Finishing processes in manufacturing

Module 5: Finishing Processes
5.1    Introduction
Similar to woven and knitted fabrics, nonwoven fabrics also undergo finishing processes in order to develop certain desirable properties. The desired properties are developed primarily looking upon the intended end-use of the produced fabric. This is said to be a procedure which adds maximum value to a semi-manufactured fabric, so it plays a vital role in the nonwoven industry. This is the last step in nonwoven fabric manufacturing technology where the final quality is established. As in woven and knitted fabrics, the term finishing in nonwoven technology is not restricted to chemical processing alone since any operation for improving the appearance or usefulness of the fabric after it leaves from bonding machines can be considered as a finishing process. As suggested by Tomasino [1], finish can be either chemicals that change the aesthetic or physical properties of the fabrics or make changes in texture or surface characteristics by physically manipulating the fabrics with mechanical devices. It can also be a combination of both. Finishing can be categorised broadly into two types as mechanical finishing and chemical finishing. Mechanical finishes are those finishes in which the fabric develops certain properties by physically manipulating the fabric with mechanical devices. By this, special changes in surface and handle properties are achieved. On the other hand, chemical finishes are those finishes which rely on the application of certain chemicals to the fabric by different methods of application.
5.2    Mechanical finishing
5.2.1    Splitting and winding
This process is used to produce nonwoven fabrics of high density, that is, high basis weight and low thickness. On the basis of economical aspects it is advantageous to produce fabric of high basis weight and then split it into required thickness. This process of separating one high basis weight and high thickness fabric into two fabrics of high basis weight and lower thickness is known as splitting. The two layers are splitted by different roller outfeed configuration and a rotating hoop knife placed between two rollers. Heavily bonded needle punched fabrics and chemically bonded fabrics can also be splitted by mechanical adjustment of the rollers responsible for splitting. This process is generally used to produce thin, supple, leather-like nonwoven fabrics that can be used as belts, shoe linings, outer materials for bags, etc. [2].
5.2.2    Perforating
This process of finishing is used to enhance the wicking property as well as softness and drape of fabrics. Perforation in nonwoven fabric can be of varying profile achieved by the help of heated needles or modified calendar rollers. For some exceptional cases like chemically bonded nonwovens, this process enhances fabric strength instead of reducing it. In this case, the penetration of hot needles promote cross-locking of the resin bonding agents.
5.2.3    Drying
During the process of manufacturing the nonwoven fabrics are subjected to tension, which cause the fabrics to distort dimensionally. That is the fabrics get stretched in length and shrunk in width. This phenomenon is more pronounced for the case of hot and wet webs. The dimensional instability of the webs sometime causes shrinkage at later stage. To overcome this problem faced by nonwoven fabrics, the process of drying has been introduced. It is also occasionally used for coagulation of thermo sensitive binders and sintering of binder powders in the fabrics. The process of drying varies in many ways based on the type of fabric being dried and also type of application for which the fabric is supposed to be used. The drying process can be realized in stenter frames, fusion ovens, can drying, drum drying, hot flue dryers, infra-red heaters, etc. In stenter frames the fabric is clipped at the edges with the frame in order to make it open width wise. This frame feeds the fabric continuously to the drying chamber. The width dimensions set can be controlled by the adjustment of rail or chain carrying the fabric. To allow length shrinkage fabric is overfed to the stenter pins. Stenters are equipped with sensors to control uniform air flow, moisture and temperature. Through air drum drying and heat setting is one of the most adopted methods for the process of drying. It can be carried in varying arrangements like single drum, twin drum or multiple drum configurations. The arrangement of the drums can be vertical or horizontal in succession. The hot air is regulated through the fabric by high capacity radial fan which regulates the hot air in and out the drum with the help of some heating elements (which are responsible for heating the air). The fabric is overfed to the drum in order to allow relaxation and shrinkage to take place. This process helps producing fabric with bulk and softness. The process is known offer have high heat and mass transfer through the fabric, making it more efficient with minimum energy consumption.
