Potential of Sudanese Tannin Materials for Leather Industry

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El Mahdi Ahmed, Haroun
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A survey was made for a large number of plant species with the objectives of evaluation of the quantity and chemical nature of their tannins and their potential for use in the leather industry in order to replace the imported polluting syntans and other pretanning and retanning agents as well as minimizing the cost of processing of upper leather. The tannins were extracted with cold water, and their presence in the extracts was detected by the gelatine salts test, while their type was determined by the iron-alum and formaldehyde-H Cl test. The tannin purity or the ratio of tannin to non-tannin was equal to or higher than 0.6 for 17 species investigated, while the extraction ratio was acceptable only for 15 species. The different chromatographic analyses gave evidence for the presence of catechin and gallic acid and also revealed the presence of some unidentified phenolics. There was a high correlation between the Folin-Denis assay and Hagerman method for determination of phenolic content. The highest astringency values obtained nearest to that of mimosa (0.16) were for three species: .Acacia senegal (0,50), A, mellifera (0.33), and Azadirachta indica (0.18). The first two species were excluded since they contained less than 10% tannin, To overcome the deficiency of the low astringency of Acacia nilotica (0.11), this is available in large areas in Sudan, different mixtures of highly and weakly astringent extracts were prepared in different proportions and their protein precipitation profiles were studied. The blend of tannin extracts from Acacia nilotica ssp tomentosa (pods) and Azadiracta indica (bark) gave a precipitation profile that was closest to the one observed for mimosa bark extracts. Acacia nilotica and Azadirchta indica extract tannin trials were carried out further for application in leather industry. Acacia nilotica extracts was used first for retanning of traditionally garad tanned crust leather and chrome tanned leather. Retanning of chrome tanned leather with A. nilotica gave the best physical properties, especially the shrinkage temperature (> 110°C), while the traditionally garad tanned crust leather gave only 84°C. This indicated the better crosslinking obtained in chrome tanning. Further semi metal tanning was conducted using A. nilotica and Azadiracta indica as vegetable tanning materials and metal salts of Al(III), Zr(IV), Ti(IV), Ti(III) as retanning materials. Aluminium gave higher results in the physical properties compared to the other metal salts. A. nilotica tannin gave better results in semi metal tanning compared to Azadiracta indica tannin. This was due to the different chemical nature of those tannins. The gallocatechol tannin gave better crosslinking with the above metals compared to the catechol type. Furthermore, Acacia nilotica and Azadiracta indica tannins were used in retanning with aldehyde crosslinkers (formaldehyde, glutaraldehyde, tetrakishy-droxymethyl phosphonium sulphate, oxazolidine). The result from Azadiracta indica trial was the best, giving high shrinkage temperature (110°C) with oxazolidine (Neosyn. TX), while A. nilotica tannin gave 90°C. This is due to the type of tannin of the Azadirachta indica (condensed tannin), while that of A. nilotica is predominantly hydrolysable tannin. The GPC results showed the molecular weight distribution of the Acacia nilotica tannin, and revealed a distribution from monomer up to the tetramers at the top of peak. Infrared spectroscopy of A. nilotica ssp tomentosa tannin showed the presence of carboxyl groups, hydroxyl groups, benzene ring, and procyinidine, an indication that this tannin is of mixed type (gallocatechol). This work confirms the garad tannins as the best of the tannins of all species investigated and as most suitable for modIfications, It is recommended to be used confidently as a pretanning and retanning agent for upper leather with the recipes outlined in the present work.
Potential of Sudanese Tannin Materials for Leather Industry