The Modern Era of Technical Textiles

Finishing Of Technical Textiles

The name textile finishing covers an extremely wide range of activities, which are performed on textiles before they reach the final customer. They may be temporary, for example the way bed sheets are pressed before packing, or they may be permanent, as in the case of a flame-retardant tenting fabric. However, all finishing processes are designed to increase the attractiveness or serviceability of the textile product. This could involve such techniques as putting a glaze on an upholstery fabric, which gives it a more attractive appearance, or the production of water repellent finishes, which improve the in-service performance of a tenting fabric.

Thus a further aim of textile finishing may be described as improvement in customer satisfaction, which finishing can bring about. This improvement in the perceived value of a product to the consumer forms the basis of modern ideas on product marketing. Technical textiles are defined as those materials with non- clothing applications. Thus the fashion aspects of textiles will be ignored, although aesthetic aspects of say upholstery and drapes will be covered.

Finishing Process

The finishing processes that are available can be divided into four main groups, which are:

Mechanical processes

These involve the passage of the material through machines whose mechanical action achieves the desired effects. A heating process, the purpose of which is usually to enhance these desired effects, frequently accompanies this.

• Calendering: compression of the fabric between two heavy ro rolls to give a flattened, smooth appearance to the surface of the fabric.
• Raising: plucking the fibres from a woven or knitted fabric to give a nap effect on the surface.
• Cropping: cutting the surface hairs from the a fabric to give a smooth appearance, often used on woollen goods where the removal of surface hair by a singeing process is not possible.
• Compressive shrinkage: the mechanical shrinking of the warp fibres in woven fabrics so that shrinkage on washing is reduced to the desired level.

Heat setting

This is a process for the stabilisation of synthetic fibres so that they do not shrink on heating.

Chemical processes

These may be described as those processes that involve the application of chemicals to the fabric. The chemicals may perform various functions such as water repellency or flame retardancy, or may be used to modify the handle of a fabric. Chemical finishes are normally applied in the form of an aqueous solution or emulsion and may be applied via a variety of techniques, the main one being the pad mangle, which is illustrated in Fig. 2.1.

After the padding or the application stage of the chemical finishing the fabric is usually dried to remove the water from the fabric and some form of fixation of the finish is then performed. This commonly takes the form of a baking process, where the fabric is subjected to a high temperature for a short period, which enables the applied chemicals to form a more durable finish on the fabric than would otherwise be achieved.

Surface coating

This is a most important part of the finishing of technical textiles.

Mechanical Finishes

Calendering

Calendering may be defined as the modification of the surface of a fabric by the action of heat and pressure. The finish is obtained by passing the fabric between heated rotating rollers (or bowls as they are frequently called), when both speed of rotation and pressure applied are variable. The surfaces of the rollers can be either smooth or engraved to provide the appropriate finish to the fabric, while the actual construction of the rollers may be varied from hardened chromium-plated steel to elastic thermoplastic rollers (Fig -2.1)

Effects which may be achieved by calendering

Calendering is done for many purposes but the main effects are:

• smoothing the surface of the fabric
• increasing the fabric lustre
• closing the threads of a woven fabric
• decreasing the air permeability
• increasing the fabric opacity
• flattening slubs
• obtaining silk-like to high gloss finishes
• surface patterning by embossing

Types of Textile Calenders

In general calendars usually have between two and seven rollers, with the most common being the three-bowl calender. Perhaps the most important factor in calender design is the composition of the rollers and the surface characteristics of these. Textile calenders are made up from alternate hard steel and elastic bowls. The      elastic bowls are made from either compressed paper or compressed cotton, however, a lot of modern calenders are made with a thermoplastic thick covering, which is usually nylon. The latter have the advantage that they are less liable to damage from knots, seams and creases than cotton and paper bowls, damage that would then mark off onto the calendered fabric.

In two-bowl calenders (Fig. 2.2) it is normal to have the steel bowl on top so that the operator can see any finish. This type of arrangement is often used with the nylon bottom bowl mentioned previously, especially where the calender is used for glazing or the embossed type of finishes. The arrangement where two steel bowls are used together only occurs in exceptional circumstances, for example, in the compaction of nonwovens. Here both bowls are usually oil heated so that some form of permanent setting occurs. Finally, the arrangement with two elastic bowls is not common but is sometimes used on cotton knitgoods to obtain a soft handle.

The three-bowl calender (Fig 2.3) was developed from the two-bowl calendar and with this type of calender it is normal to use only the top nip, with the bowls arranged steel–elastic–steel.The bottom bowl is used to keep the central elastic bowl smooth and thus assist in the finishing.The same arrangement also serves the same purpose on embossing calenders, where there is the possibility of permanent indentation from the top roller.