5.2.4    Compressive finishing
Some of nonwoven fabrics have papery like feel, hence lacking in handle and drape properties. Such properties when demanded by the consumers are deliberately introduced in the fabric to give the material more volume and softness. This process is carried out by the mechanical finish process known as compressive finish. The Micrex and the Clupak processes are the two processes which have been largely adopted by the nonwoven industry. The general principle of this process is to pass pre-wetted fabric nonwoven fabric around a heated cylinder by the help of belt or blanket. The Micrex process is illustrated in Figure 5.1. has been briefly discussed. The fabric is fed to the roller shown in figure which has got a cavity zone, where the process of compaction is carried out. There is a retarder blade which is etched with grooves to provide zones in which fabric can be compacted. The pressure plate acts as an aid to create proper contact of the fabric with the main roller and let it fall in the cavity. But here the fabric is processed in dry state and not pre-wet unlike Clupakprocess. The temperature of treatment is therefore kept relatively lower than the process of Clupak. This process provides fabric with a 3 dimensional crepe texture enhancing softness, drape, bulk and extensibility. The degree of compaction achieved can be calculated from the following formula: compaction = change in length of fabric / original length of fabric. It can be noted here that stretch is related to compression and it can be calculated from the following formula: stretch=change in length of fabric / modified length of fabric.
5.2.5    Calendering
The process of calendering can be said to be more severe form of compaction process. The purpose of calendaring is to make the fabric compact with decreased thickness and smooth surface. Calendering can be both hot and cold type. Calendering is carried out by feeding the fabric between the nip of two rollers known as calender rollers, which vary in their design. Basically there are two types of calendar roller arrangement i.e. L type and I type. In I type arrangement the rollers are placed one above the other vertically in line whereas in L type arrangement the bottom roller is placed slightly forward. The uniform distribution of nip line pressure across the width of fabric being fed is of prime importance. The rollers are designed keeping this factor in mind. There are rollers having bulge surface i.e. diameter in the middle is greater than the diameter at the edges. The rollers are also covered with resilient material for proper pressure distribution. Hot calendering is carried out by hot rollers which are heated by the help of heated oil. Belt calendering is another modified form of calendering which gives fabric less stiff in nature. In this method the nonwoven fabric is pressed against a heated drum with the aid of a tensioned belt or blanket and hence the pressure is applied. The pressure applied is slower than the conventional calendering but the contact time of the fabric with heated element (drum) is greater than the conventional calendering.
5.2.6    Singeing
The process of singeing helps in removing protruding fibres from the surface of the fabric. Singeing of fabric is normally done when smooth and clean (not hairy) surface is required like in printing or coating. The method of singeing is basically categorised as direct and indirect singeing. Direct singeing involves the exposure of fabric moving with a high speed over gas burners flames. In indirect singeing the fabric is passed over panels heated by burners. The later method is found to be more advantageous as it provides uniform singeing with lower risk of fabric damage. The process of singeing can further be varied by its process parameter like flame height and intensity, flame distance from the fabric, fabric speed and singeing angle. Singeing is followed by rapid cooling and washing in order t prevent after burn treatment and remove singeing residues.
5.2.7    Shearing
Shearing is a process of removing surface fibres from the fabric by the help of cropping or cutting. Shearing is an alternative to singeing. Unlike singeing, only partial surface fibre removal is achieved as the variation in fabric surface topography determines the cutting height. Figure 5.2 shows a shearing machine. The shearer head consists of a spiral blade revolving on its own axis in contact with a ledger blade. This creates a shearing action similar to that produced by a pair of scissors. When fibres are presented to this cutting head, they will contact the ledger blade and be cut off by the rotating blade. The fabric travels over a cloth rest in front of the ledger blade and the design is such that an acute angle is formed by the fabric. This sharp angle causes the pile to stand erect and be more easily cut. The distance between the bed and the ledger blade is adjustable so the height of the pile can be regulated.
