Fabric manufacturing (2024)

2.1 Winding

Winding is a process in which yarn from bobbins, which is the end product of ring spinning, are converted into suitable form of package. This transfer of yarn from one type of package to another package, more suitable for the subsequent process is also called winding. Main objectives of winding process are to increase the package size, clear yarn defects and produce a package suitable for subsequent process (size and shape).

The yarn packages are either parallel or tapered, with respect to shape, as shown in Fig. 3. The parallel packages may also have flanges, while tapered packages are without flange [1].

The winding process involved unwinding yarn from one package and rewinding it on to another package. The yarn may be unwound in two ways, i. e. over end and side withdrawal as shown in Fig. 4. Winding rate is the speed at which the yarn is wound on package surface, while to and fro movement of yarn when it is laid on to package is called traverse. In case of near parallel package, traverse is very slow, but in case of cross wound package traverse is quick. There is no traverse in case of parallel wound packages.

In the winding machine, yarn is taken from the bobbin / cop and is wound on the package after passing through the thread guides, balloon breaker, stop motion and yarn clearer. For cross winding, a grooved drum is also provided on the machine to traverse the yarn.

2.2 Warping

In warping process, the yarns are transferred from a number of supply packages (cones) to the warp beam in the form of a parallel sheet. The main objective of warping is to get the required number of ends as per requirement. The three main types of warping are high speed / direct warping, sectional / indirect warping and ball warping. In direct warping, the yarns are withdrawn from the single-end yarn packages (cone) on the creel and directly wound on a beam. A number of beams are warped to get the required number of ends. For example, to produce a fabric with 6040 warp ends, 8 beams will be warped, each with 755 ends. These beams are then combined into a single beam in the sizing process. The process offers only limited pattern possibilities, and is preferred for simple patterns only.

The indirect / sectional warping process completes in two steps, i. e. warping and beaming. In first step, a portion of the required number of threads (called section) is wound onto a conical drum (Fig. 5). All the sections are warped on the drum side by side, one after the other. In next step all the sections are unwinded from the drum and wound onto beam to complete the required number of threads. This beam may or may not be taken for the sizing process. The division of warp sheet into small sections provides unlimited patterning possibilities. Therefore this process is suitable for complex warp patterns. Ball warping is the process in which warping is performed in rope form on to wooden ball. The ball is wound on a special wooden core called “log”. It is also a two stage process, suitable for denim fabric manufacturing, involving rope dyeing process. Re-beaming is done to convert the rope dyed warp yarn, stored in cans, into beam.

2.3 Sizing

Sizing, also termed as slashing is the coating of warp sheet with size solution. Weaving requires the warp yarn to be strong, smooth and elastic to a certain degree. There is always a friction between metallic parts and yarn during the weaving. So, the warp yarns need to be lubricated to reduce the abrasion. The application of size material helps to improve the mechanical properties of warp, reduce abrasion and the elasticity of yarn. The amount of sizing material relates to the tenacity, hairiness and linear density of yarn, and also to its behaviour during weaving. Another major objective of this process is to get the total ends on a weavers beam, combining the ends of all warp beams. The application of sizing material results in the following properties in yarn.

1. High strength

2. Low flexibility

3. Low abrasion

4. Increased smoothness

5. Less hairiness

The process of sizing can be classified on the basis of method of application into conventional wet sizing, solvent sizing, cold sizing and hot melt sizing [3]. The main parts of conventional sizing machine include (Fig. 6) creel, sizing box, drying section, leasing section, head stock and size cooker.

In conventional wet sizing, the fundamental constituents of size recipe are the size materials and a solvent usually water. The sizing materials are broadly classified into three groups namely adhesives, softeners and auxiliaries [4].

The adhesives perform two functions; bind the constituent fibers of the yarn together and form a film over the yarn surface, resulting in increased strength, low hairiness and more even yarn. The adhesives are classified on the basis of origin into natural, synthetic and modified adhesives, produced by treating natural adhesives with certain chemicals. The natural adhesives may be obtained from plants or animals, for example maize starch, potato starch, etc. The chemical modification of natural adhesives is performed to induce the desired properties. Some common examples of modified adhesives are modified starches and carboxy methyl cellulose (CMC). The chemically synthesized polymers like poly vinyl alcohol (PVA) and acrylics fall under the category of synthetic adhesives. Starch adhesives are used most commonly because of low cost and environment safety.

