producing a non-woven fabric and its mechanical performance

1. Non-Woven Fabrics
INDA (Association of the Nonwoven Fabrics Industry located in Cary, North Carolina, USA)
defines a nonwoven as ‘a sheet, web, or batt of natural
and/or man-made fibers or filaments, excluding paper, that
is bonded by any of several means.’
EDANA (The European Disposables and Nonwovens Association located in Brussels, Belgium)
defines a nonwoven as ‘A manufactured sheet, web or batt
of directionally or randomly oriented fibers, bonded by
friction, and/or cohesion and /or adhesion, excluding paper
and products (woven, knitted, tufted, stitch-bonded)
Introduction

2. Non-Woven Fabrics
Introduction
The term nonwoven refers to web-like assemblages of fibers
wherein fiber to fiber bonding replaces twisting and interlacing.
We define a nonwoven as an engineered fabric structure that
may contain fibrous and non-fibrous elements and that is often
manufactured directly from fibers or filaments and may
incorporate other types of fabrics.
The difference primarily between a nonwoven and its more
traditional counterparts (woven, knitted and braided structures)
is the structure.

3. Non-Woven Fabrics
Introduction
The fibers or filaments in a nonwoven are not interlaced or inter-
looped and are somewhat random layered assemblies of fibers
held together by a variety of different means.
The structure of a nonwoven is defined therefore as its fiber
orientation distribution function (ODF).
Another important structural aspect to consider is the basis
weight (mass per unit area – g/m2 or more commonly referred
to as gsm) and its uniformity.

4. Non-Woven Fabrics
Introduction
ODF may dictate behavior and weight uniformity dictates failure.
The structure-property relationships in a nonwoven depends on
the process utilized to form the nonwoven.
Therefore, short review of the processes employed in the
making of nonwovens is important to describe the mechanical
properties of nonwovens.

5. Non-Woven Fabrics
Non-woven production processes
.The first question which comes to mind when asked about
nonwoven mechanical properties is:
what process was used to make it ?
Nonwovens production is usually split into two steps:
web formation and web consolidation (or bonding).

6. Non-Woven Fabrics
Web formation
.
Cross Lapper schematic

7. Non-Woven Fabrics
Web formation
.
Air Laid schematic

8. Non-Woven Fabrics
Web formation
.
Spun bonding schematic

9. Non-Woven Fabrics
Web formation
.
Melt blown schematic

10. Non-Woven Fabrics
Bonding
.
Calendering schematic

11. Non-Woven Fabrics
Bonding
.
Bond pattern schematic

12. Non-Woven Fabrics
Bonding
.
Hydroentanglement schematic

13. Non-Woven Fabrics
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Fiber orientation distribution function (ODF)
The orientation distribution function [ODF] y is a
function of the angle .

14. Non-Woven Fabrics
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The influence of the production method on anisotropy
The different web formation processes will impart different
initial ODF, which might be further modified by the bonding
process.
The opening and carding processes have a significant impact
on the orientation of the resultant web.
The carding process, by nature, imparts a high degree of
orientation to the fibers in the machine direction.
The main cylinder and the workers in the card align the fibers
parallel to the machine direction.

15. Non-Woven Fabrics
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The influence of the production method on anisotropy
Inadequate opening of the fibers creates a non-uniform fabric
that has a tendency to break easily during processing.
A fabric formed from a web with fibers mostly aligned in the
machine direction will be expected to have high strength in
the machine direction and relatively low strength in the cross
direction.
Other properties follow the same pattern

16. Non-Woven Fabrics
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The influence of the production method on anisotropy
To improve the cross direction strength requires the
rearrangement of the fibres so as to have a higher degree of
orientation in the cross direction.
This can be achieved by several mechanical methods.
One method involves stretching the web in the cross direction
prior to the consolidation or bonding step.
When the web is stretched in the cross direction, fibres are pulled
away from the machine direction and realigned in the cross
direction.

17. Non-Woven Fabrics
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The influence of the production method on anisotropy
Another method commonly employed is a cross-lapper that
takes a card feed and cross-laps it into a uniform batt before
consolidation or bonding.
Most cross-lapped webs have a bimodal fibre orientation
distribution.
The ODF in the wet lay process also has a machine direction
dependency.
Here, the ODF can be adjusted by controlling the throughput
and the speed of the belt.

