15.1.1 Approach to process control

The choice of process condition for the given product is taken by the previous history and forming new norms without affecting quality. The optimum norms may vary from unit to unit and machine to machine. This is for various reasons, such as working condition, type of machine, layout of machine, provision of utilities and variations in the quality of the fabric. Therefore every process house has to carry out their own experiments to identify their own optimum processing levels. Once the processing conditions are standardised, the implementation of these conditions during the normal course of production is carried out by maintaining required documents. However it is important to make regular inspection checks to ensure that the particular process is going on according to the norms fixed.

For quality optimisation it is necessary to know about causes for quality defects. Based on the cause and effect diagrams published by K. Ishikawa (1985) there are four critical causes for quality defects:

With machines or equipment, quality deviations may occur because of inadequate maintenance or servicing. In the textile industry, a breakdown in quality is said to have occurred if, for example, the required washing effect, degree of whiteness or absorbency of the fabrics being finished is not achieved. These are defects in the specification of the system because the expectations of the customer are not fulfilled.

A known problem in wet finishing ranges is the wear of the roller rubber, even if this happens to be outside the influence of the machine supplier. This is a potential source of risk of a reduction in quality. Two questions must be asked about a potential source of risk to quality:

Rubberised rollers are critical components in the textile finishing industry. High temperatures and large amounts of chemicals lead to rapid and uncontrollable wear. It is well known that washing efficiency at boiling temperature is considerably higher than in cold washing water. Washing processes at boiling temperature create unfavourable conditions for rubber rollers. Cold washing would improve the operating conditions but make the textile results worse. This means that the critical component, in this case the first washing unit in a washing process, will have to be modified to suit the ambient conditions. The simplest way to do this would be to avoid using rubber rollers in such situations (Ströhle, 2009).

• Temperature–time programmes,

• running partial or fully automatic processes,

• pH control,

• flow pressure/differential pressure,

• dosing of liquid chemicals.

1. The sensor detects a change in a process variable and produces a proportional output signal.

2. The controller receives the signal, compares it with the desired set value and emits a correction signal proportional to the deviation from the set value.

3. An actuator and servo device – commonly a power-operated valve – which receives the correction signal and mechanically adjusts the process variable.

1. Division of the machine into a number of functional zones mostly based on drive sections of the machine to make controls flexible.

2. Defining various control parameters (e.g. temperature, flow, etc.) for each zone.

3. Calculation of the number of controllers required in order to control each zone and thereby for the whole machine. Analogue inputs and outputs are used to control temperature, flow, pH, etc., whereas digital inputs and outputs are used for level control, drain activation, end switches, etc.

4. Connection of multiplexer to the system in order to achieve a correct communication in two directions between the computer and the controllers.

5. Connection of a standard personal computer or preferably its industrial version that presides over each machine and is connected to the multiplexer via an interface. Generally the computer is situated near the operating panel of the machine.

1. Parameter definition – each control parameter should be properly defined. A series of standard control algorithms should be available. The possibility of relating control parameters should exist. For simple definitions, the computer system should be menu-controlled.

2. Process library – a library of machine processes can be defined with chosen set points for each process parameter. When a process is chosen or changed on the status screen, the machine sets itself to the desired set points. Intervention in the process is possible on the computer and local levels.

3. Status screen – while the machine is running, an on-line status screen should be available. The status screen should display the actual status of the machine, set points, actual values as well as high and low alarms. In addition, the software should offer a number of user-selectable reports and graphs.

1. Bath temperature is measured by a PT 100 element and is controlled by a proportional valve to assure processing at the optimum temperature.

2. Cloth temperature is measured by pyrometer and is controlled to avoid over-drying.

3. Steam pressure may be controlled as an alternative to temperature control in the steamer.

4. Flow of chemicals and water is measured by an inductive flow-meter and is controlled by a proportional valve or through a measuring cylinder dosing system, in order to assure a constant flow, depending on machine production.

5. The pH is measured by a pH sensor and mainly controlled in washing of processes.

6. Washing efficiency is assessed by measuring and controlling conductivity of bath and/or cloth by a conductivity sensor.

7. Density (concentration) control is mostly used in mercerising range to achieve a constant mercerising concentration.

8. Compensator pressure control in order to run the cloth with correct cloth tension.

9. Squeezing unit pressure control, especially for heavy-duty squeezing units to assure reproducible liquor pick-up.

10. Level control in washing units and chemical tanks used for safety interlocks (temperature control), as a safety level and to fill the machine automatically.

11. Speed measurement and control as an indication for the machine efficiency in order to make production-dependent dosing of chemicals and water into the machine.

12. Control of consumption of steam, water, electricity and chemicals to maintain quality and cost-effectiveness.

15.2.1 Flow-meters

These are used to measure the flow rate of a fluid in a pipe, duct or open channel. Flow-meters are of two types – quantity and inferential. While the former measures fluid flow rate directly, the latter measures fluid velocity, and thereby fluid rate. The most commonly used inferential flow-meters are:

Valves are devices used to control the flow of a fluid by placing an obstruction in the fluid's path. There are various types of valves; the selection for a specific function depends on whether it will be used as a stop valve (only ‘off’ or ‘on’ service) or a throttling (regulating) valve. The later again may be of various types namely cocks, ball, globe, gate, diaphragm, pinch, etc. A typical automatic process control valve (which may be globe or diaphragm types) consists of four separate but integrated components – actuator, packing gland, body and valve plug. The sensing element of the valve measures a particular process parameter (flow, temperature, pressure, level, pH) and if the value deviates from the desired value, the gas (air) pressure on the diaphragm in the pressure casing is altered automatically to deflect the diaphragm to move the value stem and plug, thereby altering the flow area in the valve to change the fluid flow rate (Duckworth, 1983).

The logical objective of a process control system is to assure repeatable process quality and efficiency by controlling process variables to predetermined and preset values and to provide management information assuring proper equipment operation. To fulfil these objectives, the system should have the following characteristics (Fulmer, 1983):

The important parameters in textile processing are the quantities of chemicals, pH value, water quantity, temperature and reaction time. For many products, especially knitwear, the fabric tension also plays a decisive role. For reputed textile machine manufacturers it is necessary to accurately measure the parameters and to record these values in order to ensure long term both internal (cost reductions, quality assurance) as well as external (reliability for customers, handling of complaints) process reliability.

The chemical metering system offers the greatest potential for savings. Inductive flow rate meters ensure that expensive additives are dosed with millilitre accuracy on their way into the impregnating bath. Reproducibility can also be increased with the aid of impregnation compartments with low liquor content and a rapid rate of liquor exchange.

Modern databases for recipes or formulas also make the day-to-day work of machine operators easier. Not only the corresponding recipes for all product groups are saved, but also these recipes can be extrapolated for the specific fabric weight resulting reproducibility and quality of the highest order. The water quantity, temperature, fabric tension and pH value remain a key factor in these recipes.

One of the key factors for data management is the sensor systems, which are integrated in the machine. The process can only be started and finished correctly if the produced data are totally reliable. If a temperature sensor indicates an incorrect value then no data management system in the world will be able to save the overall result. Therefore continuous monitoring of the ‘eyes and ears’ of a system is still the best insurance policy for ensuring that, once selected, the parameters are maintained in the long term. Temperature monitoring with an external thermometer (or alternatively a second sensor), regular determination of the pH value via titration or via the Morapex process, checking the flow-meters through capacity gauging and constant checking of bearings and drives, are all important factors for guaranteeing constant fabric tension. Consequently, once modern technology is in place and the monitoring capabilities and process reliability of a state-of-the-art data management system is correctly set up, it is possible to deliver top quality in a cost-optimised framework (Kehry and Uhl, 2010).

