**Summary**

The aim of the present survey is to determine the optimal application conditions of three different fatliquors in order to get the best properties for the leathers. This paper presents the results of a comparative survey that was carried out between a sulfated triglyceride – considered to be a standard fatliquor – and three different kinds of fatliquoring agents, namely, a sulfited fish oil, a fatliquoring polymer, and a sulfited phosphoric ester.

The following properties were assessed: degree of softness, weight, thickness, compactness, intensity of colour, tensile strength, tear resistance, extractable fatty matter and volatile fat. By using an statistical design, we carried out experiments with the various fatliquors and the results were compared with those achieved with the sulfated fatliquor (reference); thus we found the optimal application surface for each property.

**Introduction**

The manufacture of soft and lightweight leathers is a required not only in the field of clothing nappa and upholstery leather, but also for items of footwear, in which nappa-like leathers have prevailed for many years. It is a well-known fact that the fatliquoring process lubricates the leather fibres, coating them with substances that reduces the internal friction. There are many studies dealing with fatliquoring, both in its classic version ^{(1,2,3,4,5,6,10)} and in its modern version with the use of polymers with fatliquoring properties.

**1. Experimental parts**

**1.1 Materials and method**

Three whole 1.4 mm wet-blue cow hides were processed. They were divided into two sides along the backbone. These sides were dried and conditioned in a chamber at a temperature of 22ºC and 62% relative humidity for two hours. Then we measured their thickness and their weight, and samples were cut in order to assess the extractable fatty matter and the volatiles.

Left halves were submitted to a standard process (described in Figure 1) using 10% of active matter of sulfated triglyceride (A). Right halves were submitted to the same process but using three different fatliquors: sulfited fish oil (B); acrylic polymer with a fatliquoring character (C) and a sulfited phosphoric ester (D), and their mixtures with a total rate of active matter of 10%.

A whole fourth leather was processed in the drums of a pilot plant, provided with the automatic control of speed and temperature. We assessed the effect of each fatliquor or of each of their mixtures by comparing their left half with their right half in accordance with equation 1 (see Leather International WNLC page 30).

We used a Simplex-Centroid design at an experimental level, adjusting it to a quadratic model. Table 1 shows the fatliquoring formulation for the seven experiments carried out and the fatliquors and the combinations tested. The statistical analysis of the results was made using Statgraphics Plus^{18} in order to find the best areas for each property.

The aim was to get a representation relating the results of the nine variables shown in Table 1 and allowing the prediction of results that would be obtained through a ternary formulation of the fatliquors B, C, and D, with a maximum 10% of active matter on wet-blue weight. A graphic is used, which is a representation of the ternary sample, and a unitary fatliquor at a 10% concentration rate is placed at each one of the triangle vertices. This means that there is no mixture at these points and the value of the property in comparison with fatliquor A is shown.

The segments formed by the three sides of the triangle are the areas where two fatliquors coexist, while the third one, belonging to the opposite vertex, shows a composition of 0%. The central point will coincide with test 7, which has 3.33% of each fatliquor.

**2. Results & discussion**

**2.1 Softness degree**

Softness was measured through a Softness Tester in accordance with IUP-36 Standard. Eleven values were determined on each area of the leather, resulting in 33 measurements and a <0.3% standard deviation. The values of the right half were compared with those of the left half (reference) by applying equation 1.

From Table 1 we can see how the sulfited phosphoric ester (fatliquor D) is the softest (12.8), followed by the sulfited fatliquor (fatliquor B) with a 9.2 value. The fatliquoring polymer gives a value of 3.6, which means that it is the least soft.

Regarding the mixtures, no mixture goes beyond the highest softness value given by one single component, which is that given by the sulfited phosphoric ester (D); therefore, we can corroborate the results achieved by other similar surveys19 that no synergistic effect exists between the fatliquors.

A quadratic equation was adjusted with a view to predicting the % variation in the values of the degree of softness (B) in the right sides belonging to fatliquors B, C and D, compared with those of the left side (Reference A). The result was R^{2} = 98,49%.

**2.2 Weight**

To assess the weight that each fatliquor brings to the leather, we recorded the weight of the sides in a wet-blue condition, dry and conditioned in a chamber at 22ºC and 62% relative humidity for 24 hours. A fourth leather was then processed under the same conditions in order to assess the weight that the rest of the process brings to the leather.

