Non woven Technology

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Hawassa University Institute of Technology
Department of Textile and Garment Engineering
Non-wovenfabric manufacturing seminar on wet-laid web preparation
By:
To: Mr. Metasebia Gizaw (Bsc)
Nov 2019
Hawassa,Ethiopia
R.NO Name ID
1 MelkamuKenito TECH/0750/09
2 Meron Tebikew TECH/ /09
3 Azeb Mekonin TECH/ /09
4 Abdulkerim Nesru TECH/ /09
5 Selamawit Asalfew TECH /09

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Table of contents
R.NO Title Page
Introduction I
1 Raw materials
2 Compatibility of raw materials
3 Production machineries
4 Process description
5 Process model
6 Special features of wet laid web products and process
7 Miscellaneous topics
8 Fabric defects
9 Merits and demerits
10 Application
11 Conclusion

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Introduction
Historical perspective:
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 wet laid 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 the roll good industry 5-
10% of nonwovens are made by using the wet laid technology.
Wet-laid paper making process:
 The wood pulp and water in the ratio of 0.003-0.007 (w/w) are mixed to make a good quality suspension
of fibers and water.
 The suspension is then pumped to the headbox which has a small opening, often called as slice.
 Through the slice, the fibre-water suspension is dropped onto the moving perforated Fourdeinier wires.
These wires contain a lot of perforations through which the water gets drained to the vacuum and the
fibres, deposited on the moving wires, formed a web. In this way, the wet-laid paper is formed.
Figure 1: Old papermaking machine

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Modified form of the machine
 By using the previous version of the machine it was not possible to process relatively long fibers as the
mentioned dilution ratio results in inadequate fiber dispersion in water.
Measures taken to solve the problem:
 The ratio of weight of fiber pulp and weight of water should be around 0.0005-0.00005.
 The inclination of the forming wire to the base is required to be equal to 20°.
 The machine was made to have large headbox (slice) opening .
This machine has been used to make papers from long fibers and subsequently the basis for making nonwovens
also.
The modified:
Figure 2: modified form of the old machine.
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. 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).

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Raw materials:
 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. Mainly cost, availability, priorities
are the factors that need to be considered in the process.
 Wood pulp is used in virtually all wet-laid nonwovens because of its ease of handling, low cost, opacity,
and chemical reactivity.
 Some natural fibers - such as cotton linters, manila hemp and cellulose staple fibers - are also used in
wet-laid process. The reason for selection of those fibers from natural fibers is due to the capability to
meet the properties needed wet laid non-woven web preparation.
 Synthetic fibers provide specialized properties, uniformity, and constancy of supply which cannot be
achieved by natural fibers. Some are used more widely than other for example, bi-component fibers,
which simultaneously provide both a structural element and a thermo-bonding capability, have been
used in specialized materials despite their high cost. 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 handling and cutting are required
 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.
Flocculation increases with increase in length to diameter ratio which is given as:
Where L = length of fibers in mm
D = diameter of fibers in mm
Tt = fiber fineness in dtex

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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).
Compatibility Of the Material With The Process:
 Whether or not a fiber is suitable for use in the web process depends on its ability to disperse in an
aqueous medium. Once the fiber is found to fulfill the condition we can use it as an input to production
an less and otherwise it have to be rejected for processing is difficult with such materials.
The dispersion behavior of a fiber depends largely on the following factors:
 The degree of fineness calculated from the length and thickness of the fiber. The finer the fiber the
easier will it be to get wet and the disperse then fineness is the crucial behavior that the material needs to
have in order to be an input of the process.
 The stiffness of the fiber in an aqueous medium (web modified)
 The kind of crimping. Crimped fibers require special dispersion and bonding techniques, but make a
very soft and bulky product. It can be taken as the measure to absorb water. The higher the crimp the
lower will be absorption(wettability) which leads to incapability to disperse.
 The wettability (ability to get wet). The higher the wettability the higher is swelling and then disperses
in aqueous medium.
 The cutting quality of the fiber. The fibers need to have ability to cut. Once the material have this
property its parts can move apart and disperse that favors the web formation process.
 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.