15.3.1 Textile finishing processes and their classification

The word ‘finish’ means all the different treatments applied to a fabric to change one or more of the following:

Finishing improves aesthetic aspects or serviceability of the fabric or imparts certain desirable characteristics.

Textile finishes are classified in several ways. According to function, these can be classified into:

Examples of aesthetic finishes are:

Examples of functional finishes are:

Some of the finishes could result in both aesthetic and functional effects. Mercerisation is considered as an aesthetic finish, as it increases lustre. It may also be considered as functional, as it increases strength. Sometimes it is even considered as a pre-treatment process, as it improves dyeability. The classification of finishes and class-wise discussion of finishing processes are, therefore, quite difficult.

Depending on the machines or chemicals used, both aesthetic and functional finishes can be further classified into:

Mechanical finishing typically refers to finishing effects ‘done by machine’ as opposed to finishes achieved through chemicals. Mechanical finishes have some advantages over chemical finishes. Mechanical finishing can process the fabric at very high speeds. The repeatability is better over time, since there are no formulations to change, and changing chemical restrictions and prices are not an issue. The mechanical finishes are now more acceptable because they are environmental friendly.

Except shrinkage control or Sanforizing process, all functional finishes mentioned above are basically chemical processes.

According to the quality, finishing may be classified into:

The types of finishing agents or finishing processes applied to various textile materials are indeed very large in number and they can be classified in various ways. It is, therefore, difficult to describe each of them systematically.

15.3.2 General process control in finishing

The number of finishes applied to various textile materials is very large and are selected as per properties desired for the end products. One or more properties are modified by each finishing agent and most of such properties can only be assessed subjectively. The situation is further complicated by the fact that many finishes are mixed or combined. Quality control during finishing is, therefore, very challenging and subjective.

15.4.1 Padding mangle

Traditionally, padders are used to apply chemical finishes in low wet pick-up techniques. In recent years such techniques have become more cost effective as the cost of energy has escalated significantly in comparison with the cost of finishing agents.

A padder consists of a trough and a pair of squeeze rolls (mangle). In the padding process, the fabric passes under a submerged roll in a trough filled with a treatment bath and then through squeeze rolls. In the simplest arrangement, a single dip is followed by a single nip. Alternately, three mangles can be used and the fabric is impregnated twice, double dipped – double nipped.

The amount of finishing solution or emulsion applied is referred to as the ‘%wet pick-up’ which is usually expressed as a percentage on the weight (wt) of the dry untreated fabric at room temperature and humidity.

[15.1]

where % conc. is the concentration of the finishing chemical in the applied solution or emulsion expressed as percentage by weight (wt/wt).

To determine the amount of supplied chemical or finish added to the fabric, the ‘% add-on’ is given by Equation [15.2]

[15.2]

[15.3]

[15.4]

Since most finishing formulas are given in terms of grams per litre (g/L), Equation [15.3] can be used to convert the g/L concentration to percentage by weight.

When the actual solid level added to the fabric is desired, the percentage of solids add-on can be calculated from % solid content in finishing chemical using Equation [15.4] (Schindler and Hauser, 2004).

Typically, for uniform application the wet pick-ups are maintained in the range of 70–100% during chemical finishing by padding. Subsequently it is necessary to remove of large amounts of water during drying. The evaporation of this water can lead to uneven finish distribution in the dried textile owing to migration of the finish to the fabric surface during drying. The high rate of evaporation at the fabric surface leads to movement of the finish solution from the wet fabric interior to the dried fabric exterior resulting in a higher concentration of the finish at the fabric surfaces with a corresponding lower concentration in the fabric interior regions.

The migration of finish may be minimised by reducing the amount of water applied initially. However, too low a wet pick-up can be equally problematic and also lead to uneven finish distribution. ‘Critical application value’ (CAV) is defined as the minimum amount of durable press finish liquid that can be applied to a given cotton fabric without producing a non-uniform distribution of cross-links after drying and curing. Cellulosic fibres, because of their inherent hydrophilicity, have CAVs in the range of 35–40% wet pick-up. Hydrophobic fibres like polyester can have CAVs of less than 5%, allowing much lower wet pick-ups than hydrophilic fibres.

There are two main types of low wet pick-up applicators. The first is the saturation–removal type where the fabric is completely saturated with the finish liquid and then the excess liquid is removed mechanically or with a vacuum before drying. With the second type, a precise amount of finish liquid is uniformly applied to the fabric using transfer roll, spray or foam techniques.

The factors controlling the amount of solution remaining on the fabric (i.e. %pick-up) after padding (Schindler and Hauser, 2004; Tomasino, 1992) and the relevant conditions for higher liquor pick-up (under bracket) are:

All fabrics have upper and lower wet pick-up limits. Within these limits, adjustments in wet pick-up can be made by increasing or decreasing the squeeze pressure.

15.4.2 Drying and curing

Drying is a process of evaporation of the embedded liquid from the fabric. While the concept of drying is simple, in practice it can be the source of unsuspected problems. For drying to take place, the liquid must be converted to vapour and the vapour must be moved away from the fabric surface. Some of the factors affecting drying rate are air temperature, relative humidity of drying air, and volume of air passing over fabric (air flow). The following factors are to be considered in controlling the cost of drying processes (Heywood, 2003):

Generally speaking, the machinery used to dry fabrics can be used for curing, provided the fabrics are capable of reaching curing temperature. In many finishing procedures, drying and curing are divided into two steps. Each step will have its own individual specified conditions. Sometimes however, no delineation between the two is made. Wet fabric enters the oven and cured fabric exits. It is well to remember that fabric temperature will not approach air temperature until all the moisture is gone. In many chemical finishing procedures, the fabric’s actual time/temperature relationship is critical in order to activate the chemical reactions. Therefore, in one-step drying and curing operations, it is important to know when the fabric is completely dry so that the residual time matches the required curing time at that particular temperature.

The thermometer probes generally indicate the temperatures. However it would be prudent to counter check the temperatures with thermal papers periodically to confirm that the temperatures shown by the probes are accurate particularly where temperature parameters are critical like in curing machines. All instruments can go wrong and there must be some verification procedure to ensure that such failures do not affect performance. Similar checks on the temperature of stenter chambers need to be monitored particularly during heat setting and thermosoling. Such precautions are necessary for all finishes involving chemical bonding or thermo-fixations at elevated temperatures.

1. drying,

2. heat setting,

3. weft straightening,

4. curing after application of finishes.

1. chamber temperatures,

2. moisture retention,

3. stretch/shrinkage (over feed),

4. fabric width,

5. fabric weight,

6. padder pressures,

7. exhaust humidity.

15.5.1 Nip pressure

It is examined by checking liquor pick-up. It should be uniform throughout the width and length. To obtain uniformity the necessary action is to check the surface of padding mangle and adjust the pressure (pneumatically or hydrolytically).

15.5.2 Bow and heading (skew) controllers

During processing visual checking is necessary. There should be no bow or heading in the fabric. The synchronisation of photo cell, heading and bowing rollers is to be checked. The hardness and alignment of bowing rollers are to be checked.

15.5.3 Chamber temperature

Checking is by dial or digital thermometer. It is to be set as per the process and quality. The necessary action is to regulate oil supply in the radiator. Proper functioning of solenoid valve and digital controls are to be checked and confirmed.

15.5.4 Dwell time

This may be checked during the finishing process with a stopwatch. Dwell time depends on the material quality and time recommended for the particular process and is to be regulated accordingly.