5.2.8    Flocking
Flocking is a process of producing three dimensional piles on the surface of a nonwoven backing fabric. In order to adhere flock to the base fabric, the base fabric is pre-treated with an adhesive resin. The resin is applied either to the whole area or to predefined areas to produce patterns. Flocking can be done mechanically by shaking or sprinkling. The process can also be done electrostatically where fibres are lined up vertically in an electrostatic field as they land on the backing fabric. Electrostatic flocking is adopted for high quality velvet like finish as it is more uniform, whereas mechanical flocking produces a randomised pile on the fabric surface. Flocking can be also be carried out over the whole surface of the fabric or decoratively in a pattern form, according to the application of the adhesive.
5.2.9    Raising
Raising is a process to create fibrous pile structures on the surface of a pre-formed fabric. The fabric is passed tangentially over a series of small rotating cylinder arranged around a large rotating drum. These small cylinders consist of fillet raising wire. The wire raises fibres from the surface of the fabric to produce dense collection of pile. The process can be varied by various process parameters like varying cylinder configuration, speed, direction and wire type. Raising increases the bulk and improves the look and feel of nonwovens.
5.2.10  Sueding
Sueding is another process, which is very similar to the process of raising. But unlike raising, fabric surface is abraded by emerising fillet and not the raising wire. This process produces dense pile with good softness and subdued appearance. Sometimes a sueder is referred to as a sander since it consists of one or more rolls covered with sand paper as the abrasive. Fabrics travelling over these rollers develop a very low pile and the material’s surface can be made to feel like suede leather. There are two basic categories of sueders, multi-cylinder and single cylinder machines. The multi-cylinder machine usually has five rotating cylinders, each independently driven and they can be rotated clockwise and counter clockwise. The single cylinder sueder has one abrasive covered metallic roller and one rubber covered pressure roller. To keep the abrasive covered cylinder from expanding from the heat generated from friction, water is circulated through the cylinder interior to keep it cool. The pressure roller presses the fabric against the abrasive cylinder and is micrometer adjustable. The abrasion of the fibres on the surface of the fabric takes place in the nip between the pressure roller and the abrasive cylinder.
5.2.11  Polishing
This process helps in improving the surface lustre in fabrics with a pile surface. The fabric carried by a blanket to bring in contact with a heated drum which is etched with deep spiralled grooves. Surface fibres are reoriented in one preferred direction thus increasing the lustre. To stabilise the finish, reducing agents are used occasionally and for the case of natural hygroscopic fibres the process is carried in wet condition. Figure 5.3 shows the schematic diagram of a polisher machine. It consists of a fluted heated cylinder driven by a variable speed motor and an endless felt blanket. The fabric passes over the endless blanket which is adjustable and brings the fabric face in contact with the heated cylinder. The serrations on the cylinder draw through the fibres to raise and parallelize them. Heat facilitates the straightening process and sets the fibres. Polished fabric appears more lustrous because the parallel fibres result in more uniform light reflection.
5.2.12  Softening
This process aims at enhancing bulk, softness and drapability of fabric by reducing its stiffness. The fabric is transported pneumatically, impacting against baffle plates to fall at the bottom of the machine and relax over there. In order to increase fabric softness further they are also sometimes treated with enzymes and chemicals. Otherwise, occasionally low pressure hydroentanglement is also carried after manufacture of fabric by thermal or chemical bonding.
5.2.13  Heat setting
Heat setting is carried out to achieve the dimensional stability of nonwoven fabric containing thermoplastic fibres. Usually the fibres undergo various kinds of stress while manufacturing and as a result, any subsequent heat treatment produces stress relaxation that show up as shrinkage or change of dimensions. The heat setting process is carried out to achieve a desired stress-free form of the fibres making up the nonwoven fabric. The pin stenter is an ideal machine for heat setting of fabric if it provides a complete control of width and length of the fabric. It is important to set the desired heat setting temperature for the fabric. Generally, the recommended heat setting temperatures for nylon is 200-220ºC, polyester 150ºC, and acrylics 140ºC. Once the heat setting temperature is reached, however, the time required is very short. Cooling before removing fabric from the pins is essential. The reduction in stress produced by setting can lead to major changes in fabric handle. However, if the same fabric were to be heat set under tension these changes would be much less noticeable, and fabric handle may even be stiffer because of fibre adhesions. As an economic consideration, the maximum safe running speed can be attained by continuous monitoring of fabric temperature. Nowadays automatic microprocessor control of speed is well established which ensure quality with economy.
