1. Introduction
At present the most of experts have agreed that tannage with chromium basic salts is a reliable tannage which could be performed with no pollution and consequences on consumers health.
However, concerns on the disposal of solid chrome leather wastes resulting from the hide processing, as well as the problem of the worn-out leather goods have led to the increasing research regarding chromium-free tannage.
Finding a chromium substitute with high perfornance characteristics is thought to be a great challenge for the future of Ieather science.
The characteristic of chrome leather to be boilfastness and low costs are the major criteria of assesment, also being the important impediments in finding the alternatives for chromium salt tannage.
One of the efficient possibilities of removing or decreasing chromium salt polluting is the wet-white technology. This technology, recognized to be the most satisfactory alternative by the most severe critics of chrome tannage alternatives, has been developing more and more last years.
The pretannage agents used may be of highly various sources, but only common feature being the lack of chromium.

2.Preparation of wet-white leathers and their characteristics

In our investigation for the preparation of wet-white leather, we used a metal heterocomplex tanning agent based on aluminium-zirconium-magnesium.
Heterocomplexing the two tanning metals in the presence of magnesium had as a first result the increase in the zirconium salt stability at a much more higher pH, as it is shown in Table 1
Table1 -Alkali fastness for aluminum sulfate,zirconium sulfate,their mixture in the 1:1 as well as for their mixture in the presence of magnesium salts solution
 
Solution PH of 
Solution as such PH  of
analytical NaOH 0.5N  NaHCO3 0.5N   Na2CO3
    pH - at the precipitation point
    initial final initial final initial final
1. Aluminum sulfate 1.75 3.06 3.59 3.91 4.14 4.14 3.65 3.85
2. Zirconium sulfate 0.02 1.05 1.24 1.81 2.83 2.83 3.32 3.32
3. Aluminum sulfate
+zirconiumsulfate 0.29 1.21 2.04 3.33 3.09 3.09 3.07 3.20
4. Aluminum sulfate +zirconiumsulfate+magnesium salt 3.27 3.84 3.86 4.06 4.05 4.14 3.95 4.07
 

When pretanning with aluminium-zirconium-magnesium heterocomplexes, a satisfactory shrinkage temperature in wet-white technology can be reached by use of comparatively low level of metallic oxides, namely 1,8 % metallic oxides based on unsplit pell weight.
Keeping the characteristics of mineral tanned leather, with usual sammying, splitting and shaving abilities is a significant advantage of the pretannage method with aluminium-zirconium-magnesium heterocomplexes.
The major chemical characteristics of the pretannage solution used were:
A1203 + Zr02, g/1.. ........75.92
MgO, g/1..... . ... . . ...     36.8
pH ...............… .. …….. 2.6
The technology used in pretanning leather with the aluminium-zirconium-magnesium complex was the fallowing:
Unsplit bovine leather for shoe uppers
%-age based on pelt weight
30 % float            25oC
6 sodium chloride            5 min.
 0,3 % Preventol WB (1:3)                                             10 min.
 0,6 % formic acid 85 % (1:5)                                         30 min.
0,6 % sulphuric acid 96 % (l:10)                                          3 h
       pH=3 .2
1,8 % metallic oxides (A1203 + ZrOz)                                 6 h
       pH=2.9
control:100 % section with chilly acetic acid
over night
the next day                                                     30 min                                
                                                                      pH=2.9
20 % water 50oC                                            30 min
pH=3.2 
1,8 % sodium bicarbonate 1:20 in installments at 30 min.
                                                                      60 min.
pH=4, Shrinkage temperature(ST) 74oC Drain. Rest

Analyses of the exhausted floates show a level of 1,55 g/l Al203 + Zr02 and 2,44 g/l MgO, that is an 89 % exhaustion of metallic oxides.
The controls were tanned as usually, with basic chromium salts.
The mechanical processes of sammying, splitting and shaving are made in the same conditions as the contols.
In the study of the wet-white leathers some specific characteristics were fallowed: chemical composition, microbiological fastness, and rehydration capability.
The chemical characteristics of the wet-white pretanned leathers are shown in the Table 2.
 
 
 

Table2-Chemical Characteristics of  Wet-White and Wet- Blue Leathers

Processing way Humidity
    % Ash
 % Fats Hide substances Cr2O3 Al2O3 ZrO2  ST
 C pH
Wet-white 56 13.9 1.79 84   ---- 3.9 74 3.9
Wet-blue 49 12.5 1.02 79 4.4  ---- 105 4.2

Mouldfastness of wet-white leathers was studied in comparison with the control wet-blue leathers in an usual curing. We also found being interesting to study the microbiological fastness of uncured wet-white leathers as compared to uncured wet-blue leathers.
The analysis method used is shown in STAS 12.697/1988 and consist in incubating samples at 28 + 2oC and 95-98 % RU. in the culture environment, specific for mould growth, for 28 days.
The results of these tests are revealed in Table 3