The softeners are added in the size recipe to lubricate the yarn and reduce abrasion / friction between adjacent yarns and between yarns and loom accessories. They also give a soft handle to the warp and size film, helping to decrease its brittleness. The softeners may be in solid form (wax group) or liquid form (oil group) and are obtained from animals, vegetables or synthesized chemically. The auxiliaries include antiseptic, antistatic, weighting, swelling agents and / or defoamers. The sized fabric must be subjected to a desizing process prior to the finishing stage. Desizing has a decisive effect on the waste water load in textile production.

2.4 Drawing In

The sized warp sheet is wound on to a beam called as the weaver’s beam. It has the required number of ends and the yarns have adequate strength to bear the tensions of weaving process on loom. This beam is either used for drawing in or knotting / tying, depending on the requirement (Fig. 7).

Style change involves the production of a new fabric style, while mass production means to continue the weaving of same fabric style just replacing the empty beam with a full beam of same type. Drawing in is the process of entering the individual yarn of warp sheet through dropper, heald eye and the reed dent (Fig. 8). The yarns can be threaded wither manually or by using automatic machines.

The yarn is now fully prepared for conversion into the fabric, which takes place at the loom. The weaver’s beam is gaited on the loom, while weft yarn is provided at right angle either from cone or bobbin depending on the picking media.

3.1 Primary Motions

The primary loom motions include the following three operations:

• Shedding: the separation of the warp sheet into two layers to form a tunnel known as the shed

• Picking: insertion of weft yarn, across the warp sheet width, through the shed

• Beat-up: pushing the newly inserted length of weft (pick) to the fell of cloth. These operations occur in a given sequence and their precise timing in relation to one another is of extreme importance.

3.2 Secondary Motions

The secondary motions facilitate the weaving of fabric in a continuous way [6]. These include:

• Let off: this motion provides warp sheet to the weaving area at the required rate and under constant tension by unwinding it from weaver’s beam

• Take-up: this motion draws fabric from the weaving area at a uniform rate to produce the required pick spacing and wind it onto a roller.

3.3 Stop Motions

These motions are used in the interest of quality and productivity; stopping the loom immediately in case of some problem. The warp stop motion will stop the loom in case any warp yarn breaks, avoiding excessive damage to the warp threads. Similarly weft stop motion will come into action in the absence of weft yarn, and stop the loom.

4.1 Tappet Shedding

This system is also termed as cam shedding. The cam is an eccentric disc mounted on the bottom shaft, rotating to lower or lift the heald frame. It is relatively simple and inexpensive system handling up to 14 heald frames [8]. But this system has very limited design possibilities and pick repeat, producing simple weaves.

4.2 Dobby Shedding

It is a relatively complex shedding system and can control up to 30 heald frames. The pick repeat to dobby system is provided by peg chain, punched papers, plastic pattern cards or computer programming, and is virtually unlimited. This system offers more design possibilities as compared to tappet shedding.

4.3 Jacquard Shedding

The jacquard shedding provides unlimited patterning possibilities. The working principle is relatively simple but involves more number of parts that make it a complex machine. Versatility of jacquard shedding is due to control over individual warp yarn. The jacquard shedding system can be either mechanical or electronic.

5.1 Shuttle Picking

It is the oldest technique of weft insertion on loom. The picking media is a wooden shuttle that traverses back and forth across the loom width. A pirn or quill having yarn wound on it is placed inside the shuttle. As the shuttle moves across, the yarn is unwound and placed in the shed. A picking stick on each side of loom helps to accelerate the shuttle by striking it. Shuttle travels on the race board, above lower portion of the warp sheet. The shuttle picking takes place from both the sides of loom.

5.2 Projectile Picking

Introduced first time by Sulzer in 1952, this machine uses a small metallic projectile along with gripper to throw the wet yarn across the loom width. The energy required for propulsion of projectile into the shed is provided by twist in the torsion rod. The projectile glides through guide teeth in the shed. It had low power consumption, versatility of yarns and a higher weft insertion rate as compared to the shuttle picking system.

