Embed Size (px)
Citation preview
NONWOVENS IN GENERAL
Definition of nonwovens
Nonwoven is a sheet, web, or batt of natural and/or man-made fibers or filaments,
excluding paper, that have not been converted into the yarns, and that are bonded together
in different ways
It may also having other names such as Shaped Fabrics (or) Yarn free Fabrics
3 stages of nonwoven production process:
– Web formation
– Web bonding
– Finishing Treatments
Needling process
Needle punching is the method of consolidation of webs by the repeated insertion of
barbed needles into the fibrous web
The fibres or filaments are reoriented into the vertical plane in a way that tufts or
stitching channels are formed. Fibre webs are characterized by:
Improved needle design
Increased needle density per working width
Increased stroke frequencies
Delivery speeds
Working widths
Principle
Needle punching is a nonwoven process by which the fibres are mechanically
entangled to produce a nonwoven fabric by repeated penetration of barbed needles
through a preformed dry fibrous web.
The needle board is mounted on a beam which is given an up and down
reciprocating motion by means of an eccentric crank mechanism. As a result, the
fibres are mechanically interlocked, thereby providing the mechanical strength.
Continuous filaments or short staple fibres are initially arranged in the form of a fibrous
web in various orientations (random, cross, parallel, or composite). This forms a three-
dimensional intermingled structure which fulfils the necessary requirements of geotextiles.
Modern needle looms operate with continuing felt transport. For optimum needling
efficiency, the following process solutions are of importance:
1) Batt feeding 2) Needling zone 3)Felt delivery
Batt feeding
Uncontrolled draft during the feed into the needle loom is causing intra fibre
migration, which leads to length dimensional changes during needling, resulting in
uneven surface mass and/or felt thickness.
A good batt feed is especially important during the first needling pass, also
referred to as “pre-needling”.
Needle
Dimensions of the barbs and their relative
arrangement vary depending on the
application and machine operation.
The nonwoven industry uses two types of
needles, known as single reduction needle and
double reduction needle.
Single reduction needle is much stiffer than
the double reduction needle. The single
reduction needle is usually made only for
coarser gauge needles. The single reduction
needle is used for stiff fibres
Appearance and degree of compression of a needle felt are mainly influenced by:
– Needle arrangement in the needle board
– Direction of needling (from top, from bottom or from both sides)
– Needle parameters (gauge, form of barb, number of barbs)
– Needling parameters (penetration depth and density, draft)
Arrangement of needle boards
Typical applications of these arrangements are filtration media, synthetic leather,
floor coverings, underlay, automotive headliners, and blankets, etc.
The crank angle to the blade is 00 on standard needles. Thus the barbs are arranged with
angles of 600 , 1800 and 3000 on the blade.
The point is the element which first penetrates the felt and suppresses the fibres to enable
the barbs to seize the fibres and transport them. In general one can say that for the
processing of finer fibres finer needles are used.
The barbs are arranged on the three edges of the triangular working blade. In general, there
are two or three at each edge.
The fibre transport at the needle’s penetration into the needle felt is mainly determined by
the barb depth and the barb length. The angle determines the barb’s ability to hold the
fibres.
The distance between the barbs and the number of barbs on each edge determine the
working blade’s length
Point
In general, the distance between needle point and
first barb is 6.4 mm. It may, however, be smaller
(3.2 mm). The point itself can be sharp or rounded.
The finenesses are also expressed in the gauge
system. The position of the structuring elements to
the needle crank is essential for the formation of
rib or velour surfaces on needle felts.
Needle barbs
Based on the spacing, the barbs are
categorized by regular barb (RB), medium
barb (MB), close barb (CB), and high
density barb (HDB).
The high density barb provides maximum
aggressive punching, followed by the close
barb, medium barb, and regular barb. At
the same time, the surface of the fabric is
found to be maximum uneven with the
high density barb, followed by the close
barb, medium barb, and regular barb.
star bladed needleTriangular bladed needle
Stitch density
The ratio Vv to nh is equivalent to the material advance Lv per stroke. Formerly one
operated with intermittent advances, which meant a felt transport, was possible
only while the felting needles were not in engagement with the material.
Critical process parameter
Punch density is defined by the number of needle penetrations per unit area of the
resulting fabric. If ξ is punch density, n is the number of needles per unit width of
needle board, A is the fibre web advance per stroke, P is the rate of production, and
m is the number of punches per unit time, then
when the needle board density is constant and for a given stroke frequency, the
punch density is determined by adjusting the fabric throughput rate.
To obtain higher punch density of a finished fabric it usually requires two or more
needle looms. Depending on the scale of production, these passages may take place
as separate operations or may form a continuous production line.
Selection of needle
Fibre linear density (denier) Needle gauge (SWG)
0.5-1.5 42
1.5-6.0 38-40
6-10 38
10-18 36-34
18-30 36-32
>30 30-coarser
- The higher is the gauge of the needle, the finer is the needle and vice-versa.
Influence factors on the needle felt’s
characteristics
Important characteristics of needle felts are the degree of felt compression, the strength-
elongation ratio and the permeability characteristics.
Fibre length:
Longer fibre lengths result in higher strength, higher felt density and less air permeability.
Length influence on the strength by the fact that when using longer fibres the movement of
the fibres into the vertical plane during transformation is minimized.
The optimum fibre length for needle felts is in the range of 50–80 mm.
Fineness & Crimp:
Finer fibres lead to smaller felt thickness and to lower air permeability
Higher crimp results in a higher tear resistance and elongation and a better dimensional
stability of the needle felts.
Machine oriented web results in a high strength in the longitudinal direction and
predominantly cross oriented webs result in a high strength in cross direction.
One-sided or double-sided needling also influence draft and thus the dimensional
change. For this reason, it is very difficult to predict draft.
As the needle felt density rises with increasing stitching density, the felt thickness
reduces.
Fiber reorientation:
Increased needle penetration:
An increase in needle penetration means an increase in the no of barbs
participating in fiber transport & increased transport distance, both of which cause
increased damage to fibers.
Fibers picked up by the barbs and transported a longer distance are more likely to
get break.
The impact of fast passing barbs on fibers is more likely to cause partial damage.
Strength characteristics:
Web consolidation after needling determines the strength and stretch characteristics
of the needle felt.
The fibres have a predominantly cross oriented position in the web, the cross
strength will be greater than the strength in machine direction.
Without fibre reorientation or at a minor reorientation the strength increase in
machine direction is bigger than in cross direction.
Other parameters influence needle felt’s characteristics:
1) Batt feeding system
2) Stripper plate design and stitching plate
3) Number of needles
4) Dwell time of needles in the felt
5) Delivery speed
6) Fibres position in the web
7) Fibre fineness
8) Fibre length
9) Length of the needling zone etc.,
Felt removal
Previously, at low stroke frequencies, the delivery of the needle felt occurred
intermittently.
The contact pressure of the delivery rolls and the contact area can be adjusted to
the product.
In general, the needle felt is being guided positively and without deformations.
The drive of the delivery rolls can either be synchronous to the stitching frequency
of the needle beam or individually controlled.
Applications
Needle-bonded nonwovens are finding a wide variety of applications, including
Geosynthetics Filter media
Synthetic leather Floor coverings
Automotive fabrics Insulation
Blankets Wipes
Waddings Padding's etc..,