Equation 2 (see Leather International WNLC page 30) was used to assess the weight percentage:

Once the weight percentage accounted for in the leather by each fatliquor was known, we subtracted the weight percentage brought to the leather by the rest of the process to identify the weight increase produced by the three fatliquors (B, C and D) compared with the reference fatliquor A, using equation 1. Without adding fatliquor, ie the fourth leather, we were able to base our calculations on these values.

Fatliquoring polymer (C) is the one that brings the least weight to the leather (-44%), followed by the sulfited phosphoric ester (-32.6%) and finally the sulfited fish oil (-11.7%). None of the mixtures gives values lower than the fatliquoring polymer (C); therefore, there is not any weight variation due to synergistic effects.

The adjustment of the quadratic equation with a view to predicting the weight variation (% PES) gives a R^{2} = to 97.52%.

**2.3 Thickness**

The thickness was measured in dry and conditioned wet-blue sides by determining its value in eleven different areas of the leather. In all, 33 measurements were taken. Standard deviation was <0.3%.

Equation 3 (see Leather International WNLC page 31) was used to assess the thickness variation (Gr):

We compared the thickness increase in each side with the reference (A). The sulfited fatliquor B reduces the thickness by 14%; the fatliquoring polymer B increases the thickness by 10.3%, while the phosphoric ester increases it by 3.3%. There were no synergistic effects either.

The quadratic equation was adjusted with a R^{2} = 99.7%. Figure 3 shows that the highest thickness is found at the vertex where there is 10% of fatliquoring polymer C.

**2.4 Compactness**

Compactness is a subjective characteristic, bringing together fullness and firmness. For this measurement various methods have been propounded by several authors^{(11, 12, 13, 14)}. In the present survey, we defined compactness by saying that it is the quotient between the thickness increase (% Gr) and the degree of softness (%Bi):

Table 1 shows the compactness variation caused by each fatliquor and their mixtures in relation to reference A. The highest compactness is given by the fatliquoring polymer C, with 28%; the sulfited fatliquor B reduces it by 15%, while the sulfited phosphoric ester increases it slightly by 2.5%. There were no synergistic effects either between the fatliquors.

The adjustment of the quadratic equation gives a R^{2} = 99.8%. The highest compactness is found at the vertex of the fatliquoring polymer C.

**2.5 Intensity of colour**

The method used was the same as for the degree of softness, that is to say, measurement of the intensity of colour (L*) in 33 different points with a standard deviation of <0.28%. The values found on the right halves were compared with reference A, applying equation 1.

Table 1 shows that the sulfited fatliquor B is the one that gives the highest intensity of colour, ie the lowest luminosity (%) L* = -12), followed by the sulfited phosphoric ester D with a -11% variation. Finally, the fatliquoring polymer C increases the colour by just 2.7%. No synergistic effects were noticed in the mixtures.

The adjustment of the quadratic equation gives a R^{2} = 99.92%. Figure 4 shows that the highest intensity of colour is found on the side of the triangle in the vertices of which the fatliquors are located.

**2.6 Tensile strength**

Several samples were cut both parallel and perpendicular and the assessment was made in accordance with the IUP-6 standard. The average value of the samples cut in a parallel way was 38% higher than those cut in the perpendicular samples, a fact due to the directionality of the fibre structure within the leather^{12}.

The application of equation 1 gives the values for tensile strength in Table 1 as the variation for each fatliquor or its mixtures with respect to reference A, taking the average between the parallel and the perpendicular values. The highest tensile resistance is given by the sulfited phosphoric ester B with 28%; followed by sulfited fatliquor B with 10% and finally a fatliquor polymer C with 8.4%. No synergistic effects were noticed either.

The adjustment of the quadratic equation gives a R2 = 99.3%. The highest tensile strength values are found on the side of the triangle in the vertices of which fatliquors B and D are located. As we move towards the centre of the triangle the resistances diminish.

**2.7 Tear resistance**

Several samples were cut parallel and perpendicular and the assessment was made in accordance with the IUP-8 standard. We noticed that the samples cut in a parallel way showed average values that were a 13% lower than those cut in the perpendicular, due to the directionality of the fibre structure of the leather, as suggested by several researchers^{(15,16,17)}.

Table 1 shows the variation rate in the tear resistance, taking the average between the parallel and the perpendicular values. The phosphoric ester D brings a 22% improvement with respect to fatliquor A; the sulfited fatliquor B increases the resistance by 4.9% while the fatliquoring polymer C does not introduce any modification. No synergistic effects were noticed.

