Carbonising of wool

Carbonising is done to remove the cellulosic impurities from wool by treatment with acid or acid producing salts. Carbonising may be carried out in loose wool or on piece goods after scouring. However, it is common practice to carbonise worsted piece goods as well as woollens. 

In a rapid carbonising process [48] wool is treated in up to 8% (w/v) sulphuric acid solution for 30 sec and time delay of about 10-15 min is introduced before drying. The locally damaged areas can greatly weaken the fibre if surface acid is not evenly distributed [49] during drying. However, in the rapid carbonising method with a delay of 10-15 min, the distribution of sulphuric acid between surface acid 

The wool burrs contain hemi-cellulose and lignin apart from cellulose. The lignin is not effected by carbonising process on acid hydrolysis, but can be split oxidatively. Salt of persulphuric acid accelerates degradation of wool burrs which enable shortening the time of reaction [51]. 

Carbonised wool are more porous than scoured wool and hence the dye uptake is higher. Uneven dyeing may result due to faulty carbonising [54]. 

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Vegetable matters of wool are normally removed by a process known as carbonising. Carbonisation of wool with inorganic acid may cause some degradation of the fibre. The replacement of carbonisation by the use of enzymes, such as cellulases, ligninases, hydrolases, lyases and oxidoreductases are reported [93]. A biochemical alternative using complex combination of enzymes to the chemical process of carbonising with sulphuric acid is also reported [94]. The amount ol sulphuric acid required for carbonisation can be reduced by the action of cellulolytic and pectinolytic enzymes [95]. 

The traditional method of carbonising with sulphuric acid is environmentally undesirable and can easily lead to fibre damage. Hence it is not surprising that research has been directed towards alternatives in which enzymes are used to remove the cellulosic impurities from wool. Cellulases and lignases are mainly used but others have been proposed [116] ... 

As already mentioned, sulphuric acid is by far the most common carbonising agent. In traditional processes, it is applied at 4-5% concentration with a dwell time of 3-5 minutes. So-called rapid processes apply 7-8% sulphuric acid with very short dwell times, typically 5 seconds. When used alone, there is a danger that localised droplets of highly concentrated sulphuric acid can be formed, with consequent damage to the wool. The critical conditions for this to occur are met when the acid concentration reaches 40-45% [286-288]. 

Large amounts of residual acid may cause damage to the wool, so that careful neutralisation after baking is an essential and important stage of the process. Carbonised fabrics allowed to accumulate without neutralisation at moderate humidity may suffer considerable damage, so it is essential that neutralisation should take place as soon as possible after carbonising. Neutralisation with ammonia or a mixture of ammonia and ammonium acetate is achieved more rapidly than with sodium carbonate or sodium acetate the ammonia is best used cold [146,286,290]. 

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