Mechanism of NaOH Action with Cotton During Scouring Process

Cotton fabrics arrive at mills with number of impurities. This includes motes, seed coat fragments, pesticides, dirt, chemical residues, metallic salts of various kinds and immature fibres.

Scouring with NaOH cleans by physical loosening fragments from the fabric and by dissolution of metallic salts and chemical residues. It softens and preconditions the seed coat fragment materials entrapped in the yarns and the fabrics. The dilute alkali swells the seed coat fragment material and opens up the cell structure to access the hydrogen peroxide in bleaching that takes place later. The swelling process helps to loosen the attachment of the seed coat fragments from the yarn and fibres. High temperature in the presence of sodium hydroxide melts some of the waxy material and converts some of it to a water-soluble form. It also converts non-cellulosic material (pectins, hemicelluloses and proteins) in the cuticle primary wall to water soluble forms to effect removal.

More precisely, the scouring process is based on the reaction between cotton impurities and alkali hydroxide. Traditional scouring implies a certain alkali consumption that determines the minimum concentration of sodium hydroxide to be used. When sodium hydroxide is brought into contact with the cotton fabric, some of the alkali absorbs, since the hydroxyl groups of cellulose have a weak acidic character. So at PH around 13-14, cellulose absorbs about 1% or 10 g/kg of sodium hydroxide. Alkali 0.5% of the sodium hydroxide concentration is required to change the pectins into water soluble salts of pectic or meta-pectic acid. Neutralisation of the amino acids obtained by hydrolysis of the protein used around 1% of sodium hydroxide. It is evident that around 3% to 4% of sodium hydroxide is necessary for the saponification of waxes and to maintain sufficient alkalinity .Literature reveals that fats are esters of fatty acids with glycerol and constitute around 37% to 47% of the total fat constituents. They have low melting points and are hydrolysed into soaps and glycerol (saponification reaction) quite easily using an aqueous solution of NaOH.

So, during the scouring process, the intra and intermolecular hydrogen bridges of the cellulose are cleaved and the polar hydroxyl groups of the polysaccharides are solvated .The fabric swells, and this facilitates transport of the impurities from the interior of the fibre to the outside. Now The removal of impurities of cotton during scouring with NaOH i.e. alkaline scouring process shown in Table

Table: The removal of impurities of cotton during alkaline scouring process 

Impurities
Mechanism of impurity removal
Fats & waxes
Saponification: The saponifiable parts of waxes (fatty acid, glycerides, and esterd) are converted into soap.
Emulsification: The non-saponifiable parts of the waxes such as alcohols and hydrocarbons are emulsified by the soap formed.
High temperature: melts some of the waxy materials and converts some of it to a water soluble form.
In extreme cases the use of solvent is necessary.

Pectin and related substances
Solubilisation: By the action of alkali, which also acts as a swelling agent to facilitate removal.
Pectins are converted to water soluble salts of pectic or meta-pectic acid.
Proteins and amino acids
Hydrolysis: Proteins are hydrolysed with the formation of soluble sodium salts of amino acid.
Hemicelluloses
Dissolution: Hemicelluloses with low DP are dissolved in NaOH.
Inorganic substances, minerals and heavy metals
Partially dissolve in NaOH.
By producing more soluble salt e.g. acid demineralisation.
By use of  sequestering or chelating agents.
Author of This Article:
Apu Das
B.Sc. in Textile Technology
Chittagong Textile Engineering College
Facebook: Apu Das

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