5.3 Rapier Picking

This picking system uses a rigid or flexible element called rapier for the insertion of weft yarn across the shed. There are two major variations in the rapier picking; single rapier and double rapier. In case of single rapier picking system, the rapier head grips the weft and carries it across the shed to receiving end. The rapier has to return empty to insert the new weft. The double rapier picking makes use of two rapiers [9]. One rapier (giver) takes yarn to the centre of machine and transfers it to the other rapier (taker), which brings the weft to other side.

5.4 Water Jet Picking

The water jet picking involves the insertion of weft yarn by highly pressurized water. This pressurized water takes the form of a coherent jet due to the surface tension viscosity of water. The flow of water has three phases: acceleration inside pump, jet outlet from nozzle and flow into the shed. The amount of water used for the insertion of one pick is less than 2 cm³. This system is mostly preferred for the synthetic yarns.

5.5 Air Jet Picking

In air jet picking system, the weft is inserted into the shed by the use of compressed air. The yarn is taken from the supply package / cone and wound on to the feeder before insertion to avoid tension variations. The weft is then passed through the main nozzle which provides initial acceleration to the yarn. The auxiliary nozzles are present at specific distance along the width to assist in weft insertion. A special type of reed, called profiled reed is used for air jet picking. The channel in the reed guides the yarn across the shed and avoids entanglement with warp. It has an extremely high weft insertion rate.

6.1 Plain

The simplest interlacing pattern for warp and weft threads is over one and under one as shown in Fig. 10. The weave design resulting from this interlacement pattern is termed as plain or 1 / 1 weave. The 1 / 1 interlacement of yarns develops more crimp and fabric produced has a tighter structure. The plain weave is produced using only two heald frames. The variations of plain weave include warp rib, weft rib and matt or basket weave.

6.2 Twill

This weave is characterized by diagonal ribs (twill line) across the fabric. It is produced in a stepwise progression of the warp yarn interlacing pattern. The interlacement pattern of each warp starts on the next filling yarn progressively. The two sub categories based on the orientation of twill line are Z- and S-twill or right hand and left hand twill, respectively. Some of the variations of twill weave include pointed, skip, and herringbone twill [11].

6.3 Satin / Sateen

The satin / sateen weave is characterized by longer floats of one yarn over several others. The satin weave is warp faced while sateen is a weft faced weave. A move number is used to determine the layout in a weave repeat of satin, and number of interlacements is kept to a minimum as shown in Fig. 10. The fabrics produced in satin weave are more lustrous as compared to corresponding weaves.

10.1 Weft Knitting

Weft-knit fabric is familiar for their comfort and shape retention properties. The fabric can be produced from a single end. The movement of yarn in the weft direction provides the stretch ability in both directions that could be engineered to achieve the required properties. The apparels, either inner or outerwear, are the most demanding area of weft-knit fabric. This technique is further classified into different machine type and structure that is given in Fig. 13. Circular knitting machines are particularly used to produce tubular fabric. Circular machines are classified into three major categories on the basis of cylinder and Dial. The first category in which machine has only one cylinder, needles are placed inside the cylinder trick that moves up and down for loop formation. Popular structures are single jersey and their derivatives. The second type of machine has both dial and cylinder. The needles are placed in both dial and cylinder. The cylinder needles move up and down while dial needle moves in to and fro manner. The major machine types are Rib and Interlock. The difference in their construction is the placement of tricks or grooves. The grooves on Rib machine of both dial and cylinder are alternative to each other whereas on interlock the grooves are exactly opposite to each other. The third class is Purl, in which the machine has two cylinders. These cylinders are superimposed. The purl fabric is also known as link-link fabric. The needle is hocked on both sides. The same needle is placed in opposite tricks of these cylinders [19]. The flat machines can produce both single and doubleknit fabrics.

10.2 Weft Knitting Machine Elements

The needle is the most important and essential part of loop formation. The needles are placed in tricks or cut of bed (flat or circular) at regular intervals so that they can move freely during the loop formation cycle. Generally, machine manufacturers prefer to use the latch needle for their machines. The latch needle is self-actuating and no auxiliary part is required. Different needle types and their parts are shown in Fig. 14.