Table3-Mouldfastness of wet-white and wet-blue leathers
           
                Sample
             Marks given after
 3 days 7 days 14 days 28 days
Unsplit antimould treated wet-white 0 0 0 0
Unsplit antimould treated wet-blue 0 0 0 0
Antimould treated wet-white split 0 0 0 0
Antimould treated wet-blue split 0 0 0 0
Untreated unsplit wet-white 0 1 2 5
Untreated unsplit wet-blue 0 2 4 5
 
 

From our studies the pretanned wet-white leather resulted to behave similarly to the wet-blue leather as rega,rding the mouldfastness, either treated or untreated, requiring no further curing. When studying the rehydration capability of wet-white pretanned leather, we dryed leather samples to a humidity of 20-22 % and then rehydrated them for variable times. The results are revealed in the Table 4, showing an usual rehydration ability.
 
 

Sample
                           HUMIDITY %
 Initial After
drying                     Rehydration
      1h     2h     3h
Unsplit wet-white
 54 22 51.7 53.3 54.8
Split wet-white
 48 20.8 58.3 57.8 58.7
 

The samples were subsequently treated according to the formulation:
Washing:100 % Water    40oC
0,2 washing agent     15 min.
Draining
Retanning: 100 % Water    40oC
 1,5 % Cr203     2 h
0,4 sodium bicarbonate (1:20)   30 min.
 0,3 o/a sodium bicarbonate ( 1:20)  60 min.

                                                            pH=3. 8
The controls were rechromated following the same formulation but with 1 % Cr203 The analysis results for the exhausted floats are shown in Table 5.

Table5-Exhausted floats from retanning
Exhausted floats    Cr2O3, g/l
Sample wet-white 0.61
Control wet-blue 2.53
 

Chromium exhaustion in this stage resulted thus to be better for samples as compared to controls. One of the phenomena arising in wet-white processing, also reported in literature, is the variation in leather thickness during processing stages, which is different from that in wet-blue leather
To clear up this phenomenon, the leather thickness was measured in eight points on a rectangle (the same for samples and controls), in various processing stages as it can be seen in Table 6.

Table6-Variation in medium thickness in wet-white and wet blue leathers during the different processing stages

      Sample

            The stages where the thickness was measured
 Tanned,shaved
(Pre-tanned,shaved)
 Tanned,shaved
(Pre-tanned,shaved)
after two days Finished
(crust)
Control 2.05 2.16 2.10
Sample 1.84 1.78 2.00
Control 1.66 1.50 1.78
Sample 1.50 1.46 1.67
Control 1.75 1.83 1.85
Sample 1.68 1.67 2.01
Control 1.82 1.92 1.94
Sample 1.84 1.83 2.10
 
 
 

It may be noticed that for wet-white leather structure becomes stable in tannage-retannage, while for wet-blue in tannage.
The need for shaving the pretanned wet-white leathers by 0,2 mm more than wet-blue leathers was reported in literature for other kinds of wet-white.retannages, too.
The results of measuring the leather surface in different manufacture stages are revealed in table 7 and they indirectly strengthen the changes in thickness.

Table7- Variation in surface of wet-white and wet-blue leather in various manufacture stages

The stage where the leather surface was measured                                Surface,dm2
      Sample     Control
 1 2 3 Total 1 2 3 Total
Pickled,sammied 125 84 141 350 123 92 137 353
Wet-white sammied 125 85 140 350    
Wet-blue sammied     110 84 126 320
Wet-white:sammied split,shaved,trimmed 116 78 121 315    
Wet-blue:sammied split shaved trimmed     107 79 122 308
Wet-white crust proc 123 80 130 333    
Wet-blue crust proc     113 84 138 335
 

In Table 7, a decrease by 9,3 % in wet-blue leather surface in pickling-tannage can be seen, as a consequence of tannage with chromium salts, while the wet white leather surface remaining unchanged.
As a result of tannage-retannage, the surface of wet-white leather is increased by 5,7 % because of the development of hide substance and increasing in thickness, and by 8,7 % for wet-blue leather.

By comparing the development of leather surface in pickling-crust stages, the conclusion can be drawn, for a Iimited number of trials, that when preparing wet-white leather a surface increase of 5,7 % can result as compared to 0,6 % for usual processing. Similar data on the improvement of surface yield in wet-white manufacture are also reported in other works. In order to understand the leather structure dynamics and implicitly its elasto-plastic properties in the pickling-pretannage-tannage range, a series of structure investigations by X-ray difraction were performed. Pickled, pretanned, tanned and sammied leathers, conditioned for 48 h at a R.U. of 50 %, were investigated.
The measurements of X-ray difraction at very small angles, specific for prevalently amorphous materials, were processed by the Guinier's method for the determination of gyratory rays, which can be assigned to the voids in collagen at the molecular level ("micro-voids").
The results of X-ray difraction measurements are shown in Table 8.

