Leather tanning process

The present invention refers to a new process meant for chrome and/or vegetable leather tanning. The process proposed by the invention comprises the steps of bating or purging, conditioning, and chrome and/or vegetable tannage. The conditioning step uses hydroxy-butaldehyde which reacts by blocking the amines of the collagen and releasing carboyxls. This reaction reduces the isoelectric point to 4.0-4.5, making the hide receptive or susceptible to effective chrome tannage by linking the carboxyls, or to tannin, by linking through hydrogen bridges. Hydroxy-butaldehyde is used in the conditioning step in a proportion of 2.5-3.0% as compared to the weight of the delimed hide, in a concentration of 40.+-.2%, with a pH of 4.2-4.4, and with the reaction time being from 3 to 4 hours.

The present invention refers to a new process for chrome (color) and/or 
vegetable leather tanning. 
BACKGROUND OF THE INVENTION 
The conventional process for leather tanning comprise the following 
operations: bating, pickling, pre-tanning and tanning. 
The bating (purging or cleaning) of leather hides consists of deliming 
(removal of lime) and then bating the hides with enzymes. The second step 
is the pickling operation, where the bated hide is treated with sulfuric 
acid and sodium chloride in order to avoid, eliminate or reduce hide 
deterioration. Afterwards, the pre-tanning step is executed with chrome 
(chromic acid) or glutaraldehyde; this is the process that gives the hide 
its pretanning characteristics, allowing some operations, such as 
splitting, and afterwards, shawing the leather. This phase results in less 
steps with the use of a smaller quantity of chemicals. Finally the tanning 
of the hide is performed with chrome (chromic acid) and/or tannins, in 
accordance with the specifications to be reached. 
SUMMARY OF THE INVENTION 
The object of the present invention is an innovative tanning process which, 
through the addition of a specific product, hydroxy-butaldehyde, to the 
bated or purged hide or skin, results in the conditioning of the hide or 
skin and, later on, chrome and/or vegetable tannage thereof. This process 
results in the shifting of the isoelectric point that, as the reaction 
time passes, varies from 3.0-3.5 to 4.0-4.5, with the pH in the same 
range. The process proposed by the invention allows a chrome tannage, 
favoring its penetration as the penetration pH range is wide, varying from 
4.0 to 6.0 and the optimum point between 4.0 and 6.0 matches with the one 
of the process of the invention. 
Advantages of the present invention over conventional tanning processes 
are: 
a. it is autobasifying, the addition of alkalis not being necessary; 
b. elimination of the pickle and pre-tanning phases with the subsequent 
removal of sulfuric acid and sodium chloride, but creating a conditioning 
phase; 
c. removal of sodium formate and of sodium bicarbonate used in the 
post-pickle treatment; 
d. simplification of the effluents treatment due to the reduction of the 
chrome quantity and the elimination of sodium chloride (main pollution 
sources of the effluents); 
e. reduction of the total tanning time, from 24-36 hours to 12-14 hours.

DETAILED DESCRIPTION 
The tanning process of the present invention comprises the steps of bating 
(purging or cleaning), conditioning with hydroxy-butaldehyde, and chrome 
and/or vegetable tannage. The bating of the hide is performed in 
accordance with conventional tanning processes. The hydroxy-butaldehyde is 
used to condition the bated hide, allowing subsequent tanning and 
retanning treatments, whenever it is the case. Conditioning comprises the 
change of the isoelectric point of the bated hide. This is due to the 
hydroxy-butaldehyde reaction with the amine. The hydroxy-butaldehyde 
blocks the amines of the collagen (proteins found in greatest number in 
the hide), releasing the carboxyls. This, in turn, reduces the isolectric 
point, making the hide receptive and otherwise susceptible to effective 
chrome tannage (linking the carboxyls), or vegetable tannage (linking 
through hydrogen bridges). The percentage of hydroxy-butaldehyde used for 
conditioning varies from 2.5 to 3.0% as compared to the delimed hide 
weight, and the reaction time is approximately 3 to 4 hours. Afterwards, 
the tanning agent is added. In the case of chrome, the tanning time is 
approximately 6 to 7 hours. For vegetable tanning, the reaction time is 
approximately 12 to 14 hours. 
Hydroxy-butaldehyde is obtained from the reaction of acetic aldehyde and 
its structural formula is the following: 
##STR1## 
The characteristics of the product are: 
mw=88.0 
pH=4.2 to 4.4 
concentration=40.+-.2% of hydroxy-butaldehyde 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained, and 
since changes may be made without departing from the spirit and scope of 
the invention, it is understood that the above description is merely 
illustrative and does not otherwise limit the scope of the invention. The 
claims that follow define the inventive scope.