Textile Savvy
Issue: Mar-Apr , 2010
Role of Scouring agents in Textile Wet-Processing

Rajendra Sanap
General Manager
(Production & Product Development)
Matex Bangladesh Limited



Cotton is based on 85-90% of cellulose and the remaining portion consists of impurities such as fats, waxes and organic compounds. Fiber materials in the form of textile fabrics, for example wovens or knits, especially cotton fabrics, normally have to be subjected to a pretreatment before they are dyed. One purpose of the pretreatment is to ensure trouble-free uniform dyeing. Ensuring that the fabric is well prepared involves removal of the natural and man-made impurities present in the greige fabric.


Soil to be removed in preparation of Knitted Cotton :

Natural Impurities (10-15%)

Added Impurities (0.5-2.0%)

Fats

Paraffin Wax

Waxes

Spinning Oils

Hemicellulose

Knitting Oils

Pectins

 

Proteins

 

Mineral Matter

 

                                                           
There are three major processes in scouring to remove these impurities.

i) Saponification : It is the process in which fats are treated with caustic which leads to the formation of hydrophilic soaps.
 
ii) Emulsification : Waxes present in the fabric cannot be removed in saponification. These are esters of higher fatty alcohol and fatty acids. Similarly mineral oils, lubricants cannot be converted to water soluble product by boiling with caustic. Thus the scouring solution must also contain an emulsifying agent. In other words, a ‘Surfactant’.

Schematic Sketch of Surfactant Molecule

A surfactant is defined as a material that can greatly reduce the surface tension of water when used in very low concentrations. A particular type of molecular structure performs as a surfactant. This molecule is made up of a water soluble (hydrophilic) and water insoluble (hydrophobic) component.   

In emulsification, the selection of surfactant or surfactant system will depend on the materials to be used and the properties desired in the end product. An emulsion can be either oil droplets suspended in water, an oil in water (O/W) emulsion, water suspended in a continuous oil phase, water in oil (W/O) emulsion, or a mixed emulsion. Selection of surfactants, order of addition and relative amounts of the two phases determine the class of emulsion.

Each of these three functions is related to the surfactant absorbing at a surface, either gas, liquid or solid with the hydrophilic ends of the molecules oriented to the water phase. The surfactants form what amounts to a protective coating around the suspended material, and these hydrophilic ends associate with the neighbouring water molecules. In addition to surfactant effects, the stability of these suspensions is related to the particle size and density of the suspended material.

As the size of the emulsified droplet becomes smaller, a condition is reached where this droplet and the surfactant micelle are the same size. At this stage, an oil droplet can be imagined as being in solution in the hydrophilic tails of the surfactant.

iii) Detergency : After removal of fats by saponification and waxes by emulsification, the remaining constituents, dust and dirt particles have to be removed by a good detergent. Detergent not only removes the dirt particles but also keep them in dispersed or suspended form in the scouring solution and does not allow them to settle on other parts of the fabric.

Simplified Illustration of Detergency

Thus in the final stage of scouring i.e. detergency, the hydrophilic end of the surfactant is strongly attracted to the water molecules and the force of attraction between the hydrophobe and water is very less. As a result the surfactant molecules align themselves at the surface and internally so that the hydrophile end is towards the water and hydrophobe is squeezed away from the water. This internal group of surfactant molecules is called ‘Micelles’.

Apart from removing these impurities, the major technical aspect is to improve the absorbency. Achieving good absorbency necessitates the thorough removal of cotton wax, which is located mainly in the fibre surface. Not all good detergents possess good wetting/re-wetting properties; hence a combination of surfactants to provide good wetting, emulsifying and re-wetting property is preferable.

Matclean RWD is designed to overcome this difficulty of adding separate surfactants in the scouring bath. It possess excellent wetting, emulsifying property and also imparts excellent absorbency to fabric required for uniform dyeing.

