New options for chrome-free leather

Abstract Dr Ramon Palop, Cromogenia-Units SA, Barcelona, discusses the demand for chrome-free leathers which is on the increase, particularly for car upholstery. This paper shows the results achieved with the optimisation of the application conditions of an aldehyde polymer (pH, time, concentration rate), assessing the physical (resistances) and chemical properties of the wet-white, particularly the free formol content. Cromogenia then selected the best retanning agent (resinic or vegetable) to achieve an chrome-free upholstery leather for cars. The following parameters were assessed in each case: * Whiteness degree (IUP-36) * Colour (L*) * Compactness (thickness) * Tear resistance (IUP-8) * Tensile strength (IUP-6) * Water drop absorption (IUP-7) * Grain produced after a standard tumbling An application process has been designed for car upholstery leather which meets all physico-chemical requirements and also enjoys excellent organoleptic properties. Introduction A wet-white pretannage technique has for a long time been used to reduce the chrome content in both the leather and wastewater. M Siegler¹ was one of the first authors to submit a paper on this subject. Since then many papers have been published^{(2,3,4,5,6,7)} on how to reduce the content of chrome in the effluent. Recently, the advantages that this process might have through chrome-free shavings were also assessed, since these shavings could be used as fertiliser or even as an animal feed due to the high nitrogen and digestible protein content. A wet-white product must have the following characteristics: * Storage stability * A simple and versatile process that allows wet-white to be chrome-retanned, if necessary, without modifying properties * Use of non-polluting, non- toxic and non-irritating chemicals (formol, phenol and other aldehydes) * Easy rewetting to avoid rehydrating difficulties if a part of the leather becomes dry during storage * Easy mechanical shaving * The finished items possess the physico-chemical characteristics required by the tanner Aim The target of this survey is to show how the use of chrome and other polluting chemicals can be reduced in order to meet the needs of the tanner. And, also how to meet the market's increasing demands for certain chrome-free items (such as those destined for car upholstery). Experimental Optimisation of the application conditions There is an ample bibliography about the use of aldehydes with a tanning capacity^{(12, 13, 14, 15)}, which include the following: formaldehyde, acetaldehyde, crotonaldehyde, acrolein, glutaraldehyde, glyoxal, methyl-glyoxal, starch aldehyde, oxazolidine, and a-hydroxyadipic aldehyde. Each one has its own chemical structure and own number of aldehyde groups. These factors give them different tanning and organoleptic capacities. Aldehydes react in contact with the basic groups of collagen that are not electrically charged⁽¹²⁾ and their combinations are strongly influenced by the pH value of the solution, the concentration rate and the time of contact with the skin. In the first part of this paper, we will consider the influence of the pH on the tanning capacity (Tc) of three different aldehydes, the tanning characteristics of which are well known, namely glutaraldehyde (40%), formaldehyde (40%), acetaldehyde (40%), and a new aldehyde polymer (40%) (A). This was the main aim of our work. The polymer (A) has two aldehyde groups, a chain length (R) containing ramified reactive groups, and a medium polymerisation degree (n) (Figure 1). As raw material, we used pickled hides at pH3, split at 3mm. The hides were trimmed to 20 x 20cm pieces. Two pieces were used for each variable and the application process was as shown in Table 1. Figure 2 shows that the glutaraldehyde at pH5 achieves a fairly high shrinkage temperature (80ºC) and rises to 84ºC when the pH is 7.5. At pH5, the formaldehyde reaches a shrinkage temperature of 72ºC, rising to 83ºC at pH7.5 with a complete penetration. The aldehyde polymer shows a slower but more uniform rising curve in the shrinkage temperature. It reaches 82ºC at pH7.5 with a thorough penetration. Finally, the acetaldehyde is the product that has a lower tanning capacity. It reaches a temperature of only 65ºC at pH7.5. Once the pH was fixed at an optimal 5.5, four different aldehyde polymer concentrations (1%, 2% 3% and 4%) were tested with 8 and 24 hour treatments (Figure 2). The application formulation was as seen in Table 2. Figure 3 shows that at a 1% concentration rate, one can already achieve shrinkage temperatures between 73-75%. As the rate increases, so does the shrinkage temperature. At 3-4% of