two bonds ( ) on the test direction

DK

Db

D�b

b

1�

�A

DdV

DDD KDLIV

Kbb2

b

T

MD

�

sinb

)sin(sinb ��

�

)cos(sinb �

�

/ 22

0 / 2

sin cos( ) ( , )K d d

� �

�

T T M D T M D M

�

� �

� �: �³ ³Assuming the fibres lying parallel to X-Y plane, therefore,

/ 2

/ 2

cos( ) ( )K d

� �

� �

� �

M D M D M

�

� �

� �: �³

b


Formulation of Micromechanical Model (Cont.)

Number of bonding points ( ) inside the volume dV:

Projection of the fibre on the test direction

where D is the fibre diameterVf is the Volume Fractionn is the real number of contacts formed in volume V

be a frequency of i-th bin of a histogram, representing this distribution as a function of the angle ( ) between the test and the fibre directions

dVn

dV

b An n

VD D

2DLn I

V

2

2 fdV

Vn K

D DS � �

1

2 cos ( )n

f i i dV ii

F nV E E F E

¦ E M D �

EfFD

fF

EfF

Bond

� �i�F EiE


Formulation of Micromechanical Model (Cont.)

where subscripts 1 and 2 refer to the components of the blend, O1 and O2are fractions of the components (O1+O2=1)

is fabric web stress per unit widthm is mass per unit area of the fabric

is fibre stressis Poisson’s ratio

DMEHEVEFED D � ¦

n

iiii

fiimKT

1

);,()(cos)(

� �> @1sin)1()1(cos)(2/12222 �� EQHHEHV ff f

f E

’fl

fl

Esinfl

Ecosfl

EH cosfl

O A

B

CB’A’

A

c

DMEHEVEFEOHEVEFEOD DD � � ¦ ¦

n

iii

n

ii

fiii

fii mKmKT

1 1222111 ;),()(cos),()(cos)(

Relationship between web and fibre strain

)(DT

fVQ


Results


Tensile Properties of Through-air Bonded Structures

• The slope of curves decreases with the angle of test, showing high degree of anisotropy of the tensile resistance.

• The ratio of tensile strength in the machine direction to the cross-machine direction of thermal bonded structures TB1 and TB2 was found to be 3.3 and 3.5, respectively.

• TB1 has lower tensile strength in comparison to TB2 in all the test directions, although the fibres (4.4 and 12 dtex) used in the production of TB2 have high tensile strength in comparison to the fibres (2.2 and 3.3 dtex) used in TB1.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80Strain (%)

Str

ess

(N/m

m) 0° 22.5°

45° 67.5°

90°

0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80Strain (%)

Str

ess

(N/m

m)

0°

22.5°

45°

67.5° 90°

TB1 TB2


Tensile Properties of Through-air Bonded Structures (Cont.)

• Ratio of 20-50 between the secant moduli in 0°and 90°directions.

• Slope of curves steeply decreases between 0 and 22.5°.

Secant moduli of thermal bonded structures TB1 and TB2 at 3.5 % strain

0

5

10

15

20

25

0 22.5 45 67.5 90

Test Angle (o)

Sec

ant m

odul

us (M

Pa) TB1

TB2


Shear Properties of Through-air Bonded Nonwoven Structures

• The first shear cycle describes as the sample installation on the frame rather than the actual material behaviour.

• Initial high shear stiffness (up to shear angles of 3°) represents bond resistance that is sufficient to prevent rotation of the fibres in the thermal bonded nonwoven structure. Later, bond resistance is overcome and the shear resistance is primarily dominated by the rotation of structural elements of the fabric.

• TB1 requires more shear force than nonwoven TB2 at higher shear angles although both fabrics have similar weight and thickness.

• The scatter of the local shear angle is high, i.e. 16°to 32°at a frame shear angle of 30°.

0

0.005

0.01

0.015

0.02

0.025

0 10 20 30 40 50

Shear Angle (o)

She

ar F

orce

per

uni

t wid

th (N

/mm

)

1st

2nd

3rd

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0 10 20 30 40 50

Shear Angle (O)

Sh

ear F

orce

(N/m

m)

TB1

TB2

Typical diagrams for the three shear cycles (TB1) Comparison of shear diagrams of TB1 and TB2


Compressional Properties of Through-air Bonded Nonwoven Structures

• Lee and Lee (1985) have reported that compressional forces are generally transmitted through the contact points.

• Three cycles produced similar results in all the three positions.• At lower pressure (up to 5 g/cm2), both through-air bonded structures

follow the same curve. However at higher pressure, the thermal bonded structure TB2 is slightly more compressible than TB1.

