Masking agents (stability and fixation of chrome)

The sensitivity of the tannage to pH variations can be greatly decreased by the use of masking agents. These are additives to the chrome tanning bath which partially stabilize the complex and which are applied at basicities of 33-35 %. It is desirable to have a tanning complex that has less affinity for the protein and which will therefore be less sensititive to the pH variations outlined above. The addition of sodium format complex accompanied by a corresponding displacement of the sulfate from the complex the longer the contact of the format with the chromium and the higher the temperature at which they are together, or the higher the mole ratio of the format the greater will be the tendency for the complex to develop. Some tanners prefer to obtain the chromium format complex before adding the chrome tanning agent (chromium sulphate) to the tanning bath; other prefer to have the formate present in the pickle as formic acid; still others add the sodium format or calcium format to the drum with the chrome tanning agent. Whatever system is used, consistency is necessary for quality production. To compensate for the greater stability of the chromium format tanning complex it is necessary to tanned at a slightly higher pH than in an unmasked tannage. Other maskings agents have been used in practical tanning these include phthalate, sulfite, oxalate and acetate. Each of these has its own special characteristics the use of format masking is by far the most common commercial method. Formate tannages, in general, are neutralized at the end of the tannage to the pH of approximately 4. Whereas basic chromium sulfates tannages are usually neutralized to pH of about 3.5 the pH at the end of the chrome tannage is one of the most critical control points in determining the quality of leather produced. The individual tanner must determine this value experimentally in order to obtain leather which will be compatible with his entire production procedure and which gives the desired results. The fixation of chrome tannage material as related to masking and pH can be seen in the figure below.
This data is compiled from various laboratory studies by the author. The fixation of unmasked chrome tanning material by the hide is very sensitive to pH. Above a critical point precipitation takes place and fixation is sharply decreased. With formate masking the reaction is less pH depended. Higher pH is needed for optimum fixation and precipitation takes place at a higher pH. With acetate masking, fixation is decreased: the pH at the precipitation point is higher the decrease in fixation of chrome is so drastic as to greatly decrease the effectiveness.