Mechanism of Removal of Impurities During Scouring Process

Scouring:
The term ‘scouring’ applies to the removal of impurities such as oils, was, gums, soluble impurities and sold dirt commonly found in textile material and produce a hydrophilic and clean cloth. Scouring is a purifying treatment of textiles. The objective of scouring is to reduce the amount of impurities sufficiently to obtain level and reproducible results in dyeing and finishing operations.
 

Mechanism of Removal of Impurities:
The action of alkaline scouring agent is to saponify any residual oils, to neutralise carboxylic acids, to solubilise any sizing materials and to cause dispersion of naturally occuring impurities in natural fibres. Table summerises the removal process involved during scouring.

Techniques for Removing Natural Impurities of Cotton During Scouring:

Impurities
Method of removal
Fats and waxes
Levels are reduced to acceptable limits by the action of alkali and surface active products, in extreme cases the use of solvent and surfactant mixtures may be necessary.
Pectins and related substances
Solubilised by the action of alkali, usually caustic soda, which also acts as a swelling agent to facilitate removal.
Minerals and heavy metals
(a) By producing more soluble salts e.g. acid demineralisation (b) By use of sequestering agents.
Amino acids or proteins
Solubilised by producing corresponding sodium salt.
Lubricants/Knitting oils
Modem mineral oil formulation usually contain their own self-emulsification system.
Natural fats, oils and lubricants (tallow) are mostly esters usually in the form of triglycerides. Being triglycerides, the lubricants can be almost hydrolysed by lipases, yielding glycerol, fatty acids and mono-and diglycerides as the reaction product. 

Glycerol is completely water soluble, fatty acid is removed during scouring and mono and diglycerides are known to be efficient surfactants or emulsifiers. Thus, a lipase treatment improve not only desizing but also the scouring processes. The esters react with sodium hydroxide to form soap and glycerine. The soap thus form can serve as an effective detergent and promote scouring. The unsaponifiable oils are emulsified by the soaps formed during hydrolysis of the saponifiable matter and are easily removed.

Wax is difficult to remove. If wax is not removed, non-uniform absorption of dyes and finishing agents will take place. In fact it is the distribution of residual wax after scouring that determines the water absorbancy.

Pectic acid is insoluble in water but soluble in alkaline solution. Proteins are situated in the central cavity of the fibre and are therefore relatively inaccessible to chemical attack. The proteins and nitrogeneous materials are hydrolysed by alkali into soluble amino acids or ammonia.

The alkali earth elements represents a major variable in cotton fibre and mainly comes from cotton seed husks. Further the use of hard water can precipitate alkali earth metal phosphates on the fibre instead of eliminating them.

The reason for reddish shade after bleaching with peroxide is the high content of manganese, Mn (2+)→colourless but Mn (7+ )→ violet. The contents of Fe, Mn, Ca and Mg varies depending upon the origin of cotton . With the demineralization treatment the mineral content of cellulosic substrates can be reduced substantially and thereby diminishing the negative influence of earth alkali and heavy metal ions in subsequent process operation. Complexion of earth alkali salts (Ca and Mg) is possible in the acid medium and heavy metals (Fe and Mn) is possible in alkaline medium. The advantages and benefits of the demineralisation are : better levelness and more brilliance in the dyeing process, lower peroxide consumption, reduction in the ash content, increase in degree of whiteness, regular decomposition of peroxide, no catalytic damage of the fibres (holes) and possibility of replacement of hypochlorite bleach. Generally good pre-treated materials should have 100-300 p.p.m, of Ca and Mg and 4-8 p.p.m, iron.

Sodium hydroxide reacts with 'motes' (cellulose of low crystallinity). Motes are swollen in alkali to form sodium cellulosates which tend to become water soluble. If any 'motes' still remain in the fibre after scouring, subesequent bleaching operation destroys them completely.

References:
  1. Chemical Technology in the Pre-Treatment Processes of Textiles by S.R. KARMAKAR
  2. http://textilelearner.blogspot.com/2013/03/an-overview-of-textile-scouring-process.html
  3. http://textilelearner.blogspot.com/2013/04/mechanism-of-naoh-action-with-cotton.html 
 

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