5.3    Chemical finishing
5.3.1    Types of chemical finishes
Antistatic agent
These finishing agent helps to prevent the static charge build up specially on synthetic fibres. It can be of two form one which increases the surface conductivity of fabric by addition of hydrophilic compounds to the surface and other generates charge opposite to the one present in the fabric.
Antimicrobial finishes
These types of finishes are used to prevent the biological degradation of fabric by bacterial or fungal growth. This fabric is specially applied to bedding components, sportswear, healthcare products, etc. This finishes also help to reduce odour emission caused by biological degradation. Mostly these finishes comprise of metallic compounds containing silver or natural biopolymers like chitosan.
Lubricants
These are mostly used to reduce friction, whether it is fibre to fbre or fibre to metal. They also help imparting softness to certain fabrics.
Flame proof finishes
Flame proof finishes helps to reduce flame propagation and suppresses smoke emission.  They are mainly made of nitrogen phosphorous compound. Many of these finishes are available in fibre form to be blended in the fabric during the process of manufacturing. They are also present in polymer dispersion form to be applied through coating. The disadvantage of applying flameproof finish is that it leads to yellowing of the fabric, decreased tensile strength, and colour change.
Water proof finishes
This type of finish is used to inhibit wetting of fabrics. They mainly comprise of silicone or fluorocarbon compounds. Conventionally wax finishes were applied which used to reduce the breathability of fabrics. The fluorocarbon compounds also provide repellency to oil diesel and gasoline to synthetic fibres. They can be applied through spraying, padding in aqueous dispersion form.
Softeners
They provide softness to the fabric and are mostly applied in fabric for which softness is of prime importance like sanitary coverstocks, wipes and other skin contact applications. Softeners which are hydrophilic in nature also increase the wettability of the fabric. 
Stiffeners
Stiffeners are the finishes which help to add stiffness to the fabric. They are introduced to increase abrasion resistance by welding of adjacent fibres. They also provide dimensional stability through welding of adjacent fibres in the fabric. They are applied in the form of polymer dispersion through padding, spraying or knife coating. They are of self-cross linking
UV stabilizers
UV stabilizers are used to protect the adhesives and polymers used as binders in the fabric from being degraded by harmful ultra violet light. The hindered amine stabilisers form complex reaction protecting the polymers from chemical break down.
5.3.2    Methods of chemical finishes
There are various methods by which the chemical finishes mentioned in section 5.3.1 (and some others also) are applied to the nonwoven fabric. These methods have been discussed briefly in the following sections.
5.3.2.1 Dyeing
Dyeing of nonwoven fabrics is carried by dyes and pigments like any woven and knitted fabrics. Dyeing of nonwoven fabric is carried out in open width form to avoid crease formation. There are principally three methods of colouration which have been discussed briefly in this section.
Dope dyeing is one of the procedures where the dye or pigments are added to the molten polymer before melt extrusion. This method provides excellent colour fastness but the colour has to be achieved at a very early stage of fabric production.
Pigments are applied to the fabric by finely grounding it in powder form and making a dispersion of it. This helps in achieving uniform colouration and minimum variation in shade. The pigments are fixed to the fabric by the help of fixation done by binders which are later removed by drying and curing. They have good fastness to perspiration but poor fastness to rubbing. The most commonly used is the common dyestuff colouration. This is done either by batch or by continuous process. Dyeing of nonwoven fabric is easier as the more fibre surface is available for dye stuff to get attached due to the open structure of nonwoven fabrics. The dye type varies based on the type of fibre the fabric is made up of i.e. natural or synthetic.
