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HOYLE
2.6 Hydraulically Entangled Webs
Hydraulic entanglement is the process of entangling individual fibers with each other by
using water jets with varying degrees
of
water pressure. The higher the water pressure,
the greater the degree of entanglement. Depending
on
the backing substrate used during
the entangling process, one can get a product that has a uniform surface
or
one that is
apertured. Most coating substrate end uses require
a
uniformly surfaced product. In a sense
one can consider hydraulic entangling to be
a
hydraulic needlepunching process. While
hydraulic entanglement produces
a
material that is flatter and thinner than a needlepunched
one, the bond formed has a lot in common with that produced by needlepunching.
It
is more like an anchor point than a rigid bond point, and the fabric made-like some
needlepunched fabrics-tends to have low elastic recovery.
2.7
Wet-Lay Mats
The majority
of
the fibers used in wet-lay processing are in the length range of
0.3-0.6
cm. In papers. the fiber used is mostly wood pulp. In nonwovens, synthetic fibers, glass
fibers, mineral fibers. and regenerated cellulose fibers are used as well
as
wood pulp. The
ones most used are polyester, polyolefin, glass, and rayon; occasionally polyamide, aramid.
vinyl chloride-vinyl acetate copolymer. and acrylic are also used. More and more, how-
ever. longer lengths are being introduced
to
increase the strength, cohesiveness, and tough-
ness of wet-lay nonwovens.
As
in dry-lay processing, the fibers have to be opened, dis-
persed, and carried to the web forming stage. The difference is that
in
dry-lay the
transportation medium is mechanical (carding or garnetting) or air, whereas in wet-lay
the transportation medium is water. This process, wet-lay, resembles more the air-lay than
the mechanical process. In both, the fiber is injected into, dispersed in. and carried by a
fluid medium to a flat or a rotary screen, where the fluid is extracted and the fiber deposited
onto the screen as a randomly oriented mat.
There are three main processes for wet-lay structuring: flat wire. inclined wire. and
rotary wire cylinder. The one
to
use depends largely on the mat weight needed, the drainage
properties of the fiber, and the fiber diameter. Wet-lay mats commonly used as coating
substrates are bonded by using latex
or
by thermal bonding. In latex bonding the latex is
either added to the fiber dispersion before web formation or flooded onto the wet-formed
web in
a
second wet deposition step on the wire as the wet fiber mat goes by the second
flooding station. These steps are normally followed by through-air drying and a series of
dry cans to complete the process. Thermal bonding is accomplished by adding a thermal
binder fiber to the initial fiber blend and following wet web deposition and drying with
a
hot calendering step to activate the binder fiber. Because of the multiplicity of fibers
present in the mat due
to
the short length
of
most wet-lay fibers, the mats are of extremely
good quality and are isotropic. Wet-lay mats tend to be very even, flat. and thin, and they
can have
MD
and
CD
strength that
is
quite high. with
a
stiff
or
soft handle, depending
on the type and amount of binder present.
2.8
Stitchbonded Materials
Stitchbonding is the process
of
bonding a web by using stitching yarns, filaments, fibers,
or just the stitching needles themselves
to
do the bonding. In many respects, a stitchbonded
fabric resembles a woven fabric in its properties, and that is because its construction is
similar in many ways to a woven fabric. In some ways, it also resembles a needlepunched