Wet pickup

A second general approach to achieve reduced wet pickup is based on the use of the vacuum slot technology (15,16). In this case the fabric may be padded conventionaUy, but is then mn against a vacuum slot (17). This vacuum slot removes a certain amount of water solution from the fabric, so that a reduced level of wet pickup is achieved prior to drying. A lower level of wet pickup is achieved on blend fabrics than it is on 100% ceUulosics using the vacuum slot technology. 

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 pad mangle is influenced by many factors such as fabric characteristics, machine settings and solution or emulsion properties. Table 2.1 summarises some of these factors. 

Fligherwet pickup with hydrophilic fibres Fligher wet pickup with low twist and/or open end yarns... 

Fligher wet pickup with loose constructions (knit vs. woven)... 

Fligher wet pickup with more easily wetted fabrics Fligher pressures lead to lower wet pickups Flarder coverings lead to lower wet pickups... 

Fligher wet pickup with longer immersion time Fligher wet pickup with higher viscosity... 

Fligher wet pickups with faster wetting solutions... 

Viscosity and surface tension change with temperature, changing wet pickups Viscosity and surface tension change with component concentrations, changing wet pickups... 

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 pickup... 

In most cases a defined chemical add-on is desired. In order to determine the pad solution or emulsion concentration of the finishing chemicals, an effective percentage wet pickup, wpu j, is calculated from Equation 2.8 ... 

T ypically, pad applications of chemical finishes yield wet pickups in the 70-100 ... 

However, too low a wet pickup can be eqnally 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 discnssing optimal wet pickups. The CAV is defined as the minimum amonnt of dnrable press finish liquid that can be applied to a given cotton fabric withont prodncing a non-uniform distribution of crosslinks 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. 

The CAV for a particular process is dependent on fibre type and fabric construction and absorbency. A finish application below the CAV may result in a non-uniform speckled treatment, while an application above the CAV could lead to finish migration. Cellulosic fibres, becanse of their inherent hydrophilicity, have CAVs in the range of 35 0 % wet pickup. Hydrophobic fibres like polyester can have CAVs of less than 5 %, allowing much lower wet pickups than hydrophilic fibres. 

There are two main types of low wet picknp applicators. The first is the saturation-removal type where the fabric is completely satnrated with the finish liquid and then the excess liqnid is removed mechanically or with a vacnnm before drying. With the second type, a precise amount of finish liqnid is nniformly applied to the fabric using transfer roll, spray or foam techniqnes. Table 2.2 gives a snrvey on low wet pickup finishing application methods, some of which will now be discussed in more detail. 

Table 2.2 Low wet pickup finish application methods (adapted from Greenwood and Holme) ...

Another relatively simple method of reducing wet pickup is the use of the Machnozzle system. Fig. 2.3, - 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. 

Another version of finish apphcation with transfer rolls is the loop transfer system, Fig. 2.5.A loop of fabric is immersed in finish liquid and then squeezed with the fabric to be treated between 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 and flame retardants, to pile fabrics (without crushing the pile). 

In order to maintain the same chemical add-on with lower wet pickups, the concentrations of the finish bath components must be increased according to Equation 2.13 ... 

A cotton fabric is to be treated with 5.0 % on weight of fabric (owf) with a chemical finish in a wet on dry padding process. If the wet pickup is 90 %, what concentration of chemical is needed ... 

This same fabric is to be treated with 5 % owf of the same chemical finish in a wet on wet pad application with entry wet pickup of 75 %, exit wet pickup of 90 % and interchange factor of 0.7. What are the effective wet pickup and the necessary pad concentration ... 

Foam technology is used to apply stain release chemicals to carpet. Liquid solutions normally used for treatment are replaced with foams. Wet pickup is lowered to as little as 8% in special applications, compared to 100% wet pickup in conventional methods [72]. The minimum pickup level for good finish penetration on cotton and cotton blends is about 45% [71], The reduction in water use at one plant was 20 million liters per year, and energy was cut 10% overall [72], However, foam and the surfactants necessary to produce it have their own set of problems in wastewater. 

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