Continuous dyeing of fabrics is carried in beam form and in jig also. Dyeing in beam form is done by spreading the fabric width wise over a beam and passing this through a chamber kept at high temperature and pressure. This also assists in increasing dimensional stability of fabric through thermal stabilisation. In jig dyeing, the fabric moves through the dye liquor as it passes between one roll and a second roll and reverse, when the receiving roll has filled its fabric capacity. Dyeing of nonwoven fabric can also be done by padding which is again of two types, cold pad batch dyeing and continuous pad steam dyeing. In cold pad batch process the fabric is immersed in dye liquor and then the excess liquor is squeezed off. After this the fabric is batched on a roll to allow dyestuff fixation. Sometimes the fabric is also heated for a predetermined time period based on the dyestuff type. Later the fabric is washed to remove unfixed dye. In continuous pad – steam dyeing the nonwoven fabric is first padded and then exposed to steam or subjected to thermo fixation. Then after the fabric is washed and dried.
5.3.2.2 Printing
Printing of nonwoven fabric is mostly carried out for its application in home furnishing area. Printing is done by pigments being applied in varying form. The various techniques of printing are rotary and flat screen printing, discharge printing, transfer printing and ink jet printing. Pigments are applied after combining suitable binder, thickener, cross linking catalysts for the softeners and binders. Printing is followed by drying and curing to remove the binders and fix the pigments to the fabric.  
5.3.2.3 Padding
In this method of finishing, finish is impregnated with treatment liquor or foam by the help of squeeze rollers. The squeezing of fabric is done to an extent where required pick-up or add-on is achieved.  The level of pick-up is predetermined based on the application and fibre type of the fabric. Padding is followed by drying in order to remove excess of water present in the liquor prepared. So the process has to be regulated to have minimum energy loss for drying and minimum chemical migration during the process of drying. Padding can be of two types wet on wet padding and wet on dry padding. In wet on wet padding the fabric to be padded is pre wet. This process must take care of sufficient interchange of treatment liquor with water present in pre wet fabric, in order to have proper chemical add on.  In wet on dry padding the fabric is in dry state and it is impregnated with pad liquor. In this process de-aerating of fabric is required as the fabric gets very less time to expel air from the fabric and interchange it with padding liquor, in order to achieve uniform application of finish. Similar to the process of calendering this process also need the roller designs to be made in a way that they maintain uniform nip line pressure. The formulation for liquor is done by the help of formula given below.

The pick-up can be established by the help of short piece of fabric. The pressure settings have to be noted for a particular pick up achieved. After deciding the treatment level (add-on) pad solution must be prepared.
5.3.2.4 Coating
Coating is the process of application of finishes in the form of solutions or dispersions. They are generally aqueous based. For the coating of nonwoven fabric special attention has to be given to the rate of fabric let-off in order to reduce stretching of fabrics. Coating is followed by drying and curing when aqueous coating is applied to the fabric. Drying can be done by various methods as discussed in the topic “Drying”. A fabric can be applied a single layer coating or multiple number of coatings depending upon the nature of coating required. Single layer coating provide more uniform and proper thickness coating. However, increasing the number of coatings helps in mending up the faults of coating in a fabric like holes or gaps.
Coating is generally applied by the aid of rotating roller (Slop padding or kiss roll) varying in its arrangement depending upon the extent of coating to be done. The slop padding roll is partially immersed in the coating and the fabric is allowed to pass over the roller. The process pick-up percentage can be varied by parameters like speed of slop padding roll, depth of roller penetration in the coating solution and direction of roller rotation. The fabric can be passed both in the direction of roller or against the direction of roller. When the fabric is passed in the direction of roller rotation penetration of coating solution is greater. For the case where fabric is passed against the direction of roller rotation, additional arrangement has to be made. The set up consists of pair of rollers one known as applicator roller and the other as support roller. The fabric is passed into the nip between these two rollers. Nip setting also governs the level of application. The application of coating is regulated by the help of scraper (which removes excess of coating), doctor blade, helically wound wire metering rods, bar profile and others. There is also knife present to evenly distribute the coating throughout the width of the fabric. The design of knife can also be varied to vary the extent of coating application. Figure 5.4 shows the schematic diagram of slop padding (kiss roller) roller.
