The actual method of finish application depends on the particular chemicals and fabrics involved and the machinery available. Chemicals that have strong affinities for fibre surfaces can be applied in batch processes by exhaustion in dyeing machines, usually after the dyeing process has been completed. Examples of these exhaust applied finishes include softeners, ultraviolet protection agents, and some soil release finishes. chemicals that do not have an affinity for fibres are applied by a variety of continuous processes that involve either immersing the textile in a solution of the finishing chemical or applying the finishing solution to the fabric by some mechanical means.
After application of the chemical finish, the fabric must be dried and if necessary, the finish must be fixed to the fibre surface, usually by additional heating in a ‘curing’ step as shown in the figure below:
In batch processes, the amount of chemical finish to be applied is usually expressed as a weight percentage based on the original fabric weight . This relationship is often abbreviated as %owf (percent on weight of fabric) or %owg (percent on weight of goods).
%OWF =(Weight of Chemical / Weight of Fabric)*100
For example, if a softener is to be applied at 3% owf to 500 kg of fabric, then 15 kg of softener will be used (3% of 500 Kg). It must be recognised that since nearly all chemical finishes are provided as an aqueous solution or emulsion, a knowledge of the actual solids concentration of the supplied chemical is needed to determine the actual increase in fabric weight after drying.
If the solids concentration is not known or provided, it can be determined by careful evaporation at moderate temperature followed by weighing the residual. But this weight ratio (residue related to the original product sample) is only the upper limit or the maximum concentration of the active finish product. the presence of dispersing or emulsifying agents, salts, unreacted components and by products may reduce the actual percentage of the active agent compared to the measured weight ratio. A low value of the active products may be determined if they are not solids but liquids and if the partially evaporate with the water during drying.
In continuous processes where a chemical solution or emulsion is applied to a fabric, the amount of chemical actually applied to the fabric depends on the amount of finishing solution applied, the concentration of the supplied chemical in the finishing solution or emulsion and the solids or active compound concentration of the supplied chemical. The amount of finishing solution or emulsion applied is referred to as the ‘wet pickup’ of the fabric and is usually expressed as a percentage on the weight of the dry untreated fabric.
%Wet Pick-up = (Wt. of Solution applied / Wt of Dry fabric)*100
To determine the amount of supplied chemical added to the fabric the % Add-on is given by following equation:
% Add-on = (% Conc. in Soln (Wt/Wt) * %Wet Pick-up) / 100
where % conc is the concentration of the finishing chemical in the applied solution or emulsion expressed as percentage by weight. Since most finishing formulas are given in terms of grams per litre, the following equation can be used to convert the ‘gpl’ concentration to weight percent:
% Conc. in Solution (Wt/Wt) = Conc in ‘gpl’ / (10 * density in grams per ml)
where the density is the applied solution or emulsion density.
PAD APPLICATION OF CHEMICALS TO DRY FABRIC
chemical finishes are often pad applied to dyed or printed fabrics after a drying step. in this situation, dry fabric is passed through the chemical finish solution and the process is called a ‘Wet on Dry’ process. The wet pickup of a chemical solution in a padding mangle is influenced by many factors such as fabric characteristics, machine settings and solution or emulsion properties. In order to obtain consistent chemical application, the nip pressure should be uniform across the fabric width, the solution level and temperature in the pad should be constant and the fabric speed should not vary throughout the application process.
The following equation can be used to determine the necessary solution feed rate to maintain a constant liquid level in the pad:
Sol. flow rate (ltr/min) = (Fabric flow (kg/min) * %WPU) / (Sol. density *100)
where fabric mass flow is defined as:
Fabric mass flow = Fabric speed (m/min) * Fabric linear density (kgs/m)
In practice, however, it is more common to maintain a constant level in a wet on dry pad application with a float valve controlling the liquid level.
PAD APPLICATION OF CHEMICALS TO WET FABRIC
To avoid the costs of a drying step after dyeing, chemical finishes are often pad applied to wet fabric in a process called ‘wet-on-wet’. In this case the wet pick-up of the fabric exiting the pad must be maintained at a higher level than that of the incoming fabric usually at least 15-20% higher. An additional complication is the fact that water entering the pad through the fabric can interchange with the finishing solution, and hence been diluting the concentration of the components and causing tailing of the finish effect. Therefore, a chemical feed more concentrated than the pad solution must be employed. In order to determine the padding liquor concentration, an effective percentage Wet Pick-up, WPUeff is calculated as below:
WPUeff = (WPUo – WPUi ) + WPUi * f
where WPUo is the percentage wet pickup of the fabric exiting the pad, WPUi is the percentage wet pickup of the fabric entering the pad and f is the interchange factor, a measure of interaction between incoming water and the pad solution, that can vary from 0 to 1 depending on the fabric and machine parameters. Typically f is between 0.7 and 0.8. An initial interchange factor is assumed and then corrected, if necessary based on analysis of the treated fabric.