There are three main types of needle

1. Latch needle

2. Spring Bearded needle

3. Compound needle

Another important part of the machine is sinker. The sinker is a thin metal plate placed on the horizontal surface at perpendicular to the needle. They move to and fro in between the needle. The sinkers get their movement from the sinker cams. The purpose of the sinker is to hold the old loop when it is cleared from the needle. The sinker is used both in weft and warp knitting [19]. Different types of sinkers are used in different machines to produce the required variety of results. The machine that has double bed construction does not need to use sinker as either bed needle hold the old loop while other bed needle is in working position. The sinker and their parts are given in Fig. 15. The sinker performs one or more of these functions:

1. Loop formation

2. Holding down

3. Knocking over

Knit Stitch

Knit stitch is formed when the needle is raised enough to engage the new yarn in the hook by the camming action and old loop is cleared. The technical back side of the knit stitch is called a purl stitch [18]. The clearing position of knit stitch is illustrated in Fig. 17.

Tuck Stitch

Tuck stitch is formed when the needle is raised to get the new yarn but not enough to clear the previous formed old loop. The needle then holds two loops when it descends as shown in Fig. 17. The needle can hold up to four loops only, and so it has to clear the previous held old loops quickly. The fabric gets thicker with tuck stitch as compared to knit stitche due to the accumulation of yarn when needle clears all the old loops. The structure becomes more open and permeable to air than knit stitches. It can also be used to get different color effect in the fabric [19].

Miss or Float Stitch

When the needle does not move upward to clear the old loop and also does not take the new yarn that presented to it then the miss or float stitch is formed. Needle is not activated in miss stitch. Moreover, it holds the old loop as shown in Fig. 17. Float stitch on the successive needles produce longer float of yarn that may cause the problem of snagging. The float is preferably used where we need to hide some color from the technical face of the fabric. The hide yarn floats at the back of the fabric. The yarn gets straighter in float stitch construction so the extensibility decreases as compared to tuck and knit stitches [19].

13.1 Loop Parts

The needle loop has different parts. The loop parts are important to understand the technical face and back side of the loop. The loop parts are given in Fig. 18.

13.2 Technical Face and Back

If the feet of the new loop cross under the legs of the old loop and legs cross over the head of the old loop, then this side is technical face or it may be defined as the side having all the face of the knit loop. Fig. 19 illustrates the interloping of the old and new loops, forming technical face side. If the feet of the new loop cross over the legs of the old loop and new loop legs pass under the head of old loop then it is said to be a technical back side. The interloping pattern of technical back is given in Fig. 19.

13.3 Wales and Courses

The series of loops that are immersed vertically are known as wales. The consecutive loops that are connected horizontally are called courses. The rows and columns of loops connected are shown in Fig. 20.

13.4 Stitch Density

The number of loops or stitch per unit area is called stitch density. This can be calculated by the product of course and wales density. In one inch square area of fabric, there are 6 loops or stitches as displayed in Fig. 20.

Stitch density of a knitted fabric is expressed as wales density and courses density. The number of wales per unit length is called the wales density, normally measured in wale / inch or wale / cm. In Fig. 20, there are three wales in one inch of fabric.

The number of courses per unit length is called the course density, normally measured in wale / inch or wale / cm. There are two complete courses in one inch of the fabric as shown in Fig. 20.

13.5 Stitch Length

The stitch length is the most important part of knitting. It is basically the length of yarn consumed to make one complete loop [16]. The knitted fabric dimensional, physical and mechanical properties are truly based on the stitch length that can be engineered to meet the requirement of the fabric. The individual loop is shown in Fig. 18.

14.1 Classification of Warp Knitting Machine

Warp knitting machines are categorized on the basis of construction of different machine parts and their operations. Tricot and Raschel are two main categories of machine. Further classification of warp knitting is given in Fig. 22, while comparison of Tricot and Raschel warp knitting is given in Table 3.

16.1 Definitions

Different definitions of nonwovens are available by different organizations. According to the ASTM D 1117–01, nonwovens can be defined as:

“A textile structure produced by the bonding or interlocking of fibers, or both, accomplished by mechanical, chemical, thermal or solvent means and combinations thereof”

According to the standard ISO-9092:1988, the nonwovens are:

“Manufactured sheet, web or batt of directionally or randomly orientated fibers, bonded by friction, and / or cohesion and / or adhesion, excluding paper and products which are woven, knitted, tufted, stitch-bonded incorporating binding yarns or filaments, or felted by wet-milling, whether or not additionally needled. The fibers may be of natural or man-made origin”.

