Synthetic retanning agents

Introduction

In practice, great difficulty exists in knowing where and how the synthetic retanning agents are distributed. In this second part the authors attempt to ‘visualise', by means of special reagents, the penetration and distribution of the synthetic agents being tested. This process will lead to the ‘anionisation' concept as a distribution factor for the synthetic retanning agents and their interaction with the dyestuff.

1. Colorimetric study

1.1 Anionicity

The chemical structures of the synthetics have in common the sulfonic groups, acting either as solubilisers (sulfonic and phenolic acid derivatives) or as part of the bond (sulfone). The ‘visualisation' of the leather in these groups, both within the corium and on the surface, provides highly valuable information concerning the synthetics' penetration and distribution. It also attempts to relate the tinctorial capacity with reference to the dyestuff penetration and distribution.

The process followed is outlined below (see right):

3 leather pieces for each variable

Process No 1

Before dyeing and fatliquoring, a leather piece is taken out and dyed with a solution of 2g methylene blue (cationic dyestuff), evaluating the penetration (Figure 1) as well as the surface distribution (Figure 2).

The method used is:

Reagent make up

Weight 2g methylene blue. Paste with 50ml water at 20°C. Add 100ml isopropyl alcohol. Stir and level at 1 litre with water at 30°C.

Leather dyeing

Cut 4 x 2 cm leather strips (it is important to cut it with a blade or cutter; if cut with scissors the cut edge is levelled, thus not showing the penetration).

Submerge in a flask containing the methylene blue solution. Take care that the solution covers the leather strip and stir with a rod for 2-3 minutes. Recover the methylene blue for future dyeing and place the flask, containing the leather strip, under a water jet until the water is clean. Take out the leather strip and the penetration and surface distribution can be observed.

Figure 1 displays the reference without retanning having a slight blue colour, while the retanned strips have a deep blue area, in the grain as in the flesh side with different penetration degrees. Generally, the flesh sides have a higher penetration degree than the grain side because the porous structure is more open.

The influence of the syntans has been photographed and has been measured either in the grain or on the flesh side. Table 1 displays the values obtained as well as the total penetration (addition of both), together with the colour intensity (L*) values.

Concerning the grain side, surface distribution can be seen from Figure 2. For colour intensity (L*): A-4 stands out for its strong anionicity and total penetration, followed by SF and NC-10, which also have good penetration results but lower anionicity (blue intensity).

2.2 Dyeing

Two dyestuffs have been selected:

Acid Brown 83 (C18 H11 Cu N6 O8 S Na) diazo acid type, Cu metallic with a molecular weight of 557.5

Acid Black 210 (C34 H27 N11 O11 (S3) triazo acid type with a molecular weight of 861.

The chemical structure of the synthetics used can be represented as follows:

Naphthale sulphonic structure (A-4)

Phenol sulphonic structure (SF)

Diphenyl sulphone (SUL)

General phenolic structure

The interaction of the structures define the dyestuff behaviour against the synthetic agents, either from the penetration or from the distribution point of view.

Acid Brown 83

Continuing with process 1, one leather strip was dyed with Acid Brown 83 under the same conditions used with methylene blue, then the penetration (Figure 3) and its distribution (Figure 4) were evaluated.

Table 2 displays the grain and flesh side values, as well as the total penetration percentage quantification. The study was carried out taking into account that some synthetics have a certain penetration unevenness, even in the same test piece cut for checking, depending on the leather structure and the synthetic performance to retain or to ‘filter' the dyestuff on the surface  (Figure 4). For the mentioned synthetics, SP-2, SUL and TRT, an average value has been chosen.

The authors established a ‘Penetration Factor' which results from the quotient between the dyestuff penetration and the anionic penetration. Lower values mean an even penetration of both dyestuffs; A-4, SF and NC-10 have a total penetration while BW and BN have only a partial one. The higher values mean that there is a higher penetration than expected, due to the anionicity, BH-10, SUL and TRT.

The anionic syntans have a lower penetration than the rest. The Reference (without retanning) has a penetration of 56%.

Concerning the dyestuff surface distribution on the grain side, Figure 7 shows that the Reference has a higher colour intensity (minimum value of L* = 41.13); A-4 has the highest colour intensity (L* = 48.56), being SP-2 the one presenting the lower intensity (L* = 60.13). The remaining synthetic agents have similar intensities, slightly varying from red to yellow.

Acid Black 210

Following process 1, one of the leather strips was dyed with Acid Black 210 under the same conditions used for the Acid Brown 83, and the penetration was evaluated (Figure 5), as well as the colour intensity (which can be seen in Figure 6).

All the synthetics have a higher penetration on the flesh side than on the grain side. The one with the highest penetration factor was BH-10 (4.1), followed by TRT (2.93) and SUL (2.25). The lowest ones are: A-4 (1.0), SF (1.09) and NC-10 (1.16).

Concerning penetration levelness, when dyeing with Acid Black 210 it was necessary to add to SUL, SP-2 and TRT (both having the lowest factor with Acid Brown 83), KR and NC-10.

The highest intensity corresponds to the Reference (L* = 28.14) followed by A-4 (L* = 28.32).

2.3 Tinctorial properties summary

a. Penetration

1) Both dyestuffs have similar penetration factors but there is a significant difference between them. Acid Brown 83 has a higher diffusion capacity, thus even when there is not a total penetration it always produces a slight dye in the middle of the leather, while the Acid Black 210 leaves a white colour where its penetration ends (comparing Figures 3 and 5).

2) Low penetration factors correspond to the naphthalene or phenol sulfonic structures (A-4, SF, NC-10), as displayed in Figure 8, where the methylene blue dyestuff penetration is poorer than the anionic dyestuffs, results being 100% for both types.

3) High penetration values (BH-10) corresponding to the polymeric phenol show a higher penetration than expected, due to the anionic penetration.

Both dyestuffs show in the cut leather, with a lighter colour in the grain and flesh sides (corresponding to the synthetic anionic part) and a deeper shade in the inner part corresponding to the non anionised area, which has been reached by the dyestuff through the polymeric net.

4) The low penetration some synthetic agents have (BW) means that a methylene blue average penetration of both dyestuffs has occurred (fig 8).

5) The penetration evenness is influenced by the leather structure and different fibre compactness; that is to say that the same synthetic agent can physically fill larger areas and facilitate the dyestuff penetration in this same spot.

For additional tables and illustrations, please see page 38 of the print version of Leather International June 2008

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