15.5.5 Overfeeding

Overfeeding is allowed during heat setting so that the fabric can recover from the stress applied on it at various stages of its production by shrinking and becoming dimensionally stable. During overfeeding fabric is fed to the stenter faster than the running speed of the fabric. If the shrinking is to be allowed in width-wise direction then the chains are kept closer and are not stretched.

Method of checking: to check % overfeed a certain length of cloth is marked before heat setting and the length between the marks is measured again after the process.

Action: the required optimum overfeed of synthetic and blended fabric of a particular variety during heat setting is to be assessed beforehand.

15.5.6 Underfeeding

During underfeeding speed of the fabric feed to the stenter machine should be less than the output. This is done to stretch the length of the fabric.

15.5.7 Expanders and uncurlers

During this process, usually the working of uncurlers is to be checked. Pneumatic uncurlers and mechanical uncurlers are generally used.

Standard: there should be no crease on the fabric.

Necessary action: the smooth working condition of the uncurlers and smooth revolution of the expanders are to be checked.

15.5.8 Blower

Proper functioning of the blower during the finishing process is to be checked.

Standard: proper air circulation.

Necessary action: for proper air circulation, the air filters are to be cleaned. The fan direction is to be checked – air is to be taken from out to in.

15.5.9 Width of the fabric

Width is decided by the distance between the chains and it is to be checked at the delivery end.

Proper working of width adjusting shaft is to be ensured.

15.5.10 Leakages of thermic fluid

There should be no leakages. If many small brown spots are seen on the fabric it means that there are small leakages where the fluid falls as a spray. Larger brown spots on the fabric may indicate bigger leakages.

15.5.11 Concentration of the chemicals

The chemicals and their respective concentrations are to be listed.

Necessary action: the required optimum concentrations are to be maintained. Higher concentrations of the chemicals will lead to white patches, called chalk marks, when scratched with nail. Chalk marks may be due to higher concentration of chemicals.

15.5.12 Temperature and viscosity of the finishing bath

Temperature and viscosity of the finishing liquor are to be kept constant throughout the process.

15.5.13 Drying efficiency

It is checked during and after the drying process with a conductometer that is with the help of transducers.

Standard: no over-drying of the fabric, which will lead to high-energy consumption and strength loss. Drying efficiency of 95–98% is expected.

15.5.14 Temperature of thermic fluid oil

The temperature of thermic fluid should be optimum and should be frequently checked during the process with digital thermometer. Standard: regulate thermic fluid circulation.

15.5.15 Clips and pins

Random inspection of clips and pins is to be done before processing. Standard: proper working of the pins and clips should be ensured.

1. Metal roller (usually steel or chilled iron), the surface of which may be ground, polished, chromium-plated or engraved and can be heated (by steam, gas, electricity) if required, with.

2. Resilient roller.

1. number of bowls,

2. arrangement of the bowls,

3. bowl composition (e.g. metal or resilient),

4. bowl pressure (50–100 tonnes, varying according to fabric type and width),

5. metal bowl temperature (up to 300 °C depending on fabric and effect),

6. running speed,

7. the degree of friction (e.g. up to 300% friction by having an inclined fabric-filled bowl or faster running metal bowl),

8. number of passages,

9. fabric construction (weave),

10. fabric state (dry, damp, wet) or moisture content,

11. fibre content,

12. finish applied.

• To change the fabric handle – softening or stiffening effect.

• To impart a smooth silky touch to the fabric.

• To compress the fabric and reduce its thickness and porosity.

• To reduce the air permeability by closing the threads.

• To change appearance – increase lustre of the fabric.

• To reduce yarn slippage.

• To increase the opacity of fabric.

• To embossing patterning on the surface.

15.6.1 Swissing, normal gloss or simple calendering

In this case, the fabric runs through the nips of several bowls having the same surface speed. As a result, the fabric achieves a lustrous paper-like finish depending on the number and composition of the bowls.

15.6.2 Chasing finish

The fabric is passed through the nips of the calender, over external rollers and back into the bottom nip of the calender. Multiple layers of fabric run through the nip resulting in a thready appearance with soft handle. This is mostly done for linen fabric.

15.6.3 Chintz, glazing or friction calendering

The main difference between a swissing calender and a glazing calender is the use of a gear system to drive the smooth metal roller faster than the softer resilient roller – the peripheral speed may be up to three times higher. The latter machine is also comparatively heavier. Apart from the lustrous effect, the cloth handle can become quite papery and thin. Incorrect fabric conditions (especially incorrect moisture content) can lead to unacceptable handle, which cannot be corrected.

15.6.4 Schreiner calendering

The problem of producing lustrous but paper thin handle by friction calender can be solved by using a ‘Schreiner’ calender. Schreiner finish, though not fast to washing, produces a fabric more attractive from marketing point of view, often called ‘silk’ finish. Extremely lustrous fabrics can be obtained with the correct type of cloth and engraving (V-shape or U-shape) of 500 lines per inch at an angle of 20° to the axis. The angle of engraving should follow the approximate angle of the line of the twist of the yarn (left or right inclination at angles of 15–25° from horizontal and 15° from vertical for weft-faced and warp-faced fabrics respectively). Imitation Schreiner finish can also be obtained using bowls with 150 to 200 lines per inch only. A usual production problem is the picking-up of lint and dirt by the fine engraving spoiling the optical effects. Schreiner finishes reduce tensile strength especially with deep V-type engravings. The darker the colour of a fabric, the better is the lustre of the fabric after Schreiner finish.

Process conditions for Schreiner calendering are:

15.6.5 Embossed calendering

This usually consists of two bowls. The top metal roller is engraved with a pattern and the softer composition roller has a surface that will accept the embossing pattern. The embossed rollers are quite expensive to produce. Originally these calenders were used to produce imitation leather cloths and book cloths. Embossed crepe design rollers easily deformed viscose fabrics to give a family of creping effects.

15.6.6 Moiré calendering

The moiré effect is an optical effect produced when a tightly woven fabric with very fine yarn is subjected to surface pressure that distorts the weave structure by yarn movement or yarn self-compression. The moiré effect can be produced only when the fibres being treated are deformable – wool fibres are not suitable for the purpose due to their high resilience.

Some of the process controlling parameters in calendering are as follows (Shah, www.sulphurdyes.com).

15.6.7 Nip pressure

The traditional method of applying pressure to the bowl set-up was by a system of levers or direct screw loading. The modern method of pneumatic or hydraulic pressure systems has altered the calender set-up. Calendering effect depends on the nip pressure, which is generally 7–9 tons on the rollers. The pressure is checked prior and during calender. The method of checking is nip pressure reading. The compressor valve and pneumatic control valve are to be regularly checked.

15.6.8 Threading

The threading through various calendering rolls depends on the finish required. It is to be checked visually before running of the machine. Proper threading of the fabric is to be ensured.

15.6.9 Damping

Controlled damping of the fabric is done before calendering. It is checked with the feel of the fabric, or with the conductivity meter. Follow-up action: adjust water spray intensity.

15.6.10 Speed of the roll

Speed is checked during the process with speedometer.

Standard: there is no standard speed, but 40–80 m/min is usually maintained.

Follow-up action: regulate speed.

15.6.11 Width of fabric

This is checked during the process by measuring with tape as per the sort or quality of the fabric. Follow-up action: ensure desired width at the delivery end.

15.6.12 Bowl surface

Prior to running it is checked visually.

Standard: there should be no crack, hole or rough surface or deposition on the roller.

Follow-up action: proper cleaning of calendering rollers is to be carried at the end of each process.

The following parameters are to be accurately controlled for consistency of the calendering process on a day-to-day basis:

Quality control test: shine or lustre may be measured by lustre meter.