The adjustment of the quadratic equation gives a R^{2} = 98.8%. The highest tear resistance values are found on the vertex where the phosphoric ester D is located. As we move away from that vertex the values diminish.

**2.8 Extractable fatty matter**

For the quantitative determination of the extractable fatty matter with methylene chloride (IUC-4), we assessed each side in a dry wet-blue condition with fatliquors A, B, C, and D, as well as the three latter in combination (Figure 2).

The extractable fatty matter values in dry wet-blue condition were very similar in the seven leathers (2.1/2.4/2.7/2.3/2.8 and 2.3%). An average value of 2.4% was taken away from the extractability values before applying equation 1 for the determination of the properties.

The lowest extractable fatty matter belongs to the polymer fatliquor C with a value of 47% lower than reference A; the sulfited fatliquor B is 10.4% less extractable, and sulfited phosphoric ester is 11% more extractable than reference A. There were no synergistic effects either.

The adjustment of the quadratic equation gives a R^{2} = 98%. The lowest extractibility values are found on the vertex where the polymer fatliquor C is located. As we move away from that vertex the extractibility values increase.

**2.9 Volatile fat**

For the determination of the volatile fat, we resorted to the method used for the assessment of the fogging effect in leathers for car upholstery. We used the Make K-20 paragraph and the test DIN-75201-B, in which the samples are submitted to a temperature of 100ºC for 16 hours. The volatile elements are deposited on an aluminium sheet, which is subsequently weighed with a view to determining the volatile fatty matter.

Each side was assessed in a dry wet-blue condition and in each side belonging to fatliquors A, B, C and D, as well as the mixtures of the three latter ones at the end of the process in a dried and conditioned state.

The volatile fat values in dry wet-blue condition were very similar with an average value of 2.8% and a deviation lower than 0.1%. These values were taken away from the value achieved with each fatliquor or with their mixtures, applying the equation 1 to these values for the determination of the properties.

Table 1 shows that the lowest quantity of volatile fat belongs to fatliquoring polymer C, with 80% less than its reference A, followed very closely by the sulfited fatliquor B (71% less) and the sulfited phosphoric ester D with 10% less. No synergistic effects were noticed.

The adjustment of the quadratic equation gives a R^{2} = 99%. Figure 5 shows that the lowest extractable fat values are found on the vertex where the fatliquors B and C are located.

**3. Process optimisation**

The determination of the best areas for each property within the graph allows us to choose which can be considered most important for each item. Obviously the more properties, the less common areas will be found. Here are three examples:

**Clothing nappa**

The main properties taken into account were the softness degree, the weight and the tear resistance. We identified the optimal areas for each one of the above-mentioned properties, as well as their common area. Within the common area we looked for the point representing the highest value for each property. In such a point the fatliquor rates were as follows: 1.5% of fatliquor B, 2% of fatliquoring polymer C, and 6.5% of fatliquor D.

**Leather for car upholstery**

This item is very demanding as far as the volatile fat property is concerned and the value ought to be very low. The other two properties were the softness degree and the tensile strength.

These requirements leave an optimal area limited by the axis C-B, while the softness degree and the physical resistances reduce it to a point whose fatliquoring values would be as follows: 8.5% of fatliquor B, 1.5% of fatliquoring polymer C (Figure 6).

**Footwear nappa**

The main properties taken into account were the degree of sotness, the compactness, and the tensile strength. The optimal fatliquoring composition would be as follows: 1.7% of fatliquor B, 3% of fatliquoring polymer C, and 5.3% of fatliquor D.

**4. Conclusions**

The fatliquor made up of sulfited phosphoric esters (D) gives the highest degree of softness and the best physical resistances. The fatliquoring polymer C brings the lowest weight, the highest thickness and compactness, the lowest extractibility and the lowest volatility.

The sulfited fatliquor B gives the higher intensity of colour and it is the softest with low volatility (fogging). No synergistic effects are produced by the mixtures of the three fatliquors.

**PRODUCTS FROM CROMOGENIA-UNITS**

A. Fosfol 50; B. Fosfol AUT C-3; C. Retanal PR-165; D. Repelan WR-10; E.Celesal DL; F. Retanal CP SUPER; G. Retanal NS; H. Retanal HD; I. Retanal RST; J. Retanal TRT