The belt is usually made of an electrically grounded conductive wire. Upon deposition, the belt discharges the filaments. This method is simple and reliable. Webs produced by spinning linearly arranged filaments through a so-called slot die eliminating the need for such bundle separating devices.
Filaments are also separated by mechanical or aerodynamic forces. The figure below illustrates a method that utilizes a rotating deflector plane to separate the filaments by depositing them in overlapping loops; suction holds the fiber mass in place.
For some applications, the filaments are laid down randomly with respect to the direction of the lay down belt. In order to achieve a particular characteristic in the final fabric, the directionality of the splayed filament is controlled by traversing the filament bundles mechanically or aerodynamically as they move toward the collecting belt. In the aerodynamic method, alternating pulses of air are supplied on either side of the filaments as they emerge from the pneumatic jet.
By proper arrangement of the spinneret blocks and the jets, lay down can be achieved predominantly in the desired direction. The production of a web with predominantly machine direction and cross-machine direction filament lay down is shown in the figure below. Highly ordered cross-lapped patterns can be generated by oscillating filament bundles, as shown.
If the lay down belt is moving and filaments are being rapidly traversed across this direction of motion, the filaments are being deposited in a zig-zag or sine-wave pattern on the surface of the moving belt. The effect of the traverse motion on the coverage and uniformity of the web has been treated mathematically. The result is that relationships between the collecting belt speed, period of traverse, and the width of filament curtain being traversed determine the appearance of the formed web. The following illustration shows the lay-down for a process where the collecting belt travels a distance equal to the width of the filament curtain x during one complete period of traverse across a belt width y. If the belt speed is Vb and the traverse speed is Vt, the number of layers deposited, z, is calculated by z = [x Vt/y Vb]. If the traverse speed is twice the belt speed and if x and y are equal, a double coverage occurs over all areas of the belt.
Many methods can be used to bond the fibers in the spun web. Although most procedures were developed for nonwoven staple fibers, they have been successfully adapted for continuous filaments. These include mechanical needling, thermal bonding, and chemical bonding. The last two may bond large regions (area bonding) or small regions (point bonding) of the web by fusion or adhesion of fibers. Point bonding results in the fusion of fibers at points, with fibers between the point bonds remaining relatively free. Other methods used with staple fiber webs, but not routinely with continuous filament webs include stitch bonding, ultrasonic fusing, and hydraulic entanglement. The last method has the potential to produce very different continuous filament structures, but is more complex and expensive. The choice of a particular bonding technique is dictated mainly by the ultimate fabric applications; occasionally a combination of two or more techniques is employed to achieve bonding.
7. SPUNBOND PROCESS SYSTEM
A number of spunbond processes can be fitted into one of these three routes with appropriate modification. The following are three successful spinning, drawing, and deposition systems merit a brief discussion.
7.1 “DOCAN SYSTEM”
This route was first developed by the Lurgi Kohle & Mineral-Oltechnik GmbH of Germany in 1970. Many nonwoven companies have licensed this route from the Lurgi Corporation for commercial production. This route (chart 2 below) is based on the melt spinning technique. The melt is forced by spin pumps through special spinnerets having a large number of holes. By suitable choice of extrusion and spinning conditions, desired filament denier is attained. The blow ducts located below individual spinnerets continuously cool the filaments with conditioned air. The force required for filament drawing and orientation is produced by a special aerodynamic system. Each continuous filament bundle is picked up by a draw-off jet operated on high pressure air and passed through a guide tube to a separator which effects separation and fanning of the filaments . Finally, the filament fan leaving the separators is deposited as a random web on a moving sieve belt. The suction below the sieve belt enhances the random lay down of the filaments.
