The effect of fatliquor sulphitation on leather quality

Introduction
During the fatliquoring process, the fatliquors are deposited or chemically bind to the leather fibres. This allows the fibres to move over each other and prevents them from sticking to each other when the leather is dried. It is therefore a well-known fact that fatliquoring impacts the tensile strength, tear resistance, impermeability of the leather to water, softness, flexibility and elasticity of the leather.

There are several different classes of fatliquor that can be used when making leather. One of these classes are the sulfited fatliquors. The properties that sulfited fatliquors give to leather are well known by tanners. What is less well known is how the degree of sulfitation affects the properties of the leather produced. This study was carried out to analyse how various degrees of sulfitation impact the softness, grain distention, tear resistance and tensile strength of the leather.

Experimental
This study made use of soyabean oil. It was oxidised and subjected to different sulfitation processes. The different anionic sulfited fatliquors were then applied to leather on a pilot scale, and the leather that was obtained was evaluated.

The oxidation of unsaturated carbon-carbon bonds in the soyabean oil was found to be most effective with air being blown into the oil heated to 115°C for 24 hours. Different degrees of sulfitation were then obtained by continuously stirring sodium metabisulphite into this treated oil at dosages of 4%, 6% and 8% (on a mass basis) for 8 hours at 70°C. These three different sulfited oils were mixed with other fatliquor components to produce three different anionic sulfited fatliquors. In this study, these three sulfited fatliquors were compared to a commercially obtained soyabean oil sulfited fatliquor.

The fatliquoring process was carried out as shown in Table 1. 

Table 1

After the fatliquoring process, the leather was dried naturally until a moisture content of 12 – 14% was obtained. The crust leather was then softened.
The leather was evaluated, in triplicate, for softness, grain distension, tear resistance, tensile strength and extension percentage and aging.

Results
The SO3 concentration found in sulfitation tests of the oxidized oils are shown in Figure 1. OS1, OS2 and OS3 are the sulfited oils produced by applying 4%, 6% and 8% of sodium metabisulphite. The higher the sodium metabisulphite dosage, the higher the degree of sulfitation of the fatliquor.

Sulfite concentrations found in the different fatliquors produced from different concentrations of sodium metabisulphite.

The results of the physical-mechanical tests carried out on the crust leather are shown in Figure 2.

Figure 2. Softness (a), grain distention (b), tear resistance (c), tensile strength (d), and elongation percentage (e) of the leather

Among the fatliquors produced in this study, fatliquor 3, with a higher degree of sulfitation, produced superior softness compared to fatliquors 1 and 2 (Figure 2a). Comparing the three fatliquors developed in this study, higher SO3 concentrations in the fatliquor resulted in higher grain distension results. The same proportionality between SO3 and grain distension has been observed in a previous study due to greater penetration of the fatliquor into the leather structure when the SO3 concentration in the fatliquor increases.

The tear resistance results (Figure 2c) show an inverse relationship compared to the grain distention results (Figure 2b). The fatliquors with lower degrees of sulfitation produced higher tear resistance (Figure 2c). Fatliquor 1 presented a higher tensile strength than the others, followed by fatliquor 3 (Figure 2d). Fatliquors 2 and the standard control fatliquor produced similar results.

The elongation percentage (Figure 2e) of the leather is inversely proportional to the percentage of SO3 present in the fatliquor.

Table 2 shows the results of colour change with accelerated aging.

Table 2

The leather sample produced with fatliquor 3 presented the best result, with the smallest colour change, followed by fatliquors 2 and 1. The colour change of the leather with the standard control fatliquor was between fatliquors 2 and 3. The accelerated aging analysis also demonstrated a correlation between the colour change of the leather and the degree of sulfitation, where the higher degree of sulfitation provided the smallest colour change. The colour change of leather is an undesirable aspect, especially in automotive upholstery leather.

Conclusion
Physical-mechanical tests of the produced leather demonstrated that different degrees of sulfitation provide distinct characteristics of softness and fibre resistance to mechanical stress. The leather produced with the lowest SO3 content (2.13%) fatliquor showed the highest results for tensile strength, tear resistance, and extension percentage. Meanwhile, the leather produced with the highest SO3 content (4.27%) fatliquor showed greater softness, grain distension, and less colour change during accelerated aging.

This article is a summary of the paper “Assessment of the Fatliquor’s Degree of Sulfitation Impact on Leather Quality” Journal of the American Leather Chemists Association (JALCA) Vol 120, 125 - 132 (2025).