The pad solution concentration is found using the desired Percentage add-on:
Pad conc. (g/l) = % Add-on * 1000 * Soln. Density / WPUeff
The concentration of the chemical feed solution must be higher than the pad concentration since the pad bath is being diluted by water on the incoming wet fabric. thus, the feed concentration needed to maintain the pad concentration is calculated as follows:
Feed conc. (g/l) = Pad conc. X WPUeff / WPUo – WPUi
Now, the feed flow rate can be found as:
Feed flow rate (ltr/min)
= Fabric mass flow (kg/min) * (WPUo – WPUi ) / Feed soln. density (g/ml) * 100
LOW WET-PICKUP METHODS
Typically, pad applications of chemical finishes yield wet pickups in the 70-100% range. These high pickups necessarily require the removal of large amounts of water during drying. the evaporation of these 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 to the at the fabric surfaces with a corresponding lower concentration in the fabric interior regions. This migration is reduced as the finish solution becomes more and more concentrated and viscous as drying progresses. Therefore, reducing the amount of water initially applied will tend to reduce finish migration.
However, too low a wet pickup can be equally problematic and also lead to uneven finish distribution if the liquid phase is discontinuous. The concept of a Critical Application Value (CAV) is useful when discussing optimal wet-pickups. The CAV is defined as the minimum amount or 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. Dye staining tests can be used to determine these distributions. For non-cellulosic fibres, other methods of finish distribution analysis can be used. Cellulosic fibres because of their inherent hydrophilicity, have CAV’s of less than 5%, allowing much lower wet pickups than hydrophilic fibres.
There are two main types of low wet-pickup applicators. The first is the saturation-removal type where the fabric is completely saturated with the finishing solution and then the excess 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. One of the simplest approaches to the saturation removal has been to place a vacuum extraction device after the pad applicator and prior to dryer entry. By pulling a vacuum through the wet fabric and returning the extracted liquid to the pad, an effective lower wet-pickup can be achieved, usually in the order of 40%. The figure below gives a typical Vacuum Extraction installation:
Another relatively simple method of reducing wet-pickup is the use of the Machnozzle system, a machine similar in principle to air-jet ejectors. in this device, high pressure steam is used to push excess liquid out of the fabric, leading to very low wet-pickups, especially for synthetic fabrics.
In the area of topical application, several methods have been used to apply chemical finishes using transfer rolls. The Kiss roll, shown in figure above, picks up the chemical finish and transfers it by direct contact to the fabric. The amount of finish picked up is dependent upon how well the finish wets the roll, the absorbency of the fabric, and to a lesser extent, the surface speed of the roll relative to the fabric speed.
Another version of finish application with transfer rolls is the Loop transfer system. A loop of fabric is immersed in finish liquid and then squeezed with the fabric to be treated between the squeeze rollers. The finish is transferred to the fabric at a much lower wet pickup than possible by direct immersion. These roll transfer techniques are especially useful for the backside application of finishes, for example hand builders, flame retardants and pile fabrics (without crushing the pile).
One interesting modification of the Kiss roll applicator is the TRIATEX MA machine which uses on-line monitoring to control wet-pickup. As the fabric passes through the system two β-gauges are used to determine the fabric weight difference before and after the fabric has passed over a kiss roll. The β-gauges measure mass per unit area based on the intensity of electrons that pass through the fabric. The kiss roll rotational speed is then automatically adjusted relative to the fabric speed to maintain the desired wet-pickup.
An engraved roll can transfer precise amounts of chemical finish to fabrics since the engravings can be made in various depths and designs. A doctor blade removes any excess liquid from the roll surface before fabric contact insuring that only the liquid in the engraved areas is transferred to the fabric. A disadvantage of engraved rolls is that a roll will deliver the same amount of finish, regardless of the fabric being treated. Therefore multiple rolls are needed if the different fabrics are to be treated to the same wet-pickup.
Chemical finishes can also be applied by spraying. By controlling the flow rate through the spray bars, the amount of applied finish can be set to the desired add-on. Care must be taken to avoid overlapping spray patterns that could lead to an unacceptable uneven finish distribution. Special care is needed with aerosols from fluorocarbon sprays.
One important application method for chemical finishes is the cause of foam to apply the finish to the fabric. By replacing part of the water in the chemical formulation with air, the amount of water added to the fabric can be significantly reduced. In addition, surfactants are included in the formulation to be foamed. Even if they are carefully selected, they may cause effect reduction of repellent finishes. the chemical formulation is mixed with air in a foam generator producing high volumes of foam that can be applied to fabrics in a number of ways. The ratio of liquid to air in a foam is referred to as the ‘Blow Ratio’, determined as follows:
Blow Ratio = 1 / Foam density
Foam densities in the order of 0.1 gm/cm³ are routinely used. The stability of foam is influenced by the components of the chemical system, the viscosity of the foam, and the method of foam preparation. The half-life of a foam is the time in which 50% of the liquid in a given foam volume has been drained from the foam.
Some of the foam application methods are shown in figure below. The one side applicators apply foam to only one side of the fabric, leaving open the possibility of two different finishes on different sides of the same fabric. The another two side foam application method shown below, employs two slots to apply foam to the fabric. Two distinctly different finishes can be applied to different sides of the same fabric simultaneously . Foam application on fabrics with large open spaces or non-uniform porosity often causes uneven finish distribution. Foam application systems also include horizontal pad mangles, kiss coating systems, knife-over-roller or knife-on-air systems, screen printing, and slot applicators.
In all these application methods, proper fabric preparation is required in order to achieve uniform finish distributions.