aldehyde polymer, the temperature may reach 82-83ºC. Regarding treatment time, the shrinkage temperature becomes stabilised after 12 hours. For comparative studies in all subsequent applications, conditions will be fixed as follows: Final pH = 5.5 Aldehyde polymer rate = 3% Treatment time = 12 hours (in bath overnight) After the optimisation of the application process of the aldehyde polymer, we have to complete its tanning properties with a view to achieving the targets established for wet white characteristics (see previous page). To reach a good rewetting capacity, several fatliquors and surfactants were tested. The best result was obtained with the use of a sulfochlorinated paraffin (C), which has an excellent fixation at neutral pH and at the same time lubricates the fibres of the wet white. In order to provide a good shaving capacity to the skin, several types of resins and syntans were tested. The best result was obtained with the use of a phenol-free syntan (D). The complete wet-white process is set out in Table 3. In order to reduce the amount of free formol in the wet-white, two products were used: 1. Organic. Naphthalenesulfonic (J) (2%) 2. Inorganic. Sodium perborate (0.5%) Values obtained are shown in Figure 4 which proves that the treatment with the neutral salt of a naphthalenesulfonic derivative reduces free formol to values of 30ppm. A further formol oxidation treatment with sodium perborate reduces that value to 10ppm. The optimised process can be seen in Table 5, page 32. In order to check the penetration and fixation of the aldehyde polymer, Nessler reactive (Figure 5) which reacts with free aldehydic groups was used. Control 1 Without polymer, it gives no coloration Control 2 There is total penetration but a small quantity of aldehyde Control 3 There is total penetration and high quantity of aldehyde Control 4 There is total penetration, and the polymer starts to fix in the leather Control 5 Aldehyde polymer is fixed (pH5.5) and the colour intensity (brown) decreases (a small quantity of free aldehyde) Control 6 Aldehyde polymer is fixed (89%) so the colour intensity decreases more and raises the Ts up to 80ºC Control 7 All the aldehyde polymer is fixed and the free formaldehyde has been removed (it gives no colour to the reactive). In order to check the penetration of anionic products, methylene blue solution was used (Table 3) which reacts with syntan anionic groups Control 8 After two hours the naphthalene sulfonic was added on the surface. Control 9 After keeping in the bath overnight, both anionic products (naphthalene sulfonic and syntan D) are completely penetrated. Choice of the retanning agents In order to carry out comparative studies on the application of the aldehyde polymer together with other treatments, it is necessary to use a retanning and fatliquoring process that may produce an item meeting certain physico-chemical and organoleptic requirements. It was decided to work out a retanning and fatliquoring process that may enable the tanner to get a chrome-free leather suitable for car upholstery, since there is a great demand for such an item. Manufacturers of cars, boats and planes are increasingly using leather upholstery, not only owing to its better quality but also because its maintenance is cheaper than that of textile materials. However, this kind of leather has some very strict requirements that must be met regarding physicochemical and organoleptic factors, such as combustibility and toxicity in case of fire, which is closely connected with the quantity of chrome present in the leather. Vegetable retanning agents Using a wet-white made with the application process mentioned in the preceding paragraph and shaved down to 0.9 mm, a series of tests were carried out according to a standard formulation, but changing only the type of vegetable extract. The retanning process is given in Table 6. The following vegetable extracts were used: AC = Acid Chestnut SC = Sweet Chestnut M = Mimosa T = Tara Q = Quebracho Ga = Gambier Table 7 offers a summary of the properties enjoyed by the leathers retanned with each one of these vegetable extracts. The highest softness value is achieved by using tara, followed by mimosa. The lightest colour is given by quebracho, followed by mimosa. The longest time in water-drop absorption is provided by sweet chestnut, followed by tara. The highest tensile strength is given by quebracho, followed by tara and gambier. The best tear resistance is obtained with the use of gambier and tara. The finest and most regular grain is produced by tara, followed by mimosa, quebracho and gambier. The greatest thickness increase is provided by sweet