18. Non-Woven Fabrics
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The role of ODF on mechanical performance
Unit cell

19. Non-Woven Fabrics
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The ODF was measured from a series of such
images captured at regular intervals of
deformation at each test direction.
Images at 0, 25% and 50% strain for 90°
direction (cross direction).
The role of ODF on mechanical performance

20. Non-Woven Fabrics
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Re-orientation with different testing directions
When the samples are tested in the cross direction (90°), the fibers
reorient significantly and the dominant orientation angle changes from
its initially preferred machine direction towards the loading direction.

21. Non-Woven Fabrics
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Re-orientation with different testing directions
In the case of samples tested in the machine direction (0°), where
the initially preferred orientation coincides with the loading direction,
the deformation-induced effect is, as expected, primarily to increase
this preference of fibers

22. Non-Woven Fabrics
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Re-orientation with different testing directions
The reorientations due to the test deformations imposed at 34° also
show similar changes in the dominant orientation angle but of a
much smaller magnitude than that obtained at 90°.

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Re-orientation with different testing directions
The reorientations due to the test deformations imposed at −34°
show similar changes in the dominant orientation angle but of a
much smaller magnitude than that obtained at 90°.

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The role of ODF on mechanical performance
Stress strain behaviour and fracture surfaces

25. Non-Woven Fabrics
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The role of ODF on mechanical performance
The samples tested at 34° and −34° directions fall
between the two cases of ‘low stress–high strain’ and
‘high stress– low strain’ failure along the cross and
machine directions, respectively.
Also, the failures are dominated by shear when the
fabrics are tested at 34° and −34°. The fracture edges are
shown for each case in Fig.
As expected, failure tends to propagate along the
dominant orientation angle.

26. Non-Woven Fabrics
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The role of ODF on mechanical performance
Shear angle as a function of strain.

27. Non-Woven Fabrics
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The role of ODF on mechanical performance
The application of a macroscopic tensile strain
produces a significant shear deformation along
the initially preferred direction in fiber ODF,
except when the two directions are either parallel
or normal to each other.

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The role of ODF on mechanical performance
Asymmetry parameter as a function of strain

29. Non-Woven Fabrics
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The role of ODF on mechanical performance
The moments are greatest when the test is performed in
directions other than the two principal directions (machine
and cross).
The fabric performance is a function of its structure or the
manner in which the fibers are arranged within the
structure.
The structural changes brought about in the structure and
the microscopic deformations are driven by the initial
orientation distribution function (ODF) of the fibers and are
similar for all structures with the same initial ODF.

30. Non-Woven Fabrics
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The role of ODF on mechanical performance
The bonding conditions only dictate the point of failure

31. Non-Woven Fabrics
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Failure Mechanisms
The main source of failure in nonwovens is defects.
Those defects can be split into two categories:
 Non-uniformities, mostly originating from the web formation,
 Over/under bonding.
Once failure is initiated, its propagation is essentially ruled by
the ODF.

32. Non-Woven Fabrics
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Failure Mechanisms
The foremost factor in this category is weight uniformity of a
nonwoven.
This refers to the degree of mass variation in a nonwoven
normally measured over a certain scale.
Local variation of the mass results in unattractive appearance,
but more importantly will lead to, and potentially dictate the
failure point of a nonwoven.
For example, tensile failure may be initiated and propagated
first in areas that are fiber-poor (region with low mass),

33. Non-Woven Fabrics
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Failure Mechanisms
Under-bonding occurs when there are an insufficient number
of chain ends in the molten state at the interface between the
two crossing fibers or there is insufficient time for them to
diffuse across the interface to entangle with the free ends of the
chains in the other fiber.
Thus, only a few tie chains exist and the bonds can be easily
pulled out or ruptured under load.
The formation of a bond requires partial melting of the crystals
to permit chain relaxation and diffusion.
In the case of underbonding, insufficient melting has occurred
or too little time for diffusion was allowed prior to cooling.

34. Non-Woven Fabrics
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Failure Mechanisms
Over-bonding occurs when many chains have diffused
across the interface and a solid, strong bond has been
formed.
The fibers within the bond have also lost some of their
molecular orientation (and strength) at the fiber bond
interface.
They may have also become flat and irregular in shape. The
bond site edge becomes a stress concentration point where
now the weaker fibers enter.

35. Non-Woven Fabrics
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Failure Mechanisms
In a fabric under load, this mechanical mismatch results in the
premature failure of the fibers at the bond periphery.
Over-bonding occurs when too much melting has occurred.
Under-bonded (left) and over-bonded (right) bond spots.