Table 8 - The gyratory rays of the structure constituents in pickled and tanned Ieather
               Sample
                   Gyratory ray, R
                           Ao
Pickled 15.8
Pre-tanned 12.3
Tanned 14.1
 

The large distances between the structure constituents in pickled leather may be assigned to the influence of water interfering in high amounts.
The results in table-8 are suggesting us that pretannage leads to a dehydration" resulting in the more close structural constituents, while the tannage is resulting in a "more steady" fibre structure simultaneously with the individualization of this structure.
The assumption of a splitting, within the leather, of the heterocomplex in smaller complexes with lower molecular weight than of chromium and in lower amounts can also account for the shorter interstructural distances.
This microstructural dynamics also leads us to the conclusion, that in the pretanned leather of the same thickness there is a higher amount of collagen, which after tannage will get a larger volume than in chromium tannage.
As regarding the order level of collagen material in the processing stages of pickling, pretannage, tannage, the following remarks can be made:
- the pickled leather has no ordered areas, the packed fibres showing a high disorder level;
- for pretanned leathers some ordered areas (lamellar) at regular intervals of 105-108 Ao a revealed;
- for tanned leather, the ordering in lamellar areas is highly diminished and the same regular intervals of 105 Ao as for pretanned leather are revealed.
The analyses with X-ray difraction at large angles also reveal the same crystalline nature, just more marked for pretanned samples as compared to the chrom tanned leather.
Although by chrome tannage and retannage the structure differences between wet-white and wetblue leathers are flattened, they still leave their mark on the characteristics of the finished leathers. This may be seen in Table 9, where the values of radial elongation, surface elongation and elongation set measured on the Bally tensiometer are shown.

Table9- Bidirectional elongation for finished leather made from wet-white and wet-blue

    Sample
 Bidirectional elongation on Bally tensiometer , %
 Radial elongation Surface elongation Elongation set
Radial     Surface
Sample 9.7 13.7 15.1 21.6 6.0 9.5
Control 11.2 15.7 17.8 25.0 7.5 11.7
Sample 6.5 9.5 10.2 14.7 3.7 5.7
Control 9.2 12.2 14.3 19.3 5.7 9.1

The data in Table 9 reveal that for finished leather made from wet-white pretanned leather the plastic constituent is diminished for the benefit of a higher elasticity as compared to the control leather made from wet-blue.
From the wet-white pretanned leathers, tanned with chromium salts at Cr203 ratios of 1,5 % and retanned as usually, various kinds of leathers for shoe upper (nappa, full grain white and black etc.) with good physico-mechanical, chemical and organoleptical properties have been made. As compared to the controls tanned with chrome salts, the leathers made from wet-white show more fullness, a more marked round touch which may be assigned to the influence of zirconium. From the wet-white leathers pretanned with aluminium-zirconium-magnesium heterocomplexes, some "chrome-free" leathers were also made by tannage and retannage with vegetable and synthetic tanning agents.
These leathers show a vegetable-like characteristic and very high and uniform fullness, a slightly more firm stand and a level dyeing.
The shrinkage temperature for these kinds of leather is 88oC, shightly higher than for vegetable tanned leather.

CONCLUSIONS

Pretannnge of  leather with alnminium-zirconium-magnesium heterocomplexes results in wet-white leather which can be deIivered as such, because of its microbiological steadfastness, rehydration capability and chemical characteristics similar to the wet-blue.
Investigations on changes in leather sizes and chemical composition correlated with structure studies made by X-ray difraction have allowed some assumption to be made regarding the structure of wet-white leather in various processing stages, as compared to wet-blue. The pretannage with aluminium-zirconium-magnesium heterocomplexes, even when followed by chrome salt tannage, acts on the elasto-plastic properties in leather. The leathers wet-white pretanned and chrome salt tanned show a higher elasticity, while their plasticity being diminished as compared to chrome tanned leathers.
From the wet-white pretanned leathers have been made various kinds of shoe upper leathers with good physico-mecanical, chemical and organoleptical characteristics, by tannage with chrome salts at ratios of 1,5 % and usual retannage.
The "chrome-free" leathers show a vegetable-like character with good and level fullness.

REFERENCES:

Heidemann E- "Disposing of offals Versees Clean Technology, Two Ways to cope the Enviromental Challenge ", Rolther. E Fundementals of Leather Manufacture, Darmstat,1993, 627

Heidemann E- Keller H. - "X-Ray Studies of Tanned Collagen" ,
The Journal of American Leather Chemists Association,65 ,512

Platon F. Gaidau C. Paduraru G. "Chrome Free Tanning of Leather " XXIII IvLTCS Congress Friedrzchshafen, May 1995

Pxentiss W.C. Prasad V.I. Siegler M. - "Practical Leather Process Using the Novel Pre-Tan Goncept", The Journal of American Leather Chemists Association ,77,1982

Ward J.G. -"Wet-White Pretanning - A Technique for Reducing Chrome Useage ", The Journal o, f 'American Leather Chemists Association,90,5,l995,142-145