*4.3.1. Two-dimensional nonwoven fabric*

The first step in the fabrication of a 2D nonwoven fabric is to prepare a short fiber web using various methods such as dry-laying, wet-laying and spun-laying. This web struc‐ ture is loosely formed without any strong binding or connection between individual fibers other than weak cohesive forces. In order to obtain a continuous fabric structure with adequate strength, this fiber web must be consolidated by entangling or binding the individual fibers together. In order to achieve this, various techniques are used including mechanical ones such as needling, stitching and water-jet entangling; chemical methods such as impregnating, coating and spraying; or cohesion-based techniques like calender‐ ing, air blowing and ultrasonic impact [57].

#### *4.3.1.1. By needling method*

Needling method uses barbed needles located vertical to the fabric plane in order to achieve entanglement between the short fibers. These needles are fixed on a reciprocating needle board located above the fiber web. The needles strike in the web catching the fibers with their barbs 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 working width [99-101].

**Figure 38.** (a) Principle of needling a fiber web [57, 98] (b) Schematic views of stitching method in which loop forma‐ tion cycle of a stitch bonding machine is shown [57].

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 calculated by using Eq. (1).

$$Ed = \frac{n\_h \cdot N\_D}{V\_v \cdot 10^4} \tag{1}$$

where, *Ed* is the stitches per area (stroke per cm2 ), *nh* is the number of lifts (per min.), *ND* is the number of needles by nonwoven fabric width (per m), and Vv is the web take-up speed (m/ min).

#### *4.3.1.2. By stitching method*

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 cross lapper and a stitch bonding machine. The stitching loop formation technique is shown in Figure 38 [56, 57]. The main elements for stitching process are the compound needle, closing wire, compound needle hook and guide. Compound needle and closing wire bar are connected to the driving cams and the knocking over sinker. Stitch bonding machines are equipped with one or two guide bars. Lapping is accomplished by the swinging and shogging movements. The swinging action is carried out by means of a rotary cam and a crank drive. Shogging allows the use of a cam disc. Working with two guide bars allows to use one guide for pillar stitch and the other for tricot-stitch. The degree of bonding is determined by the number of stitching loops per unit area which is a function of wale density (number of wales per unit length) and course density (number of courses per unit length). The density of stitching loops is determined by machine gauge and the stitch length. Twisted yarns, textured filaments and film yarns can be used for stitching.