15.7.1 Raised surface finishes

Raised surface finishes are mostly mechanical finishes. Raised fabrics are mainly of two types, namely:

The causes of defects in raised knitted fabric as described by Pehl (1991) are as follows:

The construction of knitted fabrics has an important bearing on the effectiveness of the raising process. The loops should stand erect without twisting, so twist factor (English) must not be higher than about 3.8 mm. Loop height should be 3.0–3.8 mm unlike normal terry customarily having a loop height of less than 2.5 mm. Usually raising is carried out after dyeing and drying.

15.7.2 Pre-shrinking finish

In the past relaxation drying was the only way in which fabric shrinkage could be reduced. In the so-called London shrinkage the fabric is placed in contact with a dampened wrapping material under tensionless condition for a few days followed by drying by hanging. For virtual elimination of length shrinkage, however, it is necessary to resort to the technique called ‘compressive shrinkage’.

Sanforizing and Rigmel are two methods of producing anti-shrink cellulosic materials developed by Sanford Cluett (USA) and Wrigley and Melville (UK) respectively. The methods are based on the fact that in an elastic material sinuous waveform, the convex surface is extended and concave surface is contracted. If cotton material is placed on the extended crest of the wave formed by the elastic material and moves with it into the contracted portion, being held firmly in contact with the elastic material during the movement, then the cotton material will be contracted by compression. Both methods very judiciously utilise a thick blanket for a closing up of the warp. They both allow the weft closing up to take care of itself. The fabric is not stretched weft-wise or set to an unstretched width in the stentering operation as a final stage in fabric finishing. Both processes pay particular attention to warp shrinkage, as most fabrics shrink more in the warp than in the weft and in garment warp shrinkage is usually more important (Marsh, 1979). The ‘Sanforized’ mark indicates that a fabric has been treated so that it will not shrink or stretch more than 1% or 2% on wetting.

Some process controlling parameters in a Sanforizer as described by Shah (www.sulphurdyes.com) are as follows.

15.8.1 Mercerisation

Mercerisation is a treatment that gives lustrous appearance to a cotton fabric or a cotton thread. The process is applied to materials like cotton and hemp. The process was devised in 1844 by John Mercer of Great Harwood, Lancashire, England, who treated cotton fibres with sodium hydroxide. Mercerisation alters the chemical structure of the cotton fibre. It results in the swelling of the cell wall of the cotton fibre, increase in the surface area and reflectance, and gives the fibre a softer feel.

The cellulosic materials in yarn or fabric form are treated with a concentrated solution of caustic alkali whereby the fibres are swollen and the following properties are modified:

Some controlling factors in mercerisation are described below:

15.8.2 Weight reduction of polyester

The weight reduction of polyester by chemical method confers special aesthetic properties on polyester (drape, suppleness etc.). Modification of polyester fibre has been necessitated to overcome several inherent shortcomings in the polyester fibre properties.

The drawbacks of polyester include very low moisture absorption (0.4% moisture regain) and poor dyeability due to high degree of ‘hydrophobicity’ of the fibre molecular structure and compactness of polymer structure.

The weight reduction of polyester is done by saponification of terephthallic ester by sodium hydroxide in presence of a suitable wetting agent. The partial alkaline hydrolysis causes progressive ‘weight reduction’ of polyester fibre in the surface, resulting in a small weight loss, whereas its other favourable properties remain almost unchanged or changed for the better. The weight reduction treatment is thus a major breakthrough to obtain ‘Skin care’ polyester fabric.

Important variables for weight reduction method of polyester are as follows:

To achieve 15% weight reduction, at the optimum conditions the treatment was carried with 5% caustic soda and 0.5% wetting agent at 90 °C for 60 min (Satees, www.scribd.com/). It is reported (Vigneswaran and Anbumani, 2007) that the treatment of 100% polyester rotor spun yarn (twist multiplier 3.0–4.0) with 5–15% concentrated caustic soda at 60 °C resulted in 3–12% weight loss and 7–19% strength loss and treatment at 100 °C registered 6–53% weight loss and 17–72% strength loss.

The following points are to be managed (Hayakashi, 2008) during dew-eighting of polyester:

15.9.1 Influence on textile properties

15.9.2 Influence on production parameters

15.9.3 Quality control tests

15.9.4 Cationic softeners

From an exhaust bath, the speed of exhaustion of the cationic finishing agent on to cotton fabric depends on the strength of the positive charge it carries. This, in turn, depends on the pH. At lower pH, cationic softeners carry relatively higher positive charge and therefore are exhausted more rapidly even in cold. At pH 4–5 the exhaustion is almost total. This high rate of exhaustion on cotton is also very undesirable, since it tends to develop uneven spots/stains on the fabric surface. This is due to rushing and exhaustion of the softeners into sites that are easily penetrated and relatively less or none is available for sites that are more difficult to penetrate.

Therefore for different softeners the optimum pH conditions are to be established considering temperature of application and M:L ratio. Fabric construction and geometry also will influence ease/difficulty to penetrate. Generally, weakly acidic conditions are recommended by the manufacturers/suppliers of the softeners to achieve uniform and even exhaustion. The exhaust time should normally be 20–30 min.

Secondly, the variation in pH (particularly falling on the alkaline side) on the fabric across the width/length can cause differential exhaustion on the fabric surface. Accordingly the performance in terms of the actual ‘finish’ characteristics such as softness, lubricity, feel, drape, etc. and wash fastness also will vary. It is recommended that both the fabric substrate and the bath are maintained slightly acidic with safe organic acids.

Incomplete removal of anionic soaps and detergents normally used in the earlier soaping operations results in the cationic finishing agent forming a complex with the anionic soap/detergent and causing precipitation and thus diminishing the softening effect. This point is often neglected. A proper rinsing cycle after soaping is required to minimise this problem.

Softeners are water insoluble and are marketed in the form of emulsions. Microemulsions of softeners, mainly amino-modified silicones, give special softening affects. Due to their high emulsion stability, they are very suitable for applications with high shear as in jet or package dyeing machines. Unstable emulsion deposits on equipment and fabrics are very difficult to remove. The properties of various types of emulsions are shown in Table 15.1. The macro softener has made the fabric less flexible and less hydrophilic in nature than the same with the micro. The micro softener has imparted maximum flexibility and the nano softener the best hydrophilicity (Roy Choudhury, 2012).

Some properties of softener emulsions are as follows (Schindler and Hauser, 2004):

15.10.1 Fibre properties

Fabrics made from fine cotton fibre do not wrinkle as badly as fabrics from coarse fibre. This is because the bending radius of curvature is greater for a thick fibre than for a thin one. The greater the radius of fibre, the greater is the stress on the polymer chains. Fabrics made from fine Egyptian cotton wrinkle less than those made from coarse fibres.

15.10.2 Yarn properties

Fabrics made from high twist yarns wrinkle more than those made from low twist yarns. For low twist yarns, the distortion stresses are dissipated by the physical rearrangement that takes place as adjacent fibres slip by each other. The stress is dissipated before it can affect the individual fibres.

15.10.3 Fabric properties

Tightly woven fabrics wrinkle more than loose structured fabrics. In a loosely constructed fabric, the yarns can move as they respond to the wrinkling forces. The individual fibre is spared. Woven fabrics wrinkle more than knits. The knit loops allow an even greater freedom of yarn movement again sparing the individual fibre.

Resin finishing improves wet and dry crease recovery and crease retention. The chemicals used are thermosetting resins and catalysts. The resins may be nitrogenous, such as dimethylol urea, melamine formaldehyde, DMEU, DMDHEU, etc., or non-nitrogenous, such as dichloropropanol. The catalysts used are mineral and organic acids, metal salts, etc.