The Association of Nonwoven Fabrics Industry, USA (INDA) defines nonwovens as:

“A sheet, web or batt of natural and / or man-made fibers or filaments, excluding paper, that have not been converted into yarn, and that are bonded to each other by any of several means.

To distinguish wet-laid nonwovens from wet-laid paper materials the following differentiation is made. (a) More than 50 % by mass of its fibrous content is made-up of fibers with a length to diameter ratio greater than 300. Other types of fabrics can be classified as nonwoven if, (b) More that 30 % by mass of its fibrous content is made up of fibers with a length to diameter ratio greater than 600 and/or the density of the fabric is less than 0.4 g / cm³.

The European Disposables and Nonwovens Association (EDANA) describes the nonwovens as:

“A manufactured sheet, web or batt of directionally or randomly oriented fibers, bonded by friction, and / or cohesion and / or adhesion, excluding paper and products, which are woven, knitted, tufted or stitch-bonded, or felted by wet-milling, whether or not additionally needled. The fibers may be of material or man-made origin. They may be staple or continuous filaments or be formed in situ”. To distinguish wet-laid nonwovens from wet-laid papers, a material shall be regarded as a nonwoven if, (a) More than 50 % by mass of its fibrous content is made-up of fibers with a length to diameter ratio greater than 300; or (b) More than 30 % by mass of its fibrous content is made up of fibers with a length to diameter ratio greater than 300 and its density is less than 0.40 g / cm³.”

16.2 Nonwoven Products

Nonwoven fabrics have wide area of applications depending upon the properties required in the end product. EDANA has given the nonwoven fabric applications according to the end use as shown in Fig. 23.

Nonwoven fabrics have wide range of products like disposable gowns, face masks, gloves, shoe covers, dressings, sponges, wipes, diapers, sanitary napkins, lens tissues, vacuum cleaner bags, tea and coffee bags, hand warmers, interlinings, incontinence products, floor covers, air filters, paddings, blankets, pillows, pillow cases, aprons, table cloths, hand bags, book covers, posters, banners, disk liner, sleeping bags, tarpaulins, tents, crop covers, green house shadings, weed control fabrics, golf and tennis courts, road beds, drainage, sedimentation and erosion control, soil stabilization, dam embankments, etc. [24, 25].

16.3 Raw Materials for Nonwovens

Different types of man-made and natural fibers are used in the nonwoven fabric production. Man-made fibers have 90 % share of the total fiber consumption [22].

Polymers, fibers and binders are the basic raw materials for nonwovens. Most of the fibers and binders are made of polymers. A polymer is a large molecule built up by the repetition of small single chemical units. Large molecules are called macromolecules consisting of hundreds to millions of atoms linked together by chemical bonds (typically covalent bonds) which are called primary bonds. The special properties of polymers result particularly from secondary bonds acting between the macromolecules which are known as van der Waals forces. Virtually all types of fibrous material can be used to make nonwoven fabrics depending on:

1. The required profile of fabric

2. The cost effectiveness

3. The demands of further processing

Most common fiber types used in nonwoven fabric production are: polypropylene, polyester, viscose rayon, polyamide 6 and 6.6, bicomponent fibers, surface modified fibers, superabsorbent fibers, Novoloid fibers, wool is used in felts production, cotton in hygienic goods and another quantitatively important group of fiber raw material for nonwovens is waste fiber materials.

Production of nonwovens is carried out after considering the following points:

1. Process ability in a particular technology

2. Impact on the product properties

3. Price

Thus, it is very essential to study different fiber properties for the development and production of nonwoven fabrics. According to a study carried out by Tecnon Ltd. the world consumptions of different types of fibers are as: polypropylene 63 %, polyester 23 %, viscose rayon 8 %, Acrylic 2 %, polyamide 1.5 %, others 3 %. Polypropylene fibers consumption is highest in nonwoven fabric production due to certain properties like: low density, hydrophobicity, low melting and glass transition temperature, biological degradation resistance, chemical stability, good mechanical strength [22].

