Hybrid extracts for the production of light leathers3 April 2001
In recent years the consumption of vegetable tannins for sole and technical leather tannage has decreased due to a lower worldwide production of these articles through competition from synthetic materials, fashion etc. On the other hand the usage of vegetable tannins in the production of light leathers (bags, upholstery, shoe uppers) has significantly increased. The reasons for this increase are: * requests for leathers with a warm touch and natural odour and vegetable appearance * tendency for the elimination of chrome, aldehydes and free phenol from effluent waters and residual sludge * imposition of chrome free and aldehyde free leathers for car seats, children's shoes * increases in the production of finished leathers starting from poor quality raw materials that require a strong retannage. On the other hand leathers produced in this way must stand an increasing number of physical and chemical tests which are all the time more severe and difficult to pass. Vegetable tannins have a negative effect on the light and heat fastness and they impart a strong colouration to the crust leathers which makes the dyeing operation more difficult. In this work the heat and light fastness of the main vegetable tannins used in the leather industry are compared. Preliminary trials with different tannins showed that tara and chestnut give the best results on resistance to heat and light. Chestnut has the disadvantage of giving a strong colouration to the leather. Furthermore, tara imparts a very light colouration, but being a grounded pod contains high amounts of fibrous material (insolubles) which creates many problems in its application. Starting from the above statements, the best properties of both extracts were exploited to develop an hybrid product with excellent heat and light fastness. Treatment of the tannins The methods applied were physical, physico-chemical and chemical methods. The physical methods used were filtration with active membranes, filtration with passive membranes, flocculation and clarification.The physico-chemical methods studied were absorption and adsorption. The chemical methods studies were oxidation, reduction, co-ordination and reactions with specific radicals. The best results were obtained by treatment with small amounts of special chemicals, which after natural sedimentation, gave excellent clarification of the colour. The chemical agents chosen have no flocculating effect on tara but only on the coloured fraction of the chestnut extract. In order to obtain a tara without insolubles or turbidity, extraction of the pods was carried out using a mixture of organic solvents, followed by stripping with water. The product obtained is thus a concentrated tara extract. Procedure Once a method that gave good results in laboratory trials was found it was applied in the chestnut extraction plant. Some small variants were applied to the normal production process in order to recover the sludge produced and consequently to diminish the production costs. Thus, the process was applied successfully without great and, therefore, expensive losses of product. Figure 1 outlines the production process used to develop the hybrid tannin. After having developed the 'hybrid' chestnut and tara, it was compared with the other vegetable extracts commonly used in tanneries. The analytical values of the single extracts used are given in table 1. With each single tannin a series of tannages on pickled pelt and retannages on wet-blue and wet-white were carried out. Colours and fastness were evaluated. A synthetic tannin 4',4', dihydroxi-diphenyl-sulphone, which has high tanning power, very light colour and with excellent light fastness was used as a reference. The tannages used to produce the leathers, please see attached tables. Results Determination of colour On the leathers produced with these formulations, various assessments were carried out. These were: * Visual assessment. the results are shown in figures 2, 3 and 4 * Instrumental, by means of a surface spectrophotometer. This was used to determine the reflectance values, figures 5 to 7, and bightness values, figures 8, 9 and 10. The reflectance values indicate which leathers have the lightest superficial colour; the higher the reflectance percentage, the lighter the leather colour is produced. The brightness values indicate how much darker or lighter in colour one leather is to another. Reflectance Tara tannin and the hybrid chestnut and tara show a high reflectance in all the three types of leather produced. Mimosa and quebracho tannin, which give high reflectance values in tannage and in retannage of the wet-white, give low values in the retannage of wet-blue. With wet-blue they, therefore, impart a high colouration. Sweetened chestnut extract gave medium reflectance values in the three types of leather produced. Astringent chestnut extract and gambier gave low reflectance values and, therefore, a dark colour in the three types of leather was obtained. Brightness For wet-white leathers tanned and retanned the reference was the leather tanned/retanned with the synthetic tannin. For wet-blue leathers retanned the reference leather was the pure chrome. The readings agree with what was already confirmed with the reflectance readings, and in the diagrams it can be seen that these differences are enhanced. Light and heat exposure As was mentioned in the introduction, the tests that leathers must be able to pass nowadays are more and more numerous. In particular in our research we focused on light and heat fastness. The leather samples were submitted to accelerated ageing in an oven at 100°C for 144 hours for the retanned wet-blue leathers and to 90°C for 24 hours for the retanned wet-white and the tanned pickled pelt. The results are shown in table 2. The Xenotest was carried out for 24 hours on all three leather types. The results are shown in table 3. For the heat fastness the developed hybrid tannin was at least as good as the other vegetable tannins tested. With the Xenotest, which measures the effect of light on the colour of the vegetable tanning, the hybrid tannin was again among the best tannages tested. Conclusions of the research The objectives of the research have been met successfully. This was due to the fact that the leathers produced with the new hybrid extract had technical and characteristics which can be compared with those of chestnut extract, while the colour of the articles is remarkably lighter and more stable to light, which is a characteristic of a tara tannage.