Crease resistant finishes are applied to cellulose fibres (cotton, linen and rayon) that wrinkle easily. ‘Permanent Press’ fabrics have crease resistant finishes that resist wrinkling and also help to maintain creases and pleats throughout wearing and cleaning. Wrinkle recovery is dependent on the presence of cross-links that hold adjacent molecules together and pull them back into shape when they are distorted.

Resin finish improves the following properties of fabrics:

The desirable properties of easy-care finishing are as follows:

The disadvantages of resin finishing are:

The release of carcinogenic formaldehyde during application and fabric use is highly objectionable. The liberation of formaldehyde depends on the following factors (Lewin and Sello, 1983):

The various ways of reducing liberation of formaldehyde are:

The important effects of resin finishing on fabric properties are:

The three methods of application of resin on textile materials are as follows:

Comparison of the processing parameters and the effects obtained by the above three resin finishing methods is shown in Tables 15.2 and 15.3, respectively.

The controlling factors in resin finishing are:

The efficiency of curing may be assessed by measuring the nitrogen content of a sample before and after washing (i.e. after removing unreacted resin). The efficiency should be at least 80%.

Quality control tests: crease recovery angle, tensile strength, tear strength, free-formaldehyde content.

15.11.1 Fire retardant finishes

Properties that characterise burning behaviour of the products can be listed as follows:

The physical and chemical properties of the fabric that influence burning are weight, weave, density, yarn size, twist, and ply, degree of purification of the cotton, presence of certain dyes, moisture content, characteristics of the fibres in a blend, and type of finishing agent.

The factors which promote the rate of propagation of combustion of textiles (Rouette, 2000) are:

The effect of dyes is of minor importance in this connection. It is possible that they (e.g. metal complex dyes) may occasionally have a significant wicking effect, which does not significantly increase the rate of propagation.

In testing and evaluating the burning behaviour of textiles, the following variables are important:

Quality control tests: inclined and vertical flammability testers, cigarette burning test.

15.11.2 Water repellent finishes

Many terms are used to describe the water-repellency of textile materials, particularly fabrics. However, they are mostly classified into two unambiguous groups – water-proof and water-repellent.

Treated or untreated textile materials that prevent the absorption and penetration into their structure are called water-proof. In practice, a waterproof fabric shows no penetration by water below hydrostatic pressure of 100 cm (10 kPa).

The main differences between water-repellent and water-proof fabrics are:

Water-repellent finishes resist wetting. If the fabric becomes very wet, water will eventually pass through.

When a drop of oil in contact with a textile surface forms a contact angle with it, the following three cases are possible:

The contact angle depends on the surface energy of substrate (SE) and the surface tension (ST) of the liquid. Wetting occurs only when ST is less than SE.

ST of some liquids and SE value of some fabrics are as given in Table 15.4.

Hence, it is necessary to design proper structure of water repelling agent keeping in mind the surface characteristics of the material. The structure of the fabric is also equally important from the point of view of the ability of fabric to shed water resting on its surface and its ability to resist penetration of water.

Water repellents commonly available in the market fall under different categories as follows:

For water-repellency, the fabric needs to have a rough surface and a loose structure while ensuring best water shedding properties and the right construction for offering resistance to penetration. Fabric for water repellent finish must be free from sizes, detergents, alkali and acids.

There is considerable growth in the use of more expensive fluorochemical finishes which can be applied at low add-on, for example 0.15–0.3% (on the weight of fabric). Fluorochemical-based water repellents give durable finish. The carbon–fluorine bond is extremely strong and stable. Chemical substances containing fluorinated carbons, ‘fluorochemicals’, have special physical and chemical properties such as chemical stability, thermal stability, and low surface energy. Fluorochemical products provide performance benefits not achievable with other substances. For example, textile fluorochemicals provide oil and water-repellency and stain and soil resistance without affecting the hand, appearance, or breathability of a fabric.

High fabric repellency depends on the following factors:

Three main test methods are used to evaluate the water-repellency of fabrics, namely:

Oil repellency is assessed by hydrocarbon resistance test noting contact angle or collapse time of drops of solvents falling on the fabrics. Various solvents namely n-hexadecane, n-tetradecane, n-decane, n-octane, n-heptane etc. may be used for the testing purpose.

15.12.1 Fibre variables

15.12.2 Yarn variables

15.12.3 Fabric construction

Tightly constructed knits and woven fabrics pill less than loosely constructed knits and wovens. Tighter constructions reduce the migration tendencies of the fibres within the yarns.

15.12.4 Preparation and dyeing

Some wet processes make pilling worse while others provide substantial improvement. Preparation and dyeing processes that overwork the fabric will cause excessive hairiness and lead to poor pill performance. Long preparation and dyeing cycles are especially bad. In some instances pills may arise from just these processes alone. High-temperature dyeing of fabrics containing low flex life, high shrinkage fibres improves pilling performance. Auxiliaries such as carriers also have a positive effect on these fibres.

15.12.5 Fabric finishing types

Quality control test: by pilling tester or abrasion tester

15.13.1 Antistatic finish

Static electricity is generated in various ways; however, most static generation originates from triboelectric charge generation. When two materials come in contact, for thermodynamic reasons there is a redistribution of electric charges at each surface. Rubbing is not necessary, but it will generally increase charge transfer.

Triboelectric series is the ranking of materials in order of their ability to produce static charge. Air, asbestos, glass, nylon, wool, cotton, wood can produce positive charge while rubber, acetate, polyester, acrylic, polyurethane, Teflon can produce negative charge. When a material from the first group is brought into contact with a material from the latter group both acquire respective charges. The amount of static charge build-up depends on the electrical resistances of the contacting surfaces. Static charge generation is high if the resistances are high. If conductivity is increased the charges will run back along the separating surfaces to the point of contact. In the textile industry, especially in dry or low-humidity weather, electrostatic charges may generate causing repulsion between fibres, roller lapping, ballooning of yarn while running in the machine, discomfort while wearing charged garment or walking over a carpet.

Virtually all textile fibres, except metal and carbon fibres, have high electrical resistivity when completely dry. On increase in relative humidity, the resistivity as well as static charge generation decreases. Due to very low moisture, synthetic fibres are prone to static charge generation – the problem can be minimised by blending with conductive metal or carbon fibres or by finishing with suitable antistatic agents. The antistatic agent should have the following properties (Heywood, 2003):

The chemical classes suitable for use as antistatic agents include polyglycols, fatty glycol esters, fatty amide glycol ethers, ethoxylated amines, amine oxide, quaternary polymers and ammonium salts, phosphoric ethers and esters, etc.

Antistatic effective chemicals are largely chemically inert and require thermosol or heat treatment for fixing on polyester goods. Numerous types of chemicals are used as antistatic finishes, including polyglycols, fatty polyglycol ethers, ethoxylated amines, amine oxides, quaternary compound and polymers and phosphoric esters. Thermostable antistatic agents also have a good soil-release action.

Fabrics are generally treated with antistatic agents by padding followed by drying in a stenter or a suitable oven.

Quality control test: conductivity meter (Honestometer). The surface resistivity is easy and relatively reproducible to determine. But it is a static test with no information about the important charging and discharging behaviour of textiles. Therefore, combination with a charge dissipation test is favoured.

A charge dissipation test was performed with static voltmeter. The fabric was fixed vertically and charged with direct voltage or by rubbing with a glass rod. Maximum charge generated, as well as decayed, was measured. The time necessary for charge to fall to one half of its maximum value, called field intensity half-life time, was recorded. The relation between half-life time and the antistatic property of the textile materials is shown in Table 15.5.

