Pretreatment of wool - part-2
Wool Scouring with organic solvent:
The wool cleaning system involves the use of a non-aqueous solvent (tri-chloro-ethylene) and does not use any water in the washing process.
The technique is reported to be applicable to any kind of wool. Scarcity of water is cost likely the main driving force for the implementation of this technique.
The use of water is avoided in the actual wool cleaning process. The only source of water emission is moisture introduced with the wool, steam used in vacuum ejectors and moisture recovered from sir drawn in to the equipment. This water is treated in two steps comprising a solvent air stripping unit and a residual solvent destruction unit. The residual traces destroyed using a free radical process based in the Fenton Reaction (iron and hydrogen peroxide).
Advantages of Organic Solvent Scouring:
Since pesticides adhere strongly to the solvent and are discharged with the grease, the clean wool is reported to be pesticide free. This has positive implications for the downstream processes where wool is finished.
Another positive effect of this technique is the reduction in energy consumption due to the low latent heat of evaporation of the organic solvent compared to water.
Carbonizing is a chemical treatment used to remove the vegetable impurities, which may remain on scoured wool after technical treatments.
The Textile substrates treated are:
Floc / loose fiber (only fiber used to produce fine fabric for garments, so called worsted fabrics).
Fabric ( not applied in the carpet sector).
Carbonizing may be practiced on fabrics before or after felting (depending on the quality) as well as before or after coloring. Carbonizing of raw fiber is rare as the material is then difficult to dye (e.g. after chrome dyes cannot be used or different carbonized yarns will lead to irregularities in the color affinity of the woven fabric).
Synthetic fibers (PA, PAC, PES etc) which are not damaged by the treatment, can also be carbonized; the Carbonising treatment can also be applied on wool blends.
The principle of the process of treatment using a strong acid (mainly) Sulphuric acid to transform cellulose in to mechanically removable hydrocellulose. The best effects are obtained with good pre-washed and dried material. Some processes there fore recommend pre-washing or impregnating the fabric with perchloroethylene.
The classical Carbonising treatment of fabric usually involves the following operational steps:
Impregnated with Sulphuric acid ( 6-9 %of acid).
Squeezing, exhaustion or whizzing of the surplus acid solution (5% solution relative to fabric weight may remain).
baking at 60 to 90C to concentrate the acid
baking at 105 to 130C (Carbonizing).
Rumbling and rapping (mechanical treatments) to remove the carbonized particles.
Washing and neutralizing with dilute Ammonia Solution.
The use of wetting agents leads to thorough wetting of the greige goods and reduces impregnation time. To avoid damage of the wool, an excess of sodium carbonate may be added to the neutralizing liquor.
3. Scouring and Desizing:
The combined treatment of scouring and desizing removes lubricants that are also called lubricating oils, rag pulling or batching oils and in some cases this process also removes sizing agents from woolen yarns and fabrics.
Typical substances that must be removed by scouring wool can be classified as:- Soluble in water
Insoluble in water, but able to emulsify with detergents.
Insoluble in water and non/poorly emulsifying with detergents. These substances are only removed with organic solvent.
The washing will therefore occur with water or with organic solvent (dry cleaning)
Washing (Scouring) solution for wool:-
Water washing: Sodium carbonate or bi-carbonate solution (neutral or weakly alkaline conditions).
Dry Cleaning: Perchloroethylene (most widely used), water and detergent (optional).
Worsted (or combed) wool under goes short water washing (10 to 20 dipping) with strong non-ionic detergents. The treatment usually occurs under same machine used for subsequent dyeing. Colored worsted wool is first washed with an ammoniac solution to remove non-fixed dyes. The treatment is finished with a washing in a solution of formic acid and anti-electrostatics.
Woven or knitted fabrics are commonly washed not only to remove the lubricants but also to give the fabric a special luster or handle. Further more the fabric is relieved from the tensions of the proceeding knitting or weaving. Here too ammoniac solution may be used to pre-wash the fabric.