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Production machineries:
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 traveling
screen
Continuous web formation on the screen as a result of filtration
drying and bonding of the web
(a)
(b)
Figure 3: wet-laid web machines (a) stages description (b) process integration

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Web forming device with inclined wire screen:
Wet-laid web-making machine with cylinder drier:
Figure 4: different types of wet-laid web preparation machines

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Process description:
 The fibers are mixed with water and it forms fibre-water suspension. In this case two mixing tanks for
preparation of better fibre-water suspension are needed.
 After preparation the suspension is then pumped through the headbox to the perforated wire.
 The water is drained through the perforations and the fibers are laid on the moving wire to form a web.
 The wet-laid web is then dried and bonded by using binder. It is again dried and finally wound on a roll.
Figure 5: The schematic diagram of a typical wet-lay process for making nonwovens:

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Process model:
 It is often necessary to calculate process parameters in advance with an aim to obtain a specific structure
of wet-laid nonwovens. Because of this, it is often necessary to model the wet- lay process.
Figure 6: wet laid process model

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The velocity of water in relation to the velocity of wire determines the structure of the web.
 When both the velocities are equal then the fibre lay-down is found to be practically random.
 When the velocity of wire is higher than the velocity of water then fibres are found to be preferentially
orientated in the machine direction.
 When the velocity of wire is lower than the velocity of water then fibres are found to be preferentially
orientated in the cross direction.

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Special Features Of The Wet-laid Process and Its Products:
Compared to dry web making process and its product:
 Since short fibers are required the web structure is closer, stiffer and less strong.
 The fibers in the web may be randomly or longitudinally arranged.
 The basis weight (grams/square meter) can be varied within broad limits.
Miscellaneous Topics:
Water removal from web:
 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
Calendaring:
 Calendars are often used on the product to make it dense and smooth the sheet. Creping devices are used
to soften sheets by controlled bond breakage.
Aperturing and water-jet entanglement:
 Apertures are regularly spaced holes, and can be selected for aesthetics or for performance.
 One method of aperturing uses a course forming wire, so that the sheet is formed around the protruding
"knuckles" in a regular pattern.

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Fabric defects:
There exist three types of defects in the wet-laid nonwoven fabrics:
 logs, ropes, and dumbbells
Figure 7: fabric defects
Logs:
 are characterized by bundles of fibers with aligned cut ends that are never dispersed.
 Considered to be a fiber supply problem or can be the result of remarkably low under agitation of the
initial dispersion.
Ropes:
 are characterized by assemblages of fibers, with unaligned ends, that are clearly more agglomerated than
in the general dispersion.
 Reformed when fibers are encountered a vortex that facilitates in entangling the fibers to form ropes.

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Dumbbells:
 are characterized by paired clumps of fibers connected by one or more long fibers.
 Their formation requires an excessively long fiber and a snag in the system piping.
 The good quality of dispersion of fibers in water is a key to the good quality of wet-laid webs.
Merits and demerits:
The merits of wet-lay nonwoven process are:
 High through-put rate.
 Isotropic as well as anisotropic structures can be created.
 Too brittle fibers, generally not suitable for textile applications, can be processed.
The demerits of wet-lay nonwoven process are:
 High capital intensive process.
 High energy intensive process.
 High fiber quality requirements.

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Applications:
 They have applications in diversified areas, namely surgical clothing and drapes, bed linen, table
linen, cloths, and napkins, towels, kitchen wipes.
 In addition, the wet-laid technical nonwovens are found to be used in many specialized
applications, including glass fibre roofing substrate, glass fibre mat for flooring, glass fibre mat
for printed circuit boards, wall covering, insulation materials, battery separators, RFI shielding
veils.
Table 1: some uses of wet laid webs
Special Papers Industrial nonwovens for: Nonwovens similar to textiles
Synthetic fiber paper Waterproof sheeting for roofs Surgical clothing
Dust filters Shingling Bed –linen
Filters for liquids Separators Table cloths
Overlay paper Filters Serviettes
Stencil paper Reinforcement material for plastics Towels
Tea bag paper Backing material Household cloth
Paper for wrapping
sausage and cooked
meats
Shoe uppers
Decoration, Interlinings, Sealing material,
insulation
Face clothes
Nappy (diaper), Sanitary articles

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Conclusion:
The system was thought to be applied for any types of fiber. However in practice the system is found to be
applied for those fibers capable of dispersing in the fluid. Moreover the application is constrained by different
factors. To prepare the web different machineries are applied having their own way of performing the sub
operations. The main advantage of the system is its capability to process too brittle fibers that mayn’t be
processed in other systems. The high energy intensives needed may make it limited. The products have
application in different areas including industrial and apparel uses.

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The end!