chestnut, followed by mimosa and acid chestnut. Since the most important properties of car upholstery are softness and the fineness and uniformity of grain, the best extracts for these targets are mimosa and tara, which enjoy excellent physical and organoleptic properties. Resin retanning agents Just as in the preceding paragraph, a wet-white shaved to 0.9mm and a pH adjusted to 4.5 with formic acid was used. A standard retannage was applied with 9% mimosa and 9% tara as vegetable extracts. The following resins were used: Acrylic polymer (E) (medium molecular weight) Styrene-maleic resin (F) Melaminic resin (H) Acid-stable acrylic resin (I) (high molecular weight) Table 9 shows the properties that each resin lends to the leather. It can be noted that the highest softness degree is obtained with the use of the medium molecular weight polymer (E) and the styrene-maleic resin (F). Tensile strength and tear resistance, as well as grain uniformity are also quite good. Therefore, these were choosen as the most suitable retanning agents for our process. Comparative study with glutaraldehyde Existing surveys and literature^{(13, 14, 15, and 16)} concerning both the tanning and the retanning properties provided by the use of glutaraldehyde is quite extensive. It is the most widely used product for the manufacture of wet-white. Despite some of its disadvantages, such as itching, bad smell and possibly some sanitary problems, the organoleptic characteristics of the leather regarding softness and sponginess can hardly be matched. With a view to making a comparison between the use of an aldehyde polymer or a glutaraldehyde in a wet-white process, we took a salted hide weighing 32kg and submitted it to the following process: 1. Soaking 2. First liming Flesh - Divide (3.2mm) Pelt weight = 19kg 3. Second liming 4. Deliming-Bating Aldehyde polymer pH = 5.5 Ts = 80ºC Glutaraldehyde pH = 5.5 Ts = 82ºC 5. Sammying - Shaving down to 0.9mm 6. Retanning - dyeing/fatliquoring 7. Sammying - drying 8. Mechanical operations Table 10 shows the analytical values belonging to wastewater and the properties of the related wet-white produced with aldehyde polymer or glutaraldehyde. Both the bath and the leather resulting from the use of an aldehyde polymer have values similar to those obtained in the preceding section. Compared with glutaraldehyde, the most noticeable differences are those in whiteness degree (Wi), the value of which is 15 for the aldehyde polymer (ivory white) and -25 for the glutaraldehyde (yellowish). Light fastness is higher when the aldehyde polymer is used, while heat resistance is the same in both products. Both sides were retanned, dyed and fatliquored by means of the same process. Table 12 shows the properties of the sides produced in each process, as well as the requirements of car upholstery leathers. * Free formol is very low in both processes; requirements are lower than 10ppm. This reduction in free formol in comparison with the content in wet-white is mainly due to the strong tendency of formol to give products that are stable to vegetable tanning agents. * Ashes are 0.50 and 0.48 respectively, a rate which is below the minimum required * Colour (L*) is slightly lighter (49) in glutaraldehyde * Softness is quite similar in both processes (7.7 and 7.8) * Thickness is also similar These latter values represent the average of 42 measurements in each side. * Tear resistance, tensile strength and elongation are very similar and within the required para meters * Fogging (measured in its two parameters: gravimetric and reflectometric) are also very similar and within the required rate * Light fastness is lower than required in the leather treated with glutaraldehyde, while the aldehyde polymer just meets the requirements * Heat resistance meets the requirements in both sides * Inflammability, ie the capacity to become inflamed, is in both cases below the required minimum Summary 1. The application of an aldehyde polymer used as tanning material was optimised in the following conditions: pH = 5.5, rate of concentration = 3%, time = 12 hours. This process allows a shrinkage temperature of 80ºC 2. An organic formol eliminator (J) and an inorganic one (sodium perborate) were chosen. 3. The best vegetable retanning agents for car upholstery leather were mimosa and tara. 4. A mixture of acrylic polymer of medium molecular weight (E) and a styrene-maleic resin (F) were considered the best resin-retanning agents. 5. In the comparative study between a tannage with glutaraldehyde or with an aldehyde polymer, no significant differences appeared between the processes, except for light fastness, in which the aldehyde polymer is better.