15.13.2 Soil-release finish

The purpose of soil-release finishing for textile is to facilitate the removal of soiling matter during laundering. Soils deposited on textile materials have complex composition containing both oily and particulate matters, namely clay, soot, street dirt, etc. Electrostatic attractive forces are primarily responsible for the deposition of air-borne particulate soils.

The degree of retention after laundering of a mixture of clay and six fats representative of human sebum on bleached cotton sheeting has been studied, with the conclusion that the presence of fats leads to a build-up of soiling, possibly as a result of the fats acting as adhesives for the clay (Sontag et al., 1970).

Work on the deposition of fatty soil on polyester-fibre fabric during washing resulted in the following recommendations (Grindstaff et al., 1970):

Factors affecting soiling and soil-release (Heywood, 2003) are as follows:

15.13.3 Antimicrobial finishing

Microbes, namely bacteria, virus, fungi and yeast, are present almost everywhere. Whereas human beings have an immune system to protect against accumulation of microorganisms, material such as textiles can easily be colonised by high numbers of microbes or even decomposed by them. Textiles are carriers of microorganisms, such as pathogenic bacteria, odour generating bacteria, mould and fungi. If the environment is favourable, certain bacteria can grow in a short time from a single germ to millions.

Microorganisms are small living forms of life that we cannot see with the naked eye, such as:

An ideal antimicrobial treatment of textiles should satisfy the following requirements (Vishnu and Bhagyashri, 2010):

Antimicrobial finishes may be of three types:

Some examples of antimicrobial agents are cross-linked polyethylene glycol, quaternary ammonium compounds (chains of 12–18 carbon atoms), triclosan, silver based antimicrobials, etc. Chitosan, neem, alovera and clove oil are some examples of natural antimicrobial agents.

Antibacterial efficiency may be evaluated by two types of tests:

15.13.4 UV protection finish

In recent years, consumers have become increasingly aware of the need for sun protection, which is related to the incidence of sun-induced skin damage and its relationship with increased exposure to UV light. UV radiation can lead to acute and chronic reactions and damage, such as acceleration of skin ageing and sunburn. Billions of people live on the earth, and each has his or her own colour of the skin. In the human body the skin colour depends on the quantities of melanin, carotene and oxygenated or reduced haemoglobin combined in the skin, as well as thickness, water content, etc. Among other factors, the quantity of melanin distributed in the skin determines its fairness or darkness and greatly influences the human complexion; at the same time melanin plays an important role in minimising the damage that UV rays cause in the skin.

Protection of the skin against the action of solar radiation is a relatively new objective of textile finishing, since the textile does not always guarantee adequate protection. The unfinished fabric is limited as a guarantee of adequate protection. Thus, a special additional sun protection finish is applied in the form of UV stabilisers. Electromagnetic radiation of wavelength between 150 and 400 nm are classed as ultraviolet rays. Approximately 10% of sun’s energy is in the form of ultraviolet radiation. Atmosphere absorbs most of the noxious radiations emitted by the sun, only 5% of the harmful radiations reach to the surface of the earth.

UV absorbers, such as benzotriazole and phenyl benzotriazole, molecules are able to absorb the damaging UV rays of sunlight. UV absorbers convert UV energy into harmless heat energy. This transformation is regenerative and can be repeated indefinitely.

Solar protection factor (SPF), can be defined as the ratio of the time taken for a patch of skin to develop erythema, or reddening, with and without protection. The larger the SPF, the more protective is the fabric to UV radiation. Typically, a fabric with an SPF of > 40 is considered to provide excellent protection against UV radiation. In Europe and Australia, SPF is referred to as the ultraviolet protection factor (UPF). The SPF of textile fabrics can be assessed by measuring transmission through a screen (a fabric or chemical) with the aid of a spectrophotometer, after necessary correction for the nature of the solar spectrum in comparison with instrumental characteristics and to compensate for the fact that not all portions of the spectrum are actually harmful.

Cotton and silk fibres offer little protection to UV radiation since the radiation can pass through without being markedly absorbed. Wool and polyester, on the other hand, have significantly higher SPF since these fibres will absorb UV radiation. Nylon falls in between these extremes. One factor influencing nylon and polyester absorbance is the presence of the delustrant TiO₂, a material that strongly absorbs UV radiation.

Tight microfibre fabrics provide better UV protection than fabrics made from normal sized fibres with the same specific weight and type of construction. Many dyes absorb UV radiation as well as visible light. An SPF of 50 or higher can be achieved by dyeing cotton fabric to a deep shade. Since fashion and comfort often dictate the use of lightly coloured fabrics for summer apparel, the need arose for UV absorbing materials that could be applied to fibres to provide the desired SPF in light shades. Dyestuff and auxiliary manufacturers have responded by developing a variety of materials suitable for use as UV protection finishes. Benzotriazole derivatives and oxalic acid dianilide derivatives provide UV absorbers for natural fibres, while phenyl salicylates, benzophenones, benzotriazoles, phenyltriazines and cyanoacrylates provide UV absorbers for synthetic fibres.

UV absorbers should have sufficient wash fastness and light fastness as dyestuffs. Laundering trials should be carried out with all new formulations to confirm that the claimed UV protection is actually active during the life of the garment. If a UV absorber is also present in the fibre, the brightening effect from the OBA can be greatly diminished or even absent. Proper choice of an appropriate OBA can minimise this problem.

15.14.1 Decatising of wool

Pressure decatising is a setting process for wool and wool-mix fabrics that provides conditions for achieving high levels of set. The principal objectives are:

The process involves winding the fabric under controlled tension onto a perforated beam previously covered with a wrapper) interleaved with a decatising wrapper. When the batch is complete, it is loaded into a steaming chamber, sealed and steamed under pressure. Typical conditions for wool fabrics are 0.8–1.0 bar pressure at a temperature of 120–125 °C for two minutes, followed by cooling to arrest the setting process. High levels of permanent set can be imparted, producing a ‘permanent’ finish. This is important for stability of finish (to steaming during making up), for control of fabric dimensional properties, and for handle and lustre. It is important that wool fabrics are at the correct pH and moisture content prior to all pressing and decatising procedures, particularly pressure decatising. Fabric pH is ideally around pH 6 – stronger acidity reduces set, and stronger alkalinity may cause yellowing. Moisture content for all-wool fabrics is ideally 12–15%, since low moisture levels can result in low levels of set.

Decatising effects can be influenced by the following factors (Rouette, 2000):

If these parameters act for an appropriate period they influence the surface pattern and the handle of the product according to the following rules:

Modern pressure decatising machines offer a variety of steam cycles and choice of steam directions (in to out, out to in) to produce variations in handle, lustre and finish. Computers are used to store and retrieve data, including fault monitoring. Most modern decatising machines offer continuous processing. They generally provide lower levels of set than batchwise pressure decatising (Myers, 2002).

15.14.2 Crabbing of wool

Setting or crabbing is mainly applicable to worsted fabrics and is required to relax and set the strains introduced into the yarns and fabric during spinning and weaving. If some weave structures are not set in this way, they may be susceptible to the formation of distortions (e.g. ‘crows-footing’) during subsequent wet finishing. The crabbing operation is carried out in the presence of heat and moisture, during which the intermolecular bonds in the wool are broken and then reformed in a more relaxed configuration. Setting is arrested by shock cooling.

In a traditional batch crabbing machine, the fabric is wound onto a cylinder (covered with a cotton wrapper) which is rotated, whilst half immersed in hot or boiling water, for a predetermined time. To ensure even treatment, the fabric is reversed and the treatment repeated. The fabric may then be steamed and finally passed through a tank of cold water to arrest the setting process.

