*4.3.1.3. By hydroentanglement method*

and orienting them in random in-plane and out-of-plane directions (Figure 38) [57, 98]. The most important goal of needling process is to reorient the fibers in fabric out-of-plane (i.e., thickness) direction as much as possible so that these fibers can act like a lock restraining any fiber movement and keeping the web together. It is essential to apply a certain pressure during the process in order to increase the so called "friction-lock" among fibers and to improve the degree of bonding in the felt. Important processing parameters during needling are needle design, needle density per fabric width, the stroke frequency, the delivery speeds and the

**Figure 38.** (a) Principle of needling a fiber web [57, 98] (b) Schematic views of stitching method in which loop forma‐

The fiber movement caused by needling process leads to changes in fabric dimensions and local areal mass of the fabric. Needling can also result in fiber breakages due to fiber-fiber and needle-fiber frictions. The later can be minimized by treating the fibers with a suitable finishing agent prior to needling [57]. Web feeding and take-up speeds are important process parame‐ ters. The stitch density which is the number of penetrations per square area of the felt is

> 4 . . 10 <sup>=</sup> *h D v n N Ed*

number of needles by nonwoven fabric width (per m), and Vv is the web take-up speed (m/

Stitching method involves through-the-thickness stitching of the fiber web in order to consol‐ idate the nonwoven fabric. Nonwoven production process consists of a carding machine, a

*<sup>V</sup>* (1)

), *nh* is the number of lifts (per min.), *ND* is the

working width [99-101].

112 Non-woven Fabrics

tion cycle of a stitch bonding machine is shown [57].

where, *Ed* is the stitches per area (stroke per cm2

calculated by using Eq. (1).

*4.3.1.2. By stitching method*

min).

In hydroentanglement method, water jets are used to entangle the fibers and obtain a contin‐ uous nonwoven surface. The main principle is the same as that of the needling method. Water jets strike onto the nonwoven web and reorient a portion of fibers in out-of-plane directions. These reoriented fibers wrap and lock the others in their vicinity ensuring a continuous surface. The web is soaked from the bottom side only after they have passed the jets which neutralize the part of the web densification [57, 102]. The effect of striking jets on fibers varies depending upon the position of the fiber in the web. Fibers located on the surface facing the water jets are influenced more in comparison with fibers at the bottom. The main process parameter that determines the bonding efficiency is the jet speed. The following relation can be used to find the jet speed.

$$
\upsilon\_w = a \cdot \sqrt{\frac{2 \cdot \Delta p}{\rho}} \tag{2}
$$

where, *vw* is the speed of the jet at the exit point, *Δp* is the pressure differential between the nozzle element and the surroundings, *ρ* is the density of the medium, and *a* accounts for the friction.

#### *4.3.1.4. By thermal and chemical methods*

Thermal bonding process starts with a hot-air treatment to soften the thermoplastic fibers. Then calendering and welding processes are carried out to consolidate nonwovens. Chemical bonding involves the application of binder dispersions then curing and drying of the impreg‐ nated webs [57].

#### *4.3.1.5. By electrospinning method*

Electrospinning method uses an electric energy field to spin a polymer solution from the tip of single or multiple needles to a flat or cylindrical collector. A voltage is applied to the polymer which causes a jet of the solution to be drawn towards a grounded collector. The fine jet stretches and elongates as it travels under energy field and is collected as a nonwoven nanoweb structure [103]. Various publications indicate that the voltage required to produce fibers range from 5 kV to 30 kV [104]. This range of voltage is good enough to overcome the surface tension of the polymeric solution and to produce very fine charged jets of liquid towards a grounded target. This charged jet before hitting the target undergoes splitting and drawing and forming fibers with different sizes and shapes before evaporating to form a nonwoven nano-web structure. The electrospinning system consists of two separate entities, a sprayer and a collecting device. The sprayer essentially consists of a glass spinneret, which holds the polymer solution. One of the metal electrodes from the high voltage supply is given to the solution, which serves as the positive terminal. A collector, which collects the fibers is given the other end of the electrode, which serves as the negative terminal [105].