17.1 Web Formation

After the fiber selection web formation is the major step of nonwoven fabric formation. During web formation fibrous sheet, web or batt is formed having two dimensional or three dimensional assemblies. Orientation, dimension and structural arrangement of fibers in the web greatly influence the final product properties. Fibrous sheet or web formation is classified in three areas: dry laid, wet laid and polymer laid (Spun bonded). Dry laid technique is directly related to the conventional spinning process while wet laid technique is related to the paper making industry and polymer laid technique is directly related to the polymer extrusion through spinneret.

17.2 Dry Laid Web

Dry laid web formation is concerned with carding process of spinning. Carding produces one or more webs, in which fibers are preferentially oriented in the machine direction (MD). A multilayer web is produced by using more than one card machines in a single line. The major objective of carding is to disentangle and mix the fibers to form a hom*ogeneous web of uniform mass per unit area. This purpose is achieved by the interaction of fibers with toothed rollers situated throughout the carding machine. The first and the most basic principle of carding is “working” and the second is “stripping”. The whole carding process is essentially a succession of “working” and “stripping” actions linked by incidental actions. Every card has a central cylinder or swift that is normally the largest roller and small rollers (called worker and strippers). Generally small rollers operate in pairs and situated around cylinder and carry out the basic function of working and stripping. Many cards have more than one cylinder with their own small rollers. Arrangement of rollers in a basic carding machine is shown in Fig. 26.

Worker and stripper pairs around the cylinder perform both opening and mixing functions. Doffer cylinders condense and remove the fibers from the cylinder surface in the form of a continuous web. The points of teeth on a worker roller directly oppose the points of cylinder teeth in a point-to-point relationship. The worker revolves in the opposite direction to that of the cylinder. The teeth on worker and cylinder travel in the same lateral direction at their point of interaction, causing a “working” action between the worker and cylinder. Then a stripping action between the stripper and the worker, followed by a further stripping action between the cylinder and the stripper. After removal of the web from the first card machine, it is allowed to pass under the second card machine through a conveyor, to achieve the required thickness of web. Batt drafting is done to increase the fibers orientation in the machine direction.

17.3 Wet Laid Web

The technology of wet-laid nonwovens is closely related to that of paper and papermaking which itself goes back some 2000 years, developed in China. But wet-laid nonwovens are differentiated from paper manufacturing and regarded as nonwoven: if more than 50 % by mass of its fibrous content is made up of fibers (excluding chemically digested vegetable fibers) with a length to diameter ratio of greater than 300; or more than 30 % by mass of its fibrous content is made up of fibers (excluding chemically digested vegetable fibers) with a length to diameter ratio greater than 300 and its density is less than 0.40 g / cm³.

A dilute slurry of water and fibers is deposited on a moving wire screen and drained to form a web as shown in Fig. 27. The web is further dewatered, consolidated, by pressing between rollers, and dried. Impregnation with binders is often included in a later stage of the process. Wet laid web-forming allows a wide range of fiber orientations ranging from near random to near parallel. The strength of the random oriented web is rather similar in all directions in the plane of the fabric. A wide range of natural, mineral, synthetic and man-made fibers of varying lengths can be used.

17.4 Polymer Laid Web

Polymer-laid, spun laid or “Spun melt” nonwoven fabrics are produced by extrusion spinning processes, in which filaments are directly collected to form a web instead of being formed into tows or yarns as in conventional spinning. As these processes eliminate intermediate steps, they provide opportunities for increasing production and cost reductions. In fact, melt spinning is one of the most cost efficient methods of producing fabrics. Commercially, the two main polymer-laid processes are spun bonding (spun bond) and melt blowing (melt blown). A primary factor in the production of spun bonded fabrics is the control of four simultaneous, integrated operations: filament extrusion, drawing, lay down, and bonding. The basic stages of spun bonded nonwoven fabric include [22]:

Polymer melting → Filtering and extrusion → Drawing → Laydown on forming screen → Bonding → Roll up

The first three operations are directly adapted from conventional man-made filament extrusion and constitute the spun or web formation phase of the process, while the last operation is the web consolidation or bond phase of the process, hence the generic term spun bond. All spun bond manufacturing processes have two aspects in common:

1. They all begin with a polymer resin and end with a finished fabric.

2. All spun bond fabrics are made on an integrated and continuous production line.

Melt blowing is a process in which, usually, a thermoplastic fiber forming polymer is extruded through a linear die containing several hundred small orifices. Convergent streams of hot air (exiting from the top and bottom sides of the die nosepiece) rapidly attenuate the extruded polymer streams to form extremely fine diameter fibers (1–5 μm). The attenuated fibers are subsequently blown by high-velocity air onto a collector conveyor, thus forming a fine fibered self-bonded nonwoven melt blown web as shown in Fig. 28.

