Haoming Rong & Ramaiah Kotra
Wet-laid nonwovens are nonwovens made by a modified
papermaking process. That is, the fibers to be used are suspended in water.
A major objective of wetlaid nonwoven manufacturing is to produce structures
with textile-fabric characteristics, primarily flexibility and strength, at
speeds approaching those associate with papermaking. Specialized paper
machines are used to separate the water from the fibers to form a uniform
sheet of material, which is then bonded and dried.
In 1996, 12.3% of the patents of nonwoven process were
connected to the wet-laid process. It ranked third although more than
one-half of the nonwoven process patents issued have been on extrusion
systems - melt blown and spunbonded technologies.
Paper and Textiles
The wet-laid process has its origins in the manufacture
of paper and was developed because paper manufacturers wanted to be able to
use uncut, long natural fibers and synthetic fibers in addition to the usual
raw materials without changing the process.
Two fundamental reasons account for physical property
differences between paper and textiles. The first is the difference in the
raw materials each process uses. Papermaking fibers being short and fine,
are able to pack together into a dense structure. Chemical groups attached
to their surfaces are able to form hydrogen bonds with similar groups on
neighboring fibers very easily. Textile fibers, on the other hand, tend to
be longer, stronger, and relatively inert when compared to papermaking
fibers, The second difference is the structure and the way individual
fibers are arranged by the process to make a finished product. In paper, the
fibers overlap randomly and pack densely. In textiles, there is a repeating
unit structure which provides some extensibility in all directions, but
which preserves the basic strength and stability of the fabric (whether knit
of woven). In light of the characteristics of these raw materials and
structure, you would expect paper to be weak, stiff inextensible, smooth,
and dense, while textiles would be stronger, softer, bulkier, more drapeable,
less smooth and more porous.
Manufacturers of wet-laid nonwovens desire to take
advantage of the high production rate and the ability to blend a variety of
fibers from papermaking technology. On the other hand, they must overcome
the difficulties brought on by using textile fibers and producing fabric
stiffness in wet-laying if this technology is to compete realistically with
textiles and other nonwoven products.
To alter the basic properties of paper, one must attack
paper's two problems discussed above (raw material limitations and structure
deficiencies). This has been done by including synthetic fibers in the raw
materials for wet-laid nonwovens, by bonding the fibers together (rather
than weaving, knitting, or relying on hydrogen bonding), and by using new
methods of web forming which improve the structure. The strategies have been
successful to one degree of another, either separately or in combination,
but each introduces problems for the production process.
In theory, any natural or synthetic fiber could be used
in the production of wet-laid nonwovens. However, there are practical
limitations on the use of many fibers (cost, availability, priorities, etc).
Some form of wood pulp is used in virtually all wet-laid nonwovens because
of its ease of handing, low cost, opacity, and chemical reactivity. Natural
fibers other than wood pulp remain of interest because they have valuable
properties for specialized end-uses. They suffer from unstable pricing and
supply due to variations in climate, worldwide demand, and availability of
competing fibers. Some natural fibers - such as cotton linters, manila hemp
and cellulose staple fibers - are used in wet-laid process.
Synthetic fibers provide specialized properties,
uniformity, and constancy of supply which cannot be gotten by natural
fibers. Some are used more widely than other. For example, bicomponent
fibers, which simultaneously provide both a structural element and a
thermobonding capability, have been used in specialized materials despite
their high cost. Crimped fibers require special dispersion and bonding
techniques, but make a very soft and bulky product. The use of rayon and
polyester textile fibers with lengths exceeding 1.5 inches has been reported
sporadically. "Synthetic wood pulp" made from very short, shear-precipitated
polyolefin fibers is available and results in improved wet strength and
other properties of wet-laid nonwovens.
Unfortunately, synthetic fibers for use in wet-laid
nonwovens are 20 to 50% more expensive than the same fiber in the form of
textile staple, because the market is small relative to that for textile
fibers, and special handing and cutting are required. Specialty fibers such
as low-melting bicomponent fibers are even more expensive, and their
manufacture is too small to allow economies of scale to be fully realized.
In general, man-made fibers are longer, stronger, more
uniform, and less compatible with water than natural fibers. Their
flexibility and length can mean that they entangle ("flocculate") when they
are dispersed in water, which either prevents or limits their use in
nonwovens. Several approaches have been developed to overcome this problem.
For example, synthetic fiber manufacturers offer fibers with proprietary
chemical surface treatments, which improve dispersion by overcoming the
inherent hydrophobicity of the polymers from which the fibers are made.
The general strategy for reducing flocculation of
synthetic fiber furnishes is to increase the dilution (decrease the
"consistency" of weight percent of fiber in the suspension). Papermaking
generally uses consistencies in the range of 0.3 to 0.7%, but wet-laid
nonwovens are produced using consistencies of 0.01 to 0.05%. While this
helps to separate the fibers enough to prevent or reduce flocculation, it
requires specialized machinery to form and dry the sheet because of so much
water. From an empirical case, we know that slightly less than 960,000
gallons of water per hour must be drained through the wire of a hypothetical
machine in order to deposit 2000 pounds per hour of wet-laid nonwovens.