There are many other methods of coating application. Few to name them are rotogravure coating, rotary screen coating, extrusion coating and non contact coating. Rotogravure coating is carried out to produce patterned coatings with the help of rotogravure rollers. The rollers are mostly in heated state to help fusing of thermoplastics. Rotary screen coating is used for applying hot-sealing coatings to fusible nonwoven interlinings. This process has higher production rate. Here both aqueous and non aqueous coating can be used. One can even go for using powder form of coating chemical if the size of the holes in screen is comparable to the particle size of the powder and if the particle is spherical in shape. This process can also be used for patterned coating by using fine meshed screens. In extrusion coating the coating is done by extrusion of thermoplastics. This type of coated products is often referred as laminate. This is practiced where the fabric acts as an impermeable barrier layers.  Non contacting coating is the method where the coat is applied by spraying it on the fabric without making contact with it. It is specially used for coating nonwoven fabrics which are dimensionally less stable and do not have a smooth surface. Non contacting coating can also be done by vapour deposition with sprayable metals in a high vacuum. This type of method offers lower add-on percentage to the fabric.
5.3.2.5 Lamination
Lamination is the process of joining two or more pre-formed nonwoven fabrics or nonwoven fabrics with films, scrims or any other fabric. For the process of lamination at least one of the fabrics needs to have adhesive property or an external adhesive has to be applied in order to join the fabrics permanently. Lamination of nonwoven fabric is carried out specially to inculcate two dissimilar but advantageous properties of individual fabrics. Few such properties to name are barrier properties, surface properties and dimensional stability. Lamination is of two type wet lamination and dry lamination. The lamination process in which the adhesives are applied in solvent or dispersion form is known as wet lamination. They are applied by spraying, slop padding, printing, etc. Dry lamination is carried out by the help of thermo plastic resins that are placed between two substrates to be joined. They are heated to the level the adhesive reaches their glass transition temperature and joins the two surfaces when in molten state. The process of lamination can also be categorised on the basis of the method it follows. Few types of laminating methods to name are extrusion lamination, flame lamination, flat bed lamination. In flame lamination foam is passed over an open flame. A thin layer of molten polymer is produced on the surface of the foam which helps in combination of the foam with the fabric whilst foam is in molten state. Figure 5.5 displays the schematic diagram of a flame laminator. In flat bed lamination the fabric and the adhesive layer/film are combined together and passed through a heated chamber carrying the assembly by belts or plates. Then they are cooled to solidify and get combined together. This process offers greater time for heating and pressing. They are also suitable for dimensionally less stable structure. In extrusion lamination the thermoplastic polymers are extruded to form thin continuous molten film to adhere fabric substrate on either side of itself.
5.4       Unconventional finishing
5.4.1    Ultrasonic welding
Ultrasonic bonding is the methods where the webs are bonded by the help of high frequency alternating current in the range of 20kHz. The high frequency AC is converted to mechanical vibration of a hammer. This hammer bonds the fibres lying between it and the anvil by the heat produced by the hitting of hammer with a lift of 50-100 micro metre. Normally nonwovens made of thermoplastic fibres are used so that they bond due to melting followed by cooling. This technique is mainly used for manufacture of tea bags, sanitary towels, panty liners, diapers and incontinence products.
5.4.2    Plasma
Plasma is a high energy ionised gas which is used to modify the properties of fibre surface chemically. In this method, the surface of the polymer is partially etched or surface is activated by an increase in surface energy or some material is deposited on the surface of the polymer. This treatment is mostly used to convert fabrics into hydrophilic state from hydrophobic state. This process is considered to be advantageous as it consumes low energy and has no waste causing no disposal problem.
5.4.3    Microencapsulation
Microencapsulation helps in delivery of active compounds in a controlled manner. A small amount of chemical is surrounded by a coating to form a capsule which is later delivered by various mechanisms like wall fracture and diffusion. Generally perfumes, cosmetic lotions, phase change materials (PCMs) and antimicrobial chemicals, are applied to fabrics in the form of microcapsules.
References