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Abstract
In the conventional leather production process, basic chromium sulphate (BCS) penetrates fully inside the collagen matrix during tanning and forms oligo-nuclear complexes that stabilize the skin structure during basification. Accelerated chromium sulphate complexation by basifying the BCS before full penetration leads to surface tanning and low leather qualities. Therefore, long processing times of 16 h for tanning and basification are common in modern tanneries. We used sodium hydrogen carbonate to basify BCS before penetration. However, instead of conventional tanning we performed CO2 tanning at 30 bar. The influence of accelerated chromium sulphate complexation on the leather quality was investigated. To estimate the structures of the formed chromium complexes, we considered the pH trend of the tanning solution during basification depending on the amount of basifying agent. The distribution of chromium oxide (Cr2O3) and the shrinkage temperature (Ts) in three different layers of the cross section were analysed. The analytics allowed the assessment whether the tanning agent penetrates through the whole cross section or adheres to the surface during CO2 tanning. We show that CO2-intensified tanning allows accelerated chromium sulphate complexation and a reduction in the tanning time from 16 to 4 h. The Cr2O3 is equally distributed, and Ts above 100 °C meets the standards of high-quality wet blue. However, the pH value of the tanning solution must not rise above 4.5 during basification. Otherwise, the Ts decrease dramatically. The feasibility of the process is demonstrated in a 1700-L drum, with a batch size of about 500 kg. The CO2-intensified tanning process has high potential to save time and chemicals and reduce emissions in industrial applications and is commercially competitive.Graphic abstract
Highlights
The production of chromium leather is comparatively simple and results in high leather qualities
The pH values of the tanning solutions depending on the amount of basifying agent are measured to conclude on structures of formed chromium complexes that have to penetrate through the cross section in tanning
Ten tanning solutions containing 4.5 wt.% of basic chromium sulphate (BCS) were prepared and different amounts of alkali salt were added as described in section "Determination of the pH value of the tanning solution depending on the alkali saltconcentration"
Summary
The production of chromium leather is comparatively simple and results in high leather qualities. The pre-tanning processes for the production of chromium leather include soaking, unhairing, deliming, bating and pickling. These processes remove unwanted skin components as hair, fats or non-collagen proteins and acidify the remaining collagen to a pH value of 3 (Zissel 1988). The chromium tanning agent penetrates inside the collagen matrix during tanning and cross-links single fibrils during basification. Negative charges are introduced to the functional carboxy groups This increases the reactivity of the chromium complexes towards the collagen. The introduction of two carboxy groups into the same chromium complex results in linking collagen fibrils and stabilizing of the skin structure, leading to fully tanned durable wet blue (Sundar et al 2002). The term wet blue derives from its high water content and blue colour caused by the chromium tanning agent (Heidemann 2005)
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