For some applications it may be necessary to work with
the fiber supplier to resolve compatibility problems between the dispersion
finish and other chemicals used in the system. However, mechanical problems
are now far more common than chemical ones in the dispersion step. In
especially troublesome dispersion problems, low concentrations of natural
and synthetic polymers are used to increase suspension viscosity and thus
stabilize dispersions for use on papermaking equipment.
There are three characteristic stages in the manufacture
of nonwoven bonded fabrics by the wet-laid method:
Swelling and dispersion of the fiber in water; transport of the
suspension on a continuous travelling screen;
Continuous web formation on the screen as a result of filtration;
Drying and bonding of the web.
Schematic of different stages of wet-laid process
Whether or not a fibre is suitable for use in the web
process depends on its ability to disperse in an aqueous medium. The
dispersion behavior of a fiber depends largely on the following factors:
- the degree of fineness calculated from the length and thickness of the
- the stiffness of the fiber in an aqueous medium (web modified)
- the kind of crimping
- the wettability
- the cutting quality of the fiber
After swelling and dispersion of the fibers in water, the
mixing vat are transported to the head box from where they are fed
continuously into the web-laying machine. A suitable system has been found
for creating a homogeneous web out of synthetic and long natural fibers in
which the web is formed. This includes incorporating an adequate length of
time from a prepared suspension and controlling the resistance of the web to
filtration as it is being formed.
Web formation from restrained back fiber suspension
Often, squeezing machines were used to dehydrate the web.
It is a process that began in the vacuum section of the oblique screen. The
web is compressed at the same time and consequently bonded. It is cheaper to
remove the water mechanically than thermally. Drying and bonding the web is
also an important procedure in wet-laid processing. It not only draws water
out of the web but initiates bonding. Convection, contact and radiation
dryers are used.
The advantage of low consistencies is the reduction of
defects due to premature entanglement of the fibers in the furnishing. There
are related disadvantages, however. One problem lies in finding a way to
drain these large volumes of water through the sheet of nonwoven as it is
forming without disrupting it. Second, it is desirable to control the
orientation of the fibers during web formation so that a useful ratio of
properties in the machine-direction (MD) and the cross-machine direction
(CD) is achieved. Actually papermaking machinery has been adapted to the
production of wet-laid nonwovens to deal simultaneously with the problems of
removing large amounts of water quickly without disrupting the sheet as it
forms, and controlling fiber orientation in the product. The "inclined wire
fourdrinier" and the "cylinder" machine have been in use for many years,
providing acceptable wet-laid nonwovens. Both offer many sophisticated
control mechanisms which are vital to modern wet-laid nonwoven production.
Wet-laid web-making machine with cylinder drier
Appropriate bonding of a wet-laid nonwoven is central to
nonwoven design and manufacture. Bonding agents can amount to 30% or more of
a nonwoven product. Therefore their properties are as important as those of
either the fibers or the structure. The hydrogen bonding which is typical of
paper products results in stiff structures, with little of no wet strength.
Neither of these characteristics is desirable in a nonwoven product. In
order to supply wet and dry strength and resilience, a great number of
bonding materials and processes have been tried experimentally and
The most common material used in bonding wet-laid
nonwovens is a water-based emulsion or dispersion ("latex") of a
crosslinkable synthetic polymer, such as a polyacrylate, styrene-butadiene
polymer, ethylene-vinyl acetates, vinyl chlorides and so on. Latex
manufactures sell families of products which range from stiff and strong
adhesives to materials which are soft, extensible, and somewhat weaker.
Commercial latex has been optimized for adhesion to various hydrophobic
synthetic fibers, as well as to hydrophilic materials such as wood pulp,
rayon and the like. The range of chemical modifications commercially
available is very large, and designed to meet almost any conceivable end-use
Latex can be added to a wet-laid nonwoven during its
manufacture using a size press, as a liquid or a foam, or spraying, or by
rotary screen printing. The properties of webs bonded in this way depend on
the base web structure and properties, the characteristics of the latex
polymer (relative stiffness or softness, relative strength and resilience),
the relative proportions of the bonding agent and substrate web after drying
and crosslinking, and the method of addition. Generally, size press
impregnation produces a relatively dense product with superior binder
distribution, while spray, foam and print bonding produce thicker, softer,
less dense webs that are somewhat weaker.
