Importance of neutralization and Choice of chemicals for neutralizations

Importance of neutralization

The term neutralization signifies the removal of natural salts, uncombined chromium salts and strong acid from leather or an acid exchange  with the formation of weaker acids. The free or easily liberated strong acid of chrome leather must be removed, otherwise it  may lead to trouble during the dyeing fatliquoring and retanning processes. If not properly neutralizations, colour stains and overloading of grain are caused by dyes and tanning agents existing on thee surface and smudging (dirty stain) and fat stains result from precipitation of the fat emulsion. Furthermore consequence are defects in he leather or also in the end products. An excessive neutralization may damage the chrome leather and can no longer be remedied (solve) even  by adding acid.

Contents of this  article:

Neutralization

Importance of Neutralization

chemicals for neutralization

Operational

Choice of chemicals  for neutralizations

The most widely known neutralizing agents used for mineral tanned leathers  are the weak alkalis  such  as borax or sodium bicarbonate  which are however unbuffered and are therefore  mostly use in small quantities  for surface neutralizing. But when  uniform neutralizations effect throughout the depth of leather is aimed at alkalis with buffering effect namely formats (sodium or  calcium) or sodium acetate are  used. Neutral naphthalene syntan or small particles size phenolic suntans with or without dicarboxylic acid salts , sodium sulphite basic aluminum silicate also efficient neutralizers for uniformity depth and safety.

> Operational 

Cut off a small piece of  leather if  possible from  a section  of dense texture. Then let an 0.1% indicator solution of  bromocresol green (dissolved in 50% alcohol) drip onto the  cross-section. The change of  color indicates the progressive depth of penetration of the neutralizing agent and the pH value  on the leather. 

Transition interval of bromocresol green:

It will be realized that the process of neutralization by raising the pH of chrome tanned leather 4.5-5.5 is in effect the  reversal  of cationic charge more so when organic acid/salts are employed (formate, acetate, phthalate, syntans) and  extent to which the charge is reversed depends on the amount of these  chemicals  used by tanners.  The degree of the charge reversal in hide should be carried out with the aim of regulating the depth  of penetration  of the retanning chemicals which are mostly anionic but could  be in some cases cationic as well (dye fixing agent cationic fats or  some special  retans). This  understanding is very essential  for  obtaining maximum benefit in the retannig  procedures. As a corrective measure and to offset any hardness present in water a little chelating agent  (EDTA, ethylene diamine tetra-acetic acid; NTA; nitrile triacetic acid; sodium tetraphisophate; tetra sodium prophosphate ec) can be mixed with neutralizig salt. In  order to obtain a  slight surface bleaching effect a little sodium  thiosulphate  in presence of a very little  oxalic acid are also employed followed by usual formate,  sodium acetate or bicarbonate neutralization.