Particular importance attaches here to scouring agents which are effective to provide good wettability for the textile fabrics and also good washing and cleaning effects. But the products responsible for good wettability should ideally bring about nil or only an insignificant increase in the foaming tendency of baths that contain the pretreatment products.

The tendency to form foam becomes noticeable in both continuous and batch pretreatment processes, for example in the course of a batchwise pretreatment in jet machines. This is very important not only when the pretreatment is carried out as a batch process, for example in jet machines, but also in continuous processes. The reason why no-foaming/low-foaming properties are frequently demanded is that, in many cases it is undesirable to suppress increased foaming by adding antifoams such as silicones. It may result in silicone spots on the fabric.

Matclean JET-D is especially designed to take care of such baths where silicone antifoams are undesirable. The product itself is silicone-free, hence there is no danger of developing any
silicone spots on the fabric.

Depending on the prior history and provenience of the textile fabrics or the equipment available, the pretreatment may include the measures of desizing, degreasing/scouring and bleaching the textiles. These measures may be carried out separately, but in the individual case it is also possible to integrate a plurality of these measures in a single process. Thus classic pretreatment steps such as desizing, scouring and bleaching are individual steps to be carried out, but in a number of cases it is integrated into a combined pretreatment process in order to save time, energy and cost. There is also a demand for compositions which can be used for such combined pretreatment processes.

Matclean MFD is once such multi-functional detergent that not only imparts good wettability to the textile material at the start of the pretreatment, but also good hydrophilicity at the end of the pretreatment. This good hydrophilicity leads to the good rewettability needed for the dyeing process. It also possesses inbuilt sequestering and peroxide stabilising capacity. Hence it is widely suitable for combined Scouring and bleaching baths.

Instead of the traditional scouring process, it is also possible to carry out an enzymatic scouring process (bio-scouring) to remove non-cellulosic material from cotton fibres.

The scouring of pure silk is a degumming process used to remove sericin (silk gum) from fibroin floss. Sericin is the gummy element which keeps together the fibroin floss and gives the silk a hard hand and dull appearance. It is carried out on yarn, on dyed yarn, piece –dyed fabric or on products ready for printing. The treatment , which causes loss of weight ranging between 24-28%, gives the degummed silk a lustrous appearance and a soft hand. The treatment is carried out with soapy solutions or with buffer dissolving agents. It is also possible to use enzymes (protease) which hydrolyse sericin.

On wool, the scouring process removes oils and contaminants accumulated during upstream processing steps and can be carried out on slivers, yarns and fabrics with solutions containing sodium carbonate with soap or ammonia, or anionic and non-ionic surfactants, which carry out a softer washing to avoid any damage to the fibres.

Scouring process applied to synthetic fibres removes oils, lubricants and anti-static substances, dust, contaminants and can be carried out on yarns and fabrics (when warp yarns have been bonded, the treatment is called de-bonding). It is carried out by means of surfactants, detergents and emulsifying agents.

To meet the requirements of a useful scouring process, various chemicals are widely known over the years. These includes silicates, phosphates, linear alkyl benzene sulphonates, APEO/NPEO, fatty alcohol sulphates, ethoxylated alcohols etc. The above-cited examples, although in principle are suitable for scouring, but do not have optimum properties in every regard. More particularly, the tendency to form foam, the excessive viscosity, the compatibility with enzymes and a strongly acidic pH, creates problems in a number of cases of known compositions for the pretreatment of textiles.

Also compositions containing linear alkyl benzene sulphonates, APEO/NPEO are banned in European countries as per the requirements of Directive 2003/53/EC of the European Parliament and of the Council of 18th June 2003 amending for the 26th time Council Directive 76/769/EEC relating to restrictions on the marketing and use of certain dangerous substances and preparations. Detergents containing silicates and phosphates are not readily bio-degradable. Hence, one must select an appropriate scouring agent that is APEO/NPEO free, readily bio-degradable and meets European standards.



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Bangladesh Textile Today
Issue: Mar-Apr , 2010