Meltable fibers can be added to the web for bonding and
activated by a heating step either during drying, or during a later hot
calendering step. Examples of fibers of this type include vinyon,
polypropylene, cellulose acetate, and special low melting polyester or
polyamide copolymers. If the bonding step can be combined with the normal
drying step, low melting binder fibers can be an efficient and cost
effective route to bonding. Some types of polyfibers (vinyl alcohol) swell
and partly dissolve when web temperatures reach 40 to 90
C in the drying
section of the paper machine, and have been used for many years to bond and
stiffen papers and nonwovens.
Printing latex binders onto nonwovens ("print bonding")
in a discontinuous pattern improves hand, drape, and softness. The bonded
areas provide strength, and the areas which receive no binder remain
flexible and soft. Much effort has gone into optimizing print bonding
patterns for various nonwoven substrates. A large amount of print bonded
nonwovens are made and sold each year. In general, print bonding of wet-laid
nonwovens is not done in-line commercially. Print-bonding is more generally
applied to carded or air-laid webs than to wet-laid materials.
Special features of the wet-laid process and its products
Compared to the dry web-making processes (carding,
aerodynamic and spun web methods) the distinctive features of the wet method
are its high productivity and wide range of application. It is used for
special papers, conventional wet-laid fabrics and wet-laids made from
- Since short fibers are required the web structure is closer, stiffer
and less strong than in comparable web made from longer, curled fibers in
dry processes. Special treatment is necessary to achieve comparable
- Single or multi-layered products can be made and reinforcement of the
web with a layer of threads can be applied, but in a continuous process on
only one machine.
- The fibers in the web may be randomly or longitudinally arranged.
- The GSM can be varied within broad limits.
Examples of end uses of wet-laids
Flexible sheet wet-laid material suitable for use in the
manufacture of wear-resistant laminated articles such as bearings and rotor
blades comprises particles of a low-friction substance such as graphite and
heat-resistant web-forming fibers, bound together with an organic binder.
Wet-laid crepe papers are used for a wide variety of different application
fields, especially in the hygiene market and in disposable nonwovens for
medical/surgical purposes. They can be used for wet wipes for spectacles;
wet toilet paper; dental crepe; disinfection towels; perfumed towels;
cleaning towels and many more. Wet-laid nonwovens are also
significant in filtration textiles, the products include micro-glass paper,
tea bags and coffee filters. Wet-laid nonwoven fabric can be
used for battery separators. Nylon 66 staple wet-laid
nonwovens have high surface tension, compatibility with hydrophilic
finishes, low count per filament, high dye affinity and high melting and
Water removal on drying is one of the most important
steps in the wet-laid process. Pressure, vacuum, and heat are used to remove
water from the sheet. The efficiency of the methods are determined by the
machine speed, sheet weight, and fiber compositions of the sheet. Normal
practice is to use stream heated cylinder dryers (35-75 psi steam), just as
in normal paper production. The tendency of synthetic fiber webs to stretch
during the drying is controlled by multiple dryer sections with individual
Post-treatments are common practice also. Calenders are
often used on the product to densify and smooth the sheet. Creping devices
are used to soften sheets by controlled bond breakage. If the sheet has not
been dyed in-line, it can be colored or printed off-line, after production
of the base sheet.
Other in-line treatments, include aperturing and
water-jet entanglement. Apertures are regularly spaced holes, and can be
selected for aesthetics or for performance (speeding brewing of tea from a
teabag, or improving permeability to glue for laminating a substrate). One
method of aperturing uses a course forming wire, so that the sheet is formed
around the protruding "knuckles" in a regular pattern. Another method uses
high- pressure water showers and patterned cylinders to rearrange the fiber
into the desired pattern. It is also possible to water-jet entangle
appropriate furnishes either in-line or off-line by using very small,
precise jets of high pressure of water. This technology is currently in the
development stage, but combines the high productivity of papermaking with
the latex-free fabric of the spunlaced process.
The wet-laid process has advantages of high productivity,
control of orientation of properties, and high uniformity at low basis
weight. Coupled with its ability to blend fibers into the furnish for
economy of performance, its ability to form two phase sheets, and its
flexibility in binder and chemical addition, wet-lay is a formidable
competitor with other nonwoven processes.
D.K. Smith, "Nonwovens Technologies for the 21st Century",
Nonwovens Industry, December 1997, page 38.
J. Lunenschloss, w. Albrecht, Nonwoven Bonded Fabrics, published by
Ellis Horwood Limited, 1985, page 317.
T-and-N-Materials-Research-Ltd, "Wet-laid flexible sheet materials",
International patent Classification D21H.
Asahi-Kasei-Kogyo-Kk, "Wet-laid nonwoven fabric for battery separator
its production method, and sealed-type secondary battery", Official-
Schwyn-M, "PA66 for wet laid nonwovens", Nonwovens-Report-International,
Bergmann-L, "Overview of filtration media worldwide", Nonwovens-Report-International,
James E. Williamson, et. al, Nonwovens workshop, Univ. of Tennessee,
July 30, 1996.
- Other sources.
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