Two basic types of continuous crabbing machine are available. In cylinder types, the fabric is initially wetted through a trough of hot water and then passed around a large, rotating, heated cylinder. The fabric is pressed at high pressure against the heated cylinder by a specially engineered impermeable belt. Special seals resist escape of steam and entry of air at the edges of the belt. Fabric operating temperatures as high as 135–140 °C are claimed and superheated steam is created in situ, setting the fabric. Setting is arrested by shock cooling.

Superheated water machines differ in design from the cylinder types in having no pressure belt to maintain fabric/cylinder contact and use superheated water to facilitate setting. A possible advantage for fabric quality is that yarns are claimed to fully swell with minimal fabric compacting.

The fabric enters and exits through barometric columns. The temperature of water is around 110 °C, although a series of steam battery heaters situated around the main cylinder are claimed to elevate the fabric temperature during its contact time with the cylinder, promoting fabric set.

15.14.3 Enzyme finishing

Bio-polishing is a process to remove the protruding fibres of a fabric through the action of an enzyme. This cellulase enzyme selectively acts on the protruding cellulose fibres and ceases to work after finishing the job by a simple raise in temperature of the treatment bath.

Cellulases are multi-component enzyme systems that are commonly produced by soil-dwelling fungi and bacteria. The effects achieved by treating with these enzymes are pilling reduction, increase in softness and amelioration of handle, surface structure, fabric appearance and dyeing yield. The most important cellulase-producing organisms are fungi of the genera Trichoderma, Penicillium and Fusarium. The cellulases used are chemically complex and consist of at least three enzyme systems working synergistically (Holme, 1998).

Acid cellulases exhibit the greatest activity generally in the pH range 4.5–5.5 at 45–55 °C, whereas neutral cellulases require a pH of 5.5–8.0 at 50–60 °C. Generally a treatment of 45–120 min is appropriate, as prolonged treatment time may significantly increase the fibre loss. Where acid cellulase predominates, required washing time is short (20–45 min), whereas for neutral cellulase a longer wash time (45–120 min) is required. Excessive cellulose dosage and vigorous agitation may also increase weight loss.

For cotton, restriction of the enzyme to the fibre surface is easily achieved, because cellulose is a highly crystalline material with only a small amorphous region, making the diffusion of enzymes into the interior of the fibre nearly impossible. Thus, by regulating enzyme dosage and choosing the right type of enzyme, the catalytic action of the enzyme can be confined to the surface of cotton and to the amorphous regions, leaving the fibres, as a whole, intact. Surface modification of cellulosic fabrics conferring cooler and softer feel, brighter luminous colour and more resistance to pilling using cellulases, is often known as bio-polishing – a term created by Novo Nordisk.

Bio-polishing is affected by many factors. the major ones are enzyme type, type of fabric and process variables. The predominant process variables that control bio-polishing are temperature, pH, duration of treatment, material to liquor ratio, enzyme concentration and mechanical agitation.

The bio-polishing process requires:

After treatment, the enzyme must be deactivated either by alkali treatment at pH 9–10 or by increasing the temperature to 70–80 °C and giving a treatment for ten minutes.

15.15.1 Foam properties

Foam properties are governed to a large extent by the method of generation and by the treatment liquor composition, plus other factors, such as the purpose for which the foam is generated and the method of foam application. The principal properties of foam for the textile finisher are the foaming degree, foam stability, viscosity, wetting power, and the bubble size and size distribution (Elbadawi and Pearson, 2003).

15.15.2 Foaming degree

This is the measure of the extent of foaming. It is an indication of the volume of foam in litres that has been produced from one litre of liquor. The foaming degree is normally expressed in terms of the foam density (g/mL), which is calculated from weighing a known volume of foam. Care must be exercised during sample collection by inserting the foam-discharge hose inside the container to prevent any air pockets from being trapped within the sample (Equation [15.8]).

Foam densities of between 0.07 and 0.14 g/mL and between 0.20 and 0.33 g/mL have been used for foam finishing and foam printing, respectively (Turner, 1981). The selection of the foam density to be used depends on the fabric mass per unit area and is directly proportional to it. It is reported that low foam densities yield low wet-pick-up values (Weber and Tuschen, 1980).

Another expression of the foaming degree is the blow ratio (Equation [15.9]), which is defined as the ratio of the mass of a given volume of liquid before foaming to the mass of the same volume of foam.

The blow ratio varies with the density of the liquor, whereas the foam density is independent of the liquor density. The blow ratio is altered through control of the amount and pressure of injected air or alternatively through the liquor-pump speed. Although blow ratio is widely used as a measure of the foaming degree, it is more usefully considered as a foam generation parameter than a foam property. Moreover, it assumes 100% dispersion of the injected air in the liquor, which cannot be true, because air pockets are often seen at the foam exit.

15.15.3 Foam stability

Foam stability is defined as the time that foam will maintain its initial properties as generated. Foam stability is required during generation, transportation, and application to the fabric and has to be lost thereafter. Foams that are too stable are difficult to collapse; hence penetration into the fabric is poor. On the other hand, relatively unstable foams collapse before the point of application, resulting in an uneven distribution of chemicals on the fabric.

Foam stability can be expressed in terms of foam half-life, which is the time required for half of the volume of liquid contained in the foam to revert to the bulk-liquid phase. The shorter the time, the lower is the stability of the foam. It is reported that foam half-lives vary according to the type of process for which they are used: for instance, 30–45 min for finishing, 30–180 min for dyeing, and 8–10 h for printing (Jerg, 1979).

Another method is the drop-emergence test, in which the time taken for one drop of liquid to emerge from foam is measured. For the determination of the drop-emergence time, a given volume of foam is placed in a measuring cylinder. The measuring cylinder is then inverted and the time noted until the first drop emerges.

Other less-used methods of foam stability are determination of the change in foam viscosity and measurement of the electrical conductance of the foam between two fixed electrodes (Cooke, 1983). As the foam drains, the bubble lamellae become thinner and the electrical conductance decreases, giving a measure of foam stability.

Foam stability can be attributed to several factors, which include the following.

The other factors that contribute to foam instability are:

15.15.4 Foam viscosity

This is a measure of the foam stiffness or resistance to flow when the bubble walls are still in the liquid state. The viscosity increases as the number of air bubbles increases. It is also affected by the addition of stabilisers and other foam ingredients. Higher foam viscosities result in greater stability through reduced foam-drainage rates. In practice, viscosity modifiers are added to bring about foam viscosities in the range of 2000–3000 centipoise (cP) for textile utilisation. Measurement of foam viscosity is a delicate process, since the foams may be either thixotropic (i.e. they possess the property of showing a temporary reduction in viscosity when shaken or stirred) or pseudoplastic (i.e. they show an instantaneous decrease in the apparent viscosity with an increase in the shear rate) in nature (Shaw, 1980).

There are various methods available for the measurement of foam viscosity, the selection of which depends on the type of foam produced.

Foam viscosity may be measured using a rotary Brookfield viscometer operating at a low shear rate, or by using a coaxial Brabender viscometer or a U-type Ostwald viscometer in a temperature-controlled water bath.

15.15.5 Bubble size and size distribution

Bubble size and bubble-size distribution are important criteria of foam behaviour. Foam of finer bubble size is more stable than foam of the same density with coarser bubbles. As foam ages, the bubble size increases and the distribution of sizes broadens owing to diffusion of air from the smaller (high pressure) to the larger bubbles (low pressure) through the thin lamellae. Uniform bubble size in the range of 50–100 μm in diameter is a necessity for textile applications (Ghosh, 1988).