7.2 “REICOFIL” SYSTEM
This route has been developed by Reifenhauser of Germany. Many nonwovens companies have licensed this route from the Reifenhauser GmbH for commercial production. This route (Chart 3 below), is based on the melt spinning technique. The melt is forced by spin pumps through special spinnerets having a large number of holes. The primary blow ducts, located below the spinneret block, continuously cool the filaments with conditioned air. The secondary blow ducts, located below the primary blow ducts, continuously supply the auxiliary room temperature air. Over the line's entire working width, ventilator-generated underpressure sucks filaments and mixed air down from the spinnerets and cooling chambers. The continuous filaments are sucked through a venturi (high velocity, low pressure zone) to a distributing chamber, which affects fanning and entanglement of the filaments. Finally, the entangled filaments are deposited as a random web on a moving sieve belt. The randomness is imparted by the turbulence in the air stream, but there is a small bias in the machine direction due to some directionality imparted by the moving belt. The suction below the sieve belt enhances the random lay down of the filaments.
7.3 “LUTRAVIL SYSTEM”
This route was first developed by Carl Freudenberg Company of Germany in 1965. This process is proprietary and is not available for commercial licensing. This route (Chart 4), is based on the melt spinning technique. The melt is forced by spin pumps through special spinnerets having a large number of holes. The primary blow ducts, located below the spinneret block, continuously cool the filaments with conditioned air. The secondary blow ducts, located below the primary blow ducts, continuously supply controlled room-temperature air. The filaments are passed through a special device, where high pressure tertiary air draws and orients the filaments. Finally, the filaments are deposited as a random web on a moving sieve belt .
8. CHARACTERISTICS AND PROPERTIES
The spunbonded webs represent a new class of man-made product, with a property combination falling between paper and woven fabric. Spunbonded webs offer a wide range of product characteristics ranging from very light and flexible structure to heavy and stiff structure. 
- Random fibrous structure
- Generally the web is white with high opacity per unit area
- Most spunbond webs are layered or shingled structure, the number of layers increases with increasing basis weight
- Basis weights range between 5 and 800 g/m2, typically 10-200 g/ m2
- Fiber diameters range between 1 and 50 um, but the preferred range is between 15 and 35 um
- Web thicknesses range between 0. 1 and 4.0 mm, typically 0.2-1.5mm
- High strength-to-weight ratios compared to other nonwoven, woven, and knitted structures
- High tear strength (for area bonded webs only)
- Planar isotropic properties due to random lay-down of the fibers
- Good fray and crease resistance
- High liquid retention capacity due to high void content
- High in-plane shear resistance, and low drapeability.
Spunbond fabrics are characterized by tensile, tear, and burst strengths, elongation-to-break, weight, thickness, porosity and stability to heat and chemicals. These properties reflect fabric composition and structure. Comparison of generic stress-strain curves of thermally bonded and needlepunched fabrics shows that the shape of the load-strain curves is a function of the freedom of the filaments to move when the fabric is placed under stress.
Today spunbonded webs are used throughout the automobile and in many different applications. One of the major uses of spunbonded webs in automobile is as a backing for tufted automobile floor carpets. The spunbonded webs are also used for trim parts, trunkliners, interior door panel, and seat covers.
ii) Civil Engineering
The civil engineering market segment remains the largest single market spunbond webs, constituting over 25% of the total. Spunbonded civil engineering webs cover a multiple of related uses, such as, erosion control, revestment protection, railroad beds stabilization, canal and reservoir lining protection, highway and airfield black top cracking prevention, roofing, etc.. The particular properties of spunbonded webs - which are responsible for this revolution - are chemical and physical stability, high strength/cost ratio, and their unique and highly controllable structure which can be engineered to provide desired properties .
iii) Sanitary and medical
The use of spunbond web as a coverstock for diapers and incontinence devices has grown dramatically in the past decade. This is mainly because of the unique structure of spunbond, which helps the skin of the user stay dry and comfortable . Additionally, spunbond webs are cost effective over other conventional nonwovens. Spunbond web, as coverstock, is also widely used in sanitary napkins and to a limited extent in tampons.
In medical applications many traditional materials have been replaced by high performance spunbonded webs. The particular properties of spunbonded webs, which are responsible for medical use, are: breathability; resistance to fluid penetration; lint free structure; sterilizability; and, impermeability to bacteria. Medical applications include: disposable operating room gowns, shoe covers and sterilizable packaging .
Spunbonded fabrics are widely used as packaging material where paper products and plastic films are not satisfactory. The examples include: metal-core wrap, medical sterile packaging, floppy disk liners, high performance envelopes and stationery products.
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