Pressure used in all of the above calenders can be varied between 10 and 40 tonnes, with running speeds up to 60mmin-1. However, these are very much average figures with figures as low as 6 tonnes for a 1m wide calender to as high as 120 tonnes for a 3m wide calender. In addition, running speeds of 20mmin-1 are used on an embossing calender, while on a glazing calender speeds of over 150mmin-1 have been quoted.

 

 

 

The temperatures which are used in calender rollers can, of course, vary from room temperature to 250°C.However, it must be stressed that temperature control is of vital importance, with a tolerance of ±2 °C being commonly quoted. Some generalizations can be made as follows:

• Cold bowls give a soft handle without much lustre; warm bowls (40–80 °C) give a slight
• Hot bowls (150–250 °C) give greatly improved lustre, which can be further improved by the action of friction and

Types of finish

• Swissing or normal gloss: a cold calender produces a smooth flat However, if the steel bowl of the calender is heated then in addition to smoothness the calender produces a lustrous surface. If a seven-bowl multipurpose calendar is used then a smooth fabric with surface gloss on both sides is produced.
• Chintz or glazing: this gives the highly polished surface which is associated with glazed chintz. The effect is obtained by heating the top bowl on a three- bowl calender and rotating this at a greater speed than that of the fabric. The speed of this top bowl can vary between 0 and 300% of the speed of the fabric. In certain cases where a very high gloss is required, the fabric is often pre impregnated with a wax emulsion, which further enhances the polished effect. This type of calendering is often called friction
• Embossing: in this process the heated top bowl of a two-bowl calender is engraved with an appropriate pattern which is then transferred to the fabric passing through the bowls. The effect can be made permanent by the use of thermoplastic fibres or in the case of cellulosics by the use of an appropriate crosslinking
• Schreiner or silk finishing: this is a silk-like finish on one side of the It is produced by embossing the surface of the fabric with a series of fine lines on the surface of the bowls (Fig 2.4 ). These lines are usually at an angle of about 30° to the warp threads. The effect can be made permanent by the use of thermoplastic fabric or, in the case of cotton, by the use of a resin finish. This finish is particularly popular on curtains and drapes because of the silk-like appearance this type of finish gives to the product.

Raising

Raising is the technique whereby a surface effect is produced on the fabric that gives the fabric a brushed or napped appearance. The way this was done originally was to use the seedpod of the thistle, which was known as a teasel. These teasels were nailed to a wooden board and the fabric was drawn over them to produce a fabric with a hairy surface, which had improved insulating properties. This method has largely been superseded by the use of rotating wire brushes, although where a very gentle raising action is required, such as in the case of mohair shawls, teasels are still used.

Modern raising machines

All modern raising machines use a hooked or bent steel wire to tease the fibres from the surface of fabric. The most important factor in the raising operation is the relationship between the point and the relative speed of the cloth. The raising wires or ‘card’ wires are mounted on a flexible base, which is then wrapped around a series of rollers, which are themselves mounted on a large cylindrical frame.

The raising action is brought about by the fabric passing over the rotating rollers and the wire points teasing out the individual fibres in the yarn. Because there are a large number of points acting on the fabric at any one time, the individual fibres must be sufficiently free to be raised from the fibre surface. This is a combination of the intrafibre friction and the degree of twist in the raised yarns. Thus for ‘ideal’ raising, the yarns should be of low twist and be lubricated. One further point to note is that because the fabric runs in the warp direction over the machine, only the weft threads are at right angles to the rotating raising wires and therefore only the weft threads take part in the raising process.

Raising action

From Fig. 2.5 it can be seen that both the card wire rollers and the cylinder to which these are attached may be rotated; it is the relative speed of these in relation to that of the fabric that produces the various raising effects that may be achieved. There are two basic actions in raising and these are governed by the direction in which the card wires point and the relative speed of rotation of these in relation to the fabric. These actions are called the pile and the counterpile actions. In the counterpile action, the working roller rotates in the opposite direction to that of the cylinder with the points of the wire moving in the direction of rotation. This action pulls the individual fibres free from the surface. In the pile action, the points of the wire are pointing away from the direction of movement of the fabric. This results in an action where the raised fibres are subject to a combing action which tends to tuck back the fibres into the body of the fabric.

The most common raising action uses a combination of both of these actions, producing an even raise over the whole of the fabric surface. Control of the raising action has been achieved by measurement of the surface roughness of the raised fabric. It is therefore possible to control the exact height of the nap on the surface of  fabrics.