05

101520253035404550

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Thickness (mm)

Pre

ssur

e (g

f/cm

2 )

TB1TB2

Relationship between the pressure and thickness ofthermal bonded nonwovens TB1 and TB2


Bending Properties of Through-air Bonded Nonwoven Structures

-0.2

-0.1

0

0.1

0.2

-3 -2 -1 0 1 2 3

Radius of Curvature (cm-1)

Ben

din

g M

om

ent

(gf.

cm/c

m)

TB1

TB2

a

-0.08

-0.04

0

0.04

0.08

-3 -2 -1 0 1 2 3

Radius of Curvature (cm -1)

Ben

din

g M

om

ent

(gf.

cm/c

m)

TB1

TB2

b

0.742.941.273.23Bending hysteresis, 2HB, 10-4 Nm/m

0.190.520.260.45Bending rigidity, B, 10-5

Nm2/m

Cross-Machine Direction

Machine Direction

Cross-Machine Direction

Machine Direction

TB2TB1Parameters

Bending diagrams of fabrics TB1 and TB2 in (a) machine and (b) cross-machine directions

Parameters of the bending resistance of Through-air bonded nonwoven structures


Bending Properties of Through-air Bonded Nonwoven Structures (Cont.)

• The fabrics are significantly (2-3 times) stiffer in bending in the machine (0°) direction than in the cross-machine direction, which is easily explained by the preferential orientation of the fibres in the machine direction.

• The bending hysteresis values are quite small, indicating reversibility of the bending deformation and hence strong and elastic bonds between the fibres.

• The bending diagrams are linear for the cross-machine direction and non-linear for the machine direction. This correlates well with the higher hysteresis values for the machine direction and indicates the presence of the frictional component of the bending resistance.

• The bending resistance of fabric TB1 is higher than TB2 in the cross-machine direction. The same arguments of higher density of the finer fibres and lower local porosities in the nonwoven TB1 can explain the observed difference.


Comparison of Theoretical and Experimental Results of Initial Tensile Response

a

0

0.1

0.2

0.3

0.4

0.5

0 1 2 3 4 5 6 7 8

Strain (%)

Str

ess

(N/m

m)

Experimental

Incl. Poisson’s Ratio

Excl. Poisson’s Ratio

b

0

0.2

0.4

0.6

0.8

0 1 2 3 4 5 6 7 8

Strain (%)

Str

ess

(N/m

m)

Experimental



Comparison between experimental and theoretical stress–strain curves of thermal bonded structures (a) TB1, 0°; (b) TB2, 0°; (c) TB1, 67.5°; (d) TB2, 67.5°.

c

0

0.02

0.04

0.06

0.08

0 1 2 3 4 5 6 7 8

Strain (%)

Str

ess

(N/m

m) Experimental



d

0

0.02

0.04

0.06

0 1 2 3 4 5 6 7 8

Strain (%)

Str

ess

(N/m

m) Experimental




Comparison of Theoretical and Experimental Results of Initial Tensile Response

Comparison between theoretical and experimental secant modulus of through-airstructures (a) TB1 (b) TB2 at 4.17 % strain in various test directions

a

0

5

10

15

20

0 22.5 45 67.5 90

Test Angle (o)

Sec

ant M

odul

us (M

Pa)

Experimental



b

0

5

10

15

20

0 22.5 45 67.5 90

Test Angle (o)

Sec

ant M

odul

us

(MP

a)

Experimental



There is a distinct difference between the theoretical and experimental values of secant modulus in test directions, 0 and 45°(TB1) and 22.5°(TB2). The deviations from the experimental observations may have been caused by the assumption that the distribution of in-plane orientation of the fibres to be normal. This may have exaggerated the number of fibres oriented in these directions.


Theoretical Stress-Strain Curves

Theoretical stress-strain curves of through-air bonded structure TB2 (a) including and(b) excluding the effect of Poisson’s ratio

a

0

0.1

0.2

0.3

0.4

0.5

0 1 2 3 4 5 6 7 8

Strain (%)

Str

ess

(N/m

m) 0°

22.5°

45°

67.5°

90°

b

0

0.2

0.4

0.6

0.8

0 1 2 3 4 5 6 7 8

Strain (%)

Str

ess

(N/m

m)

0°

22.5°

45°

67.5°

90°


Conclusions


• The mechanical properties namely, tension, compression, bending and shear of two through-air bonded nonwovens of similar weight and thickness have been investigated.

• The anisotropic characteristics of the properties in relation to the fibre orientation distributions have been studied.

• The observed behaviour correlates well with the directional anisotropy and with the peculiarities of the structural characteristics (different fineness of the fibres and observed porosity).

• The initial tensile response of through-air bonded nonwoven structures has been modelled based upon orientation averaging and simple fibre deformation scheme, taking into account the effect of Poisson’s ratio.


Potential Outputs


• Measurements of orientation distribution of fibres using the 2D image analysis of the fabric cross-sections are subjected to errors for fibres oriented normal to the section plane. These errors should be corrected using general hypothesis of the type of the distribution function.

• Anisotropy of tensile and bending behaviour of the through-air bonded nonwoven fabrics correlates well with the anisotropy of orientation distribution of the fibres. The tensile resistance can be reasonably estimated using simple orientation averaging approach.

• In all the mechanical tests (tension, bending and shear) the frictional losses are small, suggesting good stability and elasticity of the thermal bonds.

• The picture frame technique can be used to study the shear behaviour of nonwoven structures in the wide range of the shear angles. The second and third shear loading cycles should be used for the characterisation of the shear behaviour.

• Optical full field measurement of the strain is a promising instrument to study the unevenness of nonwoven structures.


Acknowledgements

• IWT, Flanders.• Libeltex and Centexbel, Belgium.• DWI, RWTH Aachen for performing the

KES-F tests.

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