In general, high molecular weight and broad molecular weight distribution polymers such as polypropylene, polyester and polyamide can be processed by spun bonding to produce uniform webs. Medium melt viscosity polymers, commonly used for production of fibers by melt spinning, are also used. In contrast, low molecular weight and relatively narrow molecular weight distribution polymers are preferred for melt blowing. In the past decade, the use of polyolefin, especially polypropylene, has dominated the production of melt blown and spun bonded nonwovens. One of the main reasons for the growing use of polyolefin in polymer-laid nonwovens is that the raw materials are relatively inexpensive and available throughout the world.

18.1 Thermal Bonding

Thermal bonding requires a thermoplastic component to be present in the form of a fiber, powder or as a sheath as part of a bicomponent fiber. The heat is applied until the thermoplastic component becomes viscous or melts. The polymer flows by surface tension and capillary action to fiber-to-fiber crossover points where bonding regions are formed. These bonding regions are fixed by subsequent cooling. No chemical reaction takes place between the binder and the base fiber at the bonding sites. Binder melt and flow into and around fiber crossover points, and into the surface crevices of fibers in the vicinity, and adhesive or mechanical bond is formed by subsequent cooling. Thermal bonded products are relatively soft and bulky depending upon the fibers composition. Thermal bonding process is economical, environment friendly and 100 % recycling of fibers components can be achieved. In thermal bonding technique generally hot calendar rollers are used to bind the fibrous sheet. Thermal bonding has further sub categories: point, area, infra-red, ultra-sonic and through air bonding.

18.2 Chemical Bonding

In chemical bonding, different types of chemicals or binders – rubber (latex), synthetic rubber, copolymers, acrylics, vinyl esters, styrene and different natural resins are sprayed on the nonwoven web for bonding purpose. Latex binder is most commonly used for nonwoven web bonding. During chemical bonding process chemicals are sprayed on the nonwoven web or web is allowed to pass through the chemical box. In chemical bonding different techniques are used for the web bonding. Most frequently used chemical bonding processes are spray adhesives, print bonding, saturation adhesives, discontinuous bonding and application of powders.

18.3 Mechanical Bonding

In mechanical bonding process, fibrous sheet or web is bonded together through the application of liquid or air jets, punching needles and by stitching. Depending upon the selection of any type of mechanical media, nonwovens are classified as hydro entanglement, needle punching and stitch-bonded fabrics. In hydro entanglement techniques, fibrous sheet is allowed to pass under the liquid jets provided by multiple nozzles. Through the jet pressure web is fused, consolidated and provide strength to the sheet as shown in Fig. 29. The major disadvantage of this technique is the drying of sheet after consolidation. Hydro entangled nonwoven fabrics are used in wipes and medical nonwoven industry because of their additive free, lint free, soft, strong, and cost effective characteristics.

In needle punching method, fibrous web is allowed to pass under a bar containing multiple needles. These needles pass in through the thickness direction of web and entangle the fibers to give strength to the fibrous sheet. Schematic representation of a needle punching is shown if Fig. 30. Needle punched nonwovens are used in automotive, construction, home furnishing industries, geotextiles, shoe felts, blankets, filters and insulators.

Stitch bonded nonwoven fabrics are produced by stitching the fibrous web or sheet with other fibers or yarns. The performance properties of stitch bonded nonwoven fabric depends upon the stitching yarn type, stitch density, stitch length, stitching yarn tension and machine gauge. Stitched bonded fabrics may be of one side stitched, two sides stitched or one side stitched with the projection of pile on the other side of the fabric. In order to get flexibility in the fabric Lycra yarn is used and for higher strength fabric, high performance yarns are used for stitching purpose. Commercially, two stitch bonding systems: Maliwatt and Malivlies are available. Stitch bonded fabrics are used to produce vacuum bags, geotextiles, filters, and interlining, the biggest market is shaped by home furnishing industry.