Observing and measuring the complex structure of foam is extremely difficult, as the nascent bubbles jostle one another to seek a stable arrangement before they burst and collapse. Techniques such as photomicrographs of a foam sample in a recessed microscope slide and freezing the foam have been used to determine the bubble size and distribution.

The size of the photographically recorded bubbles is measured and classified in different categories, and the distribution is characterised by the mean and standard deviation of the bubble size (Kroezen and Wassink, 1987).

A visual technique has also been reported in which foam with a narrow distribution of bubble sizes is selected by visual inspection. This is then laid on a board, frozen, and used as a standard for comparison. This technique is quite subjective.

15.15.6 Foam wetting power

Textile substrates are not readily wettable by foam. The foam has to spread out on the substrate first and then break, releasing the liquid to penetrate the fibres. The wetting power can be assessed by the spreading or penetration rate, which is governed by the foam-drainage rate.

Liquid can drain out of the lamellae films between foam bubbles owing to gravity and surface-tension differences between the least and the most sharply curved parts of the bubble. The drainage rate is affected by the foam density, viscosity, and temperature and by the film thickness.

Rapid wetting by foam, or rather by the liquid released on collapse of the foam upon contact with the textile material, is a very important property where application as textile finishing is concerned. The wetting power of the foam can be assessed by the measurement of either the spreading or the penetration rate.

Foam spreading time is evaluated by measuring the growth in the wetted area. In practice, a measuring cup is completely filled with foam and inversely placed on a standard fabric material. After a specified time, the area of the wetted-out fabric outside the circumference of the original measuring cup is recorded.

• Chemically inert (e.g. helium and argon)

• Reactive and non-polymerisable (e.g. ammonia, air and nitrogen)

• Reactive and polymerisable (e.g. tetrafluoroethylene, hexamethyldisiloxane).

• Removing the surface hairiness in yarn

• Hydrophilic enhancement for improving adhesive bonding, wetting and dyeing

• Hydrophobic enhancement of water and oil repellent textiles

• Antibacterial fabrics by deposition of silver particles in the presence of plasma

• Graft plasma polymerisation for producing fabrics with laundry-durable oleophobic, hydrophobic and stain-resistant finishes

• Atmospheric plasma-based graft polymerisation of textiles and nonwovens having different surface functional properties on the face and back side of the fabric

• Flame-retardant coating using monomer vapours (halogen and/or phosphorus) in combination with nitrogen and/or silicone

• Durable antistatic properties using PU-resin and plasma processing

• Shrink resistance of woollen fabrics and preparation of machine-washable wool by plasma processing without resin

• Electro-conductivity of textile yarns by surface plasma deposition.

• Modification can be confined to the surface layer (a few hundred angstrom units) without modifying the bulk properties of the polymer.

• Excited species in plasma can modify the surfaces of all polymers, irrespective of their structures and chemical reactivity.

• By the selection of the feed gas to plasma reactor, it is possible to achieve the desired type of chemical modification for the polymer surface.

• The use of plasma can avoid the problems encountered in wet chemical treatments, such as residual chemical in the effluent and swelling of the substrate.

• Modification is fairly uniform over the whole surface.

1) Nature of gas used

2) Flow rate

3) System pressure

4) Discharge power

5) Duration of treatment

6) Ageing of plasma-treated surface

7) Temperature change during the plasma treatment.

15.16.1 Nature of gas used

The result of plasma treatment depends strongly on the nature of the gas or the vapour used in glow discharge. Most organic, organosilicone or organometallic vapours tend to form a thin film on the surfaces, which are subjected to glow discharge, and the deposition of these films is the main factor modifying the polymer surface. On the other hand, glow discharges of non-polymerising gases, for example noble gas, nitrogen, oxygen, hydrogen, ammonia or water vapour, modify polymer surfaces through processes, such as oxidation, chemical etching, cross-linking and perhaps grafting.

Helium, neon and argon are the three inert gases commonly used in plasma technology. Because of its relatively lower cost, argon is by far the most common inert gas being used. Inert gas plasma has been used for the pre-treatment of substrates for cleaning purposes before reactive gases are applied.

If a plasma reaction is to be carried out with a high-system pressure but at a low reactive gas flow rate, an inert gas can serve as a diluent.

Oxygen and oxygen-containing plasma are most commonly employed to modify polymer surfaces, (it is well known that the oxygen plasma can react with a wide range of polymers to produce a variety of oxygen functional group, including C–O, C=O, O–C–O and C–O–O at the surface).

Nitrogen-containing plasma is widely used to improve wettability, print-ability, bondability and biocompatibility of polymer surfaces.

15.16.2 System pressure

The system pressure used may affect the energy of the plasma species. If the pressure is high, the probability of collision between plasma species will be increased leading to the loss of energy of the species before interacting with the material.

15.16.3 Discharge power

The intensity of plasma is a combined factor of pressure and discharge power. The breakdown energy necessary to produce plasma varies from one gas to another. Normally, the higher the discharge power applied, the more kinetic energy the plasma species will carry, resulting in strong intensity of plasma action.

15.16.4 Duration of treatment

The duration of treatment plays an important role in the plasma treatment. Generally speaking, the longer the duration of treatment, the more severe the modification of the material surface (e.g. sputtering or etching) will be.

A longer duration will not only affect the material surface but also provide an opportunity for the plasma species to penetrate the interior region of the material. This may alter the morphology of the polymeric material. However, when the treatment duration is too long, this will adversely affect the material and therefore careful control of treatment duration is required.

15.16.5 Ageing of the plasma-treated surface

In general, the concentration of functional groups introduced to a polymer surface by the plasma treatment may change as a function of time depending on the environment and temperature. This is because polymer chains have much greater mobility at the surface than in the bulk, allowing the surface to reorient in response to different environments.

Surface orientation can be accomplished by the diffusion of low-molecular weight oxidised materials into the bulk and the migration of polar function groups away from the surface. Ageing of plasma-treated polymer surfaces can be minimised in a number of ways.

An increase in the crystallinity and orientation of a polymer surface enhances the degree of order and thus reduces mobility of polymer chains, resulting in slower ageing. A highly cross-linked surface also restricts the mobility of polymer chains and helps to reduce the rate of ageing.

15.16.6 Effects of environment

When a polymer is exposed to the oxygen-containing plasma, the surface will change to a high-energy state with increased surface tension, as a result of the formation of polar groups. Various surface studies indicate that when the treated surface is placed in a low-energy medium, such as air or vacuum, the decrease in surface energy is caused by the rotation of the polar groups in the bulk or the migration of low-molecular weight fragments to the surface to reduce the interfacial energy. When a low-energy surface formed by treating a polymer in fluorine-containing plasma is placed in a high-energy medium, such as water, the apolar groups will tend to minimise the interfacial energy by moving away from the surface into the bulk. This phenomenon is usually described as ageing of a treated surface.

15.16.7 Temperature change during plasma treatment

The thermal decomposition of fibres under plasma condition is a major concern during the plasma treatment. The temperature increase inside the plasma reactor is the possible way of the thermal decomposition of fibre as a side effect.

• The need for higher quality, higher added value products

• Right-first-time, right-on-time, right-every-time processing

• Increased levels of automation and process control in machinery and equipment

• The increased use of process integration

• Greater emphasis on cost reduction by minimising the use of water, energy and all utilities

• The challenges posed by environmental issues, e.g. minimising discharges of chemicals onto land, into air and particularly into water

• The greater interest in multipurpose chemical finishes.

• Avoid potential sources of risk

• Built-in freedom from maintenance

• Monitoring of specified and achieved values

• Control of exceptional situations.

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