 

 

 

 

In order to replace the warp crimp and thus minimise warp shrinkage, a process known as compressive shrinkage is carried out on the fabric to replace the crimp that has been pulled out in the preparation and finishing processes. This may be illustrated in the following way. A strip of fabric is placed on a convex rubber surface and gripped at each end of the rubber. As the rubber is allowed to straighten, the length of the fabric exceeds that of the rubber. However, if the fabric could be stuck to the surface of the rubber then the fabric would be subjected to compression and warp crimp would be introduced.

This principle then is applied to the compressive shrinking machine, where the cloth is fed in a plastic state onto a thick rubber belt at point A as shown in Fig.2.6 .While the belt is in the convex position for A to B the fabric merely lies on the surface, but at point B the belt starts to wrap its way round a large  heated cylinder and thus changes from a convex to a concave shape.

Thus the surface of the rubber belt contracts and the fabric, which is held onto the surface of the rubber, is subject to a warp compression over the region C to D. The actual degree of shrinkage which takes place is controlled by the amount of fabric fed onto the rubber belt and the pressure between the heated metal cylinder and the belt, which increases or decreases the concave shape of the rubber belt. The principles of compressive shrinkage have also been reviewed.

Heat setting

The main aim of the heat setting process is to ensure that fabrics do not alter their dimensions during use. This is particularly important for uses such as timing and driving belts, where stretching of the belt could cause serious problems. It is important to examine the causes of this loss in stability so that a full understanding can be obtained of the effects that heat and mechanical forces have on the stability of fabrics. All fabrics have constraints placed on them by their construction and method of manufacture, but it is the heat-setting mechanism that occurs within the fibre that will ultimately influence fabric dimensions.

Heat-setting mechanisms

The first attempt to describe the various mechanisms of heat setting synthetic fibres was that of Hearle. He describes the various techniques which have been used to set fabrics into a given configuration and leaving aside the chemical methods of stabilisation, these techniques may be described as influencing the following:

1. chain stiffness
2. strong dipole links
3. hydrogen bonds

Hydrogen bonding is the most important of the factors which influence setting, and nylon has a strong hydrogen-bonded structure whereas polyester has not. Thus relaxation of nylon can occur in the presence of water at its boiling point. In fact one of the common tests for the nylon fabrics used in timing belts is a 5min boil in water.

Fibre structure

All fibre-forming molecules consist of long chains of molecules. In fact, a typical nylon molecule will have a length which is some 5000 times the molecule diameter. X-ray diffraction techniques have confirmed that all synthetic fibres contain crystalline and non-crystalline regions. In nylon and polyester these crystalline regions occupy about 50% of the total space in the fibre.

Polymer orientation

During the processing of both polyester and nylon, the fibres are spun through fine holes and have a structure similar. However, to develop the strength in the fibre these fibres are then cold drawn to create an orientated structure. Once the chains have been orientated then the fibres show a much greater resistance to applied loads and a greater stiffness, hence the reason for cold drawing.

Transition temperatures

In the previous section the crystalline and amorphous regions of the polymer were discussed. These do have an effect on two important parameters:

• Glass transition temperature: this represents the temperature at which molecular movement starts in the amorphous regions of the polymer, and was given the name because it is the temperature at which the polymer changes from a glassy solid to a rubbery solid. This is the temperature at which segmental loosening takes place and hence dyeing can only take place above this
• Melting point: at this point the forces holding the molecules in the crystalline regions of the fibre are overcome by the thermal energy and the polymer melts. In both polyester and nylon these temperatures are separated by about

Heat shrinkage

All textile fibres are subjected to a cold drawing process and hence when they are heated above the point at which molecular motion sets in, they will progressively shrink until they reach the point of thermodynamic equilibrium. In other words the cold drawing process is reversed by the application of heat.

Essentials of heat setting

From the previous discussion it can be seen that heat setting is a temperature dependent process and for practical purposes the heat setting temperatures vary for polyester between 190–200 °C and for nylon 6.6 between 210–220°C.The fibres must not be allowed to move during the heating process and the heating must be sufficiently long enough to allow crystallisation to take place, after which the fibre must be cooled down to well below the heat setting temperature before being released.

There is an important difference between the behaviour of the two common polyamides (nylon 6 and nylon 6.6) and polyester, because of their different behavior towards water. Polyester is non-absorbent, so the heat setting behaviour is not affected by water. However, nylon will absorb sufficient water to obtain a temporary set that is based on hydrogen bonding and is destroyed on boiling in water. The consequence of this is that to obtain a permanent set on nylon, the water has to be removed from the fibre so that crystallisation can take place. Therefore nylon tends to be heat set at a higher temperature than polyester.

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